← The Saudi aviation reference, in one place.
Aviation Maintenance Technician Handbook - General (FAA-H-8083-30B)
Reproduced for study. Always verify against the official GACAR at gaca.gov.sa.
Aviation Maintenance
Technician Handbook–General
U.S. Department of Transportation
FEDERAL AVIATION ADMINISTRATION
Flight Standards Service
iiThe Aviation Maintenance Technician Handbook–General (F AA-H-8083-30B) was developed as one of a series of three
handbooks for persons preparing for mechanic certification with airframe or powerplant ratings, or both. It is intended that
this handbook will provide basic information on principles, fundamentals, and technical procedures in the subject matter
areas common to both the airframe and powerplant ratings. Emphasis in this volume is on theory and methods of application.
The handbook is designed to aid students enrolled in a formal course of instruction preparing for FAA certification as a
maintenance technician as well as for current technicians who wish to improve their knowledge. This volume contains
information on mathematics, aircraft drawings, weight and balance, aircraft materials, processes and tools, physics, electricity,
inspection, ground operations, and FAA regulations governing the certification and work of maintenance technicians. New to
this volume is a section addressing how successful aviation maintenance technicians incorporate knowledge and awareness
of ethics, professionalism and human factors in the field.
Because there are so many different types of airframes and powerplants in use today, it is reasonable to expect that differences
exist in the components and systems of each. To avoid undue repetition, the practice of using representative systems and
units is implemented throughout the handbook. Subject matter treatment is from a generalized point of view, and should be
supplemented by reference to manufacturers’ manuals or other publications if more detail is desired. This handbook is not
intended to replace, substitute for, or supersede official regulations or the manufacturers’ instructions.
The companion handbooks to Aviation Maintenance Technician Handbook–General (F AA-H-8083-30B) are the Aviation
Maintenance Technician Handbook–Airframe (F AA-H-8083-31 (as amended)) , and the Aviation Maintenance Technician
Handbook–Powerplant (F AA-H-8083-32 (as amended)) .
This handbook is available for download, in pdf format, from www.faa.gov . Please visit this website for the latest version
of all FAA handbooks.
This handbook is published by the U.S. Department of Transportation, Federal Aviation Administration, Airman Testing
Standards Branch, AFS-630, P.O. Box 25082, Oklahoma City, OK 73125.
Comments regarding this publication should be emailed to afs630comments@faa.gov .
Preface
iiiThe Aviation Maintenance Technician Handbook–General (F AA-H-8083-30B) was produced by the Federal Aviation
Administration (FAA). The FAA wishes to acknowledge the following contributors:
• AERO Specialties, Inc. (www.aerospecialties.com) for images used in Chapter 1
• Dtom, Wikimedia Commons, for imagery used in Chapter 1
• Rama, Wikimedia Commons, Cc-by-sa-2.0-fr, for imagery used in Chapter 1
• ATA e-Business Program (www.ataebiz.org), for the ATA iSpec 2200 Standard Numbering System sample used in
Chapters 2, 6, and 10
• Avsoft International for images used in Chapter 4
• Genesys Aerosystems (www.genesys-aerosystems.com) for images used in Chapter 4
• Plustar (www.plustar.com) Fluid/Pneumatic Line Identification Tapes for images used in Chapter 4
• Burkhard Domke (http://www.b-domke.de/AviationImages.html) for imagery used in Chapter 5
• Adrian Pingstone for imagery used in Chapter 5
• Elisabeth Klimesch for imagery used in Chapter 5
• Jcmurphy, Wikimedia Commons, for imagery used in Chapter 5
• Paul Hamilton for imagery used in Chapter 5
• Larry Jackson with Jackson Aircraft Weighing Service (www.jawsscales.com) for images used in Chapter 6
• Paul New with Tennessee Aircraft Services, Inc. (www.tennesseeaircraft.net) for images used in Chapter 6
• Tysto, Wikimedia Commons, for imagery used in Chapter 6
• Immaculate Flight (www.immaculateflight.com) for images used in Chapter 8
• Jaypee, Wikimedia Commons, for imagery used in Chapter 8
• Graham Tool Co., Inc. (www.grahamtool.com) for images used in Chapter 9
• Permaswage (www.permaswage.com) for images used in Chapter 9
• Stride Tool, Inc. (www.stridetool.com) for images used in Chapter 9
• Winton Machine Company (www.wintonmachine.com) for images used in Chapter 9
• Buehler, a Division of Illinois Tool Works Inc. (www.buehler.com) for images used in Chapter 10
• Illinois Tool Works Inc., through its ITW Magnaflux Division for images used in Chapter 10
• Intertek (www.intertek.com) for images used in Chapter 10
• Mike Richman with Quality Digest (www.qualitydigest.com) for images used in Chapter 10
• Olympus (www.olympus-ims.com) for images used in Chapter 10
• The Transportation Safety Board of Canada (TSB) for images used in Chapter 10
• Victor Sloan with Victor Aviation (www.victor-aviation.com) for images used in Chapter 10
• SNAP-ON® Incorporated (store.snapon.com) for images used in Chapter 11
• The L.S. Starrett Company (www.starrett.com) for images used in Chapter 11
• Alabama Aviation College, A Unit of Enterprise State for imagery used in Chapter 12
• Binarysequence, Wikimedia Commons, for imagery used in Chapter 12
• Siglent Technologies Co., Ltd. For imagery used in Chapter 12
Acknowledgments
ivChapter 1
Safety, Ground Operations, & Servicing ........... 1-1
Shop Safety ........................................................... 1-1
Electrical Safety ................................................. 1-1
Physiological Safety ........................................ 1-1
Fire Safety ....................................................... 1-1
Safety Around Compressed Gases ................... 1-2
Safety Around Hazardous Materials .................. 1-2
Safety Around Machine Tools ............................. 1-2
Flight Line Safety ................................................... 1-4
Hearing Protection ............................................. 1-4
Foreign Object Damage (FOD) .......................... 1-4
Safety Around Airplanes ..................................... 1-4
Safety Around Helicopters .................................. 1-4
Fire Safety .......................................................... 1-5
Fire Protection ....................................................... 1-5
Requirements for Fire to Occur .......................... 1-5
Classification of Fires ......................................... 1-5
Types and Operation of Shop and Flight Line
Fire Extinguishers .............................................. 1-5
Inspection of Fire Extinguishers ......................... 1-7
Identifying Fire Extinguishers ............................. 1-8
Using Fire Extinguishers .................................... 1-8
Tie-Down Procedures ............................................ 1-8
Preparation of Aircraft ........................................ 1-8
Tie-Down Procedures for Land Planes .............. 1-8
Securing Light Aircraft ..................................... 1-8
Securing Heavy Aircraft .................................. 1-8
Tie-Down Procedures for Seaplanes ................. 1-8
Tie-Down Procedures for Ski Planes ................ 1-9
Tie-Down Procedures for Helicopters .............. 1-10
Procedures for Securing Weight-Shift-Control ..1-11
Procedures for Securing Powered Parachutes .1-11
Ground Movement of Aircraft ................................ 1-11
Engine Starting and Operation .......................... 1-11
Reciprocating Engines ...................................... 1-11
Hand Cranking Engines ................................... 1-13
Extinguishing Engine Fires ............................... 1-14
Turboprop Engines ........................................... 1-14
Turboprop Starting Procedures ........................ 1-15
Turbofan Engines ............................................. 1-15
Starting a Turbofan Engine .............................. 1-15
Auxiliary Power Units (APUs) ........................... 1-16
Unsatisfactory Turbine Engine Starts .............. 1-17
Hot Start ........................................................ 1-17False or Hung Start ....................................... 1-17
Engine Fails to Start ...................................... 1-17
Towing of Aircraft .............................................. 1-17
Taxiing Aircraft .................................................. 1-19
Taxi Signals ................................................... 1-20
Servicing Aircraft .................................................. 1-20
Servicing Aircraft Air/Nitrogen Oil & Fluids ....... 1-20
Ground Support Equipment ............................ 1-21
Electric Ground Power Units ......................... 1-21
Hydraulic Ground Power Units ...................... 1-22
Ground Support Air Units .............................. 1-23
Ground Air Heating and Air Conditioning .....1-24
Oxygen Servicing Equipment ........................... 1-24
Oxygen Hazards .............................................. 1-25
Fuel Servicing of Aircraft ...................................... 1-25
Types of Fuel and Identification ....................... 1-25
Contamination Control ..................................... 1-25
Fueling Hazards .............................................. 1-26
Fueling Procedures .......................................... 1-26
Defueling .......................................................... 1-28
Chapter 2
Regulations, Maintenance Forms, Records, &
Publications ......................................................... 2-1
Overview — Title 14 of the Code of Federal
Regulations (14 CFR) ............................................ 2-1
Title 14 CFR Part 3—General Requirements .....2-1
Maintenance-Related Regulations ..................... 2-3
14 CFR Part 1—Definitions and
Abbreviations ................................................. 2-3
14 CFR Part 21—Certification Procedures
for Products and Articles ................................. 2-3
14 CFR Part 23—Airworthiness Standards:
Normal, Utility, Acrobatic, and Commuter
Category Airplanes .......................................... 2-4
14 CFR Part 25—Airworthiness Standards:
Transport Category Airplanes ......................... 2-5
14 CFR Part 27—Airworthiness Standards:
Normal Category Rotorcraft ............................ 2-5
14 CFR Part 29—Airworthiness Standards:
Transport Category Rotorcraft ....................... 2-5
14 CFR Part 33—Airworthiness Standards:
Aircraft Engines ............................................... 2-8
14 CFR Part 35—Airworthiness Standards:
Propellers ........................................................ 2-8
Table of Contents
v14 CFR Part 39—Airworthiness Directives .....2-8
14 CFR Part 43—Maintenance, Preventive
Maintenance, Rebuilding, and Alteration ........ 2-8
14 CFR Part 45—Identification and
Registration Marking ....................................... 2-8
14 CFR Part 47—Aircraft Registration .......... 2-10
14 CFR Part 65—Certification: Airmen
Other Than Flight Crewmembers .................. 2-10
14 CFR Part 91—General Operating and
Flight Rules ................................................... 2-10
14 CFR Part 119—Certification: Air
Carriers and Commercial Operators ............. 2-10
14 CFR Part 121—Operating
Requirements: Domestic, Flag, and
Supplemental Operations ............................. 2-11
14 CFR Part 125—Certification and
Operations: Airplanes Having a Seating
Capacity of 20 or More Passengers or
a Maximum Payload Capacity of 6,000
Pounds or More; and Rules Governing
Persons on Board Such Aircraft ................... 2-12
14 CFR Part 135—Operating
Requirements: Commuter and On-Demand
Operations and Rules Governing Persons
on Board Such Aircraft ................................. 2-12
14 CFR Part 145—Repair Stations ............... 2-13
14 CFR Part 147—Aviation Maintenance
Technician Schools ....................................... 2-13
14 CFR Part 183—Representatives of the
Administrator ................................................. 2-13
Explanation of Primary Regulations (Parts 43
and 91) ................................................................ 2-14
14 CFR Part 43—Maintenance, Preventative
Maintenance Rebuilding, and Alteration .......... 2-14
Section 43.1—Applicability ........................... 2-14
Section 43.2—Records of Overhaul and
Rebuilding ..................................................... 2-14
Section 43.3—Persons authorized
to perform maintenance, preventive
maintenance, rebuilding, and alterations ......2-14
Section 43.5—Approval for return to
service after maintenance, preventive
maintenance, rebuilding, and alterations ......2-15
Section 43.7—Persons authorized to
approve aircraft, airframes, aircraft
engines, propellers, appliances, or
component parts for return to service after
maintenance, preventive maintenance,
rebuilding, or alteration ................................. 2-16
Section 43.9—Content, form and
disposition of maintenance, preventive
maintenance, rebuilding, and alteration records (except inspection performed in
accordance with parts 91 and 125, and
sections 135.411(a)(1) and 135.419 of this
chapter) ......................................................... 2-16
Section 43.10—Disposition of Life-Limited
Aircraft Parts ................................................. 2-16
Section 43.11—Content, form, and
disposition of records for inspections
conducted under parts 91 and 125, and
sections 135.411(a)(1) and 135.419 of this
chapter .......................................................... 2-17
Section 43.12—Maintenance Records:
Falsification, Reproduction, or Alteration ......2-17
Section 43.13—Performance Rules
(General) ....................................................... 2-17
Section 43.15 —Additional Performance
Rules for Inspections .................................... 2-18
Section 43.16 —Airworthiness Limitations ....2-18
Section 43.17—Maintenance, preventive
maintenance, or alterations performed
on U.S. aeronautical products by certain
Canadian persons ......................................... 2-18
Appendix A—Major Alterations, Major
Repairs, and Preventive Maintenance .......... 2-18
Appendix B—Recording of Major Repairs
and Major Alterations .................................... 2-20
Appendix C—(Reserved) .............................. 2-20
Appendix D—Scope and Detail of Items
To Be Included in Annual and 100-Hour
Inspections .................................................... 2-20
Appendix E—Altimeter System Test and
Inspection ...................................................... 2-20
Appendix F—ATC Transponder Tests and
Inspections .................................................... 2-20
14 CFR Part 91—General Operating and
Flight Rules ...................................................... 2-21
Subpart A—General ..................................... 2-21
Subpart E—Maintenance, Preventive
Maintenance, and Alterations ........................ 2-21
Civil Air Regulations (CAR) .................................. 2-23
CAR 3—Airplane Airworthiness—Normal,
Utility, Aerobatic, and Restricted Purpose
Categories ........................................................ 2-23
CAR 4a—Airplane Airworthiness ..................... 2-23
Suspected Unapproved Parts (SUP) ................... 2-24
Other FAA Documents ......................................... 2-24
Advisory Circulars (AC) .................................... 2-24
The AC Numbering System .......................... 2-26
Types of Airworthiness Directives (AD) ......... 2-26
AD Content ................................................... 2-26
AD Number ................................................... 2-26
viApplicability and Compliance ........................ 2-27
Alternative Method of Compliance ................ 2-27
Special Airworthiness Information Bulletin
(SAIB) ............................................................... 2-27
Aircraft Specifications ....................................... 2-27
Supplemental Type Certificates (STC) ............. 2-27
Type Certificate Data Sheets (TCDS) .............. 2-30
FAA Handbooks & Manuals ................................. 2-30
Non-FAA Documents ........................................... 2-30
Air Transport Association ATA iSpec 2200 ...... 2-30
Manufacturers’ Published Data ........................ 2-30
Airworthiness Limitations ................................. 2-33
Service Bulletins (SB) ...................................... 2-33
Structural Repair Manual (SRM) ...................... 2-33
Forms .................................................................. 2-33
Airworthiness Certificates ................................. 2-33
Aircraft Registration .......................................... 2-37
Radio Station License ..................................... 2-38
FAA Form 337—Major Repair and Alteration ..2-38
Records ............................................................... 2-39
Making Maintenance Record Entries ............... 2-39
Temporary Records—14 CFR Part 91
Section 91.417(a)(1) and (b)(1) ........................ 2-39
Permanent Records—14 CFR Part 91,
Section 91.417(a)(2) and (b)(2) ........................ 2-39
Electronic Records .......................................... 2-39
Light Sport Aircraft (LSA) .................................... 2-40
Maintenance ..................................................... 2-40
Aircraft Maintenance Manual (AMM) ................ 2-44
Line Maintenance, Repairs, & Alterations ........ 2-45
Major Repairs & Alterations .............................. 2-45
Chapter 3
Mathematics in Aviation Maintenance ............... 3-1
Introduction ............................................................ 3-1
Whole Numbers ..................................................... 3-1
Addition of Whole Numbers ............................... 3-1
Subtraction of Whole Numbers .......................... 3-1
Division of Whole Numbers ................................ 3-1
Fractions ................................................................ 3-2
Finding the Least Common Denominator ......... 3-2
Addition of Fractions .......................................... 3-2
Subtraction of Fractions ..................................... 3-2
Multiplication of Fractions ................................... 3-3
Division of Fractions ........................................... 3-3
Reducing Fractions ............................................ 3-3
Mixed Numbers ..................................................... 3-3
Addition of Mixed Numbers ................................ 3-4
Subtraction of Mixed Numbers ........................... 3-4
The Decimal Number System ................................ 3-4
Origin and Definition .......................................... 3-4Addition of Decimal Numbers ............................. 3-4
Subtraction of Decimal Numbers ....................... 3-4
Multiplication of Decimal Numbers ..................... 3-5
Division of Decimal Numbers ............................. 3-6
Rounding Off Decimal Numbers ......................... 3-6
Converting Decimal Numbers to Fractions ........ 3-6
Converting Fractions to Decimals ...................... 3-7
Decimal Equivalent Chart ................................... 3-7
Ratio ...................................................................... 3-7
Aviation Applications .......................................... 3-7
Proportion .............................................................. 3-8
Extremes and Means ......................................... 3-8
Solving Proportions ............................................ 3-8
Percentage .......................................................... 3-10
Expressing a Decimal Number as a
Percentage ....................................................... 3-10
Expressing a Percentage as a Decimal
Number ............................................................ 3-10
Expressing a Fraction as a Percentage ........... 3-10
Finding a Percentage of a Given Number ....... 3-10
Finding What Percentage One Number is of
Another ............................................................. 3-10
Finding a Number When a Percentage of it is
Known ............................................................... 3-11
Positive & Negative Numbers (Signed Numbers) 3-11
Addition of Positive & Negative Numbers ......... 3-11
Subtraction of Positive & Negative Numbers ....3-11
Multiplication of Positive & Negative Numbers ..3-11
Powers .................................................................. 3-11
Special Powers ............................................... 3-12
Squared ....................................................... 3-12
Cubed ........................................................... 3-12
Power of Zero ............................................... 3-12
Law of Exponents ............................................. 3-12
Powers of Ten ................................................... 3-12
Roots ................................................................ 3-12
Square Roots ................................................... 3-12
Cube Roots ...................................................... 3-13
Fractional Powers ............................................ 3-13
Functions of Numbers Chart ................................ 3-13
Scientific Notation ................................................ 3-13
Converting Numbers from Standard Notation
to Scientific Notation ........................................ 3-13
Converting Numbers from Scientific Notation
to Standard Notation ........................................ 3-16
Addition, Subtraction, Multiplication, and
Division of Scientific Numbers .......................... 3-16
Algebra ................................................................ 3-16
Equations ......................................................... 3-16
Algebraic Rules ................................................ 3-16
Solving for a Variable ....................................... 3-16
viiUse of Parentheses .......................................... 3-17
Order of Operation ........................................... 3-17
Order of Operation for Algebraic Equations .....3-18
Computing Area of Two-Dimensional Solids ....... 3-18
Rectangle ......................................................... 3-18
Square .............................................................. 3-18
Triangle ............................................................ 3-18
Parallelogram ................................................... 3-18
Trapezoid ......................................................... 3-18
Circle ................................................................ 3-19
Ellipse ............................................................... 3-19
Units of Area ..................................................... 3-20
Computing Volume of Three-Dimensional Solids 3-20
Rectangular Solid ............................................. 3-20
Cube ................................................................. 3-21
Cylinder ............................................................ 3-21
Sphere .............................................................. 3-22
Cone ................................................................. 3-23
Units of Volume ................................................ 3-23
Computing Surface Area of Three-Dimensional
Solids ................................................................... 3-23
Rectangular Solid ............................................. 3-24
Cube ................................................................. 3-24
Cylinder ............................................................ 3-24
Sphere .............................................................. 3-24
Cone ................................................................ 3-24
Trigonometric Functions ...................................... 3-24
Right Triangle, Sides, and Angles .................... 3-24
Sine, Cosine, and Tangent ............................... 3-25
Calculator Method: ........................................ 3-25
Trigonometry Table Method: ......................... 3-25
Pythagorean Theorem ...................................... 3-25
Measurement Systems ........................................ 3-26
Conventional (U.S. or English) System ............ 3-26
Metric System .................................................. 3-26
Measurement Systems & Conversions ............ 3-26
The Binary Number System ................................ 3-27
Place Values ..................................................... 3-27
Converting Binary Numbers to Decimal
Numbers ........................................................... 3-27
Converting Decimal Numbers to Binary
Numbers .......................................................... 3-27
Chapter 4
Aircraft Drawings ................................................. 4-1
Introduction ............................................................ 4-1
Computer Graphics ............................................... 4-1
Purpose & Function of Aircraft Drawings ............... 4-1
Care & Use of Drawings ........................................ 4-2
Types of Drawings ................................................. 4-2
Detail Drawing .................................................... 4-2Assembly Drawing ............................................. 4-2
Installation Drawing ............................................ 4-2
Sectional View Drawings .................................... 4-2
Full Section ..................................................... 4-2
Half Section ..................................................... 4-2
Revolved Section ............................................ 4-2
Removed Section ............................................ 4-3
Title Blocks ............................................................ 4-3
Drawing or Print Numbers .................................. 4-3
Reference and Dash Numbers ........................... 4-3
Universal Numbering System ................................ 4-3
Drawing Standards ................................................ 4-7
Bill of Material ........................................................ 4-7
Other Drawing Data ............................................... 4-8
Revision Block .................................................... 4-8
Notes .................................................................. 4-8
Zone Numbers ................................................... 4-8
Station Numbers & Location Identification on
Aircraft ................................................................ 4-8
Allowances & Tolerances ................................... 4-9
Finish Marks ....................................................... 4-9
Scale .................................................................. 4-9
Application .......................................................... 4-9
Methods of Illustration ........................................... 4-9
Applied Geometry .............................................. 4-9
Orthographic Projection Drawings .................. 4-9
Pictorial Drawings .......................................... 4-11
Diagrams ........................................................ 4-11
Flowcharts ..................................................... 4-13
Lines and Their Meanings ................................... 4-16
Centerlines ....................................................... 4-16
Dimension Lines ............................................... 4-17
Extension Lines ................................................ 4-17
Sectioning Lines ............................................... 4-17
Phantom Lines ................................................. 4-17
Break Lines ...................................................... 4-18
Leader Lines .................................................... 4-18
Hidden Lines .................................................... 4-18
Outline or Visible Lines ..................................... 4-18
Stitch Lines ....................................................... 4-18
Cutting Plane and Viewing Plane Lines ........... 4-19
Drawing Symbols ................................................. 4-19
Material Symbols .............................................. 4-19
Shape Symbols ................................................ 4-19
Electrical Symbols ............................................ 4-19
Reading and Interpreting Drawings ..................... 4-19
Drawing Sketches ................................................ 4-22
Sketching Techniques ...................................... 4-22
Basic Shapes ................................................... 4-22
Repair Sketches ............................................... 4-22
Care of Drafting Instruments ............................... 4-22
viiiGraphs & Charts .................................................. 4-23
Reading & Interpreting Graphs & Charts ......... 4-23
Nomograms ...................................................... 4-23
Microfilm & Microfiche ......................................... 4-23
Digital Images ...................................................... 4-23
Chapter 5
Physics for Aviation ............................................ 5-1
Matter ................................................................... 5-1
Characteristics of Matter ................................... 5-1
Mass & Weight ................................................ 5-1
Attraction ......................................................... 5-1
Porosity ........................................................... 5-1
Impenetrability ................................................. 5-1
Density ........................................................... 5-2
Specific Gravity .............................................. 5-2
Energy ................................................................... 5-2
Potential Energy ................................................. 5-2
Kinetic Energy .................................................... 5-3
Force, Work, Power, & Torque ............................... 5-4
Force .................................................................. 5-4
Work ................................................................... 5-4
Friction & Work ................................................... 5-4
Static Friction .................................................. 5-5
Sliding Friction ................................................ 5-5
Rolling Friction ................................................ 5-6
Power ................................................................. 5-6
Torque ................................................................ 5-6
Simple Machines ................................................... 5-7
Mechanical Advantage of Machines ................... 5-7
The Lever ........................................................... 5-8
First Class Lever ............................................. 5-8
Second Class Lever ........................................ 5-9
Third Class Lever ............................................ 5-9
The Pulley .......................................................... 5-9
Single Fixed Pulley ......................................... 5-9
Single Movable Pulley ..................................... 5-9
Block and Tackle ........................................... 5-10
The Gear .......................................................... 5-10
Inclined Plane .................................................... 5-11
Stress .................................................................. 5-12
Tension ............................................................. 5-12
Compression .................................................... 5-13
Torsion .............................................................. 5-13
Bending ............................................................ 5-13
Shear ................................................................ 5-13
Strain ................................................................ 5-14
Motion .................................................................. 5-14
Uniform Motion ................................................. 5-14
Speed and Velocity .......................................... 5-14Acceleration ..................................................... 5-15
Newton’s Law of Motion ................................... 5-16
First Law ....................................................... 5-16
Second Law .................................................. 5-16
Third Law ...................................................... 5-17
Circular Motion ................................................. 5-17
Heat ..................................................................... 5-18
Heat Energy Units ............................................ 5-18
Heat Energy and Thermal Efficiency ................ 5-19
Heat Transfer ................................................... 5-19
Conduction .................................................... 5-19
Convection .................................................... 5-20
Radiation ....................................................... 5-20
Specific Heat .................................................... 5-21
Temperature ..................................................... 5-21
Thermal Expansion/Contraction ....................... 5-22
Pressure .............................................................. 5-22
Gauge Pressure ............................................... 5-23
Absolute Pressure ............................................ 5-23
Differential Pressure ......................................... 5-23
Gas Laws ............................................................. 5-23
Boyle’s Law ...................................................... 5-24
Charles’ Law ..................................................... 5-25
General Gas Law ............................................. 5-25
Dalton’s Law ..................................................... 5-25
Fluid Mechanics ................................................... 5-26
Buoyancy ......................................................... 5-26
Fluid Pressure .................................................. 5-27
Pascal’s Law .................................................... 5-27
Bernoulli’s Principle .......................................... 5-29
Sound .................................................................. 5-30
Wave Motion .................................................... 5-30
Speed of Sound ............................................... 5-31
Mach Number ................................................... 5-31
Frequency of Sound ......................................... 5-31
Loudness .......................................................... 5-32
Measurement of Sound Intensity ..................... 5-32
Doppler Effect ................................................... 5-32
Resonance ....................................................... 5-32
The Atmosphere .................................................. 5-32
Composition of the Atmosphere ....................... 5-33
Atmospheric Pressure ...................................... 5-34
Atmospheric Density ........................................ 5-34
Water Content of the Atmosphere .................... 5-34
Absolute Humidity ......................................... 5-35
Relative Humidity .......................................... 5-35
Dew Point ...................................................... 5-36
Vapor Pressure ............................................. 5-36
Standard Atmosphere ...................................... 5-36
Aircraft Theory of Flight ....................................... 5-36
ixFour Forces of Flight ........................................ 5-36
Bernoulli’s Principle and Subsonic Flow ........... 5-37
Lift and Newton’s Third Law ............................. 5-37
Airfoils .............................................................. 5-38
Camber ......................................................... 5-38
Chord Line ..................................................... 5-38
Relative Wind ................................................ 5-38
Angle of Attack ............................................... 5-38
Boundary Layer Airflow ................................... 5-39
Boundary Layer Control ................................ 5-39
Wingtip Vortices ............................................... 5-40
Axes of an Aircraft ............................................ 5-40
Aircraft Stability ............................................... 5-40
Static Stability ............................................... 5-40
Dynamic Stability .......................................... 5-40
Longitudinal Stability ..................................... 5-41
Lateral Stability ............................................. 5-42
Directional Stability ....................................... 5-42
Dutch Roll ..................................................... 5-42
Flight Control Surfaces ..................................... 5-42
Flight Controls & the Lateral Axis .................. 5-42
Flight Controls and the Longitudinal Axis ......5-43
Flight Controls and the Vertical Axis ............. 5-43
Tabs .............................................................. 5-44
Supplemental Lift-Modifying Devices ............ 5-45
High-Speed Aerodynamics ............................... 5-46
Compressibility Effects .................................. 5-46
The Speed of Sound ..................................... 5-46
Subsonic, Transonic, and Supersonic Flight .5-47
Shock Waves ................................................ 5-47
High-Speed Airfoils ....................................... 5-48
Aerodynamic Heating .................................... 5-49
Helicopter Aerodynamics ................................. 5-49
Helicopter Structures and Airfoils .................. 5-49
Helicopter Axes of Flight ............................... 5-52
Helicopters in Flight ...................................... 5-54
Weight-Shift Control, Flexible Wing Aircraft
Aerodynamics .................................................. 5-56
Powered Parachute Aerodynamics .................. 5-58
Chapter 6
Aircraft Weight & Balance .................................. 6-1
Introduction ............................................................ 6-1
Requirements for Aircraft Weighing ....................... 6-1
Weight & Balance Terminology .............................. 6-2
Datum ................................................................. 6-2
Arm ..................................................................... 6-2
Moment .............................................................. 6-2
Center of Gravity (CG) ....................................... 6-3Maximum Weight ................................................ 6-3
Empty Weight ..................................................... 6-4
Empty Weight Center of Gravity (EWCG) .......... 6-4
Useful Load ........................................................ 6-4
Minimum Fuel ..................................................... 6-4
Tare Weight ........................................................ 6-5
Procedures for Weighing an Aircraft ...................... 6-5
General Concepts .............................................. 6-5
Weight and Balance Data ................................... 6-6
Manufacturer-Furnished Information .................. 6-6
Weight and Balance Equipment ........................... 6-7
Scales ................................................................ 6-7
Spirit Level .................................................... 6-11
Hydrometer ................................................... 6-12
Preparing an Aircraft for Weighing ................... 6-12
Fuel System .................................................. 6-13
Oil System ..................................................... 6-13
Miscellaneous Fluids ..................................... 6-14
Flight Controls ............................................... 6-14
Other Considerations .................................... 6-14
Weighing Points ............................................ 6-14
Jacking the Aircraft ........................................... 6-14
Leveling the Aircraft .......................................... 6-14
Safety Considerations ...................................... 6-15
CG Range ........................................................ 6-15
Empty Weight Center of Gravity (EWCG)
Range ........................................................... 6-15
Operating CG Range .................................... 6-16
Standard Weights Used for Aircraft Weight
and Balance ..................................................... 6-16
Example Weighing of an Airplane .................... 6-16
EWCG Formulas .............................................. 6-16
Datum Forward of the Airplane–Nosewheel
Landing Gear ................................................... 6-16
Datum Aft of the Main Wheels–Nosewheel
Landing Gear ................................................... 6-17
Location of Datum ............................................ 6-17
Datum Forward of the Main Wheels–Tail
Wheel Landing Gear ........................................ 6-17
Loading an Aircraft for Flight ................................ 6-19
Example Loading of an Airplane ................... 6-19
Adverse-Loaded CG Checks ........................... 6-20
Example Forward & Aft Adverse-Loaded CG
Checks ............................................................. 6-20
Equipment Change & Aircraft Alteration .............. 6-21
Example Calculation After an Equipment
Change ......................................................... 6-21
Use of Ballast ................................................... 6-22
Temporary Ballast ......................................... 6-22
Permanent Ballast ......................................... 6-23
Loading Graphs & CG Envelopes ....................... 6-24
xHelicopter Weight & Balance ............................... 6-25
General Concepts ............................................ 6-25
Helicopter Weighing ......................................... 6-25
Weight and Balance—Weight-Shift Control
Aircraft and Powered Parachutes ........................ 6-26
Weight-Shift Control Aircraft ............................. 6-27
Powered Parachutes ........................................ 6-28
Built-In Electronic Weighing ............................. 6-28
Mean Aerodynamic Chord ................................ 6-28
Weight & Balance Records .................................. 6-31
Chapter 7
Aircraft Materials, Hardware, & Processes ......7-1
Properties of Metals ........................................... 7-1
Hardness ......................................................... 7-1
Strength .......................................................... 7-1
Density ............................................................ 7-1
Malleability ...................................................... 7-1
Ductility ........................................................... 7-1
Elasticity .......................................................... 7-1
Toughness ....................................................... 7-1
Brittleness ....................................................... 7-1
Fusibility .......................................................... 7-2
Conductivity .................................................... 7-2
Thermal Expansion ......................................... 7-2
Ferrous Aircraft Metals ....................................... 7-2
Iron .................................................................. 7-2
Steel and Steel Alloys ..................................... 7-2
Electrochemical Test .......................................... 7-4
Nonferrous Aircraft Metals .................................. 7-5
Aluminum & Aluminum Alloys ......................... 7-5
Wrought Aluminum .......................................... 7-7
Effect of Alloying Element ............................... 7-7
Hardness Identification ................................... 7-7
Magnesium & Magnesium Alloys .................... 7-8
Titanium and Titanium Alloys .......................... 7-9
Copper and Copper Alloys ............................ 7-10
Nickel & Nickel Alloys ..................................... 7-11
Substitution of Aircraft Metals ............................ 7-11
Metalworking Processes .................................. 7-12
Hot-Working .................................................. 7-12
Internal Structure of Metals ........................... 7-14
Heat-Treating Equipment .............................. 7-14
Heating .......................................................... 7-15
Soaking ......................................................... 7-15
Cooling .......................................................... 7-16
Quenching Media .......................................... 7-16
Quenching Equipment .................................. 7-16
Heat-Treatment of Ferrous Metals ................... 7-16Behavior of Steel During Heating & Cooling .7-16
Hardening ..................................................... 7-17
Hardening Precautions ................................. 7-19
Tempering ..................................................... 7-19
Annealing ...................................................... 7-19
Normalizing ................................................... 7-19
Case Hardening ............................................ 7-20
Heat-Treatment of Nonferrous Metals .............. 7-20
Aluminum Alloys ............................................ 7-20
Alclad Aluminum ........................................... 7-21
Solution Heat-Treatment ............................... 7-21
Quenching ..................................................... 7-22
Lag Between Soaking & Quenching ............. 7-22
Reheat-Treatment ......................................... 7-22
Straightening After Solution Heat-Treatment ....7-22
Precipitation Heat-Treating ............................... 7-22
Precipitation Practices .................................. 7-23
Annealing of Aluminum Alloys .......................... 7-23
Heat-Treatment of Aluminum Alloy Rivets ........ 7-24
Heat-Treatment of Magnesium Alloys .............. 7-24
Solution Heat-Treatment ............................... 7-24
Precipitation Heat-Treatment ........................ 7-24
Heat-Treatment of Titanium .............................. 7-25
Stress Relieving ............................................ 7-25
Full Annealing ............................................... 7-25
Thermal Hardening ....................................... 7-25
Case Hardening ............................................ 7-25
Hardness Testing .............................................. 7-25
Brinell Tester ................................................. 7-26
Rockwell Tester ............................................. 7-26
Barcol Tester ................................................. 7-27
Forging ............................................................. 7-27
Casting ............................................................. 7-28
Extruding .......................................................... 7-28
Cold-Working/Hardening .................................. 7-29
Nonmetallic Aircraft Materials .............................. 7-29
Wood ................................................................ 7-29
Plastics ............................................................. 7-29
Transparent Plastics ......................................... 7-29
Composite Materials ........................................ 7-30
Advantages/Disadvantages of Composites ..7-30
Composite Safety .......................................... 7-31
Fiber Reinforced Materials ............................ 7-31
Laminated Structures .................................... 7-31
Reinforced Plastic ......................................... 7-31
Rubber ............................................................. 7-32
Natural Rubber .............................................. 7-32
Synthetic Rubber .......................................... 7-32
Shock Absorber Cord ....................................... 7-33
xiSeals .................................................................... 7-33
Packings ........................................................... 7-34
O-Ring Packings ........................................... 7-34
V-Ring Packings ............................................ 7-35
U-Ring Packings ........................................... 7-35
Gaskets ............................................................ 7-35
Wipers .............................................................. 7-35
Sealing Compounds ......................................... 7-35
One Part Sealants ......................................... 7-36
Two Part Sealants ......................................... 7-36
Aircraft Hardware ................................................. 7-36
Identification ..................................................... 7-37
Threaded Fasteners ......................................... 7-37
Classification of Threads ............................... 7-37
Aircraft Bolts ..................................................... 7-37
General Purpose Bolts .................................. 7-37
Close Tolerance Bolts ................................... 7-38
Internal Wrenching Bolts ............................... 7-38
Identification and Coding .............................. 7-38
Special-Purpose Bolts ................................... 7-39
Aircraft Nuts ..................................................... 7-41
Non-Self-Locking Nuts .................................. 7-41
Self-Locking Nuts .......................................... 7-42
Sheet Spring Nuts ......................................... 7-45
Internal & External Wrenching Nuts .............. 7-45
Identification & Coding .................................. 7-45
Aircraft Washers ............................................... 7-46
Plain Washers ............................................... 7-46
Lock Washers ............................................... 7-46
Special Washers ........................................... 7-47
Installation of Nuts, Washers, & Bolts .............. 7-47
Bolt & Hole Sizes .......................................... 7-47
Installation Practices ..................................... 7-48
Safetying of Bolts & Nuts ................................. 7-48
Repair of Damaged Internal Threads ............ 7-48
Fastener Torque ............................................... 7-49
Torque ........................................................... 7-49
Torque Wrenches ......................................... 7-49
Torque Tables ................................................ 7-50
Cotter Pin Hole Line Up ................................ 7-50
Aircraft Rivets ................................................... 7-51
Standards and Specifications ....................... 7-51
Solid Shank Rivets ........................................ 7-52
Identification .................................................. 7-54
Blind Rivets ................................................... 7-56
Mechanically-Expanded Rivets ........................ 7-57
Self-Plugging Rivets (Friction Lock) .............. 7-57
Pull-Thru Rivets ............................................ 7-57
Self-Plugging Rivets (Mechanical Lock) ....... 7-58Material ......................................................... 7-58
Head Styles ................................................... 7-59
Diameters ...................................................... 7-59
Grip Length ................................................... 7-59
Rivet Identification ......................................... 7-59
Special Shear and Bearing Load Fasteners ....7-60
Pin Rivets ...................................................... 7-60
Taper-Lok ...................................................... 7-61
HI-LOK™ Fastening System ......................... 7-61
HI-TIGUE™ Fastening System ..................... 7-63
HI-LITE™ Fastening System ........................ 7-64
Captive Fasteners ......................................... 7-64
Turn Lock Fasteners ..................................... 7-64
Dzus Fasteners ............................................. 7-64
Camloc Fasteners ......................................... 7-65
Airloc Fasteners ............................................ 7-66
Screws ............................................................. 7-66
Structural Screws .......................................... 7-66
Machine Screws ............................................ 7-67
Self-Tapping Screws ..................................... 7-67
Identification & Coding for Screws ................ 7-67
Riveted & Rivetless Nut Plates ........................ 7-68
Nut Plates ..................................................... 7-68
Rivnuts .......................................................... 7-68
Dill Lok-Skrus and Dill Lok-Rivets ................. 7-69
Deutsch Rivets .............................................. 7-70
Sealing Nut Plates ........................................ 7-70
Hole Repair & Hole Repair Hardware .............. 7-70
Repair of Damaged Holes with Acres
Fastener Sleeves .......................................... 7-71
Control Cables & Terminals ........................... 7-72
Push-Pull Tube Linkage ................................ 7-72
Safetying Methods ........................................... 7-75
Pins ............................................................... 7-75
Safety Wiring .................................................... 7-77
Nuts, Bolts, & Screws ................................... 7-77
Oil Caps, Drain Cocks, & Valves ................... 7-77
Electrical Connectors .................................... 7-79
Turnbuckles ................................................... 7-79
General Safety Wiring Rules ............................ 7-80
Cotter Pin Safetying ...................................... 7-80
Snap Rings ................................................... 7-80
Chapter 8
Cleaning & Corrosion Control ............................ 8-1
Corrosion ............................................................... 8-1
Factors Affecting Corrosion ................................ 8-1
Pure Metals ..................................................... 8-1
Climate ............................................................ 8-2
xiiGeographical Location .................................... 8-2
Foreign Material .............................................. 8-2
Micro-organisms ............................................. 8-2
Manufacturing Processes ............................... 8-2
Types of Corrosion ............................................. 8-2
Direct Chemical Attack .................................... 8-2
Electrochemical Attack .................................... 8-3
Forms of Corrosion ............................................ 8-5
Surface Corrosion ........................................... 8-5
Filiform Corrosion ........................................... 8-5
Pitting Corrosion ............................................. 8-6
Dissimilar Metal Corrosion .............................. 8-6
Concentration Cell Corrosion .......................... 8-6
Intergranular Corrosion ................................... 8-7
Exfoliation Corrosion ....................................... 8-7
Stress-Corrosion/Cracking .............................. 8-7
Fretting Corrosion ........................................... 8-9
Fatigue Corrosion ........................................... 8-9
Galvanic Corrosion ....................................... 8-10
Common Corrosive Agents .............................. 8-10
Preventive Maintenance ...................................... 8-10
Inspection ............................................................. 8-11
Corrosion Prone Areas ......................................... 8-11
Exhaust Trail Areas ........................................... 8-11
Battery Compartments and Battery Vent
Openings .......................................................... 8-12
Bilge Areas ....................................................... 8-12
Lavatories, Buffets, & Galleys .......................... 8-12
Wheel Well and Landing Gear ......................... 8-12
Water Entrapment Areas .................................. 8-12
Engine Frontal Areas & Cooling Air Vents ........ 8-13
Wing Flap & Spoiler Recesses ......................... 8-13
External Skin Areas .......................................... 8-13
Electronic & Electrical Compartments .............. 8-13
Miscellaneous Trouble Areas ........................... 8-13
Corrosion Removal .............................................. 8-14
Surface Cleaning and Paint Removal .............. 8-14
Fairing or Blending Reworked Areas ................ 8-15
Corrosion of Ferrous Metals ................................ 8-15
Mechanical Removal of Iron Rust .................... 8-15
Chemical Removal of Rust ............................... 8-16
Chemical Surface Treatment of Steel .............. 8-16
Removal of Corrosion from Highly Stressed
Steel Parts ........................................................ 8-17
Corrosion of Aluminum & Aluminum Alloys .......... 8-17
Treatment of Unpainted Aluminum Surfaces ....8-18
Treatment of Anodized Surfaces ...................... 8-19
Treatment of Intergranular Corrosion in
Heat-Treated Aluminum Alloy Surfaces ............ 8-19
Corrosion of Magnesium Alloys ........................... 8-19Treatment of Wrought Magnesium Sheet &
Forgings ........................................................... 8-19
Treatment of Installed Magnesium Castings ....8-19
Treatment of Titanium & Titanium Alloys ............. 8-20
Protection of Dissimilar Metal Contacts ............... 8-20
Contacts Not Involving Magnesium .................. 8-20
Contacts Involving Magnesium ........................ 8-20
Corrosion Limits ................................................... 8-20
Processes & Materials Used in Corrosion
Control ................................................................. 8-21
Metal Finishing ................................................. 8-21
Surface Preparation ......................................... 8-21
Chemical Treatments ........................................... 8-22
Anodizing ......................................................... 8-22
Alodizing ........................................................... 8-22
Chemical Surface Treatment and Inhibitors .....8-22
Chromic Acid Inhibitor ...................................... 8-22
Sodium Dichromate Solution ............................ 8-23
Chemical Surface Treatments .......................... 8-23
Protective Paint Finishes ..................................... 8-23
Aircraft Cleaning .................................................. 8-23
Exterior Cleaning .............................................. 8-23
Interior Cleaning ............................................... 8-24
Types of Cleaning Operations ....................... 8-24
Nonflammable Aircraft Cabin Cleaning
Agents & Solvents ......................................... 8-25
Flammable & Combustible Agents ................ 8-25
Container Controls ........................................ 8-25
Fire Prevention Precautions .......................... 8-25
Fire Protection Recommendations ................ 8-26
Powerplant Cleaning ........................................... 8-26
Solvent Cleaners ................................................. 8-27
Dry Cleaning Solvent ....................................... 8-27
Aliphatic and Aromatic Naphtha ....................... 8-27
Safety Solvent .................................................. 8-27
Methyl Ethyl Ketone (MEK) .............................. 8-27
Kerosene .......................................................... 8-27
Cleaning Compound for Oxygen Systems ....... 8-27
Emulsion Cleaners .............................................. 8-28
Water Emulsion Cleaner .................................. 8-28
Solvent Emulsion Cleaners .............................. 8-28
Soaps & Detergent Cleaners ............................... 8-28
Cleaning Compound, Aircraft Surfaces ............ 8-28
Nonionic Detergent Cleaners ........................... 8-28
Mechanical Cleaning Materials ............................ 8-28
Mild Abrasive Materials .................................... 8-28
Abrasive Papers ............................................... 8-28
Chemical Cleaners .............................................. 8-29
Phosphoric-citric Acid ....................................... 8-29
xiiiChapter 9
Fluid Lines & Fittings .......................................... 9-1
Introduction ............................................................ 9-1
Rigid Fluid Lines .................................................... 9-1
Tubing Materials ................................................. 9-1
Copper ............................................................ 9-1
Aluminum Alloy Tubing .................................... 9-1
Steel ................................................................ 9-1
Titanium 3AL–2.5V .......................................... 9-1
Material Identification ......................................... 9-1
Sizes .................................................................. 9-2
Fabrication of Metal Tube Lines ......................... 9-2
Tube Cutting .................................................... 9-2
Tube Bending .................................................. 9-2
Alternative Bending Methods ............................. 9-3
Tube Flaring .................................................... 9-3
Instructions for Rolling-Type Flaring Tools ......... 9-4
Double Flaring .................................................... 9-5
Double Flaring Instructions ............................. 9-5
Fittings ............................................................ 9-5
Flareless Fittings ............................................. 9-5
Beading ........................................................... 9-7
Fluid Line Identification ...................................... 9-7
Fluid Line End Fittings ........................................ 9-7
Universal Bulkhead Fittings ............................ 9-8
AN Flared Fittings ........................................... 9-8
MS Flareless Fittings .......................................... 9-9
Swaged Fittings .............................................. 9-9
Cryofit Fittings ................................................. 9-9
Rigid Tubing Installation and Inspection ........... 9-10
Connection & Torque ..................................... 9-10
Flareless Tube Installation ............................ 9-12
Rigid Tubing Inspection & Repair .................. 9-14
Flexible Hose Fluid Lines .................................... 9-16
Hose Materials & Construction ......................... 9-16
Buna-N ......................................................... 9-16
Neoprene ..................................................... 9-16
Butyl ............................................................. 9-16
Hose Identification ........................................ 9-16
Flexible Hose Inspection .................................. 9-18
Fabrication & Replacement of Flexible Hose ...9-18
Flexible Hose Testing .................................... 9-19
Size Designations ............................................ 9-19
Hose Fittings .................................................... 9-21
Installation of Flexible Hose Assemblies .......... 9-21
Slack ............................................................. 9-21
Flex ............................................................... 9-21
Twisting ......................................................... 9-21
Bending ......................................................... 9-21Clearance ...................................................... 9-21
Hose Clamps .................................................... 9-22
Chapter 10
Inspection Concepts & Techniques ................. 10-1
Basic Inspection ................................................. 10-1
Techniques/Practices ....................................... 10-1
Preparation ....................................................... 10-1
Aircraft Logs ........................................................ 10-1
Checklists ............................................................ 10-2
Publications ......................................................... 10-3
Manufacturers’ Service Bulletins/Instructions ...10-3
Maintenance Manual ........................................ 10-3
Overhaul Manual .............................................. 10-4
Structural Repair Manual ................................. 10-4
Illustrated Parts Catalog ................................... 10-4
Wiring Diagram Manual .................................... 10-4
Code of Federal Regulations (CFRs) .............. 10-4
Airworthiness Directives (ADs) ......................... 10-4
Type Certificate Data Sheets (TCDS) .............. 10-4
Routine/Required Inspections ............................. 10-5
Preflight/Postflight Inspections ......................... 10-5
Annual/100-Hour Inspections ........................... 10-5
Progressive Inspections ................................. 10-12
Continuous Inspections .................................. 10-12
Altimeter & Transponder Inspections ............. 10-12
Air Transport Association iSpec 2200 ................ 10-12
Special Inspections ............................................ 10-12
Hard or Overweight Landing Inspection ........ 10-14
Severe Turbulence Inspection/Over “G” ........ 10-14
Lightning Strike .............................................. 10-16
Bird Strike ...................................................... 10-16
Fire Damage .................................................. 10-16
Flood Damage ................................................ 10-16
Seaplanes ...................................................... 10-16
Aerial Application Aircraft ............................... 10-16
Special Flight Permits ........................................ 10-16
Nondestructive Inspection/Testing ..................... 10-17
Training, Qualification, & Certification ............ 10-17
Advantages & Disadvantages of NDI
Methods ......................................................... 10-17
Visual Inspection ............................................ 10-17
Surface Cracks ........................................... 10-18
Borescope ................................................... 10-18
Interpretation of Results .............................. 10-18
False Indications ......................................... 10-20
Eddy Current Inspection ................................. 10-20
Basic Principles ........................................... 10-20
Principles of Operations ................................. 10-21
Eddy Current Instruments .............................. 10-21
Ultrasonic Inspection ...................................... 10-21
xivPulse Echo .................................................. 10-22
Through-Transmission ................................ 10-23
Resonance ................................................. 10-24
Ultrasonic Instruments ................................... 10-26
Reference Standards ..................................... 10-26
Couplants ....................................................... 10-26
Inspection of Bonded Structures ....................... 10-26
Types of Defects ............................................ 10-28
Acoustic Emission Inspection ......................... 10-28
Magnetic Particle Inspection .......................... 10-29
Development of Indications ......................... 10-29
Types of Discontinuities Disclosed .............. 10-29
Preparation of Parts for Testing ................... 10-30
Effect of Flux Direction ................................ 10-30
Effect of Flux Density .................................. 10-30
Magnetizing Methods .................................. 10-31
Identification of Indications .......................... 10-31
Magnaglo Inspection ...................................... 10-32
Magnetizing Equipment .............................. 10-32
Indicating Mediums ..................................... 10-33
Demagnetizing ............................................ 10-33
Standard Demagnetizing Practice .............. 10-34
Radiographic ............................................... 10-34
Radiographic Inspection ................................. 10-34
Preparation and Exposure .......................... 10-34
Radiographic Interpretation ......................... 10-35
Radiation Hazards ...................................... 10-36
Inspection of Composites .................................. 10-36
Tap Testing ..................................................... 10-36
Electrical Conductivity .................................... 10-37
Thermography ................................................ 10-37
Inspection of Welds ........................................... 10-38
Chapter 11
Hand Tools & Measuring Devices .................... 11-1
General Purpose Tools ......................................... 11-1
Hammers & Mallets ........................................... 11-1
Screwdrivers ..................................................... 11-1
Pliers & Plier-Type Cutting Tools ....................... 11-3
Punches ............................................................ 11-3
Wrenches .......................................................... 11-4
Special Wrenches ............................................. 11-5
Torque Wrench .................................................. 11-5
Strap Wrenches ................................................ 11-6
Impact Drivers ................................................... 11-6
Metal Cutting Tools ............................................... 11-8
Hand Snips ........................................................ 11-8
Hacksaws .......................................................... 11-8
Chisels .............................................................. 11-9
Files ................................................................... 11-9Care and Use ............................................... 11-10
Most Commonly Used Files ......................... 11-10
Care of Files ................................................. 11-12
Drills ................................................................ 11-12
Twist Drills .................................................... 11-12
Reamers .......................................................... 11-13
Countersink ..................................................... 11-14
Taps and Dies ..................................................... 11-14
Layout and Measuring Tools ............................... 11-14
Rules ............................................................... 11-14
Combination Sets ............................................ 11-17
Scriber ............................................................. 11-18
Dividers and Pencil Compasses ..................... 11-19
Calipers ........................................................... 11-19
Micrometer Calipers ........................................ 11-19
Micrometer Parts ............................................. 11-19
Reading a Micrometer ..................................... 11-22
Vernier Scale ................................................... 11-23
Using a Micrometer ......................................... 11-24
Slide Calipers .................................................. 11-25
Chapter 12
Fundamentals of Electricity & Electronics ......12-1
Introduction .......................................................... 12-1
General Composition of Matter ............................ 12-1
Matter ............................................................... 12-1
Element ............................................................ 12-1
Compound ........................................................ 12-1
Molecule ........................................................... 12-1
Atom ................................................................. 12-1
Electrons, Protons, & Neutrons ........................ 12-1
Electron Shells & Energy Levels ...................... 12-2
Valence Electrons ......................................... 12-2
Ions ............................................................... 12-2
Free Electrons ............................................... 12-2
Electron Movement .......................................... 12-2
Conductors, Insulators, and Semiconductors 12-2
Conductors .................................................... 12-3
Insulators ...................................................... 12-3
Semiconductors ............................................ 12-4
Metric Based Prefixes Used for Electrical
Calculations ......................................................... 12-4
Static Electricity ................................................... 12-4
Attractive and Repulsive Forces ...................... 12-4
Electrostatic Field ............................................. 12-5
Electrostatic Discharge (ESD) Considerations .12-5
Magnetism ........................................................... 12-6
Types of Magnets ................................................ 12-9
Electromagnetism .............................................. 12-12
Conventional Flow & Electron Flow ................... 12-14
Conventional Flow .......................................... 12-14
xvElectron Flow ................................................. 12-14
Electromotive Force (Voltage) ........................... 12-14
Current ............................................................... 12-16
Ohm’s Law (Resistance) ................................... 12-17
Resistance of a Conductor ................................ 12-18
Factors Affecting Resistance .......................... 12-18
Resistance and Relation to Wire Sizing ......... 12-20
Circular Conductors (Wires/Cables) ........... 12-20
Rectangular Conductors (Bus Bars) ........... 12-20
Power and Energy ............................................. 12-20
Power in an Electrical Circuit ......................... 12-20
Power Formulas Used in the Study of
Electricity ....................................................... 12-21
Power in a Series & Parallel Circuit ............... 12-22
Energy in an Electrical Circuit ........................ 12-22
Sources of Electricity ...................................... 12-22
Pressure Source ......................................... 12-22
Chemical Source ......................................... 12-23
Thermal Sources ......................................... 12-23
Light Sources .............................................. 12-23
Schematic Representation of Electrical
Components ................................................... 12-23
Conductors .................................................. 12-23
Types of Resistors ............................................. 12-24
Fixed Resistor ................................................ 12-24
Carbon Composition ...................................... 12-24
Resistor Ratings ............................................. 12-24
Color Code .................................................. 12-24
Color Band Decoding ..................................... 12-25
Wire-Wound ................................................... 12-26
Variable Resistors .......................................... 12-26
Rheostat ...................................................... 12-26
Potentiometer .............................................. 12-27
Thermistors .................................................... 12-27
Photoconductive Cells .................................... 12-27
Circuit Protection Devices ................................. 12-27
Fuse ............................................................... 12-28
Current Limiter ............................................... 12-29
Circuit Breaker ............................................... 12-29
Arc Fault Circuit Breaker ............................. 12-29
Thermal Protectors ......................................... 12-29
Control Devices .............................................. 12-30
Switches ...................................................... 12-30
Toggle Switch .............................................. 12-30
Microswitches ............................................. 12-31
Rotary Selector Switches ............................ 12-31
Pushbutton Switches .................................. 12-31
Lighted Pushbutton Switches ...................... 12-32
Dual In-Line Parallel (DIP) Switches ........... 12-33
Switch Guards ............................................. 12-33Relays ......................................................... 12-33
Series DC Circuits ............................................. 12-33
Voltage Drops & Further Application of Ohm’s
Law ................................................................. 12-35
Voltage Sources in Series .............................. 12-36
Kirchhoff’s Voltage Law .................................. 12-36
Voltage Dividers ............................................. 12-37
Determining the Voltage Divider Formula ...... 12-38
Parallel DC Circuits ........................................... 12-40
Voltage Drops ................................................. 12-40
Total Parallel Resistance ................................ 12-40
Resistors in Parallel ....................................... 12-40
Two Resistors in Parallel ................................ 12-40
Current Source ............................................... 12-41
Kirchhoff’s Current Law .................................. 12-41
Current Dividers ............................................. 12-41
Series-Parallel DC Circuits ................................ 12-42
Determining the Total Resistance ................... 12-42
Alternating Current (AC) & Voltage .................... 12-43
AC and DC Compared ................................... 12-44
Generator Principles .......................................... 12-44
Generators of Alternating Current .................. 12-44
Position 1 .................................................... 12-45
Position 2 .................................................... 12-45
Position 3 .................................................... 12-45
Position 4 .................................................... 12-46
Position 5 .................................................... 12-46
Cycle and Frequency ..................................... 12-46
Cycle Defined .............................................. 12-46
Frequency Defined ...................................... 12-46
Period Defined ............................................ 12-47
Wavelength Defined .................................... 12-47
Phase Relationships ...................................... 12-47
In Phase Condition ...................................... 12-48
Out of Phase Condition ............................... 12-48
Values of Alternating Current .......................... 12-48
Instantaneous Value .................................... 12-48
Peak Value .................................................. 12-49
Effective Value ............................................ 12-49
Opposition to Current Flow of AC ................... 12-49
Capacitance ....................................................... 12-50
Capacitors in Direct Current ........................... 12-50
The Resistor/Capacitor (RC) Time Constant ..12-50
Units of Capacitance ...................................... 12-50
Voltage Rating of a Capacitor ........................ 12-51
Factors Affecting Capacitance ........................ 12-51
Types of Capacitors ....................................... 12-51
Fixed Capacitors ......................................... 12-51
Ceramic .......................................................... 12-51
Electrolytic ...................................................... 12-52
xviTantalum ......................................................... 12-52
Polyester Film ................................................ 12-52
Oil Capacitors ................................................. 12-53
Variable Capacitors ..................................... 12-53
Trimmers ........................................................ 12-53
Varactors ........................................................ 12-53
Capacitors in Series ....................................... 12-53
Capacitors in Parallel ..................................... 12-54
Capacitors in Alternating Current ................... 12-54
Capacitive Reactance Xc ............................... 12-54
Sample Problem: ............................................ 12-55
Solution: ......................................................... 12-55
Capacitive Reactances in Series and in
Parallel .......................................................... 12-55
Phase of Current and Voltage in Reactive
Circuits .......................................................... 12-55
Inductance ......................................................... 12-56
Characteristics of Inductance ......................... 12-56
The RL Time Constant ................................... 12-56
Physical Parameters ...................................... 12-56
Self-Inductance .............................................. 12-57
Types of Inductors .......................................... 12-57
Units of Inductance ........................................ 12-58
Inductors in Series ......................................... 12-58
Inductors in Parallel ........................................ 12-58
Inductive Reactance ....................................... 12-58
AC Circuits ......................................................... 12-59
Ohm’s Law for AC Circuits ............................. 12-59
Series AC Circuits .......................................... 12-59
Solution: ...................................................... 12-60
Solution: ...................................................... 12-61
Solution: ...................................................... 12-62
Parallel AC Circuits ........................................ 12-62
Solution: ...................................................... 12-62
Solution: ...................................................... 12-62
Resonance ..................................................... 12-63
Power in AC Circuits ...................................... 12-64
True Power Defined ....................................... 12-64
Apparent Power Defined ............................. 12-64
Solution: ...................................................... 12-65
Transformers .................................................. 12-65
Current Transformers ..................................... 12-67
Transformer Losses ....................................... 12-67
Power in Transformers ................................... 12-67
DC Measuring Instruments ................................ 12-67
D’Arsonval Meter Movement .......................... 12-68
Current Sensitivity and Resistance ................ 12-68
Damping ......................................................... 12-69
Electrical Damping ......................................... 12-69
Mechanical Damping ...................................... 12-69
A Basic Multirange Ammeter .......................... 12-69Precautions ................................................. 12-69
The Voltmeter .................................................... 12-70
Voltmeter Sensitivity ....................................... 12-70
Multiple Range Voltmeters ............................. 12-70
Voltmeter Circuit Connections ........................ 12-71
Influence of the Voltmeter in the Circuit ......... 12-71
The Ohmmeter .................................................. 12-71
Zero Adjustment ............................................. 12-71
Ohmmeter Scale ............................................ 12-71
The Multirange Ohmmeter ............................. 12-72
Megger (Megohmmeter) ................................ 12-72
AC Measuring Instruments ................................ 12-73
Electrodynamometer Meter Movement .......... 12-74
Moving Iron Vane Meter ................................. 12-74
Inclined Coil Iron Vane Meter ......................... 12-75
Varmeters ....................................................... 12-75
Wattmeter ....................................................... 12-76
Frequency Measurement/Oscilloscope .......... 12-76
Horizontal Deflection ...................................... 12-77
Vertical Deflection .......................................... 12-77
Tracing a Sine Wave ...................................... 12-77
Control Features on an Oscilloscope ............. 12-77
Flat Panel Color Displays for Oscilloscopes ..12-78
Digital Multimeter ........................................... 12-78
Basic Circuit Analysis & Troubleshooting .......... 12-78
Voltage Measurement .................................... 12-79
Current Measurement .................................... 12-80
Checking Resistance in a Circuit ................... 12-80
Continuity Checks .......................................... 12-81
Capacitance Measurement ............................ 12-81
Inductance Measurement ............................... 12-81
Troubleshooting Open Faults in a Series
Circuit ............................................................ 12-82
Tracing Opens with the Voltmeter .................. 12-82
Tracing Opens with the Ohmmeter ................ 12-82
Troubleshooting Shorting Faults in a Series
Circuit ............................................................. 12-83
Tracing Shorts with the Ohmmeter ................. 12-83
Tracing Shorts with the Voltmeter .................. 12-84
Troubleshooting Open Faults in a Parallel
Circuit ............................................................. 12-84
Tracing an Open with an Ammeter ................. 12-85
Tracing an Open with an Ohmmeter .............. 12-85
Troubleshooting Shorting Faults in Parallel
Circuits ........................................................... 12-85
Troubleshooting Shorting Faults in Series-
Parallel Circuits .............................................. 12-86
Logic in Tracing an Open ............................ 12-86
Tracing Opens with the Voltmeter .................. 12-86
Batteries ............................................................ 12-87
Primary Cell .................................................... 12-87
xviiSecondary Cell ............................................... 12-87
Battery Ratings ............................................... 12-89
Life Cycle of a Battery .................................... 12-89
Lead-Acid Battery Testing Methods ................ 12-90
Lead-Acid Battery Charging Methods ............ 12-91
Nickel-Cadmium Batteries ................................. 12-91
Chemistry and Construction ........................... 12-91
Operation of Nickel-Cadmium Cells ............... 12-92
General Maintenance and Safety
Precautions ................................................. 12-92
Sealed Lead Acid (SLA) Batteries .................. 12-92
Lithium-Ion Batteries .......................................... 12-93
Inverters ............................................................. 12-93
Rotary Inverters .............................................. 12-94
Permanent Magnet Rotary Inverter ................ 12-94
Inductor-Type Rotary Inverter ........................ 12-94
Static Inverters ............................................... 12-94
Semiconductors ................................................. 12-95
Doping ............................................................ 12-96
PN Junctions & the Basic Diode .................... 12-98
Forward Biased Diode .................................... 12-98
Reverse Biased Diode ................................... 12-99
Rectifiers .......................................................... 12-100
Half-Wave Rectifier ...................................... 12-101
Full-Wave Rectifier ....................................... 12-102
Dry Disk ........................................................ 12-102
Types of Diodes ........................................... 12-103
Power Rectifier Diodes ............................. 12-103
Zener Diodes ............................................ 12-103
Special Purpose Diodes ............................ 12-103
Light-Emitting Diode (LED) ....................... 12-103
Liquid Crystal Displays (LCD) ................... 12-104
Photodiode ................................................ 12-104
Varactors ................................................... 12-104
Schottky Diodes ........................................ 12-105
Diode Identification ....................................... 12-105
Introduction to Transistors ............................... 12-105
Classification ................................................ 12-105
Transistor Theory ......................................... 12-106
PNP Transistor Operation ............................ 12-107
Identification of Transistors .......................... 12-107
Field Effect Transistors ................................. 12-107
Metal-Oxide-Semiconductor FET (MOSFET)
Common Transistor Configurations .............. 12-108
Common-Emitter (CE) Configuration ........ 12-108
Common-Collector (CC) Configuration .....12-109
Common-Base (CB) Configuration ........... 12-109
Vacuum Tubes .................................................. 12-110
Filtering ............................................................. 12-111Filtering Characteristics of Capacitors .......... 12-111
Filtering Characteristics of Inductors ............. 12-111
Common Filter Configurations ...................... 12-111
Basic LC Filters ............................................. 12-112
Low-Pass Filter .......................................... 12-112
High-Pass Filter (HPF) ............................... 12-112
Band-Pass Filter ........................................ 12-112
Band-Stop Filter ......................................... 12-112
Amplifier Circuits ............................................... 12-113
Classification ................................................. 12-113
Class A ....................................................... 12-114
Class AB .................................................... 12-114
Class B ....................................................... 12-114
Class C ...................................................... 12-114
Methods of Coupling ..................................... 12-115
Direct Coupling .......................................... 12-115
RC Coupling ............................................... 12-115
Impedance Coupling .................................. 12-116
Transformer Coupling ................................ 12-116
Feedback ...................................................... 12-116
Operational Amplifiers (OP AMP) .................. 12-116
Applications ................................................ 12-117
Magnetic Amplifiers .......................................... 12-118
Saturable-Core Reactor ................................ 12-119
Logic Circuits .................................................... 12-119
Logic Polarity ................................................ 12-120
Positive ..................................................... 12-120
Negative .................................................... 12-120
Pulse Structure ............................................. 12-120
Basic Logic Circuits ...................................... 12-121
The Inverter Logic ..................................... 12-121
The AND Gate ........................................... 12-121
The OR Gate ............................................. 12-122
The NAND Gate ........................................ 12-122
The NOR Gate .......................................... 12-122
Exclusive OR Gate .................................... 12-122
Exclusive NOR Gate ................................. 12-122
The Integrated Circuit ................................... 12-122
Microprocessors ........................................... 12-123
DC Generators ................................................ 12-123
Theory of Operation ..................................... 12-123
Generation of a DC Voltage ......................... 12-125
Position A .................................................. 12-125
Position B .................................................. 12-126
Position C .................................................. 12-126
Position D .................................................. 12-127
The Neutral Plane ..................................... 12-127
Construction Features of DC Generators ....12-128
Field Frame ............................................... 12-128
xviiiArmature ...................................................... 12-130
Gramme-Ring Armature ............................ 12-130
Drum-Type Armature ................................. 12-130
Commutators ................................................ 12-130
Armature Reaction ....................................... 12-131
Compensating Windings .............................. 12-131
Interpoles ..................................................... 12-132
Types of DC Generators .............................. 12-132
Series Wound DC Generators .................. 12-132
Shunt Wound DC Generators ................... 12-133
Compound Wound DC Generators ........... 12-134
Generator Ratings ........................................ 12-134
Generator Terminals ..................................... 12-135
DC Generator Maintenance ............................. 12-135
Inspection ..................................................... 12-135
Condition of Generator Brushes ................... 12-136
DC Motors .................................................... 12-137
Force Between Parallel Conductors ............. 12-138
Developing Torque ....................................... 12-138
Basic DC Motor ............................................ 12-138
Position A .................................................. 12-139
Position B .................................................. 12-139
Position C .................................................. 12-139
Position D .................................................. 12-139
DC Motor Construction .................................... 12-140
Armature Assembly ...................................... 12-140
Field Assembly ............................................. 12-141
Brush Assembly ........................................... 12-141
End Frame ................................................... 12-141
Types of DC Motors ......................................... 12-141
Series DC Motor ........................................... 12-141
Shunt DC Motor ........................................... 12-142
Compound DC Motor ................................... 12-142
Counter Electromotive Force (emf) ................. 12-142
Types of Duty ............................................... 12-143
Reversing Motor Direction ............................ 12-144
Motor Speed ................................................. 12-145
Energy Losses in DC Motors .................... 12-145
Inspection and Maintenance of DC Motors ..12-146
AC Motors ........................................................ 12-147
Types of AC Motors ...................................... 12-147
Three-Phase Induction Motor ....................... 12-148
Rotating Magnetic Field ............................... 12-148
Construction of Induction Motor ................... 12-148
Induction Motor Slip ..................................... 12-149
Single-Phase Induction Motor ...................... 12-149
Shaded Pole Induction Motor ....................... 12-149
Split-Phase Motor ......................................... 12-150
Capacitor Start Motor ................................... 12-150
Direction of Rotation of Induction Motors .....12-150Synchronous Motor ...................................... 12-151
AC Series Motor ........................................... 12-152
Maintenance of AC Motors ........................... 12-153
Alternators ....................................................... 12-154
Basic Alternators & Classifications ............... 12-154
Method of Excitation ..................................... 12-154
Number of Phases ....................................... 12-154
Armature or Field Rotation ........................... 12-155
Single-Phase Alternator ............................ 12-155
Two-Phase Alternator ................................ 12-156
Three-Phase Alternator ............................. 12-156
Wye Connection (Three-Phase) ................... 12-156
Delta Connection (Three-Phase) ................. 12-156
Alternator Rectifier Unit ................................ 12-156
Brushless Alternator ..................................... 12-157
Alternator Frequency .................................... 12-158
Starter Generator ......................................... 12-158
Alternator Rating .......................................... 12-158
Alternator Maintenance ................................ 12-159
Regulation of Generator Voltage .................. 12-159
Voltage Regulation with a Vibrating-Type
Regulator ...................................................... 12-159
Three Unit Regulators .................................. 12-161
Differential Relay Switch .............................. 12-162
Overvoltage & Field Control Relays ............. 12-163
Generator Control Units (GCU) ....................... 12-164
Basic Functions of a Generator Control Unit
(GCU) ........................................................... 12-164
Voltage Regulation ....................................... 12-164
Overvoltage Protection ................................. 12-164
Parallel Generator Operations ..................... 12-164
Over-Excitation Protection ........................... 12-164
Differential Voltage ....................................... 12-164
Reverse Current Sensing ............................. 12-164
Alternator Constant Speed Drive System ........ 12-164
Hydraulic Transmission ................................... 12-165
Voltage Regulation of Alternators .................... 12-171
Alternator Transistorized Regulators ............ 12-172
Chapter 13
Mechanic Privileges & Limitations .................. 13-1
Introduction .......................................................... 13-1
Mechanic Certification: Subpart A—General (by
14 CFR Section) .................................................. 13-1
Section 65.3, Certification of Foreign Airmen
Other Than Flight Crewmembers ..................... 13-1
Section 65.11, Application and Issue ................ 13-1
Section 65.12, Offenses Involving Alcohol
and Drugs ......................................................... 13-1
Section 65.13, Temporary Certificate ............... 13-1
Section 65.14, Security Disqualification ........... 13-1
xixSection 65.15, Duration of Certificates ............. 13-1
Section 65.16, Change of Name:
Replacement of Lost or Destroyed Certificate .13-2
Section 65.17, Test: General Procedure .......... 13-2
Section 65.18, Written Tests: Cheating or
Other Unauthorized Content ............................ 13-2
Section 65.19, Retesting After Failure .............. 13-2
Section 65.20, Applications, Certificates,
Logbooks, Reports, and Records:
Falsification, Reproduction, or Alteration .......... 13-2
Section 65.21, Change of Address ................... 13-2
Refusal to Submit to a Drug or Alcohol Test .....13-2
Mechanic Certification: Subpart D—Mechanics
(by 14 CFR Section) ............................................ 13-3
Section 65.71, Eligibility Requirements:
General ............................................................ 13-3
Section 65.73, Ratings ..................................... 13-3
Section 65.75, Knowledge Requirements ........ 13-3
Section 65.77, Experience Requirements ....... 13-3
Section 65.79, Skill Requirements ................... 13-3
Section 65.80, Certificated Aviation
Maintenance ..................................................... 13-4
Technician School Students ............................. 13-4
Section 65.81, General Privileges and
Limitations ........................................................ 13-4
Section 65.83, Recent Experience
Requirements ................................................... 13-4
Section 65.85, Airframe Rating: Additional
Privileges .......................................................... 13-4
Section 65.87, Powerplant Rating: Additional
Privileges .......................................................... 13-4
Section 65.89, Display of Certificate ................ 13-4
Inspection Authorization (IA) (by 14 CFR
Section) ............................................................... 13-5
Section 65.91, Inspection Authorization ........... 13-5
Section 65.92, Inspection Authorization:
Duration ............................................................ 13-5
Section 65.93, Inspection Authorization:
Renewal ........................................................... 13-5
Section 65.95, Inspection Authorization:
Privileges and Limitations ................................ 13-6
Ethics .................................................................. 13-6
A Scenario ........................................................ 13-6
Final Observation .......................................... 13-7
Chapter 14
Human Factors .................................................. 14-1
Introduction .......................................................... 14-1
FAA Involvement .............................................. 14-1
Importance of Human Factors .......................... 14-1
Definitions of Human Factors ........................... 14-1
What are Human Factors? .................................. 14-2Elements of Human Factors ............................. 14-2
Clinical Psychology ....................................... 14-3
Experimental Psychology .............................. 14-3
Anthropometry .............................................. 14-4
Computer Science ........................................ 14-4
Cognitive Science ......................................... 14-4
Safety Engineering ....................................... 14-4
Medical Science ............................................ 14-4
Organizational Psychology ........................... 14-4
Educational Psychology ............................... 14-5
Industrial Engineering ................................. 14-5
History of Human Factors .................................... 14-6
Evolution of Maintenance Human Factors ....... 14-7
The Pear Model ................................................... 14-9
People .............................................................. 14-9
Environment ..................................................... 14-9
Physical ....................................................... 14-10
Organizational ............................................. 14-10
Actions ........................................................... 14-10
Resources ...................................................... 14-10
Human Error ...................................................... 14-13
Types of Errors ............................................... 14-13
Unintentional ............................................... 14-13
Intentional .................................................. 14-13
Active & Latent ........................................... 14-13
The “Dirty Dozen” .............................................. 14-13
Lack of Communication .................................. 14-13
Complacency ................................................. 14-14
Lack of Knowledge ......................................... 14-14
Distraction ...................................................... 14-15
Lack of Teamwork .......................................... 14-16
Fatigue ........................................................... 14-16
Lack of Resources ......................................... 14-18
Lack of Assertiveness .................................... 14-22
Stress ............................................................. 14-24
Physical Stressors ..................................... 14-24
Psychological Stressors ............................. 14-24
Physiological Stressors .............................. 14-25
Lack of Awareness ......................................... 14-26
Norms ............................................................. 14-26
Example of Common Maintenance Errors .....14-28
Where to Get Information .................................. 14-29
Federal Aviation Administration (FAA) ............ 14-30
FAA’s Maintenance Fatigue Section ........... 14-30
FAA Safety Team ......................................... 14-31
Other Resources ............................................ 14-31
System Safety Services .............................. 14-31
Human Factors & Ergonomics Society
(HFES) ........................................................ 14-31
International Ergonomics Association (IEA) 14-31
xxGlossary .............................................................. G-1
Index ...................................................................... I-1
Safety, Ground Operations, &
Servicing
Chapter 1
Aviation maintenance technicians (AMTs) devote a portion of
their aviation career to ground handling and operating aircraft.
Technicians also need to be proficient in operating ground
support equipment. The complexity of support equipment and
the hazards involved in the ground handling of aircraft require
that maintenance technicians possess a detailed knowledge of
safety procedures used in aircraft servicing, taxiing, run-up,
and in the use of ground support equipment. The information
provided in this chapter is intended as a general guide for
safely servicing and operating aircraft.
Introducing human factors to aircraft maintenance personnel
makes them aware of how it affects maintenance performance.
Although there are many human factors involved when
dealing with maintenance performance, several areas can be
considered. Some of these include fatigue, deadline pressure,
stress, distractions, poor communication skills, complacency,
and lack of information. Maintenance technicians need to
understand how human factors can impact their performance
and safety while completing maintenance tasks.
Shop Safety
Keeping the shop, hangars, and flight line clean is essential
to safety and efficient maintenance. The highest standards of
orderly work arrangements and cleanliness must be observed
during the maintenance of aircraft. Where continuous
work shifts are established, the outgoing shift removes and
properly stores personal tools, rollaway boxes, work stands,
maintenance stands, hoses, electrical cords, hoists, crates,
and boxes that were needed for the work to be accomplished.
Signs are posted to indicate dangerous equipment or hazardous
conditions. Additionally, there are signs that provide the
location of first aid and fire equipment. Safety lanes, pedestrian
walkways, and fire lanes are painted around the perimeter
inside the hangars. This is a safety measure to prevent accidents
and to keep pedestrian traffic out of work areas.
Safety is everyone’s business. However, technicians and
supervisors must watch for their own safety and for the
safety of others working around them. Communication is
key to ensuring everyone’s safety. If other personnel are
conducting their actions in an unsafe manner, communicate
with them, reminding them of their safety and that of others
around them.Electrical Safety
Physiological Safety
Working with electrical equipment poses certain physiological
safety hazards. When electricity is applied to the human body,
it can create severe burns in the area of entrance and at the
point of exit from the body. In addition, the nervous system is
affected and can be damaged or destroyed. To safely deal with
electricity, the technician must have a working knowledge
of the principles of electricity and a healthy respect for its
capability to do both work and damage.
Wearing or use of proper safety equipment can provide a
psychological assurance and physically protect the user
at the same time. The use of rubber gloves, safety glasses,
rubber or grounded safety mats, and other safety equipment
contributes to the overall safety of the technician working
on or with electrical equipment.
Two factors that affect safety when dealing with electricity are
fear and overconfidence. These two factors are major causes
of accidents involving electricity. While a certain amount of
respect for electrical equipment is healthy and a certain level
of confidence is necessary, extremes of either can be deadly.
Lack of respect is often due to lack of knowledge. Personnel
who attempt to work with electrical equipment and have
no knowledge of the principles of electricity lack the skills
to deal with electrical equipment safely. Overconfidence
leads to risk taking. The technician who does not respect
the capabilities of electricity will, sooner or later, become a
victim of electricity’s power.
Fire Safety
Anytime current flows, whether during generation or
transmission, a by-product is heat. The greater the current
flow, the greater the amount of heat created. When this heat
becomes too great, protective coatings on wiring and other
electrical devices can melt, causing shorting. That in turn
leads to more current flow and greater heat. This heat can
become so great that metals can melt, liquids vaporize, and
flammable substances ignite.
An important factor in preventing electrical fires is to keep
the area around electrical work or electrical equipment
1-2clean, uncluttered, and free of all unnecessary flammable
substances. Ensure that all power cords, wires, and lines
are free of kinks and bends that can damage the wire. Never
place wires or cords where they may be walked on or run
over by other equipment. When several wires inside a power
cord are broken, the current passing through the remaining
wires increases. This generates more heat than the insulation
coatings on the wire are designed to withstand and can
lead to a fire. Closely monitor the condition of electrical
equipment. Repair or replace damaged equipment before
further use.
Safety Around Compressed Gases
Compressed air, like electricity, is an excellent tool when
it is under control. A typical nitrogen bottle set is shown
in Figure 1-1 . The following “dos and don’ts” apply when
working with or around compressed gases:
• Inspect air hoses frequently for breaks and worn spots.
Unsafe hoses must be replaced immediately.
• Keep all connections in a “no-leak condition.”
• Maintain in-line oilers, if installed, in operating
condition.
• Ensure the system has water sumps installed and
drained at regular intervals.
• Filter air used for paint spraying to remove oil and
water.
• Never use compressed air to clean hands or clothing.
Pressure can force debris into the flesh leading to
infection.
• Never spray compressed air in the area of other
personnel.
• Straighten, coil, and properly store air hoses when not
in use.
• Many accidents involving compressed gases occur
during aircraft tire mounting. To prevent possible
personal injury, use tire dollies and other appropriate
devices to mount or remove heavy aircraft tires.
When inflating tires on any type of aircraft wheels, always
use tire cage guards. Extreme caution is required to avoid
over inflation of high-pressure tires because of possible
personal injury. Use pressure regulators on high-pressure air
bottles to eliminate the possibility of over inflation of tires.
Tire cages are not required when adjusting pressure in tires
installed on an aircraft.
Safety Around Hazardous Materials
Material safety diamonds are important with regard to
shop safety. These diamond-shaped labels are a simple
and quick way to determine the risk of hazardous material within the associated container and, if used properly with
the tags, indicate what personal safety equipment to use.
The most observable portion of the Safety Data Sheets (SDSs)
(formerly known as Material Safety Data Sheet (MSDS))
label is the risk diamond. It is a four-color segmented
diamond that represents flammability (red), reactivity
(yellow), health (blue), and special hazard (white). In the
flammability, reactivity, and health blocks, there is a number
from 0 to 4. Zero represents little or no hazard to the user,
while 4 means that the material is very hazardous. The special
hazard segment contains a word or abbreviation to represent
the specific hazard. Some examples are RAD for radiation,
ALK for alkali materials, Acid for acidic materials, and
CARC for carcinogenic materials. The letter W with a line
through it stands for high reactivity to water. [Figure 1-2]
The SDS is a more detailed version of the chemical safety
issues. These forms have the detailed breakdown of the
chemicals, including formulas and action to take if personnel
come in contact with the chemicals. All sheets have the same
information requirements; however, the exact location of the
information on the sheet may vary depending on the SDS
manufacturer. These forms are necessary for a safe shop that
meets all the requirements of the governing safety body, the
U.S. Department of Labor Occupational Safety and Health
Administration (OSHA).
Safety Around Machine Tools
Hazards in a shop increase when the operation of lathes, drill
presses, grinders, and other types of machines are used. Each
machine has its own set of safety practices. The following
discussions are necessary to avoid injury.
The drill press can be used to bore and ream holes, to do
facing, milling, and other similar types of operations. The
following precautions can reduce the chance of injury:
• Wear eye protection.
• Securely clamp all work.
• Set the proper revolutions per minute (rpm) for the
material used.
• Do not allow the spindle to feed beyond its limit of
travel while drilling.
• Stop the machine before adjusting work or attempting
to remove jammed work.
• Clean the area when finished.
Lathes are used in turning work of a cylindrical nature.
This work may be performed on the inside or outside of the
cylinder. The work is secured in the chuck to provide the
rotary motion, and the forming is done by contact with a
Figure 1-1. A typical nitrogen bottle.
W32
Figure 1-2. A risk diamond.securely mounted tool. The following precautions can reduce
the chance of injury:
• Wear eye protection.
• Use sharp cutting tools.
• Allow the chuck to stop on its own. Do not attempt to
stop the chuck by hand pressure.
• Examine tools and work for cracks or defects before
starting the work.
• Do not set tools on the lathe. Tools may be caught by
the work and thrown.
• Before measuring the work, allow it to stop in the lathe.
Milling machines are used to shape or dress; cut gear
teeth, slots, or key ways; and similar work. The following
precautions can reduce the chance of injury:
• Wear eye protection.
• Clean the work bed prior to work.
• Secure the work to the bed to prevent movement during milling.
• Select the proper tools for the job.
• Do not change the feed speed while working.
• Lower the table before moving under or away from
the work.
• Ensure all clamps and bolts are passable under
the arbor.
Grinders are used to sharpen tools, dress metal, and perform
other operations involving the removal of small amounts
of metal. The following precautions can reduce the chance
of injury:
• Wear eye protection, even if the grinder has a shield.
• Inspect the grinding wheel for defects prior to use.
• Do not force grinding wheels onto the spindle. They fit
snugly but do not require force to install them. Placing
side pressure on a wheel could cause it to explode.
• Check the wheel flanges and compression washer.
They should be one-third the diameter of the wheel.
• Do not stand in the arc of the grinding wheel while
operating in case the wheel explodes.
Welding must be performed only in designated areas. Any
part that is to be welded must be removed from the aircraft, if
possible. Repair would then be accomplished in a controlled
environment, such as a welding shop. A welding shop must
be equipped with proper tables, ventilation, tool storage, and
fire prevention and extinguishing equipment.
1-4Welding on an aircraft should be performed outside, if
possible. If welding in the hangar is necessary, observe
these precautions:
• During welding operations, open fuel tanks and work
on fuel systems are not permitted.
• Painting is not permitted.
• No aircraft are to be within 35 feet of the welding
operation.
• No flammable material is permitted in the area around
the welding operation.
• Only qualified welders are permitted to do the work.
• The welding area is to be roped off and placarded.
• Fire extinguishing equipment of a minimum rating
of 20B must be in the immediate area with 80B rated
equipment as a backup.
• Trained fire watches are to be present in the area
around the welding operation.
• The aircraft being welded must be in a towable
condition, with a tug attached, and the aircraft parking
brakes released. A qualified operator must be on the
tug and mechanics available to assist in the towing
operation should it become necessary to tow the
aircraft. If the aircraft is in the hangar, the hangar
doors are to be open.
Flight Line Safety
Hearing Protection
The flight line is a place of dangerous activity. Technicians
who perform maintenance on the flight line must constantly
be aware of what is going on around them. The noise on a
flight line comes from many places. Aircraft are only one
source of noise. There are auxiliary power units (APUs), fuel
trucks, baggage handling equipment, and so forth. Each has
its own frequency of sound. Combined all together, the noise
on the ramp or flight line can cause hearing loss.
There are many types of hearing protection available.
Hearing protection can be external or internal. Earmuffs or
headphones are considered external protection. The internal
type of hearing protection fits into the auditory canal. Both
types reduce the sound level reaching the eardrum and reduce
the chances of hearing loss.
Hearing protection is essential when working with
pneumatic drills, rivet guns, or other loud tools. Even short
duration exposure to these sounds can cause hearing loss
because of their high frequency. Continued exposure will
cause hearing loss.Foreign Object Damage (FOD)
Foreign object damage (FOD) is any damage to aircraft,
personnel, or equipment caused by any loose object. These
loose objects can be anything, such as broken runway
concrete, shop towels, safety wire, etc. To control FOD, keep
ramp and operation areas clean, have a tool control program,
and provide convenient receptacles for used hardware, shop
towels, and other consumables.
Never leave tools or other items around the intake of a turbine
engine. The modern gas turbine engine creates a low-pressure
area in front of the engine that causes any loose object to
be drawn into the engine. The exhaust of these engines
can propel loose objects great distances with enough force
to damage anything that is hit. The importance of a FOD
program cannot be overstressed when a technician considers
the cost of engines, components, or a human life.
Safety Around Airplanes
As with the previously mentioned items, it is important to
be aware of propellers. Technicians cannot assume the pilot
of a taxiing aircraft can see them and must stay within the
pilot’s view while on the ramp area. Turbine engine intakes
and exhaust can also be very hazardous areas. Smoking or
open flames are not permitted anywhere near an aircraft in
operation. Be aware of aircraft fluids that can be detrimental
to skin. When operating support equipment around aircraft,
be sure to allow space between it and the aircraft, and secure
it so it cannot roll into the aircraft. All items in the area of
operating aircraft must be stowed properly.
Safety Around Helicopters
Every type of helicopter has different features. These
differences must be learned to avoid damaging the helicopter
or injuring the technician. When approaching a helicopter
while the blades are turning, adhere to the following
guidelines to ensure safety.
• Observe the rotor head and blades to see if they
are level. This allows maximum clearance when
approaching the helicopter.
• Approach the helicopter in view of the pilot.
• Never approach a helicopter carrying anything with
a vertical height that the blades could hit. This could
cause blade damage and injury to the individual.
• Never approach a single-rotor helicopter from the rear.
The tail rotor is invisible when operating.
• Never go from one side of the helicopter to the other
by going around the tail. Always go around the nose
of the helicopter.
When securing the rotor on helicopters with elastomeric
1-5bearings, check the maintenance manual for the proper
method. Using the wrong method could damage the bearing.
Fire Safety
Performing maintenance on aircraft and their components
requires the use of electrical tools that can produce sparks,
heat-producing tools and equipment, flammable and
explosive liquids, and gases. As a result, a high potential
exists for fire to occur. Measures must be taken to prevent a
fire from occurring and to have a plan for extinguishing it.
The key to fire safety is knowledge of what causes a fire, how
to prevent it, and how to put it out. This knowledge must be
instilled in each technician, emphasized by their supervisors
through sound safety programs, and occasionally practiced.
Airport or other local fire departments can normally be
called upon to assist in training personnel and helping to
establish fire safety programs for the hangar, shops, and
flight line.
Fire Protection
Requirements for Fire to Occur
Three things are required for a fire. Remove any one of these
things and the fire extinguishes:
1. Fuel—combines with oxygen in the presence of heat,
releasing more heat. As a result, it reduces itself to
other chemical compounds.
2. Heat—accelerates the combining of oxygen with fuel,
in turn releasing more heat.
3. Oxygen—the element that combines chemically with
another substance through the process of oxidation.
Rapid oxidation, accompanied by a noticeable release
of heat and light, is called combustion or burning.
[Figure 1-3]
Classification of Fires
For commercial purposes, the National Fire Protection
Association (NFPA) has classified fires into three basic types:
Class A, Class B, and Class C.
1. Class A fires involve ordinary combustible materials,
such as wood, cloth, paper, upholstery materials, and
so forth.
2. Class B fires involve flammable petroleum products
or other flammable or combustible liquids, greases,
solvents, paints, and so forth.
3. Class C fires involve energized electrical wiring and
equipment.
A fourth class of fire, the Class D fire, involves flammable
metal. Class D fires are not commercially considered by
the NFPA to be a basic type of fire since they are caused by a Class A, B, or C fire. Usually Class D fires involve
magnesium in the shop, or in aircraft wheels and brakes, or
are the result of improper welding operations.
Any one of these fires can occur during maintenance on or
around, or operations involving aircraft. There is a particular
type of extinguisher that is most effective for each type of fire.
Types and Operation of Shop and Flight Line Fire
Extinguishers
Water extinguishers are the best type to use on Class A fires.
Water has two effects on fire. It deprives fire of oxygen and
cools the material being burned.
Since most petroleum products float on water, water-type fire
extinguishers are not recommended for Class B fires. Extreme
caution must be used when fighting electrical fires (Class C)
with water-type extinguishers. All electrical power must be
removed or shut off to the burning area. Additionally, residual
electricity in capacitors, coils, and so forth must be considered
to prevent severe injury or possibly death from electrical shock.
Never use water-type fire extinguishers on Class D fires. The
cooling effect of water causes an explosive expansion of the
metal, because metals burn at extremely high temperatures.
Water fire extinguishers are operated in a variety of ways.
Some are hand pumped, while others are pressurized. The
pressurized types of extinguishers may have a gas charge
stored in the container with the water, or it may contain a
“soda-acid” container where acid is spilled into a container
of soda inside the extinguisher. The chemical reaction of
the soda and the acid causes pressure to build inside the fire
extinguisher, forcing the water out.
Carbon dioxide (CO 2) extinguishers are used for Class A, B,
and C fires, extinguishing the fire by depriving it of oxygen.
[Figure 1-4] Additionally, like water-type extinguishers, CO 2
cools the burning material. Never use CO 2 on Class D fires.
As with water extinguishers, the cooling effect of CO 2 on
the hot metal can cause explosive expansion of the metal.
When using CO 2 fire extinguishers, all parts of the
extinguisher can become extremely cold, and remain so
for a short time after operation. Wear protective equipment
or take other precautions to prevent cold injury, such as
frostbite. Extreme caution must be used when operating CO 2
fire extinguishers in closed or confined areas. Not only can
the fire be deprived of oxygen, but so too can the operator.
CO 2 fire extinguishers generally use the self-expelling
method of operation. This means that the CO 2 has sufficient
pressure at normal operating pressure to expel itself. This
It takes three things to start a fire:
OXYGEN, HEAT, FUEL
Oxygen
Friction (heat)Fuel
Figure 1-3. Three elements of fire.
Figure 1-4. Carbon dioxide fire extinguisher.pressure is held inside the container by some type of seal
or frangible disk that is broken or punctured by a firing mechanism, usually a pin. This means that once the seal or disk is broken, pressure in the container is released and the fire extinguisher is spent, requiring replacement. [Figure 1-5]
Halogenated hydrocarbon extinguishers are most effective on Class B and C fires. They can be used on Class A and D fires, but they are less effective. Halogenated hydrocarbon, commonly called Freon™ by the industry, are numbered
according to chemical formulas with Halon™ numbers.
Carbon tetrachloride (Halon 104), chemical formula CCl
4,
has an Underwriters Laboratory (UL) toxicity rating of 3. As
such, it is extremely toxic. [Figure 1-6] Hydrochloric acid vapor, chlorine, and phosgene gas are produced whenever carbon tetrachloride is used on ordinary fires. The amount of phosgene gas is increased whenever carbon tetrachloride is brought in direct contact with hot metal, certain chemicals, or continuing electrical arcs. It is not approved for any fire extinguishing use. Old containers of Halon 104 found in or around shops or hangars should be disposed of in accordance with Environmental Protection Agency (EPA) regulations
and local laws and ordinances.
Methyl bromide (Halon 1001), chemical formula CH
3Br,
is a liquefied gas with a UL toxicity rating of 2. It is very
toxic and corrosive to aluminum alloys, magnesium, and zinc. Halon 1001 is not recommended for aircraft use. Chlorobromomethane (Halon 1011), chemical formula CH
2ClBr, is a liquefied gas with a UL toxicity rating of
3. Like methyl bromide, Halon 1011 is not recommendedfor aircraft use. Dibromodifluoromethane (Halon 1202),
chemical formula CBr
2F2, has a UL toxicity rating of 4. Halon
1202 is not recommended for aircraft use.
Bromochlorodifluoromethane (Halon 1211), chemical formula
CBrClF 2, is a liquefied gas with a UL toxicity rating of 5. It
is colorless, noncorrosive, and evaporates rapidly leaving no residue. It does not freeze or cause cold burns and does not harm fabrics, metals, or other materials it contacts. Halon 1211 acts rapidly on fires by producing a heavy blanketing mist that eliminates oxygen from the fire source. More importantly, it interferes chemically with the combustion process of the fire. Furthermore, it has outstanding properties in preventing re-flash after the fire has been extinguished.
Bromotrifluoromethane (Halon 1301), chemical formula
CF
3Br, is also a liquefied gas and has a UL toxicity rating of
6. It has all the characteristics of Halon 1211. The significant difference between the two is Halon 1211 forms a spraysimilar to CO
2, while Halon 1301 has a vapor spray that is
more difficult to direct.
Note: The EPA has restricted Halon to its 1986 production
level due to its effect on the ozone layer.
Dry powder extinguishers, while effective on Class B and
C fires, are best for use on Class D fires. The method of operation of dry powder fire extinguishers varies from gas cartridge charges, stored pressure within the container that forces the powder charge out of the container, to scooping pails or buckets of the powder from large containers or barrels to toss on the fire.
1-7Extinguishing M aterials Classes of Fire
Self-Generating
Self-Expelling
Cartridge of
N2 Cylinder
Stored
Pressure
Pump
Hand
A B C D
Water and antifreeze
Soda-acid (water)Wetting agentFoamLoaded streamMultipurpose dry chemicalCarbon dioxideDry chemical
Bromotrifluoromethane — Halon 1301
Bromochlorodifluoromethane — Halon 1211
Dry powder (metal fi res)X
XXXX
X+X
XX
X
X
XX
X
X
XXX X
XXX X
X+
X
X+
X
XXXXXXXX
X
X
+ Smaller sizes of these extinguishers are not recognized for use on these classes of fire.
Group Defin ition Examples
Gases or vapors in concentrations up to 20% by volume, for durations of
exposure of up to approximately 2 hours, do not appear to produce injury.
Gases or vapors much less toxic than Group 4, but more toxic than Group 6.
Gases or vapors in concentrations of the order of 2 to 2 ½%, for durations
of exposure of up to approximately 2 hours are lethal or produce serious injury.
Gases or vapors in concentrations of the order of 2 to 2 ½%, for durations
of exposure of the order of 1 hour are lethal or produce serious injury.
Gases or vapors in concentrations of approximately ½ to 1%, for durations
of exposure of up to approximately ½ hour are lethal or produce serious injury. 6 (Least toxic)
5a
2Carbon dioxide
Dibromodifluormethane
(Halon 1202)
Bromochloromethane
(Halon 1011)Bromotrifluoromethane
(Halon 1301)
Carbon tetrachloride
(Halon 104)
Methyl bromide
(Halon 1001)Figure 1-5. Extinguisher operation and methods of expelling.
Figure 1-6. Toxicity table.Dry powder is not recommended for aircraft use, except
on metal fires, as a fire extinguisher. The leftover chemical residues and dust often make cleanup difficult and can damage electronic or other delicate equipment.
Inspection of Fire Extinguishers
Fire extinguishers need to be checked periodically utilizing
a checklist. If a checklist is unavailable, check the following as a minimum:
•Proper location of appropriate extinguisher•Safety seals unbroken
•All external dirt and rust removed
•Gauge or indicator in operable range
•Proper weight
•No nozzle obstruction
•No obvious damage
Airport or other local fire departments can usually help in preparing or providing extinguisher checklists. In addition, these fire departments can be helpful in answering questions
1-8and assisting in obtaining repairs to or replacement of fire
extinguishers.
Identifying Fire Extinguishers
Fire extinguishers are marked to indicate suitability for a
particular class of fire. The markings on Figure 1-7 must be
placed on the fire extinguisher and in a conspicuous place
in the vicinity of the fire extinguisher. When the location is
marked, however, take extreme care to ensure that the fire
extinguisher kept at that location is in fact the type depicted
by the marking. In other words, if a location is marked for a
Class B fire extinguisher, ensure that the fire extinguisher in
that location is in fact suitable for Class B fires.
Markings must be applied by decalcomanias (decals),
painting, or similar methods. They are to be legible and as
durable as necessary for the location. For example, markings
used outside need to be more durable than those in the hangar
or office spaces.
When markings are applied to the extinguisher, they are placed
on the front of the shell, if one is installed, above or below the
extinguisher nameplate. Markings must be large enough and
in a form that is easily seen and identifiable by the average
person with average eyesight at a distance of at least 3 feet.
When markings are applied to wall panels, and so forth, in
the vicinity of extinguishers, they must be large enough and
in a form that is easily seen and identifiable by the average
person with average eyesight at a distance of at least 25 feet.
[Figure 1-8]
Using Fire Extinguishers
When using a fire extinguisher, ensure the correct type is
used for the fire. Most extinguishers have a pin to pull that
allows the handle to activate the agent. Stand back 8 feet and
aim at the base of the fire or flames. Squeeze the lever and
sweep side to side until the fire is extinguished.
Tie-Down Procedures
Preparation of Aircraft
Aircraft are to be tied down after each flight to prevent
damage from sudden storms. The direction that aircraft are
to be parked and tied down is determined by prevailing or
forecast wind direction.
Aircraft are to be headed into the wind, depending on
the locations of the parking area’s fixed tie-down points.
Spacing of tie-downs need to allow for ample wingtip
clearance. [Figure 1-9] After the aircraft is properly
located, lock the nosewheel or the tail wheel in the
fore-and-aft position.Tie-Down Procedures for Land Planes
Securing Light Aircraft
Light aircraft are most often secured with ropes tied only at
the aircraft tie-down rings provided for securing purposes.
Rope is never to be tied to a lift strut, since this practice can
bend a strut if the rope slips to a point where there is no slack.
Since manila rope shrinks when wet, about 1 inch (1") of
slack needs to be provided for movement. Too much slack,
however, allows the aircraft to jerk against the ropes. Tight
tie-down ropes put inverted flight stresses on the aircraft and
many are not designed to take such loads.
A tie-down rope holds no better than the knot. Anti-slip knots,
such as the bowline, are quickly tied and are easy to untie.
[Figure 1-10] Aircraft not equipped with tie-down fittings
must be secured in accordance with the manufacturer’s
instructions. Ropes are to be tied to outer ends of struts on
high-wing monoplanes and suitable rings provided where
structural conditions permit, if the manufacturer has not
already provided them.
Securing Heavy Aircraft
The normal tie-down procedure for heavy aircraft can be
accomplished with rope or cable tie-down. The number of
tie-downs are governed by anticipated weather conditions.
Most heavy aircraft are equipped with surface control locks
that are engaged or installed when the aircraft is secured.
Since the method of locking controls vary on different
types of aircraft, check the manufacturer’s instructions for
proper installation or engaging procedures. If high winds
are anticipated, control surface battens can also be installed
to prevent damage. Figure 1-11 illustrates four common tie-
down points on heavy aircraft.
The normal tie-down procedure for heavy aircraft includes
the following:
1. Head aircraft into prevailing wind whenever possible.
2. Install control locks, all covers, and guards.
3. Chock all wheels fore and aft. [Figure 1-12]
4. Attach tie-down reels to aircraft tie-down loops, tie-
down anchors, or tie-down stakes. Use tie-down stakes
for temporary tie-down only. If tie-down reels are not
available, 1⁄4" wire cable or 11⁄2" manila line may be
used.
Tie-Down Procedures for Seaplanes
Seaplanes can be moored to a buoy, weather permitting, or
tied to a dock. Weather causes wave action, and waves cause
the seaplane to bob and roll. This bobbing and rolling while
1-9COMBUSTIBLESORDINARY
CAPABILITYLIQUIDSFLAMMABLE
EQUIPMENTELECTRICAL
COMBUSTIBLESORDINARY
LIQUIDSFLAMMABLE
EQUIPMENTELECTRICAL
EQUIPMENTELECTRICAL
LIQUIDSFLAMMABLE
METALSCOMBUSTIBLE1. Water
2. Carbon Dioxide, Dry Chemical
Bromochlorodifluoromethane, and Bromotrifluoromethane
3. Multipurpose Dry Chemical
4. Multipurpose Dry Chemical (Insufficient Agent for ‘A’ Rating)
5. Dry PowderCOMBUSTIBLESORDINARY
LIQUIDSFLAMMABLE
EQUIPMENTELECTRICAL
METALSCOMBUSTIBLE
6'2'
3'
3'6'10'+Major axis
3'
3'
Figure 1-7. Typical extinguisher markings.Figure 1-8. Identification of fire extinguisher type location.
Figure 1-9. Diagram of tiedown dimensions.
Tie-Down Procedures for Ski Planes
Ski planes are tied down, if the securing means are available,
in the same manner as land planes. Ski-equipped airplanes can be secured on ice or in snow by using a device called a dead-man. A dead-man is any item at hand, such as a piece of pipe, log, and so forth, that a rope is attached to and buried in a snow or ice trench. Using caution to keep the free end of the rope dry and unfrozen, snow is packed in the trench. If available, pour water into the trench; when it is frozen, tie down the aircraft with the free end of the rope.tied to a dock can cause damage.
When warning of an impending storm is received and it is
not possible to fly the aircraft out of the storm area, some compartments of the seaplane can be flooded, partially sinking the aircraft. Tie down the aircraft securely to anchors. Seaplanes tied down on land have been saved from high-wind damage by filling the floats with water in addition to tying the aircraft down in the usual manner. During heavy weather, if possible, remove the seaplane from the water and tie down in the same manner as a land plane. If this is not possible, the seaplane could be anchored in a sheltered area away from the wind and waves.
Tying a Bowline Knot A
Tying a Square Knot BOverThis one must be under Up through and
around backBack down
through
Under
UnderOver
Figure 1-10. Knots commonly used for aircraft tie-down.Aft fuselage tiedown ring
Underside of wing
tiedown loop Main gear wheel tiedown loop Nose landing gear tiedown loop
Figure 1-11. Common tie-down points.
length of time the aircraft is expected to remain on the ground,
and location and characteristics of the aircraft. Wheel chocks,
control locks, rope tie-downs, mooring covers, tip socks, tie-
down assemblies, parking brakes, and rotor brakes are used Operators of ski-equipped aircraft sometimes pack soft snow
around the skis, pour water on the snow, and permit the skis
to freeze to the ice. This, in addition to the usual tie-down
procedures, aids in preventing damage from windstorms.
Caution must be used when moving an aircraft that has been
secured in this manner to ensure that a ski is not still frozen to
the ground. Otherwise, damage to the aircraft or skis can occur.
Tie-Down Procedures for Helicopters
Helicopters, like other aircraft are secured to prevent structural
damage that can occur from high-velocity surface winds.
Helicopters are to be secured in hangars, when possible. If not,
they must be tied down securely. Helicopters that are tied down
can usually sustain winds up to approximately 65 mph. If at all
possible, helicopters are evacuated to a safe area if tornadoes
or hurricanes are anticipated. For added protection, helicopters
can be moved to a clear area so that they are not damaged by
flying objects or falling limbs from surrounding trees.
If high winds are anticipated with the helicopter parked in the
open, tie down the main rotor blades. Detailed instructions for
securing and mooring each type of helicopter can be found in
the applicable maintenance manual. [Figure 1-13] Methods
of securing helicopters vary with weather conditions, the
Figure 1-12. Wheels chocked fore and aft.
to secure helicopters.
Typical mooring procedures are as follows:
1. Face the helicopter in the direction that the highest
forecast wind or gusts are anticipated.
2. Spot the helicopter slightly more than one rotor span
distance from other aircraft.
3. Place wheel chocks ahead of and behind all wheels
(where applicable). On helicopters equipped with
skids, retract the ground handling wheels, lower
the helicopter to rest on the skids, and install wheel
position lock pins or remove the ground-handling
wheels. Secure ground-handling wheels inside the
aircraft or inside the hangar or storage buildings. Do
not leave them unsecured on the flight line.
4. Align the blades and install tie-down assemblies
as prescribed by the helicopter manufacturer.
[Figure 1-14] Tie straps snugly without strain, and
during wet weather, provide some slack to avoid the
possibility of the straps shrinking, causing undue stress
on the aircraft and/or its rotor system(s).
5. Fasten the tie-down ropes or cables to the forward
and aft landing gear cross tubes and secure to ground
stakes or tie-down rings.
Procedures for Securing Weight-Shift-Control
There are many types of weight-shift-controlled aircraft—
engine powered and non-powered. These types of aircraft are
very susceptible to wind damage. The wings can be secured
in a similar manner as a conventional aircraft in light winds.
In high winds, the mast can be disconnected from the wing
and the wing placed close to the ground and secured. This
type of aircraft can also be partially disassembled or moved
into a hangar for protection. Procedures for Securing Powered Parachutes
When securing powered parachutes, pack the parachute in a
bag to prevent the chute from filling with air from the wind
and dragging the seat and engine. The engine and seat can
also be secured if needed.
Ground Movement of Aircraft
Engine Starting and Operation
The following instructions cover the starting procedures
for reciprocating, turboprop, turbofan, and APU. These
procedures are presented only as a general guide for
familiarization with typical procedures and methods. Detailed
instructions for starting a specific type of engine can be found
in the manufacturer’s instruction book.
Before starting an aircraft engine:
1. Position the aircraft to head into the prevailing wind
to ensure adequate airflow over the engine for cooling
purposes.
2. Make sure that no property damage or personal injury
occurs from the propeller blast or jet exhaust.
3. If external electrical power is used for starting, ensure
that it can be removed safely, and it is sufficient for
the total starting sequence.
4. During any and all starting procedures, a “fireguard”
equipped with a suitable fire extinguisher shall be
stationed in an appropriate place. A fireguard is
someone familiar with aircraft starting procedures.
The fire extinguisher should be a CO 2 extinguisher
of at least 5-pound capacity. The appropriate place is
adjacent to the outboard side of the engine, in view of
the pilot, and also where they can observe the engine/
aircraft for indication of starting problems.
5. If the aircraft is turbine-engine powered, the area in
front of the jet inlet must be kept clear of personnel,
property, and/or debris (FOD).
6. These “before starting” procedures apply to all aircraft
powerplants.
7. Follow manufacturer’s checklists for start procedures
and shutdown procedures.
Reciprocating Engines
The following procedures are typical of those used to start
reciprocating engines. There are, however, wide variations
in the procedures for the many reciprocating engines. Do not
attempt to use the methods presented here for actually starting
an engine. Instead, always refer to the procedures contained
in the applicable manufacturer’s instructions. Reciprocating
engines are capable of starting in fairly low temperatures
Align the blades
Figure 1-14. Securing helicopter blades and fuselage.
Figure 1-13. Example of mooring of a helicopter.
without the use of engine heating or oil dilution, depending
on the grade of oil used.
The various covers (wing, tail, flight deck, wheel, and
so forth) protecting the aircraft must be removed before
attempting to turn the engine. Use external sources of
electrical power when starting engines equipped with electric
starters, if possible or needed. This eliminates an excessive
burden on the aircraft battery. Leave all unnecessary electrical
equipment off until the generators are furnishing electrical
power to the aircraft power bus. Before starting a radial engine that has been shut down for
more than 30 minutes, check the ignition switch for off. Turn
the propeller three or four complete revolutions by hand to
detect a hydraulic lock, if one is present. Any liquid present
in a cylinder is indicated by the abnormal effort required to
rotate the propeller or by the propeller stopping abruptly
during rotation. Never use force to turn the propeller when
a hydraulic lock is detected. Sufficient force can be exerted
on the crankshaft to bend or break a connecting rod if a lock
is present.
To eliminate a lock, remove either the front or rear spark
plug from the lower cylinders and pull the propeller through.
Never attempt to clear the hydraulic lock by pulling the
propeller through in the direction opposite to normal
rotation. This tends to inject the liquid from the cylinder into
the intake pipe. The liquid is drawn back into the cylinder
with the possibility of complete or partial lock occurring
on the subsequent start.
To start the engine, proceed as follows:
1. Turn the auxiliary fuel pump on, if the aircraft is
so equipped.
2. Place the mixture control to the position recommended
for the engine and carburetor combination being
started. As a general rule, put the mixture control in
the “idle cut-off” position for fuel injection and in the
1-13“full rich” position for float-type carburetors. Many
light aircraft are equipped with a mixture control pull
rod that has no detent intermediate positions. When
such controls are pushed in flush with the instrument
panel, the mixture is set in the “full rich” position.
Conversely, when the control rod is pulled all the
way out, the carburetor is in the “idle cut-off” or “full
lean” position. The operator can select unmarked
intermediate positions between these two extremes
to achieve any desired mixture setting.
3. Open the throttle to a position that provides 1,000 to
1,200 rpm (approximately 1⁄8" to 1⁄2" from the “closed”
position).
4. Leave the pre-heat or alternate air (carburetor air)
control in the “cold” position to prevent damage
and fire in case of backfire. These auxiliary heating
devices are used after the engine warms up. They
improve fuel vaporization, prevent fouling of the
spark plugs, ice formation, and eliminate icing in
the induction system.
5. Move the primer switch to “on” intermittently (press
to prime by pushing in on the ignition switch during
the starting cycle), or prime with one to three strokes
of priming pump, depending on how the aircraft is
equipped. The colder the weather, the more priming
is needed.
6. Energize the starter and after the propeller has made
at least two complete revolutions, turn the ignition
switch on. On engines equipped with an induction
vibrator (shower of sparks, magneto incorporates a
retard breaker assembly), turn the switch to the “both”
position and energize the starter by turning the switch
to the “start” position. After the engine starts, release
the starter switch to the “both” position. When starting
an engine that uses an impulse coupling magneto,
turn the ignition switch to the “left” position. Place
the start switch to the “start” position. When the
engine starts, release the start switch. Do not crank
the engine continuously with the starter for more than
1 minute. Allow a 3- to 5-minute period for cooling
the starter (starter duty cycle) between successive
attempts. Otherwise, the starter may be burned out
due to overheating.
7. After the engine is operating smoothly, move the
mixture control to the “full rich” position if started in the
“idle cutoff” position. Carbureted engines are already
in the rich mixture position. Check for oil pressure.
8. Instruments for monitoring the engine during
operation include a tachometer for rpm, manifold
pressure gauge, oil pressure gauge, oil temperature
gauge, cylinder head temperature gauge, exhaust gas temperature gauge, and fuel flow gauge.
Hand Cranking Engines
If the aircraft has no self-starter, start the engine by turning the
propeller by hand (hand propping the propeller). The person
who is turning the propeller calls: “Fuel on, switch off, throttle
closed, brakes on.” The person operating the engine checks
these items and repeats the phrase. The switch and throttle must
not be touched again until the person swinging the prop calls
“contact.” The operator repeats “contact” and then turns on
the switch. Never turn on the switch and then call “contact.”
A few simple precautions help to avoid accidents when hand
propping the engine. While touching a propeller, always
assume that the ignition is on. The switches that control the
magnetos operate on the principle of short-circuiting the
current to turn the ignition off. If the switch is faulty, it can
be in the “off” position and still permit current to flow in
the magneto primary circuit. This condition could allow the
engine to start when the switch is off.
Be sure the ground is firm. Slippery grass, mud, grease, or loose
gravel can lead to a fall into or under the propeller. Never allow
any portion of your body to get in the way of the propeller. This
applies even when the engine is not being cranked.
Stand close enough to the propeller to be able to step away
as it is pulled down. Stepping away after cranking is a
safeguard in case the brakes fail. Do not stand in a position
that requires leaning toward the propeller to reach it. This
throws the body off balance and could cause a fall into the
blades when the engine starts.
In swinging the prop, always move the blade downward by
pushing with the palms of the hands. Do not grip the blade
with the fingers curled over the edge, since “kickback” may
break them or draw your body in the blade path. Excessive
throttle opening after the engine has fired is the principal
cause of backfiring during starting. Gradual opening of the
throttle, while the engine is cold, reduces the potential for
backfiring. Slow, smooth movement of the throttle assures
correct engine operation.
Avoid over priming the engine before it is turned over by the
starter. This can result in fires, scored or scuffed cylinders
and pistons, or engine failures due to hydraulic lock. If the
engine is inadvertently flooded or over primed, turn the
ignition switch off and move the throttle to the “full open”
position. To rid the engine of the excess fuel, turn it over by
hand or by the starter. If excessive force is needed to turn
over the engine, stop immediately. Do not force rotation of
the engine. If in doubt, remove the lower cylinder spark plugs.
1-14Immediately after the engine starts, check the oil pressure
indicator. If oil pressure does not show within 30 seconds,
stop the engine and determine the trouble. If oil pressure is
indicated, adjust the throttle to the aircraft manufacturer’s
specified rpm for engine warm up. Warm up rpm is usually
between 1,000 to 1,300 rpm.
Most aircraft reciprocating engines are air cooled and
depend on the forward speed of the aircraft to maintain
proper cooling. Therefore, particular care is necessary when
operating these engines on the ground. During all ground
running, operate the engine with the propeller in full low
pitch and headed into the wind with the cowling installed to
provide the best degree of engine cooling. Closely monitor
the engine instruments at all times. Do not close the cowl
flaps for engine warm-up, they need to be in the open
position while operating on the ground. When warming up
the engine, ensure that personnel, ground equipment that may
be damaged, or other aircraft are not in the propeller wash.
Extinguishing Engine Fires
In all cases, a fireguard should stand by with a CO 2 fire
extinguisher while the aircraft engine is being started. This
is a necessary precaution against fire during the starting
procedure. The fireguard must be familiar with the induction
system of the engine so that in case of fire, they can direct
the CO 2 into the air intake of the engine to extinguish it. A
fire could also occur in the exhaust system of the engine from
liquid fuel being ignited in the cylinder and expelled during
the normal rotation of the engine.
If an engine fire develops during the starting procedure, the
operator should continue cranking to start the engine and
extinguish the fire. If the engine does not start and the fire
continues to burn, discontinue the start attempt. The fireguard
then extinguishes the fire using the available equipment. The
fireguard must observe all safety practices at all times while
standing by during the starting procedure.
Turboprop Engines
The starting of any turbine engine consists of three steps that
must be carried out in the correct sequence. The starter turns
the main compressor to provide airflow though the engine. At
the correct speed that provides enough airflow, the igniters
are turned on and provide a hot spark to light the fuel that
is engaged next. As the engine accelerates, it reaches a self-
sustaining speed and the starter is disengaged.
The various covers protecting the aircraft must be removed.
Carefully inspect the engine exhaust areas for the presence
of fuel or oil. Make a close visual inspection of all accessible
parts of the engines and engine controls, followed by an
inspection of all nacelle areas to determine that all inspection and access plates are secured. Check sumps for water. Inspect
air inlet areas for general condition and foreign material.
Check the compressor for free rotation, when the installation
permits, by reaching in and turning the blades by hand.
The following procedures are typical of those used to start
turboprop engines. There are, however, wide variations in
the procedures applicable to the many turboprop engines.
Therefore, do not attempt to use these procedures in the actual
starting of a turboprop engine. These procedures are presented
only as a general guide for familiarization with typical
procedures and methods. For starting of all turboprop engines,
refer to the detailed procedures contained in the applicable
manufacturer’s instructions or their approved equivalent.
Turboprop engines are usually fixed turbine or free turbine.
The propeller is connected to the engine directly in a fixed
turbine, resulting in the propeller being turned as the engine
starts. This provides extra drag that must be overcome during
starting. If the propeller is not at the “start” position, difficulty
may be encountered in making a start due to high loads. The
propeller is in flat pitch at shut down and subsequently in flat
pitch during start because of this.
The free turbine engine has no mechanical connection
between the gas generator and the power turbine that is
connected to the propeller. In this type of engine, the propeller
remains in the feather position during starting and only turns
as the gas generator accelerates.
Instrumentation for turbine engines varies according to the
type of turbine engine. Turboprop engines use the normal
instruments—oil pressure, oil temperature, inter-turbine
temperature (ITT), and fuel flow. They also use instruments
to measure gas generator speed, propeller speed, and torque
produced by the propeller. [Figure 1-15] A typical turboprop
uses a set of engine controls, such as power levelers (throttle),
propeller levers, and condition levers. [Figure 1-16]
The first step in starting a turbine engine is to provide an
adequate source of power for the starter. On smaller turbine
engines, the starter is an electric motor that turns the engine
through electrical power. Larger engines need a much
more powerful starter. Electric motors would be limited by
current flow and weight. Air turbine starters were developed
that were lighter and produced sufficient power to turn the
engine at the correct speed for starting. When an air turbine
starter is used, the starting air supply may be obtained from
an APU onboard the aircraft, an external source (ground air
cart), or an engine cross-bleed operation. In some limited
cases, a low-pressure, large-volume tank can provide the air
for starting an engine. Many smaller turboprop engines are
started using the starter/generator, that is both the engine
1-15starter and the generator.
While starting an engine, always observe the following:
• Always observe the starter duty cycle. Otherwise, the
starter can overheat and be damaged.
• Assure that there is enough air pressure or electrical
capacity before attempting a start.
• Do not perform a ground start if turbine inlet
temperature (residual temperature) is above that
specified by the manufacturer.
• Provide fuel under low pressure to the engine’s fuel
pump.
Turboprop Starting Procedures
To start an engine on the ground, perform the following
operations:
1. Turn the aircraft boost pumps on.
2. Make sure that the power lever is in the “start”
position.
3. Place the start switch in the “start” position. This starts
the engine turning.
4. Place the ignition switch on. (On some engines, the
ignition is activated by moving the fuel lever.)
5. The fuel is now turned on. This is accomplished by
moving the condition lever to the “on” position.
6. Monitor the engine lights of the exhaust temperature.
If it exceeds the limits, shut the engine down.
7. Check the oil pressure and temperature.
8. After the engine reaches a self-sustaining speed, the
starter is disengaged.
9. The engine continues to accelerate up to idle.
10. Maintain the power lever at the “start” position until
the specified minimum oil temperature is reached.
11. Disconnect the ground power supply, if used.
If any of the following conditions occur during the starting
sequence, turn off the fuel and ignition switch, discontinue
the start immediately, make an investigation, and record
the findings.
• Turbine inlet temperature exceeds the specified
maximum. Record the observed peak temperature.
• Acceleration time from start of propeller rotation to
stabilized rpm exceeds the specified time.
• There is no oil pressure indication at 5,000 rpm for
either the reduction gear or the power unit.
• Torching (visible burning in the exhaust nozzle).• The engine fails to ignite by 4,500 rpm or maximum
motoring rpm.
• Abnormal vibration is noted or compressor surge
occurs (indicated by backfiring).
• Fire warning bell rings. (This may be due to either an
engine fire or overheat.)
Turbofan Engines
Unlike reciprocating engine aircraft, the turbine-powered
aircraft does not require a preflight run-up unless it is
necessary to investigate a suspected malfunction.
Before starting, all protective covers and air inlet duct covers
are removed. If possible, head the aircraft into the wind to
obtain better cooling, faster starting, and smoother engine
performance. It is especially important that the aircraft be
headed into the wind if the engine is to be trimmed.
The run-up area around the aircraft is cleared of both
personnel and loose equipment. The turbofan engine intake
and exhaust hazard areas are illustrated in Figure 1-17 .
Exercise care to ensure that the run-up area is clear of all
items, such as nuts, bolts, rocks, shop towels, or other loose
debris. Many very serious accidents have occurred involving
personnel in the vicinity of turbine engine air inlets. Use
extreme caution when starting turbine aircraft.
Check the aircraft fuel sumps for water or ice. Inspect the
engine air inlet for general condition and the presence of
foreign objects. Visually inspect the fan blades, forward
compressor blades, and the compressor inlet guide vanes for
nicks and other damage. If possible, check the fan blades for
free rotation by turning the fan blades by hand. All engine
controls must be operational. Check engine instruments and
warning lights for proper operation.
Starting a Turbofan Engine
The following procedures are typical of those used to start
many turbine engines. There are, however, wide variations
in the starting procedures used for turbine engines, and no
attempts are to be made to use these procedures in the actual
starting of an engine. These procedures are presented only as
a general guide for familiarization with typical procedures
and methods. In the starting of all turbine engines, refer
to the detailed procedures contained in the applicable
manufacturer’s instructions or their approved equivalent.
Most turbofan engines can be started by either air turbine
or electrical starters. Air-turbine starters use compressed air
from an external source as discussed earlier. Fuel is turned on
either by moving the start lever to “idle/start” position or by
opening a fuel shutoff valve. If an air turbine starter is used,
FUEL FLOW
RPH X 100 0
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FUEL FLOW
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TORQUE
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TORQUE
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PROP
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FEATHERPROPELLER
IDLE
REVERSEP
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Power levers Condition leversProp leversFigure 1-15. Typical examples of turboprop instruments.
Figure 1-16. Engine controls of a turboprop aircraft.the engine “lights off” within a predetermined time after the
fuel is turned on. This time interval, if exceeded, indicates a
malfunction has occurred and the start must be discontinued.
Most turbofan engine controls consist of a power lever,
reversing levers, and start levers. Newer aircraft have replaced
the start levers with a fuel switch. [Figure 1-18] Turbofan
engines also use all the normal instruments speeds, (percent
of total rpm) exhaust gas temperature, fuel flow, oil pressure,
and temperature. An instrument that measures the amount
of thrust being delivered is the engine pressure ratio. This
measures the ratio between the inlet pressures to the outlet
pressure of the turbine.
The following procedures are useful only as a general guide
and are included to show the sequence of events in starting
a turbofan engine.
1. If the engine is so equipped, place the power lever in
the “idle” position.
2. Turn the fuel boost pump(s) switch on.
3. A fuel inlet pressure indicator reading ensures fuel is
being delivered to engine fuel pump inlet.
4. Turn engine starter switch on. Note that the engine
rotates to a preset limit. Check for oil pressure.
5. Turn ignition switch on. (This is usually accomplished
by moving the start lever toward the “on” position.
A micro switch connected to the leveler turns on the
ignition.)
6. Move the start lever to “idle” or “start” position, this
starts fuel flow into the engine.
7. Engine start (light off) is indicated by a rise in exhaust
gas temperature.
8. If a two-spool engine, check rotation of fan or N1.
9. Check for proper oil pressure.
10. Turn engine starter switch off at proper speeds.
11. After engine stabilizes at idle, ensure that none of the
engine limits are exceeded.
12. Newer aircraft drop off the starter automatically.
Auxiliary Power Units (APUs)
APUs are generally smaller turbine engines that provide
compressed air for starting engines, cabin heating and
cooling, and electrical power while on the ground. Their
operation is normally simple. By turning a switch on and
up to the start position (spring loaded to on position), the
engine starts automatically. During start, the exhaust gas
temperature must be monitored. APUs are at idle at 100
percent rpm with no load. After the engine reaches its
operating rpm, it can be used for cooling or heating the
1-17cabin and for electrical power. It is normally used to start
the main engines.
Unsatisfactory Turbine Engine Starts
Hot Start
A hot start occurs when the engine starts, but the exhaust
gas temperature exceeds specified limits. This is usually
caused by an excessively rich air-fuel mixture entering the
combustion chamber. This condition can be caused by either
too much fuel or not enough airflow. The fuel to the engine
must be shut off immediately.
False or Hung Start
False or hung starts occur when the engine starts normally, but
the rpm remains at some low value rather than increasing to
the normal starting rpm. This is often the result of insufficient
power to the starter or the starter cutting off before the engine
starts self-accelerating. In this case, shut the engine down.
Engine Fails to Start
The engine failing to start within the prescribed time limit
can be caused by lack of fuel to the engine, insufficient or
no electrical power to the exciter in the ignition system, or
incorrect fuel mixture. If the engine fails to start within the
prescribed time, shut it down.
In all cases of unsatisfactory starts, the fuel and ignition must be
turned off. Continue rotating the compressor for approximately
15 seconds to remove accumulated fuel from the engine. If
unable to motor (rotate) the engine, allow a 30-second fuel
draining period before attempting another start.
Towing of Aircraft
Movement of large aircraft about the airport, flight line,
and hangar is usually accomplished by towing with a tow
tractor (sometimes called a “tug”). [Figure 1-19] In the case
of small aircraft, some moving is accomplished by hand
pushing on the correct areas of the aircraft. Aircraft may also
be taxied about the flight line but usually only by certain
qualified personnel.
Towing aircraft can be a hazardous operation, causing
damage to the aircraft and injury to personnel, if done
recklessly or carelessly. The following paragraphs outline
the general procedure for towing aircraft. However, specific
instructions for each model of aircraft are detailed in the
manufacturer’s maintenance instructions and are to be
followed in all instances.
Before the aircraft to be towed is moved, a qualified person
must be in the flight deck to operate the brakes in case the
tow bar fails or becomes unhooked. The aircraft can then be
stopped, preventing possible damage.Some types of tow bars available for general use can be used
for many types of towing operations. [Figure 1-20] These
bars are designed with sufficient tensile strength to pull most
aircraft, but are not intended to be subjected to torsional or
twisting loads. Many have small wheels that permit them to be
drawn behind the towing vehicle going to or from an aircraft.
When the bar is attached to the aircraft, inspect all the engaging
devices for damage or malfunction before moving the aircraft.
Additionally, some aircraft have tow steering turn limits.
Some tow bars are designed for towing various types of
aircraft. However, other special types can be used on a
particular aircraft only. Such bars are usually designed and
built by the aircraft manufacturer.
When towing the aircraft, the towing vehicle speed must be
reasonable, and all persons involved in the operation must be
alert. When the aircraft is stopped, do not rely upon the brakes
of the towing vehicle alone to stop the aircraft. The person in
the flight deck must coordinate the use of the aircraft brakes
with those of the towing vehicle. A typical smaller aircraft
tow tractor (or tug) is shown in Figure 1-21 .
The attachment of the tow bar varies on different types of
aircraft. Aircraft equipped with tail wheels are generally
towed forward by attaching the tow bar to the main landing
gear. In most cases, it is permissible to tow the aircraft in
reverse by attaching the tow bar to the tail wheel axle. Any
time an aircraft equipped with a tail wheel is towed, the tail
wheel must be unlocked or the tail wheel locking mechanism
may damage or break. Aircraft equipped with tricycle landing
gear are generally towed forward by attaching a tow bar to the
axle of the nosewheel. They may also be towed forward or
backward by attaching a towing bridle or specially designed
towing bar to the towing lugs on the main landing gear. When
an aircraft is towed in this manner, a steering bar is attached
to the nosewheel to steer the aircraft.
The following towing and parking procedures are typical of
one type of operation. They are examples and not necessarily
suited to every type of operation. Aircraft ground-handling
personnel must be thoroughly familiar with all procedures
pertaining to the types of aircraft being towed and local
operation standards governing ground handling of aircraft.
Competent persons that have been properly checked out
direct the aircraft towing team.
1. The towing vehicle driver is responsible for operating
the vehicle in a safe manner and obeying emergency
stop instructions given by any team member.
2. The person in charge assigns team personnel as wing
walkers. A wing walker is stationed at each wingtip, in
such a position that they can ensure adequate clearance
Distance in feet
Velocity in knots = K
Temperature in °F
125° 60 K
700°900 K500°500 K300°300 K200° 200 K150° 100 K
125° 40 K
150° 60 K
200° 100 K
300° 200 K200
0100º 35 K30 feet
25 feet100°12 feet
25 K
25 feet
ExhaustAir intake idle Air intake takeoff
Figure 1-17. Engine intake and exhaust hazard areas.
Figure 1-18. Turbofan engine control levers.
Figure 1-19. Example of a tow tractor. Figure 1-20. Example of a tow bar.of any obstruction in the path of the aircraft. A tail
walker is assigned when sharp turns are to be made
or when the aircraft is to be backed into position.
3. A qualified person occupies the pilot’s seat of the towed
aircraft to observe and operate the brakes as required.
When necessary, another qualified person is stationed to
watch and maintain aircraft hydraulic system pressure.
4. The person in charge of the towing operation verifies
that, on aircraft with a steerable nosewheel, the
locking scissors are set to full swivel for towing. The
locking device must be reset after the tow bar has
been removed from the aircraft. Persons stationed
in the aircraft are not to attempt to steer or turn the
nosewheel when the tow bar is attached to the aircraft.
5. Under no circumstances is anyone permitted to walk
or to ride between the nosewheel of an aircraft and the
towing vehicle, nor ride on the outside of a moving
aircraft or on the towing vehicle. In the interest of
safety, no attempt to board or leave a moving aircraft
or towing vehicle is permitted.
6. The towing speed of the aircraft is not to exceed that
of the walking team members. The aircraft’s engines
usually are not operated when the aircraft is being towed into position.
7. The aircraft brake system is to be charged before each
towing operation. Aircraft with faulty brakes are towed
into position only for repair of brake systems, and then
personnel must be standing by ready with chocks for
emergency use. Chocks must be immediately available
in case of an emergency throughout any towing
operation.
8. To avoid possible personal injury and aircraft damage
during towing operations, entrance doors are closed,
ladders retracted, and gear-down locks installed.
9. Prior to towing any aircraft, check all tires and landing
gear struts for proper inflation. (Inflation of landing gear
struts of aircraft in overhaul and storage is excluded.)
10. When moving aircraft, do not start and stop suddenly.
For added safety, aircraft brakes must never be applied
during towing, except upon command by one of the
tow team members in an emergency situation.
11. Aircraft are parked in specified areas. Generally,
the distance between rows of parked aircraft is great
enough to allow immediate access of emergency
vehicles in case of fire, as well as free movement of
equipment and materials.
12. Wheel chocks are placed fore and aft of the main
landing gear of the parked aircraft.
13. Internal or external control locks (gust locks or blocks)
are used while the aircraft is parked.
14. Prior to any movement of aircraft across runways
or taxiways, contact the airport control tower on the
appropriate frequency for clearance to proceed.
15. An aircraft parked in a hangar must be statically
grounded immediately.
Taxiing Aircraft
As a general rule, only rated pilots and qualified airframe
and powerplant (A&P) technicians are authorized to
start, run up, and taxi aircraft. All taxiing operations are
Figure 1-21. Typical smaller aircraft tow tractor.
performed in accordance with applicable local regulations.
Figure 1-22 contains the standard taxi light signals used by
control towers to control and expedite the taxiing of aircraft.
The following section provides detailed instructions on taxi
signals and related taxi instructions.
Taxi Signals
Many ground accidents have occurred as a result of improper
technique in taxiing aircraft. Although the pilot is ultimately
responsible for the aircraft until the engine is stopped, a
taxi signalman can assist the pilot around the flight line. In
some aircraft configurations, the pilot’s vision is obstructed
while on the ground. The pilot cannot see obstructions close
to the wheels or under the wings and has little idea of what
is behind the aircraft. Consequently, the pilot depends upon
the taxi signalman for directions. Figure 1-23 shows a taxi
signalman indicating his readiness to assume guidance of
the aircraft by extending both arms at full length above his
head, palms facing each other.
The standard position for a signalman is slightly ahead of
and in line with the aircraft’s left wingtip. As the signalman
faces the aircraft, the nose of the aircraft is on the left.
[Figure 1-24] The signalman must stay far enough ahead
of the wingtip to remain in the pilot’s field of vision. It is a
good practice to perform a foolproof test to be sure the pilot
can see all signals. If the signalman can see the pilot’s eyes,
the pilot can see the signals.
Figure 1-24 shows the standard aircraft taxiing signals
published in the Federal Aviation Administration (FAA)
Aeronautical Information Manual (AIM). There are other
standard signals, such as those published in Advisory Circular
00-34, as revised, and by the International Standards (ICAO)
Annex 2, Appendix 1 and the Armed Forces. Furthermore,
operation conditions in many areas may call for a modified
set of taxi signals. The signals shown in Figure 1-24 represent a minimum number of the most commonly used signals.
Whether this set of signals or a modified set is used is not
the most important consideration, as long as each flight
operational center uses a suitable, agreed-upon set of signals.
Figure 1-25 illustrates some of the most commonly used
helicopter operating signals.
The taxi signals to be used must be studied until the taxi
signalman can execute them clearly and precisely. The signals
are to be given in such a way that the pilot cannot confuse
their meaning. Remember that the pilot receiving the signals
is always some distance away and often look out and down
from a difficult angle. Thus, the signalman’s hands must be
kept well separated, and signals are to be over-exaggerated
rather than risk making indistinct signals. If there is any
doubt about a signal, or if the pilot does not appear to be
following the signals, use the “stop” sign and begin the series
of signals again.
The signalman is to always try to give the pilot an indication
of the approximate area that the aircraft is to be parked.
The signalman must glance behind himself or herself often
when walking backward to prevent backing into a propeller
or tripping over a chock, fire bottle, tie-down line, or
other obstruction.
Taxi signals are usually given at night with the aid of
illuminated wands attached to flashlights. [Figure 1-26] Night
signals are made in the same manner as day signals with the
exception of the stop signal. The stop signal used at night is
the “emergence stop” signal. This signal is made by crossing
the wands to form a lighted “X” above and in front of the head.
Servicing Aircraft
Servicing Aircraft Air/Nitrogen Oil & Fluids
Checking or servicing aircraft fluids is an important
maintenance function. Before servicing any aircraft, consult
the specific aircraft maintenance manual to determine the
proper type of servicing equipment and procedures. In
general, aircraft engine oil is checked with a dipstick or a sight
gauge. There are markings on the stick or around the sight
gauge to determine the correct level. Reciprocating engines
are to be checked after the engine has been inactive, while
the turbine engine must be checked just after shutdown. Dry
sump oil systems tend to hide oil that has seeped from the oil
tank into the gearcase of the engine. This oil does not show up
on the dipstick until the engine has been started or motored.
If serviced before this oil is pumped back into the tank, the
engine overfills. Never overfill the oil tank. Oil foams as it
is circulated through the engine. The expansion space in the
oil tank allows for this foaming (oil mixing with air). Also
the correct type of oil must be used for the appropriate engine
being serviced. Hydraulic fluid, fuel, and oil, if spilled on
clothes or skin, must be removed as soon as possible because
1-21Lights Meaning
Flashing g reen Clear ed t o taxi
Steady r ed Stop
Flashing r ed Taxi clear of run way in use
Flashing whit e Retur n to star ting point
Alternating r ed and g reen Exercise ex treme caution
Figure 1-22. Standard taxi light signals.Figure 1-23. The taxi signalman.of fire danger and health reasons.
When servicing a hydraulic reservoir, the correct fluid must
be used. Normally, this can be determined by the container
or by color. Some reservoirs are pressurized by air that must
be bled off before servicing. Efforts must be made to prevent
any type of contamination during servicing. Also, if changing
hydraulic filters, assure that the pressure is off the system
before removing the filters. After servicing the filters (if large
amounts of fluids were lost) or system quantity, air must be
purged and the system checked for leaks. While servicing
tires or struts with high-pressure nitrogen, the technician
must use caution while performing maintenance. Clean areas
before connecting filling hose and do not overinflate.
Ground Support Equipment
Electric Ground Power Units
Ground support electrical APUs vary widely in size and
type. However, they can be generally classified by towed,
stationary, or self-propelled items of equipment. Some units
are mainly for in-hangar use during maintenance. Others are
designed for use on the flight line, either at a stationary gate
area or towed from aircraft to aircraft. The stationary type
can be powered from the electrical service of the facility.
The movable type ground power unit (GPU) generally has
an onboard engine that turns a generator to produce power.
Some smaller units use a series of batteries. The towed power
units vary in size and range of available power.
The smallest units are simply high-capacity batteries used
to start light aircraft. These units are normally mounted on
wheels or skids and are equipped with an extra-long electrical
line terminated in a suitable plug-in adapter.
Larger units are equipped with generators. Providing a
wider range of output power, these power units are normally
designed to supply constant-current, variable voltage DC
electrical power for starting turbine aircraft engines and
constant-voltage DC for starting reciprocating aircraft
engines. Normally somewhat top-heavy, large towed power
units are towed at restricted speeds, and sharp turns are
avoided. An example of a large power unit is shown in Figure 1-27 .
Self-propelled power units are normally more expensive than
the towed units and, in most instances, supply a wider range
of output voltages and frequencies. The stationary power unit,
shown in Figure 1-28, is capable of supplying DC power in
varying amounts, as well as 115/200-volt, 3-phase, 400-cycle
AC power continuously for 5 minutes.
When using ground electrical power units, it is important to
position the unit to prevent collision with the aircraft being
serviced, or others nearby, in the event the brakes on the unit
fail. It must be parked so that the service cable is extended
to near its full length away from the aircraft being serviced,
but not so far that the cable is stretched or undue stress is
placed on the aircraft electrical receptacle.
Observe all electrical safety precautions when servicing an
aircraft. Additionally, never move a power unit when service
cables are attached to an aircraft or when the generator
system is engaged.
Start engines Slow downFlagman directs pilot to signalman
if traffic conditions require
Stop Come ahead Emergency stop Cut engines
Pull chocks Insert chocks
All clear (O.K.) Left turn Right turn Night operationSignalman directs towing Signalman's position
Figure 1-24. Standard F AA hand taxi signals.
Hydraulic Ground Power Units
Portable hydraulic test stands are manufactured in many sizes
and cost ranges. [Figure 1-29] Some have a limited range of
operation, while others can be used to perform all the system
tests that fixed-shop test stands are designed to perform.
Hydraulic power units, sometimes called a hydraulic mule,
provide hydraulic pressure to operate the aircraft systems
during maintenance. They can be used to:• Drain the aircraft hydraulic systems.
• Filter the aircraft hydraulic system fluid.
• Refill the aircraft hydraulic system with clean fluid.
• Check the aircraft hydraulic systems for operation
and leaks.
This type of portable hydraulic test unit is usually an
electrically-powered unit. It uses a hydraulic system
Take off Go downMove back Move forward Move right Move left
Landing direction Go up
Swing tail to right Swing tail to leftStop Engage rotor Start engine Stop rotor
Figure 1-25. Helicopter operating signals.
capable of delivering a variable volume of fluid from zero
to approximately 24 gallons per minute at variable pressures up to 3,000 psi. Operating at pressures of 3,000 psi or more, extreme caution must be used when operating hydraulic power units. At 3,000 psi, a small stream from a leak can cut like a sharp knife. Therefore, inspect lines used with the system for cuts, frays, or any other damage, and keep them free of kinks and twists. When not in use, hydraulic power unit lines are to be stored (preferably wound on a reel) and kept clean, dry, and free of contaminants.
Ground Support Air Units
Air carts are used to provide low-pressure (up to 50 psi high volume flow) air that can be used for starting the engines and heating and cooling the aircraft on the ground (using the onboard aircraft systems). It generally consists of an APU
Figure 1-26. Night operations with wands.
Figure 1-27. A mobile electrical power unit.
Figure 1-28. A stationary electrical power unit.built into the cart that provides bleed air from the APU’s
compressor for operating aircraft systems or starting engines.
[Figure 1-30]
Ground Air Heating and Air Conditioning
Most airport gates have facilities that can provide heated or
cooled air. The units that cool or heat the air are permanent
installations that connect to the aircraft’s ventilation
system by use of a large hose. Portable heating and air
conditioning units can also be moved close to the aircraft
and connected by a duct that provides air to keep the cabin
temperature comfortable.
Oxygen Servicing Equipment
Before servicing any aircraft, consult the specific aircraft
maintenance manual to determine the proper types of
servicing equipment to be used. Two personnel are required
to service an aircraft with gaseous oxygen. One person is
stationed at the control valves of the servicing equipment, and
one person is stationed where they can observe the pressure in
the aircraft system. Communication between the two people
is required in the event of an emergency.
Do not service aircraft with oxygen during fueling, defueling,
or other maintenance work that could provide a source of
ignition. Oxygen servicing of aircraft is to be accomplished outside hangars.
Oxygen used on aircraft is available in two types: gaseous and
liquid. The type to use on any specific aircraft depends on the
type of equipment in the aircraft. Gaseous oxygen is stored
in large steel cylinders, while liquid oxygen (commonly
referred to as LOX) is stored and converted into a usable gas
in a liquid oxygen converter.
Oxygen is commercially available in three general types:
aviator’s breathing, industrial, and medical. Only oxygen
marked “Aviator’s Breathing Oxygen” that meets Federal
Specification BB-0-925A, Grade A, or its equivalent is to be
used in aircraft breathing oxygen systems. Industrial oxygen
may contain impurities that could cause the pilot, crew, and/
or passengers to become sick. Medical oxygen, although pure,
contains water that can freeze in the cold temperatures found
at the altitudes where oxygen is necessary.
Figure 1-29. A portable hydraulic power unit.Figure 1-30. Aircraft air start unit.Oxygen Hazards
Gaseous oxygen is chemically stable and is nonflammable.
However, combustible materials ignite more rapidly and
burn with greater intensity in an oxygen-rich atmosphere. In
addition, oxygen combines with oil, grease, or bituminous
material to form a highly-explosive mixture that is sensitive
to compression or impact. Physical damage to, or failure
of, oxygen containers, valves, or plumbing can result in an
explosive rupture with extreme danger to life and property.
It is imperative that the highest standard of cleanliness be
observed in handling oxygen and that only qualified and
authorized persons be permitted to service aircraft gaseous
oxygen systems. In addition to aggravating the fire hazard
and because of its low temperature (it boils at −297 °F),
liquid oxygen causes severe “burns” (frostbite) if it comes
in contact with the skin.
Fuel Servicing of Aircraft
Types of Fuel and Identification
Two types of aviation fuel in general use are aviation gasoline,
also known as A VGAS, and turbine fuel, also known as JET
A fuel.
Aviation gasoline (A VGAS) is used in reciprocating engine
aircraft. Currently, there are three grades of fuel in general
use: 80/87, 100/130, and 100LL (low lead). A fourth grade,
115/145, is in limited use in the large reciprocating-engine
aircraft. The two numbers indicate the lean mixture and rich
mixture octane rating numbers of the specific fuel. In other
words, with 80/87 A VGAS, the 80 is the lean mixture rating
and 87 is the rich mixture rating number. To avoid confusing
the types of A VGAS, it is generally identified as grade 80, 100, 100LL, or 115. A VGAS can also be identified by a color
code. The color of the fuel needs to match the color band on
piping and fueling equipment. [Figure 1-31]
Turbine fuel/jet fuel is used to power turbojet and turbo-
shaft engines. Three types of turbine fuel generally used
in civilian aviation are JET A and JET A-1, made from
kerosene, and JET B, a blend of kerosene and A VGAS.
While jet fuel is identified by the color black on piping and
fueling equipment, the actual color of jet fuel can be clear
or straw colored.
Before mixing A VGAS and turbine fuel, refer to the Type
Certificate Data Sheet for the respective powerplant. Adding
jet fuel to A VGAS causes a decrease in the power developed
by the engine and could cause damage to the engine (through
detonation) and loss of life. Adding A VGAS to jet fuel can
cause lead deposits in the turbine engine and can lead to
reduced service life.
Contamination Control
Contamination is anything in the fuel that is not supposed to
be there. The types of contamination found in aviation fuel
include water, solids, and microbial growths. The control of
contamination in aviation fuel is extremely important, since
contamination can lead to engine failure or stoppage and the
loss of life. The best method of controlling contamination
is to prevent its introduction into the fuel system. Some
forms of contamination can still occur inside the fuel system.
However, the filter, separators, and screens remove most of
the contamination.
Water in aviation fuels generally take two forms: dissolved
(vapor) and free water. The dissolved water is not a major
problem until, as the temperature lowers, it becomes free
water. This then poses a problem if ice crystals form, clogging
filters and other small orifices.
1-26Color Grade
Red 80
Green 100
Blue 100LL
Purple 115
Figure 1-31. Aviation gasoline color and grade reference.Free water can appear as water slugs or entrained water.
Water slugs are concentrations of water. This is the water
that is drained after fueling an aircraft. Entrained water is
suspended water droplets. These droplets may not be visible
to the eye but give the fuel a cloudy look. The entrained water
settles out in time.
Solid contaminants are insoluble in fuel. The more common
types are rust, dirt, sand, gasket material, lint, and fragments
of shop towels. The close tolerances of fuel controls and
other fuel-related mechanisms can be damaged or blocked
by particles as small as 1⁄20 the diameter of a human hair.
Microbiological growths are a problem in jet fuel. There
are a number of varieties of micro-organisms that can
live in the free water in jet fuel. Some variations of these
organisms are airborne, others live in the soil. The aircraft
fuel system becomes susceptible to the introduction of
these organisms each time the aircraft is fueled. Favorable
conditions for the growth of micro-organisms in the fuel
are warm temperatures and the presence of iron oxide and
mineral salts in the water. The best way to prevent microbial
growth is to keep the fuel dry.
The effects of micro-organisms are:
• Formation of slime or sludge that can foul filters,
separators, or fuel controls.
• Emulsification of the fuel.
• Corrosive compounds that can attack the fuel tank’s
structure. In the case of a wet wing tank, the tank is
made from the aircraft’s structure. They can also have
offensive odors.
Fueling Hazards
The volatility of aviation fuels creates a fire hazard that has
plagued aviators and aviation engine designers since the
beginning of powered flight. V olatility is the ability of a liquid
to change into a gas at a relatively low temperature. In its
liquid state, aviation fuel does not burn. It is, therefore, the
vapor or gaseous state that the liquid fuel changes that is not
only useful in powering the aircraft, but also a fire hazard.
Static electricity is a byproduct of one substance rubbing
against another. Fuel flowing through a fuel line causes
a certain amount of static electricity. The greatest static
electricity concern around aircraft is that during flight, the
aircraft moving through the air causes static electricity to
build in the airframe. If that static electricity is not dissipated
prior to refueling, the static electricity in the airframe attempts
to return to the ground through the fuel line from the servicing
unit. The spark caused by the static electricity can ignite any
vaporized fuel.Breathing the vapors from fuel can be harmful and must
be limited. Any fuel spilled on the clothing or skin must be
removed as soon as possible.
Fueling Procedures
The proper fueling of an aircraft is the responsibility of the
owner/operator. This does not, however, relieve the person
doing the fueling of the responsibility to use the correct type
of fuel and safe fueling procedures.
There are two basic procedures when fueling an aircraft.
Smaller aircraft are fueled by the over-the-wing method.
This method uses the fuel hose to fill through fueling ports
on the top of the wing. The method used for larger aircraft is
the single point fueling system. This type of fueling system
uses receptacles in the bottom leading edge of the wing to
fill all the tanks. This decreases the time it takes to refuel
the aircraft, limits contamination, and reduces the chance
of static electricity igniting the fuel. Most pressure fueling
systems consist of a pressure fueling hose and a panel of
controls and gauges that permit one person to fuel or defuel
any or all fuel tanks of an aircraft. Each tank can be filled
to a predetermined level. These procedures are illustrated in
Figures 1-32 and 1-33 .
Prior to fueling, the person fueling must check the following:
1. Ensure all aircraft electrical systems and electronic
devices, including radar, are turned off.
2. Do not carry anything in the shirt pockets. These items
could fall into the fuel tanks.
3. Ensure no flame-producing devices are carried by
anyone engaged in the fueling operation. A moment
of carelessness could cause an accident.
4. Ensure that the proper type and grade of fuel is used.
Do not mix A VGAS and JET fuel.
5. Ensure that all the sumps have been drained.
6. Wear eye protection. Although generally not as critical
as eye protection, other forms of protection, such as
rubber gloves and aprons, can also protect the skin
from the effects of spilled or splashed fuel.
Ground wire
Figure 1-32. Refueling an aircraft by the over-the-wing method.
7. Do not fuel aircraft if there is danger of other aircraft in
the vicinity blowing dirt in the direction of the aircraft
being fueled. Blown dirt, dust, or other contaminants
can enter an open fuel tank, contaminating the entire
contents of the tank.
8. Do not fuel an aircraft when there is lightning within
5 miles.
9. Do not fuel an aircraft within 500 feet of operating
ground radar.When using mobile fueling equipment:
1. Approach the aircraft with caution, positioning the
fuel truck so that if it is necessary to depart quickly,
no backing needed.
2. Set the hand brake of the fuel truck, and chock the
wheels to prevent rolling.
3. Ground the aircraft and then ground the truck. Next,
ground or bond them together by running a connecting
wire between the aircraft and the fuel truck. This may
be done by three separate ground wires or by a “Y”
Figure 1-33. Single point refueling station of a large aircraft.cable from the fuel truck.
4. Ensure that the grounds are in contact with bare
metal or are in the proper grounding points on the
aircraft. Do not use the engine exhaust or propeller as
grounding points. Damage to the propeller can result,
and there is no way of quickly ensuring a positive bond
between the engine and the airframe.
5. Ground the nozzle to the aircraft, then open the fuel
tank.
6. Protect the wing and any other item on the aircraft from
damage caused by spilled fuel or careless handling of
the nozzle, hose, or grounding wires.
7. Check the fuel cap for proper installation and security
before leaving the aircraft.
8. Remove the grounding wires in the reverse order. If
the aircraft is not going to be flown or moved soon,
the aircraft ground wire can be left attached.
When fueling from pits or cabinets, follow the same
procedures as when using a truck. Pits or cabinets are usually
designed with permanent grounding, eliminating the need
to ground the equipment. However, the aircraft still must be
grounded, and then the equipment must be grounded to the
aircraft as it was with mobile equipment.
Defueling
Defueling procedures differ with different types of aircraft.
Before defueling an aircraft, check the maintenance/service
manual for specific procedures and cautions. Defueling can
be accomplished by gravity defueling or by pumping the
fuel out of the tanks. When the gravity method is used, it is necessary to have a method of collecting the fuel. When
the pumping method is used, care must be taken not to
damage the tanks, and the removed fuel cannot be mixed
with good fuel.
General precautions when defueling are:
• Ground the aircraft and defueling equipment.
• Turn off all electrical and electronic equipment.
• Have the correct type of fire extinguisher available.
• Wear eye protection.
Regulations, Maintenance Forms,
Records, & Publications
Chapter 2
Overview — Title 14 of the Code of Federal
Regulations (14 CFR)
Aviation-related regulations that have occurred from 1926–
1966 are reflected in Figure 2-1 . Just as aircraft continue to
evolve with ever improving technology, so do the regulations,
publications, forms, and records required to design, build,
and maintain them.
The Federal Aviation Administration (FAA) regulations that
govern today’s aircraft are found in Title 14 of the Code
of Federal Regulations (14 CFR). [Figure 2-2] There are
five volumes under Title 14, Aeronautics and Space. The
first three volumes containing 75 active regulations address
the Federal Aviation Administration. The fourth volume
deals with the Office of the Secretary of the Department
of Transportation (Aviation Proceedings) and Commercial
Space Transportation, while the fifth volume addresses the
National Aeronautics and Space Administration (NASA) and
Air Transportation System Stabilization.
These regulations can be separated into the following three
categories:
1. Administrative
2. Airworthiness Certification
3. Airworthiness Operation
Since 1958, these rules have typically been referred to as
“FARs,” short for Federal Aviation Regulations. However,
another set of regulations, Title 48, is titled “Federal
Acquisitions Regulations,” and this has led to confusion
with the use of the acronym “FAR.” Therefore, the FAA
began to refer to specific regulations by the term “14 CFR
part XX.” Most regulations and the sections within are odd
numbered, because the FAA realized in 1958 when the Civil
Aeronautics Regulations were recodified into the Federal
Aviation Regulations that it would be necessary to add
regulations later.
Over the years, the FAA has sometimes seen the need to issue
Special Federal Aviation Regulations (SFAR). [Figure 2-3]
These are frequently focused very specifically on a unique
situation and are usually given a limited length of time
for effectiveness. Note that the SFAR number is purely a sequential number and has no relevance to the regulation it
is addressing or attached to.
The remainder of this handbook focuses only on those
regulations relative to airworthiness certification. There
are 30 of these listed in Figure 2-4 , and they are shown
graphically in Figure 2-5 . A significant benefit of this chart
is the visual effect showing the interaction of the regulation
with other regulations and the placement of the regulation
relative to its impact on airworthiness. It is fundamentally
important that the definition of the term “airworthy” be
clearly understood.
Only recently did the FAA actually define the term “airworthy”
in a regulation. (Refer to the 14 CFR part 3 excerpt following
this paragraph.) Prior to this definition in part 3, the term could
be implied from reading part 21, section 21.183. The term was
defined in other non-regulatory FAA publications, and could
also be implied from the text found in block 5 of FAA Form
8100-2, Standard Airworthiness Certificate. This certificate
is required to be visibly placed on board each civil aircraft.
(Refer to “Forms” presented later in this chapter.)
Title 14 CFR Part 3—General Requirements
Definitions. The following terms have the stated meanings
when used in 14 CFR part 3, section 3.5, Statements about
products, parts, appliances and materials.
• Airworthy means the aircraft conforms to its type
design and is in a condition for safe operation.
• Product means an aircraft, aircraft engine, or aircraft
propeller.
• Record means any writing, drawing, map, recording,
tape, film, photograph or other documentary material
by which information is preserved or conveyed in any
format, including, but not limited to, paper, microfilm,
identification plates, stamped marks, bar codes or
electronic format, and can either be separate from,
attached to or inscribed on any product, part, appliance
or material.
Airworthiness can be divided into two areas: original
airworthiness as depicted in Figure 2-5, and recurrent
airworthiness as depicted in Figure 2-6. There are three
primary regulations that govern the airworthiness of an aircraft:
Comment: Civil Aeronautics Manual (CAMs) contain both regulations and advisory material in the same document.
HISTORICAL BACKGROUND OF 14 CFR PARTS 23, 25, 27, AND 29
Event
Regulating Agency
Rules and Regulations
CAR 3
CAR 4b
CAR 4b
Large A/C
CAR 4T
Transport
CAR 4b
CAR 3
CAR 4a-T
SR 422
Jets
SR 422A
SR 422B
PART 25
PART 29
PART 27
PART 23
Civil Air Regulation
CAR Part 04
Bulletin 7
Bulletin 7-A
CAR 04
<12,500 lb
CAR 4a
Small A/C
CAR 4a-T
>12,500 lb
CAR Part 6 Rotorcraft
Part 7
Transport
Rotorcraft
>6,000 lb
Part 6
Normal
Rotorcraft
Department of
Commerce
Aeronautics Branch
Air Commerce Act
Bureau of Air
Commerce
Civil Aeronautics
Administration
Federal Aviation
Agency
Federal Aviation
Administration
Consolidation of
Functions
Cutting Air Crash
Civil Aeronautics Act
First Helicopter
Certificated
Federal Aviation Act
Codification of
CARs to FARs
Agency becomes
Administration
under DOT
Figure 2-1. F AA historical background of aircraft airworthiness regulations.
1. 14 CFR part 21—Certification Procedures for Products
and Parts
2. 14 CFR part 43—Maintenance, Preventive Maintenance,
Rebuilding, and Alterations
3. 14 CFR part 91—General Operating and Flight Rules
Note that the chart in Figures 2-5 and 2-6 show most of the
other airworthiness certification regulations link to one of
these regulations.
Although the history section that opens this chapter discusses the FAA as if it was a single unit, it is important to
understand that there are various subgroups within the FAA,
and each have different responsibilities of oversight in the
aviation industry. These may vary by organizational chart or
geographic location.
The maintenance technician interacts mostly with FAA
personnel from the Flight Standards Service (AFS) and the
Flight Standards District Office (FSDO) but may also have
some interaction with FAA personnel from the Aircraft
Certification Service (AIR).
Volume 1
Chapter I
Parts 1–59FAA, DOT
Volume 2
Chapter I
Parts 60–109FAA, DOT
Volume 3
Chapter I
Parts 110–199FAA, DOT
Volume 4
Chapter II
Parts 200–399
Office of Secretary, DOT
Volume 5
Chapter V
Parts 1200–1299NASA
Subchapter D
Airmen
Parts 60, 61, 63, 65, 67, 68
Subchapter E
Airspace
Parts 71, 73, 75, 77
Subchapter F
Air Traffic and General
Operating Rules
Parts 91, 93, 95, 97, 99,
101, 103, 105,
107Subchapter G
Air Carriers and Operators
Parts 110, 117, 119, 120,
121, 125, 129, 133, 135,
136, 137, 139
Subchapter H
Schools and Other
Certificated Agencies
Parts 141, 142, 145, 147
Subchapter I
Airports
Parts 150, 151, 152, 153,
155, 156, 157, 158,
161, 169
Subchapter JNavigational FacilitiesParts 170, 171
Subchapter K
Administrative RegulationsParts 183, 185, 187, 189, 193
Subchapter L–M
Reserved
Subchapter N
War Risk InsuranceParts 198, 199
Subchapter A
Economic RegulationsParts 200–298
Subchapter B
Procedural RegulationsParts 300–331
Subchapter C
Reserved
Subchapter D
Special RegulationsParts 372–383
Subchapter E
OrganizationParts 385, 389
Subchapter F
Policy StatementsParts 398, 399Subchapter A
Definitions and
Abbreviations
Parts 1, 3, 5
Subchapter B
Procedural RulesParts 11, 13, 14, 15, 16, 17
Subchapter C
AircraftParts 21, 23, 25, 26, 27,
29, 31, 33, 34, 35, 36, 39, 43, 45, 47, 48, 49
Title 49 Transportation
Congressional Act
(Acts of Congress are Public Law)
Code of Federal Regulations (CFR)
(50 Titles)
Public Law Basis for Title 14 is PL 103-272
Title 14 Aeronautics and Space
(Five Volumes)
Subchapter A
GeneralParts 400, 401
Subchapter B
ProcedureParts 404, 405, 406
Subchapter C
LicensingParts 411–1199
Subchapter A
Office of Management
and Budget
Part 1300
Subchapter B
Air Transportation
Stabilization Board
Parts 1310–1399
Chapter III
Parts 400–1199Commercial Space Transportation, FAA
Chapter VI
Parts 1300–1399Air Transportation System Stabilization
Figure 2-2. Title 14 of the Code of Federal Regulations.
Maintenance-Related Regulations
14 CFR Part 1—Definitions and Abbreviations
This section is a very comprehensive, but certainly not all
inclusive, list of definitions that both pilots and mechanics must become familiar with. Many regulations often provide additional definitions that are unique to their use and interpretation in that specific part. Title 14 CFR part 1, section 1.2, Abbreviations and Symbols, tends to be highly focused on those abbreviations related to flight.
14 CFR Part 21—Certification Procedures for
Products and Articles
This regulation, the first of the three, identifies the
requirements of and the procedures for obtaining type
2-4Special Federal Aviation Regulations
SFAR No Title Appears in 14 CFR
Table of Contents
Special Federal Aviation Regulation No. 23Fuel venting and exhaust emission requirements for turbine engine powered airplanes. Special Flight rules in the Vicinity of the Grand Canyon National Park, AZ
Removal of this SFAR effective 10/8/2004 - Prohibition Against Certain Flights Between
the United States and Libya
Special Operating Rules for Air Tour Operators in the State of Hawaii
Robinson R-22/R-44 Special Training and Experience RequirementsProhibition Against Certain Flights Within the Territory and Airspace of IraqProhibition Against Certain Flights Within the Flight Information Region of the Democratic
People's Republic of Korea
Prohibition Against Certain Flights within the Territory and Airspace of SudanProhibition Against Certain Flights Within the Territory and Airspace of Serbia-Montenegro Airspace and Flight Operations Requirement for the Kodak Albuquerque International
Balloon Fiesta; Albuquerque, NM
Fuel Tank System Fault Tolerance Evaluation RequirementsSpecial Federal Aviation Regulation No. 97 - Special Operating Rules for the Conduct of
Instrument Flight Rules (IFR) Area Navigation (RNAV) Operations using Global Positioning Systems (GPS) in Alaska
Construction or Alteration in the Vicinity of the Private Residence of the President of the
United States
Relief for U.S. Military and Civilian Personnel Who Are Assigned Outside the United States
in Support of U.S. Armed Forces Operations
Process for Requesting Waiver of Mandatory Separation Age for Certain Federal
Aviation Administration (FAA) Air Traffic Control Specialists
Prohibition Against Certain Flights by Syrian Air Carriers to the United States
Special Federal Aviation Regulation No. 108 - Mitsubishi MU-2B Series Airplane Special
Training, Experience, and Operating Requirements
Special Requirements for Private Use Transport Category AirplanesProhibition Against Certain Flights Within the Tripoli (HLLL) Flight Information Region (FIR)Part 25Part 23Part 11Part 91Part 91
Part 91
Part 61Part 91Part 91
Part 91
Part 91Part 91
Parts 21, 25, 91, 121, 125, 129
Parts 71, 91, 95, 121, 125, 129, 135
Part 77
Parts 61, 63, 65Part 65Part 91
Part 91
Parts 21, 25, 119
Part 911323
Figure 2-3. Special Federal Aviation Regulations From 14 CFR.
certificates (TCs), supplemental type certificates (STCs),
production certificates, airworthiness certificates, and
import and export approvals. [Figure 2-5] Some of the
other major areas covered in this part are the procedures for obtaining a Part Manufacture Approval (PMA) or an authorization related to producing a Technical Standard
Order (TSO) part. Note that part 21’s greatest significance is in the original airworthiness phase, although it has minor application in recurrent airworthiness. [Figure 2-5] One
of the most important sections of this regulation is section 21.50, “Instructions for continued airworthiness and manufacturer’s maintenance manuals having airworthiness limitations sections.” When an aircraft is delivered new from the manufacturer, it comes with maintenance manuals that define the inspection and maintenance actions necessary to maintain the aircraft in airworthy condition. Also, any STC modification that was developed after 1981 must have, as part of the STC documentation, a complete set of instructions for
continued airworthiness (ICA). This ICA contains inspection and maintenance information intended to be used by the technician in maintaining that part of the aircraft that has been altered since it was new. This ICA is comprised of 16 specific subjects. [Figure 2-7] An ICA developed in accordance with
this checklist should be acceptable to the Aviation Safety
Inspector (ASI) reviewing a major alteration.
14 CFR Part 23—Airworthiness Standards:
Normal, Utility, Acrobatic, and Commuter Category Airplanes
Aircraft certificated under 14 CFR part 23 represent the
greatest portion of what the industry refers to as “general aviation.” These aircraft vary from the small two-place piston engine, propeller-driven trainers that are frequently used for flight training, to turbine-powered corporate jets used to transport business executives. Seating capacity is limited to nine or less on all aircraft, except the commuter aircraft where the maximum passenger seating is 19, excluding the pilot and copilot seats.
This part specifies the airworthiness standards that must
2-5Part 1 Definitions and Abbreviations
Part 13 Investigative and Enforcement Procedures
Part 21 Certification Procedures for Products and Parts
Part 23 Airworthiness Standards: Normal, Utility, Acrobatic, and
Commuter Category Airplanes
Part 25 Airworthiness Standards: Transport Category Airplanes
Part 27 Airworthiness Standards: Normal Category RotorcraftPart 29 Airworthiness Standards: Transport Category RotorcraftPart 31 Airworthiness Standards: Manned Free BalloonsPart 33 Airworthiness Standard: Aircraft EnginesPart 34 Fuel Venting and Exhaust Emission Requirements for
Turbine Engine Powered Airplanes
Part 35 Airworthiness Standards: PropellersPart 36
Noise Standards: Aircraft Type and
Airworthiness Certification
Part 39 Airworthiness Directives
Part 43 Maintenance, Preventive Maintenance, Rebuilding, and
Alteration
Part 45 Identification and Registration Marking
Part 47 Aircraft Registration
Part 48 Registration and Marking Requirements for Small
Unmanned Aircraft
Part 61 Certification: Pilots, Flight Instructors, and Ground
Instructors
Part 63 Certification: Flight Crewmembers Other Than Pilots
Part 65 Certification: Airmen Other Than Flight Crewmembers
Part 68 Requirements for Operating Certain Small
Aircraft Without a Medical Certificate
Part 91 General Operating and Flight Rules
Part 107 Small Unmanned Aircraft SystemsPart 119
Certification: Air Carriers and Commercial Operators
Part 121 Operating Requirements: Domestic, Flag, and
Supplemental Operations
Part 125 Certification and Operations: Airplanes Having a
Seating Capacity of 20 or More Passengers or a
Maximum Payload Capacity of 6,000 Pounds or More;
and Rules Governing Persons on Board Such Aircraft
Part 135 Operating Requirements: Commuter and On Demand
Operations and Rules Governing Persons on Board Such
Aircraft
Part 145 Repair Stations
Part 147 Aviation Maintenance Technician SchoolsPart 183 Representatives of the Administrator
Figure 2-4. List of F AA regulations relative to airworthiness
certification.
be met in order for a manufacturer to receive a TC and
for the aircraft to receive an airworthiness certificate. Part 23 aircraft are those aircraft that have a maximum certificated takeoff weight of 12,500 pounds or less, except for those aircraft in the commuter category. The maximum certificated takeoff weight limit rises to 19,000 pounds or less for these aircraft.
Part 23 has seven subparts, six of them providing detailed criteria for the design of these aircraft. The first, subpart A,
defines the applicability of this regulation. The others are:
•Subpart B—Flight
•Subpart C—Structures
•Subpart D—Design and Construction
•Subpart E—Powerplant
•Subpart F—Equipment
• Subpart G—Flightcrew Interface and Other Information
Within each of these subparts are numerous sections that specify details, such as center of gravity (CG), gust load
factors, removable fasteners, the shape of certain flight deck controls, engine and propeller requirements, fuel tank markings, flight deck instrumentation marking and placards, cabin aisle width, and flammability resistant standards.
14 CFR Part 25—Airworthiness Standards: Transport
Category Airplanes
The standards in 14 CFR part 25 apply to large aircraft with a maximum certificated takeoff weight of more than 12,500 pounds. This segment of aviation is usually referred to as “commercial aviation” and includes most of the aircraft seen at a large passenger airport, except for the commuter aircraft included in 14 CFR part 23. However, the ability to carry passengers is not a requirement for aircraft certified to 14 CFR part 25. Many of these aircraft are also used to transport cargo. This chapter is subdivided into similar design subpart categories and the same sequence as the requirements specified in 14 CFR part 23.
14 CFR Part 27—Airworthiness Standards: Normal
Category Rotorcraft
This regulation deals with the small rotor wing aircraft and is consistent with 14 CFR part 23 with limiting the passenger seating to nine or less. However, the maximum certificated weight is limited to 7,000 pounds. It contains similar design subparts identified in 14 CFR part 23 that provide the details for designing the aircraft.
14 CFR Part 29—Airworthiness Standards: Transport
Category Rotorcraft
This section specifies those standards applicable to helicopters with a maximum certified weight greater than 7,000 pounds. However, it also includes additional parameters based upon seating capacity and an additional weight limit. Those parameters are passenger seating, (nine or less, ten or more) and whether the helicopter is over or under a maximum weight of 20,000 pounds. The design subparts of part 29 are similar to those in 14 CFR parts 23, 25, and 27.
Certification
Procedures for
Products and Parts
General Operating
and Flight Rules
14 CFR §91.319
14 CFR §91.409
Fuel Venting and Exhaust Emission
Requirements for Turbine Engine Powered Airplanes
Noise Standards:
Aircraft Type and
Airworthiness Certifi cationAirworthiness Standards:
Normal, Utility, Acrobatic, and Commuter Category AirplanesInvestigative and Enforcement
Procedures
Airworthiness Standards:
Transport Category Airplanes
26Continued Airworthiness and
Safety Improvements for Transport Category Airplanes
Airworthiness Standards: Normal Category Rotorcraft
Airworthiness Standards:
Transport Category Rotorcraft
Airworthiness Standards:
Manned Free Balloons
Airworthiness Standards:
Aircraft Engines
Airworthiness Standards:
Propellers
Airworthiness Directives45
Identification and
Registration Marking
Aircraft Registration
Representatives of
the AdministratorManufacturer
43.3(j)
DER Guidance
HandbookDMIR
DER
DAR
DAS
DAR
DPE
DME
DPRE§21.9(a)(5)
Owner Operator Built PartsTC
STC
TSOAPMA
§21.8(d)8110.42
Parts Manufacturing ApprovalOriginal Airworthiness
Aircraft Certification
Office (ACO) Certificate Management
Office (CMO) Manufacturing Insp ection
District Office (MIDO)Aircraft
Certification
Service (AIR)—
FAA ORDERS:
8100.7, ACSEP
8110.4, Type Certi fication
8120.2, Production Approvals8100.8
Designee Management Handbook
Airworthiness Certification
Suspected Unapproved
Parts Program
Compliance & Enforcement8132.2
Designee Air worthiness Ce rtification
Figure 2-5. Graphic chart of F AA regulations.
2-7FAA ORDERS:
8900.1 FSIMS
General Operating
and Flight Rules
Certification
Procedures for
Products and Parts
Subpart H, L, K
Maintenance,
Preventive
Maintenance,
Rebuilding, and
Alteration
119Certification:
Air Carriers and Commercial
Operators
61Certification:
Pilots and Flight Instructors
63Certification:
Flight Crewmembers Other
Than Pilots
120Drug and Alcohol
Testing Program45
Identification and
Registration Marking
Aircraft Registration
Representatives of
the Administrator§ 43.5
No person may approve for return to service, unless...
§ 43.5
Maintenance record entry required by §43.9 or §43.11, as appropriate
§ 43.13a
Methods, techniques, and practices in current manufacturer’s maintenance manuals
§ 43.13b
Work and materials of such a quality that condition will be at least equal to the original or property altered condition
145Repair Stations
Operating
SpecificationsRepair Station
Manual
125Certification and Operations:
Airplanes Having a Seating
Capacity of 20 or More
Passengers or a Maximum
Payload Capacity of 6,000
Pounds or More
8400.10 Deviation
Designee Management Handbook
Airworthiness Certification
Suspected Unapproved
Parts Program
Compliance & Enforcement8132.2
Designee Air worthiness Ce rtificationRecurrent Airworthiness
Certificate Management
Office (CMO) Flight Standards
District Offi ce (FSDO) International
Field Office (IFO)
135Air Taxi Operators and
Commercial Operators
9 or Less 10 or More
121Certification and Operation:
Domestic, Flag, and
Supplemental Air Carriers and
Commercial Operators of
Large Aircraft
DER Guidance
Handbook14 CFR 43
Appendix B
FAA Form 337
DMIR
DER
DAR
ODA
DAR
DPE
DME
DPREFlight Standards
Service (AFS)—
65Certification:
Airmen O ther Than Flight
Crewmembers
A&P IA
Repairman Parachute RiggerDispatcher
Figure 2-6. Graphic chart of F AA regulations (continued).
2-814 CFR Part 33—Airworthiness Standards: Aircraft
Engines
Each of the four preceding 14 CFR regulations require that the
engine used in the aircraft must be type certificated. Title 14
CFR part 33 details the requirements for both reciprocating
and turbine style aircraft engines. It not only specifies the
design and construction requirements, but also the block test
requirements that subject the engine to extremely demanding
testing in order to prove its capability of enduring the stresses
of powering the aircraft.
14 CFR Part 35—Airworthiness Standards: Propellers
Just as each engine used on an aircraft must have a TC, the
propeller must also be type certificated. This part is arranged
the same way that 14 CFR part 33 is, in that subpart B
specifies design and construction while subpart C covers
tests and inspections.
Since regulations change over the years, not every aircraft
presently flying meets the current design regulations
as printed this year. When regulations are revised, they
are printed in the Federal Register and released with an
amendment number that ties them to the regulation being
revised. Aircraft are required to meet only the specifications
in force at the time the aircraft is built. Note: The preceding
statement does not apply to the mandatory requirements
imposed by Airworthiness Directives (AD), as these usually
have a compliance date included in the AD note.
14 CFR Part 39—Airworthiness Directives
In spite of all the emphasis on proper design and certification
testing, sometimes the actual day-to-day use of the aircraft
causes unanticipated wear or failure to occur. When that
happens, if the FAA determines that the wear or failure
represents an unsafe condition and that the condition is likely
to exist in other products of the same type of design, it issues
an AD. Actual AD notes are not included in 14 CFR part 39,
but rather are printed in the Federal Register and are linked
to this part as amendments to 14 CFR part 39, section 39.13.
AD notes are legally enforceable rules that apply to aircraft,
aircraft engines, propellers, and appliances.
14 CFR Part 43—Maintenance, Preventive
Maintenance, Rebuilding, and Alteration
This regulation represents the heart of aviation maintenance
and is one of the three major regulations previously identified.
The 13 rules and 6 appendices contained within 14 CFR part
43 provide the standard for maintaining all civilian aircraft
currently registered in the United States. Note that 14 CFR
part 43 has a significant relationship with part 91 and other
parts in maintaining continued airworthiness. [Figure 2-6] A
more detailed explanation of this regulation is presented later in this handbook.
14 CFR Part 45—Identification and Registration
Marking
Title 14 of the CFR part 45 includes the requirements for
the identification of aircraft, engines, propellers, certain
replacement and modification parts, and the nationality and
registration marking required on U.S.-registered aircraft. All
type-certificated products must have the following information
on a fireproof dataplate or similar approved fireproof method.
1. Builder’s name
2. Model designation
3. Builder’s serial number
4. TC number (if any)
5. Production certificate number (if any)
6. For aircraft engines, the established rating
7. Reference to compliance or exemption to 14 CFR Part
34, Fuel Venting and Exhaust Emission Requirements
for Turbine Engine Powered Airplanes
8. Any other information that the FAA determines to be
appropriate
Replacement and modification parts are produced in
accordance with a Parts Manufacturer Approvals (PMA) (14
CFR part 21, section 21.303) and must have the following
information permanently and legibly marked:
1. The letters “FAA-PMA”
2. The name, symbol, or trademark of the holder of the
PMA
3. The part number
4. The name and model designation for each type
certificated product it can be installed on
If a part has a specified replacement time, inspection interval,
or other related procedure specification in the maintenance
manual or ICA, that part must have a part number and a serial
number (or the equivalent of each).
The manufacturer of a life-limited part must either provide
marking instructions for that part, or state that the part cannot
be marked without a compromise to its integrity. Exceptions
are made for the identification of parts that are too small to
be practical to mark the required data.
Nationality and registration marks (commonly known as
the N-number for U.S.-registered aircraft) can vary in size,
depending on the year that the aircraft was built and whether
or not the aircraft has been repainted. The most common size
is at least 12 inches in height. Small aircraft built at least 30
ITEM SUBJECT
1. Introduction: Briefly describes the aircraft, engine, propeller, or com ponent that has been
altered. Include any other inform ation regarding the content, scope, purpose, arrangem ent,
applicability, definitions, abbreviations, precautions, units of m easurem ent, list of parts used,
referenced publications, and distribution of the ICA, as applicable.
2. Description: Of the m ajor alteration and its functions, including an explanation of its interface
with other system s, if any.
3. Control, operation information: Or special procedures, if any.
4. Servicing information: Such as types of fluids used, servicing points, and location of access
panels, as appropriate.
5. Maintenance instructions: Such as recom mended inspection/m aintenance periods in which
each of the major alteration com pone nts are inspected, cleaned, l ubricated, adj usted, a nd tested,
including applicable wear tolerances and work recom mended at each scheduled m aintenance
period. T his section can refe r to the manufacturers’ i nstructions for t he equipment installed
where appropriate (e.g., functional checks, repairs, inspections). It should also include any
special notes, cautions, or warnings, as applicable.
6. Troubleshooting information: Describes probable m alfunctions, how to recognize those
malfunctions, and the rem edial actions to take.
7. Removal and replacement information: Describes the order and m ethod of rem oving and
replacing products or parts, and any necessary precau tions. This section should also describe or
refer to manufacturer’s i nstructions t o make required test s, checks, alignm ent, calibrations,
center of gravity changes, lifting, or shoring, etc., if any.
8. Diagrams: Of access plates and inform ation, if needed, to gain access for inspection.
9. Special inspection requirements: Suc h as X-ra y, ultra sonic testing, or m agnetic particle
inspection, if required.
10. Application of protective treatments: To the affected area after inspection and/or
maintenance, if any.
11. Data: Relative t o structural fastene rs suc h as type, torque, and installation requirem ents, if any.
12. List of special tools: Special tools that are required, if any.
13. For commuter c ategory aircraft: Provide the following additional inform ation, as applicable:
A. Electrical loa ds
B. Methods of balancing flight controls
C. Identification of prim ary and secondary structures
D. Special repair m ethods a pplicable to t he aircraft
14. Recommended overhaul periods: Required to be noted on the ICA when an overhaul period
has been established by the m anufacturer of a com ponent or equipment. If no overhaul period
exists, the ICA shoul d state for item 14, “ No a dditional overha ul time limitations.”
15. Airw orthiness limitation section: Include s any “approve d” airwort hiness lim itations ide ntified
by the m anufacturer or FAA Type Certificate Ho lding Office (e.g., An STC incorporated in a
larger field-approved m ajor alteration m ay have an airworthiness lim itation). The FAA
inspector shoul d not establish, alter, or cancel airwort hiness lim itations without c oordi nating
with the appropriate FAA Type Certificate Holding Office. If no changes are m ade to the
airworthiness lim itations, the ICA should state for item 15, “No additional airworthiness
limitations” o r “Not Applicable.”
16. Revision: Includes inf ormation on how to revise the ICA. For exam ple, a letter will be
subm itted to the local FAA Office with a copy of the revised FAA Form 337 and revised ICA.
The FAA inspector accepts the change by signing block 3 and including the following statement:
“The attached revised/new Instructions for C ontinued Airworthiness (date______) for the above
aircraft or com ponent m ajor alteration have been accepted by the FAA, superseding the
Instructions for Continued Airworthiness (date ______).” After the revision has been accepted, a
maintenance record entry will be m ade, identifying the revision, its location, and date on the
FAA Form 337.
Figure 2-7. Instructions for Continued Airworthiness (ICA) Checklist.
SAMPLE
2-10expiration date of the new battery.
2. Section 91.213—Inoperative Instruments and
Equipment
Paragraph (a)(2)—a letter of authorization from the
FSDO authorizing the operation of the aircraft under
a Minimum Equipment List (MEL) constitutes a STC
and must be carried in the aircraft during flight.
Subpart E—Maintenance, Preventive Maintenance, and
Alterations (Sections 91.401 through 91.421)
This is the section of most interest to the technician. They
must be familiar with it, because it does carry some (indirect)
responsibility for the technician. Note that the 14 CFR part 91
icon in Figure 2-6 has a direct line to 14 CFR part 43. This
is because section 91.403(b) states, “No person may perform
maintenance, preventive maintenance, or alterations on an
aircraft other than as prescribed in this subpart and other
applicable regulations, including part 43 of this chapter.”
A more complete discussion of this regulation, especially
Subpart E—Maintenance, Preventive Maintenance, and
Alterations is presented later in this chapter.
14 CFR Part 119—Certification: Air Carriers and
Commercial Operators
In order to better understand the next three regulations
discussed here (14 CFR parts 121, 125, and 135) a brief
overview of 14 CFR part 119 is beneficial. [ Figure 2-8]
There are more than 50 Advisory Circulars (ACs) in the 120
series alone providing additional non-regulatory information
concerning the variety of procedures involved with these
operations. There are basically three different criteria
that must be analyzed in order to properly determine the
regulation that applies. These are:
1. Is the service provided for Private Carriage or
Common Carriage?
2. Is the aircraft For Hire or is it Not for Hire?
3. Is it a large or small aircraft?
AC 120-12, as revised, provides the following definition
regarding this criterion: A carrier becomes a common carrier
when it “holds itself out” to the public, or to a segment of the
public, as willing to furnish transportation within the limits
of its facilities to any person who wants it. There are four
elements in defining a common carrier:
1. A holding out of a willingness to
2. Transport persons or property
3. From place to place
4. For compensation years ago, or replicas of these, or experimental exhibition or
amateur-built aircraft may use letters at least 2 inches in height.
Only a few aircraft are authorized to display registration
markings of at least 3 inches. Note that this regulation sits
directly on the vertical line in Figure 2-5 indicating that it
applies to both original and recurrent airworthiness.
14 CFR Part 47—Aircraft Registration
This regulation provides the requirements for registering
aircraft. It includes procedures for both owner and dealer
registration of aircraft.
14 CFR Part 65—Certification: Airmen Other Than
Flight Crewmembers
Pilots, flight instructors, and ground instructors are
certificated under 14 CFR part 61. Flight crew other than
pilots are certificated under 14 CFR part 63. However,
many other people are also required to be certificated by
the FAA for the U.S. aviation fleet to operate smoothly and
efficiently. Title 14 CFR part 65 addresses many of those
other people.
• Subpart B—Air Traffic Control Tower Operators
• Subpart C—Aircraft Dispatchers
• Subpart D—Mechanics
• Subpart E—Repairmen
• Subpart F—Parachute Riggers
A more detailed discussion of this chapter with a special
emphasis on mechanics is included in Chapter 15, The
Mechanic Certificate.
Note: SFAR 100-2. Relief for U.S. Military and Civilian
Personnel who are assigned outside the United States in
support of U.S. Armed Forces Operations is a good example
of the specific nature and limited time frame that are part of
a SFAR.
14 CFR Part 91—General Operating and Flight Rules
This is the final regulation of the three major regulations
identified earlier in this chapter. Note its interaction in
Figure 2-6 with other regulations visually indicating its
“operational” involvement or “recurrent airworthiness.”
Although it is an operational regulation that is focused
toward the owner, operator, and/or pilot of the aircraft, the
maintenance technician must have an awareness of this
regulation. Two examples of these maintenance related
issues are:
1. Section 91.207—Emergency Locator Transmitters
Paragraph (c)(2)—battery replacement interval
and requirement for a logbook entry indicating the
14 CFR Part 119 Applicability of Regulations
Large A/C
Small A/CLarge A/C
Small A/C
135 121 125 91Large A/C Large A/CFor HireCommon Carriage
Not for Hire
All Other A/C91, 125, 135, 121
Private CarriageCFR Part 91,
Section 91.501(b)(5)CFR Part 119, Section 119.5
Figure 2-8. Applicability of regulations.9. Parachute operations on nonstop flights within 25 NM
from the departure airport
10. Fractional ownership in accordance with 14 CFR part
91, subpart K
14 CFR Part 121—Operating Requirements:
Domestic, Flag, and Supplemental Operations
Title 14 CFR part 121 establishes the operational rules for air
carriers flying for compensation or hire. A domestic operation
is any scheduled operation (within the 48 contiguous states,
the District of Columbia, or any territory or possession)
conducted with either a turbo-jet aircraft, an airplane having
10 or more passenger seats, or a payload capacity greater
than 7,500 pounds.
A “flag” operation means any scheduled operation (operating
in Alaska or Hawaii to any point outside of those states, or
to any territory or possession of the United States, or from
any point outside the United States to any point outside the
United States) conducted with either a turbo-jet aircraft, an
airplane having 10 or more passenger seats, or a payload
capacity greater than 7,500 pounds.
“Supplemental” operation means any common carriage
operation conducted with airplanes having more than 30
passenger seats (if less than 30, the airplane must also be
listed on the operations specifications of domestic and flag
carriers), with a payload capacity of more than 7,500 pounds.
Part 121 operators are required by 14 CFR part 119 to have
the following personnel:
• Director of SafetyThis “holding out” that makes a person a common carrier can
be done in many ways, and it does not matter how it is done.
Signs and advertising are the most direct means of “holding
out,” but are not the only ones.
Carriage for hire which does not involve "holding out" is
private carriage. Private carriers for hire are sometimes
called “contract carriers,” but the term is borrowed from the
Interstate Commerce Act and legally inaccurate when used
in connection with the Federal Aviation Act. Private carriage
for hire is carriage for one or several selected customers,
generally on a long-term basis. The number of contracts
must not be too great; otherwise, it implies a willingness to
make a contract with anybody. A carrier operating pursuant
to 18 to 24 contracts has been held to be a common carrier,
because it held itself out to serve the public generally to the
extent of its facilities. Private carriage has been found in
cases where three contracts have been the sole basis of the
operator’s business.
Operations that constitute common carriage are required to
be conducted under 14 CFR part 121 or 135. Private carriage
may be conducted under 14 CFR part 91 or 125.
The term “for hire” is not defined in any of the FAA documents
but is generally understood to mean that compensation for
both direct and indirect expenses associated with the flight,
as well as a profit margin for the operator, are collected from
the person or persons benefiting from the flight operation.
The determination of whether the aircraft is large or small is
based upon the definition provided in 14 CFR part 1. If the
aircraft has maximum certificated takeoff weight of 12,500
pounds or more, it is a large aircraft. All aircraft less than
12,500 maximum certificated takeoff weight are considered
to be small aircraft.
It may also help the reader understand when 14 CFR parts
121, 125, and 135 regulations apply, by taking a brief look at a
list of flight operations where 14 CFR part 119 does not apply.
1. Student instruction
2. Nonstop sightseeing flights with less than 30 seats and
less than 25 nautical miles (NM) from the departure
airport
3. Ferry or training flights
4. Crop dusting or other agricultural operations
5. Banner towing
6. Aerial photography or surveying
7. Fire fighting
8. Powerline or pipeline patrol
2-12• Director of Operations
• Director of Maintenance
• Chief Pilot
• Chief Inspector
There are 28 subparts and 16 appendices in this regulation.
However, only subparts J and L are of concern for the mechanic.
Subpart J—Maintenance, Preventive Maintenance, and
Alterations, identifies Special Airworthiness Requirements
that deals with many of the mechanical aspects of a passenger
or cargo aircraft. Subpart L—Maintenance, Preventive
Maintenance, and Alterations, requires that a part 121
operator have an operational manual that contains the
following information:
• Organizational chart
• List of individuals who may perform required
inspections
• Company maintenance, preventive maintenance, or
alterations
• A system to both preserve and retrieve maintenance
and inspection related information
Also, 14 CFR part 121, section 121.1105, establishes the
requirement for conducting inspections on aging aircraft.
14 CFR Part 125—Certification and Operations:
Airplanes Having a Seating Capacity of 20 or More
Passengers or a Maximum Payload Capacity of 6,000
Pounds or More; and Rules Governing Persons on
Board Such Aircraft
This regulation applies to private and noncommon carriage
when such operations are conducted in airplanes having
20 or more seats (excluding crewmembers) or having a
payload capacity of 6,000 pounds or more. There must also
be “operations specifications” issued to the operator that
include the following information:
• Kinds of operations authorized
• Types of aircraft and registration numbers of the
airplanes authorized for use
• Approval of the provisions of the operator’s manual
relating to airplane inspections, together with the
necessary conditions and limitations
• Registration numbers of the airplanes that are to
be inspected under an approved airplane inspection
program (AAIP) under 14 CFR part 125, section
• Procedures for the control of weight and balance of
airplanes• Any other item that the administrator determines is
necessary
Just as in 14 CFR part 121, subpart E identifies special
airworthiness requirements dealing mostly with the
mechanical devices of the aircraft.
14 CFR Part 135—Operating Requirements:
Commuter and On-Demand Operations and Rules
Governing Persons on Board Such Aircraft
As the title of this section states, this regulation is applicable
to short distance commercial aircraft operations or
“commuters” and nonscheduled carriers that operate “on-
demand.” These aircraft are frequently referred to as air taxi
or air charter aircraft.
Aircraft operated under 14 CFR part 135 must be operated
and maintained in accordance with the certificate holder’s
operations manual. This manual, when accepted by the
FAA, specifies how the flight crew, ground personnel, and
maintenance technicians conduct their operations.
A pivotal portion of this regulation is the first section in
subpart J, 14 CFR part 135, section 135.411, Application.
This section specifies that having a type certificated passenger
seating configuration of nine or less may be maintained in
accordance with the maintenance manual provided by the
aircraft manufacturer. Those aircraft having a type certificated
passenger seating configuration of 10 or more seats must be
maintained in accordance with a maintenance manual written
by the air carrier and must then be submitted to the FAA for
approval. The requirements for the maintenance manual are
specified in 14 CFR part 135, section 135.427. 14 CFR part
135, sections 135.415 through 135.417 and 135.423 through
135.443 specify additional maintenance requirements. 14
CFR part 135, sections 135.415 and 135.417 are applicable
regardless of the number of seats in the aircraft.
A major change in the “nine or less” aircraft maintenance
requirements occurred in February of 2005 when section
135.422, Aging Aircraft, was incorporated into 14 CFR part
135. This new subpart (note the even number) to 14 CFR
135 specifically prohibits a certificate holder from operating
certain aircraft unless the Administrator has completed the
aging aircraft inspection and records review. This inspection
requires the certificate holder to show the FAA that the
maintenance of age sensitive parts and components has been
adequate to ensure safety.
This section only applies to multi-engine aircraft in scheduled
operation with nine or fewer passenger seats. It does not
apply to aircraft operating in Alaska. The required record
review start date varies depending on the age of the aircraft.
2-13However, once initiated, the repetitive inspection intervals
are not to exceed 7 years.
The certificate holder must make both the aircraft and the
records available to the FAA for inspection and review. The
certificate holder must notify the Administrator at least 60
days in advance of the availability of the aircraft and the
records for review.
The records must include the following information:
1. Total years in service of the airplane
2. Total time in service of the airframe
3. Date of the last inspection and records review required
by this section
4. Current status of life-limited parts
5. Time since the last overhaul of all structural
components required to be overhauled on a specific
time basis
6. Current inspection status of the airplane, including the
time since the last inspection required by the inspection
program that the airplane is maintained under
7. Current status of applicable ADs, including the date
and methods of compliance, and, if the AD involves
recurring action, the time and date when the next
action is required
8. A list of major structural alterations
9. A report of major structural repairs and the current
inspection status of those repairs
14 CFR Part 145—Repair Stations
This regulation underwent a major rewrite released in 2004
and was the most comprehensive change in nearly 20 years.
It may be of interest to note an airframe and powerplant
(A&P) certificate is not necessary to be employed at a repair
station. The repair station may also employ both repairmen
(under 14 CFR part 65, subpart E) and non FAA-certificated
personnel. All work that is signed off is done so using the
repair station certificate number and must be done only by
persons authorized by 14 CFR part 65 to approve an article
for return to service (RTS). Just as other certificate holders
must have an operations manual, the repair station must have
a repair station manual that contains the following:
• An organizational chart
• Procedures for maintaining rosters
• Description of housing, facilities, and equipment
• Procedures for revising the capability list and
conducting a self-evaluation (audit)
• Procedures for revising the training program• Procedures governing work done at another location
• Procedures for working on air carrier aircraft
• Description of the required records and record keeping
• Procedures for revising the repair station manual
• Description of the system to identify and control the
sections of the manual
All records from repair station maintenance activity must
be kept a minimum of 2 years. Domestic repair station
certificates are effective until they are surrendered, suspended,
or revoked. The certificates of foreign repair stations expire,
usually after 1 or 2 years and must be renewed.
14 CFR Part 147—Aviation Maintenance Technician
Schools
Title 14 CFR part 147 defines the requirements for obtaining
a maintenance training certificate. This certificate may be for
either airframe, powerplant, or a combination of the two. The
minimum number of curriculum hours for conducting either
airframe or powerplant training independently is 1,150.
If both A&P ratings are offered, the combined total curriculum
hours are 1,900. This is because of the 1,150 hours specified
to obtain either the airframe or the powerplant rating, 400
hours are devoted to general studies. Only one set of general
studies hours is applicable to the combined total. Therefore,
400 hours can be subtracted from the implied total of 2,300
hours (1,150 × 2) to obtain the reduced figure of 1,900 hours.
Requirements are detailed as follows:
• Appendix A—Curriculum Requirements
• Appendix B—General Curriculum Subjects
• Appendix C—Airframe Curricular Subjects
• Appendix D—Powerplant Curriculum Subjects
14 CFR Part 183—Representatives of the Administrator
As the aviation industry grows and the design, manufacture,
and testing of aircraft gets more complex, the FAA faces
both budget constraints and personnel shortages. As early
as 1962, the FAA began a program to allow private sector
persons in various areas of industry to be “designees” or
“representatives of the FAA Administrator.” These people
are NOT FAA employees, but rather are designated by the
FAA to act on their behalf. Regular doctors may serve as
“aviation medical examiners,” skilled pilots can become
“pilot examiners,” and experienced airframe and/or
powerplant mechanics can become “designated mechanic
examiners (DME)” to administer the oral and practical
portion of the FAA testing.
Other lesser known designees are the designated engineering
2-14representatives (DER), the designated manufacturing
inspection representatives (DMIR), and the designated
airworthiness representatives (DAR).
• DERs approve data based upon their engineering
training and their knowledge of FAA regulations.
• DMIRs make conformity inspections only at their
employer. They are similar to “designated repairmen”
because they are only authorized to inspect parts at
their employers’ facility.
• DARs conduct aircraft certification and aircraft
inspection functions on behalf of the FAA depending
on specific functions they are authorized. They may
perform work for either manufacturing facilities or
maintenance entities depending on their designation.
Explanation of Primary Regulations (Parts 43
and 91)
14 CFR Part 43—Maintenance, Preventative
Maintenance Rebuilding, and Alteration
Section 43.1—Applicability
Paragraph (a) states quite clearly that aircraft (whether U.S.-
or foreign-registered operating under 14 CFR part 121 or
135) and component parts thereof must be maintained in
accordance with the rules set forth in this part. Although
paragraph b states quite clearly the type of aircraft that this
part does not apply to, it seems to have led to considerable
confusion within the aviation industry. If an aircraft is flying
with a Special Airworthiness—Experimental Certificate
(FAA Form 8130-7, Special Airworthiness Certificate—pink
color certificate) and that is the only airworthiness certificate
this aircraft has ever had, then 14 CFR part 43 does not apply.
Conversely, sometimes during maintenance (especially STC
modification—the STC is addressed later in this chapter),
it becomes necessary to temporarily place the aircraft into
Special Airworthiness—Experimental. This is done to show
compliance with federal regulations. These aircraft must
still be maintained in accordance with 14 CFR part 43,
because the aircraft had a different kind of airworthiness (in
this example a Standard) prior to being issued the Special
Airworthiness Certificate.
Section 43.2—Records of Overhaul and Rebuilding
These terms are not defined in 14 CFR part 1 and are given
full explanation in this subpart. Each term states that it may
not be used to describe work done on an aircraft, airframe,
aircraft engine, propeller, appliance, or component part unless
that item has been:
• Disassembled
• Cleaned• Inspected
• Repaired, as necessary
• Reassembled
• Tested
The key difference between the two terms is in determining
how the item is tested. If it is “tested in accordance with
approved standards acceptable to the Administration that
have been developed and documented by the manufacturer,
the item is said to be overhauled.” This is basically another
way of describing “service limits,” a term frequently used to
describe manufacturer specified acceptable limits for used
parts. A “rebuilt item, on the other hand, must be tested to
the same tolerances and limits as a new item.”
Section 43.3—Persons authorized to perform
maintenance, preventive maintenance, rebuilding,
and alterations
There are nine different persons who may perform
maintenance: (Reminder: Per 14 CFR part 1, the FAA
definition of a person is “an individual, firm, partnership,
corporation, association, joint-stock association, or
governmental entity. It includes a trustee, receiver, assignee,
or similar representative of any of them.”)
1. Certificated mechanic, per 14 CFR part 65
2. Certificated repairman, per 14 CFR part 65
3. Person working under the supervision of a certificated
mechanic or repairman
4. Holder of repair station certificate
5. Holder of an air carrier certificate
6. Except for holders of a sport pilot certificate, the
holder of a pilot certificate issued under part 61
may perform preventive maintenance on any aircraft
owned or operated by that pilot which is not used
under 14 CFR part 121, 129, or 135. The holder
of a sport pilot certificate may perform preventive
maintenance on an aircraft owned or operated by that
pilot and issued a special airworthiness certificate in
the light-sport category.
7. Pilot of a helicopter (when operated under 14 CFR
part 135 and in remote areas) may perform specific
preventive maintenance actions. These actions may
only be accomplished under the following conditions:
• The mechanical difficulty or malfunction
occurred en route to or in the remote area.
• The pilot has been satisfactorily trained and is
2-15authorized in writing by the certificate holder to
perform the required maintenance.
• There is no certificated mechanic available.
• The certificate holder has procedures to evaluate
the work performed when a decision for
airworthiness is required. The work done is listed
in paragraph (c) of Appendix A of this chapter.
8. Holder of part 135 certificate may allow pilots of
aircraft with nine or less passenger seats to remove and
reinstall cabin seats and stretchers and cabin mounted
medical oxygen bottles. These actions may only be
accomplished under the following conditions:
• The pilot has been satisfactorily trained and is
authorized in writing by the certificate holder to
perform the required maintenance.
• The certificate holder has written procedures
available to the pilot to evaluate the work
performed.
9. Manufacturer may inspect and rebuild any item it has
manufactured.
Section 43.5—Approval for return to service after
maintenance, preventive maintenance, rebuilding,
and alterations
Approving an aircraft component for return to service
after maintenance, preventive maintenance, rebuilding,
or alteration must be done by creating an appropriate
maintenance record entry as required by either 14 CFR part
43, section 43.9 or 43.11. This may include the use of FAA
Form 337, Major Repair and Alteration, if the maintenance
action was a major repair or a major alteration. Whenever
a maintenance action is being planned, it is critical that the
technician understands exactly:
1. What he/she is going to do.
2. How that work is classified by the FAA.
3. What type of documentation is required to support
this activity.
First consider whether this a repair or an alteration. This
should be a relatively simply decision since a repair
basically returns the aircraft to its previous or unaltered
condition (i.e., replacing magnetos, an exhaust system, tires,
or brakes). Even replacing an entire engine (although it is
a big job) is still a repair if it is the one properly specified
for that aircraft. An alteration on the other hand, always
changes or modifies the aircraft from its previous state (i.e.,
installing winglets, new avionics, or an engine that is not
listed in the aircraft TCDS).
The second question to consider is whether or not the work that to be performed constitutes a major or a minor
maintenance action. A “major” action is typically one that
might appreciably affect weight, balance, structural strength,
performance, powerplant operation, flight characteristics,
or other qualities affecting airworthiness and that are not
done according to accepted practices or cannot be done
by elementary operations. This is a much more complex
question, but it is extremely important as it drives the final
question concerning the substantiating documentation. Please
refer to 14 CFR part 1 and part 43, appendix A, for additional
clarification and examples.
The third question deals with the type of documentation
required to substantiate the work performed. Minor repairs
and alterations need only to refer to “acceptable” data, such
as manufacturers’ maintenance manuals or AC 43.13-1.
The maintenance action can simply be recorded in the
maintenance record as a logbook entry. Major repairs and
alterations require “approved data.” Some examples of
approved data are AD notes, STCs, TCDS, DER-specific
delegations, and FAA-approved manufacturer Service
Bulletins (SB).
Sometimes the repair or alteration being performed does not
have previously-approved data. In that case, the technician
may request that the FAA accomplish a “Field Approval.”
In this procedure, the technician completes the front side of
Form 337 through block 6 (leaving block 3 open for later
FAA approval) and then indicates in block 8 on the back what
work is to be done and what the substantiating reference data
is. Form 337 is then submitted to the local FAA FSDO office
for review and approval by an ASI. If necessary, this ASI may
seek input from other ASIs or FAA specialists to assist in the
review of the data. If the data is found to comply with FAA
regulations, the ASI enters one of the following statements
in block 3, depending on whether the ASI has performed
a review of the data only or has physically inspected the
aircraft:
• “The technical data identified herein has been found
to comply with applicable airworthiness requirements
and is hereby approved for use only on the above
described aircraft, subject to conformity inspection by
a person authorized in 14 CFR part 43, section 43.7.”
or
• “The alteration or repair identified herein complies
with the applicable airworthiness requirements and
is approved for use only on the above described
aircraft, subject to conformity inspection by a person
authorized in 14 CFR part 43, section 43.7.”
2-16Section 43.7—Persons authorized to approve aircraft,
airframes, aircraft engines, propellers, appliances,
or component parts for return to service after
maintenance, preventive maintenance, rebuilding,
or alteration
There are seven different persons listed in this section who
may sign RTS documentation:
1. Certificated mechanic or holder of an inspection
authorization (IA).
2. Holder of a repair station certificate.
3. Manufacturer.
4. Holder of an air carrier certificate.
5. Certificated private pilot.
6. Repairman certificated with a maintenance rating for
light sport aircraft (LSA) only.
7. Certificated sport pilot for preventive maintenance on
an aircraft owned and or operated by them.
Note that although a certificated repairman is authorized
to work on a product undergoing maintenance, preventive
maintenance, rebuilding, or alterations (refer to 14 CFR
part 43, section 43.3), they are not authorized to approve
that product for RTS. They must make the appropriate
maintenance record entry per the requirements of 14 CFR
part 43, section 43.9 or 43.11.
Section 43.9—Content, form and disposition of
maintenance, preventive maintenance, rebuilding,
and alteration records (except inspection performed
in accordance with parts 91 and 125, and sections
135.411(a)(1) and 135.419 of this chapter)
The first observation is that this section specifically
excludes inspection entries (those are covered in 14 CFR
part 43, section 43.11). This section deals exclusively with
maintenance record entries.
The next observation is that the list of maintenance actions
includes “preventive maintenance.” As stated in the
explanation of 14 CFR part 43, section 43.3, a certificated
pilot is authorized to perform preventive maintenance on the
aircraft they own or operate. Therefore, remember that the
pilot must make a record entry of the preventive maintenance
they have accomplished. There are three distinct issues to
be addressed in the maintenance entry and they answer the
questions of “what?, when?, and who?”
• What—a description of the work performed
• When—the date the work was completed
• Who—the name of the person who did the work if other than the person who approves the RTS
—the signature, certificate number, and type of
certificate of the person who is approving the
work for RTS
Note: Frequently, logbooks have a statement entered that ends
something like this: “ … and is hereby returned to service. Joe
Fixer A&P, Certificate #123456789.” As this section of the
regulation currently reads, that part of the record entry is not
required. Title 14 of the CFR part 43, section 43.9 clearly states
that “the signature constitutes the approval for return to service
only for the work performed.” Furthermore, the technician
is only signing off the work they have done. Later, 14 CFR
part 43, section 43.11 explains that an inspection write-up
usually carries a broader scope of responsibility. This section
is very clear that the entry completed in accordance with this
section only holds the technician responsible for the service
maintenance action they entered.
If the maintenance accomplished was a major repair or
alteration, the work must be documented on FAA Form 337
and requires supporting approved data. If the maintenance
action was a major repair and it was done by a certificated
repair station, a signed copy of the completed customer work
order accompanied by a signed maintenance release may be
used in lieu of the FAA Form 337.
Section 43.10—Disposition of Life-Limited Aircraft
Parts
(Note the even number again. This regulation became part
of 14 CFR part 43 in 2002.)
This section presents two terms not previously defined in
14 CFR:
1. Life-limited part means any part that has specified a
mandatory replacement limit.
2. Life status means the accumulated cycles, hours, or
any other mandatory limit of a life-limited part.
This section then goes on to specify what to do with parts
that are temporarily removed from and then reinstalled on
a type-certificated product; what to do with parts that are
removed from a type certified product and not immediately
reinstalled; and how to transfer life-limited parts from one
type-certificated product to another.
When a life-limited part is removed, the person removing it
from the type-certificated product must control the part and
ensure proper tracking of the life-limiting factor. This is to
prevent the installation of the part after it has reached its life
limit. There are seven possible methods the technician or repair
facility may choose from to comply with this requirement.
2-171. Recordkeeping
2. Tagging
3. Non-permanent marking
4. Permanent marking
5. Segregation
6. Mutilation
7. Any other method approved or accepted by the FAA
When a life-limited part is transferred, the information
concerning the life status of that part must be transferred
with it. Although regulations already did exist that required
the tracking of life-limited parts when they were installed
on an aircraft, this regulation was generated to govern the
disposition of such parts when they were removed from
the aircraft.
Section 43.11—Content, form, and disposition of
records for inspections conducted under parts 91
and 125, and sections 135.411(a)(1) and 135.419 of
this chapter
This section deals exclusively with inspection record
entries; however, the requirements are similar to 14 CFR
part 43, section 43.9 in that information of what, when, and
who is required.
• What—type of inspection, including a brief description
• When—date of the inspection and the total time in
service
• Who—the signature, certificate number, and kind of
certificate of the person approving or disapproving
the RTS
Since this is an inspection write-up and not a maintenance
entry, it is quite possible that the inspecting technician could
reject or disapprove the item being inspected for the RTS.
When that situation occurs, the regulation states in paragraph
(b) that a list of discrepancies must be given to the owner.
A reference to this list and its delivery to the aircraft owner
must be reflected in the record entry. Although the regulation
neither specifies how those discrepancies can be cleared, nor
who may do them, any appropriately-rated repair station or
certificated technician can perform the required maintenance
actions. When they are completed and the proper maintenance
record entries are generated in accordance with 14 CFR part
43, section 43.9, the aircraft is approved for RTS. It is neither
necessary to have an additional inspection, nor is it necessary
to contact the disapproving inspector.
If the aircraft is on a progressive inspection program, the
inspection statement changes slightly from the statement
referenced earlier by adding the reference to both a “routine inspection” and a “detailed inspection.” Refer
to explanatory text of 14 CFR part 43, section 43.15 for
a definition of these terms. Inspections accomplished in
accordance with other inspection program requirements
must identify that particular program and that part of the
program the inspection completed.
Section 43.12—Maintenance Records: Falsification,
Reproduction, or Alteration
The aviation community relies heavily on trust and honesty
in both oral and written communication. The maintenance
log entries described in 14 CFR part 43, sections 43.9
and 43.11 provide the documentation trail relied upon
by aircraft owners, pilots, and technicians regarding the
aircraft’s maintenance history. Falsification of these records
is potentially dangerous to the personnel who rely on the
accuracy of these records.
This section identifies that fraudulent entries are unacceptable.
If someone commits such an act, that action is the basis for
suspension or revocation of the appropriate certificate,
authorization, or approval. A technician who is encouraged
by their employer, or by anyone else, to falsify records in
any way should remember this comment: “Companies come
and go, but my signature lasts a lifetime. I will not use it
inappropriately.”
Section 43.13—Performance Rules (General)
This section deals with the specific requirements for
conducting maintenance. ( Note: This section best reflects
the relationship between the FAA’s numbering of ACs and
the regulations they are related to.) Paragraph 3 on the cover
page of AC 43.13-2B, Acceptable Methods, Techniques, and
Practices—Aircraft Alterations, dated March 3, 2008 states:
“Title 14 of the Code of Federal Regulations (14 CFR) part
43, section 43.13(a) states that each person performing
maintenance, alteration, or preventive maintenance on
an aircraft, engine, propeller, or appliance must use the
methods, techniques, and practices prescribed in the current
manufacturer’s maintenance manual or Instructions for
Continued Airworthiness prepared by its manufacturer, or
other methods techniques or practices acceptable to the
Administrator, except as noted in section 43.16.” [Figure 2-9]
Although not all ACs are linked this directly, there is a definite
relationship between ACs and companion regulations. Refer
to this chapter on ACs for additional information.
Aircraft maintenance technicians (AMTs) are highly skilled
personnel, because aviation maintenance work requires great
attention to detail. The complexity of technology on today’s
aircraft demands a significant level of communication to
2-18properly accomplish maintenance, preventive maintenance,
rebuilding, or alteration. This communication frequently
comes in written form (i.e., manufacturer’s maintenance
manuals or ICA). If neither of these documents provide the
guidance the technician needs to perform maintenance, either
AC 43.13 (AC 43.13-1 or AC 43.13-2) contain examples
of “other methods, techniques, or practices acceptable to
the Administrator” that may be sufficient. However, these
ACs specifically state that the information is applicable to
non-pressurized areas of civil aircraft weighing 12,500 lb
gross weight or less.
In addition to the documentation, the technician must also use
the proper tools, equipment, and test apparatus that ensures
that the work complies with accepted industry practices.
If the test equipment specified by the manufacturer is not
available, equipment that is determined to be equivalent and
acceptable to the Administrator may be used. The technician
should be cautious, however, as “proving” the equivalence of
test equipment may not be as simple as it seems.
Air carriers (commercial—“scheduled” airlines operating
under 14 CFR part 121, the “commuter/on demand” aircraft
operating under 14 CFR part 135, and foreign air carriers
and operators of U.S.-registered aircraft under 14 CFR
part 129) may use the maintenance manual required by the
operations specifications to comply with the requirements
of this section. The operator must provide a continuous
airworthiness maintenance and inspection program
acceptable to the Administrator.
Section 43.15 —Additional Performance Rules for
Inspections
This section presents general comments concerning the
responsibility of conducting an inspection and then provides
details of three separate conditions. They are rotorcraft,
annual and 100-hour inspections, and progressive inspections.
1. Rotorcraft—If a rotorcraft is being inspected, specific
items, such as rotor transmissions and drive shafts,
must be inspected.
2. Annual and 100-hour inspections—When performing
an annual or 100-hour inspection, a checklist must
be used. This checklist may be a personal one or one
from the manufacturer. Either way, it must include
the scope and detail of the inspection in Appendix D.
Specific engine performance is also required to be
tested (or monitored) as part of RTS for an annual or
100-hour inspection. This applies whether the aircraft
is reciprocating or turbine powered.
3. Progressive inspection—If a progressive inspection is
being conducted, it must be preceded by a complete
aircraft inspection. ( Note: A progressive inspection is the result of breaking down the large task of
conducting a major inspection into smaller tasks that
can be accomplished periodically without taking the
aircraft out of service for an extended period of time.)
Two new definitions are also presented: “routine” and
“detailed.” A routine inspection is a visual examination
or check of the item, but no disassembly is required.
A detailed inspection is a thorough examination of
the item, including disassembly. The overhaul of a
component is considered to be a detailed inspection.
If the aircraft is away from the station where
inspections are normally conducted, an appropriately
rated mechanic, a certificated repair station, or the
manufacturer of the aircraft may perform inspections
in accordance with the procedures and using the
forms of the person who would otherwise perform
the inspection.
Section 43.16 —Airworthiness Limitations
The technician performing inspection or maintenance actions
on an aircraft must be certain they have all appropriate data
available. Each person performing an inspection or other
maintenance specified in an Airworthiness Limitations section
of a manufacturer's maintenance manual or Instructions for
Continued Airworthiness shall perform the inspection or
other maintenance in accordance with that section, or in
accordance with operations specifications approved by
the Administrator under part 121 or 135, or an inspection
program approved under 14 CFR part 91, section 91.409(e).
ICAs, as required by 14 CFR part 21, section 21.50, must
also be consulted when available. Since 1998, the FAA
has required ICAs to be generated for all major alterations
that are accomplished by the field approval process. This
section specifies that the technician is responsible to perform
inspections or maintenance specified in an airworthiness
limitation in accordance with all the preceding instructions.
Section 43.17—Maintenance, preventive maintenance,
or alterations performed on U.S. aeronautical
products by certain Canadian persons
This section was significantly revised in 2005, as the result
of a Bilateral Aviation Safety Agreement (BASA) between
the United States and Canada. This section of 14 CFR
part 43 defines some terms and gives specific limitations
as to what an Aircraft Maintenance Engineer (AME is
the Canadian equivalent to the U.S. A&P) may do to
maintain U.S.-registered aircraft located in Canada. It also
provides similar limitations for an Approved Maintenance
Organization. (AMO is the Canadian equivalent to the U.S.-
certified repair stations.)
Appendix A—Major Alterations, Major Repairs, and
2-19U.S. Departmen t
of Transportation
Federal Aviat ion
Administrat ionAdvisory
Circular
Subject: Accept able M ethods,
Techniques, and Practices – Aircraft
AlterationsDate: 3/3/08
Initiated by: AFS-300 AC No: 43.13-2B
1.PURPOSE. This advisory circular (AC) cont ains m ethods, techniques, and practices
acceptable to the Ad ministrato r for the inspecti on and alteration on non- pressurized areas of civil
aircraft of 12,500 lbs gross weight or less. This AC is for use by m echanics, repair stations, and
other c ertificated e ntities. This data generally pertains to minor alterations; however, the alteration
data herein may be used as approved data for major alterations when the AC chapter, page, and
paragraph are listed in block 8 of FAA Fo rm 337 when the us er has de termined that it is:
a.Appropriate to the product being altered,
b.Directly applicab le to the alteration being made, and
c.Not contrary to manufact urer’s data.
2.CANCE LLA TION. AC 43.13-2A, Acceptable Met hods, Techniques, a nd Practices ―
Aircraft Alterations, dated January 1, 1977, is canceled.
3.REFE RENCE. Title 14 of the Code of Federal Regulatio ns (14 CFR) part 43, § 43.1 3(a)
states that each person perfor ming maintenance, alteration, or preventive maintenance on an
aircraft, engine, propeller, or appliance must use the m ethods, techniques, and practices
prescribed in the current manufacturer’s m aintenance m anual or Instructions for Continued
Airworthiness prepared by its manufacturer, or other m ethods, techniques, or practices
acceptable to the Ad ministrato r, except as no ted in § 43.16. FAA inspectors are prepared to
answer questions that may arise in this regard. Persons engage d in the inspection and alteration
of civil aircraft should be familiar with 14 CFR part 43, Mai ntenance, Preventive Maintenance,
Rebuilding, and Alterations, and part 65, subparts A, D, and E of Certification: Air men Other
than Flight Crewm embers, and applicable airw orthiness requirem ents under which the aircraft
was type-c ertificated.
4. COMMEN TS INVI TED. Comme nts regarding this AC should be directed to DOT/FAA:
ATTN: Aircraft Maintenance Division, 800 Independence Ave., SW., Washington, D C 20591,
FAX (202) 267-5115.
ORIGINAL SIGNED By
James J. Ballough
Director Flight Standards Service
Figure 2-9. AC 43.13-2B Excerpt.
2-20Preventive Maintenance
This appendix provides a comprehensive, but not exclusive,
list of subjects. For instance, paragraph (a) is titled Major
Alteration, and is further subdivided as follows:
• Airframe
• Powerplant
• Propeller
• Appliance
This same subdivision is used in paragraph (b), Major
Repairs. Paragraph (c), Preventive Maintenance, identifies
those maintenance actions that are defined as preventive
maintenance, provided the maintenance does not involve
complex assembly operations. Preventive maintenance work
may be accomplished by the holder of at least a private pilot
certificate provided they are the owner or operator of that
aircraft, and it is not operated under 14 CFR part 121, 129,
or 135.
Appendix B—Recording of Major Repairs and Major
Alterations
In most cases when a major repair or alteration is
accomplished, FAA Form 337, Major Repair or Alteration,
is completed at least in duplicate with the original going
to the aircraft owner and a copy sent to the FAA Aircraft
Registration Branch in Oklahoma City where all civil aircraft
information is compiled and retained. Note: Historically,
the second copy was sent to the local FAA FSDO within 48
hours after RTS. This copy is reviewed by an ASI and then
forwarded by the FSDO to FAA records in Oklahoma City.
However, in the fall of 2005, the FAA made a significant
change to this submittal process and now requires the
technician to submit the Form 337 directly to the Aircraft
Registration Branch in Oklahoma City. Although a third
copy is not required, it makes good business sense for the
technician or certified repair station to keep a copy of the
work that was accomplished.
However, if a certificated (part 145) repair station completes
a major repair, it may provide the customer with a signed
copy of the work order and a maintenance release signed
by an authorized representative of the repair station, instead
of the FAA Form 337. If the major repair or alteration was
done by an AME or AMO, the copy normally provided
to the FAA-FSDO is sent directly to the FAA Aircraft
Registration Branch.
However, if extended range tanks are installed in either
passenger or cargo compartments, the technician must
generate a third FAA Form 337 for the modification. This
copy must be placed and retained in the aircraft. (Refer to
14 CFR part 91, section 91.417(d).)Appendix C—(Reserved)
Appendix C is reserved for future use and therefore currently
contains no information.
Appendix D—Scope and Detail of Items To Be
Included in Annual and 100-Hour Inspections
Some important items to consider in this appendix are:
1. The list of items and areas to be inspected are exactly
the same for an annual as a 100-hour inspection.
The difference between the inspections is in who is
authorized to perform and approve the aircraft for RTS
following the inspection. Refer to 14 CFR part 65,
section 65.95(a)(2) that states that an IA must perform
an annual inspection.
2. The aircraft and engine must be cleaned prior to
conducting the inspection.
3. Any miscellaneous item not covered in the detailed
list provided must also be inspected for improper
installation and operation.
4. There are eight specific areas identified for detailed
inspection. They are the fuselage hull group, cabin/
flight deck group, engine/nacelle group, landing
gear group, wing/center section group, empennage
assembly, propeller group, and the radio group.
Appendix E—Altimeter System Test and Inspection
This is commonly referred to as “the 411 test.” Refer to 14 CFR
part 91, section 91.411 that requires that no person may operate
an aircraft in controlled airspace under IFR unless the aircraft
has had this test completed successfully within the preceding
24 months.) This section requires detailed testing of the static
pressure system, the altimeter, and the automatic pressure
altitude reporting equipment, and that the test information be
recorded in the maintenance logs and on the altimeter.
Appendix F—ATC Transponder Tests and
Inspections
This is commonly referred to as “the 413 test.” (Refer to 14
CFR part 91, section 91.413, which requires that no person
may use a transponder unless it has had this test completed
successfully within the preceding 24 months.)
This section specifies complex sets of tests, which may be
accomplished either as a bench test or by using portable
test equipment. Major categories of the testing required are
radio reply frequency, suppression, receiver sensitivity, radio
frequency peak output power, and mode S (when applicable).
Upon completion of testing, proper entries must be made in
the maintenance record.
2-2114 CFR Part 91—General Operating and Flight Rules
Subpart A—General
As mentioned in the brief overview of the regulation portion
earlier in this chapter, this part is actually addressing the
operation of the aircraft. For example, 14 CFR part 91, section
91.7(a) states “no person may operate a civil aircraft unless it
is in an airworthy condition.” We learned earlier that this term
means that the aircraft conforms to its approved type design
and is in condition for safe operation. When the pilot performs
a preflight inspection, they are making a determination
concerning the “condition for safe operation.” The pilot does
not usually determine “conformity to type design” unless
they perform a review of the maintenance records. However,
since that is fundamental to the definition of airworthy, it
is still part of their responsibility. Therefore, a professional
and ethical technician wants to help the customer understand
their responsibilities in maintaining and documenting the
airworthiness of the aircraft.
Subpart E—Maintenance, Preventive Maintenance,
and Alterations
Section 91.401—Applicability
Although this subpart describes in general the rules regarding
maintenance, preventive maintenance, and alteration, certain
sections do not apply if the aircraft is operated in accordance
with 14 CFR part 121, 125, 129, or 135.
Section 91.403—General
The owner/operator holds the primary responsibility
for maintaining the aircraft in airworthy condition. This
includes compliance with all applicable ADs and is the
reason that the FAA sends new AD notes to the registered
owners of the affected aircraft. All maintenance performed
must be accomplished in accordance with 14 CFR part 43.
Compliance with the appropriate manufacturer maintenance
manuals and ICA is also required. Mandatory replacement
times, inspection intervals, and related procedures as outlined
in the FAA-approved operations specifications must also be
complied with.
Section 91.405—Maintenance Required
The owner/operator is required to have the appropriate
inspections made, and to have discrepancies repaired in
accordance with part 43. They are also required to ensure that
the appropriate entries have been made in the maintenance
records. Any inoperative instruments or equipment must be
properly placarded as inoperative.
Section 91.407—Operation after maintenance, preventive
maintenance, or alteration
Whenever the aircraft has undergone maintenance,
preventive maintenance, rebuilding or alteration, it must have been approved for RTS and a proper entry made in
the maintenance records. If the maintenance that was done
could have appreciably changed the flight characteristics, an
appropriately rated pilot must perform an operational flight
check of the aircraft and must make an entry of the flight in
the maintenance records. If ground testing and inspection can
show conclusively that the maintenance has not adversely
affected the flight characteristics, no flight test is required.
Section 91.409—Inspections
This paragraph identifies various types of inspection
applicable to the civilian aircraft fleet. Paragraph (a) defines
the requirement for an annual inspection. However, there are
certain exceptions to this regulation:
1. An aircraft that carries a special flight permit, a current
experimental certificate, or a light-sport or provisional
airworthiness certificate;
2. An aircraft inspected in accordance with an approved
aircraft inspection program under part 125 or 135
of this chapter and so identified by the registration
number in the operations specifications of the
certificate holder having the approved inspection
program;
3. An aircraft subject to the requirements of paragraph
(d) or (e) of this section; or
4. Turbine-powered rotorcraft when the operator elects
to inspect that rotorcraft in accordance with paragraph
(e) of this section.
Annual inspections are usually the inspection method
associated with small “general aviation” aircraft. If this same
aircraft is used for hire (including flight instruction for hire),
then the aircraft must also be inspected every 100 hours of
time in service. This requirement for a 100-hour inspection
to be conducted on an aircraft may be exceeded by as much
as 10 hours if the aircraft is en route to reach a facility that
will be conducting the inspection. Any time accrued between
100 and 110 hours is subtracted from the hours remaining
before the next 100-hour inspection.
Since aircraft used for hire only generate revenue when they
are flying, any time that the aircraft is “down for inspection”
can result in a loss of income for the owner/operator.
Therefore, the FAA has made provision to minimize the
impact of the 100-hour and annual inspection requirement.
The owner/operator may petition the local FSDO for approval
of a progressive inspection program. This program breaks
the complete inspection of the aircraft into smaller, less
time-consuming steps. (Refer to 14 CFR part 43, Appendix
D.) This inspection may be either performed or supervised
by a technician holding an IA. The program must ensure at
all times that the aircraft is airworthy. The owner/operator
must submit an inspection schedule with their application
2-22to the FAA. This schedule must identify the time intervals
(hours or days) when routine and detailed inspections are to
be accomplished. (Refer to 14 CFR part 43, section 43.15.)
Just as with the 100-hour inspection, a 10-hour maximum
extension of a specified inspection interval is allowed if the
aircraft is en route. A change in the inspection interval is also
allowed for changes in service experience. If the progressive
inspection is discontinued, the aircraft is again subject to the
traditional annual and 100-hour inspections.
Other inspection programs that may be applicable to other
aircraft are a continuous airworthiness inspection program
and an approved aircraft inspection program (AAIP).
The former program is applicable to either a part 121 or
135 carrier, but the latter program is limited to part 135
operators only. Finally, the owner/operator may use either
a current inspection program recommended by the aircraft
manufacturer or one established by the owner/operator and
approved by the local FSDO. Any subsequent changes to that
program must also be approved by the local FSDO.
There may be an instance when the operator of an aircraft
wishes to change from one type of inspection program to
another. In that case, the time in service, calendar times, or
cycles of operation from the current program must be carried
over to the subsequent program.
Section 91.411—Altimeter System and Altitude Reporting
Equipment Tests and Inspections
Commonly referred to as “the 411 test,” this section specifies
the requirements for testing the static pressure system, each
altimeter instrument, and each automatic pressure altitude
reporting system every 24 calendar months. The static system
must also be tested any time it has been “opened and closed,”
except for the normal use of the system drain and alternate
static system pressure valves. If the automatic pressure
altitude reporting system of the air traffic control (ATC)
transponder is either installed or subjected to maintenance
actions, the system must also be tested per Appendix E of
14 CFR part 43.
Due to the inherent design and accuracy of this system, only
the aircraft manufacturer, a properly-rated repair station, or a
certificated airframe mechanic may perform these tests. The
airframe technician may only perform the inspection and test
of the static pressure system. Calibration and maintenance
of related instruments is specifically prohibited to the
technician by the language of 14 CFR part 65, section 65.81
and specifically allowed in 14 CFR part 145, section 145.59
for repair stations holding an instrument rating.
TSO’d items are considered to be “tested and inspected” as
of the date they were manufactured. The maximum altitude that the system was tested is the maximum altitude that
the aircraft can be flown instrument flight rules (IFR) in
controlled airspace.
Section 91.413—ATC Transponder Tests and Inspections
This “413 test” is the other test required every 24 months.
Whenever the ATC transponder is installed or has undergone
maintenance, the complete system must be tested and inspected
in accordance with Appendix E of 14 CFR part 43. The
transponder itself must be tested and inspected in accordance
with Appendix F of 14 CFR part 43. As with the 411 test,
only certain persons are authorized to conduct the tests. They
are the manufacturer of the aircraft, a properly certificated
repair station, or the holder of a continuous airworthiness
maintenance program under 14 CFR part 121 or 135.
Section 91.415—Changes to Aircraft Inspection Programs
If the FAA determines that the inspection program established
and approved under either 14 CFR part 91, section 91.409
or 91.1109 must be revised to ensure continued safety and
adequacy of the program, the owner/operator must make
the necessary changes as identified by the Administrator.
If the owner/operator desires to contest this request, they
must petition the FAA to reconsider their request to change
the program within 30 days of receiving the change request
from the FAA.
Section 91.417—Maintenance Records
The understanding and implementation of this section
is fundamental to the aircraft industry, in general, and
the aircraft owner/operator, in specific. A professional
maintenance technician must be knowledgeable of this
section and be able to help the owner/operator understand it.
[Figure 2-10] This section identifies four types of records—
two are quite specific (paragraphs a and d) and two are more
general: (a)(1) and (a)(2). Paragraph (a) refers to the 411 and
413 testing that requires testing every 24 months. Therefore,
records must be kept for that length of time. Paragraph (d)
refers to the installation of fuel tanks in the cabin or cargo
area. The FAA Form 337 authorizing this installation must
be kept on board the aircraft all the time.
Note: Other than this paragraph, there is no requirement
that the maintenance records of the aircraft be carried on the
aircraft. In fact, there are very logical reasons to not do so in
most cases. The two biggest concerns are damaged or lost
records. It is much safer to retain the logs in a filing system
in the office. It is also a very wise idea to have the logbook
copied or scanned and retained at a separate location should
a catastrophic event (fire, flood, tornado, hurricane, and so
forth) occur at the site the original records are retained.
Subparagraph (a)(1) then lists those records that are later
2-23defined in (b)(1) as being retained for 1 year or until the
work is repeated or superseded. Subparagraph (a)(2) specifies
the records that are permanent records and are identified
in subparagraph (b)(2) as those that must be transferred
with the aircraft. Refer to the chart for further clarification.
[Figure 2-10]
Paragraph (c) requires that all of the maintenance records
mandated by this section be made available upon request
to the Administrator or any authorized representative of
the NTSB. Furthermore, the owner/operator must provide
the Form 337 required to be aboard the aircraft whenever
additional fuel tanks are installed in either the passenger
compartment or the baggage compartment, per paragraph
(d), to any law enforcement officer upon request.
Section 91.419—Transfer of Maintenance Records
When an aircraft is sold, it is logical that the records are
transferred with it. They may be either in plain language or
coded. The purchaser may elect to permit the seller to retain
the actual records; however, if that occurs the purchaser (now
the current owner/operator) must still make these records
available to either the FAA or the NTSB upon request.
Section 91.421—Rebuilt Engine Maintenance Records
This section presents the term “zero time.” Although not
truly given as a definition, the wording of the regulation is
very clear that an aircraft engine, when rebuilt by the engine
manufacturer or an agency approved by the manufacturer,
may be given a new maintenance record showing no
previous operating history. This new record must include a
signed statement with the date it was rebuilt, any changes
incorporated by compliance with AD notes, and compliance
with any of the manufacturer’s SB.
Civil Air Regulations (CAR)
Prior to 1926, access to flying was uncontrolled. No licensing
or certification was required. By the middle of the 1920s, it
became obvious that unregulated private and commercial
flying was dangerous. There was a growing awareness
and acceptance that regulation could improve safety and
encourage growth in aviation. Therefore in 1926, the aviation
industry requested Congress to enact federal legislation to
regulate civil aviation. Thus, the Air Commerce Act of 1926
provided for the:
1. Establishment of airways.
2. Development of aviation aids.
3. Investigation of aviation accidents.
4. Licensing of pilots.
5. Certification of aircraft.The Civil Air Regulations (CARs) were part of the original
certification basis for aircraft first certified in the 1940s,
1950s, and 1960s by the Civil Aviation Authority (CAA).
Therefore, the CARs may still be needed as a reference for
these older aircraft or as a standard for minor changes to older
aircraft designs. [Figure 2-11]
CAR 3—Airplane Airworthiness—Normal, Utility,
Aerobatic, and Restricted Purpose Categories
As the name implies, this specific regulation is the basis for
the current 14 CFR part 23 regulation [ Figure 2-1 ]. It has
the following subpart categories:
• A—Airworthiness Requirements
• B—Flight Requirements—General
• C—Strength Requirements—General
• D—Design and Construction—General
• E—Powerplant Installations—Reciprocating Engines
• F—Equipment
Some examples of CAR 3 aircraft are Piper PA 22, PA 28,
PA 32, and Cessna 182, 195, and 310.
Note: The “CAR” acronym actually has two interpretations:
Civil Air Regulations and Canadian Aviation Regulations.
The technician must clearly understand the difference and
recognize when one or the other is appropriate.
CAR 4a—Airplane Airworthiness
This regulation was originated in 1936 and last amended on
December 15, 1952. The subparts included in this regulation
are:
• A—Airworthiness Requirements
• B—Definitions
• C—Structural Loading Conditions, General Structural
Requirements
• D—Proof of Structure
• E—Detail Design and Construction
• F—Equipment
• G—Powerplant Installation
• H—Performance
• I—Miscellaneous Requirements
Initially, this regulation was the basis for establishing the design
requirements for virtually all produced aircraft in the 1930s,
1940s, and 1950s. Eventually CAR 3 evolved as the regulatory
material specific to small aircraft, and CAR 4a and b focused on
regulatory requirements for large aircraft.
14 CFR 91.417 Maintenance Records
Sections 91.411 and 91.413.
paragraph (a): Retain for 2 years.Paragraph (d): FAA Form 337 for extended range fuel tanks in cabin or cargo. Keep on board A/C.
Paragraph (a)(1): Maintenance,Preventive Maintenance,Alterations and all Inspections.Paragraph (a)(2): Records of total time in service for A/F, each engine, each propellerand each rotor. Currents status of life-limited parts of A/F engine, prop, rotor, or app. Paragraph (b)(3) List of defects furnished to the Owner/Operatorin accordance with Section 43.11.
Description of work.Date of completion.
Signature and Certificate Number
of person approving RTS.Time since last overhaul for items that are
required to overhaul. Current inspection status. Current status of applicable ADs. Copies of Form 337.Retained until defects are repaired and the A/C is approved for RTS.
Paragraph (b)(2): Retain and transfer with
A/C.
Figure 2-10. Maintenance records.It is very important to review the TCDS for each aircraft.
For example, The Cessna 140 was certified as a landplane under CAR 3, but under CAR 4a as a ski-plane or seaplane. Another example of a more current and larger aircraft is the Gulfstream 1159 and 1159A. The former is certified under CAR 4b, but the latter is certified to 14 CFR part 25.
Suspected Unapproved Parts (SUP)
There are four types of aircraft parts:
1.Good parts with good paperwork.
2.Good parts with bad paperwork.
3.Bad parts with “good” (bogus) paperwork.
4.Bad parts with bad paperwork.
The first of those listed represents properly authorized parts that,
when properly installed, are approved parts, and the aircraft can be returned to service. The last of those listed represent unauthorized and unapproved parts. The technician should be alert for these and must never install them on an aircraft.
The center two categories of parts represent suspected
unapproved parts. If either the physical part or the paperwork associated with the part is questionable, it is best to contact the shop foreman, shift supervisor, or the assigned quality individual to discuss your concerns. Suspected unapproved parts (SUPs) should be segregated and quarantined until proper disposition can be determined. Contacting the manufacturer of the product is a good way to start gathering the facts concerning the product in question. Refer to the current version of AC 21-29, Detecting and Reporting Suspected Unapproved Parts, for additional information. Current contact information for submitting a SUP Notification can be found at www.faa.gov.
Other FAA Documents
Advisory Circulars (AC)
AC refers to a type of publication offered by the FAA
to provide guidance for compliance with airworthiness regulations. They provide guidance such as methods, procedures, and practices acceptable to the Administrator for complying with regulations. ACs may also contain explanations of regulations, other guidance material, best practices, or information useful to the aviation community. They do not create or change a regulatory requirement. The AC system became effective in 1962. It provides a single, uniform, agency-wide system that the FAA uses to deliver advisory material to FAA customers, industry, the aviation community, and the public.
Unless incorporated into a regulation by reference, the
content of ACs are not binding on the public. ACs are issued in a numbered-subject system corresponding to the subject areas of the FARs (14 CFR, Chapter 1, Federal Aviation Administration) and Chapter 3, Commercial Space Transportation, Federal Aviation Administration, Department of Transportation, Parts 400–450. An AC is issued to provide guidance and information in a designated subject area or to show a method acceptable to the Administrator for complying with a related federal aviation regulation.
Because of their close relationship to the regulations, ACs are
arranged in a numbered system that corresponds to the subject areas of the CFRs. In some series, consecutive numbers may
2-25Predecessor Regulations to the Federal Aviation Regulations (14 CFR)
Aeronautical Bulletins
7A 7F 7G 7H
7J
14 14 26 Airworthiness Requirements for Aircraft
Airworthiness Requirements for Aircraft Components and Accessories
Airworthiness Requirements for Engines and Propellers
Alteration and Repair of Aircraft
Special Requirements for Air Line Aircraft
Relative Lift Distribution in Any Biplane
Requirements for Approved Type Certificates
Design Information for Aircraft
Civil Air Regulations (CAR)
CAR 1 CAR 2 CAR 3 CAR 4a
CAR 4b
CAR 6 CAR 7 CAR 8 CAR 9 CAR 10 CAR 13 CAR 14 CAR 18 CAR 40 Special CAR 425-C
Special CAR 406 Certification, Identification, and Marking of Aircraft and Related Products
Aircraft Identification Mark
Airplane Airworthiness—Normal, Utility, Acrobatic, and Restricted Purpose Categories
Airplane Airworthiness
Airplane Airwor thiness: Transport Categories
Rotorcraft Airworthiness: Normal Category
Rotorcraft Airwor thiness: Transport Categories
Aircraft Airworthiness: Restricted Category
Aircraft Airworthiness: Limited Category
Certification and Approval of Import Aircraft and Related Products
Aircraft Engine Airworthiness
Aircraft Propeller Airworthiness
Maintenance, Repair, and Alteration of Certificated Aircraft and of Aircraft Engines, Propellers and Instruments
Scheduled Interstate Air Carrier Certification and Operation Rules
Provisional Certification and Operation of Aircraft
Application of Transport Category Performance Requirements to C-46 Type Aircraft
Civil Aeronautics Manual (CAM)
CAM 1 CAM 2 CAM 3 CAM 4a
CAM 4b
CAM 6 CAM 7 CAM 8 CAM 9 CAM 10 CAM 13 CAM 14 CAM 18 Certification, Identification, and Marking of Aircraft and Related Products
Production Certificates
Airplane Airworthiness: Normal, Utility, and Acrobatic Categories
Airplane Airworthiness
Airplane Airworthiness: Transport Categories
Rotorcraft Airworthiness
Rotorcraft Airwor thiness: Transport Categories
Aircraft Airworthiness: Restricted Category
Aircraft Airworthiness: Limited Category
Certification and Approval of Import Aircraft and Related Products
Aircraft Engine Airworthiness
Aircraft Propeller Airworthiness
Maintenance, Repair, and Alteration of Airframes, Powerplants, Propellers, and Appliances
Figure 2-11. Predecessor Regulations to the Federal Aviation Regulations (14 CFR).
2-26be missing. These numbers were either assigned to ACs
still in preparation that will be issued at a later date or were
assigned to ACs that have been canceled.
The AC Numbering System
There are three parts to an AC number, as in 25-42-C.
• The first part of the number identifies the subject
matter area of the AC. This corresponds to the part
of the FAA’s regulations. In the above example, this
would be part 25.
• The second part of the number, beginning with the
dash, is a sequential number within each subject area.
In the above example, this would be the 42nd AC
relating to part 25.
• The third part of the number is a letter assigned by
the originating office showing the revision sequence
if an AC is revised. The first version of an AC does
not have a revision letter. In the above example, this is
third revision, as designated by the “C.” [Figure 2-12]
Airworthiness Directives (AD)
In accordance with 14 CFR part 39, the FAA issues ADs in
response to deficiencies and/or unsafe conditions found in
aircraft, engines, propellers, or other aircraft parts. ADs require
that the relevant problem must be corrected on all aircraft or
aircraft parts using the same design. ADs are initiated as either
proposed, corrective, or final (telegraphic) via the Federal
Register. The Federal Register is the official daily publication
of the United States Government. It is the printed method of
informing the public of laws that are enacted or will be enacted.
Electronic versions of ADs are available from the Federal
Register and from the Regulatory and Guidance Library. You
can search by manufacturer, model, or AD number. All ADs
are “incorporated by reference” into part 39 and are considered
final. ADs must be followed to remain in compliance with
the FAA. Once an AD has been issued, a person/company is
authorized to use the affected aircraft or part only if it has been
corrected in accordance with the AD.
Types of Airworthiness Directives (AD)
Three types of ADs are issued:
• Notice of Proposed Rulemaking (NPRM), followed
by a Final Rule
• Final Rule; Request for Comments
• Emergency ADs
The standard AD process is to issue an NPRM followed
by a Final Rule. After an unsafe condition is discovered,
a proposed solution is published as an NPRM and solicits
public comment on the proposed action. After the comment
period closes, the final rule is prepared, taking into account all substantive comments received with the rule perhaps being
changed as warranted by the comments. The preamble to the
final rule AD provides response to the substantive comments
or states there were no comments received.
In certain cases, the critical nature of an unsafe condition
may warrant the immediate adoption of a rule without prior
notice and solicitation of comments. This is an exception to
the standard process. If time for the terminating action to
be accomplished is too short to allow for public comment
(that is, less than 60 days), then a finding of impracticability
is justified for the terminating action, and it can be issued
as an immediately adopted rule. The immediately adopted
rule is published in the Federal Register with a request for
comments. The Final Rule AD may be changed later if
substantive comments are received.
An Emergency AD is issued when an unsafe condition exists
that requires immediate action by an owner/operator. The
intent of an Emergency AD is to rapidly correct an urgent
safety deficiency. An Emergency AD may be distributed
by fax, letter, or other methods. It is issued and effective to
only the people who actually receive it. This is known as
“actual notice.” All known owners and operators of affected
U.S.-registered aircraft, or those aircraft that are known to
have an affected product installed, are sent a copy of an
Emergency AD. To make the AD effective to all persons, a
follow up publication of the Final Rule AD in the Federal
Register is critical. This Final Rule AD must be identical to
the Emergency AD and is normally published in the Federal
Register within 30 days of the Emergency AD issue.
AD Content
Generally, ADs include:
• A description of the unsafe condition
• The product that the AD applies to
• The required corrective action or operating limitations
or both
• The AD effective date
• A compliance time
• Where to go for more information
• Information on alternative methods of compliance
with the requirements of the AD
AD Number
ADs have a three-part number designator. The first part is the
calendar year of issuance. The second part is the biweekly
period of the year when the number is assigned. The third part
is issued sequentially within each biweekly period.
2-27Applicability and Compliance
The AD subject line specifically identifies the TC holder
of the aircraft or products affected by the AD. The specific
models affected and any special considerations, such as
specific installed part numbers or modifications, are listed
in the AD applicability section. In order to find all applicable
ADs for a specific product, you must search for ADs on
the product, aircraft, engine(s), propeller, or any installed
appliance. If there are multiple series under the aircraft or
engine model, you must also search for ADs applicable
to the model, as well as the specific series of that model.
The final determination of ADs applicable to a particular
product can only be made by a thorough examination of the
ADs and the product logbooks. No person may operate a
product that an AD applies to, except in accordance with the
requirements of the AD. Furthermore, the owner or operator
of an aircraft is required by 14 CFR part 91, section 91.403
to maintain the aircraft in compliance with all ADs. The
AD specifies a compliance time that relates to the effective
date of the AD. That compliance time determines when the
actions are required.
Alternative Method of Compliance
Different approaches or techniques that are not specified in
an AD can, after FAA approval, be used to correct an unsafe
condition on an aircraft or aircraft product. Although the
alternative was not known at the time the AD was issued,
an alternative method may be acceptable to accomplish the
intent of the AD. A compliance time that differs from the
requirements of the AD can also be approved if the revised
time period and approved alternative method provides an
acceptable level of safety as the requirements of the AD.
Special Airworthiness Information Bulletin (SAIB)
A Special Airworthiness Information Bulletin (SAIB) is an
information tool that the FAA uses to alert, educate, and make
recommendations to the aviation community. SAIBs contain
non-regulatory information and guidance that does not meet
the criteria for an AD. [Figure 2-13]
Aircraft Specifications
Specifications were originated during implementation of
the Air Commerce Act of 1926. Specifications are FAA
recordkeeping documents issued for both type-certificated
and non-type-certificated products that have been found
eligible for U.S. airworthiness certification. Although they
are no longer issued, specifications remain in effect and will
be further amended. Specifications covering type-certificated
products may be converted to a TCDS at the option of the
TC holder. However, to do so requires the TC holder to
provide an equipment list. A specification is not part of a
TC. Specifications are subdivided into five major groups
as follows:1. Group I—Type Certificate Aircraft, Engines, and
Propellers. Covering standard, restricted, and limited
types issued for domestic, foreign, and military surplus
products.
2. Group II—Aircraft, Engine, and Propeller Approvals.
Covering domestic, foreign, and military surplus
products constructed or modified between October 1,
1927, and August 22, 1938. All have met minimum
airworthiness requirements without formal type
certification. Such products are eligible for standard
airworthiness certification as though they are type-
certificated products.
3. Group III—Aircraft, Engine, and Propeller Approvals.
Covering domestic products manufactured prior to
October 1, 1927, foreign products manufactured prior
to June 20, 1931, and certain military surplus engines
and propellers. All have met minimum airworthiness
requirements of the Air Commerce Act of 1926 and
implementing Air Commerce Regulations without
formal type certification. Such products are eligible
for standard airworthiness certification as though they
are type-certificated products.
4. Group IV—Engine Ratings. Covering unapproved
engines rated for maximum power and speed only,
their use being limited to specific aircraft with
maximum gross weights less than 1,000 pounds. Such
engines are not eligible for independent airworthiness
certification. These ratings are no longer issued.
5. Group V—Engine Approvals. Covering military
surplus engines meeting CAR 13 design requirements
without formal type certification. Such engines are
eligible for airworthiness certification as though they
are type-certificated engines.
Supplemental Type Certificates (STC)
When an aircraft is designed and that design is formally
approved for manufacturing, the manufacturer is issued
a Type Certificate (TC). The TC is issued by the FAA to
signify the airworthiness of an aircraft design and may not
be changed except by formal authorization of the FAA.
This formal authorization supplements the original TC
and is called the Supplemental Type Certificate (STC).
Therefore, the STC issued by the FAA approves a product
(aircraft, engine, or propeller) modification. [Figure 2-14]
The STC defines the product design change, states how the
modification affects the existing type design, and lists serial
number effectivity. It also identifies the certification basis
listing specific regulatory compliance for the design change.
Information contained in the certification basis is helpful for
those applicants proposing subsequent product modifications
and evaluating certification basis compatibility with other
STC modifications. Refer to Figure 2-15 for a listing of how
2-28Advisory Circular Numbering System
1. General. The advisory circular numbers relate to the FAR subchapter title and correspond to the Parts, and when appropriate, to the specific
sections of the Federal Aviation Regulations.
2. General and specific subject numbers. The subject numbers and related subject areas are as follows:
General
Subject
Number
(1)Specific
Subject
Number
(2) SubjectGeneral
Subject
Number
(1)Specific
Subject
Number
(2) Subject
00 GENERAL
1 Definitions and Abbreviations
10 PROCEDURAL RULES
11 General Rule-Making Procedures
13 Investigation and Enforcement Procedures
20 AIRCRAFT
21 Certification Procedures for Products and
Parts
23 Airworthiness Standards: Normal, Utility,
and Acrobatic Category Airplanes
25 Airworthiness Standards: Transport
Category Airplanes
27 Airworthiness Standards: Normal Category
Rotorcraft
29 Airworthiness Standards: Transport
Category Rotorcraft
31 Airworthiness Standards: Manned Free
Balloons
33 Airworthiness Standards: Aircraft Engines
34 Fuel Venting and Exhaust Emission
Requirements for Turbine Engine Powered
Airplanes
35 Airworthiness Standards: Propellers
36 Noise Standards: Aircraft Type and
Airworthiness Certification
39 Airworthiness Directives
43 Maintenance, Preventive Maintenance,
Rebuilding and Alteration
45 Identification and Registration Marking
47 Aircraft Registration
49 Recording of Aircraft Titles and Security
Documents
60 AIRMEN
61 Certification: Pilots and Flight Instructors
63 Certification: Flight Crewmembers Other
Than Pilots
65 Certification: Airmen Other Than Flight
Crewmembers
67 Medical Standards and Certification
70 AIRSPACE
71 Designation of Federal Airways, Area Low
Routes, Controlled Airspace, and Reporting
Points
73 Special Use Airspace
75 Establishment of Jet Routes and Area High
Routes
77 Objects Affecting Navigable Airspace 90 AIR TRAFFIC AND GENERAL OPERATING
RULES
91 General Operating and Flight Rules
93 Special Air Traffic Rules and Airport Traffic
Patterns
95 IFR Altitudes
97 Standard Instrument Approach Procedures
99 Security Control of Air Traffic
101 Moored Balloons, Kites, Unmanned
Rockets and Unmanned Free Balloons
103 Ultralight Vehicles
105 Parachute Jumping
107 Airport Security
108 Airplane Operators Security
109 Indirect Air Carrier Security
119 CERTIFICATION: AIR CARRIERS AND
COMMERCIAL OPERATORS
120 AIR CARRIERS, AIR TRAVEL CLUBS, AND
OPERATORS FOR COMPENSATION OR
HIRE: CERTIFICATION AND OPERATIONS
121 Certification and Operations: Domestic,
Flag, and Supplemental Air Carriers and
Commercial Operators of Large Aircraft
125 Certification and Operations: Airplanes
Having a Seating Capacity of 20 or More
Passengers or a Maximum Payload
Capacity of 6,000 Pounds or More
127 Certification and Operations of Scheduled
Air Carriers with Helicopters
129 Operations of Foreign Air Carriers
133 Rotorcraft External-Load Operations
135 Air Taxi Operators and Commercial
Operators
137 Agricultural Aircraft Operations
139 Certification and Operations: Land Airports
Serving CAB-Certificated Air Carriers
140 SCHOOLS AND OTHER CERTIFICATED
AGENCIES
141 Pilot Schools
143 Ground Instructors
145 Repair Stations
147 Aviation Maintenance Technician Schools
150 AIRPORT NOISE COMPATIBILITY
PLANNING
151 Federal Aid to Airports
152 Airport Aid Program
Figure 2-12. List of advisory circular numbers.
2-29Advisory Circular Numbering System
3. Within the General Subject Number Areas, Specific selectivity in advisory circular mail lists is available corresponding to the
applicable FAR Parts. For example: under the 60 general subject area, separate mail lists for advisory circulars issued in the 61, 63,
65, or 67 series are available. An AC numbered “60” goes to all numbers in the 60 series. When the volume of circulars in a series
warrants a sub-subject breakdown, the general number is followed by a slash and a sub-subject number. Material in the 150 series,
Airports, is issued under the following sub-subjects:
4. Individual circular identification numbers. Each circular has a subject number followed either by a dash and a consecutive
number (135-15) or a period with a specific FAR section number, followed by a dash and a consecutive number (135.169-2)
identifying the individual circular. This consecutive number is not used again in the same subject series. Revised circulars have a
letter A, B, C, etc., after the consecutive number to show complete revisions. Changes to circulars have Chg. 1, Chg. 2, Chg. 3, etc.,
after the identification number on pages that have been changed. The Date on a revised page is changed to the date of the
Change transmittal. 155 Release of Airport Property from Surplus
Property Disposal Restrictions
156 State Block Grant Pilot Program
157 Notice of Construction, Alteration,
Activation, and Deactivation of Airports
158 Passenger Facilities Charges
159 National Capital Airports
159/10 Washington National Airport
159/20 Dulles International Airport
161 Notice and Approval of Airport Noise and
Access Restrictions
169 Expenditures of Federal Funds for
Nonmilitary Airports or Air Navigational
Facilities Thereon
170 NAVIGATIONAL FACILITIES
170 Establishment and Discontinuance Criteria
for Airport Traffic Control Tower Facilities
171 Non-Federal Navigation Facilities
180 ADMINISTRATIVE REGULATIONS
183 Representatives of the Administrator
185 Testimony by Employees and Production
of Records in Legal Proceedings
150/5000 Airport Planning.
150/5020 Noise Control and Compatibility Planning
for Airports.
150/5100 Federal-aid Airport Program.
150/5150 Surplus Airport Property Conveyance Programs.
150/5190 Airport Compliance Program.
150/5200 Airport Safety–General.
150/5210 Airport Safety Operations (Recommended
Training, Standards, Manning).
150/5220 Airport Safety Equipment and Facilities.
150/5230 Airport Ground Safety System. 187 Fees
189 Use of Federal Aviation Administration
Communication System
190 WITHHOLDING SECURITY INFORMATION
191 Withholding Security Information from
Disclosure Under the Air Transportation
Security Act of 1974
198 Aviation Insurance Program
210 FLIGHT INFORMATION
211 Aeronautical Charts and Flight Information
Publications
212 Publication Specification: Charts and
Publications
400 COMMERCIAL SPACE TRANSPORTATION
440 Financial Responsibility
1—Based on Federal Aviation Regulation Subchapter Titles
(Excluding the 210 series).
2—Based on Federal Aviation Regulation Part Titles (Excluding the
210 series).General
Subject
number
(1)Specific
Subject
Number
(2) SubjectGeneral
Subject
number
(1)Specific
Subject
Number
(2) Subject
150/5240 Civil Airports Emergency Preparedness.
150/5325 Influence of Aircraft Performance on Aircraft Design.
150/5335 Runway, Taxiway, and Apron Characteristics.
150/5340 Airport Visual Aids.
150/5345 Airport Lighting Equipment.
150/5360 Airport Buildings.
150/5370 Airport Construction.
150/5380 Airport Maintenance.
150/5390 Heliports.
Figure 2-12. List of advisory circular numbers (continued).
2-30TCs and STCs are numbered.
Possession of the STC document does not constitute rights to
the design data or installation of the modification. The STC
and its supporting data (drawings, instructions, specifications,
and so forth) are the property of the STC holder. You must
contact the STC holder to obtain rights for the use of the STC.
Type Certificate Data Sheets (TCDS)
The TCDS is a formal description of the aircraft, engine, or
propeller. It lists limitations and information required for type
certification including airspeed limits, weight limits, thrust
limitations, and so forth.
TCDSs and specifications set forth essential factors and
other conditions that are necessary for U.S. airworthiness
certification. Aircraft, engines, and propellers that conform
to a U.S. TC are eligible for U.S. airworthiness certification
when found to be in a condition for safe operation and
ownership requisites are fulfilled. [Figure 2-16]
TCDSs were originated and first published in January 1958.
Title 14 of the CFR part 21, section 21.41 indicates they are
part of the TC. As such, a TCDS is evidence the product has
been type certificated. Generally, TCDSs are compiled from
details supplied by the TC holder; however, the FAA may
request and incorporate additional details when conditions
warrant. [Figure 2-17]
Under federal law, no civil aircraft registered in the United
States can operate without a valid airworthiness certificate.
This certificate must be approved and issued by the FAA;
and it is only issued if the aircraft and its engines, propellers,
and appliances are found to be airworthy and meet the
requirements of an FAA-approved TC. The FAA issues a TC
when a new aircraft, engine, propeller, and so forth, is found
to meet safety standards set forth by the FAA. The TCDS
lists the specifications, conditions, and limitations that the
airworthiness requirements were met under for the specified
product, such as engine make and model, fuel type, engine
limits, airspeed limits, maximum weight, minimum crew,
and so forth. TCDSs are issued and revised as necessary
to accommodate new models or other major changes in the
certified product. TCDSs are categorized by TC holder and
product type.
FAA Handbooks & Manuals
The FAA publishes handbooks and manuals for beginners and
aviation professionals. Publications are updated periodically
to reflect new FAA regulations and technical developments.
Figure 2-18 shows a list of aircraft and aviation handbooks
and manuals available on the FAA website ( www.faa.gov ).Non-FAA Documents
Air Transport Association ATA iSpec 2200
To standardize the technical data and maintenance activities
on large and therefore complex aircraft, the ATA e-Business
Program has established a classification of maintenance
related actions. These are arranged with sequential numbers
assigned to ATA chapters. These chapters are consistent
regardless of the large aircraft that is being worked on.
[Figure 2-19]
Manufacturers’ Published Data
The original equipment manufacturer (OEM) is usually
the best source of information for the operation of and
maintenance on a particular product. If the product is a TC’d
or STC’d item, 14 CFR part 21, section 21.50 requires the
holder of the design approval to provide one set of complete
ICAs. Additional requirements for ICAs are specified in
sections 23.1529, 25.1529, 27.1529 and 29.1529. These
sections further refer the reader to 14 CFR part 23, Appendix
A; part 25, Appendix H; part 27, Appendix A; and part 29,
Appendix A. Regardless of the appendix referred to, the
requirements in the appendix for the ICA are as follows:
• General: The aircraft ICA must contain instructions
for continued airworthiness for each engine, propeller,
or appliance and the interface of those appliances and
products with the aircraft.
• Format: The ICA must be in the form of a manual or
manuals appropriate to the data being provided.
• Content: The manual contents must be in English and
must include the following:
• Introductory information, including an explanation
of the airplane’s features and data as necessary to
perform maintenance or preventive maintenance
• A description of the aircraft and its systems,
including engine, propeller, and appliances
• Basic operating information describing how the
aircraft and its components are controlled
• Servicing information with such detail as servicing
parts, tank capacities, types of fluid to be used,
applicable pressures for the various systems,
access panels for inspection and servicing,
lubrication points, and types of lubricants to be
used
The maintenance instructions must include the following
data:
• Recommended schedule for cleaning, inspecting,
adjusting, testing, and lubricating the various parts
• Applicable wear tolerances
2-311FAA
Aviation SafetySPECIAL AIRWORTHINESS
INFORMATION BULLETIN
SUBJ: FUSELAGE –Seat Belt Mounting Bracket SAIB: CE-15-13
Date: April 15, 2015
This is information only. Recommendations aren’t mandatory.
Introduction
This Special Airworthiness Information Bulletin is to alert owners, operators, and maintenance
technicians of an airworthiness concern with aluminum seat belt mounting brackets affecting all
Cessna Model s120and 140airplanes .Textron Aviation has issued Service Bulletin SEB -25-03,
dated February 17, 2015, toaddress this concern.
At this time, the airworthiness concern is not an unsafe condition that would warrant airworthiness
directive (AD) action under Title 14 of the Code of Federal Regulations (14 CFR) part 39.
Background
On July 5, 2014, an accide nt occurred in Parma, N ew Y orkwhere the pilot seat belt mounting
bracket, part number ( p/n)0425132, failed after the air plane overturned following departure from the
runway. Although cause of the f ailed bracket has not been determined and the investigati on is
ongoing, it was noted that the original Cessna seat belt installation had been replaced with a four -
point Aero Fabricators harness per Supplemental Type Certificate ( STC)SA1429GL in 2003. The
failed bracket was made of aluminum. However, Cessna now only provides steel brackets as a
replacement part for the aluminum brackets.
Recommendations
The FAA recommend sthat owners, operators, and maintenance personnel of the affected airplanes
replace aluminum brackets with steel brackets following Cessna Service Bulletin SEB -25-03 dated
February 17, 2015. To make the determination as to whether the bracket is made of a luminum or
steel, a magnet may be used or look for evidence of iron oxide (rust).
For Further Information Contact
Gary D. Park, Aerospace Engineer, ACE -118W phone: (316) 946- 4123; fax: (316) 946- 4107; e -mail:
gary.park@faa.gov.
For Related Service Information Contact
Cessna Aircraft Company, Customer Support Service, P.O. Box 7706, Wichita, Kansas; telephone:
(316) 517- 5800; fax: (316) 517- 7271.
Figure 2-13. Special Airworthiness Information Bulletin (SAIB).
SAMPLE
2-32United States Of America
Department of Transportation - Federal Aviation Administration
Supplemental Type Certificate
_______________________________________________________________________________________________________________________________ ______
Any alteration of this certificate is punishable by a fine of not exceeding $1,000, or imprisonment not exceeding 3 years, or b oth.
_______________________________________________________________________________________________________________________________ ______
FAA Form 8110-2(10-68) Page 1 of 1 This certificate may be transferred in accordance with FAR 21.47.Number SA7855SW
This Certificate issued to
certifies that the change in the type design for the following product with the limitations and conditions
therefor as specified hereon meets the airworthiness requirements of Part 23of the Federal Aviation
Regulations .
Original Product Type Certificate Number :
Description of Type Design Change:
Installation of McCauley B3D32C419/82NHA-5 Propeller on Commander Model 114
airplane in accordance with Commander Aircraft Co., Installation Instructions
dated July 13, 1990, Revision A dated August 22, 1990, or later FAA approved
revision.
Limitations and Conditions:
FAA approved Commander Aircraft Co. Flight Manual Supplement dated
August 23, 1990, must accompany this modification. The installer must determine
whether this design change is compatible with previously approved modifications.
If the holder agrees to permit another person to use this certificate to alter a
product, the holder must give the other person written evidence of that
permission.
This certificate and the supporting data which is the basis for approval shall remain in effect until
surrendered, suspended, revoked or a termination date is otherwise established by the Administrator of the
Federal Aviation Administration.Commander Premier Aircraft Corporation
20 Stanford Drive
Farmington, CT 06032
A12SO
Make : Commander
Model :114
Date reissued :March 03, 2006
Date amended :Amd. 1, September 10, 1990
By direction of the Administrator
_______________________________________________________
( Signature )
Michele M. Owsley, Manager
Airplane Certification Office,
Southwest Region
_______________________________________________________
( Title)Date of application :February 09, 1990
Date of issuance :August 23, 1990
Figure 2-14. Supplemental type certificate (STC).
SAMPLE
2-33After 1978, the POHs generally took on both roles.
• Maintenance Manuals—These manuals are often
referred to as Aircraft Maintenance Manual (AMM)
or Component Maintenance Manual (CMM).
The AMM is focused on the entire aircraft and provides the “big
picture” for the maintenance technician. It provides information
concerning the maintenance, including troubleshooting and
repair, of the aircraft and systems on the aircraft.
The CMM, on the other hand, is focused on a specific
item or component, such as hydraulic pump, generator, or
thrust reverser. It provides the bench mechanic with detail
troubleshooting information and usually serves as an overhaul
manual giving details for disassembly, cleaning, inspection,
repair as necessary, reassembly, and testing in accordance
with approved standards and technical data accepted by
the Administrator. Refer to 14 CFR part 43 section 43.2(a).
When maintenance is done according to the CMM, the
technician must always include the appropriate references
in the maintenance record entry required by 14 CFR part 43,
section 43.9 or 43.11.
Service Bulletins (SB)
Throughout the life of a product (whether TC’d or not),
manufacturing defects, changes in service, or design
improvements often occur. When that happens, the OEM
frequently uses an SB to distribute the information to the
operator of the aircraft. SBs are good information and should
be strongly considered by the owner for implementation to
the aircraft. However, SBs are not required unless they are
referred to in an AD note or if compliance is required as a
part of the authorized inspection program. Refer to section
14 CFR part 39, 39.27.
Structural Repair Manual (SRM)
As the name implies, this manual carries detail information
for the technician concerning an aircraft’s primary and
secondary structure, criteria for evaluating the severity of
the detected damage, determining the feasibility of a repair,
and alignment/inspection information. This manual is usually
a separate manual for large aircraft. On small aircraft, this
information is often included in the AMM.
Forms
Airworthiness Certificates
In addition to the registration certificate that indicates the
ownership of an aircraft, an airworthiness certificate indicates
the airworthiness of the aircraft. AC 21-12, Application
for U.S. Airworthiness Certificate, FAA Form 8130-6, is a
comprehensive guide for the completion of the application
form for this certificate. There are two certificates: standard • Recommended overhaul periods
• Details for an inspection program that identifies
both the frequency and the extent of the inspections
necessary to provide for continued airworthiness
• Troubleshooting information
• The order and method for proper removal and
replacement of parts
• Procedures for system testing during ground
operations
• Diagrams for structural access plates
• Details for application of special inspection techniques
• Information concerning the application of protective
treatments after inspection
• Information relative to the structural fasteners
• List of any special tools needed
Airworthiness Limitations
The ICA must contain a separate and clearly distinguishable
section titled “Airworthiness Limitations.” Within this section
are mandatory replacement times, structural inspection
interval, and related inspection procedures.
All of this is included in the initial release of documents
when the aircraft is delivered. However, over the course of
the life of an aircraft, various modifications can and often do
occur. Whether these are as simple as a new cabin to galley
sliding door, or as complex as a navigation related STC, any
major alteration requires that this type of maintenance data
be provided to the owner, so that subsequent maintenance,
inspection, and repair can be properly accomplished. As
aircraft and their systems become more and more complex,
and society continues its preoccupation with litigation for every
incident, it is imperative that the technician have the right
information, that it is current, and that they have the proper
tools, including those required for any special inspection, and
correct replacement parts. If any one of these items is required,
and the technician does not have it accessible, they are in
violation of 14 CFR sections 65.81(b), 43.13(a), and 43.16 if
they attempt to return the aircraft to service.
Manufacturers may provide this required information in a
variety of different manuals:
• Operating Instructions—The Airplane Flight Manual
(AFM) or the Pilot’s Operating Handbook (POH)
provides the pilot with the necessary information
to properly operate the aircraft. These manuals are
usually listed in the aircraft TCDS, and therefore
are a required item for the aircraft to be considered
airworthy. Note that the AFM is generally serial
number specific, whereas the POH is model specific.
CodeBranch
AC ASW-150 Description
Ft. Worth Airplane Certifi cation Offi ce
AK ACE-115N Anchorage Aircraft Certifi cation Offi ce
AT ACE-115A Atlanta Aircraft Certifi cation Office
BA ANM-100B Boeing Aviation Safety Oversight Office (BASOO)
BO ANE-150 Boston Aircraft Certification Office
CE ACE-112 Small Airplane Directorate
CH ACE-115C Chicago Aircraft Certifi cation Offi ce
DE ANM-100D Denver Aircraft Certification Office
EN ANE-140 Engine Certifi cation Office
GU ACE-100G Gulfstream Aviation Safety Oversight Office (GA SOO)
IB ANM-116 Transport Airplane Directorate International Branch
LA ANM-100L Los Angeles Aircraft Certification Office
MC ACE-100M Military Certification Office
NY ANE-170 New York Aircraft Certifi cation Offi ce
RC ASW-170 Ft. Worth Rotorcraft Certification Office
SC ASW-190 Ft. Worth Special Certification Office
SE ANM-100S Seattle Aircraft Certifi cation Office
Code Description
A Small Airplane
B Balloon
E Engine
G Glider
P Propeller
R Rotorcraft
S Airship
T Transport Airplane
I Experimental
Q Other, or not product
ST New Supplemental Type Certificate (STC)
AT Amended Type Certif icate
SA Amended Supplemental Type Certificate
SP Special Project (e.g. approval under section 21.305 project)Table 3–Product Type Designators
Code Description
EPD Engine-Propeller
RCD Rotorcraft
SAD Small Airplane
TAD Transport Airplane Table 4–Directorate Designators
Code Description
BOS-AEG Boston AEG
FTW-AEG Fort Worth AEG
MKC-AEG Kansas City AEG
LGB-AEG Long Beach AEG Table 5–Aircraft Evaluation Group Designators
WI ACE-115W Wichita Aircraft Certification Office SEA-AEG Seattle AEG
As an example , TC00125AT-A would be a TC project assigned by the Atlanta ACO on a small airplane with the assigned number 00125.
FAA Project Numbering and Designators
Table 1–Projec t Type Designators
Code Description
TC New Type Certif icate (TC)
PM Parts Manufacturer Approval (PMA)
Table 2–Aircraft Certification Office (ACO) DesignatorsFAA Project Numbers will use the following format:
AAnnnnnYY-XWhere:
• AA is the two-letter designator for Project Type – see table 1 below
• nnnnn is the integer sequential number for the specified ACO; e.g., 00146
• YY is the two-letter designator for the Aircraft Certification Office (ACO) – see Table 2
below
• X is the one-letter designator for the Product Type – see Table 3 below
As an example, TC00125AT-A would be a TC project assigned by the Atlanta ACO on a small airplane with the assigned number 00125.
Figure 2-15. Numbering system for type certificates (TCs) and supplemental type certificates (STCs).
and special. FAA Form 8100-2, Standard Airworthiness
Certificate, may be issued to allow operation of a type-certificated aircraft in one or more of the following categories: [Figure 2-20]
•Normal
•Utility
•Acrobatic
•Commuter
•Transport
•Manned free balloon•Special classes
FAA Form 8130-7, Special Airworthiness Certificate, may be issued to authorize the operation of an aircraft in the following categories: [Figure 2-21]
•Primary
•Restricted
•Multiple
•Limited
•Light-sport
Figure 2-16. Type certificate.
SAMPLE
2-36Page No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Rev. No. 111 101 104 97 82 97 82 99 97 101 111 104 96 101 101 110
Page No 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
Rev. No. 111 101 82 95 91 108 111 101 101 101 101 101 100 78 101 110
Page No 33 34 35 36 37 38 39 40 41 42 43
Rev. No. 104 111 110 110 111 108 111 96 100 101 101DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
A24CE
Revision 111
Beechcraft
200 A100 -1 (U-21J)
200C A200 (C -12A)
200CT A200 (C -12C)
200T A200C (UC -12B)
B200 A200CT (C -12D)
B200C A200CT (FWC -12D)
B200CT A200CT (C -12F)
B200T A200CT (RC -12D)
300 A200CT (RC -12G)
300LW A200CT (RC -12H)
B300 A200CT (RC -12K)
B300C A200CT (RC -12P)
B300C (MC -12W) A200CT (RC -12Q)
B300C (UC -12W) B200C (C -12F)
1900 B200C (UC -12M)
1900 C B200C (C -12R)
1900C (C -12J) B200C (UC -12F)
1900D B200GT
B200CGT
July21, 201 5
TYPE CERTIFICATE DATA SHEET NO. A24CE
This data sheet which is part of Type Certificate No. A24CE prescribes conditions and limitations under which the product for
which the type certificate was issued meets the airworthiness requirements of the Federal Aviation Regulations.
Type Certificate Holder: Beechcraft Corporation
10511 E. Central
Wichita, Kansas 67206
Type Certificate Holder Record: Beech Aircraft Corporation transferred to
Raytheon Aircraft Company on April 15, 1996
Raytheon Aircraft Company transferred to
Hawker Beechcraft Corporation on March 26, 2007
Hawker Beechcraft Corporation transferred to
Beechcraft Corporation on April 12, 2013
I. Model 200, Super King Air (Normal Category), Approved December 14, 1973 (See NOTES 10 and 11)
Model A200C (UC -12B), Super King Air (Normal Category), Approved February 21, 1979 (See NOTE 11)
Model 200C, Super King Air (Normal Category), Approved February 21, 1979 (See NOTE 11)
Model B200, Su per King Air (Normal Category), Approved February 13, 1981 (See NOTES 10 and 11)
Model B200C, Super King Air (Normal Category), Approved February 13, 1981 (See NOTES 10 and 11)
Model B200C (C -12F), (UC-12F), (UC-12M) and (C-12R), Super King Air (Normal C ategory), Approved
February 13, 1981, (See NOTES 10, 11, and 12)
For Notes, refer to Data Pertinent to All Model 200 Series
Engine Two United Aircraft of Canada, Ltd., or Pratt & Whitney PT6A -41
(turboprop) per Beech Specification BS 22096 (200, 200C, A200C)
Figure 2-17. Type Certificate Data Sheet.
2-37Aircraft
AviationAviation
Examiner and InspectorAircraft Weight and Balance Handbook (FAA-H-8083-1A)
Airplane Flying Handbook (FAA-H-8083-3A)
IR-M 8040-1C, Airworthiness Directives Manual
Amateur-built Aircraft & Ultralight Flight Testing Handbook
Aviation Maintenance Technician Handbook –General
(FAA-H-8083-30)
Aviation Maintenance Technician Handbook –Airframe
(FAA-H-8083-31)
Aviation Maintenance Technician Handbook –Powerplant
(FAA-H-8083-32)
Balloon Flying Handbook (FAA-H-8083-11A)
Glider Flying Handbook (FAA-H-8083-13A)
Parachute Rigger Handbook (FAA-H-8083-17)
Rotorcraft Flying Handbook (FAA-H-8083-21)
Advanced Avionics Handbook (FAA-H-8083-6)
Aerodynamics for Navy Aviators (NAVAIR 00-80T-80)
Aeronautical Information Manual
Air Quality Handbook
Airship Pilot Manual
Airship Aerodynamics Technical Manual
Aviation Instructor’s Handbook (FAA-H-8083-9A)
Balloon Safety Tips: False Lift, Shear, and Rotors ( FAA-P-8740-39)Balloon Safety Tips: Powerlines & Thunderstorms ( FAA-P-8740-34)
Banner Tow Operations ( FAA/fs-i-8700-1)
Flight Navigator Handbook ( FAA-H-8083-18)
Helicopter Flying Handbook ( FAA-H-8083-21A)
Helicopter Instructor’s Handbook ( FAA-H-8083-4)
Instrument Flying Handbook ( FAA-H-8083-15B)
Instrument Procedures Handbook ( FAA-H-8083-16)
International Flight Information Manager
MC-4 Ram Air Free-fall Personnel Parachute System Technical Manual
Pilot Safety Brochures
Pilot’s Handbook of Aeronautical Knowledge ( FAA-H-8083-25A)
Plane Sense–General Aviation Information ( FAA-H-8083-19A)
Risk Management Brochures
Risk Management Handbook ( FAA-H-8083-2)
Safety Risk Management
Seaplane, Skiplane, and Float/Ski Equipped Helicopter Operations
Handbook ( FAA-H-8083-23)
Student Pilot Guide ( FAA-H-8083-27A)
Tips on Mountain Flying ( FAA-P-8740-60)
Weight-Shift Control Aircraft Flying Handbook ( FAA-H-8083-5)
Flight Standards Information Management System (FSIMS)
(FAA Order 8900.1)
Designee Management Handbook
Guide for Aviation Medical Examiners
General Aviation Airman Designee Handbook
Figure 2-18. F AA handbooks and manuals.
• Experimental
• Special flight permit
• Provisional
Airworthiness certificates may be issued by either FAA
personnel or FAA designees. Refer to 14 CFR part 183,
sections 183.31 and 183.33. The certificate must not only
be on board the aircraft (14 CFR part 91, section 91.203(a)
(1)), but must also be “displayed at the cabin or flight deck
entrance so that it is legible to the passengers or crew” 14
CFR part 91, (section 91.203(b)). Since the ability to obtain
this certificate is based upon the requirement to inspect the
aircraft to determine that it conforms to type design and is
in condition for safe operation, it can also be revoked by
the FAA if either of those two requirements ceases to exist.Aircraft Registration
Aircraft must be registered in the United States if the
aircraft is not registered under the laws of a foreign country
and is owned by either a citizen of the United States, a
foreign citizen lawfully admitted to the United States,
or a corporation organized in and doing business under
U.S. laws and primarily based in the United States. This
registration is accomplished by using FAA Form 8050-1,
Aircraft Registration Application. The aircraft registration
form is available online at www.faa.gov. The aircraft owner
can mail in completed copy, and keep a copy of the form as
temporary authority to operate the aircraft after the fee and
evidence of ownership have been mailed or delivered to the
Registry. When carried in the aircraft with an appropriate
current airworthiness certificate or a special flight permit,
a copy of this completed application provides authority to
operate the aircraft in the United States for up to 90 days.
2-38Joint Aircraft Systems Component (JASC)/ATA Code Table
11 Placards and Markings
12 Servicing
14 Hardware
18 Helicopter Vibration
21 Air Conditioning
22 Auto Flight
23 Communications
24 Electrical Power
25 Equipment/Furnishings
26 Fire Protection
27 Flight Controls
28 Fuel
29 Hydraulic Power
30 Ice and Rain Protection
31 Instruments
32 Landing Gear
33 Lights
34 NavigationAircraft Powerplant System
Propeller/Rotor SystemsAirframe Systems35 Oxygen
36 Pneumatic
37 Vacuum
38 Water/Waste
45 Central Maintenance System (CMS)
49 Airborne Auxiliary Power
51 Standard Practices/Structures
52 Doors
53 Fuselage
54 Nacelles/Pylons
55 Stabilizers
56 Windows
57 Wings
61 Propellers/Propulsors
62 Main Rotor
63 Main Rotor Drive
64 Tail Rotor
65 Tail Rotor Drive
67 Rotors Flight Control71 PowerPlant
72 Turbine/Turboprop Engine
73 Engine Fuel and Control
74 Ignition
75 Air
76 Engine Control
77 Engine Indicating
78 Engine Exhaust
79 Engine Oil
80 Starting
81 Turbocharging
82 Water Injection
83 Accessory Gearboxes
85 Reciprocating Engine
Figure 2-19. Maintenance classification.
In addition to the completed application form, the owner must
also submit evidence of ownership (such as a bill of sale)
and a registration fee. A successful review of the application
results in the issuance of AC Form 8050-3, Certificate of
Aircraft Registration. (Note the AC prefix.)
14 CFR section 91.203(a)(2) requires that either the pink copy
of the application or the actual certificate of registration be
on board the aircraft during its operation.
If the registration is ever lost or damaged, it may be replaced
by contacting the FAA Aircraft Registration Branch and
providing them with the aircraft specific data, including
make, model, N-number, and serial number. A replacement
certificate fee and an explanation of the reason for the
replacement certificate are also required.
Radio Station License
A radio station license is required if the aircraft is equipped
with radios, and the aircraft is planned to be flown outside
the boundaries of the United States. A radio station license
is not required for aircraft that are operated domestically.
(A major change occurred on February 8, 1996, when the
telecommunications Act of 1996 was signed into law.)
The Federal Communications Commission (FCC) formerly
required that any communication transmitter installed in
aircraft be licensed. These FCC licenses were valid for 5 years. This is not an FAA requirement. FAA inspectors who
conducted ramp inspections and detected an expired radio
station license were not required to notify the FCC, nor
could they issue a violation to the owner/operator. Simply
informing the operator of the expired radio station license
was their only responsibility.
FSGA 96-06, a Flight Standards Information Bulletin (FSIB)
for General Aviation (FSGA) titled “Elimination of Aircraft
Radio Station Licenses” became effective on July 8, 1996.
Although that FSIB had an effectivity of only 1 year, the
elimination of the requirement for aircraft used only in
domestic operations continues.
FAA Form 337—Major Repair and Alteration
Refer to the current issue of AC 43.9-1, Instructions for
Completion of FAA Form 337 for help completing FAA Form
337, Major Repair and Alteration (Airframe, Powerplant,
Propeller, or Appliance). [Figure 2-22]
As the name clearly states, this form is to be used whenever
major repairs or alterations are accomplished on an aircraft.
The only exception would be that 14 CFR part 43, Appendix
B, allows for a certificated repair station to RTS an aircraft
after a major repair by using a signed and dated work order
and a signed maintenance release.
• Information in item 1 comes directly from the aircraft
dataplate, except for the tail number. That is to be
compared to the aircraft registration form.
2-39• Information in item 2 reflects the name and address
listed on AC Form 8050-3, Certificate of Registration.
• Item 3 is used when there is no existing approved data
for the intended repair or alteration. In that case, the
technician can request that the local FSDO Principal
Maintenance Inspector (PMI) review the data and
then grant a field approval, shown by completing and
signing this area. In many cases, this block is blank
because the technician has found, used, and made
reference to data already approved by the FAA.
• Item 4—If the repair or alteration is being done
to the aircraft airframe, no entry is required since
the data is identical to that in item 1. However,
if the repair or alteration is being done to an
engine, a propeller, or other appliance, entries must
include the appropriate make, model, and serial
number information.
• Item 5 should have “X” marked in either the “Repair”
or the “Alteration” column.
• Item 6—Enter appropriate data as specified and check
the proper box in B. The technician is encouraged to
carefully read the preprinted statement in subparagraph
D prior to signing this section.
• Item 7 must be completed by the IA or authorized
individual from the repair station.
• Item 8 (on the reverse side) is for the description of
the work accomplished. It must include a reference
to the approved data used to conduct the required
maintenance.
The form must be completed at least in duplicate, with the
original provided to the owner/operator and a copy to the local
FSDO within 48 hours of completing the maintenance and
RTS. If the FAA Form 337 is used to document additional
fuel tanks in the cabin or cargo, then an additional copy
must be signed and in the aircraft at all times. Maintenance
facilities and mechanics are encouraged to make a copy for
their own records.
Records
Making Maintenance Record Entries
Title 14 of the CFR part 43, sections 43.9 and 43.11 require
the technician to make appropriate entries of maintenance
actions or inspection results in the aircraft maintenance
record. How long those records must be kept is defined in
14 CFR part 91, section 91.417.
Whenever maintenance, preventive maintenance, rebuilding,
or alteration work occurs on an aircraft, airframe, aircraft
engine, propeller, appliance, or component part, a maintenance
record entry must be created. The importance of compliance with this requirement cannot be overemphasized. Complete
and organized maintenance logs for an aircraft can have
significant (and usually positive) effect during the buy/
sell negotiations of an aircraft. On the other hand, poorly
organized and incomplete logs can have a detrimental effect
upon the selling price of an aircraft.
Temporary Records—14 CFR Part 91 Section
91.417(a)(1) and (b)(1)
These are records that must be kept by the owner until
the work is repeated, superseded, or 1 year has transpired
since the work was performed. These are typically records
referring to maintenance, preventive maintenance, alteration,
and all inspections. They include a description of the work
performed (or reference to the FAA-accepted data); the date
of completion; and the name, signature and certificate number
of the person doing the RTS.
Permanent Records—14 CFR Part 91, Section
91.417(a)(2) and (b)(2)
These records must be retained by the owner during the time
they operate the aircraft. They are transferred with the aircraft
at the time of sale. Typically, these are documents relating to
total time in service, current status of life-limited parts, time
since last overhaul, current inspection status, current status of
applicable AD notes, and major alteration forms as required
by 14 CFR part 43, section 43.9.
Electronic Records
During the last 25 years, the field of aviation maintenance has
seen a significant change in the documentation requirements
for aircraft and related parts. Nowhere is that change seen as
revolutionary as the introduction of electronic data and record
retention. Just as the arrival of the personal computer placed
the possibility of the power and versatility of a computer in
the hands of the average person, it made it available to the
maintenance technician. Initially some technicians developed
their own programs for listing data (TCDS, AD notes, and
so forth), but soon commercially available programs were
developed. Basically, these were developed by either one of
the following two groups:
1. Computer literate persons who felt the aviation
industry could benefit from the computer
2. Aviation professionals who felt the aviation industry
must benefit from the computer
Some of those initial programs were either not very
user friendly (if developed by computer wizards) or not
“very sophisticated” (if developed by the maintenance
technician). Today, there is a mixture of these various
database programs. A review of the advertisement section
in any current aviation maintenance magazine offers
2-4002/04/2015 8130.2H
329. Examples of Forms. Figure s 3-1 through 3-10 of this order provide examples of forms
used in the certification process.
Figure 3- 1. Sample FAA Form 8100- 2, Standard Airworthiness Certificate,
New Aircraft (Face Side)
UNITED STATES OF AMERICA
DEPARTMENT OF TRANSPORTATION-FEDERAL AVIATION ADMINISTRATION
STANDARD AIRWORTHINESS CERTIFICATE
1NATIONALITY AND
REGISTRATION MARKS
N123452MANUFACTURER AND MODEL
Boeing 787 3AIRCRAFT SERIAL
NUMBER
432194CATEGORY
Transport
5AUTHORITY AND BASIS FOR ISSUANCE
This airworthiness certificate is issued pursuant to 49 U.S.C. 44704 and certifies that, as of the date of issuance, the aircraft to which
issued has been inspected and found to conform to the type certificate therefore, to be in condition for safe operation, and has been
shown to meet the requirements of the applicable comprehensive and detailed airworthiness code as provided by Annex 8 to the
Convention on International Civil Aviation, except as noted herein.
Exceptions:
None
6TERMS AND CONDITIONS
Unless sooner surrendered, suspended, revoked, or a termination date is otherwise established by the FAA, this airworthiness certificate
is effective as long as the maintenance, preventative maintenance, and alterations are performed in accordance with Parts 21,43, and
91 of the Federal Aviation Regulations, as appropriate, and the aircraft is registered in the United States.
DATE OF ISSUANCE FAA REPRESENTATIVE DESIGNATION NUMBER
9 Jan 2015 E.R. White E.R. White NE-XX
Any iteration, reproduction, or misuse of this certificate may be punishable by a fine not exceeding $1,000 or imprisonment not exceeding 3 years or both.
THIS CERTIFICATE MUST BE DISPLAYED IN THE AIRCRAFT IN ACCORDANCE WITH APPLICABLE FEDERAL AVIATION REGULATIONS.
FAA Form 8100- 2 (04-11)Supersedes Previous Edition
Figure 3- 2. Sample FAA Form 8100- 2, Standard Airworthiness Certificate,
Aircraft Assembled from Spare and Surplus Products and Articles (Face Side)
UNITED STATES OF AMERICA
DEPARTMENT OF TRANSPORTATION-FEDERAL AVIATION ADMINISTRATION
STANDARD AIRWORTHINESS CERTIFICATE
1NATIONALITY AND 2MANUFACTURER AND MODEL 3AIRCRAFT SERIAL 4CATEGORY
REGISTRATION MARKS NUMBER
N54321 Jackson 47G -4 3191HG Normal
5AUTHORITY AND BASIS FOR ISSUANCE
This airworthiness certificate is issued pursuant to 49 U.S.C. 44704 and certifies that, as of the date of issuance, the aircraft to which
issued has been inspected and found to conform to the type certificate therefore, to be in condition for safe operation, and has been
shown to meet the requirements of the applicable comprehensive and detailed airworthiness code as provided by Annex 8 to the
Convention on International Civil Aviation, except as noted herein.
Exceptions:
None
6TERMS AND CONDITIONS
Unless sooner surrendered, suspended, revoked, or a termination date is otherwise established by the FAA, this airworthiness certificate
is effective as long as the maintenance, preventative maintenance, and alterations are performed in accordance with Parts 21,43, and
91 of the Federal Aviat ion Regulations, as appropriate, and the aircraft is registered in the United States.
DATE OF ISSUANCE FAA REPRESENTATIVE DESIGNATION NUMBER
9 Feb 2015 E.J. Smith E.J. Smith SW-XX
Any iteration, reproduction, or misuse of this certificate may be punishable by a fine not exceeding $1,000 or imprisonment not exceeding 3 years or both.
THIS CERTIFICATE MUST BE DISPLAYED IN THE AIRCRAFT IN ACCORDANCE WITH APPLICABLE FEDERAL AVIATION REGULATIONS.
FAA Form 8100- 2 (04-11)Supersedes Previous Edition
3-23 Figure 2-20. F AA Form 8100-2, Standard Airworthiness Certificate.
SAMPLE
the reader numerous options for electronic maintenance
records. Many of these programs offer a combination of
the data research, such as ADs, SBs, STCs, and TCDSs,
required to conduct proper maintenance, inspections, and
data recording (logbook entries, AD compliance history,
length of component time in service, and so forth) desired
to improve the efficiency of the technician.
Although some large shops and certified repair stations may
have a separate group of people responsible for “records and
research,” the professional maintenance technician must
be aware of the benefits of these systems. Some factors to
consider when reviewing a system are:
• What is the typical size of the aircraft that maintenance
is being done on? (i.e., less than 12,500 pounds, more
than 12,000? Mixed?)
• Does the program have built-in templates for the
aircraft being worked on?
• What FAA forms (if any) are available in the program?
• Does it have a user-friendly template to enter the data
for the form or must data be directly entered onto the
form?
• Can it calculate weight and balance data?
• Does it have adequate word search capabilities?
• Is it networkable?
• Are the updates sent via U.S. mail or downloaded from the Internet?
• What is the maximum number of aircraft that the
system can handle?
• Can the system handle both single- and multi-engine
aircraft? Fixed and rotary wing? Piston and jet?
• Can an item removed from an aircraft be tracked?
• Is the data from this system exportable to other
electronic formats?
• Can it forecast items due for maintenance or
inspection?
Since no program can be considered the best, the technician
must learn all they can about the numerous systems that exist.
Exposure to the pros and cons of these different systems
can be one of the benefits of attending various trade shows,
maintenance seminars, or IA renewal sessions. Continuous
learning and personal improvement is the goal of every
professional maintenance technician.
Light Sport Aircraft (LSA)
Maintenance
The light sport aircraft (LSA) category includes gliders,
airplanes, gyroplanes, powered parachutes, weight-shift and
lighter-than-air aircraft. There are two general types of LSAs:
Special (SLSA) and Experimental (ELSA). The SLSA are
factory built and the ESLA are kit-built. This new category of
aircraft was added to the regulations in 2004. (Refer to 14 CFR
sections 21.190, 65.107, and 91.327, all dated July 27, 2004.)
2-4102/04/2015 8130.2H
Figure 4- 1. Sample FAA Form 8130- 7, Special Airworthiness Certificate
Front
UNITED STATES OF AMERICA
DEPARTMENT OF TRANSPORTATION - FEDERAL AVIATION ADMINISTRATION
SPECIAL AIRWORTHINESS CERTIFICATE
ACATEGORY/DESIGNATION
PURPOSE
BMANU -
FACTURER NAME
ADDRESS
C FLIGHT FROM
TO
DN- SERIAL NO.
BUILDER MODEL
EDATE OF ISSUANCE EXPIRY
OPERATING LIMITATIONS DATED ARE PART OF THIS CERTIFICATE
SIGNATURE OF FAA REPRESENTATIVE DESIGNATION OR OFFICE NO.
Any alteration, reproduction or misuse of this certificate may be punishable by a fine not exceeding $1,000 or
imprisonment not exceeding 3 years, or both. THIS CERTIFICATE MUST BE DISPLAYED IN THE AIRCRAFT
IN ACCORDANCE WITH APPLICABLE TITLE 14, CODE OF FEDERAL REGULATIONS (CFR).
FAA Form 8130-7 (04-11) Previous Edition 07/04 May be Used until Depleted SEE REVERSE SIDE NSN: 0052-00 -693-4000
Back
AThis airworthiness certificate is issued under the authority of Public Law 104-6, 49 United States Code
(USC) 44704 and Title 14 Code of Federal Regulations (CFR).
BThe airworthiness certificate authorizes the manufacturer named on the reverse side to conduct
production fight tests, and only production flight tests, of aircraft registered in his name. No person may
conduct production flight tests under this certificate: (1) Carrying persons or property for compensation or
hire: and/or (2) Carrying persons not essential to the purpose of the flight.
CThis airworthiness certificate authorizes the flight specified on the reverse side for the purpose shown in
Block A.
DThis airworthiness certificate certifies that as of the date of issuance, the aircraft to which issued has been
inspected and found to meet the requirements of the applicable CFR. The aircraft does not meet the
requirements of the applicable comprehensive and detailed airworthiness code as provided by Annex 8 to
the Convention On International Civil Aviation. No person may operate the aircraft described on the
reverse side: (1) except in accordance with the applicable CFR and in accordance with conditions and
limitations which may be prescribed by the FAA as part of this certificate; (2) over any foreign country
without the special permission of that country.
EUnless sooner surrendered, suspended, or revoked, this airworthiness certificate is effective for the
duration and under the conditions prescribed in 14 CFR, Part 21, Section 21.181 or 21.217.
4-96 Figure 2-21. F AA Form 8130-7, Special Airworthiness Certificate.
SAMPLE
Just as industry standard specifications have replaced many
of the military standards to define products that are destined
to be part of the Department of Defense (DoD) inventory,
so too have industry standards come into the FAA sights for
documenting certain information. Quality is one example.
The Society of Automotive Engineers (SAE) has developed AS 9100 and AS 9110 as auditing standards for aerospace
facilities and specifically repair stations. Likewise, ISO 9001
is being adopted by the FAA as a system of measuring their
performance. Therefore, it was logical that when the FAA
looked to develop the standards for this newest category of
aircraft, they again looked to industry, and this time it was
the American Society for Testing and Materials (ASTM).
2-42FAA Form 337 (10/06)
Federal Aviation
Administration
2.Owner(As shown on registration certificate)
3.ForFAA Use Only
4.Type 5.UnitIdent ification
(As described in Item1 abov e)
6.Conformity Statement
7.Approva lfor Return toService
Figure 2-22. F AA Form 337, Major Repair and Alteration.
SAMPLE
2-43FAA Form 337 (10/06) 8.Description of Work Acco mplished
Figure 2-22. F AA Form 337, Major Repair and Alteration (continued).
SAMPLE
2-44The ASTM developed a comprehensive list of consensus
standards for use by manufacturers, regulators, maintenance
facilities, LSA owners, and service providers. It is unique that
these standards are the first ones in over 100 years to solely
address the issue of recreational aircraft use. It is also the
first complete set of industry consensus standards covering
the design, manufacture, and use of recreational aircraft that
was developed by a non-government agency. The ASTM
committee that developed these LSA standards did so to
ensure the quality of products and services to support both the
national and the international regulatory structures for LSAs.
Over 20 standards have been generated, and more are being
developed to cover this diversity of aircraft. This handbook
only incorporates a review of F2483-05, “Standard Practice
for Maintenance and the Development of Maintenance
Manuals for Light Sport Aircraft (LSA)” a six-page document
comprised of the following 12 sections:
1. Scope
2. Referenced Documents
3. Terminology
4. Significance and Use
5. Aircraft Maintenance Manual
6. Line Maintenance, Repairs, and Alterations
7. Heavy Maintenance, Repairs, and Alterations
8. Overhaul
9. Major Repairs and Alterations
10. Task-Specific Training
11. Safety Directives
12. Keywords
The scope of that document is basically twofold:
• To provide guidelines for the qualification necessary
to accomplish various levels of maintenance on LSA.
• To provide the content and structure of maintenance
manuals for aircraft and their components that are
operated as LSAs.
Some additional definitions from section 3, Terminology,
that help to better explain the LSA concepts are:
• Annual condition inspection—defined as a detailed
inspection accomplished once a year in accordance
with instructions provided in the maintenance manual
supplied with the LSA. The purpose of this inspection
is to look for any wear, corrosion, or damage that
would cause the LSA not to be in condition for
safe operation. • Heavy maintenance—any maintenance, inspection,
repair, or alteration a manufacturer has designated that
requires specialized training, equipment, or facilities.
• Line maintenance—any repair, maintenance,
scheduled checks, servicing, inspections, or alterations
not considered heavy maintenance that are approved by
the manufacturer and is specified in the manufacturer’s
maintenance manual.
• LSA repairman–inspection—a U.S. FAA-certified
LSA repairman with an inspection rating per 14 CFR
part 65. This person is authorized to perform the 100-
hour/annual inspection of the aircraft that they own.
• LSA repairman–maintenance—a U.S. FAA-certified
LSA repairman with a maintenance rating per 14 CFR
part 65. This person is allowed to perform the required
maintenance and can also accomplish the 100-hour/
annual inspection.
• Major repair, alteration, or maintenance—any repair,
alteration, or maintenance where instructions to
complete the task are excluded from the maintenance
manual.
• Minor repair, alteration, or maintenance—any repair,
alteration, or maintenance where instructions to
complete the task are included in the maintenance
manual.
The 100-hour inspection is the same as the annual inspection,
except for the interval of time. The requirements for whether
or not the 100-hour inspection is applicable are exactly the
same as the criteria for the standard 100-hour/annual required
of non-LSA aircraft.
Aircraft Maintenance Manual (AMM)
Although these manuals do not require any FAA approval,
the regulations do require that the manual be developed in
accordance with industry standards. This ASTM sets that
standard by requiring:
• General specifications to be listed, include capacities,
servicing, lubrication, and ground handling
• An inspection checklist for the annual condition or
100-hour inspection
• A description of and the instructions for the
maintenance, repair, and overhaul of the LSA engine
• A description of and the instructions for the
maintenance, repair, and alteration of the aircraft’s
primary structure
Other items that maintenance procedures must be provided
for are:
• Fuel systems
2-45• Propeller
• Utility system
• Instruments and avionics
• Electrical system
• Structural repair
• Painting and coatings
The Inspection, Repair, and Alterations section must
specifically list any special tools and parts needed to complete
the task, as well as the type of maintenance action (line,
heavy, or overhaul) necessary to accomplish the activity.
Directly associated with that information is the requirement
to specify the level of certification needed to do the job (i.e.,
LSA repairman, A&P, or repair station). The manual may
refer to existing FAA ACs.
Line Maintenance, Repairs, & Alterations
The minimum level of certification necessary to accomplish
line maintenance is LSA inspection. Some typical tasks
considered to be line maintenance are:
• 100-hour/annual condition inspection
• Servicing of fluids
• Removing and replacing components when instructions
to do so are provided in the maintenance manual
– Batteries
– Fuel pump
– Exhaust
– Spark plugs and wires
– Floats and skis
• Repair or alteration of components when specific
instructions are provided in the maintenance manual
– Patching a hole in the fabric
– Installation of a strobe light kit
Heavy maintenance, repairs, and alterations must be
accomplished by either a certified mechanic (A or P or
A&P) or an LSA repairman—maintenance who has received
additional “task specific” training. Some examples of this
would be the removal and replacement of complete engine,
cylinder, piston and valve assemblies; primary flight controls;
and landing gear.
Heavy repair of components or structure can be accomplished
when instructions are provided in the maintenance manual
or other service directed instructions. A few examples of
this activity are:
• Repainting of control surfaces• Structural repairs
• Recovering of a dope and fabric
Heavy alterations of components can be accomplished when
instructions are provided in the maintenance manual or other
service directed instructions. Examples of this activity are
initial installation of skis and installation of new additional
pitot static instruments.
Overhaul of components can be performed only by the
manufacturer (or someone authorized to perform) of the
LSA or the component to be overhauled. An overhaul
manual is required and must be a separate manual from
the manufacturer’s maintenance manual. Items typically
considered for overhaul are engines, carburetors, starters,
generators, alternators, and instruments.
Major Repairs & Alterations
Another major difference between LSA maintenance and
traditional aircraft maintenance is that FAA Form 337,
Major Repair and Alteration, is not required to document
major repairs and alterations. Instead, any major repair or
alteration that is accomplished after the LSA has gone through
production acceptance testing must be evaluated relative to
the applicable ASTM requirements. After this evaluation has
been accomplished (either by the manufacturer or an entity
approved by them), a written affidavit must be provided
attesting that the LSA still meets the requirements of the
applicable ASTMs.
The manufacturer (or other approved entity) must provide
written instructions defining the level of certification
necessary to perform the maintenance and also include any
ground test or flight testing necessary to verify that the LSA
complies with the original LSA acceptance test standards,
and is in condition for safe operation. Proper documentation
of this maintenance activity is required to be entered in the
LSA records and is also defined by the manufacturer.
Task specific training is not required to be FAA approved.
This is solely the responsibility of the manufacturer. Some
examples of this are an engine manufacturer’s overhaul
school or the EAA Sport Air fabric covering school.
Safety directives are issued against an LSA or component
and are not issued by the FAA, but rather by the original
aircraft manufacturer. Note: If the LSA includes a product
that is TC’d by the FAA, the manufacturer is required to
issue a safety directive. Typical instructions within a safety
directive include:
• List of tools required for the task
• List of parts needed
2-46• Type of maintenance (line, heavy, overhaul)
• Level of certification needed
• Detailed instructions and diagrams
• Inspection and test methods
Safety directives are mandatory, except for experimental
use LSAs.
Mathematics in Aviation
Maintenance
Chapter 3
Introduction
Mathematics is woven into many areas of everyday life.
Performing mathematical calculations with success requires
an understanding of the correct methods, procedures,
practice, and review of these principles. Mathematics may
be thought of as a set of tools. The aviation mechanic needs
these tools to successfully complete the maintenance, repair,
installation, or certification of aircraft equipment.
Many examples of using mathematical principles by the
aviation mechanic are available. Tolerances in turbine engine
components are critical, making it necessary to measure
within a ten-thousandth of an inch. Because of these close
tolerances, it is important that the aviation mechanic can
make accurate measurements and mathematical calculations.
An aviation mechanic working on aircraft fuel systems
also uses mathematical principles to calculate volumes and
capacities of fuel tanks. The use of fractions and surface
area calculations are required to perform sheet metal repair
on aircraft structures.
Whole Numbers
Whole numbers are the numbers 0, 1, 2, 3, 4, 5, and so on.
Whole numbers can be thought of as counting numbers.
Addition of Whole Numbers
Addition is the process where the value of one number
is added to the value of another. The result is called the
sum. When working with whole numbers, it is important
to understand the principle of the place value. The place
value in a whole number is the value of the position of each
individual digit within the entire number. For example, in the
number 512, the 5 is in the hundreds column, the 1 is in the
tens column, and the 2 is in the ones column. Examples of
place values of three whole numbers are shown in Figure 3-1 .
When adding several whole numbers, such as 4,314, 122,
93,132, and 10, align them into columns according to place
value and then add.
4,314
93,132
+ 10 97,578
Therefore, 97,578 is the sum of the four whole numbers.
Subtraction of Whole Numbers
Subtraction is the process where the value of one number
is taken from the value of another. The result is called the
difference. When subtracting two whole numbers, such as
3,461 from 97,564, align them into columns according to
place value and then subtract.
97,564
– 3,461
94,103
The difference of the two whole numbers is 94,103.
Multiplication of Whole Numbers
Multiplication is the process of repeated addition. For example,
4 × 3 is the same as 4 + 4 + 4. The result is called the product.
Example: How many hydraulic system filters do you have
if there are 35 cartons in the supply room and each carton
contains 18 filters?
× 35
Therefore, there are 630 filters in the supply room.
Division of Whole Numbers
Division is the process of finding how many times one
number (called the divisor) is contained in another number
(called the dividend). The result is the quotient, and any
amount left over is called the remainder.
quotient
divisor dividend
Example: 218 landing gear bolts need to be divided between
7 aircraft. How many bolts will each aircraft receive?
3-2Place ValueTen Thousands
Thousands
Hundreds
Tens
Ones
1 2 7 4 935 shown as
269 shown as
12,749 shown as
Figure 3-1. Example of place values of whole numbers. 31
− 21
− 7
In this case, there are 31 bolts for each of the seven aircraft
with one extra remaining.
Fractions
A fraction is a number written in the form N⁄D where N is
called the numerator and D is called the denominator. The
fraction bar between the numerator and denominator shows
that division is taking place.
17 , 2 , 5Some examples of fractions are: 18 3 8
The denominator of a fraction cannot be a zero. For
example, the fraction 2⁄0 is not allowed, because dividing
by zero is undefined.
An improper fraction is a fraction in which the numerator is
equal to or larger than the denominator. For example, 4⁄4 or
15⁄8 are examples of improper fractions.
Finding the Least Common Denominator
To add or subtract fractions, they must have a common
denominator. In math, the least common denominator (LCD)
is generally used. One way to find the LCD is to list the
multiples of each denominator and then choose the smallest
number that they all have in common (can be divided by).
Example: Add 1⁄5 + 1⁄10 by finding the LCD.
Multiples of 5 are: 5, 10, 15, 20, 25, and so on. Multiples of
10 are: 10, 20, 30, 40, and so on. Notice that 10, 20, and 30
are in both lists, but 10 is the smallest or LCD. The advantage
of finding the LCD is that the final answer should be in the simplest form.
A common denominator can also be found for any group of
fractions by multiplying all the denominators together. This
number is not always the LCD, but it can still be used to add
or subtract fractions.
Example: Add 2⁄3 + 3⁄5 + 4⁄7 by finding a common denominator.
A common denominator can be found by multiplying the
denominators 3 × 5 × 7 to get 105.
( 2 + 3 + 4 )=( 70 + 63 + 60 )= 193 = 1 88 3 5 7 105 105 105 105 105
Addition of Fractions
In order to add fractions, the denominators must be the same
number. This is referred to as having “common denominators.”
Example: Add 1⁄7 to 3⁄7
1 + 3 = 1 + 3 = 4
7 7 7 7
If the fractions do not have the same denominator, then one or
all the denominators must be changed so that every fraction
has a common denominator.
Example: Find the total thickness of a panel made from
3⁄32-inch thick aluminum, that has a 1⁄64-inch thick paint
coating. To add these fractions, determine a common
denominator. The LCD for this example is 1, so only the
first fraction must be changed since the denominator of the
second fraction is already in 64ths.
( 3 + 1 )=( 3 × 2 + 1 )=( 6 + 1 )=(6 + 1)= 7 32 64 32 × 2 64 64 64 64 64
Therefore, 7⁄64 is the total thickness.
Subtraction of Fractions
To subtract fractions, they must have a common denominator.
Example: Subtract 2⁄17 from 10⁄17
10 – 2 = 10 – 2 = 8
17 17 17 17
If the fractions do not have the same denominator, then one or
all the denominators must be changed so that every fraction
has a common denominator.
3-3Example: The tolerance for rigging the aileron droop of an
airplane is 7⁄8 inch ± 1⁄5 inch. What is the minimum droop to
which the aileron can be rigged? To subtract these fractions,
first change both to common denominators. The common
denominator in this example is 40. Change both fractions to
1⁄40, as shown, then subtract.
(7 – 1)=(7 × 5 – 1 × 8)=(35 – 8 )=(35 – 8) = 27 8 5 8 × 5 5 × 8 40 40 40 40
Therefore, 27⁄40 is the minimum droop.
Multiplication of Fractions
Multiplication of fractions does not require a common
denominator. To multiply fractions, first multiply the
numerators. Then, multiply the denominators.
Example:
3 × 7 × 1 = 3 × 7 × 1 = 21
5 8 2 5 × 8 × 2 80
The use of cancellation when multiplying fractions is a
helpful technique. Cancellation divides out or cancels all
common factors that exist between the numerators and
denominators. When all common factors are cancelled before
the multiplication, the final product is in the simplest form.
Example:
(14 × 3 ) = (14 × 3) = ( 2 × 1) = 2
15 7 15 7 5 × 1 5
Division of Fractions
Division of fractions does not require a common denominator.
To divide fractions, first change the division symbol to
multiplication. Next, invert the second fraction. Then,
multiply the fractions.
Example: Divide 7⁄8 by 4⁄3
( 7 ÷ 4 ) = ( 7 × 3 ) = ( 7 × 3) = 21
8 3 8 4 8 × 4 32
Example: In Figure 3-2 , the center of the hole is in the center
of the plate. Find the distance that the center of the hole is
from the edges of the plate. To find the answer, the length
and width of the plate should each be divided in half. First,
change the mixed numbers to improper fractions:
5 7⁄16 inches = 87⁄16 inches
3 5⁄8 inches = 29⁄8 inchesThen, divide each improper fraction by 2 to find the center
of the plate.
87 ÷ 2 = 87 × 1 = 87 inches
16 1 16 2 32
29 ÷ 2 = 29 × 1 = 29 inches
8 1 8 2 16
Finally, convert each improper fraction to a mixed number:
87 = 87 ÷ 32 = 2 23 inches
32 32
29 = 29 ÷ 16 = 1 13 inches
16 16
Therefore, the distance to the center of the hole from each of
the plate edges is 2 23⁄32 inches and 1 13⁄16 inches.
Reducing Fractions
A fraction needs to be reduced when it is not in the simplest
form or “lowest terms.” Lowest term means that the numerator
and denominator do not have any factors in common. That
is, they cannot be divided by the same number (or factor).
To reduce a fraction, determine what the common factor(s)
are and divide these out of the numerator and denominator.
For example, when both the numerator and denominator are
even numbers, they can both be divided by 2.
Example: The total travel of a jackscrew is 13⁄16 inch. If the
travel in one direction from the neutral position is 7⁄16 inch,
what is the travel in the opposite direction?
13 – 7 = 13 – 7 = 6
16 16 16 16
The fraction 6⁄16 is not in lowest terms because the numerator
(6) and the denominator (16) have a common factor of 2. To
reduce 6⁄16, divide the numerator and the denominator by 2.
The final reduced fraction is 3⁄8 as shown below.
6 = 6 ÷ 2 = 3
16 16 ÷ 2 8
Therefore, the travel in the opposite direction is 3⁄8 inch.
Mixed Numbers
A mixed number is a combination of a whole number and
a fraction.
Figure 3-2. Center hole of the plate.Addition of Mixed Numbers
To add mixed numbers, add the whole numbers together. Then
add the fractions together by finding a common denominator.
The final step is to add the sum of the whole numbers to the
sum of the fractions for the final answer.
Example: The cargo area behind the rear seat of a small
airplane can handle solids that are 4 3⁄4 feet long. If the rear
seats are removed, then 2 1⁄3 feet is added to the cargo area.
What is the total length of the cargo area when the rear seats
are removed?
4 3 + 2 1 = (4 + 2) +(3 + 1) = 6 +( 9 + 4) = 6 13 =
4 3 4 3 12 12 12
7 1 feet of cargo room
Subtraction of Mixed Numbers
To subtract mixed numbers, find a common denominator
for the fractions. Subtract the fractions from each other. It
may be necessary to borrow from the larger whole number
when subtracting the fractions. Subtract the whole numbers
from each other. The final step is to combine the final whole
number with the final fraction.
Example: What is the length of the grip of the bolt shown
in Figure 3-3? The overall length of the bolt is 3 1⁄2 inches,
the shank length is 3 1⁄8 inches, and the threaded portion is
15⁄16 inches long. To find the grip, subtract the length of the
threaded portion from the length of the shank.
31⁄8 inches – 15⁄16 inches = grip length
To subtract, start with the fractions. Borrowing is necessary
because 5⁄16 is larger than 1⁄8 (or 2⁄16). From the whole number
3, borrow 1, which is actually 16⁄16. After borrowing, the first
mixed number is now 2 18⁄16. This is because, 3 1⁄8 = 3 2⁄16 = 2
+ 1 + 2⁄16 = 2 + 16⁄16 + 2⁄16 = 2 18⁄16. 3 1 – 1 5 = 3 2 – 1 5 = 2 18 – 1 5 = 113
8 16 16 16 16 16 16
Therefore, the grip length of the bolt is 113⁄16 inches.
(Note: The value for the overall length of the bolt was
given in the example, but it was not needed to solve the
problem. This type of information is sometimes referred to
as a “distracter,” because it distracts from the information
needed to solve the problem.)
The Decimal Number System
Origin and Definition
The number system that we use every day is called the
decimal system. The prefix in the word decimal, dec, is a
Latin root for the word “ten.” The decimal system probably
originated from the fact that we have ten fingers (or digits).
The decimal system has ten digits: 0, 1, 2, 3, 4, 5, 6, 7, 8 and
9. The decimal system is a base 10 system and has been in use
for over 5,000 years. A decimal is a number with a decimal
point. For example, 0.515, 0.10, and 462.625 are all decimal
numbers. Like whole numbers, decimal numbers also have
place value. The place values are based on powers of 10, as
shown in Figure 3-4 .
Addition of Decimal Numbers
To add decimal numbers, they must first be arranged so that
the decimal points are aligned vertically and according to
place value. That is, adding tenths with tenths, ones with
ones, hundreds with hundreds, and so forth.
Example: Find the total resistance for the circuit diagram
shown in Figure 3-5 . The total resistance of a series circuit is
equal to the sum of the individual resistances. To find the total
resistance, R T, the individual resistances are added together.
RT = 2.34 + 37.5 + 0.09
Arrange the resistance values in a vertical column so that the
decimal points are aligned and then add.
+ 0.09
Therefore, the total resistance, R T = 39.93 ohms.
Subtraction of Decimal Numbers
To subtract decimal numbers, they must first be arranged so
that the decimal points are aligned vertically and according
to place value. That is, subtracting tenths from tenths, ones
GripShank
Overall length37.5 Ohms2.34 Ohms
.09 OhmsMPlace ValueMillions
Hundred Thousands
Ten Thousands
Thousands
Hundreds
Tens
Ones
Tenths
Hundredths
Thousandths
Ten Thousandths
1 6 2 3 0 5 1
3 2 41,623,051
Figure 3-3. Bolt dimensions. Figure 3-5. Circuit diagram.Figure 3-4. Place values.from ones, hundreds from hundreds, and so forth.
Example: A series circuit containing two resistors has a total
resistance (R T) of 37.272 ohms. One of the resistors (R 1)
has a value of 14.88 ohms. What is the value of the other
resistor (R 2)?
R2 = R T – R 1 = 37.272 – 14.88
Arrange the decimal numbers in a vertical column so that the
decimal points are aligned and then subtract.
Therefore, the second resistor, R 2 = 22.392 ohms.
Multiplication of Decimal Numbers
To multiply decimal numbers, vertical alignment of the
decimal point is not required. Instead, align the numbers to
the right in the same way that whole numbers are multiplied
(with no regard to the decimal points or place values) and
then multiply. The last step is to place the decimal point in
the correct place in the answer. To do this, count the number
of decimal places in each of the numbers, add the total, and
then assign that number of decimal places to the result.
Example: To multiply 0.2 × 6.03, arrange the numbers
vertically and align them to the right. Multiply the numbers,
ignoring the decimal points for now.
× 0.2
1206 (ignore the decimal points, for now)
After multiplying the numbers, count the total number of
decimal places in both numbers. For this example, 6.03 has
2 decimal places and 0.2 has 1 decimal place. Together there
are a total of 3 decimal places. The decimal point for the
answer is placed 3 decimal places from the right. Therefore, the answer is 1.206.
6.03 2 decimal places
× 0.2 1 decimal place
1.206 3 decimal places
Example: Using the formula watts = amperes × voltage, what
is the wattage of an electric drill that uses 9.45 amperes from a
120-volt source? Align the numbers to the right and multiply.
After multiplying the numbers, count the total number of
decimal places in both numbers. For this example, 9.45 has 2
decimal places and 120 has no decimal place. Together there
are 2 decimal places. The decimal point for the answer is
placed 2 decimal places from the right. Therefore, the answer
is 1,134.00 watts, or simplified to 1,134 watts.
9.45 2 decimal places
× 120 no decimal place
+ 945
1,134.00 2 decimal places
3-6Division of Decimal Numbers
Division of decimal numbers is performed the same way as
whole numbers, unless the divisor is a decimal.
quotient
divisor dividend
When the divisor is a decimal, it must be changed to a whole
number before dividing. To do this, move the decimal in the
divisor to the right until there are no decimal places. At the
same time, move the decimal point in the dividend to the right
the same number of places. Then divide. The decimal in the
quotient is placed directly above the decimal in the dividend.
Example: Divide 0.144 by 0.12
0.12 0.144 = 12. 14.4
Move the decimal in the divisor (0.12) two places to the right.
The result is 12.0. Next, move the decimal in the dividend
(0.144) two places to the right. The result is 14.4. Now divide.
The result is 1.2.
Example: The wing area of an airplane is 262.6 square feet
and its span is 40.4 feet. Find the mean chord of its wing
using the formula: area ÷ span = mean chord.
40.4 262.6 = 404. 2626.0
Move the decimal in the divisor (40.4) one place to the
right. Next, move the decimal in the dividend (262.6) one
place to the right. Then divide. The mean chord length is
6.5 feet.
Rounding Off Decimal Numbers
Occasionally, it is necessary to round off a decimal number
to some value that is practical to use. For example, a
measurement is calculated to be 29.4948 inches. To use this
measurement, we can use the process of “rounding off.” A
decimal is “rounded off” by keeping the digits for a certain
number of places and discarding the rest. The degree of accuracy desired determines the number of digits to be
retained. When the digit immediately to the right of the last
retained digit is 5 or greater, round up by 1. When the digit
immediately to the right of the last retained digit is less than
5, leave the last retained digit unchanged.
Example: An actuator shaft is 2.1938 inches in diameter.
Round to the nearest tenth.
The digit in the tenths column is a 1. The digit to the right
of the 1 is a 9. Since 9 is greater than or equal to 5, “round
up” the 1 to a 2. Therefore, 2.1938 rounded to the nearest
tenth is 2.2.
Example: The outside diameter of a bearing is 3.1648
centimeters. Round to the nearest hundredth.
The digit in the hundredths column is a 6. The digit to the
right of the 6 is a 4. Since 4 is less than 5, do not round up
the 6. Therefore, 3.1648 to the nearest hundredth is 3.16.
Example: The length of a bushing is 3.7487 feet. Round to
the nearest thousandth.
The digit in the thousandths column is an 8. The digit to
the right of the 8 is a 7. Since 7 is greater than or equal to
5, “round up” the 8 to a 9. Therefore, 3.7487 to the nearest
thousandth is 3.749.
Converting Decimal Numbers to Fractions
To change a decimal number to a fraction, “read” the decimal
out loud, and then write it into a fraction just as it is read as
shown below.
Example: One oversized rivet has a diameter of 0.52 inches.
Convert 0.52 to a fraction. The decimal 0.52 is read as
“fifty-two hundredths.”
0.52 = 52 “fifty-two”
100 “hundredths”
In the above fraction of 52⁄100, we can divide 4 into each number
resulting in a fraction of 13⁄25.
A dimension often appears in a maintenance manual or on
a blueprint as a decimal instead of a fraction. To use the
dimension, it may need to be converted to a fraction. An
aviation mechanic frequently uses a steel rule that is calibrated
in units of 1⁄64 of an inch. To change a decimal to the nearest
equivalent common fraction, multiply the decimal by 64. The
product of the decimal and 64 is the numerator of the fraction
and 64 is the denominator. Reduce the fraction, if needed.
3-7Example: The width of a hex head bolt is 0.3123 inches.
Convert the decimal 0.3123 to a common fraction to decide
which socket would be the best fit for the bolt head. First,
multiply the 0.3123 decimal by 64:
0.3123 × 64 = 19.9872
Next, round the product to the nearest whole number:
19.98722 ≈ 20.
Use this whole number (20) as the numerator and 64 as the
denominator: 20⁄64.
Now, reduce 20⁄64 to 5⁄16 as 4 is common to both the numerator
and denominator. Therefore, the correct socket would be the
5⁄16 inch socket (20⁄64 reduced).
Example: When accurate holes of uniform diameter
are required for aircraft structures, they are first drilled
approximately 1⁄64 inch undersized and then reamed to the
final desired diameter. What size drill bit should be selected
for the undersized hole if the final hole is reamed to a diameter
of 0.763 inches? First, multiply the 0.763 decimal by 64.
0.763 × 64 = 48.832
Next, round the product to the nearest whole number:
48.832 ≈ 49.
Use this number (49) as the numerator and 64 as the
denominator: 49⁄64 is the closest fraction to the final reaming
diameter of 0.763 inches. To determine the drill size for the
initial undersized hole, subtract 1⁄64 inch from the finished
hole size.
49 – 1 = 48 = 3
64 64 64 4
Therefore, a 3⁄4-inch drill bit should be used for the initial
undersized holes.
Converting Fractions to Decimals
To convert any fraction to a decimal, simply divide the top
number (numerator) by the bottom number (denominator).
Every fraction has an approximate decimal equivalent.
Example:
2 = 1 ÷ 2 = 2 1.0 Therefore, 1
2 = 0.5
8 = 3 ÷ 8 = 8 3.000 Therefore, 3
8 = 0.375
Calculator tip: numerator (top number) ÷ denominator
(bottom number) = the decimal equivalent of the fraction.
Some fractions when converted to decimals produce a
repeating decimal.
Example:
0.33 1
3 = 1 ÷ 3 = 3 1.00 = 0.3 or 0.33
– 9 This decimal can be
represented with a bar, or can
be rounded. (A bar indicates
that the number(s) beneath it
are repeated to infinity.)
Other examples of repeating decimals:
0.212121… = 0.21
0.6666… = 0.7 or 0.67
0.254254… = 0.254
Decimal Equivalent Chart
Figure 3-6 is a fraction to decimal to millimeter equivalency
chart. Measurements starting at 1⁄64 inch and up to 3 inches
have been converted to decimal numbers and to millimeters.
Ratio
A ratio is the comparison of two numbers or quantities. A
ratio may be expressed in three ways: as a fraction, with a
colon, or with the word “to.” For example, a gear ratio of 5:7
can be expressed as any of the following:
5⁄7 or 5:7 or 5 to 7
Aviation Applications
Ratios have widespread application in the field of aviation.
Example: Compression ratio on a reciprocating engine is the
ratio of the volume of a cylinder with the piston at the bottom
of its stroke to the volume of the cylinder with the piston at
the top of its stroke. For example, a typical compression ratio
might be 10:1 (or 10 to 1).
Aspect ratio is the ratio of the length (or span) of an airfoil to
3-8its width (or chord). A typical aspect ratio for a commercial
airliner might be 7:1 (or 7 to 1).
Air-fuel ratio is the ratio of the weight of the air to the
weight of fuel in the mixture being fed into the cylinders of
a reciprocating engine. For example, a typical air-fuel ratio
might be 14.3:1 (or 14.3 to 1).
Glide ratio is the ratio of the forward distance traveled to
the vertical distance descended when an aircraft is operating
without power. For example, if an aircraft descends 1,000 feet
while it travels through the air for two linear miles (10,560
feet), it has a glide ratio of 10,560:1,000 which can be reduced
to 10.56: 1 (or 10.56 to 1).
Gear ratio is the number of teeth each gear represents when
two gears are used in an aircraft component. In Figure 3-7 ,
the pinion gear has 8 teeth and a spur gear has 28 teeth. The
gear ratio is 8:28. Using 7 as the LCD, 8:28 becomes 2:7.
Speed ratio is when two gears are used in an aircraft
component; the rotational speed of each gear is represented
as a speed ratio. As the number of teeth in a gear decreases,
the rotational speed of that gear increases, and vice-versa.
Therefore, the speed ratio of two gears is the inverse (or
opposite) of the gear ratio. If two gears have a gear ratio of
2:9, then their speed ratio is 9:2.
Example: A pinion gear with 10 teeth is driving a spur gear
with 40 teeth. The spur gear is rotating at 160 rpm. Calculate
the speed of the pinion gear.
Teeth in Pinion Gear = Speed of Spur Gear
Teeth in Spur Gear Speed of Pinion Gear
10 teeth = 160 rpm
40 teeth S P (speed of pinion gear)
To solve for S P, multiply 40 × 160, then divide by 10. The
speed of the pinion gear is 640 rpm.
Example: If the cruising speed of an airplane is 200 knots
and its maximum speed is 250 knots, what is the ratio of
cruising speed to maximum speed? First, express the cruising
speed as the numerator of a fraction whose denominator is
the maximum speed.
Ratio = 200
Next, reduce the resulting fraction to its simplest form.
Ratio = 200
250 = 4
5Therefore, the ratio of cruising speed to maximum speed is 4:5.
Another common use of ratios is to convert any given ratio
to an equivalent ratio with a denominator of 1.
Example: Express the ratio 9:5 as a ratio with a denominator
of 1.
R = 9
5 = ?
1 Since 9 ÷ 5 = 1.8, then 9
5 = 1.8
Therefore, 9:5 is the same ratio as 1.8:1. In other words, 9
to 5 is the same ratio as 1.8 to 1.
Proportion
A proportion is a statement of equality between two or more
ratios. For example,
4 = 6
8 or 3:4 = 6:8
This proportion is read as, “3 is to 4 as 6 is to 8.”
Extremes and Means
The first and last terms of the proportion (the 3 and 8 in
this example) are called the extremes. The second and third
terms (the 4 and 6 in this example) are called the means. In
any proportion, the product of the extremes is equal to the
product of the means.
In the proportion 2:3 = 4:6, the product of the extremes,
2 × 6, is 12; the product of the means, 3 × 4, is also 12. An
inspection of any proportion shows this to be true.
Solving Proportions
Normally when solving a proportion, three quantities are
known, and the fourth is unknown. To solve for the unknown,
multiply the two numbers along the diagonal and then divide
by the third number.
Example: Solve for X in the proportion given below.
80 = X
First, multiply 65 × 100: 65 × 100 = 6500
Next, divide by 80: 6500 ÷ 80 = 81.25
Therefore, X = 81.25.
Example: An airplane flying 300 miles used 24 gallons of
gasoline. How many gallons will it need to travel 750 miles?
The ratio here is: “miles to gallons;” therefore, the proportion
is set up as:
Miles
Gallons 300
24 = 750
G
3-9Fraction Decimal MM
1/64
1/32
3/64
1/16
5/64
3/32
7/64
1/8
9/64
5/32
11/64
3/16
13/64
7/32
15/64
1/4
17/64
9/32
19/64
5/16
21/64
11/32
23/64
3/8
25/64
13/32
27/64
7/16
29/64
15/32
31/64
1/2
33/64
17/32
35/64
37/64
19/32
39/64
5/8
41/64
21/32
43/64
11/16
45/64
23/32
47/64
3/4
49/64
25/32
51/64
13/16
53/64
27/32
55/64
7/8
57/64
29/32
59/64
15/16
61/64
31/32
63/64
25.4Fraction Decimal MM
1 1/64
1 1/32
1 3/64
1 1/16
1 5/64
1 3/32
1 7/64
1 1/8
1 9/64
1 5/32
1 11/64
1 3/16
1 13/64
1 7/32
1 15/64
1 1/4
1 17/64
1 9/32
1 19/64
1 5/16
1 21/64
1 11/32
1 23/64
1 3/8
1 25/64
1 13/32
1 27/64
1 7/16
1 29/64
1 15/32
1 31/64
1 1/2
1 33/64
1 17/32
1 35/64
1 9/16
1 37/64
1 19/32
1 39/64
1 5/8
1 41/64
1 21/32
1 43/64
1 11/16
1 45/64
1 23/32
1 47/64
1 3/4
1 49/64
1 25/32
1 51/64
1 13/16
1 53/64
1 27/32
1 55/64
1 7/8
1 57/64
1 29/32
1 59/64
1 15/16
1 61/64
1 31/32
1 63/64
50.8Fraction Decimal MM
2 1/64
2 1/32
2 3/64
2 1/16
2 5/64
2 3/32
2 7/64
2 1/8
2 9/64
2 5/32
2 11/64
2 3/16
2 13/64
2 7/32
2 15/64
2 1/4
2 17/64
2 9/32
2 19/64
2 5/16
2 21/64
2 11/32
2 23/64
2 3/8
2 25/64
2 13/32
2 27/64
2 7/16
2 29/64
2 15/32
2 31/64
2 1/2
2 33/64
2 17/32
2 35/64
2 9/16
2 37/64
2 19/32
2 39/64
2 5/8
2 41/64
2 21/32
2 43/64
2 11/16
2 45/64
2 23/32
2 47/64
2 3/4
2 49/64
2 25/32
2 51/64
2 13/16
2 53/64
2 27/32
2 55/64
2 7/8
2 57/64
2 29/32
2 59/64
2 15/16
2 61/64
2 31/32
2 63/64
Figure 3-6. Fractions, decimals, and millimeters.
2:7
Figure 3-7. Gear ratio.the denominator), and then convert the decimal number to a
percentage by multiplying by 100 as shown earlier.
Example: Express the fraction 5⁄8 as a percentage.
8 = 5 ÷ 8 = 0.625 = 62.5%
Finding a Percentage of a Given Number
This is the most common type of percentage calculation.
Here are two methods to solve percentage problems: using
algebra or using proportions. Each method is shown next to
find a percent of a given number.
Example: In a shipment of 80 wingtip lights, 15% of the
lights were defective. How many of the lights were defective?
Algebraic Method:
15% of 80 lights = N (number of defective lights)
0.15 × 80 = N
12 = N
Therefore, 12 defective lights were in the shipment.
Proportion Method:
N
80 = 15
To solve for N: N × 100 = 80 × 15
N × 100 = 1,200
N = 1,200 ÷ 100
N = 12
or
N = (80 × 15) ÷ 100
N = 12
Finding What Percentage One Number is of Another
Example: A small engine rated at 12 horsepower is found
to be delivering only 10.75 horsepower. What is the motor
efficiency expressed as a percent?
Algebraic Method:
N% of 12 rated horsepower = 10.75 actual horsepower
N% × 12 = 10.75
N% = 10.75 ÷ 12
N% = 0.8958
N = 89.58
Therefore, the motor efficiency is 89.58%.
Proportion Method:
12 = N
100 Solve for G: (750 × 24) ÷ 300 = 60
Therefore, to fly 750 miles, 60 gallons of gasoline is required.
Percentage
Percentage means “parts out of one hundred.” The
percentage sign is “%.” Ninety percent is expressed as 90%
(= 90 parts out of 100). The decimal 0.90 equals 90⁄100, or 90
out of 100, or 90%.
Expressing a Decimal Number as a Percentage
To express a decimal number in percent, move the decimal
point two places to the right (adding zeroes if necessary) and
then affix the percent symbol.
Example: Express the following decimal numbers as a percent:
0.90 = 90%
0.5 = 50%
1.25 = 125%
0.335 = 33.5%
Expressing a Percentage as a Decimal Number
Sometimes it may be necessary to express a percentage as
a decimal number. To express a percentage as a decimal
number, move the decimal point two places to the left and
drop the % symbol.
For example: Express the following percentages as decimal
numbers:
90% = 0.90
50% = 0.50
5% = 0.05
150% = 1.5
Expressing a Fraction as a Percentage
To express a fraction as a percentage, first change the
fraction to a decimal number (by dividing the numerator by
3-11To solve for N: N × 12 = 10.75 × 100
N × 12 = 1,075
N = 1,075 ÷ 12
N = 89.58
or
N = (1,075 × 100) ÷ 12
N = 89.58
Therefore, the motor efficiency is 89.58%.
Finding a Number When a Percentage of it is Known
Example: Eighty ohms represents 52% of a microphone’s
total resistance. Find the total resistance of this microphone.
Algebraic Method:
52% of N = 80 ohms
52% × N = 80
N = 80 ÷ 0.52
N = 153.846
The total resistance of the microphone is 153.846 ohms.
Proportion Method:
= 52
To solve for N: N × 52 = 80 × 100
N × 52 = 8,000
N = 8,000 ÷ 52
N = 153.846 ohms
or
N = (80 × 100) ÷ 52
N = 153.846 ohms
Positive & Negative Numbers (Signed
Numbers)
Positive numbers are numbers that are greater than
zero. Negative numbers are numbers less than zero.
[Figure 3-8] Signed numbers are also called integers.
Addition of Positive & Negative Numbers
The sum (addition) of two positive numbers is positive. The
sum (addition) of two negative numbers is negative. The
sum of a positive and a negative number can be positive or
negative, depending on the values of the numbers. A good
way to visualize a negative number is to think in terms of
debt. If you are in debt by $100 (or, −100) and you add $45
to your account, you are now only $55 in debt (or −55).
Therefore: −100 + 45 = −55.
Example: The weight of an aircraft is 2,000 pounds. A radio
rack weighing 3 pounds and a transceiver weighing 10 pounds 80
Nare removed from the aircraft. What is the new weight? For
weight and balance purposes, all weight removed from an
aircraft is given a minus sign, and all weight added is given
a plus sign.
2,000 + −3 + −10 = 2,000 + −13 = 1,987
Therefore, the new weight is 1,987 pounds.
Subtraction of Positive & Negative Numbers
To subtract positive and negative numbers, first change the
“–” (subtraction symbol) to a “+” (addition symbol), and
change the sign of the second number to its opposite (that
is, change a positive number to a negative number or vice
versa). Finally, add the two numbers together.
Example: The daytime temperature in the city of Denver
was 6° below zero (−6°). An airplane is cruising at 15,000
feet above Denver. The temperature at 15,000 feet is 20°
colder than in the city of Denver. What is the temperature
at 15,000 feet?
Subtract 20 from −6: −6 – 20 = −6 + (−20) = −26
The temperature is −26°, or 26° below zero at 15,000 feet
above the city.
Multiplication of Positive & Negative Numbers
The product of two positive numbers is always positive. The
product of two negative numbers is always positive. The
product of a positive and a negative number is always negative.
Examples:
3 × 6 = 18 −3 × 6 = −18 −3 × −6 = 18 3 × −6 = −18
Division of Positive & Negative Numbers
The quotient of two positive numbers is always positive. The
quotient of two negative numbers is always positive. The
quotient of a positive and negative number is always negative.
Examples:
6 ÷ 3 = 2 −6 ÷ 3 = −2 −6 ÷ −3 = 2 6 ÷ −3 = −2
Powers
The power (or exponent) of a number is a shorthand method
of indicating how many times a number, called the base,
is multiplied by itself. For example, 34 is read as “3 to the
power of 4.” That is, 3 multiplied by itself 4 times. The 3 is
the base and 4 is the power.
Examples:
23 = 2 × 2 × 2 = 8
3-12−5 −4 −3 −2 −1 0 +1 +2 +3 +4 +5
Figure 3-8. A scale of signed numbers.Read “two to the third power equals 8.”
105 = 10 × 10 × 10 × 10 × 10 = 100,000
Read “ten to the fifth power equals 100,000.”
Special Powers
Squared
When a number has a power of 2, it is commonly referred to
as “squared.” For example, 72 is read as “seven squared” or
“seven to the second power.” To remember this, think about
how a square has two dimensions: length and width.
Cubed
When a number has a power of 3, it is commonly referred
to as “cubed.” For example, 73 is read as “seven cubed” or
“seven to the third power.” To remember this, think about
how a cube has three dimensions: length, width, and depth.
Power of Zero
Any non-zero number raised to the zero power always equals 1.
Example:
70 = 1 1810 = 1 (–24)0 = 1
Negative Powers
A number with a negative power equals its reciprocal with
the same power made positive.
Example: The number 2-3 is read as “2 to the negative 3rd
power,” and is calculated by:
2-3 = 1
= 1
2 × 2 × 2 = 1
When using a calculator to raise a negative number to a
power, always place parentheses around the negative number
(before raising it to a power) so that the entire number gets
raised to the power.
Law of Exponents
When multiplying numbers with powers, the powers can be
added as long as the bases are the same.
Example:
32 × 34 = (3 × 3) × (3 × 3 × 3 × 3) = 3 × 3 × 3 × 3 × 3 × 3 = 36
or 32 × 34 = 3(2+4) = 36When dividing numbers with powers, the powers can be
subtracted as long as the bases are the same.
Example:
104 ÷ 102 =10 × 10 × 10 × 10
10 × 10 =10 × 10 × 10 × 10
10 × 10 =10 × 10 =102
or 104 ÷ 102 = 10(4 – 2) = 102
Powers of Ten
Because we use the decimal system of numbers, powers of
ten are frequently seen in everyday applications. For example,
scientific notation uses powers of ten. Also, many aircraft
drawings are scaled to powers of ten. Figure 3-9 gives more
information on the powers of ten and their values.
Roots
A root is a number that when multiplied by itself a specified
number of times produces a given number.
The two most common roots are the square root and the cube
root. For more examples of roots, see Figure 3-10 .
Square Roots
The square root of 25, written as 25, equals 5. That is, when
the number 5 is squared (multiplied by itself), it produces the
number 25. The symbol is called a radical sign. Finding
the square root of a number is the most common application
of roots. The collections of numbers whose square roots
are whole numbers are called perfect squares. The first ten
perfect squares are: 1, 4, 9, 16, 25, 36, 49, 64, 81, and 100.
The square root of each of these numbers is 1, 2, 3, 4, 5, 6,
7, 8, 9, and 10, respectively.
For example, 36 = 6 and 81 = 9
To find the square root of a number that is not a perfect
square, use either a calculator or the estimation method. A
longhand method does exist for finding square roots, but
with the advent of calculators and because of its lengthy
explanation, it is no longer included in this handbook. The
estimation method uses the knowledge of perfect squares to
approximate the square root of a number.
Example: Find the square root of 31. Since 31 falls between
the two perfect roots 25 and 36, we know that must be
between 25 and 36. Therefore, 31 must be greater than 5
and less than 6 because 25 = 5 and 36 = 6. If you estimate
the square root of 31 at 5.5, you are close to the correct
answer. The square root of 31 is actually 5.568.
3-13Powers
of TenExpansion Value
Positive
Exponents
1001,000,000
100,000
10,000
1,000
110 x 10 x 10 x 10 x 10 x 10
10 x 10 x 10 x 10 x 10
10 x 10 x 10 x 10
10 x 10 x 10
10 x 10
Negative
Exponents
10-61/10=0.1
1/100=0.01
1/1,000=0.001
1/10,000=0.0001
1/100,000=0.00001
1/1,000,000=0.0000011/10
1/(10 x 10)
1/(10 x 10 x 10)
1/(10 x 10 x 10 x 10)
1/(10 x 10 x 10 x 10 x 10)
1/(10 x 10 x 10 x 10 x 10 x 10)
Figure 3-9. Powers of ten.Cube Roots
The cube root of 125, written as 3125, equals 5. That is,
when the number 5 is cubed (5 multiplied by itself then
multiplying the product (25) by 5 again), it produces the
number 125. It is common to confuse the “cube” of a number
with the “cube root” of a number.
For clarification, the cube of 27 = 273 = 27 × 27 × 27 = 19,683.
However, the cube root of 27 = 327 = 3.
Fractional Powers
Another way to write a root is to use a fraction as the power
(or exponent) instead of the radical sign. The square root of a
number is written with a 1⁄2 as the exponent instead of a radical
sign. The cube root of a number is written with an exponent
of 1⁄3 and the fourth root with an exponent of 1⁄4 and so on.
Example: 31 = 311⁄2 3125 = 1251⁄3 416 = 161⁄4
Functions of Numbers Chart
The Functions of Numbers chart found in Figure 3-10
is included in this chapter for convenience in making
computations. Each column in the chart is listed below, with
new concepts explained.
• Number (N)
• N squared (N2)
• N cubed (N3)
• Square root of N ( N)
• Cube root of N (3N)
• Circumference of a circle with diameter = N.
Circumference is the linear measurement of the
distance around a circle. The circumference is
calculated by multiplying the diameter of the circle
by 3.1416 (3.1416 is the number referred to as pi,
which has the symbol π). If the diameter of a circle is
10 inches, then the circumference would be:
10 × 3.1416 = 31.4160.
• Area of a circle with diameter = N. Area of a circle is
the number of square units of measurement contained
in the circle with a diameter of N. The area of a circle
equals π multiplied by the radius squared. This is
calculated by the formula: A = π × r2. Remember that
the radius is equal to one-half of the diameter.
Example: A flight deck instrument gauge has a round
face that is 3 inches in diameter. What is the area of
the face of the gauge? From Figure 3-10 for N = 3, the
answer is 7.0686 square inches. This is calculated by:
If the diameter of the gauge is 3 inches, then the
radius = D⁄2 = 3⁄2 = 1.5 inches. Area = π × r2 = 3.1416 × 1.52 = 3.1416 × 2.25 =
7.0686 square inches.
Scientific Notation
Scientific notation is used as a type of shorthand to express very
large or very small numbers. It is a way to write numbers so
that they do not take up as much space on the page. The format
of a number written in scientific notation has two parts. The
first part is a number greater than or equal to 1 and less than
10 (for example, 2.35). The second part is a power of 10 (for
example, 106). The number 2,350,000 is expressed in scientific
notation as 2.35 × 106. It is important that the decimal point
is always placed to the right of the first digit. Notice that very
large numbers always have a positive power of 10 and very
small numbers always have a negative power of 10.
Example: The velocity of the speed of light is over 186,000
miles per second (mps). This can be expressed as 1.86 × 105 mps
in scientific notation. The mass of an electron is approximately
0.000,000,000,000,000,000,000,000,000,911 grams. This can
be expressed in scientific notation as 9.11 × 10-28 grams.
Converting Numbers from Standard Notation to
Scientific Notation
Example: Convert 1,244,000,000,000 to scientific notation
as follows. First, note that the decimal point is to the right
of the last zero. (Even though it is not usually written, it is
assumed to be there.)
3-14Number Square Cube Square Root Cube Root Circumference Area
1,024
1,089
1,156
1,225
1,296
1,369
1,444
1,521
1,600
1,681
1,764
1,849
1,936
2,025
2,116
2,209
2,304
2,401
2,5001
1,000
1,331
1,728
2,197
2,744
3,375
4,096
4,913
5,832
6,859
8,000
9,261
10,648
12,167
13,824
15,625
17,576
19,683
21,952
24,389
27,000
29,791
32,768
35,937
39,304
42,875
46,656
50,653
54,872
59,319
64,000
68,921
74,088
79,507
85,184
91,125
97,336
103,823
110,592
117,649
125,0001.000
1963.49Number (N) N Squared (N2) N Cubed (N3)Circumference
of a circle with
diameter = NArea of a
circle with
diameter = NSquare Root
of N (√N)Cube Root
of N (√N)3
Figure 3-10. Functions of numbers.
3-15Number Square Cube Square Root Cube Root Circumference Area
1002,601
2,704
2,809
2,916
3,025
3,136
3,249
3,364
3,481
3,600
3,721
3,844
3,969
4,096
4,225
4,356
4,489
4,624
4,761
4,900
5,041
5,184
5,329
5,476
5,625
5,776
5,929
6,084
6,241
6,400
6,561
6,724
6,889
7,056
7,225
7,396
7,569
7,744
7,921
8,100
8,281
8,464
8,649
8,836
9,025
9,216
9,409
9,604
9,801
10,000132,651
140,608
148,877
157,464
166,375
175,616
185,193
195,112
205,379
216,000
226,981
238,328
250,047
262,144
274,625
287,496
300,763
314,432
328,509
343,000
357,911
373,248
389,017
405,224
421,875
438,976
456,533
474,552
493,039
512,000
531,441
551,368
571,787
592,704
614,125
636,056
658,503
681,472
704,969
729,000
753,571
778,688
804,357
830,584
857,375
884,736
912,673
941,192
970,299
1,000,0007.141
314.1592042.82
7853.98Number (N) N Squared (N2) N Cubed (N3)Circumference
of a circle with
diameter = NArea of a
circle with
diameter = NSquare Root
of N (√N)Cube Root
of N (√N)3
Figure 3-10. Functions of numbers (continued).
3-161,244,000,000,000 = 1,244,000,000,000.0
To change to the format of scientific notation, the decimal
point must be moved to the position between the first and
second digits. In this case, it is between the 1 and the 2. Since
the decimal point must be moved 12 places to the left to get
there, the power of 10 is 12. Remember that large numbers
always have a positive exponent. Therefore, 1,244,000,000,000
= 1.244 × 1012 when written in scientific notation.
Example: Convert 0.000000457 from standard notation to
scientific notation. To change to the format of scientific
notation, the decimal point must be moved to the position
between the first and second numbers, which in this
case is between the 4 and the 5. Since the decimal point
must be moved 7 places to the right to get there, the
power of 10 is −7. Remember that small numbers (those
less than one) have a negative exponent. Therefore,
0.000000457 = 4.57 × 10-7 when written in scientific notation.
Converting Numbers from Scientific Notation to
Standard Notation
Example: Convert 3.68 × 107 from scientific notation to
standard notation, as follows. To convert from scientific
notation to standard notation, move the decimal place 7 places to
the right. 3.68 × 107 = 36,800,000. Another way to think about
the conversion is 3.68 × 107 = 3.68 × 10,000,000 = 36,800,000.
Example: Convert 7.1543 × 10-10 from scientific notation
to standard notation. Move the decimal place 10 places to
the left: 7.1543 × 10-10 =.00000000071543. Another way
to think about the conversion is 7.1543 × 10-10 = 7.1543 ×
0.0000000001 = 0.00000000071543
When converting, remember that large numbers always
have positive powers of ten and small numbers always have
negative powers of ten. Refer to Figure 3-11 to determine
which direction to move the decimal point.
Addition, Subtraction, Multiplication, and Division
of Scientific Numbers
To add, subtract, multiply, or divide numbers in scientific
notation, change the scientific notation number back to
standard notation. Then add, subtract, multiply or divide the
standard notation numbers. After the computation, change the
final standard notation number back to scientific notation.
Algebra
Algebra is the branch of mathematics that uses letters or
symbols to represent variables in formulas and equations.
For example, in the equation d = v × t, where distance = velocity
× time, the variables are: d, v, and t. Equations
Algebraic equations are frequently used in aviation to show
the relationship between two or more variables. Equations
normally have an equals sign (=) in the expression.
Example: The formula A = π × r2 shows the relationship
between the area of a circle (A) and the length of the radius
(r) of the circle. The area of a circle is equal to π (3.1416)
times the radius squared. The larger the radius, the larger the
area of the circle.
Algebraic Rules
When solving for a variable in an equation, you can add,
subtract, multiply, or divide the terms in the equation (you do
the same to both sides of the equals sign) to get the variable
onto one side of the equals sign.
Examples: Solve the following equations for the value N.
3N = 21
To solve for N, divide both sides by 3.
3N ÷ 3 = 21 ÷ 3
N = 7
N + 17 = 59
To solve for N, subtract 17 from both sides.
N + 17 – 17 = 59 – 17
N = 42
N – 22 = 100
To solve for N, add 22 to both sides.
N – 22 + 22 = 100 + 22
N = 122
N
5 = 50
To solve for N, multiply both sides by 5.
N × 5 = 50 × 5
N = 250
Solving for a Variable
Another application of algebra is to solve an equation for a
given variable.
Example: Using the formula given in Figure 3-12 , find the
total capacitance (C T) of the series circuit containing three
capacitors with
C1 = 0.1 microfarad
C2 = 0.015 microfarad
C3 = 0.05 microfarad
First, substitute the given values into the formula:
3-17ConversionLarge Numbers
with Positive
Powers of 10Small Numbers
with Negative
Powers of 10
From standard
notation to scientific
notation
From scientific
notation to standard
notationMove decimal
place to the right
Move decimal
place to the leftMove decimal
place to the left
Move decimal
place to the right1
+CT=1
C11
C2+1
C3
Figure 3-11. Converting between scientific and standard notation. Figure 3-12. Total capacitance in a series circuit.CT = 1
= 1
= 1
10 + 66.66 + 20 1
C1+1
C2+1
C3 1
0.1+1
0.015+1
Therefore, C T = 1⁄96.66 = 0.01034 microfarad. The microfarad
(10-6 farad) is a unit of measurement of capacitance. This is
discussed in greater length in Chapter 12, Electricity.
Use of Parentheses
In algebraic equations, parentheses are used to group numbers
or symbols together. The use of parentheses helps us to
identify the order in which we should apply mathematical
operations. The operations inside the parentheses are always
performed first in algebraic equations.
Example: Solve the algebraic equation X = (4 + 3)2.
First, perform the operation inside the parentheses, which
is, 4 + 3 = 7. Then complete the exponent calculation
X = (7)2 = 7 × 7 = 49.
When using more complex equations, which may combine
several terms and use multiple operations, grouping the
terms together helps organize the equation. Parentheses, ( ),
are most commonly used in grouping, but you may also see
brackets, [ ]. When a term or expression is inside one of these
grouping symbols, it means that any operation indicated to
be done on the group is done to the entire term or expression.
Example:
Solve the equation N = 2 × [(9 ÷ 3) + (4 + 3)2]. Start with
the operations inside the parentheses ( ), then perform the
operations inside the brackets [ ].
N = 2 × [(9 ÷ 3) + (4 + 3)2]
N = 2 × [3 + (7)2]
First, complete the operations inside the parentheses ( ).
N = 2 × [3 + 49]
N = 2 × [52]
Second, complete the operations inside the brackets [ ].
N = 104Order of Operation
In algebra, rules have been set for the order in which
operations are evaluated. These same universally accepted
rules are also used when programming algebraic equations in
calculators. When solving the following equation, the order
of operation is given below:
N = (62 – 54)2 + 62 – 4 + 3 × [8 + (10 ÷ 2)] + 25 + (42 × 2) ÷ 4 + 3⁄4
1. Parentheses. First you must do everything in parentheses,
( ), starting from the innermost parentheses. If the
expression has a set of brackets, [ ], treat these exactly
like parentheses. If you are working with a fraction, treat
the top as if it was in parentheses and the denominator
as if it were in parentheses, even if there is none shown.
From the equation above, completing the calculation in
parentheses gives the following:
N = (8)2 + 62 – 4 + 3 × [8 + (5)] + 25 + (84) ÷ 4 + 3⁄4,
then
N = (8)2 + 62 – 4 + 3 × [13] + 25 + 84 ÷ 4 + 3⁄4
2. Exponents. Next, clear any exponents. Treat any roots
(square roots, cube roots, and so forth) as exponents.
Completing the exponents and roots in the equation
gives the following:
N = 64 + 36 – 4 + 3 × 13 + 5 + 84 ÷ 4 + 3⁄4
3. Multiplication and Division. Evaluate all of the
multiplications and divisions from left to right. Multiply
and divide from left to right in one step. A common
error is to use two steps for this (that is, to clear all of
the multiplication signs and then clear all of the division
signs), but that is not the correct method. Treat fractions
as division. Completing the multiplication and division
in the equation gives the following:
N = 64 + 36 – 4 + 39 + 5 + 21 + 3⁄4
4. Addition and Subtraction. Evaluate the additions and
subtractions from left to right. Like above, addition
and subtraction are computed left to right in one
step. Completing the addition and subtraction in the
equation gives the following:
X = 161 3⁄4
A commonly used acronym, PEMDAS, is used for
remembering the order of operation in algebra. PEMDAS
is an acronym for parentheses, exponents, multiplication,
3-18division, addition, and subtraction. To remember it, many use
the sentence, “Please Excuse My Dear Aunt Sally.” Always
remember, however, to multiply/divide or add/subtract in
one sweep from left to right, not separately.
Order of Operation for Algebraic Equations
1. Parentheses
2. Exponents
3. Multiplication
4. Division
5. Addition
6. Subtraction
Computing Area of Two-Dimensional Solids
Area is a measurement of the amount of surface of an object.
Area is usually expressed in such units as square inches or
square centimeters for small surfaces or in square feet or
square meters for larger surfaces.
Figure 3-13 summarizes the formulas for computing the area
of two-dimensional solids.
Rectangle
A rectangle is a four-sided figure with opposite sides of
equal length and parallel to each other. [Figure 3-14] All of
the angles are right angles. A right angle is a 90° angle. The
rectangle is a very familiar shape in mechanics. The formula
for the area of a rectangle is:
area = length × width = l × w
Example: An aircraft floor panel is in the form of a rectangle
having a length of 24 inches and a width of 12 inches. What
is the area of the panel expressed in square inches? First,
determine the known values and substitute them in the formula.
a = l × w = 24 inches × 12 inches = 288 square inches
Square
A square is a four-sided figure with all sides of equal length
and opposite sides are parallel to each other. [Figure 3-15] All
angles are right angles. A right angle is a 90° angle. The
formula for the area of a square is:
area = length × width = l × w
Since the length and the width of a square are the same value,
the formula for the area of a square can also be written as:
area = side × side = s2
Example: What is the area of a square access plate whose side measures 25 inches? First, determine the known value
and substitute it in the formula.
a = l × w = 25 inches × 25 inches = 625 square inches
Triangle
A triangle is a three-sided figure. The sum of the three angles
in a triangle is always equal to 180°. Triangles are often
classified by their sides. An equilateral triangle has 3 sides
of equal length. An isosceles triangle has 2 sides of equal
length. A scalene triangle has three sides of differing lengths.
Triangles can also be classified by their angles. An acute
triangle has all three angles less than 90°. A right triangle
has one right angle (a 90° angle). An obtuse triangle has
one angle greater than 90°. Each of these types of triangles
is shown in Figure 3-16 .
The formula for the area of a triangle is
area = 1⁄2 × (base × height) = 1⁄2 × (b × h)
Example: Find the area of the obtuse triangle shown in
Figure 3-17 . First, substitute the known values in the
area formula.
a = 1⁄2 × (b × h) = 1⁄2 × (2'6" × 3'2")
Next, convert all dimensions to inches:
2'6" = (2 × 12") + 6" = (24 + 6) = 30 inches
3'2" = (3 × 12") + 2" = (36 + 2) = 38 inches
Now, solve the formula for the unknown value:
a = 1⁄2 × (30 inches × 38 inches) = 570 square inches
Parallelogram
A parallelogram is a four-sided figure with two pairs
of parallel sides. [Figure 3-18] Parallelograms do not
necessarily have four right angles.
The formula for the area of a parallelogram is:
area = length × height = l × h
Trapezoid
A trapezoid is a four-sided figure with one pair of parallel
sides. [Figure 3-19] The formula for the area of a trapezoid is:
area = 1⁄2 (base 1 + base 2) × height
3-19w = 12l = 24
s = 25
Figure 3-14. Rectangle. Figure 3-15. Square. Object Area Formula Figure
Rectangle
Square
Parallelogram
Trapezoid
Circle
Ellipse
Wing areaTriangle3-14
3-17a = l × w
a = l × w or a = s2
a = l × h
a = ½ (b1 + b2) × h
a = π × r2
a = π × a × b
a = s × ca = ½ (l × h) or
a = ½ (b × h) or
a = (b × h) ÷ 2length × width
length × width or side × side
length × height
½ (base1 + base2) × height
π × radius2
π × semi-axis A × semi-axis B
span × mean chord½ × (length × height) or
½ × (base × height) or
(base × height) ÷ 2
Figure 3-13. Formulas to compute area.
Example: What is the area of a trapezoid in Figure 3-19 whose
bases are 14 inches and 10 inches, and whose height (or
altitude) is 6 inches? First, substitute the known values in
the formula.
a = 1⁄2 (b1 + b 2) × h
= 1⁄2 (14 inches + 10 inches) × 6 inches
a = 1⁄2 (24 inches) × 6 inches
= 12 inches × 6 inches = 72 square inches
Circle
A circle is a closed, curved, plane figure. [Figure 3-20] Every
point on the circle is an equal distance from the center of the
circle. The diameter is the distance across the circle (through
the center). The radius is the distance from the center to the
edge of the circle. The diameter is always twice the length
of the radius. The circumference, or distance around, a circle
is equal to the diameter times π.
circumference = c = d π
The formula for the area of a circle is:area = π × radius2 = π × r2
Example: The bore, which is “inside diameter,” of a certain
aircraft engine cylinder is 5 inches. Find the area of the cross
section of the cylinder.
First, substitute the known values in the formula:
a = π × r2
The diameter is 5 inches, so the radius is 2.5 inches.
(diameter = radius × 2)
a = 3.1416 × (2.5 inches)2 = 3.1416 × 6.25 square
inches = 19.635 square inches
Ellipse
An ellipse is a closed, curved, plane figure and is commonly
called an oval. [Figure 3-21] In a radial engine, the
articulating rods connect to the hub by pins, which travel in
the pattern of an ellipse (i.e., an elliptical or orbital path).
3-20Base = 3 ft 2 inHeight = 2 ft 6 in
b1 = 14"b2 = 10"
Height = 6"LengthHeight
Figure 3-17. Obtuse triangle. Figure 3-19. Trapezoid.Figure 3-18. Parallelogram.Equilateral IsoscelesTriangles Based on Sides
Triangles Based on AnglesScalene
Acute Right ObtuseLength of all sides
are differentLength of two sides
are equalLength of all sides
are equal
Each angle is < 90° One angle is = 90° One angle is > 90°
Figure 3-16. Types of triangles.
Wing Area
To describe the shape of a wing [Figure 3-22], several terms
are required. To calculate wing area, it is necessary to know the meaning of the terms “span” and “chord.” The wingspan, S, is the length of the wing from wingtip to wingtip. The chord is the average width of the wing from leading edge to trailing edge. If the wing is a tapered wing, the average width, known as the mean chord (C), must be known to find the area. The formula for calculating wing area is:
area of a wing = span × mean chord
Example: Find the area of a tapered wing whose span is 50 feet and whose mean chord is 6'8". First, substitute the known values in the formula.
a = s × c
= 50 feet × 6 feet 8 inches
(Note: 8 inches = 8⁄12 feet = 0.67 feet)
= 50 feet × 6.67 feet
= 333.5 square feetUnits of Area
A square foot measures 1 foot by 1 foot. It also measures 12
inches by 12 inches. Therefore, one square foot also equals 144 square inches (that is, 12 × 12 = 144). To convert square feet to square inches, multiply by 144. To convert square inches to square feet, divide by 144.
A square yard measures 1 yard by 1 yard. It also measures 3
feet by 3 feet. Therefore, one square yard also equals 9 square feet (that is, 3 × 3 = 9). To convert square yards to square feet, multiply by 9. To convert square feet to square yards, divide by 9. Refer to Figure 3-23, Applied Mathematics Formula
Sheet, for a comparison of different units of area.
Computing Volume of Three-Dimensional
Solids
Three-dimensional solids have length, width, and height.
There are many three-dimensional solids, but the most common are rectangular solids, cubes, cylinders, spheres, and cones. V olume is the amount of space within a solid. V olume is expressed in cubic units. Cubic inches or cubic centimeters are used for small spaces and cubic feet or cubic meters for larger spaces.
Rectangular Solid
A rectangular solid is a three-dimensional solid with six
rectangular-shaped sides. [Figure 3-24] The volume is the
3-21Circumference
Diameter (d)
Radius (r)b
a
π = 3.1416
a = Length of one of the semi-axes
b = Length of the other semi-axis
Area = a = π x a x bCircumference = c = 2π a2 + b2
Figure 3-20. Circle.
Figure 3-21. Ellipse.
sc
a = Wing area, ft.2
c = Average chord, ft.
s = Span, ft.
Figure 3-22. Wing planform.number of cubic units within the rectangular solid. The
formula for the volume of a rectangular solid is:
volume = length × width × height = l × w × h
In Figure 3-24 , the rectangular solid is 3 feet by 2 feet by
2 feet.
The volume of the solid in Figure 3-24 is = 3 ft × 2 ft ×
2 ft = 12 cubic feet.
Example: A rectangular baggage compartment measures 5 feet
6 inches in length, 3 feet 4 inches in width, and 2 feet 3 inches
in height. How many cubic feet of baggage will it hold? First,
substitute the known values into the formula.
v = l × w × h
= 5'6" × 3'4" × 2'3"
= 5.5 ft × 3.33 ft × 2.25 ft
= 41.25 cubic feet
Cube
A cube is a solid with six square sides. [Figure 3-25] A cube
is just a special type of rectangular solid. It has the same
formula for volume as does the rectangular solid, which is
volume = length × width × height = L × W × H. Because all
of the sides of a cube are equal, the volume formula for a
cube can also be written as:
volume = side × side × side = S3
Example: A large, cube-shaped carton contains a shipment
of smaller boxes inside of it. Each of the smaller boxes is
1 ft × 1 ft × 1 ft. The measurement of the large carton is
3 ft × 3 ft × 3 ft. How many of the smaller boxes are in the large
carton? First, substitute the known values into the formula.
v = l × w × h
= 3 ft × 3 ft × 3 ft = 27 cubic feet of volume in the
large carton
Since each of the smaller boxes has a volume of 1 cubic foot,
the large carton holds 27 boxes.
Cylinder
A solid having the shape of a can, a length of pipe, or a barrel
is called a cylinder. [Figure 3-26] The ends of a cylinder are
identical circles. The formula for the volume of a cylinder is:
volume = π × radius2 × height of the cylinder = π r2 × h
One of the most important applications of the volume of
a cylinder is finding the piston displacement of a cylinder
in a reciprocating engine. Piston displacement is the total
volume (in cubic inches, cubic centimeters, or liters) swept
by all of the pistons of a reciprocating engine as they move
in one revolution of the crankshaft. The formula for piston
displacement is given as:
Piston Displacement =
π × (bore divided by 2)2 × stroke × (# cylinders)
3-22Length
2.54 centimeters
12 inches
3 feet
5,280 feet
0.0394 inches
0.62 miles25.4 millimeters
30.48 centimeters
0.9144 meters
1,760 yards1 inch
1 foot
1 yard
1 mile
1 millimeter
1 kilometer
Weight
1 ounce 28.350 grams
1 pound 16 ounces 453.592 grams 0.4536 kilograms
1 ton 2,000 pounds
1 milligram 0.001 grams
1 kilogram 1,000 grams 2.2 pounds
1 gram 0.0353 ouncesArea
6.45 square centimeters
144 square inches
9 square feet
43,560 square feet
640 acres
0.155 square inches
1.195 square yards
0.384 square miles0.093 square meters
0.836 square meters
2.59 square kilometers1 square inch
1 square foot
1 square yard
1 acre
1 square mile
1 square centimeter
1 square meter
1 square kilometer
Volume
1 fluid ounce 29.57 cubic centimeters
1 cup 8 fluid ounces
1 pint 2 cups 16 fluid ounces 0.473 liters
1 quart 2 pints 4 cups 32 fluid ounces 0.9463 liters
1 gallon 4 quarts 8 pints 16 cups 128 ounces 3.785 liters
1 gallon 231 cubic inches
1 liter 0.264 gallons 1.057 quarts
1 cubic foot 1,728 cubic inches 7.5 gallons
1 cubic yard 27 cubic feet
1 board foot 1 inch x 12 inches x 12 inches
Temperature
°F to °C
°C to °FCelsius = ⁵⁄9 × (°F − 32)
Fahrenheit = ⁹⁄5 × (°C + 32)Conversion Factors
Figure 3-23. Applied mathematics formula sheet.The bore of an engine is the inside diameter of the cylinder.
The stroke of the engine is the length the piston travels inside
the cylinder. [Figure 3-27]
Example: Find the piston displacement of one cylinder in a
multi-cylinder aircraft engine. The engine has a cylinder bore
of 5.5 inches and a stroke of 5.4 inches. First, substitute the
known values in the formula.
v = π × r2 × h = (3.1416) × (5.5 ÷ 2)2 × (5.4)
v = 23.758 × 5.4 = 128.29 cubic inches
The piston displacement of one cylinder is 128.29 cubic inches. For an eight-cylinder engine, then the total engine
displacement would be:
Total displacement for 8 cylinders = 8 × 128.29 =
1026.32 cubic inches of displacement
Sphere
A solid having the shape of a ball is called a sphere.
[Figure 3-28] A sphere has a constant diameter. The radius
(r) of a sphere is one-half of the diameter (d). The formula
for the volume of a sphere is given as:
3-23Figure 3-23. Applied mathematics formula sheet (continued).
H
LWL = 3
W= 2H = 2
Figure 3-24. Rectangular solid.
s
ss
Figure 3-25. Cube. Solid Volume Surface Area Figure
1-261-281-29l × w × h
s
π × r2 × h
/four.numerator⁄/three.denominator × π × r3
/one.numerator⁄/three.denominator × π × r2 × h2 × [(w × l) + (w × h) +
(l × h)]
6 × s2
2 × π × r2 + π × d × h
4 × π × r2
π × r × [r + (r2 + h2)½]Rectangle
Solid
Cube
Cylinder
Sphere
ConeFormulas for Area of Two-Dimensional Objects Order of Operation for Algebraic Equations
Names and Symbols for Metric Prefixes
Powers of TenPrefix Means
exa (1018)
peta (1015)
tera (1012)
giga (109)
mega (106)
kilo (103)
hecto (102)
deca (101)
deci (10−1)
centi (10−2)
milli (10−3)
micro (10−6)
nano (10−9)
pico (10−12)One quintillion times
One quadrillion timesOne trillion timesOne billion timesOne million timesOne thousand timesOne hundred timesTen timesOne tenth ofOne hundredth ofOne thousandth ofOne millionth ofOne billionth ofOne trillionth of
Powers
of TenExpansion Value
Positive
Exponents
1001,000,000
100,00010,0001,00010010110 x 10 x 10 x 10 x 10 x 1010 x 10 x 10 x 10 x 1010 x 10 x 10 x 1010 x 10 x 1010 x 1010
Negative
Exponents
10-61/10=0.1
1/100=0.011/1,000=0.0011/10,000=0.00011/100,000=0.000011/1,000,000=0.0000011/101/(10 x 10)1/(10 x 10 x 10)1/(10 x 10 x 10 x 10)1/(10 x 10 x 10 x 10 x 10)1/(10 x 10 x 10 x 10 x 10 x 10)1. P arentheses
2. Exponents
3. Multiplication
4. Division
5. Addition
6. Subtraction
c
ba
CB
A
c
ba
a2 + b2 = c2Trigonometric Equations
Pythagorean TheoremUse the acronym PEMDAS to remember the order of
operation in algebra. PEMDAS is an acronym for parentheses, exponents, multiplication, division, addition, and subtraction. To remember it, many use the sentence, “Please Excuse My Dear Aunt Sally.”
Sine (sin) of angle A =opposite side (side a)
hypotenuse (side c)
Cosine (cos) of angle A =adjacent side (side b)
hypotenuse (side c)
Tangent (tan) of angle A =opposite side (side a)
adjacent side (side b)
v = 4⁄3 × π × radius3 = 4⁄3 × π × r3 or v = 1⁄6 × πd3
Example: A pressure tank inside the fuselage of a cargo
aircraft is in the shape of a sphere with a diameter of 34 inches. What is the volume of the pressure tank?v =
4⁄3 × π × radius3 = 4⁄3 × (3.1416) × (34⁄2)3
= 1.33 × 3.1416 × 173 = 1.33 × 3.1416 × 4,913
v = 20,528.125 cubic inches
Cone
A solid with a circle as a base and with sides that gradually
taper to a point is called a cone. [Figure 3-29] The formula
for the volume of a cone is given as:
v = 1⁄3 × π × radius2 × height = 1⁄3 × π × r2 × h
Units of Volume
Since all volumes are not measured in the same units, it is
necessary to know all the common units of volume and how they are related to each other. For example, the mechanic may know the volume of a tank in cubic feet or cubic inches, but when the tank is full of gasoline, they are interested in how many gallons it contains. Refer to Figure 3-23 , Applied
Mathematics Formula Sheet, for a comparison of different units of volume.
Computing Surface Area of Three-
Dimensional Solids
The surface area of a three-dimensional solid is the sum of
the areas of the faces of the solid. Surface area is a different
hr
diameter
bore
pistond
pistond
Piston at top center Piston at bottom center
H=stroke
cylinder
Figure 3-26. Cylinder.Figure 3-27. Cylinder displacement.concept from that of volume. For example, surface area is the
amount of sheet metal needed to build a rectangular fuel tank
while volume is the amount of fuel that the tank can contain.
Rectangular Solid
The formula for the surface area of a rectangular solid
[Figure 3-24] is given as:
Surface area =
2 × [(width × length) + (width × height) + (length × height)]
= 2 × [(w × l) + (w × h) + (l × h)]
Cube
The formula for the surface area of a cube [Figure 3-25] is
given as:
Surface area = 6 × (side × side) = 6 × s2
Example: What is the surface area of a cube with a side
measure of 8 inches?
Surface area = 6 × (side × side)
= 6 × S2 = 6 × 82 = 6 × 64
= 384 square inches
Cylinder
The formula for the surface area of a cylinder [Figure 3-26] is
given as:
Surface area = 2 × π × radius2 + π × diameter × height
= 2 × π × r2 + π × d × hSphere
The formula for the surface area of a sphere [Figure 3-28] is
given as:
Surface area = 4 × π × radius2 = 4 × π × r2
Cone
The formula for the surface area of a right circular cone
[Figure 3-29] is given as:
Surface area = π × radius × [radius + √(radius2 + height2)]
= π × r × [r + √(r2 + h2)]
Figure 3-30 summarizes the formulas for computing the
volume and surface area of three-dimensional solids.
Trigonometric Functions
Trigonometry is the study of the relationship between the
angles and sides of a triangle. The word trigonometry comes
from the Greek trigonon, which means three angles, and
metro, which means measure.
Right Triangle, Sides, and Angles
In Figure 3-31 , notice that each angle is labeled with a
capital letter. Across from each angle is a corresponding
side, each labeled with a lower case letter. This triangle is
a right triangle because angle C is a 90° angle. Side “a” is
Figure 3-28. Sphere.
hs
r
Figure 3-29. Cone.opposite from angle A and is sometimes referred to as the
“opposite side.” Side “b” is next to, or adjacent to, angle
A and is therefore referred to as the “adjacent side.” Side
“c” is always across from the right angle and is referred to
as the “hypotenuse.”
Sine, Cosine, and Tangent
The three primary trigonometric functions and their
abbreviations are: sine (sin), cosine (cos), and tangent
(tan). These three functions can be found on most scientific
calculators. The three trigonometric functions are actually
ratios comparing two of the sides of the triangle as follows:
Sine (sin) of angle A = opposite side (side a)
hypotenuse (side c)
Cosine (cos) of angle A = adjacent side (side b)
hypotenuse (side c)
Tangent (tan) of angle A = opposite side (side a)
adjacent side (side b)
Example: Find the sine of a 30° angle.
Calculator Method:
Using a calculator, select the “sin” feature, enter the number
30, and press “enter.” The calculator should display the
answer as 0.5. This means that when angle A equals 30°, then
the ratio of the opposite side (a) to the hypotenuse (c) equals
0.5 to 1, so the hypotenuse is twice as long as the opposite
side for a 30° angle. Therefore, sin 30° = 0.5.
Trigonometry Table Method:
When using a trigonometry table, find 30° in the first column.
Next, find the value for sin 30° under the second column
marked “sine” or “sin.” The value for sin 30° should be 0.5.Pythagorean Theorem
The Pythagorean Theorem is named after the ancient Greek
mathematician, Pythagoras (~500 B.C.). This theorem is used
to find the third side of any right triangle when two sides are
known. The Pythagorean Theorem states that a2 + b2 = c2.
[Figure 3-32] Where “c” = the hypotenuse of a right triangle, “a”
is one side of the triangle and “b” is the other side of the triangle.
Example: What is the length of the longest side of a right
triangle, given the other sides are 7 inches and 9 inches?
The longest side of a right triangle is always side “c,” the
hypotenuse. Use the Pythagorean Theorem to solve for the
length of side “c” as follows:
a2 + b2 = c2
72 + 92 = c2
49 + 81 = c2
130 = c2
c = 130 = 11.4 inches
Therefore, side “c” = 11.4 inches.
Example: The cargo door opening in a military airplane is
a rectangle that is 51⁄2 feet tall by 7 feet wide. A section of
square steel plate that is 8 feet wide by 8 feet tall by 1 inch
thick must fit inside the airplane. Can the square section of
steel plate fit through the cargo door? It is obvious that the
square steel plate will not fit horizontally through the cargo
door. The steel plate is 8 feet wide and the cargo door is only
7 feet wide. However, if the steel plate is tilted diagonally,
will it fit through the cargo door opening?
The diagonal distance across the cargo door opening can be
calculated using the Pythagorean Theorem where “a” is the
cargo door width, “b” is the cargo door height, and “c” is the
diagonal distance across the cargo door opening.
a2 + b2 = c2
3-26 Solid Volume Surface Area Figure
3-29l × w × h
s3
π × r2 × h
⁴⁄3 × π × r3
¹⁄3 × π × r2 × h2 × [(w × l) + (w × h) +
(l × h)]
6 × s2
2 × π × r2 + π × d × h
4 × π × r2
π × r × [r + √(r2 + h2)]Rectangle
Solid
Cube
Cylinder
Sphere
Cone
c
ba
CB
Ac
ba
a2 + b2 = c2Figure 3-30. Formulas to compute volume and surface area.
Figure 3-31. Right triangle. Figure 3-32. Pythagorean Theorem. (7 ft)2 + (5.5 ft)2 = c2
49 + 30.25 = c2
79.25 = c2
c = 8.9 ft
The diagonal distance across the cargo door opening is 8.9
feet, so the 8-foot wide square steel plate fits diagonally
through the cargo door opening and into the airplane.
Measurement Systems
Conventional (U.S. or English) System
Our conventional (U.S. or English) system of measurement is
part of our cultural heritage from the days when the thirteen
colonies were under British rule. It started as a collection
of Anglo-Saxon, Roman, and Norman-French weights and
measures. For example, the inch represents the width of the
thumb and the foot is from the length of the human foot.
Tradition holds that King Henry I decreed that the yard should
be the distance from the tip of his nose to the end of his thumb.
Since medieval times, commissions appointed by various
English monarchs have reduced the chaos of measurement
by setting specific standards for some of the most important
units. Some of the conventional units of measure are: inches,
feet, yards, miles, ounces, pints, gallons, and pounds. Because
the conventional system was not set up systematically, it
contains a random collection of conversions. For example,
1 mile = 5,280 feet and 1 foot = 12 inches.
Metric System
The metric system, also known as the International System
of Units (SI), is the dominant language of measurement used today. Its standardization and decimal features make it well-
suited for engineering and aviation work.
The metric system was first envisioned by Gabriel Mouton,
Vicar of St. Paul’s Church in Lyons, France. The meter is
the unit of length in the metric system, and it is equal to one
ten-millionth of the distance from the equator to the North
Pole. The liter is the unit of volume and is equal to one cubic
decimeter. The gram is the unit of mass and is equal to one
cubic centimeter of water.
All of the metric units follow a consistent naming scheme,
which consists of attaching a prefix to the unit. For example,
since kilo stands for 1,000, one kilometer equals 1,000 meters.
Centi is the prefix for one hundredth, so one meter equals one
hundred centimeters. Milli is the prefix for one thousandths
and one gram equals one thousand milligrams. Refer to
Figure 3-33 for the names and definitions of metric prefixes.
Measurement Systems & Conversions
The United States primarily uses the conventional (U.S.
or English) system, although it is slowly integrating the
metric system (SI). A recommendation to transition to the
metric system within ten years was initiated in the 1970s.
However, this movement lost momentum, and the United
States continues to use both measurement systems. Therefore,
information to convert between the conventional (U.S.
or English) system and the metric (SI) system has been
included in Figure 3-23 , Applied Mathematics Formula
Sheet. Examples of its use are as follows:
To convert inches to millimeters, multiply the number of
inches by 25.4.
Example: 20 inches = 20 × 25.4 = 508 mm
To convert ounces to grams, multiply the number of ounces
by 28.35.
Example: 12 ounces = 12 × 28.35 = 340.2 grams
3-27PrefixMultiplier
(Exponential)Multiplier
(Numerical)Meaning Symbol
exa (1018)
peta (1015)
tera (1012)
giga (109)
mega (106)
kilo (103)
hecto (102)
deca (101)
unit
deci (10−1)
centi (10−2)
milli (10−3)
micro (10−6)
nano (10−9)
pico (10−12)
femto (10−15)
atto (10−18) quintillion
quadrillion
trillion
billion
million
thousand
hundred
ten
tenth
hundredth
thousandth
millionth
billionth
trillionth
quadrillionth
quintillionth1,000,000,000,000,000,000
1,000,000,000,000,000
1,000,000,000,000
1,000,000,000
1,000,000
1,000
0.000,001
0.000,000,001
0.000,000,000,001
0.000,000,000,000,001
0.000,000,000,000,000,001E
P
T
G
M
k
h
da
d
c
m
µ
n
p
f
aLess Than 1Greater Than 1
Figure 3-33. Names and definitions of metric prefixes.The Binary Number System
The binary number system has only two digits: 0 and 1. The
prefix in the word “binary” is a Latin root for the word “two”
and its use was first published in the late 1700s. The use of
the binary number system is based on the fact that switches
or valves have two states: open or closed (on/off).
Currently, one of the primary uses of the binary number
system is in computer applications. Information is stored as
a series of 0s and 1s, forming strings of binary numbers. An
early electronic computer, ENIAC ( Electronic Numerical
Integrator and Calculator), was built in 1946 at the University
of Pennsylvania and contained 17,000 vacuum tubes, along
with 70,000 resistors, 10,000 capacitors, 1,500 relays, 6,000
manual switches and 5 million soldered joints. Computers
obviously have changed a great deal since then, but are still
based on the same binary number system. The binary number
system is also useful when working with digital electronics
because the two basic conditions of electricity, on and off,
can be represented by the two digits of the binary number
system. When the system is on, it is represented by the digit
1, and when it is off, it is represented by the digit 0.
Place Values
The binary number system is a Base-2 system. That is,
the place values in the binary number system are based on
powers of 2. An 8-bit binary number system is shown in
Figure 3-34 .
Converting Binary Numbers to Decimal Numbers
To convert a binary number to a decimal number, add up the
place values that have a 1 (place values that have a zero do
not contribute to the decimal number conversion).Example: Convert the binary number 10110011 to a decimal
number. Using the place value chart in Figure 3-35 , add up
the place values of the ‘1s’ in the binary number (ignore the
place values with a zero in the binary number).
The binary number 10110011
= 128 + 0 + 32 + 16 + 0 + 0 + 2 + 1
= 179 in the decimal number system
Converting Decimal Numbers to Binary Numbers
To convert a decimal number to a binary number, the place
values in the binary system are used to create a sum of
numbers that equal the value of the decimal number being
converted. Start with the largest binary place value and
subtract from the decimal number. Continue this process
until all of the binary digits are determined.
Example: Convert the decimal number 233 to a binary number.
Start by subtracting 128 (the largest place value from the
8-bit binary number) from 233.
233 – 128 = 105 A “1” is placed in the first binary
digit space: 1XXXXXXX.
Continue the process of subtracting the binary number
place values:
105 – 64 = 41 A “1” is placed in the second binary
digit space: 11XXXXXX.
41 – 32 = 9 A “1” is placed in the third binary
digit space: 111XXXXX.
3-28Place Value
or 12826
or 6425
or 3224
or 1623
or 822
or 421
or 220
or 1
1 0 0 1 1 0 0 1
0 0 1 0 1 0 1 1 10011001 shown as
00101011 shown as=153
=43
Figure 3-34. Binary system.
Place Value
or 12826
or 6425
or 3224
or 1623
or 822
or 421
or 220
or 1
1 0 1 1 0 0 1 1
128 + 0 + 32 + 16 + 0 + 0 + 2 + 110110011 shown as
=179
Figure 3-35. Place value chart.
Place Value
or 12826
or 6425
or 3224
or 1623
or 822
or 421
or 220
or 1
0 0 1 0 0 0 1 1
0 1 1 1 1 1 0 0
0 1 1 0 0 0 0 0
1 1 1 1 1 1 1 1
1 1 1 0 1 0 0 135 shown as
124 shown as
96 shown as
255 shown as
233 shown as
Figure 3-36. Conversion from decimal number to binary number.Since 9 is less than 16 (the next binary place value), a “0” is
placed in the fourth binary digit space, 1110XXXX.
9 – 8 = 1 A “1” is placed in the fifth binary
digit space: 11101XXX
Since 1 is less than 4 (the next binary place value), a 0 is
placed in the sixth binary digit space: 111010XX.
Since 1 is less than 2 (the next binary place value), a 0 is
placed in the seventh binary digit space: 1110100X.
1 – 1 = 0 A “1” is placed in the eighth binary
digit space: 11101001.
The decimal number 233 is equivalent to the binary number
11101001, as shown in Figure 3-36 .
Three additional decimal number to binary number
conversions are shown in Figure 3-36.
Aircraft Drawings
Chapter 4
Introduction
The exchange of ideas is essential to everyone, regardless of
their vocation or position. This exchange is usually carried on
by oral or written word; but under some conditions, the use of
these alone is impractical. The aviation industry discovered
that it could not depend entirely upon written or spoken
words for the exchange of ideas, because misunderstanding
and misinterpretation arose frequently. A written description
of an object can be changed in meaning just by misplacing
a comma, and the meaning of an oral description can be
completely changed by using a wrong word. To avoid these
possible errors, drawings are used to describe objects. For
this reason, drawing is the draftsman’s language.
Drawing, in the aviation industry, is a method of conveying
ideas concerning the construction or assembly of objects.
This is done with the help of lines, notes, abbreviations, and
symbols. It is important that the aviation mechanic who is
to make or assemble the object understands the meaning of
the different lines, notes, abbreviations, and symbols that
are used in a drawing. (See the “Lines and Their Meanings”
section of this chapter.)
Computer Graphics
From the early days of aviation, development of aircraft,
aircraft engines, and other components relied heavily on
aircraft drawings. For most of the 20th century, drawings
were created on a drawing “board” with pen or pencil and
paper. With the introduction and advancement of computers
in the later decades of the 20th century, the way drawings
are created changed dramatically. Computers were used
not only to create drawings, but they were being used to
show items in “virtual reality,” from any possible viewing
angle. Further development of computer software programs
allowed for assembling of separately created parts to check
for proper fit and possible interferences. Additionally, with
nearly instantaneous information sharing capability through
computer networking and the Internet, it became much easier
for designers to share their work with other designers and
manufacturers virtually anytime, anywhere in the world.
Using new computer-controlled manufacturing techniques,
it became possible to design a part and have it precisely
manufactured without ever having shown it on paper. New
terms and acronyms became commonplace. The more
common of these terms are:
• Computer Graphics—drawing with the use of a
computer• Computer Aided Design (CAD)—where a computer
is used in the design of a part or product
• Computer Aided Design Drafting (CADD)—where a
computer is used in the design and drafting process
• Computer Aided Manufacturing (CAM)—where a
computer is used in the manufacturing of a part or
product
• Computer Aided Engineering (CAE)—where a
computer is used in the engineering of a part or product
As computer hardware and software continue to evolve, a
greater amount of CAE is completed in less time, at lower
cost. In addition to product design, some of the other uses
of CAE are product analysis, assembly, simulations, and
maintenance information. [Figure 4-1]
CATIA, ProEngineer, Solid Works, and Unigraphics are some
of the more popular CAD software packages used for aircraft
design and manufacturing. Most airframe manufacturers use
CATIA software to design their aircraft. The complete aircraft
is designed and assembled in the software package before
it is manufactured. Drawings of all parts of the aircraft are
available and can be accessed using the computer software.
Drawings are no longer limited to 1, 2, or 3 views. Drawings
from every angle can easily be accessed by using the
computer model of the part or product. Technicians can access
drawings and aircraft manuals on laptops or even mobile
devices when performing maintenance on the shop floor.
Purpose & Function of Aircraft Drawings
Drawings and prints are the link between the engineers who
design an aircraft and the workers who build, maintain, and
repair it. A print may be a copy of a working drawing for
an aircraft part or group of parts, or for a design of a system
or group of systems. They are made by placing a tracing of
the drawing over a sheet of chemically-treated paper and
exposing it to a strong light for a short period of time. When
the exposed paper is developed, it turns blue where the light
has penetrated the transparent tracing. The inked lines of the
tracing, having blocked out the light, show as white lines
on a blue background. With other types of sensitized paper,
prints may have a white background with colored lines or a
colored background with white lines.
Drawings created using computers may be viewed on the
computer monitor or printed out in “hard copy” by use of an
Figure 4-1. Computer graphics work station.ink jet or laser printer. Larger drawings may be printed by use
of a plotter or large format printer. Large printers can print
drawings up to 42 inches high with widths up to 600 inches
by use of continuous roll paper. [Figure 4-2]
Care & Use of Drawings
Drawings should be handled carefully as they are both
expensive and valuable. Open drawings slowly and carefully
to prevent tearing of the paper. When the drawing is open,
smooth out the fold lines instead of bending them backward.
To protect drawings from damage, never spread them on the
floor or lay them on a surface covered with tools or other
objects that may make holes in the paper. Hands should be
free of oil, grease, or other unclean matter that can soil or
smudge the print.
Never make notes or marks on a print, as they may confuse
others and lead to incorrect work. Only authorized individuals
are permitted to make notes or changes on prints, and they
must sign and date any changes they make.
When finished with a drawing, fold and return it to its proper
place. Prints are folded originally in an appropriate size for
filing. Care should be taken so that the original folds are
always used.
Types of Drawings
Drawings must give information such as size and shape of
the object and all its parts, specifications for material to be
used, how the material is to be finished, how the parts are to
be assembled, and any other information essential to making
and assembling the object. Drawings may be divided into
three classes: detail, assembly, and installation.
Detail Drawing
A detail drawing is a description of a single part, describing
bylines, notes, and symbols the specifications for size, shape,
material, and methods of manufacture to be used in making
the part. Detail drawings are usually rather simple. When
single parts are small, several detail drawings may be shown
on the same sheet or print. [Figure 4-3]
Assembly Drawing
An assembly drawing is a description of an object made up
of two or more parts. [Figure 4-4] It describes the object’s
size and shape. Its primary purpose is to show the relationship
of the various parts. An assembly drawing is usually more
complex than a detail drawing and is often accompanied by
detail drawings of various parts.Installation Drawing
An installation drawing is one that includes all necessary
information for a part or an assembly in the final installed
position in the aircraft. It shows the dimensions necessary
for the location of specific parts with relation to the other
parts and reference dimensions that are helpful in later work
in the shop. [Figure 4-5]
Sectional View Drawings
A section or sectional view is obtained by cutting away part
of an object to show the shape and construction at the cutting
plane. The part or parts cut away are shown by using section
(crosshatching) lines. Types of sections are described in the
following paragraphs.
Full Section
A full section view is used when the interior construction
or hidden features of an object cannot be shown clearly by
exterior views. For example, Figure 4-6 is a sectional view
of a cable connector and shows the internal construction of
the connector.
Half Section
In a half section, the cutting plane extends only halfway
across the object, leaving the other half of the object as an
exterior view. Half sections are used with symmetrical objects
to show both the interior and exterior. Figure 4-7 is a half
sectional view of a Capstan servo.
Revolved Section
A revolved section drawn directly on the exterior view shows
the shape of the cross section of a part, such as the spoke
of a wheel. An example of a revolved section is shown in
Figure 4-8 .
Figure 4-2. Large format printer.
Removed Section
A removed section illustrates parts of an object. It is drawn
like revolved sections, except it is placed at one side and
often drawn to a larger scale than the view indicated to bring
out pertinent details.
Figure 4-9 is an illustration of removed sections. Section
A-A shows the cross-sectional shape of the object at cutting
plane line A-A. Section B-B shows the cross-sectional shape
at cutting plane line B-B. These sectional views are drawn
to the same scale as the principal view.
Title Blocks
Every print must have some means of identification.
This is provided by a title block. [Figure 4-4A] The title
block consists of a drawing number and certain other data
concerning the drawing and the object it represents. This
information is grouped in a prominent place on the print,
usually in the lower right-hand corner. Sometimes the title
block is in the form of a strip extending almost the entire
distance across the bottom of the sheet.
Although title blocks do not follow a standard form as far
as layout is concerned, all of them present essentially the
following information:
1. A drawing number to identify the print for filing
purposes and to prevent confusing it with any
other print.
2. The name of the part or assembly
3. The drawing scale
4. The date
5. The name of the firm
6. The name of the draftsmen, the checker, and the person
approving the drawingDrawing or Print Numbers
All prints are identified by a number that appears in a number
block in the lower right corner of the title block. It may also
be shown in other places—such as near the top border line,
in the upper right corner, or on the reverse side of the print at
both ends—so that the number shows when the print is folded
or rolled. The purpose of the number is quick identification of
a print. If a print has more than one sheet and each sheet has
the same number, this information is included in the number
block, indicating the sheet number and the number of sheets
in the series. [Figure 4-4B]
Reference and Dash Numbers
Reference numbers that appear in the title block refer you
to the numbers of other prints. When more than one detail
is shown on a drawing, dash numbers are used. Both parts
would have the same drawing number plus an individual
number, such as 40267-1 and 40267-2.
In addition to appearing in the title block, dash numbers
may appear on the face of the drawing near the parts they
identify. Dash numbers are also used to identify right-hand
and left-hand parts.
In aircraft, many parts on the left side are like the
corresponding parts on the right side but in reverse. The left-
hand part is always shown in the drawing. The right-hand
part is called for in the title block. Above the title block a
notation is found, such as: 470204-1LH shown; 470204-2RH
opposite. Both parts carry the same number, but the part
called for is distinguished by a dash number. Some prints
have odd numbers for left-hand parts and even numbers for
right-hand parts.
Universal Numbering System
The universal numbering system provides a means of
identifying standard drawing sizes. In the universal
numbering system, each drawing number consists of six or
seven digits. The first digit is always 1, 2, 4, or 5 and indicates
the size of the drawing. The number 1 indicates a drawing of
8½" × 11"; number 2 indicates an 11" × 17" drawing; number
4 represents a drawing of 17" × 22"; and 5 indicates a width
of between 17 and 36 inches but on a continuous roll. Letters
are also used (and becoming more prevalent) with the most
common letters being A through E. The letter A is 8½" × 11",
B is 11" × 17", C is 17" × 22", D is 22" × 34" and E is 34"
× 44". There are additional letters, such as D1 at 24" × 36",
E1 at 30" × 42" and additional sizes unique to even larger
formats but generally reserved for inter-company operations.
The remaining digits identify the drawing. Many firms have
modified this basic system to conform to their needs. The
letter or number depicting the standard drawing size may be
4-4FLAT PATTERN
FOR REFERENCE ONLY
-03D
C
B
A ABCD
1 2 3 4 5 6 7 88 7 6 5 4 3 2 1
REVISIONS
FOR REVISION HISTORY SEE SHEET 1
SHEET 3 OF 3SWorks Drawing No.: SIZE
Figure 4-3. Detail drawing.
2-01REVISIONS
ITEM8 6 S-TEC
5 S-TEC
12 S-TEC
1 3 -03 S-TEC
1 2 -02 S-TEC
1 -01 S-TECD
C
A ABCD
1 2 3 4 5 6 7 88 7 6 5 4 3 2 1
TITLE:
SIZEDrawing No.:
SHEET 1 OF 3EC130T2
125B
CWHITWORTH
FORM (SWorks) 86359 REV -
Zone Numbers F
Allowances and ToleranceG
C. Bill of MaterialsC
Title BlockA
ScaleH
Drawing NumberB
Revision BlockD
NotesE
Figure 4-4. Assembly drawing.
4-6REF
REF1
2C
8(REF)
REF8
REF
1 REF
6D
C
B
A ABCD
1 2 3 4 5 6 7 88 7 6 5 4 3 2 1
REVISIONS
FOR REVISION HISTORY SEE SHEET 1
SHEET 3 OF 3SWorks Drawing No.: SIZEFORM 86359 REV -REV
Figure 4-5. Installation drawing.
4-7Figure 4-6. Sectional view of a cable connector.
Figure 4-7. Half section of a Capstan servo. Figure 4-8. Revolved sections.prefixed to the number, separated from it by a dash. Other
numbering systems provide a separate box preceding the
drawing number for the drawing size identifier. In another
modification of this system, the part number of the depicted
assembly is assigned as the drawing number.Drawing Standards
Drawing standards cover such items as paper sizes, notes,
numbering systems, geometric dimensions and tolerances,
abbreviations, welding symbols, roughness symbols, and
electrical symbols. These standards cover metric and inch
measurements, as well as computer-drafting standards.
Different standards for drawings are used in industry and
some of the more common ones are published by the
International Organization for Standardization (ISO) and the
American National Standards Institute (ANSI).
Bill of Material
A list of the materials and parts necessary for the fabrication
or assembly of a component or system is often included
on the drawing. The list is usually in ruled columns that
4-8A AB B
SECTION A-ASECTION B-B
Figure 4-9. Removed sections.
provide the part number, name of the part, material the part
is to be constructed of, the quantity required, and the source
of the part or material. A typical bill of material is shown
in Figure 4-4C . On drawings that do not have a bill of
material, the data may be indicated directly on the drawing.
On assembly drawings, each item is identified by a number
in a circle or square. An arrow connecting the number with
the item assists in locating it in the bill of material.
Other Drawing Data
Revision Block
Revisions to a drawing are necessitated by changes in
dimensions, design, or materials. The changes are usually
listed in ruled columns either adjacent to the title block or at
one corner of the drawing. All changes to approved drawings
must be carefully noted on all existing prints of the drawing.
When drawings contain such corrections, attention is directed to the changes by lettering or numbering them and listing those
changes against the symbol in a revision block. [Figure 4-4D]
The revision block contains the identification symbol, the
date, the nature of the revision, the authority for the change,
and the name of the draftsman who made the change.
To distinguish the corrected drawing from its previous
version, many firms are including, as part of the title block,
a space for entering the appropriate symbol to designate that
the drawing has been changed or revised.
Notes
Notes are added to drawings for various reasons. Some of these
notes refer to methods of attachment or construction. Others
give alternatives, so that the drawing can be used for different
styles of the same object. Still others list modifications that
are available. Notes may be found alongside the item that
they refer to. If the notes are lengthy, they may be placed
elsewhere on the drawing and identified by letters or numbers.
Notes are used only when the information cannot be conveyed
in the conventional manner or when it is desirable to avoid
crowding the drawing. Figure 4-4E illustrates one method
of depicting notes.
When the note refers to a specific part, a light line with an
arrowhead leads from the note to the part. If it applies to more
than one part, the note is worded to eliminate ambiguity as
to the parts it pertains to. If there are several notes, they are
generally grouped together and numbered consecutively.
Zone Numbers
Zone numbers on drawings are like the numbers and letters
printed on the borders of a map. They help locate a point.
To find a point, mentally draw horizontal and vertical lines
from the letters and numerals specified; the point where these
lines intersect is the area sought. Figure 4-4F shows the zone
numbers on a drawing.
Use the same method to locate parts, sections, and views
on large drawings, particularly assembly drawings. Parts
numbered in the title block can be located on the drawing
by finding the numbers in squares along the lower border.
Zone numbers read from right to left.
Station Numbers & Location Identification on
Aircraft
A numbering system is used on large assemblies for aircraft
to locate stations, such as fuselage frames. Fuselage station
185 indicates a location that is 185 inches from the datum of
the aircraft. The measurement is usually taken from the nose
or zero station, but in some instances, it may be taken from
the firewall or some other point chosen by the manufacturer.
Just as forward and aft locations on aircraft are made by
4-9reference to the datum, locations left and right of the aircraft’s
longitudinal axis are made by reference to the buttock line
and are called butt stations. Vertical locations on aircraft are
made in reference to the waterline.
The same station numbering system is used for wing and
stabilizer frames. The measurement is taken from the
centerline or zero station of the aircraft. Figure 4-10 shows
use of the fuselage stations (FS), waterline locations (WL),
and left and right buttock line locations (RBL and LBL).
Allowances & Tolerances
When a given dimension on a print shows an allowable
variation, the plus (+) figure indicates the maximum, and the
minus (−) figure the minimum allowable variation. The sum
of the plus and minus allowance figures is called tolerance.
[Figure 4-4G] For example, using 0.225 + 0.0025 − 0.0005, the
plus and minus figures indicate the part is acceptable if it is not
more than 0.0025 larger than the 0.225 given dimension, or not
more than 0.0005 smaller than the 0.225 dimension. Tolerance
in this example is 0.0030 (0.0025 max plus 0.0005 min).
If the plus and minus allowances are the same, you will find
them presented as 0.225 ± 0.0025. The tolerance would then
be 0.0050. Allowance can be indicated in either fractional or
decimal form. When very accurate dimensions are necessary,
decimal allowances are used. Fractional allowances are
sufficient when precise tolerances are not required. Standard
tolerances of –0.010 or −1⁄32 may be given in the title block
of many drawings, to apply throughout the drawing.
Finish Marks
Finish marks are used to indicate the surface that must
be machine finished. Such finished surfaces have a better
appearance and allow a closer fit with adjoining parts. During
the finishing process, the required limits and tolerances must
be observed. Do not confuse machined finishes with those of
paint, enamel, chromium plating, and similar coating.
Scale
Some drawings are made the same size as the drawn part;
reflecting a scale of 1:1. Other scales may be used. However,
when drawings are made on a computer, drawing sizes may
be easily increased (zoom in) or decreased (zoom out). Some
electronic printers have the same capability. Furthermore,
when a 1:1 copy of a print is made, the copy size may differ
slightly from that of the original. For accurate information,
refer to the dimensions shown on the drawing. [Figure 4-4H]
Application
When shown near or in the title block, application may
refer to a specific aircraft, assembly, sub-assembly or
unique application. For example, in Figure 4-4A the title block indicates the bracket assembly is for a Roll Servo
installation for an S-Tec Auto Pilot installation. If this
drawing pertained to a B95 Aircraft equipped with an Aero-
Tech air conditioning system and the bracket illustrated was
unique to that installation, the title block would provide that
application information. The title block may indicate Bracket
Assy., Roll Servo, with Aero-Tech air conditioner (Model
AT103-1) installed.
Methods of Illustration
Applied Geometry
Geometry is the branch of mathematics that deals with
lines, angles, figures, and certain assumed properties in
space. Applied geometry, as used in drawings, makes use
of these properties to accurately and correctly represent
objects graphically. In the past, draftsmen utilized a variety
of instruments with various scales, shapes, and curves to
make their drawings. Today, computer software graphics
programs show drawings at nearly any scale, shape, and curve
imaginable, outdating the need for additional instruments.
Several methods are used to illustrate objects graphically.
The most common are orthographic projections, pictorial
drawings, diagrams, and flowcharts.
Orthographic Projection Drawings
To show the exact size and shape of all the parts of complex
objects, several views are necessary. This is the system used
in orthographic projection.
In orthographic projection, there are six possible views of
an object, because all objects have six sides—front, top,
bottom, rear, right side, and left side. Figure 4-11A shows
an object placed in a transparent box, hinged at the edges.
The projections on the sides of the box are the views as seen
looking straight at the object through each side. If the outlines
of the object are drawn on each surface of the box, and the
box is then opened [Figure 4-11B] to lay flat [Figure 4-11C] ,
the result is a six-view orthographic projection.
It is seldom necessary to show all six views to portray an
object clearly; therefore, only those views necessary to
illustrate the required characteristics of the object are drawn.
One-, two-, and three-view drawings are the most common.
Regardless of the number of views used, the arrangement
is generally as shown in Figure 4-11 , with the front view
as principal view. If the right-side view is shown, it will be
to the right of the front view. If the left-side view is shown,
it will be to the left of the front view. The top and bottom
views, if included, will be shown in their respective positions
relative to the front view.
One-view drawings are commonly used for objects of
BL 0.0
230.0 BUTTOCK LINE (BL)BUTTOCK LINE (BL)
RBL 229.5LEMAC
FS 133.1
RBL 63.1MAC 47.7"RBL 87.7
TYPICAL LBL
LBL 63.1
LBL 229.5LBL 77.3RBL 77.3
BL 0.0WATER
LINE (WL)
FUSELAGE
STATION (FS)150.0
350.0 WL100.0
NOTE
Reference datum located
at fuselage station 0.0WL 165.5FS 350.2
FS 222.0 FS 100.0 FS 55.6
FS 38.3
FS 157.5
Figure 4-10. Station numbers and location identification on aircraft.
4-11uniform thickness, such as gaskets, shims, and plates. A
dimensional note gives the thickness as shown in Figure 4-12 .
One-view drawings are also commonly used for cylindrical,
spherical, or square parts if all the necessary dimensions can
be properly shown in one view. When space is limited and
two views must be shown, symmetrical objects are often
represented by half views, as illustrated in Figure 4-13 .
Aircraft drawings seldom show more than two principal or
complete views of an object. Instead, there will be usually one
complete view and one or more detail views or sectional views.
Detail View
A detail view shows only a part of the object, but in greater
detail and to a larger scale than the principal view. The
part that is shown in detail elsewhere on the drawing is
usually encircled by a heavy line on the principal view.
[Figure 4-14] The principal view shows the complete object,
while the detail view is an enlarged drawing of a portion of
the object.
Pictorial Drawings
A pictorial drawing is like a photograph. [Figure 4-15]
It shows an object as it appears to the eye, but it is not
satisfactory for showing complex forms and shapes. Pictorial
drawings are useful in showing the general appearance
of an object and are used extensively with orthographic
projection drawings. Pictorial drawings are used in Aircraft
Maintenance Manuals (AMM), Structural Repair Manuals
(SRM), and Illustrated Parts Catalogues (IPC). Four types
of pictorial drawings used frequently by aircraft engineers
and technicians are: perspective, isometric, oblique, and
exploded view.
Perspective Drawings
A perspective view shows an object as it appears to an
observer. [Figure 4-16A] It most closely resembles the way
an object would look in a photograph. Because of perspective,
some of the lines of an object are not parallel and therefore
the actual angles and dimensions are not accurate.
Isometric Drawings
An isometric view uses a combination of the views of an
orthographic projection and tilts the object forward so
that portions of all three views can be seen in one view.
[Figure 4-16B] This provides the observer with a three-
dimensional view of the object. Unlike a perspective drawing
where lines converge and dimensions are not true, lines in
an isometric drawing are parallel and dimensioned as they
are in an orthographic projection.Oblique Drawings
An oblique view is like an isometric view, except for one
distinct difference. In an oblique drawing, two of the three
drawing axes are always at right angles to each other.
[Figure 4-16C]
Exploded View Drawings
An exploded view drawing is a pictorial drawing of two or
more parts that fit together as an assembly. The view shows
the individual parts and their relative position to the other
parts before they are assembled. [Figure 4-17]
Exploded view drawings are often used in IPCs that are used
to order parts. The exploded view drawing has numbers and
the numbers correspond to a list of part numbers. Exploded
views are also used in Maintenance Instruction Manuals
(MIM) for the assembly and repair of aircraft components.
These drawings are often accompanied by notes that explain
the assembly process.
Diagrams
A diagram may be defined as a graphic representation of
an assembly or system, indicating the various parts and
expressing the methods or principles of operation. There are
many types of diagrams; however, those that the aviation
mechanic is concerned with during the performance of their
job may be grouped into four classes or types: installation,
schematic, block, and wiring diagrams.
Installation Diagrams
Figure 4-18 is an example of an installation diagram.
This is a diagram of the installation of the flight guidance
control components of an aircraft. It identifies each of the
components in the systems and shows their location in the
aircraft. Each number (1, 2, 3, and 4) on the detail shows the
location of the individual flight guidance system components
within the flight deck of the aircraft. Installation diagrams are
used extensively in aircraft maintenance and repair manuals,
and are invaluable in identifying and locating components
and understanding the operation of various systems.
Schematic Diagrams
Schematic diagrams do not indicate the location of the
individual components in the aircraft nor do they show
the actual size and shape of the components, but rather
locate components with respect to each other within the
system. Schematics show the principle of operation of an
aircraft system and are often used for troubleshooting and
training purposes.
Figure 4-19 illustrates a schematic diagram of an aircraft air
conditioning system. High-speed bleed air from the engine is
4-12Figure 4-12. One-view drawing.
A
B
COBJECT
ROTATED
FLAT
TOP
FRONT LEFT SIDE RIGHT SIDE REAR
BOTTOM
Figure 4-11. Orthographic projection.
the lines that lead into and out of the unit. Schematic
diagrams and installation diagrams are used extensively in
aircraft manuals.
Block Diagrams
Block diagrams are used to show a simplified relationship of a combined with cold air in the mixing chamber and distributed
via a manifold to various parts of the aircraft.
Note that each line is coded for ease of reading and tracing
the flow. Each component is identified by name, and its
location within the system can be ascertained by noting
Figure 4-13. Symmetrical object with exterior half view.
Figure 4-15. Pictorial drawing.
EE
VIEW C22 REF
8 REF
3 REF
REF
3236 REF
5D
C
B
A ABCD
1 2 3 4 5 6 7 88 7 6 5 4 3 2 1
REVISIONS
FOR REVISION HISTORY SEE SHEET 1
SHEET 4 OF 5SWorks Drawing No.: SIZEFORM 86359 REV --REV
Figure 4-14. Detail view.technicians involved with electrical repairs and installations,
a thorough knowledge of wiring diagrams and electrical
schematics is essential.
Flowcharts
Flowcharts are used to illustrate a sequence or flow of
events. There are two types of flow charts most frequently more complex system of components. [Figure 4-20] Individual
components are drawn as a rectangle (block) with lines
connecting it to other components (blocks) that it interfaces
with during operation.
Wiring Diagrams
Wiring diagrams show the electrical wiring and circuitry,
coded for identification, of all the electrical appliances and
devices used on aircraft. [Figure 4-21] These diagrams, even
for relatively simple circuits, can be quite complicated. For
4-14A B C PERSPECTIVE ISOMETRIC OBLIQUE
Figure 4-16. (A) Perspective, (B) isometric, and (C) oblique drawings.
3D
C
A ABCD
1 2 3 4 5 6 7 88 7 6 5 4 3 2 1
TITLE:
SIZEDrawing No.:
SHEET 1 OF 2B
cwhitworth
FORM (SWorks) 86359 REV -
Figure 4-17. Exploded view drawing.used in the aviation industry: troubleshooting flowcharts
and logic flowcharts.Troubleshooting Flowchart
Troubleshooting flowcharts are frequently used for the
detection of faulty components. They often consist of a
series of yes or no questions. If the answer to a question
A
AL TITUDE
TRANSDUCER
(REF) LEGEND
1. Screw
2. Cable Connector
3. Altitude/Vertical Speed Selector
4. Flight Guidance Program/Computer
FWDSerials 1005 thru 1336
and 1337 and subs
w/o PFD
Figure 4-18. Example of an installation diagram (flight guidance components).is yes, one course of action is followed. If the answer is
no, a different course of action is followed. In this simple
manner, a logical solution to a problem may be achieved.
Figure 4-22 shows a flow chart to determine the repair
options for a composite structure.
Logic Flowchart
Another type of flowchart, developed specifically for analysis
of digitally-controlled components and systems, is the logic flowchart. [Figure 4-23] A logic flowchart uses standardized
symbols to indicate specific types of logic gates and their
relationship to other digital devices in a system. Since digital
systems make use of binary mathematics consisting of 1s and
0s, voltage or no voltage, a light pulse or no light pulse, and
so forth, logic flowcharts consist of individual components
that take an input and provide an output that is either the same
as the input or opposite. By analyzing the input or multiple
inputs, it is possible to determine the digital output or outputs.
Figure 4-19. Schematic diagram of an air conditioning system for a B737 NG.Lines and Their Meanings
Every drawing is composed of lines. Lines mark the
boundaries, edges, and intersection of surfaces. Lines are
used to show dimensions and hidden surfaces and to indicate
centers. Obviously, if the same kind of line is used to show
these variations, a drawing becomes a meaningless collection
of lines. For this reason, various kinds of standardized lines
are used on aircraft drawings. [Figure 4-24] Examples of
correct line uses are shown in Figure 4-25 .Most drawings use three widths, or intensities, of lines:
thin, medium, or thick. These lines may vary somewhat
on different drawings, but there is a noticeable difference
between a thin and a thick line, with the width of the medium
line somewhere between the two.
Centerlines
Centerlines are made up of alternate long and short
dashes. They indicate the center of an object or part of an
object. Where centerlines cross, the short dashes intersect
4-17Output
Relay
AC InputAC InputAC Input
Phase A
Phase B
Phase CVoltage
SelectorComparatorReference
Voltage
DOPU
Power
Ampli fierSignal
Conditioning
and Balancing
Signal
Conditioning
and BalancingSignal
Conditioning
and Balancing
Figure 4-20. Block diagram.symmetrically. In the case of very small circles, the
centerlines may be shown unbroken.
Dimension Lines
A dimension line is a light solid line, broken at the midpoint
for insertion of measurement indications, and having opposite pointing arrowheads at each end to show origin and termination of a measurement. They are generally parallel to the line that the dimension is given for, placed outside the outline of the object, and between views if more than one view is shown.
All dimensions and lettering are placed so that they read from
left to right. The dimension of an angle is indicated by placing the degree of the angle in its arc. The dimensions of circular parts are always given in terms of the diameter of the circle and are usually marked with the letter D or the abbreviation DIA following the dimension. The dimension of an arc is given in terms of its radius and is marked with the letter R following the dimension. Parallel dimensions are placed so that the longest dimension is farthest from the outline and the shortest dimension is closest to the outline of the object. On a drawing showing several views, the dimensions are placed upon each view to show its details to the best advantage.
In dimensioning distances between holes in an object,
dimensions are usually given from center to center rather than from outside to outside of the holes. When several holes of various sizes are shown, the desired diameters are given on a leader followed by notes indicating the machining operations for each hole. If a part is to have three holes of equal size, equally spaced, this information is explicitly stated. For
precision work, sizes are given in decimals. Diameters and
depths are given for counterbored holes. For countersunk holes, the angle of countersinking and the diameters are given. [Figure 4-26]
The dimensions given for tolerances signify the amount
of clearance allowable between moving parts. A positive allowance is indicated for a part that is to slide or revolve upon another part. A negative allowance is one given for a force fit. Whenever possible, the tolerance and allowances for desired fits conform to those set up in the American Standard for Tolerances, Allowances, and Gauges for Metal Fits. The classes of fits specified in the standard may be indicated on assembly drawings.
Extension Lines
Extensions are used to extend the line showing the side or
edge of a figure for placing a dimension to that side or edge. They are very narrow and have a short break where they extend from the object and extend a short distance past the arrow of the dimensioning line.
Sectioning Lines
Sectioning lines indicate the exposed surfaces of an object in
a sectional view. They are generally thin full lines, but may vary with the kind of material shown in section.
Phantom Lines
Phantom lines indicate the alternate position of parts of the
object or the relative position of a missing part. They are composed of one long and two short evenly spaced dashes.
34 – 24 – 01WIRING DIAGRAMFLIGHT DIRECTOR
FLIGHT DIRECTORBOTH ON 2
BOTH ON 2BOTH ON 1
BOTH ON 193
J3A33
J1A
J1B
J1A
J1B
50J2A
J2A
J3A
J3A
J2A
J2A
J1
J1
J2B
J2B
J1
J1
J2A
J2AJ3A
J3AJ4A
J4A
J1A
45J1
J1
J2A
J2A
J1
J1
J2B
J2B48
J1A
J4A
J4A48
J1A
J4A
J4A
J4A
J4A
J1A
45J3A
1A
1B
2A
2B
J3A
J3ADIGITAL
FLIGHT GUIDANCE
COMPUTER-1
UIO - 212
DIGITAL
FLIGHT GUIDANCE
COMPUTER-2
UIO - 213ATTITUDE
SWITCHING UNIT
UIO - 228AIR DATA
INST
SWITCHING
UNIT
UIO - 207MM
MMS1
S2C
E
N
T
R
A
L
P
R
O
C
E
S
S
O
RC
E
N
T
R
A
L
P
R
O
C
E
S
S
O
RS1
S2NORMAL
L ON AUXNORMAL
BOTH ON 2
BOTH ON 1
NORMALR ON AUX
NORMAL
Figure 4-21. Wiring diagram.
Break Lines
Break lines indicate that a portion of the object is not shown
on the drawing. Short breaks are made by solid, freehand
lines. For long breaks, solid ruled lines with zigzags are used.
Shafts, rods, tubes, and other such parts that have a portion
of their length broken out have the ends of the break drawn
as indicated in Figure 4-25 .
Leader Lines
Leader lines are solid lines with one arrowhead. They
indicate a part or portion that a note, number, or other
reference applies.
Hidden Lines
Hidden lines indicate invisible edges or contours. Hidden
lines consist of short dashes evenly spaced and are frequently referred to as dash lines.
Outline or Visible Lines
The outline or visible line is used for all lines on the drawing
representing visible lines on the object. This is a medium-to-
wide line that represents edges and surfaces that can be seen
when the object is viewed directly.
Stitch Lines
Stitch lines are used to indicate the stitching or sewing lines
on an article and consists of a series of very short dashes,
approximately half the length of dash or hidden lines, evenly
spaced. Long lines of stitching may be indicated by a series
of stitch lines connected by phantom lines.
EXCESSIVE
QUALITY CHECK / NDTPERMANENT COMPOSITE REPAIR
ACCORDING TO APPROVED GUIDELINES
RETURN TO SERVICESUBMIT/
CHECK
SCHEME WITH
MANUFACTURER
/OEM
APPROVED
TEMPORARY
REPAIRIMPROVISE
AND
RETURN TO
REPAIR
WORKSHOPTEMPORARY
REPAIRREPAIR
TYPEDAMAGE
ASSESSMENT
SCRAP
COMPLEX REPAIR EASY REPAIR
Figure 4-22. Troubleshooting flowchart.
Cutting Plane and Viewing Plane Lines
Cutting plane lines indicate the plane where a sectional view
of the object is taken. In Figure 4-25 , plane line A indicates
the plane that section AA is taken. Viewing plane lines
indicate the plane from where a surface is viewed.
Drawing Symbols
The drawings for a component are composed largely of
symbols and conventions representing its shape and material.
Symbols are the shorthand of drawing. They graphically
portray the characteristics of a component with a minimal
amount of drawing.
Material Symbols
Section line symbols show the kind of material from which
the part is to be constructed. The material may not be indicated symbolically if its exact specification is shown
elsewhere on the drawing. In this case, the more easily
drawn symbol for cast iron is used for the sectioning, and
the material specification is listed in the bill of materials or
indicated in a note. Figure 4-27 illustrates a few standard
material symbols.
Shape Symbols
Symbols can be used to excellent advantage when needed to
show the shape of an object. Typical shape symbols used on
aircraft drawings are shown in Figure 4-28 . Shape symbols are
usually shown on a drawing as a revolved or removed section.
Electrical Symbols
Electrical symbols represent various electrical devices
rather than an actual drawing of the units. [Figure 4-29]
Having learned what the various symbols indicate, it
becomes relatively simple to look at an electrical diagram
and determine what each unit is, what function it serves, and
how it is connected in the system.
Reading and Interpreting Drawings
Aircraft technicians do not necessarily need to be accomplished
in making drawings. However, they must have a working
knowledge of the information that is to be conveyed to them.
They most frequently encounter drawings for construction
and assembly of new aircraft and components, during
modifications, and for making repairs.
A drawing cannot be read all at once any more than a whole
page of print can be read at a glance. Both must be read
one line at a time. To read a drawing effectively, follow a
systematic procedure.
Upon opening a drawing, read the drawing number and the
description of the article. Next, check the model affected,
the latest change letter, and the next assembly listed. Having
determined that the drawing is the correct one, proceed to
read the illustration(s).
In reading a multiview drawing, first get a general idea of
the shape of the object by scanning all the views. Then select
one view for a more careful study. By referring back and
forth to the adjacent view, it is possible to determine what
each line represents.
Each line on a view represents a change in the direction of
a surface, but another view must be consulted to determine
what the change is. For example, a circle on one view may
mean either a hole or a protruding boss, as in the top view
of the object in Figure 4-30 . Looking at the top view, we see
two circles. However, the other view must be consulted to
GEAR
DOOR
OPENLEFT NOSE RIGHTPROXIMITY
DETECTOR
PROXIMITY
DETECTOR
POWER
SUPPLY27 – 02
27 – 6127 – 83
32 – 1532 – 62
52 – 70OTHER PROXIMITY
UNIT SCHEMATICSRETARDED THROTTLEGROUND ISSEE 00 - 04CLOSE TARGET • LOGIC "1" • LOW VOLTAGE (TEST POINTS ONLY)
DOWN B LOCKED
DOWNUP2
UPDNLANDING GEAR
HANDLE SWITCH
S1 - 89
32 - 62LANDING GEAR WARNING
B1 - 187
AUTOSPOILER
SWITCHING UNIT
B5 - 70 27 - 61LANDING GEAR
POSITION
INDICATOR
CENTRAL AURAL
WARNING UNIT
B5 - 71
SEE 32 - 63FDAU
UIO-206 31 - 31B5 - 74
DIM & TEST UNIT-1
33 - 11B5 - 76
DIM & TEST UNIT 3
ALTERNATE GEAR
LEVER SWITCH
S1 - 466
32 - 62TURNS ON RED LIGHT
PROXIMITY SWITCH ELECTRONICS UNIT
32 - 61 - OI B5 - 80NOSE GEAR
POS INTLK
LEFT SWITCH
L2-26532-61-03 AGR
TEST DIMGR UNSAFE
DIM
B
TEST
UNIT-2
B5-75
SAFER2-262
DHDL
HOT
ON
HDL
ONSI-412 32-35NOSE GEAR WARNED LIKE MAIN PRIOR TO 1,024
GEAR
INTERLOCK
SHIP 1,024 & SUBS
OR S832 - 17BL2-632
GR
TEST DIMU
CC
DD
G
Z
B
I
Y
AO
FD
PS1939
J1AR MAIN
R GEAR UP & LATCHED2B VDC - L
L GEAR UP & LATCHEDNOSE
L MAIN
A 19
GEAR
HDL
ONONE
NOT
WIRED
RIGHT AFT RADIO RACK - SHELF 118
10E
BC
D
466B12B
A94320
27 – 61 – 11WIRING DIAGRAM
Figure 4-23. Logic flowchart.
determine what each circle represents.
A glance at the other view tells us that the smaller circle
represents a hole, and the larger circle represents a protruding
boss. In the same way, the top view must be consulted to
determine the shape of the hole and the protruding boss.
It can be seen from this example that one cannot read a print
by looking at a single view when more than one view is given.
Two views do not always describe an object and when three views are given, all three must be consulted to be sure the
shape has been read correctly.
After determining the shape of an object, determine its size.
Information on dimensions and tolerances is given so that
certain design requirements may be met. Dimensions are
indicated by figures either with or without the inch mark. If no
inch mark is used, the dimension is in inches. It is customary
to give part dimensions and an overall dimension that gives
the greatest length of the part. If the overall dimension is
4-21³⁄16 DRILL
3 HOLES
EQUALLY
SPACED
¹⁄4 DRILL
⁷⁄16 C’BORE
¹⁄8 DEEP
2 HOLES
80°0.3125 DRILL
0.3217 REAM
³⁄16 DRILL
80° C’SK
TO ⁵⁄16 DIA
0.2560 DRILL¹⁄4 DRILL, ³⁄6 C’BORE
¹⁄8 DEEP, 3 HOLES
APhantom line
Center line
Sectioning line
Outline Hidden lineBreak line
Cutting plane lineSection AADimension lineExtension lineFigure 4-26. Dimensioning holes.
Figure 4-25. Correct use of lines. Center line Thin
Dimension Thin
Extension line Thin
Break (long) Thin
Break (long) Thick
Phantom Thin
Sectioning Thin
Hidden Medium
Stitch line Medium
Visible line Thick
Datum line Thick
Cutting plane Extra thick
Cutting plane Extra thick
Complex cutting plane Extra thick
Figure 4-24. The meaning of lines.
missing, it can be determined by adding the separate part
dimensions. Many drawings used for new aircraft and
components are now using the metric system and millimeter
(mm) is the unit used for these drawings.
Drawings may be dimensioned in decimals or fractions. This
is especially true about tolerances. Instead of using plus and
minus signs for tolerances, many figures give the complete
dimension for both tolerances. For example, if a dimension is
2 inches with a plus or minus tolerance of 0.01, the drawing
would show the total dimensions as:
1.99A print tolerance (usually found in the title block) is a
general tolerance that can be applied to parts where the
dimensions are noncritical. Where a tolerance is not shown
on a dimension line, the print tolerance applies.
To complete the reading of a drawing, read the general notes
and the content of the material block, find the various changes
incorporated, and read the special information given in or
4-22CAST IRON
STEEL
BRASS, BRONZE,
AND COPPER
WOOD —ACROSS
GRAIN
CORK, FELT, FABRIC,
ASBESTOS, LEATHER,
AND FIBERRUBBER, PLASTIC
ELECTRICAL
INSULATIONMAGNESIUM,
ALUMINUM, AND
ALUMINUM ALLOYS
WOOD —WITH
GRAINBABBITT, LEAD,
ZINC, AND ALLOYS
Figure 4-27. Standard material symbols.
near views and sections.
Drawing Sketches
A sketch is a simple rough drawing that is made rapidly
and without much detail. Sketches may take many forms—
from a simple pictorial presentation to a multi-view
orthographic projection.
Just as aircraft technicians need not be highly skilled in
creating drawings, they need not be accomplished artists.
However, in many situations, they need to prepare a drawing
to present an idea for a new design, a modification, or a repair
method. The medium of sketching is an excellent way of
accomplishing this.The rules and conventional practices for making mechanical
drawings are followed to the extent that all views needed
to portray an object accurately are shown in their proper
relationship. It is also necessary to observe the rules for
correct line use and dimensioning. [Figures 4-24 and 4-25]
Sketching Techniques
To make a sketch, first determine what views are necessary
to portray the object. Then block in the views using light
construction lines. Next, complete the details, darken the
object outline, and sketch extension and dimension lines.
Complete the drawing by adding notes, dimensions, title,
date, and when necessary, the sketcher’s name. The steps in
making a sketch of an object are illustrated in Figure 4-31 .
Basic Shapes
Depending on the complexity of the sketch, basic shapes may
be drawn in freehand or by use of templates. If the sketch
is quite complicated or the technician is required to make
frequent sketches, use of a variety of templates and other
drafting tools is highly recommended.
Repair Sketches
A sketch is frequently drawn for repairs or for use in
manufacturing a replacement part. Such a sketch must
provide all necessary information to those who must make
the repair or manufacture the part.
The degree that a sketch is complete depends on its
intended use. Obviously, a sketch used only to represent
an object pictorially need not be dimensioned. If a part is
to be manufactured from the sketch, it should show all the
necessary construction details.
Care of Drafting Instruments
Good drawing instruments are expensive precision tools.
Reasonable care given to them during their use and storage
can prolong their service life.
T-squares, triangles, and scales should not be used or placed
where their surfaces or edges may be damaged. Use a drawing
board only for its intended purpose and not in a manner that
can mar the working surface.
Compasses, dividers, and pens provide better results with
less annoyance, if they are correctly shaped and sharpened
and are not damaged by careless handling. Store drawing
instruments in a place where they are not likely to be damaged
by contact with other tools or equipment. Protect compass and
divider points by inserting them into a piece of soft rubber or
similar material. Never store ink pens without first cleaning
and drying them thoroughly.
SQUARE SECTION ( METAL) SQUARE SECTION (WOOD )
ROUND SECTION ( SOLID) ROUND SECTION ( TUBULAR )
ANGLE SECTION (METAL ) CHANNEL SECTION ( METAL)
I-BEAM (METAL) SQUARE SECTION ( TABULAR )
Figure 4-28. Shape symbols.
Graphs & Charts
Graphs and charts are frequently used to convey information
graphically or information given certain conditions. They often utilize values shown on the “x” and “y” axes that can be projected up and across to arrive at a specific result. Also, when data is entered into a computer database, software programs can create a variety of different bar graphs, pie charts, and so forth, to graphically represent that data.
Reading & Interpreting Graphs & Charts
When interpreting information shown on graphs and charts,
it is extremely important that all the notes and legend information be carefully understood to eliminate any misinterpretation of the information presented.
Nomograms
A nomogram is a graph that usually consists of three
sets of data. Knowledge of any two sets of data enables the interpreter to obtain the value for the third unknown corresponding value. One type of nomogram consists of three parallel scales graduated for different variables, so that when a straight edge connects any two values, the third can be read directly. Other types may use values on the “x” and “y” axes of a graph with the third corresponding value determined by the intersection of the “x” and “y” values with one of a series of curved lines. Figure 4-32 is an example of a nomogram that shows the relationship between aviation fuels, specific weight, and temperature. Microfilm & Microfiche
The practice of recording drawings, parts catalogs, and maintenance and overhaul manuals on microfilms was utilized extensively in the past. Microfilm is available as regular 16 mm or 35 mm film. Since 35 mm film is larger, it provides a better reproduction of drawings. Microfiche is a card with pages laid out in a grid format. Microfilm and microfiche require use of special devices for both reading and printing the information.
Most modern aircraft manufacturers have replaced microfilm
and microfiche with digital storage methods utilizing CDs, DVDs, and other data storage devices. A great deal of service and repair information for older aircraft has been transferred to digital storage devices. However, there may still be a need to access information using the old methods. A well-equipped shop should have available, both the old microfilm and microfiche equipment, as well as new computer equipment.
Digital Images
Though not a drawing, a digital image created by a digital camera can be extremely helpful to aviation maintenance technicians in evaluating and sharing information concerning the airworthiness or other information about aircraft. Digital images can be rapidly transmitted over the World Wide Web as attachments to e-mail messages. Images of structural fatigue cracks, failed parts, or other flaws, as well as desired design and paint schemes, are just a few examples of the types of digital images that might be shared by any number of users over the Internet. Figure 4-33 is a digital image of damage to a composite structure taken with a simple digital camera.
4-24CONDENSERS
CONDUCTORS
SINGLEINTERSECTING
ELECTRICALLYCROSSING OVER
EACH OTHER
RELAYS
SPST
MOMENTARY
ONSPDT NORMAL OR
MOMENTARY
CONTACTSMPDT NORMAL
OR MOMENTARY
CONTACTS
CIRCUIT BREAKERS
AUTOMATIC
RESETPUSH
RESETPUSH RESET
PULL OFFSWITCH
TYPEMOMENTARY
SWITCH TYPE
SWITCHES
FUSE SPLICECONTACTS PUSHBUTTON TYPE
POLARITY GROUND
10ABASIC
ELECTRICAL DISCONNECTSPST
MPST MECHANICAL LINKAGEMOMENTARY
POSITIONNORMAL POSITION
MPDT CENTER
OFFSPST
MOMENTARY
ONSPDT
CENTER
OFFSPDT
MOMENTARY
ONSPDTSPDT CENTER
OFF NORMAL OR
MOMENTARY
CONTACTSSPDT
NORMAL OR
MOMENTARY
MOMENTARY
ONNORMAL
ON
POSITIVE NEGATIVENORMAL OR
MOMENTARY
CONTACTSPRESSUREBIMETALIC
THERMAL
CUT-OUTMOTORS
AC DCGENERATORS
AC DC
CONNECTORSMETERS
AMMETER VOLTLAMPS − RED, GREEN, WHITE
RHEOSTAT RESISTOR
BUS BATTERYCURRENT
LIMITERSOLENOID TRANSFORMER
A
B
C
DP S
FIXED REMOVABLE
P DESIGNATED PINS
S DESIGNATED SOCKETSNOT ALL
PINS SHOWALL PINS
SHOWS P
A
B
C
D
EV AM+
−G+
−M G M G OR ORR L G W
OR
+ DC −
Figure 4-29. Electrical symbols.
4-25Figure 4-30. Reading views.Block in Add Detail
Darken Views Add Dimensions
1-27-06 WEDGE RJA1¼"
1"
¾"1½"1¼"
3"
Figure 4-31. Steps in sketching.
To provide information about the extent of the damage, a
measurement scale, or other object, such as a coin, can be
placed near the area of concern before the picture is taken.
Also, within the text of the e-mail, the technician should
state the exact location of the damage, referenced to fuselage
station, wing station, and so forth.
Figure 4-33. Digital image of damage.7.5
−40 −30 −20 −10 0 10 20 30 40Specific Weight (lb/US gal)
Temperature (°C)Aviation Gasoline Grade 100/130Aviation Kerosene Jet A & Jet A1
Jet B (JP-4)NOTE: The fuel quantity indicator is calibrated for correct indication when using Aviation Kerosene Jet A and Jet A1.
When using other fuels, multiply the indicated fuel quantity in pounds by 0.99 for Jet B (JP-4) or by 0.98 for
Aviation Gasoline (100/130) to obtain actual fuel quantity in pounds.Fuel
Aviation Kerosene
Jet A and Jet A1
Jet B (JP-4)
AV Gas Grade 100/1300.7850.812
0.703Average Specific
Gravity at 15 °C (59 °F)Density Variation of Aviation Fuel
Based on Average Specific Gravity
Figure 4-32. Nomogram.
Physics for Aviation
Chapter 5
Physical science, which is most often called physics, is a very
interesting and exciting topic. For an individual who likes
technical things and is a hands-on type of person, physics
is an invaluable tool. Physics allows us to explain how
engines work, both piston and gas turbine; how airplanes
and helicopters fly; and countless other things related to the
field of aviation and aerospace. In addition to allowing us to
explain the operation of the things around us, it also allows
us to quantify them. For example, through the use of physics
we can explain what the concept of thrust means for a jet
engine, and then follow it up by mathematically calculating
the pounds of thrust being created.
Physics is the term applied to an area of knowledge regarding
the basic and fundamental nature of matter and energy. It does
not attempt to determine why matter and energy behave as
they do in their relation to physical phenomena, but rather
how they behave. The people who maintain and repair aircraft
should have knowledge of basic physics, which is sometimes
called the science of matter and energy.
Matter
Matter is the foundation, or the building blocks, for any
discussion of physics. According to the dictionary, matter is
what all things are made of; whatever occupies space, has
mass, and is perceptible to the senses in some way. According
to the Law of Conservation, matter cannot be created or
destroyed, although it is possible to change its physical state.
When liquid gasoline vaporizes and mixes with air, and then
burns, it might seem that this piece of matter has disappeared
and no longer exists. Although it no longer exists in the state
of liquid gasoline, the matter still exists in the form of the
gases given off by the burning fuel.
Characteristics of Matter
Mass & Weight
Mass is a measure of the quantity of matter in an object. In
other words, how many molecules are in the object, how
many atoms are in the object, or to be more specific, how
many protons, neutrons, and electrons are in the object. The
mass of an object does not change regardless of where you
take it in the universe, or with a change of state. The only
way to change the mass of an object is to add or take away
atoms. Mathematically, mass can be stated as follows:Mass = Weight ÷ Acceleration due to gravity
The acceleration due to gravity here on earth is 32.2 feet
per second per second (32.2 fps/s). An object weighing 32.2
pounds (lb) here on earth is said to have a mass of 1 slug.
A slug is a quantity of mass that will accelerate at a rate of
1 ft. /s2 when a force of 1 pound is applied. In other words,
under standard atmospheric condition, which is that gravity is
equal to 32.2 fps/s, a mass of one slug would be equal to 32.2 lb.
Weight is a measure of the pull from gravity acting on the
mass of an object. The more mass an object has, the more it
will weigh under the earth’s force of gravity. The only way for
an object to be weightless is for gravity to go away, because
it is not possible for the mass of an object to disappear. When
we view astronauts on the space shuttle, it appears that they
are weightless. Even though the shuttle is far from the surface
of the earth, the force of gravity has not completely gone
away, and the astronauts are not weightless. The astronauts
and the space shuttle are actually in a state of free fall, so
relative to the shuttle the astronauts appear to be weightless.
Mathematically, weight can be stated as follows:
Weight = Mass × Gravity
Attraction
Attraction is mutual force acting between particles of matter,
which tends to draw them together. Sir Isaac Newton called
this the “Law of Universal Gravitation.” Newton showed
how each particle of matter attracts every other particle,
how people are bound to the earth, and how the planets are
attracted in the solar system.
Porosity
Porosity means having pores or spaces where smaller
particles may fit when a mixture takes place. This is
sometimes referred to as granular—consisting or appearing
to consist of small grains or granules.
Impenetrability
Impenetrability means that no two objects can occupy the
same place at the same time. Thus, two portions of matter
cannot at the same time occupy the same space.
5-2Density
The density of a substance is its weight per unit volume.
The unit volume selected for use in the English system of
measurement is 1 cubic foot (ft3). In the metric system, it
is 1 cubic centimeter (cm3). Therefore, density is expressed
in pounds per cubic foot (lb⁄ft3) or in grams per cubic
centimeter (g⁄cm3).
To find the density of a substance, its weight and volume must
be known. Its weight is then divided by its volume to find
the weight per unit volume. For example, the liquid which
fills a certain container weighs 1,497.6 lb. The container
is 4 ft long, 3 ft wide and 2 ft deep. Its volume is 24 ft3
(4 ft. × 3 ft. × 2 ft.). If 24 ft3 of liquid weighs 1,497.6 lb, then
1 ft3 weighs 1,497.6 ÷ 24, or 62.4 lb. Therefore, the density
of the liquid is 62.4 lb/ft3. This is the density of water at 4 °C
(Centigrade) and is usually used as the standard for comparing
densities of other substances. In the metric system, the density
of water is 1 g⁄cm3. The standard temperature of 4 °C is used
when measuring the density of liquids and solids. Changes
in temperature will not change the weight of a substance,
but will change the volume of the substance by expansion or
contraction, thus changing its weight per unit volume.
The procedure for finding density applies to all substances;
however, it is necessary to consider the pressure when finding
the density of gases. Pressure is more critical when measuring
the density of gases than it is for other substances. The
density of a gas increases in direct proportion to the pressure
exerted on it. Standard conditions for the measurement of
the densities of gases have been established at 0 °C for
temperature and a pressure of 76 cm of mercury (Hg), which
is the average pressure of the atmosphere at sea level. Density
is computed based on these conditions for all gases.
Specific Gravity
It is often necessary to compare the density of one substance
with another substance. For this purpose, a standard is
needed. Water is the standard that physicists have chosen to
use when comparing the densities of all liquids and solids.
For gases, air is most commonly used, but hydrogen is also
sometimes used as a standard for gases. In physics, the word
“specific” implies a ratio. Thus, specific gravity is calculated
by comparing the weight of a definite volume of the given
substance with the weight of an equal volume of water. The
terms “specific weight” or “specific density” are sometimes
used to express this ratio.
The following formulas are used to find the specific gravity
of liquids and solids.
Specific Gravity = Weight of an equal volume of waterWeight of the substance
orSpecific Gravity = Density of waterDensity of the substance
The same formulas are used to find the density of gases by
substituting air or hydrogen for water.
Specific gravity is not expressed in units, but as pure numbers.
For example, if a certain hydraulic fluid has a specific gravity
of 0.8, 1 ft3 of the liquid weighs 0.8 times as much as 1 ft3
of water: 62.4 times 0.8, or 49.92 lb.
Specific gravity and density are independent of the size of
the sample under consideration and depend only upon the
substance of which it is made. See Figure 5-1 for typical
values of specific gravity for various substances.
A device called a hydrometer is used for measuring specific
gravity of liquids. This device consists of a tubular glass float
contained in a larger glass tube. [Figure 5-2] The larger glass
tube provides the container for the liquid. A rubber suction
bulb draws the liquid up into the container. There must be
enough liquid raising the float to prevent it from touching the
bottom. The float is weighted and has a vertically graduated
scale. To determine specific gravity, the scale is read at the
surface of the liquid in which the float is immersed. An
indication of 1000 is read when the float is immersed in pure
water. When immersed in a liquid of greater density, the float
rises, indicating a greater specific gravity. For liquids of lesser
density, the float sinks, indicating a lower specific gravity.
An example of the use of the hydrometer is to determine
the specific gravity of the electrolyte (battery liquid) in an
aircraft battery. When a battery is discharged, the calibrated
float immersed in the electrolyte will indicate approximately
1150. The indication of a charged battery is between 1275
and 1310. The values 1150, 1275, and 1310 represent 1.150,
1.275, and 1.310. The electrolyte in a discharged battery is
1.15 times denser than water, and in a charged battery 1.275
to 1.31 times denser than water.
Energy
Energy is typically defined as something that gives us the
capacity to perform work. As individuals, saying that we
feel full of energy is an indicator that we can perform a lot
of work. Energy can be classified as one of two types: either
as potential energy or kinetic energy.
Potential Energy
Potential energy is defined as being energy at rest, or energy
that is stored. Potential energy may be classified into three
groups: (1) energy due to position, (2) energy due to distortion
of an elastic body, and (3) energy which produces work
5-3LiquidSpecific
GravitySpecific
GravitySolidSpecific
GravityGas
Gasoline
Jet Fuel
Jp-4
Ethyl
Alcohol
Jet Fuel
Jp-5
Kerosene
Lube Oil
Synthetic
Oil
Water
Sulfuric
Acid
MercuryIce
Aluminum
Titanium
Zinc
Iron
Brass
Copper
Lead
Gold
PlatinumHydrogen
Helium
Acetylene
Nitrogen
Air
Oxygen
Carbon
Dioxide 0.917
8.9 11.4 19.3 21.5 0.0695
0.928 1.000 1.84 13.6
Figure 5-1. Specific gravity of various substances.
1,150 Discharged1,275 Charged
Figure 5-2. Hydrometer for checking battery specific gravity.through chemical action. Examples of the first group are
water in an elevated reservoir or an airplane raised off the ground with jacks; a stretched bungee cord on a Piper Tri-Pacer or compressed spring are examples of the second group; and energy in aviation gasoline, food, or storage batteries are examples of the third group.
To calculate the potential energy of an object due to its
position, as in height, the following formula is used:
Potential Energy = Weight × Height
A calculation based on this formula will produce an answer that has units of foot-pounds (ft-lb) or inch-pounds (in-lb), which are the same units that apply to work. Work, which is covered later in this chapter, is described as a force being applied over a measured distance, with the force being pounds and the distance being feet or inches. Potential energy and work have a lot in common.
Example: A Boeing 747 weighing 450,000 pounds needs to
be raised 4 feet in the air so maintenance can be done on the landing gear. How much potential energy does the airplane possess because of this raised position?
Potential Ener gy = Weight × Height
PE = 450,000 lb × 4 ft
PE = 1,800,000 ft-lb
As previously mentioned, aviation gasoline possesses potential energy because of its chemical nature. Gasoline has the potential to release heat energy, based on its British
thermal unit (BTU) content. One pound of aviation gas contains 18,900 BTU of heat energy, and each BTU is capable of 778 ft-lb of work. So, when we multiply 778 by 18,900, we find that one pound of aviation gas is capable of
14,704,200 ft-lb of work. Imagine the potential energy in the completely serviced fuel tanks of an airplane.
Kinetic Energy
Kinetic energy is defined as being energy that is in motion.
An airplane rolling down the runway or a rotating flywheel on an engine are both examples of kinetic energy. Kinetic energy has the same units as potential energy, namely foot-pounds or inch-pounds. To calculate the kinetic energy for something in motion, the following formula is used:
Kinetic Energy =
1⁄2 Mass × Velocity2
To use the formula, we will show the mass as weight divided by gravity and the velocity of the object will be in feet per second. This is necessary to end up with units in foot-pounds.
Figure 5-3. Kinetic energy (Airbus A380 taking off).Example: An Airbus A380 weighing 600,000 lb is moving
down the runway on its takeoff roll with a velocity of 200 fps.
How many foot-pounds of kinetic energy does the airplane
possess? [Figure 5-3]
Kinetic Energy = 1⁄2 Mass × Velocity2
Kinetic Energy = 1⁄2 × 600,000 ÷ 32.2 × 2002
KE = 372,670,000 ft-lb
Force, Work, Power, & Torque
Force
Before the concept of work, power, or torque can be discussed,
we need to understand what force means. According to the
dictionary, force is the intensity of an impetus, or the intensity
of an input. For example, if we apply a force to an object, the
tendency will be for the object to move. Another way to look
at it is that for work, power, or torque to exist, there must be
a force that initiates the process.
The unit for force in the English system of measurement
is pounds, and in the metric system it is newtons. One
pound of force is equal to 4.448 newtons. When we
calculate the thrust of a turbine engine, we use the formula
“Force = Mass × Acceleration,” and the thrust of the engine is
expressed in pounds. The GE90-115 turbofan engine (power
plant for the Boeing 777-300), for example, has 115,000
pounds of thrust.
Work
The study of machines, both simple and complex, can be
seen as a study of the energy of mechanical work. This is
true because all machines transfer input energy, or the work
done on the machine, to output energy, or the work done by
the machine.
Work, in the mechanical sense of the term, is done when a
resistance is overcome by force acting through a measurable
distance. Two factors are involved: (1) force and (2) movement
through a distance. As an example, suppose a small aircraft
is stuck in the snow. Two men push against it for a period of
time, but the aircraft does not move. According to the technical
definition, no work had been done when the men were pushing
against the aircraft. By definition, work is accomplished only
when an object is displaced some distance against a resistive
force. To calculate work, the following formula is used:
Work = Force (F) × distance (d)
In the English system, the force will be identified in pounds
and the distance either in feet or inches, so the units will be
foot-pounds or inch-pounds. Notice these are the same units
that were used for potential and kinetic energy. In the metric system, the force is identified in newtons (N)
and the distance in meters, with the resultant units being
joules. One pound of force is equal to 4.448 N and one meter
is equal to 3.28 feet. One joule is equal to 0.74 ft-lb.
Example: How much work is accomplished by jacking a
150,000-lb Airbus A-320 airplane a vertical height of 4 ft?
[Figure 5-4]
Work = Force × Distance
= 150,000 lb × 4 ft
= 600,000 ft-lb
Example: How much work is accomplished when a tow
tractor is hooked up to a tow bar and a Boeing 737-800
airplane weighing 130,000 lb is pushed 80 ft. into the hangar?
The force on the tow bar is 5,000 lb.
Work = Force × Distance
= 5,000 lb × 80 ft
= 400,000 ft-lb
In this last example, notice the force does not equal the weight
of the airplane. This is because the airplane is being moved
horizontally and not lifted vertically. In almost all cases, it
takes less work to move something horizontally than it does
to lift it vertically. Most people can push their car a short
distance if it runs out of gas, but they cannot get under their
car and lift it off the ground.
Friction & Work
In calculating work done, the actual resistance overcome is
measured. This is not necessarily the weight of the object
being moved. [Figure 5-5] A 900-lb load is being pulled a
distance of 200 ft. This does not mean that the work done
(force × distance) is 180,000 ft-lb (900 lb × 200 ft). This is
Figure 5-4. Airplane on jacks.because the person pulling the load is not working against the
total weight of the load, but rather against the rolling friction
of the cart, which may be no more than 90 lb.
Friction is an important aspect of work. Without friction,
it would be impossible to walk. One would have to shove
oneself from place to place, and would have to bump against
some obstacle to stop at a destination. Yet friction is a liability
as well as an asset, and requires consideration when dealing
with any moving mechanism.
In experiments relating to friction, measurement of the
applied forces reveals that there are three kinds of friction.
One force is required to start a body moving, while another
is required to keep the body moving at constant speed. Also,
after a body is in motion, a definitely larger force is required
to keep it sliding than to keep it rolling.
Thus, the three kinds of friction may be classified as:
(1) starting or static friction, (2) sliding friction, and (3)
rolling friction.
Static Friction
When an attempt is made to slide a heavy object along a
surface, the object must first be broken loose or started. Once
in motion, it slides more easily. The “breaking loose” force
is, of course, proportional to the weight of the body. The
force necessary to start the body moving slowly is designated
“F,” and “F'” is the normal force pressing the body against
the surface which is usually its weight. Since the nature of
the surfaces rubbing against each other is important, they
must be considered. The nature of the surfaces is indicated
by the coefficient of starting friction which is designated by
the letter “k.” This coefficient can be established for various
materials and is often published in tabular form. Thus, when
the load (weight of the object) is known, starting friction can
be calculated by using the following formula:
F = kF'
For example, if the coefficient of sliding friction of a
smooth iron block on a smooth, horizontal surface is 0.3,
the force required to start a 10 lb block would be 3 lb; a
40-lb block, 12 lb.
Starting friction for objects equipped with wheels and roller
bearings is much smaller than that for sliding objects. For
example, a locomotive would have difficulty getting a long
train of cars in motion all at one time. Therefore, the couples
between the cars are purposely made to have a few inches of
play. When starting the train, the engineer backs the engine
until all the cars are pushed together. Then, with a quick
start forward the first car is set in motion. This technique is employed to overcome the static friction of each wheel as
well as the inertia of each car. It would be impossible for the
engine to start all of the cars at the same instant, for static
friction, which is the resistance of being set in motion, would
be greater than the force exerted by the engine. However, once
the cars are in motion, the static friction is greatly reduced
and a smaller force is required to keep the train in motion
than was required to start it.
Sliding Friction
Sliding friction is the resistance to motion offered by an object
sliding over a surface. It pertains to friction produced after the
object has been set in motion, and is always less than starting
friction. The amount of sliding resistance is dependent on the
nature of the surface of the object, the surface over which
it slides, and the normal force between the object and the
surface. This resistive force may be computed by using the
following formula:
F = mN
In the formula above, “F” is the resistive force due to friction
expressed in pounds; “N” is the force exerted on or by the
object perpendicular (normal) to the surface over which it
slides; and “m” (mu) is the coefficient of sliding friction. On
a horizontal surface, N is equal to the weight of the object in
pounds. The area of the sliding object exposed to the sliding
surface has no effect on the results. A block of wood, for
Gravity
200 ftForce
90 lb
Resistance Work = force x distance
= 90 lb x 200 ft
= 18,000 ft-lb
Figure 5-5. The effect of friction on work.
example, will not slide any easier on one of the broad sides
than it will on a narrow side, assuming all sides have the
same smoothness. Therefore, area does not enter into the
equation above.
Rolling Friction
Resistance to motion is greatly reduced if an object is
mounted on wheels or rollers. The force of friction for
objects mounted on wheels or rollers is called rolling friction.
This force may be computed by the same equation used in
computing sliding friction, but the values of “m” will be
much smaller. For example, the value of “m” for rubber tires
on concrete or macadam is about 0.02. The value of “m” for
roller bearings is very small, usually ranging from 0.001 to
0.003 and is often disregarded.
Example: An aircraft with a gross weight of 79,600 lb is
towed over a concrete ramp. What force must be exerted
by the towing vehicle to keep the airplane rolling after once
set in motion?
F = mN
= 0.02 mu × 79,600 lb
= 1,592 lb
Power
The concept of power involves the previously discussed
topic of work, which was a force being applied over a
measured distance, but adds one more consideration—time.
In other words, how long it takes to accomplish the work.
If someone asked the average person if they could lift one
million pounds 5 feet off the ground, the answer most
assuredly would be no. This person would probably assume
that they are to lift it all at once. What if they are given 365
days to lift it, and could lift small amounts of weight at a
time? The work involved would be the same, regardless of how long it took to lift the weight, but the power required
is different. If the weight is to be lifted in a shorter period
of time, it will take more power. The formula for power is
as follows:
Power = Force × distance ÷ time
The units for power will be foot-pounds per minute, foot-
pounds per second, inch-pounds per minute or second, and
possibly mile-pounds per hour. The units depend on how
distance and time are measured.
Many years ago, there was a desire to compare the power
of the newly evolving steam engine to that of horses.
People wanted to know how many horses the steam engine
was equivalent to. The value we know currently as one
horsepower (hp) was developed, and it is equal to 550 foot-
pounds per second (ft-lb/s) because of this. It was found that
the average horse could lift a weight of 550 lb, one foot off
the ground, in one second. The values we use today, in order
to convert power to horsepower, are as follows:
1 hp = 550 ft-lb/s
1 hp = 33,000 ft-lb/min.
1 hp = 375 mile pounds per hour (mi-lb/hr.)
1 hp = 746 watts (electricity conversion)
To convert power to horsepower, divide the power by the
appropriate conversion based on the units being used.
Example: What power would be needed, and also horsepower,
to raise the GE-90 turbofan engine into position to install
it on a Boeing 777-300 airplane? The engine weighs
19,000 lb, and it must be lifted 4 ft in 2 minutes.
Power = Force × distance ÷ time
= 19,000 lb × 4 ft ÷ 2 min.
= 38,000 ft-lb/min.
Hp = 38,000 ft-lb/min. ÷ 33,000 ft-lb/min.
Hp = 1.15
The hoist that will be used to raise this engine into position
will need to be powered by an electric motor because the
average person will not be able to generate 1.15 hp in their
arms for the necessary 2 minutes.
Torque
Torque is a very interesting concept and occurrence, and it is
definitely something that needs to be discussed in conjunction
with work and power. Whereas work is described as force
acting through a distance, torque is described as force acting
along a distance. Torque is something that creates twisting
5-7and tries to make something rotate.
If we push on an object with a force of 10 lb and it moves
10 inches in a straight line, we have done 100 in-lb of
work. By comparison, if we have a wrench 10 inches long
that is on a bolt, and we push down on it with a force of 10
lb, a torque of 100 lb-in is applied to the bolt. If the bolt
was already tight and did not move as we pushed down on
the wrench, the torque of 100 lb-in would still exist. The
formula for torque is:
Torque = Force × distance
Even though this formula looks the same as the other formula
for calculating work, recognize that the distance value in this
formula is not the linear distance an object moves, but rather
the distance along which the force is applied.
Notice that with torque nothing had to move, because the
force is being applied along a distance and not through a
distance. Notice also that although the units of work and
torque appear to be the same, they are not. The units of work
were inch-pounds and the units of torque were pound-inches,
and that is what differentiates the two.
Torque is very important when thinking about how engines
work, both piston engines and gas turbine engines. Both
types of engines create torque in advance of being able
to create work or power. With a piston engine, a force in
pounds pushes down on the top of the piston and tries to
make it move. The piston is attached to the connecting rod,
which is attached to the crankshaft at an offset. That offset
would be like the length of the wrench discussed earlier,
and the force acting along that length is what creates torque.
[Figure 5-6]
For the cylinder in Figure 5-6 , there is a force of 500 lb
pushing down on the top of the piston. The connecting rod
attaches to the crankshaft at an offset distance of 4 in. The
product of the force and the offset distance is the torque, in
this case 2,000 lb-in.
In a turbine engine, the turbine blades at the back of the engine
extract energy from the high velocity exhaust gases. The energy
extracted becomes a force in pounds pushing on the turbine
blades, which happen to be a certain number of inches from
the center of the shaft they are trying to make rotate. The
number of inches from the turbine blades to the center of the
shaft would be like the length of the wrench discussed earlier.
Mathematically, there is a relationship between the
horsepower of an engine and the torque of an engine. The
formula that shows this relationship is as follows:Torque = Horsepower × 5,252 ÷ rpm
Example: A Cessna 172R has a Lycoming IO-360 engine that
creates 180 horsepower at 2,700 rpm. How many pound-feet
of torque is the engine producing?
Torque = 180 × 5,252 ÷ 2,700
= 350 lb-ft.
Simple Machines
A machine is any device with which work may be
accomplished. For example, machines can be used for any of
the following purposes, or combinations of these 5 purposes:
1. Machines are used to transform energy, as in the case
of a generator transforming mechanical energy into
electrical energy.
2. Machines are used to transfer energy from one place
to another, as in the examples of the connecting rods,
crankshaft, and reduction gears transferring energy
from an aircraft’s engine to its propeller.
3. Machines are used to multiply force; for example, a
system of pulleys may be used to lift a heavy load. The
pulley system enables the load to be raised by exerting
a force that is smaller than the weight of the load.
4. Machines can be used to multiply speed. A good
example is the bicycle, by which speed can be gained
by exerting a greater force.
5. Machines can be used to change the direction of
a force. An example of this use is the flag hoist. A
downward force on one side of the rope exerts an
upward force on the other side, raising the flag toward
the top of the pole.
There are only six simple machines. They are the lever, the
pulley, the wheel and axle, the inclined plane, the screw,
and the gear. Physicists, however, recognize only two basic
principles in machines: the lever and the inclined plane.
The pulley (block and tackle), the wheel and axle, and gears
operate on the machine principle of the lever. The wedge and
the screw use the principle of the inclined plane.
An understanding of the principles of simple machines provides
a necessary foundation for the study of compound machines,
which are combinations of two or more simple machines.
Mechanical Advantage of Machines
As identified in statements 3 and 4 under simple machines, a
machine can be used to multiply force or to multiply speed. It
cannot, however, multiply force and speed at the same time.
In order to gain one force, it must lose the other force. To do
otherwise would mean the machine has more power going
Force of 500 lbTorque = F x d
Torque = 500 x 4
Torque = 2,000 lb-in
Torque = 166.7 lb-ft
Piston
Connecting rod
Crankshaft
4"
Figure 5-6. Piston engine and torque.
out than coming in, and that is not possible.
In reference to machines, mechanical advantage is a
comparison of the output force to the input force, or the
output distance to the input distance. If there is a mechanical
advantage in terms of force, there will be a fractional
disadvantage in terms of distance. The following formulas
can be used to calculate mechanical advantage.
Mechanical Advantage = Force Out ÷ Force In
or
Mechanical Advantage = Distance Out ÷ Distance In
The Lever
The simplest machine, and perhaps the most familiar one, is
the lever. A seesaw is a familiar example of a lever, with two
people sitting on either end of a board and a pivoting point
in the middle. There are three basic parts in all levers. They
are the fulcrum “F,” a force or effort “E,” and a resistance
“R.” Shown in Figure 5-7 are the pivot point “F” (fulcrum),
the effort “E” which is applied at a distance “L” from the
fulcrum, and a resistance “R” which acts at a distance “l”
from the fulcrum. Distances “L” and “l” are the lever arms.The concept of torque was discussed earlier in this chapter,
and torque is very much involved in the operation of a lever.
When a person sits on one end of a seesaw, that person applies
a downward force in pounds which acts along the distance
to the center of the seesaw. This combination of force and
distance creates torque, which tries to cause rotation.
First Class Lever
In the first class lever, the fulcrum is located between the
effort and the resistance. As mentioned earlier, the seesaw
is a good example of a lever, and it happens to be a first
class lever. The amount of weight and the distance from
the fulcrum can be varied to suit the need. Increasing the
distance from the applied effort to the fulcrum, compared to
the distance from the fulcrum to the weight being moved,
increases the advantage provided by the lever. Crowbars,
shears, and pliers are common examples of this class of lever.
The proper balance of an airplane is also a good example,
with the center of lift on the wing being the pivot point, or
fulcrum, and the weight fore and aft of this point being the
effort and the resistance.
When calculating how much effort is required to lift a specific
weight, or how much weight can be lifted by a specific effort,
the following formula can be used.
Effort (E) × Effort Arm (L) = Resistance (R) × Resistance Arm (l)
What this formula really shows is the input torque (effort ×
effort arm) equals the output torque (resistance × resistance
arm). This formula and concept apply to all three classes of
levers and to all simple machines in general.
Example: A first class lever is to be used to lift a 500-lb
weight. The distance from the weight to the fulcrum is
12 inches and from the fulcrum to the applied effort is 60
inches. How much force is required to lift the weight?
Effort (E) × Effort Arm (L) = Resistance (R) × Resistance Arm (l)
E × 60 in = 500 lb × 12 in
E = 500 lb × 12 in ÷ 60 in
E = 100 lb
The mechanical advantage of the lever in this example
would be:
Mechanical Advantage = Force Out ÷ Force In
= 500 lb ÷ 100 lb
= 5, or 5 to 1
An interesting thing to note with this example lever is if the
applied effort moved down 10 inches, the weight on the other
end would only move up 2 inches. The weight being lifted
would only move one-fifth as far. The reason for this is the
5-9Resistance “R” Effort “E”
Fulcrum “F”“L” “l”
Resistance “R”Effort “E”
Fulcrum “F”“L”
“l”
Figure 5-7. First class lever. Figure 5-8. Second class lever.class levers. As shown in Figure 5-9 , the fulcrum is at one end
of the lever and the weight or resistance to be overcome is at
the other end, with the effort applied at some point between.
Third class levers are easily recognized because the effort is
applied between the fulcrum and the resistance. The retractable
main landing gear on an airplane is a good example of a third
class lever. The top of the landing gear, where it attaches to the
airplane, is the pivot point. The wheel and brake assembly at
the bottom of the landing gear is the resistance. The hydraulic
actuator that makes the gear retract is attached somewhere in
the middle, and that is the applied effort.
The Pulley
Pulleys are simple machines in the form of a wheel mounted
on a fixed axis and supported by a frame. The wheel, or disk,
is normally grooved to accommodate a rope. The wheel is
sometimes referred to as a “sheave,” or sometimes “sheaf.”
The frame that supports the wheel is called a block. A block and
tackle consists of a pair of blocks. Each block contains one or
more pulleys and a rope connecting the pulley(s) of each block.
Single Fixed Pulley
A single fixed pulley is really a first class lever with equal arms.
In Figure 5-10 , the arm from point “R” to point “F” is equal to
the arm from point “F” to point “E,” with both distances being
equal to the radius of the pulley. When a first class lever has
equal arms, the mechanical advantage is 1. Thus, the force of
the pull on the rope must be equal to the weight of the object
being lifted. The only advantage of a single fixed pulley is to
change the direction of the force, or pull on the rope.
Single Movable Pulley
A single pulley can be used to magnify the force exerted. In
Figure 5-11 , the pulley is movable, and both ropes extending
up from the pulley are sharing in the support of the weight.
This single, movable pulley will act like a second class lever.
The effort arm (EF) being the diameter of this pulley and the
resistance arm (FR) being the radius of this pulley. This type of
pulley would have a mechanical advantage of two because the
diameter of the pulley is double the radius of the pulley. In use,
if someone pulled in 4 ft of the effort rope, the weight would
only rise off the floor 2 ft. If the weight was 100 lb, the effort concept of work. If it allows you to lift 5 times more weight,
you will only move it 1⁄5 as far as you move the effort, because
a lever cannot have more work output than input.
Second Class Lever
The second class lever has the fulcrum at one end and the
effort is applied at the other end. The resistance is somewhere
between these points. A wheelbarrow is a good example of
a second class lever, with the wheel at one end being the
fulcrum, the handles at the opposite end being the applied
effort, and the bucket in the middle being where the weight
or resistance is placed. [Figure 5-8]
Both first and second class levers are commonly used to help
in overcoming big resistances with a relatively small effort.
The first class lever, however, is more versatile. Depending
on how close or how far away the weight is placed from the
fulcrum, the first class lever can be made to gain force or
gain distance, but not both at the same time. The second class
lever can only be made to gain force.
Example: The distance from the center of the wheel to the
handles on a wheelbarrow is 60 inches. The weight in the
bucket is 18 inches from the center of the wheel. If 300 lb is
placed in the bucket, how much force must be applied at the
handles to lift the wheelbarrow?
Effort (E) × Effort Arm (L) = Resistance (R) × Resistance Arm (l)
E × 60 inches = 300 lb × 18 in
E = 300 lb × 18 in ÷ 60 in
E = 90 lb
The mechanical advantage of the lever in this example
would be:
Mechanical Advantage = Force Out ÷ Force In
= 300 lb ÷ 90 lb
= 3.33, or 3.33 to 1
Third Class Lever
There are occasions when it is desirable to speed up the
movement of the resistance even though a large amount of
effort must be used. Levers that help accomplish this are third
5-10Resistance “R”Effort “E”
Fulcrum “F”“l”“L”
WeightR E F
Effort “E ”
Figure 5-9. Third class lever. Figure 5-10. Single fixed pulley.is an increase in force, and ultimately torque.
Bevel gears are used to change the plane of rotation, so that
a shaft turning horizontally can make a vertical shaft rotate. The size of the gears and their number of teeth determine the mechanical advantage, and whether force is being increased or rpm is being increased. If each gear has the same number of teeth, there would be no change in force or rpm. [Figure 5-14]
The worm gear has an extremely high mechanical advantage. The input force goes into the spiral worm gear, which drives the spur gear. One complete revolution of the worm gear only makes the spur gear move an amount equal to one tooth. The mechanical advantage is equal to the number of teeth on the spur gear, which in this case there are 25. This is a force gaining machine, to the tune of 25 times more output force. [Figure 5-15]
The planetary sun gear system is typical of what would be
found in a propeller reduction gearbox. The power output shaft of the engine would drive the sun gear in the middle, applied would only need to be 50 lb. With this type of pulley, the effort will always be one-half of the weight being lifted.
Block and Tackle
A block and tackle is made up of multiple pulleys, some of them fixed and some movable. In Figure 5-12, the block and
tackle is made up of four pulleys, the top two being fixed and the bottom two being movable. Viewing the figure from right to left, notice there are four ropes supporting the weight and a fifth rope where the effort is applied. The number of weight supporting ropes determines the mechanical advantage of a block and tackle, so in this case the mechanical advantage is four. If the weight was 200 lb, it would require a 50 lb effort to lift it.
The Gear
Two gears with teeth on their outer edges, as shown in
Figure 5-13, act like a first class lever when one gear drives the other. The gear with the input force is called the drive gear, and the other is called the driven gear. The effort arm is the diameter of the driven gear, and the resistance arm is the diameter of the drive gear. Notice that the two gears turn in opposite directions: the bottom one clockwise and the top one counterclockwise. The gear on top is 9 inches in diameter and has 45 teeth, and the gear on the bottom is 12 inches in diameter and has 60 teeth.
Imagine that the blue gear is driving the yellow one, which
makes the blue the drive and the yellow the driven. The mechanical advantage in terms of force would be the effort arm divided by the resistance arm, or 9 ÷ 12, which is 0.75. This would actually be called a fractional disadvantage, because there would be less force out than force in. The mechanical advantage in terms of distance, in rpm in this case, would be 12 ÷ 9, or 1.33.
This analysis tells us that when a large gear drives a small
one, the small one turns faster and has less available force. In order to be a force gaining machine, the small gear needs to turn the large one. When the terminology reduction gearbox is used, such as a propeller reduction gearbox, it means that there is more rpm going in than is coming out. The end result
WeightRE
FEffort
Weight
Support ropesEffort
Figure 5-11. Single movable pulley.Figure 5-12. Block and tackle.
which rotates the planetary gears and ultimately the ring gear.
In this example, the sun gear has 28 teeth, each planet gear has 22 teeth, and the ring gear has 82 teeth. To figure how much gear reduction is taking place, the number of teeth on the ring gear is divided by the number of teeth on the sun gear. In this case, the gear reduction is 2.93, meaning the engine has an rpm 2.93 times greater than the propeller. [Figure 5-16]
Inclined Plane
The inclined plane is a simple machine that facilitates the
raising or lowering of heavy objects by application of a small force over a relatively long distance. Some familiar examples of the inclined plane are mountain highways and a loading ramp on the back of a moving truck. When weighing a small airplane, like a Cessna 172, an inclined plane, or ramp, can be used to get the airplane on the scales by pushing it, rather than jacking it. A ramp can be seen in Figure 5-17, where a
Cessna 172 right main gear is sitting on an electronic scale. The airplane was pushed up the ramps to get it on the scales.
With an inclined plane, the length of the incline is the effort
arm and the vertical height of the incline is the resistance arm. If the length of the incline is five times greater than the height, there will be a force advantage, or mechanical advantage, of five. The Mooney M20 in Figure 5-17 weighed
1,600 lb on the day of the weighting. The ramp it is sitting on is 6 inches tall, which is the resistance arm, and the length of the ramp is 24 inches, which is the effort arm. To calculate
Figure 5-13. Spur gears.
Figure 5-14. Bevel gears.the force needed to push the airplane up the ramps, use the
same formula introduced earlier when levers were discussed,
as follows:
Effort (E) × Effort Arm (L) = Resistance (R) × Resistance Arm (l)
E × 24 in = 1,600 lb × 6 in
E = 1,600 l-b × 6 in ÷ 24 in
E = 400 lb
Bolts, screws, and wedges are also examples of devices
that operate on the principle of the inclined plane. A
bolt, for example, has a spiral thread that runs around
its circumference. As the thread winds around the bolt’s
circumference, it moves a vertical distance equal to the
space between the threads. The circumference of the bolt
is the effort arm and the distance between the threads is the
resistance arm. [Figure 5-18] Based on this analysis, a fine
threaded bolt, which has more threads per inch, has a greater mechanical advantage than a coarse threaded bolt.
A chisel is a good example of a wedge. A chisel might be
8 inches long and only 1⁄2 inch wide, with a sharp tip and
tapered sides. The 8-inch length is the effort arm and the 1⁄2-
inch width is the resistance arm. This chisel would provide
a force advantage, or mechanical advantage, of 16.
Stress
Whenever a machine is in operation, be it a simple machine
like a lever or a screw, or a more complex machine like an
aircraft piston engine or a hydraulically operated landing
gear, the parts and pieces of that machine will experience
something called stress. Whenever an external force is
applied to an object, like a weight pushing on the end of a
lever, a reaction will occur inside the object which is known
as stress. Stress is typically measured in pounds per square
foot or pounds per square inch (psi).
External force acting on an object causes the stress to
manifest itself in one of five forms, or combination of those
five. The five forms are tension, compression, torsion,
bending, and shear.
Tension
Tension is a force that tries to pull an object apart. In the
block and tackle system discussed earlier in this chapter,
the upper block that housed the two fixed pulleys was
secured to an overhead beam. The movable lower block
Ring gear Planetary gearsSun gearFigure 5-15. Worm gear.Figure 5-17. Ramp in use with a Mooney M20.
Figure 5-16. Planetary sun gear.and its two pulleys were hanging by ropes, and the weight
was hanging below the entire assembly. The weight being
lifted would cause the ropes and the blocks to be under
tension. The weight is literally trying to pull the rope apart,
and ultimately would cause the rope to break if the weight
was too great.
Compression
Compression is a force that tries to crush an object. An
excellent example of compression is when a sheet metal
airplane is assembled using the fastener known as a rivet. The
rivet passes through a hole drilled in the pieces of aluminum,
and then a rivet gun on one side and a bucking bar on the
other apply a force. This applied force tries to crush the rivet and makes it expand to fill the hole and securely hold the
aluminum pieces together. [Figure 5-19]
Torsion
Torsion is the stress an object experiences when it is
twisted, which is what happens when torque is applied to a
shaft. Torsion is made up of two other stresses: tension and
compression. When a shaft is twisted, tension is experienced
at a diagonal to the shaft and compression acts 90 degrees to
the tension. [Figure 5-20]
The turbine shaft on a turbofan engine, which connects to
the compressor in order to drive it, is under a torsion stress.
The turbine blades extract energy from the high velocity air
as a force in pounds. This force in pounds acts along the
length from the blades to the center of the shaft, and creates
the torque that causes rotation. [Figure 5-21]
Bending
An airplane in flight experiences a bending force on the wing
as aerodynamic lift tries to raise the wing. This force of lift
causes the skin on the top of the wing to compress and the
skin on the bottom of the wing to be under tension. When the
airplane is on the ground sitting on its landing gear, the force
of gravity tries to bend the wing downward, subjecting the
bottom of the wing to compression and the top of the wing
tension. [Figure 5-22] During the testing that occurs prior
to FAA certification, an airplane manufacturer intentionally
bends the wing up and down to make sure it can take the
stress without failing.
Shear
When a shear stress is applied to an object, the force tries
to cut or slice through, like a knife cutting through butter.
A clevis bolt, which is often used to secure a cable to a part
of the airframe, has shear stress acting on it. As shown in
Figure 5-23 , a fork fitting is secured to the end of the cable,
Circumference (Effort arm)Distance between threads
(Resistance arm)
Resistance armEffort arm
Rivet head
Rivet shank
Bucking barForce from rivet gun
Figure 5-18. A bolt and nut as an inclined plane.
Figure 5-19. A rivet fastener and compression.and the fork attaches to an eye on the airframe with the clevis
bolt. When the cable is put under tension, the fork tries to
slide off the eye by cutting through the clevis bolt. This bolt
would be designed to take very high shear loads.
Strain
If the stress acting on an object is great enough, it can cause
the object to change its shape or to become distorted. One
characteristic of matter is that it tends to be elastic, meaning
it can be forced out of shape when a force is applied and then
return to its original shape when the force is removed. When
an object becomes distorted by an applied force, the object
is said to be strained.
On turbine engine test cells, the thrust of the engine is
typically measured by what are called strain gages. When
the force, or thrust, of the engine is pulling out against the
strain gages, the amount of distortion is measured and then
translated into the appropriate thrust reading.
A deflecting beam style of torque wrench uses the strain on the
drive end of the wrench and the resulting distortion of the beam
to indicate the amount of torque on a bolt or nut. [Figure 5-24]
Motion
The study of the relationship between the motion of bodies
or objects and the forces acting on them is often called the
study of “force and motion.” In a more specific sense, the
relationship between velocity, acceleration, and distance is
known as kinematics.Uniform Motion
Motion may be defined as a continuing change of position
or place, or as the process in which a body undergoes
displacement. When an object is at different points in space
at different times, that object is said to be in motion, and if
the distance the object moves remains the same for a given
period of time, the motion may be described as uniform. Thus,
an object in uniform motion always has a constant speed.
Speed and Velocity
In everyday conversation, speed and velocity are often used
as if they mean the same thing. In physics, they have definite
Tension stress
CompressionRotation
Wing top is
under tension
Wing bottom is
under compression
ForceClevis Bolt
Torque applied to shaft
Turbine blades applied forceShaft experiences torsionFigure 5-20. Torsion on a rotating shaft, made up of tension and
compression.Figure 5-22. Airplane on the ground, wing under tension and
compression.
Figure 5-23. Clevis bolt, red arrows show opposing forces trying
to shear the bolt. Figure 5-21. Turbofan engine, torque creating torsion in the shaft.and distinct meanings. Speed refers to how fast an object is
moving, or how far the object will travel in a specific time.
The speed of an object tells nothing about the direction an
object is moving. For example, if the information is supplied
that an airplane leaves New York City and travels 8 hours at
a speed of 150 mph, this information tells nothing about the
direction in which the airplane is moving. At the end of 8
hours, it might be in Kansas City, or if it traveled in a circular
route, it could be back in New York City.
Velocity is that quantity in physics which denotes both the
speed of an object and the direction in which the object
moves. Velocity can be defined as the rate of motion in a
particular direction. Velocity is also described as being a
vector quantity, a vector being a line of specific length,
having an arrow on one end or the other. The length of the
line indicates the number value and the arrow indicates the
direction in which that number is acting.
Two velocity vectors, such as one representing the velocity of an airplane and one representing the velocity of the wind, can
be added together in what is called vector analysis. Figure 5-25
demonstrates this, with vectors “A” and “B” representing the
velocity of the airplane and the wind, and vector “C” being the
resultant. With no wind, the speed and direction of the airplane
would be that shown by vector “A.” When accounting for the
wind direction and speed, the airplane ends up flying at the
speed and direction shown by vector “C.”
Imagine that an airplane is flying in a circular pattern at a
constant speed. The airplane is constantly changing direction
because of the circular pattern, which means the airplane is
constantly changing velocity. The reason for this is the fact
that velocity includes direction.
To calculate the speed of an object, the distance it travels is
divided by the elapsed time. If the distance is measured in
miles and the time in hours, the units of speed will be miles
per hour (mph). If the distance is measured in feet and the
time in seconds, the units of speed will be feet per second
(fps). To convert mph to fps, divide by 1.467. Velocity is
calculated the same way, the only difference being it must
be recalculated every time the direction changes.
Acceleration
Acceleration is defined as the rate of change of velocity. If
the velocity of an object is increased from 20 mph to 30 mph,
the object has been accelerated. If the increase in velocity is
10 mph in 5 seconds, the rate of change in velocity is 10 mph
in 5 seconds, or 2 mph per second. If this were multiplied
Figure 5-24. Deflecting beam torque wrench measures strain by
distortion.
by 1.467, it could also be expressed as an acceleration of
2.93 feet per second per second (fps/s). By comparison, the
acceleration due to gravity is 32.2 fps/s.
To calculate acceleration, the following formula is used.
Acceleration (A) = Time (t)Velocity Final (Vf) − Velocity Initial (Vi)
Example: An Air Force F-15 fighter is cruising at 400 mph.
The pilot advances the throttles to full afterburner and
accelerates to 1,200 mph in 20 seconds. What is the average
acceleration in mph/s and fps/s?
A = tVf − Vi
A = 201200 − 400
A = 40 mph⁄s, or multiplying by 1.467, 58.7 fps⁄s
In the example just shown, the acceleration was found to be
58.7 fps/s. Since 32.2 fps/s is equal to the acceleration due
to gravity, divide the F-15’s acceleration by 32.2 to find out
how many G forces the pilot is experiencing. In this case,
it would be 1.82 Gs.Newton’s Law of Motion
First Law
When a magician snatches a tablecloth from a table and
leaves a full setting of dishes undisturbed, he is not displaying
a mystic art; he is actually demonstrating the principle of
inertia. Inertia is responsible for the discomfort felt when an
airplane is brought to a sudden halt in the parking area and
the passengers are thrown forward in their seats. Inertia is a
property of matter. This property of matter is described by
Newton’s first law of motion, which states:
Objects at rest tend to remain at rest and objects in motion
tend to remain in motion at the same speed and in the same
direction, unless acted on by an external force.
Second Law
Bodies in motion have the property called momentum. A body
that has great momentum has a strong tendency to remain
in motion and is therefore hard to stop. For example, a train
moving at even low velocity is difficult to stop because of
its large mass. Newton’s second law applies to this property.
It states:
When a force acts upon a body, the momentum of that body
is changed. The rate of change of momentum is proportional
to the applied force. Based on Newton’s second law, the
formula for calculating thrust is derived, which states that
force equals mass times acceleration (F = MA). Earlier in this
chapter, it was determined that mass equals weight divided by
gravity, and acceleration equals velocity final minus velocity
initial divided by time. Putting all these concepts together,
the formula for thrust is:
Force = Gravity (Time)Weight (Velocity final − Velocity initial)
Force = GtW (Vf − Vi)
Example: A turbojet engine is moving 150 lb of air per
second through the engine. The air enters going 100 fps and
leaves going 1,200 fps. How much thrust, in pounds, is the
engine creating?
F = GtW (Vf − Vi)
F = 32.2(1)150 (1200 − 100)
F = 5,124 lb of thrust
Vector C = Movement of airplaneVector A = Velocity of airplaneVector B = Wind
Figure 5-25. Vector analysis for airplane velocity and wind velocity.
Third Law
Newton’s third law of motion is often called the law of action
and reaction. It states that for every action there is an equal
and opposite reaction. This means that if a force is applied
to an object, the object will supply a resistive force exactly
equal to and in the opposite direction of the force applied.
It is easy to see how this might apply to objects at rest. In
application, as a man stands on the floor, the floor exerts a
force against his feet exactly equal to his weight. This law is
also applicable when a force is applied to an object in motion.
Forces always occur in pairs. The term “ acting force” means
the force one body exerts on a second body, and reacting
force means the force the second body exerts on the first. When an aircraft propeller pushes a stream of air backward
with a force of 500 lb, the air pushes the blades forward with
a force of 500 lb. This forward force causes the aircraft to
move forward. A turbofan engine exerts a force on the air
entering the inlet duct, causing it to accelerate out the fan
duct and the tailpipe. The air accelerating to the rear is the
action, and the force inside the engine that makes it happen
is the reaction, also called thrust.
Circular Motion
Circular motion is the motion of an object along a curved
path that has a constant radius. For example, if one end of a
string is tied to an object and the other end is held in the hand,
the object can be swung in a circle. The object is constantly
deflected from a straight (linear) path by the pull exerted on
the string, as shown in Figure 5-26 . When the weight is at
point A, due to inertia it wants to keep moving in a straight
line and end up at point B. It is forced to move in a circular
path and end up at point C because of the force being exerted
on the string.
The string exerts a centripetal force on the object, and the
object exerts an equal but opposite force on the string,
obeying Newton’s third law of motion. The force that is equal
to centripetal force, but acting in an opposite direction, is
called centrifugal force.
Centripetal force is always directly proportional to the mass
of the object in circular motion. Thus, if the mass of the
object in Figure 5-26 is doubled, the pull on the string must
be doubled to keep the object in its circular path, provided
the speed of the object remains constant.
Centripetal force is inversely proportional to the radius
of the circle in which an object travels. If the string in
Figure 5-26 is shortened and the speed remains constant,
the pull on the string must be increased since the radius is
decreased, and the string must pull the object from its linear
path more rapidly. Using the same reasoning, the pull on the
string must be increased if the object is swung more rapidly
in its orbit. Centripetal force is thus directly proportional
to the square of the velocity of the object. The formula for
centripetal force is:
Centripetal Force = Mass (Velocity2) ÷ Radius
For the formula above, mass would typically be converted
to weight divided by gravity, velocity would be in feet per
second, and the radius would be in feet.
Example: What would the centripetal force be if a 10-pound
weight was moving in a 3-ft radius circular path at a velocity
of 500 fps?
Centrifugal forceCentripetal forceB
AWT
“C”
Figure 5-26. Circular motion. Centripetal Force = Mass (Velocity2) ÷ Radius
Centripetal Force = 10 (5002) ÷ 32.2 (3)
= 25,880 lb
In the condition identified in the example, the object acts like
it weighs 2,588 times more than it actually does. It can also
be said that the object is experiencing 2,588 Gs, or force of
gravity. The fan blades in a large turbofan engine, when the
engine is operating at maximum rpm, are experiencing many
thousands of Gs for the same reason.
Heat
Heat is a form of energy. It is produced only by the conversion
of one of the other forms of energy. Heat may also be defined
as the total kinetic energy of the molecules of any substance.
Some forms of energy which can be converted into heat
energy are as follows:
• Mechanical Energy—this includes all methods of
producing increased motion of molecules such as
friction, impact of bodies, or compression of gases.
• Electrical Energy—electrical energy is converted to
heat energy when an electric current flows through
any form of resistance such as an electric iron, electric
light, or an electric blanket.
• Chemical Energy—most forms of chemical reaction
convert stored potential energy into heat. Some
examples are the explosive effects of gunpowder, the
burning of oil or wood, and the combining of oxygen
and grease.
• Radiant Energy—electromagnetic waves of certain
frequencies produce heat when they are absorbed by
the bodies they strike such as x-rays, light rays, and
infrared rays.
• Nuclear Energy—energy stored in the nucleus of
atoms is released during the process of nuclear fission
in a nuclear reactor or atomic explosion.
• Sun—all heat energy can be directly or indirectly
traced to the nuclear reactions occurring in the sun.
When a gas is compressed, work is done and the gas becomes
warm or hot. Conversely, when a gas under high pressure is
allowed to expand, the expanding gas becomes cool. In the
first case, work was converted into energy in the form of
heat; in the second case heat energy was expended. Since
heat is given off or absorbed, there must be a relationship
between heat energy and work. Also, when two surfaces are
rubbed together, the friction develops heat. However, work
was required to cause the heat, and by experimentation, it
has been shown that the work required and the amount of
heat produced by friction is proportional. Thus, heat can be regarded as a form of energy.
According to this theory of heat as a form of energy, the
molecules, atoms, and electrons in all bodies are in a continual
state of motion. In a hot body, these small particles possess
relatively large amounts of kinetic energy, but in cooler
bodies they have less. Because the small particles are given
motion, and hence kinetic energy, work must be done to
slide one body over the other. Mechanical energy apparently
is transformed, and what we know as heat is really kinetic
energy of the small molecular subdivisions of matter.
Heat Energy Units
Two different units are used to express quantities of heat
energy. They are the calorie and the BTU. One calorie is equal
to the amount of heat required to change the temperature of
1 gram of water 1 degree Centigrade.
This term “calorie” (spelled with a lower case c) is 1/1,000 of
the Calorie (spelled with a capital C) used in the measurement
of the heat energy in foods. One BTU is defined as the amount
of heat required to change the temperature of 1 lb of water
1 degree Fahrenheit (1 °F). The calorie and the gram are
seldom used in discussing aviation maintenance. The BTU,
however, is commonly referred to in discussions of engine
thermal efficiencies and the heat content of aviation fuel.
A device known as the calorimeter is used to measure
quantities of heat energy. In application, it may be used to
determine the quantity of heat energy available in 1 pound of
aviation gasoline. A given weight of the fuel is burned in the
calorimeter, and the heat energy is absorbed by a large quantity
of water. From the weight of the water and the increase in its
temperature, it is possible to compute the heat yield of the fuel.
A definite relationship exists between heat and mechanical
energy. This relationship has been established and verified by
many experiments which show that:
One BTU of heat energy = 778 ft-lb of work
As discussed earlier in this chapter under the topic “ Potential
Energy,” one pound of aviation gasoline contains 18,900 BTU
of heat energy. Since each BTU is capable of 778 ft-lb of work,
1 lb of aviation gasoline is capable of 14,704,200 ft-lb of work.
Heat Energy and Thermal Efficiency
Thermal efficiency is the relationship between the potential
for power contained in a specific heat source, and how much
usable power is created when that heat source is used. The
formula for calculating thermal efficiency is:
Thermal Efficiency =
Horsepower Produced ÷ Potential Horsepower in Fuel
For example, consider the piston engine used in a small
general aviation airplane, which typically consumes 0.5
lb of fuel per hour for each horsepower it creates. Imagine
that the engine is creating 200 hp. If we multiply 0.5 by the
horsepower of 200, we find the engine is consuming 100 lb of
fuel per hour, or 1.67 lb per minute. Earlier in this chapter, one
horsepower was found to be 33,000 ft-lb of work per minute.
The potential horsepower in the fuel burned for this example
engine would be:
Hp = 33,000 ft-lb/min 1.67 lb/minute × 18,900 BTU/lb × 778 ft lb/BTU
Hp = 744
The example engine is burning enough fuel that it has the
potential to create 744 horsepower, but it is only creating 200.
The thermal efficiency of the engine would be:
Thermal Efficiency = Hp Produced ÷ Hp in Fuel
= 200 ÷ 744
= .2688 or 26.88%
More than 70 percent of the energy in the fuel is not being
used to create usable horsepower. The wasted energy is in the form of friction and heat. A tremendous amount of heat
is given up to the atmosphere and not used inside the engine
to create power.
Heat Transfer
There are three methods by which heat is transferred from one
location to another or from one substance to another. These
three methods are conduction, convection, and radiation.
Conduction
Heat transfer always takes place by areas of high heat energy
migrating to areas of low heat energy. Heat transfer by
conduction requires that there be physical contact between
an object that has a large amount of heat energy and one that
has a smaller amount of heat energy.
Everyone knows from experience that the metal handle of
a heated pan can burn the hand. A plastic or wood handle,
however, remains relatively cool even though it is in direct
contact with the pan. The metal transmits the heat more
easily than the wood because it is a better conductor of heat.
Different materials conduct heat at different rates. Some
metals are much better conductors of heat than others.
Aluminum and copper are used in pots and pans because
they conduct heat very rapidly. Woods and plastics are used
for handles because they conduct heat very slowly.
Figure 5-27 illustrates the different rates of conduction of
various metals. Of those listed, silver is the best conductor
and lead is the poorest. As mentioned previously, copper and
aluminum are used in pots and pans because they are good
conductors. It is interesting to note that silver, copper, and
aluminum are also excellent conductors of electricity.
Liquids are poorer conductors of heat than metals. Notice
that the ice in the test tube shown in Figure 5-28 is not
melting rapidly even though the water at the top is boiling.
The water conducts heat so poorly that not enough heat
reaches the ice to melt it.
Gases are even poorer conductors of heat than liquids. It is
possible to stand quite close to a stove without being burned
because air is such a poor conductor. Since conduction is a
process whereby the increase in molecular energy is passed
along by actual contact, gases are poor conductors.
At the point of application of the heat source, the molecules
become violently agitated. These molecules strike adjacent
molecules causing them to become agitated. This process
continues until the heat energy is distributed evenly
throughout the substance. The gases are much poorer
conductors of heat because molecules are farther apart in
gases than in solids.
LEAD0.18
IRON0.22
NICKEL0.37
MAGNESIUM0.57
ALUMINUM0.94
COPPER1.00
SILVER
Water
IceSteam
Boiling water
Metal ring to keep
ice from rising
Figure 5-27. Conductivity of various metals.
Figure 5-28. Water as a poor conductor.Materials that are poor conductors are used to prevent the
transfer of heat and are called heat insulators. A wooden
handle on a pot or a soldering iron serves as a heat insulator.
Certain materials, such as finely spun glass or asbestos,
are particularly poor heat conductors. These materials are
therefore used for many types of insulation.
Convection
Convection is the process by which heat is transferred by
movement of a heated fluid (gas or liquid). For example,
an incandescent light bulb will, when heated, become
increasingly hotter until the air surrounding it begins to move.
The motion of the air is upward. This upward motion of the
heated air carries the heat away from the hot light bulb by
convection. Transfer of heat by convection may be hastened
by using a ventilating fan to move the air surrounding a hot
object. The rate of cooling of a hot electronics component,
such as the CPU in a computer, can be increased if it is
provided with copper fins that conduct heat away from the
hot surface. The fins provide large surfaces against which
cool air can be blown.
A convection process may take place in a liquid as well as in
a gas. A good example of this is a pan of water sitting on the
stove. The bottom of the pan becomes hot because it conducts
heat from the surface it is in contact with. The water on the
bottom of the pan also heats up because of conduction. As
the heated water starts to rise and cooler water moves in to
take its place, the convection process begins.
When the circulation of gas or liquid is not rapid enough to
remove sufficient heat, fans or pumps are used to accelerate
the motion of the cooling material. In some installations,
pumps are used to circulate water or oil to help cool large
equipment. In airborne installations, electric fans and blowers
are used to aid convection.
An aircraft air-cooled piston engine is a good example of
convection being used to transfer heat. The engine shown in
Figure 5-29 is a Continental IO-520, with six heavily finned
air-cooled cylinders. This engine does not depend on natural
convection for cooling, but rather forced air convection
coming from the propeller on the engine. The heat generated
inside the engine finds its way to the cylinder cooling fins by
conduction, meaning transfer within the metal of the cylinder.
Once the heat gets to the fins, forced air flowing around the
cylinders carries the heat away.
Radiation
Conduction and convection cannot wholly account for some
of the phenomena associated with heat transfer. For example,
the heat one feels when sitting in front of an open fire cannot be transferred by convection because the air currents are
moving toward the fire. It cannot be transferred through
conduction because the conductivity of the air is very small,
and the cooler currents of air moving toward the fire would
more than overcome the transfer of heat outward. Therefore,
there must be some way for heat to travel across space other
than by conduction and convection.
The existence of another process of heat transfer is still more
evident when the heat from the sun is considered. Since
conduction and convection take place only through some
medium, such as a gas or a liquid, heat from the sun must
reach the earth by another method, since space is an almost
perfect vacuum. Radiation is the name given to this third
method of heat transfer.
Figure 5-29. Aircraft piston engine cooled by convection.The term “radiation” refers to the continual emission of
energy from the surface of all bodies. This energy is known
as “radiant energy.” It is in the form of electromagnetic
waves, radio waves, or x-rays, which are all alike except
for a difference in wave length. These waves travel at the
velocity of light and are transmitted through a vacuum more
easily than through air because air absorbs some of them.
Most forms of energy can be traced back to the energy of
sunlight. Sunlight is a form of radiant heat energy that travels
through space to reach the earth. These electromagnetic
heat waves are absorbed when they come in contact with
nontransparent bodies. The result is that the motion of the
molecules in the body is increased as indicated by an increase
in the temperature of the body.
The differences between conduction, convection, and radiation
may now be considered. First, although conduction and
convection are extremely slow, radiation takes place at the
speed of light. This fact is evident at the time of an eclipse of
the sun when the shutting off of the heat from the sun takes
place at the same time as the shutting off of the light. Second,
radiant heat may pass through a medium without heating
it. In application, the air inside a greenhouse may be much
warmer than the glass through which the sun’s rays pass. Third,
although heat transfer by conduction or convection may travel
in roundabout routes, radiant heat always travels in a straight
line. For example, radiation can be cut off with a screen placed
between the source of heat and the body to be protected.
Specific Heat
One important way in which substances differ is in the
requirement of different quantities of heat to produce the
same temperature change in a given mass of the substance.
Each substance requires a quantity of heat, called its specific
heat capacity, to increase the temperature of a unit of its
mass 1 °C. The specific heat of a substance is the ratio of its
specific heat capacity to the specific heat capacity of water.
Specific heat is expressed as a number which, because it is
a ratio, has no units and applies to both the English and the
metric systems.
It is fortunate that water has a high specific heat capacity.
The larger bodies of water on the earth keep the air and
solid matter on or near the surface of the earth at a constant
temperature. A great quantity of heat is required to change
the temperature of a large lake or river. Therefore, when
the temperature falls below that of such bodies of water,
they give off large quantities of heat. This process keeps
the atmospheric temperature at the surface of the earth from
changing rapidly.
The specific heat values of some common materials are listed
in Figure 5-30 .Temperature
Temperature is a dominant factor affecting the physical
properties of fluids. It is of particular concern when
calculating changes in the state of gases.
The three temperature scales used extensively are the
Centigrade, the Fahrenheit, and the absolute or Kelvin scales.
The Centigrade scale is constructed by using the freezing and
boiling points of water, under standard conditions, as fixed
points of zero and 100, respectively, with 100 equal divisions
between. The Fahrenheit scale uses 32° as the freezing point
of water and 212° as the boiling point, and has 180 equal
divisions between. The absolute or Kelvin scale is constructed
with its zero point established as minus 273 °C, meaning
273° below the freezing point of water. The relationships of
the other fixed points of the scales are shown in Figure 5-31 .
When working with temperatures, always make sure which
system of measurement is being used and know how to
convert from one to another. The conversion formulas are
as follows:
Degrees Fahrenheit = (1.8 × Degrees Celsius) + 32
Degrees Celsius = (Degrees Fahrenheit – 32) × 5⁄9
Degrees Kelvin = Degrees Celsius + 273
Degrees Rankine = Degrees Fahrenheit + 460
For purposes of calculations, the Rankine scale is commonly
used to convert Fahrenheit to absolute. For Fahrenheit
readings above zero, 460° is added. Thus, 72 °F equals
460° plus 72°, or 532° absolute. If the Fahrenheit reading is
below zero, it is subtracted from 460°. Thus −40 °F equals
460° minus 40°, or 420° absolute. It should be stressed that
the Rankine scale does not indicate absolute temperature
readings in accordance with the Kelvin scale, but these
conversions may be used for the calculations of changes in
5-22Material Specific Heat
Lead
Mercury
Brass
Copper
Iron or Steel
Glass
Alcohol
Aluminum
Water0.031
0.0330.0940.0950.1130.1950.5470.7121.000
−273212
−460373
0Pure water boils
Pure water freezes
Celsius
(centigrade)Kelvin Rankine FahrenheitMolecular motion
ceases at absolutezero
Figure 5-30. Specific heat value for various substances. Figure 5-31. Comparison of temperature scales.the state of gases.
The Kelvin and Centigrade scales are used more extensively
in scientific work; therefore, some technical manuals may use these scales in giving directions and operating instructions. The Fahrenheit scale is commonly used in the United States, and most people are familiar with it. Therefore, the Fahrenheit scale is used in most areas of this book.
Thermal Expansion/Contraction
Thermal expansion takes place in solids, liquids, and
gases when they are heated. With few exceptions, solids will expand when heated and contract when cooled. The expansion of solids when heated is very slight in comparison to the expansion in liquids and gases because the molecules of solids are much closer together and are more strongly attracted to each other. The expansion of fluids is discussed in the study of Boyle’s law. Thermal expansion in solids must
be explained in some detail because of its close relationship to aircraft metals and materials.
It is necessary to measure experimentally the exact rate of
expansion of each one because some substances expand more than others. The amount that a unit length of any substance expands for a one degree rise in temperature is known as the coefficient of linear expansion for that substance. The coefficient of linear expansion for various materials is shown in Figure 5-32.
To estimate the expansion of any object, such as a steel rail,
it is necessary to know three things about it: its length, the rise in temperature to which it is subjected, and its coefficient of expansion. This relationship is expressed by the equation:
Expansion = (coefficient) × (length) × (rise in temperature)If a steel rod measures exactly 9 ft at 21 °C, what is its length
at 55 °C? The coefficient of expansion for steel is 11 × 10
−6.Expansion = (1 1 × 10−6) × (9 feet) × 34°
Expansion = 0.003366 feet
This amount, when added to the original length of the rod, makes the rod 9.003366 ft long. Its length has only increased by
4⁄100 of an inch.
The increase in the length of the rod is relatively small, but if the rod were placed where it could not expand freely, there would be a tremendous force exerted due to thermal expansion. Thus, thermal expansion must be taken into consideration when designing airframes, power plants, or related equipment.
Pressure
Pressure is the amount of force acting on a specific amount of surface area. The force is typically measured in pounds and the surface area in square inches, making the units of pressure pounds per square inch or psi. If a 100-lb weight was placed on top of a block with a surface area of 10 in
2,
the average weight distribution would be 10 lb for each of the square inches (100 ÷ 10), or 10 psi.
When atmospheric pressure is being measured, in addition
to psi, other means of pressure measurement can be used. These include inches or millimeters of mercury, and millibars. Standard day atmospheric pressure is equal to 14.7 psi,
29.92 inches of mercury ("Hg), 760 millimeters of mercury (mm hg), or 1013.2 millibars. The relationship between these units of measure is as follows:
1 psi = 2.04 "Hg
1 psi = 51.7 mm Hg
1 psi = 68.9 millibars
The concept behind measuring pressure in inches of mercury
5-23SubstanceCoefficient of Expansion
Per Degree Centigrade
Aluminum
Brass or Bronze
Brick
Copper
Glass (Plate)
Glass (Pyrex)
Ice
Iron or Steel
Lead
Quartz
Silver25 x 10–6
19 x 10–6
9 x 10–6
17 x 10–6
9 x 10–6
3 x 10–6
51 x 10–6
11 x 10–6
29 x 10–6
0.4 x 10–6
19 x 10–6
Figure 5-32. Coefficient of expansion for various materials.involves filling a test tube with the liquid mercury and then
covering the top. The test tube is then turned upside down and placed in an open container of mercury, and the top is uncovered. Gravity acting on the mercury in the test tube will try to make the mercury run out. Atmospheric pressure pushing down on the mercury in the open container tries to make the mercury stay in the test tube. At some point these two forces, gravity and atmospheric pressure, will equal out and the mercury will stabilize at a certain height in the test tube. Under standard day atmospheric conditions, the air in a 1-in
column extending all the way to the top of the atmosphere would weigh 14.7 lb. A 1 in2 column of mercury, 29.92 inches tall, would also weigh 14.7 lb. That is why 14.7 psi
is equal to 29.92 "Hg. Figure 5-33 demonstrates this point.
Gauge Pressure
A gauge pressure (psig) is a reading that refers to when an
instrument, such as an oil pressure gauge, fuel pressure gauge,
or hydraulic system pressure gauge, displays pressure which is over and above ambient. This can be seen on the fuel pressure gauge shown in Figure 5-34. When the oil, fuel, or
hydraulic pump is not turning, and there is no pressure being created, the gauge will read zero.
Absolute Pressure
A gauge that includes atmospheric pressure in its reading
is measuring what is known as absolute pressure, or psia. Absolute pressure is equal to gauge pressure plus atmospheric pressure. If someone hooked up a psia indicating instrument to an engine’s oil system, the gauge would read atmospheric pressure when the engine was not running. Since this would not make good sense to the typical operator, psia gauges are not used in this type of application. For the manifold pressure on a piston engine, a psia gauge does make good sense. Manifold pressure on a piston engine can read anywhere from less than atmospheric pressure if the engine is not supercharged, to more than atmospheric if it is supercharged. The only gauge that has the flexibility to show this variety of readings is the absolute pressure gauge. Figure 5-35 shows a manifold pressure gauge,
with a readout that ranges from 10 "Hg to 35 "Hg. Remember that 29.92 "Hg is standard day atmospheric.
Differential Pressure
Differential pressure, or psid, is the difference between
pressures being read at two different locations within a system. For example, in a turbine engine oil system the pressure is read as it enters the oil filter, and as it leaves the filter. These two readings are sent to a transmitter which powers a light located on the flight deck. Across anything that poses a resistance to flow, like an oil filter, there will be a drop in pressure. If the filter starts to clog, the pressure drop will become greater, eventually causing the advisory light on the flight deck to come on.
Figure 5-36 shows a differential pressure gauge for the
pressurization system on a Boeing 737. In this case, the difference in pressure is between the inside and the outside of the airplane. If the pressure difference becomes too great, the structure of the airplane could become overstressed.
Gas Laws
The simple structure of gases makes them readily adaptable to mathematical analysis from which has evolved a detailed theory of the behavior of gases. This is called the kinetic theory of gases. The theory assumes that a body of gas is composed of identical molecules which behave like minute elastic spheres, spaced relatively far apart and continuously in motion.
The degree of molecular motion is dependent upon the
temperature of the gas. Since the molecules are continuously striking against each other and against the walls of the container, an increase in temperature with the resulting increase in molecular motion causes a corresponding increase in the number of collisions between the molecules. The increased number of collisions results in an increase in pressure because a greater number of molecules strike against the walls of the container in a given unit of time.
If the container were an open vessel, the gas would expand
and overflow from the container. However, if the container is sealed and possesses elasticity, such as a rubber balloon, the increased pressure causes the container to expand. For instance, when making a long drive on a hot day, the pressure in the tires of an automobile increases, and a tire which appeared to be somewhat “soft” in cool morning temperature may appear normal at a higher midday temperature.
Such phenomena as these have been explained and set forth
in the form of laws pertaining to gases and tend to support
Vacuum
14.7 psi
Atmospheric
pressure760 mm
29.92 in
Fuel Pressure
Psi405060
Fuel
Flow
(Gal/Hr)Man.
Press.
("Hg)35 302826
104.0psi29.5
psiFigure 5-33. Atmospheric pressure as inches of mercury.
Figure 5-34. Psig read on a fuel pressure gauge.Figure 5-35. Manifold pressure gauge indicating absolute pressure.the kinetic theory.
Boyle’s Law
As previously stated, compressibility is an outstanding
characteristic of gases. The English scientist, Robert Boyle,
was among the first to study this characteristic that he called
the “springiness of air.” By direct measurement he discovered
that when the temperature of a combined sample of gas was kept constant and the absolute pressure doubled, the volume
was reduced to half the former value. As the applied absolute
pressure was decreased, the resulting volume increased.
From these observations, he concluded that for a constant
temperature the product of the volume and absolute pressure
of an enclosed gas remains constant. Boyle’s law is normally
stated: “The volume of an enclosed dry gas varies inversely
with its absolute pressure, provided the temperature remains
constant.” The following formula is used for Boyle’s law
calculations. Remember, pressure needs to be in the absolute.
V olume 1 × Pressure 1 = V olume 2 × Pressure 2
or
V1P1 = V 2P2
Example: 10 ft3 of nitrogen is under a pressure of 500 psia.
If the volume is reduced to 7 ft3, what will the new pressure
be? [Figure 5-37]
V 1P1 = V 2P2
10 (500) = 7 (P 2)
10 (500) ÷ 7 = P 2
P 2 = 714.29 psia
The useful applications of Boyle’s law are many and varied.
Some applications more common to aviation are: (1) the
carbon dioxide (CO 2) bottle used to inflate life rafts and life
vests; (2) the compressed oxygen and the acetylene tanks
used in welding; (3) the compressed air brakes and shock
absorbers; and (4) the use of oxygen tanks for high altitude
flying and emergency use.
5-25CABIN
AIR
X 1000 FT0
6PSIDEF PRESS
ALT
HORN
CUT-OUT
CABIN CLIMB
1000FEET PERMIN0.51
2.5UP
DNPRESSURE
DIFFERENCE
LIMIT:
TAKE-OFF &
LOG 0.125 PSI
Pressurization
system differential
pressure gauge
Force pushing down on gas
Pressure
increasingTemperature
held constantFigure 5-36. Differential pressure gauge.
Figure 5-37. Boyle’ s law example.Charles’ Law
The French scientist, Jacques Charles, provided much of the
foundation for the modern kinetic theory of gases. He found that all gases expand and contract in direct proportion to the change in the absolute temperature, provided the pressure is held constant. As a formula, this law is shown as follows:
V olume 1 × Absolute Temperature 2 =
V olume 2 × Absolute Temperature 1
or
V
1T2 = V 2T1
Charles’ law also works if the volume is held constant, and pressure and temperature are the variables. In this case, the formula would be as follows:
P
1T2 = P 2T1
For this second formula, pressure and temperature must be in the absolute.
Example: A 15-ft
3 cylinder of oxygen is at a temperature of
70 °F and a pressure of 750 psig. The cylinder is placed in the
sun and the temperature of the oxygen increases to 140 °F.
What would be the new pressure in psig?
70 degrees Fahrenheit = 530 degrees Rankine
140 degrees Fahrenheit = 600 degrees Rankine
750 psig + 14.7 = 764.7 psia
P1T2 = P 2T1
764.7 (600) = P 2 (530)
P2 = 764.7 (600) ÷ 530
P2 = 865.7 psia
P2 = 851 psigGeneral Gas Law
By combining Boyle’s and Charles’ laws, a single expression
can be derived which states all the information contained in both. The formula which is used to express the general gas law is as follows:
=Temperature 1 TemperaturePressure 2 (V olume 2) Pressure 1 (V olume 1)
or
P1 (V1) (T 2) = P 2 (V2) (T 1)
When using the general gas law formula, temperature and pressure must be in the absolute.
Example: 20 ft
3 of the gas argon is compressed to 15 ft3.
The gas starts out at a temperature of 60 °F and a pressure of
1,000 psig. After being compressed, its temperature is 90 °F.
What would its new pressure be in psig?
60 degrees Fahrenheit = 520 degrees Rankine
90 degrees Fahrenheit = 550 degrees Rankine
1,000 psig + 14.7 = 1,014.7 psia
P1 (V1) (T 2) = P 2 (V2) (T 1)
1,014.7 (20) (550) = P 2 (15) (520)
P2 = 1,431 psia
P2 = 1,416.3 psig
Dalton’s Law
If a mixture of two or more gases that do not combine
chemically is placed in a container, each gas expands throughout the total space and the absolute pressure of each gas is reduced to a lower value, called its partial pressure. This
5-26reduction is in accordance with Boyle’s law. The pressure of
the mixed gases is equal to the sum of the partial pressures.
This fact was discovered by Dalton, an English physicist,
and is set forth in Dalton’s law: “A mixture of several gases
which do not react chemically exerts a pressure equal to the
sum of the pressures which the several gases would exert
separately if each were allowed to occupy the entire space
alone at the given temperature.”
Fluid Mechanics
By definition, a fluid is any substance that is able to flow if it
is not in some way confined or restricted. Liquids and gases
are both classified as fluids, and often act in a very similar
way. One significant difference comes into play when a
force is applied to these fluids. In this case, liquids tend
to be incompressible and gases are highly compressible.
Many of the principles that aviation is based on, such as
the theory of lift on a wing and the force generated by a
hydraulic system, can be explained and quantified by using
the laws of fluid mechanics.
Buoyancy
A solid body submerged in a liquid or a gas weighs less than
when weighed in free space. This is because of the upward
force, called buoyant force, which any fluid exerts on a body
submerged in it. An object will float if this upward force of the
fluid is greater than the weight of the object. Objects denser
than the fluid, even though they sink readily, appear to lose
a part of their weight when submerged. A person can lift a
larger weight under water than they can possibly lift in the air.
The following experiment is illustrated in Figure 5-38 . The
overflow can is filled to the spout with water. The heavy
metal cube is first weighed in still air and weighs 10 lb. It is
then weighed while completely submerged in the water and
it weighs 3 lb. The difference between the two weights is
the buoyant force of the water. As the cube is lowered into
the overflow can, the water is caught in the catch bucket.
The volume of water which overflows equals the volume
of the cube. The volume of irregular shaped objects can
also be measured by using this method. If this experiment
is performed carefully, the weight of the water displaced by
the metal cube exactly equals the buoyant force of the water,
which the scale shows to be 7 lb.
Archimedes (287–212 B.C.) performed similar experiments.
As a result, he discovered that the buoyant force which a fluid
exerts upon a submerged body is equal to the weight of the fluid
the body displaces. This statement is referred to as Archimedes’
principle. This principle applies to all fluids, gases as well as
liquids. Just as water exerts a buoyant force on submerged
objects, air exerts a buoyant force on objects submerged in it.The amount of buoyant force available to an object can be
calculated by using the following formula:
Buoyant Force = V olume of Object × Density of Fluid Displaced
If the buoyant force is more than the object weighs, the object
will float. If the buoyant force is less than the object weighs,
the object will sink. For the object that sinks, its measurable
weight will be less by the weight of the displaced fluid.
Example: A 10-ft3 object weighing 700 lb is placed in pure
water. Will the object float? If the object sinks, what is
its measurable weight in the submerged condition? If the
object floats, how many cubic feet of its volume is below
the water line?
Buoyant Force = V olume of Object × Density of Fluid Displaced
= 10 (62.4)
= 624 lb
The object will sink because the buoyant force is less than the
object weighs. The difference between the buoyant force and
the object’s weight will be its measurable weight, or 76 lb.
Two good examples of buoyancy are a helium filled
airship and a seaplane on floats. An airship is able to float
in the atmosphere and a seaplane is able to float on water.
That means both have more buoyant force than weight.
Figure 5-39 is a DeHavilland Twin Otter seaplane, with a
gross takeoff weight of 12,500 lb. At a minimum, the floats
on this airplane must be large enough to displace a weight
in water equal to the airplane’s weight. According to Title
14 of the Code of Federal Regulations (14 CFR) part 23, the
floats must be 80 percent larger than the minimum needed
to support the airplane. For this airplane, the necessary size
of the floats would be calculated as follows:
Divide the airplane weight by the density of water.
12,500 ÷ 62.4 = 200.3 ft3
Multiply this volume by 80%.
200.3 × 80% = 160.2 ft3
Add the two volumes together to get the total volume of
the floats.
200.3 + 160.2 = 360.5 ft3
By looking at the Twin Otter in Figure 5-39 , it is obvious
that much of the volume of the floats is out of the water. This
is accomplished by making sure the floats have at least 80
percent more volume than the minimum necessary.
Some of the large Goodyear airships have a volume of
230,000 ft3. Since the fluid they are submerged in is air,
0.11 cu'10 lb
0.11 cu'
Overflow canCatch
bucket
Figure 5-38. Example of buoyancy. Figure 5-39. DeHavilland Twin Otter seaplane.to find the buoyant force of the airship, the volume of the
airship is multiplied by the density of air (.07651 lb⁄ft3). For this
Goodyear airship, the buoyant force is 17,597 lb. Figure 5-40
shows an inside view of the Goodyear airship.
The forward and aft ballonets are air chambers within the
airship. Through the air scoop, air can be pumped into the ballonets or evacuated from the ballonets in order to control the weight of the airship. Controlling the weight of the airship controls how much positive or negative lift it has. Although the airship is classified as a lighter-than-air aircraft, it is in fact flown in a condition slightly heavier than air.
Fluid Pressure
The pressure exerted on the bottom of a container by a
liquid is determined by the height of the liquid and not by the shape of the container. This can be seen in Figure 5-41, where three different shapes and sizes of containers are full of colored water. Even though they are different shapes and have different volumes of liquid, each one has a height of 231 inches. Each one would exert a pressure on the bottom of 8.34 psi because of this height. The container on the left, with a surface area of 1 in
2, contains a volume of 231 in3
(one gallon). One gallon of water weighs 8.34 lb, which is why the pressure on the bottom is 8.34 psi.Still thinking about Figure 5-41 , if the pressure was measured
half way down, it would be half of 8.34, or 4.17 psi. In other words, the pressure is adjustable by varying the height of the column. Pressure based on the column height of a fluid is known as static pressure. With liquids, such as gasoline, it is sometimes referred to as a head of pressure. For example, if a carburetor needs to have 2 psi supplied to its inlet, or head of pressure, this could be accomplished by having the fuel tank positioned the appropriate number of inches higher than the carburetor.
As identified in the previous paragraph, pressure due to the
height of a fluid column is known as static pressure. When a fluid is in motion, and its velocity is converted to pressure, that pressure is known as ram. When ram pressure and static pressure are added together, the result is known as total pressure. In the inlet of a gas turbine engine, for example, total pressure is often measured to provide a signal to the fuel metering device or to provide a signal to a gauge on the flight deck.
Pascal’s Law
The foundations of modern hydraulics and pneumatics were
established in 1653 when Pascal discovered that pressure set up in a fluid acts equally in all directions. This pressure acts at right angles to containing surfaces. When the pressure in the fluid is caused solely by the fluid’s height, the pressure against the walls of the container is equal at any given level, but it is not equal if the pressure at the bottom is compared to the pressure half way down. The concept of the pressure set up in a fluid, and how it relates to the force acting on the fluid and the surface area through which it acts, is Pascal’s law.
In Figure 5-41, if a piston is placed at the top of the cylinder
and an external force pushes down on the piston, additional
pressure will be created in the liquid. If the additional pressure is 100 psi, this 100 psi will act equally and undiminished from the top of the cylinder all the way to the bottom. The
Nose cone supportSuspension cables
Light signNeoprene cover
Control surfaces
Aft balloonet
Air scoopsEngines
Passenger carAir valves
Forward balloonetEach pressure gauge reads 8.34 psiEach container is filled with colored water to a height of 231 inches.
100 in2150 in21 in2
Figure 5-40. The Goodyear Airship and buoyancy.Figure 5-41. Fluid pressure based on column height.gauge at the bottom will now read 108.34 psi, and if a gauge
were positioned half way down the cylinder, it would read 104.17 psi, which is found by adding 100 plus half of 8.34.
Pascal’s law, when dealing with the variables of force, pressure,
and area, is dealt with by way of the following formula.
Force = Pressure × Area
In this formula, the force is in units of pounds, the pressure is in pounds per square inch (psi), and the area is in square inches. By transposing the original formula, we have two additional formulas, as follows:
Pressure = Force ÷ Area
and
Area = Force ÷ Pressure
An easy and convenient way to remember the formulas for Pascal’s law, and the relationship between the variables, is with the triangle shown in Figure 5-42. If the variable we
want to solve for is covered up, the position of the remaining two variables shows the proper math relationship. For example, if the “A,” or area, is covered up, what remains is the “F” on the top and the “P” on the bottom, meaning force divided by pressure.The simple hydraulic system in Figure 5-43 has 5 lb force
acting on a piston with a
1⁄2-in2 surface area. Based on Pascal’s
law, the pressure in the system would be equal to the force applied divided by the area of the piston, or 10 psi. As shown in Figure 5-43, the pressure of 10 psi is present everywhere
in the fluid.
The hydraulic system in Figure 5-44 is a little more complex
than the one in Figure 5-43. In Figure 5-44 , the input force
of 5 lb is acting on a
1⁄2-in2 piston, creating a pressure of
10 psi. The input cylinder and piston is connected to a second cylinder, which contains a 5-in2 piston. The pressure of 10 psi
created by the input piston pushes on the piston in the second
cylinder, creating an output force of 50 pounds.
Often, the purpose of a hydraulic system is to generate a
large output force, with the input force being much less. In Figure 5-44, the input force is 5 lb and the output force
is 50 lb, or 10 times greater. The relationship between the output force and the input force, as discussed earlier in this chapter, is known as mechanical advantage. The mechanical advantage in Figure 5-44 would be 50 divided
by 5, or 10. The following formulas can be used to calculate mechanical advantage.
Mechanical Advantage = Force Out ÷ Force In
or
Mechanical Advantage = Distance Out ÷ Distance In
Earlier in this chapter when simple machines, such as levers and gears were discussed, it was identified that no machine allows us to gain work. The same statement holds true for a hydraulic system, that we get no more work out of a hydraulic system than we put in. Since work is equal to force times distance, if we gain force with a hydraulic system, we must lose distance. We only get the same work out, if the system is 100 percent efficient.
In order to think about the distance that the output piston will
move in response to the movement of the input piston, the
5-29F
A P10 psi
Force = 5 lb10 psi
Piston area = ½ in2
Pressure = force ÷ area
Pressure = 5 ÷ ½Pressure = 10 psi
10 psi
10 psiForce = 5 lbForce = 50 lb
Piston area = ½ in2Piston area
= 5 in2
Force = pressure x area
Force = 10 psi x5 in2
Force = 50 lb
50 lb
Figure 5-42. Force, area, pressure relationship.Figure 5-43. Pressure created in a hydraulic system.
Figure 5-44. Output force created in a hydraulic system.volume of fluid displaced must be considered. In the study
of geometry, one learns that the volume of a cylinder is equal to the cylinder’s surface area multiplied by its height. So, when a piston of 2 in
2 moves down in a cylinder a distance of
10 in, it displaces a volume of fluid equal to 20 in3 (2 in2 ×
10 in). The 20 in3 displaced by the first piston is what moves
over to the second cylinder and causes its piston to move. In a simple two-piston hydraulic system, the relationship between the piston area and the distance moved is shown by the following formula.
Input Piston Area (Distance Moved) =
Output Piston Area (Distance Moved)
This formula shows that the volume in is equal to the volume out. This concept is shown in Figure 5-45 , where a small input
piston moves a distance of 20 inches, and the larger output piston only moves a distance of 1 inch.
Example: A two-piston hydraulic system, like that shown in
Figure 5-45, has an input piston with an area of
1⁄4 in2 and
an output piston with an area of 15 in2. An input force of
50 lb is applied, and the input piston moves 30 inches. What is the pressure in the system, how much force is generated by the output piston, how far would the output piston move, and what is the mechanical advantage?
Pressure = Force ÷ Area
= 50 ÷ 1⁄4
= 200 psi
Force = Pressure × Area
= 200 × 15
= 3,000 lb
Mechanical Advantage = Force Out ÷ Force In
= 3,000 ÷ 50 = 60
Input Piston Area
(Distance Moved) = Output Piston Area
(Distance Moved)
1⁄4 (30) = 15 (Distance Moved)
1⁄4 (30) ÷ 15 = Distance Moved
Distance Moved = 1⁄2 in
Part of understanding Pascal’s law and hydraulics involves utilizing formulas, and recognizing the relationship between the individual variables. Before the numbers are plugged into the formulas, it is often possible to analyze the variables in the system and come to a realization about what is happening. For example, look at the variables in Figure 5-45 and notice that the output piston is 20 times larger than the input piston, 5 in
2 compared to 1⁄4 in2. That
comparison tells us that the output force will be 20 times greater than the input force, and also that the output piston will only move
1⁄20 as far. Without doing any formula based
calculations, we can conclude that the hydraulic system in question has a mechanical advantage of 20.
Bernoulli’s Principle
Bernoulli’s principle was originally stated to explain the
action of a liquid flowing through the varying cross-sectional
Distance moved = 20 inDistance moved = 1 in
Input piston area = ¼ in2Output
piston
area =
15 in2
Piston area (distance) = Piston area (distance)
¼ in2 (20 in) = 5 in (distance)
5 ÷ 5 = Distance
Distance = 1 in
Figure 5-45. Piston movement in a hydraulic system.areas of tubes. In Figure 5-46 a tube is shown in which
the cross-sectional area gradually decreases to a minimum
diameter in its center section. A tube constructed in this
manner is called a “venturi,” or “venturi tube.” Where the
cross-sectional area is decreasing, the passageway is referred
to as a converging duct. As the passageway starts to spread
out, it is referred to as a diverging duct.
As a liquid, or fluid, flows through the venturi tube, the
gauges at points “A,” “B,” and “C” are positioned to register
the velocity and the static pressure of the liquid. The venturi
in Figure 5-46 can be used to illustrate Bernoulli’s principle,
which states that: The static pressure of a fluid, liquid or gas,
decreases at points where the velocity of the fluid increases,
provided no energy is added to nor taken away from the
fluid. The velocity of the air is kinetic energy and the static
pressure of the air is potential energy.
In the wide section of the venturi (points A and C of
Figure 5-46 ), the liquid moves at low velocity, producing a
high static pressure, as indicated by the pressure gauge. As the
tube narrows in the center, it must contain the same volume
of fluid as the two end areas. As indicated by the velocity
gauge reading high and the pressure gauge reading low, in
this narrow section, the liquid moves at a higher velocity,
producing a lower pressure than that at points A and C. A
good application for the use of the venturi principle is in a
float-type carburetor. As the air flows through the carburetor
on its way to the engine, it goes through a venturi, where the
static pressure is reduced. The fuel in the carburetor, which is
under a higher pressure, flows into the lower pressure venturi
area and mixes with the air.
Bernoulli’s principle is extremely important in understanding
how some of the systems used in aviation work, including
how the wing of an airplane generates lift or why the inlet
duct of a turbine engine on a subsonic airplane is diverging
in shape. The wing on a slow-moving airplane has a curved top surface and a relatively flat bottom surface. The curved
top surface acts like half of the converging shaped middle
of a venturi. As the air flow over the top of the wing, the
air speeds up, and its static pressure decreases. The static
pressure on the bottom of the wing is now greater than the
pressure on the top, and this pressure difference creates the
lift on the wing. Bernoulli’s principle and the concept of lift
on a wing are covered in greater depth in “Aircraft Theory
of Flight” located in this chapter.
Sound
Sound has been defined as a series of disturbances in matter
that the human ear can detect. This definition can also be
applied to disturbances which are beyond the range of human
hearing. There are three elements which are necessary for the
transmission and reception of sound. These are the source, a
medium for carrying the sound, and the detector. Anything
which moves back and forth, or vibrates, and disturbs the
medium around it may be considered a sound source.
An example of the production and transmission of sound
is the ring of a bell. When the bell is struck and begins to
vibrate, the particles of the medium, or the surrounding air, in
contact with the bell also vibrate. The vibrational disturbance
is transmitted from one particle of the medium to the next,
and the vibrations travel in a “wave” through the medium
until they reach the ear. The eardrum, acting as detector, is
set in motion by the vibrating particles of air, and the brain
interprets the eardrum’s vibrations as the characteristic sound
associated with a bell.
Wave Motion
Since sound is a wave motion in matter, it can best be
understood by first considering water waves, like a series
of circular waves travel away from the disturbance of an
object thrown into a pool. In Figure 5-47 such waves are
seen from a top perspective, with the waves traveling
out from the center. In the cross-section perspective in
Figure 5-47 , notice that the water waves are a succession
of crests and troughs. The wavelength is the distance from
the crest of one wave to the crest of the next. Water waves
are known as transverse waves because the motion of the
water molecules is up and down, or at right angles to the
direction in which the waves are traveling. This can be seen
by observing a cork on the water, bobbing up and down as
the waves pass by.
Sound travels through matter in the form of longitudinal wave
motions. These waves are called longitudinal waves because the
particles of the medium vibrate back and forth longitudinally
in the direction of propagation. [Figure 5-48] When the
tine of a tuning fork moves in an outward direction, the air
immediately in front of the tine is compressed so that its
Pressure
PressurePressure Velocity Velocity
Velocity“A” “B” “C”Low High Low High Low High Low High
Low High Low High
TroughCrest CrestPressure waves
traveling outwardRound object dropped
into the water
Figure 5-46. Bernoulli’ s principle and a venturi.Figure 5-47. Relationship between sound and waves in water.momentary pressure is raised above that at other points in the
surrounding medium. Because air is elastic, this disturbance
is transmitted progressively in an outward direction from the
tine in the form of a compression wave.
When the tine returns and moves in an inward direction,
the air in front of the tine is rarefied so that its momentary
pressure is reduced below that at other points in the
surrounding medium. This disturbance is transmitted in the
form of a rarefaction, or expansion, wave and follows the
compression wave through the medium. The progress of
any wave involves two distinct motions: (1) The wave itself
moves forward with constant speed, and (2) simultaneously,
the particles of the medium that convey the wave vibrate
harmonically. Examples of harmonic motion are the motion
of a clock pendulum, the balance wheel in a watch, and the
piston in a reciprocating engine.
Speed of Sound
In any uniform medium, under given physical conditions,
the sound travels at a definite speed. In some substances, the
velocity of sound is higher than in others. Even in the same
medium under different conditions of temperature, pressure,
and so forth, the velocity of sound varies. Density and
elasticity of a medium are the two basic physical properties
which govern the velocity of sound.
In general, a difference in density between two substances
is sufficient to indicate which one will be the faster
transmission medium for sound. For example, sound travels
faster through water than it does through air at the same
temperature. However, there are some surprising exceptions
to this rule of thumb. An outstanding example among these
exceptions involves comparison of the speed of sound in
lead and aluminum at the same temperature. Sound travels
at 16,700 fps in aluminum at 20 °C, and only 4,030 fps in lead at 20 °C, despite the fact that lead is much denser
than aluminum. The reason for such exceptions is found in
the fact, mentioned above, that sound velocity depends on
elasticity as well as density.
Using density as a rough indication of the speed of sound
in a given substance, it can be stated as a general rule that
sound travels fastest in solid materials, slower in liquids, and
slowest in gases. The velocity of sound in air at 0 °C (32 °F)
is 1,087 fps and increases by 2 fps for each Centigrade degree
of temperature rise, or 1.1 fps for each degree Fahrenheit.
Mach Number
In the study of aircraft that fly at supersonic speeds, it is
customary to discuss aircraft speed in relation to the velocity
of sound, which is approximately 761 miles per hour (mph)
at 59 °F. The term “Mach number” has been given to the
ratio of the speed of an aircraft to the speed of sound, in
honor of Ernst Mach, an Austrian scientist. If the speed of
sound at sea level is 761 mph, an aircraft flying at a Mach
number of 1.2 at sea level would be traveling at a speed of
761 mph × 1.2 = 913 mph.
Frequency of Sound
The term “pitch” is used to describe the frequency of a sound.
The outstanding recognizable difference between the tones
produced by two different keys on a piano is a difference in
pitch. The pitch of a tone is proportional to the number of
compressions and rarefactions received per second, which
in turn, is determined by the vibration frequency of the
sounding source. A good example of frequency is the noise
generated by a turbofan engine on a commercial airliner. The
AmplitudeRarefaction
Compression
WaveTuning fork
length
Figure 5-48. Sound propagation by a tuning fork.high tip speeds of the fan in the front of the engine create
a high frequency sound, and the hot exhaust creates a low
frequency sound.
Loudness
When a bell rings, the sound waves spread out in all directions
and the sound is heard in all directions. When a bell is struck
lightly, the vibrations are of small amplitude and the sound
is weak. A stronger blow produces vibrations of greater
amplitude in the bell, and the sound is louder. It is evident
that the amplitude of the air vibrations is greater when the
amplitude of the vibrations of the source is increased. Hence,
the loudness of the sound depends on the amplitude of the
vibrations of the sound waves. As the distance from the
source increases, the energy in each wave spreads out, and
the sound becomes weaker.
As the sound wave advances, variations in pressure occur
at all points in the transmitting medium. The greater the
pressure variations, the more intense the sound wave is.
The intensity is proportional to the square of the pressure
variation regardless of the frequency. Thus, by measuring
pressure changes, the intensities of sounds having different
frequencies can be compared directly.
Measurement of Sound Intensity
Sound intensity is measured in decibels, with a decibel
being the ratio of one sound to another. One decibel (dB)
is the smallest change in sound intensity the human ear can
detect. A faint whisper would have an intensity of 20 dB, and
a pneumatic drill would be 80 dB. The engine on a modern
jetliner, at takeoff thrust, would have a sound intensity of
90 dB when heard by someone standing 150 ft. away. A 110 dB noise, by comparison, would sound twice as loud as the
jetliner’s engine. Figure 5-49 shows the sound intensity from
a variety of different sources.
Doppler Effect
When sound is coming from a moving object, the object’s
forward motion adds to the frequency as sensed from the front
and takes away from the frequency as sensed from the rear.
This change in frequency is known as the Doppler Effect, and
it explains why the sound from an airplane seems different as it
approaches compared to how it sounds as it flies overhead. As
it approaches, it becomes both louder and higher pitched. As
it flies away, the loudness and pitch both decrease noticeably.
If an airplane is flying at or higher than the speed of sound,
the sound energy cannot travel out ahead of the airplane,
because the airplane catches up to it the instant it tries to
leave. The sound energy being created by the airplane piles
up, and attaches itself to the structure of the airplane. As the
airplane approaches, a person standing on the ground will not
be able to hear it until it gets past their position, because the
sound energy is actually trailing behind the airplane. When
the sound of the airplane is heard, it will be in the form of
what is called a sonic boom.
Resonance
All types of matter, regardless of whether it is a solid, liquid,
or gas, have a natural frequency at which the atoms within
that matter vibrate. If two pieces of matter have the same
natural frequency, and one of them starts to vibrate, it can
transfer its wave energy to the other one and cause it to
vibrate. This transfer of energy is known as resonance. Some
piston engine powered airplanes have an rpm range that they
are placarded to avoid, because spinning the prop at that
rpm can cause vibration problems. The difficulty lies in the
natural frequency of the metal in the prop, and the frequency
of vibration that will be set up with a particular tip speed for
the prop. At that particular rpm, stresses can be set up that
could lead to the propeller coming apart.
The Atmosphere
Aviation is so dependent upon that category of fluids called
gases and the effect of forces and pressures acting upon gases
that a discussion of the subject of the atmosphere is important
to the persons maintaining and repairing aircraft.
Data available about the atmosphere may determine whether
a flight will succeed, or whether it will even become airborne.
The various components of the air around the earth, the
changes in temperatures and pressures at different levels
above the earth, the properties of weather encountered by
aircraft in flight, and many other detailed data are considered
in the preparation of flight plans.
160140130120110100
908070605040302010
0THRESHOLD OF FEELING THRESHOLD OF AUDIBILITYLethal level
Turbojet engine at 50 ft
50 horsepower siren at 50 ft
Turbojet engine at takeoff
at 150 ft
Automobile horn
Riveting machine, as heard
by operator
An express train passing
close by
Modern turbo engine at
takeoff at 150 ft
Subway train at 20 ft
(1/2 the loudness of 110 dB noise)
Pneumatic drill at 50 ft
Vacuum cleaner at 50 ft
Nearby freeway auto traffic
Private business office
Residential area in the evening
Soft whisper at 5 ft
Radio studio
Faint whisper
Figure 5-49. Sound intensity from different sources.Pascan and Torricelli have been credited with developing
the barometer, an instrument for measuring atmospheric pressure. The results of their experiments are still used today with very little improvement in design or knowledge. They determined that air has weight which changes as altitude is changed with respect to sea level. Today scientists are also interested in how the atmosphere affects the performance of the aircraft and its equipment.
Composition of the Atmosphere
The atmosphere is a complex and ever changing mixture. Its
ingredients vary from place to place and from day to day. In addition to a number of gases, it contains quantities of foreign matter such as pollen, dust, bacteria, soot, volcanic ash, spores, and dust from outer space. The composition of the air remains almost constant from sea level up to its highest level, but its density diminishes rapidly with altitude. Six miles up, for example, it is too thin to support respiration, and 12 miles up, there is not enough oxygen to support combustion, except in some specially designed turbine engine powered airplanes. At a point several hundred miles above the earth, some gas particles spray out into space, some are dragged by gravity and fall back into the ocean of air below, while others never return. Physicists disagree as to the boundaries of the outer fringes of the atmosphere. Some think it begins 240 miles above the earth and extends to 400 miles; others place its lower edge at 600 miles and its upper boundary at 6,000 miles.
There are also certain nonconformities at various levels.
Between 12 and 30 miles, high solar ultraviolet radiation reacts with oxygen molecules to produce a thin curtain of ozone, a very poisonous gas without which life on earth could not exist. This ozone filters out a portion of the sun’s lethal ultraviolet rays, allowing only enough coming through to give us sunburn, kill bacteria, and prevent rickets. At 50 to 65 miles up, most of the oxygen molecules begin to break down under solar radiation into free atoms, and to form hydroxyl ions (OH) from water vapor. Also in this region, all the atoms become ionized.
Studies of the atmosphere have revealed that the temperature
does not decrease uniformly with increasing altitude; instead it gets steadily colder up to a height of about 7 miles, where the rate of temperature change slows down abruptly and remains almost constant at −55° Centigrade (218° Kelvin) up to about 20 miles. Then the temperature begins to rise to a peak value of 77° Centigrade (350° Kelvin) at the 55 mile level. Thereafter it climbs steadily, reaching 2,270° Centigrade (2,543° Kelvin) at a height of 250 to 400 miles. From the 50 mile level upward, a man or any other living creature, without the protective cover of the atmosphere, would be broiled on the side facing the sun and frozen on the other.
The atmosphere is divided into concentric layers or levels.
Transition through these layers is gradual and without sharply defined boundaries. However, one boundary, the tropopause, exists between the first and second layer. The tropopause is defined as the point in the atmosphere at which the decrease in temperature, with increasing altitude, abruptly ceases. The four atmosphere layers are the troposphere, stratosphere,
ionosphere, and the exosphere. The upper portion of the
stratosphere is often called the chemosphere or ozonosphere, and the exosphere is also known as the mesosphere.
The troposphere extends from the earth’s surface to about
35,000 ft at middle latitudes, but varies from 28,000 ft at the poles to about 54,000 ft at the equator. The troposphere is characterized by large changes in temperature and humidity and by generally turbulent conditions. Nearly all cloud formations are within the troposphere. Approximately three-fourths of the total weight of the atmosphere is within the troposphere. The stratosphere extends from the upper limits of the troposphere and the tropopause to an average altitude of 60 miles.
The ionosphere ranges from the 50-mile level to a level of
300 to 600 miles. Little is known about the characteristics of the ionosphere, but it is thought that many electrical phenomena occur there. Basically, this layer is characterized by the presence of ions and free electrons, and the ionization
5-34seems to increase with altitude and in successive layers.
The exosphere, or mesosphere, is the outer layer of the
atmosphere. It begins at an altitude of 600 miles and extends
to the limits of the atmosphere. In this layer, the temperature
is fairly constant at 2,500° Kelvin, and propagation of sound
is thought to be impossible due to lack of molecular substance.
Atmospheric Pressure
The human body is under pressure, since it exists at the
bottom of a sea of air. This pressure is due to the weight of the
atmosphere. On a standard day at sea level, if a 1-in2 column
of air extending to the top of the atmosphere was weighed, it
would weigh 14.7 lb. That is why standard day atmospheric
pressure is said to be 14.7 pounds per square inch (14.7 psi).
Since atmospheric pressure at any altitude is due to the weight
of air above it, pressure decreases with increased altitude.
Obviously, the total weight of air above an area at 15,000 ft
would be less than the total weight of the air above an area
at 10,000 ft.
Atmospheric pressure is often measured by a mercury
barometer. A glass tube somewhat over 30 inches in length
is sealed at one end and then filled with mercury. It is then
inverted and the open end placed in a dish of mercury.
Immediately, the mercury level in the inverted tube will drop
a short distance, leaving a small volume of mercury vapor at
nearly zero absolute pressure in the tube just above the top of
the liquid mercury column. Gravity acting on the mercury in
the tube will try to make the mercury run out. Atmospheric
pressure pushing down on the mercury in the open container
tries to make the mercury stay in the tube. At some point these
two forces (gravity and atmospheric pressure) will equilibrate
out and the mercury will stabilize at a certain height in the
tube. Under standard day atmospheric conditions, the air in a
1 square inch column extending to the top of the atmosphere
would weigh 14.7 lb. A 1-in2 column of mercury, 29.92 inches
tall, would also weigh 14.7 lb. That is why 14.7 psi is equal
to 29.92 "Hg. Figure 5-50 demonstrates this point.
A second means of measuring atmospheric pressure is with
an aneroid barometer. This mechanical instrument is a much
better choice than a mercury barometer for use on airplanes.
Aneroid barometers, or altimeters, are used to indicate
altitude in flight. The calibrations are made in thousands of
feet rather than in psi or inches of mercury. For example, the
standard pressure at sea level is 29.92 "Hg, or 14.7 psi. At
10,000 feet above sea level, standard pressure is 20.58 "Hg,
or 10.10 psi. Altimeters are calibrated so that if the pressure
exerted by the atmosphere is 10.10 psi, the altimeter will
point to 10,000 ft. [Figure 5-51]Atmospheric Density
Since both temperature and pressure decrease with altitude,
it might appear that the density of the atmosphere would
remain fairly constant with increased altitude. This is not
true, however, because pressure drops more rapidly with
increased altitude than does the temperature. The result is
that density decreases with increased altitude.
By use of the general gas law, studied earlier, it can be shown
that for a particular gas, pressure and temperature determine
the density. Since standard pressure and temperatures have
been associated with each altitude, the density of the air
at these standard temperatures and pressures must also be
considered standard. Thus, a particular atmospheric density is
associated with each altitude. This gives rise to the expression
“density altitude,” symbolized “Hd.” A density altitude of
15,000 ft is the altitude at which the density is the same as
that considered standard for 15,000 ft. Remember, however,
that density altitude is not necessarily true altitude. For
example, on a day when the atmospheric pressure is higher
than standard and the temperature is lower than standard,
the density which is standard at 10,000 ft might occur at
12,000 ft. In this case, at an actual altitude of 12,000 ft, we
have air that has the same density as standard air at 10,000 ft.
Density altitude is a calculated altitude obtained by correcting
pressure altitude for temperature.
Water Content of the Atmosphere
In the troposphere, the air is rarely completely dry. It contains
water vapor in one of two forms: (1) fog or (2) water vapor.
Fog consists of minute droplets of water held in suspension by
the air. Clouds are composed of fog. The height to which some
clouds extend is a good indication of the presence of water in
the atmosphere almost up to the stratosphere. The presence of
water vapor in the air is quite evident in Figure 5-52 , with a
military F-18 doing a high-speed fly-by at nearly Mach 1. The
temperature and pressure changes that occur as the airplane
approaches supersonic flight cause the water vapor in the air
to condense and form the vapor cloud that is visible.
As a result of evaporation, the atmosphere always contains
some moisture in the form of water vapor. The moisture in
the air is called the humidity of the air. Moisture does not
consist of tiny particles of liquid held in suspension in the air
as in the case of fog, but is an invisible vapor truly as gaseous
as the air with which it mixes. Fog and humidity both affect
the performance of an aircraft. In flight, at cruising power,
the effects are small and receive no consideration. During
takeoff, however, humidity has important effects. Two things
are done to compensate for the effects of humidity on takeoff
performance. Since humid air is less dense than dry air, the
allowable takeoff gross weight of an aircraft is generally
reduced for operation in areas that are consistently humid.
Vacuum
14.7 psi
Atmospheric
pressure760 mm
29.92 in
I00FEET
I
ALT
Figure 5-50. Atmospheric pressure as inches of mercury.Figure 5-51. An airplane’ s altimeter is an aneroid barometer.Second, because the power output of reciprocating engines
is decreased by humidity, the manifold pressure may need
to be increased above that recommended for takeoff in dry
air in order to obtain the same power output.
Engine power output is calculated on dry air. Since water
vapor is incombustible, its pressure in the atmosphere is a
total loss as far as contributing to power output. The mixture
of water vapor and air is drawn through the carburetor, and
fuel is metered into it as though it were all air. This mixture
of water vapor, air, and fuel enters the combustion chamber
where it is ignited. Since the water vapor will not burn, the
effective air-fuel ratio is enriched and the engine operates as
though it were on an excessively rich mixture. The resulting
horsepower loss under humid conditions can therefore be
attributed to the loss in volumetric efficiency due to displaced
air, and the incomplete combustion due to an excessively rich
fuel and air mixture.
The reduction in power that can be expected from humidity
is usually given in charts in the flight manual. There are
several types of charts in use. Some merely show the expected
reduction in power due to humidity; others show the boost
in manifold pressure necessary to restore full takeoff power.
The effect of fog on the performance of an engine is very
noticeable, particularly on engines with high compression
ratios. Normally, some detonation will occur during
acceleration, due to the high BMEP, which stands for brake
mean effective pressures, developed. However, on a foggy day it is difficult to cause detonation to occur. The explanation
of this lies in the fact that fog consists of particles of water
that have not vaporized. When these particles enter the
cylinders, they absorb a tremendous amount of heat energy
in the process of vaporizing. The temperature is thus lowered,
and the decrease is sufficient to prevent detonation.
Fog will generally cause a decrease in horsepower output.
However, with a supercharged engine, it will be possible to
use higher manifold pressures without danger of detonation.
Absolute Humidity
Absolute humidity is the actual amount of the water vapor
in a mixture of air and water. It is expressed either in grams
per cubic meter or pounds per cubic foot. The amount of
water vapor that can be present in the air is dependent upon
the temperature and pressure. The higher the temperatures,
the more water vapor the air is capable of holding, assuming
constant pressure. When air has all the water vapor it can
hold at the prevailing temperature and pressure, it is said to
be saturated.
Relative Humidity
Relative humidity is the ratio of the amount of water vapor
actually present in the atmosphere to the amount that
would be present if the air were saturated at the prevailing
temperature and pressure. This ratio is usually multiplied by
100 and expressed as a percentage. Suppose, for example, that
a weather report includes the information that the temperature
is 75 °F and the relative humidity is 56 percent. This indicates
that the air holds 56 percent of the water vapor required to
Figure 5-52. F-18 high-speed fly-by and a vapor cloud.
saturate it at 75 °F. If the temperature drops and the absolute
humidity remain constant, the relative humidity will increase.
This is because less water vapor is required to saturate the
air at the lower temperature.
Dew Point
The dew point is the temperature to which humid air must
be cooled at constant pressure to become saturated. If the
temperature drops below the dew point, condensation occurs.
People who wear eyeglasses have experience going from cold
outside air into a warm room and having moisture collect
quickly on their glasses. This happens because the glasses
were below the dew point temperature of the air in the room.
The air immediately in contact with the glasses was cooled
below its dew point temperature, and some of the water vapor
was condensed out. This principle is applied in determining
the dew point. A vessel is cooled until water vapor begins
to condense on its surface. The temperature at which this
occurs is the dew point.
Vapor Pressure
Vapor pressure is the portion of atmospheric pressure that
is exerted by the moisture in the air, which is expressed in
tenths of an inch of mercury. The dew point for a given
condition depends on the amount of water pressure present;
thus, a direct relationship exists between the vapor pressure
and the dew point.
Standard Atmosphere
If the performance of an aircraft is computed, either through
flight tests or wind tunnel tests, some standard reference
condition must be determined first in order to compare results with those of similar tests. The conditions in the atmosphere
vary continuously, and it is generally not possible to obtain
exactly the same set of conditions on two different days or
even on two successive flights. For this reason, a set group
of standards must be used as a point of reference. The set
of standard conditions presently used in the United States is
known as the U.S. Standard Atmosphere.
The standard atmosphere approximates the average conditions
existing at 40° latitude, and is determined on the basis of the
following assumptions. The standard sea level conditions are:
Pressure at 0 altitude (P0) = 29.92 "Hg
Temperature at 0 altitude (T0) = 15 °C or 59 °F
Gravity at 0 altitude (G0) = 32.174 fps/s
The U.S. Standard Atmosphere is in agreement with the
International Civil Aviation Organization (ICAO) Standard
Atmosphere over their common altitude range. The ICAO
Standard Atmosphere has been adopted as standard by most
of the principal nations of the world.
Aircraft Theory of Flight
Before a technician can consider performing maintenance
on an aircraft, it is necessary to understand the pieces that
make up the aircraft. Names like fuselage, empennage, wing,
and so many others, come into play when describing what
an airplane is and how it operates. For helicopters, names
like main rotor, anti-torque rotor, and autorotation come
to mind as a small portion of what needs to be understood
about rotorcraft. The study of physics, which includes basic
aerodynamics, is a necessary part of understanding why
aircraft operate the way they do.
Four Forces of Flight
During flight, there are four forces acting on an airplane.
These forces are lift, weight, thrust, and drag. [Figure 5-53 ]
Lift is the upward force created by the wing, weight is the
pull of gravity on the mass, thrust is the force created by the
airplane’s propeller or turbine engine, and drag is the friction
caused by the air flowing around the airplane.
All four of these forces are measured in pounds. Any time
the forces are not in balance, something about the airplane’s
condition is changing. The possibilities are as follows:
1. When an airplane is accelerating, it has more thrust
than drag.
2. When an airplane is decelerating, it has less thrust
than drag.
3. When an airplane is at a constant velocity, thrust and
drag are equal.
LiftWeightDrag
Thrust
Figure 5-53. Four forces acting on an airplane.4. When an airplane is climbing, it has more lift than
weight.
5. When an airplane is descending, it has more weight
than lift.
6. When an airplane is at a constant altitude, lift and
weight are equal.
Bernoulli’s Principle and Subsonic Flow
The basic concept of subsonic airflow and the resulting
pressure differentials was discovered by Daniel Bernoulli,
a Swiss physicist. Bernoulli’s principle, as we refer to it
today, states that “as the velocity of a fluid increases, the
static pressure of that fluid will decrease, provided there is
no energy added or energy taken away.” A direct application
of Bernoulli’s principle is the study of air as it flows through
either a converging or a diverging passage, and to relate the
findings to some aviation concepts.
A converging shape is one whose cross-sectional area gets
progressively smaller from entry to exit. A diverging shape is
just the opposite, with the cross-sectional area getting larger
from entry to exit. Figure 5-54 shows a converging shaped
duct, with the air entering on the left at subsonic velocity and
exiting on the right. Notice that the air exits at an increased
velocity and a decreased static pressure when looking at
the pressure and velocity gauges, and the indicated velocity
and pressure. The unit leaving must increase its velocity as
it flows into a smaller space, because a unit of air must exit
the duct when another unit enters.
In a diverging duct, just the opposite would happen. From
the entry point to the exit point, the duct is spreading out and
the area is getting larger. [Figure 5-55] With the increase
in cross-sectional area, the velocity of the air decreases
and the static pressure increases. The total energy in the
air has not changed. What has been lost in velocity, which
is kinetic energy, is gained in static pressure, which is
potential energy.
In the discussion of Bernoulli’s principle earlier in this
chapter, a venturi was shown in Figure 5-46 . In Figure 5-56 , a
venturi is shown again, only this time a wing is shown tucked
up into the recess where the venturi’s converging shape is.
There are two arrows showing airflow. The large arrow shows
airflow within the venturi, and the small arrow shows airflow
on the outside heading toward the leading edge of the wing.
In the converging part of the venturi, velocity would increase
and static pressure would decrease. The same thing would
happen to the air flowing around the wing, with the velocity
over the top increasing and static pressure decreasing.In Figure 5-56 , the air reaching the leading edge of the wing
separates into two separate flows. Some of the air goes over
the top of the wing and some travels along the bottom. The
air going over the top, because of the curvature, has farther
to travel. With a greater distance to travel, the air going over
the top must move at a greater velocity. The higher velocity
on the top causes the static pressure on the top to be less than
it is on the bottom, and this difference in static pressures is
what creates lift.
For the wing shown in Figure 5-56 , imagine it is 5 ft. wide
and 15 ft. long, for a surface area of 75 ft2 (10,800 in2). If
the difference in static pressure between the top and bottom
is 0.1 psi, there will be 1⁄10 lb of lift for each square inch of
surface area. Since there are 10,800 in2 of surface area, there
would be 1,080 lb of lift (0.1 × 10,800).
Lift and Newton’s Third Law
Newton’s third law identifies that for every force there is an
equal and opposite reacting force. In addition to Bernoulli’s
principle, Newton’s third law can also be used to explain the lift
being created by a wing. As the air travels around a wing and
leaves the trailing edge, the air is forced to move in a downward
direction. Since a force is required to make something change
direction, there must be an equal and opposite reacting force.
In this case, the reacting force is what we call lift. In order to
calculate lift based on Newton’s third law,
Newton’s second law and the formula “Force = Mass ×
Acceleration” would be used. The mass would be the
weight of air flowing over the wing every second, and
the acceleration would be the change in velocity the wing
imparts to the air.
The lift on the wing as described by Bernoulli’s principle,
and lift on the wing as described by Newton’s third law, is
AirflowSubsonic14 psi
10 psi300 mph
400 mphPressure
PressureVelocity
Velocity
Low High
Low High
Low High
Low High
AirflowSubsonic
10 psi14 psi
400 mph300 mph
PressurePressure
VelocityVelocity
Low High
Low HighLow High
Low High
Figure 5-54. Bernoulli’ s principle and a converging duct.Figure 5-55. Bernoulli’ s principle and a diverging duct.not separate or independent of each other. They are just two
different ways to describe the same thing, namely the lift on a wing.
Airfoils
An airfoil is any device that creates a force, based on
Bernoulli’s principles or Newton’s laws, when air is caused to flow over the surface of the device. An airfoil can be the wing of an airplane, the blade of a propeller, the rotor blade of a helicopter, or the fan blade of a turbofan engine. The wing of an airplane moves through the air because the airplane is in motion, and generates lift by the process previously described. By comparison, a propeller blade, helicopter rotor blade, or turbofan engine fan blade rotates through the air. These rotating blades could be referred to as rotating wings, as is common with helicopters when they are called rotary wing aircraft. The rotating wing can be viewed as a device that creates lift, or just as correctly, it can be viewed as a device that creates thrust.
In Figure 5-57 an airfoil, or wing, is shown, with some of
the terminology that is used to describe a wing. The terms
and their meaning are as follows:
Camber
The camber of a wing is the curvature which is present on top and bottom surfaces. The camber on the top is much more pronounced, unless the wing is a symmetrical airfoil, which has the same camber top and bottom. The bottom of the wing, more often than not, is relatively flat. The increased camber on top is what causes the velocity of the air to increase and the static pressure to decrease. The bottom of the wing has less velocity and more static pressure, which is why the wing generates lift.
Chord Line
The chord line is an imaginary straight line running from the wing’s leading edge to its trailing edge. The angle between the chord line and the longitudinal axis of the airplane is known as the angle of incidence.
Relative W ind
The relative wind is a relationship between the direction of airflow and the aircraft wing. In normal flight circumstances, the relative wind is the opposite direction of the aircraft flightpath.
•If the flightpath is forward then the relative wind is
backward.
•If the flightpath is forward and upward, then the
relative wind is backward and downward.
•If the flightpath is forward and downward, then the
relative wind is backward and upward.
Therefore, the relative wind is parallel to the flightpath, and travels in the opposite direction.
Angle of Attack
The angle between the chord line and the relative wind is the angle of attack. As the angle of attack increases, the lift on the wing increases. If the angle of attack becomes too great, the airflow can separate from the wing and the lift
Airflow Velocity increase
Airplane path
Relative wind
Lower camber Trailing edgeUpper camberChord lineLeading edge
Angle of attack
Figure 5-56. Venturi with a superimposed wing. Figure 5-57. Wing terminology.
will be destroyed. When this occurs, a condition known as
a stall takes place.
There are a number of different shapes, known as planforms
that a wing can have. A wing in the shape of a rectangle is very common on small general aviation airplanes. An elliptical shape or tapered wing can also be used, but these do not have as desirable a stall characteristic. For airplanes that operate at high subsonic speeds, sweptback wings are common, and for supersonic flight, a delta shape might be used.
The aspect ratio of a wing is the relationship between its
span, or a wingtip to wingtip measurement, and the chord of the wing. If a wing has a long span and a very narrow chord, it is said to have a high aspect ratio. A higher aspect ratio produces less drag for a given flight speed, and is typically found on glider type aircraft.
The angle of incidence of a wing is the angle formed by the
intersection of the wing chord line and the horizontal plane passing through the longitudinal axis of the aircraft. Many airplanes are designed with a greater angle of incidence at the root of the wing than at the tip, and this is referred to as washout. This feature causes the inboard part of the wing to stall before the outboard part, which helps maintain aileron control during the initial stages of a wing stall.
Boundary Layer Airflow
The boundary layer is a very thin layer of air lying over the
surface of the wing and, for that matter, all other surfaces of the airplane. Because air has viscosity, this layer of air tends to adhere to the wing. As the wing moves forward through the air the boundary layer at first flows smoothly over the streamlined shape of the airfoil. Here the flow is called the laminar layer.
As the boundary layer approaches the center of the wing,
it begins to lose speed due to skin friction and it becomes thicker and turbulent. Here it is called the turbulent layer. The point at which the boundary layer changes from laminar to turbulent is called the transition point. Where the boundary layer becomes turbulent, drag due to skin friction is
relatively high. As speed increases, the transition point tends to move forward. As the angle of attack increases, the transition point also tends to move forward. With higher angles of attack and further thickening of the boundary layer, the turbulence becomes so great the air breaks away from the surface of the wing. At this point, the lift of the wing is destroyed and a condition known as a stall has occurred. In Figure 5-58, view A shows a normal angle of attack and
the airflow staying in contact with the wing. View B shows an extreme angle of attack and the airflow separating and becoming turbulent on the top of the wing. In view B, the wing is in a stall.
Boundary Layer Control
One way of keeping the boundary layer air under control, or lessening its negative effect, is to make the wing’s surface as smooth as possible and to keep it free of dirt and debris. As the friction between the air and the surface of the wing increases, the boundary layer thickens and becomes more turbulent and eventually a wing stall occurs. With a smooth and clean wing surface, the onset of a stall is delayed and the wing can operate at a higher angle of attack. One of the reasons ice forming on a wing can be such a serious problem is because of its effect on boundary layer air. On a high-speed airplane, even a few bugs splattered on the wing’s leading edge can negatively affect boundary layer air.
Other methods of controlling boundary layer air include wing
leading edge slots, air suction through small holes on the wing’s upper surface, and the use of devices called vortex generators.
A wing leading edge slot is a duct that allows air to flow from
the bottom of the wing, through the duct, to the top of the wing. As the air flows to the top of the wing, it is directed along the wing’s surface at a high velocity and helps keep the boundary layer from becoming turbulent and separating from the wing’s surface.
Another way of controlling boundary layer air is to create
suction on the top of the wing through a large number of
5-40View A View B
Figure 5-58. Wing boundary layer separation.small holes. The suction on the top of the wing draws away
the slow-moving turbulent air, and helps keep the remainder
of the airflow in contact with the wing.
V ortex generators are used on airplanes that fly at high
subsonic speed, where the velocity of the air on the top of the
wing can reach Mach 1. As the air reaches Mach 1 velocity, a
shock wave forms on the top of the wing, and the subsequent
shock wave causes the air to separate from the wing’s upper
surface. V ortex generators are short airfoils, arranged in pairs,
located on the wing’s upper surface. They are positioned such
that they pull high-energy air down into the boundary layer
region and prevent airflow separation.
Wingtip Vortices
Wingtip vortices are caused by the air beneath the wing,
which is at the higher pressure, flowing over the wingtip
and up toward the top of the wing. The end result is a
spiral or vortex that trails behind the wingtip anytime lift
is being produced. This vortex is also referred to as wake
turbulence, and is a significant factor in determining how
closely one airplane can follow behind another on approach
to land. The wake turbulence of a large airplane can cause
a smaller airplane, if it is following too closely, to be
thrown out of control. V ortices from the wing and from the
horizontal stabilizer are quite visible on the MD-11 shown
in Figure 5-59.
Upwash and downwash refer to the effect an airfoil has on
the free airstream. Upwash is the deflection of the oncoming
airstream, causing it to flow up and over the wing. Downwash
is the downward deflection of the airstream after it has passed
over the wing and is leaving the trailing edge. This downward
deflection is what creates the action and reaction described
under lift and Newton’s third law.
Axes of an Aircraft
An airplane in flight is controlled around one or more of three
axes of rotation. These axes of rotation are the longitudinal,
lateral, and vertical. On the airplane, all three axes intersect
at the center of gravity. As the airplane pivots on one of these
axes, it is in essence pivoting around the center of gravity (CG). The center of gravity is also referred to as the center of rotation.
On the brightly colored airplane shown in Figure 5-60 , the
three axes are shown in the colors red (vertical axis), blue
(longitudinal axis), and orange (lateral axis). The flight
control that makes the airplane move around the axis is shown
in a matching color.
The rudder, in red, causes the airplane to move around the
vertical axis and this movement is described as being a yaw.
The elevator, in orange, causes the airplane to move around
the lateral axis and this movement is described as being
a pitch. The ailerons, in blue, cause the airplane to move
around the longitudinal axis and this movement is described
as being a roll.
Aircraft Stability
When an airplane is in straight-and-level flight at a constant
velocity, all the forces acting on the airplane are in equilibrium.
If that straight-and-level flight is disrupted by a disturbance in
the air, such as wake turbulence, the airplane might pitch up
or down, yaw left or right, or go into a roll. If the airplane has
what is characterized as stability, once the disturbance goes
away, the airplane will return to a state of equilibrium.
Static Stability
The initial response that an airplane displays after its
equilibrium is disrupted is referred to as its static stability. If
the static stability is positive, the airplane will tend to return
to its original position after the disruptive force is removed.
If the static stability is negative, the airplane will continue to
move away from its original position after the disruptive force
is removed. If an airplane with negative static stability has
the nose pitch up because of wake turbulence, the tendency
will be for the nose to continue to pitch up even after the
turbulence goes away. If an airplane tends to remain in a
displaced position after the force is removed, but does not
continue to move toward even greater displacement, its static
stability is described as being neutral.
Dynamic Stability
The dynamic stability of an airplane involves the amount of
time it takes for it to react to its static stability after it has
been displaced from a condition of equilibrium. Dynamic
stability involves the oscillations that typically occur as the
airplane tries to return to its original position or attitude. Even
though an airplane may have positive static stability, it may
have dynamic stability which is positive, neutral, or negative.
Imagine that an airplane in straight-and-level flight is
disturbed and pitches noseup. If the airplane has positive
static stability, the nose will pitch back down after the
disturbance is removed. If it immediately returns to straight-
Vertical axis
Longitudinal axis
Lateral axis
CG
Figure 5-60. The three axes intersect at the airplane’ s center of gravity. The flight control that produces motion around the indicated
axis is a matching color.Figure 5-59. Wing and horizontal stabilizer vortices on an MD-11.
and-level flight, it is also said to have positive dynamic
stability. The airplane, however, may pass through level flight
and remain pitched down, and then continue the recovery
process by pitching back up. This pitching up and then
down is known as an oscillation. If the oscillations lessen
over time, the airplane is still classified as having positive
dynamic stability. If the oscillations increase over time, the
airplane is classified as having negative dynamic stability.
If the oscillations remain the same over time, the airplane is
classified as having neutral dynamic stability.
Figure 5-61 shows the concept of dynamic stability. In view
A, the displacement from equilibrium goes through three
oscillations and then returns to equilibrium. In view B,
the displacement from equilibrium is increasing after two
oscillations, and will not return to equilibrium. In view C,
the displacement from equilibrium is staying the same with
each oscillation.Longitudinal Stability
Longitudinal stability for an airplane involves the tendency
for the nose to pitch up or pitch down, rotating around the
lateral axis, which is measured from wingtip to wingtip. If an
airplane is longitudinally stable, it will return to a properly
trimmed angle of attack after the force that upset its flightpath
is removed.
The weight and balance of an airplane, which is based on
both the design characteristics of the airplane and the way
it is loaded, is a major factor in determining longitudinal
stability. There is a point on the wing of an airplane, called
the center of pressure or center of lift, where all the lifting
forces concentrate. In flight, the airplane acts like it is being
lifted from or supported by this point. This center of lift
runs from wingtip to wingtip. There is also a point on the
airplane, called the center of gravity, where the mass or
weight of the airplane is concentrated. For an airplane to have
good longitudinal stability, the center of gravity is typically
located forward of the center of lift. This gives the airplane
a nosedown pitching tendency, which is balanced out by the
force generated at the horizontal stabilizer and elevator. The
center of gravity has limits within which it must fall. If it is
too far forward, the forces at the tail might not be able to
compensate and it may not be possible to keep the nose of
the airplane from pitching down.
In Figure 5-62 , the center of lift, center of gravity, and center
of gravity limits are shown. It can be seen that the center
of gravity is not only forward of the center of lift, it is also
forward of the center of gravity limit. At the back of the
airplane, the elevator trailing edge is deflected upward to
create a downward force on the tail, to try and keep the nose
5-42TimePositive static and
positive dynamic
stabilityPositive static and
negative dynamic
stabilityPositive static and
neutral dynamic
stability
Time TimeA B C
Aft CG limit Forward CG limit
Center of lift
Center of gravity
Figure 5-61. Dynamic stability. Figure 5-62. Longitudinal stability and balance.of the airplane up. This airplane would be highly unstable
longitudinally, especially at low speed when trying to land.
It is especially dangerous if the center of gravity is behind
the aft limit. The airplane will now have a tendency to pitch
noseup, which can lead to the wing stalling and possible loss
of control of the airplane.
Lateral Stability
Lateral stability of an airplane takes place around the
longitudinal axis, which is from the airplane’s nose to its
tail. If one wing is lower than the other, good lateral stability
will tend to bring the wings back to a level flight attitude.
One design characteristic that tends to give an airplane good
lateral stability is called dihedral. Dihedral is an upward
wing angle, with respect to the horizontal, and it is usually
just a few degrees.
Imagine a low wing airplane with a few degrees of dihedral
experiencing a disruption of its flightpath such that the left
wing drops. When the left wing drops, this will cause the
airplane to experience a sideslip toward the low wing. The
sideslip causes the low wing to experience a higher angle
of attack, which increases its lift and raises it back to a level
flight attitude. The dihedral on a wing is shown in Figure 5-63.
Directional Stability
Movement of the airplane around its vertical axis, and the
airplane’s ability to not be adversely affected by a force creating
a yaw type of motion, is called directional stability. The vertical
fin gives the airplane this stability, causing the airplane to
align with the relative wind. In flight, the airplane acts like the
weather vane we use around our home to show the direction
the wind is blowing. The distance from the pivot point on a
weather vane to its tail is greater than the distance from its
pivot point to the nose. So, when the wind blows, it creates a
greater torque force on the tail and forces it to align with the
wind. On an airplane, the same is true. With the CG being the
pivot point, it is a greater distance from the CG to the vertical
stabilizer than it is from the CG to the nose. [Figure 5-64]Dutch Roll
The dihedral of the wing tries to roll the airplane in the opposite
direction of how it is slipping, and the vertical fin will try to
yaw the airplane in the direction of the slip. These two events
combine in a way that affects lateral and directional stability. If
the wing dihedral has the greatest effect, the airplane will have
a tendency to experience a Dutch roll. A Dutch roll is a small
amount of oscillation around both the longitudinal and vertical
axes. Although this condition is not considered dangerous,
it can produce an uncomfortable feeling for passengers.
Commercial airliners typically have yaw dampers that sense
a Dutch roll condition and cancel it out.
Flight Control Surfaces
The purpose of flight controls is to allow the pilot to maneuver
the airplane, and to control it from the time it starts the takeoff
roll until it lands and safely comes to a halt. Flight controls
are typically associated with the wing and the vertical and
horizontal stabilizers, because these are the parts of the airplane
that flight controls most often attach to. In flight, and to some
extent on the ground, flight controls provide the airplane with
the ability to move around one or more of the three axes.
Flight controls function by changing the shape or aerodynamic
characteristics of the surface they are attached to.
Flight Controls & the Lateral Axis
The lateral axis of an airplane is a line that runs below
the wing, from wingtip to wingtip, passing through the
airplane’s center of gravity. Movement around this axis
is called pitch, and control around this axis is called
longitudinal control. The flight control that handles this
job is the elevator attached to the horizontal stabilizer, a
fully moving horizontal stabilizer, or on a v-tail configured
airplane, it is called ruddervators. An elevator on a Cessna
182 can be seen in Figure 5-65 . In Figure 5-66 , a fully
moving horizontal stabilizer, known as a stabilator, can
be seen on a Piper Cherokee Cruiser PA-28-140, and
Figure 5-67 shows a ruddervator on a Beechcraft Bonanza.
Depending on the airplane being discussed, movement around
the lateral axis happens as a result of the pilot moving the
Dihedral
Distance to vertical
stabilizer creates stabilityPivot point (CG)
Elevator
Figure 5-63. The dihedral of a wing.Figure 5-64. Directional stability caused by distance to vertical
stabilizer.
Figure 5-65. Elevator on a Cessna 182 provides pitch control.control wheel or yoke, the control stick, or on some airplanes,
a side stick. On the airplanes shown in Figures 5-70 and 5-71 ,
a control wheel or yoke is used.
On the Cessna 182 shown in Figure 5-65 , pulling back on
the control wheel causes the trailing edge of the elevator to
deflect upward, causing an increased downward force that
raises the nose of the airplane. Movement of the elevator
causes the nose of the airplane to pitch up or pitch down by
rotating around the lateral axis. The Cessna 182 control wheel
can be seen in Figure 5-68.
On the Piper Cherokee Cruiser PA-28-140 shown in
Figure 5-66 , pulling back on the control wheel causes the
entire horizontal surface, or stabilator, to move, with the
trailing edge deflecting upward. The anti-servo tab seen on
the Cherokee provides a control feel similar to what would
be experienced by moving an elevator. Without this tab,
the stabilator might be too easy to move and a pilot could
overcontrol the airplane.
The ruddervators shown on the Beechcraft Bonanza in
Figure 5-67 are also moved by the control wheel, with their
trailing edges deflecting upward when the control wheel is
pulled back. As the name implies, these surfaces also act as
the rudder for this airplane.
Flight Controls and the Longitudinal Axis
The longitudinal axis of the airplane runs through the
middle of the airplane, from nose to tail, passing through
the center of gravity. Movement around this axis is known
as roll, and control around this axis is called lateral control.
Movement around this axis is controlled by the ailerons,
and on jet transport airplanes, it is aided by surfaces on the
wing known as spoilers.
The ailerons move as a result of the pilot rotating the control
wheel to the left or to the right, much the same as turning the
steering wheel on an automobile. [Figure 5-68] When a pilot
turns the control wheel to the left, the airplane is being asked
to turn or bank to the left. Turning the control wheel to the
left causes the trailing edge of the aileron on the left wing to rise up into the airstream, and the aileron on the right wing
lowers down into the airstream. This increases the lift on the
right wing and decreases the lift on the left wing, causing the
right wing to move up and the airplane to bank to the left.
In Figure 5-69 , an aircraft can be seen doing an aileron roll.
Notice that the left aileron is up and the right aileron is down,
which would cause the airplane to roll around the longitudinal
axis in a counterclockwise direction.
Flight Controls and the Vertical Axis
The vertical axis of an airplane runs from top to bottom
through the middle of the airplane, passing through the center
of gravity. Movement around this axis is known as yaw, and
control around this axis is called directional control. Movement
around this axis is controlled by the rudder, or in the case of
the Beechcraft Bonanza in Figure 5-67 , by the ruddervators.
The feet of the pilot are on the rudder pedals, and pushing
on the left or right rudder pedal makes the rudder move left
or right. The trailing edge of the rudder moves to the right,
and the nose of the airplane yaws to the right, when the right
rudder pedal is pushed. The rudder pedals of a Cessna 182
can be seen in Figure 5-68 .
Even though the rudder of the airplane will make the nose
yaw to the left or the right, the rudder is not what turns the
Control wheel or yoke Rudder pedals
Figure 5-68. Cessna 182 control wheel and rudder pedals.
Ruddervators
Figure 5-67. Ruddervators on a Beechcraft Bonanza provide pitch
control.
Moving horizontal stabilizer (stabilator)
Figure 5-66. Moving horizontal stabilizer, known as a stabilator,
on a Piper Cherokee Cruiser P A-28-140 provides pitch control.
airplane. For what is called a coordinated turn to occur, both
the ailerons and rudder come into play. Let’s say we want to
turn the airplane to the right. We start by turning the control
wheel to the right, which raises the right aileron and lowers
the left aileron and initiates the banking turn. The increased
lift on the left wing also increases the induced drag on the left
wing, which tries to make the nose of the airplane yaw to the
left. To counteract this, when the control wheel is moved to
the right, a small amount of right rudder is used to keep the
nose of the airplane from yawing to the left. Once the nose of
the airplane is pointing in the right direction, pressure on the
rudder is no longer needed. The rudder of a Piper Cherokee
Arrow can be seen in Figure 5-70.
Tabs
Trim Tabs
Trim tabs are small movable surfaces that attach to the trailing
edge of flight controls. These tabs can be controlled from the
flight deck, and their purpose is to create an aerodynamic
force that keeps the flight control in a deflected position. Trim tabs can be installed on any of the primary flight controls.
A very common flight control to find fitted with a trim tab
is the elevator. In order to be stable in flight, most airplanes
have the center of gravity located forward of the center of lift
on the wing. This causes a nose heavy condition, which needs
to be balanced out by having the elevator deflect upwards
and create a downward force. To relieve the pilot of the need
to hold back pressure on the control wheel, a trim tab on the
elevator can be adjusted to hold the elevator in a slightly
deflected position. An elevator trim tab for a Cessna 182 is
shown in Figure 5-71.
Anti-servo Tab
Some airplanes, like a Piper Cherokee Arrow, do not have
a fixed horizontal stabilizer and movable elevator. The
Cherokee uses a moving horizontal surface known as a
stabilator. Because of the location of the pivot point for this
movable surface, it has a tendency to be extremely sensitive
to pilot input. To reduce the sensitivity, a full length anti-servo
tab is installed on the trailing edge of the stabilator. As the
trailing edge of the stabilator moves down, the anti-servo tab
moves down and creates a force trying to raise the trailing
edge. With this force acting against the movement of the
stabilator, it reduces the sensitivity to pilot input. The anti-
servo tab on a Piper Cherokee Arrow is shown in Figure 5-70.
Balance Tab
On some airplanes, the force needed to move the flight
controls can be excessive. In these cases, a balance tab can
be used to generate a force that assists in the movement of the
flight control. Just the opposite of anti-servo tabs, balance tabs
move in the opposite direction of the flight control’s trailing
edge, providing a force that helps the flight control move.
Servo Tab
On large airplanes, because the force needed to move the
flight controls is beyond the capability of the pilot, hydraulic
actuators are used to provide the necessary force. In the event
Rudder
Anti-servo Tab
Elevator trim tab
Aileron downAileron up
Figure 5-70. Rudder and anti-servo tab on a Piper Cherokee Arrow.
Figure 5-71. Elevator trim tab on a Cessna 182.Figure 5-69. Aircraft performing an aileron roll.
of a hydraulic system malfunction or failure, some of these
airplanes have servo tabs on the trailing edge of the primary
flight controls. When the control wheel is pulled back in an
attempt to move the elevator, the servo tab moves and creates
enough aerodynamic force to move the elevator. The servo tab
is acting like a balance tab, but rather than assisting the normal
force that moves the elevator, it becomes the sole force that
makes the elevator move. Like the balance tab, the servo tab
moves in the opposite direction of the flight control’s trailing
edge. The Boeing 727 has servo tabs that back up the hydraulic
system in the event of a failure. During normal flight, the servo
tabs act like balance tabs. [Figure 5-72]
Supplemental Lift-Modifying Devices
If the wing of an airplane was designed to produce the
maximum lift possible at low airspeed, to accommodate
takeoffs and landings, it would not be suited for higher speed
flight because of the enormous amount of drag it would
produce. To give the wing the ability to produce maximum
low speed lift without being drag prohibitive, retractable high
lift devices, such as flaps and slats, are utilized.
Flaps
The most often used lift-modifying device, for small
airplanes and large, is the wing flap. Flaps can be installed
on the leading edge or trailing edge, with the leading edge
versions used only on larger airplanes. Flaps change the
camber of the wing, and they increase both the lift and
the drag for any given angle of attack. The four different
types of flaps in use are called the plain, split, slotted, and
Fowler. [Figure 5-73]
Plain flaps attach to the trailing edge of the wing, inboard
of the ailerons, and form part of the wing’s overall surface.
When deployed downward, they increase the effective
camber of the wing and the wing’s chord line. Both of these factors cause the wing to create more lift and more drag.
The split flap attaches to the bottom of the wing, and deploys
downward without changing the top surface of the wing. This
type of flap creates more drag than the plain flap because of
the increase in turbulence.
The slotted flap is similar to the plain flap, except when it
deploys, the leading edge drops down a small amount. By
having the leading edge drop down slightly, a slot opens,
which lets some of the high-pressure air on the bottom of the
wing flow over the top of the flap. This additional airflow
over the top of the flap produces additional lift.
The Fowler flap attaches to the back of the wing using a track
and roller system. When it deploys, it moves aft in addition
to deflecting downward. This increases the total wing area,
in addition to increasing the wing camber and chord line.
This type of flap is the most effective of the four types, and
it is the type used on commercial airliners and business jets.
Leading Edge Slots
Leading edge slots are ducts or passages in the leading edge
of a wing that allow high pressure air from the bottom of the
wing to flow to the top of the wing. This ducted air flows
over the top of the wing at a high velocity and helps keep the
boundary layer air from becoming turbulent and separating
Stabilizer
Elevator
Elevator tab
Upper rudder
Anti-balance tabsInboard aileron tab
Inboard aileron
Leading edge slats (extended) Leading edge flaps (extended)Lower rudder
Ground spoilers
Inboard flap
Flight spoilers
Outboard flap
Balance tab
Outboard aileron
Figure 5-72. Boeing 727 flight controls.from the wing. Slots are often placed on the part of the wing
ahead of the ailerons, so during a wing stall, the inboard
part of the wing stalls first and the ailerons remain effective.
Leading Edge Slats
Leading edge slats serve the same purpose as slots, the
difference being that slats are movable and can be retracted
when not needed. On some airplanes, leading edge slats have
been automatic in operation, deploying in response to the
aerodynamic forces that come into play during a high angle of
attack. On most of today’s commercial airliners, the leading
edge slats deploy when the trailing edge flaps are lowered.
The flight controls of a large commercial airliner are shown
in Figure 5-72 . The controls by color are as follows:
1. All aerodynamic tabs are shown in green.
2. All leading and trailing edge high lift devices are
shown in red (leading edge flaps and slats, trailing
edge inboard and outboard flaps).
3. The tail mounted primary flight controls are in orange
(rudder and elevator).
4. The wing mounted primary flight controls are in purple
(inboard and outboard aileron). High-Speed Aerodynamics
Compressibility Effects
When air is flowing at subsonic speed, it acts like an
incompressible fluid. As discussed earlier in this chapter,
when air at subsonic speed flows through a diverging shaped
passage, the velocity decreases and the static pressure rises,
but the density of the air does not change. In a converging
shaped passage, subsonic air speeds up and its static pressure
decreases. When supersonic air flows through a converging
passage, its velocity decreases and its pressure and density
both increase. [Figure 5-74] At supersonic flow, air acts like
a compressible fluid. Because air behaves differently when
flowing at supersonic velocity, airplanes that fly supersonic
must have wings with a different shape.
The Speed of Sound
Sound, in reference to airplanes and their movement through
the air, is nothing more than pressure disturbances in the
air. As discussed earlier in this chapter, it is like dropping a
rock in the water and watching the waves flow out from the
center. As an airplane flies through the air, every point on
the airplane that causes a disturbance creates sound energy
in the form of pressure waves. These pressure waves flow
away from the airplane at the speed of sound, which at
standard day temperature of 59 °F, is 761 mph. The speed
5-47Plain flap
Split flap
Slotted flap
Fowler flap
AirflowSupersonic
Converging
Decreasing velocity
Increasing pressure
Increasing densityDiverging
Increasing velocity
Decreasing pressure
Decreasing densityFigure 5-73. Four types of wing flaps.
Figure 5-74. Supersonic airflow through a venturi.of sound in air changes with temperature, increasing as
temperature increases. Figure 5-75 shows how the speed
of sound changes with altitude.
Subsonic, Transonic, and Supersonic Flight
When an airplane is flying at subsonic speed, all of the air flowing around the airplane is at a velocity of less than the speed of sound, which is known as Mach 1. Keep in mind that the air accelerates when it flows over certain parts of the airplane, like the top of the wing, so an airplane flying at 500 mph could have air over the top of the wing reach a speed of 600 mph. How fast an airplane can fly and still be considered in subsonic flight varies with the design of the wing, but as a Mach number, it will typically be just over Mach 0.8.
When an airplane is flying at transonic speed, part of
the airplane is experiencing subsonic airflow and part is experiencing supersonic airflow. Over the top of the wing, probably about halfway back, the velocity of the air will reach Mach 1 and a shock wave will form. The shock wave forms 90 degrees to the airflow and is known as a normal shock wave. Stability problems can be encountered during transonic flight, because the shock wave can cause the airflow to separate from the wing. The shock wave also causes the center of lift to shift aft, causing the nose to pitch down. The speed at which the shock wave forms is known as the critical Mach number. Transonic speed is typically between Mach 0.80 and 1.20.
When an airplane is flying at supersonic speed, the entire
airplane is experiencing supersonic airflow. At this speed, the shock wave which formed on top of the wing during transonic flight has moved all the way aft and has attached itself to the wing trailing edge. Supersonic speed is from Mach 1.20 to 5.0. If an airplane flies faster than Mach 5, it is said to be in hypersonic flight.
Shock Waves
Sound coming from an airplane is the result of the air being disturbed as the airplane moves through it, and the resulting pressure waves that radiate out from the source of the disturbance. For a slow-moving airplane, the pressure waves travel out ahead of the airplane, traveling at the speed of sound. When the speed of the airplane reaches the speed of sound, however, the pressure waves, or sound energy, cannot get away from the airplane. At this point the sound energy starts to pile up, initially on the top of the wing, and eventually attaching itself to the wing leading and trailing edges. This piling up of sound energy is called a shock wave. If the shock waves reach the ground, and cross the path of a person, they will be heard as a sonic boom. Figure 5-76A
shows a wing in slow speed flight, with many disturbances on the wing generating sound pressure waves that are radiating outward. View B is the wing of an airplane in supersonic flight, with the sound pressure waves piling up toward the wing leading edge.
Normal Shock Wave
When an airplane is in transonic flight, the shock wave that
forms on top of the wing, and eventually on the bottom of the wing, is called a normal shock wave. If the leading edge of the wing is blunted, instead of being rounded or sharp, a normal shock wave will also form in front of the wing during
5-48AL TITUDE IN FEETSPEED OF SOUNDTEMPERATURE (°F)
1,000
2,0003,0004,0005,0006,0007,0008,0009,000
10,00015,00020,00025,00030,00035,000
* 36,089
40,00045,00050,00055,00060,00065,00070,00075,00080,00085,00090,00095,000
100,00076175875675375074874574274073773472170769267866366066066066066066066066066066066467167868459.0055.4351.8748.3044.7441.1737.6034.0430.4726.9023.34
−12.32−30.15−47.98−65.82−69.70−69.70−69.70−69.70−69.70−69.70−69.70−69.70−69.70−69.70−64.80−56.57−48.34−40.11
* Altitude at which temperature stops decreasing.
Figure 5-75. Altitude and temperature versus speed of sound.supersonic flight. Normal shock waves form perpendicular to
the airstream. The velocity of the air behind a normal shock wave is subsonic, and the static pressure and density of the air are higher. Figure 5-77 shows a normal shock wave forming
on the top of a wing.
Oblique Shock Wave
An airplane that is designed to fly supersonic will have very
sharp edged surfaces, in order to have the least amount of drag. When the airplane is in supersonic flight, the sharp leading edge and trailing edge of the wing will have shock waves attach to them. These shock waves are known as oblique shock waves. Behind an oblique shock wave the velocity of the air is lower, but still supersonic, and the static pressure and density are higher. Figure 5-78 shows an oblique shock wave on the leading and trailing edges of a supersonic airfoil.
Expansion Wave
Earlier in the discussion of high-speed aerodynamics, it was
stated that air at supersonic speed acts like a compressible fluid. For this reason, supersonic air, when given the opportunity, wants to expand outward. When supersonic air is flowing over the top of a wing, and the wing surface turns away from the direction of flow, the air will expand and follow the new direction. An expansion wave will occur at the point where the direction of flow changes. Behind the expansion wave the velocity increases, and the static pressure and density decrease. An expansion wave is not a shock wave. Figure 5-78 shows an expansion wave on a supersonic airfoil.
High-Speed Airfoils
Transonic flight is the most difficult flight regime for an airplane, because part of the wing is experiencing subsonic airflow and part is experiencing supersonic airflow. For a subsonic airfoil, the aerodynamic center, or the point of support, is approximately 25 percent of the way back from the wing leading edge. In supersonic flight, the aerodynamic center moves back to 50 percent of the wing’s chord, causing some significant changes in the airplane’s control and stability.
If an airplane designed to fly subsonic, perhaps at a Mach
number of 0.80, flies too fast and enters transonic flight, some noticeable changes will take place with respect to the airflow over the wing. Figure 5-79 shows six views of a wing, with
each view showing the Mach number getting higher.
The scenario for the six views is as follows:
A.The Mach number is fairly low, and the entire wing
is experiencing subsonic airflow.
B.The velocity has reached the critical Mach number,
where the airflow over the top of the wing is reachingMach 1 velocity.
C.The velocity has surpassed the critical Mach number ,
and a normal shock wave has formed on the top of the
wing. Some airflow separation starts to occur behindthe shock wave.
D.The velocity has continued to increase beyond the
critical Mach number, and the normal shock wave has moved far enough aft that serious airflow separationis occurring. A normal shock wave is now formingon the bottom of the wing as well. Behind the normal(MPH)
5-49A
B
Normal shock wave
Subsonic airSupersonic airSubsonic air
Oblique
shockOblique
shockExpansion
wave
Airflow
Figure 5-76. Sound energy in subsonic and supersonic flight.
Figure 5-77. Normal shock wave.Figure 5-78. Supersonic airfoil with oblique shock waves and
expansion waves.shock waves, the velocity of the air is subsonic and
the static pressure has increased.
E.T he velocity has increased to the point that both shock
waves on the wing, top and bottom, have moved tothe back of the wing and attached to the trailing edge.Some airflow separation is still occurring.
F. T he forward velocity of the airfoil is greater than Mach
1, and a new shock wave has formed just forward ofthe leading edge of the wing. If the wing has a sharpleading edge, the shock wave will attach itself to thesharp edge.
The airfoil shown in Figure 5-79 is not properly designed to handle supersonic airflow. The bow wave in front of the wing leading edge of view F would be attached to the leading edge, if the wing was a double wedge or biconvex design. These two wing designs are shown in Figure 5-80.Aerodynamic Heating
One of the problems with airplanes and high-speed flight is the heat that builds up on the airplane’s surface because of air friction. When the SR-71 Blackbird airplane is cruising at Mach 3.5, skin temperatures on its surface range from 450 °F to over 1,000 °F. To withstand this high temperature, the airplane was constructed of titanium alloy, instead of the traditional aluminum alloy. The supersonic transport Concorde was originally designed to cruise at Mach 2.2, but its cruise speed was reduced to Mach 2.0 because of structural problems that started to occur because of aerodynamic heating. If airplanes capable of hypersonic flight are going to be built in the future, one of the obstacles that will have to be overcome is the stress on the airplane’s structure caused by heat.
Helicopter Aerodynamics
The helicopter, as we know it today, falls under the
classification known as rotorcraft. Rotorcraft is also known as rotary wing aircraft, because instead of their wing being fixed like it is on an airplane, the wing rotates. The rotating wing of a rotorcraft can be thought of as a lift producing device, like the wing of an airplane, or as a thrust producing device, like the propeller on a piston engine.
Helicopter Structures and Airfoils
The main parts that make up a helicopter are the cabin, landing gear, tail boom, power plant, transmission, main rotor, and tail rotor. [Figure 5-81]
Main Rotor Systems
In the fully articulated rotor system, the blades are attached
to the hub multiple times. The blades are hinged in a way that allows them to move up and down and fore and aft, and bearings provide for motion around the pitch change axis. Rotor systems using this type of arrangement typically have three or more blades. The hinge that allows the blades to move
Mach number = 1.05
View FSupersonic flowBow
wave
Mach number = 0.88Supersonic flow
View DNormal shock
Separation
Normal shock
Mach number = 0.60Airflow over the entire
wing is subsonic
View A
Mach number = 0.82
(Critical Mach number)Airflow over the wing
reaches Mach 0.99
View B
Mach number = 0.95
View ENormal
shock
Normal
shockSupersonic flow
Mach number = 0.85SubsonicNormal shockSupersonic flow
View C
Figure 5-79. Airflow with progressively greater Mach numbers.up and down is called the flap hinge, and movement around this
hinge is called flap. The hinge that allows the blades to move
fore and aft is called a drag or lag hinge. Movement around this
hinge is called dragging, lead/lag, or hunting. These hinges and
their associated movement are shown in Figure 5-82 . The main
rotor head of a Eurocopter model 725 is shown in Figure 5-83 ,
with the drag hinge and pitch change rods visible.
The semi-rigid rotor system is used with a two-blade main
rotor. The blades are rigidly attached to the hub, with the
hub and blades able to teeter like a seesaw. The teetering
action allows the blades to flap, with one blade dropping
down while the other blade rises. The blades are able to
change pitch independently of each other. Figure 5-84
shows a Bell Jet Ranger helicopter in flight. This helicopter
uses a semi-rigid rotor system, which is evident because
of the way the rotor is tilted forward when the helicopter
is in forward flight.
With a rigid rotor system, the blades are not hinged for
movement up and down, or flapping, or for movement fore
and aft, or drag. The blades are able to move around the pitch
change axis, with each blade being able to independently
change its blade angle. The rigid rotor system uses blades
that are very strong and yet flexible. They are flexible enough
to bend when they need to, without the use of hinges or a
teetering rotor, to compensate for the uneven lift that occurs
in forward flight. The Eurocopter model 135 uses a rigid
rotor system. [Figure 5-85]
Anti-Torque Systems
Any time a force is applied to make an object rotate;
there will be equal force acting in the opposite direction.
If the helicopter’s main rotor system rotates clockwise
when viewed from the top, the helicopter will try to rotate
counterclockwise. Earlier in this chapter, it was discovered
that torque is what tries to make something rotate. For this
reason, a helicopter uses what is called an anti-torque system
to counteract the force trying to make it rotate.
One method that is used on a helicopter to counteract torque
is to place a spinning set of blades at the end of the tail boom.
These blades are called a tail rotor or anti-torque rotor, and
their purpose is to create a force, or thrust that acts in the
opposite direction of the way the helicopter is trying to rotate.
The tail rotor force, in pounds, multiplied by the distance
from the tail rotor to the main rotor, in feet, creates a torque
in pound-feet that counteracts the main rotor torque.
Figure 5-86 shows a three-bladed tail rotor on an Aerospatiale
AS-315B helicopter. This tail rotor has open tipped blades
that are variable pitch, and the helicopter’s anti-torque pedals
that are positioned like rudder pedals on an airplane, control
the amount of thrust they create. Some potential problems
BiconvexOblique
shockOblique
shockOblique
shockExpansion
wave
Airflow
Oblique
shock
Double wedge
Expansion
wave
Main rotor
Cabin
Landing gearTransmissionPowerplantTail rotor
Tail boomFigure 5-80. Double wedge and biconvex supersonic wing design.
Figure 5-81. Main components of a helicopter.with this tail rotor system are as follows:
•The spinning blades are deadly if someone walks
into them.
• When the helicopter is in forward flight and a vertical
fin may be in use to counteract torque, the tail rotor
robs engine power and creates drag.
An alternative to the tail rotor seen in Figure 5-86 is a type of
anti-torque rotor known as a fenestron, or “fan-in-tail” design as seen in Figure 5-87. The rotating blades present less of a
hazard to personnel on the ground and they create less drag in flight, because they are enclosed in a shroud.
A third method of counteracting the torque of the helicopter’s
main rotor is a technique called the “no tail rotor” system, or NOTAR. This system uses a high volume of air at low pressure, which comes from a fan driven by the helicopter’s engine. The fan forces air into the tail boom, where a portion of it exits out of slots on the right side of the boom and, in conjunction with the main rotor downwash, creates a phenomenon called the “Coanda effect.” The air coming out of the slots on the right side of the boom causes a higher velocity, and therefore lower pressure, on that side of the boom. The higher pressure on the left side of the boom creates the primary force that counteracts the torque of the main rotor.
Lead or lag Flap hinge
Drag hinge Pitch
FlapAxis of rotation
Figure 5-82. Fully articulated main rotor head.
Pitch change rod Drag hinge
Semi-rigid main rotorFigure 5-83. Eurocopter 725 main rotor head.
Figure 5-84. Bell Jet Ranger with semi-rigid main rotor.The remainder of the air travels back to a controllable rotating
nozzle in the helicopter’s tail. The air exits the nozzle at a
high velocity, and creates an additional force, or thrust, that
helps counteracts the torque of the main rotor. A NOTAR
system is shown in Figures 5-88 and 5-89 .
For helicopters with two main rotors, such as the Chinook that
has a main rotor at each end, no anti-torque rotor is needed.
For this type of helicopter, the two main rotors turn in opposite
directions, and each one cancels out the torque of the other.
Helicopter Axes of Flight
Helicopters, like airplanes, have a vertical, lateral, and
longitudinal axis that passes through the helicopter’s center
of gravity. Helicopters yaw around the vertical axis, pitch
around the lateral axis, and rotate around the longitudinal
axis. Figure 5-90 shows the three axes of a helicopter and
how they relate to the helicopter’s movement. All three axes
will intersect at the helicopter’s center of gravity, and the
helicopter pivots around this point. Notice in the figure that
the vertical axis passes almost through the center of the main
rotor, because the helicopter’s center of gravity needs to be
very close to this point.
Control Around the Vertical Axis
For a single main rotor helicopter, control around the vertical
axis is handled by the anti-torque rotor, or tail rotor, or from
the fan’s airflow on a NOTAR type helicopter. Like in an
airplane, rotation around this axis is known as yaw. The pilot
controls yaw by pushing on the anti-torque pedals located on
the cockpit floor, in the same way the airplane pilot controls
yaw by pushing on the rudder pedals. To make the nose of
Figure 5-85. Eurocopter Model 135 rigid rotor system.
Figure 5-87. Fenestron on a Eurocopter Model 135.
Figure 5-86. Aerospatiale helicopter tail rotor.the helicopter yaw to the right, the pilot pushes on the right
anti-torque pedal. When viewed from the top, if the helicopter
tries to spin in a counterclockwise direction because of the
torque of the main rotor, the pilot will also push on the right
anti-torque pedal to counteract the main rotor torque. By
using the anti-torque pedals, the pilot can intentionally make
the helicopter rotate in either direction around the vertical
axis. The anti-torque pedals can be seen in Figure 5-91.
Some helicopters have a vertical stabilizer, such as those
shown in Figures 5-90 and 5-92 . In forward flight, the vertical
stabilizer creates a force that helps counteract the torque of
the main rotor, thereby reducing the power needed to drive
the anti-torque system located at the end of the tail boom.
Control Around the Longitudinal and Lateral Axes
Movement around the longitudinal and lateral axes is handled
by the helicopter’s main rotor. In the cockpit, there are two
levers that control the main rotor, known as the collective and
cyclic pitch controls. The collective pitch lever is on the side
of the pilot’s seat, and the cyclic pitch lever is at the front of
the seat in the middle. [Figure 5-91]When the collective pitch control lever is raised, the blade
angle of all the rotor blades increases uniformly and they
create the lift that allows the helicopter to take off vertically.
The grip on the end of the collective pitch control is the
throttle for the engine, which is rotated to increase engine
power as the lever is raised. On many helicopters, the throttle
automatically rotates and increases engine power as the
collective lever is raised. The collective pitch lever may have
adjustable friction built into it, so the pilot does not have to
hold upward pressure on it during flight.
The cyclic pitch control lever, like the yoke of an airplane, can
be pulled back or pushed forward, and can be moved left and
right. When the cyclic pitch lever is pushed forward, the rotor
blades create more lift as they pass through the back half of
their rotation and less lift as they pass through the front half.
The difference in lift is caused by changing the blade angle,
or pitch, of the rotor blades. The pitch change rods that were
seen earlier, in Figures 5-82 and 5-83 , are controlled by the
cyclic pitch lever and they are what change the pitch of the rotor
blades. The increased lift in the back either causes the main
rotor to tilt forward, the nose of the helicopter to tilt downward,
or both. The end result is the helicopter moves in the forward
direction. If the cyclic pitch lever is pulled back, the rotor blade
lift will be greater in the front and the helicopter will back up.
If the cyclic pitch lever is moved to the left or the right, the
helicopter will bank left or bank right. For the helicopter to
bank to the right, the main rotor blades must create more lift
as they pass by the left side of the helicopter. Just the opposite
is true if the helicopter is banking to the left. By creating
more lift in the back than in the front, and more lift on the
Lateral axisVertical axis
Longitudinal axis
Figure 5-90. Three axes of rotation for a helicopter.
Figure 5-88. McDonnell Douglas 520 NOTAR.
Low pressure side
Air exit slotsRotating nozzle
High pressure side
Figure 5-89. Airflow for a NOTAR.
left than on the right, the helicopter can be in forward flight
and banking to the right. In Figure 5-92 , an Agusta A-109
can be seen in forward flight and banking to the right. The
rotor blade in the rear and the one on the left are both in an
upward raised position, meaning they have both experienced
the condition called flap.
Some helicopters use a horizontal stabilizer, similar to what
is seen on an airplane, to help provide additional stability
around the lateral axis. A horizontal stabilizer can be seen
on the Agusta A-109 in Figure 5-92 .
Helicopters in Flight
Hovering
For a helicopter, hovering means that it is in flight at a constant
altitude, with no forward, aft, or sideways movement. In order
to hover, a helicopter must be producing enough lift in its
main rotor blades to equal the weight of the aircraft. The
engine of the helicopter must be producing enough power
to drive the main rotor, and also to drive whatever type of
anti-torque system is being used. The ability of a helicopter to hover is affected by many things, including whether or
not it is in ground effect, the density altitude of the air, the
available power from the engine, and how heavily loaded it is.
For a helicopter to experience ground effect, it typically needs
to be no higher off the ground than one half of its main rotor
system diameter. If a helicopter has a main rotor diameter of 40
ft., it will be in ground effect up to an altitude of approximately
20 ft. Being close to the ground affects the velocity of the
air through the rotor blades, causing the effective angle of
attack of the blades to increase and the lift to increase. So, if
a helicopter is in ground effect, it can hover at a higher gross
weight than it can when out of ground effect. On a windy day,
the positive influence of ground effect is lessened, and at a
forward speed of 5 to 10 mph the positive influence becomes
less. In Figure 5-93 , an Air Force CH-53 is seen in a hover, with
all the rotor blades flapping up as a result of creating equal lift.
Forward Flight
In the early days of helicopter development, the ability to
hover was mastered before there was success in attaining
forward flight. The early attempts at forward flight resulted
in the helicopter rolling over when it tried to depart from the
hover and move in any direction. The cause of the rollover
is what we now refer to as dissymmetry of lift.
When a helicopter is in a hover, all the rotor blades are
experiencing the same velocity of airflow and the velocity
of the airflow seen by the rotor blades changes when the
helicopter starts to move. For helicopters built in the United
States, the main rotor blades turn in a counterclockwise
direction when viewed from the top. Viewed from the top, as
the blades move around the right side of the helicopter, they
are moving toward the nose; as they move around the left side
of the helicopter, they are moving toward the tail. When the
helicopter starts moving forward, the blade on the right side
is moving toward the relative wind, and the blade on the left
Cyclic pitch control
Collective pitch control Anti-torque pedals Anti-torque pedals
Figure 5-92. Agusta A-109 banking to the right.Figure 5-91. Helicopter cockpit controls.
side is moving away from the relative wind. This causes the
blade on the right side to create more lift and the blade on the
left side to create less lift. Figure 5-94 shows how this occurs.
In Figure 5-94 , blade number 2 would be called the advancing
blade, and blade number 1 would be called the retreating
blade. The advancing blade is moving toward the relative
wind, and therefore experiences a greater velocity of airflow.
The increased lift created by the blade on the right side will
try to roll the helicopter to the left. If this condition is allowed
to exist, it will ultimately lead to the helicopter crashing.
Blade Flapping
To solve the problem of dissymmetry of lift, helicopter
designers came up with a hinged design that allows the rotor
blade to flap up when it experiences increased lift, and to flap
down when it experiences decreased lift. When a rotor blade
advances toward the front of the helicopter and experiences
an increased velocity of airflow, the increase in lift causes the
blade to flap up. This upward motion of the blade changes
the direction of the relative wind in relation to the chord
line of the blade, and causes the angle of attack to decrease.
The decrease in the angle of attack decreases the lift on the
blade. The retreating blade experiences a reduced velocity
of airflow and reduced lift, and flaps down. By flapping
down, the retreating blade ends up with an increased angle of attack and an increase in lift. The end result is the lift on
the blades is equalized, and the tendency for the helicopter
to roll never materializes.
The semi-rigid and fully articulated rotor systems have
flapping hinges that automatically allow the blades to move
Blade rotation
Blade rotation
Blade
Blade experiences 300 mph airfl ow (Tip speed – airspeed)
experiences 500 mph airflow (Tip speed + airspeed)Direction
of flight
(100 mph)Direction
of relative
wind
Blade tip
speed— 400 mph1
Figure 5-94. Dissymmetry of lift for rotor blades.
Figure 5-93. Air Force CH-53 in a hover.up or down with changes in lift. The rigid type of rotor system
has blades that are flexible enough to bend up or down with changes in lift.
Advancing Blade and Retreating Blade Problems
The blade advancing toward the relative wind sees the airflow
at an ever increasing velocity as a helicopter flies forward at higher and higher speeds. Eventually, the velocity of the air over the rotor blade will reach sonic velocity, much like the critical Mach number for the wing of an airplane. When this happens, a shock wave will form and the air will separate from the rotor blade, resulting in a high-speed stall.
As the helicopters forward speed increases, the relative wind
over the retreating blade decreases, resulting in a loss of lift. The loss of lift causes the blade to flap down and the effective angle of attack to increase. At a high enough forward speed, the angle of attack will increase to a point that the rotor blade stalls. The tip of the blade stalls first, and then progresses in toward the blade root.
When approximately 25 percent of the rotor system is stalled,
due to the problems with the advancing and retreating blades, control of the helicopter will be lost. Conditions that will lead to the rotor blades stalling include high forward speed, heavy gross weight, turbulent air, high-density altitude, and steep or abrupt turns.
Autorotation
The engine on a helicopter drives the main rotor system by way
of a clutch and a transmission. The clutch allows the engine to be running and the rotor system not to be turning, while the helicopter is on the ground, and it also allows the rotor system to disconnect from the engine while in flight, if the engine fails. Having the rotor system disconnect from the engine in the event of an engine failure is necessary if the helicopter is to be capable of a flight condition called autorotation.
Autorotation is a flight condition where the main rotor blades
are driven by the force of the relative wind passing through the blades, rather than by the engine. This flight condition is similar to an airplane gliding if its engine fails while in flight. As long as the helicopter maintains forward airspeed, while decreasing altitude and the pilot lowers the blade angle on the blades with the collective pitch, the rotor blades will continue to rotate. The altitude of the helicopter, which equals potential energy, is given up in order to have enough energy, which will then be kinetic energy, to keep the rotor blades turning. As the helicopter nears the ground, the cyclic pitch control is used to slow the forward speed and to flare the helicopter for landing. With the airspeed bled off, and the helicopter now close to the ground, the final step is to use the collective pitch control to cushion the landing. The airflow through the rotor blades in normal forward flight and in an autorotation flight condition are shown in Figure 5-95.
In Figure 5-96 , a Bell Jet Ranger is shown approaching the
ground in the final stage of an autorotation.
Weight-Shift Control, Flexible Wing Aircraft
Aerodynamics
A weight-shift control, flexible wing type aircraft consists of
a fabric-covered wing, often referred to as the sail, attached to a tubular structure that has wheels, seats, and an engine and propeller. The wing structure is also tubular, with the fabric covering creating the airfoil shape. The shape of the wing varies among the different models of weight-shift control aircraft being produced, but a delta shaped wing is a very popular design. Within the weight-shift control aircraft community, these aircraft are typically referred to as trikes. [Figure 5-97]
In Figure 5-97, the trike’s mast is attached to the wing at the
hang point on the keel of the wing with a hang point bolt and safety cable. There is also a support tube, known as a king post, extending up from the top of the wing, with cables running down and secured to the tubular wing structure. The cables
Forward flight in autorotationNormal forward fl ight under power
Direction of airflow
Direction of airflow
Figure 5-96. Bell Jet Ranger in final stage of autorotation.Figure 5-95. Rotor blade airflow during normal flight and during
autorotation.running down from the king post as part of the upper rigging
are there to support the wing when the aircraft is on the ground, and to handle negative loads when in flight. The lines that run from the king post to the trailing edge of the wing are known as reflex cables. These cables maintain the shape of the wing when it is in a stalled state by holding the trailing edge of the wing up which helps raise the nose during recovery from the stall. If the aircraft goes into an inadvertent stall, having the trailing edge of the wing in a slightly raised position helps raise the nose of the aircraft and get it out of the dive. The passenger seat is centered under the wing’s aerodynamic center, with the weight of the pilot being forward of this point and the weight of the engine and propeller being aft.
Unlike a traditional airplane, the trike does not have a rudder,
elevator, or ailerons. Instead, it has a wing that can be pivoted forward or aft, and left or right. In Figure 5-98, the pilot’s hand
is on a control bar that is connected to a pivot point just forward of where the wing attaches. There are cables attached to the ends of the bar that extend up to the wing’s leading and trailing edge, and to the left and right side of the cross bar. Running from the wing leading edge to trailing edge are support pieces known as battens. The battens fit into pockets, and they give the wing its cambered shape. The names of some of the primary parts of the trike are shown in Figure 5-98 , and these parts
will be referred to when the flight characteristics of the trike are described in the paragraphs that follow.
In order to fly the trike, engine power is applied to get the
aircraft moving. As the groundspeed of the aircraft reaches a point where flight is possible, the pilot pushes forward on the control bar, which causes the wing to pivot where it attaches to the mast and the leading edge of the wing tilts up. When the leading edge of the wing tilts up, the angle of attack and the lift of the wing increase. With sufficient lift, the trike rotates and starts climbing. Pulling back on the bar reduces the angle of attack, and allows the aircraft to stop climbing and to fly straight and level. Once the trike is in level flight, airspeed can be increased or decreased by adding engine power or taking away engine power by use of the throttle.
Stability in flight along the longitudinal axis, which is a
nose to tail measurement, for a typical airplane, is achieved by having the horizontal stabilizer and elevator generate a force that balances out the airplane’s nose heavy tendency.
It must create stability along the longitudinal axis in a different
way because the trike does not have a horizontal stabilizer or elevator. The trike has a sweptback delta wing, with the trailing edge of the wingtips located well aft of the aircraft center of gravity. Pressure acting on the tips of the delta wing creates the force that balances out the nose heavy tendency. The wings of weight-shift control aircraft are designed in a way that allows them to change their shape when subjected to an external force. This is possible because the frame leading edges and the sail are flexible, which is why they are sometimes referred to as flexible wing aircraft. This produces somewhat different aerodynamic effects when compared with
Figure 5-97. Weight-shift control aircraft in level flight.a normal fixed-wing aircraft. A traditional small airplane, like
a Cessna 172, turns or banks by using the ailerons, effectively
altering the camber of the wing and thereby generating
differential lift. By comparison, weight shift on a trike
actually causes the wing to twist, which changes the angle of
attack on the wing and causes the differential lift to exist that
banks the trike. The cross-bar, or wing spreader, of the wing
frame is allowed to float slightly with respect to the keel, and
this, along with some other geometric considerations allows
the sail to “billow shift.” Billow shift can be demonstrated
on the ground by grabbing the trailing edge of one end of
the wing and lifting up on it. If this was done, the fabric on
the other end of the wing would become slightly flatter and
tighter, and the wing’s angle of attack would increase.
If the pilot pushes the bar to the right, the wing pivots with
the left wingtip dropping down and the right wingtip rising
up, causing the aircraft to bank to the left. This motion is
depicted in Figure 5-99 , showing a hang glider as an example.
The shift in weight to the left increases the wing loading on
the left, and lessens it on the right. The increased loading on
the left wing increases its washout and reduces its angle of
attack and lift. The increased load on the left wing causes the
left wing to billow, which causes the fabric to tighten on the
right wing and the angle of attack and lift to increase. The
change in lift is what banks the aircraft to the left. Billow on
the left wing is depicted in Figure 5-100.
Shifting weight to the right causes the aircraft to bank right.
The weight of the trike and its occupants acts like a pendulum,
and helps keep the aircraft stable in flight. Pushing or pulling
on the bar while in flight causes the weight hanging below
the wing to shift its position relative to the wing, which is
why the trike is referred to as a weight-shift aircraft.
Once the trike is in flight and flying straight and level, the
pilot only needs to keep light pressure on the bar that controls the wing. If the trike is properly balanced and there is no air
turbulence, the aircraft will remain stable even if the pilot’s
hands are removed from the bar. The same as with any airplane,
increasing engine power will make the aircraft climb and
decreasing power will make it descend. The throttle is typically
controlled with a foot pedal, like a gas pedal in an automobile.
A trike lands in a manner very similar to an airplane. When it
is time to land, the pilot reduces engine power with the foot-
operated throttle; causing airspeed and wing lift to decrease.
As the trike descends, the rate of descent can be controlled by
pushing forward or pulling back on the bar, and varying engine
power. When the trike is almost to the point of touchdown, the
engine power will be reduced and the angle of attack of the
wing will be increased, to cushion the descent and provide a
smooth landing. If the aircraft is trying to land in a very strong
crosswind, the landing may not be so smooth. When landing in
a cross wind, the pilot will land in a crab to maintain direction
down the runway. Touchdown is done with the back wheels
first, then letting the front wheel down.
A trike getting ready to touch down can be seen in
Figure 5-101 . The control cables coming off the control bar
can be seen, and the support mast and the cables on top of
the wing, including the luff lines, can also be seen.
Powered Parachute Aerodynamics
A powered parachute has a carriage very similar to the
weight-shift control aircraft. Its wing, however, has no
support structure or rigidity and only takes on the shape of an
airfoil when it is inflated by the blast of air from the propeller
and the forward speed of the aircraft. In Figure 5-102 , a
powered parachute is on its approach to land with the wing
fully inflated and rising up above the aircraft. Each colored
section of the inflated wing is made up of cells that are open in
the front to allow air to ram in, and closed in the back to keep
the air trapped inside. In between all the cells there are holes
that allow the air to flow from one cell to the next, in order
to equalize the pressure within the inflated wing. The wing
is attached to the carriage of the aircraft by a large number of
nylon or Kevlar lines that run from the tips of the wing all the
way to the center. The weight of the aircraft acting on these
lines and their individual lengths cause the inflated wing to
take its shape. The lines attach to the body of the aircraft at a
location very close to where the center of gravity is located,
and this attachment point is adjustable to account for balance
changes with occupants of varying weights.
As in weight-shift control aircraft, the powered parachute
does not have the traditional flight controls of a fixed-wing
airplane. When the wing of the aircraft is inflated and the
aircraft starts moving forward, the wing starts generating lift.
Once the groundspeed is sufficient for the wing’s lift greater
than the weight of the aircraft, the aircraft lifts off the ground.
Nose strut
Control bar with cables attached to the wing
ThrottleMastWing keelWing attach pointWing batten
Crossbar
Brakes
Push outDirection of turn
Weight shifted to left
Aircraft rolls to leftAircraft banks to the left
because of increased lift
on the right wing
BillowPushing the bar to
the right shifts the
weight to the leftFigure 5-98. Weight-shift control aircraft getting ready for flight.
Figure 5-99. Direction of turn based on weight shift.Figure 5-100. Weight shift to the left causing a left-hand turn.Unlike an airplane, where the pilot has a lot of control over
when the airplane rotates by deciding when to pull back on the
yoke, the powered parachute will not take off until it reaches
a specific airspeed. The powered parachute will typically lift
off the ground at a speed somewhere between 28 and 30 mph,
and will have airspeed in flight of approximately 30 mph.
Once the powered parachute is in flight, control over climbing and descending is handled with engine power. Advancing
the throttle makes the aircraft climb, and retarding the
throttle makes it descend. The inflated wing creates a lot
of drag in flight, so reducing the engine power creates a
very controllable descent of the aircraft. The throttle, for
controlling engine power, is typically located on the right-
hand side of the pilot. [Figure 5-103]
Turning of the powered parachute in flight is handled by
foot-operated pedals, or steering bars, located at the front of
the aircraft. These bars can be seen in Figure 5-103 . Each
Figure 5-102. Powered parachute with the wing inflated.
Figure 5-101. Weight-shift control aircraft landing.foot-operated pedal controls a set of lines, usually made
from nylon that runs up to the trailing edge of each wingtip.
When the right foot pedal is pushed, the line pulls down on
the tailing edge of the 8 wingtip. As the trailing edge of the
right wing drop downs, drag is increased on the right side and
the aircraft turns right. When pressure is taken off the foot
pedal, the drag in the entire airfoil equalizes and the aircraft
resumes its straight-and-level flight.
To land a powered parachute, the first action the pilot takes
is to reduce engine power and allow the aircraft to descend.
With the power reduced to idle, the aircraft will descend at a
rate of approximately 5 to 10 fps. As the aircraft approaches
the ground, the descent rate can be lessened by increasing
the engine power. Just before touchdown, the pilot pushes on
both foot-operated pedals to drop the trailing edges on both
sides of the wing. This action increases the drag on the wing
uniformly, causing the wing to pivot aft, which raises the
wing leading edge and increases the angle of attack and lift.
In Figure 5-104 , the pilot is pushing on both foot pedals and
the left and right wing trailing edges are deflected downward.
The aircraft has just touched down and the wing is trailing
behind the aircraft, caused by the high angle of attack and
the additional drag on the wing. The increase in lift reduces
the descent rate to almost nothing, and provides for a gentle
landing. If the pilot pushes on the foot pedals too soon, the
wing may pivot too far aft before touchdown resulting in an
unacceptable descent rate. In that case, it might be a relatively
hard landing.
Figure 5-104. Powered parachute wing trailing edge.Figure 5-103. Two seat powered parachute.
Aircraft Weight & Balance
Chapter 6
Introduction
The weight of an aircraft and its balance are extremely
important for operating in a safe and efficient manner. When
a manufacturer designs an aircraft and the Federal Aviation
Administration (FAA) certifies it, the specifications identify
the aircraft’s maximum weight and the limits within which
it must balance. The weight and balance system commonly
employed among aircraft consists of three equally important
elements: the weighing of the aircraft, the maintaining of
the weight and balance records, and the proper loading of
the aircraft.
The maximum weight of an aircraft is based on the amount
of lift the wings or rotors can provide under the operating
conditions for which the aircraft is designed. For example,
if a small general aviation (GA) airplane required a takeoff
speed of 200 miles per hour (mph) to generate enough lift
to support its weight, that would not be safe. Taking off and
landing at lower airspeeds is certainly safer than doing so
at higher speeds.
Aircraft balance is also a significant factor in determining
if the aircraft is safe to operate. An aircraft that does not
have good balance can exhibit poor maneuverability and
controllability, making it difficult or impossible to fly. This
could result in an accident, causing damage to the aircraft and
injury to the people on board. Safety is the primary reason
for concern about an aircraft’s weight and balance.
Another important reason for concern about weight and
balance is the efficiency of the aircraft. Improper loading
reduces the efficiency of an aircraft from the standpoint
of ceiling, maneuverability, rate of climb, speed, and fuel
consumption. If an airplane is loaded in such a way that it is
extremely nose heavy, higher than normal forces are exerted
at the tail to keep the airplane in level flight. The higher than
normal forces at the tail create additional drag, which requires
additional engine power and therefore additional fuel flow
to maintain airspeed.
The most efficient condition for an aircraft is to have the point
where it balances fall close to, or exactly at, the aircraft’s
center of lift. If this were the case, little or no flight control
force would be needed to keep the aircraft flying straight and
level. In terms of stability and safety, however, this perfectly balanced condition might not be desirable. All factors that
affect aircraft safety and efficiency, in terms of its weight and
balance, are discussed in detail in this chapter.
Requirements for Aircraft Weighing
Every aircraft type certificated by the FAA receives a weight
and balance report as part of its required aircraft records
before leaving the factory for delivery to its new owner.
The weight and balance report identifies the empty weight
of the aircraft and the location at which the aircraft balances,
known as the center of gravity (CG). The weight and balance
report must include an equipment list showing weights and
moment arms of all required and optional items of equipment
included in the certificated empty weight. If the manufacturer
chooses to do so, it can weigh every aircraft it produces and
issue the weight and balance report based on that weighing.
As an alternative, the manufacturer is permitted to weigh
an agreed upon percentage of a particular model of aircraft
produced, perhaps 10 to 20 percent, and apply the average
to all the aircraft.
After the aircraft leaves the factory and is delivered to its
owner, the requirement for placing the aircraft on scales and
reweighing it varies depending on the type of aircraft and
how it is used. For a small, GA airplane being used privately,
such as a Cessna 172, there is no FAA requirement that it be
periodically reweighed; but after each annual, the mechanic
must ensure that the weight and balance data in the aircraft
records is correct. Additionally, there is an FAA requirement
that the airplane always have a current and accurate weight
and balance report. If the weight and balance report for an
aircraft is lost, the aircraft must be weighed and a new report
must be created. When an aircraft has undergone extensive
repair, major alteration, or has new equipment installed, such
as a radio or a global positioning system, a new weight and
balance report must be created. The equipment installer may
place the airplane on scales and weigh it after the installation,
which is an acceptable way of creating the new report. If
the installer knows the exact weight and location of the new
equipment, it is also possible to create a new report by doing
a series of mathematical calculations.
Over time, almost all aircraft tend to gain weight. Examples
of how this can happen include an airplane being repainted
without the old paint being removed and the accumulation of
6-2dirt, grease, and oil in parts of the aircraft that are not easily
accessible for cleaning. When new equipment is installed,
and its weight and location are mathematically accounted
for, some miscellaneous weight might be overlooked, such
as wire and hardware. For this reason, even if the FAA does
not require it, it is a good practice to periodically place an
aircraft on scales and confirm its actual empty weight and
empty weight center of gravity (EWCG).
Some aircraft are required to be weighed and have their
CG calculated on a periodic basis, typically every 3 years.
Examples of aircraft that fall under this requirement are:
1. Air taxi and charter twin-engine airplanes operating
under Title 14 of the Code of Federal Regulations
(14 CFR) part 135, section 135.185(a).
2. Airplanes with a seating capacity of 20 or more
passengers or a maximum payload of 6,000 pounds
or more, as identified in 14 CFR part 125, section
125.91(b).
Weight & Balance Terminology
Datum
The datum is an imaginary vertical plane from which all
horizontal measurements are taken for balance purposes, with
the aircraft in level flight attitude. If the datum is viewed on a
drawing of an aircraft, it would appear as a vertical line that is
perpendicular (90 degrees) to the aircraft’s longitudinal axis.
For each aircraft make and model, the location of all items
is identified in reference to the datum. For example, the fuel
in a tank might be 60 inches (60") behind the datum, and a
radio on the flight deck might be 90" forward of the datum.
The datum is determined by the manufacturer; it is often the
leading edge of the wing or some specific distance from an
easily identified location. Typical locations for the datum are
the aircraft nose, the leading edge of the wing, the helicopter’s
mast, or a specified distance from a known point. However,
most modern helicopters, like airplanes, have the datum
located at the nose of the aircraft or a specified distance
ahead of it. Figure 6-1 shows an aircraft with the leading
edge of the wing being the datum. The distance from this
datum is measured in inches and can be either positive or
negative depending upon where the equipment is located in
relation to the datum.
The location of the datum is identified in the Aircraft
Specifications or Type Certificate Data Sheet (TCDS).
Aircraft certified prior to 1958 fell under the Civil
Aeronautics Administration and had their weight and balance
information contained in a document known as Aircraft
Specifications. Aircraft certified since 1958 fall under
the FAA and have their weight and balance information contained in a document known as a Type Certificate Data
Sheet (TCDS). The Aircraft Specifications typically included
the aircraft equipment list. For aircraft with a TCDS, the
equipment list is a separate document.
Arm
The arm is the horizontal distance from the datum to any point
within the aircraft. The arm’s distance is always measured in
inches, and it is preceded by the algebraic sign for positive (+)
or negative (−), except for a location which might be exactly
on the datum. The positive sign indicates an item is located
aft of the datum, and the negative sign indicates an item is
located forward of the datum. If the manufacturer chooses
a datum that is at the most forward location on an aircraft,
all the arms will be positive numbers. Location of the datum
at any other point on the aircraft results in some arms being
positive numbers, or aft of the datum, and some arms being
negative numbers, or forward of the datum. Figure 6-1 shows
an aircraft where the datum is the leading edge of the wing.
For this aircraft, any item (fuel, seat, radio, etc.) located
forward of the wing leading edge has a negative arm, and
any item located aft of the wing leading edge has a positive
arm. If an item is located exactly at the wing leading edge, its
arm would be zero, and mathematically it would not matter
whether its arm was positive or negative.
The arm of each item is usually included in parentheses
immediately after the item’s name or weight in the Aircraft
Specifications, TCDS, or equipment list for the aircraft. For
example, in a TCDS, the fuel quantity might be identified
as 98 gallons (gal) (+93.6) and the forward baggage limit
as 100 pounds (lb) (–22.5). These numbers indicate that the
fuel is located 93.6" aft of the datum and the nose baggage is
located 22.6" forward of the datum. If the arm for a piece of
equipment is not known, its exact location must be accurately
measured. When the arm for a piece of equipment is being
determined, the measurement is taken from the datum to the
piece of equipment’s own CG.
Moment
To understand balance, it is necessary to have a working
knowledge of the principle of moments. For those unfamiliar
with weight and balance terms, the word moment is the
product of a force or weight times a distance. The distance
used in calculating a moment is referred to as the arm or
moment arm and is usually expressed in inches. To calculate
a moment, a force (or weight) and a distance must be known.
The weight is multiplied by the distance from the datum and
the result is the moment, which is expressed in inch-pounds
(in-lb), a point through which the force acts. For the purpose
of illustration, compare an aircraft to a seesaw. Like the
seesaw, for an aircraft to be in balance, or equilibrium, the
Negative arm
Positive armDatum (leading edge of wing)
Center of gravity
Figure 6-1. Datum location and its effect on positive and negative
arms.
Arm = 80"Datum 40 " forward
of the firewall Radio (5 lb)
Center of gravity
Moment = Weight (arm)
= 5 lb (80 ")
= 400 in-lb
Figure 6-2. Moment of a radio located aft of the datum.sum of the moments on each side of the balance point must be
equal. Therefore, the same weight that is different distances
(in inches) from the datum have greater moments.
A 5 lb radio located 80" from the datum would have a moment
of 400 in-lb (5 lb × 80"). A 10-pound radio located 12" from
the datum would have a moment of 120 in-lb. Whether the
moment is preceded by a positive (+) or negative (−) sign
depends on its location in relation to the datum. Figure 6-2
shows where the moment ends up being a positive number
because the weight and arm are both positive.
The algebraic sign of the moment, based on the datum
location and whether weight is being installed or removed
[Figure 6-3] , would be as follows:
• Weight being added aft of the datum produces a
positive moment (+weight, +arm).
• Weight being added forward of the datum produces a
negative moment (+weight, −arm).
• Weight being removed aft of the datum produces a
negative moment (−weight, +arm).
• Weight being removed forward of the datum produces
a positive moment (−weight, −arm).
When dealing with positive and negative numbers, remember
that the product of like signs produces a positive answer,
and the product of unlike signs produces a negative answer.
Center of Gravity (CG)
The CG is the point at which all the weight of the aircraft
is concentrated and balanced; therefore, the aircraft can
be supported at that point (the CG). The magnitude of the
nose-heavy and tail-heavy moments are exactly equal. It is
the balance point for the aircraft and, if suspended from this
point, there would be no tendency to rotate in a noseup or
nosedown attitude.
Figure 6-4 shows a lever with the pivot point (called a
fulcrum) located at the CG for the lever. Even though the weights on either side of the fulcrum are not equal, and the
distances from each weight to the fulcrum are not equal, the
product of the weights and arms (moments) are equal, and
that is what produces a balanced condition. Therefore, the
lever would be balanced much like two persons sitting on
a seesaw who are differing weights and located at different
distances from the fulcrum.
Maximum Weight
The maximum weight is the maximum authorized weight
of the aircraft and its contents, and is indicated in the
Aircraft Specifications or TCDS. For many aircraft, there
are variations to the maximum allowable weight depending
on the purpose and conditions under which the aircraft is to
be flown. For example, a certain aircraft may be allowed a
maximum gross weight of 2,750 lb when flown in the normal
category, but when flown in the utility category, which
allows for limited aerobatics, the same aircraft’s maximum
allowable gross weight might only be 2,175 lb. There are
other variations when dealing with the concept of maximum
weight, as follows:
• Maximum Ramp Weight—the heaviest weight to
which an aircraft can be loaded while it is sitting on the
ground. This is sometimes referred to as the maximum
taxi weight.
• Maximum Takeoff Weight—the heaviest weight an
aircraft can be when it starts the takeoff roll. The
difference between this weight and the maximum
ramp weight would equal the weight of the fuel that
would be consumed prior to takeoff.
• Maximum Landing Weight—the heaviest weight an
aircraft can be when it lands. For large, wide body
commercial airplanes, it can be 100,000 lb less than
maximum takeoff weight, or even more.
• Maximum Zero Fuel Weight—the heaviest weight an
aircraft can be loaded to without having any usable
fuel in the fuel tanks. Any weight loaded above this
value must be in the form of fuel.
6-4Weight
+
+
–Arm
+
+
–Moment
+
+Rotation
Noseup
Nosedown
Nosedown
Noseup
Figure 6-3. Relationship between the algebraic signs of weight,
arms, and moments.
Distance = 70 "
Distance = 90 "Force Force
Moment = 700 lb (90 ")
= 63,000 in-lb
Moment = 900 lb (70 ")
= 63,000 in-lb
Fulcrum and CG
Figure 6-4. Center of gravity and a first class lever.Empty Weight
The empty weight of an aircraft includes all operating
equipment that has a fixed location and is actually installed in
the aircraft. It includes the weight of the airframe, powerplant,
required equipment, optional or special equipment, fixed
ballast, hydraulic fluid, and residual fuel and oil. Residual fuel
and oil are the fluids that do not normally drain out because
they are trapped in the fuel lines, oil lines, and tanks. They must
be included in the aircraft’s empty weight. For most aircraft
certified after 1978, the full capacity of the engine oil system
is also included in the empty weight. Information regarding
residual fluids in aircraft systems that must be included in the
empty weight, and whether or not full oil is included, will be
indicated in the Aircraft Specifications or TCDS.
Other terms that are used when describing empty weight
include basic empty weight, licensed empty weight, and
standard empty weight. The term “basic empty weight”
applies when the full capacity of the engine oil system is
included in the value. The term “licensed empty weight”
applies when only the weight of residual oil is included in
the value, so it generally involves only aircraft certified prior
to 1978. Standard empty weight would be a value supplied
by the aircraft manufacturer, and it would not include any
optional equipment that might be installed in an aircraft. For
most people working in the aviation maintenance field, the
basic empty weight of the aircraft is the most important one.
Empty Weight Center of Gravity (EWCG)
The EWCG for an aircraft is the point at which it balances
when it is in an empty weight condition. The concepts of
empty weight and CG were discussed earlier in this chapter,
and now they are being combined into a single concept.
One of the most important reasons for weighing an aircraft
is to determine its EWCG. All other weight and balance
calculations, including loading the aircraft for flight,
performing an equipment change calculation, and performing
an adverse condition check, begin with knowing the empty
weight and EWCG. This crucial information is part of what
is contained in the aircraft weight and balance report.Useful Load
To determine the useful load of an aircraft, subtract the
empty weight from the maximum allowable gross weight.
For aircraft certificated in both normal and utility categories,
there may be two useful loads listed in the aircraft weight and
balance records. An aircraft with an empty weight of 3,100 lb
may have a useful load of 850 lb, if the normal category
maximum weight is listed as 3,950 lb. When the aircraft is
operated in the utility category, the maximum gross weight
may be reduced to 3,700 lb, with a corresponding decrease in
the useful load to 600 lb. Some aircraft have the same useful
load regardless of the category in which they are certificated.
The useful load consists of fuel, any other fluids that are not
part of empty weight, passengers, baggage, pilot, copilot, and
crewmembers. Whether the weight of engine oil is considered
part of the useful load depends on when the aircraft was
certificated and can be determined by looking at the Aircraft
Specifications or TCDS. The payload of an aircraft is like the
useful load, except it does not include fuel.
A reduction in the weight of an item, where possible, may be
necessary to remain within the maximum weight allowed for
the category in which an aircraft is operating. Determining
the distribution of these weights is called a weight check.
Minimum Fuel
Many modern aircraft have multiple rows of seats and often
more than one baggage compartment. The weight and balance
extreme conditions represent the maximum forward and
rearward CG position for the aircraft. An aircraft has certain
fixed points, fore and aft, beyond which the CG should not
be permitted at any time during flight. A check should be
made to ensure that the CG will not shift out of limits when
crew, passengers, cargo, and expendable weights are added
or removed. If the limits are exceeded and the aircraft is
flown in this condition, it may lead to insufficient stability,
with resulting difficulty in controlling the aircraft. After any
repair or alteration that changes the weight and balance, the
Airframe and Powerplant (A&P) mechanic or repairman
6-5must ensure that no legal condition of loading can move the
CG outside of its allowable limits. To determine this, the
mechanic will deliberately attempt to calculate the aircraft
loading in such a manner as to place the CG outside the
limits of the aircraft. This is called an adverse-loading check.
For example, in a forward adverse-loaded CG check, all
useful load in front of the forward CG limit is loaded, and
all useful load behind this limit is left empty. An exception
to leaving it empty is the fuel tank. If the fuel tank is located
behind the forward CG limit, it cannot be left empty because
the aircraft cannot fly without fuel. In this case, an amount
of fuel is accounted for, which is known as minimum fuel.
Minimum fuel is the amount needed for 30 minutes of flight
at cruise power.
For weight and balance purposes, the minimum fuel is no
more than the quantity needed for one half hour of operation
at rated maximum continuous power. This is 1⁄12 gallon
for each maximum except takeoff (METO) horsepower
(hp). Because aviation gasoline (Avgas) weighs 6 pounds
per gallon (lb/gal), determine the number of pounds of
the minimum fuel by dividing the METO hp by 2. For
instance, an aircraft having a METO hp of 200 hp will have
a minimum fuel of 16.65 gallons or 99.99 pounds. An even
simpler way is to take the METO hp divided by 2, which is
100 pounds. Both methods in determining minimum fuel
are valued and result in essentially the same answer. In the
latter computation, a piston engine in cruise flight burns 1
lb of fuel per hour for each hp, or 1⁄2 lb for 30 minutes, hence
dividing the METO by 2.
For example, if a forward adverse-loaded CG check was
performed on a piston engine aircraft, with the engine having
a METO hp of 200, the minimum fuel would be 100 lb (200
METO hp ÷ 2).
For turbine engine-powered aircraft, minimum fuel is not
based on engine hp. If an adverse-loaded CG check is being
performed on a turbine engine-powered aircraft, the aircraft
manufacturer would need to supply information on minimum
fuel.
Tare Weight
When aircraft are placed on scales and weighed, it is
sometimes necessary to use support equipment to aid in
the weighing process. For example, to weigh a tail dragger
airplane, it is necessary to raise the tail to get the airplane
level. To level the airplane, a jack might be placed on the
scale and used to raise the tail. Unfortunately, the scale is now
absorbing the weight of the jack in addition to the weight of
the airplane. This extra weight is known as tare weight and
must be subtracted from the scale reading. Other examples of tare weight are wheel chocks placed on the scales and ground
locks left in place on retractable landing gear.
Procedures for Weighing an Aircraft
General Concepts
The most important reason for weighing an aircraft is to find
out its empty weight (basic empty weight) and to find out
where it balances in the empty weight condition. When an
aircraft is to be flown, the pilot-in-command must know what
the loaded weight of the aircraft is and where its loaded CG
is. For the loaded weight and CG to be calculated, the pilot
or dispatcher handling the flight must first know the empty
weight and EWCG.
Earlier in this chapter it was identified that the CG for an
object is the point about which the nose heavy and tail heavy
moments are equal. One method that could be used to find
this point would involve lifting an object off the ground twice,
first suspending it from a point near the front, and on the
second lift suspending it from a point near the back. With each
lift, a perpendicular line (90 degrees) would be drawn from
the suspension point to the ground. The two perpendicular
lines would intersect somewhere in the object, and the point
of intersection would be the CG. This concept is shown in
Figure 6-5 , where an airplane is suspended from two different
points. The perpendicular line from the first suspension point
is shown in red, and the new suspension point line is shown
as a blue plumb bob. Where the red and blue lines intersect
is the CG. If an airplane were suspended from two points,
one at the nose and one at the tail, the perpendicular drop
lines would intersect at the CG. Suspending an airplane from
the ceiling by two hooks, however, is clearly not realistic.
Even if it could be done, determining where in the airplane
the lines intersect would be difficult.
A more realistic way to find the CG for an object, especially
an airplane, is to place it on a minimum of two scales and
calculate the moment value for each scale reading. In
Figure 6-6, there is a plank that is 200" long, with the left
end being the datum (zero arm), and 6 weights placed at
various locations along the length of the plank. The purpose
of Figure 6-6 is to show how the CG can be calculated when
the arms and weights for an object are known.
To calculate the CG for the object in Figure 6-6, the moments
for all the weights need to be calculated and then summed,
and the weights need to be summed. In the four-column
table in Figure 6-7, the item, weight, and arm are listed in
the first three columns, with the information coming from
Figure 6-6. The moment value in the fourth column is the
product of the weight and arm. The weight and moment
Suspended from this point
first, with red line dropping
perpendicular to the
groundSecond suspension,
with blue line dropping
perpendicular to the
ground
Center of gravity
Figure 6-5. Center of gravity determined by two suspension points.
CG0" 30" 60" 95" 200"
106.9"125" 145" 170"50 lb 50 lb125 lb100 lb90 lb 80 lb
Weight
(lb)
495Arm
(inches)
+30
+60
+95
+125
+145
+170
+106.9Moment
(in-lb)
1,500
7,500
7,600
6,250
13,050
17,000
52,900Item
50 pound weight
125 pound weight
80 pound weight
50 pound weight
90 pound weight
100 pound weight
Total× =
Figure 6-7. Center of gravity calculation for weights on a plank
with datum at one end.Figure 6-6. Center of gravity for weights on a plank with datum
at one end.columns are summed, with the CG being equal to the total
moment divided by the total weight. The arm column is not
summed. The number appearing at the bottom of that column
is the CG. The calculation is shown in Figure 6-7.
For the calculation in Figure 6-7, the total moment is 52,900
in-lb, and the total weight is 495 lb. The CG is calculated
as follows:
CG = Total Moment ÷ Total Weight
= 52,900 in-lb ÷ 495 lb
= 106.9" (106.87 rounded to tenths)
An interesting characteristic exists for the problem in
Figure 6-6 and the table showing the CG calculation. If the
datum (zero arm) for the object was in the middle of the 200"
long plank, with 100" of negative arm to the left and 100" of
positive arm to the right, the solution would show the CG to
be in the same location. The arm for the CG would not be the
same number, but its physical location would be the same.
Figures 6-8 and 6-9 show the new calculation.
CG = Total Moment ÷ Total Weight
= 3,400 in-lb ÷ 495 lb
= 6.9" (6.87 rounded to tenths)
In Figure 6-8, the CG is 6.9" to the right of the plank’s center.
Even though the arm is not the same number, in Figure 6-6 the
CG is also 6.9" to the right of center (CG location of 106.9
with the center being 100). Because both problems are the
same in these two figures, except for the datum location, the
CG must be the same.
The definition for CG states that it is the point about which
all the moments are equal. We can prove that the CG for
the object in Figure 6-8 is correct by showing that the total
moments on either side of this point are equal. Using 6.87 as
the CG location for slightly greater accuracy, instead of the
rounded off 6.9 number, the moments to the left of the CG are
shown in Figure 6-10. The moments to the right of the CG,
shown in Figure 6-8, would be as indicated in Figure 6-11.
Disregarding the slightly different decimal value, the moment
in both previous calculations is 10,651 in-lb. Showing that
the moments are equal is a good way of proving that the CG
has been properly calculated.
Weight and Balance Data
Before an aircraft can be properly weighed and its EWCG
computed, certain information must be known. This
information is furnished by the FAA to anyone for every
certificated aircraft in the TCDS or Aircraft Specifications.
When the design of an aircraft is approved by the FAA, an
Approved Type Certificate and TCDS are issued. The TCDS includes all the pertinent specifications for the aircraft, and
at each annual or 100-hour inspection, it is the responsibility
of the inspecting mechanic or repairman to ensure that the
aircraft adheres to them.
Manufacturer-Furnished Information
When an aircraft is initially certificated, its empty weight
and EWCG are determined and recorded in the weight and
balance record, such as the one in Figure 6-12. An equipment
list is furnished with the aircraft that specifies all the required
equipment and all equipment approved for installation in the
aircraft. The weight and arm of each item is included on the
6-7CG
6.9"–100" –70" –40" –5" 100" 0" 25" 45" 70"100 lb80 lb
50 lb 50 lb90 lb125 lb
Figure 6-8. Center of gravity for weights on a plank with datum
in the middle.Weight
(lb)
495Arm
(inches)
+25
+45
+70
+6.9Moment
(in-lb)
–3,500
–5,000
1,250
4,050
7,000
3,400Item
50 pound weight
125 pound weight
80 pound weight
50 pound weight
90 pound weight
100 pound weight
Total× =
Figure 6-9. Center of gravity calculation for weights on a plank
with datum in the middle.
Weight
(lb)
255Arm
(inches)
135.61Moment
(in-lb)
3,843.50
5,858.75
10,65 1.85Item
50 pound weight
125 pound weight
80 pound weight
Total× =
Figure 6-10. Moments to the left of the center of gravity.
Weight
(lb)
240Arm
(inches)
119.39Moment
(in-lb)
3,431.70
6,313.00
10,651.25Item
50 pound weight
90 pound weight
100 pound weight
Total× =
Figure 6-11. Moments to the right of the center of gravity.list, and all equipment installed when the aircraft left the
factory is checked. When an aircraft mechanic or repairman
adds or removes any item on the equipment list, they must
change the weight and balance record to indicate the new
empty weight and EWCG, and the equipment list is revised
to show what is installed.
Figure 6-13 is an excerpt from a comprehensive equipment
list that includes all the items of equipment approved for this
model of aircraft. The Pilot’s Operating Handbook (POH) or
Airplane Flight Manual (AFM) for each individual aircraft
includes an aircraft specific equipment list of the items
from this master list. When any item is added to or removed
from the aircraft, its weight and arm are determined in the
equipment list and used to update the weight and balance
record. The POH/AFM also contains CG moment envelopes
and loading graphs.
Figures 6-14 through 6-16 shows a TCDS for a Piper twin-
engine airplane known as the Seneca (PA-34-200). The
main headings for the information contained in a TCDS
are included, but much of the information contained under
these headings has been removed if it did not directly pertain
to weight and balance. Information on only one model of
Seneca is shown, because to show all the different models
would make the document excessively long. The portion of
the TCDS that has the most direct application to weight and
balance is highlighted in yellow.
Some of the important weight and balance information found
in a TCDS is as follows:
1. Engine
2. CG range
3. Maximum weight
4. Number of seats
5. Baggage capacity
6. Fuel capacity
7. Oil capacity
8. Datum information
9. Leveling means10. Amount of oil in empty weight
11. Amount of fuel in empty weight
Weight and Balance Equipment
Scales
Weighing GA aircraft, helicopters, turboprops, corporate
jets, UA V/UAS, or transport category airliners can be
accomplished in two ways: top of jack load cells and platform
scales. Equipment selection is dependent on the operator's
needs and or equipment currently on hand, as well as the
airframe manufacturer's recommendations. Top of jack load
cells, as the name implies, can be used on top of the current
wing jacks or can be used under axle for larger jets. Platforms
are very useful for small shops that do not have jacks for
every type of aircraft.
Both types of scales feature new technologies using wireless
6-8Item
Standard empty weight
Optional equipment
Special installation
Paint
Unusable fuel
Basic empty weight Weight (lb)
1,876
1,913.4CG Arm (in)
46.0Moment (in-lb)
67,798.6
1,380
69,452.6Weight and Balance Data
Aircraft Serial #: 18259080
FAA Registration #: N42565
Date: 04-22-2005
× =
Figure 6-12. Typical weight and balance data for 14 CFR part 23 airplane.
operations with computer-based indication and cable-based
wired digital indication. Mechanical or analog meter scales
have mostly been replaced with the new wireless systems and
or digital indicators. These systems and indicators are very
accurate and easy to use, making the weighing job faster to
accomplish and providing higher quality in readings.
Platforms are available in many weight ranges and sizes.
These systems either use ramps or the aircraft can be jacked
and lowered onto the platforms during regular maintenance.
Platforms are easy to use and are a choice for many shops
that do not have jacks for the many types of aircraft to be
serviced. The limiting factors for platforms are the weight
range and the tire size, some aircraft have large tires and the
platform may be too small for the specific aircraft tire. It is
important to always use the right size scale and platform for
the aircraft type and weighing job required.
The platform scale sits on the hangar floor in a level
condition. Ramps and a tug are used to position the airplane
tire on top of the platform and centered. Built into the
platform is an electronic load cell(s) that sense the weight
being applied to it, which generates a corresponding electrical
signal. Inside the load cell is an electronic strain gauge that
measures a proportional change in electrical resistance as
the weight being applied to it increases. An electrical cable
or wireless signal runs from the platform scale to a display
unit, computer, or tablet, which interprets the resistance
change of the load cell and equates it to a specific number of
pounds. A digital readout on the display shows the weight. In
Figure 6-17, a small Piper is being weighed using wireless
platform scales that incorporate electronic load cells.
In Figure 6-18, a Cessna 182 airplane is being weighed with
portable electronic platform scales. If an aircraft is weighed on platform scales, the only way to level the aircraft is to
deflate tires and landing gear struts accordingly. This type of
scale is easy to transport and can be powered by household
current or by a battery contained in the display unit.
The display unit for the standard wired platform scales is very
easy to use. [Figure 6-19] Turn on the power and the unit
runs through the software and displays the scales in a total
mode. Pressing on the ZERO KEY (blue key not the number
key) will ZERO the channels. Once completed, the unit will
read -0- and the scale is ready to use. Select the channels by
number and pressing the PRINT/SELECT KEY . All channels
can be returned to TOTAL MODE by entering the number 4
TOTAL followed by the PRINT/SELECT KEY . If all three
scale switches are turned on at the same time, the total weight
of the airplane is displayed.
The second type of aircraft scale is a top of jack, cell-based
scale, where each jack point receives a cell-based transducer
on the top of the jack. It is very easy to use and level the
aircraft during the weighing operation. The system is easy
to transport, light weight, and simple to set up. The operator
must have a jack capable of receiving and mounting the cell.
Cells come in many weight ranges and are dependent on the
weight required per point to accomplish the weighing and
receiving the actual jack point type.
The top of the load cell has a concave shape that matches up
with the jack pad on the aircraft, with the load cell absorbing
all the weight of the aircraft at each jacking point. Each load
cell either has an electrical cable attached to it or is wireless,
which connects to the display unit or computer read out that
shows the weight transmitted to each load cell. An important
advantage of weighing an aircraft this way is that it allows
the technician to level the aircraft easily. When an aircraft
6-9Comprehensive Equipment List
This is a comprehensive list of all Cessna equipment that is available for the Model 182S airplane. It should not be confused with the airplane-specific equipment list. An
airplane-specific list is provided with each individual airplane at delivery and is typically inserted at the rear of this Pilot’s Operating Handbook. The following comprehensive
equipment list and the airplane-specific list have a similar order of listing.
The comprehensive equipment list provides the following information in column form:
In the Item No column, each item is assigned a coded number. The first two digits of the code represent the assignment of item within the ATA iSpec 2200 breakdown
(Chapter 11 for Placards, Chapter 21 for Air Conditioning, Chapter 77 for Engine Indicating, etc.). These assignments also correspond to the Maintenance Manual chapter
breakdown for the airplane. After the first two digits (and hyphen), items receive a unique sequence number (01, 02, 03, etc...). After the sequence number (and hyphen),
a suffix letter is assigned to identify equipment as a required item, a standard item or an optional item. Suffix letters are as follows:
–R = required items or equipment for FAA certification
–S = standard equipment items
–O = optional equipment items replacing required or standard items
–A = optional equipment items which are in addition to required or standard items
In the Equipment List Description column, each item is assigned a descriptive name to help identify its function.
In the Ref Drawing column, a drawing number is provided which corresponds to the item.
Note
If additional equipment is to be installed, it must be done in accordance with the reference drawing, service bulletin or a separate FAA approval.
In the Wt Lbs and Arm Ins columns, information is provided on the weight (in pounds) and arm (in inches) of the equipment item.
Notes
Unless otherwise indicated, true values (not net change values) for the weight and arm are shown. Positive arms are distances aft of the airplane datum; negative
arms are distances forward of the datum.
Asterisks (*) in the weight and arm column indicate complete assembly installations. Some major components of the assembly are listed on the lines immediately
following. The sum of these major components does not necessarily equal the complete assembly installation.
Figure 6-13. Excerpt from a typical comprehensive equipment list.
Page No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Rev No. 18 14 12 12 12 12 15 15 18 16 15 18 18 16 18 16 18 DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
A7SO
Revision 19
Piper Aircraft, Inc
PA-34-200
PA-34-200T
PA-34-220T
May 2, 2013
TYPE CERTIFICATE DATA SHEET NO. A7SO
This data sheet which is a part of type certificate No. A7 SO, prescribes conditions and limitations under which the product
for which the type certificate was issued meets the airwor thiness requirements of the Federal Aviation Regulations.
Type Certificate Holder Piper Aircraft, Inc.
2926 Piper Drive
Vero Beach, Florida 32960
Type Certificate Holder Record The New Piper Aircraft, Inc transferred TC A7SO to Piper Aircraft, Inc on August
7, 2006
I. - Model PA-34-200 (Seneca), 7 PCLM (Normal Category), Approved 7 May 1971.
Engines S/N 34-E4, 34-7250001 through 34-7250214:
1 Lycoming LIO-360-C1E6 with fuel injector,
Lycoming P/N LW-10409 or LW-12586 (right side); and
1 Lycoming IO-360-C1E6 with fuel injector,
Lycoming P/N LW-10409 or LW 12586 (left side).
S/N 34-7250215 through 34-7450220:
1 Lycoming LIO-360-C1E6 with fuel injector,
Lycoming P/N LW-12586 (right side); and
1 Lycoming IO-360-C1E6 with fuel injector,
Lycoming P/N LW-12586 (left side).
Fuel 100/130 minimum grade aviation gasoline
Engine Limits For all ope rations, 2700 r.p.m. (200 hp)
Propeller and Propeller Limits Left Engine
1 Hartzell, Hub Model HC-C2YK-2 ( ) E, Blade Model C7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) EU, Blade Model C7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) EF, Blade Model FC7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) EFU, Blade Model FC7666A-0;
1 Hartzell, Hub Model HC-C2YK-2CG (F), Blade Model (F) C7666A
(This model includes the Hartzell damper); or
1 Hartzell, Hub Model HC-C2YK-2CGU (F), Blade Model (F) C7666A
(This model includes the Hartzell damper).
Note: HC-( )2YK-( ) may be substituted by HC-( )2YR-( ) per Hartzell
Service Advisory 61.
A7SO 2 of 17
Propeller and Propeller Limits Right Engine
(continued) 1 Hartzell, Hub Mode l HC-C2YK-2 ( ) LE, Blade Model JC7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) LEU, Blade Model JC7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) LEF, Blade Model FJC7666A-0;
1 Hartzell, Hub Model HC-C2 YK-2 ( ) LEFU, Blade Model FJC7666A-0;
1 Hartzell, Hub Model HC-C2 YK-2CLG (F), Blade Model (F) JC7666A
(This model includes the Hartzell damper); or
1 Hartzell, Hub Model HC-C2 YK-2CLGU (F), Blade Model (F) JC7666A
(This model includes the Hartzell damper).
Note: HC-( )2YK-( ) may be substituted by HC-( )2YR-( ) per Hartzell
Service Advisory 61.
Pitch setting: High 79 ° to 81 °, Low 13.5° at 30" station.
Diameter: Not over 76", not under 74".
No further reduction permitted.
Spinner: Piper P/N 96388 Spinner Assembly and P/N 96836 Cap Assembly, or
P/N 78359-0 Spinner Assembly and P/N 96836-2 Cap Assembly (See NOTE 4)
Governor Assembly:
1 Hartzell hydraulic governor, Model F-6-18AL (Right);
1 Hartzell hydraulic governor, Model F-6-18A (Left).
Avoid continuous operation between 2200 and 2400 r.p.m. unl ess aircraft is
equipped with Hartzell propellers whic h incorporates Hartzell damper on both left
and right engine as noted above.
Airspeed Limits VNE (Never exceed) 217 m.p.h. (188 knots)
VNO (Maximum structural cruise) 190 m.p.h (165 knots)
VA (Maneuvering, 4200 lb.) 146 m.p.h. (127 knots)
VA (Maneuvering, 4000 lb.) 146 m.p.h. (127 knots)
VA (Maneuvering, 2743 lb.) 133 m.p.h (115 knots)
VFE (Flaps extended) 125 m.p.h (109 knots)
VLO (Landing gear operating)
Extension 150 m.p.h. (130 knots)
Retract 125 m.p.h. (109 knots)
VLE (Landing gear extended) 150 m.p.h (130 knots)
VMC (Minimum control speed) 80 m.p.h. ( 69 knots)
C.G. Range (Gear Extended) S/N 34-E4, 34-7250001 through 34-7250214 (See NOTE 3):
(+86.4) to (+94.6) at 4000 lb.
(+82.0) to (+94.6) at 3400 lb.
(+80.7) to (+94.6) at 2780 lb.
S/N 34-7250215 through 34-7450220:
(+87.9) to (+94.6) at 4200 lb.
(+82.0) to (+94.6) at 3400 lb.
(+80.7) to (+94.6) at 2780 lb.
Straight line variation between points given.
Moment change due to gear retracting landing gear (-32 in.-lb.)
Empty Weight C.G. Range None
Maximum Weight S/N 34-E4, 34-7250001 through 34-7250214:
4000 lb.- Takeoff
4000 lb. - Landing
See NOTE 3.
Page No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Rev No. 18 14 12 12 12 12 15 15 18 16 15 18 18 16 18 16 18 DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
A7SO
Revision 19
Piper Aircraft, Inc
PA-34-200
PA-34-200T
PA-34-220T
May 2, 2013
TYPE CERTIFICATE DATA SHEET NO. A7SO
This data sheet which is a part of type certificate No. A7 SO, prescribes conditions and limitations under which the product
for which the type certificate was issued meets the airwor thiness requirements of the Federal Aviation Regulations.
Type Certificate Holder Piper Aircraft, Inc.
2926 Piper Drive
Vero Beach, Florida 32960
Type Certificate Holder Record The New Piper Aircraft, Inc transferred TC A7SO to Piper Aircraft, Inc on August
7, 2006
I. - Model PA-34-200 (Seneca), 7 PCLM (Normal Category), Approved 7 May 1971.
Engines S/N 34-E4, 34-7250001 through 34-7250214:
1 Lycoming LIO-360-C1E6 with fuel injector,
Lycoming P/N LW-10409 or LW-12586 (right side); and
1 Lycoming IO-360-C1E6 with fuel injector,
Lycoming P/N LW-10409 or LW 12586 (left side).
S/N 34-7250215 through 34-7450220:
1 Lycoming LIO-360-C1E6 with fuel injector,
Lycoming P/N LW-12586 (right side); and
1 Lycoming IO-360-C1E6 with fuel injector,
Lycoming P/N LW-12586 (left side).
Fuel 100/130 minimum grade aviation gasoline
Engine Limits For all ope rations, 2700 r.p.m. (200 hp)
Propeller and Propeller Limits Left Engine
1 Hartzell, Hub Model HC-C2YK-2 ( ) E, Blade Model C7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) EU, Blade Model C7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) EF, Blade Model FC7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) EFU, Blade Model FC7666A-0;
1 Hartzell, Hub Model HC-C2YK-2CG (F), Blade Model (F) C7666A
(This model includes the Hartzell damper); or
1 Hartzell, Hub Model HC-C2YK-2CGU (F), Blade Model (F) C7666A
(This model includes the Hartzell damper).
Note: HC-( )2YK-( ) may be substituted by HC-( )2YR-( ) per Hartzell
Service Advisory 61. Figure 6-14. The Type Certificate Data Sheet (TCDS) shows various information about an aircraft to include weight and balance
information.
5 of 17 A7SO
Maximum Weight 4570 lb. - Takeoff
4342 lb. - Landing (All weight in excess of 4000 lb. must be fuel)
Zero fuel weight may be increased up to a maximum of 4077.7 lb. when approved
wing options are installed.
See NOTE 11 for optional weights.
No. of Seats 7 (2 at +85.5, 3 at +118.1, 2 at +155.7)
7 (2 at +85.5, 3 at +118.1, 2 at +157.6)
6 (2 at +85.5, *2 at +119.1, 2 at +157.6)
* - Optional Club Seats
Maximum Baggage 200 lb. (100 lb. at +22.5, 100 lb. at +178)
Fuel Capacity 98 gallons (2 wing tanks) at (+93.6) (93 gallons usable)
* 128 gallons (2 wing tanks) at (+93.6) (123 gallons usable)
* - Optional for S/N 34-7570001, 34-7670114 through 34-8170092.
See NOTE 1 for data on system fuel.
Oil Capacity 8 qts. per engi ne (5 qts. per engine usable)
See NOTE 1 for data on system oil.
Maximum Operating Altitude 25,000 feet
Control Surface Movements Ailerons (±2°) Up 35° Down 20°
Stabilator Up 12.5° ( +0°,−1°) Down 7.5° (±1°)
Rudder (±1°) Left 35° Right 35°
Stabilator Trim
Tab(±1°) Down 10.5° Up 6.5°
(Stabilator neutral)
Wing Flaps (±2°) Up 0° Down 40°
Rudder Trim
Tab (±1°) Left 25° Right 25°
(Rudder neutral)
Nose Wheel
Travel (±1°) Left 27° Right 27°
Manufacturer's Serial Number 34-7570001 through 34-8170092 (See NOTE 7).
IIIA. - Model PA-34-220T (Seneca III), 7 PCLM (Normal Category), Approved December 17, 1980.
Same as model PA-34-200T series except engines, windshield, instru ment panel, landing gear, maximum gross weight and
other minor changes.
Engines 1 Teledyne Continental TSIO-360-KB (left engine),
1 Teledyne Continental LTSIO-360-KB (right engine).
Fuel 100/100LL minimum grade aviation gasoline
Engine Limits Takeoff, 5 minutes, 2800 r.p.m. and 40" Hg. ma nifold pressure (220 hp)
Max. Continuous, 2600 r.p.m. and 40" Hg. manifold pressure (200 hp)
Propeller and Propeller Limits Left Engine
1 Hartzell, Hub Model BHC-C2 YF-2 ( ) UF, Blade Model FC8459-8R.
Right Engine
1 Hartzell, Hub Model BHC-C2YF-2 ( )L ( )UF, Blade Model FJC8459-8R.
Page No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Rev No. 18 14 12 12 12 12 15 15 18 16 15 18 18 16 18 16 18 DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
A7SO
Revision 19
Piper Aircraft, Inc
PA-34-200
PA-34-200T
PA-34-220T
May 2, 2013
TYPE CERTIFICATE DATA SHEET NO. A7SO
This data sheet which is a part of type certificate No. A7 SO, prescribes conditions and limitations under which the product
for which the type certificate was issued meets the airwor thiness requirements of the Federal Aviation Regulations.
Type Certificate Holder Piper Aircraft, Inc.
2926 Piper Drive
Vero Beach, Florida 32960
Type Certificate Holder Record The New Piper Aircraft, Inc transferred TC A7SO to Piper Aircraft, Inc on August
7, 2006
I. - Model PA-34-200 (Seneca), 7 PCLM (Normal Category), Approved 7 May 1971.
Engines S/N 34-E4, 34-7250001 through 34-7250214:
1 Lycoming LIO-360-C1E6 with fuel injector,
Lycoming P/N LW-10409 or LW-12586 (right side); and
1 Lycoming IO-360-C1E6 with fuel injector,
Lycoming P/N LW-10409 or LW 12586 (left side).
S/N 34-7250215 through 34-7450220:
1 Lycoming LIO-360-C1E6 with fuel injector,
Lycoming P/N LW-12586 (right side); and
1 Lycoming IO-360-C1E6 with fuel injector,
Lycoming P/N LW-12586 (left side).
Fuel 100/130 minimum grade aviation gasoline
Engine Limits For all ope rations, 2700 r.p.m. (200 hp)
Propeller and Propeller Limits Left Engine
1 Hartzell, Hub Model HC-C2YK-2 ( ) E, Blade Model C7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) EU, Blade Model C7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) EF, Blade Model FC7666A-0;
1 Hartzell, Hub Model HC-C2YK-2 ( ) EFU, Blade Model FC7666A-0;
1 Hartzell, Hub Model HC-C2YK-2CG (F), Blade Model (F) C7666A
(This model includes the Hartzell damper); or
1 Hartzell, Hub Model HC-C2YK-2CGU (F), Blade Model (F) C7666A
(This model includes the Hartzell damper).
Note: HC-( )2YK-( ) may be substituted by HC-( )2YR-( ) per Hartzell
Service Advisory 61.
A7SO 2 of 17
Propeller and Propeller Limits Right Engine
(continued) 1 Hartzell, Hub Mode l HC-C2YK-2 ( ) LE, Blade Model JC7666A-0;
1 Hartzell, Hub Model HC-C2 YK-2 ( ) LEU, Blade Model JC7666A-0;
1 Hartzell, Hub Model HC-C2 YK-2 ( ) LEF, Blade Model FJC7666A-0;
1 Hartzell, Hub Model HC-C2 YK-2 ( ) LEFU, Blade Model FJC7666A-0;
1 Hartzell, Hub Model HC -C2YK-2CLG (F), Blade Model (F) JC7666A
(This model includes the Hartzell damper); or
1 Hartzell, Hub Model HC-C2 YK-2CLGU (F), Blade Model (F) JC7666A
(This model includes the Hartzell dam per).
Note: HC-( )2YK-( ) may be substituted by HC-( )2YR-( ) per Hartz ell
Service Advisory 61.
Pitch setting: High 79 ° to 81 °, Low 13.5 ° at 30" station.
Diameter: Not over 76", not under 74".
No fur ther reduction permitted.
Spinner: Piper P/N 96388 Spin ner Assembly and P/N 96836 Cap Assembly, or
P/N 78359-0 Spinner A ssembly and P/N 96836-2 Cap Assembly (See NOTE 4)
Governor Assembly:
1 Hartzell hydraulic governor, Model F-6-18AL (Right);
1 Hartzell hydraulic governor, Model F-6-18A (Left).
Avoid continuous operation bet ween 2200 and 2400 r.p.m. unl ess aircraft is
equipped with Hartzell propellers whic h incorporates Hartzell damper on both left
and right engine as noted ab ove.
Airspeed Limits VNE (Never exceed) 217 m.p.h. (188 knots)
VNO (Maximum structural cruise) 190 m.p.h (165 knots)
VA (Maneuvering, 4200 lb.) 146 m.p.h. (127 knots )
VA (Maneuvering, 4000 lb.) 146 m.p.h. (127 knots)
VA (Maneuvering, 2743 lb.) 133 m.p.h (115 knots)
VFE (Flaps extended) 125 m.p.h (109 knots)
VLO (Landing gear operating)
Extension 150 m.p.h. (130 knots)
Retract 125 m.p.h. (109 knots)
VLE (Landing gear extended) 150 m.p.h (130 knots)
VMC (Minimum control speed) 80 m.p.h. ( 69 knots)
C.G. Range (Gear Extended) S/N 34-E4, 34-7250001 throug h 34-7250214 (See NOTE 3):
(+86.4) to (+94.6) at 4000 lb.
(+82.0) to (+94.6) at 3400 lb.
(+80.7) to (+94.6) at 2780 lb.
S/N 34-7250215 through 34- 7450220:
(+87.9) to (+94.6) at 4200 l b.
(+82.0) to (+94.6) at 3400 lb.
(+80.7) to (+94.6) at 2780 lb.
Straight line variation between po ints given.
Moment change due to gear retra cting landing gear (-32 in.-lb.)
Empty Weight C.G. Range None
Maximum Weight S/N 34-E4, 34-7250001 through 34-7250214:
4000 lb.- Takeoff
4000 lb. - Landing
See NOTE 3. A
3 of 17 A7SO
Maximum Weight S/N 34-7250215 through 34-7450220:
4200 lb. - Takeoff
4000 lb. - Landing
No. of Seats 7 (2 at +85.5, 3 at +118.1 , 2 at +155.7)
Maximum Baggage 200 lb. (100 lb . at +22.5, 100 lb. at +178.7)
Fuel Capacity 98 gallons (2 wing t anks) at (+93.6) (93 gallons usable)
See NOTE 1 for data on syst em fuel.
Oil Capacity 8 qts. per engi ne (6 qts. per engine usable)
See NOTE 1 for data on sys tem oil.
Control Surface Movements Ailerons (±2°) Up 30° Down 15°
Stabilator Up 12.5° ( +0,−1°) Down 7.5° (±1°)
Rudder (±1°) Left 35° Right 35°
Stabilator Trim
Tab(±1°) Down 10.5° Up 6.5°
(Stabilator neutral)
Wing Flaps (±2°) Up 0° Down 40°
Rudder Trim
Tab (±1°) Left 17° Right 22°
(Rudder neutral)
Nose Wheel S/N 34-E4, 34-7250001 through 34-7350353:
Travel (±1°) Left 21° Right 21°
Nose Wheel S/N 34-7450001 through 34-7450220:
Travel (±1°) Left 27° Right 27°
Manufacturer's Serial Number 34-E4, 34-7250001 through 3 4-7450220 (See NOTE 7).
II. - Model PA-34-200T (Seneca II), 7 PCLM (Normal Category), Approved July 18, 1974.
Same as Model PA-34-200 series except engine installa tion, maximum gross weight, and other minor changes.
Engines 1 Teledyne Continent al TS IO-360-E or TSIO-360-EB (left engine),
1 Teledyne Continental LT SIO-360-E or LTSIO-360-EB (right engine).
Fuel 100/130 minimum grade aviation gasoline
Engine Limits For all ope rations, 2575 r.p.m. and 40" Hg.
Manifold pressure, 200 hp @ S.L. and 215 hp @ 12,000 ft.
Propeller and Propeller Limits Left engine
1 Hartzell, Hub Model BHC-C2YF-2 ( )F (See NOTE 10)
or BHC-C2YF-2 ( )UF; Blade Model FC8459-8R or FC8459B-8R.
Right engine
1 Hartzell, Hub Model BHC- C2YF-2 ( )L ( )F (See NOTE 10)
or BHC-C2YF-2 ( )L ( )U F; Blade Model FJC8459-8R or FJC8459B-8R.
Pitch setting at 30" station:
Hub Serial Numbers prior t o AN3943:
High 79.3 °± 2.0°, Low 14.4 °± 0.2° or High 80.0 ° to 81.5 °, Low 14.4 °± 0.2°.
Hub Serial Numbers AN3943 an d subsequent:
High 80.0 ° to 81.5 °, Low 14.4 °± 0.2°.
Figure 6-15. Highlights of various specifications of the aircraft found within a TCDS. Note the fuel capacity of 98 gallons and its reference
to the datum (A).
15 of 17 A7SO
MODEL AFM/POH REPORT NO. APPROVED SERIAL EFFECTIVITY
PA-34-220T
(Seneca III) POH VB-1110 1/8/81 34-8133001 through
34-8633031, and
3433001 through 3433172
POH VB-1150 2/20/81 34-8133001 through
34-8633031, and
3433001 through 3433172
when Piper Kit
764-099V is installed
POH VB-1257 10/20/89 3448001 through 3448037
POH VB-1259 11/20/89 3448001 through 3448037
when Piper Kit
766-203 is installed
PA-34-220T POH VB-1556 11/5/93 3448038 through 3448079
(Seneca IV) POH VB-1558 12/6/93 3448038 through 3448079
when Piper Kit
766-283 is installed
POH VB-1615 7/12/95 3447001 through 3447029
POH VB-1620 7/12/95 3447001 through 3447029
when Piper Kit 766-608 is
installed
PA-34-220T POH VB-1638 12/6/96 3449001 and up
(Seneca V) POH VB-1649 1/23/97 3449001 and up when Piper
Kit 766-632 (or equivalent
88247-{ }) is installed
POH VB-1930 10/25/05 3449311 and 3449323 and up
when Avidyne Entegra System
is installed.
POH VB-1955 3/20/06 3449311 and 3449323 and up
when Piper kit 766-632
(or equivalent 88247-{ }) and
Avidyne Entegra System is
Installed.
POH VB-2186 9/2/2010 3449410 and up when Garmin
G600 System is installed
POH VB-2193 9/10/2010 3449410 and up when Piper
Kit 766-632 (or equivalent
88247-{ }) and Garmin G600
System is installed.
POH VB-2230 4/30/2013 3449459, 3449467 and up
when the Garmin G1000
system is installed
NOTE 1 Current Weight and Balanc e Report, incl uding list of equipment included in certificated
empty weight, and loading i nstructions when necessary, must be provided for each aircraf t at
the time of original certificatio n.
The certificated emp ty weight and corres ponding center of gravity locations must include
undrainable system oi l (not included in o il capacity) and unusable fuel as noted below:
Fuel: 30.0 lb. at (+103.0) for PA-34 series, except Model PA-34-220T
(Sen eca V), S/N 3449001 and up
Fuel: 36.0 lb. at (+103.0) for Model PA-34-220T (Seneca V), S/N 34490 01
and up
Oil: 6.2 lb. a t (+ 39.6) for Model PA-34-200
Oil: 12.0 lb. at (+ 43.7) for Models PA-34-200T and PA-34-220T
NOTE 2 All placards required in the approved Airp lane Flight Manual or Pilot's Operating Handbook
and approved Airplane Fligh t Manual of Pilot's Operating Handbook supplements must be
11 of 17 A7SO
Control Surface Movements Ailerons (±2°) Up 35° Down 20°
Stabilator Up 12.5° ( +0°,−1°) Down 7.5° (±1°)
Rudder (±1°) Left 35° Right 35°
Stabilator Trim
Tab(±1°) Down 10.5° Up 6.5°
(Stabilator neutral)
Wing Flaps Up 0° (±1°) Down 40° (±2°)
Rudder Trim
Tab (±1°) Left 26° Right 26°
(Rudder neutral)
Nose Wheel
Travel
(Maximum) Left 27° Right 27°
Manufacturer's Serial Number 3449001 and up.
DATA PERTINENT TO ALL MODELS
Datum 78.4" forward of wing leading edge from the inboard edge of the inboard fuel
tank.
Leveling Means Two screws le ft side fuselage below window.
Certification Basis Type Certificate No. A7SO issued May 7, 1971, obtained by the manufacturer
under the delegation option authorization.
Date of Type Certificate application July 23, 1968.
Model PA-34-200 (Seneca I):
FAR 23 as amended by Amendmen t 23-6 effective August 1, 1967; FAR
23.959 as amended by Amendment 23- 7 effective September 14, 1969; and
FAR 23.1557(c)(1) as amended by Amendment 23-18 effective May 2, 1977.
Compliance with FAR 23.1419 as am ended by Amendment 23-14 effective
December 20, 1973, has been esta blished with optional ice protection
provisions.
Model PA-34-200T (Seneca II):
FAR 23 as amended by Amendmen t 23-6 effective August 1, 1967; FAR
23.901, 23.909, 23.959, 23.1041, 23.1043, 23.1047, 23.1143, 23.1305(b)(c)(h)(p)
and 23.1527(b) as amended by Amendm ent 23-7 effective September 14, 1969;
and FAR 23.1557(c)(1) as amended by Amendment 23-18 effective May 2, 1977.
Model PA-34-220T (Seneca III and IV):
FAR 23 as amended by Amendmen t 23-6 effective August 1, 1967; FAR
23.207, 23.901, 23.909, 23.959, 23.1041, 23.1043, 23.1047, 23.1143,
23.1305(b)(c)(h)(p) and 23.1527 as am ended by Amendment 23-7 effective
September 14, 1969; FAR 23.201 and 23.203 as amended by Amendment
23-14 effective December 20, 1973; FAR 23.1557(c)(1) as amended by
Amendment 23-18 effective May 2, 1977; FAR 23.175(a) and 23.1581(b)(2) as
amended by Amendment 23-21 effec tive March 1, 1978; FAR 23.1545(a) as
amended by Amendment 23-23 effective December 1, 1978; and FAR 36
through Amendment 36-9 effective January 15, 1979.
Figure 6-17. Weighing a Piper Archer using electronic platform
scales.Figure 6-16. Highlights of various specifications of the aircraft found within a TCDS.
Plumb Bob
A plumb bob is a heavy metal object, cylinder or cone shape,
with a sharp point at one end and a string attached to the
other end. If the string is attached to a given point on an
aircraft, and the plumb bob can hang down so the tip just
touches the ground, the point where the tip touches will be
perpendicular to where the string is attached. An example
of the use of a plumb bob would be measuring the distance
from an aircraft’s datum to the center of the main landing
gear axle. If the leading edge of the wing was the datum, a
plumb bob could be dropped from the leading edge and a is weighed using load cells on jacks, leveling the aircraft is
done by adjusting the height with the jacks and checking the
level at the level point. Figure 6-20 shows a Gulfstream jet
on jacks with the load cells in place.
Always follow the aircraft manufacturer’s weighing and
leveling procedures and processes. All aircraft need to be
in a flight level attitude when they are weighed unless the
manufacturer’s manual specifically allows it or has a formula
in the manual to use accordingly.
Spirit Level
Before an aircraft can be weighed and reliable readings
obtained, it must be in a level flight attitude. One method that
can be used to check for a level condition is to use a spirit
level, sometimes thought of as a carpenter’s level, by placing
it on or against a specified place on the aircraft. Spirit levels
consist of a vial full of liquid, except for a small air bubble.
When the air bubble is centered between the two black lines,
a level condition is indicated.
In Figure 6-21, a spirit level is being used on a Mooney M20
to check for a flight level attitude. By looking in the TCDS,
it is determined that the leveling means is two screws on
the left side of the airplane fuselage, in line with the trailing
edge of the wing.
Figure 6-18. A Cessna 182 being weighed with portable electronic
platform scales.
Figure 6-19. M2000 platform scale digital indicator.
Figure 6-20. Airplane on jacks with load cells in use.chalk mark made on the hangar floor. The plumb bob could
also be dropped from the center of the axle on the main
landing gear, and a chalk mark made on the floor. With a tape
measure, the distance between the two chalk marks could be
determined, and the arm for the main landing gear would be
known. Plumb bobs can also be used to level an aircraft, as
described in the Helicopter Weight and Balance section of
this chapter. Figure 6-22 shows a plumb bob being dropped
from the leading edge of an aircraft wing.
Hydrometer
When an aircraft is weighed with full fuel in the tanks, the
weight of the fuel must be accounted for by mathematically
subtracting it from the scale readings. To subtract it, its
weight, arm, and moment must be known. Although the
standard weight for aviation gasoline (Avgas) is 6.0 lb/gal
and jet fuel is 6.7 lb/gal, these values are not exact for all
conditions. On a hot day versus a cold day, these values can
vary dramatically. On a hot summer day in the state of Florida,
Avgas checked with a hydrometer typically weighs between
5.85 and 5.9 lb/gal. If 100 gallons of fuel were involved in a calculation, using the actual weight versus the standard
weight would make a difference of 10 to 15 lb.
When an aircraft is weighed with fuel in the tanks, the weight
of fuel per gallon should be checked with a hydrometer. A
hydrometer consists of a weighted glass tube that is sealed
with a graduated set of markings on the side of the tube.
The graduated markings and their corresponding number
values represent units of pounds per gallon (lb/gal). When
placed in a flask with fuel in it, the glass tube floats at a
level dependent on the density of the fuel. Where the fuel
intersects the markings on the side of the tube indicates the
pounds per gallon.
Preparing an Aircraft for Weighing
Weighing an aircraft is a very important and exacting phase
of aircraft maintenance and must be carried out with accuracy
and good workmanship. Thoughtful preparation saves time
and prevents mistakes. The aircraft should be weighed inside
a hangar where wind cannot blow over the surface and cause
fluctuating or false scale readings. The aircraft should be
clean inside and out, with special attention paid to the bilge
area to be sure no water or debris is trapped. The outside of
the aircraft should be as free as possible of all mud and dirt.
To begin, assemble all the necessary equipment, such as:
1. Scales, hoisting equipment, jacks, and leveling
equipment.
2. Blocks, chocks, or sandbags for holding the airplane
Figure 6-21. Spirit level being used on a Mooney M20.
Figure 6-22. Plumb bob dropped from a wing leading edge.on the scales.
3. Straightedge, spirit level, plumb bobs, chalk line, and
a measuring tape.
4. Applicable Aircraft Specifications and weight and
balance computation forms.
Fuel System
When weighing an aircraft to determine its empty weight,
only the weight of residual (unusable) fuel should be
included. To ensure that only residual fuel is accounted for,
the aircraft should be weighed in one of the following three
conditions.
1. Weigh the aircraft with absolutely no fuel in the
aircraft tanks or fuel lines. If an aircraft is weighed in
this condition, the technician can mathematically add
the proper amount of residual fuel to the aircraft and
account for its arm and moment. The proper amount
of fuel can be determined by looking in the aircraft’s
TCDS.
2. Drain fuel from the tanks in the manner specified
by the aircraft manufacturer. If there are no specific
instructions, drain the fuel until the fuel quantity
gauges read empty and until fuel stops draining from
the tanks. The aircraft attitude may be a consideration
when draining the fuel tanks and the maintenance
manual should be consulted. In this case, the unusable
fuel will remain in the lines and system, and its
weight and arm can be determined by reference to the
aircraft’s TCDS.
3. Weigh the aircraft with the fuel tanks completely
full. If an aircraft is weighed in this condition, the
technician can mathematically subtract the weight of
usable fuel and account for its arm and moment. If
the weight of the fuel is in question, a hydrometer can also be used to determine the weight of each gallon of
fuel, while the Aircraft Specifications or TCDS can
be used to identify the fuel capacity of the aircraft. If
an aircraft is to be weighed with load cells attached to
jacks, the technician should check both the load cell
instruction manual and aircraft maintenance manual
to make sure it is permissible to jack the aircraft with
the fuel tanks full as this may add additional stress to
the aircraft structure.
Never weigh an aircraft with fuel tanks partially full, because
it will be impossible to determine exactly how much fuel to
account for.
Oil System
The empty weight for older aircraft certificated under the
Civil Air Regulations (CAR) part 3 does not include the
engine lubricating oil. The oil must be drained before the
aircraft is weighed, or its weight must be subtracted from the
scale readings to determine the empty weight.
To weigh an aircraft that does not include the engine
lubricating oil as part of the empty weight, place it in level
flight attitude, then open the drain valves and allow all the
oil that is able, to drain out. Any remaining is undrainable
oil and is part of the empty weight.
If it is impractical to drain the oil, the reservoir can be filled
to the specified level and the weight of the oil computed at 7.5
lb/gal. Then its weight and moment are subtracted from the
weight and moment of the aircraft as weighed. The amount
and arm of the undrainable oil are found in NOTE 1 of the
TCDS, and this must be added to the empty weight.
For aircraft certificated since 1978 under 14 CFR parts 23
and 25, full engine oil is typically included in an aircraft’s
6-14empty weight. This can be confirmed by looking at the TCDS.
If full oil is to be included, the oil level needs to be checked
and the oil system serviced if it is less than full.
Miscellaneous Fluids
The hydraulic fluid reservoir and all other reservoirs
containing fluids required for normal operation of the aircraft
should be full. Fluids not considered to be part of the empty
weight of the aircraft are potable (drinkable) water, lavatory
precharge water, and water for injection into the engines.
Flight Controls
The position of such items as spoilers, slats, flaps, and
helicopter rotor systems is an important factor when weighing
an aircraft. Always refer to the manufacturer’s instructions
for the proper position of these items.
Other Considerations
Inspect the aircraft to see that all items included in the
certificated empty weight are installed in the proper location.
Remove items that are not regularly carried in flight. Also,
look in the baggage compartments to make sure they are
empty. Replace all inspection plates, oil and fuel tank caps,
junction box covers, cowling, doors, emergency exits, and
other parts that have been removed during maintenance.
All doors, windows, and sliding canopies should be in the
normal flight position. Remove excessive dirt, oil, grease,
and moisture from the aircraft.
Some aircraft are not weighed with the wheels on the scales,
but are weighed with the scales placed either at the jacking
points or at special weighing points. Regardless of what
provisions are made for placing the aircraft on the scales
or jacks, be careful to prevent it from falling or rolling off,
thereby damaging the aircraft and equipment. When weighing
an aircraft with the wheels placed on the scales, release the
brakes to reduce the possibility of incorrect readings caused
by side loads on the scales.
All aircraft have leveling points or lugs, and care must be
taken to level the aircraft, especially along the longitudinal
axis. With light, fixed-wing airplanes, the lateral level is
not as critical as it is with heavier airplanes. However, a
reasonable effort should be made to level the light airplanes
along the lateral axis. Helicopters must be level longitudinally
and laterally when they are weighed. Accuracy in leveling all
aircraft longitudinally cannot be overemphasized.
Weighing Points
When an aircraft is being weighed, the arms must be known
for the points where the weight of the aircraft is being
transferred to the scales. If a tricycle gear small airplane has its three wheels sitting on floor scales, the weight transfer
to each scale happens through the center of the axle for each
wheel. If an airplane is weighed while it is on jacks, the
weight transfer happens through the center of the jack pad.
For a helicopter with skids for landing gear, determining the
arm for the weighing points can be difficult if the skids are
sitting directly on floor scales. The problem is that the skid
is in contact with the entire top portion of the scale, and it
is impossible to know exactly where the center of weight
transfer is occurring. In such a case, place a piece of pipe
between the skid and the scale, and the center of the pipe will
now be the known point of weight transfer.
The arm for each of the weighing points is the distance from
the center of the weight transfer point to the aircraft’s datum.
If the arms are not known, based on previous weighing of the
aircraft or some other source of data, they must be measured
when the aircraft is weighed. This involves dropping a
plumb bob from the center of each weighing point and from
the aircraft datum, and putting a chalk mark on the hangar
floor representing each point. The perpendicular distance
between the datum and each of the weighing points can then
be measured. In Figure 6-23, the distance from the nosewheel
centerline to the datum is being measured on an airplane.
The nosewheel sitting on an electronic scale can be seen in
the background.
Jacking the Aircraft
Aircraft are often weighed by rolling them onto ramps in
which load cells are embedded. This eliminates the problems
associated with jacking the aircraft off the ground. However,
many aircraft are weighed by jacking the aircraft up and then
lowering them onto scales or load cells. Extra care must be
used when raising an aircraft on jacks for weighing. If the
aircraft has spring steel landing gear and it is jacked at the
wheel, the landing gear will slide inward as the weight is
taken off the tire. Care must be taken to prevent the jack
from tipping over. For some aircraft, stress panels or plates
must be installed before they are raised with wing jacks to
distribute the weight over the jack pad. Be sure to follow
the recommendations of the aircraft manufacturer in detail
anytime an aircraft is jacked. When using two wing jacks,
take special care to raise them simultaneously, so the aircraft
does not slip off the jacks. As the jacks are raised, keep the
safety collars screwed down against the jack cylinder to
prevent the aircraft from tilting if one of the jacks should
lose hydraulic pressure.
Leveling the Aircraft
When an aircraft is weighed, it must be in its level flight
attitude so that all the components are at the correct distance
from the datum. This attitude is determined by information in
the TCDS. Some aircraft require a plumb line to be dropped
Figure 6-23. Measuring the nosewheel arm on an airplane.from a specified location so that the point of the weight, the
bob, hangs directly above an identifiable point. Others specify
that a spirit level be placed across two leveling lugs (special
screws on the outside of the fuselage). Other aircraft call for a
spirit level to be placed on the upper door sill. Lateral level is
not specified for all light aircraft, but provisions are normally
made on helicopters for determining both longitudinal
and lateral level. This may be done by built-in leveling
indicators or by a plumb bob that shows the conditions of
both longitudinal and lateral level. The actual adjustments
to level the aircraft using load cells are made with the jacks.
When weighing from the wheels, leveling is normally done
by adjusting the air pressure in the nosewheel shock strut.
Safety Considerations
Special precautions must be taken when raising an aircraft
on jacks.
1. Stress plates must be installed under the jack pads if
the manufacturer specifies them.
2. If anyone is required to be in the aircraft while it is
being jacked, there must be no movement.
3. The jacks must be straight under the jack pads before
beginning to raise the aircraft.
4. All jacks must be raised simultaneously and safety
devices placed against the jack cylinder to prevent the
aircraft from tipping if any jack should lose pressure.
Not all jacks have screw-down collars, some use drop
pins or friction locks.
CG Range
The CG range for an aircraft is the limits within which the
aircraft must balance. It is identified as a range and considered
an arm extending from the forward most limit to the aft most
limit usually expressed in inches. In the TCDS for the Piper
Seneca airplane, shown earlier in this chapter, the range is
given in Figure 6-24.
Because the Piper Seneca is a retractable gear airplane, the
specifications identify that the range applies when the landing
gear is extended, and that the airplane’s total moment is
decreased by 32 when the gear retracts. To know how much
the CG changes when the gear is retracted, the moment of
32 in-lb would need to be divided by the loaded weight of
the airplane. For example, if the airplane weighed 3,500 lb,
the CG would move forward 0.009" (32 ÷ 3,500).
Based on the numbers given, up to a loaded weight of
2,780 lb, the forward CG limit is +80.7" and the aft CG limit
is +94.6". As the loaded weight of the airplane increases to
3,400 lb, and eventually to the maximum of 4,000 lb, the
forward CG limit moves aft. In other words, as the loaded
weight of the airplane increases, the CG range gets smaller. The range gets smaller because of the forward limit moving
back, while the aft limit stays in the same place.
The data sheet identifies that there is a straight-line variation
between the points given. The points being referred to are
the forward and aft CG limits. From a weight of 2,780 lb to
a weight of 3,400 lb, the forward limit moves from +80.7"
to +82.0", and if plotted on a graph, that change would
form a straight line. From 3,400 lb to 4,000 lb, the forward
limit moves from +82 to +86.4", again forming a straight
line. Plotted on a graph, the CG limits would look like
Figure 6-25. When graphically plotted, the CG limits form
what is known as the CG envelope.
In Figure 6-25, the red line represents the forward limit up
to a weight of 2,780 lb. The blue and green lines represent
the straight-line variation that occurs for the forward limit
as the weight increases up to a maximum of 4,000 lb. The
yellow line represents the maximum weight for the airplane,
and the purple line represents the aft limit.
Empty Weight Center of Gravity (EWCG) Range
For some aircraft, a CG range is given for the aircraft in the
empty weight condition in the TCDS. This practice is not very
common with airplanes, but is often done for helicopters. This
range would only be listed for an airplane if the fuel tanks,
seats, and baggage compartments are so located that changes
in the fuel or occupant load have a very limited effect on the
balance of the aircraft. If the EWCG of an aircraft falls within
the EWCG limits, it is impossible to legally load the aircraft
so that its loaded CG falls outside of its allowable range. If the
TCDS lists an EWCG range and, after a repair or alteration
is completed, the EWCG falls within this range, then there is
no need to compute a fore and aft check for adverse loading.
But if the TCDS lists the EWCG range as “None” (and most
of them do), a check must be made to determine whether it
is possible by any combination of legal loading to cause the
6-16Figure 6-24. Piper Seneca airplane center of gravity range.CG Range: (Gear Extended)
S/N 34-E4, 34-7250001 through 34-7250214
(See NOTE 3)
(+86.4") to (+94.6") at 4,000 lb
(+82.0") to (+94.6") at 3,400 lb
(+80.7") to (+94.6") at 2,780 lb
Straight line variation between points given.
Moment change due to gear retracting
landing gear (–32 in-lb)aircraft CG to move outside of either its forward or aft limits.
Operating CG Range
All aircraft have CG limits identified for the operational
condition, with the aircraft loaded and ready for flight. If an
aircraft can operate in more than one category, such as normal
and utility, more than one set of limits might be listed. As
shown earlier for the Piper Seneca airplane, the limits can
change as the weight of the aircraft increases. To legally fly,
the CG for the aircraft must fall within the CG limits.
Standard Weights Used for Aircraft Weight and
Balance
Unless the specific weight for an item is known, the standard
weights used in aircraft weight and balance are as follows:
• Avgas 6 lb/gal
• Turbine fuel 6.7 lb/gal
• Lubricating oil 7.5 lb/gal
• Water 8.35 lb/gal
• Crew and passengers 170 lb per person
Example Weighing of an Airplane
In Figure 6-26, a tricycle gear airplane is being weighed by
using three floor scales. The specifications on the airplane
and the weighing specific data are shown in Figure 6-27.
By analyzing the data identified for the airplane being weighed
in Figure 6-26, the following information is determined.
• Because the airplane was weighed with the fuel tanks
full, the full weight of the fuel must be subtracted and
the unusable fuel added back in. The weight of the fuel
being subtracted is based on the pounds per gallon
determined by the hydrometer check (5.9 lb/gal).
• Because wheel chocks are used to keep the airplane
from rolling off the scales, their weight must be
subtracted from the scale readings as tare weight.
• Because the main wheel centerline is 70" behind the
datum, its arm is a +70".
• The arm for the nosewheel is the difference between
the wheelbase (100") and the distance from the datum
to the main wheel centerline (70"). Therefore, the arm
for the nosewheel is −30".
To calculate the airplane’s empty weight and EWCG, a six-
column chart is used. Figure 6-28 shows the calculation for
the airplane in Figure 6-26.
Based on the calculation shown in the chart, the CG is at
+50.1", which means it is 50.1" aft of the datum. This places the CG forward of the main landing gear, which must be the
case for a tricycle gear airplane. This number is the result of
dividing the total moment of 66,698 in-lb by the total weight
of 1,331.5 lb.
EWCG Formulas
The EWCG can be quickly calculated by using the following
formulas. There are four possible conditions and formulas
that relate the location of the CG to the datum. Notice that
the formula for each condition first determines the moment
of the noseF × L
Wwheel or tailR × L
Wwheel and then divides it
by the total weight of the airplane. The arm is then added
to or subtracted from the distance between the main wheels
and the datum (distance D).
Formula 1 Nosewheel airplanes with datum forward of
the main wheels.
F × L CG = D – () W
Formula 2 Nosewheel airplanes with the datum aft of
the main wheels.
F × L CG = – (D + ) W
Formula 3 Tail wheel airplanes with the datum forward
of the main wheels.
R × L CG = D + ( ) W
Formula 4 Tail wheel airplanes with the datum aft of
the main wheels.
R × L CG = – D + () W
Datum Forward of the Airplane–Nosewheel Landing
Gear
The datum of the airplane in Figure 6-29 is 100" forward
of the leading edge of the wing root, or 128" forward of the
6-174,200
4,100
4,000
3,900
3,800
3,700
3,600
3,500
3,400
3,200
3,000
2,900
2,800
2,700
2,600
78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98Weight (lb)
Center of Gravity (in)Forward
CG LimitAft
CG Limit
80.7 at 2,780 lb82 at 3,400 lb86.4 at 4,000 lb
Figure 6-25. Center of gravity envelope for the Piper Seneca.
main-wheel weighing points. This is distance (D). The weight
of the nosewheel (F) is 340 lb, and the distance between
main wheels and nosewheel (L) is 78". The total weight of
the airplane (W) is 2,006 lb.
The location of the CG may be determined by using this
formula:
F × L CG = D – ( ) W
340 × 78 = 128 – ( ) 2,006
= 114.8
The CG is 114.8" aft of the datum. This is 13.2" forward of
the main-wheel weighing points, which proves the location
of the datum has no effect on the location of the CG so long
as all measurements are made from the same location.
Datum Aft of the Main Wheels– Nosewheel Landing
Gear
The datum of some aircraft may be located aft of the main
wheels. The airplane in this example is the same one just
discussed, but the datum is at the intersection of the trailing
edge of the wing with the fuselage. The distance (D) between the datum of the airplane in Figure 6-30 and the main-wheel
weighing points is 75", the weight of the nosewheel (F) is 340
lb, and the distance between main wheels and nosewheel (L)
is 78". The total net weight of the airplane (W) is 2,006 lb.
The location of the CG may be determined by using this
formula:
F × L CG = – ( D + ) W
340 × 78 = – ( 75 + ) 2,006
= –88.2
The CG location is a negative value, which means it is 88.2"
forward of the datum. This places it 13.2" forward of the main
wheels, the same location as it was when it was measured
from other datum locations.
Location of Datum
It makes no difference where the datum is located if all
measurements are made from the same location.
Datum Forward of the Main Wheels–Tail Wheel
6-18Weight
(lb)
1,515Tare
(lb)
–12.5Net Wt.
(lb)
1,502.5
+6
Fu ll
1,331.5Arm
(inches)
+70
+70
+95
+98
+50.1Moment
(in-lb)
–6,675
45,150
44,450
82,925
–16,815
66,698Item
Nose
Left Main
Right Main
Subtotal
Fuel Total
Fuel Unuse
Oil
Total
Figure 6-28. Center of gravity calculation for airplane being
weighed.
Right scale reading 640 lbMain wheel centerline
Chocks
Wheel baseDatum
Left scale reading 650 lb70"
100"
Nosewheel centerlineNose scale reading 225 lb
Figure 6-26. Example airplane being weighed. The datum is 70" forward of the wing root leading edge.
Aircraft datum: Leading edge of the wing
Leveling means: Two screws, left side of fuselage
below window
Wheelbase: 100"
Fuel capacity: 30 gal aviation gasoline at +95"
Unusable fuel: 6 lb at +98"
Oil capacity: 8 qt at –38"
Note 1: Empty weight includes unusable
fuel and full oil
Left main scale reading: 650 lb
Right main scale reading: 640 lb
Nose scale reading: 225 lb
Tare weight: 5 lb chocks on left main
5 lb chocks on right main
2.5 lb chock on nose
During weighing: Fuel tanks full and oil full
Hydrometer check on
Fuel shows 5.9 lb/gal
Figure 6-27. Specifications and weighing specific data for tricycle
gear airplane.Landing Gear
Locating the CG of a tail wheel airplane is done in the
same way as locating it for a nosewheel airplane except the
formulas use R × L
W rather than F × L
W.
The distance (D) between the datum of the airplane in
Figure 6-31 and the main-gear weighing points is 7.5", the
weight of the tail wheel (R) is 67 lb, and the distance (L)
between the main-wheel and the tail wheel weighing points
is 222". The total weight of the airplane (W) is 1,218 lb.
Determine the CG by using this formula:
Datum
D = 128.0
L = 78.013.2
Figure 6-29. The datum is 100" forward of the wing root leading
edge.
Datum
D =75.0
L = 78.0
Figure 6-30. The datum is aft of the main wheels at the wing
trailing edge. R × L CG = D + ( ) W
67 × 222 = 7.5 + ( ) 1,218
= 19.7
The CG is 19.7 inches behind the datum.
Datum Aft of the Main Wheels–Tail Wheel Landing
Gear
The datum of the airplane in Figure 6-32 is located at the
intersection of the wing root trailing edge and the fuselage.
This places the arm of the main gear (D) at –80". The net
weight of the tail wheel (R) is 67 lb, the distance between
the main wheels and the tail wheel (L) is 222", and the total
net weight (W) of the airplane is 1,218 lb.
Since the datum is aft of the main wheels, use the formula:
R × L CG = – D + () W
67 × 222 = – 80 + ( ) 1,218
= –67.8
The CG is 67.8" forward of the datum, or 12.2" aft of the
main-gear weighing points. The CG is in the same location
relative to the main wheels, regardless of where the datum
is located.
Loading an Aircraft for Flight
The ultimate test of whether there is a problem with an
airplane’s weight and balance is when it is loaded and ready
to fly. The only real importance of an airplane’s empty weight
and EWCG is how it affects the loaded weight and balance
of the airplane, since an airplane does not fly when it is
empty. The pilot-in-command is responsible for the weight
and balance of the loaded airplane, and they make the final
decision on whether the airplane is safe to fly.
Example Loading of an Airplane
As an example of an airplane being loaded for flight, the
Piper Seneca twin will be used. The TCDS for this airplane
was shown earlier in this chapter, and its CG range and CG
envelope were also shown.
The information from the TCDS that pertains to this example
loading is shown in Figure 6-33.For the example loading of the airplane, the following
information applies:
• Airplane Serial Number: 34-7250816
• Airplane Empty Weight: 2,650 lb
• Airplane EWCG: +86.8"
For today’s flight, the following useful load items are
included:
• 1 pilot at 180 lb at an arm of +85.5"
• 1 passenger at 160 lb at an arm of +118.1"
• 1 passenger at 210 lb at an arm of +118.1"
• 1 passenger at 190 lb at an arm of +118.1"
• 1 passenger at 205 lb at an arm of +155.7"
• 50 lb of baggage at an arm of +22.5"
• 100 lb of baggage at an arm of +178.7"
• 80 gal of fuel at an arm of +93.6"
To calculate the loaded weight and CG of this airplane, a
four-column chart is used in Figure 6-34.
Based on the information in the TCDS, the maximum takeoff
weight of this airplane is 4,200 lb and the aft-most CG limit
Datum
142 D = –80
L = 222.0
Figure 6-32. The datum is aft of the main wheels, at the intersection
of the wing trailing edge and the fuselage.
Datum
D = 7.519.7
L = 222.0
Figure 6-31. The datum of this tail wheel airplane is the wing root
leading edge.is +94.6". The loaded airplane in Figure 6-34 is 25 lb too
heavy, and the CG is 1.82" too far aft. To make the airplane
safe to fly, the load needs to be reduced by 25 lb and some
of the load needs to be shifted forward. For example, the
baggage can be reduced by 25 lb, and a full 100 lb of it can
be placed in the more forward compartment. One passenger
can be moved to the forward seat next to the pilot, and the
aft-most passenger can then be moved forward.
With the changes made, the loaded weight is now at the
maximum allowable of 4,200 lb, and the CG has moved
forward 4.42". [Figure 6-35] The airplane is now safe to fly.
Adverse-Loaded CG Checks
Many modern aircraft have multiple rows of seats and often
more than one baggage compartment. After any repair or
alteration that changes the weight and balance, the A&P
mechanic or repairman must ensure that no legal condition of
loading can move the CG outside of its allowable limits. To
determine this, adverse-loaded CG checks must be performed
and the results noted in the weight and balance revision sheet.
During a forward adverse-loaded CG check, all useful load
items in front of the forward CG limit are loaded and all useful
load items behind the forward CG limit are left empty. So,
if there are two seats and a baggage compartment located in
front of the forward CG limit, two people weighing 170 lb
each are seated and the maximum allowable baggage is
placed in the baggage compartment. Any seat or baggage
compartment located behind the forward CG limit is left
empty. If the fuel is located behind the forward CG limit,
minimum fuel will be shown in the tank. Minimum fuel is
calculated by dividing the engine’s METO hp by 2.
During an aft adverse-loaded CG check, all useful load items
behind the aft CG limit are loaded and all useful load items
in front of the aft CG limit are left empty. Even though the
pilot’s seat will be in front of the aft CG limit, the pilot’s seat
cannot be left empty. If the fuel tank is located forward of
the aft CG limit, minimum fuel will be shown.
Example Forward & Aft Adverse-Loaded CG Checks
Using the stick airplane in Figure 6-36 as an example,
adverse forward and aft CG checks are calculated. Some
of the data for the airplane is shown in Figure 6-36 , such
as seat, baggage, and fuel information. The CG limits are
shown, with arrows pointing in the direction where maximum
and minimum weights are loaded. On the forward check,
any useful load item located in front of 89" is loaded, and
anything behind that location is left empty. On the aft check,
maximum weight is added behind 99" and minimum weight
in front of that location. For either of the checks, if fuel is not
located in a maximum weight location, minimum fuel must be accounted for. Notice that the front seats show a location
of 82" to 88", meaning they are adjustable fore and aft. In a
forward check, the pilot’s seat will be shown at 82", and in
the aft check it will be at 88". Additional specifications for
the airplane shown in Figure 6-36 are as follows:
• Airplane empty weight: 1,850 lb
• EWCG: +92.45"
• CG limits: +89" to +99"
• Maximum weight: 3,200 lb
• Fuel capacity: 45 gal at +95"
(44 usable)
40 gal at +102"
(39 usable)
In evaluating the two extreme condition checks, the following
key points should be recognized. [Figure 6-37]
• The total arm is the airplane CG and is found by
dividing the total moment by the total weight.
• For the forward check, the only thing loaded behind
the forward limit was minimum fuel.
• For the forward check, the pilot and passenger seats
were shown at the forward position of 82".
6-21Moment
(in-lb)
230,020.0
15,390.0
17,955.0
24,801.6
22,439.0
24,210.5
2,250.0
4,467.5
44,928.0
386,461.0Item
Empty Weight
Pilot
Passenger
Passenger
Passenger
Passenger
Baggage
Baggage
Fuel
TotalWeight
(lb)
2,650
4,200Arm
(inches)
+ 86.80
+85.50
+85.50
+155.01
+118.10
+118.10
+22.50
+178.70
+93.60
+92.01
Figure 6-35. Center of gravity calculation for Piper Seneca with
weights shifted.S/N 34-7250215 through 34-7450220:
(+87.9") to (+94.6") at 4,200 lb
(+82.0") to (+94.6") at 3,400 lb
(+80.7") to (+94.6") at 2,780 lb
Straight line variation between points given.
−32 in-lb moment change due to gear
retracting landing gear
None
S/N 34-7250215 through 34-7450220:
4,200 lb—Takeoff
4,000 lb—Landing
7 (2 at +85.5", 3 at +118.1", 2 at +155.7")
200 lb (100 lb at +22.5, 100 lb at +178.7)
98 gal (2 wing tanks) at (+93.6") (93 gal
usable). See NOTE 1 for data on system fuel.CG Range
(Gear Extended)
Empty Weight
CG Range
Maximum
Weight
No. of Seats
Maximum
Baggage
Fuel Capacity
Figure 6-33. Example loading information pertaining to TCDS.
Moment
(in-lb)
230,020.0
15,390.0
18,896.0
24,801.0
22,439.0
31,918.5
1,125.0
17,870.0
44,928.0
407 , 387.50Item
Empty Weight
Pilot
Passenger
Passenger
Passenger
Passenger
Baggage
Baggage
Fuel
TotalWeight
(lb)
2,650
4,225Arm
(inches)
+ 86.80
+85.50
+118.10
+118.10
+118.10
+155.70
+22.50
+178.70
+93.60
+96.42
Figure 6-34. Center of gravity calculation for Piper Seneca.• For the forward check, the CG was within limits, so
the airplane could be flown this way.
• For the aft check, the only thing loaded in front of the
aft limit was the pilot, at an arm of 88".
• For the aft check, the fuel tank at 102" was filled, which
more than accounted for the required minimum fuel.
• For the aft check, the CG was out of limits by 0.6",
so the airplane should not be flown this way.
Equipment Change & Aircraft Alteration
When the equipment in an aircraft is changed, such as the
installation of a new radar system or ground proximity warning system, or the removal of a radio or seat, the weight
and balance of an aircraft changes. An alteration performed
on an aircraft, such as a cargo door being installed or a
reinforcing plate being attached to the spar of a wing, also
changes the weight and balance of an aircraft. Any time the
equipment is changed or an alteration is performed, the new
empty weight and EWCG must be determined. This can be
accomplished by placing the aircraft on scales and weighing
it, or by mathematically calculating the new weight and
balance. The mathematical calculation is acceptable if the
exact weight and arm of all the changes are known.
Example Calculation After an Equipment Change
A small, twin-engine airplane has some new equipment
installed and some of its existing equipment removed. The
details of the equipment changes are shown in Figure 6-38.
To calculate the new empty weight and EWCG, a four-
column chart is used. [Figure 6-39] In evaluating the weight
and balance calculation shown in Figure 6-39, the following
key points should be recognized.
• The weight of the equipment needs to be identified
with a plus or minus to signify whether it is being
installed or removed.
• The sign of the moment (plus or minus) is determined
by the signs of the weight and arm.
• The strobe and the ADF are both being removed
(negative weight), but only the strobe has a negative
moment. This is because the arm for the ADF is
also negative, and two negatives multiplied together
produce a positive result.
• The total arm is the airplane’s CG and is found by
dividing the total moment by the total weight.
6-22Moment
(in-lb)
171,032.5
13,940.0
13,940.0
4,500.0
17,812.5
221,225.0Item
Empty Weight
Pilot
Passenger
Baggage
Fuel
TotalExtreme Condition Forward Check
Weight
(lb)
1,850.0
2,452.5Arm
(inches)
+ 92.45
+82.00
+82.00
+60.00
+95.00
+90.20
Moment
(in-lb)
171,032.5
14,960.0
35,700.0
42,500.0
14,000.0
23,868.0
302,060.5Item
Empty Weight
Pilot
2 Passengers
2 Passengers
Baggage
Fuel
TotalExtreme Condition Aft Check
Weight
(lb)
1,850
3,034Arm
(inches)
+ 92.45
+88.00
+105.00
+125.00
+140.00
+102.00
+99.60
Figure 6-37. Center of gravity extreme conditions check.
100 lb at 140 " 375 hp
FUEL FUEL
95" 102"2 at 125" 2 at 105" 2 at 82"–88"
Maximum weight
Forward limit 89 "Minimum weight
Aft limit 99"
Maximum weight Minimum weight75 lb at 60"
Figure 6-36. Example airplane for extreme condition checks.
• The result of the equipment change is that the
airplane’s weight was reduced by 22.5 lb and the CG
has moved forward 0.67".
Use of Ballast
Ballast is used in an aircraft to attain the desired CG balance,
when the CG is not within limits or is not at the location
desired by the operator. It is usually located as far aft or as
far forward as possible to bring the CG within limits, while
using a minimum amount of weight.
Temporary Ballast
Temporary ballast, in the form of lead bars, heavy canvas
bags of sand, or lead shot, is often carried in the baggage
compartments to adjust the balance for certain flight
conditions. The bags are marked “Ballast XX Pounds–
Removal Requires Weight and Balance Check.” Temporary
ballast must be secured so it cannot shift its location in flight,
and the structural limits of the baggage compartment must not
be exceeded. All temporary ballast must be removed before
the aircraft is weighed.
Temporary Ballast Formula
The CG of a loaded airplane can be moved into its allowable
range by shifting passengers or cargo or by adding temporary
ballast. To determine the amount of temporary ballast needed,
use this formula:
Total wt. × dist. needed to shift CG Ballast weight =
needed Dist. between ballast and desired CG
Figures 6-36 and 6-40 show an aft adverse-loaded CG check
being performed on an airplane. In this previous example,
the airplane’s CG was out of limits by 0.6". If there were a
need or a desire to fly the airplane loaded this way, one way
to make it possible would be the installation of temporary ballast in the front of the airplane. The logical choice for
placement of this ballast is the forward baggage compartment.
The CG for this airplane is 0.6" too far aft. If the forward
baggage compartment is used as a temporary ballast location,
the ballast calculation will be as shown in Figure 6-41.
Total wt. × dist. needed to shift CG Ballast weight =
needed Dist. between ballast and desired CG
6-23Moment
(in-lb)
58,045.0
–2,040.0
55,925.8Item
Empty Weight
Radio Install
GPS Install
ELT Install
Strobe Remove
ADF Remove
Seat Remove
TotalWeight
(lb)
2,350.0
+5.8
+7.3
+2.8
2,327.5Arm
(inches)
+ 24.70
+105.00
+75.00
+60.00
Figure 6-39. Center of gravity calculation after equipment change.Airplane empty weight: 2,350 lb
Airplane EWCG: +24.7"
Airplane datum: Leading edge of the wing
Radio installed: 5.8 lb at an arm of –28"
Global positioning
system installed: 7.3 lb at an arm of –26"
Emergency locater
transmitter installed: 2.8 lb at an arm of +105"
Strobe light removed: 1.4 lb at an arm of +75"
Automatic direction
finder (ADF) removed: 3 lb at an arm of –28"
Seat removed: 34 lb at an arm of +60"
Figure 6-38. Twin-engine airplane equipment changes. 3,034 lb × (0.6") = 39"
= 46.68 lb
When ballast is calculated, the answer should always be
rounded up to the next higher whole pound, or in this case, 47
lb of ballast would be used. To ensure the ballast calculation
is correct, the weight of the ballast should be plugged back
into the four-column calculation and a new CG calculated.
The aft limit for the airplane was 99", and the new CG is at
98.96", which puts it within acceptable limits. The new CG
did not fall exactly at 99" because the amount of needed
ballast was rounded up to the next whole pound. If the ballast
could have been placed farther forward, such as being bolted
to the engine firewall, less ballast would have been needed.
That is why ballast is always placed as far away from the
affected limit as possible.
In evaluating the ballast calculation shown above, the
following key points should be recognized.
• The loaded weight of the aircraft, as identified in the
formula, is what the airplane weighed when the CG
was out of limits.
• The distance the CG is out of limits is the difference
between the CG location and the CG limit, in this case
99.6" minus 99".
• The affected limit identified in the formula is the CG
limit which has been exceeded. If the CG is too far
aft, it is the aft limit that has been exceeded.
• The aft limit for this example is 99", and the ballast is being placed in the baggage compartment at
an arm of 60". The difference between the two
is 39", the quantity divided by in the formula.
Viewed as a first-class lever problem, Figure 6-42 shows
what this ballast calculation would look like. A ballast weight
of 46.68 lb on the left side of the lever multiplied by the arm
of 39" (99 minus 60) would equal the aircraft weight of 3,034
lb multiplied by the distance the CG is out of limits, which
is 0.6" (99.6 minus 99).
Permanent Ballast
If a repair or alteration causes the aircraft CG to fall outside
of its limit, permanent ballast can be installed. Usually,
permanent ballast is made of blocks of lead painted red and
marked “Permanent Ballast–Do Not Remove.” It should be
attached to the structure so that it does not interfere with any
control action, and attached rigidly enough that it cannot be
dislodged by any flight maneuvers or rough landing. The
installation of permanent ballast results in an increase in the
aircraft empty weight, and it reduces the useful load.
Three things must be known to determine the amount of
ballast needed to bring the CG within limits: the amount
the CG is out of limits, the distance between the location of
the ballast, and the limit that is affected. If an airplane with
an empty weight of 1,876 lb has been altered so its EWCG
is +32.2, and CG range for weights up to 2,250 lb is +33.0
to +46.0, permanent ballast must be installed to move the
EWCG from +32.2 to +33.0. There is a bulkhead at fuselage
station 228 strong enough to support the ballast. To determine
the amount of ballast needed, use this formula:
Aircraft empty wt. × dist. out of limits
Ballast weight = Dist. between ballast and desired CG
1,876 lb × 0.8" =
6-24Ballast weight of
46.68 lb at an
arm of 60 "
Distance out
of limits0.6"Aircraft weight of
3,034 lb at a
CG of 99.6 "
In order to balance at the aft limit of 99", the moment to the left of
the fulcrum must equal the moment to the right of the fulcrum. The
moment to the right is the weight of the airplane multiplied by 0.6".
The moment to the left is the ballast weight multiplied by 39".
Distance from aft limit to ballast = 39 "
Figure 6-42. Ballast calculation as a first class lever.Moment
(in-lb)
302,060.5
2,820.0
304,880.5Item
Loaded Weight
Ballast
TotalWeight
(lb)
3,034
3,081Arm
(inches)
+ 99.60
+ 60.00
+98.96
Figure 6-41. Ballast calculation.
Moment
(in-lb)
171,032.5
14,960.0
35,700.0
42,500.0
14,000.0
23,868.0
302,060.5Item
Empty Weight
Pilot
2 Passengers
2 Passengers
Baggage
Fuel
TotalWeight
(lb)
1,850
3,034Arm
(inches)
+ 92.45
+ 88.00
+105.00
+125.00
+140.00
+102.00
+99.60
Figure 6-40. Extreme condition check. 228 – 33
1,500.8 = 195
= 7.7 lb
A block of lead weighing 7.7 pounds attached to the bulkhead
at fuselage station 228, moves the EWCG back to its proper
forward limit of +33. This block should be painted red and
marked “Permanent Ballast– Do Not Remove.”
Loading Graphs & CG Envelopes
The weight and balance computation system, commonly
called the loading graph and CG envelope system, is an
excellent and rapid method for determining the CG location
for various loading arrangements. This method can be applied
to any make and model of aircraft, but is more often seen
with small GA aircraft.
Aircraft manufacturers using this method of weight and
balance computation prepare graphs like those shown in
Figures 6-43 and 6-44 for each make and model aircraft
at the time of original certification. The graphs become
a permanent part of the aircraft records and are typically
found in the AFM/POH. These graphs, used in conjunction
with the empty weight and EWCG data found in the weight
and balance report, allow the pilot to plot the CG for the
loaded aircraft.
The loading graph in Figure 6-43 is used to determine the
index number (moment value) of any item or weight that may
be involved in loading the aircraft. To use this graph, find the
point on the vertical scale that represents the known weight.
Project a horizontal line to the point where it intersects the
proper diagonal weight line (i.e., pilot, copilot, baggage).
Where the horizontal line intersects the diagonal, project
a vertical line downward to determine the loaded moment
(index number) for the weight being added.
After the moment for each item of weight has been determined, all weights are added and all moments are
added. The total weight and moment is then plotted on the
CG envelope. [Figure 6-44] The total weight is plotted on the
vertical scale of the graph, with a horizontal line projected out
from that point. The total moment is plotted on the horizontal
scale of the graph, with a vertical line projected up from that
point. Where the horizontal and vertical plot lines intersect
on the graph is the CG for the loaded aircraft. If the point
where the plot lines intersect falls inside the CG envelope,
the aircraft CG is within limits. In Figure 6-44, there are two
CG envelopes, one for the aircraft in the Normal Category
and one for the aircraft in the Utility Category.
The loading graph and CG envelope shown in Figures 6-43 and
6-44 are for an airplane with the following specifications and
weight and balance data.
• Number of seats: 4
• Fuel capacity (usable): 38 gal of Avgas
• Oil capacity: 8 qt (included in empty
weight)
• Baggage: 120 lb
• Empty weight: 1,400 lb
• EWCG: 38.5"
• Empty weight moment: 53,900 in-lb
An example of loading the airplane for flight and calculating
the total loaded weight and the total loaded moment is shown
−2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28Load Weight (lb)
Moment Index (Moment/1,000)Pilot & Front PassengerRear Passenger
Fuel
OilBaggage
Figure 6-43. Aircraft loading graph.in Figures 6-45 and 6-46. The use of the loading graph to
determine the moment for each of the useful load items is
shown in Figure 6-46. The color used for each useful load
item in Figure 6-45 matches the color used for the plot on
the loading graph.
The total loaded weight of the airplane is 2,258 lb and the
total loaded moment is 99,400 in-lb. These two numbers can
now be plotted on the CG envelope to see if the airplane is
within CG limits. Figure 6-47 shows the CG envelope with
the loaded weight and moment of the airplane plotted. The
CG location shown falls within the normal category envelope,
so the airplane is within CG limits for this category.
It is interesting to note that the lines that form the CG
envelope are graphic plots of the forward and aft CG limits.
In Figure 6-47, the red line is a graphic plot of the forward
limit, and the blue and green lines are graphic plots of the
aft limit for the two different categories.
Helicopter Weight & Balance
General Concepts
All the terminology and concepts that apply to airplane
weight and balance also apply generally to helicopter weight
and balance. However, there are some specific differences
that need to be identified.
Most helicopters have a much more restricted CG range
than airplanes. In some cases, this range is less than 3". The
exact location and length of the CG range is specified for
each helicopter and usually extends a short distance fore and aft of the main rotor mast or centered between the main
rotors of a dual rotor system. Whereas airplanes have a CG
range only along the longitudinal axis, helicopters have both
longitudinal and lateral CG ranges. Because the wings extend
outward from the CG, airplanes tend to have a great deal of
lateral stability. A helicopter, on the other hand, acts like a
pendulum, with the weight of the helicopter hanging from
the main rotor shaft.
Ideally, the helicopter should have such perfect balance
that the fuselage remains horizontal while in a hover. If
the helicopter is too nose heavy or tail heavy while it is
hovering, the cyclic pitch control is used to keep the fuselage
horizontal. If the CG location is too extreme, it may not be
possible to keep the fuselage horizontal or maintain control
of the helicopter.
Helicopter Weighing
When a helicopter is being weighed, the location of both
longitudinal and lateral weighing points must be known to
determine its empty weight and EWCG. This is because
helicopters have longitudinal and lateral CG limits. As with
the airplane, the longitudinal arms are measured from the
datum, with locations behind the datum being positive arms
and locations in front of the datum being negative arms.
Laterally, the arms are measured from the butt line, which is
a line from the nose to the tail running through the middle of
the helicopter. When facing forward, arms to the right of the
butt line are positive; to the left they are negative.
Before a helicopter is weighed, it must be leveled
longitudinally and laterally. This can be done with a spirit
6-26Weight (lb)
1,400
2,258Item
Aircraft Empty Weight
Pilot
Front Passengers
Rear Passengers
Baggage
Fu el
Tot alMoment (in-lb)
53,900
6,000
4,500
15,000
9,200
10,800
99,400
Figure 6-45. Aircraft load chart.2,400
2,300
2,200
2,100
2,000
1,900
1,800
1,700
1,600
1,500
50 55 60 65 70 75 80 85 90 95 100 105 110 115Load Aircraft Weight (lb)
Loaded Aircraft Moment/1,000 (in-lb)Normal Category
Utility Category
Figure 6-44. CG envelope.
level, but often it is done with a plumb bob. For example,
the Bell JetRanger has a location inside the aft cabin where a
plumb can be attached and allowed to hang down to the cabin
floor. On the cabin floor is a plate bearing cross hairs that
correspond to the horizontal and lateral axis of the helicopter.
When the point of the plumb bob falls in the middle of the
cross hairs, the helicopter is level along both axes. If the tip
of the plumb bob falls forward of this point, the nose of the
helicopter is too low; if it falls to the left of this point, the left
side of the helicopter is too low. In other words, the tip of the
plumb bob always moves toward the low point.
A Bell JetRanger helicopter is shown in Figure 6-48 with
the leveling plate depicted on the bottom right of the figure.
The helicopter has three jack pads, two at the front and one
in the back. To weigh this helicopter, three jacks would be
placed on floor scales, and the helicopter would be raised off
the hangar floor. To level the helicopter, the jacks would be
adjusted until the plumb bob point falls exactly in the middle
of the cross hairs.As an example of weighing a helicopter, consider the Bell
JetRanger in Figure 6-48 , and the following specifications
and weighing data shown in Figure 6-49.
Using six-column charts for the calculations, the empty
weight and the longitudinal and lateral CG for the helicopter
is shown in Figure 6-50. Based on the calculations in
Figure 6-50, it has been determined that the empty weight of
the helicopter is 1,985 lb, the longitudinal CG is at +108.73",
and the lateral CG is at –0.31".
Weight and Balance— Weight-Shift Control
Aircraft and Powered Parachutes
The terminology, theory, and concepts of weight and balance
that applies to airplanes also applies to weight-shift control
aircraft and powered parachutes. Weight is still weight, and
the balance point is still the balance point. However, there are
a few differences that need to be discussed. Before reading
about the specifics of weight and balance on weight-shift
control aircraft and powered parachutes, be sure to read
about their aerodynamic characteristics in Chapter 5, Physics.
Weight-shift control aircraft and powered parachutes do not
fall under the same Code of Federal Regulations that govern
certified airplanes and helicopters and, therefore, do not have
TCDS or the same type of FAA-mandated weight and balance
reports. Weight and balance information and guidelines are
left to the individual owners and the companies with which
they work in acquiring this type of aircraft. Overall, the
industry that is supplying these aircraft is regulating itself
well, and the safety record is good for those aircraft being
operated by experienced pilots.
6-272,400
2,300
2,200
2,100
2,000
1,900
1,800
1,700
1,600
1,500
50 55 60 65 70 75 80 85 90 95 100 105 110 115Load Aircraft Weight (lb)
Loaded Aircraft Moment/1,000 (in-lb)Normal Category
CG Location
Aft LimitForward Limit
Utility Category
Figure 6-47. CG envelope example plot.360
−2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28Load Weight (lb)
Moment Index (Moment/1,000)Pilot & Front PassengerRear Passenger
Fuel
OilBaggage
Figure 6-46. Example plots on a loading graph.
Weight-Shift Control Aircraft
Weight-shift control aircraft, commonly known by the name
“trikes,” have very few options for loading, because they have
very few places to put useful load items. Some trikes have
only one seat and a fuel tank, so the only variables for a flight
are amount of fuel and weight of the pilot. Some trikes have
two seats and a small storage bin in addition to the fuel tank.
The most significant factor affecting the weight and balance
of a trike is the weight of the pilot; if the aircraft has two seats,
the weight of the passenger must be considered. The trike
acts somewhat like a single main rotor helicopter because the weight of the aircraft is hanging like a pendulum under
the wing. Figure 6-51 shows a two-place trike, in which the
mast and the nose strut come together slightly below the
wing attach point. When the trike is in flight, the weight of
the aircraft is hanging from the wing attach point. The weight
of the engine and fuel is behind this point, the passenger is
almost directly below this point, and the pilot is forward of
this point. The balance of the aircraft is determined by how
all these weights compare.
The wing attach point, with respect to the wing keel, is an
adjustable location. The attach point can be loosened and
6-28moved slightly forward or slightly aft, depending on the
weight of the occupants. For example, if the aircraft is flown
by a person that weighs more, the attach point can be moved
a little farther aft, bringing the wing forward, to compensate
for the change in CG. Figure 6-52 shows a close-up of the
wing attach point, and the small amount of forward and aft
movement that is available.
Powered Parachutes
Powered parachutes have many of the same characteristics
as weight-shift aircraft when it comes to weight and balance.
They have the same limited loading, with only one or two seats
and a fuel tank. They also act like a pendulum, with the weight
of the aircraft hanging beneath the inflated wing (parachute).
The point at which the inflated wing attaches to the structure
of the aircraft is adjustable to compensate for pilots and
passengers of varying weights. With a very heavy pilot, the
wing attach point would be moved forward to prevent the
aircraft from being too nose heavy. Figure 6-53 shows the
structure of a powered parachute with the adjustable wing
attach points.
Weight & Balance for Large Airplanes
Weight and balance for large airplanes is almost identical
to what it is for small airplanes, on a much larger scale. If a
technician can weigh a small airplane and calculate its empty
weight and EWCG, that same technician should be able to
do it for a large airplane. The jacks and scales are larger, and
it may take more personnel to handle the equipment, but the
concepts and processes are the same.
Built-In Electronic Weighing
One difference that may be found with large airplanes is the
incorporation of electronic load cells in the aircraft’s landing
gear. With this type of system, the airplane can weigh itself as
it sits on the tarmac. The load cells are built into the axles of
the landing gear, or the landing gear strut, and they work in
the same manner as load cells used with jacks. This system is
currently in use on the Boeing 747-400, Boeing 777, Boeing
787, McDonnell Douglas MD-11, and the wide body Airbus
airplanes like the A-330, A-340, and A-380.
The Boeing 777 utilizes two independent systems that
provide information to the airplane’s flight management
system (FMS). If the two systems agree on the weight and
CG of the airplane, the data being provided are considered
accurate and the airplane can be dispatched based on that
information. The flight crew has access to the information
on the flight deck by accessing the FMS and bringing up the
weight and balance page.Mean Aerodynamic Chord
On small airplanes and on all helicopters, the CG location
is identified as being a specific number of inches from the
datum. The CG range is identified the same way. On larger
airplanes, from private business jets to large jumbo jets, the
CG and its range are typically identified in relation to the
width of the wing.
The width of the wing on an airplane is known as the chord.
If the leading edge and trailing edge of a wing are parallel to
each other, the chord of the wing is the same along the wing’s
length. Business jets and commercial transport airplanes
have wings that are tapered and that are swept back, so the
width of their wings is different along their entire length.
The width is greatest where the wing meets the fuselage
and progressively decreases toward the tip. In relation to the
aerodynamics of the wing, the average length of the chord
on these tapered swept-back wings is known as the mean
aerodynamic chord (MAC).
On these larger airplanes, the CG is identified as being at a
location that is a specific percent of the mean aerodynamic
chord (% MAC). For example, imagine that the MAC on
an airplane is 100", and the CG falls 20" behind the leading
edge of the MAC. That means it falls one-fifth of the way
back, or at 20 percent of the MAC.
Figure 6-54 shows a large twin-engine commercial transport
airplane. The datum is forward of the nose of the airplane,
and all the arms are being measured from that point. The CG
for the airplane is shown as an arm measured in inches. In
the lower left corner of the figure, a cross section of the wing
is shown, with the same CG information being presented.
To convert the CG location from inches to a percent of MAC,
for the airplane shown in Figure 6-54, the steps are as follows:
1. Identify the CG location, in inches from the datum.
2. Identify the leading edge of the MAC (LEMAC), in
inches from the datum.
3. Subtract LEMAC from the CG location.
4. Divide the difference by the length of the MAC.
5. Convert the result in decimals to a percentage by
multiplying by 100.
As a formula, the solution to solve for the percent of MAC
would be:
CG – LEMAC Percent of MAC = × 100 MAC
The result using the numbers shown in Figure 6-52 would be:
CG – LEMAC Percent of MAC = × 100 MAC
6-2955.16" forward of the front jack point
centerline
Plumb line from ceiling left rear cabin to
index plate on floor
+106" to +111.4" at 3,200 lb
+106" to +112.1" at 3,000 lb
+106" to +112.4" at 2,900 lb
+106" to +113.4" at 2,600 lb
+106" to +114.2" at 2,350 lb
+106" to +114.2" at 2,100 lb
Straight line variation between points
2.3" left to 3.0" right at longitudinal CG
+ 106.0"
3.0" left to 4.0" right at longitudinal CG
+108" to +114.2"
Straight line variation between points
Empty weight includes unusable fuel
and unusable oilDatum:
Leveling Means:
Longitudinal CG
Limits:
Lateral CG
Limits:
Fuel and Oil: Left Front
Scale Reading:
Left Front
Jack Point:
Right Front
Scale Reading:
Right Front
Jack Point:
After Scales
Reading:
Aft Jack Point:
Notes:650 lb
Longitudinal arm of +55.16"
Lateral arm of –25"
625 lb
Longitudinal arm of +55.16"
Lateral arm of +25"
710 lb
Longitudinal arm of +204.92"
Lateral arm of 0.0"
The helicopter was weighed with unusable
fuel and oil. Electronic scales were used
and zeroed with the jacks in place, so no
tare weight needs to be accounted for.
Figure 6-49. Specifications and weighing data for Bell JetRanger.
Forward ballast station
+13"
Forward jack point +55.16"
+25" and −25" laterally
Datum
Leveling plate
+117.7"Plumb bob
Leveling plate with crosshairsAft jack point
+204.92"Aft ballast station
+377"
Left sideRight side
NoseTail
Figure 6-48. Bell JetRanger.
Figure 6-51. Weight and balance for a weight-shift control aircraft.Longitudinal CG Calculation
Item Scale (lb) Tare Wt. (lb) Nt. Wt. (lb) Arm (inches) Moment (in-lb)
Left Front 650 0 650 +55.16 35,854.0
Right Front 625 0 625 +55.16 34,475.0
Aft 710 0 710 +204.92 145,493.2
Total 1,985 1,985 +108.73 215,822.2
Lateral CG Calculation
Item Scale (lb) Tare Wt. (lb) Nt. Wt. (lb) Arm (inches) Moment (in-lb)
Left Front 650 0 650 −25 −16,250
Right Front 625 0 625 +25 +15,625
Aft 710 0 710 0 0
Total 1,985 1,985 +.31 −625
Figure 6-50. Center of gravity calculation for Bell JetRanger.
Figure 6-52. Wing attach point for a weight-shift control aircraft.
Nosewheel steering
Throttle
Pivot bar
to control turningWing attachment point
adjustable for CG
Wing attachment point
adjustable for CG
Figure 6-53. Powered parachute structure with wing attach points.
945 – 900 = × 100 180
= 25 percent
If the CG is known in percent of MAC, and there is a need
to know the CG location in inches from the datum, the
conversion would be done as follows:
1. Convert the percent of MAC to a decimal by dividing
by 100.
2. Multiply the decimal by the length of the MAC.
Center of gravity
945"
CG at 945"Leading edge of MAC
900"
DatumTrailing edge of MAC
1,080"
LEMAC 900"
Chord line
MAC = 180"45"MAC=180"
Figure 6-54. Center of gravity location on a large commercial transport.
3. Add this number to LEMAC.
As a formula, the solution to convert a percent of MAC to
an inch value would be:
CG in inches = % MAC ÷ 100 × MAC + LEMAC
For the airplane in Figure 6-54, if the CG was at 32.5 percent
of the MAC, the solution would be:
CG in inches = % MAC ÷ 100 × MAC + LEMAC
= 32.5 ÷ 100 × 180 + 900
= 958.5
Weight & Balance Records
When a technician gets involved with the weight and balance
of an aircraft, it almost always involves a calculation of the
aircraft’s empty weight and EWCG. Only on rare occasions
are technicians involved in calculating adverse-loading CG checks, how much ballast is needed, or the loaded weight
and balance of the aircraft. Calculating the empty weight
and EWCG might involve putting the aircraft on scales and
weighing it, or a pencil and paper exercise after installing a
new piece of equipment.
The FAA requires that a current and accurate empty weight
and EWCG be known for an aircraft. This information must
be included in the weight and balance report, which is a part
of the aircraft permanent records. The weight and balance
report must be in the aircraft when it is being flown.
There is no required format for this report, but Figure 6-55
is a good example of recording the data obtained from
weighing an aircraft. As it is currently laid out, the form
would accommodate either a tricycle gear or tail dragger
airplane. Depending on the gear type, either the nose or the
tail row would be used. If an airplane is being weighed using
jacks and load cells, or if a helicopter is being weighed, the
6-32Aircraft Weight and Balanc e Rep ort
Results of A ircraft Weighing
Make Model
Serial # N#
Datum Location
Leveling Means
Scale Arms: Nose Tail Left Main Right Main
Scale Weights: Nose Tail Left Main Right Main
Tare Weights: Nose Tail Left Main Right Main
Weight and Balanc e Calcula tion
Aircraft Current Empty Weight:
Aircraft Current Empty Weight CG:
Aircraft Maximum Weight:
Aircraft Useful Load:
Computed By:
Certificate #: (print name)
(signature)
(A&P, Repair Station, etc.)
Date: Item
Nose
Tail
Left Main
Right Main
Subtotal
Fuel
Oil
Misc.
TotalScale (lb) Tare Wt. (lb) Net Wt. (lb) Arm (inches) Moment (in-lb)
Figure 6-55. Aircraft weight and balance report.
6-33item names must be changed to reflect the weight locations.
If an equipment change is being done on an aircraft, and the
new weight and balance is calculated mathematically instead
of weighing the aircraft, the same type of form shown in
Figure 6-55 can be used. The only change would be the use
of a four-column solution, instead of six columns, and there
would be no tare weight or involvement with fuel and oil.
Aircraft Materials, Hardware, &
Processes
Chapter 7
Aircraft Metals
Knowledge and understanding of the uses, strengths,
limitations, and other characteristics of structural metals
is vital to properly construct and maintain any equipment,
especially airframes. In aircraft maintenance and repair,
even a slight deviation from design specification, or the
substitution of inferior materials, may result in the loss of
both lives and equipment. The use of unsuitable materials can
readily erase the finest craftsmanship. The selection of the
correct material for a specific repair job demands familiarity
with the most common physical properties of various metals.
Properties of Metals
Of primary concern in aircraft maintenance are such general
properties of metals and their alloys as hardness, malleability,
ductility, elasticity, toughness, density, brittleness, fusibility,
conductivity contraction and expansion, and so forth. These
terms are explained to establish a basis for further discussion
of structural metals.
Hardness
Hardness refers to the ability of a material to resist abrasion,
penetration, cutting action, or permanent distortion. Hardness
may be increased by cold-working the metal and, in the case
of steel and certain aluminum alloys, by heat-treatment.
Structural parts are often formed from metals in their soft
state and are then heat-treated to harden them so that the
finished shape is retained. Hardness and strength are closely
associated properties of metals.
Strength
One of the most important properties of a material is strength.
Strength is the ability of a material to resist deformation.
Strength is also the ability of a material to resist stress without
breaking. The type of load or stress on the material affects
the strength it exhibits.
Density
Density is the weight of a unit volume of a material. In
aircraft work, the specified weight of a material per cubic
inch is preferred since this figure can be used in determining
the weight of a part before actual manufacture. Density is
an important consideration when choosing a material to be
used in the design of a part to maintain the proper weight
and balance of the aircraft.Malleability
A metal that can be hammered, rolled, or pressed into various
shapes without cracking, breaking, or leaving some other
detrimental effect, is said to be malleable. This property is
necessary in sheet metal that is worked into curved shapes,
such as cowlings, fairings, or wingtips. Copper is an example
of a malleable metal.
Ductility
Ductility is the property of a metal that permits it to be
permanently drawn, bent, or twisted into various shapes
without breaking. This property is essential for metals used in
making wire and tubing. Ductile metals are greatly preferred
for aircraft use because of their ease of forming and resistance
to failure under shock loads. For this reason, aluminum alloys
are used for cowl rings, fuselage and wing skin, and formed
or extruded parts, such as ribs, spars, and bulkheads. Chrome
molybdenum steel is also easily formed into desired shapes.
Ductility is similar to malleability.
Elasticity
Elasticity is a property that enables a metal to return to
its original size and shape when the force that causes the
change of shape is removed. This property is extremely
valuable, because it would be highly undesirable to have
a part permanently distorted after an applied load was
removed. Each metal has a point known as the elastic limit,
beyond which it cannot be loaded without causing permanent
distortion. In aircraft construction, members and parts are so
designed that the maximum loads to which they are subjected
do not stress them beyond their elastic limits. This desirable
property is present in spring steel.
Toughness
A material that possesses toughness withstands tearing or
shearing and may be stretched or otherwise deformed without
breaking. Toughness is a desirable property in aircraft metals.
Brittleness
Brittleness is the property of a metal that allows little bending
or deformation without shattering. A brittle metal is apt to
break or crack without change of shape. Because structural
metals are often subjected to shock loads, brittleness is not a
7-2very desirable property. Cast iron, cast aluminum, and very
hard steel are examples of brittle metals.
Fusibility
Fusibility is the ability of a metal to become liquid by
the application of heat. Metals are fused in welding.
Steels fuse around 2,600 °F and aluminum alloys at
approximately 1,100 °F.
Conductivity
Conductivity is the property that enables a metal to carry heat
or electricity. The heat conductivity of a metal is especially
important in welding, because it governs the amount of heat
that is required for proper fusion. Conductivity of the metal,
to a certain extent, determines the type of jig to be used to
control expansion and contraction. In aircraft, electrical
conductivity must also be considered in conjunction with
bonding to eliminate radio interference.
Thermal Expansion
Thermal expansion refers to contraction and expansion that
are reactions produced in metals as the result of heating or
cooling. Heat applied to a metal causes it to expand or become
larger. Cooling and heating affect the design of welding jigs,
castings, and tolerances necessary for hot rolled material.
Ferrous Aircraft Metals
Many different metals are required in the repair of aircraft.
This is a result of the varying needs with respect to strength,
weight, durability, and resistance to deterioration of specific
structures or parts. In addition, the particular shape or form of
the material plays an important role. In selecting materials for
aircraft repair, these factors (plus many others) are considered
in relation to the mechanical and physical properties. Among
the common materials used are ferrous metals. The term
“ferrous” applies to the group of metals having iron as their
principal element.
Iron
If carbon is added to iron in percentages ranging up to
approximately 1 percent, the product is vastly superior to iron
alone and is classified as carbon steel. Carbon steel forms
the base of those alloy steels produced by combining carbon
steel with other elements known to improve the properties of
steel. A base metal (such as iron) to which small quantities of
other metals have been added is called an alloy. The addition
of other metals changes or improves the chemical or physical
properties of the base metal for a particular use.
Steel and Steel Alloys
To facilitate the discussion of steels some familiarity with
their nomenclature is desirable. A numerical index, sponsored by the Society of Automotive Engineers (SAE) and the
American Iron and Steel Institute (AISI), is used to identify
the chemical compositions of the structural steels. In this
system, a four-numeral series is used to designate the plain
carbon and alloy steels; five numerals are used to designate
certain types of alloy steels. The first two digits indicate the
type of steel, the second digit also generally (but not always)
gives the approximate amount of the major alloying element,
and the last two (or three) digits are intended to indicate
the approximate middle of the carbon range. However, a
deviation from the rule of indicating the carbon range is
sometimes necessary.
Small quantities of certain elements are present in alloy
steels that are not specified as required. These elements are
considered as incidental and may be present to the maximum
amounts as follows: copper, 0.35 percent; nickel, 0.25
percent; chromium, 0.20 percent; molybdenum, 0.06 percent.
The list of standard steels is altered from time to time to
accommodate steels of proven merit and to provide for
changes in the metallurgical and engineering requirements
of industry. [Figure 7-1]
Metal stock is manufactured in several forms and shapes,
including sheets, bars, rods, tubing, extrusions, forgings,
and castings. Sheet metal is made in a number of sizes
and thicknesses. Specifications designate thicknesses in
thousandths of an inch. Bars and rods are supplied in a variety
of shapes, such as round, square, rectangular, hexagonal, and
octagonal. Tubing can be obtained in round, oval, rectangular,
or streamlined shapes. The size of tubing is generally
specified by outside diameter and wall thickness.
The sheet metal is usually formed cold in machines, such as
presses, bending brakes, draw benches, or rolls. Forgings are
shaped or formed by pressing or hammering heated metal
in dies. Pouring molten metal into molds produces castings.
Machining finishes the casting.
Spark testing is a common means of identifying various
ferrous metals. In this test, the piece of iron or steel is held
against a revolving grinding stone, and the metal is identified
by the sparks thrown off. Each ferrous metal has its own
peculiar spark characteristics. The spark streams vary from
a few tiny shafts to a shower of sparks several feet in length.
(Few nonferrous metals give off sparks when touched to a
grinding stone. Therefore, these metals cannot be successfully
identified by the spark test.)
Identification by spark testing is often inexact unless
performed by an experienced person or the test pieces differ
greatly in their carbon content and alloying elements.
7-3Wrought iron produces long shafts that are straw colored as
they leave the stone and white at the end. Cast iron sparks
are red as they leave the stone and turn to a straw color.
Low carbon steels give off long, straight shafts having a few
white sprigs. As the carbon content of the steel increases, the
number of sprigs along each shaft increases and the stream
becomes whiter in color. Nickel steel causes the spark stream
to contain small white blocks of light within the main burst.
Types, Characteristics, and Uses of Alloyed Steels
Steel containing carbon in percentages ranging from 0.10 to
0.30 percent is classed as low carbon steel. The equivalent
SAE numbers range from 1010 to 1030. Steels of this grade
are used for making items, such as safety wire, certain nuts,
cable bushings, or threaded rod ends. This steel in sheet
form is used for secondary structural parts and clamps and
in tubular form for moderately stressed structural parts.
Steel containing carbon in percentages ranging from 0.30 to
0.50 percent is classed as medium carbon steel. This steel
is especially adaptable for machining or forging and where
surface hardness is desirable. Certain rod ends and light
forgings are made from SAE 1035 steel.
Steel containing carbon in percentages ranging from 0.50 to
1.05 percent is classed as high carbon steel. The addition of
other elements in varying quantities adds to the hardness of
this steel. In the fully heat-treated condition, it is very hard,
withstands high shear and wear, and has little deformation. It
has limited use in aircraft. SAE 1095 in sheet form is used for
making flat springs and in wire form for making coil springs.
The various nickel steels are produced by combining nickel
with carbon steel. Steels containing from 3 to 3.75 percent
nickels are commonly used. Nickel increases the hardness,
tensile strength, and elastic limit of steel without appreciably
decreasing the ductility. It also intensifies the hardening effect
of heat-treatment. SAE 2330 steel is used extensively for
aircraft parts, such as bolts, terminals, keys, clevises, and pins.
Chromium steel is high in hardness, strength, and corrosion-
resistant properties and is particularly adaptable for heat-treated
forgings, which require greater toughness and strength than
may be obtained in plain carbon steel. It can be used for articles
such as the balls and rollers of antifriction bearings. Chrome-
nickel or stainless steels are the corrosion resistant metals.
The anticorrosive degree of this steel is determined by the
surface condition of the metal, as well as by the composition,
temperature, and concentration of the corrosive agent. The
principal alloy of stainless steel is chromium. The corrosion
resistant steel most often used in aircraft construction is known
as 18-8 steel because its content is 18 percent chromium and
8 percent nickel. One of the distinctive features of 18-8 steel is that cold-working may increase its strength.
Stainless steel may be rolled, drawn, bent, or formed to any
shape. Because these steels expand about 50 percent more
than mild steel and conduct heat only about 40 percent as
rapidly, they are more difficult to weld. Stainless steel can be
used for almost any part of an aircraft. Some of its common
applications are the fabrication of exhaust collectors, stacks
and manifolds, structural and machined parts, springs,
castings, tie rods, and control cables.
The chrome-vanadium steels are made of approximately
18 percent vanadium and about 1 percent chromium. When
heat-treated, they have strength, toughness, and resistance to
wear and fatigue. A special grade of this steel in sheet form
can be cold formed into intricate shapes. It can be folded and
flattened without signs of breaking or failure. SAE 6150 is
used for making springs; chrome-vanadium with high carbon
content, SAE 6195, is used for ball and roller bearings.
Molybdenum in small percentages is used in combination
with chromium to form chrome-molybdenum steel, which
has various uses in aircraft. Molybdenum is a strong alloying
element. It raises the ultimate strength of steel without
affecting ductility or workability. Molybdenum steels are
tough and wear resistant, and they harden throughout when
heat-treated. They are especially adaptable for welding and,
for this reason, are used principally for welded structural
parts and assemblies. This type steel has practically replaced
carbon steel in the fabrication of fuselage tubing, engine
mounts, landing gears, and other structural parts. For example,
a heat-treated SAE X4130 tube is approximately four times
as strong as an SAE 1025 tube of the same weight and size.
A series of chrome-molybdenum steel most used in aircraft
construction is that series containing 0.25 to 0.55 percent
carbon, 0.15 to 0.25 percent molybdenum, and 0.50 to 1.10
percent chromium. These steels, when suitably heat-treated,
are deep hardening, easily machined, readily welded by either
gas or electric methods, and are especially adapted to high
temperature service.
Inconel is a nickel-chromium-iron alloy closely resembling
stainless steel (corrosion resistant steel (CRES)) in
appearance. Aircraft exhaust systems use both alloys
interchangeably. Because the two alloys look very much
alike, a distinguishing test is often necessary. One method
of identification is to use an electrochemical technique, as
described in the following paragraph, to identify the nickel
(Ni) content of the alloy. Inconel has nickel content greater
than 50 percent, and the electrochemical test detects nickel.
The tensile strength of Inconel is 100,000 pounds per square
7-4 Series Designation Types
10xx Non-sulfurized carbon steels
11xx Resulfurized carbon steels (free machining)
12xx Rephosphorized and resulfurized carbon steels (free machining)
13xx Manganese 1.75%
*23xx Nickel 3.50%
*25xx Nickel 5.00%
31xx Nickel 1.25%, chromium 0.65%
33xx Nickel 3.50%, chromium 1.55%
40xx Molybdenum 0.20 or 0.25%
41xx Chromium 0.50% or 0.95%, molybdenum 0.12 or 0.20%
43xx Nickel 1.80%, chromium 0.5 or 0.80%, molybdenum 0.25%
44xx Molybdenum 0.40%
45xx Molybdenum 0.52%
46xx Nickel 1.80%, molybdenum 0.25%
47xx Nickel 1.05% chromium 0.45%, molybdenum 0.20 or 0.35%
48xx Nickel 3.50%, molybdenum 0.25%
50xx Chromium 0.25, or 0.40 or 0.50%
50xxx Carbon 1.00%, chromium 0.50%
51xx Chromium 0.80, 0.90, 0.95 or 1.00%
51xxx Carbon 1.00%, chromium 1.05%
52xxx Carbon 1.00%, chromium 1.45%
61xx Chromium 0.60, 0.80, 0.95%, vanadium 0.12%, 0.10% min., or 0.15% min.
81xx Nickel 0.30%, chromium 0.40%, molybdenum 0.12%
86xx Nickel 0.55%, chromium 0.50%, molybdenum 0.20%
87xx Nickel 0.55%, chromium 0.05%, molybdenum 0.25%
88xx Nickel 0.55%, chromium 0.05%, molybdenum 0.35%
92xx Manganese 0.85%, silicon 2.00%, chromium 0 or 0.35%
93xx Nickel 3.25%, chromium 1.20%, molybdenum 0.12%
94xx Nickel 0.45%, chromium 0.40%, molybdenum 0.12%
98xx Nickel 1.00%, chromium 0.80%, molybdenum 0.25%
*Not included in the current list of standard steels
Figure 7-1. SAE numerical index.
inch (psi) annealed, and 125,000 psi when hard rolled. It is
highly resistant to salt water and can withstand temperatures
as high as 1,600 °F. Inconel welds readily and has working
qualities like those of corrosion resistant steels.Electrochemical Test
Prepare a wiring assembly as shown in Figure 7-2 , and prepare
the two reagents (ammonium fluoride and dimethylglyoxime
solutions) placing them in separate dedicated dropper solution
7-5Aluminum rod stock9v battery
− +LED
Alligator clip
Figure 7-2. Wiring assembly schematic.bottles. Before testing, you must thoroughly clean the metal for
the electrolytic deposit to take place. You may use nonmetallic
hand scrubbing pads or 320–600 grit “crocus cloth” to remove
deposits and corrosion products (thermal oxide).
Connect the alligator clip of the wiring assembly to the bare
metal being tested. Place one drop of a 0.05 percent reagent
grade ammonium fluoride solution in deionized water on
the center of a 1 inch × 1 inch sheet of filter paper. Lay the
moistened filter paper over the bare metal alloy being tested.
Firmly press the end of the aluminum rod over the center
of the moist paper. Maintain connection for 10 seconds
while rocking the aluminum rod on the filter paper. Ensure
that the light emitting diode (LED) remains lit (indicating
good electrical contact and current flow) during this period.
Disconnect the wiring assembly and set it aside. Remove
the filter paper and examine it to determine that a light spot
appears where the connection was made.
Deposit one drop of 1.0 percent solution of reagent grade
dimethylglyoxime in ethyl alcohol on the filter paper (same
side that was in contact with the test metal). A bright,
distinctly pink spot will appear within seconds on the filter
paper if the metal being tested is Inconel. A brown spot will
appear if the test metal is stainless steel. Some stainless-steel
alloys may leave a very light pink color. However, the shade
and depth of color will be far less than would appear for
Inconel. For flat surfaces, the test spot will be circular while
for curved surfaces, such as the outside of a tube or pipe,
the test spot may appear as a streak. (Refer to Figure 7-3 for
sample test results.) This procedure should not be used in the
heat-affected zone of weldments or on nickel coated surfaces.
Nonferrous Aircraft Metals
The term “nonferrous” refers to all metals that have elements
other than iron as its base or principal constituent. This
group includes metals, such as aluminum, titanium, copper,
and magnesium, as well as alloyed metals, such as Monel
and Babbitt.
Aluminum & Aluminum Alloys
Commercially pure aluminum is a white lustrous metal,
which stands second in the scale of malleability, sixth
in ductility, and ranks high in its resistance to corrosion.
Aluminum combined with various percentages of other
metals forms alloys, which are used in aircraft construction.
Aluminum alloys with principal alloying ingredients are
manganese, chromium, or magnesium and silicon show
little attack in corrosive environments. Alloys with which
substantial percentages of copper are more susceptible to
corrosive action. The total percentage of alloying elements
is seldom more than 6 or 7 percent in the wrought alloys.Aluminum is one of the most widely used metals in modern
aircraft construction. It is vital to the aviation industry because
of its high strength-to-weight ratio and its comparative ease
of fabrication. The outstanding characteristic of aluminum
is its lightweight. Aluminum melts at the comparatively low
temperature of 1,250 °F. It is nonmagnetic and is an excellent
conductor.
Commercially pure aluminum has a tensile strength of about
13,000 psi, but rolling or other cold-working processes
may approximately double its strength. By alloying with
other metals, or by using heat-treating processes, the tensile
strength may be raised to as high as 65,000 psi or to within
the strength range of structural steel.
Aluminum alloys, although strong, are easily worked because
they are malleable and ductile. They may be rolled into
sheets as thin as 0.0017 inch or drawn into wire 0.004 inch
in diameter. Most aluminum alloy sheet stock used in aircraft
construction range from 0.016 to 0.096 inch in thickness;
however, some of the larger aircraft use sheet stock that may
be as thick as 0.356 inch.
The various types of aluminum may be divided into two
general classes:
• Casting alloys (those suitable for casting in sand,
permanent mold, or die castings)
• Wrought alloys (those which may be shaped by rolling,
drawing, or forging).
Of these two, the wrought alloys are the most widely used
in aircraft construction, being used for stringers, bulkheads,
skin, rivets, and extruded sections.
Aluminum casting alloys are divided into two basic groups.
In one, the physical properties of the alloys are determined
by the alloying elements and cannot be changed after the
metal is cast. In the other, the alloying elements make it
Figure 7-3. Electrochemical test results of Inconel (In) and stainless
steel (SS) alloys.possible to heat-treat the casting to produce the desired
physical properties.
A letter preceding the alloy number identifies the casting
alloys. When a letter precedes a number, it indicates a slight
variation in the composition of the original alloy. This
variation in composition is simply to impart some desirable
quality. For example, in casting alloy 214, the addition of zinc
to improve its pouring qualities is indicated by the letter A
in front of the number, thus creating the designation A214.
When castings have been heat-treated, the heat-treatment
and the composition of the casting is indicated by the letter
T, followed by an alloying number. An example of this
is the sand casting alloy 355, which has several different
compositions and tempers and is designated by 355-T6,
355-T51, or C355-T51.
Aluminum alloy castings are produced by one of three basic
methods: sand mold, permanent mold, or die cast. In casting
aluminum, it is important to note that in most cases different
types of alloys must be used for different types of castings.
Sand castings and die-castings require different types of
alloys than those used in permanent molds.
Sand and permanent mold castings are parts produced by
pouring molten metal into a previously prepared mold,
allowing the metal to solidify or freeze and then removing the
part. If the mold is made of sand, the part is a sand casting; if
it is a metallic mold (usually cast iron), the part is a permanent
mold casting. Sand and permanent castings are produced by
pouring liquid metal into the mold, the metal flowing under
the force of gravity alone.
The two principal types of sand casting alloys are 112 and
212. Little difference exists between the two metals in
mechanical properties, since both are adaptable to a wide
range of products.
The permanent mold process is a later development of the
sand casting process, the major difference being in the material from which the molds are made. The advantage of
this process is that there are fewer openings (called porosity)
than in sand castings. The sand and the binder, which is mixed
with the sand to hold it together, give off a certain amount of
gas, that causes porosity in a sand casting.
Permanent mold castings are used to obtain higher
mechanical properties, better surfaces, or more accurate
dimensions. There are two specific types of permanent
mold castings: permanent metal mold with metal cores, and
semi-permanent types containing sand cores. Because finer
grain structure is produced in alloys subjected to the rapid
cooling of metal molds, they are far superior to the sand type
castings. Alloys 122, A132, and 142 are commonly used in
permanent mold castings, the principal uses of which are in
internal combustion engines.
Die-castings used in aircraft are usually aluminum or
magnesium alloy. If weight is of primary importance,
magnesium alloy is used, because it is lighter than aluminum
alloy. However, aluminum alloy is frequently used because
it is stronger than most magnesium alloys.
Forcing molten metal under pressure into a metallic die and
allowing it to solidify produces a die-casting; then the die is
opened and the part removed. The basic difference between
permanent mold casting and die-casting is that in the permanent
mold process, the metal flows into the die under gravity. In the
die-casting operation, the metal is forced under great pressure.
Die-castings are used where relatively large production of
a given part is involved. Remember, any shape that can be
forged, can be cast.
Wrought aluminum and wrought aluminum alloys are divided
into two general classes: non-heat-treatable alloys and heat-
treatable alloys.
Non-heat-treatable alloys are those in which the mechanical
properties are determined by the amount of cold-work
introduced after the final annealing operation. The mechanical
properties obtained by cold-working are destroyed by any
subsequent heating and cannot be restored except by
additional cold-working, which is not always possible. The
“full hard” temper is produced by the maximum amount of
cold-work that is commercially practicable. Metal in the “as
fabricated” condition is produced from the ingot without any
subsequent controlled amount of cold-working or thermal
treatment. There is, consequently, a variable amount of strain
hardening depending upon the thickness of the section.
For heat-treatable aluminum alloys, the mechanical properties
are obtained by heat-treating to a suitable temperature,
7-7holding at that temperature long enough to allow the alloying
constituent to enter into solid solution, and then quenching
to hold the constituent in solution. The metal is left in a
supersaturated, unstable state and is then age hardened either
by natural aging at room temperature or by artificial aging at
some elevated temperature.
Wrought Aluminum
Wrought aluminum and wrought aluminum alloys are
designated by a four-digit index system. The system is broken
into three distinct groups: 1xxx group, 2xxx through 8xxx
group, and 9xxx group (which is currently unused).
The first digit of a designation identifies the alloy type. The
second digit indicates specific alloy modifications. Should the
second number be zero, it would indicate no special control
over individual impurities. Digits 1 through 9, however, when
assigned consecutively as needed for the second number in
this group, indicate the number of controls over individual
impurities in the metal.
The last two digits of the 1xxx group are used to indicate
the hundredths of 1 percent above the original 99 percent
designated by the first digit. Thus, if the last two digits were
30, the alloy would contain 99 percent plus 0.30 percent of
pure aluminum, or a total of 99.30 percent pure aluminum.
Examples of alloys in this group are:
• 1100—99.00 percent pure aluminum with one control
over individual impurities.
• 1130—99.30 percent pure aluminum with one control
over individual impurities.
• 1275—99.75 percent pure aluminum with two controls
over individual impurities.
In the 2xxx through 8xxx groups, the first digit indicates the major
alloying element used in the formation of the alloy as follows:
• 2xxx—copper
• 3xxx—manganese
• 4xxx—silicon
• 5xxx—magnesium
• 6xxx—magnesium and silicon
• 7xxx—zinc
• 8xxx—other elements
In the 2xxx through 8xxx alloy groups, the second digit in
the alloy designation indicates alloy modifications. If the
second digit is zero, it indicates the original alloy, while digits
1 through 9 indicate alloy modifications. The last two of the
four digits in the designation identify the different alloys in the group. [Figure 7-4]
Effect of Alloying Element
1000 series: 99 percent aluminum or higher, excellent
corrosion resistance, high thermal and electrical conductivity,
low mechanical properties, excellent workability. Iron and
silicon are major impurities.
2000 series: Copper is the principal alloying element.
Solution heat-treatment, optimum properties equal to mild
steel, poor corrosion resistance unclad. It is usually clad
with 6000 or high purity alloy. Its best-known alloy is 2024.
3000 series: Manganese is the principal alloying element
of this group, which is generally non-heat-treatable. The
percentage of manganese that is alloy effective is 1.5 percent.
The most popular is 3003, which is of moderate strength and
has good working characteristics.
4000 series: Silicon is the principal alloying element of
this group and lowers melting temperature. Its primary
use is in welding and brazing. When used in welding heat-
treatable alloys, this group responds to a limited amount
of heat-treatment.
5000 series: Magnesium is the principal alloying element.
It has good welding and corrosion resistant characteristics.
High temperatures (over 150 °F) or excessive cold-working
increases susceptibility to corrosion.
6000 series: Silicon and magnesium form magnesium
silicide, which makes alloys heat-treatable. It is of medium
strength, good forming qualities, and has corrosion
resistant characteristics.
7000 series: Zinc is the principal alloying element. The
most popular alloy of the series is 6061. When coupled with
magnesium, it results in heat-treatable alloys of very high
strength. It usually has copper and chromium added. The
principal alloy of this group is 7075.
Hardness Identification
Where used, the temper designation follows the alloy
designation and is separated from it by a dash (i.e., 7075-
T6, 2024-T4, and so forth). The temper designation consists
of a letter indicating the basic temper, which may be more
specifically defined by the addition of one or more digits.
These designations are as follows:
• F—as fabricated
• O—annealed, recrystallized (wrought products only)
• H—strain hardened
7-8AlloyPercentage of Alloying Elements
Aluminum and normal impurities constitute remainder
1100 — — — — — — — — —
3003 — — 1.2 — — — — — —
2011 5.5 — — — — — — 0.5 0.5
2014 4.4 0.8 0.8 0.4 — — — — —
2017 4.0 — 0.5 0.5 — — — — —
2117 2.5 — — 0.3 — — — — —
2018 4.0 — — 0.5 — 2.0 — — —
2024 4.5 — 0.6 1.5 — — — — —
2025 4.5 0.8 0.8 — — — — — —
4032 0.9 12.5 — 1.0 — 0.9 — — —
6151 — 1.0 — 0.6 — — 0.25 — —
5052 — — — 2.5 — — 0.25 — —
6053 — 0.7 — 1.3 — — 0.25 — —
6061 0.25 0.6 — 1.0 — — 0.25 — —
7075 1.6 — — 2.5 5.6 — 0.3 — —Copper Silicon Manganese Magnesium Zinc Nickel Chromium Lead Bismuth
Figure 7-4. Nominal composition of wrought aluminum alloys.• H1 (plus one or more digits)—strain hardened only
• H2 (plus one or more digits)—strain hardened and
partially annealed
• H3 (plus one or more digits)—strain hardened and
stabilized
The digit following the designations H1, H2, and H3 indicates
the degree of strain hardening, number 8 representing the
ultimate tensile strength equal to that achieved by a cold
reduction of approximately 75 percent following a full anneal,
0 representing the annealed state.
Magnesium & Magnesium Alloys
Magnesium, the world’s lightest structural metal, is a silvery
white material weighing only two-thirds as much as aluminum.
Magnesium does not possess sufficient strength in its pure state
for structural uses, but when alloyed with zinc, aluminum, and
manganese, it produces an alloy having the highest strength-
to-weight ratio of any of the commonly used metals.
Magnesium is probably more widely distributed in nature
than any other metal. It can be obtained from such ores
as dolomite and magnesite, as well as from seawater,
underground brines, and waste solutions of potash. With
about 10 million pounds of magnesium in one cubic mile of seawater, there is no danger of a dwindling supply.
Some of today’s aircraft require more than one-half ton of this
metal for use in hundreds of vital spots. Some wing panels are
fabricated entirely from magnesium alloys, weigh 18 percent
less than standard aluminum panels, and have flown hundreds
of satisfactory hours. Among the aircraft parts that have been
made from magnesium with a substantial savings in weight
are nosewheel doors, flap cover skin, aileron cover skin, oil
tanks, floorings, fuselage parts, wingtips, engine nacelles,
instrument panels, radio masts, hydraulic fluid tanks, oxygen
bottle cases, ducts, and seats.
Magnesium alloys possess good casting characteristics. Their
properties compare favorably with those of cast aluminum.
In forging, hydraulic presses are ordinarily used, although,
under certain conditions, forging can be accomplished in
mechanical presses or with drop hammers.
Magnesium alloys are subject to such treatments as annealing,
quenching, solution heat-treatment, aging, and stabilizing.
Sheet and plate magnesium are annealed at the rolling mill.
The solution heat-treatment is used to put as much of the
alloying ingredients as possible into solid solution, which
results in high tensile strength and maximum ductility.
Aging is applied to castings following heat-treatment where
7-9and liners, and miscellaneous hardware for turbine engines.
Titanium, in appearance, is like stainless steel. One quick
method used to identify titanium is the spark test. Titanium
gives off a brilliant white trace ending in a brilliant white
burst. Also, moistening the titanium and using it to draw a
line on a piece of glass can accomplish identification. This
leaves a dark line similar in appearance to a pencil mark.
Titanium falls between aluminum and stainless steel in terms
of elasticity, density, and elevated temperature strength. It
has a melting point from 2,730 °F to 3,155 °F, low thermal
conductivity, and a low coefficient of expansion. It is light,
strong, and resistant to stress corrosion cracking. Titanium is
approximately 60 percent heavier than aluminum and about
50 percent lighter than stainless steel.
Because of the high melting point of titanium, high
temperature properties are disappointing. The ultimate
yield strength of titanium drops rapidly above 800 °F.
The absorption of oxygen and nitrogen from the air at
temperatures above 1,000 °F makes the metal so brittle on
long exposure that it soon becomes worthless. However,
titanium does have some merit for short time exposure up to
3,000 °F where strength is not important. Aircraft firewalls
demand this requirement.
Titanium is nonmagnetic and has an electrical resistance
comparable to that of stainless steel. Some of the base alloys
of titanium are quite hard. Heat-treating and alloying do not
develop the hardness of titanium to the high levels of some
of the heat-treated alloys of steel. It was only recently that
a heat-treatable titanium alloy was developed. Prior to the
development of this alloy, heating and rolling was the only
method of forming that could be accomplished. However, it
is possible to form the new alloy in the soft condition and
heat-treat it for hardness.
Iron, molybdenum, and chromium are used to stabilize
titanium and produce alloys that quench-harden and age-
harden. The addition of these metals also adds ductility.
The fatigue resistance of titanium is greater than that of
aluminum or steel.
Titanium becomes softer as the degree of purity is increased.
It is not practical to distinguish between the various grades of
commercially pure or unalloyed titanium by chemical analysis;
therefore, the grades are determined by mechanical properties.
Titanium Designations
The A-B-C classification of titanium alloys was established
to provide a convenient and simple means of describing all
titanium alloys. Titanium and titanium alloys possess three maximum hardness and yield strength are desired.
Magnesium embodies fire hazards of an unpredictable nature.
When in large sections, its high thermal conductivity makes
it difficult to ignite and prevents it from burning. It does not
burn until the melting point of 1,204 °F is reached. However,
magnesium dust and fine chips are ignited easily. Precautions
must be taken to avoid this if possible. Should a fire occur, it
could be extinguished with an extinguishing powder, such as
soapstone or graphite. Water or any standard liquid or foam
fire extinguisher causes magnesium to burn more rapidly and
can cause explosions.
Magnesium alloys produced in the United States contain
varying proportions of aluminum, manganese, and zinc. A
letter of the alphabet designates these alloys, with the number
1 indicating high purity and maximum corrosion resistance.
Many of the magnesium alloys manufactured in the United
States are produced by the Dow Chemical Company and
have been given the trade name of Dow-metal™ alloys. To
distinguish between these alloys, each is assigned a letter.
Thus, we have Dow-metal™ J, Dow-metal™ M, and so forth.
Another manufacturer of magnesium alloys is the American
Magnesium Corporation, a subsidiary of the Aluminum
Company of America. This company uses an identification
system like that used for aluminum alloys, with the exception
that magnesium alloy numbers are preceded with the letters
AM. Thus, AM240C is a cast alloy, and AM240C4 is the
same alloy in the heat-treated state. AM3S0 is an annealed
wrought alloy, and AM3SRT is the same alloy rolled after
heat-treatment.
Titanium and Titanium Alloys
An English priest named Gregot discovered titanium. A crude
separation of titanium ore was accomplished in 1825. In 1906,
enough pure titanium was isolated in metallic form to permit
a study. Following this study, in 1932, an extraction process
was developed and became the first commercial method
for producing titanium. The United States Bureau of Mines
began making titanium sponge in 1946, and 4 years later the
melting process began.
The use of titanium is widespread. It is used in many
commercial enterprises and is in constant demand for such
items as pumps, screens, and other tools and fixtures where
corrosion attack is prevalent. In aircraft construction and
repair, titanium is used for fuselage skins, engine shrouds,
firewalls, longerons, frames, fittings, air ducts, and fasteners.
Titanium is used for making compressor disks, spacer rings,
compressor blades and vanes, through bolts, turbine housings
7-10currents on the surface of titanium and metallic couples are
naturally restricted. This partly accounts for good resistance
to many chemicals; also, the material may be used with some
dissimilar metals with no harmful galvanic effect on either.
Copper and Copper Alloys
Copper is one of the most widely distributed metals. It is
the only reddish-colored metal and is second only to silver
in electrical conductivity. Its use as a structural material is
limited because of its great weight. However, some of its
outstanding characteristics, such as its high electrical and heat
conductivity, in many cases overbalance the weight factor.
Because it is very malleable and ductile, copper is ideal for
making wire. It is corroded by salt water but is not affected
by fresh water. The ultimate tensile strength of copper varies
greatly. For cast copper, the tensile strength is about 25,000
psi, and when cold rolled or cold drawn, its tensile strength
increases to a range of 40,000 to 67,000 psi.
In aircraft, copper is used primarily in the electrical system
for bus bars, bonding, and as lock wire.
Beryllium copper is one of the most successful of all
the copper base alloys. It is a recently developed alloy
containing about 97 percent copper, 2 percent beryllium, and
sufficient nickel to increase the percentage of elongation.
The most valuable feature of this metal is that the physical
properties can be greatly stepped up by heat-treatment, the
tensile strength rising from 70,000 psi in the annealed state
to 200,000 psi in the heat-treated state. The resistance of
beryllium copper to fatigue and wear makes it suitable for
diaphragms, precision bearings and bushings, ball cages,
and spring washers.
Brass is a copper alloy containing zinc and small amounts
of aluminum, iron, lead, manganese, magnesium, nickel,
phosphorous, and tin. Brass with a zinc content of 30 to 35
percent is very ductile, but that containing 45 percent has
relatively high strength.
Muntz metal is a brass composed of 60 percent copper and 40
percent zinc. It has excellent corrosion-resistant qualities in
salt water. Its strength can be increased by heat-treatment. As
cast, this metal has an ultimate tensile strength of 50,000 psi,
and it can be elongated 18 percent. It is used in making bolts
and nuts, as well as parts that come in contact with salt water.
Red brass, sometimes termed “bronze” because of its tin
content, is used in fuel and oil line fittings. This metal has
good casting and finishing properties and machines freely.
Bronzes are copper alloys containing tin. The true bronzes have
up to 25 percent tin, but those with less than 11 percent are basic types of crystals: A (alpha), B (beta), and C (combined
alpha and beta). Their characteristics are:
• A (alpha)—all-around performance; good weld ability;
tough and strong both cold and hot; and resistant to
oxidation.
• B (beta)—bendability; excellent bend ductility; strong
both cold and hot, but vulnerable to contamination.
• C (combined alpha and beta for compromise
performances)—strong when cold and warm, but weak
when hot; good bendability; moderate contamination
resistance; excellent forge ability.
Titanium is manufactured for commercial use in two basic
compositions: commercially-pure titanium and alloyed
titanium. A-55 is an example of commercially-pure titanium.
It has yield strength of 55,000 to 80,000 psi and is a general-
purpose grade for moderate to severe forming. It is sometimes
used for nonstructural aircraft parts and for all types of
corrosion-resistant applications, such as tubing. Type A-70
titanium is closely related to type A-55 but has yield strength
of 70,000 to 95,000 psi. It is used where higher strength is
required, and it is specified for many moderately stressed
aircraft parts. For many corrosion applications, it is used
interchangeably with type A-55. Both type A-55 and type
A-70 is weldable.
One of the widely-used titanium base alloys is designated
as C-110M. It is used for primary structural members and
aircraft skin, has 110,000 psi minimum yield strength, and
contains 8 percent manganese.
Type A-110AT is a titanium alloy that contains 5 percent
aluminum and 2.5 percent tin. It also has high minimum yield
strength at elevated temperatures with the excellent welding
characteristics inherent in alpha-type titanium alloys.
Corrosion Characteristics
The corrosion resistance of titanium deserves special
mention. The resistance of the metal to corrosion is caused
by the formation of a protective surface film of stable oxide
or chemi-absorbed oxygen. Film is often produced by the
presence of oxygen and oxidizing agents.
Corrosion of titanium is uniform. There is little evidence of
pitting or other serious forms of localized attack. Normally,
it is not subject to stress corrosion, corrosion fatigue,
intergranular corrosion, or galvanic corrosion. Its corrosion
resistance is equal or superior to 18-8 stainless steel.
Laboratory tests with acid and saline solutions show titanium
polarizes readily. The net effect, in general, is to decrease
current flow in galvanic and corrosion cells. Corrosion
7-11most useful, especially for such items as tube fittings in aircraft.
Among the copper alloys are the copper aluminum alloys,
of which the aluminum bronzes rank very high in aircraft
usage. They would find greater usefulness in structures if
it were not for their strength-to-weight ratio as compared
with alloy steels. Wrought aluminum bronzes are almost as
strong and ductile as medium carbon steel, and they possess
a high degree of resistance to corrosion by air, salt water, and
chemicals. They are readily forged, hot or cold rolled, and
many react to heat-treatment.
These copper base alloys contain up to 16 percent of aluminum
(usually 5 to 11 percent), to which other metals, such as iron,
nickel, or manganese, may be added. Aluminum bronzes have
good tearing qualities, great strength, hardness, and resistance
to both shock and fatigue. Because of these properties, they are
used for diaphragms, gears, and pumps. Aluminum bronzes
are available in rods, bars, plates, sheets, strips, and forgings.
Cast aluminum bronzes, using about 89 percent copper, 9
percent aluminum, and 2 percent of other elements, have
high strength combined with ductility and are resistant to
corrosion, shock, and fatigue. Because of these properties,
cast aluminum bronze is used in bearings and pump parts.
These alloys are useful in areas exposed to salt water and
corrosive gases.
Manganese bronze is an exceptionally high strength, tough,
corrosion-resistant copper zinc alloy containing aluminum,
manganese, iron, and occasionally, nickel or tin. This metal
can be formed, extruded, drawn, or rolled to any desired
shape. In rod form, it is generally used for machined parts
for aircraft landing gears and brackets.
Silicon bronze is a more recent development composed of
about 95 percent copper, 3 percent silicon, and 2 percent
manganese, zinc, iron, tin, and aluminum. Although not a
bronze in the true sense because of its small tin content, silicon
bronze has high strength and great corrosion resistance.
Monel
Monel, the leading high nickel alloy, combines the properties
of high strength and excellent corrosion resistance. This metal
consists of 68 percent nickel, 29 percent copper, 0.2 percent
iron, 1 percent manganese, and 1.8 percent of other elements.
It cannot be hardened by heat-treatment.
Monel, adaptable to casting and hot or cold-working, can be
successfully welded. It has working properties like those of
steel. When forged and annealed, it has a tensile strength of
80,000 psi. This can be increased by cold-working to 125,000
psi, sufficient for classification among the tough alloys.Monel has been successfully used for gears and chains to
operate retractable landing gears and for structural parts subject
to corrosion. In aircraft, Monel is used for parts demanding
both strength and high resistance to corrosion, such as exhaust
manifolds and carburetor needle valves and sleeves.
K-Monel
K-Monel is a nonferrous alloy containing mainly nickel,
copper, and aluminum. Adding a small amount of aluminum
to the Monel formula produces it. It is corrosion resistant and
capable of being hardened by heat-treatment.
K-Monel has been successfully used for gears and structural
members in aircraft, which are subjected to corrosive
attacks. This alloy is nonmagnetic at all temperatures. Both
oxyacetylene and electric arc welding have successfully
welded K-Monel sheet.
Nickel & Nickel Alloys
There are basically two nickel alloys used in aircraft: Monel
and Inconel. Monel contains about 68 percent nickel and 29
percent copper, plus small amounts of iron and manganese.
Nickel alloys can be welded or easily machined. Some of the
nickel Monel, especially the nickel Monels containing small
amounts of aluminum, are heat-treatable to similar tensile
strengths of steel. Nickel Monel is used in gears and parts
that require high strength and toughness, such as exhaust
systems that require high strength and corrosion resistance
at elevated temperatures.
Inconel alloys of nickel produce a high strength, high
temperature alloy containing approximately 80 percent
nickel, 14 percent chromium, and small amounts of iron
and other elements. The nickel Inconel alloys are frequently
used in turbine engines because of their ability to maintain
their strength and corrosion resistance under extremely high-
temperature conditions.
Inconel and stainless steel are similar in appearance and are
frequently found in the same areas of the engine. Sometimes
it is important to identify the difference between the metal
samples. A common test is to apply one drop of cupric
chloride and hydrochloric acid solution to the unknown metal
and allow it to remain for 2 minutes. At the end of the soak
period, a shiny spot indicates the material is nickel Inconel,
and a copper-colored spot indicates stainless steel.
Substitution of Aircraft Metals
In selecting substitute metals for the repair and maintenance
of aircraft, it is very important to check the appropriate
structural repair manual. Aircraft manufacturers design
structural members to meet a specific load requirement for an
7-12aircraft. The methods of repairing these members, apparently
similar in construction, vary with different aircraft.
Four requirements must be kept in mind when selecting
substitute metals. The first and most important of these is
maintaining the original strength of the structure. The other
three are maintaining contour or aerodynamic smoothness;
maintaining original weight, if possible, or keeping added
weight to a minimum; and maintaining the original corrosion-
resistant properties of the metal.
Metalworking Processes
There are three methods of metalworking: hot-working,
cold-working, and extruding. The method used depends on
the metal involved and the part required, although in some
instances both hot and cold-working methods may be used
to make a single part.
Hot-Working
Almost all steel is hot-worked from the ingot into some form
from which it is either hot or cold-worked to the finished
shape. When an ingot is stripped from its mold, its surface is
solid, but the interior is still molten. The ingot is then placed
in a soaking pit, which retards loss of heat, and the molten
interior gradually solidifies. After soaking, the temperature
is equalized throughout the ingot, then it is reduced to
intermediate size by rolling, making it more readily handled.
The rolled shape is called a bloom when its section
dimensions are 6 inches × 6 inches or larger and square. The
section is called a billet when it is square and less than 6
inches × 6 inches. Rectangular sections, which have a width
greater than twice their thickness, are called slabs. The slab
is the intermediate shape from which sheets are rolled.
Blooms, billets, or slabs are heated above the critical range
and rolled into a variety of shapes of uniform cross section.
Common rolled shapes are sheet, bar, channel, angle, and
I-beam. As discussed later in this chapter, hot-rolled material
is frequently finished by cold rolling or drawing to obtain
accurate finish dimensions and a bright, smooth surface.
Complicated sections, which cannot be rolled, or sections of
which only a small quantity is required, are usually forged.
Forging of steel is a mechanical working at temperatures
above the critical range to shape the metal as desired. Forging
is done either by pressing or hammering the heated steel until
the desired shape is obtained.
Pressing is used when the parts to be forged are large and
heavy; this process also replaces hammering where high-
grade steel is required. Since a press is slow acting, its force
is uniformly transmitted to the center of the section, thus affecting the interior grain structure, as well as the exterior
to give the best possible structure throughout.
Hammering can be used only on relatively small pieces. Since
hammering transmits its force almost instantly, its effect is
limited to a small depth. Thus, it is necessary to use a very
heavy hammer or to subject the part to repeated blows to
ensure complete working of the section. If the force applied
is too weak to reach the center, the finished forged surface
is concave. If the center was properly worked, the surface is
convex or bulged. The advantage of hammering is that the
operator has control over both the amount of pressure applied
and the finishing temperature and can produce small parts
of the highest grade. This type of forging is usually referred
to as smith forging. It is used extensively where only a
small number of parts are needed. Considerable machining
time and material are saved when a part is smith forged to
approximately the finished shape.
Steel is often harder than necessary and too brittle for most
practical uses when put under severe internal strain. To relieve
such strain and reduce brittleness, it is tempered after being
hardened. This consists of heating the steel in a furnace to a
specified temperature and then cooling it in air, oil, water, or
a special solution. Temper condition refers to the condition of
metal or metal alloys with respect to hardness or toughness.
Rolling, hammering, or bending these alloys, or heat-treating
and aging them, causes them to become tougher and harder.
At times, these alloys become too hard for forming and must
be re-heat-treated or annealed.
Metals are annealed to relieve internal stresses, soften the metal,
make it more ductile, and refine the grain structure. Annealing
consists of heating the metal to a prescribed temperature,
holding it there for a specified length of time, and then cooling
the metal back to room temperature. To produce maximum
softness, the metal must be cooled very slowly. Some metals
must be furnace cooled; others may be cooled in air.
Normalizing applies to iron base metals only. Normalizing
consists of heating the part to the proper temperature, holding it
at that temperature until it is uniformly heated, and then cooling
it in still air. Normalizing is used to relieve stresses in metals.
Strength, weight, and reliability are three factors that
determine the requirements to be met by any material used in
airframe construction and repair. Airframes must be strong and
yet as lightweight as possible. There are very definite limits to
which increases in strength can be accompanied by increases
in weight. An airframe so heavy that it could not support a few
hundred pounds of additional weight would be of little use.
All metals, in addition to having a good strength-to-
7-13weight ratio, must be thoroughly reliable, thus minimizing
the possibility of dangerous and unexpected failures. In
addition to these general properties, the material selected
for a definite application must possess specific qualities
suitable for the purpose.
The material must possess the strength required by the
dimensions, weight, and use. The five basic stresses that
metals may be required to withstand are tension, compression,
shear, bending, and torsion.
The tensile strength of a material is its resistance to a force,
which tends to pull it apart. Tensile strength is measured in
pounds per square inch (psi) and is calculated by dividing
the load in pounds required to pull the material apart by its
cross-sectional area in square inches.
The compression strength of a material is its resistance to
a crushing force, which is the opposite of tensile strength.
Compression strength is also measured in psi. When a piece
of metal is cut, the material is subjected, as it comes in contact
with the cutting edge, to a force known as shear. Shear is the
tendency on the part of parallel members to slide in opposite
directions. It is like placing a cord or thread between the
blades of a pair of scissors (shears). The shear strength is
the shear force in psi at which a material fails. It is the load
divided by the shear area.
Bending can be described as the deflection or curving of a
member due to forces acting upon it. The bending strength of
material is the resistance it offers to deflecting forces. Torsion
is a twisting force. Such action would occur in a member fixed
at one end and twisted at the other. The torsional strength of
material is its resistance to twisting.
The relationship between the strength of a material and its
weight per cubic inch, expressed as a ratio, is known as
the strength-to-weight ratio. This ratio forms the basis for
comparing the desirability of various materials for use in
airframe construction and repair. Neither strength nor weight
alone can be used as a means of true comparison. In some
applications, such as the skin of monocoque structures,
thickness is more important than strength. In this instance,
the material with the lightest weight for a given thickness
or gauge is best. Thickness or bulk is necessary to prevent
bucking or damage caused by careless handling.
Corrosion is the eating away or pitting of the surface or the
internal structure of metals. Because of the thin sections and
the safety factors used in aircraft design and construction,
it would be dangerous to select a material possessing poor
corrosion-resistant characteristics.Another significant factor to consider in maintenance and
repair is the ability of a material to be formed, bent, or
machined to required shapes. The hardening of metals by
cold-working or forming is termed work hardening. If a piece
of metal is formed (shaped or bent) while cold, it is said to be
cold-worked. Practically all the work an aviation mechanic
does on metal is cold-work. While this is convenient, it causes
the metal to become harder and more brittle.
If the metal is cold-worked too much, that is, if it is bent
back and forth or hammered at the same place too often, it
will crack or break. Usually, the more malleable and ductile
a metal is, the more cold-working it can stand. Any process
that involves controlled heating and cooling of metals to
develop certain desirable characteristics (such as hardness,
softness, ductility, tensile strength, or refined grain structure)
is called heat-treatment or heat-treating. With steels, the term
“heat-treating” has a broad meaning and includes processes
such as annealing, normalizing, hardening, and tempering.
In the heat-treatment of aluminum alloys, only two processes
are included: the hardening and toughening process and the
softening process. The hardening and toughening process
is called heat-treating, and the softening process is called
annealing. Aircraft metals are subjected to both shock and
fatigue (vibrational) stresses. Fatigue occurs in materials that
are exposed to frequent reversals of loading or repeatedly
applied loads, if the fatigue limit is reached or exceeded.
Repeated vibration or bending ultimately causes a minute crack
to occur at the weakest point. As vibration or bending continues,
the crack lengthens until the part completely fails. This is
termed “shock and fatigue failure.” Resistance to this condition
is known as shock and fatigue resistance. It is essential that
materials used for critical parts be resistant to these stresses.
Heat-treatment is a series of operations involving the heating
and cooling of metals in the solid state. Its purpose is to
change a mechanical property, or combination of mechanical
properties, so that the metal is more useful, serviceable, and
safe for a definite purpose. By heat-treating, a metal can be
made harder, stronger, and more resistant to impact. Heat-
treating can also make a metal softer and more ductile. No one
heat-treating operation can produce all these characteristics.
In fact, some properties are often improved at the expense
of others. In being hardened, for example, a metal may
become brittle.
The various heat-treating processes are similar in that they
all involve the heating and cooling of metals. They differ,
however, in the temperatures to which the metal is heated,
the rate at which it is cooled, and, of course, in the result.
The most common forms of heat-treatment for ferrous metals
7-14are hardening, tempering, normalizing, annealing, and case
hardening. Most nonferrous metals can be annealed and
many of them can be hardened by heat-treatment. However,
there is only one nonferrous metal, titanium, that can be case
hardened, and none can be tempered or normalized.
Internal Structure of Metals
The results obtained by heat-treatment depend on the
structure of the metal and on the way the structure changes
when the metal is heated and cooled. A pure metal cannot
be hardened by heat-treatment, because there is little change
in its structure when heated. On the other hand, most alloys
respond to heat-treatment since their structures change with
heating and cooling.
An alloy may be in the form of a solid solution, a mechanical
mixture, or a combination of a solid solution and a mechanical
mixture. When an alloy is in the form of a solid solution, the
elements and compounds that form the alloy are absorbed,
one into the other, in much the same way that salt is dissolved
in a glass of water, and the constituents cannot be identified
even under a microscope.
When two or more elements or compounds are mixed but
can be identified by microscopic examination, a mechanical
mixture is formed. A mechanical mixture can be compared to
the mixture of sand and gravel in concrete. The sand and gravel
are both visible. Just as the sand and gravel are held together and
kept in place by the matrix of cement, the other constituents of
an alloy are embedded in the matrix formed by the base metal.
An alloy in the form of a mechanical mixture at ordinary
temperatures may change to a solid solution when heated.
When cooled back to normal temperature, the alloy may
return to its original structure. On the other hand, it may
remain a solid solution or form a combination of a solid
solution and mechanical mixture. An alloy, which consists
of a combination of solid solution and mechanical mixture
at normal temperatures, may change to a solid solution when
heated. When cooled, the alloy may remain a solid solution,
return to its original structure, or form a complex solution.
Heat-Treating Equipment
Successful heat-treating requires close control over all factors
affecting the heating and cooling of metals. Such control is
possible only when the proper equipment is available and
the equipment is selected to fit the job. Thus, the furnace
must be of the proper size and type and must be controlled
so that temperatures are kept within the limits prescribed
for each operation. Even the atmosphere within the furnace
affects the condition of the part being heat-treated. Further,
the quenching equipment and the quenching medium must
be selected to fit the metal and the heat-treating operation. Finally, there must be equipment for handling parts and
materials, for cleaning metals, and for straightening parts.
Furnaces & Salt Baths
There are many different types and sizes of furnaces used in
heat-treatment. As a rule, furnaces are designed to operate in
certain specific temperature ranges and attempted use in other
ranges frequently results in work of inferior quality.
In addition, using a furnace beyond its rated maximum
temperature shortens its life and may necessitate costly and
time-consuming repairs.
Fuel-fired furnaces (gas or oil) require air for proper
combustion, and an air compressor or blower is therefore
necessary. These furnaces are usually of the muffler type;
that is, the combustion of the fuel takes place outside of and
around the chamber in which the work is placed. If an open
muffler is used, the furnace should be designed to prevent
the direct impingement of flame on the work.
In furnaces heated by electricity, the heating elements are
generally in the form of wire or ribbon. Good design requires
incorporation of additional heating elements at locations
where maximum heat loss may be expected. Such furnaces
commonly operate at up to a maximum temperature of about
2,000 °F. Furnaces operating at temperatures up to about
2,500 °F usually employ resistor bars of sintered carbides.
Temperature Measurement and Control
A thermoelectric instrument, known as a pyrometer, measures
temperature in the heat-treating furnace. This instrument
measures the electrical effect of a thermocouple and, hence,
the temperature of the metal being treated. A complete
pyrometer consists of three parts: a thermocouple, extension
leads, and meter.
Furnaces intended primarily for tempering may be heated by
gas or electricity and are frequently equipped with a fan for
circulating the hot air.
Salt baths are available for operating at either tempering or
hardening temperatures. Depending on the composition of the
salt bath, heating can be conducted at temperatures as low as
325 °F to as high as 2,450 °F. Lead baths can be used in the
temperature range of 650 °F to 1,700 °F. The rate of heating
in lead or salt baths is much faster in furnaces.
Heat-treating furnaces differ in size, shape, capacity,
construction, operation, and control. They may be circular
or rectangular and may rest on pedestals or directly on the
floor. There are also pit-type furnaces, which are below the
surface of the floor. When metal is to be heated in a bath of
7-15molten salt or lead, the furnace must contain a pot or crucible
for the molten bath.
The size and capacity of a heat-treating furnace depends
on the intended use. A furnace must be capable of heating
rapidly and uniformly, regardless of the desired maximum
temperature or the mass of the charge. An oven-type furnace
should have a working space (hearth) about twice as long and
three times as wide as any part that is heated in the furnace.
Accurate temperature measurement is essential to good heat-
treating. The usual method is by means of thermocouples: the
most common base metal couples are copper-constantan (up
to about 700 °F), iron-constantan (up to about 1,400 °F), and
chromel-alumel (up to about 2,200 °F). The most common
noble metal couples (which can be used up to about 2,800 °F)
are platinum coupled with either the alloy 87 percent platinum
(13 percent rhodium) or the alloy 90 percent platinum (10
percent rhodium). The temperatures quoted are for continuous
operation.
The life of thermocouples is affected by the maximum
temperature (which may frequently exceed those given
above) and by the furnace atmosphere. Iron-constantan
is more suited for use in reducing and chromel-alumel in
oxidizing atmospheres. Thermocouples are usually encased
in metallic or ceramic tubes closed at the hot end to protect
them from the furnace gases. A necessary attachment is
an instrument, such as a millivoltmeter or potentiometer,
for measuring the electromotive force generated by the
thermocouple. In the interest of accurate control, place the
hot junction of the thermocouple as close to the work as
possible. The use of an automatic controller is valuable in
controlling the temperature at the desired value.
Pyrometers may have meters either of the indicating type
or recording type. Indicating pyrometers give direct reading
of the furnace temperature. The recording type produces a
permanent record of the temperature range throughout the
heating operation by means of an inked stylus attached to
an arm, which traces a line on a sheet of calibrated paper or
temperature chart.
Pyrometer installations on all modern furnaces provide
automatic regulation of the temperature at any desired setting.
Instruments of this type are called controlling potentiometer
pyrometers. They include a current regulator and an operating
mechanism, such as a relay.
Heating
The object in heating is to transform pearlite (a mixture of
alternate strips of ferrite and iron carbide in a single grain) to
austenite as the steel is heated through the critical range. Since this transition takes time, a relatively slow rate of heating
must be used. Ordinarily, the cold steel is inserted when the
temperature in the furnace is from 300 °F to 500 °F below
the hardening temperature. In this way, too rapid heating
through the critical range is prevented.
If temperature-measuring equipment is not available, it
becomes necessary to estimate temperatures by some other
means. An inexpensive, yet accurate method involves the
use of commercial crayons, pellets, or paints that melt at
various temperatures within the range of 125 °F to 1,600 °F.
The least accurate method of temperature estimation is by
observation of the color of the hot hearth of the furnace or
of the work. The heat colors observed are affected by many
factors, such as the conditions of artificial or natural light, the
character of the scale on the work, and so forth. Steel begins
to appear dull red at about 1,000 °F, and as the temperature
increases, the color changes gradually through various shades
of red to orange, to yellow, and finally to white. A rough
approximation of the correspondence between color and
temperature is indicated in Figure 7-5 .
It is also possible to secure some idea of the temperature of
a piece of carbon or low alloy steel, in the low temperature
range used for tempering, from the color of the thin oxide film
that forms on the cleaned surface of the steel when heated in
this range. The approximate temperature/color relationship
is indicated on the lower portion of the scale in Figure 7-5 .
It is often necessary or desirable to protect steel or cast iron
from surface oxidation (scaling) and loss of carbon from
the surface layers (decarburization). Commercial furnaces,
therefore, are generally equipped with some means of
atmosphere control. This usually is in the form of a burner for
burning controlled amounts of gas and air and directing the
products of combustion into the furnace muffle. Water vapor, a
product of this combustion, is detrimental and many furnaces
are equipped with a means for eliminating it. For furnaces
not equipped with atmosphere control, a variety of external
atmosphere generators are available. The gas so generated is
piped into the furnace and one generator may supply several
furnaces. If no method of atmosphere control is available,
some degree of protection may be secured by covering the
work with cast iron borings or chips. Since the liquid heating
medium surrounds the work in salt or lead baths, the problem
of preventing scaling or decarburization is simplified. Vacuum
furnaces also are used for annealing steels, especially when a
bright non-oxidized surface is a prime consideration.
Soaking
The temperature of the furnace must be held constant
during the soaking period, since it is during this period that
rearrangement of the internal structure of the steel takes place.
7-16Soaking temperatures for various types of steel are specified
in ranges varying as much as 100 °F. [Figure 7-6] Small parts
are soaked in the lower part of the specified range and heavy
parts in the upper part of the specified range. The length of
the soaking period depends upon the type of steel and the size
of the part. Naturally, heavier parts require longer soaking to
ensure equal heating throughout. As a general rule, a soaking
period of 30 minutes to 1 hour is sufficient for the average
heat-treating operation.
Cooling
The rate of cooling through the critical range determines the
form that the steel retains. Various rates of cooling are used to
produce the desired results. Still air is a slow cooling medium
but is much faster than furnace cooling. Liquids are the fastest
cooling media and are therefore used in hardening steels.
There are three commonly used quenching liquids: brine,
water, and oil. Brine is the strongest quenching medium,
water is next, and oil is the least. Generally, an oil quench
is used for alloy steels and brine or water for carbon steels.
Quenching Media
Quenching solutions act only through their ability to cool
the steel. They have no beneficial chemical action on
the quenched steel and in themselves impart no unusual
properties. Most requirements for quenching media are met
satisfactorily by water or aqueous solutions of inorganic salts,
such as table salt or caustic soda, or by some type of oil. The
rate of cooling is relatively rapid during quenching in brine,
somewhat less rapid in water, and slow in oil.
Brine usually is made of a 5 to 10 percent solution of salt
(sodium chloride) in water. In addition to its greater cooling
speed, brine has the ability to “throw” the scale from steel
during quenching. Their temperature considerably affects the
cooling ability of both water and brine, particularly water.
Both should be kept cold—well below 60 °F. If the volume
of steel being quenched tends to raise the temperature of the
bath appreciably, add ice or use some means of refrigeration
to cool the quenching bath.
There are many specially prepared quenching oils on the
market; their cooling rates do not vary widely. A straight
mineral oil with a Saybolt viscosity of about 100 at 100 °F
is generally used. Unlike brine and water, the oils have the
greatest cooling velocity at a slightly elevated temperature—
about 100–140 °F—because of their decreased viscosity at
these temperatures.
When steel is quenched, the liquid in immediate contact with
the hot surface vaporizes; this vapor reduces the rate of heat abstraction markedly. Vigorous agitation of the steel or the
use of a pressure spray quench is necessary to dislodge these
vapor films and thus permit the desired rate of cooling.
The tendency of steel to warp and crack during the quenching
process is difficult to overcome because certain parts of
the article cool more rapidly than others. The following
recommendations greatly reduce the warping tendency.
1. Never throw a part into the quenching bath. By
permitting it to lie on the bottom of the bath, it is apt
to cool faster on the topside than on the bottom side,
thus causing it to warp or crack.
2. Agitate the part slightly to destroy the coating of
vapor that could prevent it from cooling evenly and
rapidly. This allows the bath to dissipate its heat to the
atmosphere.
3. Immerse irregular shaped parts so that the heavy end
enters the bath first.
Quenching Equipment
The quenching tank should be of the proper size to handle
the material being quenched. Use circulating pumps and
coolers to maintain approximately constant temperatures
when doing a large amount of quenching. To avoid building
up a high concentration of salt in the quenching tank, make
provisions for adding fresh water to the quench tank used
for molten salt baths.
Tank location in reference to the heat-treating furnace is very
important. Situate the tank to permit rapid transfer of the part
from the furnace to the quenching medium. A delay of more
than a few seconds, in many instances, proves detrimental to
the effectiveness of the heat-treatment. During transfer to the
quench tank, employ guard sheets to retard the loss of heat
when heat-treating material of thin section. Provide a rinse
tank to remove all salt from the material after quenching if
the salt is not adequately removed in the quenching tank.
Heat-Treatment of Ferrous Metals
The first important consideration in the heat-treatment of a
steel part is to know its chemical composition. This, in turn,
determines its upper critical point. When the upper critical
point is known, the next consideration is the rate of heating
and cooling to be used. Carrying out these operations involves
the use of uniform heating furnaces, proper temperature
controls, and suitable quenching mediums.
Behavior of Steel During Heating & Cooling
Changing the internal structure of a ferrous metal is
accomplished by heating to a temperature above its upper
critical point, holding it at that temperature for a time
sufficient to permit certain internal changes to occur, and then
°Fahrenheit °Centigrade
°F °C
°F °CColor of Hot Body
Temper Colors2,700
2,600
2,500
2,400
2,300
2,200
2,100
2,000
1,900
1,800
1,700
1,600
1,500
1,400
1,300
1,200
1,100
1,000
1001,500
1,400
1,300
1,200
1,100
1,000
0Straw 220 430Dark Blue 290 550
Purple 270 520
Y ellow Brown 250 460White 1200 2192
Light Y ellow 1100 2012
Y ellow 1050 1922
Light Orange 980 1796
Orange 930 1706
Light Red 870 1598
810 Light cherry 1490
Cherry 760 1400
Dark Cherry 700 1292
Blood Red 650 1202
Brown Red 600 1112Approximate Temperature Colors
Figure 7-5. Temperature chart indicating conversion of Centigrade
to Fahrenheit or vice versa, color temperature scale for hardening
temperature range, and tempering temperature range.cooling to atmospheric temperature under predetermined,
controlled conditions.
At ordinary temperatures, the carbon in steel exists in the
form of particles of iron carbide scattered throughout an iron
matrix known as “ferrite.” The number, size, and distribution
of these particles determine the hardness of the steel. At
elevated temperatures, the carbon is dissolved in the iron
matrix in the form of a solid solution called “austenite,”
and the carbide particles appear only after the steel has
been cooled. If the cooling is slow, the carbide particles are
relatively coarse and few. In this condition, the steel is soft.
If the cooling is rapid, as by quenching in oil or water, the
carbon precipitates as a cloud of very fine carbide particles,
and the steel is hard. The fact that the carbide particles can
be dissolved in austenite is the basis of the heat-treatment
of steel. The temperatures at which this transformation
takes place are called the critical points and vary with the
composition of the steel. The percent of carbon in the steel has
the greatest influence on the critical points of heat-treatment.
Hardening
Pure iron, wrought iron, and extremely low carbon steels
cannot be appreciably hardened by heat-treatment, since they
contain no hardening element. Cast iron can be hardened, but
its heat-treatment is limited. When cast iron is cooled rapidly,
it forms white iron, which is hard and brittle. When cooled
slowly, it forms gray iron, which is soft but brittle under impact.
In plain carbon steel, the maximum hardness depends
almost entirely on the carbon content of the steel. As carbon
content increases, the ability of steel to harden also increases.
However, this increase in the ability to harden with an
increase in carbon content continues only to a certain point.
In practice, that point is 0.85 percent carbon content. When
the carbon content is increased beyond 0.85 percent, there
is no increase in wear resistance.
For most steels, the hardening treatment consists of heating
the steel to a temperature just above the upper critical point,
soaking or holding for the required length of time, and then
cooling it rapidly by plunging the hot steel into oil, water,
or brine. Although most steels must be cooled rapidly for
hardening, a few may be cooled in still air. Hardening increases
the hardness and strength of the steel but makes it less ductile.
Carbon steel must be cooled to below 1,000 °F in less than
1 second when hardening. Should the time required for the
temperature to drop to 1,000 °F exceed 1 second, the austenite
begins to transform into fine pearlite. This pearlite varies
in hardness, but is much harder than the pearlite formed by
annealing and much softer than the martensite desired. After
the 1,000 °F temperature is reached, the rapid cooling must
7-18Steel
NumberQuenching
Medium (n)TemperaturesTempering (drawing) Temperature for
Tensile Strength (psi)
1020 1,650–1,750 1,600–1,700 1,575–1,675 Water — — — — —
1022 (x1020) 1,650–1,750 1,600–1,700 1,575–1,675 Water — — — — —
1025 1,600–1,700 1,575–1,650 1,575–1,675 Water (a) — — — —
1035 1,575–1,650 1,575–1,625 1,525–1,600 Water 875 — — — —
1045 1,550–1,600 1,550–1,600 1,475–1,550 Oil or water 1,150 — — (n) —
1095 1,475–1,550 1,450–1,500 1,425–1,500 Oil (b) — 1,100 850 750
2330 1,475–1,525 1,425–1,475 1,450–1,500 Oil or water 1,100 950 800 — —
3135 1,600–1,650 1,500–1,550 1,475–1,525 Oil 1,250 1,050 900 750 650
3140 1,600–1,650 1,500–1,550 1,475–1,525 Oil 1,325 1,075 925 775 700
4037 1,600 1,525–1,575 1,525–1,575 Oil or water 1,225 1,100 975 — —
4130 (x4130) 1,600–1,700 1,525–1,575 1,525–1,625 Oil (c) (d) 1,050 900 700 575
4140 1,600–1,650 1,525–1,575 1,525–1,575 Oil 1,350 1,100 1,025 825 675
4150 1,550–1,600 1,475–1,525 1,550–1,550 Oil — 1,275 1,175 1,050 950
4340 (x4340) 1,550–1,625 1,525–1,575 1,475–1,550 Oil — 1,200 1,050 950 850
4640 1,675–1,700 1,525–1,575 1,500–1,550 Oil — 1,200 1,050 750 625
6135 1,600–1,700 1,550–1,600 1,575–1,625 Oil 1,300 1,075 925 800 750
6150 1,600–1,650 1,525–1,575 1,550–1,625 Oil (d)(e) 1,200 1,000 900 800
6195 1,600–1,650 1,525–1,575 1,500–1,550 Oil (f ) — — — —
NE8620 — — 1,525–1,575 Oil — 1,000 — — —
NE8630 1,650 1,525–1,575 1,525–1,575 Oil — 1,125 975 775 675
NE8735 1,650 1,525–1,575 1,525–1,575 Oil — 1,175 1,025 875 775
NE8740 1,625 1,500–1,550 1,500–1,550 Oil — 1,200 1,075 925 850
30905 — (g)(h) (i) — — — — — —
51210 1,525–1,575 1,525–1,575 1,775–1,825 (j) Oil 1,200 1,100 (k) 750 —
51335 — 1,525–1,575 1,775–1,850 Oil — — — — —
52100 1,625–1,700 1,400–1,450 1,525–1,550 Oil (f ) — — — —
Corrosion
resisting — — — — (m) — — — —
(16-2)(1)
Silicon
chromium — — 1,700–1,725 Oil — — — — —
(for springs)Normalizing
Air Cool (°F)Annealing
(°F)Hardening
(°F)100,000
(°F)125,000
(°F)150,000
(°F)180,000
(°F)200,000
(°F)
NOTES:
(a) Draw at 1,150 °F for tensile strength of 70,000 psi.
(b) For spring temper draw at 800–900 °F. Rockwell hardness C-40–45.(c) Bars or forgings may be quenched in water from 1,500–1,600 °F.(d) Air cooling from the normalizing temperature produces a tensile
strength of approximately 90,000 psi.
(e) For spring temper draw at 850–950 °F. Rockwell hardness C-40–45.(f ) Draw at 350–450 °F to remove quenching strains. Rockwell hardness
C-60–65.
(g) Anneal at 1,600–1,700 °F to remove residual stresses due to welding or
cold-work. May be applied only to steel containing titanium or
columbium.
(h) Anneal at 1,900–2,100 °F to produce maximum softness and corrosion
resistance. Cool in air or quench in water.
(i)
Harden by cold-work only.(j) Lower side of range for sheet 0.06 inch and under. Middle of range for
sheet and wire 0.125 inch. Upper side of range for forgings.
(k) Not recommended for intermediate tensile strengths because of low
impact.
(l) AN-QQ-S-770—It is recommended that, prior to tempering,
corrosion-resisting (16 Cr-2 Ni) steel be quenched in oil from a temperature of 1,875–1,900 °F, after a soaking period of 30 minutes at this temperature. To obtain a tensile strength at 115,000 psi, the tempering temperature should be approximately 525 °F. A holding time at these temperatures of about 2 hours is recommended. Tempering temperatures between 700 °F and 1,100 °F is not approved.
(m) Draw at approximately 800 °F and cool in air for Rockwell hardness of C-50.
(n) Water used for quenching shall be within the temperature range of
80–150 °F.
Figure 7-6. Heat-treatment procedures for steels.
7-19continue if the final structure is to be all martensite.
The time limit for the temperature drop to 1,000 °F increases
above the 1 second limit for carbon steels when alloys are
added to steel. Therefore, a slower quenching medium
produces hardness in alloy steels.
Because of the high internal stresses in the “as quenched”
condition, steel must be tempered just before it becomes
cold. The part should be removed from the quenching
bath at a temperature of approximately 200 °F, since the
temperature range from 200 °F down to room temperature
is the cracking range.
Hardening temperatures and quenching mediums for the
various types of steel are listed in Figure 7-6 .
Hardening Precautions
A variety of different shapes and sizes of tongs for handling
hot steels is necessary. It should be remembered that cooling
of the area contacted by the tongs is retarded and that such
areas may not harden, particularly if the steel being treated is
very shallow hardening. Small parts may be wired together
or quenched in baskets made of wire mesh.
Special quenching jigs and fixtures are frequently used to hold
steels during quenching in a manner to restrain distortion.
When selective hardening is desired, covering with alundum
cement or some other insulating material may protect portions
of the steel. Selective hardening may be accomplished by
using water or oil jets designed to direct quenching medium
on the areas to be hardened. This also is accomplished by
the induction and flame hardening procedures previously
described, particularly on large production jobs.
Shallow hardening steels, such as plain carbon and certain
varieties of alloy steels, have such a high critical cooling
rate that they must be quenched in brine or water to effect
hardening. In general, intricately-shaped sections should not
be made of shallow hardening steels because of the tendency
of these steels to warp and crack during hardening. Such
items should be made of deeper hardening steels capable of
being hardened by quenching in oil or air.
Tempering
Tempering reduces the brittleness imparted by hardening
and produces definite physical properties within the steel.
Tempering always follows, never precedes, the hardening
operation. In addition to reducing brittleness, tempering
softens the steel.
Tempering is always conducted at temperatures below the low critical point of the steel. In this respect, tempering
differs from annealing, normalizing, or hardening, all of
which require temperatures above the upper critical point.
When hardened steel is reheated, tempering begins at 212 °F
and continues as the temperature increases toward the low
critical point. By selecting a definite tempering temperature,
the resulting hardness and strength can be predetermined.
Approximate temperatures for various tensile strengths are
listed in Figure 7-6 . The minimum time at the tempering
temperature should be 1 hour. If the part is over one inch in
thickness, increase the time by 1 hour for each additional inch
of thickness. Tempered steels used in aircraft work have from
125,000 to 200,000 psi ultimate tensile strength.
Generally, the rate of cooling from the tempering temperature
has no effect on the resulting structure; therefore, the steel is
usually cooled in still air after being removed from the furnace.
Annealing
Annealing of steel produces a fine-grained, soft, ductile metal
without internal stresses or strains. In the annealed state, steel
has its lowest strength. In general, annealing is the opposite
of hardening.
Heating the metal to just above the upper critical point,
soaking at that temperature, and cooling very slowly in
the furnace accomplishes annealing of steel. (Refer to
Figure 7-6 for recommended temperatures.) Soaking time is
approximately 1 hour per inch of thickness of the material.
To produce maximum softness in steel, the metal must be
cooled very slowly. Slow cooling is obtained by shutting off
the heat and allowing the furnace and metal to cool together
to 900 °F or lower, then removing the metal from the furnace
and cooling in still air. Another method is to bury the heated
steel in ashes, sand, or other substance that does not conduct
heat readily.
Normalizing
The normalizing of steel removes the internal stresses set up
by heat-treating, welding, casting, forming, or machining.
Stress, if not controlled, leads to failure. Because of the
better physical properties, aircraft steels are often used in the
normalized state, but seldom, if ever, in the annealed state.
One of the most important uses of normalizing in aircraft work
is in welded parts. Welding causes strains to be set up in the
adjacent material. In addition, the weld itself is a cast structure
as opposed to the wrought structure of the rest of the material.
These two types of structures have different grain sizes, and
to refine the grain as well as to relieve the internal stresses, all
welded parts should be normalized after fabrication.
Heating the steel above the upper critical point and cooling in
7-20still air accomplish normalizing. The more rapid quenching
obtained by air-cooling, as compared to furnace cooling,
results in a harder and stronger material than that obtained
by annealing. Recommended normalizing temperatures for
the various types of aircraft steels are listed in Figure 7-6 .
Case Hardening
Case hardening produces a hard, wear-resistant surface or
case over a strong, tough core. Case hardening is ideal for
parts that require a wear-resistant surface and, at the same
time, must be tough enough internally to withstand the
applied loads. The steels best suited to case hardening are
the low carbon and low-alloy steels. If high-carbon steel is
case hardened, the hardness penetrates the core and causes
brittleness. In case hardening, the surface of the metal is
changed chemically by introducing a high carbide or nitride
content. The core is unaffected chemically.
When heat-treated, the surface responds to hardening while
the core toughens. The common forms of case hardening
are carburizing, cyaniding, and nitriding. Since cyaniding is
not used in aircraft work, only carburizing and nitriding are
discussed in this section.
Carburizing
Carburizing is a case hardening process in which carbon is
added to the surface of low-carbon steel. Thus, carburized
steel has a high-carbon surface and a low-carbon interior.
When the carburized steel is heat-treated, the case is hardened
while the core remains soft and tough.
A common method of carburizing is called “pack carburizing.”
When carburizing is to be done by this method, the steel
parts are packed in a container with charcoal or some other
material rich in carbon. The container is then sealed with fire
clay, placed in a furnace, heated to approximately 1,700 °F,
and soaked at that temperature for several hours. As the
temperature increases, carbon monoxide gas forms inside
the container and, being unable to escape, combines with the
gamma iron in the surface of the steel. The depth to which
the carbon penetrates depends on the length of the soaking
period. For example, when carbon steel is soaked for 8 hours,
the carbon penetrates to a depth of about 0.062 inch.
In another method of carburizing, called “gas carburizing,”
a material rich in carbon is introduced into the furnace
atmosphere. The carburizing atmosphere is produced by
using various gases or by the burning of oil, wood, or other
materials. When the steel parts are heated in this atmosphere,
carbon monoxide combines with the gamma iron to produce
practically the same results as those described under the pack
carburizing process.A third method of carburizing is that of “liquid carburizing.”
In this method, the steel is placed in a molten salt bath that
contains the chemicals required to produce a case comparable
with one resulting from pack or gas carburizing.
Alloy steels with low-carbon content, as well as low-carbon
steels, may be carburized by any of the three processes.
However, some alloys, such as nickel, tend to retard the
absorption of carbon. Thus, the time required to produce
a given thickness of case varies with the composition of
the metal.
Nitriding
Nitriding is unlike other case hardening processes in that,
before nitriding, the part is heat-treated to produce definite
physical properties. Thus, parts are hardened and tempered
before being nitrided. Most steels can be nitrided, but special
alloys are required for best results. These special alloys
contain aluminum as one of the alloying elements and are
called “nitralloys.”
In nitriding, the part is placed in a special nitriding furnace and
heated to a temperature of approximately 1,000 °F. With the
part at this temperature, ammonia gas is circulated within the
specially constructed furnace chamber. The high temperature
cracks the ammonia gas into nitrogen and hydrogen. The
ammonia, which does not break down, is caught in a water trap
below the regions of the other two gases. The nitrogen reacts
with the iron to form nitride. The iron nitride is dispersed in
minute particles at the surface and works inward. The depth
of penetration depends on the length of the treatment. Soaking
periods (as long as 72 hours) are frequently required to produce
the desired thickness during nitriding.
Nitriding can be accomplished with a minimum of distortion,
because of the low temperature at which parts are case
hardened and because no quenching is required after exposure
to the ammonia gas.
Heat-Treatment of Nonferrous Metals
Aluminum Alloys
In the wrought form, commercially-pure aluminum is known
as 1100. It has a high degree of resistance to corrosion and
is easily formed into intricate shapes. It is relatively low
in strength and does not have the properties required for
structural aircraft parts. The process of alloying generally
obtains high strengths. The resulting alloys are less easily
formed and, with some exceptions, have lower resistance to
corrosion than 1100 aluminum.
Alloying is not the only method of increasing the strength of
aluminum. Like other materials, aluminum becomes stronger
and harder as it is rolled, formed, or otherwise cold-worked.
7-21Since the hardness depends on the amount of cold-working
done, 1100 and some wrought aluminum alloys are available
in several strain-hardened tempers. The soft or annealed
condition is designated O. If the material is strain hardened,
it is said to be in the H condition.
The most widely used alloys in aircraft construction are
hardened by heat-treatment rather than by cold-work.
These alloys are designated by a somewhat different set
of symbols: T4 and W indicate solution heat-treated and
quenched but not aged, and T6 indicates an alloy in the
heat-treated, hardened condition.
• W—solution heat-treated, unstable temper
• T—treated to produce stable tempers other than F, O,
or H
• T2—annealed (cast products only)
• T3—solution heat-treated and then cold-worked
• T4—solution heat-treated
• T5—artificially aged only
• T6—solution heat-treated and then artificially aged
• T7—solution heat-treated and then stabilized
• T8—solution heat-treated, cold-worked, and then
artificially aged
• T9—solution heat-treated, artificially aged, and then
cold-worked
• T10—artificially aged and then cold-worked
Additional digits may be added to T1 through T10 to indicate
a variation in treatment, which significantly alters the
characteristics of the product.
Aluminum-alloy sheets are marked with the specification
number on approximately every square foot of material. If
for any reason this identification is not on the material, it is
possible to separate the heat-treatable alloys from the non-
heat-treatable alloys by immersing a sample of the material
in a 10 percent solution of caustic soda (sodium hydroxide).
The heat-treatable alloys turn black due to the copper content,
whereas the others remain bright. In the case of clad material,
the surface remains bright, but there is a dark area in the
middle when viewed from the edge.
Alclad Aluminum
The terms “Alclad and Pureclad” are used to designate sheets
that consist of an aluminum-alloy core coated with a layer
of pure aluminum to a depth of approximately 51⁄2 percent
on each side. The pure aluminum coating affords a dual
protection for the core, preventing contact with any corrosive
agents, and protecting the core electrolytically by preventing any attack caused by scratching or from other abrasions.
There are two types of heat-treatments applicable to
aluminum alloys: solution heat-treatment and precipitation
heat-treatment. Some alloys, such as 2017 and 2024, develop
their full properties as a result of solution heat-treatment
followed by about 4 days of aging at room temperature. Other
alloys, such as 2014 and 7075, require both heat-treatments.
The alloys that require precipitation heat-treatment (artificial
aging) to develop their full strength also age to a limited extent
at room temperature; the rate and amount of strengthening
depends upon the alloy. Some reach their maximum natural
or room temperature aging strength in a few days, and are
designated as –T4 or –T3 temper. Others continue to age
appreciably over a long period of time.
Because of this natural aging, the –W designation is specified
only when the period of aging is indicated, for example,
7075–W (1⁄2 hour). Thus, there is considerable difference in
the mechanical and physical properties of freshly quenched
(–W) material and material that is in the –T3 or –T4 temper.
The hardening of an aluminum alloy by heat-treatment
consists of four distinct steps:
1. Heating to a predetermined temperature.
2. Soaking at temperature for a specified length of time.
3. Rapidly quenching to a relatively low temperature.
4. Aging or precipitation-hardening either spontaneously
at room temperature, or because of a low temperature
thermal treatment.
The first three steps above are known as solution heat-treatment,
although it has become common practice to use the shorter term,
“heat-treatment.” Room temperature hardening is known as
natural aging, while hardening done at moderate temperatures
is called artificial aging, or precipitation heat-treatment.
Solution Heat-Treatment
Temperature
The temperatures used for solution heat-treating vary with
different alloys and range from 825 °F to 980 °F. As a rule,
they must be controlled within a very narrow range (±10 °F)
to obtain specified properties.
If the temperature is too low, maximum strength is not
obtained. When excessive temperatures are used, there is
danger of melting the low melting constituents of some
alloys with consequent lowering of the physical properties
of the alloy. Even if melting does not occur, the use of higher
than recommended temperatures promotes discoloration and
increases quenching strains.
7-22Time at Temperature
The time at temperature, referred to as soaking time, is
measured from the time the coldest metal reaches the minimum
limit of the desired temperature range. The soaking time varies,
depending upon the alloy and thickness, from 10 minutes for
thin sheets to approximately 12 hours for heavy forgings. For
the heavy sections, the nominal soaking time is approximately
1 hour for each inch of cross-sectional thickness. [Figure 7-7]
Choose the minimum soaking time necessary to develop the
required physical properties. The effect of an abbreviated
soaking time is obvious. An excessive soaking period
aggravates high-temperature oxidation. With clad material,
prolonged heating results in excessive diffusion of copper
and other soluble constituents into the protective cladding
and may defeat the purpose of cladding.
Quenching
After the soluble constituents are in solid solution, the material
is quenched to prevent or retard immediate re-precipitation.
Three distinct quenching methods are employed. The one to
be used in any instance depends upon the part, the alloy, and
the properties desired.
Cold Water Quenching
Parts produced from sheet, extrusions, tubing, small forgings,
and similar type material are generally quenched in a cold
water bath. The temperature of the water before quenching
should not exceed 85 °F.
Using a sufficient quantity of water keeps the temperature
rise under 20 °F. Such a drastic quench ensures maximum
resistance to corrosion. This is particularly important when
working with alloys, such as 2017, 2024, and 7075. This is
the reason a drastic quench is preferred, even though a slower
quench may produce the required mechanical properties.
Hot Water Quenching
Large forgings and heavy sections can be quenched in hot or
boiling water. This type of quench minimizes distortion and
alleviates cracking, which may be produced by the unequal
temperatures obtained during the quench. The use of a hot
water quench is permitted with these parts, because the
temperature of the quench water does not critically affect the
resistance to corrosion of the forging alloys. In addition, the
resistance to corrosion of heavy sections is not as critical a
factor as for thin sections.
Spray Quenching
High-velocity water sprays are useful for parts formed from
clad sheet and for large sections of almost all alloys. This type of quench also minimizes distortion and alleviates quench
cracking. However, many specifications forbid the use of
spray quenching for bare 2017 and 2024 sheet materials
because of the effect on their resistance to corrosion.
Lag Between Soaking & Quenching
The time interval between the removal of the material from
the furnace and quenching is critical for some alloys and
should be held to a minimum. The elapsed time must not
exceed 10 seconds when solution heat-treating 2017 or 2024
sheet material. The allowable time for heavy sections may
be slightly greater.
Allowing the metal to cool slightly before quenching promotes
re-precipitation from the solid solution. The precipitation
occurs along grain boundaries and in certain slip planes causing
poorer formability. In the case of 2017, 2024, and 7075 alloys,
their resistance to intergranular corrosion is adversely affected.
Reheat-Treatment
The treatment of material, which has been previously
heat-treated, is considered a reheat-treatment. The unclad
heat-treatable alloys can be solution heat-treated repeatedly
without harmful effects.
The number of solution heat-treatments allowed for clad
sheet is limited due to increased diffusion of core and
cladding with each reheating. Existing specifications allow
one to three reheat-treatments of clad sheet depending upon
cladding thickness.
Straightening After Solution Heat-Treatment
Some warping occurs during solution heat-treatment,
producing kinks, buckles, waves, and twists. Straightening and
flattening operations generally remove these imperfections.
Where the straightening operations produce an appreciable
increase in the tensile and yield strengths and a slight decrease
in the percent of elongation, the material is designated –T3
temper. When the above values are not materially affected,
the material is designated –T4 temper.
Precipitation Heat-Treating
As previously stated, the aluminum alloys are in a
comparatively soft state immediately after quenching
from a solution heat-treating temperature. To obtain their
maximum strengths, they must be either naturally aged or
precipitation-hardened.
During this hardening and strengthening operation,
precipitation of the soluble constituents from the super-
saturated solid solution takes place. The strength of the
material increases (often by a series of peaks) until a maximum
is reached, as precipitation progresses. Further aging, or over-
7-23Thickness (inch) Time (minutes)
Up to 0.032 30
0.032 to 1⁄8 30
Over ¼ 60
NOTE: Soaking time starts when the metal (or the molten bath)
reaches a temperature within the range specified above.
Figure 7-7. Typical soaking times for heat-treatment.
aging, causes the strength to steadily decline until a somewhat
stable condition is obtained. The submicroscopic particles that are precipitated provide the keys or locks within the grain structure and between the grains to resist internal slippage and distortion when a load of any type is applied. In this manner, the strength and hardness of the alloy are increased.
Precipitation-hardening produces a great increase in the
strength and hardness of the material with corresponding decreases in the ductile properties. The process used to obtain the desired increase in strength is therefore known as aging or precipitation-hardening.
The strengthening of the heat-treatable alloys by aging is
not due merely to the presence of a precipitate. The strength is due to both the uniform distribution of a finely dispersed submicroscopic precipitate and its effects upon the crystal structure of the alloy.
The aging practices used depend upon many properties
other than strength. As a rule, the artificially-aged alloys are slightly over-aged to increase their resistance to corrosion. This is especially true with the artificially-aged, high-copper content alloys that are susceptible to intergranular corrosion when inadequately aged.
The heat-treatable aluminum alloys are subdivided into
two classes: those that obtain their full strength at room temperature and those that require artificial aging.
The alloys that obtain their full strength after 4 or 5 days
at room temperature are known as natural-aging alloys. Precipitation from the supersaturated solid solution starts soon after quenching, with 90 percent of the maximum strength generally being obtained in 24 hours. Alloys 2017 and 2024 are natural-aging alloys.
The alloys that require precipitation thermal treatment
to develop their full strength are artificially-aged alloys. However, these alloys also age a limited amount at room temperature, the rate and extent of the strengthening depending upon the alloys.
Many of the artificially-aged alloys reach their maximum
natural or room temperature aging strengths after a few days. These can be stocked for fabrication in the –T4 or –T3 tempers. High zinc content alloys, such as 7075, continue to age appreciably over a long period of time. Their mechanical property changes being sufficient to reduce their formability.
The advantage of –W temper formability can be utilized;
however, in the same manner as with natural-aging alloys; that is, by fabricating shortly after solution heat-treatment or retaining formability by using refrigeration.
Refrigeration retards the rate of natural aging. At 32 °F,
the beginning of the aging process is delayed for several hours, while dry ice (−50 °F to −100 °F) retards aging for an extended period of time.
Precipitation Practices
The temperatures used for precipitation-hardening depend
upon the alloy and the properties desired, ranging from 250 °F
to 375 °F. They should be controlled within a very narrow range (±5 °F) to obtain best results. [Figure 7-8]
The time at temperature is dependent upon the temperature used, the properties desired, and the alloy. It ranges from 8 to 96 hours. Increasing the aging temperature decreases the soaking period necessary for proper aging. However, a closer control of both time and temperature is necessary when using the higher temperatures.
After receiving the thermal precipitation treatment, the
material should be air cooled to room temperature. Water quenching, while not necessary, produces no ill effects. Furnace cooling tends to produce over-aging.
Annealing of Aluminum Alloys
The annealing procedure for aluminum alloys consists
of heating the alloys to an elevated temperature, holding or soaking them at this temperature for a length of time depending upon the mass of the metal, and then cooling in still air. Annealing leaves the metal in the best condition for cold-working. However, when prolonged forming operations are involved, the metal takes on a condition known as “mechanical hardness” and resists further working. It may be necessary to anneal a part several times during the forming process to avoid cracking. Aluminum alloys should not be used in the annealed state for parts or fittings.
Clad parts should be heated as quickly and carefully as
possible, since long exposure to heat tends to cause some of the constituents of the core to diffuse into the cladding. This 1⁄8 to 1/4
7-24reduces the corrosion resistance of the cladding.
Heat-Treatment of Aluminum Alloy Rivets
Aluminum alloy rivets are furnished in the following
compositions: alloys 1100, 5056, 2117, 2017, and 2024.
Alloy 1100 rivets are used in the “as fabricated” condition
for riveting aluminum alloy sheets where a low-strength rivet
is suitable. Alloy 5056 rivets are used in the “as fabricated”
condition for riveting magnesium alloy sheets.
Alloy 2117 rivets have moderately high strength and are
suitable for riveting aluminum alloy sheets. These rivets
receive only one heat-treatment, which is performed by the
manufacturer, and are anodized after being heat-treated.
They require no further heat-treatment before they are used.
Alloy 2117 rivets retain their characteristics indefinitely after
heat-treatment and can be driven anytime. Rivets made of
this alloy are the most widely used in aircraft construction.
Alloy 2017 and 2024 rivets are high-strength rivets suitable
for use with aluminum alloy structures. They are purchased
from the manufacturer in the heat-treated condition. Since
the aging characteristics of these alloys at room temperatures
are such that the rivets are unfit for driving, they must be
reheat-treated just before they are to be used. Alloy 2017
rivets become too hard for driving in approximately 1 hour
after quenching. Alloy 2024 rivets become hardened in 10
minutes after quenching. Both alloys may be reheat-treated
as often as required; however, they must be anodized before
the first reheat-treatment to prevent intergranular oxidation
of the material. If these rivets are stored in a refrigerator at a
temperature lower than 32 °F immediately after quenching,
they remain soft enough to be usable for several days.
Rivets requiring heat-treatment are heated either in tubular
containers in a salt bath or in small screen wire baskets
in an air furnace. The heat-treatment of alloy 2017 rivets
consists of subjecting the rivets to a temperature between
930 °F to 950 °F for approximately 30 minutes and
immediately quenching in cold water. These rivets reach
maximum strength in about 9 days after being driven. Alloy
2024 rivets should be heated to a temperature of 910 °F
to 930 °F and immediately quenched in cold water. These
rivets develop greater shear strength than 2017 rivets and
are used in locations where extra strength is required. Alloy
2024 rivets develop their maximum shear strength in 1 day
after being driven.
The 2017 rivet should be driven within approximately 1 hour
and the 2024 rivet within 10 to 20 minutes after heat-treating
or removal from refrigeration. If not used within these times,
the rivets should be reheat-treated before being refrigerated.Heat-Treatment of Magnesium Alloys
Magnesium alloy castings respond readily to heat-treatment,
and about 95 percent of the magnesium used in aircraft
construction is in the cast form. The heat-treatment of
magnesium alloy castings is like the heat-treatment of
aluminum alloys in that there are two types of heat-treatment:
solution heat-treatment and precipitation (aging) heat-
treatment. Magnesium, however, develops a negligible
change in its properties when allowed to age naturally at
room temperatures.
Solution Heat-Treatment
Magnesium alloy castings are solution heat-treated to
improve tensile strength, ductility, and shock resistance. This
heat-treatment condition is indicated by using the symbol –T4
following the alloy designation. Solution heat-treatment plus
artificial aging is designated –T6. Artificial aging is necessary
to develop the full properties of the metal.
Solution heat-treatment temperatures for magnesium alloy
castings range from 730 °F to 780 °F, the exact range
depending upon the type of alloy. The temperature range
for each type of alloy is listed in Specification MIL-H-6857.
The upper limit of each range listed in the specification is
the maximum temperature to which the alloy may be heated
without danger of melting the metal.
The soaking time ranges from 10 to 18 hours, the exact time
depending upon the type of alloy as well as the thickness of the
part. Soaking periods longer than 18 hours may be necessary
for castings over 2 inches in thickness. Never heat magnesium
alloys in a salt bath as this may result in an explosion.
A serious potential fire hazard exists in the heat-treatment
of magnesium alloys. If through oversight or malfunctioning
of equipment the maximum temperatures are exceeded, the
casting may ignite and burn freely. For this reason, the furnace
used should be equipped with a safety cutoff that turns off
the power to the heating elements and blowers if the regular
control equipment malfunctions or fails. Some magnesium
alloys require a protective atmosphere of sulfur dioxide
gas during solution heat-treatment. This aids in preventing
the start of a fire even if the temperature limits are slightly
exceeded.
Air quenching is used after solution heat-treatment of
magnesium alloys since there appears to be no advantage
in liquid cooling.
Precipitation Heat-Treatment
After solution treatment, magnesium alloys may be given
an aging treatment to increase hardness and yield strength.
7-25Alloy
Temperature
(°F)Temperature
DesignationQuenchTemperature
(°F)Temperature
DesignationTime of AgingSolution Heat—Treatment Precipitation Heat—Treatment
2017 930–950 Cold water T4 T
2117 930–950 Cold water T4 T
2024 910–930 Cold water T4 T
6053 960–980 Water T4 445–455 1–2 hours T5
or
345–355 8 hours T6
6061 960–980 Water T4 315–325 18 hours T6
or
345–355 8 hours T6
7075 870 Water 250 24 hours T6
Figure 7-8. Conditions for heat-treatment of aluminum alloys.
Generally, the aging treatments are used merely to relieve
stress and stabilize the alloys to prevent dimensional changes
later, especially during or after machining. Both yield strength
and hardness are improved somewhat by this treatment at
the expense of a slight amount of ductility. The corrosion
resistance is also improved, making it closer to the “as cast”
alloy.
Precipitation heat-treatment temperatures are considerably
lower than solution heat-treatment temperatures and range
from 325 °F to 500 °F. Soaking time ranges from 4 to 18 hours.
Heat-Treatment of Titanium
Titanium is heat-treated for the following purposes:
• Relief of stresses set up during cold forming or
machining.
• Annealing after hot-working or cold-working, or
to provide maximum ductility for subsequent cold-
working.
• Thermal hardening to improve strength.
Stress Relieving
Stress relieving is generally used to remove stress concentrations
resulting from forming of titanium sheet. It is performed at
temperatures ranging from 650 °F to 1,000 °F. The time at
temperature varies from a few minutes for a very thin sheet to
an hour or more for heavier sections. A typical stress relieving
treatment is 900 °F for 30 minutes, followed by an air cool.
The discoloration or scale that forms on the surface of the
metal during stress relieving is easily removed by pickling
in acid solutions. The recommended solution contains 10 to
20 percent nitric acid and 1 to 3 percent hydrofluoric acid.
The solution should be at room temperature or slightly above.Full Annealing
The annealing of titanium and titanium alloys provides
toughness, ductility at room temperature, dimensional
and structural stability at elevated temperatures, and
improved machinability.
The full anneal is usually called for as preparation for further
working. It is performed at 1,200 to 1,650 °F. The time at
temperature varies from 16 minutes to several hours, depending
on the thickness of the material and the amount of cold-work
to be performed. The usual treatment for the commonly used
alloys is 1,300 °F for 1 hour, followed by an air cool. A full
anneal generally results in sufficient scale formation to require
the use of caustic descaling, such as sodium hydride salt bath.
Thermal Hardening
Unalloyed titanium cannot be heat-treated, but the alloys
commonly used in aircraft construction can be strengthened
by thermal treatment, usually at some sacrifice in ductility.
For best results, a water quench from 1,450 °F, followed by
reheating to 900 °F for 8 hours is recommended.
Case Hardening
The chemical activity of titanium and its rapid absorption of
oxygen, nitrogen, and carbon at relatively low temperatures
make case hardening advantageous for special applications.
Nitriding, carburizing, or carbonitriding can be used to produce
a wear-resistant case of 0.0001 to 0.0002 inch in depth.
Hardness Testing
Hardness testing is a method of determining the results of heat-
treatment, as well as the state of a metal prior to heat-treatment.
Since hardness values can be tied in with tensile strength values
and, in part, with wear resistance. Hardness tests are a valuable
7-26check of heat-treat control and of material properties.
Practically all hardness testing equipment now uses the
resistance to penetration as a measure of hardness. Included
among the better-known hardness testers are the Brinell and
Rockwell, both of which are described and illustrated in this
section. Also included is the Barcol tester, a popular portable-
type hardness tester currently in use.
Brinell Tester
The Brinell hardness tester uses a hardened spherical ball
that is forced into the surface of the metal. [Figure 7-9] This
ball is 10 millimeters (0.3937 inch) in diameter. A pressure of
3,000 kilograms is used for ferrous metals and 500 kilograms
for nonferrous metals. The pressure must be maintained at
least 10 seconds for ferrous metals and at least 30 seconds
for nonferrous metals. The load is applied by hydraulic
pressure. A hand pump or an electric motor, depending on the
model of tester, builds up the hydraulic pressure. A pressure
gauge indicates the amount of pressure. There is a release
mechanism for relieving the pressure after the test has been
made, and a calibrated microscope is provided for measuring
the diameter of the impression in millimeters. The machine
has various shaped anvils for supporting the specimen and
an elevating screw for bringing the specimen in contact with
the ball penetrator. These are attachments for special tests.
To determine the Brinell hardness number for a metal,
measure the diameter of the impression using the calibrated
microscope furnished with the tester. Then convert the
measurement into the Brinell hardness number on the
conversion table furnished with the tester.
Rockwell Tester
The Rockwell hardness tester measures the resistance to
penetration, as does the Brinell tester. [Figure 7-10] Instead
of measuring the diameter of the impression, the Rockwell
tester measures the depth, and the hardness is indicated
directly on a dial attached to the machine. The dial numbers
in the outer circle are black and the inner numbers are red.
Rockwell hardness numbers are based on the difference
between the depth of penetration at major and minor loads.
The greater this difference, the lower the hardness number
and the softer the material.
Two types of penetrators are used with the Rockwell tester:
a diamond cone and a hardened steel ball. The load, which
forces the penetrator into the metal, is called the major load
and is measured in kilograms. The results of each penetrator
and load combination are reported on separate scales
designated by letters. The penetrator, the major load, and the
scale vary with the kind of metal being tested.For hardened steels, the diamond penetrator is used; the
major load is 150 kilograms; and the hardness is read on the
“C” scale. When this reading is recorded, the letter “C” must
precede the number indicated by the pointer. The C-scale
setup is used for testing metals ranging in hardness from
C-20 to the hardest steel (usually about C-70). If the metal is
softer than C-20, the B-scale setup is used. With this setup,
the 1⁄16-inch ball is used as a penetrator; the major load is 100
kilograms; and the hardness is read on the B-scale.
In addition to the C and B scales, there are other setups for
special testing. The scales, penetrators, major loads, and dial
numbers to be read are listed in Figure 7-11 .
The Rockwell tester is equipped with a weight pan, and
two weights are supplied with the machine. One weight
is marked in red. The other weight is marked in black.
With no weight in the weight pan, the machine applies
a major load of 60 kilograms. If the scale setup calls for
a 100-kilogram load, the red weight is placed in the pan.
For a 150-kilogram load, the black weight is added to the
red weight. The black weight is always used with the red
weight; it is never used alone.
Practically all testing is done with either the B-scale setup or
the C-scale setup. For these scales, the colors may be used as
a guide in selecting the weight (or weights) and in reading
the dial. For the B-scale test, use the red weight and read the
red numbers. For a C-scale test, add the black weight to the
red weight and read the black numbers.
In setting up the Rockwell machine, use the diamond
penetrator for testing materials known to be hard. If the
hardness is unknown, try the diamond, since the steel ball
may be deformed if used for testing hard materials. If the
metal tests below C-22, then change to the steel ball.
Use the steel ball for all soft materials, those testing less than
B-100. Should an overlap occur at the top of the B-scale and
the bottom of the C-scale, use the C-scale setup.
Before the major load is applied, securely lock the test specimen
in place to prevent slipping and to seat the anvil and penetrator
properly. To do this, apply a load of 10 kilograms before the
lever is tripped. This preliminary load is called the minor load.
The minor load is 10 kilograms regardless of the scale setup.
The metal to be tested in the Rockwell tester must be ground
smooth on two opposite sides and be free of scratches and
foreign matter. The surface should be perpendicular to the
axis of penetration, and the two opposite ground surfaces
should be parallel. If the specimen is tapered, the amount of
error depends on the taper. A curved surface also causes a
slight error in the hardness test. The amount of error depends
Pressure gauge
Hydraulic
actuating unit
Penetrator
Elevating screwHand pump
Microscope0500100015002000 3000
Figure 7-9. Brinell hardness tester.ZEROMNDER
FOR RAP TESTINGc0
c5090
40c c
c
c
cc
c
cB30
B B
B
B
BB
B
B4020
ZEROMNDER
FOR RAP TESTINGc0
c5090
40c c
c
c
c
cc
c
cB30 B
B
B
B
BB
B
B40
Weights
Weight pan
Trip leverZero adjusterHand wheelElevating screwPenetrator
Specimen
Anvil
Figure 7-10. Rockwell hardness tester.on the curvature (i.e., the smaller the radius of curvature, the
greater the error). To eliminate such error, a small flat should
be ground on the curved surface if possible.
Clad aluminum alloy sheets cannot be tested directly
with any accuracy with a Rockwell hardness tester. If
the hardness value of the base metal is desired, the pure
aluminum coating must be removed from the area to be
checked prior to testing.
Barcol Tester
The Barcol tester is a portable unit designed for testing
aluminum alloys, copper, brass, or other relatively soft
materials. [Figure 7-12] It should not be used on aircraft
steels. Approximate range of the tester is 25 to 100 Brinell.
The unit can be used in any position and in any space that
allows for the operator’s hand. It is of great value in the
hardness testing of assembled or installed parts, especially
to check for proper heat-treatment. The hardness is indicated
on a dial conveniently divided into 100 graduations.The design of the Barcol tester is such that operating
experience is not necessary. It is only necessary to exert
a light pressure against the instrument to drive the spring-
loaded indenter into the material to be tested. The hardness
reading is instantly indicated on the dial.
Several typical readings for aluminum alloys are listed in
Figure 7-13 . Note that the harder the material is, the higher
the Barcol number. To prevent damage to the point, avoid
sliding or scraping when it is in contact with the material
being tested. If the point should become damaged, it must be
replaced with a new one. Do not attempt to grind the point.
Each tester is supplied with a test disk for checking the
condition of the point. To check the point, press the instrument
down on the test disk. When the downward pressure brings
the end of the lower plunger guide against the surface of the
disk, the indicator reading should be within the range shown
on the test disk.
Forging
Forging is the process of forming a product by hammering
or pressing. When the material is forged below the
recrystallization temperature, it is called cold forged. When
worked above the recrystallization temperature, it is referred
7-28A Diamond 60 Black
B 100 Red
C Diamond 150 Black
D Diamond 100 Black
E 100 Red
F 60 Red
G 150 Red
H 60 Red
K 150 RedScale
SymbolPenetratorMajor
Load (kg)Dial
Color⁄Number
Figure 7-11. Standard Rockwell hardness scales.
Figure 7-12. Barcol portable hardness tester.to as hot forged. Drop forging is a hammering process that
uses a hot ingot that is placed between a pair of formed dies in a machine called a drop hammer and a weight of several tons is dropped on the upper die. This results in the hot metal being forced to take the form of the dies. Because the process is very rapid, the grain structure of the metal is altered, resulting in a significant increase in the strength of the finished part.
Casting
Melting the metal and pouring it into a mold of the desired
shape forms casting. Since plastic deformation of the metal does not occur, no alteration of the grain shape or orientation is possible. The cooling rate, the alloys of the metal, and the thermal treatment can control the gain size of the metal. Castings are normally lower in strength and are more brittle than a wrought product of the same material. For intricate shapes or items with internal passages, such as turbine blades, casting may be the most economical process. Except for engine parts, most metal components found on an aircraft are wrought instead of cast.
All metal products start in the form of casting. Wrought
metals are converted from cast ingots by plastic deformation. For high-strength aluminum alloys, an 80 to 90 percent reduction (dimensional change in thickness) of the material is required to obtain the high mechanical properties of a fully wrought structure.
Both iron and aluminum alloys are cast for aircraft uses. Cast
iron contains 6 to 8 percent carbon and silicon.
Cast iron is a hard un-malleable pig iron made by casting or
pouring into a mold. Cast aluminum alloy has been heated to its molten state and poured into a mold to give it the desired shape.Extruding
The extrusion process involves the forcing of metal through
an opening in a die, thus causing the metal to take the shape of the die opening. The shape of the die will be the cross section of an angle, channel, tube, or some other shape. Some metals, such as lead, tin, and aluminum, may be extruded cold; however, most metals are heated before extrusion. The main advantage of the extrusion process is its flexibility. For example, because of its workability, aluminum can be economically extruded to more intricate shapes and larger sizes than is practical with other metals.
Extruded shapes are produced in very simple, as well as
extremely complex, sections. In this process, a cylinder of aluminum, for instance, is heated to 750–850 °F and is then forced through the opening of a die by a hydraulic ram. The opening is the shape desired for the cross section of the finished extrusion. The extrusion process forms many structural parts, such as channels, angles, T-sections, and Z-sections.
Aluminum is the most extruded metal used in aircraft.
Aluminum is extruded at a temperature of 700–900 °F (371–482 °C) and requires pressure of up to 80,000 psi (552 MPa). After extrusion, the product frequently is subjected to both thermal and mechanical processes to obtain the desired properties. Extrusion processes are limited to the more ductile materials.
1/16" ball
1/16" ball
1/8" ball
1/8" ball1/16" ball
1/8" ball
7-29 Alloy and Temper Barcol Number
1100-O 35
3003-O 42
3003-H14 56
2024-O 60
5052-O 62
5052-H34 75
6061-T 78
2024-T 85
Figure 7-13. Typical Barcol readings for aluminum alloy.Cold-Working/Hardening
Cold-working applies to mechanical working performed at
temperatures below the critical range. It results in a strain
hardening of the metal. In fact, the metal often becomes so
hard that it is difficult to continue the forming process without
softening the metal by annealing.
Since the errors attending shrinkage are eliminated in cold-
working, a much more compact and better metal is obtained.
The strength and hardness, as well as the elastic limit, are
increased; but the ductility decreases. Since this makes
the metal more brittle, it must be heated from time to time
during certain operations to remove the undesirable effects
of the working.
While there are several cold-working processes, the two with
which the aviation mechanic is principally concerned are cold
rolling and cold drawing. These processes give the metals
desirable qualities that cannot be obtained by hot working.
Cold rolling usually refers to the working of metal at room
temperature. In this operation, the materials that have been
rolled to approximate sizes are pickled to remove the scale,
after which they are passed through chilled finishing rolls.
This gives a smooth surface and brings the pieces to accurate
dimensions. The principal forms of cold-rolled stocks are
sheets, bars, and rods.
Cold drawing is used in making seamless tubing, wire,
streamlined tie rods, and other forms of stock. Wire is made
from hot-rolled rods of various diameters. These rods are
pickled in acid to remove scale, dipped in limewater, and
then dried in a steam room where they remain until ready
for drawing. The lime coating adhering to the metal serves
as a lubricant during the drawing operation.
The size of the rod used for drawing depends upon the
diameter wanted in the finished wire. To reduce the rod to the desired size, it is drawn cold through a die. One end of
the rod is filed or hammered to a point and slipped through
the die opening. Here it is gripped by the jaws of the drawing
block and pulled through the die. This series of operations is
done by a mechanism known as a draw bench.
To reduce the rod gradually to the desired size, it is necessary
to draw the wire through successively smaller dies. Because
each of these drawings reduces the ductility of the wire, it
must be annealed from time to time before further drawings
can be accomplished. Although cold-working reduces the
ductility, it increases the tensile strength of the wire.
In making seamless steel aircraft tubing, the tubing is cold
drawn through a ring-shaped die with a mandrel or metal bar
inside the tubing to support it while the drawing operations
are being performed. This forces the metal to flow between
the die and the mandrel and affords a means of controlling
the wall thickness and the inside and outside diameters.
Nonmetallic Aircraft Materials
The use of magnesium, plastic, fabric, and wood in aircraft
construction has nearly disappeared since the mid-1950s.
Aluminum has also greatly diminished in use, from 80 percent
of airframes in 1950 to about 15 percent aluminum and
aluminum alloys today for airframe construction. Replacing
those materials are nonmetallic aircraft materials, such as
reinforced plastics and advanced composites.
Wood
The earliest aircraft were constructed of wood and cloth.
Today, except for restorations and some home-built aircraft,
very little wood is used in aircraft construction.
Plastics
Plastics are used in many applications throughout modern
aircraft. These applications range from structural components
of thermosetting plastics reinforced with fiberglass to
decorative trim of thermoplastic materials to windows.
Transparent Plastics
Transparent plastic materials used in aircraft canopies,
such as windshields, windows and other similar transparent
enclosures, may be divided into two major classes or groups:
thermoplastic and thermosetting. These plastics are classified
according to their reaction to heat. Thermoplastic materials
soften when heated and harden when cooled. These materials
can be heated until soft and then formed into the desired
shape. When cooled, they retain this shape. The same piece
of plastic can be reheated and reshaped any number of times
without changing the chemical composition of the materials.
Thermosetting plastics harden upon heating, and reheating
7-30has no softening effect. These plastics cannot be reshaped
once being fully cured by the application of heat.
In addition to the above classes, transparent plastics are
manufactured in two forms: monolithic (solid) and laminated.
Laminated transparent plastics are made from transparent
plastic face sheets bonded by an inner layer material, usually
polyvinyl butyryl. Because of its shatter resistant qualities,
laminated plastic is superior to solid plastics and is used in
many pressurized aircraft.
Most of the transparent sheet used in aviation is manufactured
in accordance with various military specifications. A new
development in transparent plastics is stretched acrylic.
Stretched acrylic is a type of plastic, which before being
shaped, is pulled in both directions to rearrange its molecular
structure. Stretched acrylic panels have a greater resistance to
impact and are less subject to shatter; its chemical resistance
is greater, edging is simpler, and crazing and scratches are
less detrimental.
Individual sheets of plastic are covered with a heavy masking
paper to which a pressure sensitive adhesive has been added.
This paper helps to prevent accidental scratching during
storage and handling. Be careful to avoid scratches and
gouges which may be caused by sliding sheets against one
another or across rough or dirty tables.
If possible, store sheets in bins that are tilted at approximately
10° from vertical. If they must be stored horizontally, piles
should not be over 18 inches high, and small sheets should
be stacked on the larger ones to avoid unsupported overhang.
Store in a cool, dry place away from solvent fumes, heating
coils, radiators, and steam pipes. The temperature in the
storage room should not exceed 120 °F.
While direct sunlight does not harm acrylic plastic, it causes
drying and hardening of the masking adhesive, making
removal of the paper difficult. If the paper does not roll off
easily, place the sheet in an oven at 250 °F for 1 minute,
maximum. The heat softens the masking adhesive for easy
removal of the paper.
If an oven is not available, remove hardened masking paper
by softening the adhesive with aliphatic naphtha. Rub the
masking paper with a cloth saturated with naphtha. This
softens the adhesive and frees the paper from the plastic.
Sheets so treated must be washed immediately with clean
water, taking care not to scratch the surfaces.
Note: Aliphatic naphtha is not to be confused with aromatic
naphtha and other dry cleaning solvents, which have harmful
effects on plastic. However, aliphatic naphtha is flammable and all precautions regarding the use of flammable liquids
must be observed.
Composite Materials
In the 1940s, the aircraft industry began to develop synthetic
fibers to enhance aircraft design. Since that time, composite
materials have been used more and more. When composites
are mentioned, most people think of only fiberglass, or maybe
graphite or aramids (Kevlar). Composites began in aviation,
but now are being embraced by many other industries,
including auto racing, sporting goods, and boating, as well
as defense industry uses.
A “composite” material is defined as a mixture of different
materials or things. This definition is so general that it could
refer to metal alloys made from several different metals to
enhance the strength, ductility, conductivity, or whatever
characteristics are desired. Likewise, the composition of
composite materials is a combination of reinforcement, such
as a fiber, whisker, or particle, surrounded and held in place by
a resin forming a structure. Separately, the reinforcement and
the resin are very different from their combined state. Even in
their combined state, they can still be individually identified
and mechanically separated. One composite, concrete, is
composed of cement (resin) and gravel or reinforcement rods
for the reinforcement to create the concrete.
Advantages/Disadvantages of Composites
Some of the many advantages for using composite materials
are:
• High strength-to-weight ratio
• Fiber-to-fiber transfer of stress allowed by chemical
bonding
• Modulus (stiffness-to-density ratio) 3.5 to 5 times that
of steel or aluminum
• Longer life than metals
• Higher corrosion resistance
• Tensile strength 4 to 6 times that of steel or aluminum
• Greater design flexibility
• Bonded construction eliminates joints and fasteners
• Easily repairable
The disadvantages of composites include:
• Inspection methods difficult to conduct, especially
delamination detection (Advancements in technology
will eventually correct this problem.)
• Lack of long-term design database, relatively new
technology methods
• Cost
7-31• Very expensive processing equipment
• Lack of standardized system of methodology
• Great variety of materials, processes, and techniques
• General lack of repair knowledge and expertise
• Products often toxic and hazardous
• Lack of standardized methodology for construction
and repairs
The increased strength and the ability to design for the
performance needs of the product makes composites much
superior to the traditional materials used in today’s aircraft.
As more and more composites are used, the costs, design,
inspection ease, and information about strength-to-weight
advantages help composites become the material of choice
for aircraft construction.
Composite Safety
Composite products can be very harmful to the skin, eyes,
and lungs. In the long or short term, people can become
sensitized to the materials with serious irritation and health
issues. Personal protection is often uncomfortable, hot, and
difficult to wear; however, a little discomfort while working
with the composite materials can prevent serious health issues
or even death.
Respirator particle protection is very important to protecting
the lungs from permanent damage from tiny glass bubbles
and fiber pieces. At a minimum, a dust mask approved for
fiberglass is a necessity. The best protection is a respirator
with dust filters. The proper fit of a respirator or dust mask is
very important, because if the air around the seal is breathed,
the mask cannot protect the wearer’s lungs. When working
with resins, it is important to use vapor protection. Charcoal
filters in a respirator remove the vapors for a period of time.
When removing the respirator for breaks, and upon placing
the mask back on, if you can smell the resin vapors, replace
the filters immediately. Sometimes, charcoal filters last less
than 4 hours. Store the respirator in a sealed bag when not in
use. If working with toxic materials for an extended period,
a supplied air mask and hood are recommended.
Avoid skin contact with the fibers and other particles by
wearing long pants and long sleeves along with gloves or
barrier creams. The eyes must be protected using leak-proof
goggles (no vent holes) when working with resins or solvents,
because chemical damage to the eyes is usually irreversible.
Fiber Reinforced Materials
The purpose of reinforcement in reinforced plastics is to
provide most of the strength. The three main forms of fiber
reinforcements are particles, whiskers, and fibers.A particle is a square piece of material. Glass bubbles (Q-cell)
are hollow glass spheres, and since their dimensions are equal
on all axes, they are called a particle.
A whisker is a piece of material that is longer than it is wide.
Whiskers are usually single crystals. They are very strong
and used to reinforce ceramics and metals.
Fibers are single filaments that are much longer than they
are wide. Fibers can be made of almost any material and are
not crystalline like whiskers. Fibers are the base for most
composites. Fibers are smaller than the finest human hair
and are normally woven into cloth-like materials.
Laminated Structures
Composites can be made with or without an inner core of
material. Laminated structure with a core center is called
a sandwich structure. Laminate construction is strong and
stiff, but heavy. The sandwich laminate is equal in strength,
and its weight is much less; less weight is very important to
aerospace products.
The core of a laminate can be made from nearly anything. The
decision is normally based on use, strength, and fabricating
methods to be used.
Various types of cores for laminated structures include rigid
foam, wood, metal, or the aerospace preference of honeycomb
made from paper, Nomex®, carbon, fiberglass, or metal.
Figure 7-14 shows a typical sandwich structure. It is very
important to follow proper techniques to construct or repair
laminated structures to ensure the strength is not compromised.
Taking a high-density laminate or solid face and back plate and
sandwiching a core in the middle make a sandwich assembly.
The design engineer, depending on the intended application
of the part, decides the selection of materials for the face
and the back plate. It is important to follow manufacturers’
maintenance manual specific instructions regarding testing
and repair procedures as they apply to a particular aircraft.
Reinforced Plastic
Reinforced plastic is a thermosetting material used in the
manufacture of radomes, antenna covers, and wingtips, and as
insulation for various pieces of electrical equipment and fuel
cells. It has excellent dielectric characteristics that make it
ideal for radomes; however, its high strength-to-weight ratio,
resistance to mildew, rust, and rot, and ease of fabrication
make it equally suited for other parts of the aircraft.
Reinforced plastic components of aircraft are formed of either
solid laminates or sandwich-type laminates. Resins used
to impregnate glass cloths are of the contact pressure type
7-32(requiring little or no pressure during cure). These resins are
supplied as a liquid, which can vary in viscosity from water
like consistency to thick syrup. Cure or polymerization is
affected by the use of a catalyst, usually benzoyl peroxide.
Solid laminates are constructed of three or more layers of
resin impregnated cloths “wet laminated” together to form
a solid sheet facing or molded shape.
Sandwich-type laminates are constructed of two or more
solid sheet facings or a molded shape enclosing a fiberglass
honeycomb or foam-type core. Honeycomb cores are made
of glass cloths impregnated with polyester or a combination
of nylon and phenolic resins. The specific density and cell
size of honeycomb cores varies over considerable latitude.
Honeycomb cores are normally fabricated in blocks that are
later cut to the desired thickness on a band saw.
Foam-type cores are formulated from combinations of
alkyd resins and metatoluene di-isocyanate. Sandwich-
type fiberglass components filled with foam-type cores are
manufactured to exceedingly close tolerances on overall
thickness of the molded facing and core material. To achieve
this accuracy, the resin is poured into a close tolerance,
molded shape. The resin formulation immediately foams up
to fill the void in the molded shape and forms a bond between
the facing and the core.
Rubber
Rubber is used to prevent the entrance of dirt, water, or air,
and to prevent the loss of fluids, gases, or air. It is also used
to absorb vibration, reduce noise, and cushion impact loads.
The term “rubber” is as all-inclusive as the term “metal.” It is
used to include not only natural rubber, but also all synthetic
and silicone rubbers.
Natural Rubber
Natural rubber has better processing and physical properties
than synthetic or silicone rubber. These properties include
flexibility, elasticity, tensile strength, tear strength, and low
heat buildup due to flexing (hysteresis). Natural rubber is a
general-purpose product; however, its suitability for aircraft
use is somewhat limited because of its inferior resistance
to most influences that cause deterioration. Although it
provides an excellent seal for many applications, it swells
and often softens in all aircraft fuels and in many solvents
(naphthas and so forth). Natural rubber deteriorates more
rapidly than synthetic rubber. It is used as a sealing material
for water/methanol systems.
Synthetic Rubber
Synthetic rubber is available in several types, each of which
is compounded of different materials to give the desired properties. The most widely used are the butyls, Bunas,
and neoprene.
Butyl is a hydrocarbon rubber with superior resistance to gas
permeation. It is also resistant to deterioration; however, its
comparative physical properties are significantly less than
those of natural rubber. Butyl resists oxygen, vegetable oils,
animal fats, alkalies, ozone, and weathering.
Like natural rubber, butyl swells in petroleum or coal
tar solvents. It has a low water absorption rate and good
resistance to heat and low temperature. Depending on
the grade, it is suitable for use in temperatures ranging
from −65 °F to 300 °F. Butyl is used with phosphate ester
hydraulic fluids (Skydrol™), silicone fluids, gases, ketones,
and acetones.
Buna-S rubber resembles natural rubber both in processing and
performance characteristics. Buna-S is as water resistant as
natural rubber, but has somewhat better aging characteristics.
It has good resistance to heat, but only in the absence of severe
flexing. Generally, Buna-S has poor resistance to gasoline, oil,
concentrated acids, and solvents. Buna-S is normally used for
tires and tubes as a substitute for natural rubber.
Buna-N is outstanding in its resistance to hydrocarbons and
other solvents; however, it has poor resilience in solvents at
low temperature. Buna-N compounds have good resistance
to temperatures up to 300 °F and may be procured for low
temperature applications down to −75 °F. Buna-N has fair
tear, sunlight, and ozone resistance. It has good abrasion
resistance and good breakaway properties when used in
contact with metal. When used as a seal on a hydraulic piston,
it does not stick to the cylinder wall. Buna-N is used for oil
and gasoline hoses, tank linings, gaskets, and seals.
Neoprene can take more punishment than natural rubber
and has better low-temperature characteristics. It possesses
exceptional resistance to ozone, sunlight, heat, and aging.
Neoprene looks and feels like rubber. Neoprene, however,
is less like rubber in some of its characteristics than butyl
or Buna. The physical characteristics of neoprene, such as
tensile strength and elongation, are not equal to natural rubber
but do have a definite similarity. Its tear resistance, as well
as its abrasion resistance, is slightly less than that of natural
rubber. Although its distortion recovery is complete, it is not
as rapid as natural rubber.
Neoprene has superior resistance to oil. Although it is good
material for use in nonaromatic gasoline systems, it has poor
resistance to aromatic gasoline. Neoprene is used primarily
for weather seals, window channels, bumper pads, oil resistant
hose, and carburetor diaphragms. It is also recommended for
Face sheet
Honeycomb
Fabricated sandwich panelAdhesive
Face sheet
Figure 7-14. Sandwich structure.
use with Freon™ and silicate ester lubricants.
Thiokol, known also as polysulfide rubber, has the highest
resistance to deterioration but ranks the lowest in physical
properties. Petroleum, hydrocarbons, esters, alcohols,
gasoline, or water, in general, does not seriously affect
Thiokols. Thiokols are ranked low in such physical properties
as compression set, tensile strength, elasticity, and tear
abrasion resistance. Thiokol is used for oil hoses, tank linings
for aromatic aviation gasoline, gaskets, and seals.
Silicone rubbers are a group of plastic rubber materials made
from silicon, oxygen, hydrogen, and carbon. The silicones have
excellent heat stability and very low temperature flexibility.
They are suitable for gaskets, seals, or other applications where
elevated temperatures up to 600 °F are prevalent. Silicone
rubbers are also resistant to temperatures down to −150 °F.
Throughout this temperature range, silicone rubber remains
extremely flexible and useful with no hardness or gumminess.
Although this material has good resistance to oils, it reacts
unfavorably to both aromatic and nonaromatic gasoline.
Silastic, one of the best-known silicones, is used to insulate
electrical and electronic equipment. Because of its dielectric
properties over a wide range of temperatures, it remains
flexible and free from crazing and cracking. Silastic is also
used for gaskets and seals in certain oil systems.
Shock Absorber Cord
Shock absorber cord is made from natural rubber strands
encased in a braided cover of woven cotton cords treated to
resist oxidation and wear. Great tension and elongation are
obtained by weaving the jacket upon the bundle of rubber
strands while they are stretched about three times their
original length.There are two types of elastic shock absorbing cord. Type I
is a straight cord, and type II is a continuous ring known as
a “bungee.” The advantages of the type II cord are that it is
easily and quickly replaced and does not need to be secured
by stretching and whipping. Shock cord is available in
standard diameters from 1⁄4 inch to 13⁄16 inch.
Three colored threads are braided into the outer cover for
the entire length of the cord. Two of these threads are of the
same color and represent the year of manufacture; the third
thread, a different color, represents the quarter of the year in
which the cord was made. The code covers a 5-year period
and then repeats itself. This makes it easy to figure forward
or backward from the years shown in Figure 7-15 .
Seals
Seals are used to prevent fluid from passing a certain point,
as well as to keep air and dirt out of the system in which they
are used. The increased use of hydraulics and pneumatics in
aircraft systems has created a need for packings and gaskets
of varying characteristics and design to meet the many
variations of operating speeds and temperatures to which they
are subjected. No one style or type of seal is satisfactory for
all installations. Some of the reasons for this are:
• Pressure at which the system operates
• Type fluid used in the system
• Metal finish and the clearance between adjacent parts
• Type motion (rotary or reciprocating), if any
Seals are divided into three main classes: packings, gaskets,
and wipers.
7-34 Y ear Threads Color
2000 2 Black
2001 2 Green
2002 2 Red
2003 2 Blue
2004 2 Yellow
2005 2 Black
2006 2 Green
2007 2 Red
2008 2 Blue
2009 2 Yellow
2010 2 Black
Quarter Marking
Quarter Threads Color
January, February, March 1 Red
April, May, June 1 Blue
July, August, September 1 Green
October, November, December 1 Yellow
Figure 7-15. Shock absorber cord color coding.Packings
Packings are made of synthetic or natural rubber. They
are generally used as “running seals,” that is, in units that
contain moving parts, such as actuating cylinders, pumps,
selector valves, and so forth. Packings are made in the form
of O-rings, V-rings, and U-rings, each designed for a specific
purpose. [Figure 7-16]
O-Ring Packings
O-ring packings are used to prevent both internal and
external leakage. This type of packing ring seals effectively
in both directions and is the type most commonly used. In
installations subject to pressures above 1,500 psi, backup
rings are used with O-rings to prevent extrusion.
When O-ring packing is subjected to pressure from both sides,
as in actuating cylinders, two backup rings must be used (one
on either side of the O-ring). When an O-ring is subject to
pressure on only one side, a single backup ring is generally
used. In this case, the backup ring is always placed on the
side of the O-ring away from the pressure.
The materials from which O-rings are manufactured have been
compounded for various operating conditions, temperatures,
and fluids. An O-ring designed specifically for use as a static
(stationary) seal, probably will not do the job when installed
on a moving part, such as a hydraulic piston. Most O-rings
are similar in appearance and texture, but their characteristics
may differ widely. An O-ring is useless if it is not compatible
with the system fluid and operating temperature.
Advances in aircraft design have necessitated new O-ring
compositions to meet changed operating conditions. Hydraulic
O-rings were originally established under AN specification
numbers (6227, 6230, and 6290) for use in MIL-H-5606 fluid
at temperatures ranging from −65 °F to +160 °F. When new
designs raised operating temperatures to a possible 275 °F,
more compounds were developed and perfected.
Recently, a compound was developed that offered improved
low-temperature performance without sacrificing high-
temperature performance, rendering the other series obsolete.
This superior material was adopted in the MS28775 series.
This series is now the standard for MIL-H-5606 systems in
which the temperature may vary from −65 °F to +275 °F.
Manufacturers provide color-coding on some O-rings, but this
is not a reliable or complete means of identification. The color-
coding system does not identify sizes but only system fluid
or vapor compatibility and, in some cases, the manufacturer.
Color codes on O-rings that are compatible with MIL-H-5606
fluid always contains blue but may also contain red or other
colors. Packings and gaskets suitable for use with Skydrol™ fluid is always coded with a green stripe, but may also have a
blue, grey, red, green, or yellow dot as a part of the color code.
Color codes on O-rings that are compatible with hydrocarbon
fluid always contains red but never contain blue. A colored
stripe around the circumference indicates that the O-ring
is a boss gasket seal. The color of the stripe indicates fluid
compatibility: red for fuel, blue for hydraulic fluid.
The coding on some rings is not permanent. On others
it may be omitted due to manufacturing difficulties or
interference with operation. Furthermore, the color-coding
system provides no means to establish the age of the O-ring
or its temperature limitations.
Because of the difficulties with color-coding, O-rings
are available in individual hermetically-sealed envelopes
labeled with all pertinent data. When selecting an O-ring for
installation, the basic part number on the sealed envelope
provides the most reliable compound identification.
Although an O-ring may appear perfect at first glance, slight
surface flaws may exist. These flaws are often capable of
preventing satisfactory O-ring performance under the variable
operating pressures of aircraft systems; therefore, O-rings
U-ring
V-ring
MaleO-ring
U-cup
Female
Figure 7-16. Packing rings.should be rejected for flaws that affect their performance.
Such flaws are difficult to detect, and one aircraft manufacturer
recommends using a 4-power magnifying glass with adequate
lighting to inspect each ring before it is installed.
By rolling the ring on an inspection cone or dowel, the
inner diameter surface can also be checked for small cracks,
particles of foreign material, or other irregularities that cause
leakage or shorten the life of the O-ring. The slight stretching
of the ring when it is rolled inside out helps to reveal some
defects not otherwise visible.
Backup Rings
Backup rings (MS28782) made of Teflon™ do not deteriorate
with age, are unaffected by any system fluid or vapor, and can
tolerate temperature extremes in excess of those encountered
in high-pressure hydraulic systems. Their dash numbers
indicate not only their size but also relate directly to the dash
number of the O-ring for which they are dimensionally suited.
They are procurable under several basic part numbers, but
they are interchangeable; that is, any Teflon™ backup ring
may be used to replace any other Teflon™ backup ring if it is
of proper overall dimension to support the applicable O-ring.
Backup rings are not color-coded or otherwise marked and
must be identified from package labels.
The inspection of backup rings should include a check to
ensure that surfaces are free from irregularities, that the edges
are clean cut and sharp, and that scarf cuts are parallel. When
checking Teflon™ spiral backup rings, make sure that the
coils do not separate more than 1⁄4 inch when unrestrained.V-Ring Packings
V-ring packings (AN6225) are one-way seals and are always
installed with the open end of the “V” facing the pressure.
V-ring packings must have a male and female adapter to
hold them in the proper position after installation. It is also
necessary to torque the seal retainer to the value specified
by the manufacturer of the component being serviced, or the
seal may not give satisfactory service. An installation using
V-rings is shown in Figure 7-17 .
U-Ring Packings
U-ring packings (AN6226) and U-cup packings are used in
brake assemblies and brake master cylinders. The U-ring and
U-cup seal pressure in only one direction; therefore, the lip of
the packings must face toward the pressure. U-ring packings
are primarily low-pressure packings to be used with pressures
of less than 1,000 psi.
Gaskets
Gaskets are used as static (stationary) seals between two
flat surfaces. Some of the more common gasket materials
are asbestos, copper, cork, and rubber. Asbestos sheeting
is used wherever a heat-resistant gasket is needed. It is
used extensively for exhaust system gaskets. Most asbestos
exhaust gaskets have a thin sheet of copper edging to
prolong their life.
A solid copper washer is used for spark plug gaskets where it
is essential to have a non-compressible, yet semisoft gasket.
Cork gaskets can be used as an oil seal between the engine
crankcase and accessories, and where a gasket is required that
can occupy an uneven or varying space caused by a rough
surface or expansion and contraction.
Rubber sheeting can be used where there is a need for a
compressible gasket. It should not be used in any place
where it may come in contact with gasoline or oil because
the rubber deteriorates very rapidly when exposed to these
substances. Gaskets are used in fluid systems around the
end caps of actuating cylinders, valves, and other units. The
gasket generally used for this purpose is in the shape of an
O-ring, similar to O-ring packings.
Wipers
Wipers are used to clean and lubricate the exposed portions
of piston shafts. They prevent dirt from entering the system
and help protect the piston shaft against scoring. Wipers may
be either metallic or felt. They are sometimes used together,
a felt wiper installed behind a metallic wiper.
Sealing Compounds
Certain areas of all aircraft are sealed to withstand
pressurization by air, to prevent leakage of fuel, to prevent
Male V-ring adapter
Female V-ring adapter V-ring packingAdjustment nuts
Figure 7-17. V-ring installation.passage of fumes, or to prevent corrosion by sealing against
the weather. Most sealants consist of two or more ingredients
properly proportioned and compounded to obtain the best
results. Some materials are ready for use as packaged, but
others require mixing before application.
One Part Sealants
One part sealants are prepared by the manufacturer and are
ready for application as packaged. However, the consistency
of some of these compounds may be altered to satisfy a
particular method of application. If thinning is desired, use
the thinner recommended by the sealant manufacturer.
Two Part Sealants
Two part sealants are compounds requiring separate
packaging to prevent cure prior to application and are
identified as the base sealing compound and the accelerator.
Any alteration of the prescribed ratios reduces the quality of
the material. Combining equal portions, by weight, of base
compound and accelerator, mixes two part sealants.
All sealant material should be carefully weighed in accordance
with the sealant manufacturer’s recommendations. Sealant
material is usually weighed with a balance scale equipped
with weights specially prepared for various quantities of
sealant and accelerator.
Before weighing the sealant materials, thoroughly stir both
the base sealant compound and the accelerator. Do not use
accelerator, which is dried out, lumpy, or flaky. Pre-weighed
sealant kits do not require weighing of the sealant and
accelerator before mixing when the entire quantity is to be
mixed.
After determining the proper amount of base sealant
compound and accelerator, add the accelerator to the
base sealant compound. Immediately after adding the
accelerator, thoroughly mix the two parts by stirring or
folding, depending on the consistency of the material.
Carefully mix the material to prevent entrapment of air in
the mixture. Overly rapid or prolonged stirring builds up
heat in the mixture and shortens the normal application time
(working life) of the mixed sealant.
To ensure a well-mixed compound, test by smearing a small
portion on a clean, flat metal, or glass surface. If flecks or
lumps are found, continue mixing. If the flecks or lumps
cannot be eliminated, reject the batch.
The working life of mixed sealant is from 1⁄2 hour to 4 hours
(depending upon the class of sealant); therefore, apply
mixed sealant as soon as possible or place in refrigerated
storage. Figure 7-18 presents general information concerning various sealants.
The curing rate of mixed sealants varies with changes in
temperature and humidity. Curing of sealants is extremely slow
if the temperature is below 60 °F. A temperature of 77 °F with
50 percent relative humidity is the ideal condition for curing
most sealants.
Curing may be accelerated by increasing the temperature, but
the temperature should never be allowed to exceed 120 °F at
any time in the curing cycle. Heat may be applied by using
infrared lamps or heated air. If heated air is used, it must be
properly filtered to remove moisture and dirt.
Heat should not be applied to any faying surface sealant
installation until all work is completed. All faying surface
applications must have all attachments, permanent or
temporary, completed within the application limitations of
the sealant.
Sealant must be cured to a tack-free condition before applying
brush top coatings. (Tack-free consistency is the point at
which a sheet of cellophane pressed onto the sealant no
longer adheres.)
Aircraft Hardware
Aircraft hardware is the term used to describe the various
types of fasteners and miscellaneous small items used in
the manufacture and repair of aircraft. The importance of
aircraft hardware is often overlooked because of its small
size; however, the safe and efficient operation of any aircraft
is greatly dependent upon the correct selection and use of
aircraft hardware.
An aircraft, even though made of the best materials and
strongest parts, would be of doubtful value unless those parts
7-37were firmly held together. Several methods are used to hold
metal parts together; they include riveting, bolting, brazing,
and welding. The process used must produce a union that is
as strong as the parts that are joined.
Identification
Their specification number or trade name identifies most
items of aircraft hardware. Threaded fasteners and rivets are
identified by AN (Air Force-Navy) numbers, NAS (National
Aircraft Standard) numbers, or MS (Military Standard)
numbers. Quick-release fasteners are usually identified by
factory trade names and size designations.
Threaded Fasteners
Various types of fastening devices allow quick dismantling
or replacement of aircraft parts that must be taken apart and
put back together at frequent intervals. Riveting or welding
these parts each time they are serviced would soon weaken
or ruin the joint. Furthermore, some joints require greater
tensile strength and stiffness than rivets can provide. Bolts
and screws are two types of fastening devices that give the
required security of attachment and rigidity. Generally, bolts
are used where great strength is required, and screws are used
where strength is not the deciding factor. Bolts and screws
are similar in many ways. They are both used for fastening
or holding, and each has a head on one end and screw threads
on the other. Regardless of these similarities, there are several
distinct differences between the two types of fasteners. The
threaded end of a bolt is always blunt while that of a screw
may be either blunt or pointed.
The threaded end of a bolt usually has a nut screwed onto it
to complete the assembly. The threaded end of a screw may
fit into a female receptacle, or it may fit directly into the
material being secured. A bolt has a short threaded section
and a comparatively long grip length or unthreaded portion,
whereas a screw has a longer threaded section and may have
no clearly defined grip length. Turning the nut on the bolt
generally tightens a bolt assembly; the head of the bolt may
or may not be designed for turning. Turning its head always
tightens a screw.
When it becomes necessary to replace aircraft fasteners, a
duplicate of the original fastener should be used if possible.
If duplicate fasteners are not available, extreme care and
caution must be used in selecting substitutes.
Classification of Threads
Aircraft bolts, screws, and nuts are threaded in the American
National Coarse (NC) thread series, the American National
Fine (NF) thread series, the American Standard Unified
Coarse (UNC) thread series, or the American Standard
Unified Fine (UNF) thread series. There is one difference between the American National series and the American
Standard Unified series that should be pointed out. In the
1-inch diameter size, the NF thread specifies 14 threads per
inch (1-14 NF), while the UNF thread specifies 12 threads
per inch (1-12 UNF). Both types of threads are designated
by the number of times the incline (threads) rotates around a
1-inch length of a given diameter bolt or screw. For example,
a 1/4-28 thread indicates that a 1⁄4-inch (4⁄16 inch) diameter bolt
has 28 threads in 1 inch of its threaded length.
Class of fit also designates threads. The Class of a thread
indicates the tolerance allowed in manufacturing:
• Class 1 is a loose fit
• Class 2 is a free fit
• Class 3 is a medium fit
• Class 4 is a close fit
Aircraft bolts are almost always manufactured in the Class
3, medium fit.
A Class 4 fit requires a wrench to turn the nut onto a bolt,
whereas a Class 1 fit can easily be turned by hand. Generally,
aircraft screws are manufactured with a Class 2 thread fit for
ease of assembly.
Bolts and nuts are also produced with right-hand and left-hand
threads. A right-hand thread tightens when turned clockwise;
a left-hand thread tightens when turned counterclockwise.
Aircraft Bolts
Aircraft bolts are fabricated from cadmium- or zinc-plated
corrosion-resistant steel, un-plated corrosion-resistant steel,
or anodized-aluminum alloys. Most bolts used in aircraft
structures are either general purpose, AN bolts, NAS
internal wrenching or close tolerance bolts, or MS bolts. In
certain cases, aircraft manufacturers make bolts of different
dimensions or greater strength than the standard types. Such
bolts are made for a particular application, and it is of extreme
importance to use like bolts in replacement. The letter “S”
stamped on the head usually identifies special bolts.
AN bolts come in three head styles: hex head, Clevis, and
eyebolt. [Figure 7-19] NAS bolts are available in hex head,
internal wrenching, and countersunk head styles. MS bolts
come in hex head and internal wrenching styles.
General Purpose Bolts
The hex head aircraft bolt (AN-3 through AN-20) is an
all-purpose structural bolt used for general applications
involving tension or shear loads where a light drive fit is
permissible (0.006-inch clearance for a 5⁄8-inch hole and
7-38EC-801 (black)
MIL-S-7502A
Class B-2
EC-800 (red)
EC-612 P
(pink)
MIL-P-20628
PR-1302HT
(red)
MIL-S-8784
PR-727
potting
compound
MIL-S-8516B
HT-3
(grey–green)
EC-776
(clear amber)
MIL-S-4383B12 parts of
EC-807 to 100
parts of
EC-801
Use as is
Use as is
10 parts of
PR-1302HT-A
to 100 parts
of PR-1302HT
12 parts of
PR-727A to
100 parts of
PR-727
Use as isUse as is2–4 hours
8–12 hours
Indefi nite
non-drying
2–4 hours
1½ hours
minimum
Solvent
release, sets
up in 2–4
hours
8–12 hours6 months
6–9 months
6–9 months
6 months
6 months
6–9 months
Indefinite
in airtight
containers5 days at
−20 °F after
flash freeze
at −65 °F
Not applicable
Not applicable
5 days at
−20 °F after
flash freeze
at −65 °F
5 days at
−20 °F after
flash freeze
at −65 °F
Not applicable
Not applicableFaying surfaces,
fillet seals, and
packing gaps
Coating rivet
Packing voids
up to ¼"
Sealing access
door gaskets
Potting electrical
connections and
bulkhead seals
Sealing hot air
ducts passing
through
bulkheads
Top coating−65 °F to 200 °F
−65 °F to 200 °F
−40 °F to 200 °F
−65 °F to 200 °F
−65 °F to 200 °F
−60 °F to 200 °F
−65 °F to 200 °FEC-807
None
None
PR-1302HT-A
PR-727A
NoneNoneSealant BaseAccelerator
(Catalyst)Mixing Ratio
by WeightApplication
Life (Work)Storage
(Shelf) Life
After MixingStorage
(Shelf) Life
UnmixedTemperature
RangeApplication
and Limitations
Figure 7-18. General sealant information.
other sizes in proportion).
Alloy-steel bolts smaller than No. 10-32 and aluminum-
alloy bolts smaller than 1⁄4 inch in diameter are not used in
primary structures. Aluminum-alloy bolts and nuts are not used where they are repeatedly removed for purposes of maintenance and inspection. Aluminum-alloy nuts may be used with cadmium-plated steel bolts loaded in shear on land airplanes, but are not used on seaplanes due to the increased possibility of dissimilar metal corrosion.The AN-73 drilled head bolt is like the standard hex bolt, but has a deeper head, which is drilled to receive wire for safetying. The AN-3 and the AN-73 series bolts are interchangeable, for all practical purposes, from the standpoint of tension and shear strengths.
Close Tolerance Bolts
Close tolerance bolts are machined more accurately than the general-purpose bolt. Close tolerance bolts may be hex headed (AN-173 through AN-186) or have a 100° countersunk head (NAS-80 through NAS-86). They are used in applications where a tight drive fit is required. (The bolt moves into position only when struck with a 12- to 14-ounce hammer.)
Internal Wrenching Bolts
Internal wrenching bolts, (MS-20004 through MS-20024 or NAS-495) are fabricated from high-strength steel and are suitable for use in both tension and shear applications. When they are used in steel parts, the bolt hole must be slightly countersunk to seat the large corner radius of the shank at the head. In Dural material, a special heat-treated washer must be used to provide an adequate bearing surface for the head. The head of the internal wrenching bolt is recessed to allow the insertion of an internal wrench when installing or removing the bolt. Special high-strength nuts are used on these bolts. Replace an internal wrenching bolt with another internal wrenching bolt. Standard AN hex head bolts and washers cannot be substituted for them, as they do not have the required strength.
Identification and Coding
Bolts are manufactured in many shapes and varieties. A clear-cut method of classification is difficult. The shape of the head, method of securing, material used in fabrication, or the expected usage can identify bolts.
AN-type aircraft bolts can be identified by the code markings
on the bolt heads. The markings generally denote the bolt manufacturer, the material used to make the bolt, and whether the bolt is a standard AN-type or a special purpose bolt.
• AN st andard steel bolts are marked with either a raised
7-39dash or asterisk or a single raised dash.
• AN aluminum-alloy bolts are marked with two raised
dashes to indicate corrosion-resistant steel.
• Additional information, such as bolt diameter, bolt
length, and grip length, may be obtained from the bolt
part number.
For example, in the bolt part number AN3DD5A,
• The “AN” designates that it is an Air Force-Navy
standard bolt.
• The “3” indicates the diameter in sixteenths of an inch
(3⁄16).
• The “DD” indicates the material is 2024 aluminum
alloy.
• The letter “C” in place of the “DD” would indicate
corrosion-resistant steel, and the absence of the letters
would indicate cadmium-plated steel.
• The “5” indicates the length in eighths of an inch (5⁄8).
• the “A” indicates that the shank is undrilled. If the
letter “H” preceded the “5” in addition to the “A”
following it, the head would be drilled for safetying.
Close tolerance NAS bolts are marked with either a raised
or recessed triangle. The material markings for NAS bolts
are the same as for AN bolts, except that they may be either
raised or recessed. Bolts inspected magnetically (Magnaflux)
or by fluorescent means (Zyglo) are identified by means of
colored lacquer or a head marking of a distinctive type.
Special-Purpose Bolts
Bolts designed for a particular application or use is classified
as special-purpose bolts. Clevis bolts, eyebolts, Jo-bolts, and
lockbolts are special-purpose bolts.
Clevis Bolts
The head of a Clevis bolt is round and is either slotted to
receive a common screwdriver or recessed to receive a cross
point screwdriver. This type of bolt is used only where shear
loads occur and never in tension. It is often inserted as a
mechanical pin in a control system.
Eyebolt
The eyebolt is a special-purpose bolt used where external
tension loads are to be applied. The eyebolt is designed for
the attachment of devices, such as the fork of a turnbuckle,
a Clevis, or a cable shackle. The threaded end may or may
not be drilled for safetying.
Jo-Bolt
Jo-bolt is a trade name for an internally threaded three-piece rivet. The Jo-bolt consists of three parts: a threaded steel-alloy
bolt, a threaded steel nut, and an expandable stainless steel
sleeve. The parts are factory preassembled. As the Jo-bolt is
installed, the bolt is turned while the nut is held. This causes
the sleeve to expand over the end of the nut, forming the
blind head and clamping against the work. When driving is
complete, a portion of the bolt breaks off. The high shear and
tensile strength of the Jo-bolt makes it suitable for use in cases
of high stresses where some of the other blind fasteners would
not be practical. Jo-bolts are often a part of the permanent
structure of late model aircraft. They are used in areas that are
not often subjected to replacement or servicing. (Because it is
a three-part fastener, it should not be used where any part, in
becoming loose, could be drawn into the engine air intake.)
Other advantages of using Jo-bolts are their excellent resistance
to vibration, weight saving, and fast installation by one person.
Presently, Jo-bolts are available in four diameters:
• 200 series, approximately 3⁄16 inch in diameter
• 260 series, approximately 1⁄4 inch in diameter
• 312 series, approximately 5⁄16 inch in diameter
• 375 series, approximately 3⁄8 inch in diameter.
Jo-bolts are available in three head styles: F (flush), P (hex
head), and FA (flush millable).
Lockbolts
Lockbolts are used to attach two materials permanently. They
are lightweight and are equal in strength to standard bolts.
Lockbolts are manufactured by several companies and conform
to Military Standards, which specify the size of a lockbolt’s
head in relation to the shank diameter, plus the alloy used in
its construction. The only drawback to lockbolt installations is
that they are not easily removable compared to nuts and bolts.
The lockbolt combines the features of a high-strength bolt and
rivet, but it has advantages over both. The lockbolt is generally
used in wing splice fittings, landing gear fittings, fuel cell
fittings, longerons, beams, skin splice plates, and other major
structural attachments. It is more easily and quickly installed
than the conventional rivets or bolts and eliminates the use
of lock washers, cotter pins, and special nuts. Like the rivet,
the lockbolt requires a pneumatic hammer or “pull gun” for
installation. When installed, it is rigidly and permanently
locked in place. Three types of lockbolts are commonly used:
the pull type, the stump type, and the blind type. [Figure 7-20]
Pull Type
Pull-type lockbolts are used mainly in aircraft primary and
secondary structures. They are installed very rapidly and have
approximately one-half the weight of equivalent AN steel
AIR
ASSOCIATES
A RE
O
N MS
COOPERA037
4AN
C
SPEC
W
F O
X
S
Magnetically inspectedSpecial boltSpecial bolt
Drilled head bolt Special bolt NAS close tolerance bolt
Clevis bolt Reworked bolt Low strength
material boltAluminum alloy
(2024) boltAN standard steel boltAN standard steel bolt AN standard steel boltEyebolt Clevis boltCountersunk head bolt Internal hex head boltStandard head bolt Drilled hex head bolt
AN standard steel bolt AN standard steel bolt
AN standard steel bolt AN standard steel bolt AN standard steel boltAN standard steel bolt
(corrosion resistant)
Orange-dyed
magnetically inspected
AN
Figure 7-19. Aircraft bolt identification.
bolts and nuts. A special pneumatic “pull gun” is required
to install this type of lockbolt. One person can accomplish
installation since bucking is not required.
Stump Type
Stump-type lockbolts, although they do not have the extended
stem with pull grooves, are companion fasteners to pull-type lockbolts. They are used primarily where clearance does
not permit installation of the pull-type lockbolt. A standard
pneumatic riveting hammer (with a hammer set attached
for swaging the collar into the pin locking grooves) and a
bucking bar are tools necessary for the installation of stump-
type lockbolts.
Pull type Stump type Blind type
Figure 7-20. Lockbolt types.Blind Type
Blind-type lockbolts come as complete units or assemblies.
They have exceptional strength and sheet pull-together
characteristics. Blind lockbolts are used where only one side
of the work is accessible and, generally, where it is difficult
to drive a conventional rivet. This type of lockbolt is installed
in the same manner as the pull-type lockbolt.
Common Features
Common features of the three types of lockbolts are the
annular locking grooves on the pin and the locking collar,
which is swaged into the pin’s lock grooves to lock the pin
in tension. The pins of the pull- and blind-type lockbolts are
extended for pull installation. The extension is provided with
pulling grooves and a tension breakoff groove.
Composition
The pins of pull- and stump-type lockbolts are made of heat-
treated alloy steel or high-strength aluminum alloy. Companion
collars are made of aluminum alloy or mild steel. The blind
lockbolt consists of a heat-treated alloy steel pin, blind sleeve
and filler sleeve, mild steel collar, and carbon steel washer.
Substitution
Alloy-steel lockbolts may be used to replace steel high-shear
rivets, solid steel rivets, or AN bolts of the same diameter and
head type. Aluminum-alloy lockbolts may be used to replace
solid aluminum-alloy rivets of the same diameter and head
type. Steel and aluminum-alloy lockbolts may also be used to
replace steel and 2024T aluminum-alloy bolts, respectively,
of the same diameter. Blind lockbolts may be used to replace
solid aluminum-alloy rivets, stainless steel rivets, or all blind
rivets of the same diameter.
Numbering System
The numbering systems for the various types of lockbolts are
explained by the break-outs in Figure 7-21 .
Grip Range
To determine the bolt grip range required for any application,
measure the thickness of the material with a hook scale
inserted through the hole. Once this measurement is
determined, select the correct grip range by referring to the
charts provided by the rivet manufacturer. Examples of grip
range charts are shown in Figures 7-22 and 7-23.
When installed, the lockbolt collar should be swaged
substantially throughout the complete length of the collar. The
tolerance of the broken end of the pin relative to the top of the
collar must be within the dimensions given in Figure 7-24 .
When removal of a lockbolt becomes necessary, remove the collar by splitting it axially with a sharp, cold chisel. Be careful
not to break out or deform the hole. The use of a backup bar
on the opposite side of the collar being split is recommended.
The pin may then be driven out with a drift punch.
Aircraft Nuts
Aircraft nuts are made in a variety of shapes and sizes. They
are made of cadmium-plated carbon steel, stainless steel, or
anodized 2024T aluminum alloy and may be obtained with
either right- or left-hand threads. No identifying marking or
lettering appears on nuts. Only the characteristic metallic
luster or color of the aluminum, brass, or the insert can
identify them when the nut is of the self-locking type. They
can be further identified by their construction.
Aircraft nuts can be divided into two general groups: non-
self-locking and self-locking nuts. Non-self-locking nuts
are those that must be safetied by external locking devices,
such as cotter pins, safety wire, or locknuts. Self-locking nuts
contain the locking feature as an integral part.
Non-Self-Locking Nuts
Most of the familiar types of nuts, including the plain nut,
the castle nut, the castellated shear nut, the plain hex nut, the
light hex nut, and the plain check nut are the non-self-locking
type. [Figure 7-25]
7-42LC Lockbolt collar
C Material
C = 24ST aluminum alloy (green color). Use with heat-treated alloy lockbolts only.F = 61ST aluminum alloy (plain color). Use with 75ST aluminum alloy lockbolts only.R = mild steel (cadmium plated). Use with heat-treated alloy steel
lockbolts for high temperature applications only.
Diameter of a pin in 32nds of an inch
CLockbolt collar
LC C C
ALSF Head type
ASCT509 = close tolerance AN-509 C-sink head
ALSF = f lathead type
ALS509 = standard AN-509 C-sink head
ALS426 = standard AN-426 C-sink head
Pin materials
E = 75S-T6 aluminum alloy
T = heat-treated alloy steel
Body diameter in 32nds of an inch
Grip length in 16ths of an inchE
8Stump-type lockbolt
ALSF E 8 8 ALPP
H
THead type
ACT509 = close tolerance AN-509 C-sink head
ALPP = pan head
ALPB = brazier head
ALP509 = standard AN-509 C-sink head
ALP426 = standard AN-426 C-sink head
Class fit
H = hole fi lling (interference fi t)
N = non-hole filling (clearance fi t)
Pin Materials
E = 75S-T6 aluminum alloy
T = heat-treated alloy steel
Body diameter in 32nds of an inch
Grip length in 16ths of an inch8
8Pull-type lockbolt
ALPP H T 8 8
BL Blind lockbolt
Diameter in 32nds of an inch
Grip length in 16ths of an inch, ± 1⁄32 inch 8
4Blind-type lockbolt
BL 8 4
Figure 7-21. Lockbolt numbering system.The castle nut, AN310, is used with drilled shank AN hex
head bolts, Clevis bolts, eyebolts, drilled head bolts, or studs. It is rugged and can withstand large tensional loads. Slots (called castellations) in the nut are designed to accommodate a cotter pin or lock wire for safety.
The castellated shear nut, AN320, is designed for use with
devices, such as drilled Clevis bolts and threaded taper pins, which are normally subjected to shearing stress only. Like the castle nut, it is castellated for safetying. Note, however, that the nut is not as deep or as strong as the castle nut; also, that the castellations are not as deep as those in the castle nut.
The plain hex nut, AN315 and AN335 (fine and coarse
thread), is of rugged construction. This makes it suitable for carrying large tensional loads. However, since it requires an auxiliary locking device, such as a check nut or lock washer, its use on aircraft structures is somewhat limited.
The light hex nut, AN340 and AN345 (fine and coarse
thread), is a much lighter nut than the plain hex nut and must be locked by an auxiliary device. It is used for miscellaneous light tension requirements.
The plain check nut, AN316, is employed as a locking device
for plain nuts, set screws, threaded rod ends, and other devices.The wing nut, AN350, is intended for use where the desired tightness can be obtained by hand and where the assembly is frequently removed.
Self-Locking Nuts
As their name implies, self-locking nuts need no auxiliary means of safetying but have a safetying feature included as an integral part of their construction. Many types of self-locking nuts have been designed and their use has become quite widespread. Common applications are:
• At tachment of antifriction bearings and control pulleys
•Attachment of accessories, anchor nuts around
inspection holes, and small tank installation openings
•Attachment of rocker box covers and exhaust stacks
Self-locking nuts are acceptable for use on certificated aircraft subject to the restrictions of the manufacturer. Self-locking nuts are used on aircraft to provide tight connections that do not shake loose under severe vibration. Do not use self-locking nuts at joints, which subject either the nut or bolt to rotation. They may be used with antifriction bearings and control pulleys, provided the inner race of the bearing is clamped to the supporting structure by the nut and bolt. Plates must be attached to the structure in a positive manner to eliminate rotation or misalignment when tightening the bolts or screws.
7-43Grip
NumberGrip RangeGrip
NumberGrip Range
Minimum Maximum Minimum Maximum
10111213141516170.0310.0940.1560.2190.2810.3440.4060.4690.5310.5940.6560.7180.7810.8430.9060.9681.0310.0940.1560.2190.2810.3440.4060.4690.5310.5940.6560.7180.7810.8430.9060.9681.0311.094181920212223242526272829303132331.0941.1561.2191.2811.3441.4061.4691.5311.5941.6561.7181.7811.8431.9061.9682.0311.1561.2191.2811.3441.4061.4691.5311.5941.6561.7181.7811.8431.9061.9682.0312.094
Figure 7-22. Pull-and stump-type lockbolt grip ranges.¼" Diameter
Grip
NumberGrip Range Grip
NumberGrip Range
1011121314151617181920212223242523456789
1011121314151617181920212223240.0310.0940.1560.2190.2810.3440.4060.4690.5310.5940.6560.7180.7810.8430.9060.9681.0311.0941.1561.2191.2811.3441.4061.4691.5310.0940.1560.2190.2810.3440.4060.4690.5310.5940.6560.7180.7810.8430.9060.9681.0311.0941.1561.2191.2811.3431.4061.4600.1560.2190.2810.3440.4060.4690.5310.5940.6560.7180.7810.8430.9060.9681.0311.0941.1561.2191.2811.3431.4061.4691.5310.0940.1560.2190.2810.3440.4060.4690.5310.5940.6560.7180.7810.8430.9060.9681.0311.0941.1561.2191.2811.3431.4061.4691.5311.594
Minimum Maximum Minimum Maximum
Figure 7-23. Blind-type lockbolt grip ranges.
Pin diameterTolerance
Below Above
0.079 to 0.032
0.079 to 0.0500.079 to 0.0500.079 to 0.060
Figure 7-24. Pin tolerance ranges.The two general types of self-locking nuts currently in use
are the all-metal type and the fiber lock type. For the sake of simplicity, only three typical kinds of self-locking nuts are considered in this handbook: the Boots self-locking and the stainless steel self-locking nuts, representing the all-metal types; and the elastic stop nut, representing the fiber insert type.
Boots Self-Locking Nut
The Boots self-locking nut is of one piece, all-metal
construction designed to hold tight despite severe vibration. Note in Figure 7-26 that it has two sections and is essentially
two nuts in one: a locking nut and a load-carrying nut. The two sections are connected with a spring, which is an integral part of the nut.
The spring keeps the locking and load-carrying sections such
a distance apart that the two sets of threads are out of phase or spaced so that a bolt, which has been screwed through the load-carrying section, must push the locking section outward against the force of the spring to engage the threads of the locking section properly.
The spring, through the medium of the locking section, exerts
a constant locking force on the bolt in the same direction as a force that would tighten the nut. In this nut, the load-carrying section has the thread strength of a standard nut of comparable
size, while the locking section presses against the threads of the bolt and locks the nut firmly in position. Only a wrench applied to the nut loosens it. The nut can be removed and reused without impairing its efficiency.
Boots self-locking nuts are made with three different spring
styles and in various shapes and sizes. The wing type that is the most common ranges in size for No. 6 up to
1⁄4 inch, the
Rol-top ranges from 1⁄4 inch to 1⁄6 inch, and the bellows type
ranges in size from No. 8 up to 3⁄8 inch. Wing-type nuts are
made of anodized aluminum alloy, cadmium-plated carbon 5/16" Diameter
3/16
¼
5/16
3/8
7-44Top view Profile view Top view Profile view
AN310
AN320
AN315
AN335AN340
AN345
AN316
AN350Top view Profile view Top view Profile view
AN310
AN320
AN315
AN335AN340
AN345
AN316
AN350
Figure 7-25. Non-self-locking nuts.steel, or stainless steel. The Rol-top nut is cadmium-plated
steel, and the bellows type is made of aluminum alloy only.
Stainless Steel Self-Locking Nut
The stainless steel self-locking nut may be spun on and off
by hand as its locking action takes places only when the
nut is seated against a solid surface and tightened. The nut
consists of two parts: a case with a beveled locking shoulder
and key and a thread insert with a locking shoulder and
slotted keyway. Until the nut is tightened, it spins on the
bolt easily, because the threaded insert is the proper size
for the bolt. However, when the nut is seated against a solid
surface and tightened, the locking shoulder of the insert is
pulled downward and wedged against the locking shoulder
of the case. This action compresses the threaded insert and
causes it to clench the bolt tightly. The cross-sectional view
in Figure 7-27 shows how the key of the case fits into the
slotted keyway of the insert so that when the case is turned,
the threaded insert is turned with it. Note that the slot is wider
than the key. This permits the slot to be narrowed and the
insert to be compressed when the nut is tightened.
Elastic Stop Nut
The elastic stop nut is a standard nut with the height
increased to accommodate a fiber locking collar. This
fiber collar is very tough and durable and is unaffected by
immersion in hot or cold water or ordinary solvents, such
as ether, carbon tetrachloride, oils, and gasoline. It will not damage bolt threads or plating.
As shown in Figure 7-28 , the fiber locking collar is not
threaded, and its inside diameter is smaller than the largest
diameter of the threaded portion or the outside diameter of
a corresponding bolt. When the nut is screwed onto a bolt, it
acts as an ordinary nut until the bolt reaches the fiber collar.
When the bolt is screwed into the fiber collar, however,
friction (or drag) causes the fiber to be pushed upward. This
creates a heavy downward pressure on the load carrying part
and automatically throws the load carrying sides of the nut
and bolt threads into positive contact. After the bolt has been
forced all the way through the fiber collar, the downward
pressure remains constant. This pressure locks and holds the
nut securely in place even under severe vibration.
Nearly all elastic stop nuts are steel or aluminum alloy.
However, such nuts are available in practically any kind of
metal. Aluminum-alloy elastic stop nuts are supplied with an
anodized finish. Steel nuts are cadmium plated.
Normally, elastic stop nuts can be used many times with
complete safety and without detriment to their locking
efficiency. When reusing elastic stop nuts, be sure the fiber
has not lost its locking friction or become brittle. If a nut can
be turned with the fingers, replace it.
After the nut has been tightened, make sure the rounded or
Boots aircraft nut
Flexloc nut Fiber locknut Elastic stop nutElastic anchor nut
Figure 7-26. Self-locking nuts.
Tightened nutUntightened nutNut case
Threaded nut coreLocking shoulder
Keyway
Figure 7-27. Stainless steel self-locking nut.chamfered end of the bolts, studs, or screws extends at least
the full round or chamfer through the nut. Flat end bolts,
studs, or screws should extend at least 1⁄32 inch through the
nut. Bolts of 5⁄16-inch diameter and over with cotter pin holes
may be used with self-locking nuts, but only if free from
burrs around the holes. Bolts with damaged threads and
rough ends are not acceptable. Do not tap the fiber locking
insert. The self-locking action of the elastic stop nut is the
result of having the bolt threads impress themselves into the
untapped fiber.
Do not install elastic stop nuts in places where the temperature
is higher than 250 °F, because the effectiveness of the self-
locking action is reduced beyond this point. Self-locking
nuts may be used on aircraft engines and accessories when
the engine manufacturer specifies their use.
Self-locking nut bases are made in several forms and
materials for riveting and welding to aircraft structure
or parts. [Figure 7-29] Certain applications require the
installation of self-locking nuts in channels, an arrangement
that permits the attachment of many nuts with only a few
rivets. These channels are track-like bases with regularly
spaced nuts, which are either removable or non-removable.
The removable type carries a floating nut that can be snapped
in or out of the channel, thus making possible the easy
removal of damaged nuts. Nuts, such as the clinch-type and
spline-type, depend on friction for their anchorage and are
not acceptable for use in aircraft structures.
Sheet Spring Nuts
Sheet spring nuts, such as speed nuts, are used with standard
and sheet metal self-tapping screws in non-structural
locations. They find various uses in supporting line clamps,
conduit clamps, electrical equipment, access doors, and the
like and are available in several types. Speed nuts are made
from spring steel and are arched prior to tightening. This
arched spring lock prevents the screw from working loose. These nuts should be used only where originally used in the
fabrication of the aircraft.
Internal & External Wrenching Nuts
Two commercial types of high-strength internal or external
wrenching nuts are available; they are the internal and
external wrenching elastic stop nut and the Unbrako internal
and external wrenching nut. Both are of the self-locking
type, are heat-treated, and can carry high-strength bolt
tension loads.
Identification & Coding
Part numbers designate the type of nut. The common types
and their respective part numbers are:
• Plain, AN315 and AN335
• Castle, AN310
• Plain check, AN316
• Light hex, AN340 and AN345
• Castellated shear, AN320
The patented self-locking types are assigned part numbers
ranging from MS20363 through MS20367. The Boots, the
Flexloc, the fiber locknut, the elastic stop nut, and the self-
locking nut belong to this group. Part number AN350 is
assigned to the wing nut.
Letters and digits following the part number indicate such
Nut Fiber collar
Figure 7-28. Elastic stop nut.items as material, size, threads per inch, and whether the
thread is right or left hand. The letter “B” following the part
number indicates the nut material to be brass, a “D” indicates
2017-T aluminum alloy, a “DD” indicates 2024-T aluminum
alloy, a “C” indicates stainless steel, and a dash in place of
a letter indicates cadmium-plated carbon steel.
The digit (or two digits) following the dash or the material
code letter is the dash number of the nut, and it indicates the
size of the shank and threads per inch of the bolt on which
the nut fits. The dash number corresponds to the first figure
appearing in the part number coding of general purpose bolts.
A dash and the number 3, for example, indicate that the nut
fits an AN3 bolt (10-32); a dash and the number 4 means it
fits an AN4 bolt (1⁄4-28); a dash and the number 5, an AN5
bolt (5⁄16-24); and so on.
The code numbers for self-locking nuts end in three or four
digit numbers. The last two digits refer to threads per inch,
and the one or two preceding digits stand for the nut size in
16ths of an inch.
Some other common nuts and their code numbers are:
Code Number AN310D5R:
AN310 = aircraft castle nut D = 2024-T aluminum alloy
5 = 5⁄16 inch diameter
R = right-hand thread (usually 24 threads per inch)
Code Number AN320-10:
AN320 = aircraft castellated shear nut, cadmium-plated
carbon steel
10 = 5⁄8 inch diameter, 18 threads per inch (this nut is usually
right-hand thread)
Code Number AN350B1032:
AN350 = aircraft wing nut
B = brass
10 = number 10 bolt
32 = threads per inch
Aircraft Washers
Aircraft washers used in airframe repair are either plain, lock,
or special type washers.
Plain Washers
Plain washers, both the AN960 and AN970, are used under
hex nuts. [Figure 7-30] They provide a smooth bearing
surface and act as a shim in obtaining correct grip length for a
bolt and nut assembly. They are used to adjust the position of
castellated nuts in respect to drilled cotter pin holes in bolts.
Use plain washers under lock washers to prevent damage to
the surface material.
Aluminum and aluminum-alloy washers may be used under
bolt heads or nuts on aluminum alloy or magnesium structures
where corrosion caused by dissimilar metals is a factor. When
used in this manner, any electric current flow is between the
washer and the steel bolt. However, it is common practice
to use a cadmium-plated steel washer under a nut bearing
directly against a structure as this washer resists the cutting
action of a nut better than an aluminum-alloy washer.
The AN970 steel washer provides a greater bearing area than
the AN960 washer and is used on wooden structures under both
the head and the nut of a bolt to prevent crushing the surface.
Lock Washers
Lock washers, both the AN935 and AN936, are used with
machine screws or bolts where the self-locking or castellated-
type nut is not appropriate. The spring action of the washer
(AN935) provides enough friction to prevent loosening of
the nut from vibration. [Figure 7-30]
Lock washers should never be used under the following
conditions:
Boots aircraft channel assembly
Elastic stopnut channel assembly
Figure 7-29. Self-locking nut bases.
• With fasteners to primary or secondary structures
• With fasteners on any part of the aircraft where failure
might result in damage or danger to the aircraft or
personnel
• Where failure would permit the opening of a joint to
the airflow
• Where the screw is subject to frequent removal
• Where the washers are exposed to the airflow
• Where the washers are subject to corrosive conditions
• Where the washer is against soft material without a
plain washer underneath to prevent gouging the surface
Shake-Proof Lock Washers
Shake-proof lock washers are round washers designed with
tabs or lips that are bent upward across the sides of a hex nut
or bolt to lock the nut in place. There are various methods
of securing the lock washer to prevent it from turning, such
as an external tab bent downward 90° into a small hole in
the face of the unit or an internal tab that fits a keyed bolt.
Shake-proof lock washers can withstand higher heat than
other methods of safetying and can be used under high
vibration conditions safely. They should be used only once,
because the tabs tend to break when bent a second time.
Special Washers
The ball socket and seat washers, AC950 and AC955, are
special washers used where a bolt is installed at an angle to a surface or where perfect alignment with a surface is required.
These washers are used together. [Figure 7-30]
The NAS143 and MS20002 washers are used for internal
wrenching bolts of the NAS144 through NAS158 series.
This washer is either plain or countersunk. The countersunk
washer (designated as NAS143C and MS20002C) is used
to seat the bolt head shank radius, and the plain washer is
used under the nut.
Installation of Nuts, Washers, & Bolts
Bolt & Hole Sizes
Slight clearances in bolt holes are permissible wherever bolts
are used in tension and are not subject to reversal of load. A
few of the applications in which clearance of holes may be
permitted are in pulley brackets, conduit boxes, lining trim,
and miscellaneous supports and brackets.
Bolt holes are to be normal to the surface involved to provide
full bearing surface for the bolt head and nut and must not be
oversized or elongated. A bolt in such a hole carries none of
its shear load until parts have yielded or deformed enough
to allow the bearing surface of the oversized hole to contact
the bolt. In this respect, remember that bolts do not become
swaged to fill up the holes, as do rivets.
In cases of oversized or elongated holes in critical members,
obtain advice from the aircraft or engine manufacturer before
drilling or reaming the hole to take the next larger bolt.
Usually such factors as edge distance, clearance, or load
Plain AN 960
Split-Lock Internal toothlock External toothlockBall seat & socket
AC9950 & AC955Taper pin AN975
Plain AN 935 Star lock washersSpecial washers
Figure 7-30. Various types of washers.factor must be considered. Oversized or elongated holes in
noncritical members can usually be drilled or reamed to the
next larger size.
Many bolt holes, particularly those in primary connecting
elements, have close tolerances. Generally, it is permissible
to use the first lettered drill size larger than the normal bolt
diameter, except where the AN hexagon bolts are used in
light-drive fit (reamed) applications and where NAS close
tolerance bolts or AN Clevis bolts are used.
Light-drive fits for bolts (specified on the repair drawings as
0.0015 inch maximum clearance between bolt and hole) are
required in places where bolts are used in repair, or where
they are placed in the original structure.
The fit of holes and bolts cannot be defined in terms of shaft
and hole diameters; it is defined in terms of the friction between
bolt and hole when sliding the bolt into place. A tight drive fit,
for example, is one in which a sharp blow of a 12- or 14-ounce
hammer is required to move the bolt. A bolt that requires a hard
blow and sounds tight is considered too tight a fit. A light-drive
fit is one in which a bolt moves when a hammer handle is held
against its head and pressed by the weight of the body.
Installation Practices
Examine the markings on the bolt head to determine that
each bolt is of the correct material. It is extremely important
to use like bolts in replacement. In every case, refer to the
applicable Maintenance Instructions Manual and Illustrated
Parts Breakdown.
Be sure that washers are used under both the heads of bolts
and nuts unless their omission is specified. A washer guards
against mechanical damage to the material being bolted and
prevents corrosion of the structural members. An aluminum-
alloy washer should be used under the head and nut of a steel
bolt securing aluminum alloy or magnesium alloy members.
Any corrosion that occurs attacks the washer rather than the
members. Steel washers should be used when joining steel
members with steel bolts.
Whenever possible, place the bolt with the head on top or in
the forward position. This positioning tends to prevent the
bolt from slipping out if the nut is accidentally lost.
Be certain that the bolt grip length is correct. Grip length is
the length of the unthreaded portion of the bolt shank. The
grip length should equal the thickness of the material being
bolted together. However, bolts of slightly greater grip length
may be used if washers are placed under the nut or the bolt
head. In the case of plate nuts, add shims under the plate.Safetying of Bolts & Nuts
It is very important that all bolts or nuts, except the self-
locking type, be safetied after installation. This prevents
them from loosening in flight due to vibration. Methods of
safetying are discussed later in this chapter.
Repair of Damaged Internal Threads
Installation or replacement of bolts is simple when compared
to the installation or replacement of studs. Bolt heads and nuts
are cut in the open, whereas studs are installed into internal
threads in a casting or built-up assembly. Damaged threads
on bolts or nuts can be seen and only require replacement
of the defective part. If internal threads are damaged, two
alternatives are apparent: the part may be replaced or the
threads repaired or replaced. Correction of the thread problem
is usually cheaper and more convenient. Two methods of
repairing are by replacement bushings or helicoils.
Replacement Bushings
Bushings are usually special material (steel or brass spark
plug bushings into aluminum cylinder heads). A material
that resists wear is used where removal and replacement is
frequent. The external threads on the bushing are usually
coarse. The bushing is installed, a thread lock compound
may or may not be used, and staked to prevent loosening.
Many bushings have left-hand threads external and right-
hand threads internal. With this installation, removal of the
7-49bolt or stud (right-hand threads) tends to tighten the bushing.
Bushings for common installations, such as spark plugs, may
be up to 0.040 oversize (in increments of 0.005). Original
installation and overhaul shop replacements are shrunk fit:
a heated cylinder head and a frozen bushing.
Helicoils
Helicoils are precision-formed screw thread coils of 18-8
stainless steel wire having a diamond-shaped cross section.
[Figure 7-31] They form unified coarse or unified fine thread
classes 2-band 3B when assembled into (helicoil) threaded
holes. The assembled insert accommodates UNJ (controlled
radius root) male threaded members. Each insert has a driving
tang with a notch to facilitate removal of the tang after the
insert is screwed into a helicoil tapped hole.
They are used as screw thread bushings. In addition to being
used to restore damaged threads, they are used in the original
design of missiles, aircraft engines, and all types of mechanical
equipment and accessories to protect and strengthen tapped
threads in light materials, metals, and plastics, particularly in
locations that require frequent assembly and disassembly and/
or where a screw locking action is desired.
Helicoil installation is a 5 or 6 step operation, depending
upon how the last step is classed. [Figure 7-32]
Step 1: Determine what threads are damaged.
Step 2: (a) New installation of helicoil—drill out
damaged threads to minimum depth
specified.
(b) Previously installed helicoil—using proper
size extracting tool, place edge of blade in
90° from the edge of the insert. Tap with
hammer to seat tool. Turn to left, applying
pressure, until insert backs out. Threads are
not damaged if insert is properly removed.
Step 3: Tap—use the tap of required nominal thread size.
The tapping procedure is the same as standard
thread tapping. Tap length must be equal to or
exceed the requirement.
Step 4: Gauge—threads may be checked with a helicoil
thread gauge.
Step 5: Insert assembly—using proper tool, install insert
to a depth that puts end of top coil 1⁄4 to 1⁄2 turn
below the top surface of the tapped hole.
Step 6: Tang breakoff—select proper breakoff tool.
Tangs should be removed from all drilled
through holes. In blind holes, the tangs may be
removed when necessary if enough hole-depth
is provided below the tang of the installed insert.These are not to be considered specific instructions on helicoil
installation. The manufacturer’s instruction must be followed
when making an installation.
Helicoils are available for the following threads: unified
coarse, unified fine, metric, spark plug, and national taper
pipe threads.
Fastener Torque
Torque
As the speed of an aircraft increases, each structural member
becomes more highly stressed. It is therefore extremely
important that each member carry no more and no less than
the load for which it was designed. To distribute the loads
safely throughout a structure, it is necessary that proper
torque be applied to all nuts, bolts, studs, and screws.
Using the proper torque allows the structure to develop
its designed strength and greatly reduces the possibility of
failure due to fatigue.
Torque Wrenches
The three most commonly used torque wrenches are
the flexible beam, rigid frame, and the ratchet types.
[Figure 7-33] When using the flexible beam and the rigid
frame torque wrenches, the torque value is read visually on
a dial or scale mounted on the handle of the wrench.
To use the ratchet type, unlock the grip and adjust the handle
to the desired setting on the micrometer-type scale, then
relock the grip. Install the required socket or adapter to the
square drive of the handle. Place the wrench assembly on the
nut or bolt, and pull the wrench assembly on the nut or bolt
in a clockwise direction with a smooth, steady motion. (A
fast or jerky motion results in an improperly torqued unit.)
When the applied torque reaches the torque value indicated
on the handle setting, the handle automatically releases or
“breaks” and moves freely for a short distance. The release
and free travel is easily felt, so there is no doubt about when
the torqueing process is completed.
To assure getting the correct amount of torque on the
fasteners, all torque wrenches must be tested at least once a
month or more often if necessary.
Note: It is not advisable to use a handle length extension on
a flexible beam-type torque wrench at any time. A handle
extension alone has no effect on the reading of the other types.
The use of a drive end extension on any type of torque wrench
makes the use of the formula mandatory. When applying the
formula, force must be applied to the handle of the torque
wrench at the point from which the measurements were taken.
If this is not done, the torque obtained is incorrect.
Figure 7-31. Helicoil insert.
Drill Tap Gauge Install
Figure 7-32. Helicoil installation.Torque Tables
Use the standard torque table as a guide in tightening nuts,
studs, bolts, and screws whenever specific torque values are
not called out in maintenance procedures. The following
rules apply for correct use of the torque table: [Figure 7-34]
1. To obtain values in foot-pounds, divide inch-pounds
by 12.
2. Do not lubricate nuts or bolts except for corrosion-
resistant steel parts or where specifically instructed
to do so.
3. Always tighten by rotating the nut first if possible.
When space considerations make it necessary to
tighten by rotating the bolt head, approach the high
side of the indicated torque range. Do not exceed the
maximum allowable torque value.
4. Maximum torque ranges should be used only when
materials and surfaces being joined are of sufficient
thickness, area, and strength to resist breaking,
warping, or other damage.
5. For corrosion-resisting steel nuts, use torque values
given for shear-type nuts.
6. The use of any type of drive end extension on a torque
wrench changes the dial reading required to obtain
the actual values indicated in the standard torque
range tables. When using a drive end extension, the
torque wrench reading must be computed by use of
the proper formula, which is included in the handbook
accompanying the torque wrench.
Cotter Pin Hole Line Up
When tightening castellated nuts on bolts, the cotter pin holes may not line up with the slots in the nuts for the range
of recommended values. Except in cases of highly-stressed
engine parts, the nut may not be over torque. Remove
hardware and realign the holes. The torque loads specified
may be used for all unlubricated cadmium-plated steel nuts
of the fine or coarse thread series, which have approximately
equal number of threads and equal face bearing areas. These
values do not apply where special torque requirements are
specified in the maintenance manual.
If the head end, rather than the nut, must be turned in the
tightening operation, maximum torque values may be
increased by an amount equal to shank friction, provided the
latter is first measured by a torque wrench.
Basic formula F x L = T
F = Applied force
L = Lever length between centerline of drive and centerline of
applied force (F must be 90° to L)
T = Torque
Formula for use with extensions Tw =
A = Lever length of wrench
B = Lever length of wrench plus extension
Te = Required torque on bolt
Tw = Torque reading on wrench dialA
B90°
LF
T
Flexible beamRatchet type
Rigid frame90°
Te x A
B
Figure 7-33. Common torque wrenches.Aircraft Rivets
Sheets of metal must be fastened together to form the aircraft
structure, and this is usually done with solid aluminum-alloy
rivets. A rivet is a metal pin with a formed head on one
end when the rivet is manufactured. The shank of the rivet
is inserted into a drilled hole, and its shank is then upset
(deformed) by a hand or pneumatic tool. The second head,
formed either by hand or by pneumatic equipment, is called
a “shop head.” The shop head functions in the same manner
as a nut on a bolt. In addition to their use for joining aircraft
skin sections, rivets are also used for joining spar sections, for
holding rib sections in place, for securing fittings to various
parts of the aircraft, and for fastening innumerable bracing
members and other parts together. The rivet creates a bond
that is at least as strong as the material being joined.
Two of the major types of rivets used in aircraft are the
common solid shank type, which must be driven using a
bucking bar, and the special (blind) rivets, which may be
installed where it is impossible to use a bucking bar.
Aircraft rivets are not hardware store rivets. Rivets purchased
at a hardware store should never be used as a substitute for
aircraft quality rivets. The rivets may be made from very
different materials, the strength of the rivets differs greatly,
and their shear strength qualities are very different. The
countersunk heads on hardware store rivets are 78°, whereas
countersunk aircraft rivets have 100° angle heads for more
surface contact to hold it in place.
Standards and Specifications
The FAA requires that the structural strength and
integrity of type-certificated aircraft conform to all
airworthiness requirements. These requirements apply to
performance, structural strength, and integrity as well as
flight characteristics. To meet these requirements, each
aircraft must meet the same standards. To accomplish
standardization, all materials and hardware must be
manufactured to a standard of quality. Specifications and
standards for aircraft hardware are usually identified by the
organization that originated them. Some of the common
standardizing organizations include:
AMS Aeronautical Material Specifications
AN Air Force-Navy
AND Air Force-Navy Design
AS Aeronautical Standard
ASA American Standards Association
ASTM American Society for Testing Materials
MS Military Standard
NAF Naval Aircraft Factory
7-52NAS National Aerospace Standard
SAE Society of Automotive Engineers
When a MS20426-AD4-6 rivet is required, the specifications
have already been written for it in the Military Standard (MS)
specifications. That information is available to the aircraft
manufacturers, the rivet manufacturers and the mechanic.
The specifications designate the material to be used as well
as the head type, diameter, and length of the rivet. The use
of standardized materials in the production of aircraft makes
each aircraft exactly the same as the previous one and makes
them less expensive to build.
Aircraft rivets are manufactured to much higher standards and
specifications than rivets manufactured for general use. When
aircraft manufacturers started building all-metal aircraft in
the 1930s, different manufacturers had different rivet head
designs. Brazier heads, modified brazier heads, button heads,
mushroom heads, flatheads, and 78° countersunk heads
were used. As aircraft standardized, four rivet head designs
almost completely replaced all the others. Rivets exposed
to the airflow over the top of the structure are usually either
universal head MS20470 or 100° countersunk head MS20426
rivets. For rivets used in internal structures, the roundhead
MS20430 and the flathead MS20442 are generally used.
Solid Shank Rivets
Solid shank rivets are generally used in repair work. They
are identified by the kind of material of which they are made,
their head type, size of shank, and their temper condition.
The designation of the solid shank rivet head type, such as
universal head, roundhead, flathead, countersunk head, and
brazier head, depends on the cross-sectional shape of the
head. [Figure 7-35] The temper designation and strength
are indicated by special markings on the head of the rivet.
The material used for most aircraft solid shank rivets is
aluminum alloy. The strength and temper conditions of
aluminum-alloy rivets are identified by digits and letters
similar to those adopted for the identification of strength and
temper conditions of aluminum and aluminum-alloy stock.
The 1100, 2017-T, 2024-T, 2117-T, and 5056 rivets are the
five grades usually available.
The 1100 rivet, which is composed of 99.45 percent pure
aluminum, is very soft. It is for riveting the softer aluminum
alloys, such as 1100, 3003, and 5052, which are used for
nonstructural parts (all parts where strength is not a factor).
The riveting of map cases is a good example of where a rivet
of 1100 aluminum alloy may be used.
The 2117-T rivet, known as the field rivet, is used more
than any other for riveting aluminum alloy structures. The field rivet is in wide demand, because it is ready for use as
received and needs no further heat-treating or annealing. It
also has a high resistance to corrosion.
The 2017-T and 2024-T rivets are used in aluminum-alloy
structures where more strength is needed than is obtainable
with the same size 2217-T rivet. These rivets are known as
“ice box rivets,” are annealed and must be kept refrigerated
until they are to be driven. The 2017-T rivet should be driven
within approximately 1 hour and the 2024-T rivet within 10
to 20 minutes after removal from refrigeration.
The 5056 rivet is used for riveting magnesium-alloy
structures because of its corrosion-resistant qualities in
combination with magnesium.
Mild steel rivets are used for riveting steel parts. The corrosion-
resistant steel rivets are for riveting corrosion-resistant steels
in firewalls, exhaust stack brackets, and similar structures.
Monel rivets are used for riveting nickel-steel alloys. They
can be substituted for those made of corrosion-resistant steel
in some cases.
The use of copper rivets in aircraft repair is limited. Copper
rivets can be used only on copper alloys or nonmetallic
materials, such as leather.
Metal temper is an important factor in the riveting process,
especially with aluminum alloy rivets. Aluminum-alloy rivets
have the same heat-treating characteristics as aluminum-alloy
stock. They can be hardened and annealed in the same manner
as aluminum. The rivet must be soft, or comparatively soft,
before a good head can be formed. The 2017-T and 2024-T
rivets are annealed before being driven. They harden with age.
The process of heat-treating (annealing) rivets is much the
same as that for stock. Either an electric air furnace, a salt bath,
or a hot oil bath is needed. The heat-treating range, depending
on the alloy, is 625 °F to 950 °F. For convenient handling, rivets
are heated in a tray or a wire basket. They are quenched in cold
water (70 °F) immediately after heat-treating.
The 2017-T and 2024-T rivets, which are heat-treatable
rivets, begin to age harden within a few minutes after being
exposed to room temperature. Therefore, they must be used
immediately after quenching or else be placed in cold storage.
The most commonly used means for holding heat-treatable
rivets at low temperature (below 32 °F) is to keep them in a
refrigerator. They are referred to as “icebox” rivets. Under
this storage condition, they remain soft enough for driving
for up to 2 weeks. Any rivets not used within that time should
be removed for reheat-treating.
7-53Bolt, Stud, or
Screw Size On standard bolts, studs, and
screws having a tensile strength of
125,000 to 140,000 psi On bolts, studs,
and screws having a
tensile strength
of 140,000 to
160,000 psi On high-strength
bolts, studs, and
screws having a
tensile strength of
160,000 psi and over
Shear-type nuts
(AN320, AN364, or
equivalent) Tension-type nuts and
threaded machine
parts (AN-310, AN365,
or equivalent) Any nut, except shear
type Any nut, except shear
type TORQUE VALUES FOR TIGHTENING NUTS
¼–20
½–13
¾–10
7⁄8–9
11⁄8–8
1¼–88–36
¼–28
3⁄8–24
7/16–20
½–20
¾–16
11⁄8–12
1¼–127–9
140–155270–300240–290290–410300–420480–600420–540660–780700–950
1,300–1,5001,300–1,8001,500–1,80022,00–3,0002,200–3,3003,300–4,0003,000–4,2004,000–5,0005,400–6,60012–1520–2540–5050–7080–90
100–140160–185160–190235–255450–500400–480480–690500–700
800–1,000
1,100–1,3001,150–1,6002,300–2,5002,200–3,0002,500–3,0003,700–5,0003,700–5,5005,500–6,5005,000–7,0006,500–8,000
9,000–11,00014–1723–3045–4960–80
120–172173–217175–271245–342475–628440–636585–840600–845
900–1,220800–1,125
1,200–1,7301,380–1,9252,400–3,5002,600–3,5702,750–4,6504,350–5,9204,600–7,2506,000–8,650
6,000–10,2507,250–11,000
10,000–16,75015–1825–3550–6870–90
140–203185–248190–351255–428500–756480–792690–990700–990
1,000–1,440
900–1,350
1,300–2,1601,600–2,2502,500–4,5003,000–4,1403,000–6,3005,000–6,8405,500–9,000
6,500–10,8007,000–13,5008,000–14,000
11,000–22,500
Figure 7-34. Standard torque table (inch-pounds).(INCH-POUNDS)
5/16–18
3/8–16
7/16–14
9/16–12
5/8–115/16–24
9/16–18
5/8–18
7/8–14
7-54Icebox rivets attain about one-half their maximum strength
in approximately 1 hour after driving and full strength in
about 4 days. When 2017-T rivets are exposed to room
temperature for 1 hour or longer, they must be subject to
reheat-treatment. This also applies to 2024-T rivets exposed
to room temperature for a period exceeding 10 minutes.
Once an icebox rivet has been taken from the refrigerator, it
should not be mixed with the rivets still in cold storage. If
more rivets are removed from the refrigerator than can be used
in 15 minutes, they should be placed in a separate container
and stored for reheat-treatment. Heat-treatment of rivets
may be repeated a number of times if done properly. Proper
heating times and temperatures are shown in Figure 7-36 .
Most metals, and therefore aircraft rivet stock, are subject
to corrosion. Corrosion may be the result of local climatic
conditions or the fabrication process used. It is reduced
to a minimum by using metals that are highly resistant to
corrosion and possess the correct strength-to-weight ratio.
Ferrous metals placed in contact with moist salt air rust if not
properly protected. Nonferrous metals, those without an iron
base, do not rust, but a similar process known as corrosion
takes place. The salt in moist air (found in the coastal areas)
attacks the aluminum alloys. It is a common experience to
inspect the rivets of an aircraft, which has been operated near
salt water, and find them badly corroded.
If a copper rivet is inserted into an aluminum-alloy structure,
two dissimilar metals are brought in contact with each other.
Remember, all metals possess a small electrical potential.
Dissimilar metals in contact with each other in the presence of
moisture cause an electrical current to flow between them and
chemical byproducts to be formed. Principally, this results
in the deterioration of one of the metals.
Certain aluminum alloys react to each other and, therefore,
must be thought of as dissimilar metals. The commonly used
aluminum alloys may be divided into the two groups shown
in Figure 7-37 .
Members within either group A or group B can be considered
as similar to each other and will not react to others within the
same group. A corroding action will take place, however, if
any metal of group A comes in contact with a metal in group
B in the presence of moisture.
Avoid the use of dissimilar metals whenever possible. Their
incompatibility is a factor that was considered when the AN
Standards were adopted. To comply with AN Standards,
the manufacturers must put a protective surface coating on the rivets. This may be zinc chromate, metal spray, or an
anodized finish.
The protective coating on a rivet is identified by its color. A
rivet coated with zinc chromate is yellow, an anodized surface
is pearl gray, and the metal sprayed rivet is identified by a
silvery gray color. If a situation arises in which a protective
coating must be applied on the job, paint the rivet with zinc
chromate before it is used and again after it is driven.
Identification
Markings on the heads of rivets are used to classify their
characteristics. These markings may be either a raised teat,
two raised teats, a dimple, a pair of raised dashes, a raised
cross, a single triangle, or a raised dash; some other heads
have no markings.
The different markings indicate the composition of the rivet
stock. As explained previously, the rivets have different colors
to identify the manufacturers’ protective surface coating.
Roundhead rivets are used in the interior of the aircraft,
except where clearance is required for adjacent members.
The roundhead rivet has a deep, rounded top surface. The
head is large enough to strengthen the sheet around the hole
and, at the same time, resists tension.
The flathead rivet, like the roundhead rivet, is used on interior
structures. It is used where maximum strength is needed and
where there is not sufficient clearance to use a roundhead
rivet. It is seldom, if ever, used on external surfaces. The
brazier head rivet has a head of large diameter, which makes it
particularly adaptable for riveting thin sheet stock (skin). The
brazier head rivet offers only slight resistance to the airflow,
and because of this factor, it is frequently used for riveting skin
on exterior surfaces, especially on aft sections of the fuselage
and empennage. It is used for riveting thin sheets exposed to the
slipstream. A modified brazier head rivet is also manufactured;
it is simply a brazier head of reduced diameter.
The universal head rivet is a combination of the roundhead,
flathead, and brazier head. It is used in aircraft construction
and repair in both interior and exterior locations. When
replacement is necessary for protruding head rivets—
roundhead, flathead, or brazier head—they can be replaced
by universal head rivets.
The countersunk head rivet is flat topped and beveled toward
the shank so that it fits into a countersunk or dimpled hole
and is flush with the material’s surface. The angle at which
the head slopes may vary from 78° to 120°. The 100° rivet
is the most commonly used type. These rivets are used to
fasten sheets over which other sheets must fit. They are also
7-55* New specifications are for design purposes.Head Marking MaterialAN
Material
Code AN425
78°
Counter-
sunk
Head AN426
100°
Counter-
sunk Head
MS20426*AN427
100°
Counter-
sunk Head
MS20427*AN430
Round
Head
MS20470* AN435
Round
Head
MS20613*
MS20615* AN441
Flat
HeadAN442
Flat
Head
MS20470*AN455
Brazier
Head
MS20470*AN456
Brazier
Head
MS20470*AN470
Universal
Head
MS20470*Heat
Treat
Before
Use Shear
Strength
psi Bearing
Strength
psi
X
X
X
X
X
XX
X
X
X
X
X
XX
X
X
X
X
X
XX
X
X
X
X
X
XX
X
X
X
X
X
XX
X
X
X
X
X
X25,000
100,000
113,000
126,000
136,000
90,000
90,000
90,000X
X
X
XX
X
XA
AD
D
D
DD
B
F
C
M
CX
MS20613*
X
MS20613*
X
X
MS20615*
X
MS20615*X
X
X
X
X
X
X
MS20426No
No
Yes
No
Yes
No
No
No
No
No
No
No
No
No10,000
30,000
34,000
38,000
41,000
27,000
35,000
65,000
23,000
49,000
49,000
95,000
Plain
Recessed
Dot
Raised
Dot
Raised
Dot
Raised
Double Dash
Raised
Cross
Three Raised
Dashes
Recessed
Triangle
Recessed
Dash
Plain
Plain
Plain
Recessed
Large and
Small DotRecessed
Double Dots1100
2117T
2017T
2024T
5056T2017T-HD
7075-T73
Carbon Steel
Copper
Brass
TitaniumMonel (Nickel-
Copper Alloy)Corrosion
Resistant Steel
Monel
Figure 7-35. Rivet identification chart.
7-56Heating Time—A ir Furnac e
Rivet A lloyTime a t
Temper ature Hea t-Treating
Temper ature
910 °F –930 °F
925 °F –950 °F2024
20171 hour
1 hour
Heating Time—S alt B ath
201730 minut es
30 minut esRivet A lloyTime a t
Temper ature Hea t-Treating
Temper ature
910 °F –930 °F
925 °F –950 °F
Figure 7-36. Rivet heating times and temperatures.Group A
6053Group B
Figure 7-37. Aluminum groupings.used on exterior surfaces of the aircraft, because they offer
only slight resistance to the slipstream and help to minimize
turbulent airflow.
The markings on the heads of rivets indicate the material
of which they are made and, therefore, their strength.
Figure 7-37 identifies the rivet head markings and the
materials indicated by them. Although there are three
materials indicated by a plain head, it is possible to
distinguish their difference by color. The 1100 is an
aluminum color; the mild steel is a typical steel color; and
the copper rivet is a copper color. Any head marking can
appear on any head style of the same material.
A part number identifies each type of rivet so that the user
can select the correct rivet for the job. The type of rivet head
is identified by AN or MS standard numbers. The numbers
selected are in series and each series represents a particular
type of head.
The most common numbers and the types of heads they
represent are:
AN426 or MS20426—countersunk head rivets (100°)
AN430 or MS20430—roundhead rivets
AN441—flathead rivets
AN456—brazier head rivets
AN470 or MS20470—universal head rivets
There are also letters and numbers added to a part number.
The letters designate alloy content; the numbers designate
rivet diameter and length. The letters in common uses for
alloy designation are:
A—Aluminum alloy, 1100 or 3003 composition
AD—Aluminum alloy, 2117-T compositionD—Aluminum alloy, 2017-T composition
DD—Aluminum alloy, 2024-T composition
B—Aluminum alloy, 5056 composition
C—Copper
M—Monel
The absence of a letter following the AN standard number
indicates a rivet manufactured from mild steel.
The first number following the material composition letters
expresses the diameter of the rivet shank in 32nds of an inch.
For example, 3 indicates 3⁄32, 5 indicates 5⁄32, and so forth.
[Figure 7-38]
The last number(s), separated by a dash from the preceding
number, expresses the length of the rivet shank in 16ths of an
inch. For example, 3 indicates 3⁄16, 7 indicates 7⁄16, 11 indicates
11⁄16, and so forth. [Figure 7-38]
An example of identification marking of a rivet is:
AN470AD3-5—complete part number
AN—Air Force-Navy standard number
470—universal head rivet
AD—2117-T aluminum alloy
3—3⁄32 in diameter
5—5⁄16 in length
Blind Rivets
There are many places on an aircraft where access to both
sides of a riveted structure or structural part is impossible, or
where limited space does not permit the use of a bucking bar.
Also, in the attachment of many non-structural parts, such as
aircraft interior furnishings, flooring, deicing boots, and the
like, the full strength of solid shank rivets is not necessary.
For use in such places, special rivets have been designed that
can be bucked from the front. Special rivets are sometimes
lighter than solid shank rivets, yet amply strong for intended
use. These rivets are produced by several manufacturers and
7-57have unique characteristics that require special installation
tools, special installation procedures, and special removal
procedures. That is why they are called special rivets.
Because these rivets are often inserted in locations where
one head (usually the shop head) cannot be seen, they are
also called blind rivets.
Mechanically-Expanded Rivets
Two classes of mechanically-expanded rivets are discussed here:
• Non-structural—self-plugging (friction lock) rivets,
pull-thru rivets
• Mechanical lock—flush fracturing, self-plugging rivets
Self-Plugging Rivets (Friction Lock)
The self-plugging (friction lock) blind rivets are manufactured
by several companies. The same general basic information
about their fabrication, composition, uses, selection,
installation, inspection, and removal procedures apply to
all of them.
Self-plugging (friction lock) rivets are fabricated in two parts:
a rivet head with a hollow shank or sleeve, and a stem that
extends through the hollow shank. Figure 7-39 illustrates a
protruding head and a countersunk head self-plugging rivet
produced by one manufacturer.
Several events, in their proper sequence, occur when a pulling
force is applied to the stem of the rivet:
1. The stem is pulled into the rivet shank.
2. The mandrel portion of the stem forces the rivet shank
to expand.
3. When friction (or pulling action pressure) becomes
great enough, it causes the stem to snap at a breakoff
groove on the stem.
The plug portion (bottom end of the stem) is retained in the
shank of the rivet giving the rivet much greater shear strength
than could be obtained from a hollow rivet.
Self-plugging (friction lock) rivets are fabricated in two
common head styles: a protruding head like the MS20470
or universal head, and a 100° countersunk head. Other head
styles are available from some manufacturers.
The stem of the self-plugging (friction lock) rivet may have a
knot or knob on the upper portion, or it may have a serrated
portion. [Figure 7-39]
Self-plugging (friction lock) rivets are fabricated from several
materials. Rivets are available in the following material
combinations: stem 2017 aluminum alloy and sleeve 2117 aluminum alloy; stem 2017 aluminum alloy and sleeve 5056
aluminum alloy; and stem steel and sleeve steel.
Self-plugging (friction lock) rivets are designed so that
installation requires only one person; it is not necessary
to have the work accessible from both sides. The pulling
strength of the rivet stem is such that a uniform job can always
be assured. Because it is not necessary to have access to the
opposite side of the work, self- plugging (friction lock) rivets
can be used to attach assemblies to hollow tubes, corrugated
sheet, hollow boxes, and so forth. Because a hammering force
is not necessary to install the rivet, it can be used to attach
assemblies to plywood or plastics.
Factors to consider in the selection of the correct rivet for
installation are: installation location, composition of the
material being riveted, thickness of the material being riveted,
and strength desired.
If the rivet is to be installed on an aerodynamically
smooth surface, or if clearance for an assembly is needed,
countersunk head rivets should be selected. In other areas
where clearance or smoothness is not a factor, the protruding
head type rivet may be utilized.
Material composition of the rivet shank depends upon the
type of material being riveted. Aluminum alloy 2117 shank
rivets can be used on most aluminum alloys. Aluminum alloy
5056 shank rivets should be used when the material being
riveted is magnesium. Steel rivets should always be selected
for riveting assemblies fabricated from steel.
The thickness of the material being riveted determines the
overall length of the shank of the rivet. As a general rule,
the shank of the rivet should extend beyond the material
thickness approximately 3⁄64 inch to 1⁄8 inch before the stem
is pulled. [Figure 7-40]
Pull-Thru Rivets
Several companies manufacture the pull-thru blind rivets.
The same general basic information about their fabrication,
composition, uses, selection, installation, inspection, and
removal procedures apply to all of them.
Pull-thru rivets are fabricated in two parts: a rivet head with
a hollow shank or sleeve and a stem that extends through the
hollow shank. Figure 7-41 illustrates a protruding head and
a countersunk head pull-thru rivet.
Several events, in their proper sequence, occur when a pulling
force is applied to the stem of the rivet:
1. The stem is pulled through the rivet shank.
Length of rivet
Diameter of shank Diameter of shankCountersunk angle
Figure 7-38. Methods of measuring rivets.
Protruding head Countersunk head
Figure 7-39. Self-plugging (friction lock) rivets.2. The mandrel portion of the stem forces the shank to
expand forming the blind head and filling the hole.
Pull-thru rivets are fabricated in two common head styles:
protruding head like the MS20470 or universal head and a
100° countersunk head. Other head styles are available from
some manufacturers.
Pull-thru rivets are fabricated from several materials. The
most commonly used are 2117-T4 aluminum alloy, 5056
aluminum alloy, Monel. Pull-thru rivets are designed so that
installation requires only one person; it is not necessary to
have the work accessible from both sides.
Factors to consider in the selection of the correct rivet for
installation are: installation location, composition of the
material being riveted, thickness of the material being riveted,
and strength desired.
The thickness of the material being riveted determines the
overall length of the shank of the rivet. As a general rule,
the shank of the rivet should extend beyond the material
thickness approximately 3⁄64 inch to 1⁄8 inch before the stem
is pulled. [Figure 7-42]
Each company that manufactures pull-thru rivets has a code
number to help users obtain correct rivet for the grip range of
a particular installation. In addition, MS numbers are used for
identification purposes. Numbers are similar to those shown
on the preceding pages.
Self-Plugging Rivets (Mechanical Lock)
Self-plugging (mechanical lock) rivets are like self-plugging
(friction lock) rivets, except for the way the stem is retained in
the rivet sleeve. This type of rivet has a positive mechanical
locking collar to resist vibrations that cause the friction lock
rivets to loosen and possibly fall out. [Figure 7-43] Also,
the mechanical locking-type rivet stem breaks off flush with the head and usually does not require further stem trimming
when properly installed. Self-plugging (mechanical lock)
rivets display all the strength characteristics of solid shank
rivets and, in most cases, can be substituted rivet for rivet.
Bulbed CherryLOCK® Rivets
The large blind head of this fastener introduced the word
“bulb” to blind rivet terminology. In conjunction with the
unique residual preload developed by the high stem break
load, its proven fatigue strength makes it the only blind rivet
interchangeable structurally with solid rivets. [Figure 7-44]
Wiredraw CherryLOCK® Rivets
There is a wide range of sizes, materials, and strength levels
from which to select. This fastener is especially suited for
sealing applications and joints requiring an excessive amount
of sheet take-up. [Figure 7-45]
Huck® Mechanical Locked Rivets
Self-plugging (mechanical lock) rivets are fabricated in two
sections: a head and shank (including a conical recess and
locking collar in the head) and a serrated stem that extends
through the shank. Unlike the friction lock rivet, the Huck®
mechanical lock rivet has a locking collar that forms a positive
lock for retention of the stem in the shank of the rivet. This
collar is seated in position during the installation of the rivet.
Material
Self-plugging (mechanical lock) rivets are fabricated with
CA
B
A = Thickness of material (grip range)
B = 3⁄64 –1⁄8"
C = Total rivet shank length
Figure 7-40. Determining length of friction lock rivets.
Protruding head Countersunk head
Figure 7-41. Pull-thru rivets.sleeves (rivet shanks) of 2017 and 5056 aluminum alloys,
Monel, or stainless steel.
The mechanical lock type of self-plugging rivet can be
used in the same applications as the friction lock type of rivet. In addition, because of its greater stem retention characteristic, installation in areas subject to considerable vibration is recommended.
The same general requirements must be met in the
selection of the mechanical lock rivet as for the friction lock rivet. Composition of the material being joined together determines the composition of the rivet sleeve.
For example, 2017 aluminum alloy rivets for most aluminum alloys and 5056 aluminum rivets for magnesium.
Figure 7-46 depicts the sequences of a typical mechanically-
locked blind rivet. The form and function may vary slightly
between blind rivet styles and specifics should be obtained from manufacturers.
Head Styles
Self-plugging mechanical locked blind rivets are available in several head styles depending on the installation requirements. [Figure 7-47]
Diameters
Shank diameters are measured in 1⁄32-inch increments and are
generally identified by the first dash number: -3 indicates 3⁄32
inch diameter, -4 indicates 1⁄8 diameter, and so forth. Both
nominal and 1⁄64-inch oversize diameters are available.
Grip Length
Grip length refers to the maximum total sheet thickness to be riveted and is measured in
1⁄16 of an inch. This is
generally identified by the second dash number. Unless otherwise noted, most blind rivets have their grip lengths (maximum grip) marked on the rivet head and have a total grip range of
1⁄16 inch. For example, –04 grip rivet has a grip
range of 3⁄16" to 1⁄4". [Figure 7-48]
To determine the proper grip rivet to use, measure the material thickness with a grip selection gauge (available from blind rivet manufacturers). The proper use of a grip selector gauge is shown in Figure 7-49.
The thickness of the material being riveted determines the
overall length of the shank of the rivet. As a general rule, the shank of the rivet should extend beyond the material thickness approximately
3⁄64 inch to 1⁄8 inch before the stem
is pulled. [Figure 7-50]Rivet Identification
Each company that manufactures self-plugging (friction lock) rivets has a code number to help users obtain the correct rivet for the grip range or material thickness of a particular installation. In addition, MS numbers are used for identification purposes. Figures 7-51 through 7-54 contain examples of part numbers for self-plugging (friction lock) rivets that are representative of each.
CA
B
A = Thickness of material (grip range)
B = 3/64 – 1/8"
C = Total rivet shank length
Figure 7-42. Determining length of pull-thru rivets.
(Minimum grip illustrated)
Figure 7-44. Bulbed CherryLOCK® rivet.
Before installation After installation
Figure 7-43. Self-plugging (mechanical lock) rivets.Special Shear and Bearing Load Fasteners
Many special fasteners produce high strength with
lightweight and can be used in place of conventional AN
bolts and nuts. When AN bolts are tightened with the nut, the
bolt stretches, narrowing the diameter and then the bolt is no
longer tight in the hole. Special fasteners eliminate this loose
fit, because they are held in place by a collar that is squeezed
into position. These fasteners are not under the same tensile
loads as a bolt during installation. Special fasteners are also
used extensively for light sport aircraft (LSA). Always follow
the aircraft manufacturer’s recommendations.
Pin Rivets
Pin (Hi-Shear) rivets are classified as special rivets but are
not of the blind type. Access to both sides of the material
is required to install this type of rivet. Pin rivets have the
same shear strength as bolts of equal diameters, are about 40
percent of the weight of a bolt, and require only about one-
fifth as much time for installation as a bolt, nut, and washer
combination. They are approximately three times as strong
as solid shank rivets.
Pin rivets are essentially threadless bolts. The pin is headed at
one end and is grooved about the circumference at the other. A
metal collar is swaged onto the grooved end effecting a firm,
tight fit. [Figure 7-55] Pin rivets are fabricated in a variety
of materials but should be used only in shear applications.
They should never be used where the grip length is less than the shank diameter.
Part numbers for pin rivets can be interpreted to give the
diameter and grip length of the individual rivets. A typical
part number breakdown would be:
NAS177-14-17
Figure 7-45. Wiredraw CherryLOCK® rivet.
Before pulling begins 1 Stem is pulled into rivet
sleeve and starts to formbulbed blind head.2
Formation of blind head
and hole filling are completed.
Shear ring now begins to
shear from stem cone to allow
stem to pull further into rivet.4Completely installed bulbed
CherryLOCK® rivet6Clamp-up completed as
stem continues to bulbout blind head.3
Shear ring has moved down
stem cone until pulling head
automatically stops stem
break notch flush with top of
rivet head.Pulling head has inserted locking
collar, and stem has fractured flush with rivet head.5
Sheet gapStem
Rivet sleeveLocking collar
Rivet head
Shear ring
Shear ring guarantees
blind side bulbed headClamp-up and hole fill action begin.
Locking collar is now ready to be inserted.
(Maximum grip illustrated)Rivet head firmly seated
Blind side bulb head is
formed below minimum grip.
Blind side bulbed head
(In minimum grip, shear ring may not shear)
Figure 7-46. CherryLOCK® rivet installation.NAS = National Aircraft Standard
177 = 100° countersunk head rivet OR 178 = flathead rivet14 = Nominal diameter in 32nds of an inch17 = Maximum grip length in 16ths of an inch
Taper-Lok
Taper-Loks are the strongest special fasteners used in aircraft
construction. The Taper-Lok exerts a force on the walls of the hole because of its tapered shape. The Taper-Lok is designed to completely fill the hole, but unlike the rivet, it fills the hole without deforming the shank. Instead, the washer head nut squeezes the metal with tremendous force against the tapered walls of the hole. This creates radial compression around the shank and vertical compression lines as the metals are squeezed together. The combination of these forces generates
strength unequaled by any other fastener. [Figure 7-56]
HI-LOK™ Fastening System
The threaded end of the HI-LOK™ two-piece fastener
contains a hexagonal shaped recess. The hex tip of an Allen wrench engages the recess to prevent rotation of the pin while the collar is being installed. The pin is designed in two basic head styles. For shear applications, the pin is made in countersunk style and in a compact protruding head style. For tension applications, the MS24694 countersunk and regular protruding head styles are available.
The self-locking, threaded HI-LOK™ collar has an internal
counterbore at the base to accommodate variations in material thickness. At the opposite end of the collar is a wrenching device that is torqued by the driving tool until it shears off during installation, leaving the lower portion of the collar seated with the proper torque without additional torque inspection. This shear-off point occurs when a predetermined preload or clamp-up is attained in the fastener during installation.
Note: For these fasteners, “Preload” is defined as the
maximum tensile load experienced by a fastener in a joint during the fastener installation sequence. Consequently, the
100° Countersunk (MS20426)
For countersunk applications
100° Countersunk NAS 1097
For thin top sheet machine countersunk applicationsUniversal (MS20470)
For protruding head applications
Unisink
A combination of flush and protruding head
for use in very thin top sheets. Eliminates need for
double-dimpling. Not covered by NAS Standard.
156° Countersunk NAS 1097
A large diameter, shallow countersunk head providing wide
area for honeycomb applications. Not covered by NAS Standard.
Figure 7-47. CherryLOCK® rivet heads.
term “Residual Tension” is defined as the remaining tensile
load experienced by a fastener in a joint after the fastener
installation sequence is complete, and after any residual
relaxation of the joint assembly.
The advantages of HI-LOK™ two-piece fastener include its
lightweight, high fatigue resistance, high strength, and its
inability to be over-torqued. The pins, made from alloy steel,
corrosion-resistant steel, nickel, or titanium alloy, come in
many standard and oversized shank diameters. The collars are
made of aluminum alloy, corrosion-resistant steel, titanium,
or alloy steel. The collars have wrenching flats, fracture point, threads, and a recess. The wrenching flats are used to install
the collar. The fracture point has been designed to allow the
wrenching flats to shear when the proper torque has been
reached. The threads match the threads of the pins and have
been formed into an ellipse that is distorted to provide the
locking action. The recess serves as a built-in washer. This
area contains a portion of the shank and the transition area
of the fastener.
The hole shall typically be prepared so that the maximum
interference fit does not exceed 0.002-inch. This avoids
build up of excessive internal stresses in the work adjacent
¼"
Grip Min.
grip Max.
grip
Diameter3/16 "
Rivet grip number to be used: −04Read2 4 6 8
Read269C3 GaugeFigure 7-48. Typical grip length.
Figure 7-49. Grip gauge use.
CA
B
A = Thickness of material (grip range)
B = 3/64 – 1/8"C = Total rivet shank length
Figure 7-50. Determining rivet length.9SP-B Head Style9SP-B = brazier or universal head9SP-100 = 100º countersunk head
Material composition of shank
A = 2017 aluminum alloyB = 5056 aluminum alloyR = mild steel
Shank diameter in 32nds of an inch:
4 = 1⁄8" 6 = 3⁄16"
5 = 5⁄32" 8 = ¼"
Grip range (material thickness) in 16ths of an inchA
3 Huck Manufacturing Company
9S P-B A 6 3
RV ManufacturerOlympic Screw and Rivet Corporation
Rivet type
2 = self plugging (friction lock)5 = hollow pull through
Material composition of shank
0 = 2017 aluminum alloy5 = 5056 aluminum alloy7 = mild steel
Head style
0 = universal head1 = 100° countersunk
Shank diameter in 32nds of an inch:
4 = 1⁄8" 6 = 3⁄16"
5 = 5⁄32" 8 = ¼"
Grip range in 16ths of an inch2
2Olympic Screw and R ivet Corporation
RV 2 0 0 4 2 Figure 7-51. Huck Manufacturing Company codes.
Figure 7-52. Olympic Screw and Rivet Corporation codes.
CR Cherry rivet
Series number
Designates rivet material, type of rivet, and head style(163 = 2117 aluminum alloy, self-plugging(friction lock) rivet, protruding head)
Shank diameter in 32nds of an inch:
4 = 1⁄8" 6 = 3⁄16"
5 = 5⁄32" 8 = ¼"
Grip range (material thickness):Knob stem in 32nds of an inch; serrated stem in 16ths of an inch 163
6Townsend Company, Cherry Rivet Division
CR 163 6 6
Figure 7-53. Townsend Company, Cherry Rivet Division codes.to the hole. The HI-LOK™ pin has a slight radius under its
head to increase fatigue life. After drilling, deburr the edge of the hole to allow the head to seat fully in the hole. The HI-LOK™ is typically installed in interference fit holes for aluminum structure and a clearance fit for steel, titanium, and composite materials.
HI-TIGUE™ Fastening System
The HI-TIGUE™ fastener offers all the benefits of the HI-LOK™ fastening system along with a unique radius contour on the thread lead-in, or a raised bead design that enhances the fatigue performance of the structure making it ideal for situations that require a controlled interference fit. The HI-TIGUE™ fastener assembly consists of a pin and
PreloadManufacturing head
Washer nutRadial compressionLines of force
Pin
Driven collarCollar
Figure 7-56. Taper-Lok special fastenersFigure 7-55. Pin (Hi-Shear) rivet.Military Standard
Type of rivet and head style:
20600 = self-plugging (friction lock) protruding head
20600 = self-plugging (friction lock) 100º
countersunk head
Material composition of sleeve:
AD = 2117 aluminum alloy
B = 5056 aluminum alloy
Shank diameter in 32nds of an inch:
Type of stem:
K = knot head stem
W = serrated stem
Grip range (material thickness) in 16ths of an inchMilitary Standard Number
MS 20600 B 4 K 2
MS
B
K
Figure 7-54. Military Standard Numbers.
collar. These pin rivets have a radius at the transition area.
During installation in an interference fit hole, the radius area will “cold-work” the hole. These fastening systems can be easily confused, and visual reference should not be used for identification. Use part numbers to identify these fasteners. [Figure 7-57]
HI-LITE™ Fastening System
The HI-LITE™ fastener is similar in design and principle to the HI-LOK™ fastener, has the controlled radius from full diameter section to the threaded area of the HI-TIGUE™ fastener, and has a shorter transition area between the shank and the first load-bearing thread. HI-LITE™ fasteners have approximately one less thread. These differences reduce the weight of the HI-LITE™ fastener without lessening the shear strength. HI-LITE™ fasteners are available in the same materials and head configurations as the HI-LOK™ system, and can also be installed in high interference like the HI-TIGUE™ fastener. HI-LITE™ collars are also different and thus are not interchangeable with HI-LOK™ collars or HI-TIGUE™ collars.
Captive Fasteners
Captive fasteners are used for quick removal of engine nacelles, inspection panels, and areas where fast and easy access is important. A captive fastener can turn in the body in which it is mounted, but will not drop out when it is unscrewed from the part it is holding. Some of the most commonly used are the Dzus, Camloc, and Airloc.
Turn Lock Fasteners
Turn lock fasteners are used to secure inspection plates, doors, and other removable panels on aircraft. Turn lock fasteners are also referred to by such terms as quick opening, quick action, and stressed panel fasteners. The most desirable feature of these fasteners is that they permit quick and easy removal of access panels for inspection and servicing purposes. Turn lock fasteners are manufactured and supplied by several manufacturers under various trade names.
Dzus Fasteners
The Dzus turn lock fastener consists of a stud, grommet, and receptacle. Figure 7-58 illustrates an installed Dzus fastener
and the various parts.
The grommet is made of aluminum or aluminum alloy
material. It acts as a holding device for the stud. Grommets can be fabricated from 1100 aluminum tubing, if none are available from normal sources.4 = 1⁄8" 6 = 3⁄16"
5 = 5⁄32" 8 = 1⁄4"
CollarThe hex portion
breaks away once
the correct seating
torque is reached
Pin
Figure 7-57. HI-TIGUETM special fasteners.
Stud
Detachable part
Grommet
Cut-away view of complete Dzus assembly
Fixed part
Spring and rivetsStud assembly
Spring assembly
Figure 7-58. Dzus fastener.The spring is made of steel, which is cadmium plated to
prevent corrosion. The spring supplies the force that locks
or secures the stud in place when two assemblies are joined.
The studs are fabricated from steel and are cadmium plated.
They are available in three head styles: wing, flush, and oval.
Body diameter, length, and head type may be identified or
determined by the markings found on the head of the stud.
[Figure 7-59] The diameter is always measured in sixteenths
of an inch. Stud length is measured in hundredths of an inch
and is the distance from the head of the stud to the bottom
of the spring hole.
A quarter of a turn of the stud (clockwise) locks the
fastener. The fastener may be unlocked only by turning the
stud counterclockwise. A Dzus key or a specially ground
screwdriver locks or unlocks the fastener.
Camloc Fasteners
Camloc fasteners are made in a variety of styles and designs.
Included among the most commonly used are the 2600, 2700,
40S51, and 4002 series in the regular line, and the stressed
panel fastener in the heavy-duty line. The latter is used in
stressed panels, which carry structural loads.
The Camloc fastener is used to secure aircraft cowlings and fairings. It consists of three parts: a stud assembly, a grommet,
and a receptacle. Two types of receptacles are available: rigid
and floating. [Figure 7-60]
The stud and grommet are installed in the removable portion;
the receptacle is riveted to the structure of the aircraft. The
stud and grommet are installed in either a plain, dimpled,
countersunk, or counter bored hole, depending upon the
location and thickness of the material involved.
A quarter turn (clockwise) of the stud locks the fastener.
The fastener can be unlocked only by turning the stud
DZUS
6½F 0.50F = flush head
61/2= body diameter in 16ths of an inch
0.50 = length (50⁄100 of an inch)
Stud assembly
Grommet
Receptacle
Figure 7-59. Dzus identification.Figure 7-60. Camloc fastener.counterclockwise.
Airloc Fasteners
The Airloc fastener consists of three parts: a stud, a cross pin,
and a stud receptacle. [Figure 7-61] The studs are manufactured
from steel and case hardened to prevent excessive wear. The stud hole is reamed for a press fit of the cross pin.
The total amount of material thickness to be secured with
the Airloc fastener must be known before the correct length of stud can be selected for installation. The total thickness of material that each stud satisfactorily locks together is stamped on the head of the stud in thousandths of an inch (0.040, 0.070, 0.190, and so forth). Studs are manufactured in three head styles: flush, oval, and wing.
The cross pin is manufactured from chrome-vanadium steel
and heat-treated to provide maximum strength, wear, and holding power. [Figure 7-61] It should never be used the second time; once removed from the stud, replace it with a new pin.
Receptacles for Airloc fasteners are manufactured in two
types: rigid and floating. Number—No. 2, No. 5, and No. 7, classifies sizes. They are also classified by the center-to-center distance between the rivet holes of the receptacle: No. 2 is
3⁄4 inch; No. 5 is 1 inch; and No. 7 is 13⁄8 inch. Receptacles
are fabricated from high-carbon, heat-treated steel. An upper wing assures ejection of the stud when unlocked and enables the cross pin to be held in a locked position between the upper wing, cam, stop, and wing detent, regardless of the tension to which the receptacle is subjected.
Screws
Screws are the most commonly used threaded fastening
devices on aircraft. They differ from bolts because as they are generally made of lower strength materials. They can be installed with a loose-fitting thread, and the head shapes are made to engage a screwdriver or wrench. Some screws have a clearly defined grip or unthreaded portion, while others are threaded along their entire length.
Several types of structural screws differ from the standard
structural bolts only in head style. The material in them is the same, and a definite grip length is provided. The AN525 washer head screw and the NAS220 through NAS227 series are such screws.
Commonly used screws are classified in four groups:
1.Structural screws, which have the same strength as
equal size bolts.
2.Machine screws, which include most types used for
general repair.
3.Se lf-tapping screws, which are used for attaching
lighter parts.
4. Drive screws, which are not actually screws but nails.
They are driven into metal parts with a mallet or
hammer and their heads are not slotted or recessed.
Structural Screws
Structural screws are made of alloy steel, are properly heat-treated, and can be used as structural bolts. These screws
Studs
Installed fastener
Stud receptacles
Airloc cross pinCross pin Receptacle
Panel Stud
Figure 7-61. Airloc fastener.
are found in the NAS204 through NAS235 and AN509 and
AN525 series. They have a definite grip and the same shear
strength as a bolt of the same size. Shank tolerances are
similar to AN hex head bolts, and the threads are National
Fine. Structural screws are available with round, brazier,
or countersunk heads. Either a Phillips or a Reed & Prince
screwdriver drives the recessed head screws.
The AN509 (100°) flathead screw is used in countersunk
holes where a flush surface is necessary.
The AN525 washer head structural screw is used where raised
heads are not objectionable. The washer head screw provides
a large contact area.
Machine Screws
Machine screws are usually of the flathead (countersunk),
roundhead, or washer head types. These are general purpose
screws and are available in low-carbon steel, brass, corrosion-
resistant steel, and aluminum alloy.Roundhead screws, AN515 and AN520, have either slotted
or recessed heads. The AN515 screw has coarse threads, and
the AN520 has fine threads.
Countersunk machine screws are listed as AN505 and
AN510 for 82° and AN507 for 100°. The AN505 and AN510
correspond to the AN515 and AN520 roundhead in material
and usage.
The fillister head screw, AN500 through AN503, is a general
purpose screw and is used as a cap screw in light mechanisms.
This could include attachments of cast aluminum parts, such
as gearbox cover plates.
The AN500 and AN501 screws are available in low-carbon
steel, corrosion-resistant steel, and brass. The AN500 has
coarse threads, while the AN501 has fine threads. They have
no clearly defined grip length.
Screws larger than No. 6 have a hole drilled through the head
for safetying purposes.
The AN502 and AN503 fillister head screws are made of
heat-treated alloy steel, have a small grip, and are available in
fine and coarse threads. These screws are used as cap screws
where great strength is required. The coarse threaded screws
are commonly used as cap screws in tapped aluminum alloy
and magnesium castings because of the softness of the metal.
Self-Tapping Screws
Machine self-tapping screws are listed as AN504 and AN506.
The AN504 screw has a roundhead, and the AN506 is 82°
countersunk. These screws are used for attaching removable
parts, such as nameplates, to castings and parts in which the
screw cuts its own threads.
AN530 and AN531 self-tapping sheet metal screws, such
as the Parker-Kalon Z-type sheet metal screw, are blunt on
the end. They are used in the temporary attachment of metal
for riveting, and in the permanent assembly of nonstructural
assemblies. Self-tapping screws should not be used to replace
standard screws, nuts, bolts, or rivets.
Drive Screws
Drive screws, AN535, correspond to the Parker-Kalon
U-type. They are plain head self-tapping screws used as cap
screws for attaching nameplates in castings and for sealing
drain holes in corrosion proofing tubular structures. They are
not intended to be removed after installation.
Identification & Coding for Screws
The coding system used to identify screws is similar to that
used for bolts. There are AN and NAS screws. NAS screws
7-68are structural screws. Part numbers 510, 515, 550, and so
on, catalog screws into classes, such as roundhead, flathead,
washer head, and so forth. Letters and digits indicate their
material composition, length, and thickness. Examples of
AN and NAS code numbers follow.
AN501B-416-7
AN = Air Force-Navy standard
501 = fillister head, fine thread
B = brass
416 = 4⁄16-inch diameter
7 = 7⁄16-inch length
The letter “D” in place of the “B” would indicate that the
material is 2017-T aluminum alloy. The letter “C” would
designate corrosion resistant steel. An “A” placed before the
material code letter would indicate that the head is drilled
for safetying.
NAS144DH-22
NAS = National Aircraft Standard
144 = head style; diameter and thread—1⁄4-28 bolt, internal
wrenching
DH = drilled head
22 = screw length in 16ths of an inch—13⁄8 inches long
The basic NAS number identifies the part. The suffix letters
and dash numbers separate different sizes, plating material,
drilling specifications, and so forth. The dash numbers and
suffix letters do not have standard meanings. It is necessary
to refer to a specific NAS page in the Standards book for
the legend.
Riveted & Rivetless Nut Plates
When access to the back of a screw or bolt installation is
impractical, riveted or rivetless nut plates are used to secure
the connection of panels. One example in aircraft this
technique is especially useful is to secure the floorboards to
the stringers and to each other.
Nut Plates
Nuts that are made to be riveted in place in aircraft are called
nut plates. Their purpose is to allow bolts and screws to be
inserted without having to hold the nut. They are permanently
mounted to enable inspection panels and access doors to be
easily removed and installed. When many screws are used on
a panel, to make installation easier, normally floating anchor
nuts are used. The floating anchor nut fits into a small bracket,
which is riveted to the aircraft skin. The nut is free to move,
which makes it much easier to align it with the screw. For
production ease, sometimes ganged anchor nuts are used for inspection panels. Ganged anchor nuts allow the nuts to float
in a channel, making alignment with the screw easy.
Self-locking nut plates are made under several standards
and come in several shapes and sizes. Figure 7-62 shows an
MS21078 two-lug nut plate with a nonmetallic insert and an
MS21047 lightweight, all-metal, 450 °F (232 °C) nut plate.
Nut plates can also have three riveting points if the added
strength is required.
Rivnuts
This is the trade name of a hollow, blind rivet made of 6053
aluminum alloy, counter bored and threaded on the inside.
One person using a special tool, which heads the rivet on the
blind side of the material, can install Rivnuts. The Rivnut is
threaded on the mandrel of the heading tool and inserted in
the rivet hole. The heading tool is held at right angles to the
material, the handle is squeezed, and the mandrel crank is
turned clockwise after each stroke. Continue squeezing the
handle and turning the mandrel crank of the heading tool until
a solid resistance is felt, which indicates that the rivet is set.
The Rivnut is used primarily as a nut plate and in the
attachment of deicer boots to the leading edges of wings.
It may be used as a rivet in secondary structures or for
the attachment of accessories, such as brackets, fairings,
instruments, or soundproofing materials.
Rivnuts are manufactured in two head types, each with
two ends: the flathead with open or closed end and the
countersunk head with open or closed end. All Rivnuts,
except the thin head countersunk type, are available with
or without small projections (keys) attached to the head to
keep the Rivnut from turning. Keyed Rivnuts are used as a
nut plate, while those without keys are used for straight blind
riveting repairs where no torque loads are imposed. A keyway
cutter is needed when installing Rivnuts that have keys.
The countersunk style Rivnut is made with two different head
angles: the 100° with 0.048 and 0.063 inch head thickness and
the 115° with 0.063 inch head thickness. Each of these head
styles is made in three sizes: 6-32, 8-32, and 10-32. These
numbers represent the machine screw size of the threads on
the inside of the Rivnut. The actual outside diameters of the
shanks are 3⁄16 inch for the 6-32 size, 7⁄32 inch for the 8-32 size,
and 1⁄4 inch for the 10-32 size.
Open-end Rivnuts are the most widely used and are
recommended in preference to the closed end type wherever
possible. However, closed-end Rivnuts must be used in
pressurized compartments.
Rivnuts are manufactured in six grip ranges. The minimum
Two-lug anchor nutMS21078 MS21051 MS21055
NAS444 or A6195NAS680A
A1777, A1789, or A1794MS21047 MS21059One-lug anchor nut Corner anchor nut
Right-angle anchor nut
Ganged anchor nuts
U-type tinnerman nuts provide convenient
anchor points for cowlings, fairings, and panels.High-temperature
two-lug anchor nutTwo-lug floating anchor nut
Anchor type tinnerman
nuts are suitable for
nonstructural applicationsMS33737
Instrument nut
To reduce magnetic
influences in the flightdeck,
nonmagnetic mounting
nuts secure instruments
in a control panel.
Figure 7-62. Various nut plates.
grip length is indicated by a plain head and the next higher
grip length by one radial dash mark on the head. Each
succeeding grip range is indicated by an additional radial
dash mark until five marks indicate the maximum range.
Notice in Figure 7-63 that some part number codes consist
of a “6,” an “8,” or a “10,” a “dash,” and two or three more
numbers. In some, the letters “K” or “KB” replaces the
dash. The first number indicates the machine screw size of
the thread, and the last two or three numbers indicate the
maximum grip length in thousandths of an inch. A dash
between the figures indicates that the Rivnut has an open
end and is keyless; a “B” in place of the dash means it has
a closed end and is keyless; a “K” means it has an open end
and has a key; and a “KB” indicates that it has a closed end
and a key. If the last two or three numbers are divisible by
five, the Rivnut has a flathead; if they are not divisible by
five, the Rivnut has a countersunk head.
An example of a part number code is:
10KB106
10 = Grip lengthKB = Closed end and key
106 = Screw and thread size
Dill Lok-Skrus and Dill Lok-Rivets
Dill “Lok-Skru” and “Lok-Rivet” are trade names for
internally-threaded rivets. They are used for blind attachment
of accessories, such as fairings, fillets, access door covers,
door and window frames, floor panels, and the like. Lok-
Skrus and Lok-Rivets are like the Rivnut in appearance
and application; however, they come in two parts and
require more clearance on the blind side than the Rivnut to
accommodate the barrel. [Figure 7-64]
The Lok-Rivet and the Lok-Skru are alike in construction,
except the Lok-Skru is tapped internally for fastening an
accessory by using an attaching screw, whereas the Lok-Rivet is
not tapped and can be used only as a rivet. Since both Lok-Skrus
and Lok-Rivets are installed in the same manner, the following
discussion for the Lok-Skru also applies to the Lok-Rivet.
The main parts of a Lok-Skru are the barrel, the head, and
an attachment screw. The barrel is made of aluminum alloy
6B45
8B45
10B45
6K45
8K45
10K45
6KB45
8KB45
10KB456-75
6B75
8B75
10B75
6K75
8K75
10K75
6KB75
8KB75
10KB756-100
6B100
8B100
10B100
6K100
8K100
10K100
6KB100
8KB100
10KB100
6B91
8B91
10B916-121
6B121
8B121
10B1216-146
6B146
8B146
10B146
6B106
8B106
10B106
6K106
8K106
10K106
6KB106
8KB106
10KB1066-136
6B136
8B136
10B136
6K136
8K136
10K136
6KB136
8KB136
10KB1366-161
6B161
8B161
10B161
6K161
8K161
10K161
6KB161
8KB161
10KB161
Flat—0.32 H ead Thick ness
100° —0.48 H ead Thick ness
100° —0.63 H ead Thick ness
Figure 7-63. Rivnut data chart.and comes in either closed or open ends. The head is either
aluminum alloy or steel, and the attachment screw is made
of steel. All the steel parts are cadmium plated, and all of
aluminum parts are anodized to resist corrosion. When
installed, the barrel screws up over the head and grips the
metal on the blind side. The attaching screw is then inserted
if needed. There are two head types: the flathead and the
countersunk head. The Lok-Skru is tapped for 7-32, 8-32,
10-32, or 10-24 screws, and the diameters vary from 0.230
inch for 6-32 screws, to 0.292 inch for 10-32 screws. Grip
ranges vary from 0.010 inch to 0.225 inch.
Deutsch Rivets
This rivet is a high-strength blind rivet used on late model
aircraft. It has a minimum shear strength of 75,000 psi and
can be installed by one person. The Deutsch rivet consists
of two parts: the stainless-steel sleeve and the hardened steel
drive pin. [Figure 7-65] The pin and sleeve are coated with
a lubricant and a corrosion inhibitor.
The Deutsch rivet is available in diameters of 3⁄16, 1⁄4, or 3⁄8
inch. Grip lengths for this rivet range from 3⁄16 to 1 inch.
Some variation is allowed in grip length when installing the
rivet. For example, a rivet with a grip length of 3⁄16 inch can
be used where the total thickness of materials is between
0.198 and 0.228 inch.
When driving a Deutsch rivet, an ordinary hammer or a
pneumatic rivet gun and a flathead set are used. The rivet
is seated in the previously drilled hole, and then the pin is
driven into the sleeve. The driving action causes the pin to
exert pressure against the sleeve and forces the sides of the
sleeve out. This stretching forms a shop head on the end of
the rivet and provides positive fastening. The ridge on the
top of the rivet head locks the pin into the rivet as the last
few blows are struck.
Sealing Nut Plates
When securing nut plates in pressurized aircraft and in fuel
cells, a sealing nut plate is used instead of the open-ended
variety previously described. Care must be taken to use
exactly the correct length of bolt or screw. If a bolt or screw
is too short, there is not enough threads to hold the device in
place. If the bolt or screw is too long, it penetrates the back
side of the nut plate and compromises the seal. Normally, a
sealant is also used to ensure complete sealing of the nut plate.
Check the manufacturer’s specifications for the acceptable
sealant to be used for sealing nut plates.
Hole Repair & Hole Repair Hardware
Many of the blind fasteners are manufactured in oversized
diameters to accommodate slightly enlarged holes resulting
from drilling out the original fastener. When using rivets
Figure 7-64. Internally-threaded rivet (Rivnut).
Figure 7-65. Deutsch rivet.or even bolts, care must be taken to ensure the hole is not
elongated or slanted.
To reduce the chances of an incorrectly drilled rivet or bolt
hole, use a slightly smaller drill bit first, then enlarge to the
correct diameter. The last step to prepare the hole for the
fastener is to deburr the hole using either a very large drill
bit or a special deburring tool. This practice also works well
when drilling out a previously attached fastener. If the drill
bit does not exactly find the center of the rivet, bolt, or screw,
the hole can easily be elongated, but when using a smaller
drill bit, drill the head only off the fastener. Then the ring and
stem that is left can be pushed out with a pin punch of the
appropriate diameter. If an incorrectly drilled hole is found,
the options are to re-drill the hole to the next larger diameter
for an acceptable fastener or repair the hole using an Acres
fastener sleeve.
Repair of Damaged Holes with Acres Fastener
Sleeves
Acres fastener sleeves are thin-wall tubular elements with
a flared end. The sleeves are installed in holes to accept
standard bolts and rivet-type fasteners. The existing fastener
holes are drilled 1⁄64 inch oversize for installation of the sleeves. The sleeves are manufactured in 1-inch increments.
Along their length, grooves provide a place to break or cut
off excess length to match fastener grip range. The grooves
also provide a place to hold adhesive or sealing agents when
bonding the sleeve into the hole.
Advantages & Limitations
The sleeves are used in holes that must be drilled 1⁄64 inch
oversize to clean up corrosion or other damage. The oversize
hole with the sleeve installed allows the use of the original
diameter fastener in the repaired hole. The sleeves can be
used in areas of high galvanic corrosion where the corrosion
must be confined to a readily replaceable part. Oversizing of
holes reduces the net cross-sectional area of a part and should
not be done unless absolutely required.
Consult the manufacturer of the aircraft, aircraft engine, or
aircraft component prior to repair have damaged holes with
Acres sleeves.
Identification
The sleeve is identified by a standard code number that
represents the type and style of sleeve, a material code,
the fastener shank diameter, and surface finish code letter,
and grip tang for the sleeve. [Figure 7-66] The basic code
number represents the type and material of the sleeve. The
first dash number represents the diameter of the sleeve for
the fastener installed, and the second dash represents the
grip length of the sleeve. The required length of the sleeve
is determined on installation and the excess is broken off the
sleeve. A JK5512A-05N-10 is a 100° low profile head sleeve
of aluminum alloy. The diameter is for a 5⁄32-inch fastener
with no surface finish and is 5⁄8 inch in length.
Hole Preparation
Refer to Figure 7-67 for drill number for standard or close
fit holes. Inspect hole after drilling to assure all corrosion is
removed before installing the sleeve. The hole must also be
the correct shape and free from burrs. The countersink must
be enlarged to receive the flare of the sleeve, so the sleeve
is flush with the surrounding surface.
Installation
After selecting the correct type and diameter sleeve, use the
6501 sleeve breakoff tool for final installation length. Refer
to Figure 7-67 for the sleeve breakoff procedure. The sleeve
may be installed with or without being bonded in the hole.
When bonding the sleeve in a hole, use MIL-S-8802A 1⁄2
sealant. Reinstall original size fastener and torque as required.
Sleeve Removal
Sleeves not bonded in the hole may be removed by either
7-72driving them out with a drift pin of the same diameter as
the outside diameter of the sleeve, or they may be deformed
and removed with a pointed tool. Bonded sleeves may be
removed by this method, but care should be used not to
damage the structure hole. If this method cannot be used,
drill the sleeves out with a drill 0.004 to 0.008 inch smaller
than the installation drill size. The remaining portion of the
sleeve after drilling can be removed using a pointed tool and
applying an adhesive solvent to the sealant.
Control Cables & Terminals
Cables are the most widely used linkage in primary flight
control systems. Cable-type linkage is also used in engine
controls, emergency extension systems for the landing gear,
and various other systems throughout the aircraft.
Cable-type linkage has several advantages over the other
types. It is strong and lightweight, and its flexibility makes it
easy to route through the aircraft. An aircraft cable has a high
mechanical efficiency and can be set up without backlash,
which is very important for precise control.
Cable linkage also has some disadvantages. Tension must be
adjusted frequently due to stretching and temperature changes.
Aircraft control cables are fabricated from carbon steel or
stainless steel.
Cable Construction
The basic component of a cable is a wire. The diameter of
the wire determines the total diameter of the cable. Several
wires are preformed into a helical or spiral shape and then
formed into a strand. These preformed strands are laid around
a straight center strand to form a cable.
Cable designations are based on the number of strands and the
number of wires in each strand. The most common aircraft
cables are the 7 × 7 and 7 × 19.
The 7 × 7 cable consists of seven strands of seven wires
each. Six of these strands are laid around the center strand.
[Figure 7-68] This is a cable of medium flexibility and is used
for trim tab controls, engine controls, and indicator controls.
The 7 × 19 cable is made up of seven strands of 19 wires
each. Six of these strands are laid around the center strand.
[Figure 7-68] This cable is extra flexible and is used in
primary control systems and in other places where operation
over pulleys is frequent.
Aircraft control cables vary in diameter, ranging from 1⁄16 to
3⁄8 inch. The diameter is measured as shown in Figure 7-68 .Cable Fittings
Cables may be equipped with several different types of
fittings, such as terminals, thimbles, bushings, and shackles.
Terminal fittings are generally of the swaged type. They are
available in the threaded end, fork end, eye end, single shank
ball end, and double shank ball end. The threaded end, fork
end, and eye end terminals are used to connect the cable to
a turnbuckle, bell crank, or other linkage in the system. The
ball end terminals are used for attaching cables to quadrants
and special connections where space is limited. Figure 7-69
illustrates the various types of terminal fittings.
The thimble, bushing, and shackle fittings may be used
in place of some types of terminal fittings when facilities
and supplies are limited and immediate replacement of the
cable is necessary.
Turnbuckles
A turnbuckle assembly is a mechanical screw device
consisting of two threaded terminals and a threaded barrel.
[Figure 7-70]
Turnbuckles are fitted in the cable assembly for making
minor adjustments in cable length and for adjusting cable
tension. One of the terminals has right-hand threads and the
other has left-hand threads. The barrel has matching right-
and left-hand internal threads. The end of the barrel with the
left-hand threads can usually be identified by a groove or
knurl around that end of the barrel.
When installing a turnbuckle in a control system, it is
necessary to screw both terminals an equal number of turns
into the barrel. It is also essential that all turnbuckle terminals
be screwed into the barrel until not more than three threads
are exposed on either side of the turnbuckle barrel.
After a turnbuckle is properly adjusted, it must be safetied.
The methods of safetying turnbuckles are discussed later in
this chapter.
Push-Pull Tube Linkage
Push-pull tubes are used as linkage in various types of
mechanically-operated systems. This type linkage eliminates
the problem of varying tension and permits the transfer of
either compression or tension stress through a single tube.
A push-pull tube assembly consists of a hollow aluminum
alloy or steel tube with an adjustable end fitting and a check
nut at either end. [Figure 7-71] The check nuts secure the end
fittings after the tube assembly has been adjusted to its correct
length. Push-pull tubes are generally made in short lengths
to prevent vibration and bending under compression loads.
7-73Acres Sleeve TypeBasic Part
Number
JK5610
JK5511
JK5512
JK5516
JK5517
JK5533100°
509 tension head plus flange
Protruding head (shear)
100°
Low profile head
100°
Standard profile head
(509 type)
Protruding head (tension)
100°
Oversize tension head
(1⁄64 oversize bolt) 2Sleeve
Part N umberBoltSize SleeveLength
161⁄8
#6
5⁄32
#8
#10
¼
5⁄16
3⁄8JK5511( )04( )( )
JK5512( )04( )( )JK5516( )04( )( )JK5517( )04( )( )
JK5511( )45( )( )
JK5512JK5516( )45( )( )JK5517( )45( )( )
JK5511( )05( )( )
JK5512( )05( )( )JK5516( )05( )( )JK5517( )05( )( )
JK5511( )55( )( )
JK5512( )55( )( )JK5516( )55( )( )JK5517( )55( )( )JK5610( )55( )( )
JK5511( )06( )( )
JK5512( )06( )( )JK5516( )06( )( )JK5517( )06( )( )JK5610( )06( )( )
JK5511( )08( )( )
JK5512( )08( )( )JK5516( )08( )( )JK5517( )08( )( )JK5610( )08( )( )
JK5511( )10( )( )
JK5512( )10( )( )JK5516( )10( )( )JK5517( )10( )( )JK5610( )10( )( )
JK5511( )12( )( )
JK5512( )12( )( )JK5516( )12( )( )JK5517( )12( )( )JK5610( )12( )( )
SleevePart N umber BoltSize SleeveLength Acres Sleeve for 1⁄64 Oversize Bolt
JK5533( )06( )( )
JK5533( )08( )( )JK5533( )10( )( )JK5533( )12( )( )13⁄64
17⁄64
21⁄64
25⁄6412
NOTES:
Acre s sleeve, JK5533 1⁄64 ove rsize avai lable in
A286 ste el only
Acres sleeve length in sixteenth-inch increments 1
2Material Material Code
5052 Aluminum alloy ½ hard
6061 Aluminum alloy (T6 condition)
A286 Stainless steel (passivate) CBAPart N umber Breakdown
JK5511 A 04 N 08 L
JK5511
A
N
L Basic part number
Material code 1
Fastener shank diameter in 32 nds
Surface finish
N = No finish
C = C hemical film per MIL-C-554
Length in sixteenth inch increments
(Required installation length by breaking off at proper groove)
“L” at end of part number indicates cetyl alcohol lubricant
Figure 7-66. Acres sleeve identification.
DiameterDiameter
7 strands, 7 wires to each strand7 strands, 19 wires to each strand
Figure 7-68. Cable cross-sections.
A. Drill out corrosion or damage to existing hole to 1/64 oversize.
B. Select proper type and length acres sleeve for existing fastener.C. Bond sleeve in structure hole with MIL-S-8802 class A ½ sealant.Installation Procedure 1/8
#6
5/32
#8
#10
¼
5/16
3/89/64
11/64
21/64
25/640.14060.15400.17190.18000.20550.26600.32810.390828241816
17/640.14050.15200.16950.17700.20400.2656Bolt SizeStandard Fit Close Fit
Drill N umberDrill
DiameterDrill N umberDrill
DiameterHole Preparation
7/329/3211/3213/3213/6417/6421/6425/640.21870.28120.34370.4062Bolt Size Drill N umber Drill Diameter
Hole Preparation for 1⁄64 Oversize Bolt
Existing fastener
Acres sleeve
Structure 1/64 Oversize hole
Acres sleeve installation
Figure 7-67. Acres sleeve identification, installation, and breakoff procedure.
1/8 — 3/8 diameter 7 x 19
1/16 — 3/32 diameter 7 x 7
MS20663 Double shank ball end terminal
MS20664 Single shank ball end terminal
AN665 Rod end terminal
AN666 Threaded cable terminal
MS20667 Fork end cable terminal
AN668/MS20668 Eye end cable terminal
Figure 7-69. Types of terminal fittings.
Pin eye Swaged terminal BarrelLength (threads flush with ends of barrel)
Figure 7-70. Typical turnbuckle assembly.Safetying Methods
To ensure fasteners do not separate from their nuts or holding
ends, various safetying methods are used in aircraft from
heavy aircraft to gliders to recreational aircraft. Safetying is
the process of securing all aircraft, bolts, nuts, screws, pins,
and other fasteners so that they do not work loose due to
vibration. A familiarity with the various methods and means of safetying equipment on an aircraft is necessary to perform
maintenance and inspection.
There are various methods of safetying aircraft parts. The
most widely used methods are safety wire, cotter pins, lock
washers, snap rings, and special nuts, such as self-locking
nuts, pal nuts, and jam nuts. Some of these nuts and washers
have been previously described in this chapter.
Pins
The three main types of pins used in aircraft structures are
the taper pin, flathead pin, and cotter pin. Pins are used in
shear applications and for safetying. Roll pins are finding
increasing uses in aircraft construction.
Taper Pins
Plain and threaded taper pins (AN385 and AN386) are used
in joints that carry shear loads and where absence of play is
essential. The plain taper pin is drilled and usually safetied
with wire. The threaded taper pin is used with a taper pin
washer (AN975) and shear nut (safetied with a cotter pin or
safety clip) or self-locking nut.
Flathead Pin
Commonly called a Clevis pin, the flathead pin (MS20392)
is used with tie rod terminals and in secondary controls,
which are not subject to continuous operation. The pin is
customarily installed with the head up so that if the cotter
pin fails or works out, the pin remains in place.
Cotter Pins
The AN380 cadmium-plated, low-carbon steel cotter pin
is used for safetying bolts, screws, nuts, other pins, and in
various applications where such safetying is necessary. The
AN381 corrosion-resistant steel cotter pin is used in locations
where nonmagnetic material is required or in locations where
resistance to corrosion is desired.
Roll Pins
The roll pin is a pressed fit pin with chamfered ends. It is
Clevis, rod end, adjustableTube, steel, or aluminum alloy
Self-aligning, anti-friction rod
end assembly, adjustableCheck nut Rod end, threaded
Figure 7-71. Push-pull tube assembly.
Figure 7-72. Safety wiring methods.
NOTE:
The safety wire is shown
installed for right-hand
threads. The safety wire
is routed in the opposite
direction for left-hand
threads.
ValvesOil caps Drain cocks
Figure 7-73. Safety wiring attachment for plug connectors.
AN standard fillister head screw (drilled head)
BulkheadAN3102 receptacle
AN3057 adapter AN3106 plug
Figure 7-74. Safety wiring attachment for plug connectors.
tubular in shape and is slotted the full length of the tube.
The pin is inserted with hand tools and is compressed as it
is driven into place. Pressure exerted by the roll pin against
the hole-walls keeps it in place until deliberately removed
with a drift punch or pin punch.
Safety Wiring
Safety wiring is the most positive and satisfactory method of
safetying cap screws, studs, nuts, bolt heads, and turnbuckle
barrels, which cannot be safetied by any other practical
means. It is a method of wiring together two or more units
in such a manner that any tendency of one to loosen is
counteracted by the tightening of the wire.
Nuts, Bolts, & Screws
Nuts, bolts, and screws are safety wired by the single wire
or double twist method. The double twist method is the
most common method of safety wiring. The single wire
method may be used on small screws in a closely spaced
closed geometrical pattern, on parts in electrical systems,
and in places that are extremely difficult to reach. Safety
wiring should always be per conventional methods or as
required by the manufacturer, especially for Light Sport
Aircraft (LSA).
Figure 7-72 is an illustration of various methods that are
commonly used in safety wiring nuts, bolts, and screws.
Careful study of Figure 7-72 shows that:
• Examples 1, 2, and 5 illustrate the proper method of
safety wiring bolts, screws, square-head plugs, and
similar parts when wired in pairs.
• Example 3 illustrates several components wired in series.• Example 4 illustrates the proper method of wiring
castellated nuts and studs. (Note that there is no loop
around the nut.)
• Examples 6 and 7 illustrate a single-threaded component
wired to a housing or lug.
• Example 8 illustrates several components in a closely
spaced closed geometrical pattern using a single wire
method.
When drilled head bolts, screws, or other parts are grouped
together, they are more conveniently safety wired to each
other in a series rather than individually. The number of
nuts, bolts, or screws that may be safety wired together is
dependent on the application. For instance, when safety
wiring widely spaced bolts by the double twist method, a
group of three should be the maximum number in a series.
When safety wiring closely spaced bolts, the number that can
be safety wired by a 24-inch length of wire is the maximum
in a series. The wire is arranged so that if the bolt or screw
begins to loosen, the force applied to the wire is in the
tightening direction.
Parts being safety wired should be torqued to recommend
values and the holes aligned before attempting the safetying
operation. Never over torque or loosen a torqued nut to align
safety wire holes.
Oil Caps, Drain Cocks, & Valves
These units are safety wired as shown in Figure 7-73 . In
the case of the oil cap, the wire is anchored to an adjacent
fillister head screw.
A Clip-locking method
B Wire-wrapping methodStraight end
Hook lip
Hook loop
Hook endLoop endHook shoulder
Direction of pull for inspection
4 turn (min)
This applies to all
turnbuckle wrappings
Double wrap (spiral)
Double wrap
Single wrap (spiral)
Single wrap
Figure 7-75. Safetying turnbuckles: (A) clip-locking method and (B) wire-wrapping method.
7-79Cable size
(inch)Type of WrapDiameter of Safety Wire
(inch)Material
(Annealed Condition)
Single
SingleSingle
Double
SingleSingle0.0200.0400.0400.0400.0570.057Stainless steelCopper, brass
Stainless steelCopper, brass
Copper, brass1
Stainless steel
1Galvanized or tinned steel, or soft iron wires are also acceptable.5⁄32 and greater
Figure 7-76. Turnbuckle safetying guide.
Preferred Optional
Figure 7-77. Cotter pin installation.This system applies to any other unit that must be safety wired
individually. Ordinarily, anchorage lips are conveniently located near these individual parts. When such provision is not made, the safety wire is fastened to some adjacent part of the assembly.
Electrical Connectors
Under conditions of severe vibration, the coupling nut of a connector may vibrate loose and, with sufficient vibration, the connector may come apart. When this occurs, the circuit carried by the cable opens. The proper protective measure to prevent this occurrence is by safety wiring as shown in Figure 7-74. The safety wire should be as short as practicable and must be installed in such a manner that the pull on the wire is in the direction that tightens the nut on the plug.
Turnbuckles
After a turnbuckle has been properly adjusted, it must be safetied. There are several methods of safetying turnbuckles; however, only two methods are discussed in this section. These methods are illustrated in Figure 7-75 . The clip locking
method is used only on the most modern aircraft. The older type aircraft still use the type turnbuckles that require the wire wrapping method.
Double Wrap Method
Of the methods using safety wire for safetying turnbuckles,
the double wrap method is preferred, although the single wrap methods described are satisfactory. The method of double wrap safetying is shown in Figure 7-75. Use two separate lengths of the proper wire as shown in Figure 7-76.
Run one end of the wire through the hole in the barrel of the turnbuckle and bend the ends of the wire toward opposite ends of the turnbuckle. Then pass the second length of the wire into the hole in the barrel and bend the ends along the barrel on the side opposite the first. Then pass the wires at the end of the turnbuckle in opposite directions through the holes in the turnbuckle eyes or between the jaws of the turnbuckle fork, as applicable. Bend the laid wires in place before cutting off the wrapped wire. Wrap the remaining length of safety wire at least four turns around the shank and cut it off. Repeat the procedure at the opposite end of the turnbuckle.
When a swaged terminal is being safetied, pass the ends
of both wires, if possible, through the hole provided in the terminal for this purpose and wrap both ends around the shank as described above.
If the hole is not large enough to allow passage of both wires,
pass the wire through the hole, and loop it over the free end of the other wire, and then wrap both ends around the shank as described.
Single Wrap Method
The single wrap safetying methods described in the following
paragraphs are acceptable but are not the equal of the double wrap methods.
Pass a single length of wire through the cable eye or fork or
through the hole in the swaged terminal at either end of the 1⁄16
3⁄32
1⁄8
1⁄8
1⁄8
Shaft Snap ring
Single-wire method
Figure 7-78. External type snap ring with safety wire installation.turnbuckle assembly. Spiral each of the wire ends in opposite
directions around the first half of the turnbuckle barrel so
that the wires cross each other twice. Thread both wire ends
through the hole in the middle of the barrel so that the third
crossing of the wire ends is in the hole. Again, spiral the two
wire ends in opposite directions around the remaining half
of the turnbuckle, crossing them twice. Then, pass one wire
end through the cable eye or fork, or through the hole in the
swaged terminal. In the manner described above, wrap both
wire ends around the shank for at least four turns each, cutting
off the excess wire.
An alternate to the above method is to pass one length of
wire through the center hole of the turnbuckle and bend the
wire ends toward opposite ends of the turnbuckle. Then pass
each wire end through the cable eye or fork, or through the
hole in the swaged terminal and wrap each wire end around
the shank for at least four turns, cutting off the excess wire.
After safetying, no more than three threads of the turnbuckle
threaded terminal should be exposed.
General Safety Wiring Rules
When using the safety wire method of safetying, the
following general rules should be followed:1. Pigtail of 1⁄4 to 1⁄2 inch (three to six twists) should be
made at the end of the wiring. This pigtail must be bent
back or under to prevent it from becoming a snag.
2. The safety wire must be new upon each application.
3. When castellated nuts are to be secured with safety wire,
tighten the nut to the low side of the selected torque
range, unless otherwise specified, and if necessary,
continue tightening until a slot aligns with the hole.
4. All safety wires must be tight after installation, but not
under such tension that normal handling or vibration
breaks the wire.
5. The wire must be applied so that all pull exerted by
the wire tends to tighten the nut.
6. Twists should be tight and even, and the wire between
the nuts as taut as possible without over twisting.
7. The safety wire should always be installed and
twisted so that the loop around the head stays down
and does not tend to come up over the bolt head,
causing a slack loop.
Cotter Pin Safetying
Cotter pin installation is shown in Figure 7-77 . Castellated
nuts are used with bolts that have been drilled for cotter pins.
The cotter pin should fit neatly into the hole with very little
side play.
The following general rules apply to cotter pin safetying:
1. The prong bent over the bolt end should not extend
beyond the bolt diameter. (Cut it off if necessary.)
2. The prong bent down should not rest against the
surface of the washer. (Again, cut it off if necessary.)
3. If the optional wraparound method is used, the prongs
should not extend outward from the sides of the nut.
4. All prongs should be bent over a reasonable radius.
Sharp angled bends invite breakage. Tapping lightly
with a mallet is the best method of bending the prongs.
Snap Rings
A snap ring is a ring of metal, either round or flat in cross
section, which is tempered to have spring like action. This
spring like action holds the snap ring firmly seated in a groove.
The external types are designed to fit in a groove around the
outside of a shaft or cylinder and may be safety wired. Safety
wiring of an external type snap ring is shown in Figure 7-78 .
The internal types fit in a groove inside a cylinder and are
never safetied. A special type of pliers is designed to install
each type of snap ring. Snap rings can be reused as long as
they retain their shape and spring-like action.
Cleaning & Corrosion Control
Chapter 8
Corrosion
Many aircraft structures are made of metal, and the most
insidious form of damage to those structures is corrosion.
From the moment the metal is manufactured, it must be
protected from the deleterious effects of the environment that
surrounds it. This protection can be the introduction of certain
elements into the base metal, creating a corrosion-resistant
alloy, or the addition of a surface coating of a chemical
conversion coating, metal, or paint. While in use, additional
moisture barriers, such as viscous lubricants and protectants,
may be added to the surface.
The introduction of airframes built primarily of composite
components has not eliminated the need for careful
monitoring of aircraft with regard to corrosion. The airframe
itself may not be subject to corrosion; however, the use
of metal components and accessories within the airframe
means the aviation maintenance technician (AMT) must be
on the alert for the evidence of corrosion when inspecting
any aircraft.
This chapter provides an overview to the problems associated
with aircraft corrosion. For more in-depth information on
the subject, refer to the latest edition of the Federal Aviation
Administration (FAA) Advisory Circular (AC) 43-4, Corrosion
Control for Aircraft. The AC is an extensive handbook that
deals with the sources of corrosion particular to aircraft
structures, as well as steps the AMT can take in the course
of maintaining aircraft that have been attacked by corrosion.
Metal corrosion is the deterioration of the metal by chemical
or electrochemical attack. This type of damage can take place
internally, as well as on the surface. As in the rotting of wood,
this deterioration may change the smooth surface, weaken
the interior, or damage or loosen adjacent parts.
Water or water vapor containing salt combines with oxygen
in the atmosphere to produce the main source of corrosion
in aircraft. Aircraft operating in a marine environment, or
in areas where the atmosphere contains industrial fumes
that are corrosive, are particularly susceptible to corrosive
attacks. [Figure 8-1]
If left unchecked, corrosion can cause eventual structural
failure. The appearance of corrosion varies with the metal. [Figure 8-2] On the surface of aluminum alloys and
magnesium, it appears as pitting and etching and is often
combined with a gray or white powdery deposit. On copper
and copper alloys, the corrosion forms a greenish film; on
steel, a reddish corrosion byproduct commonly referred to
as rust. When the gray, white, green, or reddish deposits
are removed, each of the surfaces may appear etched and
pitted, depending upon the length of exposure and severity
of attack. If these surface pits are not too deep, they may not
significantly alter the strength of the metal; however, the
pits may become sites for crack development, particularly if
the part is highly stressed. Some types of corrosion burrow
between the inside of surface coatings and the metal surface,
spreading until the part fails.
Factors Affecting Corrosion
Many factors affect the type, speed, cause, and seriousness of
metal corrosion. Some of these factors that influence metal
corrosion and the rate of corrosion are:
1. Type of metal
2. Heat-treatment and grain direction
3. Presence of a dissimilar, less corrodible metal
4. Anodic and cathodic surface areas (in galvanic
corrosion)
5. Temperature
6. Presence of electrolytes (hard water, salt water, battery
fluids, etc.)
7. Availability of oxygen
8. Presence of biological organisms
9. Mechanical stress on the corroding metal
10. Time of exposure to a corrosive environment
11. Lead/graphite pencil marks on aircraft surface metals
Pure Metals
Most pure metals are not suitable for aircraft construction and
are used only in combination with other metals to form alloys.
Most alloys are made up entirely of small crystalline regions
called grains. Corrosion can occur on surfaces of those
regions that are less resistant and also at boundaries between
regions, resulting in the formation of pits and intergranular
corrosion. Metals have a wide range of corrosion resistance.
Figure 8-1. Seaplane operations.The most active metals (those that lose electrons easily), such
as magnesium and aluminum, corrode easily. The most noble
metals (those that do not lose electrons easily), such as gold
and silver, do not corrode easily.
Climate
The environmental conditions that an aircraft is maintained
and operated under greatly affects corrosion characteristics.
In a predominately marine environment (with exposure to sea
water and salt air), moisture-laden air is considerably more
detrimental to an aircraft than it would be if all operations
were conducted in a dry climate. Temperature considerations
are important, because the speed of electrochemical attack is
increased in a hot, moist climate.
Geographical Location
The flight routes and bases of operation expose some
airplanes to more corrosive conditions than others. The
operational environment of an aircraft may be categorized
as mild, moderate, or severe with respect to the corrosion
severity of the operational environment. The corrosion
severity of the operational environments in North America are
identified in Figure 8-3. Additional maps for other locations
around the world are published in AC 43-4.
The corrosion severity of any particular area may be increased
by many factors, including airborne industrial pollutants,
chemicals used on runways and taxiways to prevent ice
formation, humidity, temperatures, prevailing winds from a
corrosive environment, etc. Suggested intervals for cleaning,
inspection, lubrication, and preservation when located in mild
zones are every 90 days, moderate zones every 45 days, and
severe zones every 15 days.
Foreign Material
Among the controllable factors that affect the onset and
spread of corrosive attack is foreign material that adheres to
the metal surfaces. Such foreign material includes:• Soil and atmospheric dust
• Oil, grease, and engine exhaust residues
• Salt water and salt moisture condensation
• Spilled battery acids and caustic cleaning solutions
• Welding and brazing flux residues
Micro-organisms
Slimes, molds, fungi and other living organisms (some
microscopic) can grow on damp surfaces. Once they are
established, the area tends to remain damp, increasing the
possibility of corrosion.
Manufacturing Processes
Manufacturing processes, such as machining, forming, welding,
or heat-treatment, can leave stresses in aircraft parts. The
residual stress can cause cracking in a corrosive environment
when the threshold for stress corrosion is exceeded.
It is important that aircraft be kept clean. How often and to
what extent an aircraft must be cleaned depends on several
factors, including geographic location, model of aircraft, and
type of operation.
Types of Corrosion
There are two general classifications of corrosion that
cover most of the specific forms: direct chemical attack and
electrochemical attack. In both types of corrosion, the metal
is converted into a metallic compound, such as an oxide,
hydroxide, or sulfate. The corrosion process involves two
simultaneous changes: the metal that is attacked or oxidized
suffers what is called anodic change, and the corrosive agent
is reduced and is considered as undergoing cathodic change.
Direct Chemical Attack
Direct chemical attack, or pure chemical corrosion, is an
attack resulting from direct exposure of a bare surface to
caustic liquid or gaseous agents. Unlike electrochemical
attack where anodic and cathodic changes take place a
measurable distance apart, the changes in direct chemical
attack occur simultaneously at the same point. The most
common agents causing direct chemical attack on aircraft
are: spilled battery acid or fumes from batteries; residual
flux deposits resulting from inadequately cleaned, welded,
brazed, or soldered joints; and entrapped caustic cleaning
solutions. [Figure 8-4]
With the introduction of sealed lead-acid batteries and the use
of nickel-cadmium batteries, spilled battery acid is becoming
less of a problem. The use of these closed units lessens the
hazards of acid spillage and battery fumes.
Many types of fluxes used in brazing, soldering, and welding
8-3Alloy Type of attack to which alloy is susceptible
Highly susceptible to pitting
Surface oxidation and pitting, surface, and intergranular
Surface pitting, intergranular, exfoliation stress–
corrosion and fatigue cracking, and fretting
Highly corrosion resistant; extended or repeated
contact with chlorinated solvents may result in degradation of the metal’s structural properties at high temperature
Uniform surface corrosion; used as sacrificial plating
to protect steel
Crevice corrosion; some pitting in marine environments;
corrosion cracking; intergranular corrosion (300 series); surface corrosion (400 series)
Generally has good corrosion resistant qualities;
susceptible to pitting in sea water
Surface and intergranular corrosionPitting (promotes rusting of steel where pits occur in
plating)
Will tarnish in the presence of sulfurHighly corrosion resistantSubject to whisker growthMagnesium
Low alloy steel
(4,000–8,000 series)
Aluminum
Titanium
Cadmium
Stainless steels
(300–400 series)
Nickel–base
(Inconel, Monel)
Copper–base Brass,
Bronze
Chromium (Plate)
Silver
Gold
TinAppearance of corrosion product
White, powdery, snow-like mounds and white spots on
surface
Reddish–brown oxide (rust)
White–to–grey powder
No visible corrosion products at low temperature.
Colored surface oxides develop above 700 °F (370 °C)
From white powdery deposit to brown or black mottling
of the surface
Rough surface; sometimes a uniform red, brown, stain
Green powdery deposit
Blue or blue–green powdery deposit
No visible corrosion products; blistering of plating due to
rusting and lifting
Brown–to–black fi lm
Deposits cause darkening of refl ective surfaces
Whisker–like deposit
Figure 8-2. Corrosion of metals.
are corrosive, chemically attacking the metals or alloys that
they are used with. Therefore, it is important to remove residual flux from the metal surface immediately after the joining operation. Flux residues are hygroscopic in nature, absorbing moisture, and unless carefully removed, tend to cause severe pitting.
Caustic cleaning solutions in concentrated form are
kept tightly capped and as far from aircraft as possible. Some cleaning solutions used in corrosion removal are, in themselves, potentially corrosive agents. Therefore, particular attention must be directed toward their complete removal after use on aircraft. Where entrapment of the cleaning solution is likely to occur, use a noncorrosive cleaning agent, even though it is less efficient.
Electrochemical Attack
Corrosion is a natural occurrence that attacks metal by chemical or electrochemical action, converting it back to a metallic compound. The following four conditions must exist before electrochemical corrosion can occur. [Figure 8-5]
1.A metal subject to corrosion (anode)
2.A dissimilar conductive material (cathode) that has
less tendency to corrode3.Presence of a continuous, conductive liquid path
(electrolyte)
4.Electrical contact between the anode and the cathode
(usually in the form of metal to metal contact, suchas rivets, bolts, and corrosion)
Elimination of any one of these conditions stops electrochemical corrosion.
NOTE: Paint can mask the initial stages of corrosion.
Since corrosion products occupy more volume than the original metal, painted surfaces must be inspected often for irregularities, such as blisters, flakes, chips, and lumps.
An electrochemical attack may be likened chemically to
the electrolytic reaction that takes place in electroplating, anodizing, or in a dry cell battery. The reaction in this corrosive attack requires a medium, usually water, that is capable of conducting a tiny current of electricity. When a metal comes in contact with a corrosive agent and is also connected by a liquid or gaseous path that electrons flow through, corrosion begins as the metal decays by oxidation. [Figure 8-5] During
the attack, the quantity of corrosive agent is reduced and, if not renewed or removed, may completely react with the
Figure 8-4. Direct chemical attack in a battery compartment.
CORROSION SEVERITY ZONE
MILD
MODERATE
SEVEREANCHORAGEFAIRBANKS
MONTREAL
NEW YORK
ATLANTA
MIAMI
HAVANA
JAMAICAHAITI
ST. DOMINGOPUERTO RICOCHICAGO
DENVERSEATTLE
LOS ANGELES
MEXICOHOUSTONDALLAS
Figure 8-3. North America corrosion severity chart.
metal becoming neutralized. Different areas of the same
metal surface have varying levels of electrical potential and,
if connected by a conductor such as salt water, sets up a series
of corrosion cells and corrosion will commence.
All metals and alloys are electrically active and have a
specific electrical potential in a given chemical environment.
This potential is commonly referred to as the metal’s
“nobility.” [Figure 8-6] The less noble a metal is, the more
easily it can be corroded. The metals chosen for use in aircraft
structures are a studied compromise with strength, weight,
corrosion resistance, workability, and cost balanced against
the structure’s needs.
The constituents in an alloy also have specific electrical
potentials that are generally different from each other.
Exposure of the alloy surface to a conductive, corrosive
medium causes the more active metal to become anodic and
the less active metal to become cathodic, thereby establishing
conditions for corrosion. These are called local cells. The
greater the difference in electrical potential between the two
metals, the greater the severity of a corrosive attack if the
proper conditions are allowed to develop.
8-5Anodic area Cathodic areaSimplified corrosion cell showing conditions that must exist for electrochemical corrosion.
Elimination of corrosion by application of an organic film to metal surface.
Anodic areaCathodic areaNo contact between electrolyte
and anode and cathodeContinuous liquid path (electrolyte)
Current fl ow
Continuous liquid path (electrolyte)Electron flow
Electron conductor metal
Electron conductor metalUnbroken paint film
Figure 8-5. Electrochemical attack.The conditions for these corrosion reactions are the presence
of a conductive fluid and metals having a difference in potential. If, by regular cleaning and surface refinishing, the medium is removed and the minute electrical circuit eliminated, corrosion cannot occur. This is the basis for effective corrosion control. The electrochemical attack is responsible for most forms of corrosion on aircraft structure and component parts.
Forms of Corrosion
There are many forms of corrosion. The form of corrosion
depends on the metal involved, its size and shape, its specific function, atmospheric conditions, and the corrosion producing agents present. Those described in this section are the more common forms found on airframe structures.
Surface Corrosion
General surface corrosion (also referred to as uniform etch or uniform attack corrosion) is the most common form of corrosion. Surface corrosion appears as a general roughening, etching, or pitting of the surface of a metal, frequently accompanied by a powdery deposit of corrosion products. Surface corrosion may be caused by either direct chemical or electrochemical attack. Sometimes corrosion spreads under the surface coating and cannot be recognized by either the roughening of the surface or the powdery deposit. Instead, closer inspection reveals the paint or plating is lifted off the surface in small blisters that result from the pressure of the underlying accumulation of corrosion products. [Figure 8-7]
Filiform Corrosion
Filiform corrosion is a special form of oxygen concentration cell that occurs on metal surfaces having an organic coating system. It is recognized by its characteristic worm-like trace of corrosion products beneath the paint film. [Figure 8-8] Polyurethane finishes are especially susceptible
to filiform corrosion. Filiform occurs when the relative humidity of the air is between 78–90 percent, and the surface is slightly acidic. This corrosion usually attacks steel and aluminum surfaces. The traces never cross on steel, but they
8-6Metal most
likely to corrode
(anodic)
Metal least likely
to corrode
(cathodic)
Magnesium
Magnesium alloy
Zinc
Aluminum (1100)
Cadmium
Aluminum 2024-T4
Steel or iron
Cast iron
Chromium-iron (active)
Ni-Resist cast iron
Type 304 stainless steel (active)
Type 316 stainless steel (active)
Lead-tin solder
Lead
Tin
Nickel (active)
Inconel nickel-chromium alloy
(active)
Hastelloy alloy C (active)
Brass
Copper
Bronze
Copper-nickel alloy
Monel nickel-copper alloy
Silver solder
Nickel (passive)
Inconel nickel-chromium alloy
(passive)
Chromium-iron (passive)
Type 304 stainless steel (passive)
Type 316 stainless steel (passive)
Hastelloy alloy C (passive)
Silver
Titanium
Graphite
Gold
Platinum
Figure 8-6. The galvanic series of metals and alloys.cross under one another on aluminum, making the damage
deeper and more severe for aluminum. If the corrosion is not
removed, the area treated, and a protective finish applied,
the corrosion can lead to intergranular corrosion, especially
around fasteners and at seams.
Filiform corrosion can be removed using glass bead blasting
material with portable abrasive blasting equipment or
sanding. Filiform corrosion can be prevented by storing
aircraft in an environment with a relative humidity below 70
percent, using coating systems having a low rate of diffusion
for oxygen and water vapors, and by washing the aircraft to
remove acidic contaminants from the surface, such as those
created by pollutants in the air.
Pitting Corrosion
Pitting corrosion is one of the most destructive and intense
forms of corrosion. It can occur in any metal but is most
common on metals that form protective oxide films, such
as aluminum and magnesium alloys. It is first noticeable
as a white or gray powdery deposit, similar to dust, which
blotches the surface. When the deposit is cleaned away, tiny
holes or pits can be seen in the surface. These small surface
openings may penetrate deeply into structural members and
cause damage completely out of proportion to its surface
appearance. [Figure 8-9]
Dissimilar Metal Corrosion
Extensive pitting damage may result from contact between
dissimilar metal parts in the presence of a conductor. While
surface corrosion may or may not be taking place, a galvanic
action, not unlike electroplating, occurs at the points or areas
of contact where the insulation between the surfaces has
broken down or been omitted. This electrochemical attack
can be very serious because, in many instances, the action
is taking place out of sight, and the only way to detect it
prior to structural failure is by disassembly and inspection.
[Figure 8-10]
The contamination of a metal’s surface by mechanical means
can also induce dissimilar metal corrosion. The improper use
of steel cleaning products, such as steel wool or a steel wire
brush on aluminum or magnesium, can force small pieces
of steel into the metal being cleaned, causing corrosion and
ruining the adjoining surface. Carefully monitor the use of
nonwoven abrasive pads, so that pads used on one type of
metal are not used again on a different metal surface.
Concentration Cell Corrosion
Concentration cell corrosion, (also known as crevice
corrosion) is corrosion of metals in a metal-to-metal joint,
corrosion at the edge of a joint even though the joined metals
are identical, or corrosion of a spot on the metal surface
covered by a foreign material. Metal ion concentration cells,
Figure 8-8. Filiform corrosion.
Figure 8-7. Surface corrosion.oxygen concentration cells, and active-passive cells are three
general types of concentration cell corrosion.
Metal Ion Concentration Cells
The solution may consist of water and ions of the metal that
are in contact with water. A high concentration of metal ions
normally exists under faying surfaces where the solution is
stagnant and a low concentration of metal ions exist adjacent
to the crevice, created by the faying surface. [Figure 8-11] An
electrical potential exists between the two points: the area
of the metal in contact with the low concentration of metal
ions is anodic and corrodes; the area in contact with the high
metal ion concentration is cathodic and does not show signs
of corrosion.
Oxygen Concentration Cells
The solution in contact with the metal surface normally
contains dissolved oxygen. An oxygen cell can develop
at any point where the oxygen in the air is not allowed to
diffuse into the solution, thereby creating a difference in
oxygen concentration between two points. Typical locations
of oxygen concentration cells are under gaskets, wood,
rubber, and other materials in contact with the metal surface.
Corrosion occurs at the area of low oxygen concentration
(anode). Alloys such as stainless steel are particularly
susceptible to this type of crevice corrosion. [Figure 8-12]
Active-Passive Cells
Metals that depend on a tightly adhering passive film, usually
an oxide for corrosion protection, are prone to rapid corrosive
attack by active-passive cells. The corrosive action usually
starts as an oxygen concentration cell. The passive film is
broken beneath the dirt particle exposing the active metal to
corrosive attack. An electrical potential will develop between
the large area of the passive film and the small area of the
active metal, resulting in rapid pitting. [Figure 8-13]Intergranular Corrosion
This type of corrosion is an attack along the grain boundaries of
an alloy and commonly results from a lack of uniformity in the
alloy structure. Aluminum alloys and some stainless steels are
particularly susceptible to this form of electrochemical attack.
[Figure 8-14] The lack of uniformity is caused by changes
that occur in the alloy during the heating and cooling process
of the material’s manufacturing. Intergranular corrosion
may exist without visible surface evidence. High-strength
aluminum alloys, such as 2014 and 7075, are more susceptible
to intergranular corrosion if they have been improperly heat-
treated and then exposed to a corrosive environment.
Exfoliation Corrosion
Exfoliation corrosion is an advanced form of intergranular
corrosion and shows itself by lifting up the surface grains
of a metal by the force of expanding corrosion products
occurring at the grain boundaries just below the surface.
[Figure 8-15] It is visible evidence of intergranular corrosion
and is most often seen on extruded sections where grain
thickness is usually less than in rolled forms. This type of
corrosion is difficult to detect in its initial stage. Extruded
components, such as spars, can be subject to this type of
corrosion. Ultrasonic and eddy current inspection methods
are being used with a great deal of success.
Stress-Corrosion/Cracking
This form of corrosion involves a constant or cyclic stress
acting in conjunction with a damaging chemical environment.
The stress may be caused by internal or external loading.
[Figure 8-16] Internal stress may be trapped in a part of
structure during manufacturing processes, such as cold-
working or by unequal cooling from high temperatures.
Most manufacturers follow these processes with a stress
relief operation. Even so, sometimes stress remains trapped.
The stress may be externally introduced in part structure by
riveting, welding, bolting, clamping, press fit, etc. If a slight
mismatch occurs or a fastener is over-torqued, internal stress
8-8High oxygen concentration
Low oxygen concentration
OXYGEN CONCENTRATION CELL
Figure 8-12. Oxygen concentration cell.
Low metal ion concentration
High metal ion concentration
METAL ION CONCENTRATION CELL
Figure 8-11. Metal ion concentration cell. TROUGH PITS
SIDEWAY PITSNarrow, deep
Wide, shallow
Elliptical
Vertical
Subsurface
Undercutting
Horizontal
Figure 8-9. Types of pitting corrosion.
Figure 8-10. Dissimilar metal corrosion.is present. Internal stress is more important than design stress,
because stress corrosion is difficult to recognize before it has
overcome the design safety factor. The level of stress varies
from point to point within the metal. Stresses near the yield
strength are generally necessary to promote stress corrosion
cracking. However, failures may occur at lower stresses.
Specific environments have been identified that cause stress
corrosion cracking of certain alloys.
1. Salt solutions and sea water cause stress corrosion
cracking of high-strength, heat-treated steel and
aluminum alloys.
2. Methyl alcohol-hydrochloric acid solutions cause
stress corrosion cracking of some titanium alloys.
3. Magnesium alloys may stress corrode in moist air.
Stress corrosion may be reduced by applying protective
coatings, stress relief heat-treatments, using corrosion
inhibitors, or controlling the environment. Shot peening a
metal surface increases resistance to stress corrosion cracking
by creating compressive stresses on the surface which should
be overcome by applied tensile stress before the surface
sees any tension load. Therefore, the threshold stress level
is increased.
Figure 8-15. Exfoliation corrosion.
Cathode AnodeElectrolyte enters through cracks in paint fi lm
Cladding Cladding
7075-T6 Aluminum Steel fastenerPaint fi lm
Preferential
corrosion along grain boundaries
Intergranular corrosion
Figure 8-14. Intergranular corrosion of 7075-T6 aluminum adjacent to steel fastener.Foreign material
creates a low oxygen
region that prevents
the re-formation of
passive film
ACTIVE – PASSIVE CELLPassive film protects
exposed surface
Active metal
Figure 8-13. Active-passive cell.
environment. Metals may withstand cyclic stress for an
infinite number of cycles so long as the stress is below the endurance limit of the metal. Once the limit has been exceeded, the metal eventually cracks and fails from metal fatigue. However, when the part or structure undergoing cyclic stress is also exposed to a corrosive environment, the stress level for failure may be reduced many times. Thus, failure occurs at stress levels that can be dangerously low depending on the number of cycles assigned to the life-limited part.
Fatigue corrosion failure occurs in two stages. During the
first stage, the combined action of corrosion and cyclic stress damages the metal by pitting and crack formations to such a degree that fracture by cyclic stress occurs, even if the corrosive environment is completely removed. The second stage is essentially a fatigue stage where failure proceeds by propagation of the crack (often from a corrosion pit or pits). It is controlled primarily by stress concentration effects and the physical properties of the metal. Fracture of a metal part due to fatigue corrosion generally occurs at a stress level far Fretting Corrosion
Fretting corrosion is a particularly damaging form of corrosive attack that occurs when two mating surfaces, normally at rest with respect to one another, are subject to slight relative motion. It is characterized by pitting of the surfaces and the generation of considerable quantities of finely divided debris. Since the restricted movements of the two surfaces prevent the debris from escaping very easily, an extremely localized abrasion occurs. [Figure 8-17]
The presence of water vapor greatly increases this type of deterioration. If the contact areas are small and sharp, deep grooves resembling brinell markings or pressure indentations may be worn in the rubbing surface. As a result, this type of corrosion on bearing surfaces has also been called false brinelling. The most common example of fretting corrosion is the smoking rivet found on engine cowling and wing skins. This is one corrosion reaction that is not driven by an electrolyte, and in fact, moisture may inhibit the reaction. A smoking rivet is identified by a black ring around the rivet.
Fatigue Corrosion
Fatigue corrosion involves cyclic stress and a corrosive
Figure 8-17. Fretting corrosion.
Figure 8-16. Stress corrosion cracking.
extremely vulnerable. Exposure of airframe materials
to salts or their solutions is extremely undesirable.
• Atmosphere—the major atmospheric corrosive agents
are oxygen and airborne moisture. Corrosion often
results from the direct action of atmospheric oxygen
and moisture on metal, and the presence of additional
moisture often accelerates corrosive attack, particularly
on ferrous alloys. However, the atmosphere may
also contain other corrosive gases and contaminants,
particularly industrial and marine salt spray.
• Water—the corrosiveness of water depends on the
type and quantity of dissolved mineral and organic
impurities and dissolved gasses (particularly oxygen)
in the water. One characteristic of water that determines
its corrosiveness is the conductivity or ability to act as
an electrolyte and conduct a current. Physical factors,
such as water temperature and velocity, also have a
direct bearing on its corrosiveness.
Preventive Maintenance
Much has been done to improve the corrosion resistance
of aircraft, such as improvements in materials, surface
treatments, insulation, and modern protective finishes. All of
these have been aimed at reducing the overall maintenance
effort, as well as improving reliability. In spite of these
improvements, corrosion and its control is a very real problem
that requires continuous preventive maintenance. During any
corrosion control maintenance, consult the Safety Data Sheet
(SDS) for information on any chemicals used in the process.
Corrosion preventive maintenance includes the following
specific functions:
1. Adequate cleaning
2. Thorough periodic lubrication
3. Detailed inspection for corrosion and failure of below the fatigue limit of an uncorroded part, even though
the amount of corrosion is relatively small.
Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals
make electrical contact in the presence of an electrolyte.
[Figure 8-18] The rate which corrosion occurs depends on
the difference in the activities. The greater the difference
in activity, the faster corrosion occurs. The rate of galvanic
corrosion also depends on the size of the parts in contact. If
the surface area of the corroding metal is smaller than the
surface area of the less active metal, corrosion is rapid and
severe. When the corroding metal is larger than the less active
metal, corrosion is slow and superficial.
Common Corrosive Agents
Substances that cause corrosion of metals are called corrosive
agents. The most common corrosive agents are acids, alkalies,
and salts. The atmosphere and water, the two most common
media for these agents, may also act as corrosive agents.
• Acids—moderately strong acids severely corrode
most of the alloys used in airframes. The most
destructive are sulfuric acid (battery acid), halogen
acids (hydrochloric, hydrofluoric, and hydrobromic),
nitrous oxide compounds, and organic acids found in
the wastes of humans and animals.
• Alkalies—as a group, alkalies are not as corrosive
as acids. Aluminum and magnesium alloys are
exceedingly prone to corrosive attack by many
alkaline solutions unless the solutions contain a
corrosion inhibitor. Substances particularly corrosive
to aluminum are washing soda, potash (wood ashes),
and lime (cement dust). Ammonia, an alkali, is
an exception because aluminum alloys are highly
resistant to it.
• Salts—most salt solutions are good electrolytes and
can promote corrosive attack. Some stainless-steel
alloys are resistant to attack by salt solutions but
aluminum alloy, magnesium alloys, and other steels are
Figure 8-18. Galvanic corrosion.protective systems
4. Prompt treatment of corrosion and touch up of
damaged paint areas
5. Accurate record keeping and reporting of material or
design deficiencies to the manufacturer and the FAA
6. Use of appropriate materials, equipment, technical
publications, and adequately-training personnel
7. Maintenance of the basic finish systems
8. Keeping drain holes free of obstructions
9. Daily draining of fuel cell sumps
10. Daily wipe down of exposed critical areas
11. Sealing of aircraft against water during foul weather
and proper ventilation on warm, sunny days
12. Replacing deteriorated or damaged gaskets and
sealants to avoid water intrusion and/or entrapment
13. Maximum use of protective covers on parked aircraft
After any period where regular corrosion preventive
maintenance is interrupted, the amount of maintenance
required to repair accumulated corrosion damage and bring
the aircraft back up to standard is usually quite high.
Inspection
Inspection for corrosion is a continuing problem and must
be handled daily. Overemphasizing a particular corrosion
problem when it is discovered and then forgetting about
corrosion until the next crisis is an unsafe, costly, and
troublesome practice. Most scheduled maintenance checklists
are complete enough to cover all parts of the aircraft or
engine, thus no part of the aircraft goes uninspected. Use these
checklists as a general guide when an area is to be inspected
for corrosion. Through experience, one learns that most
aircraft have trouble areas where, despite routine inspection
and maintenance, corrosion still sets in.
All corrosion inspections start with a thorough cleaning of the area to be inspected. A general visual inspection of the
area follows using a flashlight, inspection mirror, and a 5–
l0X magnifying glass. The general inspection is to look for
obvious defects and suspected areas. A detailed inspection
of damage or suspected areas found during the general
inspection follows.
Visual inspection is the most widely used technique and
is an effective method for the detection and evaluation of
corrosion. Visual inspection employs the eyes to look directly
at an aircraft surface or at a low angle of incidence to detect
corrosion. Using the sense of touch is also an effective
inspection method for the detection of hidden, well-developed
corrosion. Other tools used during the visual inspection are
mirrors, optical micrometers, and depth gauges.
Sometimes the inspection areas are obscured by structural
members, equipment installations, or for some reason are
awkward to check visually. Adequate access for inspection
must be obtained by removing access panels and adjacent
equipment, cleaning the area as necessary, and removing
loose or cracked sealants and paints. Mirrors, borescopes, and
fiber optics are useful in providing the means of observing
obscure areas.
In addition to visual inspection, there are several NDI
methods, such as liquid penetrant, magnetic particle, eddy
current, x-ray, ultrasonic, and acoustical emission, that may
be of value in the detection of corrosion. These methods have
limitations and must be performed only by qualified and
certified NDI personnel. Eddy current, x-ray, and ultrasonic
inspection methods require properly calibrated (each time
used) equipment and a controlling reference standard to
obtain reliable results.
In addition to routine maintenance inspections, amphibians
or seaplanes must be checked daily and critical areas cleaned
or treated, as necessary.
Corrosion Prone Areas
Discussed briefly in this section are most of the corrosion
problem areas common to all aircraft. These areas should
be cleaned, inspected, and treated more frequently than less
corrosion prone areas. This information is not necessarily
complete and may be amplified and expanded to cover
the special characteristics of the particular aircraft model
involved by referring to the applicable maintenance manual.
Exhaust Trail Areas
Both jet and reciprocating engine exhaust deposits are very
corrosive and give particular trouble where gaps, seams,
hinges, and fairings are located downstream from the exhaust
pipes or nozzles. [Figure 8-19] Deposits may be trapped
Figure 8-19. Exhaust nozzle area.and not reached by normal cleaning methods. Pay special
attention to areas around rivet heads and in skin lap joints
and other crevices. Remove and inspect fairings and access
plates in the exhaust areas. Do not overlook exhaust deposit
buildup in remote areas, such as the empennage surfaces.
Buildup in these areas is slower and may not be noticed until
corrosive damage has begun.
Battery Compartments and Battery Vent Openings
Despite improvements in protective paint finishes and in
methods of sealing and venting, battery compartments
continue to be corrosion prone areas. Fumes from overheated
electrolyte are difficult to contain and spread to adjacent
cavities, causing a rapid corrosive attack on all unprotected
metal surfaces. Battery vent openings on the aircraft skin
should be included in the battery compartment inspection and
maintenance procedure. If aircraft batteries with electrolytes,
sulfuric acid, or potassium hydroxide are in use, their leakage
will cause corrosion. Regular cleaning and neutralization of
acid deposits minimizes corrosion from this cause. Consult
the applicable maintenance manuals for the particular
aircraft to determine the type of battery installed and the
recommended maintenance.
Bilge Areas
These are natural collection points for waste hydraulic fluids,
water, dirt, and odds and ends of debris. Residual oil quite
often masks small quantities of water that settle to the bottom
and set up a hidden chemical cell.
Instead of using chemical treatments for the bilge water,
current float manufacturers recommend the diligent
maintenance of the internal coatings applied to the float’s
interior during manufacture. In addition to chemical
conversion coatings applied to the surface of the sheet metal
and other structural components and to sealants installed in
lap joints during construction, the interior compartments are
painted to protect the bilge areas. When seaplane structures
are repaired or restored, this level of corrosion protection
must be maintained.
Lavatories, Buffets, & Galleys
These areas, particularly deck areas behind lavatories,
sinks, and ranges, where spilled food and waste products
may collect if not kept clean, are potential trouble spots.
Even if some contaminants are not corrosive in themselves,
they attract and retain moisture and, in turn, cause corrosive
attack. Pay attention to bilge areas located under galleys and
lavatories. Clean these areas frequently and maintain the
protective sealant and paint finishes.Wheel Well and Landing Gear
More than any other area on the aircraft, this area probably
receives more punishment due to mud, water, salt, gravel,
and other flying debris. [Figure 8-20] Because of the many
complicated shapes, assemblies, and fittings, complete
area paint film coverage is difficult to attain and maintain.
A partially applied preservative tends to mask corrosion
rather than prevent it. Due to heat generated by braking
action, preservatives cannot be used on some main landing
gear wheels.
During inspection of this area, pay particular attention to the
following trouble spots:
1. Magnesium wheels, especially around bolt heads, lugs,
and wheel web areas, for the presence of entrapped
water or its effects
2. Exposed rigid tubing, especially at B-nuts and ferrules,
under clamps and tubing identification tapes
3. Exposed position indicator switches and other
electrical equipment
4. Crevices between stiffeners, ribs, and lower skin
surfaces that are typical water and debris traps
5. Axle interiors
6. Exposed surfaces of struts, oleos, arms, links, and
attaching hardware (bolts, pins, etc.)
Water Entrapment Areas
Design specifications require that aircraft have drains installed
in all areas where water may collect. Daily inspection of low
point drains is a standard requirement. If this inspection is
neglected, the drains may become ineffective because of
accumulated debris, grease, or sealants.
Figure 8-20. The landing gear area should be cleaned and inspected
more frequently than other areas.Engine Frontal Areas & Cooling Air Vents
These areas are being constantly abraded with airborne dirt
and dust, bits of gravel from runways, and rain erosion,
leading to removal of the protective finish. Furthermore,
cores of radiator coolers, reciprocating engine cylinder
fins, etc., may not be painted due to the requirement for
heat dissipation. Engine accessory mounting bases usually
have small area of unpainted magnesium or aluminum on
the machined-mounted surfaces. Inspection of these areas
must include all sections in the cooling air path, with special
attention to places where salt deposits may be built up during
marine operations. It is imperative that incipient corrosion be
inhibited and that paint touchup and hard film preservative
coatings are maintained on seaplane and amphibian engine
surfaces at all times.
Wing Flap & Spoiler Recesses
Dirt and water may collect in flap and spoiler recesses
unnoticed, because they are normally retracted. For this
reason, these recesses are potential corrosion problem areas.
Inspect these areas with the spoilers and flaps in the fully
deployed position.
External Skin Areas
External aircraft surfaces are readily visible and accessible
for inspection and maintenance. Even here, certain types
of configurations or combinations of materials become
troublesome under certain operating conditions and require
special attention.
Relatively little corrosion trouble is experienced with
magnesium skins if the original surface finish and insulation
are adequately maintained. Trimming, drilling, and riveting
destroy some of the original surface treatment and can
never be completely restored by touchup procedures. Any
inspection for corrosion must include all magnesium skin
surfaces with special attention to edges, areas around
fasteners, and cracked, chipped, or missing paint.
Piano-type hinges are prime spots for corrosion due to the
dissimilar metal contact between the steel pin and aluminum
hinge. They are also natural traps for dirt, salt, and moisture.
Inspection of hinges must include lubrication and actuation
through several cycles to ensure complete lubricant
penetration. Use water-displacing lubricants when servicing
piano hinges. [Figures 8-21 and 8-22]
Corrosion of metal skins joined by spot welding is the result
of the entrance and entrapment of corrosive agents between
the layers of metal. This type of corrosion is evidenced
by corrosion products appearing at the crevices where the
corrosive agents enter. More advanced corrosive attack
causes skin buckling and eventual spot weld fracture. Skin buckling in its early stages may be detected by sighting along
spot welded seams or by using a straightedge. The only
technique for preventing this condition is to keep potential
moisture entry points, including seams and holes created
by broken spot welds, filled with a sealant or a suitable
preservative compound.
Electronic & Electrical Compartments
Electronic and electrical compartments cooled by ram air or
compressor bleed air are subjected to the same conditions
common to engine and accessory cooling vents and engine
frontal areas. While the degree of exposure is less, because
a lower volume of air passing through and special design
features incorporated to prevent water formation in enclosed
spaces, this is still a trouble area that requires special attention.
Circuit breakers, contact points, and switches are extremely
sensitive to moisture and corrosive attack, thus inspection is
required for these conditions as thoroughly as design permits.
If design features hinder examination of these items while
in the installed condition, inspection is accomplished after
component removal for other reasons.
Miscellaneous Trouble Areas
Helicopter rotor heads and gearboxes, in addition to being
constantly exposed to the elements, contain bare steel
surfaces, many external working parts, and dissimilar metal
contacts. Inspect these areas frequently for evidence of
corrosion. The proper maintenance, lubrication, and the use
of preservative coatings can prevent corrosion in these areas.
All control cables, whether plain carbon steel or corrosion-
resistant steel, are to be inspected to determine their condition
at each inspection period. In this process, inspect cables for
corrosion by random cleaning of short sections with solvent
soaked cloths. If external corrosion is evident, relieve tension
Bare steel hinge pin Al alloy extrusions
Hidden corrosion occurs here. Joint freezes and lugs
break off when hinge is actuated.
Figure 8-22. Hinge corrosion points.
Figure 8-21. Piano hinge.and check the cable for internal corrosion. Replace cables
that have internal corrosion. Remove light external corrosion with a nonwoven abrasive pad lightly soaked in oil or, alternatively, a steel wire brush. When corrosion products have been removed, recoat the cable with preservative.
Corrosion Removal
In general, any complete corrosion treatment involves cleaning and stripping of the corroded area, removing as much of the corrosion products as practicable, neutralizing any residual materials remaining in pits and crevices, restoring protective surface films, and applying temporary or permanent coatings or paint finishes.
Repair of corrosion damage includes removal of all corrosion
and corrosion products. When the corrosion damage is severe and exceeds the damage limits set by the aircraft or parts manufacturer, the part must be replaced. The following paragraphs deal with the correction of corrosive attack on aircraft surface and components where deterioration has not progressed to the point requiring rework or structural repair of the part involved.
Several standard methods are available for corrosion
removal. The methods normally used to remove corrosion are mechanical and chemical. Mechanical methods include hand sanding using abrasive mat, abrasive paper, or metal wool, and powered mechanical sanding, grinding, and buffing, using abrasive mat, grinding wheels, sanding discs, and abrasive rubber mats. However, the method used depends upon the metal and the degree of corrosion.
Surface Cleaning and Paint Removal
The removal of corrosion includes removal of surface finishes
covering the attacked or suspected area. To assure maximum efficiency of the stripping compound, the area must be cleaned of grease, oil, dirt, or preservatives. This preliminary cleaning operation is also an aid in determining the extent of the spread of the corrosion, since the stripping operation is held to the minimum consistent with full exposure of the corrosion damage. Extensive corrosion spread on any panel is to be corrected by fully treating the entire section.
The selection of the type of materials to be used in cleaning
depends on the nature of the matter to be removed. Modern environmental standards encourage the use of water-based, non-toxic cleaning compounds whenever possible. In some locations, local or state laws may require the use of such products, and prohibit the use of solvents that contain volatile organic compounds (VOCs). Where permitted, dry cleaning solvent (P-D-680) may be used for removing oil, grease, or soft preservative compounds. For heavy-duty removal of thick or dried preservatives, other compounds of the solvent emulsion type are available.
The use of a general purpose, water soluble stripper can be
used for most applications. There are other methods for paint removal that have minimal impact upon the aircraft structure, and are considered “environmentally friendly.”
Wherever practicable, chemical paint removal from any
large area is to be accomplished outside (in open air) and preferably in shaded areas. If inside removal is necessary, adequate ventilation must be assured. Synthetic rubber surfaces, including aircraft tires, fabric, and acrylics, must be thoroughly protected against possible contact with paint remover. Care must be exercised in using paint remover, especially around gas or watertight seam sealants, since the stripper tends to soften and destroy the integrity of these sealants.
8-15Mask off any opening that would permit the stripping
compound to get into aircraft interiors or critical cavities.
Paint stripper is toxic and contains ingredients harmful to
both skin and eyes. Therefore, wear rubber gloves, aprons
of acid repellent material, and goggle type eyeglasses. The
following is a general stripping procedure:
1. Brush the entire area to be stripped with a cover of
stripper to a depth of 1⁄32" to 1⁄16". Any paintbrush makes
a satisfactory applicator, except that the bristles will be
loosened by the effect of paint remover on the binder,
and the brush must not be used for other purposes after
being exposed to paint remover.
2. Allow the stripper to remain on the surface for a
sufficient length of time to wrinkle and lift the paint.
This may be from 10 minutes to several hours,
depending on temperature, humidity, and the condition
of the paint coat being removed. Scrub the surface with
a bristle brush saturated with paint remover to further
loosen finish that may still be adhering to the metal.
3. Reapply the stripper as necessary in areas where the
paint remains tightly adhered or where the stripper has
dried, and repeat the above process. Only nonmetallic
scrapers are to be used to assist in removing persistent
paint finishes. Nonwoven abrasive pads intended for
paint stripping may also prove to be useful in removing
the loosened paint.
4. Remove the loosened paint and residual stripper by
washing and scrubbing the surface with water and
a broom, brush, or fresh nonwoven abrasive pad. If
water spray is available, use a low to medium pressure
stream of water directly on the area being scrubbed.
If steam-cleaning equipment is available and the area
is sufficiently large, cleaning may be accomplished
using this equipment together with a solution of steam-
cleaning compound. On small areas, any method may
be used that assures complete rinsing of the cleaned
area. Use care to dispose of the stripped residue in
accordance with environmental laws.
Fairing or Blending Reworked Areas
All depressions resulting from corrosion rework must be
faired or blended with the surrounding surface. Fairing can
be accomplished as follows:
1. Remove rough edges and all corrosion from the
damaged area. All dish-outs must be elliptically
shaped with the major axis running spanwise on wings
and horizontal stabilizers, longitudinally on fuselages,
and vertically on vertical stabilizers.
2. In critical and highly stressed areas, all pits remaining
after the removal of corrosion products must be
blended out to prevent stress risers that may cause stress corrosion cracking. [Figure 8-23] On a non-
critical structure, it is not necessary to blend out pits
remaining after removal of corrosion products by
abrasive blasting, since this results in unnecessary
metal removal.
Rework depressions by forming smoothly blended dish-outs,
using a ratio of 20:1, length to depth. [Figure 8-24] In areas
having closely-spaced, multiple pits, intervening material
must be removed to minimize surface irregularity or
waviness. [Figure 8-25] Steel nut-plates and steel fasteners
are to be removed before blending corrosion out of aluminum
structure. Steel or copper particles embedded in aluminum
can become a point of future corrosion. All corrosion products
must be removed during blending to prevent reoccurrence
of corrosion.
Corrosion of Ferrous Metals
One of the most familiar types of corrosion is ferrous oxide
(rust), generally resulting from atmospheric oxidation of
steel surfaces. Some metal-oxides protect the underlying
base metal, but rust is not a protective coating in any sense
of the word. Its presence actually promotes additional attack
by attracting moisture from the air and acting as a catalyst
for additional corrosion. If complete control of the corrosive
attack is to be realized, all rust must be removed from steel
surfaces.
Rust first appears on bolt heads, hold-down nuts, or other
unprotected aircraft hardware. [Figure 8-26] Its presence in
these areas is generally not dangerous and has no immediate
effect on the structural strength of any major components.
The residue from the rust may also contaminate other ferrous
components, promoting corrosion of those parts. The rust is
indicative of a need for maintenance and of possible corrosive
attack in more critical areas. It is also a factor in the general
appearance of the equipment. When paint failures occur or
mechanical damage exposes highly-stressed steel surfaces
to the atmosphere, even the smallest amount of rusting is
potentially dangerous in these areas and must be removed
and controlled. Rust removal from structural components,
followed by an inspection and damage assessment, must be
done as soon as feasible. [Figure 8-27]
Mechanical Removal of Iron Rust
The most practicable means of controlling the corrosion
of steel is the complete removal of corrosion products
by mechanical means and restoring corrosion preventive
coatings. Except on highly-stressed steel surfaces, the use
of abrasive papers and compounds, small power buffers and
buffing compounds, hand wire brushing, or steel wool are
all acceptable cleanup procedures. However, it should be
recognized that in any such use of abrasives, residual rust
ACCEPTABLE NOT ACCEPTABLECROSS SECTIONTOP VIEW
TRUE PERSPECTIVELocation of corrosion pitsWidth of cleaned up area
(10 times depth min)
Length of cleaned up area
(20 times depth min)
Depth of cleaned up of corrosion
Figure 8-23. Blending or blending corrosion damage.
LONGITUDINALCORROSION DAMAGE BEFORE REWORKPit has been cleaned up to the
extent that all loose corrosion
products have been removed.
Rough edges have been
smoothed and all corrosion
has been removed. However,
depression has not been shaped.Dish-out
after blending
Figure 8-24. Blend of corrosion as a single depression. usually remains in the bottom of small pits and other crevices.
It is practically impossible to remove all corrosion products
by abrasive or polishing methods alone. As a result, once a
part cleaned in such a manner has rusted, it usually corrodes
again more easily than it did the first time.
The introduction of variations of the nonwoven abrasive pad
has also increased the options available for the removal of
surface rust. [Figure 8-28] Flap wheels, pads intended for
use with rotary or oscillating power tools, and hand-held
nonwoven abrasive pads all can be used alone or with light
oils to remove corrosion from ferrous components.
Chemical Removal of Rust
As environmental concerns have been addressed in recent
years, interest in noncaustic chemical rust removal has
increased. A variety of commercial products that actively
remove the iron oxide without chemically etching the
base metal are available and can be considered for use. If
at all possible, the steel part is removed from the airframe
for treatment, as it can be nearly impossible to remove all
residue. The use of any caustic rust removal product requires
the isolation of the part from any nonferrous metals during
treatment and probably inspection for proper dimensions. Chemical Surface Treatment of Steel
There are approved methods for converting active rust to
phosphates and other protective coatings. Other commercial
preparations are effective rust converters where tolerances
are not critical and where thorough rinsing and neutralizing
of residual acid is possible. These situations are generally
not applicable to assembled aircraft, and the use of chemical
Figure 8-26. Rust. Figure 8-27. Rust on structural components.
Damage removed and surface smoothed
with shallow elliptical dish-out Bottom of depression
after corrosion removalCorrosion damage
before removal
LONGITUDINALTRANSVERSE
5D MIN10D MIN
Figure 8-25. Blend out of multiple pits in a corroded area.
inhibitors on installed steel parts is not only undesirable, but
also very dangerous. The danger of entrapment of corrosive
solutions and the resulting uncontrolled attack, that could
occur when such materials are used under field conditions,
outweigh any advantages to be gained from their use.
Removal of Corrosion from Highly Stressed Steel
Parts
Any corrosion on the surface of a highly-stressed steel part is
potentially dangerous, and the careful removal of corrosion
products is required. Surface scratches or change in surface
structure from overheating can also cause sudden failure of
these parts. Corrosion products must be removed by careful
processing, using mild abrasive papers, such as rouge or fine
grit aluminum oxide or fine buffing compounds on cloth
buffing wheels. Nonwoven abrasive pads can also be used.
It is essential that steel surfaces not be overheated during
buffing. After careful removal of surface corrosion, reapply
protective paint finishes immediately. The use of chemical
corrosion removers is prohibited without engineering
authorization, because high-strength steel parts are subject
to hydrogen embrittlement.Corrosion of Aluminum & Aluminum Alloys
Aluminum and aluminum alloys are the most widely used
material for aircraft construction. Aluminum appears high
in the electro-chemical series of elements and corrodes
very easily. However, the formation of a tightly-adhering
oxide film offers increased resistance under most corrosive
conditions. Most metals in contact with aluminum form
couples that undergo galvanic corrosion attack. The alloys
of aluminum are subject to pitting, intergranular corrosion,
and intergranular stress corrosion cracking. In some cases,
the corrosion products of metal in contact with aluminum are
corrosive to aluminum. Therefore, aluminum and its alloys
must be cleaned and protected.
Corrosion on aluminum surfaces is usually quite obvious,
since the products of corrosion are white and generally more
voluminous than the original base metal. Even in its early
stages, aluminum corrosion is evident as general etching,
pitting, or roughness of the aluminum surfaces.
NOTE: Aluminum alloys commonly form a smooth surface
oxidation that is from 0.001" to 0.0025" thick. This is not
considered detrimental. The coating provides a hard-shell
barrier to the introduction of corrosive elements. Such
oxidation is not to be confused with the severe corrosion
discussed in this paragraph.
General surface attack of aluminum penetrates relatively
slowly, but speeds up in the presence of dissolved salts.
Considerable attack can usually take place before serious
loss of structural strength develops.
At least three forms of attack on aluminum alloys are
particularly serious: the penetrating pit-type corrosion
through the walls of aluminum tubing, stress-corrosion
cracking of materials under sustained stress, and intergranular
corrosion, which is characteristic of certain improperly heat-
treated aluminum alloys.
In general, corrosion of aluminum can be more effectively
Figure 8-28. Nonwoven abrasive pads.
treated in place compared to corrosion occurring on other
structural materials used in aircraft. Treatment includes the
mechanical removal of as much of the corrosion products
as practicable and the inhibition of residual materials by
chemical means, followed by the restoration of permanent
surface coatings.
Treatment of Unpainted Aluminum Surfaces
Relatively pure aluminum has considerably more corrosion
resistance when compared with the stronger aluminum alloys.
To take advantage of this characteristic, a thin coating of
relatively pure aluminum is applied over the base aluminum
alloy. The protection obtained is good and the pure-aluminum
clad surface, commonly called “Alclad,” can be maintained in
a polished condition. In cleaning such surfaces, however, care
must be taken to prevent staining and marring of the exposed
aluminum. More important from a protection standpoint,
avoid unnecessary mechanical removal of the protective
Alclad layer and the exposure of the more susceptible
aluminum alloy base material. A typical aluminum corrosion
treatment sequence follows:
1. Remove oil and surface dirt from the aluminum
surface using any suitable mild cleaner. Use caution
when choosing a cleaner. Many commercial consumer
products are actually caustic enough to induce
corrosion if trapped between aluminum lap joints.
Choose a neutral pH product.
2. Hand polish the corroded areas with fine abrasives
or with metal polish. Metal polish intended for use
on clad aluminum aircraft surfaces must not be used
on anodized aluminum, since it is abrasive enough
to actually remove the protective anodized film. It
effectively removes stains and produces a highly
polished, lasting surface on unpainted Alclad. If a
surface is particularly difficult to clean, a cleaner
and brightener compound for aluminum can be used before polishing to shorten the time and lessen the
effort necessary to get a clean surface.
3. Treat any superficial corrosion present using an
inhibitive wipe down material. An alternate treatment
is processing with a solution of sodium dichromate and
chromium trioxide. Allow these solutions to remain
on the corroded area for 5 to 20 minutes, and then
remove the excess by rinsing and wiping the surface
dry with a clean cloth.
4. Overcoat the polished surfaces with waterproof wax.
Aluminum surfaces that are to be subsequently painted can be
exposed to more severe cleaning procedures and can also be
given more thorough corrective treatment prior to painting.
The following sequence is generally used:
1. Thoroughly clean the affected surfaces of all soil
and grease residues prior to processing. Any general
aircraft cleaning procedure may be used.
2. If residual paint film remains, strip the area to be
treated. Procedures for the use of paint removers and
the precautions to observe were previously mentioned
in this chapter under “Surface Cleaning and Paint
Removal.”
3. Treat superficially corroded areas with a 10 percent
solution of chromic acid and sulfuric acid. Apply the
solution by swab or brush. Scrub the corroded area
with the brush while it is still damp. While chromic
acid is a good inhibitor for aluminum alloys, even
when corrosion products have not been completely
removed, it is important that the solution penetrate to
the bottom of all pits and underneath any corrosion
that may be present. Thorough brushing with a stiff
fiber brush loosens or removes most existing corrosion
and assures complete penetration of the inhibitor into
crevices and pits. Allow the chromic acid to remain
in place for at least 5 minutes, and then remove the
excess by flushing with water or wiping with a wet
cloth. There are several commercial chemical surface
treatment compounds similar to the type described
above that may also be used.
4. Dry the treated surface and restore recommended
permanent protective coatings, as required in
accordance with the aircraft manufacturer’s procedures.
Restoration of paint coatings must immediately follow
any surface treatment performed. In any case, make
sure that corrosion treatment is accomplished or is
reapplied on the same day that paint refinishing is
scheduled.
8-19Treatment of Anodized Surfaces
As previously stated, anodizing is a common surface
treatment of aluminum alloys. When this coating is damaged
in service, it can only be partially restored by chemical
surface treatment. Therefore, avoid destruction of the oxide
film in the unaffected area when performing any corrosion
correction of anodized surfaces. Do not use steel wool or steel
wire brushes. Do not use severe abrasive materials.
Nonwoven abrasive pads have generally replaced aluminum
wool, aluminum wire brushes, or fiber bristle brushes as the
tools used for cleaning corroded anodized surfaces. Care must
be exercised in any cleaning process to avoid unnecessary
breaking of the adjacent protective film. Take every
precaution to maintain as much of the protective coating as
practicable. Otherwise, treat anodized surfaces in the same
manner as other aluminum finishes. Chromic acid and other
inhibitive treatments can be used to restore the oxide film.
Treatment of Intergranular Corrosion in Heat‑Treated
Aluminum Alloy Surfaces
As previously described, intergranular corrosion is an attack
along grain boundaries of improperly or inadequately heat-
treated alloys, resulting from precipitation of dissimilar
constituents following heat-treatment. In its most severe form,
actual lifting of metal layers (exfoliation) occurs. [Figure 8-15]
More severe cleaning is a must when intergranular corrosion
is present. The mechanical removal of all corrosion products
and visible delaminated metal layers must be accomplished
to determine the extent of the destruction and to evaluate the
remaining structural strength of the component. Corrosion
depth and removal limits have been established for some
aircraft. Any loss of structural strength must be evaluated
prior to repair or replacement of the part. If the manufacturer’s
limits do not adequately address the damage, a designated
engineering representative (DER) can be brought in to assess
the damage.
Corrosion of Magnesium Alloys
Magnesium is the most chemically active of the metals used
in aircraft construction and is the most difficult to protect.
When a failure in the protective coating does occur, the
prompt and complete correction of the coating failure is
imperative if serious structural damage is to be avoided.
Magnesium attack is probably the easiest type of corrosion
to detect in its early stages, since magnesium corrosion
products occupy several times the volume of the original
magnesium metal destroyed. The beginning of attack shows
as a lifting of the paint film and white spots on the magnesium
surface. These rapidly develop into snow-like mounds or
even “white whiskers.” [Figure 8-29] Reprotection involves
the removal of corrosion products, the partial restoration of surface coatings by chemical treatment, and a reapplication
of protective coatings.
Treatment of Wrought Magnesium Sheet & Forgings
Magnesium skin corrosion usually occurs around edges
of skin panels, underneath washers, or in areas physically
damaged by shearing, drilling, abrasion, or impact. If
the skin section can be removed easily, do so to assure
complete inhibition and treatment. If insulating washers
are involved, loosen screws sufficiently to permit brush
treatment of the magnesium under the insulating washer.
Complete mechanical removal of corrosion products is to
be practiced insofar as practicable. Limit such mechanical
cleaning to the use of stiff, hog bristle brushes and similar
nonmetallic cleaning tools (including nonwoven abrasive
pads), particularly if treatment is to be performed under field
conditions. Like aluminum, under no circumstances are steel
or aluminum tools; steel, bronze, or aluminum wool; or other
cleaning abrasive pads used on different metal surfaces to
be used in cleaning magnesium. Any entrapment of particles
from steel wire brushes or steel tools, or contamination of
treated surfaces by dirty abrasives, can cause more trouble
than the initial corrosive attack.
Corroded magnesium may generally be treated as follows:
1. Clean and strip the paint from the area to be treated.
Paint stripping procedures were discussed earlier in
this chapter and are also addressed in FAA AC 43.13-1,
Acceptable Methods, Techniques, and Practices—
Aircraft Inspection and Repair.
2. Use a stiff, hog-bristle brush or nonwoven abrasive
pad to break loose and remove as much of the
corrosion products as practicable. Steel wire brushes,
carborundum abrasives, or steel cutting tools must not
be used.
3. Treat the corroded area liberally with a chromic acid
solution that sulfuric acid has been added to. Work
the solution into pits and crevices by brushing the
area while still wet with chromic acid, again using a
nonmetallic brush.
4. Allow the chromic acid to remain in place for 5 to
20 minutes before wiping up the excess with a clean,
damp cloth. Do not allow the excess solution to dry
and remain on the surface, as paint lifting is caused
by such deposits.
5. As soon as the surfaces are dry, restore the original
protective paint.
Treatment of Installed Magnesium Castings
Magnesium castings, in general, are more porous and prone
to penetrating attack than wrought magnesium skins. For
Figure 8-29. Magnesium corrosion.all practical purposes, however, treatment is the same for
all magnesium areas. Engine cases, bellcranks, fittings,
numerous covers, plates, and handles are the most common
magnesium castings.
When attack occurs on a casting, the earliest practicable
treatment is required if dangerous corrosive penetration is
to be avoided. In fact, engine cases submerged in saltwater
overnight can be completely penetrated. If it is at all
practicable, separate parting surfaces to effectively treat the
existing attack and prevent its further progress. The same
general treatment sequence in the preceding paragraph for
magnesium skin is to be followed.
If extensive removal of corrosion products from a structural
casting is involved, a decision from the manufacturer may
be necessary to evaluate the adequacy of structural strength
remaining. Specific structural repair manuals usually include
dimensional tolerance limits for critical structural members
and must be referred to if any question of safety is involved.
Treatment of Titanium & Titanium Alloys
Attack on titanium surfaces is generally difficult to detect.
Titanium is, by nature, highly corrosion resistant, but it may
show deterioration from the presence of salt deposits and metal
impurities, particularly at high temperatures. Therefore, the
use of steel wool, iron scrapers, or steel brushes for cleaning or
for the removal of corrosion from titanium parts is prohibited.
If titanium surfaces require cleaning, hand polishing with
aluminum polish or a mild abrasive is permissible if fiber brushes only are used and if the surface is treated following
cleaning with a suitable solution of sodium dichromate. Wipe
the treated surface with dry cloths to remove excess solution,
but do not use a water rinse.
Protection of Dissimilar Metal Contacts
Certain metals are subject to corrosion when placed in contact
with other metals. This is commonly referred to as electrolytic
or dissimilar metals corrosion. Contact of different bare
metals creates an electrolytic action when moisture is
present. If this moisture is salt water, the electrolytic action
is accelerated. The result of dissimilar metal contact is
oxidation (decomposition) of one or both metals. The chart
shown in Figure 8-30 lists the metal combinations requiring
a protective separator. The separating materials may be
metal primer, aluminum tape, washers, grease, or sealant,
depending on the metals involved.
Contacts Not Involving Magnesium
All dissimilar joints not involving magnesium are protected
by the application of a minimum of two coats of zinc
chromate or, preferably, epoxy primer in addition to normal
primer requirements. Primer is applied by brush or spray and
allowed to air dry 6 hours between coats.
Contacts Involving Magnesium
To prevent corrosion between dissimilar metal joints in which
magnesium alloy is involved, each surface is insulated as
follows:
At least two coats of zinc chromate or, preferably, epoxy
primer are applied to each surface. Next, a layer of pressure
sensitive vinyl tape 0.003" thick is applied smoothly and
firmly enough to prevent air bubbles and wrinkles. To avoid
creep back, the tape is not stretched during application.
When the thickness of the tape interferes with the assembly
of parts, where relative motion exists between parts or when
service temperatures above 250 °F are anticipated, the use
of tape is eliminated and extra coats (minimum of three) of
primer are applied.
Corrosion Limits
Corrosion, however slight, is damage. Therefore, corrosion
damage is classified under the four standard types, as is any
other damage. These types are negligible damage, damage
repairable by patching, damage repairable by insertion, and
damage necessitating replacement of parts.
The term “negligible” does not imply that little or nothing is
to be done. The corroded surface must be cleaned, treated,
and painted as appropriate. Negligible damage, generally, is
corrosion that has scarred or eaten away the surface protective
coats and begun to etch the metal. Corrosion damage
8-21Aluminum alloy
Calcium plate
Zinc plate
Carbon and alloy steels
Lead
Tin coating
Copper and alloys
Nickel and alloys
Titanium and alloys
Chromium plate
Corrosion resisting steel
Magnesium alloy
Aluminum alloy
Cadmium plate
Zinc plate
Carbon and alloy steels
Lead
Tin coating
Copper and alloys
Nickel and alloys
Titanium and alloys
Chromium plate
Corrosion resisting steel
Magnesium alloysContacting Metals
Orange areas indicate dissimilar metal contact
Figure 8-30. Dissimilar metal contacts that will result in electrolytic corrosion.extending to classifications of “repairable by patching” and
“repairable by insertion” must be repaired in accordance
with the applicable structural repair manual. When corrosion
damage exceeds the damage limits to the extent that repair is
not possible, the component or structure must be replaced.
Processes & Materials Used in Corrosion
Control
Metal Finishing
Aircraft parts are almost always given some type of surface
finish by the manufacturer. The main purpose is to provide
corrosion resistance; however, surface finishes may also be
applied to increase wear resistance or to provide a suitable
base for paint.
In most instances, the original finishes described in the
following paragraphs cannot be restored in the field due
to unavailable equipment or other limitations. However,
an understanding of the various types of metal finishes is
necessary if they are to be properly maintained in the field
and if the partial restoration techniques used in corrosion
control are to be effective.
Surface Preparation
Original surface treatments for steel parts usually include
a cleaning treatment to remove all traces of dirt, oil,
grease, oxides, and moisture. This is necessary to provide
an effective bond between the metal surface and the final finish. The cleaning process may be either mechanical or
chemical. In mechanical cleaning, the following methods are
employed: wire brush, steel wool, emery cloth, sandblasting,
or vapor blasting.
Chemical cleaning is preferred over mechanical since none
of the base metal is removed by cleaning. There are various
chemical processes now in use, and the type used depends
on the material being cleaned and the type of foreign matter
being removed.
Steel parts are pickled to remove scale, rust, or other foreign
matter, particularly before plating. The pickling solution can
be either muriatic (hydrochloric) or sulfuric acid. Cost wise,
sulfuric acid is preferable, but muriatic acid is more effective
in removing certain types of scale. The pickling solution is
kept in a stoneware tank and is usually heated by means
of a steam coil. Parts not to be electroplated after pickling
are immersed in a lime bath to neutralize the acid from the
pickling solution.
Electrocleaning is another type of chemical cleaning used to
remove grease, oil, or organic matter. In this cleaning process,
the metal is suspended in a hot alkaline solution containing
special wetting agents, inhibitors, and materials to provide
the necessary electrical conductivity. An electric current is
then passed through the solution in a manner similar to that
used in electroplating.
8-22Aluminum and magnesium parts are also cleaned by using
some of the foregoing methods. Blast cleaning, using abrasive
media, is not applicable to thin aluminum sheets, particularly
Alclad. Steel grits are not used on aluminum or corrosion
resistant metals.
Polishing, buffing, and coloring of metal surfaces play a very
important part in the finishing of metal surfaces. Polishing
and buffing operations are sometimes used when preparing
a metal surface for electroplating, and all three operations
are used when the metal surface requires a high luster finish.
Chemical Treatments
Anodizing
Anodizing is the most common surface treatment of
nonclad aluminum alloy surfaces. It is typically done in
specialized facilities in accordance with MIL-DTL-5541F or
AMS-C-5541A. The aluminum alloy sheet or casting is the
positive pole in an electrolytic bath in which chromic acid or
other oxidizing agent produces an aluminum oxide film on
the metal surface. Aluminum oxide is naturally protective.
Anodizing merely increases the thickness and density of the
natural oxide film. When this coating is damaged in service, it
can only be partially restored by chemical surface treatments.
Therefore, when an anodized surface is cleaned including
corrosion removal, the technician must avoid unnecessary
destruction of the oxide film. The anodized coating provides
excellent resistance to corrosion. The coating is soft and
easily scratched, making it necessary to use extreme caution
when handling it prior to coating it with primer.
Aluminum wool, nylon webbing impregnated with aluminum
oxide abrasive, fine grade, nonwoven abrasive pads, or fiber
bristle brushes are the approved tools for cleaning anodized
surfaces. The use of steel wool, steel wire brushes, or harsh
abrasive materials on any aluminum surface is prohibited.
Producing a buffed or wire brush finish by any means is also
prohibited. Otherwise, anodized surfaces are treated in much
the same manner as other aluminum finishes.
In addition to its corrosion resistant qualities, the anodic
coating is also an excellent bond for paint. In most cases, parts
are primed and painted as soon as possible after anodizing.
The anodic coating is a poor conductor of electricity;
therefore, if parts require bonding, the coating is removed
where the bonding wire is to be attached. Alclad surfaces that
are to be left unpainted require no anodic treatment; however,
if the Alclad surface is to be painted, it is usually anodized
to provide a bond for the paint.Alodizing
Alodizing is a simple chemical treatment for all aluminum
alloys to increase their corrosion resistance and to improve
their paint bonding qualities. Because of its simplicity, it is
rapidly replacing anodizing in aircraft work.
The process consists of precleaning with an acidic or alkaline
metal cleaner that is applied by either dipping or spraying.
The parts are then rinsed with fresh water under pressure
for 10 to 15 seconds. After thorough rinsing, Bonderite® is
applied by dipping, spraying, or brushing. A thin, hard coating
results, ranging in color from light, bluish green with a slight
iridescence on copper free alloys to an olive green on copper
bearing alloys. The Bonderite® is first rinsed with clear, cold
or warm water for a period of 15 to 30 seconds. An additional
10 to 15 second rinse is then given in a Deoxylyte® bath.
This bath is to counteract alkaline material and to make the
Bonderite® aluminum surface slightly acid on drying.
Chemical Surface Treatment and Inhibitors
As previously described, aluminum and magnesium alloys
in particular are protected originally by a variety of surface
treatments. Steels may have been treated on the surface
during manufacture. Most of these coatings can only be
restored by processes that are completely impractical in the
field. However, corroded areas where such protective films
have been destroyed require some type of treatment prior
to refinishing.
The labels on the containers of surface treatment chemicals
provide warnings if a material is toxic or flammable. However,
the label might not be large enough to accommodate a list of
all the possible hazards that may ensue if the materials are
mixed with incompatible substances. The Safety Data Sheet
(SDS) should also be consulted for information. For example,
some chemicals used in surface treatments react violently if
inadvertently mixed with paint thinners. Chemical surface
treatment materials must be handled with extreme care and
mixed exactly according to directions.
Chromic Acid Inhibitor
A 10 percent solution by weight of chromic acid, activated
by a small amount of sulfuric acid, is particularly effective in
treating exposed or corroded aluminum surfaces. It may also
be used to treat corroded magnesium. This treatment tends
to restore the protective oxide coating on the metal surface.
Such treatment must be followed by regular paint finishes as
soon as practicable and never later than the same day as the
latest chromic acid treatment. Chromium trioxide flake is a
powerful oxidizing agent and a fairly strong acid. It must be
stored away from organic solvents and other combustibles.
Either thoroughly rinse or dispose of wiping cloths used in
chromic acid pickup.
Figure 8-31. Protective paint finishes are the most effective means of preventing corrosion.Sodium Dichromate Solution
A less active chemical mixture for surface treatment of
aluminum is a solution of sodium dichromate and chromic
acid. Entrapped solutions of this mixture are less likely to
corrode metal surfaces than chromic acid inhibitor solutions.
Chemical Surface Treatments
Several commercial, activated chromate acid mixtures are
available under Specification MIL-C-5541 for field treatment
of damaged or corroded aluminum surfaces. Take precautions
to make sure that sponges or cloths used are thoroughly rinsed
to avoid a possible fire hazard after drying.
Protective Paint Finishes
A good, intact paint finish is the most effective barrier between
metal surfaces and corrosive media. [Figure 8-31] The most
common finishes include catalyzed polyurethane enamel,
waterborne polyurethane enamel, and two-part epoxy
paint. As new regulations regarding the emission of volatile
organic compounds (VOCs) are put into effect, the use of
waterborne paint systems have increased in popularity.
Also, still available are nitrate and butyrate dope finishes
for fabric-covered aircraft. In addition, high visibility
fluorescent materials may also be used, along with a variety
of miscellaneous combinations of special materials. There
may also be rain erosion resistant coatings on metal leading
edges and several different baked enamel finishes on engine
cases and wheels.
Aircraft Cleaning
Cleaning an aircraft and keeping it clean are extremely
important. From an AMT's viewpoint, it should be considered
a regular part of aircraft maintenance. Keeping the aircraft
clean can mean more accurate inspection results, and may
even allow a flight crewmember to spot an impending
component failure. A cracked landing gear fitting covered with mud and grease may be easily overlooked. Dirt can
hide cracks in the skin. Dust and grit cause hinge fittings to
wear excessively. If left on the aircraft’s outer surface, a film
of dirt reduces flying speed and adds extra weight. Dirt or
trash blowing or bouncing around the inside of the aircraft is
annoying and dangerous. Small pieces of dirt blown into the
eyes of the pilot at a critical moment can cause an accident. A
coating of dirt and grease on moving parts makes a grinding
compound that can cause excessive wear. Salt water has a
serious corroding effect on exposed metal parts of the aircraft
and must be washed off immediately.
There are many kinds of cleaning agents approved for use in
cleaning aircraft. It is impractical to cover each of the various
types of cleaning agents since their use varies under different
conditions, such as the type of material to be removed, the
aircraft finish, and whether the cleaning is internal or external.
In general, the types of cleaning agents used on aircraft are
solvents, emulsion cleaners, soaps, and synthetic detergents.
Their use must be in accordance with the applicable
maintenance manual. The types of cleaning agents named
above are also classed as light- or heavy-duty cleaners.
The soap and synthetic detergent-type cleaners are used for
light-duty cleaning, while the solvent and emulsion-type
cleaners are used for heavy-duty cleaning. The light-duty
cleaners that are nontoxic and nonflammable must be used
whenever possible. As mentioned previously, cleaners that
can be effectively rinsed and neutralized must be used, or an
alkaline cleaner may cause corrosion within the lap joints of
riveted or spot-welded sheet metal components.
Exterior Cleaning
There are three methods of cleaning the aircraft exterior:
wet wash, dry wash, and polishing. Polishing can be further
broken down into hand polishing and mechanical polishing.
8-24The type and extent of soiling and the final desired appearance
determine the cleaning method to be used.
Wet wash removes oil, grease, carbon deposits, and most
soils, with the exception of corrosion and oxide films. The
cleaning compounds used are generally applied by spray or
mop. Then high-pressure running water is used as a rinse.
Either alkaline or emulsion cleaners can be used in the wet
wash method.
Dry wash is used to remove airport film, dust, and small
accumulations of dirt and soil when the use of liquids is
neither desirable nor practical. This method is not suitable for
removing heavy deposits of carbon, grease, or oil, especially
in the engine exhaust areas. Dry wash materials are applied
with spray, mops, or cloths and removed by dry mopping or
wiping with clean, dry cloths.
Polishing restores the luster to painted and unpainted surfaces
of the aircraft and is usually performed after the surfaces have
been cleaned. Polishing is also used to remove oxidation and
corrosion. Polishing materials are available in various forms
and degrees of abrasiveness. It is important that the aircraft
manufacturer’s instructions be used in specific applications.
The washing of aircraft should be performed in the shade
whenever possible, as cleaning compounds tend to streak
the surface if applied to hot metal or are permitted to dry
on the area. Install covers over all openings where water
or cleaners might enter and cause damage. Pay particular
attention to instrument system components, such as pitot-
static fittings and ports.
Various areas of aircraft, such as the sections housing radar
and the area forward of the flight deck that are finished with
a flat-finish paint, must not be cleaned more than necessary
and never scrubbed with stiff brushes or coarse rags. A soft
sponge or cheesecloth with a minimum of manual rubbing
is advisable. Any oil or exhaust stains on the surface must
first be removed with a solvent, such as kerosene or other
petroleum-based solvent. Rinse the surfaces immediately after
cleaning to prevent the compound from drying on the surface.
Before applying soap and water to plastic surfaces, flush the
plastic surfaces with fresh water to dissolve salt deposits and
wash away dust particles. Plastic surfaces are to be washed
with soap and water, preferably by hand.
Rinse with fresh water and dry with a chamois, synthetic wipes
designed for use on plastic windshields, or absorbent cotton.
In view of the soft surface, do not rub plastic with a dry cloth
since this is not only likely to cause scratches, but it also builds
up an electrostatic charge that attracts dust particles to the surface. The charge, as well as the dust, may be removed by
patting or gently blotting with a clean, damp chamois. Do not
use scouring powder or other material that can mar the plastic
surface. Remove oil and grease by rubbing gently with a cloth
wet with soap and water. Do not use acetone, benzene, carbon
tetrachloride, lacquer thinners, window cleaning sprays,
gasoline, fire extinguisher, or deicer fluid on plastics, because
they soften the plastic and cause crazing. Finish cleaning the
plastic by coating with a plastic polish intended for aircraft
windows and windshields. These polishes can minimize
small surface scratches and also help keep static charges from
building up on the surface of the windows.
Surface oil, hydraulic fluid, grease, or fuel can be removed
from aircraft tires by washing with a mild soap solution. After
cleaning, lubricate all grease fittings, hinges, and so forth,
where removal, contamination, or dilution of the grease is
suspected during washing of the aircraft.
Interior Cleaning
Keeping the interior of the aircraft clean is just as important as
maintaining a clean exterior surface. Corrosion can establish
itself on the inside structure to a greater degree, because it is
difficult to reach some areas for cleaning. Nuts, bolts, bits of
wire, or other metal objects carelessly dropped and neglected,
combined with moisture and dissimilar metal contact, can
cause electrolytic corrosion.
When performing structural work inside the aircraft, clean
up all metal particles and other debris as soon as possible.
To make cleaning easier and prevent the metal particles and
debris from getting into inaccessible areas, use a drop cloth
in the work area to catch this debris. A vacuum cleaner can
be used to pick up dust and dirt from the interior of the flight
deck and cabin.
Aircraft interior present certain problems during cleaning
operations due to the fact that aircraft cabin compartments
are relatively small enclosures. The possibility of restricted
ventilation and quick buildup of flammable vapor/air
mixtures can occur when there is any indiscriminate use of
flammable cleaning agents or solvents. Additionally, there
may also exist the possibility of an ignition source from
concurrent maintenance work in the form of an electrical
fault, friction or static spark, an open flame device, etc.
Wherever possible, use nonflammable agents in these operations
to reduce to the minimum the fire and explosion hazards.
Types of Cleaning Operations
The principal areas of aircraft cabins that may need periodic
cleaning are:
1. Aircraft passenger cabin areas (seats, carpets, side
8-25panels, headliners, overhead racks, curtains, ash trays,
windows, doors, decorative panels of plastic, wood,
or similar materials)
2. Aircraft flight station areas (similar materials to those
found in passenger cabin areas plus instrument panels,
control pedestals, glare shields, flooring materials,
metallic surfaces of instruments and flight control
equipment, electrical cables and contacts, and so forth)
3. Lavatories and buffets (similar materials to those
found in passenger cabin areas plus toilet facilities,
metal fixtures and trim, trash containers, cabinets,
wash and sink basins, mirrors, ovens, and so forth)
Nonflammable Aircraft Cabin Cleaning Agents &
Solvents
1. Detergents and soaps—These have widespread
application for most aircraft cleaning operations
involving fabrics, headliners, rugs, windows, and
similar surfaces that are not damageable by water
solutions since they are colorfast and nonshrinkable.
Care is frequently needed to prevent leaching of water-
soluble fire retardant salts that may have been used
to treat such materials in order to reduce their flame
spread characteristics. Allowing water laced with fire
retardant salts to come in contact with the aluminum
framework of seats and seat rails can induce corrosion.
Be careful to ensure only the necessary amount of
water is applied to the seat materials when cleaning.
2. Alkaline cleaners—Most of these agents are water-
soluble and thus have no fire hazard properties. They
can be used on fabrics, headliners, rugs, and similar
surfaces in the same manner as detergent and soap
solutions with only minor added limitations resulting
from their inherent caustic character. This may
increase their efficiency as cleaning agents, but results
in somewhat greater deteriorating effects on certain
fabrics and plastics.
3. Acid solutions—A number of proprietary acid
solutions are available for use as cleaning agents.
They are normally mild solutions designed primarily
to remove carbon smut or corrosive stains. As water-
based solutions, they have no flash point, but may
require more careful and judicious use to prevent
damage to fabrics, plastics, or other surfaces and
protect the skin and clothing of those using the
materials.
4. Deodorizing or disinfecting agents—A number of
proprietary agents useful for aircraft cabin deodorizing
or disinfecting are nonflammable. Most of these are
designed for spray application (aerosol type) and have a nonflammable pressurizing agent, but it is best to
check this carefully as some may contain a flammable
compressed gas for pressurization.
5. Abrasives—Some proprietary nonflammable mild
abrasive materials are available for rejuvenating
painted or polished surfaces. They present no fire
hazard.
6. Dry cleaning agents—Perchlorethylene and
trichlorethylene as used at ambient temperatures are
examples of nonflammable dry cleaning agents. These
materials do have a toxicity hazard requiring care in
their use, and in some locations due to environmental
laws, their use may be prohibited or severely
restricted. In the same way, water-soluble agents can
be detrimental. Fire retardant treated materials may
be adversely affected by the application of these dry
cleaning agents.
Flammable & Combustible Agents
1. High flash point solvents—Specially refined petroleum
products, first developed as “Stoddard solvent” and
now sold under a variety of trade names by different
companies, have solvent properties approximating
gasoline, but have fire hazard properties similar to
those of kerosene as commonly used (not heated).
Most of these are stable products having a flash point
from 100 °F to 140 °F with a comparatively low degree
of toxicity.
2. Low flash point solvents—Class I (flash point at
below 100 °F) flammable liquids are not to be used for
aircraft cleaning or refurbishing. Common materials
falling into this “class” are acetone, aviation gasoline
(A VGAS), methyl ethyl ketone, naphtha, and toluol.
In cases where it is absolutely necessary to use a
flammable liquid, use high flash point liquids (those
having a flash point of 100 °F or more).
3. Mixed liquids—Some commercial solvents are
mixtures of liquids with differing rates of evaporation,
such as a mixture of one of the various naphthas and a
chlorinated material. The different rates of evaporation
may present problems from both the toxicity and fire
hazard viewpoints. Such mixtures must not be used,
unless they are stored and handled with full knowledge
of these hazards and appropriate precautions taken.
Container Controls
Flammable liquids should be handled only in approved
containers or safety cans appropriately labeled.
Fire Prevention Precautions
During aircraft cleaning or refurbishing operations where
8-26flammable or combustible liquids are used, the following
general safeguards are recommended:
1. Aircraft cabins are to be provided with ventilation
sufficient at all times to prevent the accumulation of
flammable vapors. To accomplish this, doors to cabins
shall be open to secure maximum advantage of natural
ventilation. Where such natural ventilation is not
sufficient, approved mechanical ventilation equipment
shall be provided and used. The accumulation of
flammable vapors above 25 percent of the lower
flammability limit of the particular vapor being used,
measured at a point 5 feet from the location of use,
shall result in emergency revisions of operations in
progress.
2. All open flame and spark producing equipment or
devices that may be brought within the vapor hazard
area must be shut down and not operated during the
period when flammable vapors may exist.
3. Electrical equipment of a hand portable nature, used
within an aircraft cabin, shall be of the type approved
for use in Class I, Group D, Hazardous Locations as
defined by the National Electrical Code.
4. Switches to aircraft cabin lighting and to the aircraft
electrical system components within the cabin area
must not be worked on or switched on or off during
cleaning operations.
5. Suitable warning signs must be placed in conspicuous
locations at aircraft doors to indicate that flammable
liquids are being or have been used in the cleaning or
refurbishing operation in progress.
Fire Protection Recommendations
During aircraft cleaning or refurbishing operations where
flammable liquids are used, the following general fire
protection safeguards are recommended:
1. Aircraft undergoing such cleaning or refurbishing
must preferably be located outside of the hangar
buildings when weather conditions permit. This
provides for added natural ventilation and normally
assures easier access to the aircraft in the event of fire.
2. It is recommended that during such cleaning or
refurbishing operations in an aircraft outside of the
hangar that portable fire extinguishers be provided
at cabin entrances having a minimum rating of 20-B.
Additionally, at minimum, a booster hose line with an
adjustable water spray nozzle capable of reaching the
cabin area for use pending the arrival of airport fire
equipment must be available. As an alternate to the
previous recommendations, a Class A fire extinguisher
having a minimum rating of 4-A plus or a Class B fire
extinguisher having a minimum rating of 20-B must be placed at aircraft cabin doors for immediate use if
required.
NOTE 1: All-purpose ABC (dry chemical) type
extinguishers are not to be used in situations where
aluminum corrosion is a problem, if the extinguisher
is used.
NOTE 2: Portable and semi-portable fire detection and
extinguishing equipment has been developed, tested,
and installed to provide protection to aircraft during
construction and maintenance operations. Operators
are urged to investigate the feasibility of utilizing
such equipment during aircraft cabin cleaning and
refurbishing operations.
3. Aircraft undergoing such cleaning or refurbishing
where the work is to be done under cover must be
in hangars equipped with automatic fire protection
equipment.
Powerplant Cleaning
Cleaning the powerplant is an important job and must be done
thoroughly. Grease and dirt accumulations on an air-cooled
engine provide an effective insulation against the cooling
effect of air flowing over it. Such an accumulation can also
cover up cracks or other defects.
When cleaning an engine, open or remove the cowling as
much as possible. Beginning with the top, wash down the
engine and accessories with a fine spray of kerosene or
solvent. A bristle brush may be used to help clean some of
the surfaces.
Fresh water, soap, and approved cleaning solvents may be used
for cleaning propeller and rotor blades. Except in the process
of etching, caustic material must not be used on a propeller.
Scrapers, power buffers, steel brushes, or any tool or substances
that mar or scratch the surface must not be used on propeller
blades, except as recommended for etching and repair.
Water spray, rain, or other airborne abrasive material strikes
a whirling propeller blade with such force that small pits are
formed in the blade’s leading edge. If preventive measures are
not taken, corrosion causes these pits to rapidly grow larger.
The pits may become so large that it is necessary to file the
blade’s leading edge until it is smooth.
Steel propeller blades have more resistance to abrasion and
corrosion than aluminum alloy blades. Steel blades, if rubbed
down with oil after each flight, retain a smooth surface for
a long time.
Examine the propellers regularly, because cracks in steel or
aluminum alloy blades can become filled with oil that tends to
oxidize. This can readily be seen when the blade is inspected.
8-27Keeping the surface wiped with oil serves as a safety feature
by helping to make cracks more obvious.
Propeller hubs must be inspected regularly for cracks and
other defects. Unless the hubs are kept clean, defects may not
be found. Clean steel hubs with soap and fresh water or with
an approved cleaning solvent. These cleaning solvents may
be applied by cloths or brushes. Avoid tools and abrasives
that scratch or otherwise damage the plating.
In special cases where a high polish is desired, the use
of a good grade of metal polish is recommended. Upon
completion of the polishing, all traces of polish must be
removed immediately, the blades cleaned, and then coated
with clean engine oil. All cleaning substances must be
removed immediately after completion of the cleaning of any
propeller part. Soap in any form can be removed by rinsing
repeatedly with fresh water. After rinsing, all surfaces must
be dried and coated with clean engine oil. After cleaning the
powerplant, all control arms, bellcranks, and moving parts
must be lubricated according to instructions in the applicable
maintenance manual.
Solvent Cleaners
In general, solvent cleaners used in aircraft cleaning must
have a flashpoint of not less than 105 °F, if explosion proofing
of equipment and other special precautions are to be avoided.
Chlorinated solvents of all types meet the nonflammable
requirements, but are toxic. Safety precautions must be
observed in their use. Use of carbon tetrachloride is to be
avoided. The SDS for each solvent must be consulted for
handling and safety information.
AMTs must review the SDS available for any chemical,
solvent, or other materials they may come in contact with
during the course of their maintenance activities. In particular,
solvents and cleaning liquids, even those considered
“environmentally friendly,” can have varied detrimental
effects on the skin, internal organs, and/or nervous system.
Active solvents, such as methyl ethyl ketone (MEK) and
acetone, can be harmful or fatal if swallowed, inhaled, or
absorbed through the skin in sufficient quantities.
Particular attention must be paid to recommended protective
measures including gloves, respirators, and face shields. A
regular review of the SDS keeps the AMT updated on any
revisions that may be made by chemical manufacturers or
government authorities.
Dry Cleaning Solvent
Stoddard solvent is the most common petroleum base solvent
used in aircraft cleaning. Its flashpoint is slightly above
105 °F and can be used to remove grease, oils, or light soils. Dry cleaning solvent is preferable to kerosene for all cleaning
purposes, but like kerosene, it leaves a slight residue upon
evaporation that may interfere with the application of some
final paint films.
Aliphatic and Aromatic Naphtha
Aliphatic naphtha is recommended for wipe down of cleaned
surfaces just before painting. This material can also be used
for cleaning acrylics and rubber. It flashes at approximately
80 °F and must be used with care. Aromatic naphtha must
not be confused with the aliphatic material. It is toxic,
attacks acrylics and rubber products, and must be used with
adequate controls.
Safety Solvent
Safety solvent, trichloroethane (methyl chloroform), is used
for general cleaning and grease removal. It is nonflammable
under ordinary circumstances and is used as a replacement
for carbon tetrachloride. The use and safety precautions
necessary when using chlorinated solvents must be observed.
Prolonged use can cause dermatitis on some persons.
Methyl Ethyl Ketone (MEK)
MEK is also available as a solvent cleaner for metal surfaces
and paint stripper for small areas. This is a very active solvent
and metal cleaner with a flashpoint of about 24 °F. It is toxic
when inhaled, and safety precautions must be observed during
its use. In most instances, it has been replaced with safer to
handle and more environmentally-friendly cleaning solvents.
Kerosene
Kerosene is mixed with solvent emulsion-type cleaners for
softening heavy preservative coatings. It is also used for
general solvent cleaning, but its use must be followed by a
coating or rinse with some other type of protective agent.
Kerosene does not evaporate as rapidly as dry cleaning
solvent and generally leaves an appreciable film on cleaned
surfaces that may actually be corrosive. Kerosene films may
be removed with safety solvent, water emulsion cleaners, or
detergent mixtures.
Cleaning Compound for Oxygen Systems
Cleaning compounds for use in the oxygen system are
anhydrous (waterless) ethyl alcohol or isopropyl (anti-
icing fluid) alcohol. These may be used to clean accessible
components of the oxygen system, such as crew masks and
lines. Fluids must not be put into tanks or regulators.
Do not use any cleaning compounds that may leave an
oily film when cleaning oxygen equipment. Instructions
of the manufacturer of the oxygen equipment and cleaning
compounds must be followed at all times.
8-28Emulsion Cleaners
Solvent and water emulsion compounds are used in general
aircraft cleaning. Solvent emulsions are particularly useful in
the removal of heavy deposits, such as carbon, grease, oil, or
tar. When used in accordance with instructions, these solvent
emulsions do not affect good paint coatings or organic finishes.
Water Emulsion Cleaner
Material available under Specification MIL-C-22543A is a
water emulsion cleaning compound intended for use on both
painted and unpainted aircraft surfaces. This material is also
acceptable for cleaning fluorescent painted surfaces and is
safe for use on acrylics. However, these properties vary with
the material available. A sample application must be checked
carefully before general uncontrolled use.
Solvent Emulsion Cleaners
One type of solvent emulsion cleaner is nonphenolic and
can be safely used on painted surfaces without softening the
base paint. Repeated use may soften acrylic nitrocellulose
lacquers. It is effective, however, in softening and lifting
heavy preservative coatings. Persistent materials are to be
given a second or third treatment as necessary.
Another type of solvent emulsion cleaner has a phenolic
base that is more effective for heavy-duty application, but it
also tends to soften paint coatings. It must be used with care
around rubber, plastics, or other nonmetallic materials. Wear
rubber gloves and goggles for protection when working with
phenolic base cleaners.
Soaps & Detergent Cleaners
A number of materials are available for mild cleaning use. In
this section, some of the more common materials are discussed.
Cleaning Compound, Aircraft Surfaces
Specification MIL-C-5410 Type I and II materials are used
in general cleaning of painted and unpainted aircraft surfaces
for the removal of light to medium soils, operational films,
oils, or greases. They are safe to use on all surfaces, including
fabrics, leather, and transparent plastics. Nonglare (flat)
finishes are not to be cleaned more than necessary and must
never be scrubbed with stiff brushes.
Nonionic Detergent Cleaners
These materials may be either water-soluble or oil-soluble.
The oil-soluble detergent cleaner is effective in a 3 to 5
percent solution in dry cleaning solvent for softening and
removing heavy preservative coatings. This mixture’s
performance is similar to the emulsion cleaners mentioned
previously.Mechanical Cleaning Materials
Mechanical cleaning materials must be used with care and
in accordance with directions given, if damage to finishes
and surfaces is to be avoided.
Mild Abrasive Materials
No attempt is made in this section to furnish detailed
instructions for using various materials listed. Some “do’s
and don’ts” are included as an aid in selecting materials for
specific cleaning jobs.
The introduction of various grades of nonwoven abrasive
pads has given the AMT a clean, inexpensive material for
the removal of corrosion products and for other light abrasive
needs. The pads can be used on most metals (although the
same pad should not be used on different metals) and are
generally the first choice when the situation arises. A very
open form of this pad is also available for paint stripping
when used in conjunction with wet strippers.
Powdered pumice can be used for cleaning corroded
aluminum surfaces. Similar mild abrasives may also be used.
Impregnated cotton wadding material is used for removal
of exhaust gas stains and polishing corroded aluminum
surfaces. It may also be used on other metal surfaces to
produce a high reflectance.
Aluminum metal polish is used to produce a high luster,
long lasting polish on unpainted aluminum clad surfaces. It
must not be used on anodized surfaces, because it removes
the oxide coat.
Three grades of aluminum wool, coarse, medium, and fine are
used for general cleaning of aluminum surfaces. Impregnated
nylon webbing material is preferred over aluminum wool for
the removal of corrosion products and stubborn paint films
and for the scuffing of existing paint finishes prior to touchup.
Lacquer rubbing compound material can be used to remove
engine exhaust residues and minor oxidation. Avoid heavy
rubbing over rivet heads or edges where protective coatings
may be worn thin.
Abrasive Papers
Abrasive papers used on aircraft surfaces must not contain
sharp or needlelike abrasives that can imbed themselves in
the base metal being cleaned or in the protective coating
being maintained. The abrasives used must not corrode the
material being cleaned. Aluminum oxide paper, 300 grit or
finer, is available in several forms and is safe to use on most
surfaces. Type I, Class 2 material under Federal Specification
8-29P-C-451 is available in 11⁄2" and 2" widths. Avoid the use
of carborundum (silicon carbide) papers, particularly on
aluminum or magnesium. The grain structure of carborundum
is sharp and the material is so hard that individual grains
penetrate and bury themselves, even in steel surfaces. The use
of emery paper or crocus cloth on aluminum or magnesium can
cause serious corrosion of the metal by imbedded iron oxide.
Chemical Cleaners
Chemical cleaners must be used with great care in cleaning
assembled aircraft. The danger of entrapping corrosive
materials in faying surfaces and crevices counteracts any
advantages in their speed and effectiveness. Any materials
used must be relatively neutral and easy to remove. It is
emphasized that all residues must be removed. Soluble salts
from chemical surface treatments, such as chromic acid or
dichromate treatment, liquefy and promote blistering in the
paint coatings.
Phosphoric‑citric Acid
A phosphoric-citric acid mixture (Type I) for cleaning
aluminum surfaces is available and is ready to use as
packaged. Type II is a concentrate that must be diluted with
mineral spirits and water. Wear rubber gloves and goggles
to avoid skin contact. Any acid burns may be neutralized by
copious water washing, followed by treatment with a diluted
solution of baking soda (sodium bicarbonate).
Baking Soda
Baking soda may be used to neutralize acid deposits in
lead-acid battery compartments and to treat acid burns from
chemical cleaners and inhibitors.
Fluid Lines & Fittings
Chapter 9
Introduction
Aircraft fluid lines are usually made of metal tubing or
flexible hose. Metal tubing (also called rigid fluid lines) is
used in stationary applications and where long, relatively
straight runs are possible. They are widely used in aircraft
for fuel, oil, coolant, oxygen, instrument, and hydraulic lines.
Flexible hose is generally used with moving parts or where
the hose is subject to considerable vibration.
Occasionally, it may be necessary to repair or replace
damaged aircraft fluid lines. Very often the repair can be made
simply by replacing the tubing. However, if replacements are
not available, the needed parts may have to be fabricated.
Replacement tubing should be of the same size and material
as the original tubing. All tubing is pressure tested prior to
initial installation and is designed to withstand several times
the normal operating pressure to which it is subjected. If a
tube bursts or cracks, it is generally the result of excessive
vibration, improper installation, or damage caused by
collision with an object. All tubing failures should be
carefully studied and the cause of the failure determined.
Rigid Fluid Lines
Tubing Materials
Copper
In the early days of aviation, copper tubing was used
extensively in aviation fluid applications. In modern aircraft,
aluminum alloy, corrosion-resistant steel, or titanium tubing
have generally replaced copper tubing.
Aluminum Alloy Tubing
Tubing made from 1100 H14 (1⁄2-hard) or 3003 H14 (1⁄2-hard)
is used for general purpose lines of low or negligible fluid
pressures, such as instrument lines and ventilating conduits.
Tubing made from 2024-T3, 5052-O, and 6061-T6 aluminum
alloy materials is used in general purpose systems of low and
medium pressures, such as hydraulic and pneumatic 1,000
to 1,500 psi systems, and fuel and oil lines.
Steel
Corrosion-resistant steel tubing, either annealed CRES 304,
CRES 321, or CRES 304-1⁄8-hard, is used extensively in
high-pressure hydraulic systems (3,000 psi or more) for the
operation of landing gear, flaps, brakes, and in fire zones. Its higher tensile strength permits the use of tubing with thinner
walls; consequently, the final installation weight is not much
greater than that of the thicker wall aluminum alloy tubing.
Steel lines are used where there is a risk of foreign object
damage (FOD) (i.e., the landing gear and wheel well areas).
Swaged or MS flareless fittings are used with corrosion-
resistant tubing. Although identification markings for steel
tubing differ, each usually includes the manufacturer’s name
or trademark, the Society of Automotive Engineers (SAE)
number, and the physical condition of the metal.
Titanium 3AL–2.5V
Titanium 3AL–2.5V tubing and fitting is used extensively in
transport category and high-performance aircraft hydraulic
systems for pressures above 1,500 psi. Titanium is 30 percent
stronger than steel and 50 percent lighter than steel. Cryofit
fittings or swaged fittings are used with titanium tubing.
Do not use titanium tubing and fittings in any oxygen
system assembly. Titanium and titanium alloys are oxygen
reactive. If a freshly formed titanium surface is exposed in
gaseous oxygen, spontaneous combustion could occur at
low pressures.
Material Identification
Before making repairs to any aircraft tubing, it is important to
make accurate identification of tubing materials. Aluminum
alloy, steel, or titanium tubing can be identified readily by
sight where it is used as the basic tubing material. However,
it is difficult to determine whether a material is carbon
steel or stainless steel, or whether it is 1100, 3003, 5052-O,
6061-T6, or 2024-T3 aluminum alloy. To positively identify
the material used in the original installation, compare
code markings of the replacement tubing with the original
markings on the tubing being replaced.
On large aluminum alloy tubing, the alloy designation is
stamped on the surface. On small aluminum tubing, the
designation may be stamped on the surface; but more often it
is shown by a color code, not more than 4" in width, painted
at the two ends and approximately midway between the ends
of some tubing. When the band consists of two colors, one-
half the width is used for each color. [Figure 9-1]
If the code markings are hard or impossible to read, it may
9-2 Aluminum Alloy Number Color of Band
1100 White
3003 Green
2014 Gray
2024 Red
5052 Purple
6053 Black
6061 Blue and Yellow
7075 Brown and Yellow
Figure 9-1. Painted color codes used to identify aluminum
alloy tubing.be necessary to test samples of the material for hardness by
hardness testing.
Sizes
Metal tubing is sized by outside diameter (OD), which is
measured fractionally in sixteenths of an inch. For example,
number 6 tubing is 6⁄16" (or 3⁄8") and number 8 tubing is 8⁄16"
(or 1⁄2") and so forth. The tube diameter is printed on all
rigid tubing. In addition to other classifications or means
of identification, tubing is manufactured in various wall
thicknesses. Thus, it is important when installing tubing to
know not only the material and outside diameter, but also
the thickness of the wall. The wall thickness is printed on
the tubing in thousandths of an inch. To determine the inside
diameter (ID) of the tube, subtract twice the wall thickness
from the outside diameter. For example, a number 10 piece of
tubing with a wall thickness of 0.063" has an inside diameter
of 0.625" – 2(0.063") = 0.499".
Fabrication of Metal Tube Lines
Damaged tubing and fluid lines should be repaired with
new parts whenever possible. Unfortunately, sometimes
replacement is impractical and repair is necessary. Scratches,
abrasions, or minor corrosion on the outside of fluid lines
may be considered negligible and can be smoothed out with
a burnishing tool or aluminum wool. Limitations on the
amount of damage that can be repaired in this manner are
discussed in this chapter under “Rigid Tubing Inspection
and Repair.” If a fluid line assembly is to be replaced, the
fittings can often be salvaged; then the repair involves only
tube forming and replacement.
Tube forming consists of four processes: cutting, bending,
flaring, and beading. If the tubing is small and made of soft
material, the assembly can be formed by hand bending during
installation. If the tube is 1⁄4" diameter or larger, hand bending
without the aid of tools is impractical.
Tube Cutting
When cutting tubing, it is important to produce a square
end, free of burrs. Tubing may be cut with a tube cutter or a
hacksaw. The cutter can be used with any soft metal tubing,
such as copper, aluminum, or aluminum alloy. Correct use of
the tube cutter is shown in Figure 9-2. Special chipless cutters
are available for cutting aluminum 6061-T6, corrosion-
resistant steel, and titanium tubing.
A new piece of tubing should be cut approximately 10 percent
longer than the tube to be replaced to provide for minor
variations in bending. Place the tube in the cutting tool with
the cutting wheel at the point where the cut is to be made.
Rotate the cutter around the tubing, applying light pressure to
the cutting wheel by intermittently twisting the thumbscrew. Too much pressure on the cutting wheel at one time could
deform the tubing or cause excessive burring. After cutting
the tubing, carefully remove any burrs from inside and outside
the tube. Use a knife or the burring edge attached to the tube
cutter. The deburring operation can be accomplished by the
use of a deburring tool. [Figure 9-3] This tool is capable of
removing both the inside and outside burrs by just turning
the tool end for end.
When performing the deburring operation, use extreme care
that the wall thickness of the end of the tubing is not reduced
or fractured. Very slight damage of this type can lead to
fractured flares or defective flares, which do not seal properly.
Use a fine-tooth file to file the end square and smooth.
If a tube cutter is not available, or if tubing of hard material
is to be cut, use a fine-tooth hacksaw, preferably one having
32 teeth per inch. The use of a saw decreases the amount of
work hardening of the tubing during the cutting operation.
After sawing, file the end of the tube square and smooth,
removing all burrs.
An easy way to hold small diameter tubing, when cutting it, is
to place the tube in a combination flaring tool and clamp the
tool in a vise. Make the cut about one-half inch from the flaring
tool. This procedure keeps sawing vibrations to a minimum
and prevents damage to the tubing if it is accidentally hit with
the hacksaw frame or file handle while cutting. Be sure all
filings and cuttings are removed from the tube.
Tube Bending
The objective in tube bending is to obtain a smooth bend
without flattening the tube. Tubing under 1⁄4" in diameter
usually can be bent without the use of a bending tool. For
larger sizes, either portable hand benders or production
benders are usually used. Figure 9-4 shows preferred methods
and standard bend radii for bending tubing by tube size.
Rotate toward open side of cutter
Tube Clean cutCut
Figure 9-2. Tube cutting.
Figure 9-3. Deburring tool.Using a hand bender, insert the tubing into the groove of
the bender so that the measured end is left of the form
block. Align the two zeros and align the mark on the tubing
with the L on the form handle. If the measured end is on
the right side, then align the mark on the tubing with the
R on the form handle. With a steady motion, pull the form
handle until the zero mark on the form handle lines up
with the desired angle of bend, as indicated on the radius
block. [Figure 9-5]
Hand benders come in different sizes that correspond to the
tube diameter. Make sure to select the correct bender for
the desired tube diameter. Figure 9-6 shows hand benders
available for different sizes of tubing. Typically, the tubing
size is stamped in the bender. [Figure 9-7]
Bend the tubing carefully to avoid excessive flattening,
kinking, or wrinkling. A small amount of flattening in bends is
acceptable, but the small diameter of the flattened portion must
not be less than 75 percent of the original outside diameter.
Tubing with flattened, wrinkled, or irregular bends should
not be installed. Wrinkled bends usually result from trying to
bend thin wall tubing without using a tube bender. Excessive
flattening causes fatigue failure of the tube. Examples of
correct and incorrect tubing bends are shown in Figure 9-8.
Tube bending machines for all types of tubing are generally
used in repair stations and large maintenance shops.
With such equipment, proper bends can be made on large
diameter tubing and on tubing made from hard material. The
production CNC™ tube bender is an example of this type of
machine. [Figure 9-9]The ordinary production tube bender accommodates tubing
ranging from 1⁄4" to 11⁄2" outside diameter. Benders for larger
sizes are available, and the principle of their operation is
similar to that of the hand tube bender. The radius blocks are
so constructed that the radius of bend varies with the tube
diameter. The radius of bend is usually stamped on the block.
Alternative Bending Methods
When hand or production tube benders are not available or
are not suitable for a particular bending operation, a filler of
metallic composition or of dry sand may be used to facilitate
bending. When using this method, cut the tube slightly longer
than required. The extra length is for inserting a plug (which
may be wooden) in each end. The tube can also be closed
by flattening the ends or by soldering metal disks in them.
After plugging one end, fill and pack the tube with fine, dry
sand and plug tightly. Both plugs must be tight so they are not
forced out when the bend is made. After the ends are closed,
bend the tubing over a forming block shaped to the specified
radius. In a modified version of the filler method, a fusible
alloy is used instead of sand. In this method, the tube is filled
under hot water with a fusible alloy that melts at 160 °F. The
alloy-filled tubing is then removed from the water, allowed
to cool, and bent slowly by hand around a forming block or
with a tube bender. After the bend is made, the alloy is again
melted under hot water and removed from the tubing. When
using either filler methods, make certain that all particles of
the filler are removed. Visually inspect with a borescope to
make certain that no particles are carried into the system in
which the tubing is installed. Store the fusible alloy filler
where it is free from dust or dirt. It can be re-melted and
reused as often as desired. Never heat this filler in any other
way than the prescribed method, as the alloy will stick to the
inside of the tubing, making them both unusable.
Tube Flaring
Two kinds of flares are generally used in aircraft tubing: the
9-4Type Bender AB AB B B B BC B BC B BC C BC C
Tube OD ¹⁄8" ³⁄16" ¹⁄4" ⁵⁄16" ³⁄8" ³⁄8" ⁷⁄16" ¹⁄2" ¹⁄2" ⁵⁄8" ⁵⁄8" ³⁄4" ³⁄4"
Standard Bend ³⁄8" ⁷⁄16" ⁹⁄16" ¹¹⁄16" ¹¹⁄16" ¹⁵⁄16" 1³⁄8" 1¹⁄2" 1¹⁄4" 2" 1¹⁄2" 2¹⁄2" 1³⁄4"
Type Bender C B C C C C C C C C C C C
Tube OD ⁷⁄8" 1" 1 " 1 ¹⁄8" 1¹⁄4" 1³⁄8" 1³⁄8" 1¹⁄2" 1¹⁄2" 1³⁄4" 2" 2¹⁄2" 3"
Standard Bend 2" 3¹⁄2" 3" 3¹⁄2" 3³⁄4" 5" 6" 5" 6" 7" 8" 10" 12"A– Hand B–Portable hand benders C–Production bender
Figure 9-4. Standard bend radii to which bending tools form the various sizes of tubes.
Figure 9-6. Hand benders.
Figure 9-7. Size identification.
Figure 9-5. Tube bending.single flare and the double flare. [Figure 9-10A and B] Flares
are frequently subjected to extremely high pressures; therefore,
the flare on the tubing must be properly shaped or the
connection leaks or fails. A flare made too small produces a
weak joint, which may leak or pull apart; if made too large,
it interferes with the proper engagement of the screw thread
on the fitting and causes leakage. A crooked flare is the result
of the tubing not being cut squarely. If a flare is not made
properly, flaws cannot be corrected by applying additional
torque when tightening the fitting. The flare and tubing must be
free from cracks, dents, nicks, scratches, or any other defects.
The flaring tool used for aircraft tubing has male and female
dies ground to produce a flare of 35° to 37°. Under no
circumstance is it permissible to use an automotive-type
flaring tool that produces a flare of 45°. [Figure 9-11]
The single-flare hand flaring tool, similar to that shown in
Figure 9-12, is used for flaring tubing. The tool consists of
a flaring block or grip die, a yoke, and a flaring pin. The
flaring block is a hinged double bar with holes corresponding
to various sizes of tubing. These holes are countersunk on
one end to form the outside support against which the flare is
formed. The yoke is used to center the flaring pin over the end
of the tube to be flared. Two types of flaring tools are used to
make flares on tubing: the impact type and the rolling type.
Instructions for Rolling-Type Flaring Tools
Use these tools only to flare soft copper, aluminum, and brass
tubing. Do not use with corrosion-resistant steel or titanium. Cut the tube squarely and remove all burrs. Slip the fitting
nut and sleeve on the tube. Loosen clamping screw used for
locking the sliding segment in the die holder. This permits
their separation. The tools are self-gauging; the proper size
GoodPerfect bend Flattened bend
Wrinkled bend Kinked bend
Figure 9-8. Correct and incorrect tubing bends.
Figure 9-9. CNC™ tube bending machine.
flare is produced when tubing is clamped flush with the top
of the die block. Insert tubing between the segments of the
die block that correspond to the size of the tubing to be flared.
Advance the clamp screw against the end segment and tighten
firmly. Move the yoke down over the top of the die holder
and twist it clockwise to lock it into position. Turn the feed
screw down firmly, and continue until a slight resistance is
felt. This indicates an accurate flare has been completed.
Always read the tool manufacturer’s instructions, because
there are several different types of rolling-type flaring tools
that use slightly different procedures.
Double Flaring
A double flare is used on soft aluminum alloy tubing 3⁄8"
outside diameter and under. This is necessary to prevent
cutting off the flare and failure of the tube assembly under
operating pressures. A double flare is smoother and more
concentric than a single flare and therefore seals better. It is
also more resistant to the shearing effect of torque.
Double Flaring Instructions
Deburr both the inside and outside of the tubing to be flared.
Cut off the end of the tubing if it appears damaged. Anneal
brass, copper, and aluminum by heating to a dull red and cool rapidly in cold water. Open the flaring tool by unscrewing
both clamping screws. Select the hole in the flaring bar that
matches the tubing diameter and place the tubing with the
end you have just prepared, extending above the top of the
bar by a distance equal to the thickness of the shoulder of
the adapter insert. Tighten clamping screws to hold tubing
securely. Insert pilot of correctly sized adapter into tubing.
Slip yoke over the flaring bars and center over adapter.
Advance the cone downward until the shoulder of the adapter
rests on the flaring bar. This bells out the end of the tubing.
Next, back off the cone just enough to remove the adapter.
After removing the adapter, advance the cone directly into the
belled end of the tubing. This folds the tubing on itself and
forms an accurate double flare without cracking or splitting
the tubing. To prevent thinning out of the flare wall, do not
overtighten. [Figure 9-13]
Fittings
Rigid tubing may be joined to either an end item (such as a
brake cylinder), another section of either rigid tubing, or to a
flexible hose (such as a drain line). In the case of connection
to an end item or another tube, fittings are required, which
may or may not necessitate flaring of the tube. In the case of
attachment to a hose, it may be necessary to bead the rigid
tube so that a clamp can be used to hold the hose onto the tube.
Flareless Fittings
Although the use of flareless tube fittings eliminates all
tube flaring, another operation, referred to as presetting,
is necessary prior to installation of a new flareless tube
assembly. Flareless tube assemblies should be preset with the
proper size presetting tool or operation. Figure 9-14 (steps
37°
Figure 9-11. Flaring tool.
Figure 9-12. Hand flaring tool.
A. Single-flared end
B. Double-flared end
Figure 9-10. Cutaway view of single-flared (A) and double-
flared (B) tube ends.
1, 2, and 3) illustrates the presetting operation, which is
performed as follows:
1. Cut the tube to the correct length, with the ends
perfectly square. Deburr the inside and outside of
the tube. Slip the nut, then the sleeve, over the tube
(step 1), lubricate the threads of the fitting and nut
with hydraulic fluid.
2. Place the fitting in a vise (step 2), and hold the tubing
firmly and squarely on the seat in the fitting. (The
tube must bottom firmly in the fitting.) Tighten the
nut until the cutting edge of the sleeve grips the tube.
To determine this point, slowly turn the tube back
and forth while tightening the nut. When the tube no
longer turns, the nut is ready for tightening. 3. Final tightening depends upon the tubing (step 3). For
aluminum alloy tubing up to and including 1⁄2" outside
diameter, tighten the nut from 1 to 11⁄6 turns. For steel
tubing and aluminum alloy tubing over 1⁄2" outside
diameter, tighten from 11⁄6 to 11⁄2 turns.
After presetting the sleeve, disconnect the tubing from the
fitting and check the following points: The tube should extend
3⁄32" to 1⁄8" beyond the sleeve pilot; otherwise, blowoff may
occur. The sleeve pilot should contact the tube or have a
maximum clearance of 0.005" for aluminum alloy tubing or
0.015" for steel tubing. A slight collapse of the tube at the
sleeve cut is permissible. No movement of the sleeve pilot,
except rotation, is permissible.
Figure 9-13. Double flare tool.Beading
Tubing may be beaded with a hand beading tool, with
machine beading rolls, or with grip dies. The method to be
used depends on the diameter and wall thickness of the tube
and the material from which it was made.
The hand beading tool is used with tubing having 1⁄4" to
1" outside diameter. [Figure 9-15] The bead is formed by
using the beader frame with the proper rollers attached. The
inside and outside of the tube is lubricated with light oil to
reduce the friction between the rollers during beading. The
sizes, marked in sixteenths of an inch on the rollers, are for
the outside diameter of the tubing that can be beaded with
the rollers.
Separate rollers are required for the inside of each tubing size,
and care must be taken to use the correct parts when beading.
The hand beading tool works somewhat like the tube cutter in
that the roller is screwed down intermittently while rotating
the beading tool around the tubing. In addition, a small vise
(tube holder) is furnished with the kit.
Other methods and types of beading tools and machines are
available, but the hand beading tool is used most often. As a
rule, beading machines are limited to use with large diameter
tubing, over 115⁄16", unless special rollers are supplied. The
grip-die method of beading is confined to small tubing.
Fluid Line Identification
Fluid lines in aircraft are often identified by markers made
up of color codes, words, and geometric symbols. These
markers identify each line’s function, content, and primary
hazard. Figure 9-16 illustrates the various color codes and
symbols used to designate the type of system and its contents.
Fluid lines are marked, in most instances, with 1" tape or
decals. [Figure 9-17A] On lines 4" in diameter (or larger),
lines in oily environment, hot lines, and on some cold lines, steel tags may be used in place of tape or decals.
[Figure 9-17B] Paint is used on lines in engine compartments
where there is the possibility of tapes, decals, or tags being
drawn into the engine induction system.
In addition to the above-mentioned markings, certain lines
may be further identified regarding specific function within a
system (e.g., drain, vent, pressure, or return). Lines conveying
fuel may be marked FLAM [Figure 9-17] ; lines containing
toxic materials are marked TOXIC in place of FLAM. Lines
containing physically dangerous materials, such as oxygen,
nitrogen, or FreonTM, may be marked PHDAN.
Aircraft and engine manufacturers are responsible for the
original installation of identification markers, but the aviation
mechanic is responsible for the replacement when it becomes
necessary. Tapes and decals are generally placed on both
ends of a line and at least once in each compartment through
which lines run. In addition, identification markers are placed
immediately adjacent to each valve, regulator, filter, or other
accessories within a line. Where paint or tags are used,
location requirements are the same as for tapes and decals.
Fluid Line End Fittings
Depending on the type and use, fittings have either pipe
threads or machine threads. Pipe threads are similar to those
used in ordinary plumbing and are tapered, both internal and
external. External threads are referred to as male threads and
internal threads are female threads.
When two fittings are joined, a male into a female, the
thread taper forms a seal. Some form of pipe thread lubricant
approved for particular fluid application should be used when
joining pipe threads to prevent seizing and high-pressure
leakage. Use care when applying thread lubricant so that the
lubricant does not enter and contaminate the system. Do not
use lubricants on oxygen lines. Oxygen reacts with petroleum
products and can ignite (special lubricants are available or
oxygen systems).
Fitting
Tube Sleeve pilot
Sleeve cutting edgeSleeve Tube nutSTEP 1
STEP 2
Slight deformation permissible
.005" maximum aluminum alloy tubing
.015" maximum corrosion-resistant steel tubingSTEP 3
Figure 9-14. Presetting flareless tube assembly.Machine threads have no sealing capability and are similar
to those used on common nuts and bolts. This type of fitting is used only to draw connections together or for attachment through bulkheads. A flared tube connection, a crush washer, or a synthetic seal is used to make the connection fluid tight. Machine threads have no taper and do not form a fluid-tight seal. The size of these fittings is given in dash numbers, which equal the nominal outside diameter in sixteenths of an inch.
Universal Bulkhead Fittings
When a fluid line passes through a bulkhead, and it is desired to secure the line to the bulkhead, a bulkhead fitting should be used. The end of the fitting that passes through the bulkhead is longer than the other end(s), which allows a locknut to be installed, securing the fitting to the bulkhead.
Fittings attach one piece of tubing to another or to system
units. There are four types: (1) bead and clamp, (2) flared fittings, (3) flareless fittings, and (4) permanent fittings (Permaswage™, Permalite™, and Cyrofit™). The amount of pressure that the system carries and the material used are usually the deciding factors in selecting a connector.
The beaded type of fitting, which requires a bead and a section
of hose and hose clamps, is used only in low- or medium-pressure systems, such as vacuum and coolant systems. The flared, flareless, or permanent-type fittings may be used as connectors in all systems, regardless of the pressure.
AN Flared Fittings
A flared tube fitting consists of a sleeve and a nut. [Figure 9-18] The nut fits over the sleeve and, when
tightened, draws the sleeve and tubing flare tightly against a male fitting to form a seal. Tubing used with this type of fitting must be flared before installation. The male fitting has a cone-shaped surface with the same angle as the inside of the flare. The sleeve supports the tube so that vibration does not concentrate at the edge of the flare and distributes the shearing action over a wider area for added strength.Fitting combinations composed of different alloys should be avoided to prevent dissimilar metal corrosion. As with all fitting combinations, ease of assembly, alignment, and proper lubrication should be assured when tightening fittings during installation.
Standard AN fittings are identified by their black or blue color.
All AN steel fittings are colored black, all AN aluminum fittings are colored blue, and aluminum bronze fittings are cadmium plated and natural in appearance. A sampling of AN fittings is shown in Figure 9-19. Figure 9-20 contains
3/32" to 1/8"
Figure 9-15. To use the hand beading tool, cut with tube cutter (1), deburr (2), oil (3), and revolve tool around tube while tightening
handle (4).
connection that is virtually maintenance free. Swaged
fittings are used to join hydraulic lines in areas where routine
disconnections are not required and are often used with
titanium and corrosion-resistant steel tubing. The fittings are
installed with portable hydraulically-powered tooling, which
is compact enough to be used in tight spaces. [Figure 9-24]
If the fittings need to be disconnected, cut the tubing with a
tube cutter. Special installation tooling is available in portable
kits. Always use the manufacturer’s instructions to install
swaged fittings. Typical Permaswage™ fittings are shown
in Figure 9-25.
One of the latest developments is the Permalite™ fitting.
Permalite™ is a tube fitting that is mechanically attached to
the tube by axial swaging. Permalite™ works by deforming
the fitting into the tube being joined by moving a ring, a
component of the Permalite™ fitting, axially along the fitting
length using a Permaswage™ Axial swage tool. Typical
Permalite™ fittings are shown in Figure 9-26 .
Cryofit Fittings
Many transport category aircraft use Cryofit fittings to join
hydraulic lines in areas where routine disconnections are not
required. Cryofit fittings are standard fittings with a cryogenic
sleeve. The sleeve is made of a shape memory alloy, Tinel™. additional information on sizes, torques, and bend radii. AN
flared fittings are different from MS flareless fittings and
they are not interchangeable. AN flared fittings are easily
recognized, because they have a cone at the end of the fitting
while the MS flareless fitting has a straight end. [Figure 9-21]
MS Flareless Fittings
MS flareless fittings are designed primarily for high-pressure
(3,000 psi) hydraulic systems that may be subjected to severe
vibration or fluctuating pressure. [Figure 9-22] Using this
type of fitting eliminates all tube flaring, yet provides a safe
and strong, dependable tube connection. The fitting consists
of three parts: a body, a sleeve, and a nut. [Figure 9-23] The
internal design of the body causes the sleeve to cut into the
outside of the tube when the body and nut are joined. The
counterbore shoulder within the body is designed with a
reverse angle of 15° for steel connectors and 45° for aluminum
fittings. This reverse angle prevents inward collapse of the
tubing when tightened and provides a partial sealing force
to be exerted against the periphery of the body counterbore.
Swaged Fittings
A popular repair system for connecting and repairing
hydraulic lines on transport category aircraft is the use of
Permaswage™ fittings. Swaged fittings create a permanent
TubingAN819 sleeve
AN818 nut
Figure 9-18. Flared tube fitting.ELECTRICAL
CONDUIT
ELECTRICAL
CONDUIT
ELECTRICAL
CONDUIT
ELECTRICAL
CONDUIT
ELECTRICAL
CONDUITWATER
WATER
WATER
WATER
WATER
WATERFUEL
FUEL
FUEL
FUEL
FUEL
FUEL
FUELLUBRICATION
LUBRICATION
LUBRICATION
LUBRICATION
LUBRICATION
LUBRICATION
LUBRICATIONPNEUMATIC
PNEUMATIC
PNEUMATIC
PNEUMATIC
PNEUMATIC
PNEUMATICX
X
X
X
X
X
X
X
X
X
X
XHYDRAULIC
HYDRAULIC
HYDRAULIC
HYDRAULIC
HYDRAULIC
HYDRAULIC
COOLANT
COOLANT
COOLANT
COOLANT
COOLANT
COOLANT
COOLANTAIR
CONDITION
AIR
CONDITION
AIR
CONDITION
AIR
CONDITION
COMPRESSED
GAS
COMPRESSED
GAS
COMPRESSED
GAS
COMPRESSED
GAS
COMPRESSED
GASINSTRUMENT
AIR
INSTRUMENT
AIR
INSTRUMENT
AIR
INSTRUMENT
AIR
INSTRUMENT
AIRBREATHING
OXYGEN
BREATHING
OXYGEN
BREATHING
OXYGEN
BREATHING
OXYGEN
BREATHING
OXYGENFIRE
PROTECTION
FIRE
PROTECTION
FIRE
PROTECTION
FIRE
PROTECTION
FIRE
PROTECTION
DE-ICING
DE-ICING
DE-ICING
DE-ICING
DE-ICING
DE-ICINGVACUUM
VACUUM
VACUUM
VACUUM
VACUUM
VACUUMINERTING
FLUID
INERTING
FLUID
INERTING
FLUID
INERTING
FLUID
INERTING
FLUID
WARNING
SYMBOL
Figure 9-16. Identification of aircraft fluid lines.A
B
FUEL
FLAMFUEL
FUELFLAM
FLAM
Figure 9-17. Fluid line identification using: tape and decals (A)
and metal tags (B).
Rigid Tubing Installation and Inspection
Before installing a line assembly in an aircraft, inspect the line
carefully. Remove dents and scratches, and be sure all nuts
and sleeves are snugly mated and securely fitted by proper
flaring of the tubing. The line assembly should be clean and
free of all foreign matter.
Connection & Torque
Never apply compound to the faces of the fitting or the
flare, as it destroys the metal-to-metal contact between the
fitting and flare, a contact which is necessary to produce
the seal. Be sure that the line assembly is properly aligned
before tightening the fittings. Do not pull the installation
into place with torque on the nut. Correct and incorrect
methods of installing flared tube assemblies are illustrated in
Figure 9-28. Proper torque values are given in Figure 9-20 .
Remember that these torque values are for flared-type fittings
only. Always tighten fittings to the correct torque value when
installing a tube assembly. Overtightening a fitting may badly
The sleeve is manufactured 3 percent smaller, frozen in
liquid nitrogen, and expanded to 5 percent larger than the
line. During installation, the fitting is removed from the
liquid nitrogen and inserted onto the tube. During a 10 to 15
second warming up period, the fitting contracts to its original
size (3 percent smaller), biting down on the tube, forming
a permanent seal. Cryofit fittings can only be removed by
cutting the tube at the sleeve, though this leaves enough
room to replace it with a swaged fitting without replacing
the hydraulic line. It is frequently used with titanium tubing.
The shape memory technology is also used for end fittings,
flared fittings, and flareless fittings. [Figure 9-27]
9-11ELBOW AN821 ELBOW AN823 TEE AN825
ELBOW AN822 TEE AN824 TEE AN826CROSS AN827 UNION AN832
ELBOW AN833 TEE AN834ELBOW AN837ELBOW AN838 ELBOW AN839TEE AN804 PLUG AN814PLUG AN806 UNION AN815
NUT AN817
NUT AN818SLEEVE AN819
CAP AN820 NIPPLE AN816 ADAPTOR AN807BOLT AN774 BOLT AN775 ELBOW AN776 ELBOW AN777 ELBOW AN778 TEE AN779AN744 to AN932
Material:
Aluminum alloy........................................................................................................................................... (code D)
Steel ........................................................................................................................................................... (code, absence of letter)
Brass .......................................................................................................................................................... (code B)
Aluminum bronze ....................................................................................................................................... (code Z–for AN819 sleeve)
Size:
The dash number following the AN number indicates the size of the tubing (or hose) for which the fitting is made in 16ths of an inch.
This size measures the outer diameter of tubing and the inner diameter of hose. Fittings that have pipe threads are coded by a dash
number, indicating the pipe size in 8ths of an inch. The material code letter, as noted above, follows the dash number.
Figure 9-19. AN standard fittings.
9-121¹⁄8
³⁄16
¹⁄4
⁵⁄16
³⁄8
¹⁄2
⁵⁄8
³⁄4
⁷⁄8
1¹⁄4
1¹⁄2
1³⁄4
2²¹⁄32
⁷⁄8
1¹⁄8
1⁵⁄16
1³⁄4
2³⁄16
2⁵⁄8
3¹⁄2
4³⁄8
5¹⁄4
6¹⁄8
7³⁄8
⁷⁄16
⁹⁄16
³⁄4
¹⁵⁄16
1¹⁄4
1¹⁄2
1³⁄4
3³⁄4
1,15070
900–1,000
1,000–1,100
1,200–1,400
1,200–1,400
1,500–1,80020–30
850–1,050
950–1,150–2
–32Tubing
Outer Diameter
(inches)Fitting Bolt
or Nut SizeAluminum
Alloy Tubing,
Bolt, Fitting,
or Nut
Torque
(in–lb)Steel
Tubing, Bolt
Fitting, or
Nut Torque
(in–lb)Hose End Fittings and
Hose Assemblies
MS28740 or Equivalent
End FittingMinimum Bend
Radii (inches)
Steel
Minimum MaximumAlum. Alloy
1100-H14
Figure 9-20. Flared fitting data.
Figure 9-21. AN flared (left) and MS flareless fitting (right).
damage or completely cut off the tube flare, or it may ruin the
sleeve or fitting nut. Failure to tighten sufficiently also may
be serious, as this condition may allow the line to blow out
of the assembly or to leak under system pressure. The use of
torque wrenches and the prescribed torque values prevents
overtightening or undertightening. If a tube fitting assembly is
tightened properly, it may be removed and retightened many
times before reflaring is necessary.
Flareless Tube Installation
Tighten the nut by hand until an increase in resistance to
turning is encountered. Should it be impossible to run the nut
down with the fingers, use a wrench, but be alert for the first
signs of bottoming. It is important that the final tightening commence at the point where the nut just begins to bottom.
Use a wrench and turn the nut one-sixth turn (one flat on a
hex nut). Use a wrench on the connector to prevent it from
turning while tightening the nut. After the tube assembly
is installed, the system should be pressure tested. It is
permissible to tighten the nut an additional one-sixth turn
(making a total of one-third turn), should a connection leak.
If leakage still occurs after tightening the nut a total of one-
third turn, remove the assembly and inspect the components
for scores, cracks, presence of foreign material, or damage
from overtightening. Several aircraft manufacturers include
torque values in their maintenance manuals to tighten the
flareless fittings.
9-13TEE MS21909 TEE MS21910 TEE MS21911 ELBOW
MS21908
SLEEVE
MS21922NUT
MS21917BUSHING
MS21915PLUG
MS21913
UNION
MS21924TEE
MS21912ADAPTER
MS21923NUT
MS21921REDUCER
MS21916CAP
MS21914
ELBOW
MS21925ELBOW
MS21926TEE MS21905SLEEVE
MS20819UNION
MS21902ELBOW
MS21904
UNION
MS21903ADAPTER
MS21900ADAPTER
MS21901ELBOW
MS21907CROSS
MS21906
Material: aluminum alloy
Size of fitting in 16ths inch – 4⁄16 inch
Design part number: adapter, flareless tube to AN flared tube
Prefix: military specificationMS 21900 –4 D
Figure 9-22. Typical MS flareless tube fittings.
Nut Body
Sleeve
Figure 9-23. Flareless fitting.
Lower die/holder assembly
Power unit
Waged FittingHydraulic Tubing
DisconnectHead assembly
Figure 9-24. Swaged fitting tooling.The following notes, cautions, and faults apply to the
installation of rigid tubing.
Note: Overtightening a flareless tube nut drives the cutting
edge of the sleeve deeply into the tube, causing the tube to be
weakened to the point where normal in-flight vibration could
cause the tube to shear. After inspection (if no discrepancies
are found), reassemble the connections and repeat the
pressure test procedures.
Caution: Never tighten the nut beyond one-third turn (two
flats on the hex nut); this is the maximum the fitting may be
tightened without the possibility of permanently damaging
the sleeve and nut.
Common faults: Flare distorted into nut threads; sleeve
cracked; flare cracked or split; flare out of round; inside of
flare rough or scratched; and threads of nut or union dirty,
damaged, or broken.
Rigid Tubing Inspection & Repair
Minor dents and scratches in tubing may be repaired.
Scratches or nicks not deeper than 10 percent of the wall
thickness in aluminum alloy tubing, which are not in the
heel of a bend, may be repaired by burnishing with hand
tools. The damage limits for hard, thin-walled corrosion-
resistant steel and titanium tubing are considerably less
than for aluminum tubing and might depend on the aircraft
manufacturer. Consult the aircraft maintenance manual for
damage limits. Replace lines with severe die marks, seams,
or splits in the tube. Any crack or deformity in a flare is
unacceptable and is cause for rejection. A dent of less than
20 percent of the tube diameter is not objectionable, unless
it is in the heel of a bend. To remove dents, draw a bullet of
proper size through the tube by means of a length of cable,
or push the bullet through a short straight tube by means of
a dowel rod. In this case, a bullet is a ball bearing or slug
Figure 9-25. PermaswageTM fitting.
Figure 9-26. Permalite™ fitting.
Figure 9-27. Cryofit fittings.
Incorrect—will damage
flare or threads, or cause
sleeve to crack under
vibration if tightened
Incorrect—may pull off or
distort flare if tightened
Correctly fitted and
tightened
.025 clearance between flare
and shoulder before tightening
Do not deflect into place.
Replace tube assembly.
Figure 9-28. Correct and incorrect methods of tightening flared fittings.normally made of steel or some other hard metal. In the case
of soft aluminum tubing, a hard wood slug or dowel may even
be used as a bullet. [Figure 9-29] A severely damaged line
should be replaced. However, the line may be repaired by
cutting out the damaged section and inserting a tube section of
the same size and material. Flare both ends of the undamaged
and replacement tube sections and make the connection by
using standard unions, sleeves, and tube nuts. Aluminum
6061-T6, corrosion-resistant steel 304-1/8h and Titanium
3AL-2.5V tubing can be repaired by swaged fittings. If
the damaged portion is short enough, omit the insert tube
and repair by using one repair union. [Figure 9-30] When
repairing a damaged line, be very careful to remove all chips
and burrs. Any open line that is to be left unattended for a
period of time should be sealed, using metal, wood, rubber,
9-16Cable
BulletDent
Figure 9-29. Dent removal using a bullet.or plastic plugs or caps.
When repairing a low-pressure line using a flexible fluid
connection assembly, position the hose clamps carefully
to prevent overhang of the clamp bands or chafing of the
tightening screws on adjacent parts. If chafing can occur,
the hose clamps should be repositioned on the hose.
Figure 9-31 illustrates the design of a flexible fluid connection
assembly and gives the maximum allowable angular and
dimensional offset.
When replacing rigid tubing, ensure that the layout of the new
line is the same as that of the line being replaced. Remove the
damaged or worn assembly, taking care not to further damage
or distort it, and use it as a forming template for the new part.
If the old length of tubing cannot be used as a pattern, make
a wire template, bending the pattern by hand as required for
the new assembly. Then bend the tubing to match the wire
pattern. Never select a path that does not require bends in
the tubing. A tube cannot be cut or flared accurately enough
so that it can be installed without bending and still be free
from mechanical strain. Bends are also necessary to permit
the tubing to expand or contract under temperature changes
and to absorb vibration. If the tube is small (under 1⁄4") and
can be hand formed, casual bends may be made to allow
for this. If the tube must be machine formed, definite bends
must be made to avoid a straight assembly. Start all bends a
reasonable distance from the fittings because the sleeves and
nuts must be slipped back during the fabrication of flares and
during inspections. In all cases, the new tube assembly should
be so formed prior to installation that it is not necessary to
pull or deflect the assembly into alignment by means of the
coupling nuts.
Flexible Hose Fluid Lines
Flexible hose is used in aircraft fluid systems to connect
moving parts with stationary parts in locations subject to
vibration or where a great amount of flexibility is needed. It
can also serve as a connector in metal tubing systems.
Hose Materials & Construction
Pure rubber is never used in the construction of flexible fluid
lines. To meet the requirements of strength, durability, and
workability, among other factors, synthetics are used in place
of pure rubber. Synthetic materials most commonly used in
the manufacture of flexible hose are Buna-N, neoprene, butyl,
ethylene propylene diene rubber (EPDM) and Teflon™.
While Teflon™ is in a category of its own, the others are
synthetic rubber.
Buna-N
Buna-N is a synthetic rubber compound that has excellent
resistance to petroleum products. Do not confuse with Buna-S. Do not use for phosphate ester base hydraulic fluid
(Skydrol™).
Neoprene
Neoprene is a synthetic rubber compound that has an
acetylene base. Its resistance to petroleum products is not
as good as Buna-N, but it has better abrasive resistance. Do
not use for phosphate ester base hydraulic fluid (Skydrol™).
Butyl
Butyl is a synthetic rubber compound made from petroleum
raw materials. It is an excellent material to use with phosphate
ester base hydraulic fluid (Skydrol™). Do not use with
petroleum products.
Flexible rubber hose consists of a seamless synthetic rubber
inner tube covered with layers of cotton braid and wire braid
and an outer layer of rubber-impregnated cotton braid. This
type of hose is suitable for use in fuel, oil, coolant, and
hydraulic systems. The types of hose are normally classified
by the amount of pressure they are designed to withstand
under normal operating conditions: low, medium, and high.
• Low pressure—below 250 psi. Fabric braid
reinforcement.
• Medium pressure—up to 3,000 psi. One wire braid
reinforcement. Smaller sizes carry up to 3,000 psi.
Larger sizes carry pressure up to 1,500 psi.
• High pressure—all sizes up to 3,000 psi operating
pressures.
Flexible hoses used for brake systems have sometimes a
stainless steel wire braid installed over the hose to protect
the hose from damage. [Figure 9-32]
Hose Identification
Lay lines and identification markings consisting of
lines, letters, and numbers are printed on the hose.
[Figure 9-33] Most hydraulic hose is marked to identify
its type, the quarter and year of manufacture, and a 5-digit
code identifying the manufacturer. These markings are in
contrasting colored letters and numerals that indicate the
9-17Type of Failure Repair Method
a. Make 1 or 2 cuts, as necessary, to remove damaged
section. If 2 cuts are required, the distance between them
shall not exceed 0.30". If distance is more than 0.30", go to
repair method 2.
b. Swage 1 tube-to-tube union in tube section under repair.
a. Make 2 cuts to enable removal of damaged section.
b. Remove damaged section and duplicate.
c. Swage replacement section into tubing under repair using 2
tube-to-tube unions.
a. Cut out defective tee or elbow.
b. Duplicate tubing sections for each branch.
c. Swage splice sections to tee or elbow.
d. Connect each splice section to tubing under repair using a
tube-to-tube union.
a. Cut tubing to remove defective fitting.
b. Swage appropriate end fitting to tube end.
c. Connect new end fitting to mating connection, torquing nut
as required.1. Pin hole leak or circumferential crack in tubing.
2. Longitudinal crack in tubing (crack length in excess of 0.30").
3. Leaking tee or elbow (permanent tube connection type).
4. Leaking flared, flareless, or lipseal end fittings.Not to exceed 0.30"
Original tubingNew section
Figure 9-30. Permaswage™ repair.
natural lay (no twist) of the hose and are repeated at intervals
of not more than 9 inches along the length of the hose. Code
markings assist in replacing a hose with one of the same
specifications or a recommended substitute. Hose suitable
for use with phosphate ester base hydraulic fluid is marked
Skydrol™ use. In some instances, several types of hose
may be suitable for the same use. Therefore, to make the
correct hose selection, always refer to the applicable aircraft
maintenance or parts manual.
Teflon™ is the DuPont trade name for tetrafluoroethylene
resin. It has a broad operating temperature range (−65 °F to
+450 °F). It is compatible with nearly every substance or
agent used. It offers little resistance to flow; sticky, viscous
materials do not adhere to it. It has less volumetric expansion
than rubber, and the shelf and service life is practically limitless. Teflon™ hose is flexible and designed to meet the
requirements of higher operating temperatures and pressures
in present aircraft systems. Generally, it may be used in the
same manner as rubber hose. Teflon™ hose is processed and
extruded into tube shape to a desired size. It is covered with
stainless steel wire, which is braided over the tube for strength
and protection. Teflon™ hose is unaffected by any known
fuel, petroleum, or synthetic base oils, alcohol, coolants,
or solvents commonly used in aircraft. Teflon™ hose has
the distinct advantages of a practically unlimited storage
time, greater operating temperature range, and broad usage
(hydraulic, fuel, oil, coolant, water, alcohol, and pneumatic
systems). Medium-pressure Teflon™ hose assemblies are
sometimes preformed to clear obstructions and to make
connections using the shortest possible hose length. Since
preforming permits tighter bends that eliminate the need for
Clamps
Fitting Hose
Minimum gap “G” shall be ½" or Tube OD/4TubingG. ref¼" Min.
" Max. offset3° Max. angular deviation
Figure 9-31. Flexible fluid connection assembly.
Figure 9-32. Flexible hose with stainless braid.
MIL-H-8794: SIZE-6-2/90 Mfg SymbolTwo cotton braids—impregnated with synthetic compound
Hose number Hose size
Synthetic inner tubeSingle wire braid
Quarter and year of manufacturer(A) MIL-H-8794 Type Hose
(D) MIL-H-27267 Type Hose (E) MIL-H-83797 Type HoseMfg Symbol MIL-H-6000-SIZE-3-4/90
Quarter and year of manufacturerHose size(B) MIL-H-6000 Type Hose
(Views showing opposite side of hose)
MIL-H-5593-6-4/90 Mfg SymbolLay lines
Quarter and year of manufacturer(C) MIL-H-5593 Type Hose
Hose size
Figure 9-33. Hose identification markings.special elbows, preformed hose assemblies save space and
weight. Never straighten a preformed hose assembly. Use a
support wire if the hose is to be removed for maintenance.
[Figure 9-34]
Flexible Hose Inspection
Check the hose and hose assemblies for deterioration at each
inspection period. Leakage, separation of the cover or braid
from the inner tube, cracks, hardening, lack of flexibility, or
excessive “cold flow” are apparent signs of deterioration and
reason for replacement. The term “cold flow” describes the
deep, permanent impressions in the hose produced by the
pressure of hose clamps or supports.
When failure occurs in a flexible hose equipped with swaged
end fittings, the entire assembly must be replaced. Obtain a
new hose assembly of the correct size and length, complete
with factory installed end fittings. When failure occurs in hose equipped with reusable end fittings, a replacement line can be
fabricated with the use of such tooling as may be necessary to
comply with the assembly instructions of the manufacturer.
Fabrication & Replacement of Flexible Hose
To make a hose assembly, select the proper size hose and
end fitting. [Figure 9-35] MS-type end fittings for flexible
hose are detachable and may be reused if determined to be
serviceable. The inside diameter of the fitting is the same
as the inside diameter of the hose to which it is attached.
[Figure 9-36]
Support wire
Figure 9-34. Suggested handling of preformed hose.
1. Place hose in vise and cut
to desired length using
fine tooth hacksaw or cut
off wheel.2. Locate length of hose to
be cut off and slit cover
with knife to wire braid,
taking care to not damage
underlying materials. After
slitting cover, twist off with
pair of pliers. (See note
below.)
3. Place hose in vise and
screw socket on hose
counterclockwise.4. *Lubricate inside of hose
and nipple threads liberally.
5. Screw nipple into socket
using wrench on hex of
nipple and leave .005" to
.031" clearance between
nipple hex and socket.NOTE: Hose assemblies
fabricated per MIL-H-8790
must have the exposed wire
braid coated with a special
sealant.
NOTE: Step 2 applies to high-
pressure hose only.
*CAUTION: Do not use any
petroleum product with hose
designed for synthetic fluids
(Skydrol™ and/or HYJET
product). For a lubricant
during assembly, use a
vegetable soap liquid.
Disassemble in reverse order.
Figure 9-35. Assembly of MS fitting to flexible hose.
Figure 9-36. MS-type end fitting.Flexible Hose Testing
All flexible hose must be proof-tested after assembly and
applying pressure to the inside of the hose assembly. The
proof-test medium may be a liquid or gas. For example,
hydraulic, fuel, and oil lines are generally tested using
hydraulic oil or water, whereas air or instrument lines
are tested with dry, oil-free air or nitrogen. When testing
with a liquid, all trapped air is bled from the assembly
prior to tightening the cap or plug. Hose tests, using a gas,
are conducted underwater. In all cases, follow the hose
manufacturer’s instructions for proof-test pressure and
fluid to be used when testing a specific hose assembly.
[Figure 9-37]
When a flexible hose has been repaired or overhauled using
existing hardware and new hose material, and before the hose
is installed on the aircraft, it is recommended that the hose be
tested to at least 1.5 system pressure. A hydraulic hose burst
test stand is used for testing flexible hose. [Figure 9-38] A
new hose can be operationally checked after it is installed in
the aircraft using system pressure.
Size Designations
Hose is also designated by a dash number according to its
size. The dash number is stenciled on the side of the hose and
indicates the size tubing with which the hose is compatible.
It does not denote inside or outside diameter. When the dash
number of the hose corresponds with the dash number of
the tubing, the proper size hose is being used. [Figure 9-33]
9-20 MIL-H-8794-3-L ³⁄16 ¹⁄8 .45 3,000 12,000 6,000 3.00
MIL-H-8794-4-L ¹⁄4 ³⁄16 .52 3,000 12,000 6,000 3.00
MIL-H-8794-5-L ⁵⁄16 ¹⁄4 .58 3,000 10,000 5,000 3.38
MIL-H-8794-6-L ³⁄8 ⁵⁄16 .67 2,000 9,000 4,500 4.00
MIL-H-8794-8-L ¹⁄2 ¹³⁄32 .77 2,000 8,000 4,000 4.63
MIL-H-8794-10-L ⁵⁄8 ¹⁄2 .92 1,750 7,000 3,500 5.50
MIL-H-8794-12-L ³⁄4 ⁵⁄8 1.08 1,750 6,000 3,000 6.50
MIL-H-8794-16-L 1 ⁷⁄8 1.23 800 3,200 1,600 7.38
MIL-H-8794-20-L 1 ¹⁄4 1 ¹⁄8 1.50 600 2,500 1,250 9.00
MIL-H-8794-24-L 1 ¹⁄2 1 ³⁄8 1.75 500 2,000 1,000 11.00
MIL-H-8794-32-L 2 1 ¹³⁄16 2.22 350 1,400 700 13.25
MIL-H-8794-40-L 2 ¹⁄2 2 ³⁄8 2.88 200 1,000 300 24.00
MIL-H-8794-48-L 3 3 3.56 200 800 300 33.00 MIL. Part No.Tube Size
OD
(inches)Hose Size
ID
(inches)Hose Size
OD
(inches)Recomm.
Operating
Pressure (PSI)Min. Burst
Pressure
(PSI)Max. Proof
Pressure
(PSI)Min. Bend
Radius
(inches)
MIL. Part No.Tube Size
OD
(inches)Hose Size
ID
(inches)Hose Size
OD
(inches)Recomm.
Operating
Pressure (PSI)Min. Burst
Pressure
(PSI)Max. Proof
Pressure
(PSI)Min. Bend
Radius
(inches)Single Wire Braid Fabric Covered
MIL-H-8788- 4-L ¹⁄4 ⁷⁄32 .63 3,000 16,000 8,000 3.00
MIL-H-8788- 5-L ⁵⁄16 ⁹⁄32 .70 3,000 14,000 7,000 3.38
MIL-H-8788- 6-L ³⁄8 ¹¹⁄32 .77 3,000 14,000 7,000 5.00
MIL-H-8788- 8-L ¹⁄2 ⁷⁄16 .86 3,000 14,000 7,500 5.75
MIL-H-8788-10-L ⁵⁄8 ⁹⁄16 1.03 3,000 12,000 6,000 6.50
MIL-H-8788-12-L ³⁄4 ¹¹⁄16 1.22 3,000 12,000 6,000 7.75
MIL-H-8788-16-L 1 ⁷⁄8 1.50 3,000 10,000 5,000 9.63 Multiple Wire Braid Rubber CoveredConstruction: Seamless synthetic rubber inner tube reinforced
with one fiber braid, one braid of high tensile steel wire and
covered with an oil resistant rubber impregnated fiber braid.
Identification: Hose is identified by specification number,
size number, quarter year and year, hose manufacturer’s
identification.
Uses: Hose is approved for use in aircraft hydraulic, pneumatic,
coolant, fuel, and oil systems.Operating Temperatures:
Sizes 3 through 12: − 65 °F to + 250 °F
Sizes 16 through 48: − 40 °F to + 275 °F
Note: Maximum temperatures and pressures should not be
used simultaneously.
Construction: Seamless synthetic rubber inner tube reinforced
with one fiber braid, two or more steel wire braids, and
covered with synthetic rubber cover (for gas applications
request perforated cover).
Identification: Hose is identified by specification number,
size number, quarter year and year, hose manufacturer’s
identification.Uses: High pressure hydraulic, pneumatic, coolant, fuel and oil.
Operating Temperatures: − 65 °F to + 200 °F
Figure 9-37. Aircraft hose specifications.
Figure 9-38. Hydraulic hose burst test stand.
SocketNippleSpur
Reinforcement Sheath
Figure 9-39. Reusable fittings for medium-pressure hose.
stripe running along its length. This stripe should not spiral
around the hose.
Bending
To avoid sharp bends in the hose assembly, use elbow
fittings, hose with elbow-type end fittings, or the appropriate
bend radii. Bends that are too sharp reduce the bursting
pressure of flexible hose considerably below its rated value.
[Figure 9-40]
Clearance
The hose assembly must clear all other lines, equipment, and
adjacent structure under every operating condition.
Flexible hose should be installed so that it is subject to a
minimum of flexing during operation. Although hose must
be supported at least every 24 inches, closer supports are
desirable. Flexible hose must never be stretched tightly
between two fittings. If clamps do not seal at specified
tightening, examine hose connections and replace parts as
necessary. The above is for initial installation and should not
be used for loose clamps.
For retightening loose hose clamps in service, proceed as
follows:
• Non-self-sealing hose—if the clamp screw cannot
be tightened with the fingers, do not disturb unless
leakage is evident. If leakage is present, tighten one-
fourth turn.
• Self-sealing hose—if looser than finger-tight, tighten
to finger-tight and add one-fourth turn. [Figure 9-41]Hose Fittings
Flexible hose may be equipped with either swaged fittings or
detachable fittings, or they may be used with beads and hose
clamps. Hoses equipped with swaged fittings are ordered by
correct length from the manufacturer and ordinarily cannot
be assembled by the mechanic. They are swaged and tested
at the factory and are equipped with standard fittings. The
detachable fittings used on flexible hoses may be detached
and reused if they are not damaged; otherwise, new fittings
must be used. [Figure 9-39]
Installation of Flexible Hose Assemblies
Slack
Hose assemblies must not be installed in a manner that causes
a mechanical load on the hose. When installing flexible hose,
provide slack or bend in the hose line from 5 to 8 percent of
its total length to provide for changes in length that occurs
when pressure is applied. Flexible hose contracts in length
and expands in diameter when pressurized. Protect all flexible
hoses from excessive temperatures, either by locating the lines
so they are not affected or by installing shrouds around them.
Flex
When hose assemblies are subject to considerable vibration
or flexing, sufficient slack must be left between rigid fittings.
Install the hose so that flexure does not occur at the end
fittings. The hose must remain straight for at least two hose
diameters from the end fittings. Avoid clamp locations that
restrict or prevent hose flexure.
Twisting
Hoses must be installed without twisting to avoid possible
rupture of the hose or loosening of the attaching nuts. Use of
swivel connections at one or both ends relieve twist stresses.
Twisting of the hose can be determined from the identification
Wrong
WrongWrong
WrongRight
RightRight
RightPlanning Hose Line Installations
1. Provide slack or bend in the hose line to provide for changes
in length that will occur when pressure is applied.2. Observe linear stripe. The hose must not be twisted. High
pressures applied to a twisted hose may cause failure or
loosen the nut.
3. Relieve sharp bends, avoid strain or hose collapse, and make
cleaner installations by using Aeroquip elbows or other
adapter fittings. Provide as large a bend radius as possible.
Never use less than the recommended minimum bend
radius specified for the hose.
4. Provide additional bend radius when lines are subject to
flexing and remember that the metal end fittings are not
flexible. Place line support clamps so as not to restrict
hose flexing.
Figure 9-40. Flexible hose installation.
Self-sealing hose
approximately
15 in-lb
All other
aircraft hose
approximately
25 in-lbFinger-tight plus 2
complete turns
Finger-tight plus
1¼ complete
turnsFinger-tight plus
2½ complete
turns
Finger-tight plus 2
complete turnsInitial
Installation
OnlyWorm screw type
clamp (10 threads
per inch)Clamps—
radial and other
type (28 threads
per inch)
Figure 9-41. Hose clamp tightening.Hose Clamps
To ensure proper sealing of hose connections and to prevent
breaking hose clamps or damaging the hose, follow the hose
clamp tightening instructions carefully. When available, use
the hose clamp torque-limiting wrench. These wrenches
are available in calibrations of 15 and 25 in-lb limits. In the
absence of torque-limiting wrenches, follow the finger-tight-
plus-turns method. Because of the variations in hose clamp
design and hose structure, the values given in Figure 9-41 are
approximate. Therefore, use good judgment when tightening
hose clamps by this method. Since hose connections are subject
to “cold flow” or a setting process, a follow-up tightening
check should be made for several days after installation.
Support clamps are used to secure the various lines to the
airframe or powerplant assemblies. Several types of support
clamps are used for this purpose. The most commonly
used clamps are the rubber-cushioned and plain. The
rubber-cushioned clamp is used to secure lines subject to
vibration; the cushioning prevents chafing of the tubing. [Figure 9-42] The plain clamp is used to secure lines in areas
not subject to vibration.
A Teflon™-cushioned clamp is used in areas where the
deteriorating effect of Skydrol™, hydraulic fluid, or fuel
is expected. However, because it is less resilient, it does
not provide as good a vibration-damping effect as other
cushion materials.
GrommetSupport tube at least 1/4" from edge of hole
Figure 9-42. Rubber-cushioned clamp.
Tube OD (in.)Distance Between Supports (in.)
Aluminum Alloy Steel
¹⁄8 9¹⁄2 11¹⁄2
³⁄16 12 14
¹⁄4 13¹⁄2 16
⁵⁄16 15 18
³⁄8 16¹⁄2 20
¹⁄2 19 23
⁵⁄8 22 25¹⁄2
³⁄4 24 27¹⁄2
1 26¹⁄2 30
Figure 9-43. Maximum distance between supports for fluid tubing.Use bonded clamps to secure metal hydraulic, fuel, or oil lines
in place. Unbonded clamps should be used only for securing
wiring. Remove any paint or anodizing from the portion of the
tube at the bonding clamp location. Make certain that clamps
are of the correct size. Clamps or supporting clips smaller
than the outside diameter of the hose may restrict the flow
of fluid through the hose. All fluid lines must be secured at
specified intervals. The maximum distance between supports
for rigid tubing is shown in Figure 9-43.
Inspection Concepts & TechniquesChapter 10
Inspections are visual examinations and manual checks to
determine the condition of an aircraft or component. An
aircraft inspection can range from a casual walk around to a
detailed inspection involving complete disassembly and the
use of complex inspection aids.
An inspection system consists of several processes, including
reports made by mechanics, the pilot, or crew flying an
aircraft and regularly scheduled inspections of an aircraft. An
inspection system is designed to maintain an aircraft in the
best possible condition. Thorough and repeated inspections
must be considered the backbone of a good maintenance
program. Irregular and haphazard inspections invariably
result in gradual and certain deterioration of an aircraft. The
time spent repairing an abused aircraft often totals far more
than any time saved in hurrying through routine inspections
and maintenance.
It has been proven that regularly scheduled inspections and
preventive maintenance assure airworthiness. Operating
failures and malfunctions of equipment are appreciably
reduced if excessive wear or minor defects are detected
and corrected early. The importance of inspections and the
proper use of records concerning these inspections cannot
be overemphasized.
Airframe and engine inspections may range from preflight
inspections to detailed inspections. The time intervals for the
inspection periods vary with the models of aircraft involved
and the types of operations being conducted. The airframe
and engine manufacturer’s instructions should be consulted
when establishing inspection intervals.
Aircraft may be inspected using a flight hours inspection
system, a calendar inspection system, or a combination of
both. Under the calendar inspection system, the appropriate
inspection is performed on the expiration of a specified
number of calendar weeks. The calendar inspection system
is an efficient system from a maintenance management
standpoint. Scheduled replacement of components with stated
hourly operating limitations is normally accomplished during
the calendar inspection falling nearest the hourly limitation.
In some instances, a flight hour limitation is established
to limit the number of hours that may be flown during the
calendar interval.Aircraft operating under the flight hour system are inspected
when a specified number of flight hours are accumulated.
Components with stated hourly operating limitations are
normally replaced during the inspection that falls nearest
the hourly limitation.
Basic Inspection
Techniques/Practices
Before starting an inspection, be certain all plates, access
doors, fairings, and cowling have been opened or removed
and the structure cleaned. When opening inspection plates
and cowling, and before cleaning the area, take note of any
oil or other evidence of fluid leakage.
Preparation
In order to conduct a thorough inspection, a great deal of
paperwork and/or reference information must be accessed
and studied before proceeding to the aircraft to conduct the
inspection. The aircraft logbooks must be reviewed to provide
background information and a maintenance history of the
particular aircraft. The appropriate checklist or checklists
must be utilized to ensure that no items are forgotten or
overlooked during the inspection. Also, many additional
publications must be available, either in hard copy or in
electronic format, to assist in the inspections. These additional
publications may include information provided by the aircraft
and engine manufacturers, appliance manufacturers, parts
vendors, and the Federal Aviation Administration (FAA).
Aircraft Logs
“Aircraft logs,” as used in this handbook, is an inclusive
term that applies to the aircraft logbook and all supplemental
records concerned with the aircraft. They may come in a
variety of formats. For a small aircraft, the log may indeed
be a small 5" × 8" logbook. For larger aircraft, the logbooks
are often larger and in the form of a three-ring binder. Aircraft
that have been in service for a long time are likely to have
several logbooks.
The aircraft logbook is the record where all data concerning
the aircraft is recorded. Information gathered in this log is
used to determine the aircraft condition, date of inspections,
time on airframe, engines, and propellers. It reflects a
10-2history of all significant events occurring to the aircraft, its
components, and accessories. Additionally, it provides a place
for indicating compliance with FAA airworthiness directives
(ADs) or manufacturers’ service bulletins (SB). The more
comprehensive the logbook, the easier it is to understand the
aircraft’s maintenance history.
When the inspections are completed, appropriate entries
must be made in the aircraft logbook certifying that the
aircraft is in an airworthy condition and may be returned to
service. When making logbook entries, exercise special care
to ensure that the entry can be clearly understood by anyone
having a need to read it in the future. Also, if making a
hand-written entry, use good penmanship and write legibly.
To some degree, the organization, comprehensiveness, and
appearance of the aircraft logbooks have an impact on the
value of the aircraft. High quality logbooks can mean a
higher value for the aircraft.
Checklists
Always use a checklist when performing an inspection.
The checklist may be of your own design, one provided
by the manufacturer of the equipment being inspected, or
one obtained from some other source. The checklist should
include the following:
1. Fuselage and Hull Group
a. Fabric and skin—for deterioration, distortion,
other evidence of failure, and defective or
insecure attachment of fittings.
b. Systems and components—for proper installation,
apparent defects, and satisfactory operation.
c. Envelope gas bags, ballast tanks, and related
parts—for condition.
2. Cabin and Cockpit Group
a. General—for cleanliness and loose equipment
that needs to be secured.
b. Seats and safety belts—for condition and security.
c. Windows and windshields—for deterioration and
breakage.
d. Instruments—for condition, mounting, marking,
and (where practicable) for proper operation.
e. Flight and engine controls—for proper installation
and operation.
f. Batteries—for proper installation and charge.
g. All systems—for proper installation, general
condition, apparent defects, and security of
attachment.
3. Engine and Nacelle Groupa. Engine section—for visual evidence of excessive
oil, fuel, hydraulic leaks, and sources of such
leaks.
b. Studs and nuts—for proper torquing and obvious
defects.
c. Internal engine—for cylinder compression and
for metal particles or foreign matter on screens
and sump drain plugs. If cylinder compression is
weak, check for improper internal condition and
improper internal tolerances.
d. Engine mount—for cracks and looseness of
mounting.
e. Flexible vibration dampeners—for condition and
deterioration.
f. Engine controls—for defects, proper travel, and
proper safetying.
g. Lines, hoses, and clamps—for leaks, condition,
and looseness.
h. Exhaust stacks—for cracks, defects, and proper
attachment.
i. Accessories—for apparent defects in security of
mounting.
j. All systems—for proper installation, general
condition defects, and secure attachment.
k. Cowling—for cracks and defects.
l. Ground run-up and functional check—check
all powerplant controls and systems for correct
response, all instruments for proper operation
and indication.
4. Landing Gear Group
a. All units—for condition and security of
attachment.
b. Shock absorbing devices—for proper oleo fluid
level.
c. Linkage, trusses, and members—for undue or
excessive wear, fatigue, and distortion.
d. Retracting and locking mechanism—for proper
operation.
e. Hydraulic lines—for leakage.
f. Electrical system—for chafing and proper
operation of switches.
g. Wheels—for cracks, defects, and condition of
bearings.
h. Tires—for wear and cuts.
i. Brakes—for proper adjustment.
j. Floats and skis—for security of attachment and
10-3obvious defects.
5. Wing and Center Section
a. All components—for condition and security.
b. Fabric and skin—for deterioration, distortion,
other evidence of failure, and security of
attachment.
c. Internal structure (spars, ribs, compression
members)—for cracks, bends, and security.
d. Movable surfaces—for damage or obvious
defects, unsatisfactory fabric or skin attachment,
and proper travel.
e. Control mechanism—for freedom of movement,
alignment, and security.
f. Control cables—for proper tension, fraying, wear,
and proper routing through fairleads and pulleys.
6. Empennage Group
a. Fixed surfaces—for damage or obvious defects,
loose fasteners, and security of attachment.
b. Movable control surfaces—for damage or
obvious defects, loose fasteners, loose fabric, or
skin distortion.
c. Fabric or skin—for abrasion, tears, cuts, defects,
distortion, and deterioration.
7. Propeller Group
a. Propeller assembly—for cracks, nicks, bends, and
oil leakage.
b. Bolts—for proper torquing and safe tying.
c. Anti-icing devices—for proper operation and
obvious defects.
d. Control mechanisms—for proper operation,
secure mounting, and travel.
8. Communication and Navigation Group
a. Radio and electronic equipment—for proper
installation and secure mounting.
b. Wiring and conduits—for proper routing, secure
mounting, and obvious defects.
c. Bonding and shielding—for proper installation
and condition.
d. Antennas—for condition, secure mounting, and
proper operation.
9. Miscellaneous
a. Emergency and first aid equipment—for general
condition and proper stowage.b. Parachutes, life rafts, flares, and so forth—
inspect in accordance with the manufacturer’s
recommendations.
c. Autopilot system—for general condition, security
of attachment, and proper operation.
Publications
Aeronautical publications are the sources of information
for guiding aviation mechanics in the operation and
maintenance of aircraft and related equipment. The
proper use of these publications greatly aid in the
efficient operation and maintenance of all aircraft. These
include manufacturers’ SBs, manuals, and catalogs; FAA
regulations; ADs; advisory circulars (ACs); and aircraft,
engine, and propeller specifications.
Manufacturers’ Service Bulletins/Instructions
Service bulletins or service instructions are two of several
types of publications issued by airframe, engine, and
component manufacturers. The bulletins may include:
purpose for issuing the publication; name of the applicable
airframe, engine, or component; detailed instructions for
service, adjustment, modification or inspection, and source
of parts, if required; and estimated number of man-hours
required to accomplish the job.
Maintenance Manual
The manufacturer’s aircraft maintenance manual contains
complete instructions for maintenance of all systems and
components installed in the aircraft. It contains information
for the mechanic who normally works on components,
assemblies, and systems while they are installed in the
aircraft, but not for the overhaul mechanic. A typical aircraft
maintenance manual contains:
• A description of the systems (i.e., electrical, hydraulic,
fuel, control)
• Lubrication instructions setting forth the frequency
and the lubricants and fluids that are to be used in the
various systems
• Pressures and electrical loads applicable to the various
systems
• Tolerances and adjustments necessary to proper
functioning of the airplane
• Methods of leveling, raising, and towing
• Methods of balancing control surfaces
• Identification of primary and secondary structures
• Frequency and extent of inspections necessary to the
proper operation of the airplane
10-4• Special repair methods applicable to the airplane
• Special inspection techniques requiring x-ray,
ultrasonic, or magnetic particle inspection
• A list of special tools
Overhaul Manual
The manufacturer’s overhaul manual contains brief
descriptive information and detailed step-by-step instructions
covering work normally performed on a unit that has been
removed from the aircraft. Simple, inexpensive items, such
as switches and relays where overhaul is uneconomical, are
not covered in the overhaul manual.
Structural Repair Manual
The structural repair manual contains the manufacturer’s
information and specific instructions for repairing primary
and secondary structures. Typical skin, frame, rib, and stringer
repairs are covered in this manual. Also, included are material
and fastener substitutions and special repair techniques.
Illustrated Parts Catalog
The illustrated parts catalog presents component breakdowns
of structure and equipment in disassembly sequence. Also,
included are exploded views or cutaway illustrations for all
parts and equipment manufactured by the aircraft manufacturer.
Wiring Diagram Manual
The wiring diagram manual is a collection of diagrams,
drawings, and lists that define the wiring and hook up of
associated equipment installed on airplanes. The data is
organized in accordance with the Air Transport Association
ATA iSPec 2200 specification.
Code of Federal Regulations (CFRs)
The Code of Federal Regulations (CFRs) were established
by law to provide for the safe and orderly conduct of
flight operations and to prescribe airmen privileges and
limitations. A knowledge of the CFRs is necessary during the
performance of maintenance, since all work done on aircraft
must comply with CFR provisions.
Airworthiness Directives (ADs)
A primary safety function of the FAA is to require correction
of unsafe conditions found in an aircraft, aircraft engine,
propeller, or appliance when such conditions exist and are
likely to exist or develop in other products of the same
design. The unsafe condition may exist because of a design
defect, maintenance, or other causes. Title 14 of the CFR
part 39, Airworthiness Directives, defines the authority
and responsibility of the administrator for requiring the
necessary corrective action. The ADs are published to
notify aircraft owners and other interested persons of unsafe conditions and to prescribe the conditions that the
product may continue to be operated. Furthermore, these
are federal aviation regulations and must be complied with
unless specific exemption is granted.
There are two categories of ADs:
1. Those of an emergency nature requiring immediate
compliance upon receipt.
2. Those of a less urgent nature requiring compliance
within a relatively longer period of time.
Also, ADs may be a one-time compliance item or a recurring
item that requires future inspection on an hourly basis (accrued
flight time since last compliance) or a calendar time basis.
The contents of ADs include the aircraft, engine, propeller, or
appliance model and serial numbers affected. Also, included
are the compliance time or period, a description of the
difficulty experienced, and the necessary corrective action.
Type Certificate Data Sheets (TCDS)
The type certificate data sheet (TCDS) describes the type
design and sets forth the limitations prescribed by the
applicable CFR part. It also includes any other limitations
and information found necessary for type certification of a
particular model aircraft. [Figure 10-1]
All TCDS are numbered in the upper right corner of each
page. This number is the same as the type certificate number.
The name of the type certificate holder, together with all of
the approved models, appears immediately below the type
certificate number. The issue date completes this group. This
information is contained within a bordered text box to set it off.
The TCDS is separated into one or more sections. Each
section is identified by a Roman numeral followed by the
model designation of the aircraft that the section pertains. The
category or categories that the aircraft can be certificated in
are shown in parentheses following the model number. Also,
included is the approval date shown on the type certificate.
The data sheet contains information regarding:
1. Model designation of all engines that the aircraft
manufacturer obtained approval for use with this
model aircraft.
2. Minimum fuel grade to be used.
3. Maximum continuous and takeoff ratings of the
approved engines, including manifold pressure (when
used), rotations per minute (rpm), and horsepower (hp).
4. Name of the manufacturer and model designation for
each propeller that the aircraft manufacturer obtained
10-5approval is shown together with the propeller limits
and any operating restrictions peculiar to the propeller
or propeller engine combination.
5. Airspeed limits in both miles per hour (mph) and knots.
6. Center of gravity (CG) range for the extreme loading
conditions of the aircraft is given in inches from the
datum. The range may also be stated in percent of
mean aerodynamic chord (%MAC) for transport
category aircraft.
7. Empty weight center of gravity (EWCG) range (when
established) is given as fore and aft limits in inches
from the datum. If no range exists, the word “none”
is shown following the heading on the data sheet.
8. Location of the datum.
9. Means provided for leveling the aircraft.
10. All pertinent maximum weights.
11. Number of seats and their moment arms.
12. Oil and fuel capacity.
13. Control surface movements.
14. Required equipment.
15. Additional or special equipment found necessary for
certification.
16. Information concerning required placards.
It is not within the scope of this handbook to list all the items
that can be shown on the TCDS. Those items listed above
serve only to acquaint aviation mechanics with the type of
information generally included on the data sheets. TCDS
may be many pages in length.
When conducting a required or routine inspection, it is
necessary to ensure that the aircraft and all the major items
on it are as defined in the TCDS. The inspector ensures that
all installed aircraft equipment conforms to the TCDS. This
is called a conformity check and verifies that the aircraft
conforms to the specifications of the aircraft as it was
originally certified. Sometimes alterations are made that are
not specified or authorized in the TCDS. When that condition
exists, a supplemental type certificate (STC) is issued. STCs
are considered a part of the permanent records of an aircraft
and should be maintained as part of that aircraft’s logs.
Routine/Required Inspections
For the purpose of determining their overall condition, 14
CFR provides for the inspection of all civil aircraft at specific
intervals, depending generally upon the type of operations
that they are engaged in. The pilot-in-command (PIC)
of a civil aircraft is responsible for determining whether
that aircraft is in a condition for safe flight. Therefore, the aircraft must be inspected before each flight. More detailed
inspections must be conducted by aviation maintenance
technicians (AMTs at least once each 12 calendar months,
while inspection is required for others after each 100 hours
of flight. In other instances, an aircraft may be inspected in
accordance with a system set up to provide for total inspection
of the aircraft over a calendar or flight time period. These
include phase-type inspections.
To determine the specific inspection requirements and
rules for the performance of inspections, refer to the CFR
that prescribes the requirements for the inspection and
maintenance of aircraft in various types of operations.
Preflight/Postflight Inspections
Pilots are required to follow a checklist contained within
the Pilot’s Operating Handbook (POH) when operating
aircraft. The first section of the checklist is entitled “Preflight
Inspection.” The preflight inspection checklist includes
a “walk-around” section listing items that the pilot is to
visually check for general condition as they walk around
the airplane. Also, the pilot must ensure that fuel, oil, and
other items required for flight are at the proper levels and not
contaminated. Additionally, it is the pilot’s responsibility to
review the aircraft maintenance records, and other required
paperwork to verify that the aircraft is indeed airworthy. After
each flight, it is recommended that the pilot or mechanic
conduct a postflight inspection to detect any problems that
might require repair or servicing before the next flight.
Annual/100-Hour Inspections
The basic requirements for annual and 100-hour inspections are
discussed in 14 CFR part 91. With some exceptions, all aircraft
must have a complete inspection annually. Aircraft that are used
for commercial purposes (carrying any person, other than a
crewmember, for hire or flight instruction for hire) and are likely
to be used more frequently than noncommercial aircraft must
have this complete inspection every 100 hours. The scope and
detail of items to be included in annual and 100-hour inspections
is included as Appendix D to part 43. [Figure 10-2]
A properly written checklist, such as the one shown earlier in
this chapter, includes all the items of Appendix D. Although
the scope and detail of annual and 100-hour inspections are
identical, there are two significant differences. One difference
involves persons authorized to conduct them. A certified
airframe and powerplant (A&P) maintenance technician can
conduct a 100-hour inspection, whereas an annual inspection
must be conducted by a certified A&P maintenance technician
with inspection authorization (IA). The other difference
involves authorized overflight of the maximum 100 hours
before inspection. An aircraft may be flown up to 10 hours
beyond the 100-hour limit if necessary to fly to a destination
10-6Page No. 1 2 3 4 5 6
Rev. No. 4 - - 4 2 4
DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
A27EU
Revision 4
AIRBUS DEFENCE AND SPACE GMBH
EADS DEUTSCHLAND GMBH
DAIMLER CHRYSLER AEROSPACE AG
DAIMLER -BENZ AEROSPACE AG
DEUTSCHE AEROSPACE AG
MESSERSCHMITT -BÖLKOW -BLOHM AG
MESSERSCHMITT -BÖL KOW -BLOHM GMBH
BO-209-150 FV & RV
BO-209-160 FV & RV
BO-209-150 FF
July 9 , 2015
TYPE CERTIFICATE DATA SHEET NO. A27EU
This data sheet, which is a part of Type Certificate No. A27EU, prescribes conditions and limitations under which the product
for which the Type Certificate was issued meets the airworthiness requirements of the Federal Aviation Regulations.
Type Certificate Holder Airbus Defence and Space GmbH
Willy -Messerschmitt -Strasse 1
85521 Ottobrunn
Germany
Type Certificate Ownership Record Messerschmitt -Bölkow -Blohm GmbH transferred TC A27EU to Messerschmitt -Bölkow -
Blohm AG on April 1, 1992 (See NOTE 4.)
Messerschmitt -Bölkow -Blohm AG transferred TC A27EU to Deutsche Aerospace AG on
November 30, 1992
Deutsche Aerospace AG transferred TC A27E Uto Daimler -Benz Aerospace AG on
January 2, 1995
Daimler -Benz Aerospace AG transferred TC A27EU to Daimler Chrysler Aerospace AG
on November 17, 1998
Daimler Chrysler Aerospace AG transferred TC A27EU to EADS Deutschland GmbH on
July 10, 2000
EADS Deu tschland GmbH transferred TC A27EU to Airbus Defence and Space GmbH
on July 1, 2014
(See NOTE 7.)
I-Model BO -209-150 FV and RV, 2 PCLM (Normal and Utility Category), approved 9 July 1971
(FV model has fixed nose L.g.; RV model has retractable nose L .g.).
Engine Lycoming O -320-E1C or O-320-E1F
Fuel 80/87 minimum grade aviation gasoline
Engine limits For all operations, 2700 r.p.m. (150 hp.)
Propeller and Hartzell HC -C2YL -1B/7663A -6
propeller limits Diameter: 70 in. no further reduction pe rmitted
Pitch setting at 30 in. radius: High 27°
Low 12°12'
Spinner: MBB P/N 209-61056
Governor: Woodward P/N T210452 or P/N 210681
Figure 10-1. Type certificate data sheet (TCDS).
10-72 A27EU
Airspeed limits (CAS) Normal and Utility Category
Never exceed 173 knots (199 m.p.h.)
Maximum structural cruising 135 knots (155 m.p.h.)
Maneuvering 117 knots (135 m.p.h.)
Flaps extended 88 knots (101 m.p.h.)
*Landing gear operation 104 knots (120 m.p.h.)
*Landing gear extended 173 knots (199 m.p.h.)
(*Applies only to the RV model).
C.G. range Normal Category
(85.47) to (89.37) at 1265 lb. or less
(86.92) to (89.37) at 1808 lb.
Utility Category
(85.47) to (89.37) at 1265 lb. or less
(86.25) to (89.37) at 1565
Maximum weight 1808 lb., for Normal Category
1565 lb., for Utility Category
No. of seats 2 at (+ 90.7)
Maximum baggage 110 lb. at (+114.2)
Fuel capacity 39.2 gal. total (38.6 gal. usable; two 19.6 gal., wing tanks at + 90.7)
Oil capacity 8 qt. (+3.94)
See NOTE 1 for unusable fuel and undrainable oil data.
II-Model BO -209-160 FV and RV, 2 PCLM (Normal and Utility Category), approved 9 July 1971
(FV model has fixed nose L.g.; RV model has retractable nose L.g.).
Engine Lycoming IO -320-D1A or IO-320-D1B
Fuel 100/130 minimum grade aviation gasoline
Engine limits For all operations, 2700 r.p.m. (160 hp.)
Propeller and Hartzell HC -C2YL -1B/7663A -6
propeller limits Diameter: 70 in. no further reduction permitted
Pitch setting at 30 in. radius: High 27°
Low 14°57'
Spinner: MBB P/N 209-61056
Governor: Woodward P/N T210452 or P/N 210681
Airspeed limits (C AS) Normal and Utility Category
Never exceed 173 knots (199 m.p.h.)
Maximum structural cruising 135 knots (155 m.p.h.)
Maneuvering 117 knots (135 m.p.h.)
Flaps extended 88knots (101 m.p.h.)
*Landing gear operation 104 knots (120 m.p.h.)
*Landing gear extended 173 knots (199 m.p.h.)
(*Applies only to the RV model).
C.G. range Normal Category
(85.47) to (89.37) at 1265 lb. or less
(86.92) to (89.37) at 1808 lb.
Utility Category
(85.47) to (89.37) at 1265 lb. or less
(86.25) to (89.37) at 1565 lb.
Figure 10-1. Type certificate data sheet (TCDS) (continued).
10-83 A27EU
Maximum weight 1808 lb. for Normal Category
1565 lb. for Utility Category
No. of seats 2 at (+ 90.7)
Maximum baggage 110 lb. at (+ 114.2)
Fuel capacity 39.2 gal. total (38.6 gal. usable; two 19.6 gal., wing tanks at + 90.7)
Oil capacity 8 qt. (+3.94)
See NOTE 1 for unusable fuel and undrainable oil data.
III-Model BO -209-150 FF, 2 PCLM (Normal and Utility Category), approved 9 July 1971
(fixed nose L.g.).
Engine Lycoming O -320-E2C or O-320-E2F
Fuel 80/87 minimum grade aviation gasoline.
Engine limits For all operations, 2700 r.p.m. (150 hp.)
Propeller and McCauley 1C172MGM -70.5-60 or -66
propeller limits Static r.p.m. at maximum permissible throttle setting:
Not over 2400, not under 2100
No additional tolerance permitted.
Diameter: Maximum 70.5 in., minimum for repairs 70 in.
No further reduction permitted
Spinner: MBB P/N 209-61156
Airspeed limits (CAS) Normal and Utility Category
Never exceed 173knots (199 m.p.h.)
Maximum structural cruising 135 knots (155 m.p.h.)
Maneuvering 117 knots (135 m.p.h.)
Flaps extended 88 knots (101 m.p.h.)
C.G. range Normal Category
(85.47) to (89.37) at 1265 lb. or less
(86.92) to (89.37) at 1808 lb.
Utility Category
(85.47) to (89.37) at 1265 lb. or less
(86.25) to (89.37) at 1565 lb.
Maximum weight 1808 lb., for Normal Category
1565 lb., for Utility Category
No. of seats 2 at (+ 90.7)
Maximum baggage 110 lb. at (+114.2)
Fuel capacity 39.2 gal. total (38.6 gal. usable; two 19.6 gal., wing tanks at + 90.7)
Oil capacit y 8 qt. (+3.94)
See NOTE 1 for unusable fuel and undrainable oil data.
Figure 10-1. Type certificate data sheet (TCDS) (continued).
10-94 A27EU
DATA PERTINENT TO ALL MODELS
Control Surface Movements Ailerons Up 29°+1° Down 14° +1°
Wing flaps Down 35° + 0°
-3°
Stabilator Up 18°+1° Down 9°+1°
Rudder Left 28°+2° Right 28° +2°
Stabilator trim, distance measured between trailing edge of trim tab
and trailing edge of stabilator with stabilator in the neutral position.
tab neutral: 0.32 in. Down, +0.08 in.
nose down: 0.20 in. Up, +0.08 in.
nose up: 0.66 in. Down, +0.08 in.
total travel: 0.86 in. +0.16 in.
Datum 75.51 in. forward of wing leading edge at split line of the wing/wing stub fairing.
Leveling means Two leveling points on left side of fuselage.
Serial Nos. eligible Serial Numbers 121 and subsequent.
The Federal Republic of Germany Government Certificate of Airworthiness for Export
endorsed as noted below under "Import Requirements" must be submitted for each
individual aircraft for which application for airworthiness certification is made.
Certification basis FAR 21.29 and FAR 23 dated 1 February 1965 as amended by Amendments 23-1
through 23-9 inclusive. Type Certificate No. A27EU, issued 9 July 1971.
Date of Application for Type Certificate: 11 May 1970.
The Luftfahrt Bundesamt originally type certificated this aircraft under its type certificate
Number 680. The FAA validated this product under U.S. Type Certificate Number
A27EU. Effective September 28, 2003, the European Aviation Safety Agency (EASA)
began oversight of this product on behalf of Germany.
The EASA type certificate for the BO-209 models is EASA.A.357.
Import Requireme nts The FAA can issue a U.S. airworthiness certificate based on an NAA Export Certificate
of Airworthiness (Export C of A) signed by a representative of the Luftfahrt Bundesamt
on behalf of the European Community. The Export C of A should contain the following
statement: ‘The aircraft covered by this certificate has been examined, tested, and found
to comply with U.S. airworthiness regulations 14 CFR Part 23 approved under U.S.
Type Certificate No . A27EU and to be in a condition for safe operation.’
Servi ce Information Each of the documents listed below must state that it is approved by the European
Aviation Safety Agency (EASA) or – for approvals made before September 28, 2003 –
by the Luftfahrt Bundesamt .
• Service bulletins,
• Structural repair m anuals,
• Vendor manuals,
• Aircraft flight manuals, and
• Overhaul and maintenance manuals.
The FAA accepts such documents and considers them FAA -approved unless one of the
following conditions exists:
• The documents change the limitations , performance, or procedures of the FAA
approved manuals; or
•The documents make an acoustical or emissions changes to this product’s U.S. type
certificate as defined in 14 CFR § 21.93.
Figure 10-1. Type certificate data sheet (TCDS) (continued).
10-105 A27EU
Service Information, cont'd The FAA uses the post type validation procedures to approve these documents. The
FAA may delegate on case-by -case to EASA to approve on behalf of the FAA for the
U.S. type certificate. If this is the case it will be noted on the document.
Equipment The basic required equipment as prescribed in the applicable airworthiness regulations
(see Certification Basis) must be installed in the aircraft for certification. In addition,
the following items of equipment are required:
1. Stall Warning System.
2. LBA -approved Model BO -209 Approved Flight Manual, Ref. No.LF
37E-7/71 dated July 1971 or later LBA-approved revision.
3. Airplanes S/N 121 through 130 must be modified in accordance with MBB
Technical Note TN 9 -71 to provide an alternate static system source
and an aural landing gear warning system. (These systems are incorporated
in production on S/Ns 131 and subsequent).
NOTE 1. Current weight and balance report including list of equipment in certificated empty weight, and loading
instructions when necessary, must be provided for each airplane at the time of original airworthiness
certification. The certificated empty weight and corresponding center of gravity must include
undrainable oil of 0 lbs. at +39.4 and unusable fuel of 3.6 lb. at +90.7.
NOTE 2. The following placard must be displayed in front and in clear view of the pilot:
"This airplane must be operated as a Normal or Utility Category airplane in compliance with their
operating limitations stated in the form of placards, m arkings, and manuals."
In addition, all placards required in the LBA -approved Airplane Flight Manual must be installed
in the appropriate location.
NOTE 3. Information essential for proper maintenance of the airplane is contained in the Messerschmit t-Bolkow -
Blohm GmbH., Model BO -209 Maintenance Manual included in MBB document Ref. LF 37E -7/71.
NOTE 4. The airplane manufacturer is:
Waggon- und Maschienenbau A.G.
Donauworth, Laupheim
Federal Republic of Germany
(A division of Messerschmitt-Bolkow -Blohm).
NOTE 5. Installation of a Tost tow coupling (ring type), LBA approval No. 60.230.4 may be approved when
installed in accordance with MBB Drwg. 209-85003 (for glider towing) or MBB Drwgs. 209-85003
and 209-8700 (for banner towing).
NOTE 6. For issuance of an airworthiness certificate in accordance with 14 CFR Part 21.182(c), the Luftfahrt Bundesamt of
Germany must certify that the airplane conforms to the type design and is in a condition for safe operation. In that
regard, the Luftfahrt Bundesamt of Germany will certify that the airplane complies with all applicable mandatory
continuing airworthiness information (MCAI) it has issued. For issuance of an airworthiness certificate in
accordance with 14 CFR Part 21.182(d) the certific ating inspector, or other authorized person, must find, among
other things, that the product is in a condition for safe operation. In order to make that finding, the certificating
inspector or other authorized person should contact ACE -112, Federal Aviation Administration, Small Airplane
Directorate, prior to issuance to determine whether showing airplane compliance with certain MCAI is necessary
to support a finding that the airplane is in a condition for safe operation.
Figure 10-1. Type certificate data sheet (TCDS) (continued).
10-116 A27EU
NOTE 7. Some of these transfers were not notified to the FAA and so in some instances the actual type certificates were
not reissued.
.....END.....
Figure 10-1. Type certificate data sheet (TCDS) (continued).
10-12where the inspection is to be conducted.
Progressive Inspections
Because the scope and detail of an annual inspection is
very extensive and could keep an aircraft out of service
for a considerable length of time, alternative inspection
programs designed to minimize down time may be utilized.
A progressive inspection program allows an aircraft to be
inspected progressively. The scope and detail of an annual
inspection is essentially divided into segments or phases
(typically four to six). Completion of all the phases completes
a cycle that satisfies the requirements of an annual inspection.
The advantage of such a program is that any required segment
may be completed overnight and thus enable the aircraft to
fly daily without missing any revenue earning potential.
Progressive inspection programs include routine items, such
as engine oil changes, and detailed items, such as flight
control cable inspection. Routine items are accomplished
each time the aircraft comes in for a phase inspection, and
detailed items focus on detailed inspection of specific areas.
Detailed inspections are typically done once each cycle. A
cycle must be completed within 12 months. If all required
phases are not completed within 12 months, the remaining
phase inspections must be conducted before the end of the
12th month from when the first phase was completed.
Each registered owner or operator of an aircraft desiring to
use a progressive inspection program must submit a written
request to the FAA Flight Standards District Office (FSDO)
having jurisdiction over the area that the applicant is located.
Section 91.409(d) of 14 CFR part 91 establishes procedures
to be followed for progressive inspections. [Figure 10-3]
Continuous Inspections
Continuous inspection programs are similar to progressive
inspection programs, except that they apply to large or
turbine-powered aircraft and are therefore more complicated.
Like progressive inspection programs, they require approval
by the FAA Administrator. The approval may be sought
based upon the type of operation and the CFR parts that
the aircraft is operated under. The maintenance program
for commercially operated aircraft must be detailed in
the approved operations specifications (OpSpecs) of the
commercial certificate holder.
Airlines utilize a continuous maintenance program that
includes both routine and detailed inspections. However, the
detailed inspections may include different levels of detail.
Often referred to as “checks,” the A-checks, B-checks,
C-checks, and D-checks involve increasing levels of detail.
A-checks are the least comprehensive and occur frequently.
D-checks, on the other hand, are extremely comprehensive,
involving major disassembly, removal, overhaul, and inspection of systems and components. They might occur
only three to six times during the service life of an aircraft.
Altimeter & Transponder Inspections
Aircraft that are operated in controlled airspace under
instrument flight rules (IFR) must have each altimeter and
static system tested in accordance with procedures described
in 14 CFR part 43, Appendix E, within the preceding 24
calendar months. Aircraft having an air traffic control (ATC)
transponder must also have each transponder checked within
the preceding 24 months. All these checks must be conducted
by appropriately certified individuals.
Air Transport Association iSpec 2200
In an effort to standardize the format in which maintenance
information is presented in aircraft maintenance manuals,
Air Transport Association (now Airlines for America)issued
specifications for Manufacturers’ Technical Data. The
original specification was called ATA Spec 100. Over the
years, Spec 100 has been continuously revised and updated.
Eventually, ATA Spec 2100 was developed for electronic
documentation. These two specifications evolved into one
document called ATA iSpec 2200, developed and managed
by the ATA e-Business Program, a consensus-based industry
standards organization administered by Airlines for America
(A4A). As a result of this standardization, maintenance
technicians can always find information regarding a particular
system in the same section of an aircraft maintenance
manual, regardless of manufacturer. For example, if seeking
information about the electrical system on any aircraft, that
information is always found in section (chapter) 24.
The ATA iSpec 2200 divides the aircraft into systems, such
as air conditioning, that covers the basic air conditioning
system (ATA 21). Numbering in each major system provides
an arrangement for breaking the system down into several
subsystems. [Figure 10-4] Late model aircraft, both over and
under the 12,500-pound designation, have their parts manuals
and maintenance manuals arranged according to the ATA-
coded system. The following abbreviated table of ATA System,
Subsystem, and Titles is included for familiarization purposes.
Keep in mind that not all aircraft have all these systems
installed. Small and simple aircraft have fewer systems than
larger, more complex aircraft.
Special Inspections
During the service life of an aircraft, occasions may arise
when something out of the ordinary care and use of an aircraft
could possibly affect its airworthiness. When these situations
are encountered, special inspection procedures, also called
conditional inspections, are followed to determine if damage
to the aircraft structure has occurred. The procedures
10-13Figure 10-2. Title 14 CFR Appendix D to Part 43—Scope and detail of items (as applicable to the particular aircraft) to be included in
annual and 100-hour inspections. Appendix D to Part 43—Scope and Detail of Items (as Applicable to the Particular Aircraft) To Be Included in Annual
and 100-Hour Inspectionscondition, defects, and insecure attachment.
(11) Cowling—for cracks, and defects.
(e) Each person performing an annual or 100-hour inspection
shall inspect (where applicable) the following components of
the landing gear group:
(1) All units—for poor condition and insecurity of
attachment.
(2) Shock absorbing devices—for improper oleo fluid level.
(3) Linkages, trusses, and members—for undue or excessive
wear fatigue, and distortion.
(4) Retracting and locking mechanism—for improper
operation.
(5) Hydraulic lines—for leakage.
(6) Electrical system—for chafing and improper operation
of switches.
(7) Wheels—for cracks, defects, and condition of bearings.
(8) Tires—for wear and cuts.
(9) Brakes—for improper adjustment.
(10) Floats and skis—for insecure attachment and obvious or
apparent defects.
(f) Each person performing an annual or 100-hour inspection
shall inspect (where applicable) all components of the wing
and center section assembly for poor general condition, fabric
or skin deterioration, distortion, evidence of failure, and
insecurity of attachment.
(g) Each person performing an annual or 100-hour inspection shall
inspect (where applicable) all components and systems that
make up the complete empennage assembly for poor general
condition, fabric or skin deterioration, distortion, evidence of
failure, insecure attachment, improper component installation,
and improper component operation.
(h) Each person performing an annual or 100-hour inspection
shall inspect (where applicable) the following components of
the propeller group:
(1) Propeller assembly—for cracks, nicks, binds, and oil
leakage.
(2) Bolts—for improper torquing and lack of safetying.
(3) Anti-icing devices—for improper operations and obvious
defects.
(4) Control mechanisms—for improper operation, insecure
mounting, and restricted travel.
(i) Each person performing an annual or 100-hour inspection
shall inspect (where applicable) the following components of
the radio group:
(1) Radio and electronic equipment—for improper
installation and insecure mounting.
(2) Wiring and conduits—for improper routing, insecure
mounting, and obvious defects.
(3) Bonding and shielding—for improper installation and
poor condition.
(4) Antenna including trailing antenna—for poor condition,
insecure mounting, and improper operation.
(j) Each person performing an annual or 100-hour inspection
shall inspect (where applicable) each installed miscellaneous
item that is not otherwise covered by this listing for improper
installation and improper operation. (a) Each person performing an annual or 100-hour inspection
shall, before that inspection, remove or open all necessary
inspection plates, access doors, fairing, and cowling. He shall
thoroughly clean the aircraft and aircraft engine.
(b) Each person performing an annual or 100-hour inspection
shall inspect (where applicable) the following components
of the fuselage and hull group:
(1) Fabric and skin—for deterioration, distortion, other
evidence of failure, and defective or insecure attachment
of fittings.
(2) Systems and components—for improper installation,
apparent defects, and unsatisfactory operation.
(3) Envelope, gas bags, ballast tanks, and related parts—for
poor condition.
(c) Each person performing an annual or 100-hour inspection
shall inspect (where applicable) the following components
of the cabin and cockpit group:
(1) Generally—for uncleanliness and loose equipment that
might foul the controls.
(2) Seats and safety belts—for poor condition and apparent
defects.
(3) Windows and windshields—for deterioration and
breakage.
(4) Instruments—for poor condition, mounting, marking,
and (where practicable) improper operation.
(5) Flight and engine controls—for improper installation
and improper operation.
(6) Batteries—for improper installation and improper
charge.
(7) All systems—for improper installation, poor general
condition, apparent and obvious defects, and insecurity
of attachment.
(d) Each person performing an annual or 100-hour inspection
shall inspect (where applicable) components of the engine
and nacelle group as follows:
(1) Engine section—for visual evidence of excessive oil,
fuel, or hydraulic leaks, and sources of such leaks.
(2) Studs and nuts—for improper torquing and obvious
defects.
(3) Internal engine—for cylinder compression and for
metal particles or foreign matter on screens and sump
drain plugs. If there is weak cylinder compression,
for improper internal condition and improper internal
tolerances.
(4) Engine mount—for cracks, looseness of mounting, and
looseness of engine to mount.
(5) Flexible vibration dampeners—for poor condition and
deterioration.
(6) Engine controls—for defects, improper travel, and
improper safetying.
(7) Lines, hoses, and clamps—for leaks, improper condition
and looseness.
(8) Exhaust stacks—for cracks, defects, and improper
attachment.
(9) Accessories—for apparent defects in security of
mounting.
(10) All systems—for improper installation, poor general
10-14Figure 10-3. Title 14 CFR Section 91.409(d), Progressive Inspection.§ 91.409 Inspections.
(3) Enough housing and equipment for necessary
disassembly and proper inspection of the aircraft; and
(4) Appropriate current technical information for the
aircraft.
The frequency and detail of the progressive inspection shall
provide for the complete inspection of the aircraft within each
12 calendar months and be consistent with the manufacturer's
recommendations, field service experience, and the kind of
operation in which the aircraft is engaged. The progressive
inspection schedule must ensure that the aircraft, at all times,
will be airworthy and will conform to all applicable FAA
aircraft specifications, type certificate data sheets, airworthiness
directives, and other approved data. If the progressive inspection
is discontinued, the owner or operator shall immediately notify
the local FAA Flight Standards district office, in writing, of
the discontinuance. After the discontinuance, the first annual
inspection under §91.409(a)(1) is due within 12 calendar months
after the last complete inspection of the aircraft under the
progressive inspection. The 100-hour inspection under §91.409(b)
is due within 100 hours after that complete inspection. A complete
inspection of the aircraft, for the purpose of determining when
the annual and 100-hour inspections are due, requires a detailed
inspection of the aircraft and all its components in accordance with
the progressive inspection. A routine inspection of the aircraft and
a detailed inspection of several components is not considered to
be a complete inspection.(d) Progressive inspection. Each registered owner or operator of
an aircraft desiring to use a progressive inspection program
must submit a written request to the FAA Flight Standards
district office having jurisdiction over the area in which the
applicant is located, and shall provide—
(1) A certificated mechanic holding an inspection
authorization, a certificated airframe repair station, or
the manufacturer of the aircraft to supervise or conduct
the progressive inspection;
(2) A current inspection procedures manual available
and readily understandable to pilot and maintenance
personnel containing, in detail—
(i) An explanation of the progressive inspection,
including the continuity of inspection
responsibility, the making of reports, and the
keeping of records and technical reference
material;
(ii) An inspection schedule, specifying the intervals
in hours or days when routine and detailed
inspections will be performed and including
instructions for exceeding an inspection interval
by not more than 10 hours while en route and
for changing an inspection interval because of
service experience;
(iii) Sample routine and detailed inspection forms
and instructions for their use; and
(iv) Sample reports and records and instructions for
their use;
outlined on the following pages are general in nature and
are intended to acquaint the aviation mechanic with the areas
to be inspected. As such, they are not all inclusive. When
performing any of these special inspections, always follow
the detailed procedures in the aircraft maintenance manual.
In situations where the manual does not adequately address
the situation, seek advice from other maintenance technicians
who are highly experienced with them. The following
paragraphs describe some typical types of special inspections.
Hard or Overweight Landing Inspection
The structural stress induced by a landing depends not only
upon the gross weight at the time, but also upon the severity
of impact. The hard landing inspection is for hard landings at
or below the maximum design landing limits. An overweight
landing inspection must be performed when an airplane lands at
a weight above the maximum design landing weight. However,
because of the difficulty in estimating vertical velocity at the
time of contact, it is hard to judge whether or not a landing
has been sufficiently severe to cause structural damage. For
this reason, a special inspection is performed after a landing is
made at a weight known to exceed the design landing weight or
after a rough landing, even though the latter may have occurred
when the aircraft did not exceed the design landing weight.Wrinkled wing skin is the most easily detected sign of
an excessive load having been imposed during a landing.
Another indication easily detected is fuel leakage along
riveted seams. Other possible locations of damage are spar
webs, bulkheads, nacelle skin and attachments, firewall skin,
and wing and fuselage stringers. If none of these areas show
adverse effects, it is reasonable to assume that no serious
damage has occurred. If damage is detected, a more extensive
inspection and alignment check may be necessary.
Severe Turbulence Inspection/Over “G”
When an aircraft encounters a gust condition, the airload
on the wings exceeds the normal wingload supporting the
aircraft weight. The gust tends to accelerate the aircraft while
its inertia acts to resist this change. If the combination of gust
velocity and airspeed is too severe, the induced stress can
cause structural damage.
A special inspection is performed after a flight through severe
turbulence. Emphasis is placed upon inspecting the upper and
lower wing surfaces for excessive buckles or wrinkles with
permanent set. Where wrinkles have occurred, remove a few
rivets and examine the rivet shanks to determine if the rivets
10-15ATA iSpec 2200 Systems Sample
Systems Subsystems Title
21 AIR CONDITIONING
21 -00 General
21 -10 Compression
21 -20 Distribution
21 -30 Pressurization Control
21 -40 Heating
21 -50 Cooling
21 -60 Temperature Control
21 -70 Moisture/Air Contaminate Control
22 AUTO FLIGHT
23 COMMUNICATIONS
24 ELECTRICAL POWER
25 EQUIPMENT/FURNISHINGS
26 FIRE PROTECTION
27 FLIGHT CONTROLS
28 FUEL
29 HYDRAULIC POWER
30 ICE AND RAIN PROTECTION
31 INDICATING/RECORDING SYSTEMS
32 LANDING GEAR
33 LIGHTS
34 NAVIGATION
35 OXYGEN
36 PNEUMATIC
37 VACUUM
38 WATER/WASTE Systems Subsystems Title
42 INTEGRATED MODULAR AVIONICS
51 STANDARD PRACTICES AND
STRUCTURES - GENERAL
52 DOORS
53 FUSELAGE
54 NACELLES/PYLONS
55 STABILIZERS
56 WINDOWS
57 WINGS
60 STANDARD PR ACTICES -
PROPELLER/ROTOR
61 PROPELLERS/PROPULSION
62 ROTOR(S)
71 POWER PLANT
72 ENGINE TURBINE/TURBOPROP DUCTED
FAN/UNDUCTED FAN
72 -20 AIR INLET SECTION
73 ENGINE FUEL AND CONTROL
74 IGNITION
75 AIR
76 ENGINE CONTROLS
77 ENGINE INDICATING
78 EXHAUST
79 OIL
80 STARTING
81 TURBINES
82 WATER INJECTION
83 ACCESSORY GEAR-BOXES
This figure shows a representative number of systems/subsystems for demonstration purposes only. It is not all-encompassing and
should be viewed as a learning aid to understand the numbering method of the ATA iSpec 2200 System. Consult the specific aircraft
maintenance manuals or Airlines for America (A4A) for a complete description of the systems and subsystems.
Figure 10-4. ATA iSpec 2200 Systems.have sheared or were highly loaded in shear.
Through the inspection doors and other accessible openings,
inspect all spar webs from the fuselage to the tip. Check for
buckling, wrinkles, and sheared attachments. Inspect for
buckling in the area around the nacelles and in the nacelle
skin, particularly at the wing leading edge. Check for fuel
leaks. Any sizeable fuel leak is an indication that an area
may have received overloads that have broken the sealant
and opened the seams.If the landing gear was lowered during a period of severe
turbulence, inspect the surrounding surfaces carefully
for loose rivets, cracks, or buckling. The interior of the
wheel well may give further indications of excessive gust
conditions. Inspect the top and bottom fuselage skin. An
excessive bending moment may have left wrinkles of a
diagonal nature in these areas.
Inspect the surface of the empennage for wrinkles, buckling,
or sheared attachments. Also, inspect the area of attachment
of the empennage to the fuselage. These inspections cover
10-16the critical areas. If excessive damage is noted in any of the
areas mentioned, the inspection must be continued until all
damage is detected.
Lightning Strike
Although lightning strikes to aircraft are extremely rare,
if a strike has occurred, the aircraft is carefully inspected
to determine the extent of any damage that might have
occurred. When lightning strikes an aircraft, the electrical
current must be conducted through the structure and be
allowed to discharge or dissipate at controlled locations.
These controlled locations are primarily the aircraft’s static
discharge wicks, or on more sophisticated aircraft, null field
dischargers. When surges of high-voltage electricity pass
through good electrical conductors, such as aluminum or
steel, damage is likely to be minimal or nonexistent. When
surges of high-voltage electricity pass through non-metallic
structures, such as a fiberglass radome, engine cowl or fairing,
glass or plastic window, or a composite structure that does
not have built-in electrical bonding, burning and more serious
damage to the structure could occur. Visual inspection of
the structure is required. Look for evidence of degradation,
burning, or erosion of the composite resin at all affected
structures, electrical bonding straps, static discharge wicks,
and null field dischargers.
Bird Strike
When the aircraft is hit by birds during flight, the external
areas of the airplane are inspected in the general area of the
bird strike. If the initial inspection shows structural damage,
then the internal structure of the airplane must be inspected
as well. Also, inspect the hydraulic, pneumatic, and any other
systems in the area of the bird strike.
Fire Damage
Inspection of aircraft structures that have been subjected to fire
or intense heat can be relatively simple if visible damage is
present. Visible damage requires repair or replacement. If there
is no visible damage, the structural integrity of an aircraft may
still have been compromised. Since most structural metallic
components of an aircraft have undergone some sort of heat-
treatment process during manufacture, an exposure to high
heat not encountered during normal operations could severely
degrade the design strength of the structure. The strength and
airworthiness of an aluminum structure that passes a visual
inspection, but is still suspect, can be further determined by use
of a conductivity tester. This is a device that uses eddy current
and is discussed later in this chapter. Since strength of metals is
related to hardness, possible damage to steel structures might
be determined by use of a hardness tester, such as a Rockwell
C hardness tester. [Figure 10-5]Flood Damage
Like aircraft damaged by fire, aircraft damaged by water
can range from minor to severe. This depends on the level
of the flood water, whether it was fresh or salt water, and
the elapsed time between the flood occurrence and when
repairs were initiated. Any parts that were totally submerged
are completely disassembled, thoroughly cleaned, dried, and
treated with a corrosion inhibitor. Many parts might have to be
replaced, particularly interior carpeting, seats, side panels, and
instruments. Since water serves as an electrolyte that promotes
corrosion, all traces of water and salt must be removed before
the aircraft can again be considered airworthy.
Seaplanes
Because they operate in an environment that accelerates
corrosion, seaplanes must be carefully inspected for corrosion
and conditions that promote corrosion. Inspect bilge areas
for waste hydraulic fluids, water, dirt, drill chips, and
other debris. Additionally, since seaplanes often encounter
excessive stress from the pounding of rough water at high
speeds, inspect for loose rivets and other fasteners; stretched,
bent or cracked skins; damage to the float attach fitting; and
general wear and tear on the entire structure.
Aerial Application Aircraft
Two primary factors that make inspecting these aircraft
different from other aircraft are the corrosive nature of
some of the chemicals used and the typical flight profile.
Damaging effects of corrosion may be detected in a much
shorter period of time than normal use aircraft. Chemicals
may soften the fabric or loosen the fabric tapes of fabric-
covered aircraft. Metal aircraft may need to have the paint
stripped, cleaned, and repainted and corrosion treated
annually. Leading edges of wings and other areas may
require protective coatings or tapes. Hardware may require
more frequent replacement.
During peak use, these aircraft may fly up to 50 cycles
(takeoffs and landings) or more in a day, most likely from
an unimproved or grass runway. This can greatly accelerate
the failure of normal fatigue items. Landing gear and related
items require frequent inspections. Because these aircraft
operate almost continuously at very low altitudes, air filters
tend to become obstructed more rapidly.
Special Flight Permits
For an aircraft that does not currently meet airworthiness
requirements because of an overdue inspection, damage,
expired replacement times for time-limited parts, or other
reasons, but is capable of safe flight, a special flight permit
may be issued. Special flight permits, often referred to as
ferry permits, are issued for the following purposes:
Figure 10-5. Rockwell C Hardness Tester.• Flying the aircraft to a base where repairs, alterations, or
maintenance are to be performed or to a point of storage
• Delivering or exporting the aircraft
• Production flight testing new production aircraft
• Evacuating aircraft from areas of impending danger
• Conducting customer demonstration flights in new
production aircraft that have satisfactorily completed
production flight tests
Additional information about special flight permits may be
found in 14 CFR part 21. Application forms for special flight
permits may be requested from the nearest FAA FSDO.
Nondestructive Inspection/Testing
The preceding information in this chapter provided general
details regarding aircraft inspection. The remainder of
this chapter deals with several methods often used on
specific components or areas on an aircraft when carrying
out the more specific inspections. They are referred to as
nondestructive inspection (NDI) or nondestructive testing
(NDT). The objective of NDI and NDT is to determine the
airworthiness of a component, without damaging it, that
would render it unairworthy. Some of these methods are simple, requiring little additional expertise, while others are
highly sophisticated and require that the technician be highly
trained and specially certified.
Training, Qualification, & Certification
The product manufacturer or the FAA generally specifies the
particular NDI method and procedure to be used in inspection.
These NDI requirements are specified in the manufacturer’s
inspection, maintenance, or overhaul manual, FAA ADs,
supplemental structural inspection documents (SSID), or SBs.
The success of any NDI method and procedure depends
upon the knowledge, skill, and experience of the NDI
personnel involved. The person(s) responsible for detecting
and interpreting indications, such as eddy current, x-ray, or
ultrasonic NDI, must be qualified and certified to specific
FAA or other acceptable government or industry standards,
such as MIL-STD-410, Nondestructive Testing Personnel
Qualification and Certification or ATA iSpec 105, Guidelines
for Training and Qualifying Personnel in Nondestructive
Testing Methods. The person must be familiar with the test
method, know the potential types of discontinuities peculiar
to the material, and be familiar with their effect on the
structural integrity of the part. Additional information on NDI
may be found by referring to Chapter 5 of FAA AC 43.13-1,
Acceptable Methods, Techniques, and Practices—Aircraft
Inspection and Repair.
Advantages & Disadvantages of NDI Methods
Figure 10-6 provides a table of the advantages and
disadvantages of common NDI methods. This table could
be used as a guide for evaluating the most appropriate NDI
method when the manufacturer or the FAA has not specified
a particular NDI method to be used.
General Techniques
Before conducting NDI, it is necessary to follow preparatory
steps in accordance with procedures specific to that type of
inspection. Generally, the parts or areas must be thoroughly
cleaned. Some parts must be removed from the aircraft or
engine. Others might need to have any paint or protective
coating stripped. A complete knowledge of the equipment
and procedures is essential and, if required, calibration and
inspection of the equipment must be current.
Visual Inspection
Visual inspection can be enhanced by looking at the suspect
area with a bright light, a magnifying glass, and a mirror. Some
defects might be so obvious that further inspection methods are
not required. The lack of visible defects does not necessarily
mean further inspection is unnecessary. Some defects may lie
beneath the surface or may be so small that the human eye, even
with the assistance of a magnifying glass, cannot detect them.
10-18Surface Cracks
When searching for surface cracks with a flashlight, direct
the light beam at a 5 to 45 degree angle to the inspection
surface towards the face. [Figure 10-7] Do not direct the light
beam at such an angle that the reflected light beam shines
directly into the eyes. Keep the eyes above the reflected light
beam during the inspection. Determine the extent of any
cracks found by directing the light beam at right angles to
the crack and tracing its length. Use a 10-power magnifying
glass to confirm the existence of a suspected crack. If this is
not adequate, use other NDI techniques, such as penetrant,
magnetic particle, or eddy current to verify cracks.
Borescope
Inspection by use of a borescope is essentially a visual
inspection. A borescope is a device that enables the inspector
to see inside areas that could not otherwise be inspected
without disassembly. Borescopes are used in aircraft and
engine maintenance programs to reduce or eliminate the
need for costly teardowns. Aircraft turbine engines have
access ports that are specifically designed for borescopes.
Borescopes are also used extensively in a variety of aviation
maintenance programs to determine the airworthiness of
difficult to reach components. Borescopes typically are used
to inspect interiors of hydraulic cylinders and valves for
pitting, scoring, porosity, and tool marks; search for cracked
cylinders in aircraft reciprocating engines; inspect turbojet
engine turbine blades and combustion cans; verify the proper
placement and fit of seals, bonds, gaskets, and subassemblies
in difficult to reach areas; and assess foreign object damage
(FOD) in aircraft, airframe, and powerplants. Borescopes
may also be used to locate and retrieve foreign objects in
engines and airframes.
Borescopes are available in two basic configurations. The
simpler of the two is a rigid type, small diameter telescope
with a tiny mirror at the end that enables the user to see around
corners. The other type uses fiber optics that enable greater
flexibility. [Figure 10-8] Many borescopes provide images
that can be displayed on a computer or video monitor for
better interpretation of what is being viewed and to record
images for future reference. Most borescopes also include a
light to illuminate the area being viewed.
Liquid Penetrant Inspection
Penetrant inspection is a nondestructive test for defects open to
the surface in parts made of any nonporous material. It is used
with equal success on such metals as aluminum, magnesium,
brass, copper, cast iron, stainless steel, and titanium. It may
also be used on ceramics, plastics, molded rubber, and glass.
Penetrant inspection detects defects, such as surface cracks or porosity. These defects may be caused by fatigue cracks,
shrinkage cracks, shrinkage porosity, cold shuts, grinding and
heat-treat cracks, seams, forging laps, and bursts. Penetrant
inspection also indicates a lack of bond between joined metals.
The main disadvantage of penetrant inspection is that the
defect must be open to the surface in order to let the penetrant
get into the defect. For this reason, if the part in question is
made of material that is magnetic, the use of magnetic particle
inspection is generally recommended.
Penetrant inspection uses a penetrating liquid that enters a
surface opening and remains there, making it clearly visible
to the inspector. It calls for visual examination of the part
after it has been processed, increasing the visibility of the
defect so that it can be detected. Visibility of the penetrating
material is increased by the addition of one or two types of
dye: visible or fluorescent.
The visible penetrant kit consists of dye penetrant, dye
remover emulsifier, and developer. The fluorescent penetrant
inspection kit contains a black light assembly, as well as spray
cans of penetrant, cleaner, and developer. The light assembly
consists of a power transformer, a flexible power cable, and
a hand-held lamp. Due to its size, the lamp may be used in
almost any position or location.
The steps for performing a penetrant inspection are:
1. Clean the metal surface thoroughly.
2. Apply penetrant.
3. Remove penetrant with remover emulsifier or cleaner.
4. Dry the part.
5. Apply the developer.
6. Inspect and interpret results.
Interpretation of Results
The success and reliability of a penetrant inspection depends
upon the thoroughness that the part was prepared with.
Several basic principles applying to penetrant inspection are:
1. The penetrant must enter the defect in order to form
an indication. It is important to allow sufficient time
so the penetrant can fill the defect. The defect must
be clean and free of contaminating materials so that
the penetrant is free to enter.
2. If all penetrant is washed out of a defect, an indication
cannot be formed. During the washing or rinsing
operation, prior to development, it is possible that the
penetrant is removed from within the defect, as well
as from the surface.
3. Clean cracks are usually easy to detect. Surface
openings that are uncontaminated, regardless of how
10-19Visual
Penetrant Dye
Eddy Current
UltrasonicMagnetic
Particle
X-Ray
Radiography
Isotope
RadiographyMethod Advantages
• Inexpensive
• Highly portable• Immediate results• Minimum training• Minimum part preparation
• Portable
• Inexpensive• Sensitive to very small discontinuities• 30 minutes or less to accomplish• Minimum skill required
• Can be portable
• Inexpensive• Sensitive to small discontinuities• Immediate results• Moderate skill required• Detects surface and subsurface discontinuities• Relatively fast
• Portable
• Detects surface and subsurface discontinuities• Moderate speed• Immediate results• Sensitive to small discontinuities• Thickness sensitive• Can detect many variables
• Portable
• Inexpensive• Sensitive to very small discontinuities• Immediate results• Little part preparation• Wide range of materials and thickness can be inspected
• Detects surface and internal flaws
• Can inspect hidden areas
• Permanent test record obtained
• Minimum part preparation
• Portable
• Less inexpensive than x-ray
• Detects surface and internal flaws
• Can inspect hidden areas
• Permanent test record obtained
• Minimum part preparationDisadvantages
• Surface discontinuities only
• Generally only large discontinuities• Misinterpretation of scratches
• Locate surface defects only
• Rough or porous surfaces interfere with test
• Part preparation required (removal of finishes and sealant, etc.)
• High degree of cleanliness required
• Direct visual detection on results required
• Surface must be accessible
• Rough surfaces interfere with test
• Part preparation required (removal of finishes and sealant, etc.)
•Semi-directional requiring general orie ntati on of field to
discontinuity
• Ferro-magnetic materials only
• Part must be demagnetized after test
• Surface must be accessible to probe
• Rough surfaces interfere with test• Electrically conductive materials• Skill and training required• Time consuming for large areas
• Surface must be accessible to probe
• Rough surfaces interfere with test• Highly sensitive to sound beam discontinuity orientation• High degree of skill and experience required for exposure and
interpretation
• Depth of discontinuity not indicated
• Safety hazard
• Very expensive (slow process)
• Highly directional, sensitive to flaw orientation
• High degree of skill and experience required for exposure
and interpretation
• Depth of discontinuity not indicated
• Safety hazard
• Must conform to federal and state regulations for handling and use
• Highly directional, sensitive to fl aw orientation
• High degree of skill and experience required for exposure and
interpretation
• Depth of discontinuity not indicated
Figure 10-6. Advantages and disadvantages of NDI methods.
fine, are seldom difficult to detect with the penetrant
inspection.
4.The smaller the defect, the longer the penetrating time.
Fine crack-like apertures require a longer penetratingtime than defects such as pores.
5.When the part to be inspected is made of a material
susceptible to magnetism, it should be inspected by amagnetic particle inspection method if the equipment is available.
6.V isible penetrant-type developer, when applied to the
surface of a part, dries to a smooth, white coating.As the developer dries, bright red indications appearwhere there are surface defects. If no red indicationsappear, there are no surface defects.
7.When conducting the fluorescent penetrant-type
inspection, the defects show up (under black light)as a brilliant yellow-green color and the sound areasappear deep blue-violet.
8.It is possible to examine an indication of a defect and
to determine its cause as well as its extent. Such anappraisal can be made if something is known aboutthe manufacturing processes that the part has beensubjected to.
The size of the indication, or accumulation of penetrant, shows the extent of the defect and the brilliance is a measure of its depth. Deep cracks hold more penetrant and are
Fay45°KEEP EYE ABOVE REFLECTED
LIGHT BEAM
Crack open to surfaceIncandescent light beam
Line of sightReflected
light beam
Figure 10-7. Using a flashlight to inspect for cracks.
Rigid
Flexible
Figure 10-8. Rigid and flexible borescopes.
broader and more brilliant. Very fine openings can hold
only small amounts of penetrants and appear as fine lines.
[Figure 10-9]
False Indications
With the penetrant inspection, there are no false indications in
the sense that they occur in the magnetic particle inspection.
There are, however, two conditions that may create
accumulations of penetrant that are sometimes confused with
true surface cracks and discontinuities.
The first condition involves indications caused by poor
washing. If all the surface penetrant is not removed in
the washing or rinsing operation following the penetrant
dwell time, the unremoved penetrant is visible. Evidences
of incomplete washing are usually easy to identify since
the penetrant is in broad areas rather than in the sharp
patterns found with true indications. When accumulations
of unwashed penetrant are found on a part, the part must be
completely reprocessed. Degreasing is recommended for
removal of all traces of the penetrant.
False indications may also be created where parts press fit
to each other. If a wheel is press fit onto a shaft, penetrant
shows an indication at the fit line. This is perfectly normal
since the two parts are not meant to be welded together.
Indications of this type are easy to identify since they are
regular in form and shape.
Eddy Current Inspection
Electromagnetic analysis is a term describing the broad
spectrum of electronic test methods involving the intersection
of magnetic fields and circulatory currents. The most widely
used technique is the eddy current. Eddy currents are
composed of free electrons under the influence of an induced
electromagnetic field that are made to “drift” through metal. Eddy current is used to detect surface cracks, pits, subsurface
cracks, corrosion on inner surfaces, and to determine alloy
and heat-treat condition.
Eddy current is used in aircraft maintenance to inspect
jet engine turbine shafts and vanes, wing skins, wheels,
bolt holes, and spark plug bores for cracks, heat, or
frame damage. Eddy current may also be used in repair
of aluminum aircraft damaged by fire or excessive heat.
Different meter readings are seen when the same metal is
in different hardness states. Readings in the affected area
are compared with identical materials in known unaffected
areas for comparison. A difference in readings indicates
a difference in the hardness state of the affected area.
In aircraft manufacturing plants, eddy current is used to
inspect castings, stampings, machine parts, forgings, and
extrusions. Figure 10-10 shows a technician performing an
eddy current inspection on a fan blade.
Basic Principles
When an alternating current (AC) is passed through a coil,
it develops a magnetic field around the coil, which in turn
induces a voltage of opposite polarity in the coil and opposes
the flow of original current. If this coil is placed in such a
way that the magnetic field passes through an electrically
conducting specimen, eddy currents are induced into the
specimen. The eddy currents create their own field that varies
the original field’s opposition to the flow of original current.
The specimen’s susceptibility to eddy currents determines
the current flow through the coil.
The magnitude and phase of this counter field is dependent
primarily upon the resistance and permeability of the specimen
under consideration and enables us to make a qualitative
determination of various physical properties of the test
material. The interaction of the eddy current field with the
original field results is a power change that can be measured
by utilizing electronic circuitry similar to a Wheatstone bridge.
Figure 10-9. Dye penetrant inspection.
Principles of Operations
Eddy currents are induced in a test article when an AC is
applied to a test coil (probe). The AC in the coil induces an
alternating magnetic field in the article, causing eddy currents
to flow in the article. [Figure 10-11]
Flaws in or thickness changes of the test-piece influence
the flow of eddy currents and change the impedance of the
coil accordingly. [Figure 10-12] Instruments display the
impedance changes either by impedance plane plots or by
needle deflection. [Figure 10-13]
The specimen is either placed in or passed through the field
of an electromagnetic induction coil, and its effect on the
impedance of the coil or on the voltage output of one or more
test coils is observed. The process that involves electric fields
made to explore a test piece for various conditions involves
the transmission of energy through the specimen much like
the transmission of x-rays, heat, or ultrasound.
Eddy current inspection can frequently be performed without
removing the surface coatings, such as primer, paint, and
anodized films. It can be effective in detecting surface and
subsurface corrosion, pots, and heat-treat condition.
Eddy Current Instruments
A wide variety of eddy current test instruments are
available. The eddy current test instrument performs three
basic functions: generating, receiving, and displaying. The
generating portion of the unit provides an alternating current
to the test coil. The receiving section processes the signal
from the test coil to the required form and amplitude for
display. Instrument outputs or displays consist of a variety
of visual, audible, storage, or transfer techniques utilizing
meters, video displays, chart recorders, alarms, magnetic
tape, computers, and electrical or electronic relays.
A reference standard is required for the calibration of eddy
current test equipment. A reference standard is made from the same material as the item is to be tested. A reference
standard contains known flaws or cracks and could include
items, such as a flat surface notch, a fastener head, a fastener
hole, or a countersink hole. Figures 10-14, 10-15, and 10-16
show typical surface cracks, subsurface cracks, and structural
corrosion that can be detected with eddy current techniques.
Ultrasonic Inspection
Ultrasonic inspection is an NDI technique that uses sound
energy moving through the test specimen to detect flaws.
The sound energy passing through the specimen is displayed
on a cathode ray tube (CRT), a liquid crystal display (LCD)
computer data program, or video/camera medium. Indications
of the front and back surface and internal/external conditions
appear as vertical signals on the CRT screen or nodes of data
in the computer test program. [Figure 10-17] There are three
types of display patterns: “A” scan, “B” scan, and “C” scan.
Each scan provides a different picture or view of the specimen
being tested. [Figure 10-18]
Ultrasonic detection equipment makes it possible to locate
defects in all types of materials. Minute cracks, checks, and
voids too small to be seen by x-ray can be located by ultrasonic
inspection. An ultrasonic test instrument requires access to
only one surface of the material to be inspected and can be
used with either straight line or angle beam testing techniques.
Two basic methods are used for ultrasonic inspection. The
first of these methods is immersion testing. In this method
of inspection, the part under examination and the search unit
are completely immersed in a liquid couplant, such as water
or other suitable fluids.
The second method is called contact testing. It is readily
adapted to field use and is the method discussed in this
chapter. In this method, the part under examination and the
search unit are coupled with a viscous material, liquid, or a
paste that wets both the face of the search unit and the material
under examination.
Conductor Probe coilAlternating current
Primary magnetic field
Eddy currente
e
e
e
Figure 10-11. Generating an eddy current.
Eddy current inspection on crankshaft
Eddy current inspection on fan blade
Figure 10-10. Eddy current inspection.
There are three basic ultrasonic inspection methods: pulse
echo, through-transmission, and resonance. Through-
transmission and pulse echo are shown in Figure 10-19 .
Pulse Echo
Flaws are detected by measuring the amplitude of
signals reflected and the time required for these signals
to travel between specific surfaces and the discontinuity.
[Figure 10-20]
The time base, triggered simultaneously with each transmission pulse, causes a spot to sweep across the screen
of the CRT or LCD. The spot sweeps from left to right across
the face of the scope 50 to 5,000 times per second or higher
if required for high-speed automated scanning. Due to the
speed of the cycle of transmitting and receiving, the picture
on the oscilloscope appears to be stationary.
A few microseconds after the sweep is initiated, the rate
generator electrically excites the pulser, and the pulser in turn
emits an electrical pulse. The transducer converts this pulse
into a short train of ultrasonic sound waves. If the interfaces
ConductorP
S
The alternating current flowing
through the coil at a chosen
frequency generates a
magnetic field around the coil.A
When the coil is placed close to an
electrically conductive material, eddy current is included in the material.B
If a flaw in the conductive material
disturbs the eddy current circulation,
the magnetic coupling with the
probe is changed and a defect signal
can be read by measuring the coil
impedance variation. C
Probe coilAlternating current
Primary magnetic field
Secondary magnetic fieldEddy current
Figure 10-12. Detecting an eddy current.
Figure 10-13. Impedance plane test.
of the transducer and the specimen are properly oriented, the
ultrasound is reflected back to the transducer when it reaches
the internal flaw and the opposite surface of the specimen. The time interval between the transmission of the initial impulse and the reception of the signals from within the specimen are measured by the timing circuits. The reflected pulse received by the transducer is amplified, transmitted to, and displayed on the instrument screen. The pulse is displayed in the same relationship to the front and back pulses as the flaw is in relation to the front and back surfaces of the specimen. [Figure 10-21]
Pulse-echo instruments may also be used to detect flaws not
directly underneath the probe by use of the angle beam testing method. Angle beam testing differs from straight beam testing only in the manner that the ultrasonic waves pass through the material being tested. As shown in Figure 10-22 , the
beam is projected into the material at an acute angle to the surface by means of a crystal cut at an angle and mounted in plastic. The beam, or a portion thereof, reflects successively from the surfaces of the material or any other discontinuity, including the edge of the piece. In straight beam testing, the horizontal distance on the screen between the initial pulse and the first back reflection represents the thickness of the piece; while in angle beam testing, this distance represents the width of the material between the searching unit and the opposite edge of the piece.
Through-Transmission
Through-transmission inspection uses two transducers, one to generate the pulse and another placed on the opposite surface
Skin
Chord
Angle
CrackSkin
Skin Splice plateChordSkin gapUpper member crack Second or deeper member crackFastener hole crack, fastener in place
Figure 10-15. Typical subsurface cracks.
Crack
CrackFastener removed
Figure 10-14. Typical surface cracks.
to receive it. A disruption in the sound path indicates a flaw and
is displayed on the instrument screen. Through-transmission
is less sensitive to small defects than the pulse-echo method. Resonance
This system differs from the pulse method in that the
frequency of transmission may be continuously varied.
The resonance method is used principally for thickness
Corrosion
WebStringerSkinSkin
Chord
Bonded doubler
Body skin lap splice Skin and bonded doublerSkin and chord web
Figure 10-16. Typical structural corrosion.
Figure 10-17. Ultrasonic inspection.
measurements when the two sides of the material being tested
are smooth and parallel and the backside is inaccessible. The
point where the frequency matches the resonance point of
the material being tested is the thickness determining factor.
It is necessary that the frequency of the ultrasonic waves
corresponding to a particular dial setting be accurately
known. Checks are made with standard test blocks to guard
against possible drift of frequency.
If the frequency of an ultrasonic wave is such that its wavelength
is twice the thickness of a specimen (fundamental frequency),
then the reflected wave arrives back at the transducer in the
same phase as the original transmission so that strengthening
of the signal occurs. This results from constructive interference or a resonance and is shown as a high amplitude value on
the indicating screen. If the frequency is increased such that
three times the wavelength equals four times the thickness,
the reflected signal returns completely out of phase with the
transmitted signal and cancellation occurs. Further increase
of the frequency causes the wavelength to be equal to the
thickness again and gives a reflected signal in phase with the
transmitted signal and a resonance once more. By starting at the
fundamental frequency and gradually increasing the frequency,
the successive cancellations and resonances can be noted and
the readings used to check the fundamental frequency reading.
[Figure 10-23]
In some instruments, the oscillator circuit contains a motor-
driven capacitor that changes the frequency of the oscillator.
[Figure 10-24] In other instruments, the frequency is
changed by electronic means. The change in frequency
is synchronized with the horizontal sweep of a CRT. The
horizontal axis represents a frequency range. If the frequency
range contains resonances, the circuitry is arranged to present
these vertically. Calibrated transparent scales are then placed
in front of the tube and the thickness can be read directly.
The instruments normally operate between 0.25 millicycle
(mc) and 10 mc in four or five bands.
The resonance thickness instrument can be used to test the
thickness of such metals as steel, cast iron, brass, nickel,
copper, silver, lead, aluminum, and magnesium. In addition,
areas of corrosion or wear on tanks, tubing, airplane wing skins,
10-26TEST SPECIMEN
Plan viewC-scan
Amplitude
X
B-scan
PATH OF THE PROBE (FRONT TO BACK)X
X
(Material Thickness)BackFront
A-scanFrontBack
Signal amplitudeFlaw
Figure 10-18. Typical structural corrosion.
and other structures or products can be located and evaluated.
Direct reading dial-operated units are available that measure
thickness between 0.025 inch and 3 inches with an accuracy
of better than ±1 percent. Ultrasonic inspection requires a
skilled operator who is familiar with the equipment being
used, as well as the inspection method to be used for the
many different parts being tested. [Figure 10-25]
Ultrasonic Instruments
A portable, battery-powered ultrasonic instrument is used
for field inspection of airplane structure. The instrument
generates an ultrasonic pulse, detects and amplifies the
returning echo, and displays the detected signal on a
CRT or similar display. Piezoelectric transducers produce
longitudinal or shear waves, the most commonly used
waveforms for aircraft structural inspection.
Reference Standards
Reference standards are used to calibrate the ultrasonic
instrument. Reference standards serve two purposes: to
provide an ultrasonic response pattern that is related to the
part being inspected and to establish the required inspection
sensitivity. To obtain a representative response pattern, the
reference standard configuration is the same as that of the
test structure or is a configuration that provides an ultrasonic
response pattern representative of the test structure. The
reference standard contains a simulated defect (notch) that is positioned to provide a calibration signal representative
of the expected defect. The notch size is chosen to establish
inspection sensitivity (response to the expected defect size).
The inspection procedure gives a detailed description of the
required reference standard.
Couplants
Inspection with ultrasonics is limited to the part in contact
with the transducer. A layer of couplant is required to couple
the transducer to the test piece, because ultrasonic energy
does not travel through air. Some typical couplants used are
water, glycerin, motor oils, and grease.
Inspection of Bonded Structures
Ultrasonic inspection is finding increasing application in
aircraft bonded construction and repair. Many configurations
and types of bonded structures are in use in aircraft. All of
these variations complicate the application of ultrasonic
inspections. An inspection method that works well on one
part or one area of the part may not be applicable for different
parts or areas of the same part. Some of the variables in the
types of bonded structures are as follows:
• Top skin material is made from different materials and
thickness
• Different types and thickness of adhesives are used in
bonded structures
Amplifier
RF pulser
Rate generator
Timing circuit
Cathode ray oscilloscopeSpecimen
FlawTransducer
Figure 10-20. Block diagram of basic pulse-echo system.WATER YOKE HANDHELD
Pulse Echo
Through-transmission Ultrasonic (TTU)
0 1 2 3 4 5 6 7 8 9 1010
DEPTHSIGNAL STRENGTH
DELAMINATION
0 1 2 3 4 5 6 7 8 9 1010
DEPTHSIGNAL STRENGTH
NORMALINSPECTION – NDI OVERVIEW
Figure 10-19. Through-transmission and pulse echo indications.
FlawFlaw Transducer
SpecimenT
FB
Cathode ray tube
Figure 10-21. Pulse-echo display in relationship to flaw detection.
A
B
C
DTransducer
incident wave Reflective wave
Reflective
surface
Material
under testT = Wavelength
F = F1 (Fundamental frequency)
T = W F= 2F1 (2nd Harmonic)
T = 11/2 W F= 3F1 (3rd Harmonic)
Figure 10-23. Conditions of ultrasonic resonance in a metal plate.
8-23It is necessary that the frequency of the ultrasonic
waves corresponding to a particular dial setting beaccurately known. Checks should be made with standard test blocks to guard against possible drift of frequency.
If the frequency of an ultrasonic wave is such that its
wavelength is twice the thickness of a specimen (funda -
mental frequency), then the reflected wave will arrive back at the transducer in the same phase as the originaltransmission so that strengthening of the signal will occur. This results from constructive interference or aresonance and is shown as a high amplitude value on the indicating screen. If the frequency is increased such that three times the wavelength equals four times the thickness, the reflected signal will return completely out of phase with the transmitted signal and cancella-tion will occur. Further increase of the frequency causes the wavelength to be equal to the thickness again and gives a reflected signal in phase with the transmitted signal and a resonance once more.
By starting at the fundamental frequency and gradually
increasing the frequency, the successive cancellations and resonances can be noted and the readings used to check the fundamental frequency reading. [Figure 8-10]
In some instruments, the oscillator circuit contains a
motor driven capacitor which changes the frequency
of the oscillator. [Figure 8-11] In other instruments, the frequency is changed by electronic means.
The change in frequency is synchronized with the
horizontal sweep of a CRT. The horizontal axis thus represents a frequency range. If the frequency range contains resonances, the circuitry is arranged to pres-ent these vertically. Calibrated transparent scales are then placed in front of the tube, and the thickness can be read directly. The instruments normally operate Coaxial cable
Quartz crystal
Figure 8-9. Pulse-echo
A
B
D
Figure 8-10. Conditions of ultrasonic resonance
in a metal plate.
45°
Coaxial cable
Quartz crystal
DefectMaterial
Figure 10-22. Pulse-echo angle beam testing.
•Underlying structures contain differences in core
material, cell size, thickness, height, back skin material
and thickness, doublers (material and thickness), closure member attachments, foam adhesive, steps in skins, internal ribs, and laminates (number of layers, layer thickness, and layer material)
•The top only or top and bottom skin of a bonded
structure may be accessible
Types of Defects
Defects can be separated into five general types to represent
the various areas of bonded and laminate structures as follows:
1.Type I—disbonds or voids in an outer skin-to-
adhesive interface.
2. Type II—disbonds or voids at the adhesive-to-core
interface.
3. Type III—voids between layers of a laminate.
4.Type IV—voids in foam adhesive or disbonds between
the adhesive and a closure member at core-to-closure member joints.
5. Type V—water in the core.
Acoustic Emission Inspection
Acoustic emission is an NDI technique that involves the
placing of acoustic emission sensors at various locations on an aircraft structure and then applying a load or stress. The materials emit sound and stress waves that take the form of ultrasonic pulses. Cracks and areas of corrosion in the stressed airframe structure emit sound waves that are registered by the sensors. These acoustic emission burstscan be used to locate flaws and to evaluate their rate of growth as a function of applied stress. Acoustic emission testing has an advantage over other NDI methods in that itcan detect and locate all of the activated flaws in a structure in one test. Because of the complexity of aircraft structures,
T = 2W F= 4F1 (4th Harmonic)
10-29CRT
H. F.
oscillatorPulse
amplifier
Transducer
Tuning capacitor
Horizontal time-base generatorContacts
Material
Motor
Figure 10-24. Block diagram of resonance thickness measuring
system.
Figure 10-25. Ultrasonic inspection of a composite structure.
application of acoustic emission testing to aircraft has
required a new level of sophistication in testing technique
and data interpretation.
Magnetic Particle Inspection
Magnetic particle inspection is a method of detecting invisible
cracks and other defects in ferromagnetic materials, such as
iron and steel. It is not applicable to nonmagnetic materials.
In rapidly rotating, reciprocating, vibrating, and other highly-
stressed aircraft parts, small defects often develop to the
point that they cause complete failure of the part. Magnetic
particle inspection has proven extremely reliable for the
rapid detection of such defects located on or near the surface.
With this method of inspection, the location of the defect is
indicated and the approximate size and shape are outlined.
The inspection process consists of magnetizing the part and
then applying ferromagnetic particles to the surface area to be
inspected. The ferromagnetic particles (indicating medium)
may be held in suspension in a liquid that is flushed over the
part; the part may be immersed in the suspension liquid; or the particles, in dry powder form, may be dusted over the
surface of the part. The wet process is more commonly used
in the inspection of aircraft parts.
If a discontinuity is present, the magnetic lines of force
are disturbed and opposite poles exist on either side of the
discontinuity. The magnetized particles thus form a pattern in
the magnetic field between the opposite poles. This pattern,
known as an “indication,” assumes the approximate shape
of the surface projection of the discontinuity. A discontinuity
may be defined as an interruption in the normal physical
structure or configuration of a part, such as a crack, forging
lap, seam, inclusion, porosity, and the like. A discontinuity
may or may not affect the usefulness of a part.
Development of Indications
When a discontinuity in a magnetized material is open to
the surface and a magnetic substance (indicating medium) is
available on the surface, the flux leakage at the discontinuity
tends to form the indicating medium into a path of higher
permeability. (Permeability is a term used to refer to the
ease that a magnetic flux can be established in a given
magnetic circuit.) Because of the magnetism in the part and
the adherence of the magnetic particles to each other, the
indication remains on the surface of the part in the form of an
approximate outline of the discontinuity that is immediately
below it. The same action takes place when the discontinuity
is not open to the surface, but since the amount of flux leakage
is less, fewer particles are held in place and a fainter and less
sharply defined indication is obtained.
If the discontinuity is very far below the surface, there
may be no flux leakage and no indication on the surface.
The flux leakage at a transverse discontinuity is shown
in Figure 10-26 . The flux leakage at a longitudinal
discontinuity is shown in Figure 10-27 .
Types of Discontinuities Disclosed
The following types of discontinuities are normally detected
by the magnetic particle test: cracks, laps, seams, cold shuts,
inclusions, splits, tears, pipes, and voids. All of these may
affect the reliability of parts in service.
Cracks, splits, bursts, tears, seams, voids, and pipes are
formed by an actual parting or rupture of the solid metal.
Cold shuts and laps are folds that have been formed in the
metal, interrupting its continuity.
Inclusions are foreign material formed by impurities in the
metal during the metal processing stages. They may consist,
for example, of bits of furnace lining picked up during the
melting of the basic metal or of other foreign constituents.
Inclusions interrupt the continuity of the metal, because they
Figure 10-26. Flux leakage at transverse discontinuity.
Figure 10-27. Flux leakage at longitudinal discontinuity.prevent the joining or welding of adjacent faces of the metal.
Preparation of Parts for Testing
Grease, oil, and dirt must be cleaned from all parts before
they are tested. Cleaning is very important since any grease
or other foreign material present can produce nonrelevant
indications due to magnetic particles adhering to the foreign
material as the suspension drains from the part.
Grease or foreign material in sufficient amount over a
discontinuity may also prevent the formation of a pattern
at the discontinuity. It is not advisable to depend upon the
magnetic particle suspension to clean the part. Cleaning by
suspension is not thorough and any foreign materials so
removed from the part contaminates the suspension, thereby
reducing its effectiveness.
In the dry procedure, thorough cleaning is absolutely
necessary. Grease or other foreign material holds the
magnetic powder, resulting in nonrelevant indications and
making it impossible to distribute the indicating medium
evenly over the part’s surface. All small openings and oil
holes leading to internal passages or cavities must be plugged
with paraffin or other suitable nonabrasive material.
Coatings of cadmium, copper, tin, and zinc do not interfere
with the satisfactory performance of magnetic particle
inspection, unless the coatings are unusually heavy or the
discontinuities to be detected are unusually small.
Chromium and nickel plating generally do not interfere with
indications of cracks open to the surface of the base metal, but
prevent indications of fine discontinuities, such as inclusions.
Because it is more strongly magnetic, nickel plating is more
effective than chromium plating in preventing the formation
of indications.
Effect of Flux Direction
To locate a defect in a part, it is essential that the magnetic
lines of force pass approximately perpendicular to the defect.
It is, therefore, necessary to induce magnetic flux in more
than one direction, since defects are likely to exist at any
angle to the major axis of the part. This requires two separate
magnetizing operations, referred to as circular magnetization
and longitudinal magnetization. The effect of flux direction
is illustrated in Figure 10-28.
Circular magnetization is the induction of a magnetic field
consisting of concentric circles of force about and within
the part. This is achieved by passing electric current through
the part, locating defects running approximately parallel
to the axis of the part. Figure 10-29 illustrates circular
magnetization of a crankshaft. In longitudinal magnetization, the magnetic field is produced in a direction parallel to the
long axis of the part. This is accomplished by placing the part
in a solenoid excited by electric current. The metal part then
becomes the core of an electromagnet and is magnetized by
induction from the magnetic field created in the solenoid.
In longitudinal magnetization of long parts, the solenoid
must be moved along the part in order to magnetize it.
[Figure 10-30] This is necessary to ensure adequate field
strength throughout the entire length of the part.
Solenoids produce effective magnetization for approximately
12 inches from each end of the coil, thus accommodating parts
or sections approximately 30 inches in length. Longitudinal
magnetization equivalent to that obtained by a solenoid may
be accomplished by wrapping a flexible electrical conductor
around the part. Although this method is not as convenient,
it has an advantage in that the coils conform more closely to
the shape of the part, producing a somewhat more uniform
magnetization. The flexible coil method is also useful for
large or irregularly-shaped parts when standard solenoids
are not available.
Effect of Flux Density
The effectiveness of the magnetic particle inspection also
depends on the flux density or field strength at the surface of the
part when the indicating medium is applied. As the flux density
in the part is increased, the sensitivity of the test increases,
because of the greater flux leakages at discontinuities and the
resulting improved formation of magnetic particle patterns.
Longitudinal magnetization
Attraction of particles at defects Attraction of particles at defectsCircular magnetization
A B
Figure 10-28. Effect of flux direction on strength of indication.
Figure 10-29. Circular magnetization of a crankshaft.Excessively high flux densities may form nonrelevant
indications, such as patterns of the grain flow in the material.
These indications interfere with the detection of patterns
resulting from significant discontinuities. It is therefore
necessary to use a field strength high enough to reveal all
possible harmful discontinuities, but not strong enough to
produce confusing nonrelevant indications.
Magnetizing Methods
When a part is magnetized, the field strength in the part
increases to a maximum for the particular magnetizing force
and remains at this maximum as long as the magnetizing
force is maintained.
When the magnetizing force is removed, the field strength
decreases to a lower residual value depending on the magnetic
properties of the material and the shape of the part. These
magnetic characteristics determine whether the continuous
or residual method is used in magnetizing the part.
In the continuous inspection method, the part is magnetized
and the indicating medium applied while the magnetizing
force is maintained. The available flux density in the part
is thus at a maximum. The maximum value of flux depends
directly upon the magnetizing force and the permeability of
the material that the part is made of.
The continuous method may be used in practically all circular
and longitudinal magnetization procedures. The continuous
procedure provides greater sensitivity than the residual
procedure, particularly in locating subsurface discontinuities.
The highly critical nature of aircraft parts and assemblies and
the necessity for subsurface inspection in many applications
have resulted in the continuous method being more
widely used. Since the continuous procedure reveals more
nonsignificant discontinuities than the residual procedure, careful and intelligent interpretation and evaluation of
discontinuities revealed by this procedure are necessary.
The residual inspection procedure involves magnetization of
the part and application of the indicating medium after the
magnetizing force has been removed. This procedure relies on
the residual or permanent magnetism in the part and is more
practical than the continuous procedure when magnetization
is accomplished by flexible coils wrapped around the part. In
general, the residual procedure is used only with steels that
have been heat-treated for stressed applications.
Identification of Indications
The correct evaluation of the character of indications is
extremely important but is sometimes difficult to make
from observation of the indications alone. The principal
distinguishing features of indications are shape, buildup,
width, and sharpness of outline. These characteristics
are more valuable in distinguishing between types of
discontinuities than in determining their severity. Careful
observation of the character of the magnetic particle pattern
Figure 10-30. Longitudinal magnetization of camshaft (solenoid
method).must always be included in the complete evaluation of the
significance of an indicated discontinuity.
The most readily distinguished indications are those produced
by cracks open to the surface. These discontinuities include
fatigue cracks, heat-treat cracks, shrink cracks in welds and
castings, and grinding cracks. An example of a fatigue crack
is shown in Figure 10-31.
Magnaglo Inspection
Magnaglo inspection is similar to the preceding method,
but differs in that a fluorescent particle solution is
used and the inspection is made under black light.
[Figure 10-32] Efficiency of inspection is increased by the
neon-like glow of defects allowing smaller flaw indications
to be seen. This is an excellent method for use on gears,
threaded parts, and aircraft engine components. The reddish-
brown liquid spray or bath that is used consists of Magnaglo
paste mixed with a light oil at the ratio of 0.10 to 0.25 ounce
of paste per gallon of oil. After inspection, the part must be
demagnetized and rinsed with a cleaning solvent.
Magnetizing Equipment
Fixed (Nonportable) General Purpose Unit A fixed, general
purpose unit provides direct current (DC) for wet, continuous,
or residual magnetization procedures. [Figure 10-33]
Circular or longitudinal magnetization may be used, and
it may be powered with rectified AC, as well as DC. The
contact heads provide the electrical terminals for circular
magnetization. One head is fixed in position with its contact
plate mounted on a shaft surrounded by a pressure spring
so that the plate may be moved longitudinally. The plate
is maintained in the extended position by the spring until
pressure transmitted through the work from the movable
head forces it back.The motor-driven movable head slides horizontally in
longitudinal guides and is controlled by a switch. The spring
allows sufficient overrun of the motor-driven head to avoid
jamming it and also provides pressure on the ends of the
work to ensure good electrical contact.
A plunger-operated switch in the fixed head cuts out the
forward motion circuit of the movable head motor when
the spring has been properly compressed. In some units,
the movable head is hand operated, and the contact plate is
sometimes arranged for operation by an air ram. Both contact
plates are fitted with various fixtures for supporting the work.
The magnetizing circuit is closed by depressing a pushbutton
on the front of the unit. It is set to open automatically, usually
after about one-half second. The strength of the magnetizing
current may be set manually to the desired value by means
of the rheostat or increased to the capacity of the unit by
the rheostat short circuiting switch. The current utilized is
indicated on the ammeter. Longitudinal magnetization is
produced by the solenoid that moves in the same guide rail
as the movable head and is connected in the electrical circuit
by means of a switch.
The suspension liquid is contained in a sump tank and is
agitated and circulated by a pump. The suspension is applied
to the work through a nozzle. The suspension drains from the
work through a nonmetallic grill into a collecting pan that
leads back to the sump. The circulating pump is operated by
a pushbutton switch.
Portable General Purpose Unit
It is often necessary to perform the magnetic particle inspection
at locations where fixed general purpose equipment is not
available or to perform an inspection on members of aircraft
structures without removing them from the aircraft. It is
particularly useful for inspecting landing gear and engine
mounts suspected of having developed cracks in service.
Portable units supply both AC and DC magnetization.
This unit is a source of magnetizing and demagnetizing
current but does not provide a means for supporting the work
or applying the suspension. It operates on 200 volt, 60 cycle
AC and contains a rectifier for producing DC when required.
[Figure 10-34]
The magnetizing current is supplied through the flexible cables
with prods or contact clamps, as shown in Figure 10-35 .
The cable terminals may be fitted with prods or with contact
clamps. Circular magnetization may be developed by using
either the prods or clamps.
Longitudinal magnetization is developed by wrapping the
Figure 10-31. Fatigue crack on the bottom end fitting of a Hydrosorb
shock absorber.
Figure 10-32. Magnaglo inspection.
Figure 10-33. Fixed general-purpose magnetizing unit.cable around the part. The strength of the magnetizing current
is controlled by an eight-point tap switch, and the duration
that it is applied is regulated by an automatic cutoff similar
to that used in the fixed general purpose unit.
This portable unit also serves as a demagnetizer and
supplies high amperage, low-voltage AC for this purpose.
For demagnetization, the AC is passed through the part and
gradually reduced by means of a current reducer.
In testing large structures with flat surfaces where current
must be passed through the part, it is sometimes impossible
to use contact clamps. In such cases, contact prods are used.
Prods can be used with the fixed general purpose unit, as well
as the portable unit. The part or assembly being tested may
be held or secured above the standard unit and the suspension
hosed onto the area, while excess suspension drains into the
tank. The dry procedure may also be used.
Prods are held firmly against the surface being tested. There
is a tendency for a high-amperage current to cause burning
at contact areas, but with proper care, such burning is
usually slight. For applications where prod magnetization is acceptable, slight burning is normally acceptable.
Indicating Mediums
The various types of indicating mediums available for
magnetic particle inspection may be divided into two general
material types: wet and dry. The basic requirement for any
indicating medium is that it produce acceptable indications
of discontinuities in parts.
The contrast provided by a particular indicating medium on
the background or part surface is particularly important. The
colors most extensively used are black and red for the wet
procedure and black, red, and gray for the dry procedure.
For acceptable operation, the indicating medium must be
of high permeability and low retentivity. High permeability
ensures that a minimum of magnetic energy is required to
attract the material to flux leakage caused by discontinuities.
Low retentivity ensures that the mobility of the magnetic
particles is not hindered by the particles themselves becoming
magnetized and attracting one another.
Demagnetizing
The permanent magnetism remaining after inspection must
be removed by a demagnetization operation if the part is
to be returned to service. Parts of operating mechanisms
must be demagnetized to prevent magnetized parts from
attracting filings, grindings, or chips inadvertently left in the
system or steel particles resulting from operational wear. An
accumulation of such particles on a magnetized part may cause
scoring of bearings or other working parts. Parts of the airframe
must be demagnetized so they do not affect instruments.
Demagnetization between successive magnetizing operations
Figure 10-34. Portable magnetic particle inspection equipment. is not normally required unless experience indicates that
omission of this operation results in decreased effectiveness
for a particular application. Demagnetization may be
accomplished in a number of different ways. A convenient
procedure for aircraft parts involves subjecting the part to a
magnetizing force that is continually reversing in direction
and, at the same time, gradually decreasing in strength.
As the decreasing magnetizing force is applied first in one
direction and then the other, the magnetization of the part
also decreases.
Standard Demagnetizing Practice
The basic procedure for developing a reversing and gradually
decreasing magnetizing force in a part involves the use of a
solenoid coil energized by AC. As the part is moved away
from the alternating field of the solenoid, the magnetism in
the part gradually decreases.
A demagnetizer whose size approximates that of the work
is used. For maximum effectiveness, small parts are held as
close to the inner wall of the coil as possible. Parts that do
not readily lose their magnetism are passed slowly in and
out of the demagnetizer several times and, at the same time,
tumbled or rotated in various directions. Allowing a part to
remain in the demagnetizer with the current on accomplishes
very little practical demagnetization.
The effective operation in the demagnetizing procedure is
that of slowly moving the part out of the coil and away from
the magnetizing field strength. As the part is withdrawn, it is
kept directly opposite the opening until it is 1 or 2 feet from
the demagnetizer. The demagnetizing current is not cut off
until the part is 1 or 2 feet from the opening as the part may
be remagnetized if current is removed too soon. Another
procedure used with portable units is to pass AC through
the part being demagnetized, while gradually reducing the
current to zero.Radiographic
Because of their unique ability to penetrate material and
disclose discontinuities, X and gamma radiations have
been applied to the radiographic (x-ray) inspection of metal
fabrications and nonmetallic products.
The penetrating radiation is projected through the part to be
inspected and produces an invisible or latent image in the film.
When processed, the film becomes a radiograph or shadow
picture of the object. This inspection medium and portable
unit provides a fast and reliable means for checking the
integrity of airframe structures and engines. [Figure 10-36]
Radiographic Inspection
Radiographic inspection techniques are used to locate
defects or flaws in airframe structures or engines with little
or no disassembly. This is in marked contrast to other types
of nondestructive testing that usually require removal,
disassembly, and stripping of paint from the suspected
part before it can be inspected. Due to the radiation risks
associated with x-ray, extensive training is required to become
a qualified radiographer. Only qualified radiographers are
allowed to operate the x-ray units.
Three major steps in the x-ray process discussed in subsequent
paragraphs are: exposure to radiation, including preparation;
processing of film; and interpretation of the radiograph.
Preparation and Exposure
The factors of radiographic exposure are so interdependent
that it is necessary to consider all factors for any particular
radiographic exposure. These factors include, but are not
limited to, the following:
• Material thickness and density
• Shape and size of the object
• Type of defect to be detected
• Characteristics of x-ray machine used
• The exposure distance
• The exposure angle
• Film characteristics
• Types of intensifying screen, if used
Knowledge of the x-ray unit’s capabilities form a background
for the other exposure factors. In addition to the unit rating
in kilovoltage, the size, portability, ease of manipulation,
and exposure particulars of the available equipment must
be thoroughly understood. Previous experience on similar
objects is also very helpful in the determination of the overall
exposure techniques. A log or record of previous exposures
provides specific data as a guide for future radiographs. After
Prod set
Contact clamp
Figure 10-35. Magnetic particle inspection accessories.
exposure to x-rays, the latent image on the film is made
permanently visible by processing it successively through a
developer chemical solution, an acid bath, and a fixing bath,
followed by a clear water wash.
Radiographic Interpretation
From the standpoint of quality assurance, radiographic
interpretation is the most important phase of radiography.
It is during this phase that an error in judgment can
produce disastrous consequences. The efforts of the whole
radiographic process are centered in this phase, where the
part or structure is either accepted or rejected. Conditions
of unsoundness or other defects that are overlooked, not
understood, or improperly interpreted can destroy the purpose
and efforts of radiography and can jeopardize the structural
integrity of an entire aircraft. A particular danger is the false
sense of security imparted by the acceptance of a part or
structure based on improper interpretation.
As a first impression, radiographic interpretation may seem
simple, but a closer analysis of the problem soon dispels this
impression. The subject of interpretation is so varied and
complex that it cannot be covered adequately in this type of
document. Instead, this chapter gives only a brief review of
basic requirements for radiographic interpretation, including
some descriptions of common defects.
Experience has shown that, whenever possible, it is
important to conduct radiographic interpretation close to
the radiographic operation. When viewing radiographs, it
is helpful to have access to the material being tested. The
radiograph can thus be compared directly with the material
being tested, and indications due to such things as surface condition or thickness variations can be immediately
determined. The following paragraphs present several factors
that must be considered when analyzing a radiograph.
There are three basic categories of flaws: voids, inclusions,
and dimensional irregularities. The last category, dimensional
irregularities, is not pertinent to these discussions, because its
prime factor is one of degree and radiography is not exact.
V oids and inclusions may appear on the radiograph in a
variety of forms ranging from a two-dimensional plane to
a three-dimensional sphere. A crack, tear, or cold shut most
nearly resembles a two-dimensional plane, whereas a cavity
looks like a three-dimensional sphere. Other types of flaws,
such as shrink, oxide inclusions, porosity, and so forth, fall
somewhere between these two extremes of form.
It is important to analyze the geometry of a flaw, especially for
items such as the sharpness of terminal points. For example,
in a crack-like flaw, the terminal points appear much sharper
in a sphere-like flaw, such as a gas cavity. Also, material
strength may be adversely affected by flaw shape. A flaw
having sharp points could establish a source of localized stress
concentration. Spherical flaws affect material strength to a far
lesser degree than do sharp-pointed flaws. Specifications and
reference standards usually stipulate that sharp-pointed flaws,
such as cracks, cold shuts, and so forth, are cause for rejection.
Material strength is also affected by flaw size. A metallic
component of a given area is designed to carry a certain load
plus a safety factor. Reducing this area by including a large
flaw weakens the part and reduces the safety factor. Some
flaws are often permitted in components due to these safety
factors. In this case, the interpreter must determine the degree
Dark
areaLight
area
After processingFilmVoidRadiation sourceDark areaSpecimen
Figure 10-36. Radiograph.
of tolerance or imperfection specified by the design engineer.
Both flaw size and flaw shape are considered carefully, since
small flaws with sharp points can be just as bad as large flaws
with no sharp points.
Another important consideration in flaw analysis is flaw
location. Metallic components are subjected to numerous and
varied forces during their effective service life. Generally, the
distribution of these forces is not equal in the component or
part, and certain critical areas may be rather highly stressed.
The interpreter must pay special attention to these areas.
Another aspect of flaw location is that certain types of
discontinuities close to one another may potentially serve
as a source of stress concentrations creating a situation that
must be closely scrutinized.
An inclusion is a type of flaw that contains entrapped
material. Such flaws may be of greater or lesser density than
the item being radiographed. The foregoing discussions on
flaw shape, size, and location apply equally to inclusions
and to voids. In addition, a flaw containing foreign material
could become a source of corrosion.
Radiation Hazards
Radiation from x-ray units and radioisotope sources is
destructive to living tissue. It is universally recognized that
in the use of such equipment, adequate protection must be
provided. Personnel must keep outside the primary x-ray
beam at all times.
Radiation produces change in all matter that it passes through.
This is also true of living tissue. When radiation strikes the
molecules of the body, the effect may be no more than to dislodge a few electrons, but an excess of these changes
could cause irreparable harm. When a complex organism is
exposed to radiation, the degree of damage, if any, depends
on the body cells that have been changed.
Vital organs in the center of the body that are penetrated by
radiation are likely to be harmed the most. The skin usually
absorbs most of the radiation and reacts earliest to radiation.
If the whole body is exposed to a very large dose of radiation,
death could result. In general, the type and severity of the
pathological effects of radiation depend on the amount of
radiation received at one time and the percentage of the
total body exposed. Smaller doses of radiation could cause
blood and intestinal disorders in a short period of time.
The more delayed effects are leukemia and other cancers.
Skin damage and loss of hair are also possible results of
exposure to radiation.
Inspection of Composites
Composite structures are inspected for delamination
(separation of the various plies), debonding of the skin from
the core, and evidence of moisture and corrosion. Previously
discussed methods including ultrasonic, acoustic emission,
and radiographic inspections may be used as recommended
by the aircraft manufacturer. The simplest method used in
testing composite structures is the tap test. Newer methods,
such as thermography, have been developed to inspect
composite structures.
Tap Testing
Tap testing, also referred to as the ring test or coin test, is
widely used as a quick evaluation of any accessible surface
Tap hammer
Panel surface38 mm
(1.50 in)
(approximately)25 – 38 mm
(1.00 – 1.50 in)
(approximately)
Figure 10-37. Tap testing using hammer.to detect the presence of delamination or debonding. The
testing procedure consists of lightly tapping the surface with a
light weight hammer (maximum weight of 2 ounces), a coin,
or other suitable device. The acoustic response or “ring” is
compared to that of a known good area. A “flat” or “dead”
response indicates an area of concern. Tap testing is limited
to finding defects in relatively thin skins, less than 0.080"
thick. On honeycomb structures, both sides need to be tested.
Tap testing on one side alone would not detect debonding on
the opposite side. [Figure 10-37]
Electrical Conductivity
Composite structures are not inherently electrically
conductive. Some aircraft, because of their relatively low
speed and type of use, are not affected by electrical issues.
Manufacturers of other aircraft, such as high-speed, high-
performance jets, are required to utilize various methods of
incorporating aluminum or copper into their structures to
make them conductive. The aluminum or copper (aluminum
is used with fiberglass and Kevlar, while copper is used with
carbon fiber) is imbedded within the plies of the lay-ups either
as a thin wire mesh, screen, foil, or spray. When damaged
sections of the structure are repaired, care must be taken to
ensure that the conductive path be restored. Not only is it
necessary to include the conductive material in the repair,
but the continuity of the electrical path from the original
conductive material to the replacement conductor and back to
the original must be maintained. Electrical conductivity may
be checked by use of an ohmmeter. Specific manufacturer’s
instructions must be carefully followed.
Thermography
Thermography is an NDI technique often used with thin
composite structures that use radiant electromagnetic thermal
energy to detect flaws. Most common sources of heat are
heat lamps or heater blankets. The basic principle of thermal inspection consists of measuring or mapping of surface
temperatures when heat flows from, to, or through a test
object. All thermographic techniques rely on differentials in
thermal conductivity between normal, defect-free areas and
those having a defect. Normally, a heat source is used to
elevate the temperature of the article being examined while
observing the surface heating effects. Because defect-free
areas conduct heat more efficiently than areas with defects,
the amount of heat that is either absorbed or reflected
indicates the quality of the bond. The type of defects that
affect the thermal properties include disbonds, cracks, impact
damage, panel thinning, and water ingress into composite
materials and honeycomb core. Thermal methods are most
effective for thin laminates or for defects near the surface.
The most widely used thermographic inspection technique
uses an infrared (IR) sensing system to measure temperature
distribution. This type of inspection can provide rapid,
one-sided, non-contact scanning of surfaces, components,
or assemblies. The heat source can be as simple as a heat
lamp, so long as the appropriate heat energy is applied to
the inspection surface. The induced temperature rise is
a few degrees and dissipates quickly after the heat input
is removed. The IR camera records the IR patterns. The
resulting temperature data is processed to provide more
quantitative information. An operator analyzes the screen
and determines whether a defect was found. Because IR
thermography is a radiometric measurement, it can be
done without physical contact. Depending on the spatial
resolution of the IR camera and the size of the expected
damage, each image can be of a relatively large area.
Furthermore, as composite materials do not radiate heat
nearly as much as aluminum and have higher emissivity,
thermography can provide better definition of damage
with smaller heat inputs. Understanding of structural
arrangement is imperative to ensure that substructure is not
10-38Current voltage
and speed
normal
low low high high
Profile Viewsslow fastCurrent Voltage SpeedC B A
Figure 10-38. Examples of poor welds: too rapidly (A), improper penetration and cold laps (B), and irregular edges and considerable
variation (C).mistaken for defects or damage.
Inspection of Welds
A discussion of welds in this chapter is confined to judging
the quality of completed welds by visual means. Although the appearance of the completed weld is not a positive indication of quality, it provides a good clue about the care used in making it. A properly designed joint weld is stronger than the base metal that it joins. The characteristics of a properly welded joint are discussed in the following paragraphs.
A good weld is uniform in width; the ripples are even and
well feathered into the base metal and show no burn due to overheating. [Figure 10-38] The weld has good penetration
and is free of gas pockets, porosity, or inclusions. The edges of the bead are not in a straight line, yet the weld is good since penetration is excellent.
Penetration is the depth of fusion in a weld. Thorough fusion
is the most important characteristic contributing to a sound weld. Penetration is affected by the thickness of the material to be joined, the size of the filler rod, and how it is added. In a butt weld, the penetration should be 100 percent of the thickness of the base metal. On a fillet weld, the penetration requirements are 25 to 50 percent of the thickness of the base metal. The width and depth of bead for a butt weld and fillet weld are shown in Figure 10-39.
To assist further in determining the quality of a welded joint,
several examples of incorrect welds are discussed in the following paragraphs.The weld in Figure 10-38A was made too rapidly. The long
and pointed appearance of the ripples was caused by an excessive amount of heat or an oxidizing flame. If the weld were cross-sectioned, it would probably disclose gas pockets, porosity, and slag inclusions.
Figure 10-38B illustrates a weld that has improper penetration
and cold laps caused by insufficient heat. It appears rough and
irregular, and its edges are not feathered into the base metal.The puddle tends to boil during the welding operation if an excessive amount of acetylene is used. This often leaves slight bumps along the center and craters at the finish of the weld. Cross-checks are apparent if the body of the weld is sound. If the weld were cross-sectioned, pockets and porosity are visible. [Figure 10-38C]
A bad weld has irregular edges and considerable variation in the depth of penetration. It often has the appearance of a cold weld.
10-39A B
Leg 2 to 3 T
25 to 50% T100% PenetrationApprox. ½ TBead width 3 to 5TReinforcement ¼ to ½ T Throat 11/3 to 1½ T
T
Figure 10-39. Butt weld (A) and fillet weld (B), showing width and depth of bead.
Hand Tools & Measuring Devices
Chapter 11
The aviation maintenance technician (AMT) spends a major
portion of each day using a wide variety of hand tools to
accomplish maintenance tasks. This chapter contains an
overview of some of the hand tools an AMT can expect to use.
An AMT encounters many special tools as their experience
widens. For example, large transport category aircraft have
different maintenance tasks from those of a light airplane,
and special hand tools are often required when working on
complex aircraft.
This chapter outlines the basic knowledge required
when using the most common hand tools and measuring
instruments used in aircraft repair work. This information,
however, cannot replace sound judgment on the part of the
individual, nor additional training as the need arises. There
are many times when ingenuity and resourcefulness can
supplement these basic rules. Sound knowledge is required
of these basic rules and of the situations in which they apply.
The use of tools may vary, but good practices for safety, care,
and storage of tools remain the same.
General Purpose Tools
Hammers & Mallets
Figure 11-1 shows some of the hammers that the aviation
mechanic may be required to use. Metal head hammers
are usually sized according to the weight of the head alone
without the handle.
Occasionally, it is necessary to use a soft-faced hammer,
which has a striking surface made of wood, brass, lead,
rawhide, hard rubber, or plastic. These hammers are intended
for use in forming soft metals and striking surfaces that are
easily damaged. Soft-faced hammers should not be used
for striking punch heads, bolts, or nails, as using one in this
fashion quickly ruins this type of hammer.
A mallet is a hammer-like tool with a head made of hickory,
rawhide, or rubber. It is handy for shaping thin metal parts
without causing creases or dents with abrupt corners. Always
use a wooden mallet when pounding a wood chisel or a gouge.
When using a hammer or mallet, choose the one best suited
for the job. Ensure that the handle is tight. When striking a
blow with the hammer, use the forearm as an extension of
the handle. Swing the hammer by bending the elbow, not the wrist. Always strike the work squarely with the full face of
the hammer. When striking a metal tool with a metal hammer,
the use of safety glasses or goggles is strongly encouraged.
Always keep the faces of hammers and mallets smooth and free
from dents, chips, or gouges to prevent marring of the work.
Screwdrivers
The screwdriver can be classified by its shape, type of blade,
and blade length. [ Figure 11-2] It is made for only one purpose,
loosening or tightening screws or screw head bolts. When using
the common screwdriver, select the largest screwdriver whose
blade makes a good fit in the screw that needs to be turned.
A common screwdriver must fill at least 75 percent of the
screw slot. If the screwdriver is the wrong size, it cuts and
burrs the screw slot, making it unusable. The damage may be
so severe that the use of a screw extractor may be required. A
screwdriver with the wrong size blade may slip and damage
adjacent parts of the structure as well.
The common screwdriver is used only where slotted head
screws or fasteners are found on aircraft. An example of a
fastener that requires the use of a common screwdriver is
the camlock style fastener that is used to secure the cowling
on some aircraft.
The two types of recessed head screws for common use are
the Phillips and the Reed & Prince. Both the Phillips and
Reed & Prince recessed heads are optional on several types
of screws. As shown in Figure 11-2 , the Reed & Prince
recessed head forms a perfect cross. The screwdriver used
with this screw is pointed on the end. Since the Phillips
screw has a slightly larger center in the cross, the Phillips
screwdriver is blunt on the end. The Phillips screwdriver
is not interchangeable with the Reed & Prince. The use of
the wrong type of screwdriver results in mutilation of the
screwdriver and the screw head. When turning a recessed
head screw, use only the proper recessed head screwdriver of
the correct size. The most common crosspoint screwdrivers
are the Number 1 and Number 2 Phillips. Each of these are
designed to be used for specific sized screws. A Number 1
Phillips screwdriver is used on 2, 3, and 4 screws, while a
Number 2 Phillips is used for screw sizes 5, 6, 7, 8, and 9.
An offset screwdriver may be used when vertical space
11-2Ball peen Straight peen Cross peen Tinner’s mallet Riveting hammer
Figure 11-1. Hammers.
Phillips screwdriver
Offset screwdriver
Flat blade screwdriverPhillips Reed & Prince
Figure 11-2. Typical screwdrivers.is limited. Offset screwdrivers are constructed with both
ends bent 90° to the shank handle. By using alternate ends, most screws can be seated or loosened even when the swinging space is limited. Offset screwdrivers are made for both standard and recessed head screws. Ratcheting right angle screwdrivers are also available and often prove to be indispensable when working in close quarters.
A screwdriver should not be used for chiseling or prying.
Do not use a screwdriver to check an electric circuit since an electric arc will burn the tip and make it ineffective. In some cases, an electric arc may fuse the blade to the unit being checked, creating a short circuit.
When using a screwdriver on a small part, always hold the
part in the vise or rest it on a workbench. Do not hold the part in the hand, as the screwdriver may slip and cause serious personal injury.
Replaceable tip screwdrivers, commonly referred to as
“10 in 1” screwdrivers, allow for the quick changing of a screwdriver tip and economical replacement of the tip when it becomes worn. A wide variety of screwdriver tips, including flat, crosspoint (Reed & Prince, Phillips), Torx (6-point star-shaped pattern), and square drive tips are available for use with the handles. [Figure 11-3]
The cordless hand-held power screwdriver has replaced most automatic or spiral screwdrivers for the removal of multiple screws from an airframe. Care must be exercised when using a power screwdriver. If the slip clutch is set for too high a setting when installing a screw, the screwdriver tip will slip and rotate on top of the screw head, damaging it. The screw should be started by hand to avoid driving the screw into the nut or nut plate in a cross-threaded manner. To avoid damaging the slot or receptacle in the head of the screw, the use of cordless power drills fitted with a removable tip driver to remove or install screws is not recommended, as the drill does not have a slip-clutch installed.
Torx Flat Square drive
tipCrosspoint
(Phillips)
Figure 11-3. Replaceable tip screwdriver.Pliers & Plier-Type Cutting Tools
As shown in Figure 11-4 , the pliers used most frequently
in aircraft repair work are the diagonal, needle-nose, and
duckbill. The size of pliers indicates their overall length,
usually ranging from 5 to 12 inches.
Roundnose pliers are used to crimp metal. They are not made
for heavy work because too much pressure springs the jaws,
which are often wrapped to prevent scarring the metal.
Needle-nose pliers have half round jaws of varying lengths.
They are used to hold objects and make adjustments in
tight places.
Duckbill pliers resemble a “duck’s bill” in that the jaws are
thin, flat, and have the shape of a duck’s bill. They are used
exclusively for twisting safety wire.
Diagonal pliers are usually referred to as diagonals or
“dikes.” The diagonal is a short-jawed cutter with a blade
set at a slight angle on each jaw. This tool can be used to
cut wire, rivets, small screws, and cotter pins, besides being
practically indispensable in removing or installing safety
wire. The duckbill pliers and the diagonal cutting pliers are
used extensively in aviation for the job of safety wiring.
Two important rules for using pliers:
1. Do not make pliers work beyond their capacity. The
long-nosed variety is especially delicate. It is easy to
spring or break them or nick the edges. If this occurs,
they are practically useless.
2. Do not use pliers to turn nuts. In just a few seconds,
a pair of pliers can damage a nut more than years
of service.
Punches
Punches are used to locate centers for drawing circles, to start
holes for drilling, to punch holes in sheet metal, to transfer
location of holes in patterns, and to remove damaged rivets,
pins, or bolts.
Solid or hollow punches are the two types generally used.
Solid punches are classified according to the shape of their
points. Figure 11-5 shows several types of punches.
Prick punches are used to place reference marks on metal.
This punch is often used to transfer dimensions from a paper
pattern directly on the metal. To do this, first place the paper
pattern directly on the metal. Then go over the outline of the
pattern with the prick punch, tapping it lightly with a small
hammer and making slight indentations on the metal at the
major points on the drawing. These indentations can then
be used as reference marks for cutting the metal. A prick punch should never be struck a heavy blow with a hammer
because it may bend the punch or cause excessive damage
to the material being worked.
Large indentations in metal, which are necessary to start a
twist drill, are made with a center punch. It should never be
struck with enough force to dimple the material around the
indentation or to cause the metal to protrude through the other
side of the sheet. A center punch has a heavier body than a
prick punch and is ground to a point with an angle of about 60°.
The drive punch, which is often called a tapered punch, is used
for driving out damaged rivets, pins, and bolts that sometimes
bind in holes. The drive punch is therefore made with a flat
face instead of a point. The size of the punch is determined
by the width of the face, which is usually 1⁄8 inch to 1⁄4 inch.
Pin punches, often called drift punches, are similar to drive
punches and are used for the same purposes. The difference
between the two is that the sides of a drive punch taper all
the way to the face while the pin punch has a straight shank.
Pin punches are sized by the diameter of the face, in thirty-
seconds of an inch, and range from 1⁄16 to 3⁄8 inch in diameter.
In general practice, a pin or bolt that is to be driven out is
usually started and driven with a drive punch until the sides
of the punch touch the side of the hole. A pin punch is then
used to drive the pin or bolt the rest of the way out of the
hole. Stubborn pins may be started by placing a thin piece of
scrap copper, brass, or aluminum directly against the pin and
then striking it with a hammer until the pin begins to move.
Never use a prick punch or center punch to remove objects
from holes because the point of the punch spreads the object
and causes it to bind even more.
The transfer punch is usually about 4 inches long. It has a
point that tapers and then turns straight for a short distance
in order to fit a drill locating hole in a template. The tip has
a point similar to that of a prick punch. As its name implies,
the transfer punch is used to transfer the location of holes
through the template or pattern to the material.
11-4Auto center punch
Prick punch
Starting punch
Pin punch
Aligning punch
Drift pin
Drive punchCenter punch
Figure 11-5. Punches.
Diagonal cutter
Duckbill
Needle-nose
Roundnose
Figure 11-4. Pliers.
Wrenches
The wrenches most often used in aircraft maintenance are
classified as open-end, box-end, socket, adjustable, ratcheting
and special wrenches. The Allen wrench, although seldom
used, is required on one special type of recessed screw. One
of the most widely used metals for making wrenches is
chrome-vanadium steel. Wrenches made of this metal are
almost indestructible. Solid, nonadjustable wrenches with
open parallel jaws on one or both ends are known as open-end
wrenches. These wrenches may have their jaws parallel to
the handle or at an angle up to 90°; most are set at an angle
of 15°. The wrenches are designed to fit a nut, bolt head, or
other object, which makes it possible to exert a turning action.
Box-end wrenches are popular tools because of their
usefulness in close quarters. They are called box wrenches
since they box, or completely surround, the nut or bolt head.
Practically all well-manufactured box-end wrenches are
made with 12 points so they can be used in places having as
little as 15° swing. In Figure 11-6 , point A on the illustrated
double-broached hexagon wrench is nearer the centerline of
the head and the wrench handle than point B and also the
centerline of nut C. If the wrench is inverted and installed on nut C, point A will be centered over side “Y” instead of
side “X.” The centerline of the handle will now be in the
dotted line position. It is by reversing (turning the wrench
over) the position of the wrench that a 15° arc may be made
with the wrench handle.
Although box-end wrenches are ideal to break loose tight
nuts or pull tight nuts tighter, time is lost turning the nut off
the bolt once the nut is broken loose. Only when there is
sufficient clearance to rotate the wrench in a complete circle
can this tedious process be avoided.
After a tight nut is broken loose, it can be completely backed
off or unscrewed more quickly with an open-end than with a
box-end wrench. In this case, a combination wrench can be
A B
XY
C
Figure 11-6. Box-end wrench use.used. A combination wrench has a box end on one end and
an open-end wrench of the same size on the other.
Another option for removing a nut from a bolt is the
ratcheting box-end wrench, which can be swung back and
forth to remove the nut or bolt. The box-end, combination,
and ratcheting wrenches are shown in Figure 11-7 .
A socket wrench is made of two parts: the socket, which is
placed over the top of a nut or bolt head; and a handle, which
is attached to the socket. Many types of handles, extensions,
and attachments are available to make it possible to use socket
wrenches in almost any location or position. Sockets are made
with either fixed or detachable handles. Socket wrenches
with fixed handles are usually furnished as an accessory to a
machine. They have a four, six, or twelve-sided recess to fit
a nut or bolt head that needs regular adjustment. Sockets
with detachable handles usually come in sets and fit several
types of handles, such as the T, ratchet, screwdriver grip, and
speed handle. Socket wrench handles have a square lug on
one end that fits into a square recess in the socket head. The
two parts are held together by a light, spring-loaded poppet.
Two types of sockets, a set of handles, and an extension bar
are shown in Figure 11-8 .
The adjustable wrench is a handy utility tool that has smooth
jaws and is designed as an open-end wrench. One jaw is
fixed, but the other may be moved by a thumbscrew or spiral
screwworm adjustment in the handle. The width of the jaws
may be varied from 0 to 1⁄2 inch or more. The angle of the
opening to the handle is 221⁄2 degrees on an adjustable wrench.
One adjustable wrench does the work of several open-end
wrenches. Although versatile, they are not intended to replace
the standard open-end, box-end, or socket wrenches. When
using any adjustable wrench, always exert the pull on the
side of the handle attached to the fixed jaw of the wrench.
To minimize the possibility or rounding off the fastener, use
care to fit the wrench to the bolt or nut to be turned.
Special Wrenches
The category of special wrenches includes the crowfoot,
flare nut, spanner, torque, and Allen wrenches.
[Figure 11-9 and 11-10]
The crowfoot wrench is normally used when accessing nuts
that must be removed from studs or bolts that cannot be
accessed using other tools.
The flare nut wrench has the appearance of a box-end wrench
that has been cut open on one end. This opening allows the
wrench to be used on the B-nut of a fuel, hydraulic, or oxygen
line. Since it mounts using the standard square adapter, like
the crowfoot wrench, it can be used in conjunction with a torque wrench.
The hook spanner is for a round nut with a series of notches
cut in the outer edge. This wrench has a curved arm with a
hook on the end that fits into one of the notches on the nut.
The hook is placed in one of these notches with the handle
pointing in the direction the nut is to be turned.
Some hook spanner wrenches are adjustable and fit nuts of
various diameters. U-shaped hook spanners have two lugs
on the face of the wrench to fit notches cut in the face of the
nut or screw plug. End spanners resemble a socket wrench,
but have a series of lugs that fit into corresponding notches
in a nut or plug. Pin spanners have a pin in place of a lug, and
the pin fits into a round hole in the edge of a nut. Face pin
spanners are similar to the U-shaped hook spanners except
that they have pins instead of lugs.
Most headless setscrews are the hex-head Allen type and
must be installed and removed with an Allen wrench. Allen
wrenches are six-sided bars in the shape of an L, or they can
be hex-shaped bars mounted in adapters for use with hand
ratchets. They range in size from 3⁄64 to 1⁄2 inch and fit into a
hexagonal recess in the setscrew.
Torque Wrench
There are times when definite pressure must be applied to a
nut or bolt as it is installed. In such cases, a torque wrench
must be used. The torque wrench is a precision tool consisting
of a torque indicating handle and appropriate adapter or
attachments. It measures the amount of turning or twisting
force applied to a nut, bolt, or screw.
Speed handle
Socket and universal joint combined Extension barHinge handleRatchet handle
Socket
Figure 11-8. Socket wrench set.
Box-endOpen-end
Combination wrenchRatcheting wrench
Box-end wrench12 mm3/8 3/8
10 mm
10 mm 11 mm
Figure 11-7. Ratcheting, box-end, and combination wrenches.
Before each use, the torque wrench should be visually
inspected for damage. If a bent pointer, cracked or broken
glass (dial type), or signs of rough handling are found, the
wrench must be tested. Torque wrenches must be tested at
periodic intervals to ensure accuracy.
Calibrating a torque wrench is the process in which the
manufacturers of the torque wrench set ensure a precise
torque occurs on a standard and consistent basis. Regular
torque wrench calibration ensures repeatable accuracy and
adherence to standards. A torque wrench is a precision
tool and should be treated and maintained like a delicate
measuring instrument. A torque wrench must be properly calibrated and maintained on a preventative maintenance
and calibration schedule. In order to maintain accuracy,
it is crucial that a torque wrench and other measuring
equipment be calibrated regularly. Some wrenches or tools
may recommend six (6) month calibration intervals, while
others may schedule it at twelve (12) months.
The three most commonly used torque wrenches are the
deflecting beam, dial indicating, and micrometer setting types.
[Figure 11-10] When using the deflecting beam and the dial
indicating torque wrenches, the torque is read visually on a dial
or scale mounted on the handle of the wrench. The micrometer
setting torque wrench is preset to the desired torque. When
this torque is reached, the operator notices a sharp impulse
or breakaway “click.” For additional information on the
installation of fasteners requiring the use of a torque wrench,
refer to “Installation of Nuts, Washers, and Bolts” located in
Chapter 7, Aircraft Materials, Processes and Hardware.
Strap Wrenches
The strap wrench can prove to be an invaluable tool for the
AMT. By their very nature, aircraft components, such as
tubing, pipes, small fittings, and round or irregularly-shaped
components, are built to be as light as possible while still
retaining enough strength to function properly. The misuse of
pliers or other gripping tools can quickly damage these parts.
If it is necessary to grip a part to hold it in place, or to rotate it
to facilitate removal, consider using a strap wrench that uses
a plastic covered fabric strap to grip the part. [Figure 11-11]
Impact Drivers
In certain applications, the use of an impact driver may be
required. Struck with a mallet, the impact driver uses cam
action to impart a high amount of torque in a sharp impact
to break loose a stubborn fastener. The drive portion of the
impact driver can accept a number of different drive bits
Allen wrench Hook spannerFlare nutCrowfoot
C
EBAC
E
DAF
A = Outside diameter
B = Wall openingC = Center to centerD = LengthE = Head depthF = Total depth
Dial indicating torque wrench Micrometer “click-type” torque wrench
Electronic torque wrench Deflecting beam torque wrench
Figure 11-9. Special wrenches.
Figure 11-10. Torque wrenches.
Figure 11-11. Strap wrench.
Figure 11-12. Impact driver.and sockets. The use of special bits and sockets specifically
manufactured for use with an impact driver is required.
[Figure 11-12]
Metal Cutting Tools
Hand Snips
There are several kinds of hand snips, each of which serves
a different purpose. Straight, curved, hawksbill, and aviation
snips are in common use. Straight snips are used for cutting
straight lines when the distance is not great enough to use a
squaring shear and for cutting the outside of a curve. The other
types are used for cutting the inside of curves or radii. Snips
should never be used to cut heavy sheet metal. [Figure 11-13]
Aviation snips are designed especially for cutting heat-treated
aluminum alloy and stainless steel. They are also adaptable
for enlarging small holes. The blades have small teeth on the
cutting edges and are shaped for cutting very small circles and
irregular outlines. The handles are the compound leverage
type, making it possible to cut material as thick as 0.051
inch. Aviation snips are available in two types: those which
cut from right to left and those which cut from left to right.
Unlike the hacksaw, snips do not remove any material when
the cut is made, but minute fractures often occur along the
cut. Therefore, cuts should be made about 1⁄32 inch from the
layout line and finished by hand filing down to the line.
Hacksaws
The common hacksaw has a blade, a frame, and a handle.
The handle can be obtained in two styles: pistol grip and
straight. [Figure 11-14]
Hacksaw blades have holes in both ends; they are mounted
on pins attached to the frame. When installing a blade in
a hacksaw frame, mount the blade with the teeth pointing
forward, away from the handle.
Blades are made of high-grade tool steel or tungsten steel
and are available in sizes from 6 to 16 inches in length. The
10-inch blade is most commonly used. There are two types:
the all-hard blade and the flexible blade. In flexible blades,
only the teeth are hardened.
Selection of the best blade for the job involves finding the
right type and pitch. An all-hard blade is best for sawing
brass, tool steel, cast iron, and heavy cross-section materials.
A flexible blade is usually best for sawing hollow shapes and
metals having a thin cross-section.
The pitch of a blade indicates the number of teeth per inch.
Pitches of 14, 18, 24, and 32 teeth per inch are available. A
blade with 14 teeth per inch is preferred when cutting machine steel, cold-rolled steel, or structural steel. A blade with 18
teeth per inch is preferred for solid stock aluminum, bearing
metal, tool steel, and cast iron. Use a blade with 24 teeth per
inch when cutting thick-walled tubing, pipe, brass, copper,
channel, and angle iron. Use the 32 teeth per inch blade for
cutting thin-walled tubing and sheet metal. When using a
hacksaw, observe the following procedures:
1. Select an appropriate saw blade for the job.
2. Assemble the blade in the frame so that the cutting
edge of the teeth points away from the handle.
3. Adjust tension of the blade in the frame to prevent the
saw from buckling and drifting.
4. Clamp the work in the vise in such a way that provides
as much bearing surface as possible and engages the
greatest number of teeth.
5. Indicate the starting point by nicking the surface with
the edge of a file to break any sharp corner that might
strip the teeth. This mark also aids in starting the saw
at the proper place.
6. Hold the saw at an angle that keeps at least two teeth
in contact with the work at all times. Start the cut with
a light, steady, forward stroke just outside the cutting
line. At the end of the stroke, relieve the pressure and
draw the blade back. (The cut is made only on the
forward stroke.)
7. After the first few strokes, make each stroke as long
as the hacksaw frame allows. This prevents the blade
from overheating. Apply just enough pressure on the
Pistol grip
StraightFigure 11-13. Typical snips.
Figure 11-14. Hacksaws.Chisels are usually made of eight-sided tool steel bar stock,
carefully hardened and tempered. Since the cutting edge
is slightly convex, the center portion receives the greatest
shock when cutting, and the weaker corners are protected.
The cutting angle should be 60° to 70° for general use, such
as for cutting wire, strap iron, or small bars and rods. When
using a chisel, hold it firmly in one hand. With the other
hand, strike the chisel head squarely with a ball peen hammer.
When cutting square corners or slots, a special cold chisel
called a cape chisel should be used. It is like a flat chisel except
the cutting edge is very narrow. It has the same cutting angle
and is held and used in the same manner as any other chisel.
Rounded or semicircular grooves and corners that have
fillets should be cut with a roundnose chisel. This chisel is
also used to re-center a drill that has moved away from its
intended center.
The diamond point chisel is tapered square at the cutting end,
and then ground at an angle to provide the sharp diamond
point. It is used for cutting B-grooves and inside sharp angles.
Files
Most files are made of high-grade tool steels that are hardened
and tempered. Files are manufactured in a variety of shapes
and sizes. They are known either by the cross section, the
general shape, or by their particular use. The cuts of files
must be considered when selecting them for various types
of work and materials.
Files are used to square ends, file rounded corners, remove
burrs and slivers from metal, straighten uneven edges, smooth
rough edges, and file holes and slots.
Files have three distinguishing features:
1. Their length, measured exclusive of the tang
[Figure 11-16] ;
2. Their kind or name, such as a hand file shown in
Figure 11-16 , that has reference to the relative
coarseness of the teeth; and
3. Their cut, such as a single- or double-cut file.
Files are usually made in two types of cuts: single cut and
double cut. The single cut file has a single row of teeth
extending across the face at an angle of 65° to 85° with
the length of the file. The size of the cuts depends on the
coarseness of the file. The double cut file has two rows of
teeth that cross each other. For general work, the angle of
the first row is 40° to 45°. The first row is generally referred
to as “overcut,” and the second row as “upcut;” the upcut is forward stroke to cause each tooth to remove a small
amount of metal. The strokes should be long and
steady with a speed not more than 40 to 50 strokes
per minute.
8. After completing the cut, remove chips from the blade,
loosen tension on the blade, and return the hacksaw
to its proper place.
Chisels
A chisel is a hard steel cutting tool that can be used for cutting
and chipping any metal softer than the chisel itself. It can be
used in restricted areas and for such work as shearing rivets,
or splitting seized or damaged nuts from bolts. [Figure 11-15]
The size of a flat cold chisel is determined by the width of
the cutting edge. Lengths vary, but chisels are seldom under
5 inches or over 8 inches long.
Flat cold chisel
Single bevel point
Double bevel point
Roundnose
Diamond point
60°–70°
Convex
Figure 11-15. Chisels.
Half-Round Files
These files cut on both the flat and round sides. They may
be single or double cut. Their shape permits them to be used
where other files would be unsatisfactory. [Figure 11-17]
Lead-Float Files
These are especially designed for use on soft metals. They
are single cut and are made in various lengths. [Figure 11-17]
Warding File
Rectangular in section and tapers to narrow point in width.
This file is used for narrow space filing where other files somewhat finer than and not as deep as the overcut.
Care and Use
Files and rasps are catalogued in three ways:
• Length—Measuring from the tip to the heel of the file.
The tang is never included in the length.
• Shape—Refers to the physical configuration of the
file (circular, rectangular, triangular, or a variation
thereof).
• Cut—Refers to both the character of the teeth or the
coarseness—rough, coarse, and bastard for use on
heavier classes of work and second cut, smooth, and
dead smooth for finishing work.
Most Commonly Used Files
Hand Files
These are parallel in width and tapered in thickness. They
have one safe edge (smooth edge) that permits filing in
corners and on other work where a safe edge is required.
Hand files are double cut and used principally for finishing
flat surfaces and similar work. [Figure 11-17]
Flat Files
These files are slightly tapered toward the point in both width
and thickness. They cut on both edges, as well as on the sides.
They are the most common files in use. Flat files are double
cut on both sides and single cut on both edges. [Figure 11-17]
Mill Files
These are usually tapered slightly in thickness and in width
for about one-third of their length. The teeth are ordinarily
single cut. These files are used for draw filing and to some
extent for filing soft metals. [Figure 11-17]
Square Files
These files may be tapered or blunt and are double cut. They
are used principally for filing slots and key seats and for
surface filing. [Figure 11-17]
Round or Rattail Files
These are circular in cross section and may be either tapered
or blunt and single or double cut. They are used principally for
filing circular openings or concave surfaces. [Figure 11-17]
Triangular and Three Square Files
These files are triangular in cross section. Triangular files
are single cut and are used for filing the gullet between saw
teeth. Three square files, which are double cut, may be used
for filing internal angles, clearing out corners, and filing taps
and cutters. [Figure 11-17]
Length
Tang
Heel Face Edge Point
Hand—taper width, parallel thickness
Mill—taper width, parallel thickness
Pillar—taper thickness, parallel width
Warding—much taper width, parallel thickness
Square, round, and three–square—taper
Half–round—taper
Knife—taper
Vixen—parallel edges and sides
Figure 11-16. Hand file.
Figure 11-17. Types of files.cannot be used. [Figure 11-17]
Knife File
Knife blade section. This file is used by tool and die makers
on work having acute angles. [Figure 11-17]
Wood File
Same section as flat and half-round files. This file has
coarser teeth and is especially adaptable for use on wood.
[Figure 11-17]
Vixen (Curved-Tooth Files)
Curved-tooth files are especially designed for rapid filing and
smooth finish on soft metals and wood. The regular cut is
adapted for tough work on cast iron, soft steel, copper, brass,
aluminum, wood, slate, marble, fiber, rubber, and so forth. The
fine cut gives excellent results on steel, cast iron, phosphor
bronze, white brass, and all hard metals. The smooth cut is
used where the amount of material to be removed is very
slight, but where a superior finish is desired. [Figure 11-17]
The following methods are recommended for using files:
1. Crossfiling. Before attempting to use a file, place
a handle on the tang of the file. This is essential
for proper guiding and safe use. In moving the file
endwise across the work (commonly known as
crossfiling), grasp the handle so that its end fits into
and against the fleshy part of the palm with the thumb
lying along the top of the handle in a lengthwise
direction. Grasp the end of the file between the thumb
and first two fingers. To prevent undue wear of the
file, relieve the pressure during the return stroke.
2. Drawfiling. A file is sometimes used by grasping it
at each end, crosswise to the work, then moving it
lengthwise with the work. When done properly, work
may be finished somewhat finer than when cross filing
with the same file. In draw filing, the teeth of the file
produce a shearing effect. To accomplish this shearing
effect, the angle at which the file is held with respect
to its line of movement varies with different files,
depending on the angle at which the teeth are cut. Pressure should be relieved during the backstroke.
3. Rounding corners. The method used in filing a rounded
surface depends upon its width and the radius of the
rounded surface. If the surface is narrow or only a
portion of a surface is to be rounded, start the forward
stroke of the file with the point of the file inclined
downward at approximately a 45° angle. Using a
rocking chair motion, finish the stroke with the heel
of the file near the curved surface. This method allows
use of the full length of the file.
4. Removing burred or slivered edges. Practically every
cutting operation on sheet metal produces burrs or
slivers. These must be removed to avoid personal
injury and to prevent scratching and marring of parts
to be assembled. Burrs and slivers prevent parts from
fitting properly and should always be removed from
11-12the work as a matter of habit.
Lathe filing requires that the file be held against the work
revolving in the lathe. The file should not be held rigid or
stationary but should be stroked constantly with a slight
gliding or lateral motion along the work. A standard mill file
may be used for this operation, but the long angle lathe file
provides a much cleaner shearing and self-clearing action.
Use a file with “safe” edges to protect work with shoulders
from being marred.
Care of Files
There are several precautions that any good craftsman takes
in caring for files.
1. Choose the right file for the material and work to be
performed.
2. Keep all files racked and separated so they do not bear
against each other.
3. Keep the files in a dry place—rust corrodes the teeth
points, dulling the file.
4. Keep files clean. Tap the end of the file against the
bench after every few strokes to loosen and clear the
filings. Use the file card to keep files clean—a dirty
file is a dull file. A dirty file can also contaminate
different metals when the same file is used on multiple
metal surfaces.
Particles of metal collect between the teeth of a file and may
make deep scratches in the material being filed. When these
particles of metal are lodged too firmly between the teeth and
cannot be removed by tapping the edge of the file, remove
them with a file card or wire brush. Draw the brush across
the file so that the bristles pass down the gullet between the
teeth. [Figure 11-18]
Drills
The four types of portable drills used in aviation for holding
and turning twist drills are the hand drill, breast drill, electric
power drill, and pneumatic power drill. Holes 1⁄4 inch in
diameter and under can be drilled using a hand drill. This
drill is commonly called an “egg beater.” The breast drill is
designed to hold larger size twist drills than the hand drill.
Also, a breastplate is affixed at the upper end of the drill
to permit the use of body weight to increase the cutting
power of the drill. Electric and pneumatic power drills are
available in various shapes and sizes to satisfy almost any
requirement. Pneumatic drills are preferred for use around
flammable materials, since sparks from an electric drill are
a fire or explosion hazard. Twist Drills
A twist drill is a pointed tool that is rotated to cut holes in
material. It is made of a cylindrical hardened steel bar having
spiral flutes, or grooves, running the length of the body and
a conical point with cutting edges formed by the ends of
the flutes.
Twist drills are made of carbon steel or high-speed alloy steel.
Carbon steel twist drills are satisfactory for the general run
of work and are relatively inexpensive. The more expensive
high-speed twist drills are used for the tough materials, such
as stainless steels. Twist drills have from one to four spiral
flutes. Drills with two flutes are used for most drilling.
Whereas those with three or four flutes are used principally
to follow smaller drills or to enlarge holes.
The principal parts of a twist drill are the shank, the body, and
the heel. [Figure 11-19] The drill shank is the end that fits
into the chuck of a hand or power drill. The two shank shapes
most commonly used in hand drills are the straight shank and
the square or bit stock shank. The straight shank generally
is used in hand, breast, and portable electric or pneumatic
drills. The square shank is made to fit into a carpenter’s
brace. Tapered shanks generally are used in machine shop
drill presses. [Figure 11-20]
The metal column forming the core of the drill is the body.
The body clearance area lies just back of the margin. It is
slightly smaller in diameter than the margin to reduce the
friction between the drill and the sides of the hole. The angle
at which the drill point is ground is the lip clearance angle.
On standard drills used to cut steel and cast iron, the angle
should be 59° from the axis of the drill. For faster drilling of
soft materials, sharper angles are used.
The diameter of a twist drill may be given in one of three
ways: by fractions, letters, or numbers. Fractionally, they are
classified by sixteenths of an inch (from 1⁄16 to 31⁄2 inches), by
thirty-secondths (from 1⁄32 to 21⁄2 inches), or by sixty-fourths
(from 1⁄64 to 11⁄4 inches). For a more exact measurement, a
letter system is used with decimal equivalents: A (0.234 inch)
to Z (0.413 inch). The number system of classification is most
accurate: No. 80 (0.0314 inch) to No. 1 (0.228 inch). Drill
sizes and their decimal equivalents are shown in Figure 11-21 .
The twist drill should be sharpened at the first sign of
dullness. For most drilling, a twist drill with a cutting angle
of 118° (59° on either side of center) is sufficient. However,
when drilling soft metals, a cutting angle of 90° may be
more efficient.
Typical procedures for sharpening drills are as follows:
[Figure 11-22]
Square shank ( used in brace )Taper shankSize stamped hereStraight shankPoint Margin Flute
Lip clearance (125° – 135°)
Shank BodyLip or cutting edge
Heel angle (12° – 15°)
Lip angle (normally 59°)
Lip or cutting edgesHeelFlute
LandFigure 11-18. File card.
Figure 11-20. Drill types.Figure 11-19. Twist drill.1.Adjust the grinder tool rest to a convenient height for
resting the back of the hand while grinding.
2.Hold the drill between the thumb and index finger of
the right or left hand. Grasp the body of the drill near
the shank with the other hand.
3.Place the hand on the tool rest with the centerline of
the drill making a 59° angle with the cutting face ofthe grinding wheel. Lower the shank end of the drillslightly.
4.Slowly place the cutting edge of the drill against the
grinding wheel. Gradually lower the shank of the drill as you twist the drill in a clockwise direction. Maintain pressure against the grinding surface only until youreach the heel of the drill.
5.Check the res ults of grinding with a gauge to determine
whether or not the lips are the same length and at a59° angle.
Alternatively, there are commercially available twist drill grinders available, as well as attachments for bench grinders that ensure consistent, even sharpening of twist drills.
Reamers
Reamers are used to smooth and enlarge holes to exact size.
Hand reamers have square end shanks so that they can be turned with a tap wrench or similar handle. The various types of reamers are illustrated in Figure 11-23.
A hole that is to be reamed to exact size must be drilled about 0.003 to 0.007 inch undersize. A cut that removes more than 0.007 inch places too much load on the reamer and should not be attempted.
Reamers are made of either carbon tool steel or high-speed
steel. The cutting blades of a high-speed steel reamer lose their original keenness sooner than those of a carbon steel reamer; however, after the first super keenness is gone, they are still serviceable. The high-speed reamer usually lasts much longer than the carbon steel type.
Reamer blades are hardened to the point of being brittle and
11-14must be handled carefully to avoid chipping them. When
reaming a hole, rotate the reamer in the cutting direction only.
Do not back a reamer out of a hole by rotating it opposite
the cutting direction. Turn the reamer steadily and evenly to
prevent chattering, or marking and scoring of the hole walls.
Reamers are available in any standard size. The straight fluted
reamer is less expensive than the spiral fluted reamer, but
the spiral type has less tendency to chatter. Both types are
tapered for a short distance back of the end to aid in starting.
Bottoming reamers have no taper and are used to complete
the reaming of blind holes.
For general use, an expansion reamer is the most practical.
This type is furnished in standard sizes from 1⁄4 inch to 1 inch,
increasing in diameter by 1⁄32-inch increments.
Taper reamers, both hand and machine operated, are used to
smooth and true taper holes and recesses.
Countersink
A countersink is a tool that cuts a cone-shaped depression
around the hole to allow a rivet or screw to set flush with the
surface of the material. Countersinks are made with various
angles to correspond to the various angles of the countersunk
rivet and screw heads. The angle of the standard countersink
shown in Figure 11-24 is 100°.
Special stop countersinks are available. Stop countersinks
are adjustable to any desired depth, and the cutters are
interchangeable so that holes of various countersunk angles
may be made. Some stop countersinks have a micrometer set
arrangement (in increments of 0.001 inch) for adjusting the
cutting depths. [Figure 11-24]
When using a countersink, care must be taken not to remove
an excessive amount of material, since this reduces the
strength of flush joints.
Taps and Dies
A tap is used to cut threads on the inside of a hole, while a
die is for cutting external threads on round stock. They are
made of hard tempered steel and ground to an exact size.
There are four types of threads that can be cut with standard
taps and dies: National Coarse, National Fine, National Extra
Fine, and National Pipe.
Hand taps are usually provided in sets of three taps for each
diameter and thread series. Each set contains a taper tap, a
plug tap, and a bottoming tap. The taps in a set are identical
in diameter and cross section and the only difference is the
amount of taper. [Figure 11-25]
The taper tap is used to begin the tapping process, because it is tapered back for 6 to 7 threads. This tap cuts a complete
thread when it is cutting above the taper. It is the only tap
needed when tapping holes that extend through thin sections.
The plug tap supplements the taper tap for tapping holes in
thick stock.
The bottoming tap is not tapered. It is used to cut full threads
to the bottom of a blind hole.
Dies may be classified as adjustable round split die and plain
round split die. The adjustable split die has an adjusting
screw that can be tightened so that the die is spread slightly.
By adjusting the die, the diameter and fit of the thread can
be controlled. [Figure 11-26]
Solid dies are not adjustable. Therefore, a variety of thread
fits cannot be obtained with this type. There are many types
of wrenches for turning taps, as well as turning dies. The
T-handle, the adjustable tap wrench, and the diestock for round
split dies shown in Figure 11-27 are a few of the more common
types. Information on thread sizes, fits, types, and drill speeds
are shown in shown in Figure 11-28 through 11-30 .
Layout and Measuring Tools
Layout and measuring devices are precision tools. They are
carefully machined, accurately marked and, in many cases,
are made up of very delicate parts. When using these tools,
be careful not to drop, bend, or scratch them. The finished
product is no more accurate than the measurements or the
layout; therefore, it is very important to understand how to
read, use, and care for these tools.
Rules
Rules are made of steel and are either rigid or flexible. The
flexible steel rule bends, but it should not be bent intentionally
as it may be broken rather easily. In aircraft work, the unit
of measure most commonly used is the inch. The inch may
be divided into smaller parts by means of either common or
decimal fraction divisions.
The fractional divisions for an inch are found by dividing
the inch into equal parts: halves (1⁄2), quarters (1⁄4), eighths
(1⁄8), sixteenths (1⁄16), thirty-secondths (1⁄32), and sixty-fourths
(1⁄64). The fractions of an inch may be expressed in decimals,
called decimal equivalents of an inch. For example, 1⁄8
inch is expressed as 0.0125 (one hundred twenty-five ten-
thousandths of an inch).
11-15Millimeter
Fractional
NumberDecimal
Equivalent
Millimeter
Fractional
NumberDecimal
Equivalent
Millimeter
Fractional
NumberDecimal
Equivalent
Millimeter
Fractional
NumberDecimal
Equivalent
0.1 0.0039 — 0.0410 59 2.2 0.0866 — 0.1470 26
0.15 0.0059 1.05 0.0413 2.25 0.0885 3.75 0.1476
0.2 0.0079 — 0.0420 58 — 0.0890 43 — 0.1495 25
0.25 0.0098 — 0.0430 57 2.3 0.0905 3.8 0.1496
0.3 0.0118 1.1 0.0433 2.35 0.0925 — 0.1520 24
— 0.0135 80 1.15 0.0452 — 0.0935 42 3.9 0.1535
0.35 0.0138 — 0.0465 56 2.38 0.0937 ³⁄32 — — 0.1540 23
— 0.0145 79 1.19 0.0469 ³⁄64 — 2.4 0.0945 3.97 0.1562 ⁵⁄32 —
0.39 0.0156 ¹⁄64 — 1.2 0.0472 — 0.0960 41 — 0.1570 22
0.4 0.0157 1.25 0.0492 2.45 0.0964 4.0 0.1575
— 0.0160 78 1.3 0.0512 — 0.0980 40 — 0.1590 21
0.45 0.0177 — 0.0520 55 2.5 0.0984 — 0.1610 20
— 0.0180 77 1.35 0.0531 — 0.0995 39 4.1 0.1614
0.5 0.0197 — 0.0550 54 — 0.1015 38 4.2 0.1654
— 0.0200 76 1.4 0.0551 2.6 0.1024 — 0.1660 19
— 0.0210 75 1.45 0.0570 — 0.1040 37 4.25 0.1673
0.55 0.0217 1.5 0.0591 2.7 0.1063 4.3 0.1693
— 0.0225 74 — 0.0595 53 — 0.1065 36 — 0.1695 18
0.6 0.0236 1.55 0.0610 2.75 0.1082 4.37 0.1719 ¹¹⁄64 —
— 0.0240 73 1.59 0.0625 ¹⁄16 — 2.78 0.1094 ⁷⁄64 — — 0.1730 17
— 0.0250 72 1.6 0.0629 — 0.1100 35 4.4 0.1732
0.65 0.0256 — 0.0635 52 2.8 0.1102 — 0.1770 16
— 0.0260 71 1.65 0.0649 — 0.1110 34 4.5 0.1771
— 0.0280 70 1.7 0.0669 — 0.1130 33 — 0.1800 15
0.7 0.0276 — 0.0670 51 2.9 0.1141 4.6 0.1811
— 0.0292 69 1.75 0.0689 — 0.1160 32 — 0.1820 14
0.75 0.0295 — 0.7000 50 3.0 0.1181 4.7 0.1850 13
— 0.0310 68 1.8 0.0709 — 0.1200 31 4.75 0.1870
0.79 0.0312 ¹⁄32 — 1.85 0.0728 3.1 0.1220 4.76 0.1875 ³⁄16 —
0.8 0.0315 — 0.0730 49 3.18 0.1250 ¹⁄8 — 4.8 0.1890 12
— 0.0320 67 1.9 0.0748 3.2 0.1260 — 0.1910 11
— 0.0330 66 — 0.0760 48 3.25 0.1279 4.9 0.1929
0.85 0.0335 1.95 0.0767 — 0.1285 30 — 0.1935 10
— 0.0350 65 1.98 0.0781 ⁵⁄64 — 3.3 0.1299 — 0.1960 9
0.9 0.0354 — 0.0785 47 3.4 0.1338 5.0 0.1968
— 0.0360 64 2.0 0.0787 — 0.1360 29 — 0.1990 8
— 0.0370 63 2.05 0.0807 3.5 0.1378 5.1 0.2008
0.95 0.0374 — 0.0810 46 — 0.1405 28 — 0.2010 7
— 0.0380 62 — 0.0820 45 3.57 0.1406 ⁹⁄64 5.16 0.2031 ¹³⁄64 —
— 0.0390 61 2.1 0.0827 3.6 0.1417 — 0.2040 6
1.0 0.0394 2.15 0.0846 — 0.1440 27 5.2 0.2047
— 0.0400 60 — 0.0860 44 3.7 0.1457 — — 0.2055 5
Figure 11-21. Drill sizes.
11-16Millimeter
Fractional
NumberDecimal
Equivalent
Millimeter
Fractional
NumberDecimal
Equivalent
Millimeter
Fractional
NumberDecimal
Equivalent
Millimeter
FractionalDecimal
Equivalent
5.25 0.2067 7.25 0.2854 9.5 0.3740 16.5 0.6496
5.3 0.2086 7.3 0.2874 9.53 0.3750 ³⁄8 — 16.67 0.6562 ²¹⁄32
— 0.2090 4 — 0.2900 L — 0.3770 V 17.0 0.6693
5.4 0.2126 7.4 0.2913 9.6 0.3780 17.06 0.6719 ⁴³⁄64
— 0.2130 — 0.2950 M 9.7 0.3819 17.46 0.6875 ¹¹⁄16
5.5 0.2165 7.5 0.2953 9.75 0.3838 17.5 0.6890
5.56 0.2187 ¹⁄32 — 7.54 0.2968 ¹⁹⁄64 — 9.8 0.3858 17.86 0.7031 ⁴⁵⁄64
5.6 0.2205 7.6 0.2992 — 0.3860 W 18.0 0.7087
— 0.2210 2 — 0.3020 N 9.9 0.3898 18.26 0.7187 ²³⁄32
5.7 0.2244 7.7 0.3031 9.92 0.3906 ²⁵⁄64 — 18.5 0.7283
5.75 0.2263 7.75 0.3051 10.0 0.3937 18.65 0.7344 ⁴⁷⁄64
— 0.2280 1 7.8 0.3071 0.3970 X 19.0 0.7480
5.8 0.2283 7.9 0.3110 — 0.4040 Y 19.05 0.7500 ³⁄4
5.9 0.2323 7.94 0.3125 ⁵⁄16 — 10.32 0.4062 ¹³⁄32 — 19.45 0.7656 ⁴⁹⁄64
— 0.2340 A 8.0 0.3150 — 0.4130 Z 19.5 0.7677
5.95 0.2344 ¹⁵⁄64 — — 0.3160 O 10.5 0.4134 19.84 0.7812 ²⁵⁄32
6.0 0.2362 8.1 0.3189 10.72 0.4219 ²⁷⁄64 20.0 0.7874
— 0.2380 B 8.2 0.3228 11.0 0.4330 20.24 0.7969 ⁵¹⁄64
6.1 0.2401 — 0.3230 P 11.11 0.4375 ⁷⁄16 20.5 0.8071
— 0.2420 C 8.25 0.3248 11.5 0.4528 20.64 0.8125 ¹³⁄16
6.2 0.2441 8.3 0.3268 11.51 0.4531 ²⁹⁄64 21.0 0.8268
6.25 0.2460 D 8.33 0.3281 ²¹⁄64 — 11.91 0.4687 ¹⁵⁄32 21.03 0.8281 ⁵³⁄64
6.3 0.2480 8.4 0.3307 12.0 0.4724 21.43 0.8437 ²⁷⁄32
6.35 0.2500 ¹⁄4 E — 0.3320 Q 12.30 0.4843 ³¹⁄64 21.5 0.8465
6.4 0.2520 8.5 0.3346 12.5 0.4921 21.83 0.8594 ⁵⁵⁄64
6.5 0.2559 8.6 0.3386 12.7 0.5000 ¹⁄2 22.0 0.8661
— 0.2570 F — 0.3390 R 13.0 0.5118 22.23 0.8750 ⁷⁄8
6.6 0.2598 8.7 0.3425 13.10 0.5156 ³³⁄64 22.5 0.8858
— 0.2610 G 8.73 0.3437 ¹¹⁄32 — 13.49 0.5312 ¹⁷⁄32 22.62 0.8906 ⁵⁷⁄64
6.7 0.2638 8.75 0.3445 13.5 0.5315 23.0 0.9055
6.75 0.2657 ¹⁷⁄64 — 8.8 0.3465 13.89 0.5469 ³⁵⁄64 23.02 0.9062 ²⁹⁄32
6.75 0.2657 — 0.3480 S 14.0 0.5512 23.42 0.9219 ⁵⁹⁄64
— 0.2660 H 8.9 0.3504 14.29 0.5625 ⁹⁄16 23.5 0.9252
6.8 0.2677 9.0 0.3543 14.5 0.5709 23.81 0.9375 ¹⁵⁄16
6.9 0.2716 — 0.3580 T 14.68 0.5781 ³⁷⁄64 24.0 0.9449
— 0.2720 I 9.1 0.3583 15.0 0.5906 24.21 0.9531 ⁶¹⁄64
7.0 0.2756 9.13 0.3594 ²³⁄64 — 15.08 0.5937 ¹⁹⁄32 24.5 0.9646
— 0.2770 J 9.2 0.3622 15.48 0.6094 ³⁹⁄32 24.61 0.9687 ³¹⁄32
7.1 0.2795 9.25 0.3641 15.5 0.6102 25.0 0.9843
— 0.2811 K 9.3 0.3661 15.88 0.6250 ⁵⁄8 25.03 0.9844 ⁶³⁄64
7.14 0.2812 ⁹⁄32 — — 0.3680 U 16.0 0.6299 25.4 1.0000 1
7.2 0.2835 9.4 0.3701 16.27 0.6406 ⁴¹⁄64
Figure 11-21. Drill sizes (continued).
512°-15°
59° 59°
59°
Figure 11-22. Drill sharpening procedures.
Adjustable reamerExpansion reamerSpiral taper reamer (power) Straight reamer (power)Straight reamer (hand)
Taper reamer
(hand)
Figure 11-23. Reamers.
Rules are manufactured in two basic styles — those divided or
marked in common fractions and those divided or marked in decimals or divisions of one one-hundredth of an inch. A rule may be used either as a measuring tool or as a straightedge. [Figure 11-31]
Combination Sets
The combination set, as its name implies, is a tool that has
several uses. It can be used for the same purposes as an ordinary tri-square, but it differs from the tri-square in that the head slides along the blade and can be clamped at any desired place. Combined with the square or stock head are a level and scriber. The head slides in a central groove on the blade or scale, which can be used separately as a rule. [Figure 11-32]The spirit level in the stock head makes it convenient to square a piece of material with a surface and at the same time tell whether one or the other is plumb or level. The head can be used alone as a simple level.
The combination of square head and blade can also be used
as a marking gauge to scribe lines at a 45° angle, as a depth gauge, or as a height gauge. A convenient scriber is held frictionally in the head by a small brass bushing.
Adjusting screw
Adjustable round split die
Plain round split die
Figure 11-26. Types of dies.
Plug
BottomingTaper
Figure 11-25. Hand taps.
100°
100°
Cutter shaft
Cutter
Pilot
Body
Lock nut
Stop
Chip opening
Fiber collarStop countersink Standard countersink
Profile view
Top view
Figure 11-24. Countersinks.
The center head is used to find the center of shafts or other
cylindrical work. The protractor head can be used to check
angles and also may be set at any desired angle to draw lines.
Scriber
The scriber is designed to serve the aviation mechanic in
the same way a pencil or pen serves a writer. In general, it is
used to scribe or mark lines on metal surfaces. The scriber is
made of tool steel, 4 to 12 inches long, and has two needle
pointed ends. One end is bent at a 90° angle for reaching and marking through holes. [Figure 11-33]
Before using a scriber, always inspect the points for
sharpness. Be sure the straightedge is flat on the metal
and in position for scribing. Tilt the scriber slightly in the
direction toward which it will be moved, holding it like a
pencil. Keep the scriber’s point close to the guiding edge of
the straightedge. The scribed line should be heavy enough to
be visible, but no deeper than necessary to serve its purpose.
DiestockTap wrenches
Figure 11-27. Diestock and tap wrenches.
It is very important to use a scribe only where it is defining
a line to be cut as scribed lines may introduce stress points
where failures can occur.
Dividers and Pencil Compasses
Dividers and pencil compasses have two legs joined at the
top by a pivot. They are used to scribe circles and arcs and
for transferring measurements from the rule to the work.
Pencil compasses have one leg tapered to a needle point. The
other leg has a pencil or pencil lead inserted. Dividers have
both legs tapered to needle points.
When using pencil compasses or dividers, the following
procedures are suggested:
1. Inspect the points to make sure they are sharp.
2. To set the dividers or compasses, hold them with the
point of one leg in the graduations on the rule. Turn the
adjustment nut with the thumb and forefinger. Adjust
the dividers or compasses until the point of the other
leg rests on the graduation of the rule that gives the
required measurement.
3. To draw an arc or circle with either the pencil
compasses or dividers, hold the thumb attachment on
the top with the thumb and forefinger. With pressure
exerted on both legs, swing the compass in a clockwise
direction and draw the desired arc or circle.
4. The tendency for the legs to slip is avoided by inclining
the compasses or dividers in the direction in which
they are being rotated. In working on metals, the
dividers are used only to scribe arcs or circles that are
later removed by cutting. All other arcs or circles are
drawn with pencil compasses to avoid scratching the
material.
5. On paper layouts, the pencil compasses are used for describing arcs and circles. Dividers should be used to
transfer critical measurements because they are more
accurate than a pencil compass.
Calipers
Calipers are used for measuring diameters and distances or
for comparing distances and sizes. The three common types
of calipers are inside, outside, and hermaphrodite calipers,
such as gear tool calipers. [Figure 11-34]
Outside calipers are used for measuring outside dimensions—
for example, the diameter of a piece of round stock. Inside
calipers have outward curved legs for measuring inside
diameters, such as diameters of holes, the distance between
two surfaces, the width of slots, and other similar jobs. A
hermaphrodite caliper is generally used as a marking gauge in
layout work. It should not be used for precision measurement.
Micrometer Calipers
There are four types of micrometer calipers, each designed
for a specific use: outside micrometer, inside micrometer,
depth micrometer, and thread micrometer.
Micrometers are available in a variety of sizes, either 0 to
1⁄2 inch, 0 to 1 inch, 1 to 2 inch, 2 to 3 inch, 3 to 4 inch, 4 to
5 inch, or 5 to 6 inch sizes. In addition to the micrometer
inscribed with the measurement markings, micrometers
equipped with electronic digital liquid crystal display (LCD)
readouts are also in common use.
The AMT uses the outside micrometer more often than any
other type. It may be used to measure the outside dimensions
of shafts, thickness of sheet metal stock, the diameter of drills,
and for many other applications. [Figure 11-35]
The smallest measurement that can be made with the use
of the steel rule is one sixty-fourth of an inch in common
fractions and one one-hundredth of an inch in decimal
fractions. To measure more closely than this (in thousandths
and ten-thousandths of an inch), a micrometer is used. If a
dimension given in a common fraction is to be measured
with the micrometer, the fraction must be converted to its
decimal equivalent.
All four types of micrometers are read in the same way. The
method of reading an outside micrometer is discussed later
in this chapter.
Micrometer Parts
The fixed parts of a micrometer are the frame, barrel, and anvil.
The movable parts of a micrometer are the thimble and spindle.
The thimble rotates the spindle, which moves in the threaded
portion inside the barrel. Turning the thimble provides an
11-20National Coarse Thread Series
Medium Fit Class 3 (NC)
Size
and
ThreadsDiameter
of Body
for ThreadBody
DrillPreferred
Diameter
of HoleNearest
Standard
Drill SizeTap Drill
Size
and
ThreadsDiameter
of Body
for ThreadBody
DrillPreferred
Diameter
of HoleNearest
Standard
Drill SizeTap DrillNational Fine Thread Series
Medium Fit Class 3 (NF)
0-80 0.060 52 0.0472 ³⁄64"
1-64 0.073 47 0.0575 #53 1-72 0.073 47 0.0591 #53
2-56 0.086 42 0.0682 #51 2-64 0.086 42 0.0700 #50
3-48 0.099 37 0.078 ⁵⁄64 3-56 0.099 37 0.0810 #46
4-40 0.112 31 0.0866 #44 4-48 0.112 31 0.0911 #42
5-40 0.125 29 0.0995 #39 5-44 0.125 25 0.1024 #38
6-32 0.138 27 0.1063 #36 6-40 0.138 27 0.113 #33
8-32 0.164 18 0.1324 #29 8-36 0.164 18 0.136 #29
10-24 0.190 10 0.1472 #26 10-32 0.190 10 0.159 #21
12-24 0.216 2 0.1732 #17 12-28 0.216 2 0.180 #15
¹⁄4-20 0.250 ¹⁄4 0.1990 #8 ¹⁄4-28 0.250 F 0.213 #3
⁵⁄16-18 0.3125 ⁵⁄16 0.2559 #F ⁵⁄16-24 0.3125 ⁵⁄16 0.2703 I
³⁄8-16 0.375 ³⁄8 0.3110 ⁵⁄16" ³⁄8-24 0.375 ³⁄8 0.332 Q
⁷⁄16-14 0.4375 ⁷⁄16 0.3642 U ⁷⁄16-20 0.4375 ⁷⁄16 0.386 W
¹⁄2-13 0.500 ¹⁄2 0.4219 ²⁷⁄64" ¹⁄2-20 0.500 ¹⁄2 0.449 ⁷⁄16"
⁹⁄16-12 0.5625 ⁹⁄16 0.4776 ³¹⁄64" ⁹⁄16-18 0.5625 ⁹⁄16 0.506 ¹⁄2"
⁵⁄8-11 0.625 ⁵⁄8 0.5315 ¹⁷⁄64" ⁵⁄8-18 0.625 ⁵⁄8 0.568 ⁹⁄16"
³⁄4-10 0.750 ³⁄4 0.6480 ⁴¹⁄64" ³⁄4-16 0.750 ³⁄4 0.6688 ¹¹⁄16"
⁷⁄8-9 0.875 ⁷⁄8 0.7307 ⁴⁹⁄64" ⁷⁄8-14 0.875 ⁷⁄8 0.7822 ⁵¹⁄64"
1-8 1.000 1.0 0.8376 ⁷⁄8" 1-14 1.000 1.0 0.9072 ⁴⁹⁄64"
Figure 11-28. American (National) screw thread sizes.
Nominal
Size
InchesNumber
of
Threads
Per InchA
(inches)B
(inches)L2
(inches)L1
(inches)Pitch Diameter Size and Threads
Pipe
OD
(inches)Depth
of
Thread
(inches)Size
DrillMinor Diameter
Small End of PipeTap Drill for Pipe Threads
¹⁄8 27 0.36351 0.37476 0.2638 0.180 0.405 0.02963 0.33388 R
¹⁄4 18 0.47739 0.48989 0.4018 0.200 0.540 0.04444 0.43294 ⁷⁄16
³⁄8 18 0.61201 0.62701 0.4078 0.240 0.675 0.04444 0.56757 ³⁷⁄64
¹⁄2 14 0.75843 0.77843 0.5337 0.320 0.840 0.05714 0.70129 ²³⁄32
³⁄4 14 0.96768 0.98887 0.5457 0.339 1.050 0.5714 0.91054 ⁵⁹⁄64
1 11 ¹⁄2 1.21363 1.23863 0.6828 0.400 1.315 0.06957 1.14407 1 ⁵⁄32
1 ¹⁄4 11 ¹⁄2 1.55713 1.58338 0.7068 0.420 1.660 0.06957 1.48757 1 ¹⁄2
1 ¹⁄2 11 ¹⁄2 1.79609 1.82234 0.7235 0.420 1.900 0.06957 1.72652 1 ⁴⁷⁄64
2 11 ¹⁄2 2.26902 2.29627 0.7565 0.436 2.375 0.06957 2.19946 1 ⁷⁄32
2 ¹⁄2 8 2.71953 2.76216 1.1375 0.682 2.875 0.10000 2.61953 2 ⁵⁄8
3 8 3.34062 3.8850 1.2000 0.766 3.500 0.10000 3.24063 3 ¹⁄4
3 ¹⁄2 8 3.83750 3.88881 1.2500 0.821 4.000 0.10000 3.73750 3 ³⁄4
4 8 4.33438 4.38712 1.3000 0.844 4.500 0.10000 4.23438 4 ¹⁄4
Figure 11-29. American (National) pipe thread dimensions and tap drill sizes.
11-21¹⁄32⁶⁄32¹⁄64³⁄64⁵⁄64
⁷⁄64
¹⁄8¹⁄16³⁄16⁵⁄16⁷⁄16⁹⁄16
¹¹⁄16
¹³⁄16
¹⁵⁄16
¹⁄4
³⁄4⁷⁄811 1 2
³⁄8
⁵⁄8¹⁄2²⁄32⁵⁄32
³⁄32⁴⁄32
Figure 11-31. Rules.Soft
Metals
300 FPMPlastic
and Hard
Rubber
200 FPMAnnealed
Cast Iron
140 FPMMild Steel
100 FPMMalleable
Iron
90 FPMHard Cast
Iron
80 FPMTool or
Hard
Steel
60 FPMAlloy
Steel
Cast Steel
40 FPMDiameter of Drill
¹⁄16 (No. 53 – 80) 18,320 12,217 8,554 6,111 5,500 4,889 3,667 2,445
³⁄32 (No. 42 – 52) 12,212 8,142 5,702 4,071 3,666 3,258 2,442 1,649
¹⁄8 (No. 31– 41) 9,160 6,112 4,278 3,056 2,750 2,445 1,833 1,222
⁵⁄32 (No. 23 – 30) 7,328 4,888 3,420 2,444 2,198 1,954 1,465 977
³⁄16 (No. 13 – 22) 6,106 4,075 2,852 2,037 1,833 1,630 1,222 815
⁷⁄32 (No. 1– 12) 5,234 3,490 444 1,745 1,575 1,396 1,047 698
¹⁄4 (A – F) 4,575 3,055 2,139 1,527 1,375 1,222 917 611
⁹⁄32 (G – K) 4,071 2,715 1,900 1,356 1,222 1,084 814 542
⁹⁄16 (L – N) 3,660 2,445 1,711 1,222 1,100 978 7,333 489
¹¹⁄32 (O – R) 3,330 2,220 1,554 1,110 1,000 888 666 444
³⁄8 (S – U) 3,050 2,037 1,426 1,018 917 815 611 407
¹³⁄32 (V – Z) 2,818 1,878 1,316 939 846 752 563 376
⁷⁄16 2,614 1,746 1,222 873 786 698 524 349
¹⁵⁄32 2,442 1,628 1,140 814 732 652 488 326
¹⁄2 2,287 1,528 1,070 764 688 611 458 306
⁹⁄16 2,035 1,357 950 678 611 543 407 271
³⁄8 1,830 1,222 856 611 550 489 367 244
1 ¹⁄16 1,665 1,110 777 555 500 444 333 222
³⁄4 1,525 1,018 713 509 458 407 306 204
Figure 11-30. Drill speeds.
Spring outside calipersFirm joint screw
adjusting outside
calipersFirm joint screw
adjusting inside
calipersHermaphrodite calipers
Spring inside calipers
Figure 11-34. Calipers.
3 4 5 8 9 10Scriber
Stock head
Center headProtractor head Level
Figure 11-32. Combination set.
Figure 11-33. Scriber.
opening between the anvil and the end of the spindle where
the work is measured. The size of the work is indicated by the
graduations on the barrel and thimble. [Figure 11-36]
Reading a Micrometer
The lines on the barrel marked 1, 2, 3, 4, and so forth,
indicate measurements of tenths, or 0.100 inch, 0.200 inch,
0.300 inch, 0.400 inch, respectively. [Figure 11-37]
Each of the sections between the tenths divisions (between 1,
2, 3, 4, and so forth) is divided into four parts of 0.025 inch
each. One complete revolution of the thimble (from zero on
the thimble around to the same zero) moves it one of these
divisions (0.025 inch) along the barrel.
The bevel edge of the thimble is divided into 25 equal parts.
Each of these parts represents one twenty-fifth of the distance
the thimble travels along the barrel in moving from one of
the 0.025 inch divisions to another. Thus, each division on
the thimble represents one one-thousandth (0.001) of an inch.
These divisions are marked for convenience at every five
spaces by 0, 5, 10, 15, and 20. When 25 of these graduations
have passed the horizontal line on the barrel, the spindle
(having made one revolution) has moved 0.025 inch.
The micrometer is read by first noting the last visible figure
on the horizontal line of the barrel representing tenths of an
inch. Add to this the length of barrel between the thimble and
the previously noted number. (This is found by multiplying
Figure 11-35. Outside micrometers.
Anvil Spindle
Barrel
Thimble Ratchet stopThimble capThread play adjusting nut
Fixed nut
Clamp ringMicrometer screw
FrameMeasuring faces
Figure 11-36. Outside micrometer parts.the number of graduations by 0.025 inch.) Add to this the
number of divisions on the bevel edge of the thimble that
coincides with the line of the graduation. The total of the three
figures equals the measurement. [Figure 11-38]
Vernier Scale
Some micrometers are equipped with a vernier scale that
makes it possible to directly read the fraction of a division
that is indicated on the thimble scale. Typical examples of
the vernier scale as it applies to the micrometer are shown
in Figure 11-39 .
All three scales on a micrometer are not fully visible
without turning the micrometer, but the examples shown
in Figure 11-38 are drawn as though the barrel and thimble
of the micrometer were laid out flat so that all three scales
can be seen at the same time. The barrel scale is the lower
horizontal scale, the thimble scale is vertical on the right,
and the long horizontal lines (0 through 9 and 0) make up
the vernier scale.
In reading a micrometer, an excellent way to remember
the relative scale values is to remember that the 0.025 inch
barrel scale graduations are established by the lead screw (40
threads per inch). Next, the thimble graduations divide the
0.025 inch into 25 parts, each equal to 0.001 inch. Then, the
vernier graduations divide the 0.001 inch into 10 equal parts,
each equal to 0.0001 inch. Remembering the values of the
various scale graduations, the barrel scale reading is noted.
The thimble scale reading is added to it, then the vernier
scale reading is added to get the final reading. The vernier
scale line to be read is always the one aligned exactly with any thimble graduation.
In the first example in Figure 11-39 , the barrel reads
0.275 inch and the thimble reads more than 0.019 inch. The
number 1 graduation on the thimble is aligned exactly with
the number 4 graduation on the vernier scale. Thus, the final
reading is 0.2944 inch.
In the second example in Figure 11-39, the barrel reads 0.275
= 0.15005
0.100 0.025 0.025
0.100 0.025 0.025 = 0.1510.150 in.
0.151 in.
0.160 in.
0.175 in.A
C
DB1
Figure 11-38. Reading a micrometer.
0.1000.2000.300
0 1 2Barrel Thimble
Horizontal line ¹⁄10 of an inch
Figure 11-37. Micrometer measurements.inch, and the thimble reads more than 0.020 inch and less than
0.021 inch. On the Vernier scale, the number 0 graduation
coincides closest with the line on the thimble. This means
that the thimble reading would be 0.020 inch. Adding this to
the barrel reading of 0.275 inch gives a total measurement
of 0.2950 inch.
The third and fourth examples in Figure 11-39 are additional
readings that would require use of the Vernier scale for
accurate readings to ten-thousandths of an inch.
Using a Micrometer
The micrometer must be handled carefully. If it is dropped,
its accuracy may be permanently affected. Continually sliding
work between the anvil and spindle may wear the surfaces. If
the spindle is tightened too much, the frame may be sprung
permanently and inaccurate readings will result. In any
event, follow the manufacturer’s instructions for calibration
procedures and types of gauges to be used, such as gauge
blocks, gauge pins, or ring gauges.
To measure a piece of work with the micrometer, hold the
frame of the micrometer in the palm of the hand with the
little finger or third finger, whichever is more convenient.
This allows the thumb and forefinger to be free to revolve
the thimble for adjustment.
A variation of the micrometer is the dial indicator, which
measures variations in a surface by using an accurately
machined probe mechanically-linked to a circular hand whose
movement indicates thousandths of an inch or is displayed
on a LCD screen. [Figure 11-40]
A typical example would be using a dial indicator to
measure the amount of runout, or bend, in a shaft. If a bend
is suspected, the part can be rotated while resting between a
pair of machined V-blocks. A dial indicator is then clamped
to a machine table stand, and the probe of the indicator is
positioned so it lightly contacts the surface. The outer portion
09876543210Barrel scale Vernier Scale0.2944 0.2950
0.2153 0.1002Example 1 Example 2
Example 3 Example 4
Figure 11-39. Vernier scale readings.
Figure 11-40. Dial indicator.of the dial is then rotated until the needle is pointed at zero.
The part is then rotated, and the amount of bend, or run out,
is displayed on the dial as the needle fluctuates. The total
amount of the fluctuation is the runout.
Another common use for the dial indicator is to check for a
warp in a rotating component, such as a brake disc. In some
cases, this can be done with the brake disc installed on the
airplane, with the base clamped to a stationary portion of
the structure.
In either case, it is imperative that the dial indicator be
securely fastened so that movement of the indicator itself
induces no errors in measurement.
Slide Calipers
Often used to measure the length of an object, the slide
caliper provides greater accuracy than the ruler. It can, by
virtue of its specially formed jaws, measure both inside and
outside dimensions. As the tool’s name implies, the slide
caliper jaw is slid along a graduated scale, and its jaws then
contact the inside or outside of the object to be measured. The
measurement is then read on the scale located on the body
of the caliper or on the LCD screen. [Figure 11-41] Some
slide calipers also contain a depth gauge for measuring the
depth of blind holes.
11-26Figure 11-41. Electronic and dial indicator slide calipers.
Vernier caliper
Digital caliper
Dial caliper
Fundamentals of Electricity &
Electronics
Chapter 12
Introduction
This chapter addresses the fundamental concepts that are
the building blocks for advanced electrical knowledge and
practical troubleshooting. Some of the questions addressed
are: How does energy travel through a copper wire and
through space? What is electric current and electromotive
force? What makes a landing light turn on or a hydraulic
pump motor run? Each of these questions requires an
understanding of many basic principles. By adding one
basic idea on top of other basic ideas, it becomes possible
to answer most of the interesting and practical questions
about electricity or electronics.
Our understanding of electrical current must begin with the
nature of matter. All matter is composed of molecules. All
molecules are made up of atoms, which are themselves made
up of electrons, protons, and neutrons.
General Composition of Matter
Matter
Matter can be defined as anything that has mass and volume
and is the substance of which physical objects are composed.
Essentially, it is anything that can be touched. Matter is what
all things are made of; whatever occupies space, has mass,
and is perceptible to the senses in some way. Weight is an
indirect method of determining mass, but it is not the same.
Weight is a measure of the pull of gravity acting on the mass
of an object. The more mass an object has, the more it weighs
under the earth’s force of gravity. Mathematically, weight can
be stated as follows:
Weight = Mass × Gravity
Categories of matter are ordered by molecular activity.
The four categories or states are: solids, liquids, gases, and
plasma. For the purposes of the aircraft technician, only
solids, liquids, and gases are considered.
Element
An element is a substance that cannot be reduced to a
simpler form by chemical means. Iron, gold, silver, copper,
and oxygen are examples of elements. Beyond this point of
reduction, the element ceases to be what it is.Compound
A compound is a chemical combination of two or more
elements. Water is one of the most common compounds and
is made up of two hydrogen atoms and one oxygen atom.
Molecule
The smallest particle of matter that can exist and still retain
its identity, such as water (H 2O), is called a molecule.
[Figure 12-1] Substances composed of only one type of atom
are called elements. But most substances occur in nature as
compounds, that is, combinations of two or more types of
atoms. It would no longer retain the characteristics of water if
it were compounded of one atom of hydrogen and two atoms
of oxygen. If a drop of water is divided and then divided again
and again until it cannot be divided any longer, it is still water.
Atom
The atom is considered to be the most basic building block
of all matter. Atoms are composed of three subatomic
particles: protons, neutrons, and electrons. These three
particles determine the properties of the specific atoms.
Elements are substances composed of the same atoms with
specific properties. Oxygen is an example of this. The main
property that defines each element is the number of neutrons,
protons, and electrons. Hydrogen and helium are examples
of elements. Both of these elements have neutrons, protons,
and electrons but differ in the number of those items. This
difference alone accounts for the variations in chemical and
physical properties of these two different elements. There are
over 100 known elements in the periodic table. [Figure 12-2]
They are categorized according to their properties on that
table. The kinetic theory of matter also states that the
particles that make up the matter are always moving. Thermal
expansion is considered in the kinetic theory and explains
why matter contracts when it is cool and expands when it is
hot, with the exception of water/ice.
Electrons, Protons, & Neutrons
At the center of the atom is the nucleus, which contains
protons and neutrons. Protons are positively-charged
particles, and neutrons are neutrally-charged particles. A
neutron has approximately the same mass as the proton. The
third particle of the atom is the electron that is a negatively-
Oxygen atom
Hydrogen atomsNucleus Electrons
Figure 12-1. A water molecule.charged particle with a very small mass compared to the
proton. The proton’s mass is approximately 1,837 times
greater than the electron. Due to the proton and the neutron
location in the central portion of the atom (nucleus) and the
electron’s position at the distant periphery of the atom, it
is the electron that undergoes the change during chemical
reactions. Since a proton weighs approximately 1,845 times
as much as an electron, the number of protons and neutrons
in its nucleus determines the overall weight of an atom. The
weight of an electron is not considered in determining the
weight of an atom. Indeed, the nature of electricity cannot be
defined clearly because it is not certain whether the electron
is a negative charge with no mass (weight) or a particle of
matter with a negative charge.
Hydrogen represents the simplest form of an atom.
[Figure 12-3] At the nucleus of the hydrogen atom is one
proton and at the outer shell is one orbiting electron. At a
more complex level is the oxygen atoms, which has eight
electrons in two shells orbiting the nucleus with eight protons
and eight neutrons. [Figure 12-4] When the total positive
charge of the protons in the nucleus equals the total negative
charge of the electrons in orbit around the nucleus, the atom
is said to have a neutral charge.
Electron Shells & Energy Levels
Electrons require a certain amount of energy to stay in an
orbit. This particular quantity is called the electron’s energy
level. By its motion alone, the electron possesses kinetic
energy, while the electron’s position in orbit determines its
potential energy. The total energy of an electron is the main
factor that determines the radius of the electron’s orbit.
Electrons of an atom appear only at certain definite energy
levels (shells). The spacing between energy levels is such
that when the chemical properties of the various elements are
cataloged, it is convenient to group several closely spaced
permissible energy levels together into electron shells. The maximum number of electrons that can be contained in any
shell or sub-shell is the same for all atoms and is defined as
electron capacity = 2n2. In this equation, n represents the
energy level in question. The first shell can only contain two
electrons; the second shell can only contain eight electrons;
the third, 18, and so on until we reach the seventh shell for
the heaviest atoms, which have six energy levels. Because the
innermost shell is the lowest energy level, the shell begins to
fill up from the shell closest to the nucleus and fill outward
as the atomic number of the element increases. However,
an energy level does not need to be completely filled before
electrons begin to fill the next level. The Periodic Table
of Elements should be checked to determine an element’s
electron configuration.
Valence Electrons
Valence is the number of chemical bonds an atom can
form. Valence electrons are electrons that can participate in
chemical bonds with other atoms. The number of electrons
in the outermost shell of the atom is the determining factor
in its valence. Therefore, the electrons contained in this shell
are called valence electrons.
Ions
Ionization is the process by which an atom loses or gains
electrons. Dislodging an electron from an atom causes the
atom to become positively charged. This net positively-
charged atom is called a positive ion or a cation. An atom
that has gained an extra number of electrons is negatively
charged and is called a negative ion or an anion. When atoms
are neutral, the positively-charged proton and the negatively-
charged electron are equal.
Free Electrons
Valence electrons are found drifting midway between two
nuclei. Some electrons are more tightly bound to the nucleus
of their atom than others and are positioned in a shell or sphere
closer to the nucleus, while others are more loosely bound and
orbit at a greater distance from the nucleus. These outermost
electrons are called “free” electrons because they can be easily
dislodged from the positive attraction of the protons in the
nucleus. Once freed from the atom, the electron can then travel
from atom to atom, becoming the flow of electrons commonly
called current in a practical electrical circuit.
Electron Movement
Conductors, Insulators, and Semiconductors
The valence of an atom determines its ability to gain or
lose an electron, which ultimately determines the chemical
and electrical properties of the atom. These properties can
be categorized as being a conductor, semiconductor, or
insulator, depending on the ability of the material to produce
free electrons. When a material has a large number of free
Electron
Nucleus (1 Proton)
8 Protons
8 Neutrons
Figure 12-3. Hydrogen atom.
Figure 12-4. Oxygen atom.Figure 12-2. Periodic table of elements.
electrons available, a greater current can be conducted in
the material.
Conductors
Elements such as gold, copper, and silver possess many free
electrons and make good conductors. The atoms in these
materials have a few loosely bound electrons in their outer
orbits. Energy in the form of heat can cause these electrons
in the outer orbit to break loose and drift throughout the
material. Copper and silver have one electron in their outer
orbits. At room temperature, a piece of silver wire has billions of free electrons.
Insulators
Insulators are materials that do not conduct electrical current
very well or not at all, such as glass, ceramic, and plastic.
12-4Under normal conditions, atoms in these materials do not
produce free electrons. The absence of free electrons means
that electrical current cannot be conducted through the
material. Only when the material is in an extremely strong
electrical field will the outer electrons be dislodged. This
action is called breakdown and usually causes physical
damage to the insulator.
Semiconductors
The material properties of semiconductors fall in between
conductors and insulators. In their pure state, they are not
good at conducting or insulating. Semiconductors can operate
like a conductor or insulator depending on what external load
is placed on the material. Semiconductors are used to make
transistors and integrated circuits. Silicon and germanium
are the most widely used semiconductor materials. For a
more detailed explanation on this topic, refer to page 12-98
in this chapter.
Metric Based Prefixes Used for Electrical
Calculations
In any system of measurements, a single set of units is usually
not sufficient for all the computations involved in electrical
repair and maintenance. Small distances, for example, can
usually be measured in inches, but larger distances are
more meaningfully expressed in feet, yards, or miles. Since
electrical values often vary from numbers that are a millionth
part of a basic unit of measurement to very large values, it is
often necessary to use a wide range of numbers to represent
the values of units, such as volts, amperes, or ohms. A series
of prefixes that appear with the name of the unit have been
devised for the various multiples or submultiples of the basic
units. There are 12 of these prefixes, which are also known as
conversion factors. Four of the most commonly used prefixes
in electrical work are:
Mega (M) means one million (1,000,000).
Kilo (k) means one thousand (1,000).
Milli (m) means one-thousandth (1⁄1,000).
Micro (μ) means one-millionth (1⁄1,000,000).
Kilo is one of the most extensively used conversion factors. It
explains the use of prefixes with basic units of measurement.
Kilo means 1,000, and when used with volts, is expressed as
kilovolt, meaning 1,000 volts. The symbol for kilo is the letter
“k.” Thus, 1,000 volts is one kilovolt or 1 kV . Conversely,
one volt would equal one-thousandth of a kV , or 1 ⁄1,000 kV .
This could also be written 0.001 kV .
Similarly, the word “milli” means one-thousandth, and
thus, 1 millivolt equals one-thousandth (1⁄1000) of a volt.
Figure 12-5 contains a complete list of the multiples used to express electrical quantities, together with the prefixes and
symbols used to represent each number.
Static Electricity
Electricity is often described as being either static or dynamic.
The difference between the two is based simply on whether
the electrons are at rest (static) or in motion (dynamic). Static
electricity is a buildup of an electrical charge on the surface
of an object. It is considered “static” due to the fact that there
is no current flowing as in alternate current (AC) or direct
current (DC) electricity. Static electricity is usually caused
when non-conductive materials, such as rubber, plastic, or
glass, are rubbed together causing a transfer of electrons,
which results in an imbalance of charges between the two
materials. The fact that there is an imbalance of charges
between the two materials means that the objects will exhibit
an attractive or repulsive force.
Attractive and Repulsive Forces
One of the most fundamental laws of static electricity, as
well as magnetics, deals with attraction and repulsion. Like
charges repel each other and unlike charges attract each other.
All electrons possess a negative charge and as such repel each
other. Similarly, all protons possess a positive charge and
as such repel each other. Electrons (negative) and protons
(positive) are opposite in their charge and attract each other.
For example, if two pith balls are suspended, as shown in
Figure 12-6 , and each ball is touched with the charged glass
rod, some of the charge from the rod is transferred to the balls.
The balls now have similar charges and, consequently, repel
each other as shown in part B of Figure 12-6 . If a plastic rod
is rubbed with fur, it becomes negatively charged and the
fur is positively charged. By touching each ball with these
differently charged sources, the balls obtain opposite charges
and attract each other as shown in part C of Figure 12-6 .
Although most objects become charged with static electricity
by means of friction, a charged substance can also influence
objects near it by contact. [Figure 12-7] If a positively-charged
rod touches an uncharged metal bar, it draws electrons from
the uncharged bar to the point of contact. Some electrons enter
the rod, leaving the metal bar with a deficiency of electrons
(positively charged) and making the rod less positive than it
was or, perhaps, even neutralizing its charge completely.
A method of charging a metal bar by induction is demonstrated
in Figure 12-8 . A positively-charged rod is brought near, but
does not touch, an uncharged metal bar. Electrons in the metal
bar are attracted to the end of the bar nearest the positively-
charged rod, leaving a deficiency of electrons at the opposite
end of the bar. If this positively-charged end is touched by
a neutral object, electrons will flow into the metal bar and
12-5Number Prefix Symbol
1,000,000,000,000 tera t
1,000,000,000 giga g
1,000,000 mega M
1,000 kilo k
100 hecto h
10 deka dk
0.1 deci d
0.01 centi c
0.001 milli m
0.000001 micro µ
0.000000001 nano n
0.000000000001 pico pFigure 12-5. Prefixes and symbols for multiples of basic quantities.neutralize the charge. The metal bar is left with an overall
excess of electrons.
Electrostatic Field
A field of force exists around a charged body. This field is
an electrostatic field (sometimes called a dielectric field) and
is represented by lines extending in all directions from the
charged body and terminating where there is an equal and
opposite charge.
To explain the action of an electrostatic field, lines are used
to represent the direction and intensity of the electric field
of force. As illustrated in Figure 12-9 , the intensity of the
field is indicated by the number of lines per unit area, and
the direction is shown by arrowheads on the lines pointing
in the direction in which a small test charge would move (or
tend to move) if acted upon by the field of force.
Either a positive or negative test charge can be used, but it
has been arbitrarily agreed that a small positive charge is
always used in determining the direction of the field. Thus,
the direction of the field around a positive charge is always
away from the charge because a positive test charge would
be repelled. [Figure 12-9] On the other hand, the direction of
the lines about a negative charge is toward the charge, since
a positive test charge is attracted toward it.
Figure 12-10 illustrates the field around bodies having like
charges. Positive charges are shown, but regardless of the
type of charge, the lines of force would repel each other if the
charges were alike. The lines terminate on material objects
and always extend from a positive charge to a negative
charge. These are imaginary lines used to show the direction
a real force takes.
It is important to know how a charge is distributed on an
object. Figure 12-11 shows a small metal disk on which a concentrated negative charge has been placed. By using
an electrostatic detector, it can be shown that the charge is
spread evenly over the entire surface of the disk. Since the
metal disk provides uniform resistance everywhere on its
surface, the mutual repulsion of electrons results in an even
distribution over the entire surface.
Another example, shown in Figure 12-12 , is the charge on
a hollow sphere. Although the sphere is made of conducting
material, the charge is evenly distributed over the outside
surface. The inner surface is completely neutral. This
phenomenon is used to safeguard operating personnel of the
large Van de Graaff static generators used for atom smashing.
The safest area for the operators is inside the large sphere,
where millions of volts are being generated.
The distribution of the charge on an irregularly-shaped object
differs from that on a regularly-shaped object. Figure 12-13
shows that the charge on such objects is not evenly
distributed. The greatest charge is at the points, or areas of
sharpest curvature, of the objects.
Electrostatic Discharge (ESD) Considerations
One of the most frequent causes of damage to a solid-
state component or integrated circuits is the electrostatic
discharge (ESD) from the human body when one of these
devices is handled. Careless handling of line replaceable
units (LRUs), circuit cards, and discrete components can
cause unnecessarily time consuming and expensive repairs.
This damage can occur if a technician touches the mating
pins for a card or box. Other sources for ESD can be the top
of a toolbox that is covered with a carpet. Damage can be
avoided by discharging the static electricity from your body
by touching the chassis of the removed box, by wearing
a grounding wrist strap, and exercising good professional
handling of the components in the aircraft. This can include
placing protective caps over open connectors and not placing
an ESD-sensitive component in an environment that causes
damage. Parts that are ESD sensitive are typically shipped
in bags specially designed to protect components from
electrostatic damage.
Other precautions that should be taken with working with
electronic components are:
1. Always connect a ground between test equipment and
circuit before attempting to inject or monitor a signal.
2. Ensure test voltages do not exceed maximum
allowable voltage for the circuit components and
transistors.
3. Ohmmeter ranges that require a current of more than
one milliampere in the test circuit should not be used
for testing transistors.
12-6A
B
CPith ballsCharge rod
Repulsion
Attraction
Electrons are attracted
by positive chargePositively-charged
rod almost touching
uncharged bar
The rod is now less
positively chargedWhen rod touches bar,
electrons enter rod
Metal bar now has
positive charge
Electrons are attracted
toward charged rod
Rod is removed and
excess electrons remainElectrons are attracted off
finger and enter bar
Finger is removed. Positive
and negative charges are
mostly neutralizedFigure 12-6. Reaction of like and unlike charges.Figure 12-7. Charging by contact.
Figure 12-8. Charging a bar by induction.4. The heat applied to a diode or transistor, when
soldering is required, should be kept to a minimum
by using low-wattage soldering irons and heat sinks.
5. Do not pry components of a circuit board.
6. Power must be removed from a circuit before replacing
a component.
7. When using test probes on equipment and the space
between the test points is very close, keep the exposed
portion of the leads as small as possible to prevent
shorting.
Magnetism
Magnetism is defined as the property of an object to attract
certain metallic substances. In general, these substances are
ferrous materials; that is, materials composed of iron or iron
Inside surface neutral
Greatest charge
Figure 12-9. Direction of electric field around positive and negative
charges.
Figure 12-12. Charge on a hollow sphere.
Figure 12-13. Charge on irregularly-shaped objects.Figure 12-10. Field around two positively-charged bodies.
Figure 12-11. Even distribution of charge on metal disk.alloys, such as soft iron, steel, and alnico. These materials,
sometimes called magnetic materials, include at least three
nonferrous materials: nickel, cobalt, and gadolinium, which
are magnetic to a limited degree. All other substances are
considered nonmagnetic. A few of these non-magnetic
substances can be classified as diamagnetic since they are
repelled by both poles of a magnet.
Magnetism is an invisible force, the ultimate nature of which
has not been fully determined. It can best be described by
the effects it produces. Examination of a simple bar magnet
similar to that illustrated in Figure 12-14 discloses some basic
characteristics of all magnets. If the magnet is suspended to swing freely, it aligns itself with the earth’s magnetic poles.
One end is labeled “N,” meaning the North seeking end or
pole of the magnet. If the “N” end of a compass or magnet
is referred to as North seeking rather than North, there is no
conflict in referring to the pole it seeks, which is the North
magnetic pole. The opposite end of the magnet, marked “S”
is the South seeking end and points to the South magnetic
pole. Since the earth is a giant magnet, its poles attract
the ends of the magnet. These poles are not located at the
geographic poles.
The somewhat mysterious and completely invisible force
of a magnet depends on a magnetic field that surrounds the
magnet. [Figure 12-15] This field always exists between
the poles of a magnet and arranges itself to conform to the
shape of any magnet.
The theory that explains the action of a magnet holds that each
molecule making up the iron bar is itself a tiny magnet, with
both North and South poles as illustrated in Figure 12-16A .
These molecular magnets each possess a magnetic field,
but in an unmagnetized state, the molecules are arranged at
random throughout the iron bar. If a magnetizing force, such
as stroking with a lodestone, is applied to the unmagnetized
North Pole
South
Pole
Lines of force
N
S
N SFigure 12-14. One end of magnetized strip points to the magnetic
North pole.
Figure 12-15. Magnetic field around magnets.bar, the molecular magnets rearrange themselves in line with
the magnetic field of the lodestone, with all North ends of the
magnets pointing in one direction and all South ends in the
opposite direction. [Figure 12-16B] In such a configuration,
the magnetic fields of the magnets combine to produce the
total field of the magnetized bar.
When handling a magnet, avoid applying direct heat,
hammering, or dropping it. Heating or sudden shock creates
misalignment of the molecules, causing the strength of a
magnet to decrease. When a magnet is to be stored, devices
known as “keeper bars” are installed to provide an easy path
for flux lines from one pole to the other. This promotes the
retention of the molecules in their North-South alignment.
The presence of the magnetic force or field around a magnet
can best be demonstrated by the experiment illustrated in
Figure 12-17 . A sheet of transparent material, such as glass
or Lucite™, is placed over a bar magnet and iron filings are
sprinkled slowly on this transparent shield. If the glass or
Lucite™ is tapped lightly, the iron filings arrange themselves
in a definite pattern around the bar, forming a series of lines
from the North to South end of the bar to indicate the pattern
of the magnetic field.
As shown, the field of a magnet is made up of many individual
forces that appear as lines in the iron filing demonstration.
Although they are not “lines” in the ordinary sense, this word
is used to describe the individual nature of the separate forces
making up the entire magnetic field. These lines of force are
also referred to as magnetic flux.
They are separate and individual forces, since one line will
never cross another; indeed, they actually repel one another.
They remain parallel to one another and resemble stretched
rubber bands, since they are held in place around the bar by
the internal magnetizing force of the magnet.
The demonstration with iron filings further shows that the
magnetic field of a magnet is concentrated at the ends of
the magnet. These areas of concentrated flux are called the
North and South poles of the magnet. There is a limit to the
number of lines of force that can be crowded into a magnet of
a given size. When a magnetizing force is applied to a piece
of magnetic material, a point is reached where no more lines
of force can be induced or introduced. The material is then
said to be saturated.
The characteristics of the magnetic flux can be demonstrated
by tracing the flux patterns of two bar magnets with like poles
together. [Figure 12-18] The two like poles repel one another
because the lines of force will not cross each other. As the
arrows on the individual lines indicate, the lines turn aside as the two like poles are brought near each other and travel in a
path parallel to each other. Lines moving in this manner repel
each other, causing the magnets as a whole to repel each other.
By reversing the position of one of the magnets, the attraction
of unlike poles can be demonstrated. [Figure 12-19] As the
unlike poles are brought near each other, the lines of force
rearrange their paths and most of the flux leaving the North
pole of one magnet enters the South pole of the other. The
tendency of lines of force to repel each other is indicated by
the bulging of the flux in the air gap between the two magnets.
To further demonstrate that lines of force do not cross
one another, a bar magnet and a horseshoe magnet can be
positioned to display a magnetic field similar to that of
Unmagnetized Magnetized
N S
N SN S
N SN S
N SN S
N S
A B
N
SN
S
N
SN S N
SNS
N
SSNS
NSN
SN
Figure 12-16. Arrangement of molecules in a piece of magnetic material.
of one. The nonferrous metals with a permeability greater
than one, such as nickel and cobalt, are called paramagnetic.
The term ferromagnetic is applied to iron and its alloys,
which have by far the greatest permeability. Any substance,
such as bismuth, having a permeability of less than one, is
considered diamagnetic.
Reluctance, the measure of opposition to the lines of force
through a material, can be compared to the resistance of an
electrical circuit. The reluctance of soft iron, for instance, is
much lower than that of air. Figure 12-24 demonstrates that a
piece of soft iron placed near the field of a magnet can distort
the lines of force, which follow the path of lowest reluctance
through the soft iron.
The magnetic circuit can be compared in many respects
to an electrical circuit. The magnetomotive force, causing
lines of force in the magnetic circuit, can be compared to
the electromotive force (emf) or electrical pressure of an
electrical circuit. The magnetomotive force is measured in
gilberts, symbolized by the capital letter “F.” The symbol for
the intensity of the lines of force, or flux, is the Greek letter
phi, and the unit of field intensity is the gauss. An individual
line of force, called a maxwell, in an area of one square
centimeter produces a field intensity of one gauss. Using
reluctance rather than permeability, the law for magnetic
circuits can be stated: a magnetomotive force of one gilbert
causes one maxwell, or line of force, to be set up in a material
when the reluctance of the material is one.
Types of Magnets
Magnets are either natural or artificial. Since naturally
occurring magnets or lodestones have no practical use, all
magnets considered in this chapter are artificial or manmade.
Artificial magnets can be further classified as permanent
magnets, which retain their magnetism long after the
magnetizing force has been removed, and temporary magnets,
which quickly lose most of their magnetism when the external
magnetizing force is removed.Figure 12-20 . The magnetic fields of the two magnets do
not combine, but are rearranged into a distorted flux pattern.
The two bar magnets may be held in the hands and the North
poles brought near each other to demonstrate the force of
repulsion between like poles. In a similar manner, the two
South poles can demonstrate this force. The force of attraction
between unlike poles can be felt by bringing a South and a
North end together. [Figure 12-21]
Figure 12-22 illustrates another characteristic of magnets.
If the bar magnet is cut or broken into pieces, each piece
immediately becomes a magnet itself, with a North and South
pole. This feature supports the theory that each molecule
is a magnet, since each successive division of the magnet
produces more magnets.
Since the magnetic lines of force form a continuous loop,
they form a magnetic circuit. It is impossible to say where in
the magnet they originate or start. Arbitrarily, it is assumed
that all lines of force leave the North pole of any magnet and
enter at the South pole.
There is no known insulator for magnetic flux, or lines of
force, since they pass through all materials. However, they do
pass through some materials more easily than others. Thus,
it is possible to shield items, such as instruments, from the
effects of the flux by surrounding them with a material that
offers an easier path for the lines of force. Figure 12-23 shows
an instrument surrounded by a path of soft iron, which offers
very little opposition to magnetic flux. The lines of force
take the easier path, the path of greater permeability, and are
guided away from the instrument.
Materials such as soft iron and other ferrous metals are
said to have a high permeability, the measure of the ease
with which magnetic flux can penetrate a material. The
permeability scale is based on a perfect vacuum with a
rating of one. Air and other nonmagnetic materials are so
close to this that they are also considered to have a rating
MagnetMagnet
Tracing out a magnetic field
by means of iron filings
How the iron filings
arrange themselvesIron filings
Lucite
Figure 12-17. Tracing out a magnetic field with iron filings.
Air gap
S N
S N
N S
S NAir gap
Figure 12-18. Like poles repel.
Figure 12-19. Unlike poles attract.
N S
N
S
SNSNSNSNSNSN
RADIO
COMPASS
Soft ironFigure 12-20. Bypassing flux lines.Figure 12-22. Magnetic poles in a broken magnet.
Figure 12-23. Magnetic shield.
Repulsion Repulsion Attraction
SN
NS
NS
SN
SN
SN
Figure 12-21. Repulsion and attraction of magnet poles.Modern permanent magnets are made of special alloys that
have been found through research to create increasingly
better magnets. The most common categories of magnet
materials are made out of aluminum-nickel-cobalt
(alnicos), strontium-iron (ferrites, also known as ceramics),
neodymium-iron-boron (neo magnets), and samarium-
cobalt. Alnico, an alloy of iron, aluminum, nickel and
cobalt, is considered one of the very best. Others with
excellent magnetic qualities are alloys such as Remalloy™
and Permendur™.
The ability of a magnet to hold its magnetism varies greatly
with the type of metal and is known as retentivity. Magnets
made of soft iron are very easily magnetized but quickly
lose most of their magnetism when the external magnetizing
force is removed. The small amount of magnetism remaining,
called residual magnetism, is of great importance in such
electrical applications as generator operation.
Horseshoe magnets are commonly manufactured in two forms.
[Figure 12-25] The most common type is made from a long
bar curved into a horseshoe shape, while a variation of this
type consists of two bars connected by a third bar, or yoke.
Magnets can be made in many different shapes, such as balls,
cylinders, or disks. One special type of magnet is the ring
magnet, or Gramme ring, often used in instruments. This is
a closed loop magnet, similar to the type used in transformer
cores, and is the only type that has no poles.Sometimes special applications require that the field of force
lie through the thickness rather than the length of a piece of
metal. Such magnets are called flat magnets and are used as
pole pieces in generators and motors.
S N
Soft iron
NN
N
SS
S
N S
Figure 12-24. Effect of a magnetic substance in a magnetic field.Figure 12-25. Two forms of horseshoe magnets.Electromagnetism
In 1820, the Danish physicist, Hans Christian Oersted,
discovered that the needle of a compass brought near a current
carrying conductor would be deflected. When the current flow
stopped, the compass needle returned to its original position.
This important discovery demonstrated a relationship between
electricity and magnetism that led to the electromagnet and to
many of the inventions on which modern industry is based.
Oersted discovered that the magnetic field had no connection
with the conductor in which the electrons were flowing,
because the conductor was made of nonmagnetic copper. The
electrons moving through the wire created the magnetic field
around the conductor. Since a magnetic field accompanies a
charged particle, the greater the current flow, the greater the
magnetic field. Figure 12-26 i llustrates the magnetic field
around a current carrying wire. A series of concentric circles
around the conductor represent the field, which if all the lines
were shown would appear more as a continuous cylinder of
such circles around the conductor.
As long as current flows in the conductor, the lines of force
remain around it. [Figure 12-27] If a small current flows
through the conductor, there will be a line of force extending
out to circle A. If the current flow is increased, the line of
force increases in size to circle B, and a further increase in
current expands it to circle C. As the original line (circle) of
force expands from circle A to B, a new line of force appears
at circle A. As the current flow increases, the number of
circles of force increases, expanding the outer circles farther
from the surface of the current carrying conductor.
If the current flow is a steady nonvarying direct current, the
magnetic field remains stationary. When the current stops,
the magnetic field collapses and the magnetism around the
conductor disappears.
A compass needle is used to demonstrate the direction of
the magnetic field around a current carrying conductor.
Figure 12-28A shows a compass needle positioned at right angles to, and approximately one inch from, a current
carrying conductor. If no current were flowing, the North
seeking end of the compass needle would point toward the
earth’s magnetic pole. When current flows, the needle lines
itself up at right angles to a radius drawn from the conductor.
Since the compass needle is a small magnet, with lines of
force extending from South to North inside the metal, it turns
until the direction of these lines agrees with the direction
of the lines of force around the conductor. As the compass
needle is moved around the conductor, it maintains itself in
a position at right angles to the conductor, indicating that the
magnetic field around a current carrying conductor is circular.
As shown in Figure 12-28B , when the direction of current
flow through the conductor is reversed, the compass needle
points in the opposite direction, indicating the magnetic field
has reversed its direction.
A method used to determine the direction of the lines of force
when the direction of the current flow is known is shown in
Figure 12-29 . If the conductor is grasped in the left hand,
with the thumb pointing in the direction of current flow, the
fingers will be wrapped around the conductor in the same
direction as the lines of the magnetic field. This is called the
left-hand rule.
Although it has been stated that the lines of force have direction,
this should not be construed to mean that the lines have motion
in a circular direction around the conductor. Although the
lines of force tend to act in a clockwise or counterclockwise
direction, they are not revolving around the conductor.
Since current flows from negative to positive, many
illustrations indicate current direction with a dot symbol
on the end of the conductor when the electrons are flowing
toward and a plus sign when the current is flowing away
from the observer. [Figure 12-30]
When a wire is bent into a loop and an electric current flows
through it, the left-hand rule remains valid. [Figure 12-31]
If the wire is coiled into two loops, many of the lines of force
become large enough to include both loops. Lines of force
Magnetic fields
Conductor
Magnetic lines
Conductor
A
B
C
Magnetic field
Compass
Current carrying conductorChecking direction of magnetic field,
using N S compass
and left-hand rule
A BFigure 12-26. Magnetic field formed around a conductor in which
current is flowing.Figure 12-27. Expansion of magnetic field as current increases.
Figure 12-28. Magnetic field around a current-carrying conductor.go through the loops in the same direction, circle around the
outside of the two coils, and come in at the opposite end.
[Figure 12-32]
When a wire contains many such loops, it is called a coil. The
lines of force form a pattern through all the loops causing
a high concentration of flux lines through the center of the
coil. [Figure 12-33]
In a coil made from loops of a conductor, many of the
lines of force are dissipated between the loops of the coil.
By placing a soft iron bar inside the coil, the lines of force
are concentrated in the center of the coil, since soft iron
has a greater permeability than air. [Figure 12-34] This
combination of an iron core in a coil of wire loops, or turns,
is called an electromagnet, since the poles (ends) of the coil
possess the characteristics of a bar magnet.
The addition of the soft iron core does two things for the
current carrying coil. First, the magnetic flux is increased.
Second, the flux lines are more highly concentrated.
When direct current flows through the coil, the core becomes
magnetized with the same polarity (location of North and
South poles) as the coil would have without the core. If the
current is reversed, the polarity is also reversed.
The polarity of the electromagnet is determined by the
left-hand rule in the same manner as the polarity of the
coil without the core was determined. If the coil is grasped
in the left hand in such a manner that the fingers curve
Direction
of field
Direction of current
Dot indicates current is
flowing out of the conductorCross indicates current is
flowing into the conductor
+Figure 12-29. Left-hand rule.
Figure 12-30. Direction of current flow in a conductor. Figure 12-31. Magnetic field around a looped conductor.around the coil in the direction of electron flow (minus to
plus), the thumb points in the direction of the North pole.
[Figure 12-35]
The strength of the magnetic field of the electromagnet
can be increased by either increasing the flow of current or
the number of loops in the wire. Doubling the current flow
approximately doubles the strength of the field. In a similar
manner, doubling the number of loops approximately doubles
magnetic field strength. Finally, the type of metal in the core
is a factor in the field strength of the electromagnet.
A soft iron bar is attracted to either pole of a permanent
magnet and, likewise, is attracted by a current carrying coil.
The lines of force extend through the soft iron, magnetizing
it by induction and pulling the iron bar toward the coil. If
the bar is free to move, it is drawn into the coil to a position
near the center where the field is strongest. [Figure 12-36]
Electromagnets are used in electrical instruments, motors,
generators, relays, and other devices. Some electromagnetic
devices operate on the principle that an iron core held away
from the center of a coil is rapidly pulled into a center position
when the coil is energized. This principle is used in the
solenoid, also called solenoid switch or relay, in which the
iron core is spring-loaded off center and moves to complete
a circuit when the coil is energized.Conventional Flow & Electron Flow
Today’s technician will find that there are two competing
schools of thought and analytical practices regarding the flow
of electricity. The two are called the conventional current
theory and the electron theory.
Conventional Flow
Of the two, the conventional current theory was the first to
be developed and, through many years of use, this method
has become ingrained in electrical publications. The theory
was initially advanced by Benjamin Franklin who reasoned
that current flowed out of a positive source into a negative
source or an area that lacked an abundance of charge. The
notation assigned to the electric charges was positive (+)
for the abundance of charge and negative (−) for a lack of
charge. It then seemed natural to visualize the flow of current
as being from the positive (+) to the negative (−).
Electron Flow
Later discoveries were made that proved just the opposite
is true. Electron flow is what actually happens when an
abundance of electrons flow out of the negative (−) source
to an area that lacks electrons or the positive (+) source. Both
conventional flow and electron flow are used in industry.
Many publications in current use employ both electron flow
and conventional flow methods. From the practical standpoint
of the technician troubleshooting a system, it makes little
to no difference which way current is flowing as long as it
is used consistently in the analysis. The Federal Aviation
Administration (FAA) officially defines current flow using
electron theory (negative to positive).
Electromotive Force (Voltage)
Unlike current, which is easy to visualize as a flow, voltage
is a variable that is determined between two points. Often,
we refer to voltage as a value across two points. It is the
electromotive force (emf) or the push or pressure felt in a
Direction of
current
S NFigure 12-32. Magnetic field around a conductor with two loops.
Figure 12-33. Magnetic field of a coil.
S N
Figure 12-34. Electromagnet.
12-16S N
N S
N SFigure 12-35. Left-hand rule applied to a coil.
Figure 12-36. Solenoid with iron core.conductor that ultimately moves the electrons in a flow. The
symbol for emf is the capital letter “E.”
Across the terminals of the typical aircraft battery, voltage
can be measured as the potential difference of 12 volts or
24 volts. That is to say that between the two terminal posts
of the battery, there is an emf of 12 or 24 volts available to
push current through a circuit. Relatively free electrons in
the negative terminal move toward the excessive number
of positive charges in the positive terminal. Recall from the
discussion on static electricity that like charges repel each
other but opposite charges attract each other. The net result
is a flow or current through a conductor. There cannot be a
flow in a conductor unless there is an applied voltage from
a battery, generator, or ground power unit. The potential
difference, or the voltage across any two points in an electrical
system, can be determined by:
Where
E = E
Q
E = Potential difference in volts
E = Energy expanded or absorbed in joules (J)
Q = Charge measured in coulombs
Figure 12-37 illustrates the flow of electrons of electric
current. Two interconnected water tanks demonstrate that when a difference of pressure exists between the two tanks,
water flows until the two tanks are equalized. The illustration
shows the level of water in tank A to be at a higher level,
reading 10 psi (higher potential energy) than the water level
in tank B, reading 2 psi (lower potential energy). Between
the two tanks, there is 8-psi potential difference. If the valve
in the interconnecting line between the tanks is opened,
water flows from tank A into tank B until the level of water
(potential energy) of both tanks is equalized. It is important
to note that it was not the pressure in tank A that caused the
water to flow; rather, it was the difference in pressure between
tank A and tank B that caused the flow.
This comparison illustrates the principle that electrons move,
when a path is available, from a point of excess electrons
(higher potential energy) to a point deficient in electrons
(lower potential energy). The force that causes this movement
is the potential difference in electrical energy between the
two points. This force is called the electrical pressure or
the potential difference or the electromotive force (electron
moving force).
Current
Electrons in motion make up an electric current. This electric
current is usually referred to as “current” or “current flow,”
no matter how many electrons are moving. Current is a
measurement of a rate at which a charge flows through some
region of space or a conductor. The moving charges are the
free electrons found in conductors, such as copper, silver,
aluminum, and gold. The term “free electron” describes a
condition in some atoms where the outer electrons are loosely
bound to their parent atom. These loosely bound electrons
can be easily motivated to move in a given direction when
an external source, such as a battery, is applied to the circuit.
These electrons are attracted to the positive terminal of the
battery, while the negative terminal is the source of the
electrons. The greater amount of charge moving through the
conductor in a given amount of time translates into a current.
Current = Charge
Time
or
I = Q
t
Where:
I = current in amperes (A)
Q = charge in coulombs (C)
T = time
A B
Figure 12-37. Difference of pressure.The System International (SI) unit for current is the ampere
(A), where
1A = 1 C
s
One ampere (A) of current is equivalent to 1 coulomb (C)
of charge passing through a conductor in 1 second. One
coulomb of charge equals 6.28 billion electrons. The symbol
used to indicate current in formulas or on schematics is the
capital letter “I.”
When current flow is one direction, it is called direct
current (DC). Later in the handbook, the form of current
that periodically oscillates back and forth within the circuit
is discussed. The present discussion is only concerned with
the use of DC.
The velocity of the charge is actually an average velocity
and is called drift velocity. To understand the idea of drift
velocity, think of a conductor in which the charge carriers
are free electrons. These electrons are always in a state of
random motion similar to that of gas molecules. When a
voltage is applied across the conductor, an emf creates an
electric field within the conductor and a current is established.
The electrons do not move in a straight direction but undergo
repeated collisions with other nearby atoms. These collisions
usually knock other free electrons from their atoms, and
these electrons move on toward the positive end of the
conductor with an average velocity called the drift velocity,
which is relatively a slow speed. To understand the nearly
instantaneous speed of the effect of the current, it is helpful
to visualize a long tube filled with steel balls as shown in
Figure 12-38 . It can be seen that a ball introduced in one end
of the tube, which represents the conductor, will immediately
cause a ball to be emitted at the opposite end of the tube.
Thus, electric current can be viewed as instantaneous, even
though it is the result of a relatively slow drift of electrons.Ohm’s Law (Resistance)
The two fundamental properties of current and voltage
are related by a third property known as resistance. In any
electrical circuit, when voltage is applied to it, a current
results. The resistance of the conductor determines the
amount of current that flows under the given voltage. In most
cases, the greater the circuit resistance, the less the current.
If the resistance is reduced, then the current increases. This
relation is linear in nature and is known as Ohm’s Law.
By having a linearly proportional characteristic, it is meant
that if one unit in the relationship increases or decreases by
a certain percentage, the other variables in the relationship
increase or decrease by the same percentage. An example
would be if the voltage across a resistor is doubled, then
the current through the resistor doubles. It should be added
that this relationship is true only if the resistance in the
circuit remains constant. If the resistance changes, current
also changes. A graph of this relationship is shown in
Figure 12-39 , which uses a constant resistance of 20Ω. The
relationship between voltage and current in this example
shows voltage plotted horizontally along the X axis in values
from 0 to 120 volts, and the corresponding values of current
are plotted vertically in values from 0 to 6.0 amperes along
the Y axis. A straight line drawn through all the points where
the voltage and current lines meet represents the equation I
= E⁄20 and is called a linear relationship.
If E = 10 V
Then 10 V
20 Ω = 0.5 A
If E = 60 V
Then 60 V
20 Ω = 3 A
If E = 120 V
Then 120V
20 Ω = 6 A
Ohm’s Law may be expressed as an equation, as follows:
Equation 1
I = E
R
I = current in amperes (A)
E = voltage (V)
R = resistance (Ω)
Where I is current in amperes, E is the potential difference
measured in volts, and R is the resistance measured in ohms.
Figure 12-38. Electron movement.If any two of these circuit quantities are known, the third
may be found by simple algebraic transposition. With this
equation, we can calculate current in a circuit if the voltage
and resistance are known. This same formula can be used to
calculate voltage. By multiplying both sides of the equation
1 by R, we get an equivalent form of Ohm’s Law, which is:
Equation 2
E = I (R)
Finally, if we divide equation 2 by I, we solve for resistance,
This relationship is true only if the resistance in the
circuit remains constant. If the resistance changes, current
also changes. A graph of this relationship is shown in
Figure 12-39 , which uses a constant resistance of 20Ω. The
relationship between voltage and current in this example
shows voltage plotted horizontally along the X axis in values
from 0 to 120 volts. The corresponding values of current are
plotted vertically in values from 0 to 6.0 amperes along the
Y axis. A straight line drawn through all the points where the
voltage and current lines meet represents the equation I = E⁄20
and is called a linear relationship.
Equation 3
R = E
I
All three formulas presented in this section are equivalent
to each other and are simply different ways of expressing
Ohm’s Law.
The various equations, which may be derived by transposing
the basic law, can be easily obtained by using the triangles
in Figure 12-40 .
The triangles containing E, R, and I are divided into two
parts, with E above the line and I × R below it. To determine
an unknown circuit quantity when the other two are known,
cover the unknown quantity with a thumb. The location of
the remaining uncovered letters in the triangle indicate the
mathematical operation to be performed. For example, to find
I, refer to Figure 12-40A , and cover I with the thumb. The
uncovered letters indicate that E is to be divided by R, or I =
E⁄R. To find R, refer to Figure 12-40B , and cover R with the
thumb. The result indicates that E is to be divided by I, or R = E⁄I. To find E, refer to Figure 12-40C , and cover E with the
thumb. The result indicates I is to be multiplied by R, or E =
I × R. This chart is useful when learning to use Ohm’s Law.
It should be used to supplement the beginner’s knowledge
of the algebraic method.
Resistance of a Conductor
While wire of any size or resistance value may be used, the
word “conductor” usually refers to materials that offer low
resistance to current flow, and the word “insulator” describes
materials that offer high resistance to current. There is no
distinct dividing line between conductors and insulators; under
the proper conditions, all types of material conduct some
current. Materials offering a resistance to current flow midway
between the best conductors and the poorest conductors
(insulators) are sometimes referred to as “semiconductors,”
and find their greatest application in the field of transistors.
The best conductors are materials, chiefly metals, which
possess a large number of free electrons; conversely,
insulators are materials having few free electrons. The best
conductors are silver, copper, gold, and aluminum; but
some nonmetals, such as carbon and water, can be used as
conductors. Materials such as rubber, glass, ceramics, and
plastics are such poor conductors that they are usually used
as insulators. The current flow in some of these materials
is so low that it is usually considered zero. The unit used
to measure resistance is called the ohm. The symbol for the
ohm is the Greek letter omega (Ω). In mathematical formulas,
the capital letter “R” refers to resistance. The resistance of a
conductor and the voltage applied to it determine the number
of amperes of current flowing through the conductor. Thus,
1 ohm of resistance limits the current flow to 1 ampere in a
conductor to which a voltage of 1 volt is applied.
Factors Affecting Resistance
1. The resistance of a metallic conductor is dependent on
the type of conductor material. It has been pointed out
that certain metals are commonly used as conductors
because of the large number of free electrons in their
outer orbits. Copper is usually considered the best
available conductor material, since a copper wire of a
particular diameter offers a lower resistance to current
flow than an aluminum wire of the same diameter.
However, aluminum is much lighter than copper, and
for this reason, as well as cost considerations, aluminum
is often used when the weight factor is important.
2. The resistance of a metallic conductor is directly
proportional to its length. The longer the length
of a given size of wire, the greater the resistance.
Figure 12-41 shows two wire conductors of different
lengths. If 1 volt of electrical pressure is applied across
10 20 30 40 50 60 70 80 90 100 110 120
Volts (E)Amperes (I)
R = 20 Ohms
(Constant)
E
I X R
E
I X R
E
I X RA
B
CTo find I (amperes),
place thumb over I
and divide E by R
as indicated.
To find R (ohms),
place thumb over
R and divide as
indicated.
To find E (volts),
place thumb over
E and multiply as
indicated.Figure 12-39. Voltage vs. current in a constant-resistance circuit.
Figure 12-40. Ohm’ s law chart.the two ends of the conductor that is 1 foot in length,
and the resistance to the movement of free electrons
is assumed to be 1 ohm, the current flow is limited
to 1 ampere. If the same size conductor is doubled in
length, the same electrons set in motion by the 1 volt
applied now find twice the resistance; consequently,
the current flow is reduced by one-half.
3. The resistance of a metallic conductor is inversely
proportional to the cross-sectional area. This area may
be triangular or even square, but is usually circular.
If the cross-sectional area of a conductor is doubled,
the resistance to current flow is reduced in half. This
is true because of the increased area in which an
electron can move without collision or capture by an
atom. Thus, the resistance varies inversely with the
cross-sectional area of a conductor.
4. The fourth major factor influencing the resistance of a
conductor is temperature. Although some substances,
such as carbon, show a decrease in resistance as the
ambient (surrounding) temperature increases, most
materials used as conductors increase in resistance as
temperature increases. The resistance of a few alloys,
such as constantan and Manganin™, change very little
as the temperature changes. The amount of increase in
the resistance of a 1 ohm sample of a conductor, per
degree rise in temperature above 0° Centigrade (C), the
assumed standard, is called the temperature coefficient
of resistance. For each metal, this is a different value.
For example, for copper the value is approximately 0.00427 ohm. Thus, a copper wire having a resistance
of 50 ohms at a temperature of 0 °C has an increase
in resistance of 50 × 0.00427, or 0.214 ohm, for each
degree rise in temperature above 0 °C. The temperature
coefficient of resistance must be considered where there
is an appreciable change in temperature of a conductor
during operation. Charts listing the temperature
coefficient of resistance for different materials are
available. Figure 12-42 shows a table for “resistivity”
of some common electric conductors.
The resistance of a material is determined by four properties:
material, length, area, and temperature. The first three
properties are related by the following equation at T = 20 °C
(room temperature):
(ρ × 1)
AR =
Where
R = resistance in ohms
ρ = resistivity of the material in circular
mil-ohms per foot
+
+2 feet
0.5 amp ( 2 ohms)
1 foot
1 amp (1 ohm)Conductor Material Resistivity (ohm meters @ 20 °C)
Silver 1.64 × 10-8
Copper 1.72 × 10-8
Aluminum 2.83 × 10-8
Tungsten 5.50 × 10-8
Nickel 7.80 × 10-8
Iron 12.0 × 10-8
Constantan 49.0 × 10-8
Nichrome II 110 × 10-8
Figure 12-41. Resistance varies with length of conductor. Figure 12-42. Resistivity table. l = length of the sample in feet
A = area in circular mils
Resistance and Relation to Wire Sizing
Circular Conductors (Wires/Cables)
Because it is known that the resistance of a conductor is directly
proportional to its length, and if we are given the resistance of
the unit length of wire, we can readily calculate the resistance
of any length of wire of that particular material having the
same diameter. Also, because it is known that the resistance of
a conductor is inversely proportional to its cross-sectional area,
and if we are given the resistance of a length of wire with unit
cross-sectional area, we can calculate the resistance of a similar
length of wire of the same material with any cross-sectional
area. Therefore, if we know the resistance of a given conductor,
we can calculate the resistance for any conductor of the same
material at the same temperature. From the relationship:
(ρ × 1)
AR =
It can also be written:
R1
R2 = 11
12 = A1
A2
If we have a conductor that is 1 meter (m) long with a cross-
sectional area of 1 (millimeter) mm2 and has a resistance of
0.017 ohm, what is the resistance of 50 m of wire from the
same material but with a cross-sectional area of 0.25 mm2?
R1
R2 = 11
12 = A1
A2
R2 = 0.017 Ω × 50 m
1 m × 1 mm2
0.25 mm2 = 3.4 Ω
While the SI units are commonly used in the analysis of
electric circuits, electrical conductors in North America are still being manufactured using the foot as the unit length and
the mil (one thousandth of an inch) as the unit of diameter.
Before using the equation R = (ρ × l)⁄A to calculate the resistance
of a conductor of a given American wire gauge (AWG) size,
the cross-sectional area in square meters must be determined
using the conversion factor 1 mil = 0.0254 mm. The most
convenient unit of wire length is the foot. Using these
standards, the unit of size is the mil-foot. Thus, a wire has
unit size if it has a diameter of 1 mil and length of 1 foot.
In the case of using copper conductors, we are spared the
task of tedious calculations by using a table as shown in
Figure 12-43 . Note that cross-sectional dimensions listed on
the table are such that each decrease of one gauge number
equals a 25 percent increase in the cross-sectional area.
Because of this, a decrease of three gauge numbers represents
an increase in cross-sectional area of approximately 2:1.
Likewise, change of ten wire gauge numbers represents a 10:1
change in cross-sectional area—also, by doubling the cross-
sectional area of the conductor, the resistance is cut in half.
A decrease of three wire gauge numbers cuts the resistance
of the conductor of a given length in half.
Rectangular Conductors (Bus Bars)
To compute the cross-sectional area of a conductor in square
mils, the length in mils of one side is squared. In the case of a
rectangular conductor, the length of one side is multiplied by
the length of the other. For example, a common rectangular
bus bar (large, special conductor) is 3⁄8 inch thick and 4 inches
wide. The 3⁄8-inch thickness may be expressed as 0.375
inch. Since 1,000 mils equal 1 inch, the width in inches can
be converted to 4,000 mils. The cross-sectional area of the
rectangular conductor is found by converting 0.375 to mils
(375 mils × 4,000 mils = 1,500,000 square mils).
Power and Energy
Power in an Electrical Circuit
This section covers power in the DC circuit and energy
consumption. Whether referring to mechanical or electrical
systems, power is defined as the rate of energy consumption
12-21AWG Number Diameter in mils Ohms per 1,000 ft.
0000 460.0 0.04901
000 409.6 0.06180
00 364.8 0.07793
0 324.9 0.09827
1 289.3 0.1239
2 257.6 0.1563
3 229.4 0.1970
4 204.3 0.2485
5 181.9 0.3133
6 162.0 0.3951
8 128.5 0.6282
10 101.9 0.9989
12 80.81 1.588
14 64.08 2.525
16 50.82 4.016
18 40.30 6.385
20 31.96 10.15
22 25.35 16.14
24 20.10 25.67
26 15.94 40.81
28 12.64 64.9
30 10.03 103.2
Figure 12-43. Conversion table when using copper conductors.or conversion within that system—that is, the amount of
energy used or converted in a given amount of time.
From the scientific discipline of physics, the fundamental
expression for power is:
E
tP =
Where
P = power measured in watts (W)
E = energy ( E is a script E) measured in joules (J)
and
t = time measured in seconds (s)
The unit measurement for power is the watt (W), which refers
to a rate of energy conversion of 1 joule (J)/second. Therefore,
the number of joules consumed in 1 second is equal to the
number of watts. A simple example is given below.
Suppose 300 joules of energy is consumed in 10 seconds.
What would be the power in watts?
General formula energy
timeP =
300 J
10 sP =
P = 30 W
The watt is named for James Watt, the inventor of the steam
engine. Watt devised an experiment to measure the power of
a horse in order to find a means of measuring the mechanical
power of his steam engine. One horsepower is required to
move 33,000 pounds 1 foot in 1 minute. Since power is the
rate of doing work, it is equivalent to the work divided by
time. Stated as a formula, this is:
33,000 ft-lb
60 sPower =
P = 550 ft-lb/s
Electrical power can be rated in a similar manner. For
example, an electric motor rated as a 1 horsepower motor
requires 746 watts of electrical energy.
Power Formulas Used in the Study of Electricity
When current flows through a resistive circuit, energy is
dissipated in the form of heat. Recall that voltage can be
expressed in the terms of energy and charge as given in the
expression:
E
QE =
Where E = potential difference in volts
E = energy expanded or absorbed in joules (J)
Q = charge measured in coulombs
Current I, can also be expressed in terms of charge and time
as given by the expression:
Charge
TimeCurrent =
or
Q
TI =
Where:
I = current in amperes (A)
Q = charge in coulombs (C)
T = time
When voltage W⁄Q and current Q⁄t are multiplied, the charge
Q is divided out leaving the basic expression from physics:
E × I = E
Q × E
t = E
t = power
For a simple DC electrical system, power dissipation
12-22can then be given by the equation:
General Power Formula P = I (E)
Where
P = Power
I = Current
E = V olts
If a circuit has a known voltage of 24 volts and a current of
2 amps, then the power in the circuit is:
P = I (E)
P = 2A (24V)
P = 48W
Now recall Ohm’s Laws that state E = I(R). If we now
substitute IR for E in the general formula, we get a formula
that uses only current I and resistance R to determine the
power in a circuit.
P = I (IR)
Second Form of Power Equation
P = I2R
If a circuit has a known current of 2 amps and a resistance
of 100 Ω, then the power in the circuit is:
P = I2R
P = (2A)2 100 Ω
P = 400 W
Using Ohm’s Law, which can be stated as I = E⁄R, we can again
make a substitution such that power can be determined by
knowing only the voltage (E) and resistance (R) of the circuit.
E
RP = ( )(E)
Third Form of Power Equation
E2
RP =
If a circuit has a known voltage of 24 volts and a resistance
of 20 Ω, then the power in the circuit is:
E2
RP =
(24 V)2
20 ΩP =
P = 28.8 WPower in a Series & Parallel Circuit
The total power dissipated in both a series and parallel circuit
is equal to the sum of the power dissipated in each resistor
in the circuit. Power is simply additive and can be stated as:
PT = P 1 + P 2 + P 3 + ……P N
Figure 12-44 provides a summary of all the possible
transpositions of the Ohm’s Law formula and the power
formula.
Energy in an Electrical Circuit
Energy is defined as the ability to do work. Because power
is the rate of energy usage, power used over a span of time
is actually energy consumption. If power and time are
multiplied together, we get energy.
The joule is defined as a unit of energy. There is another unit
of measure which is perhaps more familiar. Because power
is expressed in watts and time in seconds, a unit of energy
can be called a watt-second (Ws) or more recognizable from
the electric bill, a kilowatt-hour (kWh). Refer to Chapter 5,
Physics, for further discussion on energy.
Sources of Electricity
Electrical energy can be produced in a number of methods. The
four most common are pressure, chemical, thermal, and light.
Pressure Source
This form of electrical generation is commonly known as
piezoelectric (piezo or piez taken from Greek: to press;
pressure; to squeeze) is a result of the application of
mechanical pressure on a dielectric or non-conducting crystal.
The most common piezoelectric materials used today are
crystalline quartz and Rochelle salt. However, Rochelle salt is
being superseded by other materials, such as barium titanate.
The application of a mechanical stress produces an electric
polarization, which is proportional to this stress. This
polarization establishes a voltage across the crystal. If a
circuit is connected across the crystal, a flow of current can
be observed when the crystal is loaded (pressure is applied).
An opposite condition can occur, where an application of a
voltage between certain faces of the crystal can produce a
mechanical distortion. This effect is commonly referred to
as the piezoelectric effect.
Piezoelectric materials are used extensively in transducers
for converting a mechanical strain into an electrical signal.
Such devices include microphones, phonograph pickups, and
vibration-sensing elements. The opposite effect, in which a
mechanical output is derived from an electrical signal input,
is also widely used in headphones and loudspeakers.
E2
R
E2 PI2R
IE
IRRE
IP
I2PEP
IERP
PRP I
E R
Figure 12-44. Ohm’ s Law formula.Chemical Source
Chemical energy can be converted into electricity; the
most common form of this is the battery. A primary battery
produces electricity using two different metals in a chemical
solution like alkaline electrolyte, where a chemical reaction
between the metals and the chemicals frees more electrons
in one metal than in the other. One terminal of the battery is
attached to one of the metals, such as zinc; the other terminal
is attached to the other metal, such as manganese oxide. The
end that frees more electrons develops a positive charge
and the other end develops a negative charge. If a wire is
attached from one end of the battery to the other, electrons
flow through the wire to balance the electrical charge.
Thermal Sources
The most common source of thermal electricity found in the
aviation industry comes from thermocouples. Thermocouples
are widely used as temperature sensors. They are cheap
and interchangeable, have standard connectors, and can
measure a wide range of temperatures. Thermocouples
are pairs of dissimilar metal wires joined at least at one
end, which generate a voltage between the two wires that
is proportional to the temperature at the junction. This is
called the Seebeck effect, in honor of Thomas Seebeck who
first noticed the phenomena in 1821. It was also noticed
that different metal combinations have a different voltage
difference. Thermocouples are used to measure cylinder head
temperatures and Turbine Inlet Temperature (TIT).
Light Sources
A solar cell or a photovoltaic cell is a device that converts light energy into electricity. Fundamentally, the device
contains certain chemical elements that, when exposed to
light energy, release electrons.
Photons in sunlight are taken in by the solar panel or cell,
where they are absorbed by semiconducting materials, such
as silicon. Electrons in the cell are broken loose from their
atoms, allowing them to flow through the material to produce
electricity. The complementary positive charges that are also
created are called holes (absence of electron) and flow in the
direction opposite of the electrons in a silicon solar panel.
Solar cells have many applications and have historically been
used in earth orbiting satellites or space probes, handheld
calculators, and wrist watches.
Schematic Representation of Electrical
Components
The schematic is the most common place where the technician
finds electronic symbols. The schematic is a diagram that
depicts the interconnection and logic of an electronic or
electrical circuit. Many symbols are employed for use in
the schematic drawings, blueprints, and illustrations. This
section briefly outlines some of the more common symbols
and explains how to interpret them.
Conductors
The schematic depiction of a conductor is simple enough.
This is generally shown as a solid line. However, the line
types may vary depending on who drew the schematics and
what exactly the line represents. While the solid line is used
to depict the wire or conductor, schematics used for aircraft
modifications can also use other line types, such as a dashed
to represent “existing” wires prior to modification and solid
lines for “new” wires.
There are two methods employed to show wire crossovers
and wire connections. Figure 12-45 shows the two methods
of drawing wires that cross: version A and version B.
Figure 12-46 shows the two methods for drawing wire
that connect version A and version B. If version A in
Figure 12-45 is used to depict crossovers, then version A for
wire connections in Figure 12-46 is used. The same can then
be said about the use of version B methods. The technician
encounters both in common use.
Figure 12-47 shows a few examples of the more common
wire types that the technician encounters in schematics. They
are the single wire, single shielded, shielded twisted pair or
double, and the shielded triple. This is not an exhaustive list of
wire types but a fair representation of how they are depicted.
Figure 12-47 also shows the wires having a wire number.
These are shown for the sake of illustration and vary from one
12-24A B
A B
Single
wireConductorTypical Wire No.
SX001A22
12PB501-*APJ356
NM200A22
RF300A22Shielded
single wire
Shielded
double
wire
Shielded
triple
wire
ShieldingFigure 12-45. Unconnected crossover wires.
Figure 12-46. Connected wires.
Figure 12-47. Common wire types.installation agency to another. For further understanding of a
wire numbering system, consult the appropriate wiring guide
published by the agency that drew the prints. Regardless of
the specifics of the wire numbering system, organizations
that exercise professional wiring practices have the installed
wire marked in some manner. This is an aid to the technician
who has to troubleshoot or modify the system at a later date.
Types of Resistors
Fixed Resistor
Fixed resistors have built into the design a means of opposing
current. [Figure 12-48] The general use of a resistor in a
circuit is to limit the amount of current flow. There are a
number of methods used in construction and sizing of a
resistor that control properties, such as resistance value, the
precision of the resistance value, and the ability to dissipate
heat. While in some applications the purpose of the resistive
element is used to generate heat, such as in propeller anti-ice
boots, heat typically is the unwanted loss of energy.
Carbon Composition
The carbon composed resistor is constructed from a mixture
of finely grouped carbon/graphite, an insulation material
for filler, and a substance for binding the material together.
The amount of graphite in relation to the insulation material
determines the ohmic or resistive value of the resistor. This
mixture is compressed into a rod, which is fitted with axial
leads or “pigtails.” The finished product is sealed in an
insulating coating for isolation and physical protection.
There are other types of fixed resistors in common use.
Included in this group are:
• Carbon film
• Metal-oxide
• Metal film
• Metal glaze
The construction of a film resistor is accomplished by
depositing a resistive material evenly on a ceramic rod. This
resistive material can be graphite for the carbon film resistor,
nickel chromium for the metal film resistor, metal and glass
for the metal glaze resistor, and metal and an insulating oxide
for the metal-oxide resistor.
Resistor Ratings
Color Code
It is very difficult to manufacture a resistor to an exact standard
of ohmic values. Fortunately, most circuit requirements are
not extremely critical. For many uses, the actual resistance in
ohms can be 20 percent higher or lower than the value marked
on the resistor without causing difficulty. The percentage variation between the marked value and the actual value of
a resistor is known as the “tolerance” of a resistor. A resistor
coded for a 5 percent tolerance is not more than 5 percent
higher or lower than the value indicated by the color code.
The resistor color code is made up of a group of colors,
numbers, and tolerance values. Each color is represented by a
number, and in most cases, by a tolerance value. [Figure 12-49]
When the color code is used with the end-to-center band marking
system, the resistor is normally marked with bands of color at
one end of the resistor. The body or base color of the resistor
has nothing to do with the color code, and in no way indicates
a resistance value. To prevent confusion, this body is never
the same color as any of the bands indicating resistance value.
12-25Figure 12-48. Fixed resistor schematic.Resistor color code
Color Number Tolerance
Black 0 —
Brown 1 1%
Red 2 2%
Orange 3 3%
Yellow 4 4%
Green 5 5%
Blue 6 6%
Violet 7 7%
Gray 8 8%
White 9 9%
Gold — 5%
Silver — 10%
No color — 20%
Figure 12-49. Resistor color code.Color Band Decoding
When the end-to-center band marking system is used, either
three or four bands mark the resistor.
1. The first color band (nearest the end of the resistor)
indicates the first digit in the numerical resistance
value. This band is never gold or silver in color.
2. The second color band always indicates the second
digit of ohmic value. It is never gold or silver in color.
[Figure 12-50]
3. The third color band indicates the number of zeros to
be added to the two digits derived from the first and
second bands, except in the following two cases:
(a) If the third band is gold in color, the first two
digits must be multiplied by 10 percent.
(b) If the third band is silver in color, the first two
digits must be multiplied by 1 percent.
4. If there is a fourth color band, it is used as a multiplier
for percentage of tolerance, as indicated in the color
code chart in Figure 12-49 . If there is no fourth band,
the tolerance is understood to be 20 percent.
Figure 12-50 illustrates the rules for reading the resistance
value of a resistor marked with the end-to-center band system.
This resistor is marked with three bands of color, which must
be read from the end toward the center.
There is no fourth color band; therefore, the tolerance is
understood to be 20 percent. 20 percent of 250,000 Ω, equals
50,000 Ω.
Since the 20 percent tolerance is plus or minus,
Maximum resistance
= 250,000 Ω + 50,000 Ω
= 300,000 Ω
Minimum resistance
= 250,000 Ω − 50,000 Ω
= 200,000 Ω
The following paragraphs provide a few extra examples of
resistor color band decoding. Figure 12-51 contains a resistor
with another set of colors. This resistor code should be read
as follows:The resistance of this resistor is 86,000 ±10 percent ohms.
The maximum resistance is 94,600 ohms, and the minimum
resistance is 77,400 ohms.
Another example is the resistance of the resistor in Figure 12-52
is 960 ±5 percent ohms. The maximum resistance is 1,008
ohms, and the minimum resistance is 912 ohms.
Sometimes circuit considerations dictate that the tolerance
must be smaller than 20 percent. Figure 12-53 shows
an example of a resistor with a 2 percent tolerance. The
resistance value of this resistor is 2,500 ±2 percent ohms.
The maximum resistance is 2,550 ohms, and the minimum
resistance is 2,450 ohms.
Figure 12-54 contains an example of a resistor with a black
third color band. The color code value of black is zero, and
the third band indicates the number of zeros to be added to
the first two digits.
In this case, a zero number of zeros must be added to the first
two digits; therefore, no zeros are added. Thus, the resistance
value is 10 ±1 percent ohms. The maximum resistance is
10.1 ohms, and the minimum resistance is 9.9 ohms. There
are two exceptions to the rule stating the third color band
indicates the number of zeros. The first of these exceptions
is illustrated in Figure 12-55 . When the third band is gold in
color, it indicates that the first two digits must be multiplied
by 10 percent. The value of this resistor in this case is:
10 × 0.10 ±2% = 1 = 0.02 ohms
When the third band is silver, as is the case in Figure 12-56 ,
the first two digits must be multiplied by 1 percent. The value
GrayBlue Orange
Silver
8 6000 10%
WhiteBlue Brown
Gold
9 6 05%
RedGreen Red
Red
2 5 00 2%
BrownBlack Black
Brown
1 0 1%Figure 12-51. Resistor color code example.
Figure 12-52. Resistor color code example.
Figure 12-53. Resistor with two percent tolerance.
Figure 12-54. Resistor with black third color band.
1st Band2nd Band
3rd Band
Red Green Yellow
Color Numerical Value Significance
1st band — red 2 1st digit
2nd band — green 5 2nd digit
3rd band — yellow 4 No. of zeroes to add
Figure 12-50. End-to-center band marking.
YellowGreen Silver (Multiplier)
Silver
4 51% 10%
BrownBlack Gold (Multiplier)
Red
1 010% 2%
Figure 12-56. Resistor with a silver third band.Figure 12-55. Resistor with gold third band.of the resistor is 0.45 ±10 percent ohms.
Wire-Wound
Wire-wound resistors typically control large amounts of
current and have high power ratings. Resistors of this type are
constructed by winding a resistance wire around an insulating
rod usually made of porcelain. The windings are coated with an
insulation material for physical protection and heat conduction.
Both ends of the windings are connected to terminals, which
are used to connect the resistor to a circuit. [Figure 12-57]
A wire-wound resistor with tap is a special type of fixed
resistor that can be adjusted. These adjustments can be
made by moving a slide bar tap or by moving the tap to a
preset incremental position. While the tap may be adjustable,
the adjustments are usually set at the time of installation
to a specific value and then operated in service as a fixed
resistor. Another type of wire-wound resistor is constructed
of Manganin wire, used when high precision is needed.
Variable Resistors
Variable resistors are constructed so that the resistive value
can be changed easily. This adjustment can be manual or
automatic, and the adjustments can be made while the system
that it is connected to is in operation. There are two basic
types of manual adjustors: the rheostat and the potentiometer.
Rheostat
A rheostat is a variable resistor used to vary the amount of
current flowing in a circuit. [Figure 12-58] Figure 12-59
shows a rheostat connected in series with an ordinary
resistance in a series circuit. As the slider arm moves
from point A to B, the amount of rheostat resistance (AB)
is increased. Since the rheostat resistance and the fixed
resistance are in series, the total resistance in the circuit also
increases, and the current in the circuit decreases. On the
other hand, if the slider arm is moved toward point A, the total
resistance decreases and the current in the circuit increases.
Figure 12-57. Wire-wound resistors.
Figure 12-58. Rheostat schematic symbol.Potentiometer
The potentiometer is considered a three-terminal device.
[Figure 12-60] As illustrated, terminals 1 and 2 have the
entire value of the potentiometer resistance between them.
Terminal 3 is the wiper or moving contact. Through this wiper,
the resistance between terminals 1 and 3 or terminals 2 and 3
can be varied. While the rheostat is used to vary the current
in a circuit, the potentiometer is used to vary the voltage in a
circuit. A typical use for this component can be found in the
volume controls on an audio panel and input devices for flight
data recorders, among many other applications.
In Figure 12-61A , a potentiometer is used to obtain a variable
voltage from a fixed voltage source to apply to an electrical
load. The voltage applied to the load is the voltage between
points 2 and 3. When the slider arm is moved to point 1, the
entire voltage is applied to the electrical device (load); when
the arm is moved to point 3, the voltage applied to the load
is zero. The potentiometer makes possible the application of
any voltage between zero and full voltage to the load.
The current flowing through the circuit of Figure 12-61
leaves the negative terminal electron flow of the battery and
divides one part flowing through the lower portion of the
potentiometer (points 3 to 2) and the other part through the
load. Both parts combine at point 2 and flow through the
upper portion of the potentiometer (points 2 to 1) back to the
positive terminal of the battery. In View B of Figure 12-61 , a
potentiometer and its schematic symbol are shown.
In choosing a potentiometer resistance, the amount of current
drawn by the load should be considered, as well as the current
flow through the potentiometer at all settings of the slider
arm. The energy of the current through the potentiometer is
dissipated in the form of heat.
It is important to keep this wasted current as small as
possible by making the resistance of the potentiometer as
large as practicable. In most cases, the resistance of the
potentiometer can be several times the resistance of the load.
Figure 12-62 shows how a potentiometer can be wired to
function as a rheostat.
Linear Potentiometers
In a linear potentiometer, the resistance between both terminal
and the wiper varies linearly with the position of the wiper.
To illustrate, one quarter of a turn on the potentiometer results
in one quarter of the total resistance. The same relationship
exists when one-half or three-quarters of potentiometer
movement. [Figure 12-63]
Tapered Potentiometers
Resistance varies in a nonlinear manner in the case of the tapered potentiometer. [Figure 12-64] Keep in mind that
one-half of full potentiometer travel does not necessarily
correspond to one-half the total resistance of the potentiometer.
Thermistors
The thermistor is a type of a variable resistor that is
temperature sensitive. [Figure 12-65] This component
has a negative temperature coefficient, which means that
as the sensed temperature increases, the resistance of the
thermistor decreases.
Photoconductive Cells
The photoconductive cell is similar to the thermistor. Like the
thermistor, it has a negative temperature coefficient. Unlike
the thermistor, the resistance is controlled by light intensity.
This kind of component can be found in radio control heads
where the intensity of the ambient light is sensed through
the photoconductive cell resulting in the backlighting of the
control heads to adjust to the flight deck lighting conditions.
[Figure 12-66]
Circuit Protection Devices
Perhaps the most serious trouble in a circuit is a direct short.
The term “direct short” describes a situation in which some
point in the circuit, where full system voltage is present,
comes in direct contact with the ground or return side of the
circuit. This establishes a path for current flow that contains
no resistance other than that present in the wires carrying the
current, and these wires have very little resistance.
Most wires used in aircraft electrical circuits are small
gauge, and their current carrying capacity is quite limited.
The size of the wires used in any given circuit is determined
A
B1
3Load
ShaftTerminals
Resistive stripWiper
Rotary potentiometer construction
A
25%70 Ω
30 Ω100 Ω
B
50%30 Ω
70 Ω100 Ω
C
75%10 Ω
90 Ω100 ΩFigure 12-61. Potentiometer and schematic symbol.
Figure 12-62. Potentiometer wired to function as rheostat.
Figure 12-63. Linear potentiometer schematic.
A B
23Figure 12-59. Rheostat connected in series.
Figure 12-60. Potentiometer schematic symbol.
by the amount of current the wires are expected to carry
under normal operating conditions. Any current flow in
excess of normal, such as the case of a direct short, would
cause a rapid generation of heat. If the excessive current
flow caused by the short is left unchecked, the heat in the
wire could cause a portion of the wire to melt and at the
very least, open the circuit.
To protect aircraft electrical systems from damage and failure
caused by excessive current, several kinds of protective
devices are installed in the systems. Fuses, circuit breakers,
thermal protectors, and arc fault circuit breakers are used
for this purpose.
Circuit protective devices, as the name implies, all have a
common purpose—to protect the units and the wires in the
circuit. Some are designed primarily to protect the wiring
and to open the circuit in such a way as to stop the current
flow when the current becomes greater than the wires can
safely carry. Other devices are designed to protect a unit in
the circuit by stopping the current flow to it when the unit
becomes excessively warm.
Fuse
Fuses are used to protect the circuit from over current
conditions. [Figure 12-67A] The fuse is installed in the circuit
so that all the current in the circuit passes through it. In most
fuses, the strip of metal is made of an alloy of tin and bismuth,
which melts and opens the circuit when the current exceeds
T
A
25%70 Ω
30 Ω100 Ω
B
50%30 Ω
70 Ω100 Ω
C
75%10 Ω
90 Ω100 Ω
Figure 12-65. Schematic symbol for thermistor.Figure 12-66. Photoconductive cell schematic symbol component.
Figure 12-64. Tapered potentiometer.the rated capacity of the fuse. For example, if a 5-amp fuse
is placed into a circuit, the fuse allows currents up to 5 amps
to pass. Because the fuse is intended to protect the circuit,
it is quite important that its capacity match the needs of the
circuit in which it is used.
When replacing a fuse, consult the applicable manufacturer’s
instructions to be sure a fuse of the correct type and capacity
is installed. Fuses are installed in two types of fuse holders
in aircraft. “Plug-in holders” or in-line holders are used for
small and low capacity fuses. “Clip” type holders are used
for heavy high capacity fuses and current limiters.
Current Limiter
The current limiter is very much like the fuse. However, the
current limiter link is usually made of copper and will stand
a considerable overload for a short period of time. Like the
fuse, it opens up in an over current condition in heavy current
circuits such as 30 amp or greater. These are used primarily
to sectionalize an aircraft circuit or bus. Once the limiter is
opened, it must be replaced. The schematic symbol for the
current limiter shows two triangles pointing to each other
with a line on both sides of the triangles. [ Figure 12-67B ].
Circuit Breaker
The circuit breaker is commonly used in place of a fuse and is
designed to break the circuit and stop the current flow when
the current exceeds a predetermined value. Unlike the fuse,
the circuit breaker can be reset; whereas the fuse or current
limiter must be replaced. [Figure 12-68] There are several types of circuit breakers in general use in
aircraft systems. One is a magnetic type. When excessive
current flows in the circuit, it makes an electromagnet strong
enough to move a small armature, which trips the breaker.
Another type is the thermal overload switch or breaker.
This consists of a bimetallic strip which, when it becomes
overheated from excessive current, bends away from a catch
on the switch lever and permits the switch to trip open.
Most circuit breakers must be reset by hand. If the overload
condition still exists, the circuit breaker trips again to prevent
damage to the circuit. At this point, it is usually not advisable
to continue resetting the circuit breaker, but to initiate
troubleshooting to determine the cause. Repeated resetting
of a circuit breaker can lead to circuit or component damage
or worse, the possibility of a fire or explosion. Automatic
reset type circuit breakers are not allowed in aircraft. Circuit
breakers are commonly grouped on a circuit breaker panel that
is accessible to the flight crew. Figure 12-69 shows a circuit
breaker and a circuit breaker panel.
Arc Fault Circuit Breaker
In recent years, the arc fault circuit breaker has begun to
provide an additional layer of protection beyond that of the
thermal protection already provided by conventional circuit
breakers. The arc fault circuit breaker monitors the circuit
for an electrical arcing signature, which can indicate possible
wiring faults and unsafe conditions. These conditions can
lead to fires or loss of power to critical systems. The arc fault
circuit breaker is only beginning to make an appearance in
the aircraft industry and is not widely used like the thermal
type of circuit breaker.
Thermal Protectors
A thermal protector, or switch, is used to protect a motor. It
is designed to open the circuit automatically whenever the
temperature of the motor becomes excessively high. It has
two positions—open and closed. The most common use for
a thermal switch is to keep a motor from overheating. If a
Figure 12-68. Schematic symbol for circuit breaker.A
B
Figure 12-67. Schematic symbol for fuse (A) and current limiter (B).
malfunction in the motor causes it to overheat, the thermal
switch breaks the circuit intermittently.
The thermal switch contains a bimetallic disk, or strip, that
bends and breaks the circuit when it is heated. This occurs
because one of the metals expands more than the other when
they are subjected to the same temperature. When the strip
or disk cools, the metals contract and the strip returns to its
original position and closes the circuit.
Control Devices
Components in the electrical circuits are typically not all
intended to operate continuously or automatically. Most of
them are meant to operate at certain times, under certain
conditions, to perform very definite functions. There must
be some means of controlling their operation. Either a
switch, or a relay, or both may be included in the circuit
for this purpose.Switches
Switches control the current flow in most aircraft electrical
circuits. A switch is used to start, to stop, or to change the
direction of the current flow in the circuit. The switch in
each circuit must be able to carry the normal current of the
circuit and must be insulated heavily enough for the voltage
of the circuit. Figure 12-70 shows various switches used in
aircraft electrical systems.
An understanding of some basic definitions of the switch is
necessary before any of the switch types are discussed. The
number of poles, throws, and positions they have designates
toggle switches, as well as some other type of switches.
Pole—the switch’s movable blade or contactor. The number
of poles is equal to the number of circuits, or paths for
current flow, that can be completed through the switch at
any one time.
Throw—indicates the number of circuits, or paths for current,
that it is possible to complete through the switch with each
pole or contactor.
Positions—indicates the number of places at which the
operating device (toggle, plunger, and so forth) comes to
rest and at the same time open or close one or more circuits.
Toggle Switch
Single-Pole, Single-Throw (SPST)
The single-pole, single-throw (SPST) switch allows a
connection between two contacts. One of two conditions
exist. Either the circuit is open in one position or closed in
the other position. [Figure 12-71]
Single-Pole, Double-Throw (SPDT)
The single-pole, double-throw (SPDT) switch is shown in
Figure 12-72 . With this switch, contact between one contact
can be made between one contact and the other.
Double-Pole, Single-Throw (DPST)
The double-pole, single-throw (DPST) switch connection
can be made between one set of contacts and either of two
other sets of contacts. [Figure 12-73]
Double-Pole, Double-Throw (DPDT)
The schematic symbol for the double-pole, double-throw
(DPDT) switch is shown in Figure 12-74 . This type of switch
makes a connection from one set of contacts to either of two
other sets of contacts.
A toggle switch that is spring-loaded to the OFF position
Figure 12-69. Circuit breaker assembly for aircraft electrical
system.
Figure 12-70. Various types of switches used in modern aircraft.
and must be held in the ON position to complete the circuit
is a momentary contact two-position switch. One that comes
to rest at either of two positions, opening the circuit in one position and closing it in another, is a two-position switch. A
toggle switch that comes to rest at any one of three positions
is a three-position switch.
A switch that stays open, except when it is held in the closed
position, is a normally open switch (usually identified as
NO). One that stays closed, except when it is held in the
open position is a normally closed switch (NC). Both kinds
are spring loaded to their normal position and return to that
position as soon as they are released.
Locking toggles require the operator to pull out on the switch
toggle before moving it in to another position. Once in the new
position, the switch toggle is release back into a lock, which
then prevents the switch from inadvertently being moved.
Microswitches
A microswitch opens or closes a circuit with a very small
movement of the tripping device (1⁄16 inch or less). This is
what gives the switch its name, since micro means small.
Microswitches are usually pushbutton switches. They are
used primarily as limit switches to provide automatic control
of landing gears, actuator motors, and the like. Figure 12-75
shows a normally closed microswitch in cross-section and
illustrates how these switches operate. When the operating
plunger is pressed in, the spring and the movable contact are
pushed, opening the contacts and the circuit. Figure 12-76
shows a pushbutton microswitch.
Rotary Selector Switches
A rotary selector switch takes the place of several switches.
When the knob of the switch is rotated, the switch opens one
circuit and closes another. Ignition switches and voltmeter
selector switches are typical examples of this kind of switch.
[Figure 12-77]
Pushbutton Switches
Pushbutton switches have one stationary contact and one
Figure 12-74. Double-pole, double-throw switch schematic symbol.
Figure 12-72. Single-pole, double-throw switch schematic symbol.
Figure 12-73. Double-pole, single-throw switch schematic symbol.
Figure 12-71. Single-pole, single-throw switch schematic symbol.
movable contact. The movable contact is attached to the
pushbutton. The pushbutton is either an insulator itself or is
insulated from the contact. This switch is spring loaded and
designed for momentary contact.
Lighted Pushbutton Switches
Another more common switch found in today’s aircraft is
the lighted pushbutton switch. This type of switch takes the
form of a 5⁄8-inch to 1-inch cube with incandescent or LED
lights to indicate the function of the switch. Switch designs
come in a number of configurations; the two most common
are the alternate action and momentary action and usually
have a two-pole or four-pole switch body. Other less common switch actions are the alternate and momentary holding coil
configurations. The less known holding or latching coil switch
bodies are designed to have a magnetic coil inside the switch
body that is energized through two contacts in the base of the
switch. When the coil is energized and the switch is pressed,
the switch contacts remain latched until power is removed from
the coil. This type of design allows for some degree of remote
control over the switch body. [Figure 12-78]
The display optics of the lighted pushbutton switch provide the
crew with a clear message that is visible under a wide range of
lighting conditions with very high luminance and wide viewing
angles. While some displays are simply a transparent screen
that is backlit by an incandescent light, the higher quality
and more reliable switches are available in sunlight readable
displays and night vision (NVIS) versions. Due to the sunlight
environment of the flight deck, displays utilizing standard
lighting techniques “washout” when viewed in direct sunlight.
Sunlight readable displays are designed to minimize this effect.
Lighted pushbutton switches can also be used in applications
where a switch is not required and the optics are only for
indications. This type of an indicator is commonly called
an annunciator.
Stationary contact
Moveable contactOperating plunger
Grooved anchor Three-bladed spring
Figure 12-75. Cross-section of a microswitch.
Figure 12-76. Pushbutton microswitch.Figure 12-77. Rotary switches.
Dual In-Line Parallel (DIP) Switches
The acronym “DIP” switch is defined as Dual In-Line Parallel
switch in reference to the physical layout. DIP switches are
commonly found in card cages, and line replaceable units
(LRU) and are used in most cases to adjust gains, control
configurations, and so forth. Each one of the switches is
generally an SPST slide or rocker switch. The technician may
find this switch in packages ranging in size from DIP2 through
DIP32. Some of the more common sizes are DIP4 and DIP8.
Switch Guards
Switch guards are covers that protect a switch from
unintended operation. Prior to the operation of the switch, the
guard is usually lifted. Switch guards are commonly found
on systems such as fire suppression and override logics for
various systems. Figure 12-79A shows a traditional switch
with a guard while Figure 12-79B shows a pushbutton switch
with a guard. The guard needs to be moved before the switch
can be pushed.
Relays
A relay is simply an electromechanical switch where a small
amount of current can control a large amount of current.
[Figure 12-80] When a voltage is applied to the coil of the relay, the electromagnet is energized due to the current. When
energized, an electromagnetic field pulls the common (C) or
arm of the relay down. When the arm or common is pulled
down, the circuit between the arm and the normally closed
(NC) contacts is opened and the circuit between the arm and the
normally open (NO) contacts are closed. When the energizing
voltage is removed, the spring returns the arm contacts back to
the normally closed (NC) contacts. The relay usually has two
connections for the coil. The (+) side is designated as X1 and
the ground-side of the coil is designated as X2.
Series DC Circuits
The series circuit is the most basic electrical circuit and
provides a good introduction to basic circuit analysis. The
series circuit represents the first building block for all of
the circuits to be studied and analyzed. Figure 12-81 shows
this simple circuit with nothing more than a voltage source
or battery, a conductor, and a resistor. This is classified as a
series circuit because the components are connected end-to-
end, so that the same current flows through each component
equally. There is only one path for the current to take and the
battery and resistor are in series with each other. Next is to
make a few additions to the simple circuit in Figure 12-81 .
Figure 12-82 shows an additional resistor and a little more
12-34Figure 12-79. (A) Red guarded switch. (B) Pushbutton switch with
a guard.
A
BFigure 12-78. Lighted pushbutton switches.
detail regarding the values. With these values, we can now
begin to learn more about the nature of the circuit. In this
configuration, there is a 12-volt DC source in series with two
resistors, R 1 = 10 Ω and R 2 = 30 Ω. For resistors in a series
configuration, the total resistance of the circuit is equal to
the sum of the individual resistors. The basic formula is:
R T = R 1 + R 2 + R 3 + ………R N
For Figure 12-82 , this will be:
R T = 10 Ω + 30 Ω
R T = 40 Ω
Now that the total resistance of the circuit is known, the
current for the circuit can be determined. In a series circuit,
the current cannot be different at different points within the
circuit. The current through a series circuit is always the
same through each element and at any point. Therefore, the
current in the simple circuit can now be determined using
Ohm’s Law:
Formula, E = I (R)
Solve for current, E
RI =
The variables, E = 12 V and R T = 40 Ω
Substitute variables, 12 V
40 ΩI =
Current in circuits, I = 0.3 A
Ohm’s Law describes a relationship between the variables
of voltage, current, and resistance that is linear and easy to
illustrate with a few extra calculations. First is the act of
changing the total resistance of the circuit while the other
two remain constant. In this example, the R T of the circuit
in Figure 12-82 is doubled.
The effects on the total current in the circuit are: Formula, E = I (R)
Solve for current, E
RI =
The variables, E = 12 V and R T = 80 Ω
Substitute variables, 12 V
80 ΩI =
Current in circuits, I = 0.15 A
It can be seen quantitatively and intuitively that when the
resistance of the circuit is doubled, the current is reduced by
half the original value.
Next, reduce the R T of the circuit in Figure 12-82 to half of
its original value. The effects on the total current are:
Multiple poles
Schematic Symbol for a Relay
Relay IllustrationA2B2
A3
A1B3
B1C
C
X1
X2
Normally closed (NC)
Normally open (NO)
Magnetic fieldCommon (C) or Arm
Electromagnet pulls
contact downNC
NONC
NO
X1
X2
X1
X2(+)
(−)N
S
+
−
+
12V
30ΩR210ΩR1
−Figure 12-80. Basic relay.
Figure 12-81. Simple DC circuit.
Figure 12-82. Simple DC circuit with additional resistor. Formula, E = I (R)
Solve for current, E
RI =
The variables, E = 12 V and R T = 20 Ω
Substitute variables, 12 V
20 ΩI =
Current in circuits, I = 0.6 A
Voltage Drops & Further Application of Ohm’s Law
The example circuit in Figure 12-83 is used to illustrate the
idea of voltage drop. It is important to differentiate between voltage and voltage drop when discussing series circuits.
V oltage drop refers to the loss in electrical pressure or emf
caused by forcing electrons through a resistor. Because there
are two resistors in the example, there are separate voltage
drops. Each drop is associated with each individual resistor.
The amount of electrical pressure required to force a given
number of electrons through a resistance is proportional to
the size of the resistor.
In Figure 12-83 , the values used to illustrate the idea of
voltage drop are:
Current, I = 1 mA
R 1 = 1 kΩ
R 2 = 3 kΩ
R 3 = 5 kΩ
The voltage drop across each resistor is calculated using
Ohm’s Law. The drop for each resistor is the product of each
resistance and the total current in the circuit. Keep in mind
that the same current flows through series resistor.
Formula: E = I (R)
V oltage across R 1: E1 = I T (R1)
E1 = 1 mA (1 kΩ) = 1 volt
V oltage across R 2: E2 = I T (R2)
E2 = 1 mA (3 kΩ) = 3 volt
V oltage across R 3: E3 = I T (R3)
E3 = 1 mA (5 kΩ) = 5 volt
The source voltage can now be determined, which can then
be used to confirm the calculations for each voltage drop.
Using Ohm’s Law:
Formula: E = I (R)
Source voltage = current × the total resistance
ES = I (R T)
12-36+
5 kΩR31 kΩR1IT 1 mA
3 kΩR2
−
Figure 12-83. Example of three resistors in series. RT = 1 kΩ + 3 kΩ + 5 kΩ
RT = 9 kΩ
Now: ES = I (R T)
Substitute ES = 1 mA (9 kΩ)
ES = 9 volts
Simple checks to confirm the calculation and to illustrate
the concept of the voltage drop add up the individual values
of the voltage drops and compare them to the results of the
above calculation.
1 volt + 3 volts + 5 volts = 9 volts
Voltage Sources in Series
A voltage source is an energy source that provides a constant
voltage to a load. Two or more of these sources in series equals
the algebraic sum of all the sources connected in series. The
significance of pointing out the algebraic sum is to indicate that
the polarity of the sources must be considered when adding up
the sources. The polarity is indicated by a plus or minus sign
depending on the source’s position in the circuit.
In Figure 12-84, all of the sources are in the same direction
in terms of their polarity. All of the voltages have the same
sign when added up. In the case of Figure 12-84 , three cells
of a value of 1.5 volts are in series with the polarity in the
same direction. The addition is simple enough:
ET = 1.5v + 1.5v + 1.5v = +4.5 volts
However, in Figure 12-85 , one of the three sources has been
turned around, and the polarity opposes the other two sources.
Again the addition is simple:
ET = + 1.5v − 1.5v + 1.5v = +1.5 volts
Kirchhoff’s Voltage Law
A law of basic importance to the analysis of an electrical
circuit is Kirchhoff’s V oltage Law. This law simply states
that the algebraic sum of all voltages around a closed path
or loop is zero. Another way of saying it: the sum of all the
voltage drops equals the total source voltage. A simplified
formula showing this law is shown below:
With three resistors in the circuit:
E S – E 1 – E 2 – E 3 … –E N = 0 volts
Notice that the sign of the source is opposite that of the
individual voltage drops. Therefore, the algebraic sum equals
zero. Written another way: E S = E 1 + E 2 + E 3 … +E N
The source voltage equals the sum of the voltage drops. The
polarity of the voltage drop is determined by the direction
of the current flow. When going around the circuit, notice
that the polarity of the resistor is opposite that of the source
voltage. The positive on the resistor is facing the positive
on the source, and the negative on the resistor is facing the
negative on the source.
Figure 12-86 illustrates the very basic idea of Kirchhoff’s
V oltage Law. There are two resistors in this example. One
has a drop of 14 volts and the other has a drop of 10 volts.
The source voltage must equal the sum of the voltage drops
around the circuit. By inspection, it is easy to determine the
source voltage as 24 volts.
Figure 12-87 shows a series circuit with three voltage
drops and one voltage source rated at 24 volts. Two of the
voltage drops are known. However, the third is not known.
Using Kirchhoff’s V oltage Law, the third voltage drop can
be determined.
With three resistors in the circuit:
E S – E 1 – E 2 – E 3 = 0 volts
Substitute the known values:
24v – 12v – 10v − E 3 = 0
Collect known values: 2v – E 3 = 0
Solve for the unknown: E 3 = 2 volts
Determine the value of E 4 in Figure 12-88 . For this example,
I = 200mA.
First, the voltage drop across each of the individual resistors
must be determined.
E1 = I (R 1)
E1 = (200 mA) (10 Ω)
12-37+1.5V
' + ' + '1.5V 1.5V
+1.5V
− +− +−1.5V 1.5V+
ES=24V
−14V
10V+−+−
+
10VE2 24VES
−+
−+12VE1
−
+
?E3−Figure 12-84. Voltage sources in series add algebraically.
Figure 12-85. Voltage sources add algebraically; one source
reversed.Figure 12-86. Kirchhoff’ s Voltage Law.
Figure 12-87. Determine the unknown voltage drop.V oltage drop across R 1 E1 = 2 volts
E2 = I (R 2)
E2 = (200 mA) (50 Ω)
V oltage drop across R 2 E2 = 10 volts
E3 = I (R 3)
E3 = (200 mA) (100 Ω)
V oltage drop across R 3 E3 = 20 volts
Kirchhoff’s V oltage Law is now employed to determine the
voltage drop across E 4.
With four resistors in the circuit
E S – E 1 – E 2 – E 3 – E 4 = 0 volts
Substituting values:
100v – 2v – 10v – 20v – E 4 = 0
Combine: 68v – E 4 = 0
Solve for unknown: E 4 = 68v
Using Ohm’s Law and substituting in E 4, the value for R 4
can now be determined. Ohm’s Law: E
IR =
Specific application: E4
IR4 =
Substitute values: 68 V
200 mAR4 =
Value for R 4: R 4 = 340 Ω
Voltage Dividers
V oltage dividers are devices that make it possible to obtain
more than one voltage from a single power source. A voltage
divider usually consists of a resistor, or resistors connected in
series, with fixed or movable contacts and two fixed terminal
contacts. As current flows through the resistor, different
voltages can be obtained between the contacts.
Series circuits are used for voltage dividers. The voltage divider
rule allows the technician to calculate the voltage across one
or a combination of series resistors without having to first
calculate the current in the circuit. [Figure 12-89] Because
the current flows through each resistor, the voltage drops are
proportional to the ohmic values of the constituent resistors.
To understand how a voltage divider works, examine
Figure 12-90 carefully and observe the following:
Each load draws a given amount of current: I 1, I2, I3. In
12-38++
100 ΩR310 ΩR1
50 ΩR2
−+
−−
+ −
?R4+ −100 VES
C B A
R1R2R3Figure 12-88. Determine the unknown voltage drop.
Figure 12-89. A voltage divider circuit.addition to the load currents, some bleeder current (I B) flows.
The current (I T) is drawn from the power source and is equal
to the sum of all currents.
The voltage at each point is measured with respect to a
common point. Note that the common point is the point at
which the total current (I T) divides into separate currents (I 1,
I2, I3). Each part of the voltage divider has a different current
flowing in it. The current distribution is as follows:
Through R 1 — bleeder current (I B)
Through R 2 — I B + I 1
Through R 3 — I B + I 1, + I 2
The voltage across each resistor of the voltage divider is:
90 volts across R 1
60 volts across R 2
50 volts across R 3
The voltage divider circuit discussed up to this point has had
one side of the power supply (battery) at ground potential. In
Figure 12-91, the common reference point (ground symbol)
has been moved to a different point on the voltage divider.
The voltage drop across R 1 is 20 volts; however, since tap A is
connected to a point in the circuit that is at the same potential
as the negative side of the battery, the voltage between tap
A and the reference point is a negative (−) 20 volts. Since
resistors R 2 and R 3 are connected to the positive side of the
battery, the voltages between the reference point and tap B
or C are positive.
The following rules provide a simple method of determining
negative and positive voltages: (1) If current enters a
resistance flowing away from the reference point, the voltage
drop across that resistance is positive in respect to the
reference point; (2) if current flows out of a resistance toward
the reference point, the voltage drop across that resistance is
negative in respect to the reference point. It is the location
of the reference point that determines whether a voltage is
negative or positive.
Tracing the current flow provides a means for determining
the voltage polarity. Figure 12-92 shows the same circuit
with the polarities of the voltage drops and the direction of
current flow indicated.
The current flows from the negative side of the battery to R 1.
Tap A is at the same potential as the negative terminal of the
battery since the slight voltage drop caused by the resistance
of the conductor is disregarded; however, 20 volts of the
source voltage are required to force the current through R 1
and this 20-volt drop has the polarity indicated. Stated another
way, there are only 80 volts of electrical pressure left in the circuit on the ground side of R 1.
When the current reaches tap B, 30 more volts have been used
to move the electrons through R 2, and in a similar manner
the remaining 50 volts are used for R 3. But the voltages
across R 2 and R 3 are positive voltages, since they are above
ground potential.
Figure 12-93 shows the voltage divider used previously. The
voltage drops across the resistances are the same; however,
the reference point (ground) has been changed. The voltage
between ground and tap A is now a negative 100 volts, or
the applied voltage.
The voltage between ground and tap B is a negative 80 volts,
and the voltage between ground and tap C is a negative 50 volts.
Determining the Voltage Divider Formula
Figure 12-94 shows the example network of four resistors and
a voltage source. With a few simple calculations, a formula
for determining the voltage divisions in a series circuit can
be determined.
The voltage drop across any particular resistor shall be called
EX, where the subscript x is the value of a particular resistor
(1, 2, 3, or 4). Using Ohm’s Law, the voltage drop across any
12-39C B A
100 VR3=100 Ω R2=60 Ω R1=40 Ω50 V 30 V 20 VC B A
100 VR3R2R1
+50 V
+ − + − + −+30 V −20 V200v
150v
90v
ITIT
IBI1I2I3R1R2 ER3
Load Load Load
Figure 12-91. Positive and negative voltage on a voltage divider.Figure 12-92. Current flow through a voltage divider.Figure 12-90. A typical voltage divider.
C B A
100 VR3R2R150 V 30 V 20 V
Figure 12-93. Voltage divider with changed ground.resistor can be determined.
Ohm’s Law: E X = I (R X)
As seen earlier in the handbook, the current is equal to the
source voltage divided by the total resistance of the series
circuit.
Current: ES
RTI =
The current equation can now be substituted into the
equation for Ohm’s Law.
Substitute: ES
RTEX = ( )(RX)
Algebraic rearrange: RX
RTEX = ( )(ES)
This equation is the general voltage divider formula. The explanation of this formula is that the voltage drop across
any resistor or combination of resistors in a series circuit is
equal to the ratio of the resistance value to the total resistance,
divided by the value of the source voltage. Figure 12-95
illustrates this with a network of three resistors and one
voltage source.
RX
RTEX = ( ) ES
R T = 100 Ω + 300 Ω + 600 Ω = 1,000 Ω
E S = 10 V
V oltage drop over 100 Ω resistor is:
100 Ω
1,000 ΩEX = ( ) 100 V
E 100Ω = 10 V
V oltage drop over 300 Ω resistor is:
300 Ω
1,000 ΩEX = ( ) 100 V
E 100 Ω = 30 V
V oltage drop over 600 Ω resistor is:
600 Ω
1,000 ΩEX = ( ) 100 V
E 100Ω = 60 V
12-40+
−+
−
+
−
+
−
+
−R1
R2
ES
R3
R4E3
E4E2E1
100 Ω
300 Ω
600 Ω+
−+
−
+
−
+
−R1
R2 ES100 v
R3
Figure 12-94. Four resistor voltage divider. Figure 12-95. Network of three resistors and one voltage source. Checking work
E T = 10 V + 30 V + 60 V = 100 V
Parallel DC Circuits
A circuit in which two of more electrical resistances or loads
are connected across the same voltage source is called a
parallel circuit. The primary difference between the series
circuit and the parallel circuit is that more than one path is
provided for the current in the parallel circuit. Each of these
parallel paths is called a branch. The minimum requirements
for a parallel circuit are the following:
• A power source
• Conductors
• A resistance or load for each current path
• Two or more paths for current flow
Figure 12-96 depicts the most basic parallel circuit. Current
flowing out of the source divides at point A in the diagram
and goes through R 1 and R 2. As more branches are added to
the circuit, more paths for the source current are provided.
Voltage Drops
The first point to understand is that the voltage across any
branch is equal to the voltage across all of the other branches.
Total Parallel Resistance
The parallel circuit consists of two or more resistors connected
in such a way as to allow current flow to pass through all of the
resistors at once. This eliminates the need for current to pass
one resistor before passing through the next. When resistors
are connected in parallel, the total resistance of the circuit decreases. The total resistance of a parallel combination is
always less than the value of the smallest resistor in the circuit.
In the series circuit, the current has to pass through the resistors
one at a time. This gave a resistance to the current equal the sum
of all the resistors. In the parallel circuit, the current has several
resistors that it can pass through, actually reducing the total
resistance of the circuit in relation to any one resistor value.
The amount of current passing through each resistor varies
according to its individual resistance. The total current of
the circuit is the sum of the current in all branches. It can
be determined by inspection that the total current is greater
than that of any given branch. Using Ohm’s Law to calculate
the total resistance based on the applied voltage and the total
current, it can be determined that the total resistance is less
than any branch.
An example of this is if there was a circuit with a 100 Ω
resistor and a 5 Ω resistor; while the exact value must be
calculated, it still can be said that the combined resistance
between the two is less than the 5 Ω.
Resistors in Parallel
The formula for the total parallel resistance is as follows:
RT= + + + ... 1
R1 1
R2 1
R3 1
RN
If the reciprocal of both sides is taken, then the general
formula for the total parallel resistance is:
RT =
+ + + ... 1
R1 1
R2 1
R3 1
RN 1
Two Resistors in Parallel
Typically, it is more convenient to consider only two resistors
at a time because this setup occurs in common practice. Any
number of resistors in a circuit can be broken down into pairs.
Therefore, the most common method is to use the formula
12-41+
−+
−+
−R1A
BR2ES
Figure 12-96. Basic parallel circuit.for two resistors in parallel.
RT =
+ 1
R1 1
R2 1
Combining the terms in the denominator and rewriting:
RT =R1 + R2 R1R2
Put in words, this states that the total resistance for two
resistors in parallel is equal to the product of both resistors
divided by the sum of the two resistors. In the formula below,
calculate the total resistance.
General formula RT =R1 + R2 R1R2
Known values R1 = 500 Ω
R2 = 400 Ω
RT =500 Ω + 400 Ω 500 Ω 400 Ω
RT =900 Ω 200,000 Ω
RT = 222.22 Ω
Current Source
A current source is an energy source that provides a constant
value of current to a load even when the load changes in
resistive value. The general rule to remember is that the total
current produced by current sources in parallel is equal to the
algebraic sum of the individual sources.
Kirchhoff’s Current Law
Kirchhoff’s Current Law can be stated as: the sum of the
currents into a junction or node is equal to the sum of the
currents flowing out of that same junction or node. A junction
can be defined as a point in the circuit where two or more
circuit paths come together. In the case of the parallel circuit,
it is the point in the circuit where the individual branches join. General formula I T = I 1 + I 2 + I 3
Refer to Figure 12-97 for an example. Point A and point B
represent two junctions or nodes in the circuit with three
resistive branches in between. The voltage source provides
a total current I T into node A. At this point, the current must
divide, flowing out of node A into each of the branches
according to the resistive value of each branch. Kirchhoff’s
Current Law states that the current going in must equal that
going out. Following the current through the three branches
and back into node B, the total current I T entering node B
and leaving node B is the same as that which entered node
A. The current then continues back to the voltage source.
Figure 12-98 shows that the individual branch currents are:
I 1 = 5 mA
I 2 = 12 mA
The total current flow into the node A equals the sum of the
branch currents, which is: IT = I 1 + I 2
Substitute I T = 5 mA + 12 mA
IT = 17 mA
The total current entering node B is also the same.
Figure 12-99 illustrates how to determine an unknown current
in one branch. Note that the total current into a junction of
the three branches is known. Two of the branch currents are
known. By rearranging the general formula, the current in
branch two can be determined.
General formula IT = I 1 + I 2 + I 3
Substitute 75 mA = 30 mA + I 2 + 20 mA
Solve I 2 I2 = 75 mA – 30 mA – 20 mA
I2 = 25 mA
Current Dividers
It can now be easily seen that the parallel circuit is a current
divider. As shown in Figure 12-96 , there is a current through
each of the two resistors. Because the same voltage is applied
across both resistors in parallel, the branch currents are
inversely proportional to the ohmic values of the resistors.
Branches with higher resistance have less current than those
with lower resistance. For example, if the resistive value of
R2 is twice as high as that of R 1, the current in R 2 is half of
that of R 1. All of this can be determined with Ohm’s Law.
By Ohm’s Law, the current through any one of the branches
can be written as:
12-42+
−+
−+
−+
−R1R2A
BR3ESIT
ITI1I3
+
−+
−+
−+
−A
BESI1=30 mA I3=20 mA
I2=?
IT=75 mAIT=75 mA+
−+
−+
−A
BESI2=12 mA
R2R1
IT=17 mAIT=17 mA
I1=5 mAFigure 12-97. Kirchhoff’ s Current Law.
Figure 12-99. Determining an unknown circuit in branch 2.Figure 12-98. Individual branch currents. I X = E S/RX
The voltage source appears across each of the parallel resistors
and R X represents any one the resistors. The source voltage
is equal to the total current times the total parallel resistance.
E S = I TRT
Substituting I TRT for E S ITRT
RXIX =
Rearranging RT
RXIX = ( ) IT
R2
RTI2 = ( ) IT
And R1
RTI1 = ( ) IT
This formula is the general current divider formula. The
current through any branch equals the total parallel resistance
divided by the individual branch resistance, multiplied by
the total current.
Series-Parallel DC Circuits
Most of the circuits that the technician encounters will not
be a simple series or parallel circuit. Circuits are usually a
combination of both, known as series-parallel circuits, which
are groups consisting of resistors in parallel and in series. An
example of this type of circuit can be seen in Figure 12-100 .
While the series-parallel circuit can initially appear to be
complex, the same rules that have been used for the series
and parallel circuits can be applied to these circuits.
The voltage source provides a current out to resistor R 1,
then to the group of resistors R 2 and R 3 and then to the next
resistor R 4 before returning to the voltage source. The first
step in the simplification process is to isolate the group R 2
and R 3 and recognize that they are a parallel network that
can be reduced to an equivalent resistor. Using the formula
for parallel resistance,
R23 =R2 + R3 R2R3
R2 and R 3 can be reduced to R 23. Figure 12-101 now shows
an equivalent circuit with three series connected resistors. The
total resistance of the circuit can now be simply determined
by adding up the values of resistors R 1, R23, and R 4.
Determining the Total Resistance
A more quantitative example for determining total resistance
and the current in each branch in a combination circuit is
shown in the following example. [Figure 12-102]
The first step is to determine the current at junction A,
leading into the parallel branch. To determine the I T, the total
12-43+
−R1
R2ESR3A
B
R4+
−R1
R23ES
R4
Figure 12-100. Series-parallel circuits.Figure 12-101. Equivalent circuit with three series connected
resistors.
1k Ω
2k Ω 3k Ω+
−R1
R2ES=24 VR3A
B
Figure 12-102. Determining total resistance.resistance R T of the entire circuit must be known. The total
resistance of the circuit is given as:
RT = R 1 + R 23
Where R23 = ( ) Parallel network R2 + R 3 R2R3
Find R EQ R23 =2k Ω + 3k Ω 2k Ω 3k Ω
Solve for R EQ R23 =5k Ω 6,000k Ω
R23 = 1.2k Ω
Solve for R T RT = 1k Ω + 1.2k Ω
RT = 2.2k Ω
With the total resistance R T now determined, the total I T can
be determined. Using Ohm’s Law:
IT =RT ES
Substitute values IT =2.2k Ω 24 V
IT = 10.9 mA
The current through the parallel branches of R 2 and R 3 can
be determined using the current divider rule discussed earlier
in this handbook.
Recall Parallel Branch Resistance:
R23 = 5,000 Ω 6,000 Ω
R23 = 1.2k Ω
Substitute values for I 2: I2 = x (10.9 mA) 2,000 Ω1,200 Ω
I2 = 6.54 mA
Now using Kirchhoff’s Current Law, the current in the branch
with R 3 can be determined.
I 2 + I 3 = IT
Recall that I T = 10.9 mA
I T – I 2 = I 3
Also recall that I 2 = 6.54 mA
Subtract I 2 from I T to get I 3
I 3 = 4.36 mA
Alternating Current (AC) & Voltage
Alternating current (AC) has largely replaced direct current
(DC) in commercial power systems for a number of reasons.
It can be transmitted over long distances more readily and
more economically than DC, since AC voltages can be
12-44increased or decreased by means of transformers.
Because more and more units are being operated electrically
in airplanes, the power requirements are such that a number
of advantages can be realized by using AC. Space and
weight can be saved since AC devices, especially motors, are
smaller and simpler than DC devices. In most AC motors,
no brushes are required, and commutation trouble at high
altitude is eliminated. Circuit breakers operate satisfactorily
under load at high altitudes in an AC system, whereas arcing
is so excessive on DC systems that circuit breakers must be
replaced frequently. Finally, most airplanes using a 24-volt
DC system have special equipment that requires a certain
amount of 400-cycle AC current.
AC and DC Compared
Many of the principles, characteristics, and effects of AC
are similar to those of DC. Similarly, there are a number of
differences. DC flows constantly in only one direction with
a constant polarity. It changes magnitude only when the
circuit is opened or closed, as shown in the DC waveform
in Figure 12-103 . AC changes direction at regular intervals,
increases in value at a definite rate from zero to a maximum
positive strength, and decreases back to zero; then it flows
in the opposite direction, similarly increasing to a maximum
negative value, and again decreasing to zero. DC and AC
waveforms are compared in Figure 12-103 .
Since AC constantly changes direction and intensity, the
following two effects (to be discussed later) take place in
AC circuits that do not occur in DC circuits:
1. Inductive reactance
2. Capacitive reactance
Generator Principles
After the discovery that an electric current flowing through
a conductor creates a magnetic field around the conductor,
there was considerable scientific speculation about whether
a magnetic field could create a current flow in a conductor.
In 1831, Faraday discovered that this could be accomplished.
To show how an electric current can be created by a magnetic
field, a demonstration similar to Figure 12-104 can be used.
Several turns of a conductor are wrapped around a cylindrical
form, and the ends of the conductor are connected together
to form a complete circuit, which includes a galvanometer.
If a simple bar magnet is plunged into the cylinder, the
galvanometer can be observed to deflect in one direction
from its zero (center) position. [Figure 12-104A]
When the magnet is at rest inside the cylinder, the
galvanometer shows a reading of zero, indicating that no current is flowing. [Figure 12-104B]
In Figure 12-104C , the galvanometer indicates a current
flow in the opposite direction when the magnet is pulled
from the cylinder.
The same results may be obtained by holding the magnet
stationary and moving the cylinder over the magnet, indicating
that a current flows when there is relative motion between the
wire coil and the magnetic field. These results obey a law first
stated by the German scientist, Heinrich Lenz. Lenz’s Law
states that the induced current caused by the relative motion
of a conductor and a magnetic field always flows in such a
direction that its magnetic field opposes the motion.
When a conductor is moved through a magnetic field, an emf
is induced in the conductor. [Figure 12-105] The direction
(polarity) of the induced emf is determined by the magnetic
lines of force and the direction the conductor is moved
through the magnetic field. The generator left-hand rule
(not to be confused with the left-hand rules used with a coil)
can be used to determine the direction of the induced emf.
[Figure 12-106] The left-hand rule is summed up as follows:
The first finger of the left hand is pointed in the direction
of the magnetic lines of force (North to South), the thumb
is pointed in the direction of movement of the conductor
through the magnetic field, and the second finger points in
the direction of the induced emf.
When a loop conductor is rotated in a magnetic field, a
voltage is induced in each side of the loop. [Figure 12-107]
The two sides cut the magnetic field in opposite directions,
and although the current flow is continuous, it moves in
opposite directions with respect to the two sides of the loop.
If sides A and B and the loop are rotated half a turn and
the sides of the conductor have exchanged positions, the
induced emf in each wire reverses its direction, since the wire
formerly cutting the lines of force in an upward direction is
now moving downward.
The value of an induced emf depends on three factors:
1. Number of wires moving through the magnetic field
2. Strength of the magnetic field
3. Speed of rotation
Generators of Alternating Current
Generators used to produce an alternating current are called
AC generators or alternators.
The simple generator constitutes one method of generating
an alternating voltage. [Figure 12-108] It consists of
Closed
switchOpen
switchOperation of circuitWaveform for DCVolts
TimeWaveform for AC Volts +
Time−0° 90° 180° 270° 360°
Figure 12-103. DC and AC voltage curves.
NS
NS
NSMotion of magnet Galvanometer
A
B
CMotion of magnetCoil
Magnet at restII
II
Figure 12-104. Inducing a current flow.a rotating loop, marked A and B, placed between two
magnetic poles, N and S. The ends of the loop are connected
to two metal slip rings (collector rings), C 1 and C 2. Current
is taken from the collector rings by brushes. If the loop is
considered as separate wires A and B, and the left-hand
rule for generators is applied, then it can be observed that
as wire A moves up across the field, a voltage is induced
which causes the current to flow inward. As wire B moves
down across the field, a voltage is induced which causes the
current to flow outward. When the wires are formed into a
loop, the voltages induced in the two sides of the loop are
combined. Therefore, for explanatory purposes, the action
of either conductor, A or B, while rotating in the magnetic
field is similar to the action of the loop.
Figure 12-109 illustrates the generation of AC with a simple
loop conductor rotating in a magnetic field. As it is rotated
in a counterclockwise direction, varying values of voltages
are induced in it.
Position 1
The conductor A moves parallel to the lines of force. Since
it cuts no lines of force, the induced voltage is zero. As the
conductor advances from position 1 to position 2, the voltage
induced gradually increases.
Position 2
The conductor is now moving perpendicular to the flux
and cuts a maximum number of lines of force; therefore, a
maximum voltage is induced. As the conductor moves beyond
position 2, it cuts a decreasing amount of flux at each instant,
and the induced voltage decreases.
Position 3
At this point, the conductor has made one-half of a revolution
and again moves parallel to the lines of force, and no voltage
is induced in the conductor. As the A conductor passes
position 3, the direction of induced voltage now reverses
since the A conductor is moving downward, cutting flux
S
N
S
N
Inducted emf
Flux forward
InductedemfConductor
moved up
Figure 12-106. An application of the generator left-hand rule.
N
S
AB
Figure 12-107. Voltage induced in a loop.
N
S
Motion of conductoremf
Figure 12-105. Inducing an emf in a conductor.
in the opposite direction. As the A conductor moves across
the South pole, the induced voltage gradually increases in a
negative direction, until it reaches position 4.
Position 4
Like position 2, the conductor is again moving perpendicular
to the flux and generates a maximum negative voltage. From
position 4 to 5, the induced voltage gradually decreases until
the voltage is zero, and the conductor and wave are ready to
start another cycle.
Position 5
The curve shown at position 5 is called a sine wave. It
represents the polarity and the magnitude of the instantaneous
values of the voltages generated. The horizontal base line is
divided into degrees, or time, and the vertical distance above
or below the base line represents the value of voltage at each
particular point in the rotation of the loop.Cycle and Frequency
Cycle Defined
A cycle is a repetition of a pattern. Whenever a voltage
or current passes through a series of changes, returns to
the starting point, and then again starts the same series of
changes, the series is called a cycle. The cycle is represented
by the symbol of a wavy line in a circle ~. In the cycle of
voltage shown in Figure 12-110 , the voltage increases from
zero to a maximum positive value, decreases to zero; then
increases to a maximum negative value, and again decreases
to zero. At this point, it is ready to go through the same series
of changes. There are two alternations in a complete cycle:
the positive alternation and the negative. Each is half a cycle.
Frequency Defined
The frequency is the number of cycles of AC per second (1
N
S
N
SBrushes
C1C2AB+
In current Out current
Figure 12-108. Simple generator.
second). The standard unit of frequency measurement is the
hertz (Hz). [Figure 12-111] In a generator, the voltage and
current pass through a complete cycle of values each time
a coil or conductor passes under a North and South pole of
the magnet. The number of cycles for each revolution of the
coil or conductor is equal to the number of pairs of poles.
The frequency, then, is equal to the number of cycles in
one revolution multiplied by the number of revolutions per
second (rps).
Expressed in equation form:
F = ×2 60 Number of poles rpm
where P⁄2 is the number of pairs of poles, and rpm/60 the
number of revolutions per second. If in a 2-pole generator,
the conductor is turning at 3,600 rpm, the revolutions per
second are:
rps = = 60 revolutions per second60 3,600
Since there are 2 poles, P⁄2 is 1, and the frequency is 60 cycles
per second (cps). In a 4-pole generator with an armature speed
of 1,800 rpm, substitute in the equation:
F = × as follows2 60 P rpm
F = × 2 60 4 1,800
F = 2 × 30 F = 60 cps
Period Defined
The time required for a sine wave to complete one full cycle
is called a period. [Figure 12-110] The period of a sine
wave is inversely proportional to the frequency: the higher
the frequency, the shorter the period. The mathematical
relationship between frequency and period is given as:
Period is t =f 1
Frequency is f =t 1
Wavelength Defined
The distance that a waveform travels during a period is
commonly referred to as a wavelength and is indicated by
the Greek letter lambda ( l). The measurement of wavelength
is taken from one point on the waveform to a corresponding
point on the next waveform. [Figure 12-110]
Phase Relationships
In addition to frequency and cycle characteristics, alternating
voltage and current also have a relationship called “phase.”
In a circuit that is fed (supplied) by one alternator, there
must be a certain phase relationship between voltage and
current if the circuit is to function efficiently. In a system
fed by two or more alternators, not only must there be a
certain phase relationship between voltage and current of
one alternator, but there must be a phase relationship between
the individual voltages and the individual currents. Also, two
separate circuits can be compared by comparing the phase
SB
Position 2 Quarter turn completed
Conductors cutting directly across the magnetic field as conductor
A passes across the North (N) magnetic pole and B passes across
the South (S) magnetic pole.Maximum positive voltage0° 90° 180° 270° 360° C1C2
N
0° 90° 180° 270° 360°
S
Position 4 Three quarters turn completed
Conductors again moving directly across magnetic field A passes
across South (S) magnetic pole and B across North (N) magnetic pole.C1C2
Maximum negative voltage
N0° 90° 180° 270° 360°
N
SB
Voltage drops to zero
Position 3 One half turn completed
Conductor again moving parallel to magnetic field, cutting minimum
lines of force.C1C2
N
SAC1C2
0° 90° 180° 270° 360°
Zero voltage
Position 1
Rotating conductors moving parallel to magnetic field, cutting
minimum lines of force.Magnetic field
0° 90° 180° 270° 360°
N
SA
Position 5Zero voltage
Full turn completed
Conductor A has made one complete cycle and is in same position
as in position A. The generator has generated one complete cycle
of alternating voltage or current.C1C2
Figure 12-109. Generation of a sine wave.
characteristics of one to the phase characteristics of the other.
In Phase Condition
Figure 12-112A shows a voltage signal and a current signal
superimposed on the same time axis. Notice that when the
voltage increases in the positive alternation that the current
also increases. When the voltage reaches its peak value, so
does the current. Both waveforms then reverse and decrease
back to a zero magnitude, then proceed in the same manner
in the negative direction as they did in the positive direction.
When two waves, such as these in Figure 12-112A , are exactly
in step with each other, they are said to be in phase. To be in
phase, the two waveforms must go through their maximum and
minimum points at the same time and in the same direction.
Out of Phase Condition
When two waveforms go through their maximum and
minimum points at different times, a phase difference exists
between the two. In this case, the two wave-forms are said
to be out of phase with each other. The terms lead and lag
are often used to describe the phase difference between
waveforms. The waveform that reaches its maximum or
minimum value first is said to lead the other waveform.
Figure 12-112B shows this relationship. V oltage source
one starts to rise at the 0° position and voltage source two starts to rise at the 90° position. Because voltage source one
begins its rise earlier in time (90°) in relation to the second
voltage source, it is said to be leading the second source.
On the other hand, the second source is said to be lagging
the first source. When a waveform is said to be leading or
lagging, the difference in degrees is usually stated. If the
two waveforms differ by 360°, they are said to be in phase
with each other. If there is a 180° difference between the two
signals, then they are still out of phase even though they are
both reaching their minimum and maximum values at the
same time. [Figure 12-112C]
A practical note of caution: When encountering an aircraft that
has two or more AC busses in use, it is possible that they may
be split and not synchronized to be in phase with each other.
When two signals that are not locked in phase are mixed, much
damage can occur to aircraft systems or avionics.
Values of Alternating Current
There are three values of AC: instantaneous, peak, and
effective root mean square (RMS).
Instantaneous Value
An instantaneous value of voltage or current is the induced
voltage or current flowing at any instant during a cycle. The
sine wave represents a series of these values. The instantaneous
One cycle
One period
(Time)
One wavelength
(Distance)Second cycle
(Repeated)Vertical scale (voltage)Horizontal scale
(Time)Positive alternation
1T 2T 3T 4T
Negative
alternation
Cycle is defined as a repetitive pattern.
360°1 second timePositive
alternation1T0° 90° 180° 270°
Frequency = 2 cycles per second
270°180°
0°90°Frequency = 2 cycles per second
1 second time
Frequency = 8 cycles per second360°Negative
alternation
Figure 12-110. Cycle of voltage.
Figure 12-111. Frequency in cycles per second.value of the voltage varies from zero at 0° to maximum at 90°,
back to zero at 180°, to maximum in the opposite direction at 270°, and to zero again at 360°. Any point on the sine wave is considered the instantaneous value of voltage.
Peak Value
The peak value is the largest instantaneous value. The largest single positive value occurs when the sine wave of voltage is at 90°, and the largest single negative value occurs when it is at 270°. Maximum value is 1.41 times the effective value. These are called peak values.
Effective Value
The effective value is also known as the RMS value or root mean square, which refers to the mathematical process by which the value is derived. Most AC voltmeters display the effective or RMS value when used. The effective value is less than the maximum value, being equal to .707 times the maximum value.
The effective value of a sine wave is actually a measure of
the heating effect of the sine wave. Figure 12-113 illustrates
what happens when a resistor is connected across an AC voltage source. In Figure 12-113A , a certain amount of heat is
generated by the power in the resistor. Figure 12-113B shows the same resistor now inserted into a DC voltage source. The value of the DC voltage source can now be adjusted so that the resistor dissipates the same amount of heat as it did when it was in the AC circuit. The RMS or effective value of a sine wave is equal to the DC voltage that produces the same amount of heat as the sinusoidal voltage.
The peak value of a sine wave can be converted to the
corresponding RMS value using the following relationship.
Vrms = ( 0.5
) × Vp
Vrms = 0.707 × VpThis can be applied to either voltage or current.Algebraically rearranging the formula and solving for Vp can
also determine the peak voltage. The resulting formula is:
Vp = 1.414 × Vrms
Thus, the 110 volt value given for AC supplied to homes is only 0.707 of the maximum voltage of this supply. The maximum voltage is approximately 155 volts (110 × 1.41 = 155 volts maximum).
In the study of AC, any values given for current or voltage
are assumed to be effective values unless otherwise specified. In practice, only the effective values of voltage and current are used. Similarly, AC voltmeters and ammeters measure the effective value.
Opposition to Current Flow of AC
There are three factors that can create an opposition to the flow
of electrons (current) in an AC circuit. Resistance, similar to resistance of DC circuits, is measured in ohms and has a direct influence on AC regardless of frequency. Inductive reactance and capacitive reactance, on the other hand, oppose current flow only in AC circuits, not in DC circuits. Since AC constantly changes direction and intensity, inductors and capacitors may also create an opposition to current flow in AC circuits. It should also be noted that inductive reactance and capacitive reactance may create a phase shift between the voltage and current in an AC circuit. Whenever analyzing an AC circuit, it is very important to consider the resistance, inductive reactance, and the capacitive reactance. All three have an effect on the current of that circuit.
CurrentVoltage
0° 90° 180° 270° 360°
Voltage source 1 (leads source 2)
90°
Difference0° 90° 180° 270° 360°Voltage source 2 (lags source 1)
Voltage source 1
0° 90° 180° 270° 360°Voltage source 2A. Voltage and current are in phase
B. Two voltage waves, 90° out of phase
C. Two voltage waves, 180 ° out of phase
Figure 12-112. In phase and out of phase conditions.
Capacitance
Another important property in AC circuits, besides resistance
and inductance, is capacitance. While inductance is
represented in a circuit by a coil, capacitance is represented by
a capacitor. In its most basic form, the capacitor is constructed
of two parallel plates separated by a nonconductor called
a dielectric. In an electrical circuit, a capacitor serves as a
reservoir or storehouse for electricity.
Capacitors in Direct Current
When a capacitor is connected across a source of DC, such as a
storage battery in the circuit shown in Figure 12-114A , and the
switch is then closed, the plate marked B becomes positively charged, and the A plate negatively charged. Current flows in
the external circuit during the time the electrons are moving
from B to A. The current flow in the circuit is at a maximum
the instant the switch is closed, but continually decreases
thereafter until it reaches zero. The current becomes zero as
soon as the difference in voltage of A and B becomes the same
as the voltage of the battery. If the switch is opened as shown in
Figure 12-114B , the plates remain charged. Once the capacitor
is shorted, it discharges quickly as shown Figure 12-114C .
It should be clear that during the time the capacitor is being
charged or discharged, there is current in the circuit, even
though the circuit is broken by the gap between the capacitor
plates. Current is present only during the time of charge and
discharge, and this period of time is usually short.
The Resistor/ Capacitor (RC) Time Constant
The time required for a capacitor to attain a full charge is
proportional to the capacitance and the resistance of the
circuit. The resistance of the circuit introduces the element
of time into the charging and discharging of a capacitor.
When a capacitor charges or discharges through a resistance,
a certain amount of time is required for a full charge or
discharge. The voltage across the capacitor does not change
instantaneously. The rate of charging or discharging is
determined by the time constant of the circuit. The time
constant of a series resistor/capacitor (RC) circuit is a time
interval that equals the product of the resistance in ohms
and the capacitance in farad and is symbolized by the Greek
letter tau ( τ).
τ = RC
The time in the formula is the time required to charge to 63
percent of the voltage of the source. The time required to
bring the charge to about 99 percent of the source voltage is
approximately 5 τ. [Figure 12-115]
The measure of a capacitor’s ability to store charge is its
capacitance. The symbol used for capacitance is the letter C.
As can be seen from Figure 12-115, there can be no continuous
movement of DC through a capacitor. A good capacitor blocks
DC and passes the effects of pulsing DC or AC.
Units of Capacitance
Electrical charge, which is symbolized by the letter Q, is
measured in units of coulombs. The coulomb is given by
the letter C, as with capacitance. Unfortunately, this can be
confusing. One coulomb of charge is defined as a charge
having 6.28 × 1018 electrons. The basic unit of capacitance
is the farad and is given by the letter f. By definition, one
A
BEqual heat dissipation
in both circuits
+
−R Vrms=Vdc
R Vrms
Figure 12-113. Sine wave effective value.
farad is one coulomb of charge stored with one volt across the
plates of the capacitor. The general formula for capacitance
in terms of charge and voltage is:
Where C =E Q
C = capacitance measured in farads
E = applied voltage measured in volts
Q = charge measured in coulombs
In practical terms, one farad is a large amount of capacitance.
Typically, in electronics, much smaller units are used. The two
more common smaller units are the microfarad (μF), which is
10-6 farad, and the picofarad (pF), which is 10-12 farad.
Voltage Rating of a Capacitor
Capacitors have their limits as to how much voltage can be
applied across the plates. The aircraft technician must be
aware of the voltage rating, which specifies the maximum
DC voltage that can be applied without the risk of damage
to the device. This voltage rating is typically called the
breakdown voltage, the working voltage, or simply the
voltage rating. If the voltage applied across the plates is
too great, the dielectric breaks down and arcing occurs
between the plates. The capacitor is then short circuited,
and the possible flow of DC through it can cause damage
to other parts of the equipment.
A capacitor that can be safely charged to 500 volts DC cannot
be safely subjected to AC or pulsating DC whose effective
values are 500 volts. An alternating voltage of 500 volts (RMS)
has a peak voltage of 707 volts, and a capacitor to which it is
applied should have a working voltage of at least 750 volts. The
capacitor should be selected so that its working voltage is at
least 50 percent greater than the highest voltage to be applied.The voltage rating of the capacitor is a factor in determining
the actual capacitance, because capacitance decreases as the
thickness of the dielectric increases. A high-voltage capacitor
that has a thick dielectric must have a larger plate area in
order to have the same capacitance as a similar low voltage
capacitor having a thin dielectric.
Factors Affecting Capacitance
1. The capacitance of parallel plates is directly
proportional to their area. A larger plate area produces
a larger capacitance and a smaller area produces less
capacitance. If we double the area of the plates, there
is room for twice as much charge. The charge that a
capacitor can hold at a given potential difference is
doubled, and since C = Q/E, the capacitance is doubled.
2. The capacitance of parallel plates is inversely
proportional to their spacing.
3. The dielectric material affects the capacitance of
parallel plates. The dielectric constant of a vacuum is
defined as 1, and that of air is very close to 1. These
values are used as a reference, and all other materials
have values specified in relation to air (vacuum).
The strength of some commonly used dielectric materials is
listed in Figure 12-116 . The voltage rating also depends on
frequency because the losses, and the resultant heating effect,
increase as the frequency increases.
Types of Capacitors
Capacitors come in all shapes and sizes and are usually
marked with their value in farads. They may also be divided
into two groups: fixed and variable. The fixed capacitors,
which have approximately constant capacitance, may then
be further divided according to the type of dielectric used.
Some varieties are: paper, oil, mica, electrolytic and ceramic
capacitors. Figure 12-117 shows the schematic symbols for
a fixed and variable capacitor.
Fixed Capacitors
Mica Capacitors
The fixed mica capacitor is made of metal foil plates that are
separated by sheets of mica, which form the dielectric. The
whole assembly is covered in molded plastic, which keeps out
moisture. Mica is an excellent dielectric and withstands higher
voltages than paper without allowing arcing between the plates.
Common values of mica capacitors range from approximately
50 microfarads to about 0.02 microfarads. [Figure 12-118]
Ceramic
The ceramic capacitor is constructed with materials, such
as titanium acid barium for a dielectric. Internally these
63%100%
90%
80%
70%
60%
50%
40%
30%
20%
10%86%95%98%Voltage
Current
1t
Charging curve for a capacitor2t 3t 4t 5t99%
Figure 12-115. Capacitance discharge curve.
A
B
C+
−++++
−−−−Open
B
ACapacitor being charged
Capacitor retains charge
Capacitor discharges+
−++++
−−−−Close Charge
B
A
+
−++
−−Close
Short RemovedB
A
Figure 12-114. Capacitors in direct current.
capacitors are not constructed as a coil, so they are well
suited for use in high-frequency applications. They are shaped
like a disk, available in very small capacitance values, and
very small sizes. This type is fairly small, inexpensive, and
reliable. Both the ceramic and the electrolytic are the most
widely available and used capacitor.Electrolytic
Two kinds of electrolytic capacitors are in use: wet electrolytic
and dry electrolytic. The wet electrolytic capacitor is designed
of two metal plates separated by an electrolyte with an
electrolyte dielectric, which is basically conductive salt in
solvent. For capacitances greater than a few microfarads, the
plate areas of paper or mica capacitors must become very
large; thus, electrolytic capacitors are usually used instead.
These units provide large capacitance in small physical sizes.
Their values range from 1 to about 1,500 microfarads. Unlike
the other types, electrolytic capacitors are generally polarized,
with the positive lead marked with a “+” and the negative
lead marked with a “−” and should only be subjected to direct
voltage or pulsating direct voltage only.
The electrolyte in contact with the negative terminal, either
in paste or liquid form, comprises the negative electrode. The
dielectric is an exceedingly thin film of oxide deposited on
the positive electrode of the capacitor. The positive electrode,
which is an aluminum sheet, is folded to achieve maximum
area. The capacitor is subjected to a forming process during
manufacture in which current is passed through it. The flow
of current results in the deposit of the thin coating of oxide
on the aluminum plate.
The close spacing of the negative and positive electrodes
gives rise to the comparatively high-capacitance value, but
allows greater possibility of voltage breakdown and leakage
of electrons from one electrode to the other.
The electrolyte of the dry electrolytic unit is a paste contained
in a separator made of an absorbent material, such as gauze or
paper. The separator not only holds the electrolyte in place but
also prevents it from short circuiting the plates. Dry electrolytic
capacitors are made in both cylindrical and rectangular block
form and may be contained either within cardboard or metal
covers. Since the electrolyte cannot spill, the dry capacitor
may be mounted in any convenient position. [Figure 12-119]
Tantalum
Similar to the electrolytic, these capacitors are constructed
with a material called tantalum, which is used for the
electrodes. They are superior to electrolytic capacitors,
having better temperature and frequency characteristics.
When tantalum powder is baked in order to solidify it, a crack
forms inside. This crack is used to store an electrical charge.
Like electrolytic capacitors, the tantalum capacitors are also
polarized and are indicated with the “+” and “−” symbols.
Polyester Film
In this capacitor, a thin polyester film is used as a dielectric.
These components are inexpensive, temperature stable, and
widely used. Tolerance is approximately 5–10 percent. It
Fixed Variable
Dielectric KDielectric Strength
(volts per .001 inch)
Air 1.0 80
Paper
(1) Paraffined 2.2 1,200
(2) Beeswaxed 3.1 1,800
Glass 4.2 200
Castor Oil 4.7 380
Bakelite 6.0 500
Mica 6.0 2,000
Fiber 6.5 50Figure 12-117. Schematic symbols for a fixed and variable capacitor.
Figure 12-118. Fixed capacitors. Figure 12-116. Strength of some dielectric materials.can be quite large depending on capacity or rated voltage.
Oil Capacitors
In radio and radar transmitters, voltages high enough to cause
arcing, or breakdown, of paper dielectrics are often used.
Consequently, in these applications capacitors that use oil or
oil impregnated paper for the dielectric material are preferred.
Capacitors of this type are considerably more expensive than
ordinary paper capacitors, and their use is generally restricted
to radio and radar transmitting equipment. [Figure 12-120]
Variable Capacitors
Variable capacitors are mostly used in radio tuning circuits, and
they are sometimes called “tuning capacitors.” They have very
small capacitance values, typically between 100 pF and 500 pF.
Trimmers
The trimmer is actually an adjustable or variable capacitor,
which uses ceramic or plastic as a dielectric. Most of them are
color coded to easily recognize their tunable size. The ceramic
type has the value printed on them. Colors are: yellow (5 pF),
blue (7 pF), white (10 pF), green (30 pF), and brown (60 pF).
Varactors
A voltage-variable capacitor or varactor is also known as a
variable capacitance diode or a varicap. This device utilizes
the variation of the barrier width in a reversed-biased diode.
Because the barrier width of a diode acts as a non-conductor,
a diode forms a capacitor when reversed biased. Essentially,
the N-type material becomes one plate and the junctions
are the dielectric. If the reversed-bias voltage is increased,
then the barrier width widens, effectively separating the two
capacitor plates and reducing the capacitance.
Capacitors in Series
When capacitors are placed in series, the effective plate
separation is increased and the total capacitance is less
than that of the smallest capacitor. Additionally, the series combination is capable of withstanding a higher total
potential difference than any of the individual capacitors.
Figure 12-121 is a simple series circuit. The bottom plate
of C 1 and the top plate of C 2 is charged by electrostatic
induction. The capacitors charge as current is established
through the circuit. Since this is a series circuit, the current
must be the same at all points. Since the current is the rate
of flow of charge, the amount of charge (Q) stored by each
capacitor is equal to the total charge.
Q T = Q 1 + Q 2 + Q 3
According to Kirchhoff’s V oltage Law, the sum of the
voltages across the charged capacitors must equal the total
voltage, E T. This is expressed as:
E T = E 1 + E 2 + E 3
Equation E = Q/C can now be substituted into the voltage
equation where we now get:
= + + CT C1 C2 C3 QT Q1 Q2 Q3
Since the charge on all capacitors is equal, the Q terms can
be factored out, leaving us with the equation:
= + + CT C1 C2 C3 1 1 1 1
Figure 12-119. Electrolytic capacitors. Figure 12-120. Oil capacitor.Consider the following example:
If C 1 = 10 μF, C 2 = 5 μF and C 3 = 8 μF
Then = + + CT10 μF 5 μF 8 μF 1 1 1 1
CT = = 2.35 μF 0.425 μF 1
Capacitors in Parallel
When capacitors are connected in parallel, the effective plate
area increases, and the total capacitance is the sum of the
individual capacitances. Figure 12-122 shows a simplified
parallel circuit. The total charging current from the source
divides at the junction of the parallel branches. There is
a separate charging current through each branch so that
a different charge can be stored by each capacitor. Using
Kirchhoff’s Current Law, the sum of all of the charging
currents is then equal to the total current. The sum of the
charges (Q) on the capacitors is equal to the total charge.
The voltages (E) across all of the parallel branches are equal.
With all of this in mind, a general equation for capacitors in
parallel can be determined as:
Q T = Q 1 + Q 2 + Q 3
Because Q = CE: C TET = C 1E1 + C 2E2 + C 3E3
V oltages can be factored out because:
E T = E 1 + E 2 + E 3Leaving us with the equation for capacitors in parallel:
C T = C 1 + C 2 + C 3
Consider the following example:
If C 1 = 330 μF, C 2 = 220 μF
Then C T = 330 μF + 220 μF = 550 μF
Capacitors in Alternating Current
If a source of AC is substituted for the battery, the capacitor
acts quite differently than it does with DC. When AC is
applied in the circuit, the charge on the plates constantly
changes. [Figure 12-123] This means that electricity
must flow first from Y clockwise around to X, then from
X counterclockwise around to Y , then from Y clockwise
around to X, and so on. Although no current flows through
the insulator between the plates of the capacitor, it constantly
flows in the remainder of the circuit between X and Y . In
a circuit where there is only capacitance, current leads the
applied voltage as contrasted with a circuit in which there is
inductance, where the current lags the voltage.
Capacitive Reactance Xc
The effectiveness of a capacitor in allowing an AC flow to pass
depends upon the capacitance of the circuit and the applied
frequency. To what degree a capacitor allows an AC flow to
pass depends largely upon the capacitive value of the capacitor
given in farads (f). The greater the capacitance of the capacitor,
the greater the number of electrons, measured in Coulombs,
necessary to bring the capacitor to a fully charged state. Once
the capacitor approaches or actually reaches a fully charged
condition, the polarity of the capacitor opposes the polarity of
the applied voltage, essentially acting then as an open circuit.
To further illustrate this characteristic and how it manifests
itself in an AC circuit, consider the following. If a capacitor has
a large capacitive value, meaning that it requires a relatively
large number of electrons to bring it to a fully charged state,
+
− +
−+
−+
− C1
C2
C3
+
−+
−+
−+
− C1C2C3
Figure 12-121. Simple series circuit. Figure 12-122. Simplified parallel circuit.then a rather high-frequency current can alternate through the
capacitor without the capacitor ever reaching a full charge. In
this case, if the frequency is high enough and the capacitance
large enough that there is never enough time for the capacitor
to ever reach a full charge, it is possible that the capacitor
may offer very little or no resistance to the current. However,
the smaller the capacitance, the fewer electrons are required
to bring it up to a full charge and it is more likely that the
capacitor will build up enough of an opposing charge that it
can present a great deal of resistance to the current if not to the
point of behaving like an open circuit. In between these two
extreme conditions lies a continuum of possibilities of current
opposition depending on the combination of applied frequency
and the selected capacitance. Current in an AC circuit can be
controlled by changing the circuit capacitance in a similar
manner that resistance can control the current. The actual AC
reactance Xc, which just like resistance, is measured in ohms
(Ω). Capacitive reactance Xc is determined by the following:
Xc =2πfC 1
Where Xc = capacitive reactance
f = frequency in cps
C = capacity in farads
2π = 6.28
Sample Problem:
A series circuit is assumed in which the impressed voltage
is 110 volts at 60 cps, and the capacitance of a condenser is
80 Mf. Find the capacitive reactance and the current flow.
Solution:
To find capacitive reactance, the equation Xc = 1/(2πfC)
is used. First, the capacitance, 80 Mf, is changed to farads
by dividing 80 by 1,000,000, since 1 million microfarads is
equal to 1 farad. This quotient equals 0.000080 farad. This is substituted in the equation and:
Xc =6.28 × 60 × 0.000080 1
Xc = 33.2 ohms reactance
Once the reactance has been determined, Ohm’s Law can
then be used in the same manner as it is used in DC circuits
to determine the current.
Current = , orCapacitive reactance V oltage
I = Xc E
Find the current flow:
I = Xc E
I = 33.2 110
I = 3.31 amperes
Capacitive Reactances in Series and in Parallel
When capacitors are connected in series, the total reactance
is equal to the sum of the individual reactances. Thus,
Xct = (Xc) 1 + (Xc) 2
The total reactance of capacitors connected in parallel is
found in the same way total resistance is computed in a
parallel circuit:
(Xc)t =
+ +1
(Xc) 11
(Xc) 21
(Xc) 3 1
Phase of Current and Voltage in Reactive Circuits
Unlike a purely resistive circuit, the capacitive and inductive
reactance has a significant effect on the phase relationship
between the applied AC voltage and the corresponding
X
YAC generator
Figure 12-123. Capacitor in an AC circuit. current in the circuit.
In review, when current and voltage pass through zero and
reach maximum value at the same time, the current and
voltage are said to be in phase. [Figure 12-124A] If the current
and voltage pass through zero and reach the maximum values
at different times, the current and voltage are said to be out
of phase. In a circuit containing only inductance, the current
reaches a maximum value later than the voltage, lagging the
voltage by 90°, or one-fourth cycle. [Figure 12-124B]
In a circuit containing only capacitance, the current reaches
its maximum value ahead of the voltage and the current leads
the voltage by 90°, or one-fourth cycle. [Figure 12-124C] The
amount the current lags or leads the voltage in a circuit
depends on the relative amounts of resistance, inductance,
and capacitance in the circuit.
Inductance
Characteristics of Inductance
Michael Faraday discovered that by moving a magnet through
a coil of wire, a voltage was induced across the coil. If a
complete circuit was provided, then a current was also induced.
The amount of induced voltage is directly proportional to
the rate of change of the magnetic field with respect to the
coil. The simplest of experiments can prove that when a bar
magnet is moved through a coil of wire, a voltage is induced
and can be measured on a voltmeter. This is commonly known
as Faraday’s Law or the Law of Electromagnetic Induction,
which states that the induced emf or electromagnetic force in
a closed loop of wire is proportional to the rate of change of
the magnetic flux through a coil of wire.
Conversely, current flowing through a coil of wire produces
a magnetic field. When this wire is formed into a coil, it then
becomes a basic inductor. The magnetic lines of force around
each loop or turn in the coil effectively add to the lines of
force around the adjoining loops. This forms a strong magnetic
field within and around the coil. Figure 12-125A shows a coil
of wire strengthening a magnetic field. The magnetic lines
of force around adjacent loops are deflected into an outer
path when the loops are brought close together. This happens because the magnetic lines of force between adjacent loops
are in opposition with each other. The total magnetic field for
the two loops is shown in Figure 12-125B . As more loops
are added close together, the strength of the magnetic field
increases. Figure 12-125C illustrates the combined effects
of many loops of a coil. The result is a strong electromagnet.
The primary aspect of the operation of a coil is its property
to oppose any change in current through it. This property
is called inductance. When current flows through any
conductor, a magnetic field starts to expand from the center
of the wire. As the lines of magnetic force grow outward
through the conductor, they induce an emf in the conductor
itself. The induced voltage is always in the direction opposite
to the direction of the current flow. The effects of this
countering emf are to oppose the immediate establishment
of the maximum current. This effect is only a temporary
condition. Once the current reaches a steady value in the
conductor, the lines of magnetic force no longer expand and
the countering emf is no longer present.
At the starting instant, the countering emf nearly equals the
applied voltage, resulting in a small current flow. However,
as the lines of force move outward, the number of lines
cutting the conductor per second becomes progressively
smaller, resulting in a diminished counter emf. Eventually,
the counter emf drops to zero and the only voltage in
the circuit is the applied voltage and the current is at its
maximum value.
The RL Time Constant
Because the inductors basic action is to oppose a change in
its current, it then follows that the current cannot change
instantaneously in the inductor. A certain time is required
for the current to make a change from one value to another.
The rate at which the current changes is determined by a time
constant represented by the Greek letter τ. The time constant
for the RL circuit is:
τ =R L
Where τ = seconds
L = inductance (H)
R = resistance (Ω)
In a series RL circuit, the current increases to 63 percent of
its full value in 1 time constant after the circuit is closed. This
buildup is similar to the buildup of voltage in a capacitor
when charging an RC circuit. Both follow an exponential
curve and reach 99 percent value after the 5th time constant.
[Figure 12-126]
Physical Parameters
Some of the physical factors that affect inductance are:
Current and voltage in phase
Effect of inductance
Effect of capacitanceA
B
C0° 180° 360°emf
0°
Lag180° 90° 360° 270°emf
0°
Lead180° 90° 360° 270°emf
Figure 12-124. Phase of current and voltage. 1. The number of turns: Doubling the number of turns
in a coil produces a field twice as strong if the same
current is used. As a general rule, the inductance varies
as the square of the number of turns.
2. The cross-sectional area of the coil: The inductance
of a coil increases directly as the cross-sectional area
of the core increases. Doubling the radius of a coil
increases the inductance by a factor of four.
3. The length of a coil: Doubling the length of a coil,
while keeping the same number of turns, halves the
value of inductance.
4. The core material around which the coil is formed:
Coils are wound on either magnetic or nonmagnetic
materials. Some nonmagnetic materials include air,
copper, plastic, and glass. Magnetic materials include
nickel, iron, steel, or cobalt, which have a permeability
that provides a better path for the magnetic lines of
force and permit a stronger magnetic field.
Self-Inductance
The characteristic of self-inductance was summarized by
German physicist Heinrich Lenz in 1833, and gives the direction
of the induced emf resulting from electromagnetic induction.
This is commonly known as Lenz’s Law, which states: The emf
induced in an electric circuit always acts in such a direction
that the current it drives around a closed circuit produces a
magnetic field, which opposes the change in magnetic flux.
Self-inductance is the generation of a voltage in an electric
circuit by a changing current in the same circuit. Even
a straight piece of wire has some degree of inductance
because current in a conductor produces a magnetic field.
When the current in a conductor changes direction, there is
a corresponding change in the polarity of the magnetic field
around the conductor. Therefore, a changing current produces
a changing magnetic field around the wire. To further
intensify the magnetic field, the wire can be rolled into a
coil, which is called an inductor. The changing magnetic field
around the inductor induces a voltage across the coil. This
induced emf is called self-inductance and tends to oppose
any change in current within the circuit. This property is
usually called inductance and symbolized with the letter L.
Types of Inductors
Inductors used in radios can range from a straight wire at UHF
to large chokes and transformers used for filtering the ripple
from the output of power supplies and in audio amplifiers.
Figure 12-127 shows the schematic symbols for common
inductors. Values of inductors range from nano-henries to
tens of henries.
Inductors are classified by the type of core and the method of winding them. The number of turns in the inductor winding
and the core material determine the capacity of the inductor.
Cores made of dielectric material like ceramics, wood, and
paper provide small amounts of stored energy while cores
made of ferrite substances have a much higher degree of
stored energy. The core material is usually the most important
aspect of the inductors construction. The conductors typically
used in the construction of an inductor offer little resistance
to the flow of current. However, with the introduction of a
core, resistance is introduced in the circuit and the current
now builds up in the windings until the resistance of the core
is overcome. This buildup is stored as magnetic energy in
the core. Depending on the core resistance, the buildup soon
reaches a point of magnetic saturation, and it can be released
when necessary. The most common core materials are: air,
solid ferrite, powdered ferrite, steel, toroid, and ferrite toroid.
63%100%
90%
80%
70%
60%
50%
40%
30%
20%
10%86%95% 98%
Applied
voltageCurrent
1t
Counter emf
Current, counter emf, and applied voltage in an inductive circuit.2t 3t 4t 5t99%
Figure 12-126. Inductor curve.
Opposing magnetic fields
Combined magnetic fieldsCoils with some separationA
Coils without separationB
Strong magnetic field in a coilCCurrent eee eee
Current eee eee
Current
eeeeeeSouth North
Figure 12-125. Many loops of a coil.
Units of Inductance
The henry is the basic unit of inductance and is symbolized
with the letter H. An electric circuit has an inductance of one
henry when current changing at the rate of one ampere per
second induces a voltage of one volt into the circuit. In many
practical applications, millihenries (mH) and microhenries
(μH) are more common units. The typical symbol for an
inductor is shown in Figure 12-127 .
Inductors in Series
If we connect two inductors in series, the same current flows
through both inductors and, therefore, both are subject to
the same rate of change of current. [Figure 12-128] When
inductors are connected in series, the total inductance L T, is
the sum of the individual inductors. The general equation for
n number of inductors in series is:
LT = L 1 + L 2 + L 3 + … L NInductors in Parallel
When two inductors are connected in parallel, each must
have the same potential difference between the terminals.
[Figure 12-129] When inductors are connected in parallel,
the total inductance is less than the smallest inductance. The
general equation for n number of inductors in parallel is:
LT =
+ +1
L11
L21
L31
LN 1
+ . . .
A simple example would be:
L 1 = 10 mH, L 2 = 5 mH, L 3 = 2 mH
LT =
+ +1
10 mH1
5 mH1
2 mH 1
LT =0.8 mH 1
L T = 1.25 mH
Inductive Reactance
Alternating current is in a constant state of change; the effects
of the magnetic fields are a continuously inducted voltage
opposition to the current in the circuit. This opposition
is called inductive reactance, symbolized by X L, and is
measured in ohms just as resistance is measured. Inductance
is the property of a circuit to oppose any change in current and
is measured in henries. Inductive reactance is a measure of
how much the countering emf in the circuit opposes current
Inductor
+L1L2
−
Figure 12-127. Typical symbol for an inductor. Figure 12-128. Two inductors in series.variations.
The inductive reactance of a component is directly
proportional to the inductance of the component and the
applied frequency to the circuit. By increasing either the
inductance or applied frequency, the inductive reactance
likewise increases and presents more opposition to current
in the circuit. This relationship is given as:
X L = 2πfL
Where: X L = inductive reactance in ohms
f = frequency in cycles per second
π = 3.1416
L = inductance
In Figure 12-130 , an AC series circuit is shown in which the
inductance is 0.146 henry and the voltage is 110 volts at a
frequency of 60 cps. Inductive reactance is determined by
the following method.
X L = 2π × f × L
X L = 6.28 × 60 × 0.146
X L = 55 ohm
In any circuit where there is only resistance, the expression
for the relationship of voltage and current is given by Ohm’s
Law: I = E/R. Similarly, when there is inductance in an AC
circuit, the relationship between voltage and current can be
expressed as:
Current = or I = V oltage E
Reactance XL
Where:
X L = inductive reactance of the circuit in ohms
I = E
XL
I = 110
I = 2 amperes
In AC series circuits, inductive reactances are added like
resistances in series in a DC circuit. [Figure 12-131] Thus,
the total reactance in the illustrated circuit equals the sum of the individual reactances.
The total reactance of inductors connected in parallel is found
the same way as the total resistance in a parallel circuit.
[Figure 12-132] Thus, the total reactance of inductances
connected in parallel, as shown, is expressed as:
(XL)T =
+ +1
(XL)11
(XL)21
(XL)3 1
AC Circuits
Ohm’s Law for AC Circuits
The rules and equations for DC circuits apply to AC circuits
only when the circuits contain resistance alone, as in the case
of lamps and heating elements. In order to use effective values
of voltage and current in AC circuits, the effect of inductance
and capacitance with resistance must be considered.
The combined effects of resistance, inductive reactance,
and capacitive reactance make up the total opposition to
current flow in an AC circuit. This total opposition is called
impedance and is represented by the letter Z. The unit for
the measurement of impedance is the ohm.
Series AC Circuits
If an AC circuit consists of resistance only, the value of the
impedance is the same as the resistance, and Ohm’s Law for
an AC circuit, I = E/Z, is exactly the same as for a DC circuit.
In Figure 12-133, a series circuit containing a lamp with 11
ohms resistance connected across a source is illustrated. To
find how much current flows if 110 volts DC is applied and
how much current flows if 110 volts AC are applied, the
following examples are solved:
I = E
R I = (where Z = R) E
Z
I = 110 V
11 W I = 110 V
11 W
I = 10 amperes DC I = 10 amperes AC
When AC circuits contain resistance and either inductance
or capacitance, the impedance, Z, is not the same as the
110V AC
60 cycles
A
XL1
XL2Figure 12-130. AC circuit containing inductance.
Figure 12-131. Inductances in series.
+
L2
−L1
Figure 12-129. Two inductors in parallel.
resistance, R. The impedance of a circuit is the circuit’s
total opposition to the flow of current. In an AC circuit,
this opposition consists of resistance and reactance, either
inductive or capacitive or elements of both.
Resistance and reactance cannot be added directly, but they
can be considered as two forces acting at right angles to
each other. Thus, the relation between resistance, reactance,
and impedance may be illustrated by a right triangle.
[Figure 12-134]
Since these quantities may be related to the sides of a right
triangle, the formula for finding the impedance, or total
opposition to current flow in an AC circuit, can be found
by using the law of right triangles. This theorem, called the
Pythagorean theorem, applies to any right triangle. It states
that the square of the hypotenuse is equal to the sum of the
squares of the other two sides. Thus, the value of any side of a
right triangle can be found if the other two sides are known. If
an AC circuit contains resistance and inductance, as shown in
Figure 12-135 , the relation between the sides can be stated as:
Z2 = R2 + X L2
The square root of both sides of the equation gives
Z = R2 + X L2
This formula can be used to determine the impedance when
the values of inductive reactance and resistance are known. It
can be modified to solve for impedance in circuits containing
capacitive reactance and resistance by substituting X C in the
formula in place of X L. In circuits containing resistance with
both inductive and capacitive reactance, the reactances can be
combined, but because their effects in the circuit are exactly
opposite, they are combined by subtraction:
X = X L − X C or X = X C − X L (the smaller number is
always subtracted from the larger)
In Figure 12-135 , a series circuit consisting of resistance and inductance connected in series is connected to a source of
110 volts at 60 cps. The resistive element is a lamp with 6
ohms resistance, and the inductive element is a coil with an
inductance of 0.021 henry. What is the value of the impedance
and the current through the lamp and the coil?
Solution:
First, the inductive reactance of the coil is computed:
X L = 2π × f × L
X L = 6.28 × 60 × 0.021
X L = 8 ohms inductive reactance
Next, the total impedance is computed:
Z = R2 + X L2
Z = 62 + 82
Z = 36 + 64
Z = 100
Z = 10 ohms impedance
Then the current flow,
I = E
Z
I = 110
I = 11 amperes current
XL2XL1
110 V
Reactance
ResistanceZ
RImpedance
XL − XC
Figure 12-132. Inductances in parallel.Figure 12-133. Applying DC and AC to a circuit.
Figure 12-134. Impedance triangle.The voltage drop across the resistance (ER) is:
ER = I × R
ER = 11 × 6 = 66 volts
The voltage drop across the inductance (EX L) is:
EX L = I × X L
EX L = 11 × 8 = 88 volts
The sum of the two voltages is greater than the impressed
voltage. This results from the fact that the two voltages are
out of phase and, as such, represent the maximum voltage.
If the voltage in the circuit is measured by a voltmeter, it is
approximately 110 volts, the impressed voltage. This can be
proved by the equation:
E = (ER)2 + (EX L)2
E = 662 + 882
E = 4,356 + 7,744
E = 12,100
E = 110 volts
In Figure 12-136 , a series circuit is illustrated in which a
capacitor of 200 µf is connected in series with a 10 ohm
lamp. What is the value of the impedance, the current flow,
and the voltage drop across the lamp?
Solution:
First, the capacitance is changed from microfarads to farads.
Since 1 million microfarads equal 1 farad, then:
200 μf = = 0.000200 farads 200
1,000,000
XC = 1
2πfC
XC = 1
6.28 × 60 × 0.000200 farads
XC = 1
0.07536 X C = 13 ohms capacitive reactance
To find the impedance,
Z = R2 + X C2
Z = 102 + 132
Z = 100 + 169
Z = 269
Z = 16.4 ohms capacitive reactance
To find the current,
I = E
Z
I = 110
I = 6.7 amperes
The voltage drop across the lamp (ER) is:
ER = 6.7 × 10
ER = 67 volts
The voltage drop across the capacitor (EX C) is
EX C = I × X C
EX C = 6.7 × 13
EX C = 86.1 volts
110 V AC
60 cycles10 Ω
200 μf
A
Figure 12-136. A circuit containing resistance and capacitance.110V AC
60 cycles0.021 henries
A6 Ω
Figure 12-135. A circuit containing resistance and inductance.
The sum of these two voltages does not equal the applied
voltage, since the current leads the voltage. To find the
applied voltage, use the following formula:
ET = (ER)2 + (EX C)2
ET = 672 + 86.12
ET = 4,489 + 7,413
ET = 11,902
E T = 110 volts
When the circuit contains resistance, inductance, and
capacitance, the following equation is used to find the
impedance:
Z = R2 + (X L – XC)2
Example: What is the impedance of a series circuit,
consisting of a capacitor with a reactance of 7 ohms, an
inductor with a reactance of 10 ohms, and a resistor with a
resistance of 4 ohms? [Figure 12-137]
Solution:
Z = R2 + (X L – XC)2
Z = 42 + (10 – 7)2
Z = 42 + 32
Z = 25
Z = 5 ohms
Assuming that the reactance of the capacitor is 10 ohms and
the reactance of the inductor is 7 ohms, then X C is greater
than X L. Thus,
Z = R2 + (X L – XC)2
Z = 42 + (7 – 10)2
Z = 42 + (–3)2
Z = 16 + 9
Z = 25
Z = 5 ohmsParallel AC Circuits
The methods used in solving parallel AC circuit problems
are basically the same as those used for series AC circuits.
Out of phase voltages and currents can be added by using
the law of right triangles. However, in solving circuit
problems, the currents through the branches are added since
the voltage drops across the various branches are the same
and are equal to the applied voltage. In Figure 12-138 , a
parallel AC circuit containing an inductance and a resistance
is shown schematically. The current flowing through the
inductance, I L, is 0.0584 ampere, and the current flowing
through the resistance is 0.11 ampere. What is the total
current in the circuit?
Solution:
IT = IL2 + I R2
= (0.0584)2 + (0.11)2
= 0.0155
= 0.1245 ampere
Since inductive reactance causes voltage to lead the current,
the total current, which contains a component of inductive
current, lags the applied voltage. If the current and voltages
are plotted, the angle between the two, called the phase angle,
illustrates the amount the current lags the voltage.
In Figure 12-139 , a 112-volt generator is connected to a load
consisting of a 2 µf capacitance and a 10,000-ohm resistance
in parallel. What is the value of the impedance and total
current flow?
Solution:
First, find the capacitive reactance of the circuit:
XC = 1
2πfC
Changing 2 μf to farads and entering the values into the
formula given:
110 V AC
60 cycles4 Ω
10 Ω7 Ω
Figure 12-137. A circuit containing resistance, inductance, and
capacitance. = 1
2 × 3.14 × 60 × 0.000002
= or 1 10,000
0.00075360 7.536
= 1,327 X C capacitive reactance
To find the impedance, the impedance formula used in a
series AC circuit must be modified to fit the parallel circuit:
Z = R2 + X C2RX C
= (10,000)2 + (1,327)210,000 × 1,327
= 0.1315 W (approximately)
To find the current through the capacitance:
IC = E
XC
IC = 110
1,327
= 0.0829 ampere
To find the current flowing through the resistance:
IR = E
R
= 110
10,000
= 0.011 ampere
To find the total current in the circuit:
IT2 = IR2 + I C2
IT = IL2 + I R2
= 0.0836 ampere (approximately)
Resonance
It has been shown that both inductive reactance:
(X L = 2πfL)
and capacitive reactance:
XC = 1
2πfC
are functions of an AC frequency. Decreasing the frequency
decreases the ohmic value of the inductive reactance, but a
decrease in frequency increases the capacitive reactance. At
some particular frequency, known as the resonant frequency,
the reactive effects of a capacitor and an inductor is equal. Since these effects are the opposite of one another, they will
cancel, leaving only the ohmic value of the resistance to
oppose current flow in a circuit. If the value of resistance is
small or consists only of the resistance in the conductors, the
value of current flow can become very high.
In a circuit where the inductor and capacitor are in series,
and the frequency is the resonant frequency, or frequency
of resonance, the circuit is said to be “in resonance” and
is referred to as a series resonant circuit. The symbol for
resonant frequency is Fn.
If, at the frequency of resonance, the inductive reactance is
equal to the capacitive reactance, then:
XL = X C, or
2πfL = 1
2πfC
Dividing both sides by 2 fL,
Fn2 = 1
(2π) 2LC
Extracting the square root of both sides gives:
Fn = 1
2π LC
Where Fn is the resonant frequency in cps, C is the
capacitance in farads, and L is the inductance in henries.
With this formula, the frequency at which a capacitor and
inductor is resonant can be determined.
To find the inductive reactance of a circuit use:
XL = 2πfL
The impedance formula used in a series AC circuit must be
modified to fit a parallel circuit.
12-642 μf 110V G 10,000 Ω
Figure 12-139. A parallel AC circuit containing capacitance and
resistance.ILGIR5 henries
1000 Ω
Figure 12-138. AC parallel circuit containing inductance and
resistance.
Z = R2 = X L2XL
To find the parallel networks of inductance and capacitive
reactors, use:
X = XL + X CXL + X C
To find the parallel networks with resistance capacitive and
inductance, use:
Z = XL2 XC2 + (RX L – RX C)2R X L XC
Since at the resonant frequency X L cancels X C, the current can
become very large, depending on the amount of resistance. In
such cases, the voltage drop across the inductor or capacitor
is often higher than the applied voltage.
In a parallel resonant circuit, the reactances are equal and
equal currents flow through the coil and the capacitor.
[Figure 12-140]
Since the inductive reactance causes the current through the
coil to lag the voltage by 90°, and the capacitive reactance causes the current through the capacitor to lead the voltage
by 90°, the two currents are 180° out of phase. The canceling
effect of such currents would mean that no current would
flow from the generator and the parallel combination of
the inductor and the capacitor would appear as infinite
impedance. In practice, no such circuit is possible, since some
value of resistance is always present, and the parallel circuit,
sometimes called a tank circuit, acts as very high impedance.
It is also called an antiresonant circuit, since its effect in a
circuit is opposite to that of a series resonant circuit, in which
the impedance is very low.
Power in AC Circuits
In a DC circuit, power is obtained by the equation, P = EI,
(watts equal volts × amperes). Thus, if 1 ampere of current
flows in a circuit at a pressure of 200 volts, the power is 200
watts. The product of the volts and the amperes is the true
power in the circuit.
True Power Defined
True power of any AC circuit is commonly referred to as
the working power of the circuit. True power is the power
consumed by the resistance portion of the circuit and is
measured in watts. True power is symbolized by the letter P
and is indicated by any wattmeter in the circuit. True power
is calculated by the formula:
P = I2 × Z
Apparent Power Defined
Apparent power in an AC circuit is sometimes referred to as
the reactive power of a circuit. Apparent power is the power
consumed by the entire circuit, including both the resistance
and the reactance. Apparent power is symbolized by the letter
S and is measured in volt-amps (V A). Apparent power is a
product of the effective voltage multiplied by the effective
current. Apparent power is calculated by the formula:
S = I2 × Z
Only when the AC circuit is made up of pure resistance is
the apparent power equal to the true power. [Figure 12-141]
When there is capacitance or inductance in the circuit, the
current and voltage are not exactly in phase, and the true
power is less than the apparent power. The true power is
obtained by a wattmeter reading. The ratio of the true power
to the apparent power is called the power factor and is usually
expressed in percent. In equation form, the relationship is:
Power Factor (PF) = 100 × watts (True Power)
volts × amperes (Apparent Power)
Example: A 220-volt AC motor takes 50 amperes from the
line, but a wattmeter in the line shows that only 9,350 watts
Figure 12-140. A parallel resonant circuit.are taken by the motor. What are the apparent power and the
power factor?
Solution:
Apparent power = V olts × Amperes
Apparent power = 220 × 50 = 11,000 watts or
volt-amperes.
(PF) = Watts (True Power) × 100
V A (Apparent Power)
(PF) = 9,350 × 100
11,000
(PF) = 85, or 85%
Transformers
A transformer changes electrical energy of a given voltage
into electrical energy at a different voltage level. It consists of
two coils that are not electrically connected, but are arranged
so that the magnetic field surrounding one coil cuts through
the other coil. When an alternating voltage is applied to
(across) one coil, the varying magnetic field set up around that
coil creates an alternating voltage in the other coil by mutual
induction. A transformer can also be used with pulsating DC,
but a pure DC voltage cannot be used, since only a varying
voltage creates the varying magnetic field that is the basis
of the mutual induction process.
A transformer consists of three basic parts. [Figure 12-142]
These are an iron core, which provides a circuit of low
reluctance for magnetic lines of force; a primary winding,
which receives the electrical energy from the source of applied
voltage; and a secondary winding, which receives electrical
energy by induction from the primary coil.
The primary and secondary of this closed core transformer
are wound on a closed core to obtain maximum inductive
effect between the two coils.
There are two classes of transformers: voltage transformers,
used for stepping up or stepping down voltages; and
current transformers used in instrument circuits. In voltage
transformers, the primary coils are connected in parallel
across the supply voltage. [Figure 12-143A] The primary
windings of current transformers are connected in series in
the primary circuit. [Figure 12-143B] Of the two types, the
voltage transformer is the more common.
There are many types of voltage transformers. Most of these
are either step-up or step-down transformers. The factor
that determines whether a transformer is a step-up or step-
down type is the “turns” ratio. The turns ratio is the ratio of
the number of turns in the primary winding to the number
of turns in the secondary winding. For example, the turns
ratio of the step-down transformer is 5 to 1, since there are five times as many turns in the primary as in the secondary.
[Figure 12-144A] The step-up transformer has a 1 to 4 turns
ratio. [Figure 12-144B]
The ratio of the transformer input voltage to the output
voltage is the same as the turns ratio if the transformer is
100 percent efficient. Thus, when 10 volts are applied to
the primary of the transformer, two volts are induced in the
secondary. [Figure 12-144A] If 10 volts are applied to the
primary of the transformer, the output voltage across the
terminals of the secondary is 40 volts. [Figure 12-144B]
No transformer can be constructed that is 100 percent
efficient, although iron core transformers can approach this
figure. This is because all the magnetic lines of force set up
in the primary do not cut across the turns of the secondary
coil. A certain amount of the magnetic flux, called leakage
flux, leaks out of the magnetic circuit. The measure of how
well the flux of the primary is coupled into the secondary
is called the “coefficient of coupling.” For example, if it is
assumed that the primary of a transformer develops 10,000
lines of force and only 9,000 cut across the secondary, the
coefficient of coupling would be 0.9. Stated another way, the
transformer would be 90 percent efficient.
When an AC voltage is connected across the primary terminals
of a transformer, an AC flows and self induces a voltage in the
primary coil that is opposite and nearly equal to the applied
voltage. The difference between these two voltages allows
just enough current in the primary to magnetize its core. This
is called the exciting, or magnetizing, current. The magnetic
field caused by this exciting current cuts across the secondary
coil and induces a voltage by mutual induction.
If a load is connected across the secondary coil, the load current
flowing through the secondary coil produces a magnetic field
that tends to neutralize the magnetic field produced by the
primary current. This reduces the self-induced (opposition)
voltage in the primary coil and allows more primary current
Primary coil
Secondary coilS
P
Iron core
Figure 12-142. An iron-core transformer.
Reactive power
WattsTrue powerApparent power
volts x amperes
Figure 12-141. Power relations in AC circuit.to flow. The primary current increases as the secondary load
current increases, and decreases as the secondary load current
decreases. When the secondary load is removed, the primary
current is again reduced to the small exciting current sufficient
only to magnetize the iron core of the transformer.
If a transformer steps up the voltage, it steps down the current
by the same ratio. This should be evident if the power formula
is considered, for the power (I × E) of the output (secondary)
electrical energy is the same as the input (primary) power
minus that energy loss in the transforming process. Thus,
if 10 volts and 4 amps (40 watts of power) are used in the
primary to produce a magnetic field, there is 40 watts of
power developed in the secondary (disregarding any loss).
If the transformer has a step-up ratio of 4 to 1, the voltage
across the secondary is 40 volts and the current is 1 amp. The
voltage is 4 times greater and the current is one-fourth the
primary circuit value, but the power (I × E value) is the same.
When the turns ratio and the input voltage are known, the
output voltage can be determined as follows:
E2
E1 = N2
N1
Where E is the voltage of the primary, E 2 is the output voltage
of the secondary, and N 1 and N 2 are the number of turns of
the primary and secondary, respectively.
Transposing the equation to find the output voltage gives:
E2 = E1 N2
N1
The most commonly used types of voltage transformers are:
1. Power transformers are used to step up or step down
voltages and current in many types of power supplies.
They range in size from the small power transformer
[Figure 12-145] used in a radio receiver to the large
transformers used to step down high power line voltage to the 110–120 volt level used in homes.
Figure 12-146 shows the schematic symbol for an
iron core transformer. In this case, the secondary is
made up of three separate windings. Each winding
supplies a different circuit with a specific voltage,
which saves the weight, space, and expense of three
separate transformers. Each secondary has a midpoint
connection called a “center tap,” which provides a
selection of half the voltage across the whole winding.
The leads from the various windings are color coded
by the manufacturer. [Figure 12-146] This is a standard
color code, but other codes or numbers may be used.
2. Audio transformers resemble power transformers. They
have only one secondary and are designed to operate
over the range of audio frequencies (20 to 20,000 cps).
3. RF transformers are designed to operate in equipment
that functions in the radio range of frequencies. The
symbol for the RF transformer is the same as for an RF
choke coil. It has an air core as shown in Figure 12-147 .
4. Autotransformers are normally used in power circuits;
however, they may be designed for other uses. Two
different symbols for autotransformers used in power
or audio circuits are shown in Figure 12-148 . If
used in an RF communication or navigation circuit
[Figure 12-148B] , it is the same, except there is no
symbol for an iron core. The autotransformer uses
part of a winding as a primary; and, depending on
whether it is step up or step down, it uses all or part
of the same winding as the secondary. For example,
the autotransformer shown in Figure 12-148A could
use the following possible choices for primary and
secondary terminals.
AC power supply
Load
AC power supply
MeterTo load
A B
10 turns
primary2 turns
secondary
8 turns
secondary2 turns
primary
A
BFigure 12-143. Voltage and current transformers.
Figure 12-144. A step-down and a step-up transformer.Current Transformers
Current transformers are used in AC power supply systems
to sense generator line current and to provide a current,
proportional to the line current, for circuit protection and
control devices.
The current transformer is a ring-type transformer using a
current carrying power lead as a primary (either the power
lead or the ground lead of the AC generator). The current in the primary induces a current in the secondary by magnetic
induction. The sides of all current transformers are marked
“H1” and “H2” on the unit base. The transformers must
be installed with the “H1” side toward the generator in the
circuit in order to have proper polarity. The secondary of the
transformer should never be left open while the system is being
operated; to do so could cause dangerously high voltages and
could overheat the transformer. Therefore, the transformer
output connections should always be connected with a jumper
when the transformer is not being used but is left in the system.
Transformer Losses
In addition to the power loss caused by imperfect coupling,
transformers are subject to “copper” and “iron” losses. The
resistance of the conductor comprising the turns of the coil
causes copper loss. The iron losses are of two types: hysteresis
loss and eddy current loss. Hysteresis loss is the electrical
energy required to magnetize the transformer core, first in
one direction and then in the other, in step with the applied
alternating voltage. Eddy current loss is caused by electric
currents (eddy currents) induced in the transformer core by
the varying magnetic fields. To reduce eddy current losses,
cores are made of laminations coated with an insulation,
which reduces the circulation of induced currents.
Power in Transformers
Since a transformer does not add any electricity to the circuit
but merely changes or transforms the electricity that already
exists in the circuit from one voltage to another, the total
amount of energy in a circuit must remain the same. If it were
possible to construct a perfect transformer, there would be no
loss of power in it; power would be transferred undiminished
from one voltage to another.
Since power is the product of volts times amperes, an increase
in voltage by the transformer must result in a decrease in
current and vice versa. There cannot be more power in the
secondary side of a transformer than there is in the primary.
The product of amperes times volts remains the same.
The transmission of power over long distances is accomplished
by using transformers. At the power source, the voltage is
stepped up in order to reduce the line loss during transmission.
At the point of utilization, the voltage is stepped down, since
it is not feasible to use high voltage to operate motors, lights,
or other electrical appliances.
DC Measuring Instruments
Understanding the functional design and operation of
electrical measuring instruments is very important, since
they are used in repairing, maintaining, and troubleshooting
electrical circuits. The best and most expensive measuring
instrument is of no use unless the technician knows what is
Red
RedRed - Yellow
YellowBlackIron core
High-voltage
winding
5-volt winding
6-volt winding
Secondary windingsBlackPrimary
Yellow
Green
GreenGreen - YellowYellow - Blue
2 Input
Output
3Primary
2–3used with
“ ”
“ ”
“ ”
“ ”
“ ”
“ ”Secondary
A BFigure 12-146. Schematic symbol for an iron-core power
transformer.
Figure 12-147. An air-core transformer.
Figure 12-148. Autotransformers.
Figure 12-145. Power supply transformer.being measured and what each reading indicates. The purpose
of the meter is to measure quantities existing in a circuit. For
this reason, when a meter is connected to a circuit, it must
not change the characteristics of that circuit.
Meters are either self-excited or externally excited. Those that
are self-excited operate from a power source within the meter.
Externally-excited meters get their power source from the
circuit that they are connected to. The most common analog
meters in use today are the voltmeter, ammeter, and ohmmeter.
All of which operate on the principles of electromagnetism.
The fundamental principle behind the operation of the meter
is the interaction between magnetic fields created by a current
gathered from the circuit in some manner. This interaction is
between the magnetic fields of a permanent magnet and the
coils of a rotating magnet. The greater the current through the
coils of the rotating magnet, the stronger the magnetic field
produced. A stronger field produces greater rotation of the coil.
While some meters can be used for both DC and AC circuit
measurement, only those used as DC instruments are discussed
in this section. The meters used for AC, or for both AC and
DC, are discussed in the study of AC theory and circuitry.
D’Arsonval Meter Movement
This basic DC type of meter movement—first employed
by the French scientist, d’Arsonval, in making electrical
measurement—is a current measuring device, which is
used in the ammeter, voltmeter, and ohmmeter. The pointer
is deflected in proportion to the amount of current through
the coil. Basically, both the ammeter and the voltmeter are
current measuring instruments, the principal difference being
the method in which they are connected in a circuit. While an
ohmmeter is also basically a current measuring instrument,
it differs from the ammeter and voltmeter in that it provides
its own source (self-excited) of power and contains other
auxiliary circuits.Current Sensitivity and Resistance
The current sensitivity of a meter movement is the amount of
current required to drive the meter movement to a full-scale
deflection. A simple example would be a meter movement
that has 1 mA sensitivity. What this indicates is that meter
movement requires 1 mA of current to move the needle to
a full-scale indication. Likewise, a half-scale deflection
requires only 0.5 mA of current. Additionally, what is called
12-69movement resistance is the actual DC resistance of the wire
used to construct the meter coil.
In a standard d’Arsonval meter, movement may have a
current sensitivity of 1 mA and a resistance of 50 Ω. If the
meter is going to be used to measure more than 1 mA, then
additional circuitry is required to accomplish the task. This
additional circuitry is a simple shunt resistor. The purpose
of the shunt resistor is to bypass current that exceeds the
1 mA limitation of the meter movement. To illustrate this,
assume that the 1 mA meter in question is needed to measure
10 mA. The shunt resistor used should carry 9 mA while
the remaining 1 mA is allowed to pass through the meter.
[Figure 12-149]
To determine the proper shunt resistance for this situation:
R SH = Shunt resistance
R M = Meter resistance = 50 Ω
Because the shunt resistance and the 50 Ω meter resistance are
in parallel, the voltage drop across both of them is the same.
E SH = E M
Using Ohm’s Law, this relationship can be rewritten as:
E SH = I SH × R SH
E M = I M × R M
I SH × R SH = I M × R M
Simply solve for R SH
RSH =IM × R M
ISH
Substituting the values
RSH = = 5.56 ΩImA × 50 Ω
9 mA
Damping
To make meter readings quickly and accurately, it is
desirable that the moving pointer overshoot its proper
position only a small amount and come to rest after not more
than one or two small oscillations. The term “damping” is
applied to methods used to bring the pointer of an electrical
meter to rest after it has been set in motion. Damping may
be accomplished by electrical means, by mechanical means,
or by a combination of both.Electrical Damping
A common method of damping by electrical means is to wind
the moving coil on an aluminum frame. As the coil moves
in the field of the permanent magnet, eddy currents are set
up in the aluminum frame. The magnetic field produced by
the eddy currents opposes the motion of the coil. The pointer
therefore swings more slowly to its proper position and comes
to rest quickly with very little oscillation.
Mechanical Damping
Air damping is a common method of damping by mechanical
means. As shown in Figure 12-150 , a vane is attached to the
shaft of the moving element and enclosed in an air chamber.
The movement of the shaft is retarded because of the
resistance that the air offers to the vane. Effective damping is
achieved if the vane nearly touches the walls of the chamber.
A Basic Multirange Ammeter
Building upon the basic meter previously discussed is the more
complex and useful multirange meter, which is more practical.
The basic idea of a multirange ammeter is to make the meter
usable over a wide range of voltages. In order to accomplish
this, each range must utilize a different shunt resistance. The
example given in this handbook is that of a two-range meter.
However, once the basics of a two-range multirange ammeter
are understood, the concepts can easily be transferred to the
design of meters with many selectable ranges.
Figure 12-151 shows the schematic of an ammeter with two
selectable ranges. This example builds upon the previous
10 mA range meter by adding a 100 mA range. With the
switch selected to the 10 mA range, the meter indicates
10 mA when the needle is deflected to full-scale and likewise
indicates 100 mA at full-scale when selected to 100 mA. The
value of the 100 mA shunt resistor is determined the same
way the 10 mA shunt resistor was determined. Recall that
the meter movement can only carry 1 mA. This means that
in a 100 mA range the remaining current of 99 mA must pass
through the shunt resistor.
RSH =IM × R M
ISH
Substituting the values:
RSH = = 0.51 ΩImA × 50 Ω
99 mA
Precautions
The precautions to observe when using an ammeter are
summarized as follows:
1. Always connect ammeter in series with the element
Meter movement
[+] [−]
RSHISHIMM
Meter movement
Basic ammeter[+] [−]
RSHISH1 mA
10 mA9 mA
Figure 12-149. Basic meter drawing.through which the current flow is to be measured.
2. Never connect an ammeter across a source of voltage,
such as a battery or generator. Remember that the
resistance of an ammeter, particularly on the higher
ranges, is extremely low and that any voltage, even a
volt or so, can cause very high current to flow through
the meter, causing damage to it.
3. Use a range large enough to keep the deflection less
than full-scale. Before measuring a current, form
some idea of its magnitude. Then switch to a large
enough scale or start with the highest range and
work down until the appropriate scale is reached. The
most accurate readings are obtained at approximately
half-scale deflection. Many milliammeters have been
ruined by attempts to measure amperes. Therefore,
be sure to read the lettering either on the dial or on
the switch positions and choose proper scale before
connecting the meter in the circuit.
4. Observe proper polarity in connecting the meter in the
circuit. Current must flow through the coil in a definite
direction in order to move the indicator needle up
scale. Current reversal because of incorrect connection
in the circuit results in a reversed meter deflection and
frequently causes bending of the meter needle. Avoid
improper meter connections by observing the polarity
markings on the meter.The Voltmeter
The voltmeter uses the same type of meter movement as
the ammeter but employs a different circuit external to the
meter movement.
As shown before, the voltage drop across the meter coil
is a function of current and the coil resistance. In another
example, 50 μA × 1,000 Ω = 50 mV . In order for the meter to
be used to measure voltages greater than 50 mV , there must
be added a series resistance to drop any excess voltage greater
than that which the meter movement requires for a full-scale
deflection. The case of the voltmeter, this resistance is called
multiplier resistance and is designated as R M. [Figure 12-152]
The voltmeter only has one multiplier resistor for use in one
range. In this example, the full-scale reading is 1 volt. R M is
determined in the following way:
The meter movement drops 50 mV at a full-scale deflection of
50 μA. The multiplying resistor R M must drop the remaining
voltage of 1 V − 50 mV = 950 mV . Since R M is in series with
the movement, it also carries 50 μA at full scale.
RM = = 19k Ω 950 mV
50 μA
Therefore, for 1 volt full-scale deflection, the total resistance
of the voltmeter is 20k Ω. That is, the multiplier resistance
and the coil resistance.
Voltmeter Sensitivity
V oltmeter sensitivity is defined in terms of resistance per
volt (Ω/V). The meter used in the previous example has a
sensitivity of 20k Ω and a full-scale deflection of 1 volt.
Multiple Range Voltmeters
The simplified voltmeter in Figure 12-152 has only one range
(1 volt), which means that it can measure voltages from 0
volts to 1 volt. In order for the meter to be more useful,
additional multiplier resistors must be used. One resistor
must be used for each desired range.
For a 50 μA movement, the total resistance required is 20k
Ω for each volt of full-scale reading. In other words, the
sensitivity for a 50 μA movement is always 20k Ω regardless
of the selected range. The full-scale meter current is 50 μA at
any range selection. To find the total meter resistance, multiply
the sensitivity by the full-scale voltage for that particular range.
For example for a 10 volt range, R T = (20k Ω/V) (10V) = 200k Ω.
The total resistance for the 1 volt range is 20k Ω, so R M for
a 10 V range is 200k Ω − 20k Ω = 180k Ω. [Figure 12-153]
Closed air chamber
Light vane swinging in
chamber with small
clearance
1mA, 50 ohm movement
[+][−]10mA
100mA5.56Ω
5.51ΩFigure 12-150. Air damping.
Figure 12-151. Ammeter with two ranges.Voltmeter Circuit Connections
When voltmeters are used, they are connected in parallel
with a circuit. If unsure about the voltage to be measured,
take the first reading at the high value on the meter and then
progressively move down through the range until a suitable
read is obtained. Observe that the polarity is correct before
connecting the meter to the circuit or damage occurs by
driving the movement backwards.
Influence of the Voltmeter in the Circuit
When a voltmeter is connected across two points in a circuit,
current is shunted. If the voltmeter has low resistance, it draws
off a significant amount of current. This lowers the effective
resistance of the circuit and change the voltage readings.
When making a voltage measurement, use a high resistance
voltmeter to prevent shunting of the circuit.The Ohmmeter
The meter movement used for the ammeter and the voltmeter
can also be used for the ohmmeter. The function of the
ohmmeter is to measure resistance. A simplified one-stage
ohmmeter is illustrated in Figure 12-154 , which shows that
the basic ohmmeter contains a battery and a variable resistor
in series with the meter movement. To measure resistance, the
leads of the meter are connected across an external resistance,
which is to be measured. By doing this, the ohmmeter circuit
is completed. This connection allows the internal battery
to produce a current through the movement coil, causing
a deflection of the pointer proportional to the value of the
external resistance being measured.
Zero Adjustment
When the ohmmeter leads are open, the meter is at a full-scale
deflection, indicating an infinite (∞) resistance or an open
circuit. [Figure 12-155] When the leads are shorted as shown
in figure “zero adjust,” the pointer is at the full right-hand
position, indicating a short circuit or zero resistance. The
purpose of the variable resistor in this figure is to adjust the
current so that the pointer is at exactly zero when the leads
are shorted. This is used to compensate for changes in the
internal battery voltage due to aging.
Ohmmeter Scale
Figure 12-156 shows a typical analog ohmmeter scale.
Between zero and infinity (∞), the scale is marked to indicate
various resistor values. Because the values decrease from left
to right, this scale is often called a back-off scale.
In the case of the example given, assume that a certain
ohmmeter uses a 50 μA, 1,000 Ω meter movement and has
an internal 1.5 volt battery. A current of 50 μA produces a
full-scale deflection when the test leads are shorted. To have
50 μA, the total ohmmeter resistance is 1.5 V/50 μA = 30k Ω.
Therefore, since the coil resistance is 1k Ω, the variable zero
adjustment resistor must be set to 30k Ω – 1k Ω = 29k Ω.
Now consider that a 120k Ω resistor is connected to
the ohmmeter leads. Combined with the 30k Ω internal
resistance, the total R is 150k Ω. The current is 1.5 V/150k
Ω = 10 μA, which is 20 percent of the full-scale current and
appears on the scale shown in Figure 12-156 .
Now consider further that a 120k Ω resistor is connected to
the ohmmeter leads. This results in a current of 1.5 V/75k Ω =
10 μA, which is 40 percent of the full-scale current and
marked on the scale. Additional calculations of this type
show that the scale is nonlinear. It is more compressed
toward the left side than the right side. The center scale point
corresponds to the internal meter resistance of 30k Ω. The
reason is as follows:
1k ohm movement
[+][−]
10 V 1 VRM2
180k Ω 19k ΩRM10 ∞
Basic voltmeter
Figure 12-153. Two range voltmeter.
50A, 1k ohm movement
[+] [−]RM
Figure 12-152. Basic voltmeter.With 30k Ω connected to the leads, the current is 1.5 V/60k Ω
= 25 μA, which is half of the full-scale current of 50 μA.
The Multirange Ohmmeter
A practical ohmmeter has several operational ranges. These
typically are indicated by R × 1, R × 10, R × 100, R × 1k,
R × 100k and R × 1M. These range selections are interpreted
in a different manner than that of an ammeter or voltmeter.
The reading on the ohmmeter scale is multiplied by the factor
indicated by the range setting. For example, if the pointer is
set on the scale and the range switch is set at R × 100, the
actual resistance measurement is 20 × 100 or 2k Ω.
To measure small resistance values, the technician must use
a higher ohmmeter current than is needed for measuring
large resistance values. Shunt resistors are needed to provide
multiple ranges on the ohmmeter to measure a range of
resistance values from the very small to very large. For each
range, a different value of shunt resistance is switched in.
The shunt resistance increases for higher ohm ranges and
is always equal to the center scale reading on any selected
range. In some meters, a higher battery voltage is used for
the highest ohm range. [Figure 12-157]
Megger (Megohmmeter)
The megger, or megohmmeter, is a high range ohmmeter
containing a hand-operated generator. It is used to measure
insulation resistance and other high-resistance values. It is
also used for ground, continuity, and short-circuit testing of
electrical power systems. The chief advantage of the megger
over an ohmmeter is its capacity to measure resistance with a
high potential, or “breakdown” voltage. This type of testing
ensures that insulation or a dielectric material will not short
or leak under potential electrical stress.
The megger consists of two primary elements, both of which
are provided with individual magnetic fields from a common
permanent magnet: a hand-driven DC generator, G, which
supplies the necessary current for making the measurement;
and the instrument portion, which indicates the value of the
resistance being measured. The instrument portion is of the
opposed coil type. Coils A and B are mounted on the movable
member with a fixed angular relationship to each other and
are free to turn as a unit in a magnetic field. Coil B tends to move the pointer counterclockwise and coil A, clockwise. The
coils are mounted on a light, movable frame that is pivoted in
jewel bearings and free to move about axis 0. [Figure 12-158]
Coil A is connected in series with R3 and the unknown
resistance, R X, to be measured. The series combination of
coil A, R3, and R X is connected between the + and − brushes
of the DC generator. Coil B is connected in series with R2,
and this combination is also connected across the generator.
There are no restraining springs on the movable member of
the instrument portion of the megger. When the generator is
not in operation, the pointer floats freely and may come to
rest at any position on the scale.
If the terminals are open circuited, no current flows in coil
A, and the current in coil B alone controls the movement
of the moving element. Coil B takes a position opposite
the gap in the core (since the core cannot move and coil B
can), and the pointer indicates infinity on the scale. When a
resistance is connected between the terminals, current flows
in coil A, tending to move the pointer clockwise. At the same
time, coil B tends to move the pointer counterclockwise.
Therefore, the moving element, composed of both coils and
the pointer, comes to rest at a position at which the two forces
are balanced. This position depends upon the value of the
external resistance, which controls the relative magnitude of
current of coil A. Because changes in voltage affect both coils
A and B in the same proportion, the position of the moving
element is independent of the voltage. If the terminals are
short circuited, the pointer rests at zero because the current
in A is relatively large. The instrument is not damaged under
these circumstances because the current is limited by R3.
There are two types of hand-driven meggers: the variable
type and the constant pressure type. The speed of the variable
pressure megger is dependent on how fast the hand crank
12-7300 Ω
∞40% full-scale 45k Ω
50% full-scale 30k Ω
Full-scale
Simplified Ohmmeter Scale
Meter movement
[+]+
[−]−
OpenAdjuster
0 ∞
Meter movement
[+]+
[−]−
ShortedAdjuster
Internal battery0 ∞
Meter movement
[+]+
[−]−
Test probes0 ∞
Meter movement
[+]+
[−]−
External resistorCurrentInternal batteryInternal batteryBasic voltmeter
Basic voltmeter0 ∞
Figure 12-156. Ohm scale.Figure 12-155. Zero adjustment.
Figure 12-154. Basic ohmmeter.
is turned. The constant pressure megger uses a centrifugal
governor, or slip clutch. The governor becomes effective only when the megger is operated at a speed above its slip speed, at which speed its voltage remains constant.
AC Measuring Instruments
A DC meter, such as an ammeter, connected in an AC circuit indicates zero, because the meter movements used in a d’Arsonval type movement is restricted to DC. Since the field of a permanent magnet in the d’Arsonval type meter remains constant and in the same direction at all times, the moving coil follows the polarity of the current. The coil attempts to move in one direction during half of the AC cycle and in the reverse direction during the other half when the current reverses.
The current reverses direction too rapidly for the coil to
follow, causing the coil to assume an average position. Since 20% full-scale 120k Ω
12-74Meter movement
[+] [−]−+Adjuster
R x 1
Range switchR x 10
R x 100
R x 1k
R x 10k0 ∞
Figure 12-157. Multirange ohmmeter.the current is equal and opposite during each half of the AC
cycle, the DC meter indicates zero, which is the average value.
Thus, a meter with a permanent magnet cannot be used to
measure alternating voltage and current. For AC measurements
of current and voltage, additional circuitry is required. The
additional circuitry has a rectifier, which converts AC to DC.
There are two basic types of rectifiers: the half-wave rectifier
and the full-wave rectifier. [Figure 12-159]
Figure 12-159 also shows a simplified block diagram of an
AC meter. In this depiction, the full-wave rectifier precedes
the meter movement. The movement responds to the average
value of the pulsating DC. The scale can then be calibrated
to show anything the designer wants. In most cases, it is the
root mean square (RMS) value or peak value.
Electrodynamometer Meter Movement
The electrodynamometer can be used to measure alternating or
direct voltage and current. It operates on the same principles
as the permanent magnet moving coil meter, except that the
permanent magnet is replaced by an air core electromagnet.
The field of the electrodynamometer is developed by the same
current that flows through the moving coil. [Figure 12-160]
Because this movement contains no iron, the
electrodynamometer can be used as a movement for both AC
and DC instruments. AC can be measured by connecting the
stationary and moving coils in series. Whenever the current
in the moving coil reverses, the magnetic field produced
by the stationary coil reverses. Regardless of the direction
of the current, the needle moves in a clockwise direction.
However, for either voltmeter or ammeter applications,
the electrodynamometer is too expensive to economically
compete with the d’Arsonval-type movement.
Moving Iron Vane Meter
The moving iron vane meter is another basic type of meter.
It can be used to measure either AC or DC. Unlike the
d’Arsonval meter, which employs permanent magnets, it
depends on induced magnetism for its operation. It utilizes the
principle of repulsion between two concentric iron vanes, one
fixed and one movable, placed inside a solenoid. A pointer is
attached to the movable vane. [Figure 12-161]
When current flows through the coil, the two iron vanes
become magnetized with North poles at their upper ends and
South poles at their lower ends for one direction of current
through the coil. Because like poles repel, the unbalanced
component of force, tangent to the movable element, causes
it to turn against the force exerted by the springs.
The movable vane is rectangular in shape and the fixed
vane is tapered. This design permits the use of a relatively
uniform scale.When no current flows through the coil, the movable vane
is positioned so that it is opposite the larger portion of the
tapered fixed vane, and the scale reading is zero. The amount
of magnetization of the vanes depends on the strength of
the field, which, in turn, depends on the amount of current
flowing through the coil.
The force of repulsion is greater opposite the larger end of
the fixed vane than it is nearer the smaller end. Therefore,
the movable vane moves toward the smaller end through an
angle that is proportional to the magnitude of the coil current.
The movement ceases when the force of repulsion is balanced
by the restraining force of the spring.
Because the repulsion is always in the same direction (toward
the smaller end of the fixed vane), regardless of the direction
of current flow through the coil, the moving iron vane
instrument operates on either DC or AC circuits.
Mechanical damping in this type of instrument can be obtained
by the use of an aluminum vane attached to the shaft so that,
as the shaft moves, the vane moves in a restricted air space.
When the moving iron vane meter is used as an ammeter, the
coil is wound with relatively few turns of large wire in order
to carry the rated current. When the moving iron vane meter is
used as a voltmeter, the solenoid is wound with many turns of
small wire. Portable voltmeters are made with self-contained
series resistance for ranges up to 750 volts. Higher ranges are
obtained by the use of additional external multipliers.
Hand
generator
OhmmeterAxis O
−+R2
R3RX
0Inf.M
MGACB
Half-wave rectifier
Full-wave rectifier
Simple AC voltmeter
Full-wave
rectifierAC In Pulsing DC
AC In Pulsing DC
AC In 000 0
0AC converted to pulsing DC
on every positive half-cycle
AC converted to pulsing DC
on both positive and
negative half-cycleFigure 12-158. Simplified megger circuit.
Figure 12-159. Simplified block diagram of AC meter.The moving iron vane instrument may be used to measure
DC but has an error due to residual magnetism in the vanes.
Reversing the meter connections and averaging the readings
may minimize the error. When used on AC circuits, the
instrument has an accuracy of 0.5 percent. Because of its
simplicity, relatively low cost, and the fact that no current is
conducted to the moving element, this type of movement is
used extensively to measure current and voltage in AC power
circuits. However, because the reluctance of the magnetic
circuit is high, the moving iron vane meter requires much
more power to produce full-scale deflection than is required
by a d’Arsonval meter of the same range. Therefore, the
moving iron vane meter is seldom used in high-resistance
low-power circuits.
Inclined Coil Iron Vane Meter
The principle of the moving iron vane mechanism is applied
to the inclined coil type of meter, which can be used to
measure both AC and DC. The inclined coil, iron vane
meter has a coil mounted at an angle to the shaft. Attached obliquely to the shaft, and located inside the coil, are two
soft iron vanes. When no current flows through the coil, a
control spring holds the pointer at zero, and the iron vanes
lie in planes parallel to the plane of the coil. When current
flows through the coil, the vanes tend to line up with magnetic
lines passing through the center of the coil at right angles to
the plane of the coil. Thus, the vanes rotate against the spring
action to move the pointer over the scale.
The iron vanes tend to line up with the magnetic lines
regardless of the direction of current flow through the coil.
Therefore, the inclined coil, iron vane meter can be used to
measure either AC or DC. The aluminum disk and the drag
magnets provide electromagnetic damping.
Like the moving iron vane meter, the inclined coil type requires
a relatively large amount of current for full-scale deflection and
is seldom used in high-resistance low-power circuits.
As in the moving iron vane instruments, the inclined coil
instrument is wound with few turns of relatively large wire
when used as an ammeter and with many turns of small wire
when used as a voltmeter.
Varmeters
Multiplying the volts by the amperes in an AC circuit gives
the apparent power: the combination of the true power (which
does the work) and the reactive power (which does no work
and is returned to the line). Reactive power is measured in
units of vars (volt-amperes reactive) or kilovars (kilovolt-
amperes reactive (kV AR). When properly connected,
wattmeters measure the reactive power. As such, they are
called varmeters. [Figure 12-162]
Fixed coils
Shunt Movable coilsA
A
B
CBC
Pointer
Control spring
Cylindrical coil
Moving iron segment Stationary iron segment
Figure 12-160. Simplified diagram of an electrodynamometer
movement.Figure 12-161. Moving iron vane meter.Wattmeter
Electric power is measured by means of a wattmeter. Because
electric power is the product of current and voltage, a
wattmeter must have two elements, one for current and the
other for voltage. For this reason, wattmeters are usually of
the electrodynamometer type. [Figure 12-163]
The movable coil with a series resistance forms the voltage
element, and the stationary coils constitute the current
element. The strength of the field around the potential coil
depends on the amount of current that flows through it. The
current, in turn, depends on the load voltage applied across
the coil and the high resistance in series with it. The strength
of the field around the current coils depends on the amount of
current flowing through the load. Thus, the meter deflection is
proportional to the product of the voltage across the potential
coil and the current through the current coils. The effect is almost the same (if the scale is properly calibrated) as if the
voltage applied across the load and the current through the
load were multiplied together.
If the current in the line is reversed, the direction of current in
both coils and the potential coil is reversed, the net result is
that the pointer continues to read up scale. Therefore, this type
of wattmeter can be used to measure either AC or DC power.
Frequency Measurement/ Oscilloscope
The oscilloscope is by far one of the more useful electronic
measurements available. The viewing capabilities of the
oscilloscope make it possible to see and quantify various
waveform characteristics, such as phase relationships,
amplitudes, and durations. While oscilloscopes come in
a variety of configurations and presentations, the basic
operation is typically the same. Most oscilloscopes in general
bench or shop applications use a cathode-ray tube (CRT),
which is the device or screen that displays the waveforms.
The CRT is a vacuum instrument that contains an electron gun,
which emits a very narrow and focused beam of electrons. A
phosphorescent coat applied to the back of the screen forms
the screen. The beam is electronically aimed and accelerated
so that the electron beam strikes the screen. When the electron
beam strikes the screen, light is emitted at the point of impact.
Figure 12-164 shows the basic components of the CRT
with a block diagram. The heated cathode emits electrons.
The magnitude of voltage on the control grid determines
the actual flow of electrons and thus controls the intensity
of the electron beam. The acceleration anodes increase the
speed of the electrons, and the focusing anode narrows the
beam down to a fine point. The surface of the screen is also
an anode and assists in the acceleration of the electron beam.
LoadCurrent
coilVarmeter
ReactorVoltage
coilM
Figure 12-162. A varmeter connected in an AC circuit.
The purpose of the vertical and horizontal deflection plates
is to bend the electron beam and position it to a specific
point of the screen. [Figure 12-165] By providing a neutral
or zero voltage to a deflection plate, the electron beam is
unaffected. By applying a negative voltage to a plate, the
electron beam is repelled and driven away from the plate.
Finally, by applying a positive voltage, the electron beam is
drawing to the plate. Figure 12-165 provides a few possible
plate voltage combinations and the resultant beam position.
Horizontal Deflection
To get a visual representation of the input signal, an internally
generated saw-tooth voltage is generated and then applied
to the horizontal deflection plates. Figure 12-166 illustrates
that the saw-tooth is a pattern of voltage applied, which
begins at a negative voltage and increases at a constant rate
to a positive voltage. This applied varying voltage draws or
traces the electron beam from the far left of the screen to the
far right side of the screen. The resulting display is a straight
line, if the sweep rate is fast enough. This saw-tooth applied
voltage is a repetitive signal so that the beam is repeatedly
swept across the tube. The rate at which the saw-tooth voltage
goes from negative to positive is determined by the frequency.
This rate then establishes the sweep rate of the beam. When
the saw-tooth reaches the end of its sweep from left to right,
the beam then rapidly returns to the left side and is ready to
make another sweep. During this time, the electron beam is
stopped or blanked out and does not produce any kind of a
trace. This period of time is called flyback.
Vertical Deflection
If this same signal were applied to the vertical plates, it would
also produce a vertical line by causing the beam to trace from
the down position to the up position.
Tracing a Sine Wave
Reproducing the sine wave on the oscilloscope combines
both the vertical and horizontal deflection patterns.
[Figure 12-167] If the sine wave voltage signal is applied
across the vertical deflection plates, the result will be the
vertical beam oscillation up and down on the screen. The
amount that the beam moves above the centerline depends on the peak value of the voltage.
While the beam is being swept from the left to the right by
the horizontal plates, the sine wave voltage is being applied
to the vertical plates, causing the form of the input signal to
be traced out on the screen.
Control Features on an Oscilloscope
While there are many different styles of oscilloscopes, which
range from the simple to the complex, they all have some
controls in common. Apart from the screen and the ON/OFF
switch, some of these controls are listed as follows:
• Horizontal Position—allows for the adjustment of the
neutral horizontal position of the beam. Use this control
to reposition the waveform display in order to have a
better view of the wave or to take measurements.
• Vertical Position—moves the traced image up or down
allowing better observations and measurements.
• Focus—controls the electron beam as it is aimed and
converges on the screen. When the beam is in sharp
focus, it is narrowed down to a very fine point and
does not have a fuzzy appearance.
• Intensity—essentially the brightness of the trace.
Controlling the flow of electrons onto the screen varies
the intensity. Do not keep the intensity too high for
extended testing or when the beam is motionless and
forms a dot on the screen. This can damage the screen.
• Seconds/Division—a time-based control that sets the
horizontal sweep rate. Basically, the switch is used
to select the time interval that each division on the
horizontal scale represents. These divisions can be
seconds, milliseconds, or even microseconds. A simple
example would be if the technician had the seconds/
division control set to 10 μS. If this technician is
viewing a waveform that has a period of 4 divisions
on the screen, then the period would be 40 μS. The
frequency of this waveform can then be determined
by taking the inverse of the period. In this case, 1⁄40 μS
equals a frequency of 25 kHz.
• V olts/Division—used to select the voltage interval
that each division on the vertical scale represents.
For example, suppose each vertical division was set
to equal 10 mV . If a waveform was measured and
had a peak value of 4 divisions, then the peak value
in voltage would be 40 mV .
• Trigger—The trigger control provides synchronization
between the saw-tooth horizontal sweep and the applied
signal on the vertical plates. The benefit is that the
waveform on the screen appears to be stationary and
fixed and not drifting across the screen. A triggering
circuit is used to initiate the start of a sweep rather than
CRT
base
AnodeScreenMating
connector
Horizontal deflection plates (H) Preaccelerating anode (A1)Cathode (K) & heaterControl grid Vertical deflection plates (V)
Accelerating anode (A3)
Focus anode (A2)Electron gun
e e e e e e e e e e e e e e e e e e e e
Figure 12-164. Basic components of the CRT with a block diagram.R Current coilMPotential
coil
Load
Figure 12-163. Simplified electrodynamometer wattmeter circuit.
the fixed saw-tooth sweep rate. In a typical oscilloscope,
this triggering signal comes from the input signal
itself at a selected point during the signal’s cycle. The
horizontal signal goes through one sweep, retraces back
to the left side and waits there until it is triggered again
by the input signal to start another sweep.
Flat Panel Color Displays for Oscilloscopes
While the standard CRT design of oscilloscope is still in
service, the technology of display and control has evolved
into use of the flat panel monitors. Furthermore, the newer
oscilloscopes can even be integrated with the common
personal computer (PC). [Figure 12-168] Some of the features
of this technology include easy data capture, data transfer,
documentation, and data analysis. Hand-held oscilloscopes are
now available that can perform the functions of larger bench type equipment but are mobile and great tools for trouble
shooting. [Figure 12-169]
Digital Multimeter
Traditionally, the meters that technicians have used have been
the analog voltmeter, ammeter, and the ohmmeter. These
have usually been combined into the same instrument and
called a multimeter or a VOM (volt-ohm-milliammeter). This
approach has been both convenient and economical. Digital
multimeters (DMM) and digital voltmeters (DVM) are more
common due to their ease of use. These meters are easier to
read and provide greater accuracy when compared to the
older analog units with needle movement. The multimeter’s
single-coil movement requires a number of scales, which are
not always easy to read accurately. In addition, the loading
characteristics due to the internal resistance sometimes
affect the circuit and the measurements. Not only does the
DVM offer greater accuracy and less ambiguity, but also
higher input resistance, which has less of a loading effect
and influence on a circuit.
Basic Circuit Analysis & Troubleshooting
Troubleshooting is the systematic process of recognizing
the symptoms of a problem, identifying the possible cause,
and locating the failed component or conductor in the
circuit. To be proficient at troubleshooting, the technician
must understand how the circuit operates and know how to
properly use the test equipment. There are many ways in
which a system can fail and to cover all of the possibilities is
beyond the scope of this handbook. However, there are some
basic concepts that enable the technician to handle many of
the common faults encountered in the aircraft.
Before starting a discussion on basic circuits and
troubleshooting, the following definitions are given.
• Short circuit—an unintentional low resistance path
between two components in a circuit or between a
Applied (0) volts(+) voltsHorizontal
deflection
plates (H)Vertical
deflection plates (V)
(0) voltsThe horizontal
beam position
will depend on
the applied
saw-tooth
voltage
Voltage starts at a maximum
negative value and increases to a
maximum positive value, tracing
the beam from the left to the right(−) volts
Figure 12-166. Saw-tooth applied voltage.
Vertical
deflection plates (v)
Horizontal
deflection plates (h) Screen
Beam on center(0) volts (−) voltsElectrons repelled down
(−) voltsElectrons repelled to left
(0) volts
(0) volts (0) volts(0) volts
(+) voltsElectrons attracted up
(+) voltsElectrons attracted to right
(0) volts
Figure 12-165. Possible plate voltage combinations and the resultant beam position.
component/conductor and ground. It usually creates
high current flow, which burns out or causes damage
to the circuit conductor or components.
• Open circuit—a circuit that is not a complete or
continuous path. An open circuit represents an
infinitely large resistance. Switches are common
devices used to open and close a circuit. Sometimes
a circuit opens due to a component failure, such as a
light bulb or a burned out resistor.
• Continuity—the state of being continuous,
uninterrupted or connected together; the opposite of
a circuit that is not broken or does not have an open.
• Discontinuity—the opposite of continuity, indicating
that a circuit is broken or not continuous.
Voltage Measurement
V oltage is measured across a component with a voltmeter or
the voltmeter position on a multimeter. Usually, there is a DC
and an AC selection on the meter. Before the meter is used
for measurements, make sure that the meter is selected for
the correct type of voltage. When placing the probes across a
component to take a measurement, take care to ensure that the
polarity is correct. [Figure 12-170] Standard practice is for
the red meter lead to be installed in the positive (+) jack and
the black meter lead to be installed in the negative meter jack
(−). Then when placing the probes across or in parallel with
a component to measure the voltage, the leads should match
the polarity of the component. The red lead is on the positive
AppliedApplied
(0) volts(+) voltsHorizontal deflection plates (h)
(−) voltsVertical deflection plates (v)
(0) volts
(–) volts(+) volts
(0) volts
Input signal
Figure 12-167. Sine wave voltage signal.
side of the component and the black is on the negative side,
which prevents damage to the meter or incorrect readings.
All meters have some resistance and will shunt some of the
current. This has the effect of changing the characteristic of
the circuit because of this change in current. This is typically
more of a concern with older analog type meters. If there
are any questions about the magnitude of the voltage across
a component, then the meter should be set to measure on
the highest voltage range. This prevents the meter from
“pegging” and possible damage. The range should then be
selected to low values until the measured voltage is read at
the mid-scale deflection. Readings taken at mid-scale are
the most accurate.
Current Measurement
Current is measured with the ammeter connected in the
current path by opening or breaking the circuit and inserting
the meter in series. [Figure 12-170] Standard practice is for
the red meter lead to be installed in the positive (+) jack and
the black meter lead to be installed in the negative meter jack (−). The positive side of the meter is connected towards the
positive voltage source. Ideally, the meter should not alter
the current and influence the circuit and the measurements.
However, the meter does have some effect because of its
internal resistance that is connected with the rest of the circuit
in series. The resistance is rather small and for most practical
purposes, this can be neglected.
Checking Resistance in a Circuit
The ohmmeter is used to measure the resistance. In its more
basic form, the ohmmeter consists of a variable resistor in
series with a meter movement and a voltage source. The
meter must first be adjusted before use.
Refer to Figure 12-171 for meter configurations during
adjustments. When the meter leads are not connected (open),
the needle points to the full left-hand position, indicating
infinite resistance or and open circuit. With the lead placed
together, the circuit is shorted as shown with the meter
needle to the full right-hand position. When a connection
is made, the internal battery is allowed to produce a current
through the movement coil, causing a deflection of the
Figure 12-168. Oscilloscope with flat panel display.
Figure 12-169. Handheld oscilloscope.needle in proportion to the value of the external resistance.
In this case, the resistance is zero because the leads are
shorted.
The purpose of the variable resistor in the meter is to adjust
the current so that the pointer reads exactly zero when the
leads are shorted. This is needed because as the battery
continues to be used, the voltage changes, thus requiring an
adjustment. The meter should be “zeroed” before each use.
To check the value of a resistor, the resistor must be
disconnected from the circuit. This prevents any possible
damage to the ohmmeter, and it prevents the possibility of
any inaccurate readings due to the circuit being in parallel
with the resistor in question. [Figure 12-172]
Continuity Checks
In many cases, the ohmmeter is not used for measuring the
resistance of a component but to simply check the integrity
of a connection from one portion of a circuit to another. If
there is a good connection, then the ohmmeter reads a near
zero resistance or a short. If the circuit is open or has a very
poor connection at some point like an over-crimped pin in
a connector, then the ohmmeter reads infinity or some very
high resistance. Keep in mind that while any measurement
is being taken, contact with the circuit or probes should be
avoided. Contact can introduce another parallel path and
provide misleading indications.
Capacitance Measurement
Figure 12-173 illustrates a basic test of a capacitor with an
ohmmeter. There are usually two common modes of failure
for a capacitor. One is a complete failure characterized
by short circuit through the capacitor due to the dielectric
breaking down or an open circuit. The more insidious failure
occurs due to degradation, which is a gradual deterioration
of the capacitor’s characteristics.
If a problem is suspected, remove the capacitor from the circuit
and check with an ohmmeter. The first step is to short the two
leads of the capacitor to ensure that it is entirely discharged.
Next, connect the two leads as shown in Figure 12-173 across
the capacitor and observe the needle movement. At first, the
needle should indicate a short circuit. Then as the capacitor
begins to charge, the needle should move to the left or infinity
and eventually indicate an open circuit. The capacitor takes
its charge from the internal battery of the ohmmeter. The
greater the capacitance, the longer it takes to charge. If the
capacitor is shorted, then the needle remains at a very low or
shorted resistance. If there is some internal deterioration of the
dielectric, then the needle never reaches a high resistance but
some intermediate value, indicating a current.Inductance Measurement
The common mode of failure in an inductor is an open. To
check the integrity of an inductor, it must be removed from
the circuit and tested as an isolated component just like the
capacitor. If there is an open in the inductor, a simple check
with an ohmmeter shows it as an open circuit with infinite
resistance. If in fact the inductor is in good condition, then
the ohmmeter indicates the resistance of the coil.
On occasions, the inductor fails due to overheating. When
the inductor is overheated, it is possible for the insulation
covering the wire in the coil to melt, causing a short. The
effects of a shorted coil are that of reducing the number of
turns. At this point, further testing of the inductor must be
Ammeter symbol Voltmeter symbol
Meter symbols
Simplified use of metersOhmmeter symbol
R1++
−−
3k ΩOpen the circuit
and insert ammeter in
series with the loadA
R1++
− −3k ΩVoltage is measured
across a component V
R1++
− −3k ΩResistance is
measured across a
componentΩA V Ω
Figure 12-170. Current, voltage, and resistance measurement.done with test equipment not covered in this handbook.
Troubleshooting Open Faults in a Series Circuit
One of the most common modes of failure is the “open”
circuit. A component, such as a resistor, can overheat due to the power rating being exceeded. Other more frustrating problems can happen when a “cold” solder joint cracks leaving a wire disconnected from a relay or connector. This type of damage can occur during routine maintenance after a technician has accessed an area for inspections. In many cases, there is no visual indication that a failure has occurred, and the soon-to-be-frustrated technician is unaware that there is a problem until power is reapplied to the aircraft in the final days leading up to aircraft delivery and scheduled operations.
The first example is a simplified diagram shown in
Figures 12-174 through 12-176 . The circuit depicted in
Figure 12-174 is designed to cause current to flow through a
lamp, but because of the open resistor, the lamp will not light. To locate this open, a voltmeter or an ohmmeter should be used.Tracing Opens with the Voltmeter
A general procedure to follow in this case is to measure the
voltage drop across each component in the circuit, keeping in mind the following points. If there is an open in a series circuit, then the voltage drops on sides of the component. In this case, the total voltage must appear across the open resistor as per Kirchhoff’s V oltage Law.
If a voltmeter is connected across the lamp, as shown in
Figure 12-175 , the voltmeter reads zero. Since no current can
flow in the circuit because of the open resistor, there is no voltage drop across the lamp indicating that the lamp is good.
Next, the voltmeter is connected across the open resistor, as
shown in Figure 12-176. The voltmeter has closed the circuit by shunting (paralleling) the burned out resistor, allowing current to flow. Current flows from the negative terminal of the battery, through the switch, through the voltmeter and the lamp, back to the positive terminal of the battery. However, the resistance of the voltmeter is so high that only a very small current flows in the circuit. The current is too small to light the lamp, but the voltmeter reads the battery voltage.
Tracing Opens with the Ohmmeter
A simplified circuit, as shown in Figures 12-177 and 12-178,
illustrates how to locate an open in a series circuit using the
ohmmeter. A general rule to keep in mind when troubleshooting with an ohmmeter is: when an ohmmeter is properly connected across a circuit component and a resistance reading is obtained, the component has continuity and is not open.
When an ohmmeter is used, the circuit component to be tested
must be isolated and the power source removed from the circuit. In this case, these requirements can be met by opening the circuit switch as shown in Figure 12-177 . The ohmmeter
is zeroed and across all good components is zero. The voltage drop across the open component equals the total voltage across the series combination. This condition happens because the open component prevents current to pass through the series circuit. With there being no current, there can be no voltage drop across any of the good components. Because the current is zero, it can be determined by Ohm’s Law that E = IR = 0 volts across a component. The voltage is the same on both places across (in parallel with) the lamp. In this testing configuration, some value of resistance is read indicating that the lamp is in good condition and is not the source of the open in the circuit.
Now the technician should move to the resistor and place
the ohmmeter probe across it as shown in Figure 12-178.
When the ohmmeter is connected across the open resistor, it indicates infinite resistance, or a discontinuity. Thus, the circuit open has now been located.
12-83+
−
Black lead
Open circuitRed lead0Simplified ohmmeter
∞
+
−
Black lead
Shorted circuitRed lead0Simplified ohmmeter
∞
+
−
Black lead
R1Red lead
Isolate from circuit
connect across resistor0Simplified ohmmeter
∞
R2R3+
−
Figure 12-171. Meter configurations during adjustments.Figure 12-172. Meter adjustment.
Troubleshooting Shorting Faults in a Series Circuit
An open fault can cause a component or system not to
work, which can be critical and hazardous. A shorting
fault can potentially be more of a severe nature than the
open type of fault. A short circuit, or “short,” causes the
opposite effect. A short across a series circuit produces a
greater than normal current flow. Faults of this type can
develop slowly when a wire bundle is not properly secured
and is allowed to chafe against the airframe structure or
other systems, such as hydraulic lines. Shorts can also
occur due to a careless technician using incorrect hardware
when installing an interior. If screws that are too long are
used to install trim, it is possible to penetrate a wire bundle
immediately causing numerous shorts. Worse yet, are the shorts that are not immediately seen but “latent” and do not
show symptoms until the aircraft is in service. Another point
to keep in mind is when closing panels. Wires can become
pinched between the panel and the airframe causing either
a short or a latent, intermittent short. The simplified circuit,
shown in Figures 12-179 through 12-182 is used to illustrate
troubleshooting a short in a series circuit.
In Figure 12-179 , a circuit is designed to light a lamp. A
resistor is connected in the circuit to limit current flow. If the
resistor is shorted, as shown in the illustration, the current
flow increases and the lamp becomes brighter. If the applied
voltage were high enough, the lamp would burn out, but in
this case the fuse would protect the lamp by opening first.
Usually a short circuit produces an open circuit by either
blowing (opening) the fuse or burning out a circuit
component. But in some circuits, there may be additional
resistors which do not allow one shorted resistor to increase
the current flow enough to blow the fuse or burn out a
component. [Figure 12-180] Thus, with one resistor shorted
out, the circuit still functions since the power dissipated by
the other resistors does not exceed the rating of the fuse.
Tracing Shorts with the Ohmmeter
The shorted resistor can be located with an ohmmeter.
[Figure 12-181] First the switch is opened to isolate the
12-84Break
Break
VFigure 12-174. An open circuit.
Figure 12-175. Voltmeter across a lamp in an open circuit.+
−
Black leadNeedle moves
full-scale (short)
Conventional current
Initial charging of capacitor behaves like a shorted circuit.Red lead
0Simplified ohmmeter
∞
+
−
+−Black leadNeedle moves
full-scale (open)
Full charge no current
Once charged, the capacitor behaves like an open circuit.Red lead
0Simplified ohmmeter
∞
Figure 12-173. Basic test of a capacitor with an ohmmeter.
circuit components. In Figure 12-181 , this circuit is shown
with an ohmmeter connected across each of the resistors.
Only the ohmmeter connected across the shorted resistor
shows a zero reading, indicating that this resistor is shorted.
Tracing Shorts with the Voltmeter
To locate the shorted resistor while the circuit is functioning,
a voltmeter can be used. Figure 12-182 illustrates that when
a voltmeter is connected across any of the resistors that are
not shorted, a portion of the applied voltage is indicated on
the voltmeter scale. When it is connected across the shorted
resistor, the voltmeter reads zero.Troubleshooting Open Faults in a Parallel Circuit
The procedures used in troubleshooting a parallel circuit are
sometimes different from those used in a series circuit. Unlike a
series circuit, a parallel circuit has more than one path in which
current flows. A voltmeter cannot be used, since, when it is placed
across an open resistor, it reads the voltage drop in a parallel
branch. But an ammeter or the modified use of an ohmmeter
can be employed to detect an open branch in a parallel circuit.
If the open resistor shown in Figure 12-183 was not visually
apparent, the circuit might appear to be functioning properly,
because current would continue to flow in the other two
branches of the circuit. To determine that the circuit is not
operating properly, a determination must be made as to how the
circuit should behave when working properly. First, the total
resistance, total current, and the branch currents of the circuit
should be calculated as if there were no open in the circuit.
In this case, the total resistance can be simply determined by:
RT =R
N
Where RT is the total circuit resistance
N is the number of resistors
R is the resistor value
12-85BreakV
Break
ΩBreakΩ
Figure 12-176. Voltmeter across a resistor in an open circuit.
Figure 12-177. Using an ohmmeter to check a circuit component.Figure 12-178. Using an ohmmeter to locate an open in a circuit
component.
RT = = 10 Ω30 Ω
The total current of the circuit can now be determined
by using Ohm’s Law:
IT =ES
RT
Where IT is the total current
ES is the source voltage across the
parallel branch
RT is the total resistance of the
parallel branch
IT = = 3 amperes (total current)30 v
10 Ω
Each branch current should be determined in a similar
manner. For the first branch, the current is:
I1 =ES
R1
Where I1 is the current in the first branch
ES is the source voltage across the
parallel branch
R1 is the resistance of the first branch
I1 = = 1 ampere30 v
30 Ω
Because the other two branches are of the same resistive
value, then the current in each of those branches is 1 ampere
also. Adding up the amperes in each branch confirms the initial calculation of total current being 3 amperes.
Tracing an Open with an Ammeter
If the technician now places an ammeter in the circuit, the
total current would be indicated as 2 amperes as shown in
Figure 12-183 instead of the calculated 3 amperes. Since 1
ampere of current should be flowing through each branch,
it is obvious that one branch is open. If the ammeter is then
connected into the branches, one after another, the open
branch is eventually located by a zero ammeter reading.
Tracing an Open with an Ohmmeter
A modified use of the ohmmeter can also locate this type
of open. If the ohmmeter is connected across the open
resistor, as shown in Figure 12-184 , an erroneous reading
of continuity would be obtained. Even though the circuit
switch is open, the open resistor is still in parallel with R 1
and R 2, and the ohmmeter would indicate the open resistor
had a resistance of 15 ohms, the equivalent resistance of
the parallel combination of R 1 and R 2.
Therefore, it is necessary to open the circuit as shown in
Figure 12-185 in order to check the resistance of R 3. In this
way, the resistor is not shunted (paralleled) by R 1 and R 2. The
reading on the ohmmeter now indicates infinite resistance,
which means the open component has been isolated.
Troubleshooting Shorting Faults in Parallel Circuits
As in a series circuit, a short in a parallel circuit usually
causes an open circuit by blowing the fuse. But, unlike a
series circuit, one shorted component in a parallel circuit
stops current flow by causing the fuse to open. Refer to the
circuit in Figure 12-186 . If resistor R 3 is shorted, a path of
almost zero resistance is offered the current, and all the circuit
current flows through the branch containing the shorted
resistor. Since this is practically the same as connecting a
wire between the terminals of the battery, the current rises to
an excessive value, and the fuse opens. Since the fuse opens
almost as soon as a resistor shorts out, there is no time to
V
VSome voltage
ShortZero
voltage
Figure 12-182. Voltmeter connected across resistors.
Ω
Ω
Ω Ω40 Ω
50 ΩR1
R3R2
R4
10 Ω100 V
0 ΩFigure 12-179. A shorted resistor.
Figure 12-180. A short that does not open the circuit.
Figure 12-181. Using an ohmmeter to locate a shorted resistor.perform a current or voltage check. Thus, troubleshooting
a parallel DC circuit for a shorted component should be
accomplished with an ohmmeter. But, as in the case of
checking for an open resistor in a parallel circuit, a shorted
resistor can be detected with an ohmmeter only if one end
of the shorted resistor is disconnected and isolated from the
rest of the circuit.
Troubleshooting Shorting Faults in Series-Parallel
Circuits
Logic in Tracing an Open
Troubleshooting a series-parallel resistive circuit involves
locating malfunctions similar to those found in a series or
a parallel circuit. Figures 12-187 through 12-189 illustrate
three points of failure in a series-parallel circuit and their
generalized effects.
1. In the circuit shown in Figure 12-187 , an open has
occurred in the series portion of the circuit. When
the open occurs anywhere in the series portion of a
series-parallel circuit, current flow in the entire circuit
stops. In this case, the circuit does not function, and
the lamp, L 1, is not lit.
2. If the open occurs in the parallel portion of a series-
parallel circuit, as shown in Figure 12-188 , part of the
circuit continues to function. In this case, the lamp
continues to burn, but its brightness diminishes, since
the total resistance of the circuit has increased and the
total current has decreased.
3. If the open occurs in the branch containing the lamp,
as shown in Figure 12-189 , the circuit continues to
function with increased resistance and decreased
current, but the lamp does not light.
Tracing Opens with the Voltmeter
To explain how the voltmeter and ohmmeter can be used
to troubleshoot series-parallel circuits, the circuit shown in
Figure 12-190 has been labeled at various points. A point-
to-point description is listed below with expected results:
1. By connecting a voltmeter between points A and D,
the battery and switch can be checked for opens.
2. By connecting the voltmeter between points A and
B, the voltage drop across R 1 can be checked. This
voltage drop is a portion of the applied voltage.
3. If R 1 is open, the reading between B and D is zero.
4. By connecting a voltmeter between A and E, the
continuity of the conductor between the positive
terminal of the battery and point E, as well as the fuse,
30 Ω 30 ΩR1 30 VR2
30 ΩR3
A2aBreak
30 Ω 30 ΩR1R2
30 ΩR3BreakFigure 12-183. Finding an open branch in a parallel circuit.
Figure 12-184. A misleading ohmmeter indication.can be checked. If the conductor or fuse is open, the
voltmeter reads zero.
5. If the lamp is burning, it is obvious that no open
exists in the branch containing the lamp, and the
voltmeter could be used to detect an open in the branch
containing R 2 by removing lamp, L 1, from the circuit.
Troubleshooting the series portion of a series-parallel circuit
presents no difficulties, but in the parallel portion of the
circuit, misleading readings can be obtained.
Batteries
Primary Cell
The dry cell is the most common type of primary-cell battery
and is similar in its characteristics to that of an electrolytic
cell. This type of a battery is basically designed with a metal
electrode or graphite rod acting as the cathode (+) terminal,
immersed in an electrolytic paste. This electrode⁄electrolytic
build-up is then encased in a metal container, usually made
of zinc, which itself acts as the anode (−) terminal. When the
battery is in a discharge condition an electrochemical reaction
takes place resulting in one of the metals being consumed.
Because of this consumption, the charging process is not
reversible. Attempting to reverse the chemical reaction in a
primary cell by way of recharging is usually dangerous and
can lead to a battery explosion.
These batteries are commonly used to power items such as
flashlights. The most common primary cells today are found
in alkaline batteries, silver-oxide, and lithium batteries. The
earlier carbon-zinc cells, with a carbon post as cathode and a
zinc shell as anode were once prevalent but are not as common.
Secondary Cell
A secondary cell is any kind of electrolytic cell in which the
electrochemical reaction that releases energy is reversible.
The lead-acid car battery is a secondary-cell battery. The electrolyte is sulfuric acid (battery acid), the positive
electrode is lead peroxide, and the negative electrode is
lead. A typical lead-acid battery consists of six lead-acid
cells in a case. Each cell produces 2 volts, so the whole
battery produces a total of 12 volts.
Other commonly used secondary cell chemistry types are
nickel-cadmium (Ni-Cad), nickel-metal hydride (NiMH),
lithium-ion (Li-ion), and Lithium-ion polymer (Li-ion
polymer).
Lead-acid batteries used in aircraft are similar to automobile
batteries. The lead acid battery is made up of a series of
identical cells each containing sets of positive and negative
plates. Figure 12-191 illustrates each cell contains positive
plates of lead dioxide (PbO 2), negative plates of spongy lead,
and electrolyte (sulfuric acid and water). A practical cell is
constructed with many more plates than just two in order to get
the required current output. All positive plates are connected
together as well as all the negatives. Because each positive
plate is always positioned between two negative plates, there
are always one or more negative plates than positive plates.
Between the plates are porous separators that keep the
positive and negative plates from touching each other and
shorting out the cell. The separators have vertical ribs on
the side facing the positive plate. This construction permits
the electrolyte to circulate freely around the plates. In
addition, it provides a path for sediment to settle to the
bottom of the cell.
Each cell is seated in a hard rubber casing through the top
of which are terminal posts and a hole into which a nonspill
vent cap is screwed. The hole provides access for testing the
strength of the electrolyte and adding water. The vent plug
permits gases to escape from the cell with a minimum of
leakage of electrolyte, regardless of the position the airplane
might assume. [Figure 12-192] In level flight, the lead weight
R2L1Break
R2R1
L1
Break
R2R1
L1BreakFigure 12-187. An open in the series portion of a series-parallel
circuit.
Figure 12-188. An open in the parallel portion of a series-parallel
circuit.
Figure 12-189. An open lamp in a series-parallel circuit.
R1A
BR2R3
BreakOpen
30 Ω 30 ΩR1 30 VR2
30 ΩR3Figure 12-185. Opening a branch circuit to obtain an accurate
ohmmeter reading.
Figure 12-186. A shorted component causes the fuse to open.
permits venting of gases through a small hole. In inverted
flight, this hole is covered by the lead weight.
The individual cells of the battery are connected in series by
means of cell straps. [Figure 12-193] The complete assembly
is enclosed in an acid resisting metal container (battery box),
which serves as electrical shielding and mechanical protection.
The battery box has a removable top. It also has a vent tube
nipple at each end. When the battery is installed in an airplane, a
vent tube is attached to each nipple. One tube is the intake tube
and is exposed to the slipstream. The other is the exhaust vent
tube and is attached to the battery drain sump, which is a glass
jar containing a felt pad moistened with a concentrated solution
of sodium bicarbonate (baking soda). With this arrangement,
the airstream is directed through the battery case where battery
gases are picked up, neutralized in the sump, and then expelled
overboard without damage to the airplane.
To facilitate installation and removal of the battery in some
aircraft, a quick disconnect assembly is used to connect the
power leads to the battery. This assembly attaches the battery
leads in the aircraft to a receptacle mounted on the side of the
battery. [Figure 12-194] The receptacle covers the battery
terminal posts and prevents accidental shorting during the installation and removal of the battery. The plug consists of
a socket and a handwheel with a course pitch thread. It can
be readily connected to the receptacle by the handwheel.
Another advantage of this assembly is that the plug can be
installed in only one position, eliminating the possibility of
reversing the battery leads.
The voltage of lead acid cell is approximately two volts in order
to attain the voltage required for the application. Each cell is
then connected in series with heavy gauge metal straps to form
a battery. In a typical battery, such as that used in an aircraft for
starting, the voltage required is 12 or 24 volts. This voltage is
achieved by connecting six cells or twelve cells respectively
together in series and enclosing them in one plastic box.
12-89R2R1
L1A B C
D E F
Figure 12-190. Using the voltmeter to troubleshoot a series-parallel
circuit.
Vent cap
Terminal post
Supporting ribsCell container
PlatesCell cover
Separators
Figure 12-191. Lead-acid cell construction.Each cell containing the plates are filled with an electrolyte
composed of sulfuric acid and distilled water with a specific
gravity of 1.270 at 60 °F. This solution contains positive
hydrogen ions and negative sulfate (SO 4) ions that are free to
combine with other ions and form a new chemical compound.
When the cell is discharged, electrons leave the negative
plate and flow to the positive plates where they cause the
lead dioxide (PbO 2) to break down into negative oxygen
ions and positive lead ions. The negative oxygen ions join
with positive hydrogen ions from the sulfuric acid and form
water (H 2O). The negative sulfate ions join with the lead
ions in both plates and form lead sulfate (PbSO 4). After the
discharge, the specific gravity changes to about 1.150.
Battery Ratings
The voltage of a battery is determined by the number of
cells connected in series to form the battery. Although the
voltage of one lead-acid cell just removed from a charger is
approximately 2.2 volts, a lead-acid cell is normally rated at
approximately 2 volts. A battery rated at 12 volts consists of
6 lead-acid cells connected in series, and a battery rated at
24 volts is composed of 12 cells.
The most common battery rating is the ampere-hour rating.
This is a unit of measurement for battery capacity. It is
determined by multiplying a current flow in amperes by the
time in hours that the battery is being discharged.
A battery with a capacity of 1 ampere-hour should be able
to continuously supply a current of 1 amp to a load for
exactly 1 hour, or 2 amps for 1⁄2 hour, or 1⁄3 amp for 3 hours,
etc., before becoming completely discharged. Actually, the
ampere-hour output of a particular battery depends on the
rate at which it is discharged. Heavy discharge current heats
the battery and decreases its efficiency and total ampere-hour
output. For airplane batteries, a period of 5 hours has been
established as the discharge time in rating battery capacity.
However, this time of 5 hours is only a basis for rating and
does not necessarily mean the length of time during which the battery is expected to furnish current. Under actual service
conditions, the battery can be completely discharged within
a few minutes, or it may never be discharged if the generator
provides sufficient charge.
The ampere-hour capacity of a battery depends upon its total
effective plate area. Connecting batteries in parallel increases
ampere-hour capacity. Connecting batteries in series increases
the total voltage but not the ampere-hour capacity.
Life Cycle of a Battery
Battery life cycle is defined as the number of complete charge/
discharge cycles a battery can perform before its normal charge
capacity falls below 80 percent of its initial rated capacity.
Battery life can vary anywhere from 500 to 1,300 cycles.
Various factors can cause deterioration of a battery and shorten
its service life. The first is over-discharging, which causes
excess sulfation; second, too-rapid charging or discharging
that results in overheating of the plates and shedding of active
material. The accumulation of shed material, in turn, causes
shorting of the plates and results in internal discharge. A battery
that remains in a low or discharged condition for a long period
of time may be permanently damaged. The deterioration can
continue to a point where cell capacity can drop to 80 percent
after 1,000 cycles. In many cases, the cell can continue working
to nearly 2,000 cycles but with a diminished capacity of 60
percent of its original state.
Vent plug
Cell strapCells
Figure 12-193. Connection of storage battery.
Figure 12-194. A battery quick-disconnect assembly.
Inverted position
electrolyte
Vent
VentUpright position gases escape
Lead weight seated not sealed Lead weight seated
Figure 12-192. Nonspill battery vent plug.Lead-Acid Battery Testing Methods
The state of charge of a storage battery depends upon
the condition of its active materials, primarily the plates.
However, the state of charge of a battery is indicated by the
density of the electrolyte and is checked by a hydrometer,
an instrument that measures the specific gravity (weight as
compared with water) of liquids.
The most commonly used hydrometer consists of a small
sealed glass tube weighted at its lower end so it floats upright.
[Figure 12-195] Within the narrow stem of the tube is a paper
scale with a range of 1.100 to 1.300. When a hydrometer
is used, a quantity of electrolyte sufficient to float the
hydrometer is drawn up into the syringe. The depth to which
the hydrometer sinks into the electrolyte is determined by the
density of the electrolyte, and the scale value indicated at the
level of the electrolyte is its specific gravity. The more dense
the electrolyte, the higher the hydrometer floats; therefore,
the highest number on the scale (1.300) is at the lower end
of the hydrometer scale.
In a new, fully-charged aircraft storage battery, the electrolyte
is approximately 30 percent acid and 70 percent water (by
volume) and is 1.300 times as heavy as pure water. During
discharge, the solution (electrolyte) becomes less dense and
its specific gravity drops below 1.300. A specific gravity
reading between 1.300 and 1.275 indicates a high state of
charge; between 1.275 and 1.240, a medium state of charge;
and between 1.240 and 1.200, a low state of charge. Aircraft
batteries are generally of small capacity but are subject to
heavy loads. The values specified for state of charge are
therefore rather high. Hydrometer tests are made periodically
on all storage batteries installed in aircraft. An aircraft battery
in a low state of charge may have perhaps 50 percent charge
remaining, but is nevertheless considered low in the face of
heavy demands that would soon exhaust it. A battery in such a state of charge is considered in need of immediate recharging.
When a battery is tested using a hydrometer, the temperature
of the electrolyte must be taken into consideration. The
specific gravity readings on the hydrometer vary from
the actual specific gravity as the temperature changes. No
correction is necessary when the temperature is between
70 °F and 90 °F, since the variation is not great enough to
consider. When temperatures are greater than 90 °F or less
than 70 °F, it is necessary to apply a correction factor. Some
hydrometers are equipped with a correction scale inside the
tube. With other hydrometers, it is necessary to refer to a chart
provided by the manufacturer. In both cases, the corrections
should be added to, or subtracted from the reading shown
on the hydrometer.
The specific gravity of a cell is reliable only if nothing has
Low charge Medium charge High charge
Figure 12-195. Hydrometer (specific gravity readings).been added to the electrolyte except occasional small amounts
of distilled water to replace that lost as a result of normal
evaporation. Always take hydrometer readings before adding
distilled water, never after. This is necessary to allow time
for the water to mix thoroughly with the electrolyte and to
avoid drawing up into the hydrometer syringe a sample that
does not represent the true strength of the solution.
Exercise extreme care when making the hydrometer test of
a lead-acid cell. Handle the electrolyte carefully because
sulfuric acid burns clothing and skin. If the acid does contact
the skin, wash the area thoroughly with water and then apply
bicarbonate of soda.
Lead-Acid Battery Charging Methods
Passing direct current through the battery in a direction
opposite to that of the discharge current may charge a
storage battery. Because of the internal resistance (IR) in the
battery, the voltage of the external charging source must be
greater than the open circuit voltage. For example, the open
circuit voltage of a fully charged 12 cell, lead-acid battery is
approximately 26.4 volts (12 × 2.2 volts), but approximately
28 volts are required to charge it. This larger voltage is needed
for charging because of the voltage drop in the battery caused
by the internal resistance. Hence, the charging voltage of a
lead-acid battery must equal the open circuit voltage plus the
IR drop within the battery (product of the charging current
and the internal resistance).
Batteries are charged by either the constant voltage or
constant current method. In the constant voltage method
[Figure 12-196A] , a motor generator set with a constant,
regulated voltage forces the current through the battery. In
this method, the current at the start of the process is high but
automatically tapers off, reaching a value of approximately
1 ampere when the battery is fully charged. The constant
voltage method requires less time and supervision than does
the constant current method.
In the constant current method [Figure 12-196B] , the current
remains almost constant during the entire charging process.
This method requires a longer time to charge a battery fully
and, toward the end of the process, presents the danger of
overcharging, if care is not exercised.
In the aircraft, the storage battery is charged by direct current
from the aircraft generator system. This method of charging
is the constant voltage method, since the generator voltage
is held constant by use of a voltage regulator.
When a storage battery is being charged, it generates a
certain amount of hydrogen and oxygen. Since this is an explosive mixture, it is important to take steps to prevent
ignition of the gas mixture. Loosen the vent caps and
leave in place. Do not permit open flames, sparks, or other
sources of ignition in the vicinity. Before disconnecting
or connecting a battery to the charge, always turn off the
power by means of a remote switch. Figure 12-197 shows
battery charging equipment.
Nickel-Cadmium Batteries
Chemistry and Construction
Active materials in nickel-cadmium cells (Ni-Cad) are nickel
hydroxide (NiOOH) in the charged positive plate (Anode)
and sponge cadmium (Cd) in the charged negative plate
(Cathode). The electrolyte is a potassium hydroxide (KOH)
solution in concentration of 20–34 percent by weight pure
KOH in distilled water.
Sintered nickel-cadmium cells have relatively thin sintered
nickel matrices forming a plate grid structure. The grid
structure is highly porous and is impregnated with the active
positive material (nickel-hydroxide) and the negative material
(cadmium-hydroxide). The plates are then formed by sintering
nickel powder to fine-mesh wire screen. In other variations
of the process, the active material in the sintered matrix is
converted chemically, or thermally, to an active state and
then formed. In general, there are many steps to these cycles
of impregnation and formation. Thin sintered plate cells are
ideally suited for very high rate charge and discharge service.
Pocket plate nickel-cadmium cells have the positive or negative
active material, pressed into pockets of perforated nickel-plated
steel plates or into tubes. The active material is trapped securely
in contact with a metal current collector so active material
shedding is largely eliminated. Plate designs vary in thickness
A
B++
Constant current
charging circuitConstant voltage
charging circuit
Rheostat
A
OFFONV
RMotor
generator
Figure 12-196. Battery charging methods.depending upon cycling service requirements. The typical open
circuit cell voltage of a nickel-cadmium battery is about 1.25
volts. Figure 12-198 shows a nickel-cadmium aircraft battery.
Operation of Nickel-Cadmium Cells
When a charging current is applied to a nickel-cadmium
battery, the negative plates lose oxygen and begin forming
metallic cadmium. The active material of the positive plates,
nickel-hydroxide, becomes more highly oxidized. This
process continues while the charging current is applied or
until all the oxygen is removed from the negative plates and
only cadmium remains.
Toward the end of the charging cycle, the cells emit gas. This
also occurs if the cells are overcharged. This gas is caused by
decomposition of the water in the electrolyte into hydrogen
at the negative plates and oxygen at the positive plates. The
voltage used during charging, as well as the temperature,
determines when gassing occurs. To completely charge a
nickel-cadmium battery, some gassing, however slight, must
take place; thus, some water is used.
The chemical action is reversed during discharge. The
positive plates slowly give up oxygen, which is regained by
the negative plates. This process results in the conversion of
the chemical energy into electrical energy. During discharge,
the plates absorb a quantity of the electrolyte. On recharge,
the level of the electrolyte rises and, at full charge, the
electrolyte is at its highest level. Therefore, water should be
added only when the battery is fully charged.
The nickel-cadmium battery is usually interchangeable with
the lead-acid type. When replacing a lead-acid battery with
a nickel-cadmium battery, the battery compartment must be
clean, dry, and free of all traces of acid from the old battery.
The compartment must be washed out and neutralized with
ammonia or boric acid solution, allowed to dry thoroughly,
and then painted with an alkali resisting varnish.
The pad in the battery sump jar should be saturated with a
three percent (by weight) solution of boric acid and water
before connecting the battery vent system.
General Maintenance and Safety Precautions
Refer to the battery manufacturer for detailed service
instructions. Below are general recommendations for
maintenance and safety precautions. For vented nickel-
cadmium cells, the general maintenance requirements are:
1. Hydrate cells to supply water lost during overcharging.
2. Maintain inter-cell connectors at proper torque values.
3. Keep cell tops and exposed sides clean and dry.
Electrolyte spillage can form grounding paths. White moss around vent cap seals is potassium carbonate (K 2CO 3).
Clean up these surfaces with distilled water and dry. While
handling the caustic potassium hydroxide electrolyte, wear
safety goggles to protect the eyes. The technician should also
wear plastic gloves and an apron to protect skin and clothes.
In case of spillage on hands or clothes, neutralize the alkali
immediately with vinegar or dilute boric acid solution (one
pound per gallon of water); then rinse with clear water.
During overcharging conditions, explosive mixtures of
hydrogen and oxygen develop in nickel-cadmium cells.
When this occurs, the cell relief valves vent these gases
to the atmosphere, creating a potentially explosive hazard.
Additionally, room ventilation should be such as to prevent
a hydrogen build up in closed spaces from exceeding one
percent by volume. Explosions can occur at concentrations
above four percent by volume in air.
Sealed Lead Acid (SLA) Batteries
In many applications, sealed lead acid (SLA) batteries
are gaining in use over flooded lead acid and Ni-Cad
batteries. One leading characteristic of Ni-Cad batteries is
that they perform well in low voltage, full-discharge, high
cycle applications. However, they do not perform as well
Figure 12-197. Battery charger.Figure 12-198. Nickel-Cadmium aircraft battery.in extended standby applications, such as auxiliary or as
emergency battery packs used to power inertial reference
units or stand-by equipment (attitude gyro).
It is typical during the servicing of a Ni-Cad battery to
match as many as twenty individual cells in order to prevent
unbalance and thus cell reversal during end of discharge.
When a Ni-Cad does reverse, very high pressure and heat
can result. The result is often pressure seal rupture, and in
the worst case, a cell explosion. With SLA batteries, cell
matching is inherent in each battery. Ni-Cads also have an
undesirable characteristic caused by constant overcharge
and infrequent discharges, as in standby applications. It is
technically known as “voltage depression” and commonly
but erroneously called “memory effect.” This characteristic
is only detectable when a full discharge is attempted. Thus,
it is possible to believe a full charge exists, while in fact it
does not. SLA batteries do not have this characteristic voltage
depression (memory) phenomenon, and therefore do not
require scheduled deep cycle maintenance as do Ni-Cads.
The Ni-Cad emergency battery pack requires relatively
complicated test equipment due to the complex characteristics
of the Ni-Cad. Sealed lead acid batteries do not have these
temperamental characteristics and therefore it is not necessary
to purchase special battery maintenance equipment. Some
manufacturers of SLA batteries have included in the battery
packs a means by which the battery can be tested while still
installed on the aircraft. Ni-Cads must have a scheduled
energy test performed on the bench due to the inability to
measure their energy level on the aircraft, and because of
their notable “memory” shortcoming.
The SLA battery can be designed to alert the technician if a
battery is failing. Furthermore, it may be possible to test the
failure detection circuits by activating a Built in Test (BITE)
button. This practice significantly reduces FAA paperwork and
maintenance workload. Figure 12-199 shows a SLA battery.Lithium-Ion Batteries
Lithium-ion batteries are the primary type of battery for
many consumer type of equipment, such as cell phones,
battery-powered tools, and computers, but now they are also
being used in commercial and military aircraft. The FAA has
certified lithium-ion batteries to be used on aircraft and one
of the first aircraft to utilize the lithium-ion battery is the
Boeing 787. The three primary functional components of a
lithium-ion battery are the positive and negative electrodes and
electrolyte. Generally, the negative electrode of a conventional
lithium-ion cell is made from carbon. The positive electrode is
a metal-oxide, and the electrolyte is a lithium salt in an organic
solvent. The electrochemical roles of the electrodes reverse
between anode and cathode, depending on the direction of
current flow through the cell. Lithium-ion batteries can be
dangerous under some conditions and can pose a safety
hazard since they contain, unlike other rechargeable batteries,
a flammable electrolyte and are also kept pressurized. Under
certain conditions, they can overheat and a fire can occur.
The Boeing 787 aircraft utilizes two large 32V 8 cell lithium-
ion batteries. These batteries are much lighter and more
powerful than Ni-Cad batteries used in similar-sized aircraft.
These batteries can produce 150 A for airplane power up.
Figure 12-200 shows a B787 battery.
Inverters
An inverter is used in some aircraft systems to convert a
portion of the aircraft’s DC power to AC. This AC is used
mainly for instruments, radio, radar, lighting, and other
accessories. These inverters are usually built to supply
current at a frequency of 400 cps, but some are designed to
provide more than one voltage; for example, 26 volt AC in
one winding and 115 volts in another.
There are two basic types of inverters: the rotary and the
static. Either type can be single-phase or multiphase. The
multiphase inverter is lighter for the same power rating than
the single-phase, but there are complications in distributing
multiphase power and in keeping the loads balanced.
Figure 12-199. Sealed battery.Rotary Inverters
There are many sizes, types, and configurations of rotary
inverters. Such inverters are essentially AC generators and
DC motors in one housing. The generator field, or armature,
and the motor field, or armature, are mounted on a common
shaft that rotates within the housing. One common type of
rotary inverter is the permanent magnet inverter.
Permanent Magnet Rotary Inverter
A permanent magnet inverter is composed of a DC motor
and a permanent magnet AC generator assembly. Each has a
separate stator mounted within a common housing. The motor
armature is mounted on a rotor and connected to the DC supply
through a commutator and brush assembly. The motor field
windings are mounted on the housing and connected directly
to the DC supply. A permanent magnet rotor is mounted at
the opposite end of the same shaft as the motor armature, and
the stator windings are mounted on the housing, allowing
AC to be taken from the inverter without the use of brushes.
Figure 12-201 shows an internal wiring diagram for this
type of rotary inverter. The generator rotor has six poles,
magnetized to provide alternate North and South poles about
its circumference.
When the motor field and armature are excited, the rotor
begins to turn. As the rotor turns, the permanent magnet rotates
within the AC stator coils, and the magnetic flux developed
by the permanent magnets are cut by the conductors in the AC
stator coils. An AC voltage is produced in the windings whose
polarity changes as each pole passes the windings.
This type inverter may be made multiphase by placing more
AC stator coils in the housing in order to shift the phase the
proper amount in each coil.
As the name of the rotary inverter indicates, it has a revolving
armature in the AC generator section. Figure 12-202 shows
the diagram of a revolving armature, three phase inverter.
The DC motor in this inverter is a four pole, compound wound
motor. The four field coils consist of many turns of fine wire,
with a few turns of heavy wire placed on top. The fine wire
is the shunt field, connected to the DC source through a filter
and to ground through a centrifugal governor. The heavy
wire is the series field, which is connected in series with the
motor armature. The centrifugal governor controls the speed
by shunting a resistor that is in series with the shunt field
when the motor reaches a certain speed.
The alternator is a three-phase, four-pole, star-connected AC
generator. The DC input is supplied to the generator field
coils and connected to ground through a voltage regulator. The output is taken off the armature through three slip rings
to provide three-phase power. The inverter would be a single-
phase inverter if it had a single armature winding and one
slip ring. The frequency of this type unit is determined by
the speed of the motor and the number of generator poles.
Inductor-Type Rotary Inverter
Inductor-type inverters use a rotor made of soft iron laminations
with grooves cut laterally across the surface to provide poles
that correspond to the number of stator poles. [Figure 12-203]
The field coils are wound on one set of stationary poles and the
AC armature coils on the other set of stationary poles. When
DC is applied to the field coils, a magnetic field is produced.
The rotor turns within the field coils and, as the poles on the
rotor align with the stationary poles, a low reluctance path for
flux is established from the field pole through the rotor poles
to the AC armature pole and through the housing back to the
field pole. In this circumstance, there is a large amount of
magnetic flux linking the AC coils.
When the rotor poles are between the stationary poles, there
is a high reluctance path for flux, consisting mainly of air;
then, there is a small amount of magnetic flux linking the AC
coils. This increase and decrease in flux density in the stator
induces an alternating current in the AC coils.
The number of poles and the speed of the motor determine
the frequency of this type of inverter. The DC stator field
current controls the voltage. A cutaway view of an inductor-
type rotary inverter is shown in Figure 12-204 .
Figure 12-205 is a simplified diagram of a typical aircraft
AC power distribution system, utilizing a main and a standby
rotary inverter system.
Static Inverters
In many applications where continuous DC voltage must be
converted to alternating voltage, static inverters are used in
place of rotary inverters or motor generator sets. The rapid
progress made by the semiconductor industry is extending
Figure 12-200. Boeing 787 lithium-ion battery.the range of applications of such equipment into voltage and
power ranges that would have been impractical a few years
ago. Some such applications are power supplies for frequency
sensitive military and commercial AC equipment, aircraft
emergency AC systems, and conversion of wide frequency
range power to precise frequency power. [Figure 12-206]
The use of static inverters in small aircraft also has increased
rapidly in the last few years, and the technology has advanced to
the point that static inverters are available for any requirement
filled by rotary inverters. For example, 250 V A emergency AC
supplies operated from aircraft batteries are in production,
as are 2,500 V A main AC supplies operated from a varying
frequency generator supply. This type of equipment has certain
advantages for aircraft applications, particularly the absence
of moving parts and the adaptability to conduction cooling.
Static inverters, referred to as solid-state inverters, are
manufactured in a wide range of types and models that can
be classified by the shape of the AC output waveform and
the power output capabilities. One of the most commonly
used static inverters produces a regulated sine wave output. A
block diagram of a typical regulated sine wave static inverter
is shown in Figure 12-207 . This inverter converts a low DC
voltage into higher AC voltage. The AC output voltage is
held to a very small voltage tolerance, a typical variation
of less than 1 percent with a full input load change. Output
taps are normally provided to permit selection of various
voltages; for example, taps may be provided for 105, 115,
and 125 volt AC outputs. Frequency regulation is typically
within a range of one cycle for a 0–100 percent load change.
Variations of this type of static inverter are available, many
of which provide a square wave output.
Since static inverters use solid-state components, they are considerably smaller, more compact, and much lighter in
weight than rotary inverters. Depending on the output power
rating required, static inverters that are no larger than a typical
airspeed indicator can be used in aircraft systems. Some of
the features of static inverters are:
1. High efficiency
2. Low maintenance, long life
3. No warmup period required
4. Capable of starting under load
5. Extremely quiet operation
6. Fast response to load changes
Static inverters are commonly used to provide power for
such frequency sensitive instruments as the attitude gyro and
directional gyro. They also provide power for autosyn and
magnesyn indicators and transmitters, rate gyros, radar, and
other airborne applications. Figure 12-208 is a schematic of
a typical small jet aircraft auxiliary battery system. It shows
the battery as input to the inverter and the output inverter
circuits to various subsystems.
Semiconductors
To understand why solid-state devices function as they do,
it is necessary to examine the composition and nature of
semiconductors. The two most common materials used for
semiconductors are germanium and silicon. The essential
characteristic of these elements is that each atom has four
valence electrons to share with adjacent atoms in forming
bonds. While both elements are used in semiconductor
construction, silicon is preferred in most modern applications
due to its ability to operate over a wider range of temperatures.
The nature of a bond between two silicon atoms is such that
each atom provides one electron to share with the other.
The two electrons shared are in fact shared equally between
the two atoms. This form of sharing is known as a covalent
bond. Such bonds are very stable and hold the two atoms
together very tightly requiring much energy to break this
bond. [Figure 12-209] In this case, all of the outer electrons
are used to make covalent bonds with other silicon atoms.
In this condition, because all of the outer shell atoms are
used, silicon takes on the characteristic of a good insulator,
due to the fact that there are no open positions available for
electrons to migrate through the orbits.
For the silicon crystal to conduct electricity, there must be
some means available to allow some electrons to move from
place to place within the crystal, regardless of the covalent
bonds present between the atoms. One way to accomplish this
is to introduce an impurity, such as arsenic or phosphorus, into
the crystal structure, which either provides an extra electron
or create a vacant position in the outer shell for electrons
AC coilField coil
Field coilCommutator
Permanent magnet rotor
FilterAC output DC inputA B
G
R A
B
Figure 12-201. Internal wiring diagram of single-phase permanent magnet rotary inverter.
to pass though. The method used to create this condition is
called doping.
Doping
Doping is the process by which small amounts of additives
called impurities are added to the semiconductor material
to increase their current flow by adding a few electrons or
a few holes. Once the material is doped, it then falls into
one of two categories: the N-type semiconductor and the
P-type semiconductor.
An N-type semiconductor material is one that is doped with
an N-type or a donor impurity. Elements such as phosphorus,
arsenic, and antimony are added as impurities and have five
outer electrons to share with other atoms. This causes the
semiconductor material to have an excess electron. Due to the surplus of electrons, the electrons are then considered the
majority current carriers. This electron can easily be moved
with only a small applied electrical voltage. Current flow
in an N-type silicon material is similar to conduction in a
copper wire. That is, with voltage applied across the material,
electrons will move through the crystal towards the positive
terminal just like current flows in a copper wire.
A P-type semiconductor is one that is doped with a P-type or
an acceptor impurity. Elements such as boron, aluminum, and
gallium have only three electrons in the valence shell to share
with the silicon atom. Those three electrons form covalent
bonds with adjacent silicon atoms. However, the expected
fourth bond cannot be formed and a complete connection
is impossible here, leaving a “hole” in the structure of the
crystal. There is an empty place where an electron would
naturally go, and often an electron moves into that space.
AC field40 ohms500 ohms
1250 ohms
Brush
connections
DC
shunt
fieldDC Filter
MotorGovernor
AlternatorSlip ringsTerminal
board
Voltage
regulatorGrounded
can
Noise
slideRectifierPlug rear view27.5 volt inputCommon ground inverter
A
A
B
CD1
4B
CD115 volt 3 phase 400 cycle AC
output phase sequence ACB
DC
series
field+
+–+
–AC
condenser
Figure 12-202. Internal wiring diagram of three-phase, revolving armature.
However, the electron filling the hole left a covalent bond
behind to fill this empty space, which leaves another hole
behind as it moves. Another electron may then move into
that particular hole, leaving another hole behind. As this
progression continues, holes appear to move as positive
charges throughout the crystal. This type of semiconductor
material is designated P-type silicon material. Figure 12-210
shows the progression of a hole moving through a number
of atoms. Notice that the hole illustrated at the far left of
top depiction of Figure 12-210 attracts the next valence
electron into the vacancy, which then produces another vacancy called a hole in the next position to the right. Once
again, this vacancy attracts the next valence electron. This
exchange of holes and electrons continues to progress and
can be viewed in one of two ways. The first way that this
flow can be seen as that of electron movement. The electron
is shown in Figure 12-210 as moving from the right to the
left through a series of holes. Likewise, the second depiction
in Figure 12-210 of the motion of the vacated hole can be
seen as migrating from the left to the right. This view is often
called hole movement. The valence electron in the structure
progresses along a path detailed by the arrows. Holes,
12-98AC output Field
winding
DC
Flux lines
Magnetic fluxN
NS
S
NN
SS
Figure 12-203. Diagram of basic inductor-type inverter.
however, move along a path opposite that of the electrons.
PN Junctions & the Basic Diode
A single type of semiconductor material by itself is not very
useful. Useful applications are developed only when a single
component contains both P-type and N-type materials. The
semiconductor diode is also known as a PN junction diode.
This is a two-element semiconductor device that makes
use of the rectifying properties of a PN junction to convert
alternating current into direct current by permitting current
flow in one direction only.
Figure 12-211 illustrates the electrical characteristics of
an unbiased diode, which means that no external voltage
is applied. The P-side in the illustration is shown to have
many holes, while the N-side shows many electrons. The
electrons on the N-side tend to diffuse out in all directions.
When an electron enters the P region, it becomes a minority
carrier. By definition, a minority carrier is an electron or hole,
whichever is the less dominant carrier in a semiconductor
device. In P-type materials, electrons are the minority carrier
and in N-type material, the hole is considered the minority
carrier. With so many holes around the electron, the electron
soon drops into a hole. When this occurs, the hole then
disappears, and the conduction band electron becomes a
valence electron.Each time an electron crosses the PN junction, it creates a
pair of ions. Figure 12-211 shows this area outlined by dashed
lines. The circled plus signs and the circled negative signs
are the positive and negative ions, respectively. These ions
are fixed in the crystal and do not move around like electrons
or holes in the conduction band. Thus, the depletion zone
constitutes a layer of a fixed charge. An electrostatic field,
represented by a small battery in Figure 12-211 , is established
across the junction between the oppositely charged ions.
The junction barrier is an electrostatic field, which has been
created by the joining of a section of N-type and P-type
material. Because holes and electrons must overcome this
field to cross the junction, the electrostatic field is usually
called a barrier. Because there is a lack or depletion of free
electrons and holes in the area around the barrier, this area is
called the depletion region. [Figure 12-211] As the diffusion
of electrons and holes across the junction continue, the
strength of the electrostatic field increases until it is strong
enough to prevent electrons or holes from crossing over.
At this point, a state of equilibrium exists, and there is no
further movement across the junction. The electrostatic field
created at the junction by the ions in the depletion zone is
called a barrier.
Forward Biased Diode
Figure 12-212 illustrates a forward biased PN junction. When
an external voltage is applied to a PN junction, it is called
bias. In a forward biased PN junction or diode, the negative
voltage source is connected to the N-type material and the
positive voltage source is connected to the P-type material.
In this configuration, the current can easily flow. If a battery
is used to bias the PN junction and it is connected in such a
way that the applied voltage opposes the junction field, it has
the effect of reducing the junction barrier and consequently
aids in the current flow through the junction.
The electrons move toward the junction and the right end
of the diode becomes slightly positive. This occurs because
electrons at the right end of the diode move toward the
junction and leave positively charged atoms behind. The
positively charged atoms then pull electrons into the diode
from the negative terminal of the battery.
When electrons on the N-type side approach the junction,
they recombine with holes. Basically, electrons are flowing
into the right end of the diode, while the bulk of the electrons
in the N-type material move toward the junctions. The
left edge of this moving front of electrons disappears by
dropping into holes at the junction. In this way, there is a
continuous current of electrons from the battery moving
toward the junction.
Housing, stator and coil assembly
Armature Resistor
Head assembly
Fan
Yoke assembly
ResistorBrush assemblyCommutator
Condenser
Brush retainer capRotor
Figure 12-204. Cutaway view of inductor-type rotary inverter.
When the electrons hit the junction, they then become
valence electrons. Once a valence electron, they can then
move through the holes in the P-type material. When the
valence electrons move through the P-type material from
the right to the left, a similar movement is occurring with
the holes by moving from the left side of the P-type material
to the right. Once the valence electron reaches the end of
the diode, it then flows back into the positive terminal of
the battery.
In summary:
1. Electron leaves negative terminal of the battery and
enters the right end (N-type material) of the diode.
2. Electron then travels through the N-type material.
3. The electron nears the junction and recombines and
becomes a valence electron.
4. The electron now travels through the P-type material
as a valence electron.
5. The electron then leaves the diode and flows back to
the positive terminal of the battery.Reverse Biased Diode
When the battery is turned around as shown in Figure 12-213 ,
then the diode is reverse biased and current does not flow. The
most noticeable effect seen is the widened depletion zone.
The applied battery voltage is in the same direction as the
depletion zone field. Because of this, holes and electrons tend
to move away from the junction. Simply stated, the negative
terminal attracts the holes away from the junction, and the
positive terminal attracts the electrons away from the barrier.
Therefore, the result is a wider depletion zone. This action
increases the barrier width because there are more negative
ions on the P-side of the junction and more positive ions on
the N-side of the junction. This increase in the number of
ions at the junction prevents current flow across the barrier
by the majority carriers.
To summarize, the important thing to remember is that these
PN junction diodes offer very little resistance to current when
the diode is forward biased. Maximum resistance happens
AC switching
relayAC primary bus
AC secondary bus
MAIN
StdbyOff
Left
generator
DC bus
Inverter switch
DC powerMain
inverter
power
relayMain
inverter
AC bus tie
breaker
Weather radar
From main
inverter only
From standby
inverter onlyCircuit energized by
standby inverter only with
inverter switch set to main
and radar switch set to on.
RMI card
RMI pointer 26 VAC busor
Engine oil pressure ratio
Oil pressure Standby
inverterSecondary
inverter
power
relay
Right
generator
DC bus
115 VAC power 26 VAC power
Figure 12-205. A typical aircraft AC power distribution system using main and standby rotary inverters.
Figure 12-206. Static inverter.
when the diode is reversed biased. Figure 12-214 shows a
graph of the current characteristics of a diode that is biased
in both directions.
Rectifiers
Many devices in an aircraft require high amperage, low
voltage DC for operation. This power may be furnished by
DC engine-driven generators, motor generator sets, vacuum tube rectifiers, or dry disk or solid-state rectifiers.
In aircraft with AC systems, a special DC generator is not
desirable since it would be necessary for the engine accessory
section to drive an additional piece of equipment. Motor
generator sets, consisting of air-cooled AC motors that drive
DC generators, eliminate this objection because they operate
directly off the AC power system. Vacuum tube or various
types of solid-state rectifiers provide a simple and efficient
method of obtaining high voltage DC at low amperage.
Dry disk and solid-state rectifiers, on the other hand, are an
excellent source of high amperage at low voltage.
A rectifier is a device that transforms AC into DC by limiting
or regulating the direction of current flow. The principal
types of rectifiers are dry disk and solid state. Solid-state,
or semiconductor, rectifiers have replaced virtually all
other types; and, since dry disk and motor generators are
largely limited to older model aircraft, the major part of the
study of rectifiers is devoted to solid-state devices used for
rectification. The two methods discussed in this handbook
Gear control
Flaps
Spoiler
Wheel master
Nose steering relay
Yaw trim
Altitude gyroAuxiliary
batteryNormally
closed relay
Directional gyroRoll trim
ADFOn
(MOM)
OffGear, flap, spoiler
switch
Pilot’s audio
Copilot’s map light
Transceiver
On
OffGyros
Battery
Inverter
Right fwd bus
Right fwd bus
Transceiver light
Altitude
gyro lightDirectional
gyro light
Figure 12-208. Auxiliary battery system using static inverter.
Resonant filter
regulator
Output keyer
Buffer amplifier
DC regulator
Square wave
oscillatorDC inputAC output
Figure 12-207. Regulated sine wave static inverter.
are the half-wave rectifier and the full-wave rectifier.
Half-Wave Rectifier
Figure 12-215 illustrates the basic concept of a half-wave
rectifier. When an AC signal is on a positive swing as shown
in Figure 12-215A , the polarities across the diode and the load
resistor are also positive. In this case, the diode is forward
biased and can be replaced with a short circuit as shown in
the figure. The positive portion of the input signal appears across the load resistor with no loss in potential across the
series diode.
Figure 12-215B now shows the input signal being reversed.
Note that the polarities across the diode and the load resistor are
also reversed. In this case, the diode is now reverse biased and
can be replaced with an equivalent open circuit. The current
in the circuit is now 0 amperes and the voltage drop over the
load resistor is 0 volts. The resulting waveform for a complete
12-102Si
SiSi Si
Si Si
Si Si SiSi
A
B
Electron movement
Silicon atom showing one of the electrons in its
valence shell
Silicon atom in which one electron has broken out of
its valence shell and left a hole
Electron moving from one silicon atom to another
and leaving a holeHole movement
N-type P-type
N-typeDepletion zone
Represents electrostatic fieldP-type
Figure 12-209. Valence electrons.
Figure 12-210. A hole moving through atoms.Figure 12-211. Depletion region.
sinusoidal input can be seen at the far right of Figure 12-215 .
The output waveform is a reproduction of the input waveform
minus the negative voltage swing of the wave. For this reason,
this type of rectifier is called a half-wave rectifier.Full-Wave Rectifier
Figure 12-216 illustrates a more common use of the diode as
a rectifier. This type of a rectifier is called a full-wave bridge
rectifier. The term “full-wave” indicates that the output is a
continuous sequence of pulses rather than having gaps that
appear in the half-wave rectifier.
Figure 12-216C shows the initial condition, during which, a
positive portion of the input signal is applied to the network.
Note the polarities across the diodes. Diodes D2 and D4 are
reverse biased and can be replaced with an open circuit.
Diodes D1 and D3 are forward biased and act as an open
circuit. The current path through the diodes is clear to see, and
the resulting waveform is developed across the load resistor.
During the negative portion of the applied signal, the diodes
reverse their polarity and bias states. The result is a network
shown in Figure 12-216D . Current now passes through diodes
D4 and D2, which are forward biased, while diodes D1 and
D3 are essentially open circuits due to being reverse biased.
Note that during both alternations of the input waveform, the
current passes through the load resistor in the same direction.
This results in the negative swing of the waveform being
flipped up to the positive side of the time line.
Dry Disk
Dry disk rectifiers operate on the principle that electric
current flows through a junction of two dissimilar conducting
materials more readily in one direction than it does in the
opposite direction. This is true because the resistance to
current flow in one direction is low, while in the other
direction it is high. Depending on the materials used, several
amperes may flow in the direction of low resistance but only
12-103N-typeNarrow depletion zone
P-type
+ –Holes Electrons
Electron flowe –N-typeWide depletion zoneP-type
+ –
Figure 12-212. Forward biased PN junction.Figure 12-213. Reversed diode.
a few milliamperes in the direction of high resistance.
Three types of dry disk rectifiers may be found in aircraft:
the copper oxide rectifier, the selenium rectifier, and the
magnesium copper-sulfide rectifier. The copper oxide
rectifier consists of a copper disk upon which a layer of
copper oxide has been formed by heating. [Figure 12-217]
It may also consist of a chemical copper oxide preparation
spread evenly over the copper surface. Metal plates, usually
lead plates, are pressed against the two opposite faces of the
disk to form a good contact. Current flow is from the copper
to the copper oxide.
The selenium rectifier consists of an iron disk, similar to a
washer, with one side coated with selenium. Its operation is
similar to that of the copper oxide rectifier. Current flows
from the selenium to the iron.
The magnesium copper sulfide rectifier is made of washer-
shaped magnesium disks coated with a layer of copper
sulfide. The disks are arranged similarly to the other types.
Current flows from the magnesium to the copper sulfide.
Types of Diodes
Today, there are many varieties of diodes that can be grouped
into one of several basic categories.
Power Rectifier Diodes
The rectifier diode is usually used in applications that require
high current, such as power supplies. The range in which
the diode can handle current can vary anywhere from one
ampere to hundreds of amperes. One common example
of diodes is the series of diodes, part numbers 1N4001
to 1N4007. The “1N” indicates that there is only one PN
junction, or that the device is a diode. The average current
carrying range for these rectifier diodes is about one ampere
with a peak inverse voltage between 50 volts to 1,000 volts.
Larger rectifier diodes can carry currents up to 300 amperes
when forward biased and have a peak inverse voltage of 600
volts. A recognizable feature of the larger rectifier diodes
is that they are encased in metal in order to provide a heat sink. [Figure 12-218]
Zener Diodes
Zener diodes (sometimes called “breakdown diodes”) are
designed so that they break down (allow current to pass)
when the circuit potential is equal to or in excess of the
desired reverse bias voltage. The range of reverse bias
breakdown-voltages commonly found can range from 2
volts to 200 volts depending on design. Once a specific
reverse bias voltage has been reached, the diode conducts
and behaves like a constant voltage source. Within the
normal operating range, the zener functions as a voltage
regulator, waveform clipper, and other related functions.
Below the desired voltage, the zener blocks the circuit like
any other diode biased in the reverse direction. Because
the zener diode allows free flow in one direction when it
is used in an AC circuit, two diodes connected in opposite
directions must be used. This takes care of both alternations
of current. Power ratings of these devices range from about
250 milliwatts to 50 watts.
Special Purpose Diodes
The unique characteristics of semiconductor material have
allowed for the development of many specialized types of
diodes. A short description of some of the more common diode
types is given for general familiarization. [Figure 12-219]
Light-Emitting Diode (LED)
In a forward biased diode, electrons cross the junction
and fall into holes. As the electrons fall into the valence
band, they radiate energy. In a rectifier diode, this energy
is dissipated as heat. However, in the light-emitting diode
(LED), the energy is dissipated as light. By using elements
such as gallium, arsenic, and phosphorous, an LED can be
designed to radiate colors, such as red, green, yellow, blue,
and infrared light. LEDs that are designed for the visible light
portion of the spectrum are useful for instruments, indicators,
and even cabin lighting. The advantages of the LED over the
incandescent lamps are longer life, lower voltage, faster on
and off operations, and less heat.
A
BA
BRL
Forward bias (Short circuit)++
−−
+
−
RL
++
−−
+−
Reverse bias (Open circuit)V–V+
Time120 v
–120 vV–V+
Time120 v
V–V+
TimeInput Signal Circuit Depiction Output Wave-form Across Load ResistorHalf-wave Rectification
Figure 12-215. Basic concept of half-wave rectifier.120v 90v 60v 30v
1v 2v 3v 4v 5v60 mA
50 mA
40 mA
30 mA
20 mA
10 mA
600 Aμ
1200 Aμ
Junction diode characteristics
Figure 12-214. Diode biased in both directions.
Liquid Crystal Displays (LCD)
The liquid crystal display (LCD) has an advantage over the
LED in that it requires less power to operate. Where LEDs
commonly operate in the milliwatt range, the LCD operates in the microwatt range. The liquid crystal is encapsulated
between two glass plates. When voltage is not applied to the
LCD, the display is clear. However, when a voltage is applied,
the result is a change in the orientation of the atoms of the
crystals. The incident light is then reflected in a different
direction. A frosted appearance results in the regions that have
voltage applied and permits distinguishing of numeric values.
Photodiode
Thermal energy produces minority carriers in a diode. The
higher the temperature, the greater the current in a reverse
current diode. Light energy can also produce minority
carriers. By using a small window to expose the PN junction,
a photodiode can be built. When light falls upon the junction
of a reverse-biased photodiode, electrons-hole pairs are
created inside the depletion layer. The stronger the light,
the greater the number of light-produced carriers, which in
turn causes a greater magnitude of reverse-current. Because
of this characteristic, the photodiode can be used in light
detecting circuits.
Varactors
The varactor is simply a variable-capacitance diode. The
reverse voltage applied controls the variable-capacitance
of the diode. The transitional capacitance decreases as
the reverse voltage is increasingly applied. In many
12-105DB A
CRLD2 D1
D3 D4+
−+−
RLD1
D3+
−++
+−−
−RLD2
D4+
−++
+−−
−V–V+
Time120 v
–120 vInput signal
V–V+
Time120 v 120 vOutput wave-form
Across load resistorCircuit depiction
Current
CurrentCurrent
Current
Figure 12-216. Full-wave bridge rectifier.applications, the varactor has replaced the old mechanically
tuned capacitors. Varactors can be placed in parallel with
an inductor and provide a resonant tank circuit for a tuning
circuit. By simply varying the reverse voltage across the
varactor, the resonant frequency of the circuit can be adjusted.
Schottky Diodes
Schottky diodes are designed to have metal, such as gold,
silver, or platinum, on one side of the junction and doped
silicon, usually an N-type, on the other side of the junction.
This type of a diode is considered a unipolar device because
free electrons are the majority carrier on both sides of the
junction. The Schottky diode has no depletion zone or charge
storage, which means that the switching time can be as high as
300 MHz. This characteristic exceeds that of the bipolar diode.
Diode Identification
Figure 12-218 illustrates a number of methods employed
for identifying diodes. Typically manufacturers place some
form of an identifier on the diode to indicate which end is
the anode and which end is the cathode. Dots, bands, colored
bands, the letter ‘k’ or unusual shapes indicate the cathode
end of the diode.Introduction to Transistors
The transistor is a three-terminal device primarily used
to amplify signals and control current within a circuit.
[Figure 12-220] The basic two-junction semiconductor must
have one type of region sandwiched between two of the other
type. The three regions in a transistor are the collector (C),
which is moderately doped, the emitter (E), which is heavily
doped, and the base (B), which is significantly less doped.
The alternating layers of semiconductor material type provide
the common commercial name for each type of transistor. The
interface between the layers is called a junction. Selenium
and germanium diodes previously discussed are examples
of junction diodes. Note that the sandwiched layer or base
is significantly thinner than the collector or the emitter. In
general, this permits a “punching through” action for the
carriers passing between the collector and emitter terminals.
Classification
The transistors are classified as either NPN or PNP according
to the arrangement of their N and P-materials. The NPN
transistor is formed by introducing a thin region of P-material
between two regions of N-type material. The opposite is true
for the PNP configuration.
The two basic types of transistors along with their circuit
Approx. 0.75"Schematic symbol
General purpose signal diodes
Approx. 0.1"
(+) Anode Cathode (−)(+) Anode Cathode (−)
Cathode (−) (+) AnodeApprox. 0.2"Approx. 1.5"
Rectifier diodes 35 ampere silicon rectifier
Figure 12-218. General purpose diodes.
CopperLead
Insulating washer
Insulating tube
Pressure plateCopper oxide
Direction of
circuit flow
Figure 12-217. Copper oxide dry disk rectifier.
symbols are shown in Figure 12-221 . Note that the two
symbols are different. The horizontal line represents the base,
and two angular lines represent the emitter and collector. The
angular line with the arrow on it is the emitter, while the line
without is the collector. The direction of the arrow on the
emitter determines whether or not the transistor is a PNP or
an NPN type. If the arrow is pointing in, the transistor is a PNP. On the other hand, if the arrow is pointing out, then it
is an NPN type.
Transistor Theory
As discussed in the section on diodes, the movement of
the electrons and holes can be considered current. Electron
current moves in one direction, while hole current travels in
the opposite direction. In transistors, both electrons and holes
act as carriers of current.
A forward biased PN junction is comparable to a low-
resistance circuit element, because it passes a high current
for a given voltage. On the other hand, a reverse-biased PN
junction is comparable to a high-resistance circuit element.
By using Ohm’s Law formula for power (P = I2R) and
assuming current is held constant through both junctions, it
can be concluded that the power developed across the high
resistance junction is greater than that developed across a low
resistance junction. Therefore, if a crystal were to contain
two PN junctions, one forward biased and the other reverse
biased, and a low-power signal injected into the forward
biased junction, a high-power signal could be produced at
the reverse-biased junction.
To use the transistor as an amplifier, some sort of external
bias voltage must modify each of the junctions. The first PN
junction (emitter-base) is biased in the forward direction.
This produces a low resistance. The second junction, which
is the collector-base junction, is reverse biased to produce a
high resistance. [Figure 12-222]
Schottky diode
Step-recovery diode
ZenerLED
Photodiode
Varactor
Figure 12-219. Schematic symbols for special purpose diodes.With the emitter-base junction biased in the forward direction,
electrons leave the negative terminal of the battery and enter
the N-material. These electrons pass easily through the
emitter, cross over the junction, and combine with the hole
in the P-material in the base. For each electron that fills a
hole in the P-material, another electron leaves the P-material,
which creates a new hole and enters the positive terminal of
the battery.
The second PN junction, which is the base-collector junction,
is reverse biased. This prevents the majority carriers from
crossing the junction, thus creating a high-resistance circuit.
It is worth noting that there still is a small current passing
through the reversed PN junction in the form of minority
carriers—that is, electrons in the P-material and holes in the
N-material. The minority carriers play a significant part in
the operation of the NPN transistor.
Figure 12-223 illustrates the basic interaction of the NPN
junction. There are two batteries in the circuit used to bias the
NPN transistor. Vbb is considered the base voltage supply,
rated in this illustration at 1 volt, and the battery voltage
Vcc, rated at 6 volts, is called the collector voltage supply.
Current within the external circuit is simply the movement
of free electrons originating at the negative terminal of the
battery and flowing to the N-material. [Figure 12-223]
As the electrons enter the N-material, they become the
majority carrier and move through the N-material to the
emitter-base PN junction. This emitter-base junction is
forward biased at about 0.65 to 0.7 volts positive with
respect to the emitter and presents no resistance to the flow of
electrons from the emitter into the base, which is composed of
P-material. As these electrons move into the base, they drop
into available holes. For every electron that drops into a hole,
another electron exits the base by way of the base lead and
becomes the base current or Ib. Of course, when one electron
leaves the base, a new hole is formed. From the standpoint of
the collector, these electrons that drop into holes are lost and of no use. To reduce this loss of electrons, the transistor is
designed so that the base is very thin in relation to the emitter
and collector, and the base is lightly doped.
Most of the electrons that move into the base fall under the
influence of the reverse bias of the collector. While collector-
base junction is reverse biased with respect to the majority
carriers, it behaves as if it is forward biased to the electrons
or minority carriers in this case. The electrons are accelerated
through the collector-base junction and into the collector. The
collector is comprised of the N-type material; therefore, the
electrons once again become the majority carrier. Moving
easily through the collector, the electrons return to the
positive terminal of the collector supply battery Vcc, which
is shown in Figure 12-223 as Ic.
Because of the way this device operates to transfer current
(and its internal resistances) from the original conduction path
to another, its name is a combination of the words “transfer”
and “resistor”—transistor.
PNP Transistor Operation
The PNP transistor generally works the same way as the
NPN transistor. The primary difference is that the emitter,
base, and collector materials are made of different material
than the NPN. The majority and minority current carriers
are the opposite in the PNP to that of the NPN. In the case
of the PNP, the majority carriers are the holes instead of the
electrons in the NPN transistor. To properly bias the PNP, the
polarity of the bias network must be reversed.
Identification of Transistors
Figure 12-224 illustrates some of the more common transistor
lead identifications. The methods of identifying leads vary
due to a lack of a standard and require verification using
manufacturer information to properly identify. However, a
short description of the common methods is discussed below.
Figure 12-224D shows an oval-shaped transistor. The
collector lead in this case is identified by the wide space
between it and the lead for the base. The final lead at the
far left is the emitter. In many cases, colored dots indicate
the collector lead, and short leads relative to the other leads
indicate the emitter. In a conventional power diode, as seen
in Figure 12-224E , the collector lead is usually a part of the
mounting bases, while the emitter and collector are leads or
tines protruding from the mounting surface.
Field Effect Transistors
Another transistor design that has become more important
than the bipolar transistor is the field-effect transistor (FET).
The primary difference between the bipolar transistor and the
FET is that the bipolar transistor has two PN junctions and
P N P Emitter Collector
BasePN junctions
N P N Emitter Collector
BasePN junctions
P N PEmitter Collector
Base
N P NBase
Emitter Collector
BaseBaseFigure 12-220. Transistor.
Figure 12-221. Two basic transistors with circuit symbols.is a current-controlled device, while the FET has only one
PN junction and is a voltage-controlled device. Within the
FET family, there are two general categories of components.
One category is called the junction FET (JFET), which has
only one PN junction. The other category is known as the
enhancement-type or metal-oxide JET (MOSFET).
Figure 12-225 shows the basic construction of the JFET and
the schematic symbol. In this figure, it can be seen that the
drain (D) and source (S) are connected to an N-type material,
and the gate (G) is connected to the P-type material. With
gate voltage Vgg set to 0 volts and drain voltage Vdd set to
some positive voltage, a current flows between the source
and the drain, through a narrow band of N-material. If then,
Vgg is adjusted to some negative voltage, the PN junction
is reverse biased, and a depletion zone (no charge carriers)
is established at the PN junction. By reducing the region of
noncarriers, it has the effect of reducing the dimensions of the
N-channel, resulting in a reduction of source to drain current.
Because JFETs are voltage-controlled devices, they have some
advantages over the bipolar transistor. One such advantage
is that because the gate is reverse biased, the circuit that it
is connected to sees the gate as a very high resistance. This
means that the JFET has less of an insertion influence in the
circuit. The high resistance also means that less current is used.
Like many other solid-state devices, careless handling and
static electricity can damage the JFET. Technicians should
take all precautions to prevent such damage.
Metal-Oxide-Semiconductor FET (MOSFET)
Figure 12-226 illustrates the general construction and the
schematic symbol of the MOSFET transistor. The biasing
arrangement for the MOSFET is essentially the same as that
for the JFET. The term “enhancement” comes from the idea
that when there is no bias voltage applied to the gate (G),
then there is no channel for current conduction between the source (S) and the drain (D). By applying a greater voltage on
the gate (G), the P-channel begins to materialize and grow in
size. Once this occurs, the source (S) to drain (D) current Id
increases. The schematic symbol reflects this characteristic
by using a broken line to indicate that the channel does not
exist without a gate bias.
Common Transistor Configurations
A transistor may be connected in one of three different
configurations: common-emitter (CE), common-base (CB),
and common-collector (CC). The term “common” is used
to indicate which element of the transistor is common
to both the input and the output. Each configuration has
its own characteristics, which makes each configuration
suitable for particular applications. A way to determine what
configuration you may find in a circuit is to first determine
which of the three transistor elements is used for the input
signal. Then, determine the element used for the output
signal. At that point, the remaining element, (base, emitter,
or collector) is the common element to both the input and
output, and thus you determine the configuration.
Common-Emitter (CE) Configuration
This is the configuration most commonly used in amplifier
+
−
+
−
EmitterVcc = 6 voltsCollector Reverse biased junctionHole [blue]
Electron [green]
lbVbb = 1 voltForward biased
junction
lelc
N
P
NBase
+
−+
−Figure 12-222. NPN transistor.
Figure 12-223. NPN Junction.circuits because they provide good gains for voltage, current,
and power. The input signal is applied to the base-emitter
junction, which is forward biased (low resistance), and the
output signal is taken off the collector-emitter junction,
which is reverse biased (high resistance). Then the emitter
is the common element to both input and output circuits.
[Figure 12-227]
When the transistor is connected in a CE configuration, the
input signal is injected between the base and emitter, which
is a low-resistance, low-current circuit. As the input signal
goes positive, it causes the base to go positive relative to
the emitter. This causes a decrease in the forward bias,
which in turn reduces the collector current I C and increases
the collector voltage (E C being more negative). During the
negative portion of the input signal, the voltage on the base
is driven more negative relative to the emitter. This increases
the forward bias and allows an increase in collector current IC and a decrease in collector voltage (E C being less negative
and going positive). The collector current, which flows
through the reverse-biased junction, also flows through a
high-resistance load resulting in a high level of amplification.
Because the input signal to the CE goes positive when the
output goes negative, the two signals are 180° out of phase.
This is the only configuration that provides a phase reversal.
The CE is the most popular of the three configurations
because it has the best combination of current and voltage
gain. Gain is a term used to indicate the magnitude of
amplification. Each transistor configuration has its unique
gain characteristics even though the same transistors are used.
Common-Collector (CC) Configuration
This transistor configuration is usually used for impedance
matching. It is also used as a current driver due to its high
current gain. It is also very useful in switching circuits
since it has the ability to pass signals in either direction.
[Figure 12-227]
In the CC circuit, the input signal is applied to the base, and the
output signal is taken from the emitter, leaving the collector as
the common point between the input and the output. The input
resistance of the CC circuit is high, while the output resistance
is low. The current gain is higher than that in the CE, but it has
a lower power gain than either the CE or CB configuration. Just
like the CB configuration, the output signal of the CC circuit
is in phase with the input signal. The CC is typically referred
to as an emitter-follower because the output developed on the
emitter follows the input signal applied to the base.
Common-Base (CB) Configuration
The primary use of this configuration is for impedance
matching because it has low input impedance and high
output resistance. Two factors, however, limit the usefulness
of this circuit application. First is the low-input resistance
and second is its lack of current, which is always below 1.
Since the CB configuration gives voltage amplification, there
are some applications for this circuit, such as microphone
amplifiers. [Figure 12-227]
In the CB circuit, the input signal is applied to the emitter and
the output signal is taken from the collector. In this case, both
the input and the output have the base as a common element.
When an input signal is applied to the emitter, it causes the
emitter-base junction to react in the same manner as that in
the CE circuit. When an input adds to the bias, it increases
the transistor current; conversely, when the signal opposes
the bias, the current in the transistor decreases.
The signal adds to the forward bias, since it is applied to
the emitter, causing the collector current to increase. This
increase in I C results in a greater voltage drop across the
+
Gate (G)Gate (G)
Drain (D)Drain (D)ID
IS
VGG
VDD
Source (S)Source (S)N-material
P-materialCharge free or depletion zone
P-channel N-channel
JFET symbolJFET construction
Drain (D) Source (S)Gate (G)−
+−+
+−
Figure 12-225. JFET and the schematic symbol.
Color dot
Color dotE
E
EB B
CCBC
EB C
A B C D E
E
B
Figure 12-224. Common transistor lead identifications.
load resistor RL, thus lowering the collector voltage E C.
The collector voltage, in becoming less negative, swings in a
positive direction and is therefore in phase with the incoming
positive signal.
Vacuum Tubes
The use of vacuum tubes in aircraft electrical and electronic
systems has rapidly declined due to the many advantages
of using transistors. However, some systems still employ
vacuum tubes in special applications, and possibly some
older model aircraft still in service are equipped with devices
that use vacuum tubes. While these components may still
be in service, their infrequent occurrence does not warrant
a detailed discussion.
Originally, vacuum tubes were developed for radio work.
They are used in radio transmitters as amplifiers for
controlling voltage and current, as oscillators for generating
audio and radio frequency signals, and as rectifiers for
converting AC into DC. While there are many types of
vacuum tubes for a variety of applications, the most common
types fall into one of the following families: (1) diode, (2)
triode, (3) tetrode, and (4) pentode. Each of these vacuum
tube types operates on the following fundamental principles.
When a piece of metal is heated, the speed of the electrons
in the metal is increased. If the metal is heated to a high
enough temperature, the electrons are accelerated to the
point where some of them actually leave the surface of the
metal. In a vacuum tube, electrons are supplied by a piece of
metal called a cathode, which is heated by an electric current.
Within limits, the hotter the cathode, the greater the number
of electrons it gives off or emits.
To increase the number of electrons emitted, the cathode is usually coated with special chemical compounds. If an
external field does not draw the emitted electrons away, they
form about the cathode into a negatively-charged cloud called
the space charge. The accumulation of negative electrons
near the emitter repels others coming from the emitter. The
emitter, if insulated, becomes positive because of the loss of
electrons. This establishes an electrostatic field between the
cloud of negative electrons and the now positive cathode. A
balance is reached when only enough electrons flow from the
cathode to the area surrounding it to supply the loss caused
by diffusion of the space charge.
Gate (G)Applied voltage +
Drain (D) Source (S)
P-material
MOSFET symbolMOSFET construction+ −
N-MOSFETDrain (D) Source (S)Gate (G)
P-MOSFETDrain (D) Source (S)Gate (G)
N N
NPN
InputOutputPNP
InputOutput
NPN
InputOutput
NPN
Input OutputPNP
Input OutputPNP
InputOutputCommon-emitter
Common-collector
Common-baseFigure 12-226. General construction and schematic symbol of
MOSFET transistor.
Figure 12-227. Transistor configurations (common-emitter,
common-collector, common-base).Filtering
One of the more common uses of the capacitor and inductor
that the technician may find in the field is that of the filter.
Filtering Characteristics of Capacitors
The nature of capacitance opposes a voltage change across its
terminal by storing energy in its electrostatic field. Whenever
the voltage tends to rise, the capacitor converts this voltage
change to stored energy. When the voltage tends to fall, the
capacitor converts this stored energy back to voltage. The use
of a capacitor for filtering the output of a rectifier is illustrated
in Figure 12-228 . The rectifier is shown as a block, and the
capacitor C 1 is connected in parallel with the load R 1.
The capacitor C 1 is chosen to offer very low impedance to
the AC ripple frequency and very high impedance to the
DC component. The ripple voltage is therefore bypassed
to ground through the low impedance path of the capacitor,
while the DC voltage is applied unchanged to the load. The
effect of the capacitor on the output of the rectifier can be seen
in the waveshapes shown in Figure 12-229 . Dotted lines show
the rectifier output, while the solid lines show the effect of
the capacitor. In this example, full-wave rectifier outputs are
shown. The capacitor C 1 charges when the rectifier voltage
output tends to increase and discharges when the voltage
output tends to decrease. In this manner, the voltage across
the load R 1 is kept fairly constant.Filtering Characteristics of Inductors
The inductance provided by an inductor may be used as a
filter, because it opposes a change in current through it by
storing energy in its electromagnetic field. Whenever the
current increases, the stored energy in the electromagnetic
field increases. When the current through the inductor
decreases, the inductor supplies the energy back into the
circuit in order to maintain the existing flow of current.
The use of an inductor for filtering the output of a rectifier
is shown in Figure 12-230. Note that in this network the
inductor L 1 is in series with the load R 1.
The inductance L 1 is selected to offer high impedance to the
AC ripple voltage and low impedance to the DC component.
The result is a very large voltage drop across the inductor
and a very small voltage drop across the load R 1. For the
DC component, however, a very small voltage drop occurs
across the inductor and a very large voltage drop across the
load. The effect of an inductor on the output of a full-wave
rectifier in the output waveshape is shown in Figure 12-231 .
Common Filter Configurations
Capacitors and inductors are combined in various ways to
provide more satisfactory filtering than can be obtained
with a single capacitor or inductor. These are referred to
collectively as LC filters. Several combinations are shown
schematically in Figure 12-232 . Note that the L, or inverted
L-type, and the T-type filter sections resemble schematically
12-112the corresponding letters of the alphabet. The pi-type filter
section resembles the Greek letter pi (π) schematically.
All the filter sections shown are similar in that the inductances
are in series and the capacitances are in parallel with the
load. The inductances must, therefore, offer very high
impedance and the capacitors very low impedance to the
ripple frequency. Since the ripple frequency is comparatively
low, the inductances are iron core coils having large values
of inductance (several henries). Because they offer such
high impedance to the ripple frequency, these coils are called
chokes. The capacitors must also be large (several microfarads)
to offer very little opposition to the ripple frequency. Because
the voltage across the capacitor is DC, electrolytic capacitors
are frequently used as filter capacitors. Always observe the
correct polarity in connecting electrolytic capacitors.
LC filters are also classified according to the position of
the capacitor and inductor. A capacitor input filter is one in
which the capacitor is connected directly across the output
terminals of the rectifier. A choke input filter is one in which
a choke precedes the filter capacitor.
If it is necessary to increase the applied voltage to more than
a single rectifier can tolerate, the usual solution is to stack
them. These rectifiers are similar to resistors added in series.
Each resistor drops a portion of the applied voltage rather than
the total voltage. The same theory applies to rectifiers added
in series or stacked. Series stacking increases the voltage
rating. If, for example, a rectifier is destroyed with an applied
voltage exceeding 50 volts, and it is to be used in a circuit
with an applied voltage of 150 volts, stacking of diodes can
be employed. The result is shown in Figure 12-233 .
Basic LC Filters
Analog filters are circuits that perform signal processing
functions, specifically intended to remove unwanted signal
components, such as ripple, and enhance desired signals. The
simplest analog filters are based on combinations of inductors
and capacitors. The four basic categories of filters discussed
are: low-pass, high-pass, band-pass and band-stop. All these
types are collectively known as passive filters, because they
do not depend on any external power source.
The operation of a filter relies on the characteristic of variable
inductive and capacitive reactance based on the applied
frequency. In review, the inductor blocks high-frequency
signals (high reactance) and conducts low-frequency
signals (low reactance), while capacitors do the reverse.
A filter in which the signal passes through an inductor, or
in which a capacitor provides a path to earth, presents less
attenuation (reduction) to a low-frequency signal than to a
high-frequency signal and is considered a low-pass filter. If
the signal passes through a capacitor, or has a path to ground through an inductor, then the filter presents less attenuation
to high-frequency signals than low-frequency signals and
is then considered a high-pass filter. Typically after an AC
signal is rectified, the pulses of voltage are changed to usable
form of DC by way of filtering.
Low-Pass Filter
A low-pass filter is a filter that passes low frequencies well,
but attenuates (reduces) higher frequencies. The so-called
cutoff frequency divides the range of frequencies that are
passed and the range of frequencies that are stopped. In other
words, the frequency components higher than the cutoff
frequency are stopped by a low-pass filter. The actual amount
of attenuation for each frequency varies by filter design.
An inductive low-pass filter inserts an inductor in series with
the load, where a capacitive low-pass filter inserts a resistor
in series and a capacitor in parallel with the load. The former
filter design tries to block the unwanted frequency signal
while the latter tries to short it out. Figure 12-234 illustrates
this type of circuit and the frequency/current flow response.
High-Pass Filter (HPF)
A high-pass filter (HPF) is a filter that passes high frequencies
well, but attenuates (reduces) frequencies lower than the
cutoff frequency. The actual amount of attenuation for each
frequency varies once again depending on filter design. In
some cases, it is called a low-cut filter. A HPF is essentially
the opposite of a low-pass filter.
It is useful as a filter to block any unwanted low frequency
components of a signal while passing the desired higher
frequencies. Figure 12-235 illustrates this type of circuit and
the frequency/current flow response.
Band-Pass Filter
A band-pass filter is basically a combination of a high-
pass and a low-pass. There are some applications where
a particular range of frequencies need to be singled out or
filtered from a wider range of frequencies. Band-pass filter
circuits are designed to accomplish this task by combining
the properties of low-pass and high-pass into a single
filter. Figure 12-236 illustrates this type of circuit and the
frequency/current flow response.
Band-Stop Filter
In signal processing, a band-stop filter or band-rejection filter
is a filter that passes most frequencies unaltered, but attenuates
those in a range to very low levels. It is the opposite of a band-
pass filter. A notch filter is a band-stop filter with a narrow
stopband (high Q factor). Notch filters are used in live sound
reproduction (public address (PA) systems) and in instrument
DC C1R1AC inputRectifier
Voltage across C1 with
large load circuitVoltage across C1 with
small load circuit
L1
R1AC inputRectifierLoad
Figure 12-228. A capacitor used as a filter.
Figure 12-229. Half-wave and full-wave rectifier outputs using
capacitor filter.
Figure 12-230. An inductor used as a filter. Figure 12-231. Output of an inductor filter rectifier.amplifier (especially amplifiers or preamplifiers for acoustic
instruments, such as acoustic guitar, mandolin, bass instrument
amplifier, etc.) to reduce or prevent feedback, while having
little noticeable effect on the rest of the frequency spectrum.
Other names include “band limit filter,” “T-notch filter,” “band-
elimination filter,” and “band-rejection filter.”
Typically, the width of the stop-band is less than 1 to 2
decades (that is, the highest frequency attenuated is less than
10 to 100 times the lowest frequency attenuated). In the audio
band, a notch filter uses high and low frequencies that may
be only semitones apart.
A band-stop filter is the general case. A notch filter is a specific
type of band-stop filter with a very narrow range. Also called
band-elimination, band-reject, or notch filters, this kind of
filter passes all frequencies above and below a particular
range set by the component values. Not surprisingly, it can
be made out of a low-pass and a high-pass filter, just like
the band-pass design, except that this time we connect the
two filter sections in parallel with each other instead of in
series. Figure 12-237 illustrates this type of circuit and the frequency/current flow response.
Amplifier Circuits
An amplifier is a device that enables an input signal to control
an output signal. The output signal has some or all of the
characteristics of the input signal but generally is a greater
magnitude than the input signal in terms of voltage, current,
or power. Gain is the basic function of all amplifiers. Because
of this gain, we can expect the output signal to be greater than
the input signal. For example, if we have an input signal of
1 volt and an output signal of 10 volts, then the gain factor
can be determined by:
Gain = Signal out /signal in
Gain = 10 V/1 V = 10
V oltage gain is usually used to describe the operation of a
small gain amplifier. In this type of an amplifier, the output
signal voltage is larger than the input signal voltage. Power
gain, on the other hand, is usually used to describe the
operation of large signal amplifiers. In the case of power gain
amplifiers, the gain is not based on voltage but on watts. A
power amplifier is an amplifier in which the output signal
power is greater than the input signal power. Most power
amplifiers are used as the final stage of amplification and
drive the output device. The output device could be a flight
deck or cabin speaker, an indicator, or antenna. Whatever the
device, the power to make it work comes from the final stage
of amplification. Drivers for autopilot servos are sometimes
contained in line replaceable units (LRUs) called autopilot
amplifiers. These units take the low signal commands from
the flight guidance system and amplify the signals to a level
usable for driving the servo motors.
Classification
The classification of a transistor amplifier circuit is
determined by the percentage of the time that the current
flows through the output circuit in relation to the input signal.
There are four classifications of operation: A, AB, B, and C.
Each class of operation has a certain use and characteristic.
No individual class of amplifiers is considered the best. The
best use of an amplifier is a matter of proper selection for
the particular operation desired.
150 V 150 V50 V
50 V
50 V
Figure 12-233. Stacking diodes in a circuit.
A
B
C
DC1
L-filterLoad
C1
Inverted L-filterLoad
T -filterLoad C1
π-filterLoad C1C2
Figure 12-232. LC filters.
Class A
In the Class A operation, the current in the transistor
flows for 100 percent or 360° of the input signal.
[Figure 12-238] Class A operation is the least efficient class
of operation but provides the best fidelity. Fidelity simply
means that the output signal is a good reproduction of the
input signal in all respects other than the amplitude, which is
amplified. In some cases, there may be some phase shifting
between the input signal and the output signal. Typically,
the phase difference is 180°. If the output signal is not a
good reproduction of the input signal, then the signal is said
to be distorted. Distortion is any undesired change to the
signal from the input to the output.
The efficiency of an amplifier refers to the amount of power delivered to the output compared to the power supplied to the
circuit. Every device in the circuit consumes power in order
to operate. If the amplifier operates for 360° of input signal,
then it is using more power than if it was using only 180°
of input signal. The more power consumed by the amplifier,
the less there is available for the output signal. Usually the
Class A amplifier is used where efficiency is of little concern
and where fidelity in reproduction is desired.
Class AB
In the Class AB operation, the transistor current flows for more
than 50 percent but less than 100 percent of the input signal.
[Figure 12-239] Unlike the Class A amplifier, the output
signal is distorted. A portion of the output circuit appears to
be truncated. This is due to the lack of current through the
transistor during this point of operation. When the emitter in this
case becomes positive enough, the transistor cannot conduct
because the base to emitter junction is no longer forward
biased. The input signal going positive beyond this point does
not produce any further output and the output remains level.
The Class AB amplifier has a better efficiency and a poorer
fidelity than the Class A amplifier. These amplifiers are used
when an exact reproduction of the input is not required but
both the positive and negative portions of the input signals
need to be available on the output.
Class B
In Class B operation, the transistor current flows for only 50
percent of the input signal. [Figure 12-240] In this illustration,
the base-emitter bias does not allow the transistor to conduct
whenever the input signal is greater than zero. In this case, only
the negative portion of the input signal is reproduced. Unlike
the rectifier, the Class B amplifier does not only reproduce half
of the input signal, but it also amplifies it. Class B amplifiers
are twice as efficient as the Class A amplifier because the
amplifying device only uses power for half of the input signal.
Class C
In Class C operations, transistor current flows for less than
50 percent of the input signal. [Figure 12-241] This class of
Low-pass filterLoadCurrent
Frequency
Frequency response
High-pass filterLoadCurrent
Frequency
Frequency response
Band-pass filterLoadBand-pass filterLoadCurrent
Frequency
Frequency responseFigure 12-234. Low-pass filter.
Figure 12-235. High-pass filter.Figure 12-236. Band-pass filter.operation is the most efficient. Because the transistor does
not conduct except during a small portion of the input signal,
this is the most efficient class of amplifier. The distortion of
the Class C amplifier is greater (poor fidelity) than the Class
A, AB, and B amplifiers because a small portion of the input
signal is reproduced on the output. Class C amplifiers are used
when the output signal is used for only small portions of time.Methods of Coupling
Coupling is used to transfer a signal from one stage on an
amplifier to another stage. Regardless of whether an amplifier
is a single stage or one in a series of stages, there must be a
method for the signal to enter and leave the circuit. Coupling
is the process of transferring the energy between circuits.
There are a number of ways for making this transfer and to
discuss these methods in detail goes beyond the scope of this
handbook. However, four methods are listed below with a
brief description of their operation.
Direct Coupling
Direct coupling is the connection of the output of one stage
directly to the input of the next stage. Direct coupling provides
a good frequency response because no frequency-sensitive
components, such as capacitors and inductors, are used. Yet
this method is not used very often due to the complex power
supply requirements and the impedance matching problems.
RC Coupling
RC coupling is the most common method of coupling and
12-116Band-pass filterLoadBand-pass filterLoadCurrent
Frequency
Frequency response
Figure 12-237. Band-stop filter.uses a coupling capacitor and signal developing resistors.
[Figure 12-242] In this circuit, R1 acts as a load resistor
for Q1 and develops the output signal for that stage. The
capacitor C1 blocks the DC bias signal and passes the AC
output signal. R2 then becomes the load over which the
passes AC signal is developed as an input to the base of Q2.
This arrangement allows for the bias voltage of each stage to
be blocked, while the AC signal is passed to the next stage.
Impedance Coupling
Impedance coupling uses a coil as a load for the first
stage but otherwise functions just as an RC coupling.
[Figure 12-243] This method is similar to the RC coupling
method. The difference is that R1 is replaced with inductor
L1 as the output load. The amount of signal developed on the
output load depends on the inductive reactance of the coil. In
order for the inductive reactance to be high, the inductance
must be large; the frequency must be high or both. Therefore,
load inductors should have relatively large amounts of
inductance and are most effective at high frequencies.
Transformer Coupling
Transformer coupling uses a transformer to couple the signal from one stage to the next. [Figure 12-244] The transformer
action of T1 couples the signal from the first stage to the second
stage. The primary coil of T1 acts as a load for the output of
the first stage while the secondary coil acts as the developing
impedance for the second stage Q2. Transformer coupling is very
efficient and the transformer can aid in impedance matching.
Feedback
Feedback occurs when a small portion of the output signal is
sent back to the input signal to the amplifier. There are two
types of feedback in amplifiers:
1. Positive (regenerative)
2. Negative (degenerative)
The main difference between these two signals is whether
the feedback signal adds to the input signal or if the feedback
signal diminishes the input signal.
When the feedback is positive, the signal being returned to
the input is in phase with the input signal and thus interferes
constructively. Figure 12-245 illustrates this concept applied
in the amplified circuit through a block diagram. Notice
that the feedback signal is in phase with the input signal,
which regenerates the input signal. This results in an output
signal with amplitude greater than would have been without
the constructive, positive feedback. This type of positive
feedback is what causes an audio system to squeal.
Figure 12-245 also illustrates with a block diagram how
negative or degenerative feedback occurs. In this case, the
feedback signal is out of phase with the input signal. This
causes destructive interference and degenerates the input
signal. The result is a lower amplitude output signal than
would have occurred without the feedback.
Operational Amplifiers (OP AMP)
An operational amplifier (OP AMP) is designed to be used
with other circuit components and performs either computing
functions or filtering. [Figure 12-246] Operational amplifiers
are usually high-gain amplifiers with the amount of gain
governed by the amount of feedback.
Operational amplifiers were originally developed for analog
computers and used to perform mathematical functions.
Today many devices use the operational amplifier for DC
amplifiers, AC amplifiers, comparators, oscillators, and filter
circuits. The widespread use is due to the fact that the OP
AMP is a versatile device, small, and inexpensive. Built into
the integrated chip, the operational amp is used as a basic
building block of larger circuits.
There are two inputs to the operational amplifier, inverting (−)
and non-inverting (+), and there is one output. The polarity
NPN
Input
Class A amplifierOutputVCC
VEE+
−
−
NPN
Input
Class AB amplifierOutputVCC
VEE+
−
−
NPN
Input
Class B amplifierOutputVCC
VEE+
−
−Figure 12-238. Simplified Class A amplifier circuit. Figure 12-239. Simplified Class AB amplifier circuit.
Figure 12-240. Simplified Class B amplifier circuit.of a signal applied to the inverting input (−) is reversed at the
output. A signal applied to the non-inverting (+) input retains
its polarity on the output. To be classified as an operational
amplifier, the circuit must have certain characteristics:
1. Very high gain
2. Very high input impedance
3. Very high output impedance
This type of a circuit can be made up of discrete components,
such as resistors and transistors. However, the most common
form of an operational amplifier is found in the integrated
circuit. This integrated circuit or chip contains the various
stages of the operational amplifier and can be treated as if it
were a single stage.
Applications
The number of applications for OP AMPs is too numerous to
detail in this handbook. However, the technician occasionally
comes across these devices in modern aircraft and should be
able to recognize their general purpose in a circuit. Some of
the basic applications are:
1. Go/no-go detectors
2. Square wave circuits
3. Non-inverting amplifier
4. Inverting amplifier5. Half-wave rectifier
Input
Impedance coupled amplifierOutput
Q1Q2 C1
VCCL1
Figure 12-243. Simplified impedance coupling circuit.
NPN
Input
Class C amplifierOutputVCC
VEE+
−
−
Input
RC coupled amplifierOutput
Q1Q2 C1
VCCR1 R2
Figure 12-241. Simplified Class C amplifier circuit.Figure 12-242. Simplified RC coupling circuit.
Magnetic Amplifiers
Magnetic amplifiers do not amplify magnetism but use
electromagnetism to amplify a signal. Essentially, the
magnetic amplifier is a power amplifier with a very limited
frequency response. The frequency range most commonly
associated with the magnetic amplifier is 100 Hz and less,
which places it in the audio range. As a technical point, the
magnetic amplifier is a low-frequency amplifier.
Advantages of the magnetic amplifier are:
1. Very high efficiency, on the order of approximately
90 percent
2. High reliability
3. Very rugged, able to withstand vibrations, moisture,
and overloads
4. No warm-up time
Some of the disadvantages of the magnetic amplifier are:
1. Incapacity to handle low-voltage signals
2. Not usable in high-frequency applications
3. Time delay associated with magnetic affects
4. Poor fidelity
The basic operating principles of the magnetic amplifier are
fairly simple. Keep in mind that all amplifiers are current
control devices. In this particular case, power that is delivered to the load is controlled by a variable inductance.
If an AC voltage is applied to the primary winding of an
iron core transformer, the iron core is magnetized and
demagnetized at the same frequency as that of the applied
voltage. This, in turn, induces a voltage in the transformers
secondary winding. The output voltage across the terminals
of the secondary depends on the relationship of the number
of turns in the primary and the secondary of the transformer.
The iron core of the transformer has a saturation point
after which the application of a greater magnetic force
produces no change in the intensity of magnetization.
Hence, there is no change in transformer output, even if the
input is greatly increased. The magnetic amplifier circuit
in Figure 12-247 is used to explain how a simple magnetic
amplifier functions.
1. Assume that there is 1 ampere of current in coil A,
which has 10 turns of wire. If coil B has 10 turns of
wire, an output of 1 ampere is obtained if coil B is
properly loaded.
2. By applying direct current to coil C, the core of the
magnetic amplifier coil can be further magnetized.
Assume that coil C has the proper number of turns and,
upon the application of 30 milliamperes, that the core
Input
Transformer coupled amplifierOutput
Q1Q2 T1
VCC
Input
Positive feedback
Negative feedbackOutput
Input Output
Figure 12-244. Simplified transformer coupling circuit.Figure 12-245. Feedback.is magnetized to the point where 1 ampere on coil A
results in only 0.24 ampere output from coil B.
3. By making the DC input to coil C a continuous variable
from 0 to 30 milliamperes and by maintaining an
input of 1 ampere on coil A, it is possible to control
the output of coil B to any point between 0.24 ampere
and 1 ampere in this example.
The term “amplifier” is used for this arrangement because,
by use of a few milliamperes, control of an output of 1 or
more amperes is obtained.
Saturable-Core Reactor
The same procedure can be used with the circuit shown
in Figure 12-248 . A saturable-core reactor is a magnetic-
core coil whose reactance is controlled by changing the
permeability of the core. Varying the unidirectional flux
controls the permeability of the core.
By controlling the extent of magnetization of the iron ring,
it is possible to control the amount of current flowing to
the load, since the amount of magnetization controls the
impedance of the AC input winding. This type of magnetic
amplifier is called a simple saturable reactor circuit.
Adding a rectifier to such a circuit would remove half the
cycle of the AC input and permit DC to flow to the load. The
amount of DC flowing in the load circuit is controlled by a
DC control winding (sometimes referred to as bias). This type
of magnetic amplifier is referred to as being self-saturating.
To use the full AC input power, a circuit such as that shown
in Figure 12-249 may be used. This circuit uses a full-wave
bridge rectifier. The load receives a controlled DC by using
the full AC input. This type of circuit is known as a self-
saturating, full-wave magnetic amplifier.
In Figure 12-250, it is assumed that the DC control winding
is supplied by a variable source, such as a sensing circuit. To control such a source and use its variations to control the AC
output, it is necessary to include another DC winding that has
a constant value. This winding, referred to as the reference
winding, magnetizes the magnetic core in one direction.
The DC control winding, acting in opposition to the reference
winding, either increases (degenerative) or decreases
(regenerative) the magnetization of the core to change the
amount of current flowing through the load. This is essentially
a basic preamplifier.
Logic Circuits
Logic is considered the science of reasoning—the development
of a reasonable conclusion based on known information.
Human reasoning tells us that certain propositions are true if
certain conditions or premises are true. An annunciator being
lit in the master warning panel is an example of a proposition,
which is either true or false. For example, predetermined and
designed conditions must be met in order for an annunciator
in a master warning panel to be lit. A “LOW HYDRAULIC
PRESS” annunciator may have a simple set of conditions
that cause it to be illuminated. If the conditions are met, such
as a hydraulic reservoir that is low on fluid causing the line
press to be low, then the logic is true and the annunciator
lights. Several propositions, when combined, form a logical
function. In the example above, the “LOW HYDRAULIC
PRESS” annunciator is on if the LED is not burned out and
the hydraulic press is low or if the LED is not burned out
and the annunciator test is being asserted.
This section on logic circuits only serves as an introduction
to the basic concepts. The technician encounters many
Inputs
Power supply requirementsOutput+VCC
−VEE
+VCC
−VEE−
+
InputsOutput−
+
InputsOutput−
+Input/output requirements
Feedback
Figure 12-246. Schematic symbol for the operational amplifier.
situations or problems in everyday life that can be expressed
in some form of a logical function. Many problems and
situations can be condensed down to simple yes⁄no or
true⁄false statements that, if logically ordered, can filter
a problem down to a reasonable answer. The digital logic
circuits are well suited for this task and have been employed
in today’s integrated circuits found in virtually all of the
devices that we take for granted in modern aircraft. These
logical circuits are used to carry out the logical functions for
such things as navigation and communications. There are
several fundamental elements that form the building blocks
of the complex digital systems found in line replaceable
units (LRUs) and avionics card cages. The following is
a very basic outline of what those elements are and what
logic conditions they process. It is far beyond the scope of this handbook to cover digital logic systems due to the vast
body of knowledge that it represents. However, this serves
as an introduction and, in some limited cases, is useful in
reading system block diagrams that use logic symbols to aid
the technician in understanding how a given circuit operates.
Logic Polarity
Electrical pulses can represent two logic conditions and
any two differing voltages can be used for this purpose. For
example, a positive voltage pulse could represent a true or 1
condition and a negative voltage pulse could then represent a
false or 0 logic condition. The condition in which the voltage
changes to represent a true or 1 logic is known as the logic
polarity. Logic circuits are usually divided into two broad
classes: positive polarity and negative polarity. The voltage
levels used and a statement indicating the use of positive
or negative logic is usually specified in the logic diagrams
provided by the original equipment manufacturers (OEMs).
Positive
When a signal that activates a circuit to a 1, true or high
condition, has an electrical level that is relatively more
positive than the other 0 or false condition, then the logic
polarity is said to be positive. An example would be:
Active State: 1 or True = +5 volts direct current (VDC)
0 or False = −5 VDC
Negative
When the signal that actives a circuit to a 1, true or high
condition, has an electrical level that is relatively more
negative than the other 0 or false condition, then the logic
polarity is said to be negative. An example would be:
Active State: 1 or True = 0 VDC
0 or False = +5 VDC
Pulse Structure
Figure 12-251 illustrates the positive and negative pulse in an
idealized form. In both forms, the pulse is composed of two
edges—one being the leading edge and the other the trailing
edge. In the case of the positive pulse logic, the positive
transition from a lower state to a higher state is the leading
edge and the trailing edge is the opposite. In the case of the
negative logic pulse, the negative transition from a higher
state to a lower state is the leading edge while the rise from
the lower state back to the higher state is the trailing edge.
Figure 12-251 is considered an ideal pulse because the rise
and fall times are instantaneous. In reality, these changes
take time, although in actual practice, the rise and fall can
be assumed as instantaneous. Figure 12-252 shows the non-
ideal pulse and its characteristics. The time required for a
pulse to go from a low state to a high state is called the rise
AC input AC output
DCA B
C
Load
AC
inputDC Load
ACDC Figure 12-247. Magnetic amplifier circuit.
Figure 12-248. Saturable CORE reactor circuit. Figure 12-249. Self-saturating, full-wave magnetic amplifier.time, and the time required for the pulse to return to zero is
called the fall time. It is common practice to measure the rise
and fall time between 10 percent amplitude and 90 percent
amplitude. The reason for taking the measurements in these
points is due to the non-linear shape of the pulse in the first
10 percent and final 90 percent of the rise and fall amplitudes.
The pulse width is defined as the duration of the pulse. To be
more specific, it is the time between the 50 percent amplitude
point on both the pulse rise and fall.
Basic Logic Circuits
Boolean logic is a symbolic system used in representing the
truth value of statements. It is employed in the binary system
used by digital computers primarily because the only truth
values (true and false) can be represented by the binary digits
1 and 0. A circuit in computer memory can be open or closed,
depending on the value assigned to it. The fundamental
operations of Boolean logic, often called Boolean operators,
are “and,” “or,” and “not;” combinations of these make up 13
other Boolean operators. Six of these operators are discussed.
The Inverter Logic
The inverter circuit performs a basic logic function called
inversion. The purpose of the inverter is to convert one logic state into the opposite state. In terms of a binary digit, this
would be like converting a 1 to a 0 or a 0 to a 1. When a
high voltage is applied to the inverter input, low voltage is
the output. When a low voltage is applied to the input, a high
voltage is on the output. This operation can be put into what
is known as a logic or truth table. The standard logic symbol
is shown in Figure 12-253 . Figure 12-254 shows the possible
logic states for this gate. This is the common symbol for an
amplifier with a small circle on the output. This type of logic
can also be considered a NOT gate.
The AND Gate
The AND gate is made up of two or more inputs and a single
output. The logic symbol is shown in Figure 12-255 . Inputs
are on the left and the output is on the right in each of the
depictions. Gates with two, three, and four inputs are shown;
however, any number of inputs can be used in the AND logic
as long as the number is greater than one. The operation of
the AND gate is such that the output is high only when all
of the inputs are high. If any of the inputs are low, the output
is also low. Therefore, the basic purpose of an AND gate is
to determine when certain conditions have been met at the
same time. A high level on all inputs produces a high level
on the output. Figure 12-256 shows a simplified diagram of
the AND logic with two switches and a light bulb. Notice that
both switches need to be closed in order for the light bulb to
turn on. Any other combination of switch positions is an open
circuit and the light does not turn on. An example of AND
logic could possibly be engage logic found in an autopilot. In
this case, the autopilot would not be allowed to be engaged
unless certain conditions are first met. Such conditions could
be: Vertical gyro is valid AND directional gyro is valid AND
all autopilot control knobs are in detents AND servo circuits
are operational. Only when these conditions are met does the
12-122Load
ACDC RefDC
Control
Figure 12-250. Basic preamplifier circuit.
autopilot engage. [Figure 12-257]
The OR Gate
The OR gate has two or more inputs and one output and
is normally represented by the standard logic symbol and
truth table. [Figure 12-258] Note that the OR gate can have
any number of inputs as long as it is greater than one. The
operation of the OR gate is such that a high on any one of
the inputs produces a high on the output. The only time that a
low is produced on the output is if there are no high levels on
any input. Figure 12-259 is a simplified circuit that illustrates
the OR logic. The example used is a “DOOR UNSAFE”
annunciator. Let’s say in this case that the plane has one
cabin door and a baggage door. In order for the annunciator
light on the master warning panel to extinguish, both doors
must be closed and locked. If any one of the doors is not
secured properly, the baggage door OR the cabin door, then
the “DOOR UNSAFE” annunciator illuminates. In this case,
two switches are in parallel with each other. If either one of
the two switches is closed, the light bulb lights up. The lamp
is off only when both switches are open.
The NAND Gate
The term NAND is a combination of the NOT-AND gate
and indicates an AND function with an inverted output. A
standard logic symbol for a two input NAND gate is shown
in Figure 12-260 . Notice that an equivalent AND gate with
an inverter is also shown. The logical operation of the NAND
gate is such that a low output occurs only if all inputs are high.
If any of the inputs are low, the output is high. An example
of a two input NAND gate and its corresponding truth table
are shown in Figure 12-261 .The NOR Gate
The term NOR is a combination of the NOT and OR and
indicates an OR function with an inverted output. The
standard logic symbol for a two-inputs NOR gate is shown
in Figure 12-263 . Notice that an equivalent AND gate with
an inverter is also shown. The logical operation of the NOR
gate is such that a low output happens when any of its inputs
are high. Only when all of its inputs are low is the output
high. The logic of this gate produces resultant outputs that
are the opposite of the OR gate. In the NOR gate, the low
output is the active output level. Figure 12-263 illustrates
the logical operation of a two-input NOR gate for all of its
possible combinations and the truth table.
Exclusive OR Gate
The exclusive OR gate is a modified OR gate that produces a
1 output when only one of the inputs is a 1. The abbreviation
often used is X-OR. It is different from the standard OR gate
in that when both inputs are a 1, then the output remains at
a 0. The standard symbol and truth table for the X-OR gate
are shown in Figure 12-264 .
Exclusive NOR Gate
The exclusive NOR (X-NOR) gate is nothing more than an
X-OR gate with an inverted output. It produces a 1 output
when all inputs are 1s and also when all inputs are 0s. The
standard symbol is shown in Figure 12-265 .
The Integrated Circuit
All of the logic functions so far discussed plus many other
components are available in some form of an integrated
circuit. The digital systems found in today’s aircraft owe
their existence to a large extent to the design of the integrated
circuit (IC). In most cases, the IC has an advantage over the
use of discrete components in that they are smaller, consume
less power, are very reliable, and are inexpensive. The most
noticeable characteristic of the IC is its size and in comparison
to the discrete semiconductor component, can easily be on
the order of thousands of times smaller. [Figure 12-266]\
A monolithic integrated circuit is an electronic circuit
that is constructed entirely on a single chip or wafer of
semiconductor material. All of the discrete components,
such as resistors, transistors, diodes, and capacitors, can
be constructed on these small pieces of semiconductor
material and are an integral part of the chip. There are
a number of levels of integration. Those levels are:
small-scale integration, medium-scale integration, large-
scale integration, and microprocessors. The small-scale
integration is considered the least complex design of
the digital ICs. These ICs contain the basic components,
such as the AND, OR, NOT, NOR and NAND gates.
[Figure 12-267] The medium-scale integration can contain
12-123High
Leading
edge
LowTrailing
edge
Positive logic pulse
Negative logic pulseTrailing
edgeHigh
Leading
edge
Low
Pulse width
Pulse characteristic from a databusRise
time50% 5 VPulse
amplitude
(10 volts)90% 9 V
10% 1 V
Fall
timeInput = 1
(High)Output = 0
(Low)
A
X
X
XB
A
BA
B
C
C
DInput Output
High
LowLow
HighFigure 12-251. Positive and negative pulse in an idealized form.
Figure 12-252. Non-ideal pulse and its characteristics.Figure 12-253. Standard logic symbol.
Figure 12-255. AND gate logic symbol.Figure 12-254. Possible logic states.the same components as found in the small-scale design but
in larger numbers ranging from 12 to 100. The medium-
scale designs are house circuits that are more complex, such
as encoders, decoders, registers, counters, multiplexers,
smaller memories, and arithmetic circuits. [Figure 12-268]
The large-scale integrated circuits contain even more logic
gates, larger memories than the medium-scale circuits, and
in some cases microprocessors.
Microprocessors
The microprocessor is a device that can be programmed
to perform arithmetic and logical operations and other
functions in a preordered sequence. The microprocessor is
usually used as the central processing unit (CPU) in today’s
computer systems when it is connected to other components,
such as memory chips and input/output circuits. The basic
arrangement and design of the circuits residing in the
microprocessor is called the architecture.DC Generators
Theory of Operation
In the study of alternating current, basic generator principles
were introduced to explain the generation of an AC voltage
by a coil rotating in a magnetic field. Since this is the basis
for all generator operation, it is necessary to review the
principles of generation of electrical energy.
When lines of magnetic force are cut by a conductor passing
through them, voltage is induced in the conductor. The
strength of the induced voltage is dependent upon the speed
of the conductor and the strength of the magnetic field. If
the ends of the conductor are connected to form a complete
circuit, a current is induced in the conductor. The conductor
and the magnetic field make up an elementary generator.
This simple generator is illustrated in Figure 12-269 ,
together with the components of an external generator
12-124Ahrs
+28 vdc
Ahrs
+28 vdc
A/p controller
+28 vdc+28 vdc
+28 vdc
+28 vdc
+28 vdc
+28 vdc
+28 vdc
+28 vdc
+28 vdc
Pitch servo
valid +28
Roll servo
valid +28
Yaw damper
valid +28
Trim servo
valid +28
Attitude valid +28
Heading valid +28Attitude/heading
reference system
(Ahrs)Autopilot
computer
(engage circuit)
Autopilot controller
Pitch wheel centered
Turn knob centeredWhen all input circuits are valid,
then the a/p can be engaged
Autopilot Engage Logic+28 vdc
Figure 12-257. AND logic of system found in the aircraft wiring diagrams.
OR gate input/output
Truth tableA
f = A + B
B
A
1B
1f
Figure 12-258. OR gate.
A B
+
−
A B
+
−
Figure 12-256. Simplified diagram of the AND logic.
circuit which collect and use the energy produced by the
simple generator. The loop of wire [Figure 12-269A and B]
is arranged to rotate in a magnetic field. When the plane of
the loop of wire is parallel to the magnetic lines of force,
the voltage induced in the loop causes a current to flow in
the direction indicated by the arrows in Figure 12-269 . The
voltage induced at this position is maximum, since the wires
are cutting the lines of force at right angles, thus cutting
more lines of force per second than in any other position
relative to the magnetic field. As the loop approaches the
vertical position shown in Figure 12-270 , the induced
voltage decreases because both sides of the loop (A and B)
are approximately parallel to the lines of force and the rate of cutting is reduced. When the loop is vertical, no lines
of force are cut since the wires are momentarily traveling
parallel to the magnetic lines of force, and there is no
induced voltage. As the rotation of the loop continues, the
number of lines of force cut increases until the loop has
rotated an additional 90° to a horizontal plane. As shown
in Figure 12-271 , the number of lines of force cut and the
induced voltage once again are maximum. The direction
of cutting, however, is in the opposite direction to that
occurring in Figures 12-269 and 12-270 , so the direction
(polarity) of the induced voltage is reversed. As rotation
of the loop continues, the number of lines of force having
been cut again decreases, and the induced voltage becomes
zero at the position shown in Figure 12-272 , since the wires
A and B are again parallel to the magnetic lines of force.
If the voltage induced throughout the entire 360° of rotation
A
F
B
NAND gate
A (1)
(0)(0)
(1)F
B
A
F
B
Equivalent circuit NAND gate input/output
NAND gate input/output
NAND gate truth table0
A
1B
1f
OR gate input/output
Truth tableA
f = A + B
B
A
1B
1f
Door
unsafeDoor warnLight onCabin door not secured
Baggage door secured +28 VDC
Door
unsafeDoor warnLight offCabin door secured
Baggage door secured +28 VDC
Figure 12-260. Standard logic symbol for two input NAND gate.Figure 12-261. Two input NAND gate and corresponding truth table.
Figure 12-262. Standard logic symbol for two inputs OR gate.Figure 12-259. Simplified circuit that illustrates OR logic.
is plotted, the curve shown in Figure 12-273 results. This
voltage is called an alternating voltage because of its reversal
from positive to negative value, first in one direction and
then in the other.
To use the voltage generated in the loop for producing a current
flow in an external circuit, some means must be provided to
connect the loop of wire in series with the external circuit.
Such an electrical connection can be effected by opening the
loop of wire and connecting its two ends to two metal rings,
called slip rings, against which two metal or carbon brushes
ride. The brushes are connected to the external circuit. By
replacing the slip rings of the basic AC generator with two
half cylinders, called a commutator, a basic DC generator is
obtained. [Figure 12-274] In this illustration, the black side of
the coil is connected to the black segment, and the white side
of the coil to the white segment. The segments are insulated
from each other. The two stationary brushes are placed on
opposite sides of the commutator and are so mounted that each
brush contacts each segment of the commutator as the latter
revolves simultaneously with the loop. The rotating parts of a DC generator (coil and commutator) are called an armature.
The generation of an emf by the loop rotating in the magnetic
field is the same for both AC and DC generators, but the action
of the commutator produces a DC voltage.
Generation of a DC Voltage
Figure 12-275 illustrates in an elementary, step-by-step
manner, how a DC voltage is generated. This is accomplished
by showing a single wire loop rotating through a series of
positions within a magnetic field.
Position A
The loop starts in position A and is rotating clockwise.
However, no lines of force are cut by the coil sides,
which means that no emf is generated. The black brush is
shown coming into contact with the black segment of the
commutator, and the white brush is just coming into contact
with the white segment.
X-NOR gate input/output Truth tableA
f = A + B0f0 0
BT0 T1 T2 T3 T4
A
1B
1f
A1 0T0 T1 T2 T3 T4
B0 1 1
Figure 12-265. Standard Symbol for X-NOR gate.
X-OR gate input/output Truth tableA
f0
f1 0 0
BT0 T1 T2 T3 T4 T5
A
1B
1f
A1 0 0T0 T1 T2 T3 T4 T4
B0 0 1 1
Figure 12-264. Standard symbol and truth table for X-OR gate.
NOR gate input/output
NOR gate truth table0
A
1B
1f
Figure 12-263. Logical operation of two-input NOR gate and truth
table.Position B
In position B, the flux is now being cut at a maximum rate,
which means that the induced emf is maximum. At this time,
the black brush is contacting the black segment, and the
white brush is contacting the white segment. The deflection
of the meter is toward the right, indicating the polarity of
the output voltage.
Position C
At position C, the loop has completed 180° of rotation. Like
position A, no flux lines are being cut and the output voltage
is zero. The important condition to observe at position C is
the action of the segments and brushes. The black brush at the
180° angle is contacting both black and white segments on
one side of the commutator, and the white brush is contacting
both segments on the other side of the commutator. After the
Typical integrated logic circuit0.280" max
0.200" max0.785" max16151413121110 9
4Y4A4BGND3A3B3Y
1Y1A1BVCC2A2B2Y
Quad exclusive-OR circuit
Input Output
A
1Y
0B
1Q2Q2QEnableGnd3Q3Q4Q
1Q1D2DEnableVcc3D4D4Q
DQ
G
Q
QD
QG
DQ
G
Q
QD
QG
D-registerFigure 12-266. Integrated circuit.
Figure 12-267. Small-scale integration schematic form.
Figure 12-268. Medium-scale integration schematic form.loop rotates slightly past the 180° point, the black brush is
contacting only the white segment, and the white brush is
contacting only the black segment.
Because of this switching of commutator elements, the
black brush is always in contact with the coil side moving
downward, and the white brush is always in contact with the
coil side moving upward. Though the current actually reverses
its direction in the loop in exactly the same way as in the AC
generator, commutator action causes the current to flow always
in the same direction through the external circuit or meter.
Position D
At position D, commutator action reverses the current in
the external circuit, and the second half cycle has the same
waveform as the first half cycle. The process of commutation
is sometimes called rectification, since rectification is the
converting of AC voltage to DC voltage.
The Neutral Plane
At the instant that each brush is contacting two segments on
the commutator [ Figure 12-275A, C, and E], a direct short
circuit is produced. If an emf were generated in the loop at
this time, a high current would flow in the circuit, causing
an arc and thus damaging the commutator. For this reason,
the brushes must be placed in the exact position where the
short occurs when the generated emf is zero. This position is
called the neutral plane. If the brushes are installed properly,
no sparking occurs between the brushes and the commutator.
Sparking is an indication of improper brush placement, which
is the main cause of improper commutation.
The voltage generated by the basic DC generator in
Figure 12-275 varies from zero to its maximum value twice
for each revolution of the loop. This variation of DC voltage
is called “ripple,” and may be reduced by using more loops,
or coils, as shown in Figure 12-276A . As the number of loops is increased, the variation between maximum and minimum
values of voltage is reduced [Figure 12-276B] , and the output
voltage of the generator approaches a steady DC value. In
Figure 12-276A, the number of commutator segments is
increased in direct proportion to the number of loops; that
N
SA
B
N
SB
AFigure 12-271. Inducing maximum voltage in the opposite direction.
Figure 12-272. Inducing a minimum voltage in the opposite
direction.
N
SAArmature
B
LoadSlip ringPole piece
Brush
N
SA
B
Figure 12-269. Inducing maximum voltage in an elementary
generator.Figure 12-270. Inducing minimum voltage in an elementary
generator.is, there are two segments for one loop, four segments for
two loops, and eight segments for four loops.
The voltage induced in a single turn loop is small. Increasing
the number of loops does not increase the maximum value
of generated voltage, but increasing the number of turns
in each loop increases this value. Within narrow limits,
the output voltage of a DC generator is determined by the
product of the number of turns per loop, the total flux per
pair of poles in the machine, and the speed of rotation of
the armature.
An AC generator, or alternator, and a DC generator are
identical as far as the method of generating voltage in the
rotating loop is concerned. However, if the current is taken
from the loop by slip rings, it is an alternating current, and
the generator is called an AC generator, or alternator. If the
current is collected by a commutator, it is direct current, and
the generator is called a DC generator.
Construction Features of DC Generators
Generators used on aircraft may differ somewhat in design,
since various manufacturers make them. All, however, are of
the same general construction and operate similarly. The major
parts, or assemblies, of a DC generator are a field frame (or
yoke), a rotating armature, and a brush assembly. The parts
of a typical aircraft generator are shown in Figure 12-277 .
Field Frame
The field frame is also called the yoke, which is the
foundation or frame for the generator. The frame has two
functions: It completes the magnetic circuit between the
poles and acts as a mechanical support for the other parts of
the generator. In Figure 12-278A , the frame for a two-pole
generator is shown in a cross-sectional view. A four-pole
generator frame is shown in Figure 12-278B .
12-129– Voltage + VoltageMaximum
Minimum0°
Maximum90° 270°
360°360° 1 cycle
180°
S N
Induced EMF
1 Revolution
A E B C DFigure 12-273. Output of an elementary generator.
Figure 12-274. Basic DC generator.
Figure 12-275. Operation of a basic DC generator.In small generators, the frame is made of one piece of iron,
but in larger generators, it is usually made up of two parts
bolted together. The frame has high magnetic properties and,
together with the pole pieces, forms the major part of the
magnetic circuit. The field poles are bolted to the inside of
the frame and form a core on which the field coil windings
are mounted. [Figure 12-278]
The poles are usually laminated to reduce eddy current
losses and serve the same purpose as the iron core of an
electromagnet; that is, they concentrate the lines of force
produced by the field coils. The entire frame, including field
poles, is made from high-quality magnetic iron or sheet steel.
A practical DC generator uses electromagnets instead of
permanent magnets. To produce a magnetic field of the
necessary strength with permanent magnets would greatly
increase the physical size of the generator.
The field coils are made up of many turns of insulated wire
and are usually wound on a form that fits over the iron core of
the pole to which it is securely fastened. [Figure 12-279] The exciting current, which is used to produce the magnetic field
and which flows through the field coils, is obtained from an
external source or from the generated DC of the machine.
No electrical connection exists between the windings of the
field coils and the pole pieces.
Most field coils are connected so that the poles show
alternate polarity. Since there is always one North pole for
each South pole, there must always be an even number of
poles in any generator.
Note that the pole pieces in Figure 12-278 project from the
frame. Because air offers a great amount of reluctance to the
magnetic field, this design reduces the length of the air gap
between the poles and the rotating armature and increases the
efficiency of the generator. When the pole pieces are made to
project they are called salient poles. [Figure 12-278]
A B C D E
0 1/4 1/2 3/4 1Induced EMF
Revolutions
S N
A
B
Figure 12-276. Increasing the number of coils reduces the ripple
in the voltage.Armature
The armature assembly of a generator consists of many
armature coils wound on an iron core, a commutator, and
associated mechanical parts. These additional loops of wire
are actually called windings and are evenly spaced around
the armature so that the distance between each winding is
the same. Mounted on a shaft, it rotates through the magnetic
field produced by the field coils. The core of the armature acts
as an iron conductor in the magnetic field and, for this reason,
is laminated to prevent the circulation of eddy currents.
Gramme-Ring Armature
There are two general kinds of armatures: the ring and the
drum. Figure 12-280 shows a ring-type armature made up of
an iron core, an eight-section winding, and an eight-segment
commutator. The disadvantage of this arrangement is that
the windings, located on the inner side of the iron ring, cut
few lines of flux. As a result, they have very little voltage
induced in them. For this reason, the Gramme ring armature
is not widely used.
Drum-Type Armature
A drum-type armature is shown in Figure 12-281 . The
armature core is in the shape of a drum and has slots cut into
it where the armature windings are placed. The advantage is
that each winding completely surrounds the core so that the
entire length of the conductor cuts through the magnetic flux.
The total induced voltage in this arrangement is far greater
than that of the Gramme ring-type armature.
Drum-type armatures are usually constructed in one of
two methods: lap winding and the wave winding. Each
method having its own advantage. Lap windings are used in
generators that are designed for high current. The windings
are connected in parallel paths and for this reason require
several brushes. The wave winding is used in generators that
are designed for high voltage outputs. The two ends of each
coil are connected to commutator segments separated by the
distance between poles. This results in a series arrangement
of the coils and is additive of all the induced voltages.
Commutators
Figure 12-282 shows a cross-sectional view of a typical
commutator. The commutator is located at the end of an
armature and consists of wedge shaped segments of hard drawn
copper, insulated from each other by thin sheets of mica. The
segments are held in place by steel V-rings or clamping flanges
fitted with bolts. Rings of mica insulate the segments from the
flanges. The raised portion of each segment is called a riser,
and the leads from the armature coils are soldered to the risers.
When the segments have no risers, the leads are soldered to
short slits in the ends of the segments.
The brushes ride on the surface of the commutator, forming the electrical contact between the armature coils and the
external circuit. A flexible, braided copper conductor,
commonly called a pigtail, connects each brush to the external
circuit. The brushes, usually made of high-grade carbon and
held in place by brush holders insulated from the frame, are
free to slide up and down in their holders in order to follow
any irregularities in the surface of the commutator. The
brushes are usually adjustable so that the pressure of the
brushes on the commutator can be varied and the position
of the brushes with respect to the segments can be adjusted.
The constant making and breaking of connections to the coils
in which a voltage is being induced necessitates the use of
material for brushes, which has a definite contact resistance.
Also, this material must be such that the friction between the
commutator and the brush is low, to prevent excessive wear.
For these reasons, the material commonly used for brushes is
high-grade carbon. The carbon must be soft enough to prevent
undue wear of the commutator and yet hard enough to provide
reasonable brush life. Since the contact resistance of carbon
is fairly high, the brush must be quite large to provide a large
area of contact. The commutator surface is highly polished to
reduce friction as much as possible. Oil or grease must never
be used on a commutator, and extreme care must be used
when cleaning it to avoid marring or scratching the surface.
Connector ligs
Field frame Drive end frameSealed ball bearings
Drive shaft
Field winding Screw Pole shoeBrush and holderSteel ringCommutatorBrush connector barsAir scoop
Armature
Drive endField frameCommutator
end frame
Figure 12-277. Typical 24-volt aircraft generator.Armature Reaction
Current flowing through the armature sets up electromagnetic
fields in the windings. These new fields tend to distort or
bend the magnetic flux between the poles of the generator
from a straight-line path. Since armature current increases
with load, the distortion becomes greater with an increase in
load. This distortion of the magnetic field is called armature
reaction. [Figure 12-283]
Armature windings of a generator are spaced so that, during
rotation of the armature, there are certain positions when the
brushes contact two adjacent segments, thereby shorting the
armature windings to these segments. When the magnetic
field is not distorted, there is usually no voltage being
induced in the shorted windings, and therefore no harmful
results occur from the shorting of the windings. However,
when the field is distorted, a voltage is induced in these
shorted windings, and sparking takes place between the
brushes and the commutator segments. Consequently, the
commutator becomes pitted, the wear on the brushes becomes
excessive, and the output of the generator is reduced. To
correct this condition, the brushes are set so that the plane of
the coils, which are shorted by the brushes, is perpendicular
to the distorted magnetic field, which is accomplished by
moving the brushes forward in the direction of rotation. This
operation is called shifting the brushes to the neutral plane or plane of commutation. The neutral plane is the position
where the plane of the two opposite coils is perpendicular
to the magnetic field in the generator. On a few generators,
the brushes can be shifted manually ahead of the normal
neutral plane to the neutral plane caused by field distortion.
On nonadjustable brush generators, the manufacturer sets
the brushes for minimum sparking.
Compensating windings or interpoles may be used to
counteract some of the effects of field distortion, since
shifting the brushes is inconvenient and unsatisfactory,
especially when the speed and load of the generator are
changing constantly.
Compensating Windings
The compensating windings consist of a series of coils
embedded in slots in the pole faces. These coils are also
connected in series with the armature. Consequently, this series
connection with the armature produces a magnetic field in the
compensating windings that varies directly with the armature
current. The compensating windings are wound in such a
manner that the magnetic field produced by them counteracts
the magnetic field produced by the armature. As a result, the
neutral plane remains stationary any magnitude of armature
current. With this design, once the brushes are set correctly,
they do not need to be moved again. Figure 12-284A illustrates
how the windings are set into the pole faces.
N S
Figure 12-280. An eight-section, ring-type armature.
NS
NN
SSA
BFlux
Pole
Field windingFrame
Flux
Pole
Field windingFrame
Figure 12-278. A two-pole and a four-pole frame assembly.Figure 12-279. A field coil removed from a field pole.
Interpoles
An interpole is a pole placed between the main poles of a
generator. An example of interpole placement is shown in
Figure 12-284B . This is a simple two-pole generator with
two interpoles.
An interpole has the same polarity as the next main pole in
the direction of rotation. The magnetic flux produced by
an interpole causes the current in the armature to change
direction as an armature winding passes under it. This
cancels the electromagnetic fields about the armature
windings. The magnetic strength of the interpoles varies with
the load on the generator; and since field distortion varies
with the load, the magnetic field of the interpoles counteracts
the effects of the field set up around the armature windings
and minimizes field distortion. Thus, the interpole tends to
keep the neutral plane in the same position for all loads on the generator; therefore, field distortion is reduced by the
interpoles, and the efficiency, output, and service life of the
brushes are improved.
Types of DC Generators
There are three types of DC generators: series wound, shunt
wound, and shunt series or compound wound. The difference
in type depends on the relationship of the field winding to
the external circuit.
Series Wound DC Generators
The field winding of a series generator is connected in series
with the external circuit called the load. [Figure 12-285] The
field coils are composed of a few turns of large wire; the
magnetic field strength depends more on the current flow rather
than the number of turns in the coil. Series generators have very
poor voltage regulation under changing load, since the greater
the current through the field coils to the external circuit, the
greater the induced emf and the greater the terminal or output
voltage. Therefore, when the load is increased, the voltage
increases; likewise, when the load is decreased, the voltage
Mica V-ring
Front V-ring
Commutator bar
Mica
Iron shell
Back V-ring with mica inner and outer rings for insulation SlotsTightening nut
Iron ring
Commutator bars
Mica insulation between bars
Commutator Slots
Shaft
Coils
Figure 12-282. Commutator with portion removed to show construction.Figure 12-281. A drum-type armature.
decreases. The output voltage of a series wound generator
may be controlled by a rheostat in parallel with the field
windings. [Figure 12-285A] Since the series wound generator
has such poor regulation, it is never employed as an airplane
generator. Generators in airplanes have field windings, which
are connected either in shunt or in compound.
Shunt Wound DC Generators
A generator having a field winding connected in parallel
with the external circuit is called a shunt generator.
[Figure 12-286A and B] The field coils of a shunt generator
contain many turns of small wire; the magnetic strength
is derived from the large number of turns rather than the
current strength through the coils. If a constant voltage is
desired, the shunt wound generator is not suitable for rapidly fluctuating loads. Any increase in load causes a decrease
in the terminal or output voltage, and any decrease in load
causes an increase in terminal voltage; since the armature
and the load are connected in series, all current flowing in
the external circuit passes through the armature winding.
Because of the resistance in the armature winding, there is
a voltage drop (IR drop = current × resistance). As the load
increases, the armature current increases and the IR drop in
the armature increases. The voltage delivered to the terminals
is the difference between the induced voltage and the voltage
drop; therefore, there is a decrease in terminal voltage. This
decrease in voltage causes a decrease in field strength,
because the current in the field coils decreases in proportion
to the decrease in terminal voltage; with a weaker field, the
voltage is further decreased. When the load decreases, the
output voltage increases accordingly, and a larger current
flows in the windings. This action is cumulative, so the output
voltage continues to rise to a point called field saturation,
after which there is no further increase in output voltage.
The terminal voltage of a shunt generator can be controlled
by means of a rheostat inserted in series with the field
windings. [Figure 12-286A] As the resistance is increased,
the field current is reduced; consequently, the generated
voltage is reduced also. For a given setting of the field
rheostat, the terminal voltage at the armature brushes is
approximately equal to the generated voltage minus the IR
N SRotation
+
+
+
+a
bNeutral plane
Rotation
++
+
+
+
+
+
+
+
++
S
N
N SRotation
+
+
+
+++
+
+
+
+
+
++++a
bA
B
CField excited, armature unexcited
Armature excited, field unexcited
Both field and armature excited
Figure 12-283. Armature reaction.drop produced by the load current in the armature; thus, the
voltage at the terminals of the generator drops as the load is
applied. Certain voltage sensitive devices are available that
automatically adjust the field rheostat to compensate for
variations in load. When these devices are used, the terminal
voltage remains essentially constant.
Compound Wound DC Generators
A compound wound generator combines a series winding
and a shunt winding in such a way that the characteristics
of each are used to advantage. The series field coils are
made of a relatively small number of turns of large copper
conductor, either circular or rectangular in cross section, and
are connected in series with the armature circuit. These coils
are mounted on the same poles on which the shunt field coils
are mounted and, therefore, contribute a magnetomotive
force which influences the main field flux of the generator.
A diagrammatic and a schematic illustration of a compound
wound generator is shown in Figure 12-287A and B.If the ampere turns of the series field act in the same direction
as those of the shunt field, the combined magnetomotive force
is equal to the sum of the series and shunt field components.
Load is added to a compound generator in the same manner
in which load is added to a shunt generator, by increasing
the number of parallel paths across the generator terminals.
Thus, the decrease in total load resistance with added load
is accompanied by an increase in armature circuit and series
field circuit current. The effect of the additive series field is
that of increased field flux with increased load. The extent of
the increased field flux depends on the degree of saturation
of the field as determined by the shunt field current. Thus, the
terminal voltage of the generator may increase or decrease with
load, depending on the influence of the series field coils. This
influence is referred to as the degree of compounding. A flat
compound generator is one in which the no load and full load
voltages have the same value; whereas an under compound
generator has a full load voltage less than the no load value,
and an over compound generator has a full load voltage which
is higher than the no load value. Changes in terminal voltage
with increasing load depend upon the degree of compounding.
If the series field aids the shunt field, the generator is said
to be cumulative compounded. If the series field opposes
the shunt field, the machine is said to be differentially
compounded or is called a differential generator. Compound
generators are usually designed to be overcompounded.
This feature permits varied degrees of compounding by
connecting a variable shunt across the series field. Such a
shunt is sometimes called a diverter. Compound generators
are used where voltage regulation is of prime importance.
Differential generators have somewhat the same characteristics
as series generators in that they are essentially constant
current generators. However, they generate rated voltage at
no load, the voltage dropping materially as the load current
increases. Constant current generators are ideally suited as
power sources for electric arc welders and are used almost
universally in electric arc welding.
If the shunt field of a compound generator is connected across
both the armature and the series field, it is known as a long
shunt connection, but if the shunt field is connected across
the armature alone, it is called a short shunt connection.
These connections produce essentially the same generator
characteristics. A summary of the characteristics of the various
types of generators discussed is shown in Figure 12-288 .
Generator Ratings
A generator is rated in power output. Since a generator is
designed to operate at a specified voltage, the rating usually
is given as the number of amperes the generator can safely
Generator
+−S
NS N
External load
Generator
−−
−
−
−
−−++
+
++
+
++−
−
−
−
−
−
−+
+
+
+
+
+
+
CompensatingA
InterpoleBMain field windingsArmature windings
Main field
windingsCompensating
windingsCompensating
windings
Interpole windings
Interpole
windingsA
BN S
Load−
+Field rheostat
Field coils
To load
Arm
Figure 12-284. Simple two-pole generator with two interpoles.Figure 12-285. Diagram and schematic of a series wound generator.
supply at its rated voltage. Generator rating and performance
data are stamped on the nameplate attached to the generator.
When replacing a generator, it is important to choose one of
the proper rating.
The rotation of generators is termed either clockwise or
counterclockwise, as viewed from the driven end. Usually,
the direction of rotation is stamped on the data plate. If
no direction is stamped on the plate, the rotation may be
marked by an arrow on the cover plate of the brush housing.
It is important that a generator with the correct direction of
rotation be used; otherwise, the voltage is reversed.
The speed of an aircraft engine varies from idle rpm to takeoff
rpm; however, during the major portion of a flight, it is at a
constant cruising speed. The generator drive is usually geared
to revolve the generator between 11⁄8 and 11⁄2 times the engine
crankshaft speed. Most aircraft generators have a speed at which they begin to produce their normal voltage. Termed
the “coming in” speed, it is usually about 1,500 rpm.
Generator Terminals
On most large 24-volt generators, electrical connections
are made to terminals marked B, A, and E. The positive
armature lead in the generator connects to the B terminal.
The negative armature lead connects to the E terminal. The
positive end of the shunt field winding connects to terminal A,
and the opposite end connects to the negative terminal brush.
Terminal A receives current from the negative generator brush
through the shunt field winding. This current passes through
the voltage regulator and back to the armature through the
positive brush. Load current, which leaves the armature
through the negative brushes, comes out of the E lead and
passes through the load before returning to the armature
through the positive brushes.
DC Generator Maintenance
Inspection
The following information about the inspection and
maintenance of DC generator systems is general in nature
because of the large number of differing aircraft generator
systems. These procedures are for familiarization only.
Always follow the applicable manufacturer’s instructions
for a given generator system.
12-136A
BN S
+−
Load
Series field coil
To load
ArmShunt
field
coil
Compound wound
Figure 12-287. Compound wound generator.A
BN S
+−Field
rheostat
Load
Shunt circuit Main circuit
Arm To load Field coils
Figure 12-286. Shunt wound generator.
In general, the inspection of the generator installed in the
aircraft should include the following items:
1. Security of generator mounting
2. Condition of electrical connections
3. Dirt and oil in the generator—if oil is present, check
engine oil seal. Blow out dirt with compressed air.
4. Condition of generator brushes
5. Generator operation
6. V oltage regulator operation
Condition of Generator Brushes
Sparking of brushes quickly reduces the effective brush area
in contact with the commutator bars. The degree of such
sparking should be determined. Excessive wear warrants a
detailed inspection.
The following information pertains to brush seating, brush
pressure, high mica condition, and brush wear. Manufacturers
usually recommend the following procedures to seat brushes
that do not make good contact with slip rings or commutators.
Lift the brush sufficiently to permit the insertion of a strip of No. 000, or finer, sandpaper under the brush, rough side
out. [Figure 12-289] Pull the sandpaper in the direction
of armature rotation, being careful to keep the ends of the
sandpaper as close to the slip ring or commutator surface as
possible in order to avoid rounding the edges of the brush.
When pulling the sandpaper back to the starting point, raise
the brush so it does not ride on the sandpaper. Sand the brush
only in the direction of rotation.
After the generator has run for a short period, brushes should
be inspected to make sure that pieces of sand have not become
embedded in the brush and are collecting copper.
Under no circumstances should emery cloth or similar
abrasives be used for seating brushes (or smoothing
commutators), since they contain conductive materials that
cause arcing between brushes and commutator bars.
Excessive pressure causes rapid wear of brushes. Too
little pressure, however, allows “bouncing” of the brushes,
resulting in burned and pitted surfaces.
A carbon, graphite, or light metalized brush should exert a
pressure of 11⁄2 to 21⁄2 psi on the commutator. The pressure
recommended by the manufacturer should be checked by
the use of a spring scale graduated in ounces. Brush spring
Output current or load 0 100%100%Output voltageSeries connectionDifferential compoundingShunt connectionCumulative compounding
Figure 12-288. Generator characteristics.tension is usually adjusted between 32 to 36 ounces; however,
the tension may differ slightly for each specific generator.
When a spring scale is used, the measurement of the pressure
that a brush exerts on the commutator is read directly on the
scale. The scale is applied at the point of contact between the
spring arm and the top of the brush, with the brush installed
in the guide. The scale is drawn up until the arm just lifts
off the brush surface. At this instant, the force on the scale
should be read.
Flexible low resistance pigtails are provided on most heavy
current carrying brushes, and their connections should be
securely made and checked at frequent intervals. The pigtails
should never be permitted to alter or restrict the free motion
of the brush.
The purpose of the pigtail is to conduct the current, rather
than subjecting the brush spring to currents that would alter
its spring action by overheating. The pigtails also eliminate
any possible sparking to the brush guides caused by the
movement of the brushes within the holder, thus minimizing
side wear of the brush.
Carbon dust resulting from brush sanding should be
thoroughly cleaned from all parts of the generators after a
sanding operation. Such carbon dust has been the cause of
several serious fires, as well as costly damage to the generator.
Operation over extended periods of time often results in
the mica insulation between commutator bars protruding
above the surface of the bars. This condition is called “high
mica” and interferes with the contact of the brushes to
the commutator. Whenever this condition exists, or if the armature has been turned on a lathe, carefully undercut the
mica insulation to a depth equal to the width of the mica, or
approximately 1.20 inch.
Each brush should be a specified length to work properly.
If a brush is too short, the contact it makes with the
commutator will be faulty, which can also reduce the spring
force holding the brush in place. Most manufacturers specify
the amount of wear permissible from a new brush length.
When a brush has worn to the minimum length permissible,
it must be replaced.
Some special generator brushes should not be replaced
because of a slight grooving on the face of the brush. These
grooves are normal and will appear in AC and DC generator
brushes which are installed in some models of aircraft
generators. These brushes have two cores made of a harder
material with a higher expansion rate than the material used
in the main body of the brush. Usually, the main body of
the brush face rides on the commutator. However, at certain
temperatures, the cores extend and wear through any film
on the commutator.
DC Motors
Most devices in an airplane, from the starter [Figure 12-290]
to the automatic pilot, depend upon mechanical energy
furnished by DC motors. A DC motor is a rotating machine,
which transforms DC energy into mechanical energy. It
consists of two principal parts—a field assembly and an
armature assembly. The armature is the rotating part in which
current carrying wires are acted upon by the magnetic field.
Whenever a current carrying wire is placed in the field of
a magnet, a force acts on the wire. The force is not one of
attraction or repulsion; however, it is at right angles to the
wire and also at right angles to the magnetic field set up by
the magnet. The action of the force upon a current carrying
wire placed in a magnetic field is shown in Figure 12-291 .
A wire is located between two permanent magnets. The lines
of force in the magnetic field are from the North pole to the
South pole. When no current flows, no force is exerted on the
wire, but when current flows through the wire, a magnetic
field is set up about it. [Figure 12-291] The direction of the
field depends on the direction of current flow. Current in one
direction creates a clockwise field about the wire, and current
in the other direction, a counterclockwise field.
Since the current carrying wire produces a magnetic field,
a reaction occurs between the field about the wire and the
magnetic field between the magnets. When the current flows
Unseated brush
000 sandpaper (sand side next to brush)
Properly seated brush1/32" to 1/16"
Figure 12-289. Seating brushes with sandpaper.in a direction to create a counterclockwise magnetic field
about the wire, this field and the field between the magnets
add or reinforce at the bottom of the wire because the lines
of force are in the same direction. At the top of the wire,
they subtract or neutralize, since the lines of force in the two
fields are opposite in direction. Thus, the resulting field at the
bottom is strong and the one at the top is weak. Consequently,
the wire is pushed upward. [Figure 12-291C] The wire is
always pushed away from the side where the field is strongest.
If current flow through the wire were reversed in direction,
the two fields would add at the top and subtract at the bottom.
Since a wire is always pushed away from the strong field,
the wire would be pushed down.
Force Between Parallel Conductors
Two wires carrying current in the vicinity of one another exert
a force on each other because of their magnetic fields. An
end view of two conductors is shown in Figure 12-292 . In
Figure 12-292A , electron flow in both conductors is toward
the reader, and the magnetic fields are clockwise around the
conductors. Between the wires, the fields cancel because the
directions of the two fields oppose each other. The wires are
forced in the direction of the weaker field, toward each other.
This force is one of attraction. In Figure 12-292B , the electron
flow in the two wires is in opposite directions.
The magnetic fields are, therefore, clockwise in one and counterclockwise in the other, as shown. The fields reinforce
each other between the wires, and the wires are forced in the
direction of the weaker field, away from each other. This
force is one of repulsion.
To summarize: conductors carrying current in the same
direction tend to be drawn together; conductors carrying current
in opposite directions tend to be repelled from each other.
Developing Torque
If a coil in which current is flowing is placed in a magnetic
field, a force is produced which causes the coil to rotate. In
the coil shown in Figure 12-293 , current flows inward on
side A and outward on side B. The magnetic field about B is
clockwise and that about A, counterclockwise. As previously
explained, a force develops which pushes side B downward.
At the same time, the field of the magnets and the field
about A, in which the current is inward, adds at the bottom
and subtracts at the top. Therefore, A moves upward. The
coil rotates until its plane is perpendicular to the magnetic
lines between the North and South poles of the magnet, as
indicated in Figure 12-293 by the white coil at right angles
to the black coil.
The tendency of a force to produce rotation is called torque.
When the steering wheel of a car is turned, torque is applied.
The engine of an airplane gives torque to the propeller. Torque
is developed also by the reacting magnetic fields about the
current carrying coil just described. This is the torque, which
turns the coil.
The right-hand motor rule can be used to determine the
direction a current carrying wire moves in a magnetic field.
As illustrated in Figure 12-294 , if the index finger of the
right hand is pointed in the direction of the magnetic field
and the second finger in the direction of current flow, the
thumb indicates the direction the current carrying wire moves.
The amount of torque developed in a coil depends upon several
factors: the strength of the magnetic field, the number of turns
in the coil, and the position of the coil in the field. Magnets are
made of special steel that produces a strong field. Since there
is torque acting on each turn, the greater the number of turns
on the coil, the greater the torque. In a coil carrying a steady
current located in a uniform magnetic field, the torque varies
at successive positions of rotation. [Figure 12-295] When the
plane of the coil is parallel to the lines of force, the torque is
zero. When its plane cuts the lines of force at right angles, the
torque is 100 percent. At intermediate positions, the torque
ranges between 0 and 100 percent.
Basic DC Motor
A coil of wire through which the current flows rotates when
placed in a magnetic field. This is the technical basis governing
the construction of a DC motor. [Figure 12-296] However,
A B C
Wire without current located
in a magnetic fieldWire with current and
accompanying fieldResultant field and
direction of force on wire
Figure 12-290. DC series starter motor.
Figure 12-291. Force on a current carrying wire.if the connecting wires from the battery were permanently
fastened to the terminals of the coil and there was a flow of
current, the coil would rotate only until it lined itself up with
the magnetic field. Then, it would stop, because the torque
at that point would be 0.
A motor, of course, must continue rotating. It is therefore
necessary to design a device that reverses the current in the
coil just at the time the coil becomes parallel to the lines of
force. This creates torque again and causes the coil to rotate.
If the current reversing device is set up to reverse the current
each time the coil is about to stop, the coil can be made to
continue rotating as long as desired.
One method of doing this is to connect the circuit so that, as
the coil rotates, each contact slides off the terminal to which it
connects and slides onto the terminal of opposite polarity. In
other words, the coil contacts switch terminals continuously
as the coil rotates, preserving the torque and keeping the
coil rotating. In Figure 12-296 , the coil terminal segments
are labeled A and B. As the coil rotates, the segments slide
onto and past the fixed terminals or brushes. With this
arrangement, the direction of current in the side of the coil
next to the North-seeking pole flows toward the reader, and
the force acting on that side of the coil turns it downward.
The part of the motor that changes the current from one wire
to another is called the commutator.
Position A
When the coil is positioned as shown in Figure 12-296A ,
current flows from the negative terminal of the battery to the
negative (−) brush, to segment B of the commutator, through
the loop to segment A of the commutator, to the positive (+)
brush, and then back to the positive terminal of the battery.
By using the right-hand motor rule, it is seen that the coil
rotates counterclockwise. The torque at this position of the
coil is maximum, since the greatest number of lines of force
is being cut by the coil.Position B
When the coil has rotated 90° to the position shown
in Figure 12-296B , segments A and B of the commutator no
longer make contact with the battery circuit and no current
can flow through the coil. At this position, the torque has
reached a minimum value, since a minimum number of lines
of force are being cut. However, the momentum of the coil
carries it beyond this position until the segments again make
contact with the brushes, and current again enters the coil;
this time, though, it enters through segment A and leaves
through segment B. However, since the positions of segments
A and B have also been reversed, the effect of the current is
as before, the torque acts in the same direction, and the coil
continues its counterclockwise rotation.
Position C
On passing through the position shown in Figure 12-296C ,
the torque again reaches maximum.
Position D
Continued rotation carries the coil again to a position of
minimum torque as in Figure 12-296D . At this position, the
brushes no longer carry current, but once more the momentum
rotates the coil to the point where current enters through
N
SA
BDirection of currentMagnetic fieldTorque
CurrentMotion
Magnetic fluxFigure 12-293. Developing a torque.
Figure 12-294. Right-hand motor rule.
+A
B
Figure 12-292. Fields surrounding parallel conductors.segment B and leaves through A. Further rotation brings the
coil to the starting point and, thus, one revolution is completed.
The switching of the coil terminals from the positive to the
negative brushes occurs twice per revolution of the coil.
The torque in a motor containing only a single coil is neither
continuous nor very effective, for there are two positions
where there is actually no torque at all. To overcome this, a
practical DC motor contains a large number of coils wound on
the armature. These coils are so spaced that, for any position
of the armature, there are coils near the poles of the magnet.
This makes the torque both continuous and strong. The
commutator, likewise, contains a large number of segments
instead of only two.
The armature in a practical motor is not placed between
the poles of a permanent magnet but between those of an
electromagnet, since a much stronger magnetic field can be
furnished. The core is usually made of a mild or annealed
steel, which can be magnetized strongly by induction. The
current magnetizing the electromagnet is from the same
source that supplies the current to the armature.DC Motor Construction
The major parts in a practical motor are the armature
assembly, the field assembly, the brush assembly, and the
end frame. [Figure 12-297]
Armature Assembly
The armature assembly contains a laminated, soft-iron core,
coils, and a commutator, all mounted on a rotatable steel shaft.
Laminations made of stacks of soft iron, insulated from each
other, form the armature core. Solid iron is not used, since a
solid iron core revolving in the magnetic field would heat and
use energy needlessly. The armature windings are insulated
copper wire, which are inserted in slots insulated with fiber
paper (fish paper) to protect the windings. The ends of the
windings are connected to the commutator segments. Wedges
or steel bands hold the windings in place to prevent them
from flying out of the slots when the armature is rotating at
high speeds. The commutator consists of a large number of
copper segments insulated from each other and the armature
shaft by pieces of mica. Insulated wedge rings hold the
segments in place.
S
N 45°71%
100%
90°
0°Zero torque
Figure 12-295. Torque on a coil at various angles of rotation.
S N
A BBrushTorque
S N
ABNo torque
S N
B ATorque
S N
BANo torqueA
B
C
D
Figure 12-296. Basic DC motor operation.Field Assembly
The field assembly consists of the field frame, the pole pieces,
and the field coils. The field frame is located along the inner
wall of the motor housing. It contains laminated, soft-steel pole
pieces on which the field coils are wound. A coil, consisting of
several turns of insulated wire, fits over each pole piece and,
together with the pole, constitutes a field pole. Some motors
have as few as two poles, others as many as eight.
Brush Assembly
The brush assembly consists of the brushes and their holders.
The brushes are usually small blocks of graphitic carbon, since
this material has a long service life and also causes minimum
wear to the commutator. The holders permit some play in the
brushes so they can follow any irregularities in the surface
of the commutator and make good contact. Springs hold the
brushes firmly against the commutator. [Figure 12-298]
End Frame
The end frame is the part of the motor opposite the
commutator. Usually, the end frame is designed so that it
can be connected to the unit to be driven. The bearing for the
drive end is also located in the end frame. Sometimes the end
frame is made a part of the unit driven by the motor. When
this is done, the bearing on the drive end may be located in
any one of a number of places.
Types of DC Motors
There are three basic types of DC motors: series motors, shunt
motors, and compound motors. They differ largely in the
method in which their field and armature coils are connected.
Series DC Motor
In the series motor, the field windings, consisting of a
relatively few turns of heavy wire, are connected in series
with the armature winding. Both a diagrammatic and a
schematic illustration of a series motor are shown in
Figure 12-299 . The same current flowing through the field
winding also flows through the armature winding. Any
increase in current, therefore, strengthens the magnetism of
12-142both the field and the armature.
Because of the low resistance in the windings, the series
motor is able to draw a large current in starting. This starting
current, in passing through both the field and armature
windings, produces a high starting torque, which is the series
motor’s principal advantage.
The speed of a series motor is dependent upon the load. Any
change in load is accompanied by a substantial change in
speed. A series motor runs at high speed when it has a light
load and at low speed with a heavy load. If the load is removed
entirely, the motor may operate at such a high speed that the
armature falls apart. If high starting torque is needed under
heavy load conditions, series motors have many applications.
Series motors are often used in aircraft as engine starters and for
raising and lowering landing gears, cowl flaps, and wing flaps.
Shunt DC Motor
In the shunt motor, the field winding is connected in parallel
or in shunt with the armature winding. [Figure 12-300] The
resistance in the field winding is high. Since the field winding
is connected directly across the power supply, the current
through the field is constant. The field current does not vary
with motor speed, as in the series motor and, therefore, the
torque of the shunt motor varies only with the current through
the armature. The torque developed at starting is less than
that developed by a series motor of equal size.
The speed of the shunt motor varies very little with changes
in load. When all load is removed, it assumes a speed slightly
higher than the loaded speed. This motor is particularly
suitable for use when constant speed is desired and when
high starting torque is not needed.
Compound DC Motor
The compound motor is a combination of the series and shunt
motors. There are two windings in the field: a shunt winding
and a series winding. [Figure 12-301] The shunt winding
is composed of many turns of fine wire and is connected in
parallel with the armature winding. The series winding
consists of a few turns of large wire and is connected in series
with the armature winding. The starting torque is higher than
in the shunt motor but lower than in the series motor. Variation
of speed with load is less than in a series wound motor but
greater than in a shunt motor. The compound motor is used
whenever the combined characteristics of the series and shunt
motors are desired.
Like the compound generator, the compound motor has both
series and shunt field windings. The series winding may
either aid the shunt wind (cumulative compound) or oppose
the shunt winding (differential compound). The starting and load characteristics of the cumulative compound motor
are somewhere between those of the series and those of the
shunt motor.
Because of the series field, the cumulative compound motor
has a higher starting torque than a shunt motor. Cumulative
compound motors are used in driving machines, which are
subject to sudden changes in load. They are also used where a
high starting torque is desired, but a series motor cannot be
used easily.
In the differential compound motor, an increase in load
creates an increase in current and a decrease in total flux in
this type of motor. These two tend to offset each other and
the result is a practically constant speed. However, since an
increase in load tends to decrease the field strength, the speed
characteristic becomes unstable. Rarely is this type of motor
used in aircraft systems.
A graph of the variation in speed with changes of load of
the various types of DC motors is shown in Figure 12-302 .
Counter Electromotive Force (emf)
The armature resistance of a small, 28-volt DC motor
is extremely low, about 0.1 ohm. When the armature is
connected across the 28-volt source, current through the
armature is apparently:
I = = = 280 amperesE 28
R 0.1
This high value of current flow is not only impracticable but
also unreasonable, especially when the current drain, during
normal operation of a motor, is found to be about 4 amperes.
This is because the current through a motor armature during
operation is determined by more factors than ohmic resistance.
When the armature in a motor rotates in a magnetic field, a
voltage is induced in its windings. This voltage is called the
back or counter emf and is opposite in direction to the voltage
applied to the motor from the external source.
Counter emf opposes the current, which causes the armature
to rotate. The current flowing through the armature, therefore,
decreases as the counter emf increases. The faster the
armature rotates, the greater the counter emf. For this reason,
a motor connected to a battery may draw a fairly high current
on starting, but as the armature speed increases, the current
flowing through the armature decreases. At rated speed, the
counter emf may be only a few volts less than the battery
voltage. Then, if the load on the motor is increased, the motor
slows down, less counter emf is generated, and the current
drawn from the external source increases. In a shunt motor,
the counter emf affects only the current in the armature, since
End frame
Field coils
Armature
Brush riggingBearing
CoverCommutator
Tube type brush
Box type brushFigure 12-297. Cutaway view of practical DC motor.
Figure 12-298. Commutator and brushes.the field is connected in parallel across the power source. As
the motor slows down and the counter emf decreases, more
current flows through the armature, but the magnetism in the
field is unchanged. When the series motor slows down, the
counter EMF decreases and more current flows through the
field and the armature, thereby strengthening their magnetic fields. Because of these characteristics, it is more difficult to
stall a series motor than a shunt motor.
Types of Duty
Electric motors are called upon to operate under various
conditions. Some motors are used for intermittent operation;
12-144CommutatorArmature
Shunt fieldShunt field
A
BDiagrammatic
Schematic
Figure 12-300. Shunt motor.Commutator
ArmatureWire
A
BDiagrammatic
Schematic
Figure 12-299. Series motor.others operate continuously. Motors built for intermittent
duty can be operated for short periods only and, then, must
be allowed to cool before being operated again. If such a
motor is operated for long periods under full load, the motor
becomes overheated. Motors built for continuous duty may
be operated at rated power for long periods.
Reversing Motor Direction
By reversing the direction of current flow in either the armature
or the field windings, the direction of a motor’s rotation may be
reversed. This reverses the magnetism of either the armature or
the magnetic field in which the armature rotates. If the wires
connecting the motor to an external source are interchanged,
the direction of rotation is not reversed, since changing these
wires reverses the magnetism of both field and armature and
leaves the torque in the same direction as before.
One method for reversing direction of rotation employs
two field windings wound in opposite directions on the
same pole. This type of motor is called a split field motor.
[Figure 12-303] The single pole, double throw switch
makes it possible to direct current through either of the two
windings. When the switch is placed in the lower position,
current flows through the lower field winding, creating a North pole at the lower field winding and at the lower
pole piece, and a South pole at the upper pole piece. When
the switch is placed in the upper position, current flows
through the upper field winding, the magnetism of the
field is reversed, and the armature rotates in the opposite
direction. Some split field motors are built with two separate
field windings wound on alternate poles. The armature in
such a motor, a four pole reversible motor, rotates in one
direction when current flows through the windings of one set
of opposite pole pieces, and in the opposite direction when
current flows through the other set of windings.
Another method of direction reversal, called the switch
method, employs a double pole, double throw switch which
changes the direction of current flow in either the armature
or the field. In the illustration of the switch method shown in
Figure 12-304 , current direction may be reversed through the
field but not through the armature. When the switch is thrown
to the “up” position, current flows through the field winding
to establish a North pole at the right side of the motor and a
South pole at the left side of the motor. When the switch is
thrown to the “down” position, this polarity is reversed and
the armature rotates in the opposite direction.
12-145Shunt
field+
–Series
field
M
Output power or load 0 100%100%Speed of rotationSeries connectionDifferential compounding
Shunt connection
Cumulative compounding
Figure 12-301. Compound motor.Figure 12-302. Load characteristics of DC motors.Motor Speed
Motor speed can be controlled by varying the current in the
field windings. When the amount of current flowing through
the field windings is increased, the field strength increases,
but the motor slows down since a greater amount of counter
EMF is generated in the armature windings. When the field
current is decreased, the field strength decreases, and the
motor speeds up because the counter EMF is reduced. A
motor in which speed can be controlled is called a variable
speed motor. It may be either a shunt or series motor.
In the shunt motor, speed is controlled by a rheostat in series
with the field windings. [Figure 12-305] The speed depends
on the amount of current that flows through the rheostat to the
field windings. To increase the motor speed, the resistance in
the rheostat is increased, which decreases the field current.
As a result, there is a decrease in the strength of the magnetic
field and in the counter EMF . This momentarily increases the
armature current and the torque. The motor then automatically
speeds up until the counter EMF increases and causes the
armature current to decrease to its former value. When this
occurs, the motor operates at a higher fixed speed than before.
To decrease the motor speed, the resistance of the rheostat
is decreased. More current flows through the field windings
and increases the strength of the field; then, the counter EMF
increases momentarily and decreases the armature current.
As a result, the torque decreases and the motor slows down
until the counter EMF decreases to its former value; then the
motor operates at a lower fixed speed than before.
In the series motor, the rheostat speed control is connected
either in parallel or in series with the motor field, or in
parallel with the armature. When the rheostat is set for
maximum resistance, the motor speed is increased in the
parallel armature connection by a decrease in current. When
the rheostat resistance is maximum in the series connection,
motor speed is reduced by a reduction in voltage across the
motor. For above normal speed operation, the rheostat is in
parallel with the series field. Part of the series field current is bypassed and the motor speeds up. [Figure 12-306]
Energy Losses in DC Motors
Losses occur when electrical energy is converted to
mechanical energy (in the motor), or mechanical energy
is converted to electrical energy (in the generator). For
the machine to be efficient, these losses must be kept to a
minimum. Some losses are electrical; others are mechanical.
Electrical losses are classified as copper losses and iron
losses; mechanical losses occur in overcoming the friction
of various parts of the machine.
Copper losses occur when electrons are forced through
the copper windings of the armature and the field. These
losses are proportional to the square of the current. They are
sometimes called I2R losses, since they are due to the power
dissipated in the form of heat in the resistance of the field
and armature windings.
Iron losses are subdivided in hysteresis and eddy current
losses. Hysteresis losses are caused by the armature revolving
in an alternating magnetic field. It, therefore, becomes
magnetized first in one direction and then in the other. The
residual magnetism of the iron or steel of which the armature
is made causes these losses. Since the field magnets are
always magnetized in one direction (DC field), they have
no hysteresis losses.
Eddy current losses occur because the iron core of the
armature is a conductor revolving in a magnetic field. This
sets up an EMF across portions of the core, causing currents
to flow within the core. These currents heat the core and, if
they become excessive, may damage the windings. As far as
the output is concerned, the power consumed by eddy currents
is a loss. To reduce eddy currents to a minimum, a laminated
12-146Double pole
Double throw switch
DownUp
Figure 12-304. Switch method of reversing motor direction.AB
Figure 12-303. Split field series motor.core usually is used. A laminated core is made of thin sheets
of iron electrically insulated from each other. The insulation
between laminations reduces eddy currents, because it is
“transverse” to the direction in which these currents tend to
flow. However, it has no effect on the magnetic circuit. The
thinner the laminations, the more effectively this method
reduces eddy current losses.
Inspection and Maintenance of DC Motors
Use the following procedures to make inspection and
maintenance checks:
1. Check the operation of the unit driven by the motor
in accordance with the instructions covering the
specific installation.
2. Check all wiring, connections, terminals, fuses, and
switches for general condition and security.
3. Keep motors clean and mounting bolts tight.
4. Check brushes for condition, length, and spring
tension. Minimum brush lengths, correct spring
tension, and procedures for replacing brushes are
given in the applicable manufacturer’s instructions.
5. Inspect commutator for cleanness, pitting, scoring,
roughness, corrosion, or burning. Check for high
mica ( IF the copper wears down below the mica, the
mica insulates the brushes from the commutator.)
Clean dirty commutators with a cloth moistened with
the recommended cleaning solvent. Polish rough
or corroded commutators with fine sandpaper (000
or finer) and blow out with compressed air. Never
use emery paper since it contains metallic particles that may cause shorts. Replace the motor if the
commutator is burned, badly pitted, grooved, or worn
to the extent that the mica insulation is flush with the
commutator surface.
6. Inspect all exposed wiring for evidence of overheating.
Replace the motor if the insulation on leads or
windings is burned, cracked, or brittle.
7. Lubricate only if called for by the manufacturer’s
instructions covering the motor. Most motors used
in today’s airplanes require no lubrication between
overhauls.
8. Adjust and lubricate the gearbox, or unit which the
motor drives, in accordance with the applicable
manufacturer’s instructions covering the unit.
When trouble develops in a DC motor system, check first to
determine the source of the trouble. Replace the motor only
when the trouble is due to a defect in the motor itself. In most
cases, the failure of a motor to operate is caused by a defect
in the external electrical circuit or by mechanical failure in
the mechanism driven by the motor.
Check the external electrical circuit for loose or dirty
12-147RheostatFrame
Pole
Commutator
ArmatureShunt led
Figure 12-305. Shunt motor with variable speed control.connections and for improper connection of wiring. Look
for open circuits, grounds, and shorts by following the
applicable manufacturer’s circuit testing procedure. If the
fuse is not blown, failure of the motor to operate is usually
due to an open circuit. A blown fuse usually indicates an
accidental ground or short circuit. A low battery usually
causes the chattering of the relay switch, which controls the
motor. When the battery is low, the open circuit voltage of
the battery is sufficient to close the relay, but with the heavy
current draw of the motor, the voltage drops below the level
required to hold the relay closed. When the relay opens, the
voltage in the battery increases enough to close the relay
again. This cycle repeats and causes chattering, which is very
harmful to the relay switch due to the heavy current causing
an arc which burns the contacts.
Check the unit driven by the motor for failure of the unit
or drive mechanism. If the motor has failed as a result of a
failure in the driven unit, the fault must be corrected before
installing a new motor. If it has been determined that the
fault is in the motor itself (by checking for correct voltage
at the motor terminals and for failure of the driven unit),
inspect the commutator and brushes. A dirty commutator
or defective or binding brushes may result in poor contact
between brushes and commutator. Clean the commutator,
brushes, and brush holders with a cloth moistened with the
recommended cleaning solvent. If brushes are damaged or
worn to the specified minimum length, install new brushes in accordance with the applicable manufacturer’s instructions
covering the motor. If the motor still fails to operate, replace
it with a serviceable motor.
AC Motors
Because of their advantages, many types of aircraft motors
are designed to operate on alternating current. In general,
AC motors are less expensive than comparable DC motors.
In many instances, AC motors do not use brushes and
commutators so sparking at the brushes is avoided. AC
motors are reliable and require little maintenance. They are
also well suited for constant speed applications and certain
types are manufactured that have, within limits, variable
speed characteristics. Alternating current motors are designed
to operate on polyphase or single-phase lines and at several
voltage ratings.
The speed of rotation of an AC motor depends upon the number
of poles and the frequency of the electrical source of power:
rpm =120 × Frequency
Number of poles
Since airplane electrical systems typically operate at 400
cycles, an electric motor at this frequency operates at about
seven times the speed of a 60-cycle commercial motor with
the same number of poles. Because of this high speed of
rotation, 400-cycle AC motors are suitable for operating
small, high-speed rotors, through reduction gears, in
lifting and moving heavy loads, such as the wing flaps, the
retractable landing gear, and the starting of engines. The
400-cycle induction type motor operates at speeds ranging
from 6,000 rpm to 24,000 rpm. Alternating current motors
are rated in horsepower output, operating voltage, full load
current, speed, number of phases, and frequency. Whether
the motors operate continuously or intermittently (for short
intervals) is also considered in the rating.
Types of AC Motors
There are two general types of AC motors used in aircraft
systems: induction motors and synchronous motors. Either type
may be single-phase, two-phase, or three-phase. Three-phase
induction motors are used where large amounts of power are
required. They operate such devices as starters, flaps, landing
gears, and hydraulic pumps. Single-phase induction motors
are used to operate devices such as surface locks, intercooler
shutters, and oil shutoff valves in which the power requirement
is low. Three-phase synchronous motors operate at constant
synchronous speeds and are commonly used to operate flux
gate compasses and propeller synchronizer systems. Single-
phase synchronous motors are common sources of power to
operate electric clocks and other small, precision equipment.
They require some auxiliary method to bring them up to
12-148Below normal speed Normal speed Above normal speedMRheostatSeries
filed
Fast Slow
–+ +
MRheostatFast Slow
–+
MFast
Slow
Figure 12-306. Controlling the speed of a series DC motor.
synchronous speeds; that is, to start them. Usually the starting
winding consists of an auxiliary stator winding.
Three-Phase Induction Motor
The three-phase AC induction motor is also called a squirrel
cage motor. Both single-phase and three-phase motors
operate on the principle of a rotating magnetic field. A
horseshoe magnet held over a compass needle is a simple
illustration of the principle of the rotating field. The needle
takes a position parallel to the magnetic flux passing between
the two poles of the magnet. If the magnet is rotated, the
compass needle follows. A rotating magnetic field can be
produced by a two-or three-phase current flowing through
two or more groups of coils wound on inwardly projecting
poles of an iron frame. The coils on each group of poles
are wound alternately in opposite directions to produce
opposite polarity, and each group is connected to a separate
phase of voltage. The operating principle depends on a
revolving, or rotating, magnetic field to produce torque.
The key to understanding the induction motor is a thorough
understanding of the rotating magnetic field.
Rotating Magnetic Field
The field structure shown in Figure 12-307A has poles
whose windings are energized by three AC voltages: a, b,
and c. These voltages have equal magnitude but differ in
phase. [Figure 12-307B] At the instant of time shown as
0, the resultant magnetic field produced by the application
of the three voltages has its greatest intensity in a direction extending from pole 1 to pole 4. Under this condition, pole
1 can be considered as a North pole and pole 4 as a South
pole. At the instant of time shown as 1, the resultant magnetic
field has its greatest intensity in the direction extending from
pole 2 to pole 5. In this case, pole 2 can be considered as a
North pole and pole 5 as a South pole. Thus, between instant
0 and instant 1, the magnetic field has rotated clockwise. At
instant 2, the resultant magnetic field has its greatest intensity
in the direction from pole 3 to pole 6, and the resultant
magnetic field has continued to rotate clockwise. At instant
3, poles 4 and 1 can be considered as North and South poles,
respectively, and the field has rotated still farther. At later
instants of time, the resultant magnetic field rotates to other
positions while traveling in a clockwise direction, a single
revolution of the field occurring in one cycle. If the exciting
voltages have a frequency of 60 cps, the magnetic field makes
60 revolutions per second, or 3,600 rpm. This speed is known
as the synchronous speed of the rotating field.
Construction of Induction Motor
The stationary portion of an induction motor is called a stator,
and the rotating member is called a rotor. Instead of salient
poles in the stator, as shown in Figure 12-307A , distributed
windings are used. These windings are placed in slots
around the periphery of the stator. It is usually impossible
to determine the number of poles in an induction motor by
visual inspection, but the information can be obtained from
the nameplate of the motor. The nameplate usually gives the
number of poles and the speed at which the motor is designed
12-149to run. This rated, or nonsynchronous, speed is slightly less
than the synchronous speed. To determine the number of
poles per phase on the motor, divide 120 times the frequency
by the rated speed. Written as an equation, it is:
P =120 × f
N
Where: P is the number of poles per phase
f is the frequency in cps
N is the rated speed in rpm, and
120 is a constant
The result is nearly equal to the number of poles per phase.
For example, consider a 60-cycle, three-phase motor with a
rated speed of 1,750 rpm. In this case:
P = = = 4.1120 × 60 7,200
1,750 1,750
Therefore, the motor has four poles per phase. If the number
of poles per phase is given on the nameplate, the synchronous
speed can be determined by dividing 120 times the frequency
by the number of poles per phase. In the example used above,
the synchronous speed is equal to 7,200/4, or 1,800 rpm.
The rotor of an induction motor consists of an iron core
having longitudinal slots around its circumference in which
heavy copper or aluminum bars are embedded. These bars
are welded to a heavy ring of high conductivity on either
end. The composite structure is sometimes called a squirrel
cage, and motors containing such a rotor are called squirrel
cage induction motors. [Figure 12-308]
Induction Motor Slip
When the rotor of an induction motor is subjected to the
revolving magnetic field produced by the stator windings,
a voltage is induced in the longitudinal bars. The induced
voltage causes a current to flow through the bars. This current,
in turn, produces its own magnetic field, which combines
with the revolving field so that the rotor assumes a position in
which the induced voltage is minimized. As a result, the rotor
revolves at very nearly the synchronous speed of the stator
field, the difference in speed being just sufficient enough to
induce the proper amount of current in the rotor to overcome
the mechanical and electrical losses in the rotor. If the rotor
were to turn at the same speed as the rotating field, the rotor
conductors would not be cut by any magnetic lines of force,
no EMF would be induced in them, no current could flow, and
there would be no torque. The rotor would then slow down.
For this reason, there must always be a difference in speed
between the rotor and the rotating field. This difference in
speed is called slip and is expressed as a percentage of the
synchronous speed. For example, if the rotor turns at 1,750 rpm and the synchronous speed is 1,800 rpm, the difference in speed
is 50 rpm. The slip is then equal to 50/1,800 or 2.78 percent.
Single-Phase Induction Motor
The previous discussion has applied only to polyphase
motors. A single-phase motor has only one stator winding.
This winding generates a field, which merely pulsates, instead
of rotating. When the rotor is stationary, the expanding and
collapsing stator field induces currents in the rotor. These
currents generate a rotor field opposite in polarity to that of
the stator. The opposition of the field exerts a turning force
on the upper and lower parts of the rotor trying to turn it 180°
from its position. Since these forces are exerted through the
center of the rotor, the turning force is equal in each direction.
As a result, the rotor does not turn. If the rotor starts turning,
it continues to rotate in the direction in which it started, since
the turning force in that direction is aided by the momentum
of the rotor.
Shaded Pole Induction Motor
The first effort in the development of a self-starting,
single-phase motor was the shaded pole induction motor.
[Figure 12-309] This motor has salient poles, a portion
of each pole being encircled by a heavy copper ring. The
presence of the ring causes the magnetic field through the
ringed portion of the pole face to lag appreciably behind
that through the other part of the pole face. The net effect
is the production of a slight component of rotation of the
field, sufficient to cause the rotor to revolve. As the rotor
accelerates, the torque increases until the rated speed is
obtained. Such motors have low starting torque and find
their greatest application in small fan motors where the initial
torque required is low.
In Figure 12-309 , a diagram of a pole and the rotor is
shown. The poles of the shaded pole motor resemble those
of a DC motor.
A low resistance, short-circuited coil or copper band is placed
across one tip of each small pole from which the motor gets
the name of shaded pole. The rotor of this motor is the squirrel
cage type. As the current increases in the stator winding, the
flux increases. A portion of this flux cuts the low resistance
shading coil. This induces a current in the shading coil, and
by Lenz’s Law, the current sets up a flux that opposes the
flux inducing the current. Hence, most of the flux passes
through the unshaded portion of the poles. [Figure 12-310]
When the current in the winding and the main flux reaches
a maximum, the rate of change is zero; thus, no emf is
induced in the shading coil. A little later, the shading coil
current, which causes the induced emf to lag, reaches zero,
and there is no opposing flux. Therefore, the main field flux
12-150a
a0 1 2 3 4 5 6
a1
b
b
bc
c
c180°
TimeA
B
Figure 12-307. Rotating magnetic field developed by application
of three-phase voltages.
passes through the shaded portion of the field pole. The
main field flux, which is now decreasing, induces a current
in the shading coil. This current sets up a flux that opposes
the decrease of the main field flux in the shaded portion of
the pole. The effect is to concentrate the lines of force in
the shaded portion of the pole face. In effect, the shading
coil retards, in time phase, the portion of the flux passing
through the shaded part of the pole. This lag in time phase
of the flux in the shaded tip causes the flux to produce the
effect of sweeping across the face of the pole, from left to
right in the direction of the shaded tip. This behaves like a
very weak rotating magnetic field, and sufficient torque is
produced to start a small motor. The starting torque of the
shaded pole motor is exceedingly weak, and the power factor
is low. Consequently, it is built in sizes suitable for driving
such devices as small fans.Split-Phase Motor
There are various types of self-starting motors, known as
split-phase motors. Such motors have a starting winding
displaced 90 electrical degrees from the main or running
winding. In some types, the starting winding has a fairly
high resistance, which causes the current in this winding
to be out of phase with the current in the running winding.
This condition produces, in effect, a rotating field and
the rotor revolves. A centrifugal switch disconnects the
starting winding automatically after the rotor has attained
approximately 25 percent of its rated speed.
Capacitor Start Motor
With the development of high-capacity electrolytic
capacitors, a variation of the split-phase motor known as the
capacitor start motor, has been made. Nearly all fractional
horsepower motors in use today on refrigerators and other
similar appliances are of this type. [Figure 12-311] In this
adaptation, the starting winding and running winding have
the same size and resistance value. The phase shift between
currents of the two windings is obtained by using capacitors
connected in series with the starting winding.
Capacitor start motors have a starting torque comparable to
their torque at rated speed and can be used in applications
where the initial load is heavy. Again, a centrifugal switch
is required for disconnecting the starting winding when the
rotor speed is approximately 25 percent of the rated speed.
Although some single-phase induction motors are rated as
high as 2 horsepower (hp), the major field of application is
1 hp, or less, at a voltage rating of 115 volts for the smaller
sizes and 110 to 220 volts for one-fourth hp and up. For even
larger power ratings, polyphase motors generally are used,
since they have excellent starting torque characteristics.
Direction of Rotation of Induction Motors
The direction of rotation of a three-phase induction motor
can be changed by simply reversing two of the leads to the
motor. The same effect can be obtained in a two-phase motor
by reversing connections to one phase. In a single-phase
motor, reversing connections to the starting winding reverses
the direction of rotation.
Most single-phase motors designed for general application
have provision for readily reversing connections to the
starting winding. Nothing can be done to a shaded pole motor
to reverse the direction of rotation because the direction is
determined by the physical location of the copper shading
ring. If, after starting, one connection to a three-phase motor
is broken, the motor continues to run but delivers only
one-third the rated power. Also, a two-phase motor runs at
one-half its rated power if one phase is disconnected. Neither
12-151A B
Aluminum or copper barsShaftWeld at all joints Welds holding copper or
aluminum bars to end ring
Aluminum or
copper end ring
Iron core
Figure 12-308. Squirrel cage rotor for an AC induction motor.
Shaded poles
Shaded poles
Figure 12-309. Shaded pole induction motor.motor starts under these abnormal conditions.
Synchronous Motor
The synchronous motor is one of the principal types of AC
motors. Like the induction motor, the synchronous motor
makes use of a rotating magnetic field. Unlike the induction
motor, however, the torque developed does not depend on
the induction of currents in the rotor. Briefly, the principle of
operation of the synchronous motor is as follows: A multiphase
source of AC is applied to the stator windings, and a rotating
magnetic field is produced. A direct current is applied to the
rotor winding, and another magnetic field is produced. The
synchronous motor is so designed and constructed that these
two fields react to each other in such a manner that the rotor
is dragged along and rotates at the same speed as the rotating
magnetic field produced by the stator windings.
An understanding of the operation of the synchronous
motor can be obtained by considering the simple motor of
Figure 12-312 . Assume that poles A and B are being rotated
clockwise by some mechanical means in order to produce
a rotating magnetic field. They induce poles of opposite
polarity in the soft iron rotor, and forces of attraction exist
between corresponding North and South poles.
Consequently, as poles A and B rotate, the rotor is dragged
along at the same speed. However, if a load is applied to the
rotor shaft, the rotor axis momentarily falls behind that of the
rotating field but, thereafter, continues to rotate with the field
at the same speed as long as the load remains constant. If the
load is too large, the rotor pulls out of synchronism with the
rotating field and, as a result, no longer rotates with the field
at the same speed. Thus, the motor is said to be overloaded.
Such a simple motor as shown in Figure 12-312 is never used.
The idea of using some mechanical means of rotating the poles is impractical because another motor would be required to
perform this work. Also, such an arrangement is unnecessary
because a rotating magnetic field can be produced electrically
by using phased AC voltages. In this respect, the synchronous
motor is similar to the induction motor.
The synchronous motor consists of a stator field winding
similar to that of an induction motor. The stator winding
produces a rotating magnetic field. The rotor may be a
permanent magnet, as in small, single-phase synchronous
motors used for clocks and other small precision equipment,
or it may be an electromagnet, energized from a DC source of
power and fed through slip rings into the rotor field coils, as
in an alternator. In fact, an alternator may be operated either
as an alternator or a synchronous motor.
Shaded pole
Rotor
Figure 12-310. Diagram of a shaded pole motor.Since a synchronous motor has little starting torque, some
means must be provided to bring it up to synchronous speed.
The most common method is to start the motor at no load,
allow it to reach full speed, and then energize the magnetic
field. The magnetic field of the rotor locks with the magnetic
field of the stator and the motor operates at synchronous speed.
The magnitude of the induced poles in the rotor shown in
Figure 12-313 is so small that sufficient torque cannot be
developed for most practical loads. To avoid such a limitation
on motor operation, a winding is placed on the rotor and
energized with DC. A rheostat placed in series with the DC
source provides the operator of the machine with a means
of varying the strength of the rotor poles, thus placing the
motor under control for varying loads.
The synchronous motor is not a self-starting motor. The
rotor is heavy and, from a dead stop, it is impossible to
bring the rotor into magnetic lock with the rotating magnetic
field. For this reason, all synchronous motors have some
kind of starting device. One type of simple starter is
another motor, either AC or DC, which brings the rotor up
to approximately 90 percent of its synchronous speed. The
starting motor is then disconnected, and the rotor locks in
step with the rotating field. Another starting method is a
second winding of the squirrel cage type on the rotor. This
induction winding brings the rotor almost to synchronous
speed, and when the DC is connected to the rotor windings,
the rotor pulls into step with the field. The latter method is
the more commonly used.
AC Series Motor
An AC-series motor is a single-phase motor, but is not an
induction or synchronous motor. It resembles a DC motor in that it has brushes and a commutator. The AC-series motor
operates on either AC or DC circuits. Remember that the
direction of rotation of a DC-series motor is independent
of the polarity of the applied voltage, provided the field
and armature connections remain unchanged. Hence, if a
DC-series motor is connected to an AC source, a torque is
developed that tends to rotate the armature in one direction.
However, a DC-series motor does not operate satisfactorily
from an AC supply for the following reasons:
1. The alternating flux sets up large eddy current and
hysteresis losses in the unlaminated portions of the
magnetic circuit and causes excessive heating and
reduced efficiency.
2. The self-induction of the field and armature windings
causes a low power factor.
3. The alternating field flux establishes large currents in
the coils, which are short circuited by the brushes; this
action causes excessive sparking at the commutator.
To design a series motor for satisfactory operation on AC,
the following changes are made:
1. The eddy current losses are reduced by laminating the
field poles, frame, and armature.
2. Hysteresis losses are minimized by using high
permeability, transformer-type, silicon steel
laminations.
3. The reactance of the field windings is kept satisfactorily
low by using shallow pole pieces, few turns of wire,
low frequency (usually 25 cycles for large motors),
low flux density, and low reluctance (a short air gap).
4. The reactance of the armature is reduced by using a
compensating winding embedded in the pole pieces. If
the compensating winding is connected in series with
the armature, as shown in Figure 12-314 , the armature
is conductively compensated.
If the compensating winding is designed as shown
in Figure 12-315 , the armature is inductively
compensated. If the motor is designed for operation
on both DC and AC circuits, the compensating winding
is connected in series with the armature. The axis of
the compensating winding is displaced from the main
field axis by an angle of 90°. This arrangement is
similar to the compensating winding used in some DC
motors and generators to overcome armature reaction.
The compensating winding establishes a counter
magnetomotive force, neutralizing the effect of the
armature magnetomotive force, preventing distortion
of the main field flux, and reducing the armature
reactance. The inductively compensated armature
acts like the primary of a transformer, the secondary
12-153NS
S NAlternatorCentrifugal switch
Winding 2 (Running winding)Winding 1 (Starting winding)
Capacitor
Figure 12-311. Single-phase motor with capacitor starting winding.
Rotating pole
structure
Rotating pole
structureRotor
Rotor axis
BSNA
S
NInduced poles Rotor shaft
Figure 12-312. Illustrating the operation of a synchronous motor.of which is the shorted compensating winding. The
shorted secondary receives an induced voltage by the
action of the alternating armature flux, and the resulting
current flowing through the turns of the compensating
winding establishes the opposing magnetomotive force,
neutralizing the armature reactance.
5. Sparking at the commutator is reduced by the use of
preventive leads P 1, P2, P3, and so forth, as shown in
Figure 12-316 , where a ring armature is shown for
simplicity. When coils at A and B are shorted by the
brushes, the induced current is limited by the relatively
high resistance of the leads. Sparking at the brushes
is also reduced by using armature coils having only a
single turn and multipolar fields. High torque is obtained
by having a large number of armature conductors and
a large diameter armature. Thus, the commutator has
a large number of very thin commutator bars and the
armature voltage is limited to about 250 volts.
Fractional horsepower AC series motors are called universal
motors. They do not have compensating windings or
preventive leads. They are used extensively to operate fans
and portable tools, such as drills, grinders, and saws.
Maintenance of AC Motors
The inspection and maintenance of AC motors is very simple.
The bearings may or may not need frequent lubrication. If
they are the sealed type, lubricated at the factory, they require
no further attention. Be sure the coils are kept dry and free
from oil or other abuse. The temperature of a motor is usually
its only limiting operating factor. A good rule of thumb is that a temperature too hot for the hand is too high for safety. Next
to the temperature, the sound of a motor or generator is the
best trouble indicator. When operating properly, it should hum
evenly. If it is overloaded it “grunts.” A three-phase motor
Three-phase winding produces
a rotating magnetic field
Slip rings+N
NS S
Figure 12-313. Synchronous motor.with one lead disconnected refuses to turn and “growls.” A
knocking sound generally indicates a loose armature coil,
a shaft out of alignment, or armature dragging because of
worn bearings. In all cases, the inspection and maintenance
of all AC motors should be performed in accordance with
the applicable manufacturer’s instructions.
Alternators
Basic Alternators & Classifications
An electrical generator is a machine that converts mechanical
energy into electrical energy by electromagnetic induction.
A generator that produces alternating current is referred to
as an AC generator and, through combination of the words
“alternating” and “generator,” the word “alternator” has come
into widespread use. In some areas, the word “alternator” is
applied only to small AC generators. This handbook treats
the two terms synonymously and uses the term “alternator”
to distinguish between AC and DC generators.
The major difference between an alternator [Figure 12-317]
and a DC generator is the method of connection to the
external circuit. The alternator is connected to the external
circuit by slip rings, but the DC generator is connected by
a commutator.Method of Excitation
One means of classification is by the type of excitation system
used. In alternators used on aircraft, excitation can be affected
by one of the following methods:
1. A direct connected, DC generator. This system consists
of a DC generator fixed on the same shaft with the
AC generator. A variation of this system is a type of
alternator that uses DC from the battery for excitation,
after which the alternator is self-excited.
2. By transformation and rectification from the AC
system. This method depends on residual magnetism
for initial AC voltage buildup, after which the field is
supplied with rectified voltage from the AC generator.
3. Integrated brushless type. This arrangement has a DC
generator on the same shaft with an AC generator.
The excitation circuit is completed through silicon
rectifiers rather than a commutator and brushes. The
rectifiers are mounted on the generator shaft and
their output is fed directly to the AC generator’s main
rotating field.
Number of Phases
Another method of classification is by the number of phases
of output voltage. AC generators may be single-phase,
two-phase, three-phase, or even six-phase and more. In the
Compensating
winding
Main field
Compensating
winding
Main fieldFigure 12-314. Conductively compensated armature of AC series
motor.
Figure 12-315. Inductively compensated armature of AC series
motor.electrical systems of aircraft, the three-phase alternator is by
far the most common.
Armature or Field Rotation
Still another means of classification is by the type of stator
and rotor used. From this standpoint, there are two types of
alternators: the revolving armature-type and the revolving
field-type. The revolving armature alternator is similar in
construction to the DC generator in that the armature rotates
through a stationary magnetic field. The revolving armature
alternator is found only in alternators of low-power rating
and generally is not used. In the DC generator, the EMF
generated in the armature windings is converted into a
unidirectional voltage (DC) by means of the commutator.
In the revolving armature-type of alternator, the generated
AC voltage is applied unchanged to the load by means of
slip rings and brushes.
The revolving field type of alternator has a stationary
armature winding (stator) and a rotating field winding
(rotor). [Figure 12-318] The advantage of having a stationary
armature winding is that the armature can be connected
directly to the load without having sliding contacts in the
load circuit. A rotating armature would require slip rings
and brushes to conduct the load current from the armature
to the external circuit. Slip rings have a relatively short
service life and arc over is a continual hazard; therefore, high
voltage alternators are usually of the stationary armature,
rotating field-type. The voltage and current supplied to the
rotating field are relatively small, and slip rings and brushes
for this circuit are adequate. The direct connection to the
armature circuit makes possible the use of large cross-section
conductors, adequately insulated for high voltage. Since the
rotating field alternator is used almost universally in aircraft
systems, this type is explained in detail, as a single-phase,
two-phase, and three-phase alternator.
Single-Phase Alternator
Since the EMF induced in the armature of a generator is
alternating, the same sort of winding can be used on an
alternator as on a DC generator. This type of alternator is
known as a single-phase alternator, but since the power
delivered by a single-phase circuit is pulsating, this type of
circuit is objectionable in many applications.
A single-phase alternator has a stator made up of a number of
windings in series, forming a single circuit in which an output
voltage is generated. [Figure 12-319] The stator has four polar
groups evenly spaced around the stator frame. The rotor has
four poles with adjacent poles of opposite polarity. As the rotor
revolves, AC voltages are induced in the stator windings. Since
one rotor pole is in the same position relative to a stator winding
as any other rotor pole, all stator polar groups are cut by equal numbers of magnetic lines of force at any time.
As a result, the voltages induced in all the windings have
the same amplitude, or value, at any given instant. The four
stator windings are connected to each other so that the AC
voltages are in phase or “series adding.” Assume that rotor
pole 1, a South pole, induces a voltage in the direction
indicated by the arrow in stator winding 1. Since rotor
pole 2 is a North pole, it induces a voltage in the opposite
direction in stator coil 2 with respect to that in coil 1. For
the two induced voltages to be in series addition, the two
coils are connected as shown in Figure 12-319 . Applying
the same reasoning, the voltage induced in stator coil
3 (clockwise rotation of the field) is the same direction
(counterclockwise) as the voltage induced in coil 1.
A
BP9
P8
P7
P6
P5P4P3P2P1
Figure 12-316. Preventive coils in AC series motor.
Figure 12-317. Belt driven alternator for small single-engine
aircraft.Similarly, the direction of the voltage induced in winding
4 is opposite to the direction of the voltage induced in coil
1. All four stator coil groups are connected in series so that
the voltages induced in each winding add to give a total
voltage that is four times the voltage in any one winding.
Two-Phase Alternator
Two-phase alternators have two or more single-phase windings
spaced symmetrically around the stator. In a two-phase
alternator, there are two single-phase windings spaced physically
so that the AC voltage induced in one is 90° out of phase with
the voltage induced in the other. The windings are electrically
separate from each other. When one winding is being cut by
maximum flux, the other is being cut by no flux. This condition
establishes a 90° relation between the two phases.
Three-Phase Alternator
A three-phase, or polyphase circuit, is used in most aircraft
alternators, instead of a single or two-phase alternator. The
three-phase alternator has three single-phase windings spaced
so that the voltage induced in each winding is 120° out of
phase with the voltages in the other two windings. A schematic
diagram of a three-phase stator showing all the coils becomes
complex and difficult to see what is actually happening.
A simplified schematic diagram showing each of three
phases is illustrated in Figure 12-320 . The rotor is omitted for
simplicity. The waveforms of voltage are shown to the right of
the schematic. The three voltages are 120° apart and are similar
to the voltages that would be generated by three single-phase
alternators whose voltages are out of phase by angles of 120°.
The three phases are independent of each other.Wye Connection (Three-Phase)
Rather than have six leads from the three-phase alternator,
one of the leads from each phase may be connected to form
a common junction. The stator is then called wye or star
connected. The common lead may or may not be brought out
of the alternator. If it is brought out, it is called the neutral lead.
The simplified schematic shows a wye connected stator with
the common lead not brought out. [Figure 12-321A] Each
load is connected across two phases in series. Thus, RAB is
connected across phases A and B in series; RAC is connected
across phases A and C in series; and RBC is connected across
phases B and C in series. Therefore, the voltage across each
load is larger than the voltage across a single phase. The total
voltage, or line voltage, across any two phases is the vector
sum of the individual phase voltages. For balanced conditions,
the line voltage is 1.73 times the phase voltage. Since there is
only one path for current in a line wire and the phase to which
it is connected, the line current is equal to the phase current.
Delta Connection (Three-Phase)
A three-phase stator can also be connected so that the phases
are connected end to end. [Figure 12-321B] This arrangement
is called a delta connection. In a delta connection, the voltages
are equal to the phase voltages; the line currents are equal
to the vector sum of the phase currents; and the line current
is equal to 1.73 times the phase current when the loads are
balanced. For equal loads (equal output), the delta connection
supplies increased line current at a value of line voltage equal
to phase voltage, and the wye connection supplies increased
line voltage at a value of line current equal to phase current.
Alternator Rectifier Unit
A type of alternator used in the electrical system of many
aircraft weighing less than 12,500 pounds is shown in
Figure 12-322 . This type of power source is sometimes called
a DC generator, since it is used in DC systems. Although its
output is a DC voltage, it is an alternator rectifier unit. This
type of alternator rectifier is a self-excited unit but does not
contain a permanent magnet. The excitation for starting is
12-1570+e
Time axis
–eN
NS
S1 1 3 3
Figure 12-319. Single-phase alternator.
Rotating field
S NArmature Circuit
Slip rings
To exciter– +
Figure 12-318. Alternator with stationary armature and rotating
field.obtained from the battery; immediately after starting, the unit
is self-exciting. Cooling air for the alternator is conducted
into the unit by a blast air tube on the air inlet cover.
The alternator is directly coupled to the aircraft engine by
means of a flexible drive coupling. The output of the alternator
portion of the unit is three-phase alternating current, derived
from a three-phase, delta connected system incorporating a
three-phase, full-wave bridge rectifier. [Figure 12-323] This
unit operates in a speed range from 2,100 to 9,000 rpm, with
a DC output voltage of 26–29 volts and 125 amperes.
Brushless Alternator
This design is more efficient because there are no brushes to
wear down or to arc at high altitudes. This generator consists of
a pilot exciter, an exciter, and the main generator system. The
need for brushes is eliminated by using an integral exciter with
a rotating armature that has its AC output rectified for the main
AC field, which is also of the rotating type. [Figure 12-324]
The pilot exciter is an 8-pole, 8,000 rpm, 533 cps, AC
generator. The pilot exciter field is mounted on the main
generator rotor shaft and is connected in series with the main
generator field. The pilot exciter armature is mounted on the
main generator stator. The AC output of the pilot exciter is
supplied to the voltage regulator, where it is rectified and
controlled, and is then impressed on the exciter field winding
to furnish excitation for the generator.
The exciter is a small AC generator with its field mounted on the main generator stator and its three-phase armature
mounted on the generator rotor shaft. Included in the exciter
field are permanent magnets mounted on the main generator
stator between the exciter poles.
The exciter field resistance is temperature compensated by a
thermistor. This aids regulation by keeping a nearly constant
resistance at the regulator output terminals. The exciter
output is rectified and impressed on the main generator
field and the pilot exciter field. The exciter stator has a
stabilizing field, which is used to improve stability and to
prevent voltage regulator over-corrections for changes in
generator output voltage.
The AC generator shown in Figure 12-324 is a 6-pole, 8,000
rpm unit having a rating of 31.5 kilovoltamperes (kV A),
115⁄200 volts, 400 cps. This generator is three-phase, 4 wire,
wye connected with grounded neutrals. By using an integral
AC exciter, the necessity for brushes within the generator
has been eliminated. The AC output of the rotating exciter
armature is fed directly into the three-phase, full-wave,
rectifier bridge located inside the rotor shaft, which uses
high-temperature silicon rectifiers. The DC output from the
rectifier bridge is fed to the main AC generator rotating field.
V oltage regulation is accomplished by varying the strength
of the AC exciter stationary fields. Polarity reversals of the
AC generator are eliminated and radio noise is minimized
by the absence of the brushes. A noise filter mounted on
the alternator further reduces any existing radio noise.
The rotating pole structure of the generator is laminated
from steel punchings, containing all six poles and a
connecting hub section. This provides optimum magnetic
and mechanical properties.
Some alternators are cooled by circulating oil through steel
tubes. The oil used for cooling is supplied from the constant
speed drive assembly. Ports located in the flange connecting
the generator and drive assemblies make oil flow between
the constant speed drive and the generator possible.
Figure 12-322. Exploded view of alternator rectifier.A
B
CA
0+e
–eB CWye connection Delta connectionRBRA A C
BRC
RBCRABA C
BRAC
A B
Figure 12-320. Simplified schematic of three-phase alternator with
output waveforms.Figure 12-321. Wye and delta connected alternators.V oltage is built up by using permanent magnet interpoles in
the exciter stator. The permanent magnets assure a voltage
buildup, precluding the necessity of field flashing. The rotor
of the alternator may be removed without causing loss of the
alternator’s residual magnetism.
Alternator Frequency
The frequency of the alternator voltage depends upon the
speed of rotation of the rotor and the number of poles.
The faster the speed, the higher the frequency; the lower
the speed, the lower the frequency. The more poles on the
rotor, the higher the frequency for a given speed. When a
rotor has rotated through an angle so that two adjacent rotor
poles (a North and a South pole) have passed one winding,
the voltage induced in that winding has varied through one
complete cycle. For a given frequency, the greater the number
of pairs of poles, the lower the speed of rotation. A two-pole
alternator rotates at twice the speed of a four-pole alternator
for the same frequency of generated voltage. The frequency
of the alternator in cycles per minute (cpm) is related to the
number of poles and the speed, as expressed by the equation:
F = × = P N PN
N 60 120
Where: P is the number of poles per phase
f is the frequency in cps
N is the rated speed in rpm
For example, a 2-pole, 3,600 rpm alternator has a frequency of:
= 60 cps 2 × 3,600
A 4-pole, 1,800 rpm alternator has the same frequency; a
6-pole, 500 rpm alternator has a frequency of:
= 25 cps 6 × 500
A 12-pole, 4,000 rpm alternator has a frequency of: = 400 cps12 × 4,000
Starter Generator
Many turbine-powered aircraft use a starter generator that
acts like a starter during the start of the engine and when the
engine is online it acts like a Generator. [Figure 12-325] The
main advantage of the starter generator is saving weight by
eliminating a separate starter that is only used during the start.
Initially used on small turboprops and light jets but large units
are now installed on the B787 aircraft engines to power the
main engines and power the electrical system.
Alternator Rating
The maximum current that can be supplied by an alternator
depends upon the maximum heating loss (I2R power loss) that
can be sustained in the armature and the maximum heating loss
that can be sustained in the field. The armature current of an
alternator varies with the load. This action is similar to that of A
12 pole, 4,000 rpm alternator has a frequency of DC generators.
In AC generators, however, lagging power factor loads tend
to demagnetize the field of an alternator, and terminal voltage
is maintained only by increasing DC field current. For this
reason, AC generators are usually rated according to kV A,
power factor, phases, voltage, and frequency. One generator,
for example, may be rated at 40 kV A, 208 volts, 400 cycles,
three phase, at 75 percent power factor. The kV A indicates the
apparent power. This is the kV A output, or the relationship
between the current and voltage at which the generator is
intended to operate. The power factor is the expression of the
12-159E
B
AG1 L1
C3
C2CR4
CR5
CR6CR1
CR2
CR3+
+
+–
Figure 12-323. Wiring diagram of alternator-rectifier unit.ratio between the apparent power (volt-amperes) and the true
or effective power (watts). The number of phases is the number
of independent voltages generated. Three-phase generators
generate three voltages 120 electrical degrees apart.
Alternator Maintenance
Maintenance and inspection of alternator systems is similar
to that of DC systems. Check the exciter brushes for wear and
surfacing. On most large aircraft with two or four alternator
systems, each power panel has three signal lights, one
connected to each phase of the power bus, so the lamp lights
when the panel power is on. The individual buses throughout
the airplane can be checked by operating equipment from that
particular bus. Consult the manufacturer’s instructions on
operation of equipment for the method of testing each bus.
Alternator test stands are used for testing alternators and
constant speed drives in a repair facility. They are capable of
supplying power to constant speed drive units at input speeds
varying from 2,400 rpm to 9,000 rpm.
A typical test stand motor uses 220/440 volt, 60 cycle,
three-phase power. Blowers for ventilation, oil coolers,
and necessary meters and switches are integral parts of the
test stand. A load bank supplies test circuits. An AC motor
generator set for ground testing is shown in Figure 12-326 .
A typical, portable, AC electrical system test set is an
analyzer, consisting of a multirange ohmmeter, a multirange
combination AC DC voltmeter, an ammeter with a clip-on
current transformer, a vibrating reed type frequency meter, and an unmounted continuity light.
A portable load bank unit furnishes a load similar to that
on the airplane for testing alternators, either while mounted
in the airplane or on the shop test stand. A complete unit
consists of resistive and reactive loads controlled by selector
switches and test meters mounted on a control panel. This
load unit is compact and convenient, eliminating the difficulty
of operating large loads on the airplane while testing and
adjusting the alternators and control equipment.
Proper maintenance of an alternator requires that the unit be
kept clean and that all electrical connections are tight and
in good repair. If the alternator fails to build up voltage as
designated by applicable manufacturer’s technical instructions,
test the voltmeter first by checking the voltages of other
alternators, or by checking the voltage in the suspected
alternator with another voltmeter and comparing the results.
If the voltmeter is satisfactory, check the wiring, the brushes,
and the drive unit for faults. If this inspection fails to reveal
the trouble, the exciter may have lost its residual magnetism.
Residual magnetism is restored to the exciter by flashing the
field. Follow the applicable manufacturer’s instructions when
flashing the exciter field. If, after flashing the field, no voltage
is indicated, replace the alternator, since it is probably faulty.
Clean the alternator exterior with an approved fluid; smooth
a rough or pitted exciter commutator or slip ring with 000
sandpaper; then clean and polish with a clean, dry cloth. Check
the brushes periodically for length and general condition.
Consult the applicable manufacturer’s instructions on the
specific alternator to obtain information on the correct brushes.
Regulation of Generator Voltage
Efficient operation of electrical equipment in an airplane
depends on a constant voltage supply from the generator.
Among the factors, which determine the voltage output of a
generator, only one, the strength of the field current, can be
conveniently controlled. To illustrate this control, refer to the
diagram in Figure 12-327 , showing a simple generator with
a rheostat in the field circuit. If the rheostat is set to increase
the resistance in the field circuit, less current flows through
the field winding and the strength of the magnetic field in
which the armature rotates decreases. Consequently, the
voltage output of the generator decreases. If the resistance in
the field circuit is decreased with the rheostat, more current
flows through the field windings, the magnetic field becomes
stronger, and the generator produces a greater voltage.
Voltage Regulation with a Vibrating-Type Regulator
Refer to Figure 12-328 . With the generator running at
normal speed and switch K open, the field rheostat is
adjusted so that the terminal voltage is about 60 percent of
Stator Terminal block
Terminal block
Oil inlet cover
Rotor
Filter assemblyBall bearing
F
SNS
Stabilizing
field Radio
noise
filter
ThermistorExciter
fieldE1E2 T1T4T3
T6
T5
Main AC
generator
stator
windings Main AC
generator
field Rotor
Pilot exciter
armature
Exciter
armature
3 phase bridge rectifier Pilot exciter
field
Figure 12-324. A typical brushless alternator.
normal. Solenoid S is weak and contact B is held closed
by the spring. When K is closed, a short circuit is placed
across the field rheostat. This action causes the field current
to increase and the terminal voltage to rise.
When the terminal voltage rises above a certain critical value,
the solenoid downward pull exceeds the spring tension and
contact B opens, thus reinserting the field rheostat in the field
circuit and reducing the field current and terminal voltage.When the terminal voltage falls below a certain critical voltage,
the solenoid armature contact B is closed again by the spring,
the field rheostat is now shorted, and the terminal voltage starts
to rise. The cycle repeats with a rapid, continuous action. Thus,
an average voltage is maintained with or without load change.
The dashpot P provides smoother operation by acting as a
damper to prevent hunting. The capacitor C across contact B
Figure 12-325. Starter generator for small business jet.eliminates sparking. Added load causes the field rheostat to
be shorted for a longer period of time and, thus, the solenoid
armature vibrates more slowly. If the load is reduced and the
terminal voltage rises, the armature vibrates more rapidly and
the regulator holds the terminal voltage to a steady value for
any change in load, from no load to full load, on the generator.
Vibrating-type regulators cannot be used with generators,
which require a high-field current, since the contacts pit or
burn. Heavy-duty generator systems require a different type
of regulator, such as the carbon pile voltage regulator.
Three Unit Regulators
Many light aircraft employ a three unit regulator for their
generator systems. [Figure 12-329] This type of regulator
includes a current limiter and a reverse current cut-out in
addition to a voltage regulator.
The action of the voltage regulator unit is similar to the
vibrating-type regulator described earlier. The second of the
three units is a current regulator to limit the output current
of the generator. The third unit is a reverse current cut-out
that disconnects the battery from the generator. If the battery
is not disconnected, it discharges through the generator
armature when the generator voltage falls below that of the
battery, thus driving the generator as a motor. This action is
called “motoring” the generator and, unless it is prevented,
it discharges the battery in a short time.
The operation of a three unit regulator is described in the
following paragraphs. [Figure 12-330]
The action of vibrating contact C1 in the voltage regulator
unit causes an intermittent short circuit between points R1 and
L2. When the generator is not operating, spring S1 holds C1
closed; C2 is also closed by S2. The shunt field is connected
directly across the armature.
When the generator is started, its terminal voltage rises as
the generator comes up to speed, and the armature supplies
the field with current through closed contacts C2 and C1.As the terminal voltage rises, the current flow through L1
increases and the iron core becomes more strongly magnetized.
At a certain speed and voltage, when the magnetic attraction
on the movable arm becomes strong enough to overcome the
tension of spring S1, contact points C1 are separated. The field
current now flows through R1 and L2. Because resistance is
added to the field circuit, the field is momentarily weakened
and the rise in terminal voltage is checked. Also, since the
L2 winding is opposed to the L1 winding, the magnetic pull
of L1 against S1 is partially neutralized, and spring S1 closes
contact C1. Therefore, R1 and L2 are again shorted out of
the circuit, and the field current again increases; the output
voltage increases, and C1 is opened because of the action of
L1. The cycle is rapid and occurs many times per second. The
terminal voltage of the generator varies slightly, but rapidly,
above and below an average value determined by the tension
of spring S1, which may be adjusted.
The purpose of the vibrator-type current limiter is to limit the
output current of the generator automatically to its maximum
rated value in order to protect the generator. As shown in
Figure 12-330 , L3 is in series with the main line and load.
Thus, the amount of current flowing in the line determines
when C2 is opened and R2 placed in series with the generator
field. By contrast, the voltage regulator is actuated by line
voltage, whereas the current limiter is actuated by line current.
Spring S2 holds contact C2 closed until the current through
the main line and L3 exceeds a certain value, as determined
by the tension of spring S2, and causes C2 to be opened.
The increase in current is due to an increase in load. This
action inserts R2 into the field circuit of the generator and
decreases the field current and the generated voltage. When
the generated voltage is decreased, the generator current is
reduced. The core of L3 is partly demagnetized and the spring
closes the contact points. This causes the generator voltage
and current to rise until the current reaches a value sufficient
to start the cycle again. A certain minimum value of load
current is necessary to cause the current limiter to vibrate.
The purpose of the reverse current cut-out relay is to
automatically disconnect the battery from the generator when
the generator voltage is less than the battery voltage. If this
device were not used in the generator circuit, the battery
would discharge through the generator. This would tend
to make the generator operate as a motor, but because the
generator is coupled to the engine, it could not rotate such
a heavy load. Under this condition, the generator windings
may be severely damaged by excessive current.
There are two windings, L4 and L5, on the soft iron core. The
current winding, L4, consisting of a few turns of heavy wire,
is in series with the line and carries the entire line current.
Exciter
(if used)
Voltage controlStart-stop pushbutton
Reset buttonVM
5WV A W
AM
5WAlternator
Motor starting panel Alternator control panel
Input terminal Output terminalMotor
Terminal connections
Current flowLoadShunt field
RheostatB
+
– A
EFigure 12-326. AC motor generator set for ground testing.
Figure 12-327. Regulation of generator voltage by field rheostat.The voltage winding, L5, consisting of a large number of
turns of fine wire, is shunted across the generator terminals.
When the generator is not operating, the contacts, C3 are held
open by the spring S3. As the generator voltage builds up, L5
magnetizes the iron core. When the current (as a result of the
generated voltage) produces sufficient magnetism in the iron
core, contact C3 is closed, as shown. The battery then receives
a charging current. The coil spring, S3, is so adjusted that the
voltage winding does not close the contact points until the
voltage of the generator is in excess of the normal voltage of
the battery. The charging current passing through L4 aids the
current in L5 to hold the contacts tightly closed. Unlike C1
and C2, contact C3 does not vibrate. When the generator slows
down or, for any other cause, the generator voltage decreases
to a certain value below that of the battery, the current reverses
through L4 and the ampere turns of L4 oppose those of L5.
Thus, a momentary discharge current from the battery reduces
the magnetism of the core and C3 is opened, preventing the
battery from discharging into the generator and motoring it.
C3 does not close again until the generator terminal voltage
exceeds that of the battery by a predetermined value.Differential Relay Switch
Aircraft electrical systems normally use some type of reverse
current relay switch, which acts not only as a reverse current
relay cut-out but also serves as a remote control switch by
which the generator can be disconnected from the electrical
system at any time. One type of reverse current relay switch
operates on the voltage level of the generator, but the type
most commonly used on large aircraft is the differential
relay switch, which is controlled by the difference in voltage
12-163Load
PKC
B
S
Voltage
regulator
Armature
Shunt
field Storage
batteryCurrent
limiterReverse
current
cut-out
Loads
+
–+
–S3
L4L3L1
L2
C1C2C3
S1 S2 R1
R2L5AFigure 12-328. Vibrating-type voltage regulator.
Figure 12-329. Three unit regulator.
Figure 12-330. Three unit regulator for variable speed generators.between the battery bus and the generator.
The differential type relay switch connects the generator to
the main bus bar in the electrical system when the generator
voltage output exceeds the bus voltage by 0.35 to 0.65 volt.
It disconnects the generator when a nominal reverse current
flows from the bus to the generator. The differential relays on
all the generators of a multiengine aircraft do not close when
the electrical load is light. For example, in an aircraft having
a load of 50 amperes, only two or three relays may close.
If a heavy load is applied, the equalizing circuit lowers the
voltage of the generators already on the bus and, at the same
time, raise the voltage of the remaining generators, allowing
their relays to close. If the generators have been paralleled
properly, all the relays stay closed until the generator control
switch is turned off or until the engine speed falls below the
minimum needed to maintain generator output voltage.
The differential generator control relay shown in Figure 12-331
is made up of two relays and a coil-operated contactor. One
relay is the voltage relay and the other is the differential relay.
Both relays include permanent magnets that pivot between
the pole pieces of temporary magnets wound with relay coils.
V oltages of one polarity set up fields about the temporary
magnets with polarities that cause the permanent magnet to
move in the direction necessary to close the relay contacts;
voltages of the opposite polarity establish fields that cause
the relay contacts to open. The differential relay has two
coils wound on the same core. The coil-operated contactor,
called the main contactor, consists of movable contacts that
are operated by a coil with a movable iron core.
Closing the generator switch on the control panel connects
the generator output to the voltage relay coil. When generator
voltage reaches 22 volts, current flows through the coil
and closes the contacts of the voltage relay. This action
completes a circuit from the generator to the battery through
the differential coil.When the generator voltage exceeds the bus voltage by
0.35 volt, current flows through the differential coil, the
differential relay contact closes and, thus, completes the main
contractor coil circuit. The contacts of the main contactor
close and connect the generator to the bus.
When the generator voltage drops below the bus (or battery)
voltage, a reverse current weakens the magnetic field about
the temporary magnet of the differential relay. The weakened
field permits a spring to open the differential relay contacts,
breaking the circuit to the coil of the main contactor relay,
opening its contacts, and disconnecting the generator from
the bus. The generator battery circuit may also be broken
by opening the flight deck control switch, which opens the
contacts of the voltage relay, causing the differential relay
coil to be de-energized.
Overvoltage & Field Control Relays
Two other items used with generator control circuits are the
overvoltage control and the field control relay. As its name
implies, the overvoltage control protects the system when
12-164excessive voltage exists. The overvoltage relay is closed
when the generator output reaches 32 volts and completes a
circuit to the trip coil of the field control relay. The closing of
the field control relay trip circuit opens the shunt field circuit
and completes it through a resistor, causing generator voltage
to drop; also, the generator switch circuit and the equalizer
circuit (multiengine aircraft) are opened. An indicator light
circuit is completed, warning that an overvoltage condition
exists. A “reset” position of the flight deck switch is used
to complete a reset coil circuit in the field control relay,
returning the relay to its normal position.
Generator Control Units (GCU)
Basic Functions of a Generator Control Unit (GCU)
The generator control unit (GCU) is more commonly found
on turbine power aircraft. The most basic GCU perform a
number of functions related to the regulation, sensing, and
protection of the DC generation system. [Figure 12-332]
Voltage Regulation
The most basic of the GCU functions is that of voltage
regulation. Regulation of any kind requires the regulation unit
to take a sample of an output and to compare that sample with
a controlled reference. If the sample taken falls outside of the
limits set by the reference, then the regulation unit must provide
an adjustment to the unit generating the output so as to diminish
or increase the output levels. In the case of the GCU, the output
voltage from a generator is sensed by the GCU and compared
to a reference voltage. If there is any difference between the
two, the error is usually amplified and then sent back to the field
excitation control portion of the circuit. The field excitation
control then makes voltage⁄excitation adjustments in the field
winding of the generator in order to bring the output voltage
back into required bus tolerances.
Overvoltage Protection
Like the voltage regulation feature of the GCU, the
overvoltage protection system compares the sampled voltage
to reference voltage. The output of the overvoltage protection
circuit is used to open the relay that controls the output for the
field excitation. These types of faults can occur for a number
of reasons. The most common, however, is the failure of the
voltage regulation circuit in the GCU.
Parallel Generator Operations
The paralleling feature of the GCU allows for two or more
GCU/generator systems to work in a shared effort to provide
current to the aircraft electrical system. Comparing voltages
between the equalizer bus and the interpole/compensator
voltage, and amplifying the differences accomplishes the
control of this system. The difference is then sent to the
voltage regulation circuit, where adjustments are then made
in the regulation output. These adjustments continue until all of the busses are equalized in their load sharing.
Over-Excitation Protection
When a GCU in a paralleled system fails, a situation can
occur where one of the generators becomes overexcited and
tries to carry more than its share of the load, if not all of
the loads. When this condition is sensed on the equalizing
bus, the faulted generation control system shuts down
by receiving a de-excitation signal. This signal is then
transmitted to the overvoltage circuit, and then opens the
field excitation output circuit.
Differential Voltage
When the GCU allows the logic output to close the generator
line contactor, the generator voltage must be within a close
tolerance of the load bus. If the output is not within the
specified tolerance, then the contactor is not allowed to
connect the generator to the bus.
Reverse Current Sensing
If the generator is unable to maintain the required voltage
level, it eventually begins to draw current instead of providing
it. In this case, the faulty generator is seen as a load to the
other generators and will need to be removed from the
bus. Once the generator is off-line, it is not permitted to be
reconnected to the bus until such time that the generator faults
are cleared and the generator is capable of providing a current
to the bus. In most cases, the differential voltage circuit and
the reverse current sensing circuit are one in the same.
Alternator Constant Speed Drive System
Alternators are not always connected directly to the airplane
engine like DC generators. Since the various electrical
devices operating on AC supplied by alternators are designed
to operate at a certain voltage and at a specified frequency,
the speed of the alternators must be constant; however, the
speed of an airplane engine varies. Therefore, the engine,
through a constant speed drive installed between the engine
and the alternator, drives some alternators.
A typical hydraulic-type drive is shown in Figure 12-333 .
The following discussion of a constant speed drive system
is based on such a drive found on large multiengine aircraft.
The constant speed drive is a hydraulic transmission that may
be controlled either electrically or mechanically.
The constant speed drive assembly is designed to deliver an
output of 6,000 rpm, provided the input remains between
2,800 and 9,000 rpm. If the input, which is determined by
engine speed, is below 6,000 rpm, the drive increases the
speed in order to furnish the desired output. This stepping
up of speed is known as overdrive.
12-165Gen. Batt.
N
S N
NS
S
SWNReverse current coilMain contractor
Permanent magnet
Temporary magnetDifferential coilPivotPivot
Differential relay contacts Voltage relay contacts
Temporary magnetVoltage relay coil
Permanent magnet
Figure 12-331. Differential generator control relay.
In overdrive, an automobile engine operates at about the
same rpm at 60 mph as it does in conventional drive at 49
mph. In aircraft, this principle is applied in the same manner.
The constant speed drive enables the alternator to produce
the same frequency at slightly above engine idle rpm as it
would at takeoff or cruising rpm.
With the input speed to the drive set at 6,000 rpm, the output
speed is the same. This is known as straight drive and might
be compared to an automobile in high gear. However, when
the input speed is greater than 6,000 rpm, it must be reduced
to provide an output of 6,000 rpm. This is called underdrive,
which is comparable to an automobile in low gear. Thus, the
large input, caused by high engine rpm, is reduced to give
the desired alternator speed.
As a result of this control by the constant speed drive, the
frequency output of the generator varies from 420 cps at no
load to 400 cps under full load. This, in brief, is the function
of the constant speed drive assembly. Before discussing
the various units and circuits, the overall operation of the
transmission should be discussed as follows.
Hydraulic Transmission
The transmission is mounted between the generator and the
aircraft engine. Its name denotes that hydraulic oil is used,
although some transmissions may use engine oil. Refer to the
cutaway view of such a transmission in Figure 12-334. The
input shaft D is driven from the drive shaft on the accessory
section of the engine. The output drive F, on the opposite end
of the transmission, engages the drive shaft of the generator.
The input shaft is geared to the rotating cylinder block gear, which it drives, as well as to the makeup and scavenger
gear pumps E.
The makeup (charge) pump delivers oil (300 psi) to the pump
and motor cylinder block, to the governor system, and to the
pressurized case, whereas the scavenger pump returns the oil
to the external reservoir.
The rotating cylinder assembly B consists of the pump and
motor cylinder blocks, which are bolted to opposite sides of
a port plate. The two other major parts are the motor wobbler
A and the pump wobbler C. The governor system is the unit
at the top of the left side in Figure 12-334 .
The cylinder assembly has two primary units. The block
assembly of one of the units, the pump, contains 14 cylinders,
each of which has a piston and pushrod. Charge pressure from
the makeup pump is applied to each piston in order to force
it outward against the pushrod. It, in turn, is pushed against
the pump wobble plate.
If the plate remained as shown in Figure 12-335A , each of the
14 cylinders would have equal pressure, and all pistons would
be in the same relative position in their respective cylinders.
But with the plate tilted, the top portion moves outward and
the lower portion inward. [Figure 12-335B] As a result, more
oil enters the interior of the upper cylinder, but oil is forced
from the cylinder of the bottom piston.
If the pump block were rotated while the plate remained
stationary, the top piston would be forced inward because of
the angle of the plate. This action would cause the oil confined
within the cylinder to be subjected to increased pressure great
Figure 12-332. Generator Control Unit (GCU).
enough to force it into the motor cylinder block assembly.
Before explaining what the high-pressure oil in the motor
unit does, it is necessary to know something about this part
of the rotating cylinder block assembly. The motor block
assembly has 16 cylinders, each with its piston and pushrod.
These are constantly receiving charge pressure of 300 psi.
The position of the piston depends upon the point at which
each pushrod touches the motor wobble plate. These rods
cause the wobble plate to rotate by the pressure they exert
against its sloping surface.
The piston and pushrod of the motor are pushed outward as
oil is forced through the motor valve plate from the pump
cylinder. The pushrods are forced against the motor wobble
plate, which is free to rotate but cannot change the angle at
which it is set. Since the pushrods cannot move sideways,
the pressure exerted against the motor wobble plate’s sloping
face causes it to rotate.
In the actual transmission, there is an adjustable wobble plate.
The control cylinder assembly determines the tilt of the pump
wobble plate. For example, it is set at an angle, which causes
the motor cylinders to turn the motor wobble plate faster than
the motor assembly if the transmission is in overdrive. The greater pressure in the pump and motor cylinders produces
the result described.
With the transmission in underdrive, the angle is arranged so
there is a reduction in pumping action. The subsequent slippage
between the pushrods and motor wobble plate reduces the
output speed of the transmission. When the pump wobble plate
is not at an angle, the pumping action is at a minimum and
the transmission has what is known as hydraulic lock. For this
condition, the input and output speed is about the same, and
the transmission is considered to be in straight drive.
To prevent the oil temperature from becoming excessively
high within the cylinder block, the makeup pressure pump
forces oil through the center of this block and the pressure
relief valve. From this valve, the oil flows into the bottom
of the transmission case. A scavenger pump removes the
oil from the transmission case and circulates it through the
oil cooler and filters before returning it to the reservoir. At
the start of the cycle, oil is drawn from the reservoir, passed
through a filter, and forced into the cylinder block by the
makeup pressure pump.
The clutch, located in the output gear and clutch assembly,
is an overrunning one way, sprag-type device. Its purpose
is to ratchet if the alternator becomes motorized; otherwise,
the alternator might turn the engine. Furthermore, the clutch
provides a positive connection when the transmission is
driving the alternator.
There is another unit of the drive that must be discussed—the
governor system. The governor system, which consists of a
hydraulic cylinder with a piston, is electrically controlled. Its
duty is to regulate oil pressure flowing to the control cylinder
assembly. [Figure 12-336]
The center of the system’s hydraulic cylinder is slotted so the
arm of the pump wobble plate can be connected to the piston.
As oil pressure moves the piston, the pump wobble plate is
placed in either overspeed, underspeed, or straight drive.
Figure 12-337 shows the electrical circuit used to
govern the speed of the transmission. First, the main
points of the complete electrical control circuit are
discussed. [Figures 12-337 and 12-338] For simplification,
two portions, the overspeed circuit and the load division
circuit, are considered as individual circuits.
Note, in Figure 12-337 , that the circuit has a valve and
solenoid assembly (O) and a control cylinder (E), and that
it contains such units as the tachometer generator (D), the
rectifier (C), and adjustable resistor (B), rheostat (A), and
the control coil (Q).
Figure 12-333. Constant speed drive.
Since it is driven by a drive gear in the transmission, the
tachometer (often called tach) generator, a three-phase unit,
has a voltage proportional to the speed of the output drive.
The rectifier changes its voltage from AC to DC. After
rectification, the current flows through the resistor, rheostat,
and valve and solenoid. Each of these units is connected in
series. [Figure 12-338]
Under normal operating conditions, the output of the tach
generator causes just enough current to enter the valve and
solenoid coil to set up a magnetic field of sufficient strength
to balance the spring force in the valve. When the alternator
speed increases as the result of a decrease in load, the tach
generator output also increases. Because of the greater output,
the coil in the solenoid is sufficiently strengthened to overcome
the spring force. Thus, the valve moves and, as a result, oil
pressure enters the reduced speed side of the control cylinder.
In turn, the pressure moves the piston, causing the angle of
the pump wobble plate to be reduced. The oil on the other
side of the piston is forced back through the valve into the
system return. Since the angle of the pump wobble plate is
smaller, there is less pumping action in the transmission. The
result is decreased output speed. To complete the cycle, the procedure is reversed.
With the output speed reduction, tach generator output
decreases; consequently, the flow of current to the solenoid
diminishes. Therefore, the magnetic field of the solenoid
becomes so weak that the spring is able to overcome it and
reposition the valve.
If a heavy load is put on the AC generator, its speed decreases.
The generator is not driven directly by the engine; the
hydraulic drive allows slippage. This decrease causes the
output of the tach generator to taper off and, as a result,
weakens the magnetic field of the solenoid coil. The spring
in the solenoid moves the valve and allows oil pressure to
enter the increase side of the control cylinder and the output
speed of the transmission is raised.
There are still two important circuits that must be discussed:
the overspeed circuit and the load division circuit. The
generator is prevented from overspeeding by a centrifugal
switch (S) in Figure 12-339 and the overspeed solenoid coil
(R), which is located in the solenoid and valve assembly.
The centrifugal switch is on the transmission and is driven
through the same gear arrangement as the tach generator.
A B
EC
DF
Figure 12-334. Cutaway of a hydraulic transmission.
The aircraft DC system furnishes the power to operate the
overspeed coil in the solenoid and coil assembly. If the
output speed of the transmission reaches a speed of 7,000
to 7,500 rpm, the centrifugal switch closes the DC circuit
and energizes the overspeed solenoid. This component then
moves the valve and engages the latch that holds the valve
in the underdrive position. To release the latch, energize the
underdrive release solenoid.
The load division circuit’s function is to equalize the loads
placed on each of the alternators, which is necessary to
assure that each alternator assumes its share; otherwise,
one alternator might be overloaded while another would be
carrying only a small load.
In Figure 12-340 , one phase of the alternator provides power
for the primary in transformer (G), whose secondary supplies
power to the primaries of two other transformers (J 1 and J2). Rectifiers (K) then change the output of the transformer
secondaries from AC to DC.
The function of the two capacitors (L) is to smooth out the
DC pulsations.
The output of the current transformer (F) depends upon the
amount of current flowing in the line of one phase. In this
way, it measures the real load of the generator. The output
voltage of the current transformer is applied across resistor
(H). This voltage is added vectorially to the voltage applied
to the upper winding of transformer (J) by the output of
transformer (F). At the same time as it adds vectorially to
the upper winding of transformer (J), it subtracts vectorially
from the voltage applied to the lower winding of (J).
This voltage addition and subtraction depends on the real load
of the generator. The amount of real load determines the phase
12-169A B
Figure 12-335. Wobble plate position.
angle and the amount of voltage impressed across resistor (H).
The greater the real load, the greater the voltage across (H), and
hence, the greater the difference between the voltages applied
to the two primaries of transformer (J). The unequal voltages
applied to resistor (M) by the secondaries of transformer (J)
cause a current flow through the control coil (P).
The control coil is wound so that its voltage supplements
the voltage for the control coil in the valve and solenoid
assembly. The resulting increased voltage moves the valve
and slows down the generator’s speed. Why should the speed
be decreased if the load has been increased? Actually, systems
using only one generator would not have decreased speed,
but for those having two or more generators, a decrease is
necessary to equalize the loads.
The load division circuit is employed only when two or more
generators supply power. In such systems, the control coils
are connected in parallel. If the source voltage for one of these
becomes higher than the others, it determines the direction of
current flow throughout the entire load division circuit. As
explained before, the real load on the generator determines
the amount of voltage on the control coil; therefore, the
generator with the highest real load has the highest voltage.
As shown in Figure 12-341 , current through No. 1 control
coil, where the largest load exists, aids the control coil of the
valve and solenoid, thereby slowing down the generator. (The
source voltage of the control coils is represented by battery
symbols in Figure 12-341 .) The current in the remaining
control coils opposes the control coil of the valve and
solenoid, in order to increase the speed of the other generators so the load is more evenly distributed.
On some drives, instead of an electrically-controlled
governor, a flyweight-type governor is employed, which
consists of a recess-type revolving valve driven by the
output shaft of the drive, flyweights, two coil springs, and
a nonrotating valve stem. Centrifugal force, acting on the
governor flyweights, causes them to move outward, lifting
the valve stem against the opposition of a coil spring.
The valve stem position controls the directing of oil to the
two oil outlines. If the output speed tends to exceed 6,000
rpm, the flyweights lift the valve stem to direct more oil to
the side of the control piston, causing the piston to move in a
direction to reduce the pump wobble plate angle. If the speed
drops below 6,000 rpm, oil is directed to the control piston
so that it moves to increase the wobble plate angle.
Overspeed protection is installed in the governor. The drive
starts in the underdrive position. The governor coil springs
are fully extended and the valve stem is held at the limit of
its downward travel. In this condition, pressure is directed
to the side of the control piston giving minimum wobble
plate angle. The maximum angle side of the control piston
is open to the hollow stem. As the input speed increases, the
flyweights start to move outward to overcome the spring bias.
This action lifts the valve stem and starts directing oil to the
maximum side of the control piston, while the minimum side
is opened to the hollow stem.
At about 6,000 rpm, the stem is positioned to stop drainage
of either side, and the two pressures seek a balance point as
12-170Generator
EDCBA
OPQRLatch
Oil out
Oil inConstant
speed drive
Figure 12-337. Electrical hydraulic control circuit.
Figure 12-336. Control cylinder.
the flyweight force is balanced against the spring bias. Thus,
a mechanical failure in the governor causes an underdrive
condition. The flyweight’s force is always tending to move
the valve stem to the decrease speed position so that, if the coil spring breaks and the stem moves to the extreme position
in that direction, output speed is reduced. If the input to the
governor fails, the spring forces the stem all the way to the
start position to obtain minimum output speed.
12-171Generator
To loadFE
GHDCBA
S
J
JKK
N
N
OPQR
LLLatch
Oil out
Oil inConstant
speed drive
Figure 12-338. Speed control circuit.
An adjustment screw on the end of the governor regulates
the output speed of the constant speed drive. This adjustment
increases or decreases the compression of a coil spring,
opposing the action of the flyweights. The adjustment
screws turn in an indented collar, which provides a means of
making speed adjustments in known increments. Each “click”
provides a small change in generator frequency.
The constant speed drive (CSD) can be an independent unit
or mounted within the alternator housing. When the CSD and
the alternator are contained within one unit, the assembly is
known as an integrated drive generator (IDG).
Voltage Regulation of Alternators
The problem of voltage regulation in an AC system does not
differ basically from that in a DC system. In each case, the
function of the regulator system is to control voltage, maintain
a balance of circulating current throughout the system, and
eliminate sudden changes in voltage (anti-hunting) when a
load is applied to the system. However, there is one important
difference between the regulator system of DC generators
and alternators operated in a parallel configuration. The load
carried by any particular DC generator in either a two or four
generator system depends on its voltage as compared with the bus voltage, while the division of load between alternators
depends upon the adjustments of their speed governors,
which are controlled by the frequency and droop circuits
discussed in the previous section on alternator constant-speed
drive systems.
When AC generators are operated in parallel, frequency and
voltage must both be equal. Where a synchronizing force is
required to equalize only the voltage between DC generators,
synchronizing forces are required to equalize both voltage and
speed (frequency) between AC generators. On a comparative
basis, the synchronizing forces for AC generators are much
greater than for DC generators. When AC generators are of
sufficient size and are operating at unequal frequencies and
terminal voltages, serious damage may result if they are
suddenly connected to each other through a common bus. To
avoid this, the generators must be synchronized as closely as
possible before connecting them together.
Regulating the voltage output of a DC exciter, which
supplies current to the alternator rotor field, best controls
the output voltage of an alternator. This is accomplished
by the regulation of a 28-volt system connected in the field
circuit of the exciter. A regulator controls the exciter field
current and thus regulates the exciter output voltage applied
12-172OPQRSLatch
Oil out
Oil in
Figure 12-339. Overspeed circuit.
to the alternator field.
Alternator Transistorized Regulators
Many aircraft alternator systems use a transistorized voltage
regulator to control the alternator output. Before studying
this section, a review of transistor principles may be helpful.
A transistorized voltage regulator consists mainly of
transistors, diodes, resistors, capacitors, and, usually, a
thermistor. In operation, current flows through a diode
and transistor path to the generator field. When the proper
voltage level is reached, the regulating components cause the transistor to cut off conduction to control the
alternator field strength. The regulator operating range is
usually adjustable through a narrow range. The thermistor
provides temperature compensation for the circuitry. The
transistorized voltage regulator shown in Figure 12-342
will be referred to in explaining the operation of this type
of regulator.
The AC output of the generator is fed to the voltage regulator,
where it is compared to a reference voltage, and the difference
is applied to the control amplifier section of the regulator. If
the output is too low, field strength of the AC exciter generator
is increased by the circuitry in the regulator. If the output is
too high, the field strength is reduced.
The power supply for the bridge circuit is CR1, which
provides full-wave rectification of the three phase output
from transformer T1. The DC output voltages of CR1 are
proportional to the average phase voltages. Power is supplied
from the negative end of the power supply through point B,
R2, point C, zener diode (CR5), point D, and to the parallel
hookup of V1 and R1. Takeoff point C of the bridge is located
between resistor R2 and the zener diode. In the other leg of
the reference bridge, resistors R9, R7, and the temperature
compensating resistor RT1 are connected in series with V1
and R1 through points B, A, and D. The output of this leg of
the bridge is at the wiper arm of R7.
As generator voltage changes occur, for example, if the
voltage lowers, the voltage across R1 and V1 (once V2
starts conducting) remains constant. The total voltage change
occurs across the bridge circuit. Since the voltage across the
zener diode remains constant (once it starts conducting), the
total voltage change occurring in that leg of the bridge is
across resistor R2. In the other leg of the bridge, the voltage
change across the resistors is proportional to their resistance
values. Therefore, the voltage change across R2 is greater
than the voltage change across R9 to wiper arm of R7. If
the generator output voltage drops, point C is negative with
respect to the wiper arm of R7. Conversely, if the generator
voltage output increases, the polarity of the voltage between
the two points is reversed.
The bridge output, taken between points C and A, is
connected between the emitter and the base of transistor
Q1. With the generator output voltage low, the voltage from
the bridge is negative to the emitter and positive to the base.
This is a forward bias signal to the transistor, and the emitter
to collector current therefore increases. With the increase of
current, the voltage across emitter resistor R11 increases.
This, in turn, applies a positive signal to the base of transistor
12-173Generator
To LoadFE
GHJ1
J2KK
M
N
OPQR
LLLatch
Oil out
Oil inConstant
speed drive
Figure 12-340. Droop circuit.
Figure 12-341. Relative direction of current in droop coil circuit
with unequal loads.
Q4, increasing its emitter to collector current and increasing
the voltage drop across the emitter resistor R10.
This gives a positive bias to the base of Q2, which increases
its emitter to collector current and increase the voltage drop
across its emitter resistor R4. This positive signal controls
output transistor Q3. The positive signal on the base of Q3
increases the emitter to collector current.
The control field of the exciter generator is in the collector
circuit. Increasing the output of the exciter generator
increases the field strength of the AC generator, which
increases the generator output.
To prevent exciting the generator when the frequency is at a
low value, there is an underspeed switch located near the F+
terminal. When the generator reaches a suitable operating
frequency, the switch closes and allows the generator to be excited.
Another item of interest is the line containing resistors R27,
R28, and R29 in series with the normally closed contacts
of the K1 relay. The operating coil of this relay is found in
the lower left part of the schematic. Relay K1 is connected
across the power supply (CR4) for the transistor amplifier.
When the generator is started, electrical energy is supplied
from the 28-volt DC bus to the exciter generator field to
“flash the field” for initial excitation. When the field of the
exciter generator has been energized, the AC generator starts
to produce, and as it builds up, relay K1 is energized, opening
the “field flash” circuit.
12-174R11K1T1T2 T3
T1
Y-R10 R4R6R29
R28
R27
K1R9R7R1
RT1
A
AFF+
A–A+N
Switch
underspeed+28V
Special bus
AC gen.
exciter
Gen. neg. busR
GC
BDV1
CR5
R2
R3 R8
Q1 Q4 Q2 Q3
CR3CR2CR4CR1
CR4
Figure 12-342. Transistorized voltage regulator.
Mechanic Privileges & Limitations
Chapter 13
Introduction
Since Title 14 of the Code of Federal Regulations (14 CFR)
part 65 was covered only briefly in Chapter 2, Regulations,
Maintenance Forms, Records, and Publications, it is discussed
in greater detail in this chapter. This chapter discusses the
Federal Aviation Administration (FAA) regulation governing
the certification of airmen other than flight crew members.
This chapter is based on the material contained in 14 CFR
part 65, which has the following subparts:
• Subpart A—General
• Subpart B—Air Traffic Control Tower Operators
• Subpart C—Aircraft Dispatchers
• Subpart D—Mechanics
• Subpart E—Repairmen
• Subpart F—Parachute Riggers
This chapter only focuses on the certification of maintenance
technicians and, therefore, subparts B, C, E, and F are
not addressed.
The FAA certificates two separate categories of maintenance
technicians: mechanic and repairman. The fundamental
difference between these two is that the mechanic certificate
is transportable, is issued to the technician based upon
their training and knowledge, and is not dependent on the
technician’s location. Although the repairman certificate is
also based upon the training and knowledge of the technician,
it is specifically issued to that technician while they are
employed at a distinct location of a specific company. This
certificate carries a literal address where the technician
is authorized to work using their repairman skills. When
the technician is no longer employed there, the repairman
certificate must be returned to the Flight Standards District
Office (FSDO) that issued it.
Mechanic Certification: Subpart A—General
(by 14 CFR Section)
Section 65.3, Certification of Foreign Airmen Other
Than Flight Crewmembers
Normally, the FAA issues these certificates only to United
States (U.S.) citizens or resident aliens residing in the United
States. However, if the FAA determines that the issuance of a certificate to a person located outside of the United States
is necessary for the operation and continued airworthiness
of a U.S.-registered civil aircraft, it will issue a certificate to
that person, providing they meet the necessary requirements.
Section 65.11, Application and Issue
Any person who meets the criteria for obtaining a mechanic
certificate must apply by means of FAA Form 8610-2, Airman
Certificate and/or Rating Application. If a mechanic has had
a certificate suspended, they may not apply for additional
ratings during the time of suspension. A revocation of a
mechanic certificate prevents that person from applying for
a certificate within a period of 1 year after the revocation.
Section 65.12, Offenses Involving Alcohol and
Drugs
Any person, who has been convicted of violating federal or
state statutes relating to drug offenses, can be denied their
application for a certificate or rating up to 1 year after the date
of conviction. The violation can be relating to any one or more
of the following actions: growing, processing, manufacturing,
selling, disposing, possessing, transporting, or importing
narcotic drugs, marijuana, depressants, or stimulants. They
may also face the suspension or revocation of any certificate
that they currently hold.
Section 65.13, Temporary Certificate
A qualified applicant who successfully passes all required
tests with a minimum score of 70 percent may be issued a
temporary certificate, which is valid for not more than 120
days. During this time, the FAA will review the application
and any supplementary documentation and will issue the
official certificate and rating.
Section 65.14, Security Disqualification
This section was added following the terrorist attacks of
September 11, 2001. It basically states that anyone determined
by the Transportation Security Administration (TSA) to be a
security threat will either have their application held if they
are applying for a certificate, or have the certificate that they
do hold revoked.
Section 65.15, Duration of Certificates
Mechanic’s certificates are effective until they are surrendered,
suspended, or revoked. The difference in these terms can be
summarized in the following manner:
• Surrendered means given up voluntarily.
• Suspended means the FAA temporarily removes the
certificate from the holder.
• Revoked means the FAA permanently removes the
certificate from the holder.
Section 65.16, Change of Name: Replacement of
Lost or Destroyed Certificate
An application for a change of name on a certificate issued
under this part must be accompanied by the applicant's
current certificate and the marriage license, court order, or
other document verifying the change.
If the technician changes their name, or is seeking a
replacement certificate, an application must be submitted to
the FAA at the following address:
Federal Aviation Administration
Airmen Certification Branch (AFB-720)
P.O. Box 25082
Oklahoma City, OK 73125
It should be noted that there is a nominal charge for this service.
Section 65.17, Test: General Procedure
The FAA has designated certain persons to administer
tests associated with obtaining a mechanic certificate. The
minimum passing score for these tests is 70 percent.
Section 65.18, Written Tests: Cheating or Other
Unauthorized Content
If the mechanic or repairmen applicant is determined to be
cheating, or otherwise involved in unauthorized conduct, they
are not eligible for any certificate or rating under this chapter
for a period of 1 year. Furthermore, current ratings the person
already holds may also be suspended or revoked. Examples
of unacceptable conduct for written tests are:
• Copying or intentionally removing the test.
• Giving or receiving any part of a copy of the test.
• Giving or receiving help during the test taking period.
• Take any part of the test on behalf of another person.
• Using any material or aid during the test taking period
that is not provided by authorized test administrators.
• Intentionally causing, assisting, or participating in any
of the previous acts.
Section 65.19, Retesting After Failure
Should the mechanic or repairman fail to achieve the required
minimum passing grade, there are two options they may consider when desiring to apply for retesting:
• Wait a period of 30 days after the date of test failure
and then take the test again.
• Seek additional instruction in the subject matter
areas failed and provide a signed statement from the
certificated technician providing the instruction stating
the applicant has received necessary instruction and
is ready for testing.
Section 65.20, Applications, Certificates, Logbooks,
Reports, and Records: Falsification, Reproduction,
or Alteration
14 CFR part 43, sections 43.9 and 43.11 define the
requirements for a technician to make appropriate entries
in the maintenance/inspection records for the work
performed. This proper documentation is fundamental to
safe and efficient operation of the U.S. civil aircraft fleet.
Therefore, the FAA takes strong action against those who
would participate in the falsification of those records. The
following actions are the basis for suspending or revoking
any certificate or rating held by the person who:
• Makes fraudulent or intentionally false statement on
an application.
• Makes fraudulent or intentionally false statement
in any logbook, record, or report required to show
compliance with any certificate requirements.
• Reproduces a certificate or rating for fraudulent
purposes.
• Alters any certificate or rating under this part.
Section 65.21, Change of Address
If the technician changes their address, the FAA (at the
address shown below) must be notified in writing within 30
days after the change of permanent residence:
Federal Aviation Administration
Airmen Certification Branch (AFB-720)
P.O. Box 25082
Oklahoma City, OK 73125
Refusal to Submit to a Drug or Alcohol Test
Any technician who refuses to submit to a drug test, which
is required by 14 CFR part 120, section 120.15, is subject
to denial by the FAA of any application for additional
certification or ratings, as well as suspension or revocation
of any existing certificate or rating they currently hold. Part
120, section 120.117, Implementing a Drug Testing Program,
requires a urine sample from the employee. Part 120, section
120.37, Misuse of Alcohol, requires that the employee
submit to a breath test. Each section contains a “Definitions”
section and a section titled “Employees who must be tested.”
Persons involved with “Aircraft maintenance or preventative
maintenance duties” are listed in both sections. There are
various types (or rather times) when testing is required:
• Pre-employment
• Periodic
• Random
• Post-accident
• Testing based upon reasonable cause
• Return to duty testing
• Follow-up testing
The numerous test methods and the harsh penalty imposed
by the FAA on those who involve themselves with these
unauthorized substances or abuse the allowable use of
alcohol indicates the concern that the FAA has for the
possible impairment of technicians. Aviation maintenance
is a professional career choice that demands the highest
caliber technical person to be capable of functioning at their
maximum potential. There is no room in this profession
for a person to be involved with substance abuse. By
doing so, the technician not only endangers themselves,
but their co-workers, and ultimately the customer who is
expecting to have an airworthy aircraft delivered following
a maintenance activity.
Mechanic Certification: Subpart D—Mechanics
(by 14 CFR Section)
Section 65.71, Eligibility Requirements: General
The requirements for obtaining a mechanic certificate are:
• Be at least 18 years of age.
• Be able to read, write, speak, and understand the
English language. ( Note: If the applicant does not
meet this requirement and is employed outside the
United States by a U.S. carrier, the certificate will be
endorsed “valid only outside the United States.”)
• Have passed all the required tests (written, oral,
and practical) within the preceding 24 months from
application.
• Possess and demonstrate the appropriate knowledge
and skill for the certificate rating being sought.
If a technician has one of the ratings and desires to add the
other, they must meet the requirements set forth in section
65.77, and take the written, oral, and practical tests within
24 months.
Section 65.73, Ratings
The FAA recognizes two ratings: airframe and powerplant.
These may be attained by a person upon successful application and testing either individually or as a combined certificate.
Any person holding an aircraft (A) or aircraft engine (E)
certificate prior to June 15, 1952, and which was valid on
that date, may exchange it for the corresponding current
certificate. If both ratings were held, the A & E certificate
may be exchanged for an Airframe and Powerplant (A&P).
Section 65.75, Knowledge Requirements
Any applicant meeting the experience requirements listed
in section 65.77 must pass a written test (minimum passing
score of 70% as described in section 65.17) covering the
construction and maintenance of aircraft. There are three
separate tests that the applicant for the A&P certificate must
pass: General (60 questions), Airframe (100 questions), and
Powerplant (100 questions). Applicable portions of 14 CFR
43 and 91 are also included in the testing. Basic principles for
the installation and maintenance of propellers are included
with the testing that is administered for the powerplant
rating. Successful completion of the written test is required
before the candidate may apply for the oral and practical
tests identified in section 65.79.
Section 65.77, Experience Requirements
Each mechanic applicant must have a certificate of completion
from a certificated aviation maintenance technician school
(AMTS) (14 CFR part 147) or provide documented evidence
of a minimum of 18 months practical experience related
to either airframe or powerplant maintenance (30 months
required if applying for certification for both airframe and
powerplant).
Section 65.79, Skill Requirements
Oral and practical tests to determine the applicant’s basic
knowledge and skills necessary for the certificate or rating
sought are required to be completed after the applicant has
successfully completed the written test. The practical test
additionally requires minor repairs and minor alterations to
propellers to be demonstrated as part of the powerplant rating.
To assist the applicant, the Aviation Mechanic Practical
Test Standards (PTS) have been published by the FAA to
provide standards for testing in which the applicant for the
A&P certificate should be familiar. The Aviation Mechanic
PTS include the subject areas of knowledge and skill for
the issuance of an aviation mechanic certificate and/or the
addition of a rating. The subject areas are the topics in which
aviation mechanic applicants must have knowledge and/or
demonstrate skill. The PTSs are available on the FAA website
at www.faa.gov.
Section 65.80, Certificated Aviation Maintenance
Technician School Students
Whenever satisfactory evidence is shown to the FAA that a
student enrolled in an aviation maintenance technician school
(certificated under part 147) is making satisfactory progress,
that student may take the oral and practical tests required
by section 65.79, prior to completing the school’s approved
curriculum (as required by section 65.77) and prior to taking
the written test required by section 65.75.
Section 65.81, General Privileges and Limitations
Once a technician becomes a certificated mechanic, they
may perform or supervise the maintenance, preventive
maintenance, or alterations of an aircraft or appliance (or
part thereof) for which they are rated. However, they are
not permitted to perform major repair or major alterations
to propellers nor accomplish any repair to or alteration of
instruments. These activities are reserved for certificated
repairmen at an authorized repair station. Also, they may
not supervise the maintenance, preventive maintenance,
or alteration of any aircraft or appliance (or part thereof)
for which they are rated, unless they have satisfactorily
performed this work at an earlier date. This is where the
benefit of keeping an on the job training (OJT) log cannot be
overemphasized. Whether the technician attends a part 147
maintenance training school or receives the required number
of months as practical experience, they have only scratched
the surface of the tremendously complex world of aviation
maintenance. The technician must either work with someone
(like a shop mentor) or must perform the task satisfactorily
for the FAA. The certified mechanic must have and be able to
comprehend the maintenance manuals and/or instructions for
continued airworthiness for the task they are accomplishing.
Section 65.83, Recent Experience Requirements
In addition to having the proper documentation, the mechanic
is required by this regulation to have recent and relevant
work experience. Although, as it was stated earlier in this
chapter, the A&P certificate is valid until it is surrendered,
suspended, or revoked, it may not be exercised if the holder
has not been actively working as a mechanic for at least 6 of
the preceding 24 months.
This activity can be any one or a combination of the
following:
• Served as a mechanic under the certificate and rating
• Technically supervised other mechanics
• Supervised (in an executive capacity) the maintenance
or alteration of an aircraftSection 65.85, Airframe Rating: Additional Privileges
A mechanic who holds an airframe rating may approve and
return to service an airframe, an appliance, or any related
part after they have performed, supervised, or inspected
minor repairs or alterations. They may also perform the
maintenance actions required for a major repair or alteration,
and should initiate the appropriate form (FAA Form 337,
Major Repair and Alteration) associated with that work.
However, the return to service action must be accomplished
by a certificated A&P technician holding an Inspection
Authorization (IA). (Refer to 14 CFR section 65.95.) The
airframe mechanic is also authorized to perform the 100-hour
inspection (if required per 14 CFR part 91 section 91.409)
on the airframe.
A certificated mechanic with an airframe rating can approve
and return to service the airframe of an aircraft with a special
airworthiness certificate, in the light-sport category (refer
to 14 CFR part 21, section 21.190) after performing and
inspecting a major repair or major alteration. The work must
have been done on products that are not produced under FAA
approval (i.e., are not type certificated) and must have been
performed in accordance with instructions developed by the
manufacturer or person acceptable to the FAA.
Section 65.87, Powerplant Rating: Additional
Privileges
Similarly, a mechanic holding a powerplant rating has the
same limitations imposed regarding the powerplant and
propeller as the airframe technician has on the airframe
rating. They may perform and return to service minor repairs
or alterations. They may also accomplish the work activities
required for a major repair or alteration, but the work must
be signed off for return to service by an IA. The privilege of
performing a 100-hour inspection (if required by 14 CFR part
91) on a powerplant or propeller is also authorized.
A certificated mechanic with a powerplant rating can
approve and return to service the powerplant or propeller
of an aircraft with a special airworthiness certificate, in
the light-sport category (refer to 14 CFR part 21, section
21.190) after performing and inspecting a major repair or
major alteration. The work must have been done on products
that are not produced under FAA approval (i.e., are not type
certificated) and must have been performed in accordance
with instructions developed by the manufacturer or person
acceptable to the FAA.
Section 65.89, Display of Certificate
Once a technician receives their mechanic certificate, the
certificate must be kept in the immediate area where they
normally conduct work and exercises the privileges of the
certificate. When requested, the technician is required to
present the certificate for inspection to the FAA, or any
authorized representation from the National Transportation
Safety Board (NTSB), or any federal, state, or local law
enforcement officer.
Inspection Authorization (IA) (by 14 CFR
Section)
Section 65.91, Inspection Authorization
An A&P mechanic who has held their certificate for at least
3 years, and has been active for the last 2 years, may submit
application using FAA Form 8610-1, Mechanic’s Application
for Inspection Authorization, to the FAA for consideration
as an IA. In addition to the preceding time requirements, the
IA candidate must have:
• A fixed base of operation where they can be located
in person or by phone during a normal working week
but it need not be the place where they will exercise
their inspection authority.
• Available equipment, facilities, and inspection
data necessary to properly inspect the airframe,
powerplants, propellers, or any related part or
appliance they are approving for return to service.
The applicant who meets all the above criteria must then
pass a written test (or computerized version of the test) to
determine their ability to inspect the airworthiness of an
aircraft following either a major repair or alteration action
or the performance of an annual or progressive inspection.
The minimum passing score for the computer test is 70
percent. If the applicant fails the test, retesting cannot be
attempted until a minimum of 90 days have elapsed from the
failure date. Unlike the A&P test, there is no reduction in this
time if the applicant receives additional training.
Section 65.92, Inspection Authorization: Duration
An IA certificate expires on March 31 of each odd-numbered
year, but may only be exercised during the time the technician
holds a currently effective mechanic certificate. The IA ceases
to be effective if:
• The technician surrenders it, or it is suspended or
revoked.
• The technician no longer has a fixed base of operations.
• The technician no longer has the required facilities,
equipment, or inspection data available.
Whenever the certificate is suspended or revoked, the
technician must return it to the Administrator when requested
by the FAA to do so. Section 65.93, Inspection Authorization: Renewal
An IA certificate may be renewed in one of the following
ways each year the technician is seeking renewal:
• The performance of at least one annual inspection for
each 90 days the technician has held the IA rating.
• The performance of the inspections of at least
two major repairs or alterations for each 90 days
the technician has held the IA rating. ( Note: The
inspections can be counted regardless of the approval
or disapproval of the work.)
• The performance (or supervision) and approval of at
least one progressive inspection.
• The attendance and successful completion of a
refresher course (acceptable to the Administrator) that
is at least 8 hours of instruction. This can be either a
single day seminar or a combination of individual
classes acceptable to the Administrator. Some seminars
are sponsored by the FAA FSDO and are free; others
are low cost. Private industry also frequently conducts
one-day sessions and usually charge for their efforts.
Regardless of who is conducting the seminar, it is
usually an excellent way to accomplish renewal, learn
about new issues, and develop a network among peers.
• Passed an oral test by an FAA inspector to determine
that the applicant's knowledge of applicable regulations
and standards is current.
Because all IA certificates expire in the first quarter of each
calendar year (March 31), and the regulation states that
anyone holding an IA for less than 90 days need not meet
the preceding renewal requirements, no renewal is required
for someone who received the IA during the first quarter of
the calendar year.
The technician with IA should note that regulations clearly
state the number of annual inspections (four) and major
repair or alteration inspections (eight) are required for each
90-day period prior to March 31st. This does not mean in each
previous 90-day period the technician must have conducted
either an annual or two major repair or alteration inspections,
but rather their cumulative number by March 31st. Therefore,
an IA could actually go 11 months without performing any
inspection activity relative to renewal. Then in March, they
could conduct all four necessary annual inspections, or all
eight 337-related inspections. However, the regulations do
not provide for the mixing of any of these renewal activities
(i.e., two annual inspections and four Major Repair and
Alteration forms).
Another method of renewal is to meet with the FAA-assigned
FSDO inspector who will determine that the applicant
possesses current knowledge of the applicable regulations
and standards. Although this is often considered the renewal
method of last resort, it should not be considered a negative
experience. If the IA has been performing their activities
in a professional manner throughout the year, this session
can be considered a professional follow-up or consultation.
Proper IA-to-FSDO inspector interaction can be enhanced
with such a meeting.
Section 65.95, Inspection Authorization: Privileges
and Limitations
The IA may perform an annual inspection or perform or
supervise a progressive inspection. They may also approve
for return to service any aircraft-related part or appliance that
has undergone a major repair or alteration (except aircraft
maintained in accordance with a continuous airworthiness
program operated under 14 CFR part 121).
The IA must keep their certificate available for inspection
by any one of the following persons:
• Aircraft owner
• A&P technician
• FAA Administrator
• Authorized representative of the NTSB
• Any federal, state, local, or law enforcement officer
If the holder of an IA moves their fixed base of operation,
they must notify in writing the FSDO responsible for the
location they are moving to before beginning to exercise the
privileges of an IA. Although it is not required, good business
etiquette and professional responsibility would suggest that
a similar letter be written to the responsible FAA Principal
Maintenance Inspector (PMI) at the FSDO in the area they
are leaving.
Ethics
This is a tremendously broad and diverse area of study. It is
also an area that is coming under more scrutiny by consumers,
individual watchdog groups, and government review
committees. Ethics, or more appropriately the lack of ethics,
has caused the loss of millions of dollars through fraudulent
accounting practices, shoddy workmanship, etc. This chapter
examines some definitions of ethics and some examples of
poor business ethics in order to raise the awareness of the
technician to the importance of ethics.
The word “ethics” is actually a philosophical term that comes
from the Greek word “ethos,” which means character or
custom. So, it is logical that a current definition of ethics
is “the study of standards of conduct and moral judgment.” Although situations involving questionable ethics can exist
wherever and whenever business decisions are made, the
scope of this discussion is limited to areas with which the
technician is probably associated.
A Scenario
The following incident illustrates one way that both personal
ethics and technician knowledge of regulations can work
together to provide them with the ability to make the right
decision. Unfortunately, others in the shop did not appear as
concerned as the technician sharing the incident.
A technician working for an airline was involved in a situation
that required a repair or replacement of a fuselage ice
shield. The computer inventory indicated that a replacement
part was in stock, so the technician removed the damaged
component. It was then found that the replacement part was
not actually in stock. At this point, a crucial decision was to
be made: Can the damaged item be reinstalled? The steps in
properly documenting a maintenance event are to record the
removal of the damaged part, then document the installation
of an airworthy part. Once the technician has committed to
removing the damaged part, it becomes unairworthy and
cannot be reinstalled regardless of its deferability in the
minimum equipment list (MEL).
The actual sequence of events is as follows:
• Significant impact damage to the ice shield was
observed and recorded.
• The company inspector reviewed and instructed the
technician to replace the ice shield.
• Availability of the replacement part was confirmed by
computer.
• The damaged part was removed, and the technician
prepared the surface for the replacement part.
• The new part was ordered from inventory, but the part
was not in stock (inventory error).
• The inspector instructed the technician to reinstall the
old one.
• The technician refused.
• The inspector instructed the technician to repair it.
• The technician researched the structural repair
manual (SRM) and found that the facility did not
have the proper facility authorization to repair the
damaged part.
• The company inspector told the technician to apply
5-minute epoxy to the area, sand it down, and paint
it.
• The technician walked away.
• The company inspector found someone else to
compromise standards. The aircraft departed on
time—illegally and unairworthy.
This happens more often than one would like, is probably
overlooked by many people, and, unfortunately, might be
considered standard operating procedure (SOP) for some
maintenance facilities. It is the responsibility of the mechanic
to follow regulations and to question the actions of their
supervisors if the policy is circumvented to make an on-time
departure.
This incident provides some valuable insights into how day-
to-day events can lead to pressure to produce and ultimately
compromise the decision-making.
1. The incident occurred while working for a commercial
airline. The pressure for getting the aircraft in the air
is tremendous in this environment.
2. Inventory error added to the pressure. The damaged
part had been removed because the technician
had queried and believed a replacement part was
immediately available.
3. The company inspector was either unaware of
regulatory requirements or simply did not care.
4. The second technician was either unaware of
regulatory requirements or simply did not care.
Final Observation
The underlying company culture was apparently lacking
concern for ethical decisions and regulatory compliance.
An effective organizational culture should always encourage
ethical behavior and discourage unethical behavior. This
means that not only does the upper management of an
organization say that they conduct themselves ethically,
they must do it consistently; employees, customers, vendors,
and even competitors should know this company has “high
ethical standards.”
This latter issue may sometimes have painful consequences,
if the businesses are competing for a customer’s business.
The ethical company may estimate the maintenance activities
to take 8 weeks and quotes that time frame to the customer.
The unethical company may also know the work takes 8
weeks, but tells the customer only 6 weeks, hoping to get
the job. Once the plane is “captured” and maintenance has
begun, explanations and excuses extend the original time
estimate of 6 weeks to the actual 8 weeks or longer. Although
the customer would be disappointed in this situation, few
customers would be able to remove an aircraft undergoing
maintenance. This “bait and switch” tactic is often used by unscrupulous companies to get an aircraft into their shop no
matter what it takes. Although the shop’s retention of clients is
frequently very low, there always seem to be new ones willing
to accept a shorter-than-normal turnaround time quote. Often
these same shops underbid the job, and then continually
add extra costs as the work progresses. The technician is
encouraged to avoid employment at maintenance facilities
that do not think twice about trying to deceive the customer.
Since companies are usually in business to make money, the
“bottom line” mentality frequently drives management and,
ultimately, technician decisions. But short-term, quick-fix
solutions that focus only on immediate financial success
promote the idea that everything boils down to monetary
gain. Ethical behavior is not about monetary gain.
In addition to monetary gain, there are other common ways
that unethical behavior is rationalized:
• Pretending that the behavior is not unethical or illegal.
• Excusing the behavior by saying it is really in the
organization’s (or the technician’s) best interest.
• Assuming the behavior is okay, because no one else
would even be expected to find out about it.
• Expecting your superiors to support and protect you
if anything should go wrong (Gellerman 1986).
This latter point often leads to a significant surprise for the
individual technician if they compromised their standards
at the encouragement of management to get the job done.
Should there be a problem with maintenance and subsequent
airworthiness of the aircraft, the very same managers or
superiors who directed that technician to shortcut proper
maintenance procedures would testify in court that they
always encouraged their employees to work “by the book”
and never encouraged unauthorized shortcuts.
Ultimately, every organization establishes a climate or
culture regarding honesty, integrity, and ethical behavior.
This corporate climate sets the tone for decision making at
all levels and in all circumstances. This leads to the second
business example, the Aircraft Brake Scandal. Although this
incident occurred at the B.F. Goodrich Wheel and Brake
Plant in Troy, Ohio, and is therefore focused on the design,
manufacture, and test of wheels and brakes for the U.S. Air
Force A-7D, it is a classic case of both personal ethics and
“whistle blowing.” A brief review of the pertinent facts in
the incident follows.
A young engineering technician is in charge of conducting
the required qualification testing for a newly designed
brake and rotor system awarded to the B.F. Goodrich Co.
by L.T.V . Aerospace. An aggressive time schedule and an
upper management mindset of not wanting to hear bad news
(i.e., the brakes are failing test), a senior engineer who is not
willing to have their computations challenged, and a project
manager who states the brake will be qualified “no matter
what,” ultimately lead to a congressional oversight hearing
in 1969. Along the way, the brake system is tested (and fails
14 times), no one wants to write the required test report, low
level employees seek legal advice, and the aircraft suffers
serious damage during landing while conducting initial
flight testing due to unsatisfactory braking. (The reader is
encouraged to look up this now famous case on the Internet
to obtain more details.)
Some of the ethical conflicts that are evident in this situation
are:
• Young engineer (newly hired) feels intimidated by
senior level engineer.
• Early brake failure during development testing is
excused away because “they are not representative
of the final design.”
• A company culture of intimidation and distrust.
Most of these conflicts could have easily occurred in the
maintenance realm if the specifics are broadened, even a little.
• Change the word “engineer” to “maintenance
technician.”
• Instead of brake failure during development testing,
think of component test failure (with the shop norm
of “we don’t follow the manual on this step; we have
developed our own (unauthorized) procedure here.”)
• The existence of a company culture of intimidation
and distrust transcends all lines of business.
For a company to nurture a healthy ethical climate and long-
term success, the element of trust is fundamental both inside
and outside the organization. This trust boosts employee
morale and usually boosts productivity and, therefore,
profitability. It also aids and enhances long-term business
relationships with customers and vendors.
When differences of opinion do exist, ethical organizations
pay close attention to those who are dissenting. Those
companies that are committed to promoting an ethical climate
encourage rather than punish dialogue and debate about
policies and practices.
It is encouraging to note that more and more institutions of
learning, whether business schools or technical colleges, are
adding ethics courses into their required curriculum. More
and more organizations are developing a corporate “code of
ethics.” Some are using the following seven-step checklist to help employees deal with an ethical decision:
1. Recognize and clarify the dilemma.
2. Get all the possible facts.
3. List options—all of them.
4. Test each option by asking such questions as:
—Is it legal?
—Is it right?
—Is it beneficial?
5. Make your decision.
6. Double check your decision by asking:
—How would I feel if my family found out about this?
—How would I feel if my decision is printed in the
local newspaper?
7. Take action (Schermerhorn 1989).
Finally, the technician is encouraged to read the following code
of ethics developed by Professional Aviation Maintenance
Association (PAMA), Inc. and consider adopting it as their
own.
“As a certified technician, my performance is a public service
and, as such, I have a responsibility to the United States
Government and its citizens. I must ensure that all citizens
have confidence in my integrity, and that I will perform my
work according to the highest principles of ethical conduct.
Therefore, I swear that I shall hold in sacred trust the rights
and privileges conferred upon me as a certified technician.
The safety and lives of others are dependent on my skill and
judgment; therefore, I shall never knowingly subject others
to risks which I would not be willing to assume for myself
or those who are dear to me.”
“As a certified technician, I am aware that it is not possible
to have knowledge and skill in every aspect of aviation
maintenance for every airplane, so I pledge that I will never
undertake work or approve work which I believe to be beyond
the limits of my knowledge. I shall not allow any superior to
persuade me to approve aircraft or equipment as airworthy
when there is doubt in my mind as to the validity of my action.
Under no circumstances will I permit the offer of money or
other personal favors to influence me to act contrary to my
best judgment, nor to pass as airworthy aircraft or equipment
about which I am in doubt.”
“The responsibility that I have accepted as a certified
technician demands that I exercise my judgment on the
airworthiness of aircraft and equipment; therefore, I pledge
unyielding adherence to these precepts for the advancement
of aviation and for the dignity of my vocation.”
Human Factors
Chapter 14
Introduction
FAA Involvement
The FAA has had a formal involvement in this issue since
1988. That was the year the first Human Factors Issues in
Aviation Maintenance and Inspection National Conference
was conducted, and that effort reflects a working relationship
between government research and industry activity. This
yearly event includes airlines, suppliers, manufacturers,
schools, and government agencies. There is also an FAA
website for human factors at hf.faa.gov which is a tremendous
resource.
Importance of Human Factors
The greatest impact in aircraft safety in the future will
not come from improving the technology. Rather it will
be from educating the employee to recognize and prevent
human error. A review of accident related data indicates that
approximately 75–80 percent of all aviation accidents are the
result of human error. Of those accidents, about 12 percent
are maintenance related . Although pilot/co-pilot errors tend
to have immediate and highly visible effects, maintenance
errors tend to be more latent and less obvious. However, they
can be just as lethal.
Definitions of Human Factors
Human factors are concerned with optimizing performance
… including reducing errors so that the highest level of safety
is achieved and maintained.
—Ron LoFaro, PhD
FAA
Human factors is the study of how people interact with their
environments.
—FAA-H-8083-25,
Pilot’ s Handbook of Aeronautical Knowledge
Human factors are those elements that affect our behavior
and performance, especially those that may cause us to make
errors.
—Canadian Department of Defense (video)
Our focus is on human factors as it relates to improper actions.
Note, however, that human factors exist in both proper and
improper actions. [Figure 14-1] Since improper actions
usually result in human error, we should also define that term.Human error is the unintentional act of performing a task
incorrectly that can potentially degrade the system. There
are three types of human error:
1. Omission: not performing an act or task.
2. Commission: accomplishing a task incorrectly.
3. Extraneous: performing a task not authorized.
There are also four consequences of human error:
1. Little or no effect.
2. Damage to equipment/hardware.
3. Personal injury.
4. Catastrophic.
Why are human conditions, such as fatigue, complacency,
and stress, so important in aviation maintenance? These
conditions, along with many others, are called human
factors. Human factors directly cause or contribute to many
aviation accidents. It is universally agreed that 80 percent
of maintenance errors involve human factors. If they are not
detected, they can cause events, worker injuries, wasted time,
and even accidents. [Figure 14-2]
Aviation safety relies heavily on maintenance. When it is
not done correctly, it contributes to a significant proportion
of aviation accidents and incidents. Some examples of
maintenance errors are parts installed incorrectly, missing
parts, and necessary checks not being performed. In
comparison with many other threats to aviation safety, the
mistakes of an aviation maintenance technician (AMT) can be
more difficult to detect. Often, these mistakes are present but
not visible and have the potential to remain latent, affecting
the safe operation of aircraft for extended periods of time.
AMTs are confronted with a set of human factors unique
within aviation. They can be working in the evening or early
morning hours, in confined spaces, on high platforms, and in a
variety of adverse temperature/humidity conditions. The work
can be physically strenuous, yet it also requires attention to
detail. [Figure 14-3] Because of the nature of maintenance
tasks, AMTs commonly spend more time preparing for a
task than actually carrying it out. Proper documentation of
14-2Figure 14-1. Human factors exist in both proper and improper actions.Human Factors
Active Failure Latent Failure
Assertiveness Leadership
Asynchronous Communication Maintenance Resource Management
Authoritarian Leader Mental Model
Communication Norms
Complacency Participatory Leader
Crew Resource Management Safety Culture
Dirty Dozen* Situational Awareness
Egalitarian Stressor
Ergonomics Synchronous Communication
Human Factors Team
Inter-team Team Situational Awareness
Intra-team Teamwork
Instructional Systems Design
* 1. Lack of Communication 2. Complacency 3. Lack of Knowledge 4. Distraction
5. Lack of Teamwork 6. Fatigue 7. Lack of Resources 8. Pressure 9. Lack of Assertiveness
10. Stress 11. Lack of Awareness 12. Norms
Human Factors
Proper Actions Improper ActionsHF
applies
to bothall maintenance work is a key element, and AMTs typically
spend as much time updating maintenance logs as they do
performing the work. [Figure 14-4]
Human factors awareness can lead to improved quality, an
environment that ensures continuing worker and aircraft
safety, and a more involved and responsible work force.
The reduction of even minor errors can provide measurable
benefits including cost reductions, fewer missed deadlines,
reduction in work related injuries, reduction of warranty
claims, and reduction in more significant events that can be
traced back to maintenance error. Within this chapter, the
many aspects of human factors are discussed in relation to
aviation maintenance. The most common human factors are
introduced along with ways to mitigate the risk to stop them
from developing into a problem. Several Federal Aviation
Administration (FAA) human factor resources are provided,
including a direct link to aviation maintenance human factors
are at hf.faa.gov .
What are Human Factors?
The term “human factors” has grown increasingly popular
as the commercial aviation industry realizes that human
error, rather than mechanical failure, underlies most
aviation accidents and incidents. Human factors science
or technologies are multidisciplinary fields incorporating
contributions from psychology, engineering, industrial
design, statistics, operations research, and anthropometry. It is
a term that covers the science of understanding the properties
of human capability, the application of this understanding
to the design, development, and deployment of systems and services, and the art of ensuring successful application
of human factor principles into the maintenance working
environment.
The spectrum of human factors that can affect aviation
maintenance and work performance is broad. They
encompass a wide range of challenges that influence
people very differently as humans do not all have the
same capabilities, strengths, weaknesses, or limitations.
Unfortunately, aviation maintenance tasks that do not take
into account the vast amount of human limitations can result
in technical error and injuries. Figure 14-5 shows some of
the human factors that affect AMTs. Some are more serious
than others but, in most cases, when you combine three or
four of the factors, they create a problem that contributes to
an accident or incident.
Elements of Human Factors
Human factors are comprised of many disciplines.
This section discusses ten of those disciplines: Clinical
Psychology, Experimental Psychology, Anthropometrics,
Computer Science, Cognitive Science, Safety Engineering,
Medical Science, Organizational Psychology, Educational
Psychology, and Industrial Engineering. [Figure 14-6]
The study and application of human factors is complex
because there is not just one simple answer to fix or
change how people are affected by certain conditions or
situations. The overall goal of aviation maintenance human
factors research is to identify and optimize the factors that
affect human performance in maintenance and inspection.
14-3Figure 14-3. Aviation maintenance technicians (AMTs) are confronted with many human factors due to their work environments.
Figure 14-2. Human factors and how they affect people are very
important to aviation maintenance.
Human
CapabilitiesHuman
Capabilities
Human
Limitations
Environmental
ConditionsEnvironmental
Conditions
Human-Machine
InterfaceHuman-Machine
InterfaceHuman Factors Human Factors
Mental
StateMental
State
Physical
StatePhysical
StateEmotional
StateEmotional
State
The focus initiates on the technician but extends to the
entire engineering and technical organization. Research is optimized by incorporating the many disciplines that affect
human factors in an effort to understand how people can work
more efficiently and maintain work performance.
By understanding each of the disciplines and applying them
to different situations or human behaviors, we can correctly
recognize potential human factors and address them before
they develop into a problem or create a chain of problems
that result in an accident or incident.
Clinical Psychology
Clinical psychology includes the study and application of
psychology for the purpose of understanding, preventing, and
relieving psychologically-based distress or dysfunction and
to promote subjective well-being and personal development.
It focuses on the mental well-being of the individual. Clinical
psychology can help individuals deal with stress, coping
mechanisms for adverse situations, poor self-image, and
accepting criticism from coworkers.
Experimental Psychology
Experimental psychology includes the study of a variety of
basic behavioral processes, often in a laboratory environment.
These processes may include learning, sensation, perception,
human performance, motivation, memory, language,
14-4Figure 14-4. AMT documenting repair work.
thinking, and communication, as well as the physiological
processes underlying behaviors, such as eating, reading, and
problem solving. In an effort to test the efficiency of work
policies and procedures, experimental studies help measure
performance, productivity, and deficiencies.
Anthropometry
Anthropometry is the study of the dimensions and abilities of
the human body. This is essential to aviation maintenance due
to the environment and spaces that AMTs have to work with.
For example, a man who is 6 feet 3 inches and weighs 230
pounds may be required to fit into a small crawl space of an
aircraft to conduct a repair. Another example is the size and
weight of equipment and tools. Men and women are generally
on two different spectrums of height and weight. Although
both are equally capable of completing the same task with a
high level of proficiency, someone who is smaller may be able
to perform more efficiently with tools and equipment tailored
to their size. In other words, one size does not fit all and the
term “average person” does not apply when employing such
a diverse group of people.
Computer Science
The technical definition for computer science is the study of
the theoretical foundations of information and computation
and of practical techniques for their implementation and
application in computer systems. Yet how this relates to
aviation maintenance is simpler to explain. As mentioned
earlier, AMTs spend as much time documenting repairs
as they do performing them. It is important that they have
computer work stations that are comfortable and reliable.
Software programs and computer-based test equipment
should be easy to learn and use, and not intended only for
those with a high levels of computer literacy.
Cognitive Science
Cognitive science is the interdisciplinary scientific study of minds as information processors. It includes research on how
information is processed (in faculties such as perception,
language, reasoning, and emotion), represented, and
transformed in a nervous system or machine (e.g., computer).
It spans many levels of analysis from low-level learning
and decision mechanisms to high-level logic and planning.
AMTs must possess a great ability to problem solve quickly
and efficiently. They are constantly required to troubleshoot
situations and quickly react to them. This can be a vicious
cycle creating an enormous amount of stress. The discipline
of cognitive science helps us understand how to better assist
AMTs during situations that create high levels of stress so
that their mental process does not get interrupted and affect
their ability to work.
Safety Engineering
Safety engineering ensures that a life-critical system
behaves as needed even when the component fails. Ideally,
safety engineers take an early design of a system, analyze
it to find what faults can occur, and then propose safety
requirements in design specifications up front and changes
to existing systems to make the system safer. Safety cannot
be stressed enough when it comes to aviation maintenance,
and everyone deserves to work in a safe environment.
Safety engineering plays a big role in the design of aviation
maintenance facilities, storage containers for toxic materials,
equipment used for heavy lifting, and floor designs to ensure
no one slips, trips, or falls. In industrial work environments,
the guidelines of the Occupational Safety and Health
Administration (OSHA) are important.
Medical Science
Medicine is the science and art of healing. It encompasses
a variety of health care practices evolved to maintain and
restore health by the prevention and treatment of illness.
Disposition and physical well-being are very important and
directly correlated to human factors. Just like people come
in many shapes and sizes, they also have very different
reactions to situations due to body physiology, physical
structures, and biomechanics.
Organizational Psychology
Organizational psychologists are concerned with relations
between people and work. Their interests include
organizational structure and organizational change, workers’
productivity and job satisfaction, consumer behavior,
and the selection, placement, training, and development
of personnel. Understanding organizational psychology
helps aviation maintenance supervisors learn about the
points listed below that, if exercised, can enhance the work
environment and productivity.
• Rewards and compensations for workers with good
FatigueBoring
repetitive jobs
Incomplete
or incorrect
documentationLack of
spare parts
Lack of tools
and equipmentPoor trainingSmelly fumes
Personal life
problemsSubstance
abuse
Loud noises
SnowUnrealistic
deadlines
Poorly
designed testing
for skill and
knowledgeSlippery
floors
Poor
communicationPoor
instructions
Poor tool
control
Figure 14-5. A list of human factors that affect AMTs.
safety records.
• Motivation for workers to want to do well and work
safely.
• Unifying work teams and groups so they get along
and work together to get the job done right.
• Treating all workers equally.
Educational Psychology
Educational psychologists study how people learn and
design the methods and materials used to educate people of
all ages. Everyone learns differently and at a different pace.
Supervisors should design blocks of instruction that relate
to a wide variety of learning styles. Industrial Engineering
Industrial engineering is the organized approach to the study
of work. It is important for supervisors to set reasonable work
standards that can be met and exceeded. Unrealistic work
standards create unnecessary stressors that cause mistakes.
It is also beneficial to have an efficient facility layout so that
there is room to work. Clean and uncluttered environments
enhance work performance. Another aspect of industrial
engineering that helps in the understanding of human factors
is the statistical analysis of work performance. Concrete data
of work performance, whether good or bad, can show the
contributing factors that may have been present when the
work was done.
Cognitive Science
Organization Psychology
30Not Complex Flight
Exercise Caution
Area of ConcernEndangermentLow Risk
Educational Psychology
Safety Engineering
Medical Science
Computer Science
Anthropometric Science
Experimental Psychology
Clinical Psychology
Industrial Engineering
Human Factors
Figure 14-6. Human factor disciplines.
are required to read back instructions or clearances given by
air traffic control (ATC) to ensure that the pilot receives the
correct instructions and gives ATC an opportunity to correct
if the information is wrong. Frank and Lillian Gilbreth also
are known for their research on fatigue.
Also in the early 1900s, Orville and Wilbur Wright were
the first to fly a powered aircraft and also pioneered many
human factors considerations. While others were trying
to develop aircraft with a high degree of aerodynamic
stability, the Wrights intentionally designed unstable aircraft
with cerebralized control modeled after the flight of birds.
Between 1901 and 1903, the brothers worked with large
gliders at Kill Devil Hills, near Kitty Hawk, North Carolina,
to develop the first practical human interactive controls for
aircraft pitch, roll, and yaw. On December 17, 1903, they
made four controlled powered flights over the dunes at
Kitty Hawk with their Wright Flyer. [Figure 14-11] They
later developed practical in-flight control of engine power,
plus an angle of attack sensor and stick pusher that reduced
pilot workload. The brothers’ flight demonstrations in the
United States and Europe during 1908-1909 awakened the
world to the new age of controlled flight. Orville was the first
aviator to use a seat belt and also introduced a rudder boost/
trim control that gave the pilot greater control authority. The
Wrights’ flight training school in Dayton, Ohio included a
flight simulator of their own design. The Wrights patented History of Human Factors
Around 1487, Leonardo da Vinci began research in the
area of anthropometrics. The Vitruvian Man, one of his
most famous drawings, can be described as one of the
earliest sources presenting guidelines for anthropometry.
[Figure 14-7] Around the same time, he also began to
study the flight of birds. He grasped that humans are too
heavy and not strong enough to fly using wings simply
attached to the arms. Therefore, he sketched a device in
which the aviator lies down on a plank and works two large,
membranous wings using hand levers, foot pedals, and a
system of pulleys. [Figure 14-8] Today, anthropometry plays
a considerable role in the fields of computer design, design
for access and maintainability, simplicity of instructions, and
ergonomic issues.
In the early 1900s, industrial engineers Frank and Lillian
Gilbreth were trying to reduce human error in medicine.
[Figures 14-9 and 14-10] They developed the concept of
using call backs when communicating in the operating
room. For example, the doctor says “scalpel” and the nurse
repeats “scalpel” and then hands it to the doctor. That is
called the challenge-response system. Speaking out loud
reinforces what tool is needed and provides the doctor with an
opportunity to make corrections if it is not the necessary tool.
This same verbal protocol is used in aviation today. Pilots
Figure 14-7. Vitruvian Man, one of Leonardo da Vinci’ s most famous
anthropometric drawings.
Figure 14-8. Leonardo da Vinci’ s rendering of a flying device
for man. their practical airplane and flight control concepts, many of
which are still in use today.
Prior to World War I, the only test of human to machine
compatibility was that of trial and error. If the human
functioned with the machine, he was accepted, if not he was
rejected. There was a significant change in the concern for
humans during the American Civil War. The U.S. Patent
Office was concerned about whether the mass-produced
uniforms and new weapons could be used effectively by the
infantry men.
Evolution of Maintenance Human Factors
With the onset of World War I (1914–1918), more
sophisticated equipment was being developed and the
inability of personnel to use such systems led to an increased
interest in human capability. Up to this point, the focus of
aviation psychology was on the pilot, but as time progressed,
the focus shifted onto the aircraft. Of particular concern
was the design of the controls and displays, the effects of
altitude, and environmental factors on the pilot. The war
also brought on the need for aeromedical research and the
need for testing and measurement methods. By the end of
World War I, two aeronautical labs were established, one at
Brooks Air Force Base, Texas, and the other at Wright Field
outside of Dayton, Ohio.
Another significant development was in the civilian sector,
where the effects of illumination on worker productivity were
examined. This led to the identification of the Hawthorne
Effect, which suggested that motivational factors could
significantly influence human performance.
With the onset of World War II (1939–1945), it was becoming
increasingly harder to match individuals to pre-existing jobs.
Now the design of equipment had to take into account human
limitations and take advantage of human capabilities. This
change took time as there was a lot of research still to be
done to determine the human capabilities and limitations. An
example of this is the 1947 study done by Fitts and Jones on
the most effective configuration of control knobs to be used
in aircraft flight decks. Much of this research transitioned
into other equipment with the aim of making the controls
and displays easier for the operators to use.
Unfortunately, all the “lessons learned” in the WWII studies
of group dynamics, and flight crew communication were
seemingly forgotten after the war. Post WWII aircrew studies
continued to focus primarily on flight crews, especially pilot
selection, simulator training, and cockpit layout and design.
Subsequent studies of the technician focused on individual
competency and included equipment design (ergonomics). The Vietnam Conflict brought the quest for greater safety,
and with that, came a systematic approach for error reduction.
This increased attention brought both good and bad changes.
It led to the “Zero Defects” quality programs in maintenance
and manufacturing. Generally, this had a positive effect.
However, it also led to “crackdown programs” which were
one-way communication from management (the infamous
“my way or the highway” approach). This concept is more
Figure 14-9. Frank Gilbreth – Industrial Engineer.
Figure 14-10. Lillian Gilbreth – Industrial Engineer.
Figure 14-11. The Wright Brothers on December 17, 1903, flying
over the dunes at Kitty Hawk with their Wright Flyer. dictatorial than democratic, and typically had a long-term
negative effect on the company. This “crackdown” approach
for behavior control is based upon fear and punishment,
which creates a problem. Errors are driven into hiding, and then become apparent later, usually at a more critical
time (“Murphy’s Law”). Additional attempts to develop
“foolproof” equipment designs were added to the zero-defect
manufacturing goal and began to find recognition in the
maintenance world as well. Subsequent efforts focused on
effects of positive rather than negative motivators. The results
of this effort were a reversal of the “crackdown” method,
and motivation due to increased morale often improved
maintenance safety performance. Studies have shown that
motivation resulting from negative sources seldom achieved
the same effect. This led to a “Participative Management”
style recognized by some U.S. industry and a few airlines,
but did not reach maintenance operations until much later.
The Airline Deregulation (1978 –1988) effort had a profound
effect upon the aviation community. Prior to 1978, the
airline industry was regulated by the Civil Aeronautics Act
of 1938. This resulted in peaceful markets, stable routes,
and consistent air fares. However, there was a downside
consisting of two major problems: wasteful management
practices and excessively high wages compared to other
comparable skilled-labor industries. The Airline Deregulation
Act brought in competitive business practices, with routes
and fares controlled by their profitability. This led to a new
style of airline management in which a CEO was more of a
business person and less knowledgeable of aviation. Existing
airlines developed new routes and added new kinds of service
and style. Start-up airlines brought other innovative ideas.
The numerous mergers and acquisitions added an increasing
pressure to focus on the financial bottom line. Doing more
with less became the byline. In the 1980s, maintenance
departments were not immune to the pressures of mergers
and staff reductions. However, fleets were extremely
reliable at that time, and significant savings were aided by a
reduction in number of maintenance technicians. Other new
ways of conducting business included leasing of aircraft
and outsourcing of maintenance. A result of deregulation
was change for the maintenance programs (both personnel
and departmental) and the pressure to produce and adjust.
The problem, however, was that human factors for aviation
maintenance was still stuck in the 1960s model.
14-9physiological, psychological, and psychosocial factors.
[Figure 14-12] The programs must focus on individuals, their
physical capabilities, and the factors that affect them. They
also should consider their mental state, cognitive capacity,
and conditions that may affect their interaction with others. In
most cases, human factors programs are designed around the
people in the company’s existing workforce. You cannot apply
identical strength, size, endurance, experience, motivation,
and certification standards equally to all employees. The
company must match the physical characteristics of each
person to the tasks each performs.
The company must consider factors like each person’s size,
strength, age, eyesight, and more to ensure each person is
physically capable of performing all the tasks making up the
job. A good human factors program considers the limitations
of humans and designs the job accordingly. An important
element when incorporating human factors into job design is
planned rest breaks. People can suffer physical and mental
fatigue under many work conditions. Adequate breaks and rest
periods ensure the strain of the task does not overload their
capabilities. Another “People” consideration, which also is
related to “E” for “Environment,” is ensuring there is proper
lighting for the task, especially for older workers. Annual
vision testing and hearing exams are excellent proactive
interventions to ensure optimal human physical performance.
Attention to the individual does not stop at physical abilities.
A good human factors program must address physiological
and psychological factors that affect performance. Companies
should do their best to foster good physical and mental health.
Offering educational programs on health and fitness is one
way to encourage good health. Many companies have reduced
sick leave and increased productivity by making healthy
meals, snacks, and drinks available to their employees.
Companies also should have programs to address issues
associated with chemical dependence, including tobacco
and alcohol. Another “People” issue involves teamwork
and communication. Safe and efficient companies find ways
to foster communication and cooperation among workers,
managers, and owners. For example, workers should be
rewarded for finding ways to improve the system, eliminate
waste, and help ensure continuing safety.
Environment
There are at least two environments in aviation maintenance.
There is the physical workplace on the ramp, in the hangar,
or in the shop. In addition, there is the organizational
environment that exists within the company. A human
factors program must pay attention to both environments.
[Figure 14-13]A detailed review of aviation literature published between
1976 and 1987 had very little to say about maintenance. Out
of 50 published articles, only 15 even mention maintenance.
Most of these articles deal with ergonomics, one article
examines military engine design to “solider proof” the
maintenance duties, and one U.S. Navy article advocated
more management control.
As human factors awareness progressed, a “culture change”
occurred in U.S. carriers in the 1990s. Management
behavior began to change; there were practical applications
of systems thinking; organization structure was revised;
and new strategy, policy, and values emerged. Virtually
all of these involved communication and collaboration.
One example is in 1991, when Continental Airlines began
“CRM type” training in maintenance. They saw the
importance of improving communication, teamwork, and
participative decision making. A second example is when
United Airlines instituted a change in organization and the
job of design of inspectors. They remained more accessible
during heavy maintenance and overhaul and stayed in closer
communication with mechanics during normal repairs.
This resulted in fewer turnbacks and higher quality. A third
example is when Southwest Airlines created and sustained a
strong and clear organizational structure led by the CEO. This
resulted in open and positive communication between the
maintenance and other departments. A final example is when
TWA instituted a new program to improve communication
between the maintenance trade union and maintenance
management. This resulted in improved quality.
The Pear Model
There are many concepts related to the science and practice
of human factors. However, from a practical standpoint, it is
most helpful to have a unified view, or a model of the things
we should be concerned about when considering aviation
maintenance human factors. For more than a decade, the term
“PEAR” has been used as a memory jogger, or mnemonic,
to characterize human factors in aviation maintenance.
The PEAR mode prompts recall of the four important
considerations for human factors programs, which are listed
below.
• People who do the job.
• Environment in which they work.
• Actions they perform.
• Resources necessary to complete the job.
People
Aviation maintenance human factors programs focus on
the people who perform the work and address physical,
14-10Physical Physiological
Psychological Psychosocial• Physical size
• Sex
• Age
• Strength
• Sensory limitations• Workload
• Experience
• Knowledge
• Training
• Attitude
• Mental or emotional state
• Nutritional Factors
• Health
• Lifestyle
• Fatigue
• Chemical dependency• Interpersonal conflicts
PEOPLE
Figure 14-12. People who do the job. job. Many regulatory authorities require that the JTA serve
as the basis for the company’s general maintenance manual
and training plan. Many human factors challenges associated
with use of job cards and technical documentation fall under
“Actions.” Clearly understandable documentation of actions
ensures instructions and checklists are correct and useable.
[Figure 14-14]
Resources
The final PEAR letter is “R” for “Resources.” [Figure 14-15]
It is sometimes difficult to separate resources from the
other elements of PEAR. In general, the characteristics of
the people, environment, and actions dictate the resources.
Many resources are tangible, such as lifts, tools, test
equipment, computers, technical manuals, and so forth. Other
resources are less tangible. Examples include the number
and qualifications of staff to complete a job, the amount of
time allocated, and the level of communication among the
crew, supervisors, vendors, and others. Resources should be
viewed (and defined) from a broad perspective. A resource
is anything a technician (or anyone else) needs to get the job
done. For example, protective clothing is a resource. A mobile
phone can be a resource. Rivets can be resources. What is
important to the “Resource” element in PEAR is focusing
on identifying the need for additional resources.
Another major human factors tool for use in investigation of
maintenance problems is the Boeing developed Maintenance
Error Decision Aid (MEDA). This is based on the idea that
errors result from a series of factors or incidents. The goal of
using MEDA is to investigate errors, understand root causes,
and prevent accidents, instead of simply placing blame on the
maintenance personnel for the errors. Traditional efforts to
investigate errors are often designed to identify the employee
who made the error. In this situation, the actual factors that Physical
The physical environment is obvious. It includes ranges of
temperature, humidity, lighting, noise control, cleanliness,
and workplace design. Companies must acknowledge
these conditions and cooperate with the workforce to either
accommodate or change the physical environment. It takes a
corporate commitment to address the physical environment.
This topic overlaps with the “Resources” component of
PEAR when it comes to providing portable heaters, coolers,
lighting, clothing, and good workplace and task design.
Organizational
The second, less tangible, environment is the organizational
one. The important factors in an organizational environment
are typically related to cooperation, communication, shared
values, mutual respect, and the culture of the company.
An excellent organizational environment is promoted with
leadership, communication, and shared goals associated with
safety, profitability, and other key factors. The best companies
guide and support their people and foster a culture of safety. A
safe culture is one where there is a shared value and attitude
toward safety. In a safe culture, each person understands
their individual role is contributing to overall mission safety.
Actions
Successful human factors programs carefully analyze all the
actions people must perform to complete a job efficiently
and safely. Job task analysis (JTA) is the standard human
factors approach to identify the knowledge, skills, and
attitudes necessary to perform each task in a given job.
The JTA helps identify what instructions, tools, and other
resources are necessary. Adherence to the JTA helps ensure
each worker is properly trained and each workplace has the
necessary equipment and other resources to perform the
14-11Physical Organizational
• Weather
• Location inside/outside
• Workspace
• Shift
• Lighting
• Sound level
• Safety• Personnel
• Supervision
• Labor-management relations
• Pressures
• Crew structure
• Size of company
• Profitability
• Morale
• Corporate culture
ENVIRONMENT
Figure 14-13. Environment in which they work.contributed to the errors or accident remain unchanged, and
the mistake is likely to recur. In an effort to break this “blame
and train” cycle, MEDA investigators learn to look for the
factors that contributed to the error, instead of the employee
who made the error. The MEDA concept is based on the
following three principles:
• Positive employee intent (In other words, maintenance
technicians want to do the best job possible and do
not make intentional errors.)
• Contribution of multiple factors (There is often a series
of factors that contribute to an error.)
• Manageability of errors (Most of the factors that
contribute to an error can be managed.)
When a company is willing to adopt these principles, then the
MEDA process can be implemented to help the maintenance
organization achieve the dual goals of identifying those
factors that contribute to existing errors, and avoiding future
errors. In creating this five-step process, Boeing initially
worked with British Airways, Continental Airlines, United
Airlines, a maintenance worker labor union, and the FAA.
The five steps are:
1. Event: the maintenance organization must select
which error that caused events will be investigated.
2. Decision: was the event maintenance related? If the
answer is yes, then the MEDA investigation continues.
3. Investigation: using the MEDA results form,
the operator conducts an investigation to record
general information about the airplane—when the
maintenance and the event occurred, what event
initiated the investigation, the error that caused the
event, the factors contributing to the error, and a list
of possible presentation strategies.
4. Prevention strategies: the operator reviews,
prioritizes, implements, and then tracks the process
improvements (prevention strategies) in order to avoid or reduce the likelihood of similar errors in the future.
5. Feedback: the operator provides feedback to the
maintenance workplace so technicians know that
changes have been made to the maintenance system
as a result of this MEDA process.
The implantation and continuous use of MEDA is a long-
term commitment and not a “quick fix.” However, airline
operators and maintenance facilities frequently decide to use
the MEDA approach to investigate serious, high visibility
events which have caused significant cost to the company.
The desire to do this is based upon the potential “payback”
of such an investigation.
This may ultimately be counterproductive because a highly
visible event may not really be the best opportunity to
investigate errors. Those involved in the process may be
intimidated by the attention coming from upper management
and various regulatory authorities.
By using the MEDA process properly, the organization can
investigate the factors that contributed to an error, discover
exactly what led to that error, and fix those factors. Successful
implementation of MEDA will allow the organization to
avoid rework, lost revenue, and potentially dangerous
situations related to events caused by maintenance errors.
The “SHEL” model is another concept for investigating and
evaluating maintenance errors. [Figure 14-16] As with other
human factors tools, its goal is to determine not only what the
problem is, but where and why it exists. SHEL was initiated
by Professor Elwyn Edwards (Professor Emeritus, Aston
University, Birmingham, U.K.) in 1972. It was later modified
slightly by the late Capt. Frank Hawkins, a Human Factors
consultant to KLM, in 1975. The acronym SHEL represents:
• Software
• Hardware
14-12• Steps to perform a task
• Sequence of activity
• Number of people involved
• Information control requirements• Knowledge requirements
• Skill requirements
• Altitude requirements
• Certification requirements
• Inspection requirements
ACTIONS
Figure 14-14. Actions they perform.
• Procedures/work cards
• Technical manuals
• Other people
• Test equipment
• Tools
• Computers/software
• Paperwork/signoffs• Ground Handling equipment
• Work stands and lifts
• Fixtures
• Materials
• Task lighting
• Training
• Quality systems
RESOURCES
Figure 14-15. Resources necessary to complete the job.
• Environment
• Liveware
The model examines interaction with each of the four SHEL
components, and does not consider interactions not involving
human factors. The term “software” is not referring to the
common use of the term as applied to computer programs.
Instead it includes a broader view of manual layout,
checklist layout, symbology, language (both technical and
nontechnical), and computer programs. Hardware includes
such things as the location of components, the accessibility
of components and tooling. Environment takes temperature,
humidity, sound, light, and time of day factors into account.
Liveware relates technician interaction with other people,
both on the job and off. These include managers, peers,
family, friends, and self.
No discussion of human factors is complete without reference
to James Reasons’ Model of Accident Causation. This
diagram, which was introduced in 1990, and revised by Dr.
Reason in 1993, is often referred to as the Swiss cheese model
and shows how various “holes” in different systems must be
aligned in order for an error to occur. Only when the holes
are all aligned can the incident take place.There are two types of failure which can occur—active
and latent. An active failure is one in which the effects are
immediate. An example of this type would be an aircraft
slipping off one of the lifting jacks due to improper placement
by the technician. In this example, the aircraft jack is the
approved item of ground support equipment, and it has been
properly maintained.
A latent failure occurs as a result of a decision or action
made long before the incident or accident actually occurs.
The consequences of such a decision may remain dormant
for a long time. An example of a latent failure could also
involve the aircraft slipping off a joint, but in this case, it
could be an unapproved jack being used because funding had
not been approved to purchase the correct ground support
equipment (GSE).
The field of human factors, especially in aviation maintenance,
is a growing field of study. This section of this chapter has
presented only a small segment of the numerous observations
and presentations about the topic. If the technician desires to
learn more, numerous books exist and a review of Internet
data will provide an abundant supply of information.
S
(Procedures)
Worker
H
(Machines)
E
(Ambient)
L
(Personnel)SHEL
Software • Hardware • Environment • Liveware
Figure 14-16. SHEL model. A good place to start researching would be the FAA’s own
website at hf.faa.gov . This site, titled “ Human Factors on
Aviation Maintenance and Inspection (HFAMI)” provides
access to products of the Federal Aviation Administration
Flight Standards Service Human Factors in Aviation
Maintenance and Inspection Program. Many aviation
maintenance industry trade magazines include a section or at
least a page devoted to human factors. “The Human Factors
and Ergonomics Society” is a national organization composed
of 22 technical groups, including one devoted to aerospace
systems, which address both civilian and military issues of
safety and performance.
Human Error
Human error is defined as a human action with unintended
consequences. When you couple error with aviation
maintenance and the negative consequences that it
produces, it becomes extremely troublesome. Training, risk
assessments, safety inspections, etc., should not be restricted
to an attempt to avoid errors but rather to make them visible
and identify them before they produce damaging and
regrettable consequences. Simply put, human error is not
avoidable but it is manageable. [Figure 14-17]
Types of Errors
Unintentional
An unintentional error is an accidental wandering or deviation
from accuracy. This can include an error in your action
(a slip), opinion, or judgment caused by poor reasoning,
carelessness, or insufficient knowledge (a mistake). For
example, an AMT reads the torque values from a job card
and unintentionally transposed the number 26 to 62. They
did not mean to make that error but unknowingly and
unintentionally did. An example of an unintentional mistake
would be selecting the wrong work card to conduct a specific
repair or task. Again, it is not an intentional mistake but a
mistake nonetheless.
Intentional
In aviation maintenance, an intentional error should really be
considered a violation. If someone knowingly or intentionally
chooses to do something wrong, it is a violation, which
means that one has purposely deviated from safe practices,
procedures, standards, or regulations.
Active & Latent
An active error is the specific individual activity that is an
obvious event. A latent error is the company issues that lead
up to the event. For example, an AMT climbs up a ladder to
do a repair knowing that the ladder is broken. In this example,
the active error was falling from the ladder. The latent error
was the broken ladder that someone should have replaced. The “Dirty Dozen”
Due to a large number of maintenance-related aviation
accidents and incidents that occurred in the late 1980s and
early 1990s, Transport Canada identified twelve human
factors that degrade people’s ability to perform effectively
and safely, which could lead to maintenance errors. These
twelve factors, known as the “dirty dozen,” were eventually
adopted by the aviation industry as a straightforward means to
discuss human error in maintenance. It is important to know
the dirty dozen, how to recognize their symptoms, and most
importantly, know how to avoid or contain errors produced
by the dirty dozen. Understanding the interaction between
organizational, work group, and individual factors that may
lead to errors and accidents, AMTs can learn to prevent or
manage them proactively in the future.
Lack of Communication
Lack of communication is a key human factor that can
result in suboptimal, incorrect, or faulty maintenance.
[Figure 14-18] Communication occurs between the AMT
and many people (i.e., management, pilots, parts suppliers,
aircraft servicers). Each exchange holds the potential
for misunderstanding or omission. But communication
between AMTs may be the most important of all. Lack
of communication between technicians could lead to a
maintenance error and result in an aircraft accident. This
is especially true during procedures where more than one
technician performs the work on the aircraft. It is critical
that accurate, complete information be exchanged to ensure
that all work is completed without any step being omitted.
Knowledge and speculation about a task must be clarified and
not confused. Each step of the maintenance procedure must
be performed according to approved instructions as though
only a single technician did the work.
Figure 14-17. Safety awareness will help foresee and mitigate the
risk of human error. A common scenario where communication is critical and a
lack thereof can cause problems, is during shift change in an
airline or fixed base operator (FBO) operation. A partially
completed job is transferred from the technician finishing
their workday to the technician coming on duty. Many
steps in a maintenance procedure are not able to be seen or
verified once completed due to the installation of components
hiding the work. No steps in the procedure can be omitted
and some steps still to be performed may be contingent on
the work already completed. The departing technician must
thoroughly explain what has occurred so that the arriving
technician can correctly complete the job. A recounting of
critical steps and any difficulties encountered gives insight.
A lack of communication at this juncture could result in the
work being continued without certain required operations
having been performed.
The approved steps of a maintenance procedure must be
signed off by the technician doing the work as it is performed.
Continuing a job that has been started by someone else should
only occur after a face-to-face meeting of technicians. The
applicable paperwork should be reviewed, the completed
work discussed, and attention drawing to the next step.
Absence of either a written or oral turnover serves as warning
that an error could occur.
It is vital that work not be continued on a project without
both oral and written communication between the technician
who started the job and the technician continuing it. Work
should always be done in accordance with the approved
written procedure and all of the performed steps should
bear the signature of the technician who accomplishes the
work. If necessary, a phone call can be made to obtain an
oral turnover when technicians cannot meet face-to-face at
the work area. In general, the technician must see their role
as part of a greater system focused on safe aircraft operation
and must communicate well with all those in that system
to be effective.
Complacency
Complacency is a human factor in aviation maintenance that
typically develops over time. [Figure 14-18] As a technician
gains knowledge and experience, a sense of self satisfaction
and false confidence may occur. A repetitive task, especially
an inspection item, may be overlooked or skipped because
the technician has performed the task a number of times
without ever finding a fault. The false assumption might be
made that inspection of the item is not important. However,
even if rare, a fault may exist. The consequences of the fault
not being detected and corrected could cause an incident or
accident. Routine tasks performed over and over allow time
for the technician’s mind to wander, which may also result
in a required task not being performed. When a technician performs work without documentation,
or documents work that was not performed, it is a sign that
complacency may exist. Approved, written maintenance
procedures should be followed during all maintenance
inspections and repairs. Executing the proper paperwork
draws attention to a work item and reinforces its significance.
To combat complacency, a technician must be trained to
expect to find the fault that created the inspection item in
the first place. The technician must stay mentally engaged
in the task being performed. All inspection items must be
treated with equal importance, and it must never be assumed
that an item is acceptable when it has not been inspected.
A technician should never sign for any work that has not
been performed. Prior to the pen touching the paper for a
signature, the technician should read the item before signing
and confirm it has been performed.
Lack of Knowledge
A lack of knowledge when performing aircraft maintenance
can result in a faulty repair that can have catastrophic results.
[Figure 14-20] Differences in technology from aircraft to
aircraft and updates to technology and procedures on a single
aircraft also make it challenging to obtain the knowledge
required to perform airworthy maintenance.
All maintenance must be performed to standards specified
in approved instructions. These instructions are based on
knowledge gained from the engineering and operation of
the aircraft equipment. Technicians must be sure to use the
latest applicable data and follow each step of the procedure
as outlined. They must also be aware that differences exist in
the design and maintenance procedures on different aircraft.
It is important for technicians to obtain training on different
types of aircraft. When in doubt, a technician with experience
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Properly use logbooks and
worksheets to communicate
work accomplishments.Never assume that the work
has been completed.Ensure that maintenance
personnel are discussing
exactly what has been and
needs to be completed
to the next shift.
Maintainers must communicate with one another and explain what work has and has not been
completed when changing shifts.
Figure 14-18. Lack of communication.on the aircraft should be consulted. If one is not available, or
the consulted technician is not familiar with the procedure, a
manufacturer’s technical representative should be contacted.
It is better to delay a maintenance procedure than to do it
incorrectly and cause an accident.
Distraction
A distraction while performing maintenance on an aircraft
may disrupt the procedure. [Figure 14-21] When work resumes, it is possible that the technician skips over a
detail that needs attention. It is estimated that 15 percent of
maintenance related errors are caused by distractions.
Distractions can be mental or physical in nature. They can
occur when the work is located on the aircraft or in the
hangar. They can also occur in the psyche of the technician
independent of the work environment. Something as simple
as a cell phone call or a new aircraft being pushed into the
14-16hangar can disrupt the technician’s concentration on a job.
Less visible is a difficult family or financial matter or other
personal issues that may occupy the technician’s thought
process as work is performed. This can make performance
of the required maintenance less effective.
Whatever their nature, numerous distractions can occur during
the course of maintaining an aircraft. The technician must
recognize when attention to the job at hand is being diverted
and assure that work continues correctly. A good practice
is to go back three steps in the work procedure from when
distraction occurred and resume the job from that point. Using
of a detailed step-by-step written procedure and signing off
each step only after it is completed also helps. Incomplete
work can be marked or tagged, especially when the technician
is pulled from the work by a distraction, and it is unknown
when work will be resumed and by whom. Disconnect any
connector and leave it plainly visible if an installation is not
complete. There is a tendency to think a job is finished when
a component is “hooked up.” Similarly, when a step in the
maintenance procedure is complete, be sure to immediately
lock wire or torque the fasteners if required. This can be used
as an indication that all is well up to that point in the procedure.
Lack of Teamwork
A lack of teamwork may also contribute to errors in aircraft
maintenance. [Figure 14-22] Closely related to the need for
communication, teamwork is required in aviation maintenance
in many instances. Sharing of knowledge between technicians,
coordinating maintenance functions, turning work over from
shift to shift, and working with flight personnel to troubleshoot
and test aircraft are all are executed better in an atmosphere
of teamwork. Often associated with improved safety in the
workplace, teamwork involves everyone understanding
and agreeing on actions to be taken. A gear swing or other
operational check involves all the members of a team working
together. Multiple technicians contribute to the effort to ensure
a single outcome. They communicate and look out for one
another as they do the job. A consensus is formed that the
item is airworthy or not airworthy.
The technician primarily deals with the physical aspect of
the aircraft and its airworthiness. Others in the organization
perform their roles and the entire company functions as a
team. Teams can win or lose depending on how well everyone
in the organization works together toward a common
objective. A lack of teamwork makes all jobs more difficult
and, in maintenance, could result in a miscommunication
that affects the airworthiness of the aircraft.
Fatigue
Fatigue is a major human factor that has contributed
to many maintenance errors resulting in accidents. [Figure 14-23] Fatigue can be mental or physical in
nature. Emotional fatigue also exists and affects mental
and physical performance. A person is said to be fatigued
when a reduction or impairment in any of the following
occurs: cognitive ability, decision-making, reaction time,
coordination, speed, strength, or balance. Fatigue reduces
alertness and often reduces a person’s ability to focus on the
task being performed.
Symptoms of fatigue can also include short-term memory
problems, channeled concentration on unimportant issues
while neglecting more important ones, and failure to maintain
a situational overview. A fatigued person may be easily
distracted or may be nearly impossible to distract. They
may experience abnormal mood swings. Fatigue results in
an increase in mistakes, poor judgment, and poor decisions
or perhaps no decisions at all. A fatigued person may also
lower their standards.
Tiredness is a symptom of fatigue. However, sometimes a
fatigued person may feel wide awake and engaged in a task.
The primary cause of fatigue is a lack of sleep. Good restful
sleep, free from drugs or alcohol is a human necessity to
prevent fatigue. Fatigue can also be caused by stress and
overworking. A person’s mental and physical state also
naturally cycles through various levels of performance each
day. Variables such as body temperature, blood pressure,
heart rate, blood chemistry, alertness, and attention rise and
fall in a pattern daily. This is known as circadian rhythm.
[Figure 14-24] A person’s ability to work (and rest) rises and
falls during this cycle, and performance counter to circadian
rhythm can be difficult. Until it becomes extreme, a person
may be unaware that they are fatigued. It is easier recognized
by another person or in the results of tasks being performed.
This is particularly dangerous in aviation maintenance since
the lives of people depend on maintenance procedures
performed at a high level of proficiency. Working alone when
fatigued is particularly dangerous.
The best remedy for fatigue is to get enough sleep on a
regular basis. The technician must be aware of the amount
and quality of sleep obtained. Time off is justified when
too little sleep has occurred and errors are probable during
maintenance. Countermeasures to fatigue are often used, but
their effectiveness can be short lived and many can make
fatigue worse. Caffeine is a common fatigue countermeasure.
Pseudoephedrine found in sinus medicine and amphetamines
are also used. While they can be effective for short periods, the
underlying fatigue remains and due to this drug use, the person
may have trouble getting the rest needed once off the job.
Suggestions to help mitigate the problems caused by fatigue
include looking for symptoms of fatigue in oneself and in
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Always expect to find
something wrong.Always double check
your work.Never sign off on something
that you did not fully check.
People tend to become overconfident after becoming proficient in a certain task, which can mask
the awareness of dangers.
Figure 14-19. Complacency.
others. Have others check your work, even if an inspector sign
off is not required. Avoid complex tasks during the bottom
of your circadian rhythm. Sleep and exercise daily. Eight to
nine hours of daily sleep are recommended to avoid fatigue.
AMTs in airline operations are part of a system in which
most maintenance is performed at night. Fleet aircraft are
operated primarily during daytime hours to generate company
revenue. Therefore, shift work is required to maintain the
fleet. It is already known that turning work over to other
technicians during shift changes can lead to errors due to
lack of communication. But shift work alone is a cause of fatigue that can degrade performance and also lead to errors.
Shift work requires technicians to work during low cycles
of their natural circadian rhythm. It also makes sleep more
difficult when not on the job. Furthermore, regular night shift
work makes a person’s body more sensitive to environmental
disturbances. It can degrade performance, morale, and
safety. It can also affect one’s physical health. All of these
can be reflected in degraded maintenance performance—a
dangerous situation.
The technician must be aware that shift work is the norm in
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Only fix parts that you are
trained to fix.If you do not know how to
fix something, ask for help
from someone who does.Ensure that the maintenance
manual you are using is up
to date.
In a world of ever-changing technology, maintainers must remain up to date on current equipment
and how to fix it.
Figure 14-20. Lack of knowledge.
aviation. Avoidance of fatigue is part of the job. Title 14 of
the Code of Federal Regulations (14 CFR) part 121, section
377, only requires 24 hours time off during a week of work.
Since this is obviously not enough, it is up to companies and
technicians to regulate shift work and time off to reduce the
potential for errors. Most importantly, each technician must
monitor and control their sleep habits to avoid fatigue. Lack of Resources
A lack of resources can interfere with a person’s ability to
complete a task because of a lack of supplies and support.
[Figure 14-25] Low quality products also affect one’s ability
to complete a task. Aviation maintenance demands proper
tools and parts to maintain a fleet of aircraft. Any lack of
resources to safely carry out a maintenance task can cause
both non-fatal and fatal accidents. For example, if an aircraft
is dispatched without a functioning system that is typically
nonessential for flight but suddenly becomes needed, this
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Once returning to the job, go
back through all of the steps
to ensure where you left off.Never leave tools or parts
lying around. Secure them
before leaving the area.Use a detailed checklist.
A distraction could be anything that takes your mind off the task that is being done. Any distraction
while working can cause us to think we are further ahead in the process than we actually are.
Figure 14-21. Distraction.
could create a problem.
Parts are not the only resources needed to do a job properly,
but all too frequently parts become a critical issue. AMTs can
try to be proactive by checking suspected areas or tasks that
may require parts at the beginning of the inspection. Aircraft
on ground (AOG) is a term in aviation maintenance indicating
that a problem is serious enough to prevent an aircraft from
flying. In these cases, there is a rush to acquire the parts to
put the aircraft back into service and prevent further delays or cancellations of the planned itinerary. AOG applies to any
aviation materials or spare parts that are needed immediately
for an aircraft to return to service. AOG suppliers refer
qualified personnel and dispatch the parts required to repair
the aircraft for an immediate return to service. AOG also is
used to describe critical shipments for parts or materials for
aircraft “out of service” (OTS) at a location.
If the status of an aircraft is AOG and materials required are
not on hand, parts and personnel must be driven, flown, or
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Ensure that lines of
communication are open
between personnel.Always look out for
co-workers with safety
in mind.Discuss specific duties when
jobs require more than one
person to eliminate any
questions.
Personality differences in the workplace must be left at the door. Organizations should emphasize
that a lack of teamwork can ultimately affect the safety of maintenance work.
Figure 14-22. Lack of teamwork.
sailed to the location of the grounded aircraft. Usually the
problem is escalated through an internal AOG desk, then
the manufacturer’s AOG desk, and finally competitors’
AOG desks. All major air carriers have an AOG desk that is
manned 24 hours a day, 7 days a week by personnel trained
in purchasing, hazardous materials shipping, and parts
manufacturing and acquisition processes.
Within an organization, making sure that personnel have the
correct tools for the job is just as important as having the proper parts when they are needed. Having the correct tools
means not having to improvise. For example, an aircraft that
had received a new interior needed to be weighed prior to
being released to fly. Two days before the planned release, the
aircraft was weighed without the proper electronic load cells
placed between the aircraft jack and the aircraft. Because the
correct equipment was not used, the aircraft slipped off of
one of the load cells and the jack point creased the spar. The
cost of improvising can be very steep. The right tools for the
job need to be used at all times, and if they are broken, out of
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Be aware of the symptoms
and look for them in yourself
and coworkers.Eating healthy, exercising
and maintaining regular sleep
patterns can prevent fatigue.Forfeit complex tasks if you
know you are exhausted.
Occupations that require an individual to work long hours or stay up overnight can lead to fatigue.
Fatigue can cause a decreased attentiveness and a decreased level of consciousness, which can be
very dangerous when conducting maintenance.
Figure 14-23. Fatigue.
calibration, or missing, they need to be repaired, calibrated,
or found as soon as possible.
Technical documentation is another critical resource that can
lead to problems in aviation maintenance. When trying to
find out more about the task at hand or how to troubleshoot
and repair a system, the needed information often cannot be
found because the manuals or diagrams are not available.
If the information is unavailable, personnel should ask
a supervisor or speak with a technical representative or technical publications department at the appropriate
aircraft manufacturer. Most manuals are in a constant state
of revision and, if organizations do not identify missing
information in the manuals, then nothing is done to correct the
documentation. Resources such as publications departments
and manufacturers’ technical support are available and should
be used rather than ignoring the problem.
Another valuable resource that the maintenance department
should rely on is the flight crew. Organizations should
9 12 15 18 21 24 3 6 920
0Alertness Level Multiple Sleep Latency Test (MSLT)Peak alertness
Dangerously
drowsy
Time of Day1-3 AM
Reduced
alertnessSlightly
impaired
Figure 14-24. Many human variables rise and fall daily due to one’ s natural circadian rhythm.
encourage open communication between flight crews and
maintenance crews. The flight crew can provide valuable
information when dealing with a defective part or problem.
Figure 14-26 shows a number of questions that flight crews can
be asked to help resolve and understand maintenance issues.
When the proper resources are available for the task at hand,
there is a much higher probability that maintenance will do
a better, more efficient job and higher likelihood that the job
will be done correctly the first time. Organizations must learn
to use all of the resources that are available and, if the correct
resources are not available, make the necessary arrangements
to get them in a timely manner. The end result saves time
and money, and enables organizations to complete the task
knowing the aircraft is airworthy.
Pressure
Aviation maintenance tasks require individuals to perform
in an environment with constant pressure to do things
better and faster without making mistakes and letting things
fall through the cracks. Unfortunately, these types of job
pressures can affect the capabilities of maintenance workers
to get the job done right. [Figure 14-27] Airlines have
strict financial guidelines, as well as tight flight schedules,
that pressure mechanics to identify and repair mechanical
problems quickly so that the airline industry can keep
moving. Most important, aircraft mechanics are responsible
for the overall safety of everyone who uses flying as a mode
of transportation.
Organizations must be aware of the time pressures that are put
on aircraft mechanics and help them manage all of the tasks
that need to be completed so that all repairs, while done in a timely manner, are completed correctly with safety being the
ultimate goal. Sacrificing quality and safety for the sake of time
should not be tolerated or accepted. Likewise, AMTs need to
recognize on their own when time pressures are clouding their
judgments and causing them to make unnecessary mistakes.
Self-induced pressures are those occasions where one takes
ownership of a situation that was not of their doing.
In an effort to combat self-induced pressure, technicians
should ask for help if they feel overwhelmed and under a
time constraint to complete a repair. Another method is to
have someone check the repair thoroughly to ensure that all
maintenance tasks were completed correctly.
Lastly, if given a repair with a specific time limitation that
you feel is unrealistic or compromises safety, bring it to
the attention of the organization’s management and openly
discuss a different course of action.
Lack of Assertiveness
Assertiveness is the ability to express your feelings, opinions,
beliefs, and needs in a positive, productive manner and should
not be confused with being aggressive. [Figure 14-28] It
is important for AMTs to be assertive in issues relating to
aviation repair rather than choosing not to or not being
allowed to voice their concerns and opinions. Not being
assertive could ultimately cost people their lives. The
following are examples of how a lack of assertiveness can
be offset:
1. Address managers and supervisors directly by stating
the problem.
Example: “John, I have a concern with how this repair
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Maintain a sufficient supply
of parts and order any
anticipated parts before they
are required.Preserve all equipment
through proper maintenance.Never replace a part with one
that is not compatible for the
sake of getting the job done.
When there is a lack of resources available to properly fix something, a decision should be made
to cease maintenance until the proper parts are available.
Figure 14-25. Lack of resources.
is being rushed.”
2. Explain what the consequences will be.
Example: “If we continue, the result will be that the
part will break sooner rather than later.”
3. Propose possible solutions to the problem.
Example: “We could try doing things another way or
you may want to try this way.”
4. Always solicit feedback and include other opinions. Example: “John, what do you think?”
When being assertive with co-workers or management,
deal with one issue at a time rather than trying to tackle a
number of problems at once. It is also important to have
documentation and facts to back up your argument, which can
give people a visual account of what you are trying to explain.
A lack of assertiveness in failing to speak up when things
do not seem right has resulted in many fatal accidents. This
can easily be changed by promoting good communication
When and where did the event occur?
Were there any indications prior to failure?
Did the system surge or flicker?
How often does the system cycle?
What was the range of transmission or reception?
What was the time of retractions or extensions?
Were there noises in the aircraft or headsets?
Were there vibrations or stiffness of system controls?
Was irregular trim required?
Was there ease or lack of control?
Were smoke or fumes present?
Was there a loss of amperage and/or voltage?
Figure 14-26. Questions that technicians can ask flight crews in an
effort to resolve and understand maintenance issues.between co-workers and having an open relationship with
supervisors and management. Maintenance managers must be
familiar with the behavior styles of the people they supervise
and learn to utilize their talents, experience, and wisdom.
As the employees become aware of behavior styles and
understand their own behavior, they see how they unwittingly
contribute to some of their own problems and how they can
make adjustments. Assertive behavior may not be a skill that
comes naturally to every individual, but it is a critical skill
to achieve effectiveness. AMTs should give supervisors and
management the kind of feedback required to ensure that they
will be able to assist mechanics in doing their job.
Stress
Aviation maintenance is a stressful task due to many factors.
[Figure 14-29] Aircraft must be functional and flying in order
for airlines to make money, which means that maintenance
must be done within a short timeframe to avoid flight delays
and cancellations. Fast-paced technology that is always
changing can add stress to technicians. This demands that
AMTs stay trained on the latest equipment. Other stressors
include working in dark, tight spaces, lack of resources to
get the repair done correctly, and long hours. The ultimate
stress of aviation maintenance is knowing that the work they
do, if not done correctly, could result in tragedy.
Everyone handles stress differently and particular situations
can bring about different degrees of difficulty for different
people. For example, working under a strict timeline can be
a stressor for one person and normal for another. The causes
of stress are referred to as “stressors” and are categorized as
physical, psychological, or physiological. Following is a list
of each and how they may affect maintenance.
Physical Stressors
Physical stressors add to a person’s workload and make their
work environment uncomfortable.
• Temperature—high temperatures in the hangar
increase perspiration and heart rate causing the body
to overheat. Low temperatures can cause the body to
feel cold, weak, and drowsy.
• Noise—hangars that have high noise levels (due to
aircraft taking off and landing close by) can make
it difficult for maintenance personnel to focus
and concentrate.
• Lighting—poor lighting within a work space makes
it difficult to read technical data and manuals.
Likewise, working inside an aircraft with poor lighting
increases the propensity to miss something or to repair
something incorrectly.
• Confined spaces—small work spaces make it very difficult to perform tasks, as technicians are often
contorted into unusual positions for a long period
of time.
Psychological Stressors
Psychological stressors relate to emotional factors, such
as a death or illness in the family; business worries; poor
interpersonal relationships with family, co-workers, or
supervisors; and financial worries.
• Work-related stressors—over anxiousness can hinder
performance and speed while conducting maintenance
if there is any apprehension about how to do a repair
or concerns about getting it done on time.
• Financial problems—impending bankruptcy,
recession, loans, and mortgages are a few examples
of financial problems that can create stressors.
• Marital problems—divorce and strained relationships
can interfere with one’s ability to perform their job
correctly.
• Interpersonal problems—problems with superiors
and colleagues due to miscommunication or
perceived competition and backstabbing can cause
a hostile work environment.
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Ensure that the pressure is
not self-induced.Ask for extra help if time is
an issue.Communicate if you think
you will need more time to
complete a repair rather than
rush through it.
Pressure to get things repaired is always present in aviation. Maintainers must not let the
pressures of time constraints get in the way with safely finishing a repair.
Figure 14-27. Pressure.
Physiological Stressors
Physiological stressors include fatigue, poor physical
condition, hunger, and disease.
• Poor physical condition—trying to work when ill
or not feeling well can force the body to use more
energy fighting the illness, leaving less energy to
perform vital tasks.
• Proper meals—not eating enough, or eating foods lacking the proper nutrition, can result in low energy
and induce symptoms like headaches and shaking.
• Lack of sleep—a fatigued AMT is unable to perform
to standard for long periods of time and can become
sloppy with repairs and make significant mistakes.
• Conflicting shift schedules—the effect of changing
sleep patterns on the body’s circadian cycle can lead
to a degradation of performance.
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Provide clear feedback when
a risk or danger is perceived.Allow co-workers to give
their opinions and always
accept corrective criticisms.Never compromise your
standards.
Lack of assertiveness in failing to alert others when something does not seem right can result in many
fatal accidents. Do not let something that you know is wrong continue by ignoring it.
Figure 14-28. Lack of assertiveness.
People cope with stress in many different ways. Specialists
say that the first step is to identify stressors and the
symptoms that occur after exposure to those stressors. Other
recommendations involve development or maintenance of a
healthy lifestyle with adequate rest and exercise, a healthy
diet, limited consumption of alcoholic drinks, and avoidance
of tobacco products.
Lack of Awareness
Lack of awareness is defined as a failure to recognize all the
consequences of an action or lack of foresight. [Figure 14-30]
In aviation maintenance, it is not unusual to perform the same
maintenance tasks repeatedly. After completing the same
task multiple times, it is easy for technicians to become less
vigilant and develop a lack of awareness of what they are
doing and what is around them. Each time a task is completed
it must be treated as if it were the first time. Norms
Norms is short for “normal,” or the way things are normally
done. [Figure 14-31] They are unwritten rules that are
followed or tolerated by most organizations. Negative
norms can detract from the established safety standard and
cause an accident to occur. Norms are usually developed
to solve problems that have ambiguous solutions. When
faced with an ambiguous situation, an individual may use
another’s behavior as a frame of reference around which to
form their own reactions. As this process continues, group
norms develop and stabilize. Newcomers to the situation are
then accepted into the group based on adherence to norms.
Very rarely do newcomers initiate change in a group with
established norms.
Some norms are unsafe in that they are non-productive or
detract from the productivity of the group. Taking shortcuts in
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Take time off or a short
break if you are feeling
stressed.Healthy eating, exercise, and
a sufficient amount of rest
can reduce stress levels.Discuss with a co-worker and
ask them to monitor your
work.
Stress is the subconscious response to the demands placed on a person.
Figure 14-29. Stress.aircraft maintenance, working from memory, or not following
procedures are examples of unsafe norms. Newcomers are
better able to identify these unsafe norms than long-standing
members of the group. On the other hand, the newcomer’s
credibility depends on their assimilation into the group. The
newcomer’s assimilation, however, depends on adherence
to the group norms. Everyone should be aware of the
perceptiveness of newcomers in identifying unhealthy norms
and develop a positive attitude toward the possibility that
norms may need to be changed. Finally, as newcomers become
assimilated into the group structure, they build credibility with
others. Once this has been done, a relative newcomer may
begin to institute change within the group. Unfortunately,
such actions are often difficult to do and rely heavily on the group’s perception of the newcomer’s credibility.
Norms have been identified as one of the dirty dozen in
aviation maintenance and a great deal of anecdotal evidence
points to the use of unsafe norms on the line. The effect of
unsafe norms may range from the relatively benign, such as
determining accepted meeting times, to the inherently unsafe,
such as signing off on incomplete maintenance tasks. Any
behavior commonly accepted by the group, whether as a
standard operating procedure (SOP) or not, can be a norm.
Supervisors need to ensure that everyone adheres to the same
standards and that unsafe norms are not tolerated. AMTs
should pride themselves on following procedure, rather than
unsafe norms that may have been adopted as regular practice.
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Check to see if what you are
working on conflicts with an
existing modification or
repair. Even if you are highly
proficient in a task, always
have someone check
your work.Always ask co-workers to
check your work.
After completing the same tasks multiple times, maintainers can develop a lack of awareness for
what is around them. They tend to lack common sense and vigilance because they have
completed the same task so many times.
Figure 14-30. Lack of awareness.
Example of Common Maintenance Errors
In an effort to identify the most frequently occurring
maintenance discrepancies, the United Kingdom Civil
Aviation Authority (CAA) conducted in-depth studies of
maintenance sites on aviation maintenance operations. The
following list is what they found to be the most common
occurring maintenance errors.
1. Incorrect installation of components.
2. Fitting of wrong parts.
3. Electrical wiring discrepancies to include crossing
connections. [Figure 14-32]4. Forgotten tools and parts.
5. Failure to lubricate. [Figure 14-33]
6. Failure to secure access panels, fairings, or cowlings.
7. Fuel or oil caps and fuel panels not secured.
8. Failure to remove lock pins. [Figure 14-34]
All of the maintenance discrepancies listed above can be
avoided if the proper procedures are followed on the job
card that is being used. [Figure 14-35] Regardless of how
many times the task has been completed, each time you
THE DIRTY DOZEN
Twelve human factors for aircraft maintenance proficiency
Lack of Teamwork
Fatigue
Lack of Resources
Pressure
Lack of Assertiveness
Stress
Lack of Awareness
Norms
Complacency
Lack of Knowledge
Distraction
Lack of Communication
MITIGATING THE RISK
Ensure that everyone follows
the same standard.The easiest way of
accomplishing something
may not be the standard.Be aware that just because it
seems normal does not make
it correct.
Norms is short for “normal, ” or the way things are normally done. They are unwritten rules that
are followed or tolerated by most of the organization. Negative norms can detract from the
established safety standard and cause an accident to occur.
Figure 14-31. Norms.
pick up a job card, treat it like it is the first time you have
ever completed the task, and complete it with diligence and
complete accuracy.
Historically, twenty percent of all accidents are caused by
a machine failure, and eighty percent by human factors.
[Figure 14-36] Originally focusing on the pilot community,
human factors awareness has now spread into the training
sphere of maintenance technicians. An in-depth review of
an aviation incident reveals time and again that a series of
human errors (known also as a chain of events) was allowed
to build until the accident occurred. If the chain of events is broken at the maintenance level, the likelihood of the accident
occurring can be drastically decreased. Figure 14-37 is a list
of maintenance-related incidents/accidents and their causes.
It is easy to see how many of the “Dirty Dozen” contributed
to the causes or were considered contributing factors.
Where to Get Information
Following is a list of websites and references that are good
sources of information on human factors.
On March 20, 2001, a Lufthansa Airbus A320 almost
crashed shortly after takeoff because of reversed wiring in
the captain's sidestick flight control. Quick action by the
co-pilot, whose sidestick was not faulty, prevented a crash.
The investigation has focused on maintenance on the
captain's controls carried out by Lufthansa Technik just
before the flight. During the previous flight, a problem with
one of the two elevator/aileron computers (ELAC) had
occurred. An electrical pin in the connector was found to
be damaged and was replaced. It has been confirmed that
two pairs of pins inside the connector had accidentally
been crossed during the repair. This changed the polarity
in the sidestick and the respective control channels
“bypassing” the control unit, which might have sensed the
error and would have triggered a warning. Clues might
have been seen on the electronic centralized aircraft
monitor (ECAM) screen during the flight control checks,
but often pilots only check for a deflection indication, not
the direction. Before the aircraft left the hangar, a flight
control check was performed by the mechanic, but only
using the first officer’s sidestick.Incident
Cause
Lock pin
Alaska Airlines Flight 261, a McDonnell Douglas MD-83
aircraft, experienced a fatal accident on January 31, 2000,
in the Pacific Ocean. The two pilots, three cabin
crewmembers, and 83 passengers on board were killed and
the aircraft was destroyed.
The subsequent investigation by the National
Transportation Safety Board (NTSB) determined that
inadequate maintenance led to excessive wear and
catastrophic failure of a critical flight control system during
flight. The probable cause was stated to be “a loss of
airplane pitch control resulting from the in-flight failure of
the horizontal stabilizer trim system jackscrew assembly’s
acme nut threads. The thread failure was caused by
excessive wear resulting from Alaska Airlines’ insufficient
lubrication of the jackscrew assembly.”Accident
CauseFigure 14-32. A description of a Lufthansa Airbus A320 that almost
crashed due to reversed wiring of the flight controls.Figure 14-34. Lock pins located on the landing gear of an aircraft.
Figure 14-33. A description of Alaska Airlines Flight 261 that
crashed due to insufficient lubrication of the jackscrew assembly.
MAINTENANCE JOB CARD WORKNO A26
AD-NOTE 1 OF 2 -4001 OF 1 97471
FZE
1 ZONE 221-222
SOLID STATE FLIGHT DATA RECORDER (IF APPLICABLE)
MS 31-31-00-007-002-500
(AMM 31-31-01) F1 SSFDR DATA COPY
NOTE: THIS IS A DUPLICATE INSPECTION!
PERFORMANCE AND INSPECTION MUST BE DONE BY TWO PERSONS.
PERFORMANCE = B1
INSPECTION = B1
Make a copy of the FDR data with the portable interface (PI) unit
A General
(1) This taskwork the hand-hold portable interface (PI) unit to make a
copy of the data from the flight data recorder (FDR) when the FDR
is in the airplane
(a) The PI puts the data on a removable PC card (PCMCIA).
(b) The data on the PC card can then be analyzed by the applicable
airline personnel.1 0.00 0.00SSFOR - PORTABLE INTERFACE UNITJOB
4AD-4
FRAWB41
PACUZ0030
Figure 14-35. A sample picture of a maintenance job card that
explains the steps of each maintenance task.
Federal Aviation Administration (FAA)
There are a number of human factors resources within the
FAA. The most direct link for aviation maintenance human
factors is the FAA Human Factors website at hf.faa.gov . It
offers document access and services, including most of the
FAA maintenance human factors documents dating back to
the 1988 start of FAA’s maintenance human factors research
and development program. New documents include videos,
PowerPoint presentations, and other media.FAA’s Maintenance Fatigue Section
The FAA has sponsored a multi-disciplinary subject matter
expert work group involving industry, labor, research,
and government to investigate the issues associated with
maintenance fatigue, and the practical science-based methods
that can be used to manage fatigue risk. For more information,
visit the website at www.mxfatigue.com .
TECHNICAL CAUSES HUMAN CAUSES
1903 Present100
0Accidents
TimeAccidents in Aviation
Figure 14-36. Statistical illustration showing that 80 percent of all aviation accidents are caused by human factors.
FAA Safety Team
The FAA Safety Team has a dedicated website that provides
up-to-date information safety concerns, upcoming seminars,
featured courses and resources. For more information, visit
the website at www.faasafety.gov .
Other Resources
System Safety Services
The mission of System Safety Services is to assist clients
in developing the best possible safety system to meet their
needs. They have an experienced and professional team of
individuals with years of experience in aviation and human
factors. The website provides a lot of information on human
factors including articles, upcoming events, presentations,
safety videos, training aids and workshops. For more
information, visit their website at www.system-safety.com .
Human Factors & Ergonomics Society (HFES)
The Human Factors and Ergonomics Society (HFES) is the
only organization in the United States dedicated specifically
to the human factors profession. The HFES was formed in
1957 and typically maintains about 5,000 members. For more
information, visit their website at www.hfes.org .International Ergonomics Association (IEA)
The International Ergonomics Association (IEA) is a
federation of over 40 ergonomics and human factors societies
located all over the world. All members of the HFES are
automatically also members of the IEA. The main contact
point within the IEA is through the office of their Secretary
General. For more information, visit their website at iea.cc .
14-32Figure 14-37. A list of maintenance-related incidents/accidents and their causes.
August 26, 1993, an Excalibur Airways Airbus 320 took off from London-Gatwick Airport (LGW) and exhibited an undemanded roll
to the right on takeoff, a condition which persisted until the aircraft landed back at LGW 37 minutes later. Control of the aircraft
required significant left sidestick at all times and the flight control system was degraded by the loss of spoiler control.
Technicians familiar with Boeing 757 flap change procedures lacked the knowledge required to correctly lock out the spoilers on the
Airbus during the flap change work that was done the day before the flight. Turnover to technicians on the next shift compounded
the problem. No mention of incorrect spoiler lockout procedure was given since it was assumed that the 320 was like the 757. The
flap change was operationally checked, but the spoiler remained locked out incorrectly and was not detected by the flight crew
during standard functional checks. The lack of knowledge on Airbus procedures was considered a primary cause of this incident.
April 26, 2001, an Emery Worldwide Airlines DC-8-71F left main landing gear would not extend for landing.
Probable cause was failure of maintenance to install the correct hydraulic landing gear extension component and the failure of
inspection to comply with post-maintenance test procedures. No injuries.
On May 25, 2002, China Airlines Flight 611 Boeing 747 broke into pieces in mid-air and crashed, killing all 225 people on board.
The accident was the result of metal fatigue caused by inadequate maintenance after a previous incident.
On August 26, 2003, a Colgan Air Beech 1900D crashed just after takeoff from Hyannis, Massachusetts. Both pilots were killed.
The improper replacement of the forward elevator trim cable and subsequent inadequate functional check of the maintenance
performed resulted in a reversal of the elevator trim system and a loss of control in flight. Factors were the flight crew’s failure to
follow the checklist procedures and the aircraft manufacturer’s erroneous depiction of the elevator trim drum in the maintenance
manual.
On September 28, 2007, American Airlines Flight 1400 DC-9 experienced an in-flight engine fire during departure climb from
Lambert St. Louis International Airport (STL). During the return to STL, the nose landing gear failed to extend, and the flight crew
executed a go-around, during which the crew extended the nose gear using the emergency procedure. The flight crew conducted
an emergency landing, and the 2 flight crewmembers, 3 flight attendants, and 138 passengers deplaned on the runway. No
occupant injuries were reported, but the airplane sustained substantial damage from the fire.
American Airlines’ maintenance personnel’s use of an inappropriate manual engine-start procedure, which led to the uncom -
manded opening of the left engine air turbine starter valve, and a subsequent left engine fire.INCIDENT
CAUSE
INCIDENT
CAUSE
ACCIDENT
CAUSE
ACCIDENT
CAUSE
ACCIDENT
CAUSE
G-1A
Absolute humidity. The actual amount of the water vapor
in a mixture of air and water.
Absolute pressure. Equal to gauge pressure plus atmospheric
pressure. Also known as psia.
Absolute temperature. Temperature measured relative to
absolute zero. Absolute temperature scales include Kelvin
and Rankine.
Acceleration due to gravity. The acceleration of an object
caused by gravity. On earth, it is measured as 32.2 feet per
second per second (32.2 fps/s).
Addition. The process in which the value of one number is
added to the value of another.
Advisory Circulars (AC). Issued to inform the aviation
public in a systematic way of nonregulatory material. An
AC is issued to provide guidance and information in a
designated subject area or to show a method acceptable to
the Administrator for complying with a related 14 CFR part.
Aircraft Specifications. FAA recordkeeping documents
issued for both type-certificated and non-typecertificated
products which have been found eligible for U.S. airworthiness
certification.
Airfoil. Any device that creates a force, or lift, based on
Bernoulli’s principles or Newton’s laws, when air is caused
to flow over the surface of the device.
Airworthiness certificate. A document required to be
onboard an aircraft that indicates the aircraft conforms to
type design and is in condition for safe operation.
Airworthiness directive (AD). Issued by the FAA in
response to deficiencies and/or unsafe conditions found
in aircraft, engines, propellers, or other aircraft parts.
Compliance with an AD is mandatory.
Alclad aluminum. Used to designate sheets that consist of
an aluminum alloy core coated with a layer of pure aluminum
to a depth of approximately 51⁄2 percent on each side.
Glossary
Algebra. The branch of mathematics that uses letters or
symbols to represent numbers in formulas and equations.
Allowance. The difference of the upper and lower variation
of a part.
Alodizing. A simple chemical treatment for all aluminum
alloys to increase their corrosion resistance and to improve
their paint bonding qualities.
Alteration. A change or modification to an aircraft from its
previous state
Alternating current. An electric current that reverses
direction in a circuit at regular intervals.
Ammeter. An instrument for measuring electric current in
amperes.
Ampere. A unit of measure of the rate of electron flow or
current in an electrical conductor. One ampere of current
represents one coulomb of electrical charge (6.24 × 1018
charge carriers) moving past a specific point in one second.
Annealing. The process of heating a metal to a prescribed
temperature, holding it there for a specified length of time,
and then cooling the metal back to room temperature.
Annual inspection. An inspection required by the FAA
once every 12 calendar months if other suitable inspections
do not occur within that timeframe. An A&P technician with
inspection authorization must perform this inspection.
Anodizing. The most common surface treatment of nonclad
aluminum alloy surfaces. The aluminum alloy sheet or casting
is the positive pole in an electrolytic bath in which chromic
acid or other oxidizing agent produces an aluminum oxide
film on the metal surface. Aluminum oxide is naturally
protective, and anodizing merely increases the thickness and
density of the natural oxide film.
Apparent power. That power apparently available for use
in an AC circuit containing a reactive component. It is the
product of effective voltage times the effective current,
expressed in volt-amperes.
Archimedes’ principle. The buoyant force that a fluid exerts
G-2upon a submerged body is equal to the weight of the fluid
the body displaces.
Area. A measurement of the amount of surface inside a two-
dimensional object.
Arm. The horizontal distance that a part of the aircraft or a
piece of equipment is located from the datum.
Armature. The rotating part of an electric generator or motor.
Aspect ratio. The relationship of the length (wingtip to
wingtip), or span, of an airfoil to its width, or chord.
Assembly drawing. A description of an object made up of
two or more parts.
Atom. The smallest particle composed of a nucleus that
contains protons, neutrons, and electrons, which revolve
around the nucleus.
B
Ballast. A weight installed or carried in an aircraft to move
the center of gravity to a location within its allowable limits.
Base. In mathematics, used to refer to a particular mathematical
object that is used as a building block. A base-a system is
one that uses a as a new unit from which point counting starts
again. (See decimal system.) In the mathematical expression
an, read as “a to the nth power,” a is the base.
Basic empty weight. Standard empty weight plus optional
equipment.
Bernoulli’s principle. Equivalent to the principle of
conservation of energy, this principle states that the static
pressure of a fluid (liquid or gas) decreases at points where
the velocity of the fluid increases, provided no energy is
added to or taken away from the fluid.
Binary number system. The binary number system is a
number system that has only two digits, 0 (zero) and 1.
Binary numbers are made from a series of zeros and ones. An
example of an 8-bit binary number is 11010010. The prefix
“bi” in the word binary is a Latin root for the word “two.”
Block diagrams. Used to show a simplified relationship of
a more complex system of components.
Borescope. A device that enables the inspector to see
inside areas that could not otherwise be inspected without
disassembly.Boyle’s law. States that the volume of an enclosed dry gas
varies inversely with its absolute pressure, provided the
temperature remains constant.
Break lines. Line on a drawing indicating that a portion of
the object is not shown on the drawing.
British thermal unit (Btu). The amount of heat required
to change the temperature of 1 pound of water by 1 degree
Fahrenheit.
Buoyancy. The upward force that any fluid exerts on a body
submerged in it.
Buttock line (BL). The longitudinal axis of the aircraft that
serves as the reference location for positions to the left and
right of center. The positions are usually dimensioned in
inches.
C
Calorie. The amount of heat required to change the
temperature of 1 gram of water by 1 degree Centigrade.
Camber. The curvature of a wing as viewed by cross section.
A wing has upper camber on its top surface and lower camber
on its bottom surface. The upper camber is more pronounced;
the lower camber is comparatively flat. This causes the
velocity of the airflow immediately above the wing to be
much higher than that below the wing.
Capacitance (C). The property of an electric conductor that
characterizes its ability to store an electric charge.
Capacitive reactance (X c). The measure of a capacitor’s
opposition to alternating current.
Capacitor. An electrical component that stores an electric
charge.
Case hardening. A process in which the surface of a metal is
changed chemically by introducing a high carbide or nitride
content. Case hardening produces a hard, wear-resistant
surface, or case, over a strong, tough core.
Center of gravity (CG). The point about which the
nose-heavy and tail-heavy moments are exactly equal in
magnitude.
Center of gravity range. The center of gravity range for an
aircraft is the limits within which the aircraft must balance.
It is identified as a forward-most limit (arm) and an aft-most
limit (arm).
G-3Centrifugal force. The apparent force occurring in
curvilinear motion acting to deflect objects outward from the
axis of rotation. For instance, when pulling out of a dive, it
is the force pushing the pilot down in their seat.
Centripetal force. The force in curvilinear motion acting
toward the axis of rotation. For instance, when pulling out
of a dive, it is the force that the seat exerts on the pilot to
offset the centrifugal force.
Charles’ Law. States that all gases expand and contract in
direct proportion to the change in the absolute temperature,
provided the pressure is held constant.
Chemical energy. Energy released from chemical reactions.
Circular magnetization. The induction of a magnetic field
consisting of concentric circles of force about and within a
part, which is achieved by passing electric current through
the part.
Circumference (of a circle). The linear measurement of the
distance around a circle. The circumference is calculated by
multiplying the diameter of the circle by 3.1416.
Code of Federal Regulations (CFR). Established by law to
provide for the safe and orderly conduct of flight operations
and to prescribe airmen privileges and limitations.
Compression ratio. The ratio of the volume of a cylinder
with the piston at the bottom of its stroke to the volume of
the cylinder with the piston at the top of its stroke.
Computer aided design (CAD). Using a computer in the
design of a product.
Computer aided design drafting (CADD). Using a
computer in the design and drafting process.
Computer aided engineering (CAE). Using a computer in
the engineering of a product.
Computer aided manufacturing (CAM). Using a computer
in the manufacturing of a product.
Computer graphics. Drawing with the use of a computer.
Conduction. The transfer of heat which requires physical
contact between an object that has a large amount of heat
energy and one that has a smaller amount of heat energy.
Conductor. A material that will carry electric current.Convection. The process by which heat is transferred by
movement of a heated fluid (gas or liquid).
Corrosion. The deterioration of metal by chemical or
electrochemical attack.
Cosine (cos). A trigonometric function comparing two sides
of a right triangle as follows:
Cos =adjacent side
hypotenuse
Coulomb. A measure of electrical output. One coulomb is
6.24 × 1018 electrons.
Countersink. A tool that cuts a cone-shaped depression
around a hole in order to allow a rivet or screw to set flush
with the surface of the material.
Current. The flow of electrical charge.
D
Dalton’s Law. States that a mixture of several gases which
do not react chemically exerts a pressure equal to the sum of
the pressures which the several gases would exert separately
if each were allowed to occupy the entire space alone at the
given temperature.
Datum. An imaginary vertical plane or line from which all
measurements of arm are taken. The datum is established
by the manufacturer. Once the datum has been selected, all
moment arms and the location of CG range are measured
from this point.
Debonding. Separation of the bond between the skin
laminates and the core of a composite structure.
Decibels. The unit for measuring sound intensity. One
decibel is the smallest change in sound intensity the human
ear can detect.
Decimal system. The number system, also called the base-ten
system, based on the number 10. Consisting of ten symbols,
or digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9), the main principle is that
10 is considered as a new unit from which point counting
starts again.
Degradation. The alteration of material properties (e.g.,
strength, modulus, coefficient of expansion) which may result
from deviations in manufacturing or from repeated loading
and/or environmental exposure.
Delamination. Separation of the bond between the individual
plies of a laminated composite structure.
G-4Denominator. The lower part of a fraction (represented by
the letter D in N⁄D), the quantity by which the numerator is
divided.
Density. The weight of a substance per unit volume.
Detail drawing. A description of a single part, given in
such a manner as to describe bylines, notes, and symbols
the specifications for size, shape, material, and methods of
manufacture to be used in making the part.
Detailed inspection. A thorough examination of an item
including disassembly. The overhaul of a component is
considered to be a detailed inspection.
Detonation. Uncontrolled burning of fuel in the cylinder of
a reciprocating engine. Detonation causes explosive burning
of the fuel which creates an increased cylinder pressure,
excessive cylinder head temperature, and decreased engine
performance.
Dew point. The temperature to which humid air must be
cooled at constant pressure to become saturated.
Dial indicator. Measures variations in a surface by using an
accurately machined probe mechanically linked to a circular
hand whose movement indicates thousandths of an inch, or
is displayed on a liquid crystal display (LCD) screen.
Diameter (circle). The length of a line passing directly
through the center of a circle. Twice the radius of the circle.
Die. Used for cutting external threads on round stock.
Difference. The answer to a subtraction problem.
Direct current (DC). Electricity that flows in one direction
at all times.
Directional stability. Stability about the vertical axis of an
aircraft, whereby an aircraft tends to return, on its own, to
flight aligned with the relative wind when disturbed from
the equilibrium state.
Discontinuity. An interruption in the normal physical
structure or configuration of a part, such as a crack, forging
lap, seam, inclusion, porosity, and the like. A discontinuity
may or may not affect the usefulness of a part.
Dissimilar metal corrosion. Caused by contact between
dissimilar metal parts in the presence of a conductor.Dividend. In a division problem, the number to be divided
by the divisor. In 6 ÷ 2 = 3, the dividend is 6.
Division. The process of finding how many times one number
(the divisor) is contained in another number (the dividend).
Divisor. In a division problem, the number by which dividend
is to be divided. In 6 ÷ 2 = 3, the divisor is 2.
Doping. The process by which small amounts of additives
called impurities are added to the semiconductor material
to increase their current flow by adding a few electrons or
a few holes.
Dynamic stability. The property of an aircraft that causes
it, when disturbed from straight-and level flight, to develop
forces or moments that restore the original condition of
straight and level.
E
Eddy current inspection. An inspection method where eddy
currents are induced into the material to be tested. In aircraft
manufacturing plants, eddy current is used to inspect castings,
stampings, machine parts, forgings, and extrusions.
Electrical energy. Electrical energy is converted to heat
energy when an electric current flows through any form
of resistance such as an electric iron, electric light, or an
electric blanket.
Electromotive force (EMF). The pressure or force that
causes electrons to flow in an electrical circuit.
Electrostatic field. A field of force that exists around a
charged body.
Empennage. The section of the airplane that consists of the
vertical stabilizer, the horizontal stabilizer, and the associated
control surfaces.
Empty-weight center of gravity range. The distance
between the allowable forward and aft empty-weight CG
limits.
Empty-weight center of gravity. The center of gravity of
an aircraft when it contains only the items specified in the
aircraft empty weight.
Empty weight. See standard empty weight.
Energy. The capacity of a physical system to perform work.
There are two types of energy, kinetic and potential.
G-5Exploded view drawing. A pictorial drawing of two or more
parts that fit together as an assembly. The view shows the
individual parts and their relative position to the other parts
before they are assembled.
Exponent (power). A shorthand method of indicating how
many times a number, called the base, is multiplied by itself.
For example, in the number 43, the 3 is the power or exponent
and 4 is the base. That is, 43 is equal to 4 × 4 × 4 = 64.
Extension lines. Used to extend the line showing the side
or edge of a figure for the purpose of placing a dimension
to that side or edge.
F
FAA Form 337. This form must be completed when a major
repair or alteration is accomplished.
Ferrous metals. Metals having iron as their principal
constituent.
Force. The intensity of an impetus, or the intensity of an
input.
Foreign object damage (FOD). Any damage caused by
any loose object to aircraft, personnel, or equipment. These
loose objects can be anything from broken runway concrete
to shop towels and safety wire.
Fraction. A number written in the form N⁄D in which N is
the numerator and D is the denominator. For example, 5⁄16
is a fraction.
Frequency. The number of cycles (on/off) completed per
unit of time. Usually expressed in Hertz.
Fretting corrosion. Occurs when two mating surfaces,
normally at rest with respect to one another, are subject to
slight relative motion.
Friction. The opposition to movement between objects.
Fuel grade. The rating system used for aviation gasoline.
It rates fuel according to its antidetonation characteristics.
Fuse. A protective device containing a special wire that melts
when current exceeds the rated value for a definite period.
Fuselage stations (FS). Reference locations, usually given
in inches, used to determine forward and aft positions on an
aircraft. FS − 0 is the datum.H
Heat. The total kinetic energy of the molecules of any
substance.
Henry. The basic unit of inductance, symbolized with the
letter H. An electric circuit has an inductance of one henry
when current changing at the rate of one ampere per second
induces a voltage of one volt into the circuit.
Hermaphrodite caliper. Generally used as a marking
gauge in layout work. It should not be used for precision
measurement.
Hidden lines. Indicates invisible edges or contours.
Horsepower. A measure of power equal to 550 foot-pounds
per second or 33,000 foot-pounds per minute and 746 Watts.
Hot start. Occurs when the engine starts, but the exhaust
gas temperature exceeds specified limits. This is usually
caused by an excessively rich air-fuel mixture entering the
combustion chamber.
Humidity. The amount of water vapor in the air.
Hung start. Occurs when the engine starts normally, but the
rpm. remains at some low value rather than increasing to the
normal starting rpm. This is often the result of insufficient
power to the starter, or the starter cutting off before the engine
starts self-accelerating.
Hydrometer. An instrument for determining the specific
gravity of liquids.
Hypotenuse. The side of a right triangle that is opposite
the right angle. The hypotenuse is the longest side of a right
triangle.
I
Improper fraction. A fraction with the numerator equal to
or greater than the denominator.
Inside calipers. Calipers with outward curved legs for
measuring inside diameters, such as diameters of holes.
Installation drawing. A drawing that includes all necessary
information for a part or an assembly in the final installed
position in the aircraft.
Intergranular corrosion. An attack along the grain
boundaries of an alloy that commonly results from a lack of
uniformity in the alloy structure.
G-6Inductance. The ability of a coil or conductor to oppose a
change in current flow.
Inductive reactance. The opposition to the flow of current
which inductances put in a circuit.
Inductor. A coil of wire that produces inductance in an
electrical circuit.
Insulator. A material that does not conduct electrical current
very well or not at all. Examples are glass, ceramic, and
plastic.
Ion. An atom or group of atoms in which the number of
electrons is different from the number of protons. It is a
positive ion if the number of electrons is less than the number
of protons, and a negative ion if the number of electrons is
greater than the number of protons.
Isometric drawings. A drawing that uses a combination of
the views of an orthographic projection and tilts the object
forward so that portions of all three views can be seen in
one view.
J
Joule. The amount of work done by a force of one newton
when it acts through a distance of one meter.
K
Kinetic energy. Energy due to motion, defined as one half
mass times velocity squared.
Kirchhoff’s Law (voltage). A basic law of electrical currents
stating that the algebraic sum of the applied voltage and the
voltage drop around any closed circuit is zero.
L
Lateral stability. The stability about the longitudinal axis of
an aircraft; the rolling stability, or the ability of an airplane
to return to level flight due to a disturbance that causes one
of the wings to drop.
Lever. The simplest machine. There are three basic parts
in all levers: the fulcrum “F,” a force or effort “E,” and a
resistance “R.”
Load cell. A component in an electronic weighing system that
is placed between the jack and the jack pad on the aircraft.
The load cell contains strain gauges whose resistance changes
with the weight on the cell.Longitudinal magnetization. The magnetic field is produced
in a direction parallel to the long axis of the part. This is
accomplished by placing the part in a solenoid excited by
electric current.
Longitudinal stability. The tendency for an aircraft nose
to pitch up or pitch down, rotating around the lateral axis
(wingtip to wingtip).
M
Magnetic particle inspection. A method of detecting
invisible cracks and other defects in ferromagnetic materials
such as iron and steel. The inspection process consists
of magnetizing the part and then applying ferromagnetic
particles to the surface area to be inspected.
Maintenance. This includes inspection, overhaul, repair,
preservation, and the replacement of parts, but excludes
preventive maintenance.
Major alteration. An alteration not listed in the aircraft,
aircraft engine, or propeller specifications: (1) that might
appreciably affect weight, balance, structural strength,
performance, powerplant operation, flight characteristics,
or other qualities affecting airworthiness; or (2) that is not
done according to accepted practices or cannot be done by
elementary operations.
Major repairs. A repair that (1) if improperly done, might
appreciably affect weight, balance, structural strength,
performance, powerplant operation, flight characteristics,
or other qualities affecting airworthiness, or (2) is not
done according to accepted practices, or cannot be done by
elementary operations.
Malfunction or Defect Report. A report (FAA Form 8010-
4) providing the FAA and industry with a very essential
service record of mechanical difficulties encountered in
aircraft operations. Such reports contribute to the correction
of conditions or situations which otherwise will continue
to prove costly and/or adversely affect the airworthiness
of aircraft.
Manufacturer’s maintenance manual. A manual
provided by an aircraft manufacturer that outlines the
methods, techniques, and practices prescribed for each
person performing maintenance, alteration, or preventive
maintenance on an aircraft, engine, propeller, or appliance.
Mass. A measure of the quantity of matter in an object.
Matter. Any substance that has mass and takes up space.
G-7Maximum landing weight. The heaviest weight an aircraft
can have when it lands. For large wide body commercial
airplanes, it can be 100,000 pounds less than maximum
takeoff weight, or even more.
Maximum ramp weight. The heaviest weight to which
an aircraft can be loaded while it is sitting on the ground,
sometimes referred to as the maximum taxi weight.
Maximum takeoff weight. The heaviest weight an aircraft
can have when it starts the takeoff roll. The difference
between this weight and the maximum ramp weight would
equal the weight of the fuel that would be consumed prior
to takeoff.
Maximum weight. The maximum authorized weight of
the aircraft and its contents, and is indicated in the Aircraft
Specifications or Type Certificate Data Sheet.
Maximum zero fuel weight. The heaviest weight an aircraft
can be loaded to without having any usable fuel in the fuel
tanks. Any weight loaded above this value must be in the
form of fuel.
Mean aerodynamic chord (MAC). The average distance
from the leading edge to the trailing edge of the wing.
Mechanical advantage. A ratio of the resistance force to
the effort force.
Mechanical energy. This includes all methods of producing
increased motion of molecules such as friction, impact of
bodies, or compression of gases.
METO horsepower. The maximum power allowed to
be continuously produced by an engine. Takeoff power is
usually limited to a given amount of time, such as 1 minute
or 5 minutes.
Mixed number. A combination of a whole number and a
fraction. For example, 53⁄8 is a mixed number.
Molecule. The smallest particle of an element or compound
that retains the chemical properties of the element or
compound.
Moment. In determining weight and balance, the moment is
the product of a weight multiplied by its arm.
MS flareless fittings. Designed primarily for highpressure
(3,000 psi) hydraulic systems that may be subjected to severe
vibration or fluctuating pressure. Using this type of fitting
eliminates all tube flaring, yet provides a safe and strong, dependable tube connection. The fitting consists of three
parts: a body, a sleeve, and a nut.
Multiplication. The process of repeated addition.
N
Negative number. A number that is less than zero.
Nomogram. A graph that usually consists of three sets of
data. Knowledge of any two sets of data enables the reader
to determine the third set.
Normalizing. The process of heating the part to the proper
temperature, holding it at that temperature until it is uniformly
heated, and then cooling it in still air.
Nuclear energy. Energy stored in the nucleus of atoms is
released during the process of nuclear fission in a nuclear
reactor or atomic explosion.
Numerator. The upper part of a fraction (represented by
the letter N in N⁄D).
O
Oblique view. A view that is similar to an isometric view
except with two of the three drawing axes always at right
angles to each other.
Ohm’s Law. Explains the relationship between voltage,
current, and resistance in an electrical circuit, and states that
current flow in an electrical circuit is directly proportional to
the amount of voltage applied to the circuit.
Ohmmeter. A current measuring instrument that provides
its own source (self-excited) of power.
Ohm. The standard unit used to measure resistance.
One-hundred-hour inspection. A complete inspection
that is required for all aircraft operated for hire every 100
hours. An annual inspection must be conducted by an A&P
mechanic with Inspection Authorization.
Operating center of gravity range. The center of gravity
for an aircraft loaded and ready for flight.
Orthographic projection. A method of showing all six
possible views of an object: front, top, bottom, rear, right
side, and left side.
Outside calipers. Used for measuring outside dimensions,
such as the diameter of a piece of round stock.
G-8P
Parallel circuit. A circuit in which two or more electrical
resistances or loads are connected across the same voltage
source.
Pascal’s Law. The law that states that pressure applied
anywhere to a body of fluid causes a force to be transmitted
equally in all directions; the force acts at right angles to any
surface in contact with the fluid.
Percentage. Used to express a number as a fraction of 100.
Using the percentage sign, %, 90 percent is expressed as 90%.
Permeability. Used to refer to the ease with which a magnetic
flux can be established in a given magnetic circuit.
Perspective view. A drawing that shows a three-dimensional
object (portraying height, width, and depth) as it appears to an
observer. It most closely resembles the way an object would
look in a photograph.
Phantom line. Composed of one long and two short evenly
spaced dashes, indicates an alternate position of parts of the
object or the relative position of a missing part.
Pictorial drawing. A drawing that is similar to a photograph.
It shows an object as it appears to the eye, but it is not
satisfactory for showing complex forms and shapes.
Pitch (aircraft maneuver). Rotation of an aircraft about
its lateral axis.
Pitch (rivet layout dimension). The distance between the
centers of adjacent rivets installed in the same row.
Pitch (thread dimension). The linear distance, measured
parallel to the length of a threaded fastener, between
corresponding points on two adjacent threads.
Pitch angle (helicopter rotor blade). The angle between
the chord line of a rotor blade and the reference plane of the
main rotor hub, or the plane of rotation of the rotor.
Pitch angle (propeller specification). The angle between
the chord line of a propeller blade and the plane of rotation.
Pitch axis (aircraft axis). The lateral axis of an aircraft that
extends from wing tip to wing tip and passes through the
center of gravity. This is the axis about which the aircraft
pitches.
Pitch distribution (propeller specification). The gradual
twist in the propeller blade from shank to tip.Plumb bob. A heavy metal object, cylinder- or coneshaped,
with a sharp point at one end that is suspended by a string
to produce a vertical reference line useful in aircraft
measurements.
Positive number. A number that is greater than zero.
Potential difference. A difference in electrical pressure.
Potential energy. Energy that is stored.
Potentiometer. A variable tapped resistor that can be used
as a voltage divider.
Power (exponent). A shorthand method of indicating how
many times a number, called the base, is multiplied by itself.
For example, in the number 43, 3 is the power, or exponent,
and 4 is the base. That is, 43 is equal to 4 × 4 × 4 = 64.
Power. Power is the time rate at which work is done or
energy is transferred.
Powers of ten. Also called scientific notation. It is a shorthand
method of depicting very large or very small numbers.
Pressure. The amount of force acting on a specific amount
of surface area, typically measured in pounds per square
inch or psi.
Preventive maintenance. Simple or minor preservation
operations and the replacement of small standard parts not
involving complex assembly operations.
Product. The result of multiplication.
Progressive inspection. Breaking down the large task of
conducting a major inspection into smaller tasks which can
be accomplished periodically without taking the aircraft out
of service for an extended period of time.
Proportion. A proportion is a statement of equality between
two or more ratios. The example of A is to B as C is to D can
be represented A:B = C:D or A/B = C/D.
Pythagorean Theorem. An equation used to find the length of
a third side of any right triangle when the lengths of two sides
are known. The Pythagorean Theorem states that a2 + b2 = c2.
The square of the hypotenuse (side opposite the right angle) is
equal to the sum of the squares of the other two sides (a and b).
Q
Quenching. The rapid cooling of metal in the heat-treatment
G-9process.
Quotient. The result of dividing two numbers.
Radiant energy. Electromagnetic waves of certain
frequencies produce heat when they are absorbed by the
bodies they strike such as x-rays, light rays, and infrared rays.
R
Radiation. The continuous emission of energy from the
surface of all bodies.
Radical sign. The symbol √, used to indicate the root of a
number.
Radiographic inspection. Inspection using radiography to
locate defects or flaws in airframe structures or engines with
little or no disassembly.
Radius (circle). Equal to one-half the diameter of a circle.
Ratio. The comparison of two numbers or quantities.
Reamers. Tools made of either carbon tool steel or high-
speed steel that are used to smooth and enlarge holes to
exact size.
Rectifier. A device for converting alternating current to
direct current.
Relative humidity. The ratio of the amount of water vapor
actually present in the atmosphere to the amount that
would be present if the air were saturated at the prevailing
temperature and pressure.
Remainder. The leftover number in the process of division.
Repair. The restoration of an aircraft component to its
previous state.
Repair station. A maintenance facility certificated under 14
CFR part 145 to perform maintenance functions.
Resistance. The opposition a device or material offers to
the flow or current.
Resonance. The increase in amplitude of vibrations of
an electric or mechanical system exposed to a periodic
force whose frequency is equal or very close to the natural
frequency of the system.
Retentivity. The ability of a material to hold its magnetism
after the magnetizing field has been removed.Rheostat. A variable resistor used to vary the amount of
current flowing in a circuit.
Root. A number that when multiplied by itself a specified
number of times will produce a given number. The two most
commonly used roots are the square root and the cube root.
Routine inspection. A visual examination or check of an
item in which no disassembly is required.
S
Schematic diagram. A diagram that locates components
with respect to each other within a system.
Scientific notation. Used as a type of shorthand to express
very large or very small numbers. For example, to express
1,250,000,000,000 in scientific notation is 1.25 × 1012.
Sea level pressure. The atmospheric pressure at sea level.
Average sea level pressure is 29.92 inches of mercury, or
1013.25 millibars.
Sectional view. A view obtained by cutting away part of an
object to show the shape and construction at the cutting plane.
Semiconductor. Any device based on either preferred
conduction through a solid in one direction, as in rectifiers,
or on a variation in conduction characteristics through a
partially conductive material, as in a transistor.
Series circuit. The most basic electrical circuit in which
there is only one possible path for current to flow. Current
must pass through the circuit components, the battery and
the resistor, one after the other, or “in series.”
Series-parallel DC circuits. A grouping of parallel resistors
connected in series with other resistors.
Signed numbers. A signed number can be either a positive
or negative number. A positive number is a number that is
greater than zero. A negative number is a number that is
less than zero.
Sine. A trigonometric function comparing two sides of a
right triangle as follows:
Sine =opposite side
hypotenuse
Sine wave. A continuous waveform with a constant
frequency and amplitude.
Sketch. A simple rough drawing that is made rapidly and
without much detail.
G-10Slide caliper. Often used to measure the length of an object.
It provides greater accuracy than a ruler.
Solenoid. A loop of wire, often wrapped around a metal core,
which produces a magnetic field when an electrical current
is passed through it.
Specific gravity. The ratio of the mass of a solid or liquid
to the mass of an equal volume of water.
Specific heat. The quantity of heat necessary to increase the
temperature of a unit of the mass of a substance 1 °C. The
specific heat of a substance is the ratio of its specific heat
capacity to the specific heat capacity of water.
Speed of sound. The speed of sound at sea level under
standard temperature and pressure conditions is 1,108 feet
per second or 658 knots.
Spirit level. A leveling instrument placed on or against a
specified place on the aircraft. Spirit levels have vials that
are full of liquid, except for a small air bubble. When the
air bubble is centered between the two black lines, a level
condition is indicated.
Square root. A non-negative number that must be multiplied
by itself to equal a given number.
Standard empty weight. The weight of the airframe,
engines, all permanently installed equipment, and unusable
fuel. Depending upon the part of the Federal regulations under
which the aircraft was certificated, either the undrainable oil
or full reservoir of oil is included.
Standard weights. Values used in weight and balance
calculations if specific weight for an item is unknown. The
following are examples:
• Aviation gasoline 6 pounds per gallon
• Crew and passengers 170 pounds per person
• Lubricating oil 7.5 pounds per gallon
• Turbine fuel 6.7 pounds per gallon
• Water 8.35 pounds per gallon
Static stability. The initial response that an airplane displays
after its equilibrium is disrupted.
Strain. A deformity or change in an object due to stress.
Stress corrosion. Occurs as the result of the combined effect
of sustained tensile stresses and a corrosive environment.Stress. The internal resistance of an object to external
forces attempting to strain or deform that object. Measured
in pounds per square foot or pounds per square inch (psi).
Subtraction. The process where the value of one number is
taken from the value of another.
Sum. The resulting answer in the addition process.
Supplemental Type Certificates (STC). A document
issued by the FAA approving a product (aircraft, engine, or
propeller) modification.
Surface corrosion. Caused by either direct chemical or
electrochemical attack, it appears as a general roughening,
etching, or pitting of the surface of a metal, frequently
accompanied by a powdery deposit of corrosion products.
Swaged Fittings. These fittings create a permanent
connection that is virtually maintenance free. Swaged
fittings are used to join hydraulic lines in areas where routine
disconnections are not required and are often used with
titanium and corrosion resistant steel tubing.
T
Tangent (tan). A trigonometric function comparing two
sides of a right triangle as follows:
Tan =opposite side
adjacent side
Tap. Instrument used to cut threads on the inside of a hole.
Tare weight. The weight of any chocks or devices used
to hold an aircraft on scales when it is weighed. The tare
weight must be subtracted from the scale reading to get the
net weight of the aircraft.
Tempering. Process that reduces the brittleness imparted by
hardening and produces definite physical properties within
the steel. Tempering always follows, never precedes, the
hardening operation.
Thermal expansion. The increase in size of a material as
temperature increases.
Tolerance. The sum of the plus and minus allowance figures.
Torque. The tendency of a force to cause or change rotational
motion of a body.
Transformer. A device that changes electrical energy of a
given voltage into electrical energy at a different voltage level.
G-11It consists of two coils that are not electrically connected,
but arranged so that the magnetic field surrounding one coil
cuts through the other coil.
Transistor. A three-terminal device primarily used to
amplify signals and control current within a circuit.
Trapezoid. A four-sided figure with one pair of parallel sides.
True power. The power dissipated in the resistance of a
circuit, or the power actually used in the circuit.
Triangle. A three-sided figure in which the sum of the three
angles equal 180°.
Trigonometry. The study of the relationships between the
angles and sides of a triangle.
Type Certificate Data Sheet (TCDS). The FAA issues a
type certificate when a new aircraft, engine, propeller, etc.,
is found to meet safety standards set forth by the FAA. The
TCDS lists the specifications, conditions and limitations
under which airworthiness requirements were met for the
specified product, such as engine make and model, fuel type,
engine limits, airspeed limits, maximum weight, minimum
crew, etc.
U
Ultrasonic inspection. Uses high frequency sound energy to
conduct examinations and make measurements. Ultrasonic
inspection can be used for flaw detection/evaluation,
dimensional measurements, and material characterization.
Useful load. Fuel, any other fluids that are not part of
empty weight, passengers, baggage, pilot, copilot, and
crewmembers. It is determined by subtracting the empty
weight from the maximum allowable gross weight.
V
Vapor pressure. The portion of atmospheric pressure that
is exerted by the moisture in the air (expressed in tenths of
an inch of mercury).
Volt. The basic unit of electrical potential or electromotive
force. A potential of one volt appears across a resistance of
one ohm when a current of one ampere flows through that
resistance.
Voltmeter. A current-measuring instrument, designed to
indicate voltage by measuring the current flow through a
resistance of known value.Volume. The amount of space within a three-dimensional
solid.
W
Waterline (WL). A horizontal reference plane used to locate
vertical positions on an aircraft. Positions are usually given
in inches above or below the waterline.
Watt. A unit of power equal to one joule per second.
Weighing points. Locations on an aircraft that the
manufacturer designates for the placement of scales when
weighing aircraft.
Weight. A measure of the pull of gravity acting on the mass
of an object.
Whole numbers. The numbers: 0, 1, 2, 3, 4, 5, and so on.
Wiring diagrams. A diagram that shows the electrical wiring
and circuitry, coded for identification, of all the electrical
appliances and devices used on aircraft.
Work. The amount of energy transferred by a force.
Z
Zero fuel weight. The weight of an aircraft without fuel.
Zone numbers. On drawings, these are similar to the
numbers and letters printed on the borders of a map, used
for locating a particular point in the drawing.
I-1
Index
Symbols
100-Hour Inspections ................................................... 10-5
A
Abrasive Papers ........................................................... 8-28
Acceleration ................................................................. 5-15
AC Circuits ................................................................ 12-59
AC Motors ............................................................... 12-147
Direction of Rotation of Induction ....................... 12-150
Induction Motor Slip ............................................ 12-149
Rotating Magnetic Field ....................................... 12-148
Shaded Pole Induction .......................................... 12-149
Single-Phase Induction ......................................... 12-149
Split-Phase ............................................................ 12-150
Synchronous ......................................................... 12-151
Three-Phase Induction .......................................... 12-148
Acoustic Emission ..................................................... 10-28
AC Series ................................................................. 12-152
Acting force ................................................................. 5-17
Advancing Blade .......................................................... 5-56
Adverse-Loaded CG Checks ........................................ 6-20
Advisory Circulars (ACs) ............................................ 2-10
Aerodynamic Heating .................................................. 5-49
Aeronautical Information Manual (AIM) .................... 1-20
After Solution ............................................................... 7-22
Aircraft Certification Service (AIR) .............................. 2-2
Aircraft Drawings .......................................................... 4-1
Aircraft Listings ........................................................... 2-27
Aircraft Logs ................................................................ 10-1
Aircraft Maintenance Manual (AMM) .....2-33, 2-44, 4-11
Aircraft Maintenance Technicians (AMTs) ................... 1-1
Aircraft on ground (AOG) ......................................... 14-19
Aircraft Registration .................................................... 2-37
Aircraft Theory of Flight ............................................. 5-36
Aircraft Weighing .......................................................... 6-1
Airfoils ......................................................................... 5-38
Airplane Flight Manual (AFM) ............................ 2-33,6-7
Air traffic control (ATC) ............................................. 2-22
Air Transport Association (ATA) ................................ 10-4
Air Transport Association (ATA) 100 ......................... 2-30
Airworthiness Directives (AD) .................................... 2-26
Airworthiness Directives (ADs) .................................. 10-4
Airworthiness Limitations ........................................... 2-33
Alclad Aluminum ......................................................... 7-21
Aliphatic and Aromatic Naphtha ................................. 8-27
Allowances ..................................................................... 4-9Allowances and Tolerances ........................................... 4-9
Alloying ......................................................................... 7-7
Alodizing ...................................................................... 8-22
Alternating Current (AC) ........................................... 12-43
Alternators ................................................... 12-154 ,12-171
Alternator Transistorized Regulators ....................... 12-172
Altimeter .................................................................... 10-12
Aluminum ...................................................................... 7-5
Aluminum Alloy Rivets ............................................... 7-24
Aluminum Alloys ........................................ 7-5, 7-20, 7-23
Aluminum Alloy Tubing ................................................ 9-1
American Iron and Steel Institute (AISI) ....................... 7-2
American National Fine (NF) ...................................... 7-37
American National Standards Institute (ANSI) ............. 4-7
Amplifier Circuits .................................................... 12-113
Classification ........................................................ 12-113
Class A .............................................................. 12-114
Class AB ........................................................... 12-114
Class B .............................................................. 12-114
Class C .............................................................. 12-114
AN Flared Fittings ......................................................... 9-8
Angle of Attack ............................................................ 5-38
Annealing ..................................................................... 7-23
Anodizing ..................................................................... 8-22
Anti-Torque Systems ................................................... 5-50
Apparent Power ......................................................... 12-64
Approved Airplane Inspection Program (AAIP) ......... 2-12
Arc Fault Circuit Breaker ........................................... 12-29
Arm ................................................................................ 6-2
Armatur .................................................................... 12-155
Armature ..................................................... 12-130 ,12-131
Drum-Type ....................................................... 12-130
Gramme-Ring ................................................... 12-130
Atom ............................................................................ 12-1
Attraction ....................................................................... 5-1
Autorotation ................................................................. 5-56
Auxiliary Power Units (APUs) ............................. 1-4,1-16
Aviation gasoline (AVGAS) ........................................ 1-25
Aviation Gasoline (AVGAS) ....................................... 1-25
Aviation Safety Inspector (ASI) .................................... 2-4
Axes of an Aircraft ....................................................... 5-40
B
Baking Soda ................................................................. 8-29
Ballast .......................................................................... 6-22
Barcol Tester ................................................................ 7-27
I-2Basic Inspection ........................................................... 10-1
Preparation ................................................................ 10-1
Techniques/Practices ................................................ 10-1
Batteries ..................................................................... 12-87
Lead-Acid ............................................................... 12-91
Primary Cell ........................................................... 12-87
Secondary Cell ....................................................... 12-87
Bearing Load Fasteners ................................................ 7-60
Bending ........................................................................ 9-21
Bernoulli’s Principle .......................................... 5-29, 5-37
Bilateral Aviation Safety Agreement (BASA) ............ 2-18
Bilge Areas ................................................................... 8-12
Bill of Material ............................................................... 4-7
Binary Number System ................................................ 3-27
Blade Flapping ............................................................. 5-55
Bolts ................................................................... 7-37, 7-47
Close Tolerance .................................................... 7-38
General Purpose .................................................... 7-37
Identification and Coding ..................................... 7-38
Internal Wrenching ............................................... 7-38
Special-Purpose .................................................... 7-39
Bonded ....................................................................... 10-26
Borescope ................................................................... 10-18
Boundary Layer Airflow .............................................. 5-39
Boyle’s Law ....................................................... 5-22, 5-24
Brinell Tester ............................................................... 7-26
British Thermal Unit (BTU) .......................................... 5-3
Brushless Alternator ................................................. 12-157
Buna-N ......................................................................... 9-16
Buoyancy ..................................................................... 5-26
Butyl ............................................................................. 9-16
C
Calipers ...................................................................... 11-19
Camber ......................................................................... 5-38
Capacitance ................................................................ 12-50
Alternating Current ................................................. 12-54
Capacitive Reactance Xc ........................................ 12-54
Direct Current ......................................................... 12-50
Parallel .................................................................... 12-54
Reactances in Series and in Parallel ....................... 12-55
Series ...................................................................... 12-53
Capacitor .................................................................... 12-50
Capacitor Start Motor .............................................. 12-150
Capstan servo ................................................................. 4-2
Carbon dioxide (CO 2) extinguishers .............................. 1-5
Casting ......................................................................... 7-28
Center of gravity (CG) .................................. 2-5, 5-40, 6-3
Centigrade scale ........................................................... 5-21
Centripetal force ........................................................... 5-17
Cessna 172 ................................................................... 5-11CG Envelopes .............................................................. 6-24
CG Range ..................................................................... 6-15
Charles’ Law ................................................................ 5-25
Checklists ..................................................................... 10-2
Chemical Cleaners ....................................................... 8-29
Chemical Energy .......................................................... 5-18
Chisels .......................................................................... 11-9
Chord Line ................................................................... 5-38
Circuit Breaker ........................................................... 12-29
Circuit Protection Devices ......................................... 12-27
Circular Conductors ................................................... 12-20
Circular Motion ............................................................ 5-17
Civil Air Regulations (CAR) ....................................... 2-23
Civil Aviation Authority (CAA) ....................... 2-23,14-28
Cleaning .............................................................. 8-23,8-24
Exterior ..................................................................... 8-23
Interior ...................................................................... 8-24
Container Controls ................................................ 8-25
Fire Prevention Precautions .................................. 8-25
Fire Protection Recommendations ........................ 8-26
Flammable and Combustible Agents .................... 8-25
Nonflammable Aircraft Cabin Cleaning Agents and
Solvents ................................................................. 8-25
Types of Cleaning Operations .............................. 8-24
Code of Federal Regulations (CFRs) ........................... 10-4
Commutators ............................................................ 12-130
Compensating Windings .......................................... 12-131
Component Maintenance Manual (CMM) ................... 2-33
Composite Materials .................................................... 7-30
Compressibility Effects ................................................ 5-46
Computer Graphics ........................................................ 4-1
Computer Aided Design (CAD) ................................. 4-1
Computer Aided Design Drafting (CADD) ............... 4-1
Computer Aided Engineering (CAE) ......................... 4-1
Computer Aided Manufacturing (CAM) .................... 4-1
Computing Area of Two-Dimensional Solids ............. 3-18
Computing Surface Area of Three-Dimensional Solids 3-23
Computing Volume of Three-Dimensional Solids ......3-20
Conduction ................................................................... 5-19
Conductors ..................................................... 12-18 , 12-23
Contamination Control ................................................. 1-25
Convection ................................................................... 5-20
Conventional Flow ..................................................... 12-14
Cooling Air Vents ........................................................ 8-13
Copper .................................................................. 7-10, 9-1
Copper Alloys .............................................................. 7-10
Corrosion ........................................................................ 8-1
Forms .......................................................................... 8-5
Concentration Cell .................................................. 8-6
Active-Passive ..................................................... 8-7
Metal Ion Concentration ...................................... 8-7
I-3Oxygen Concentration ......................................... 8-7
Dissimilar Metal ..................................................... 8-6
Exfoliation .............................................................. 8-7
Fatigue .................................................................... 8-9
Filiform ................................................................... 8-5
Fretting .................................................................... 8-9
Galvanic ................................................................ 8-10
Intergranular ........................................................... 8-7
Pitting ...................................................................... 8-6
Stress-Corrosion/Cracking ...................................... 8-7
Surface .................................................................... 8-5
Types .......................................................................... 8-2
Direct Chemical Attack .......................................... 8-2
Electrochemical Attack ........................................... 8-3
Corrosion Control ................................................. 8-1,8-21
Corrosion Limits .......................................................... 8-20
Corrosion Prone Areas ................................................. 8-11
Corrosion Removal ...................................................... 8-14
Counter Electromotive Force (emf) ......................... 12-142
Countersink ................................................................ 11-14
Couplants ................................................................... 10-26
Coupling ................................................................... 12-115
Direct .................................................................... 12-115
Impedance ............................................................ 12-116
RC ......................................................................... 12-115
Transformer .......................................................... 12-116
Cube Roots ................................................................... 3-13
Current ....................................................................... 12-16
Current Dividers ......................................................... 12-41
Current Limiter .......................................................... 12-29
Current Transformers ................................................. 12-67
D
Dalton’s Law ................................................................ 5-25
Datum ................................................................... 6-2, 6-17
DC Generator .............................................. 12-123 ,12-135
Compound Wound ............................................... 12-134
Series Wound ....................................................... 12-132
Shunt Wound ........................................................ 12-133
DC Motor ...................................... 12-137 , 12-138 , 12-140
Armature ............................................................... 12-140
Brush .................................................................... 12-141
End Frame ............................................................ 12-141
Field ...................................................................... 12-141
DC Voltage .............................................................. 12-125
decibel (dB) .................................................................. 5-32
Defueling ...................................................................... 1-28
Delta Connection (Three Phase) .............................. 12-156
Density ........................................................................... 5-2
Department of Defense (DoD) ..................................... 2-41
Department of Transportation ........................................ 2-1Designated Airworthiness Representatives (DAR) .....2-14
Designated Engineering Representatives (DER) ......... 2-13
Designated Manufacturing Inspection Representatives
(DMIR) ........................................................................ 2-14
Dies ............................................................................ 11-14
Differential Relay Switch ........................................ 12-162
Digital Images .............................................................. 4-23
Digital Multimeter ..................................................... 12-78
Dihedral ........................................................................ 5-42
Doping ........................................................................ 12-96
Doppler Effect .............................................................. 5-32
Double Flaring ............................................................... 9-5
Beading ....................................................................... 9-7
Double Flaring Instructions ........................................ 9-5
Fittings ........................................................................ 9-5
Flareless Fittings ........................................................ 9-5
Drain Cocks ................................................................. 7-77
Drills .......................................................................... 11-12
Dual In-Line Parallel (DIP) Switches ........................ 12-33
E
Eddy Current .............................................................. 10-20
Electrical Conductivity .............................................. 10-37
Electrical Energy .......................................................... 5-18
Electrical Safety ............................................................. 1-1
Fire Safety .................................................................. 1-1
Physiological Safety ................................................... 1-1
Electrochemical Test ...................................................... 7-4
Electrodynamometer Meter Movement ..................... 12-74
Electrolytic ................................................................. 12-52
Electromagnetism ...................................................... 12-12
Electromotive Force (Voltage) .................................. 12-14
Electron ........................................................................ 12-2
Conductors ............................................................ 12-3
Conductors, Insulators, and Semiconductors ........ 12-2
Insulators ............................................................... 12-3
Semiconductors ..................................................... 12-4
Electron Flow ............................................................. 12-14
Electronic Numerical Integrator and Calculator .......... 3-27
Electronic Weighing .................................................... 6-28
Electron Shells ............................................................. 12-2
Electrons, Protons, and Neutrons ................................. 12-1
Electrostatic Field ........................................................ 12-5
Element ........................................................................ 12-1
Elements of Human Factors ......................................... 14-2
Anthropometry ......................................................... 14-4
Clinical Psychology .................................................. 14-3
Cognitive Science ..................................................... 14-4
Computer Science ..................................................... 14-4
Educational Psychology ........................................... 14-5
Experimental Psychology ......................................... 14-3
I-4Industrial Engineering .............................................. 14-5
Medical Science ....................................................... 14-4
Organizational Psychology ....................................... 14-4
Safety Engineering ................................................... 14-4
Empty Weight ................................................................ 6-4
Empty Weight Center of Gravity (EWCG) ........... 6-2, 6-4
Empty Weight Center of Gravity (EWCG) Range ......6-15
Emulsion Cleaners ....................................................... 8-28
Solvent Emulsion ..................................................... 8-28
Water Emulsion ........................................................ 8-28
Energy ................................................................. 5-2,12-20
Engine Frontal .............................................................. 8-13
Environmental Protection Agency (EPA) ...................... 1-6
Ethics ............................................................................ 13-6
Exosphere ..................................................................... 5-33
Extinguishers .................................................................. 1-5
Bromochlorodifluoromethane (Halon 1211) .............. 1-6
Bromotrifluoromethane (Halon 1301) ........................ 1-6
Carbon dioxide (CO 2) ................................................. 1-5
Chlorobromomethane (Halon 1011) .......................... 1-6
Dibromodifluoromethane (Halon 1202) ..................... 1-6
Halogenated hydrocarbon ........................................... 1-6
Methyl bromide (Halon 1001) .................................... 1-6
Extruding ...................................................................... 7-28
F
FAA .............................................................................. 2-30
FAA Involvement ........................................................ 14-1
Fasteners
Airloc ........................................................................ 7-66
Camloc ..................................................................... 7-65
Captive ..................................................................... 7-64
Hi-Tigue ................................................................... 7-61
Taper-Lok ................................................................. 7-61
Turn Lock ................................................................. 7-64
Federal Aviation Administration (FAA) ........... 1-20,14-30
FAA Safety Team ................................................... 14-31
FAA’s Maintenance Fatigue Section ..................... 14-30
Federal Communications Commission (FCC) ............. 2-38
Ferrous ........................................................................... 7-2
Ferrous Metals ............................................................. 7-16
Fiber Reinforce ............................................................ 7-31
Field Effect Transistors ............................................ 12-107
Field Frame .............................................................. 12-128
Field Rotation ........................................................... 12-155
Files .............................................................................. 11-9
Flat .......................................................................... 11-10
Half-Round ............................................................. 11-10
Hand ....................................................................... 11-10
Knife ....................................................................... 11-11
Lead-Float .............................................................. 11-10Mill ......................................................................... 11-10
Round or Rattail ..................................................... 11-10
Square ..................................................................... 11-10
Triangular and Three Square .................................. 11-10
Vixen (Curved-Tooth Files) ................................... 11-11
Warding .................................................................. 11-10
Wood ...................................................................... 11-11
Filtering .................................................................... 12-111
Fire Extinguishers ................................................... 1-7,1-8
Fire Protection ................................................................ 1-5
Using Fire Extinguishers ............................................ 1-8
Fire Safety ...................................................................... 1-5
Fittings ........................................................................... 9-1
Fixed base operator (FBO) ......................................... 14-14
Fixed Resistor ............................................................ 12-24
Flex .............................................................................. 9-21
Flexible Hose ...................................................... 9-16,9-18
Flexible Wing Aircraft ................................................. 5-56
Flight Controls ............................................................. 6-14
Flight Management System (FMS) .............................. 6-28
Flight Standards District Office (FSDO) ............ 2-2,10-12
Flight Standards Service (AFS) ..................................... 2-2
Float-type carburetor .................................................... 5-30
Fluid Lines .................................................. 9-7, 9-16, 9-17
Fluid Pressure ............................................................... 5-27
Fluids ............................................................................ 6-14
Flux Density ............................................................... 10-30
Force .............................................................................. 5-4
Foreign Object Damage (FOD) ............................. 1-4, 9-1
Forging ......................................................................... 7-27
Form 337, Major Repair and Alteration ...................... 2-38
Forms ........................................................................... 2-33
Forward Biased Diode ............................................... 12-98
Forward Flight ............................................................. 5-54
four forces of flight .............................................. 5-4, 5-36
Power .......................................................................... 5-6
Torque ........................................................................ 5-6
Work ........................................................................... 5-4
Rolling .................................................................... 5-6
Sliding ..................................................................... 5-5
Static ....................................................................... 5-5
Fractional Powers ......................................................... 3-13
Fractions ......................................................................... 3-2
Free Electrons .............................................................. 12-2
Freon .............................................................................. 1-6
Frequency ................................................................. 12-158
Frequency Measurement ............................................ 12-76
Frequency of Sound ..................................................... 5-31
Fueling Hazards ........................................................... 1-26
Fueling Procedures ....................................................... 1-26
Fuel System .................................................................. 6-13
I-5Fundamentals of Electricity and Electronics ............... 12-1
Fuse ............................................................................ 12-28
Fuselage stations (FS) .................................................... 4-9
G
Gaskets ......................................................................... 7-35
Gas Laws ...................................................................... 5-23
Gauge pressure (psig) .................................................. 5-23
Gear .............................................................................. 5-10
General Gas Law .......................................................... 5-25
Generator Brushes .................................................... 12-136
Generator Control Units (GCU) ............................... 12-164
Generator Terminals ................................................ 12-135
Gravity ........................................................................... 5-2
Ground Air Heating ..................................................... 1-24
Ground Movement of Aircraft ..................................... 1-11
ground power unit (GPU) ............................................ 1-21
Ground Support Air Units ............................................ 1-23
Ground Support Equipment (GSE) ............................ 14-12
H
Hacksaws ..................................................................... 11-8
Halogenated hydrocarbon extinguishers ........................ 1-6
Hammers ...................................................................... 11-1
Hand Tool .................................................................... 11-1
Hardening ..................................................................... 7-29
Hardness Testing .......................................................... 7-25
Hearing Protection ......................................................... 1-4
Heat .............................................................................. 5-18
Heat Transfer ............................................................... 5-19
Heat Treatment .................................................... 7-16,7-24
Annealing ................................................................. 7-19
Case Hardening ........................................................ 7-20
Carburizing ........................................................... 7-20
Nitriding ................................................................ 7-20
Hardening ................................................................. 7-17
Hardening Precautions ............................................. 7-19
Normalizing .............................................................. 7-19
Heat Treatment of Magnesium Alloys ......................... 7-24
Heat Treatment of Nonferrous Metals ......................... 7-20
Heat Treatment of Titanium ......................................... 7-25
Full Annealing .......................................................... 7-25
Stress Relieving ........................................................ 7-25
Thermal Hardening ................................................... 7-25
Helicoils ....................................................................... 7-49
Helicopters ................................................................... 1-10
Helicopter Structures ................................................... 5-49
Helicopter Weighing .................................................... 6-25
Helicopter Weight and Balance ................................... 6-25
High-Speed Aerodynamics .......................................... 5-46
High-Speed Airfoils ..................................................... 5-48Hole Repair .................................................................. 7-70
Push-Pull Tube Linkage ....................................... 7-72
Repair of Damaged Holes with Acres Fastener
Sleeves .................................................................. 7-71
Hole Repair Hardware ................................................. 7-70
Horizontal Deflection ................................................. 12-77
Hose Clamps ................................................................ 9-22
Hose Fittings ................................................................ 9-21
Human Error .............................................................. 14-13
Human Factors ............................................ 14-1,14-2,14-6
Human Factors and Ergonomics Society (HFES) .....14-31
Human Factors on Aviation Maintenance and Inspection
(HFAMI) .................................................................... 14-13
Hydraulic Ground Power Units ................................... 1-22
Hydraulic lock .............................................................. 1-12
Hydraulic Transmission ........................................... 12-165
Hydrochloric acid vapor ................................................ 1-6
Hydrometer .................................................................. 6-12
I
Illustrated Parts Catalogues (IPC) ................................ 4-11
Impact Drivers ............................................................. 11-6
Impenetrability ............................................................... 5-1
Inclined Coil Iron Vane Meter ................................... 12-75
Inclined Plane ............................................................... 5-11
Inductance .................................................................. 12-56
Inductor-Type Rotary Inverter ................................... 12-94
Inspection ..................................................................... 8-11
Inspection authorization (IA) ....................................... 2-16
Inspection Authorization (IA) (by 14 CFR Section) ....13-5
Section 65.91, Inspection Authorization .................. 13-5
Section 65.92, Inspection Authorization: Duration ..13-5
Section 65.93, Inspection Authorization: Renewal ..13-5
Section 65.95, Inspection Authorization: Privileges and
Limitations ............................................................... 13-6
Inspection of Composites ........................................... 10-36
Inspection of Welds ................................................... 10-38
Instructions for Continued Airworthiness (ICA) ........... 2-4
Instrument Flight Rules (IFR) ...................................... 2-22
Integrated Circuit ..................................................... 12-122
International Ergonomics Association (IEA) ............. 14-31
International Organization for Standardization (ISO) ...4-7
Interpoles .................................................................. 12-132
Inter-turbine temperature (ITT) ................................... 1-14
Inverters ..................................................................... 12-93
Involving Magnesium .................................................. 8-20
Ionosphere .................................................................... 5-33
Ions ............................................................................... 12-2
Iron Vane Meter ......................................................... 12-74
ISO 9001 ...................................................................... 2-41
I-6J
Jacking ......................................................................... 6-14
JET A ........................................................................... 1-25
JET A-1 ........................................................................ 1-25
JET B ........................................................................... 1-25
Job task analysis (JTA) .............................................. 14-10
K
Kelvin scales ................................................................ 5-21
Kerosene ...................................................................... 8-27
Kinetic Energy ............................................................... 5-3
Kinetic theory ............................................................... 5-23
Kirchhoff’s Current Law ............................................ 12-41
Kirchhoff’s Voltage Law ........................................... 12-36
K-Monel ....................................................................... 7-11
L
Laminated Structures ................................................... 7-31
Landing Gear ...................................................... 6-17,8-12
Land Planes .................................................................... 1-8
Lateral .......................................................................... 5-53
Lateral Axis .................................................................. 5-42
Law of Conservation ...................................................... 5-1
Law of Exponents ........................................................ 3-12
LC Filters ................................................................. 12-112
Band-Pass ......................................................... 12-112
Band-Stop ......................................................... 12-112
High-Pass (HPF) ............................................... 12-112
Low-Pass ........................................................... 12-112
Leading edge of the MAC (LEMAC) .......................... 6-28
Least common denominator (LCD) ............................... 3-2
Leveling ....................................................................... 6-14
Lever .............................................................................. 5-8
First Class Lever ..................................................... 5-8
Second Class Lever ................................................. 5-9
Third Class Lever ................................................... 5-9
Lighted Pushbutton Switches ..................................... 12-32
Light Emitting Diode (LED) .......................................... 7-5
Light-Emitting Diode (LED) ................................... 12-103
Light Sport Aircraft (LSA) ......................... 2-16,2-40,7-77
Liquid Crystal Displays (LCD) ................................ 12-104
Liquid Penetrant ......................................................... 10-18
Lithium Ion Batteries ................................................. 12-93
Loading Graphs ............................................................ 6-24
Logic Circuits ........................................................... 12-119
.............................................................................. 12-121
AND Gate ............................................................. 12-121
Exclusive NOR Gate ............................................ 12-122
Exclusive OR Gate ............................................... 12-122
Inverter Logic ....................................................... 12-121Logic Polarity ....................................................... 12-120
NAND Gate .......................................................... 12-122
NOR Gate ............................................................. 12-122
OR Gate ................................................................ 12-122
Longitudinal ................................................................. 5-53
Longitudinal Axis ........................................................ 5-43
Loudness ...................................................................... 5-32
M
Mach Number .............................................................. 5-31
Magnesium ..................................................................... 7-8
Magnesium Alloys ......................................................... 7-8
Magnetic Amplifiers ................................................ 12-118
Magnetic Particle ....................................................... 10-29
Magnetism .................................................................... 12-6
Magnets ........................................................................ 12-9
Main Rotor Systems ..................................................... 5-49
Maintenance Error Decision Aid (MEDA) ................ 14-10
Maintenance Instruction Manuals (MIM) .................... 4-11
Maintenance Manual .................................................... 10-3
Main Wheels ................................................................ 6-17
Mallets .......................................................................... 11-1
Manifold pressure gauge .............................................. 5-23
Manufacturers’ Service Bulletin .................................. 10-3
Mass and Weight ............................................................ 5-1
Material Safety Data Sheet (MSDS) .............................. 1-2
Matter ................................................................... 5-1, 12-1
Maximum except takeoff (METO) ................................ 6-5
Maximum Weight .......................................................... 6-3
Mean Aerodynamic Chord ........................................... 6-28
Mean aerodynamic chord (MAC) ................................ 6-28
Mechanical Energy ...................................................... 5-18
Mechanic Certification: Subpart A - General (by 14 CFR
Section) ........................................................................ 13-1
Refusal to Submit to a Drug or Alcohol Test ........... 13-2
Section 65.3, Certification of Foreign Airmen Other
Than Flight Crewmembers ....................................... 13-1
Section 65.11, Application and Issue ....................... 13-1
Section 65.12, Offenses Involving Alcohol and
Drugs ........................................................................ 13-1
Section 65.13, Temporary Certificate ...................... 13-1
Section 65.14, Security Disqualification .................. 13-1
Section 65.15, Duration of Certificates .................... 13-1
Section 65.16, Change of Name: Replacement of Lost or
Destroyed Certificate ................................................ 13-2
Section 65.17, Test: General Procedure ................... 13-2
Section 65.18, Written Tests: Cheating or Other
Unauthorized Content ............................................... 13-2
Section 65.19, Retesting After Failure ..................... 13-2
Section 65.20, Applications, Certificates, Logbooks,
Reports, and Records: Falsification, Reproduction, or
Alteration .................................................................. 13-2
I-7Section 65.21, Change of Address ........................... 13-2
Mechanic Certification: Subpart D - Mechanics (by 14
CFR Section) ................................................................ 13-3
Section 65.71, Eligibility Requirements: General ....13-3
Section 65.73, Ratings .............................................. 13-3
Section 65.75, Knowledge Requirements ................ 13-3
Section 65.77, Experience Requirements ................. 13-3
Section 65.79, Skill Requirements ........................... 13-3
Section 65.80, Certificated Aviation Maintenance
Technician School Students ..................................... 13-4
Section 65.81, General Privileges and Limitations ..13-4
Section 65.83, Recent Experience Requirements .....13-4
Section 65.85, Airframe Rating: Additional
Privileges .................................................................. 13-4
Section 65.87, Powerplant Rating: Additional
Privileges .................................................................. 13-4
Section 65.89, Display of Certificate ....................... 13-4
Megger (Megohmmeter) ............................................ 12-72
Mercury barometer ....................................................... 5-34
Metal-Oxide-Semiconductor FET (MOSFET) ........ 12-108
Metal Tube Lines ........................................................... 9-2
Methyl Ethyl Ketone (MEK) ....................................... 8-27
Metric System .............................................................. 3-26
Microfiche .................................................................... 4-23
Microfilm ..................................................................... 4-23
Micrometer Calipers .................................................. 11-19
Micro-organisms ............................................................ 8-2
Microprocessors ....................................................... 12-123
Microswitches ............................................................ 12-31
Mild Abrasive Materials .............................................. 8-28
Minimum Fuel ............................................................... 6-4
Molecule ...................................................................... 12-1
Moment .......................................................................... 6-2
Mooney M20 ................................................................ 5-11
Motion .......................................................................... 5-14
MS Flareless Fittings ..................................................... 9-9
Cryofit ........................................................................ 9-9
Swaged ....................................................................... 9-9
MS (Military Standard) numbers ................................. 7-37
Multirange Ohmmeter ................................................ 12-72
N
NAS (National Aircraft Standard) numbers ................ 7-37
National Aeronautics and Space Administration (NASA) .
........................................................................................ 2-1
National Coarse (NC) .................................................. 7-37
National Fire Protection Association (NFPA) ............... 1-5
National Transportation Safety Board (NTSB) ........... 13-5
Nautical miles (NM) .................................................... 2-11
NDI Methods ............................................................. 10-17
Negative Powers .......................................................... 3-12Neoprene ...................................................................... 9-16
Newton’s Law of Motion ............................................. 5-16
First ........................................................................... 5-16
Second ...................................................................... 5-16
Third ............................................................... 5-17, 5-37
Nickel-Cadmium Batteries ......................................... 12-91
Nomograms .................................................................. 4-23
Nosewheel .................................................................... 6-17
Notice of Proposed Rulemaking (NPRM) ................... 2-26
Nuclear Energy ............................................................ 5-18
Nut Plates ..................................................................... 7-68
Nuts .................................................................... 7-41, 7-47
Identification and Coding ..................................... 7-45
Internal and External Wrenching .......................... 7-45
Non-Self-Locking ................................................. 7-41
Self-Locking ......................................................... 7-42
Sheet Spring .......................................................... 7-45
O
Occupational Safety and Health Administration (OSHA) ..
...................................................................................... 14-4
Ohmmeter .................................................................. 12-71
Ohm’s Law ..................................................... 12-17 , 12-59
Oil Capacitors ............................................................ 12-53
Oil Caps ....................................................................... 7-77
Oil System .................................................................... 6-13
Original equipment manufacturer (OEM) ................... 2-30
Orthographic .................................................................. 4-9
Oscilloscope ............................................................... 12-76
Over-Excitation Protection ...................................... 12-164
Over “G” .................................................................... 10-14
Overhaul Manual ......................................................... 10-4
Overvoltage Protection ............................................ 12-164
Oxygen Hazards ........................................................... 1-25
Oxygen Servicing Equipment ...................................... 1-24
P
Parallel ....................................................................... 12-62
Parallel Conductors .................................................. 12-138
Parallel DC Circuits ................................................... 12-40
Parallel Generator Operations .................................. 12-164
Part Manufacture Approval (PMA) ............................... 2-4
Pascal’s Law ................................................................ 5-27
Percentage .................................................................... 3-10
Permanent Ballast ........................................................ 6-23
Permanent Magnet Rotary Inverter ............................ 12-94
Phantom Lines ............................................................. 4-17
phosgene gas .................................................................. 1-6
Phosphoric-citric Acid ................................................. 8-29
Photoconductive Cells ............................................... 12-27
Physical Parameters ................................................... 12-56
I-8Pins ............................................................................... 7-75
Cotter ................................................................. 7-75
Flathead ............................................................. 7-75
Roll .................................................................... 7-75
Tape ................................................................... 7-75
Plastics ......................................................................... 7-29
Pliers ............................................................................ 11-3
Plumb Bob ................................................................... 6-11
PN Junctions .............................................................. 12-98
PNP Transistor ......................................................... 12-107
Polyester Film ............................................................ 12-52
Porosity .......................................................................... 5-1
Potential Energy ................................................... 5-2, 5-19
Power .................................................................. 5-4,12-20
Power and Energy ...................................................... 12-20
Powered Parachute ........................... 1-11, 5-58, 6-26, 6-28
Power of Zero .............................................................. 3-12
Power Rectifier Diodes ............................................ 12-103
Powers .......................................................................... 3-11
Powers of Ten .............................................................. 3-12
Precipitation Practices .................................................. 7-23
Preflight ........................................................................ 10-5
Pressure ........................................................................ 5-22
Absolute ................................................................... 5-23
Differential ............................................................... 5-23
Gauge ........................................................................ 5-23
Preventive Maintenance ............................................... 8-10
Proportion ...................................................................... 3-8
Pulley ............................................................................. 5-9
Block and Tackle .................................................. 5-10
Single Fixed Pulley ................................................. 5-9
Single Movable Pulley ............................................ 5-9
Pulse Structure ......................................................... 12-120
Punches ........................................................................ 11-3
Pure Metals .................................................................... 8-1
Push-Pull Tube Linkage ............................................... 7-72
Pythagorean Theorem .................................................. 3-25
Q
Quenching .................................................................... 7-22
R
Radiant Energy ............................................................. 5-18
Radiation ...................................................................... 5-20
Radiation Hazards ...................................................... 10-36
Radiographic .............................................................. 10-34
Radiographic Interpretation ....................................... 10-35
Radio Station License .................................................. 2-38
Ratio ............................................................................... 3-7
Reamers ...................................................................... 11-13
Reciprocating Engines ................................................. 1-11Rectangular Conductors ............................................. 12-20
Rectifiers .................................................................. 12-100
Diode ....................................................... 12-103 ,12-105
Light-Emitting Diode (LED) ............................ 12-103
Liquid Crystal Displays (LCD) ........................ 12-104
Photodiode ........................................................ 12-104
Power Rectifier Diodes ..................................... 12-103
Schottky Diodes ................................................ 12-105
Varactors ........................................................... 12-104
Zener Diodes ..................................................... 12-103
Full-Wave Rectifier .............................................. 12-102
Half-Wave Rectifier ............................................. 12-101
Rectifier Unit ........................................................... 12-156
Reheat Treatment ......................................................... 7-22
Reinforced Plastic ........................................................ 7-31
Relative Wind .............................................................. 5-38
Relays ......................................................................... 12-33
Resistance .................................................................. 12-18
Resistance of a Conductor ......................................... 12-18
Resistance and Relation to Wire Sizing ................. 12-20
Resistors ......................................................... 12-24 , 12-50
Variable Resistors ................................................... 12-26
Potentiometer ...................................................... 12-27
Rheostat .............................................................. 12-26
Wire-Wound ........................................................... 12-26
Resistors in Parallel .................................................... 12-40
Resonance ......................................................... 5-32,12-63
Retreating Blade ........................................................... 5-56
Reverse Current Sensing .......................................... 12-164
Rigid Fluid Lines ........................................................... 9-1
Riveted and Rivetless Nut Plates ................................. 7-68
Deutsch ................................................................. 7-70
Dill Lok-Skrus and Dill Lok-Rivets ..................... 7-69
Rivets .................................................................. 7-51,7-57
Blind ..................................................................... 7-56
Dzus ...................................................................... 7-64
Friction Lock ........................................................ 7-57
Mechanical Lock .................................................. 7-58
Bulbed CherryLOCK Rivets ............................. 7-58
Huck Mechanical Locked Rivets ....................... 7-58
Wiredraw CherryLOCK Rivets ......................... 7-58
Pin ......................................................................... 7-60
Pull-Thru ............................................................... 7-57
Solid Shank ........................................................... 7-52
Standards and Specifications ................................ 7-51
Rockwell Tester ........................................................... 7-26
Rolling-Type Flaring ..................................................... 9-4
Roots ............................................................................ 3-12
Rotary Inverters ......................................................... 12-94
Rotary Selector Switches ........................................... 12-31
Rubber .......................................................................... 7-32
I-9Ruddervators ................................................................ 5-42
Rules .......................................................................... 11-14
S
Safety Around Hazardous Materials .............................. 1-2
Acid ............................................................................ 1-2
ALK ............................................................................ 1-2
CARC ......................................................................... 1-2
RAD ........................................................................... 1-2
Safety Data Sheets (SDSs) ............................................. 1-2
Saturable-Core Reactor ............................................ 12-119
Scales ............................................................................. 6-7
Screwdrivers ................................................................ 11-1
Screws .......................................................................... 7-66
Identification and Coding ..................................... 7-67
Machine ................................................................ 7-67
Structural ............................................................... 7-66
Scriber ........................................................................ 11-18
Sealed Lead Acid (SLA) Batteries ............................. 12-92
Seals ............................................................................. 7-33
Packings ................................................................... 7-34
O-Ring .................................................................. 7-34
V-Ring .................................................................. 7-35
Sealing Compounds .................................................. 7-35
One Part ................................................................ 7-36
Two Part ................................................................ 7-36
Wipers ...................................................................... 7-35
Seaplanes ............................................................. 1-8,10-16
Self-Inductance .......................................................... 12-57
Semiconductors .......................................................... 12-95
Reverse Biased Diode ............................................ 12-99
Series .......................................................................... 12-59
Series Circuit ................................................... 12-82 ,12-83
Series DC Circuits ...................................................... 12-33
Series-Parallel Circuits ............................................... 12-86
Series-Parallel DC Circuits ........................................ 12-42
Service Bulletins (SB) .................................................. 2-33
Servicing Aircraft ......................................................... 1-20
Ground Support Equipment ..................................... 1-21
Electric Ground Power Units ................................ 1-21
Shock Absorber Cord ................................................... 7-33
Shock Waves ................................................................ 5-47
Expansion .......................................................... 5-48
Normal Shock .................................................... 5-47
Oblique Shock ................................................... 5-48
Shop Safety .................................................................... 1-1
Sine Wave .................................................................. 12-77
Single-Phase Alternator ........................................... 12-155
Ski Planes ....................................................................... 1-9
Slack ............................................................................. 9-21
Slide Calipers ............................................................. 11-25Society for Testing and Materials (ASTM). ................ 2-41
Society of Automotive Engineers (SAE) .............. 2-41,7-2
soda-acid ........................................................................ 1-5
Sodium Dichromate Solution ....................................... 8-23
Solution Heat Treatment .............................................. 7-21
Solvent Cleaners .......................................................... 8-27
Sound ................................................................. 5-30, 5-46
Sound Intensity ............................................................ 5-32
Sources of Electricity ................................................. 12-22
Chemical ................................................................. 12-23
Light ....................................................................... 12-23
Pressure .................................................................. 12-22
Thermal .................................................................. 12-23
Special Airworthiness Information Bulletin (SAIB) ...2-27
Special Federal Aviation Regulations (SFAR) .............. 2-1
Special Flight Permits ................................................ 10-16
Special Inspections ..................................................... 10-12
Special Wrenches ......................................................... 11-5
Specific Gravity ............................................................. 5-2
Specific Heat ................................................................ 5-21
Speed ............................................................................ 5-14
Speed of Sound ............................................................ 5-31
Spirit Level ................................................................... 6-11
Split-Phase Motor .................................................... 12-150
Spoiler Recesses ........................................................... 8-13
Square Roots ................................................................ 3-12
Stability ........................................................................ 5-40
Directional ............................................................ 5-42
Dutch Roll ............................................................. 5-42
Dynamic ................................................................ 5-40
Lateral ................................................................... 5-42
Longitudinal .......................................................... 5-41
Static ..................................................................... 5-40
Static Electricity ........................................................... 12-4
Static Inverters ........................................................... 12-94
Steel ................................................................................ 9-1
Stitch Lines .................................................................. 4-18
Strap Wrenches ............................................................ 11-6
Stratosphere .................................................................. 5-33
Stress ............................................................................ 5-12
Bending .................................................................... 5-13
Compression ............................................................. 5-13
Shear ......................................................................... 5-13
Strain ........................................................................ 5-14
Tension ..................................................................... 5-12
Torsion ..................................................................... 5-13
Structural Repair Manual .......................... 2-33, 4-11, 10-4
Subsonic ....................................................................... 5-47
Subsonic Flow .............................................................. 5-37
Sun ............................................................................... 5-18
Supersonic .................................................................... 5-47
I-10Supplemental Lift-Modifying Devices ........................ 5-45
Flaps .................................................................. 5-45
Slats ................................................................... 5-46
Slots ................................................................... 5-45
Supplemental Type Certificates (STC) ................. 2-4,2-27
Suspected Unapproved Parts (SUP) ............................. 2-24
Switches ..................................................................... 12-30
Synthetic Rubber .......................................................... 7-32
System Safety Services .............................................. 14-31
T
Tabs .............................................................................. 5-44
Anti-servo ................................................................. 5-44
Balance ..................................................................... 5-44
Servo ......................................................................... 5-44
Trim .......................................................................... 5-44
Tantalum .................................................................... 12-52
Tap ................................................................. 10-36 , 11-14
Taps and Dies ............................................................. 11-14
Tare Weight ................................................................... 6-5
Taxiing ......................................................................... 1-19
Taxi Signals ................................................................. 1-20
Technical Standard Order (TSO) ................................... 2-4
Temperature ................................................................. 5-21
Temporary Ballast ........................................................ 6-22
The Atmosphere ........................................................... 5-32
Density ..................................................................... 5-34
Pressure .................................................................... 5-34
The “Dirty Dozen” ..................................................... 14-13
Complacency .......................................................... 14-14
Distraction .............................................................. 14-15
Fatigue .................................................................... 14-16
Lack of Assertiveness ............................................. 14-22
Lack of Awareness ................................................. 14-26
Lack of Communication ......................................... 14-13
Lack of Knowledge ................................................ 14-14
Lack of Resources .................................................. 14-18
Lack of Teamwork ................................................. 14-16
Norms ..................................................................... 14-26
Pressure .................................................................. 14-22
Stress ...................................................................... 14-24
Physical ............................................................... 14-24
Physiological ....................................................... 14-25
Psychological ...................................................... 14-24
The Neutral Plane .................................................... 12-127
The Pear Model ............................................................ 14-9
The Pilot in command (PIC) ........................................ 10-5
The RL Time Constant ............................................... 12-56
Thermal Efficiency ...................................................... 5-19
Thermal Expansion ...................................................... 5-22
Thermal Protectors ..................................................... 12-29Thermistors ................................................................ 12-27
Thermography ............................................................ 10-37
Three-Phase Alternator ............................................ 12-156
Three-Phase Induction Motor .................................. 12-148
Three Unit Regulators .............................................. 12-161
Tie-Down Procedures .................................................... 1-8
Time Constant ............................................................ 12-50
Titanium ................................................................ 7-9,8-20
Titanium Alloys .................................................... 7-9,8-20
Title 14 of the Code of Federal Regulations (14 CFR) ..2-1
Part 1 Definitions and Abbreviations ......................... 2-3
Part 3 General Requirements ...................................... 2-1
Part 21 Certification Procedures for Products and
Parts ............................................................................ 2-3
Part 23 Airworthiness Standards: Normal, Utility,
Acrobatic, and Commuter Category Airplanes .......... 2-4
Part 25 Airworthiness Standards: Transport Category
Airplanes .................................................................... 2-5
Part 27 Airworthiness Standards: Normal Category
Rotorcraft .................................................................... 2-5
Part 29 Airworthiness Standards: Transport Category
Rotorcraft .................................................................... 2-5
Part 33 Airworthiness Standards: Aircraft Engines ...2-8
Part 35 Airworthiness Standards: Propellers .............. 2-8
Part 39 Airworthiness Directives ............................... 2-8
part 43 Maintenance, Preventive Maintenance,
Rebuilding, and Alterations ........................................ 2-2
Part 45 Identification and Registration Marking ........ 2-8
Part 47 Aircraft Registration .................................... 2-10
Part 65 Certification: Airmen Other Than Flight
Crewmembers ........................................................... 2-10
part 91 General Operating and Flight Rules ............... 2-2
Part 119 Certification: Air Carriers and Commercial
Operators .................................................................. 2-10
Part 121 Operating Requirements: Domestic, Flag, and
Supplemental Operations ......................................... 2-11
Part 125 Certification and Operations: Airplanes Having
a Seating Capacity of 20 or More Passengers or a
Maximum Payload Capacity of 6,000 Pounds or More;
and Rules Governing Persons on Board Such
Aircraft ..................................................................... 2-12
Part 135 Operating Requirements: Commuter and On-
Demand Operations and Rules Governing Persons on
Board Such Aircraft .................................................. 2-12
Part 145 Repair Stations ........................................... 2-13
Part 147 Aviation Maintenance Technician Schools 2-13
Part 183 Representatives of the Administrator ......... 2-13
Toggle Switch ............................................................ 12-30
Double-Pole, Double-Throw (DPDT) .................... 12-30
Double-Pole, Single-Throw (DPST) ...................... 12-30
Single-Pole, Double-Throw (SPDT) ...................... 12-30
Single-Pole, Single-Throw (SPST) ........................ 12-30
I-11Tolerances ...................................................................... 4-9
Torque ...................................................... 5-4, 7-49,12-138
Cotter Pin Hole Line Up ....................................... 7-50
Torque ................................................................... 7-49
Torque Tables ....................................................... 7-50
Torque Wrenches .................................................. 7-49
Torque Wrench ............................................................ 11-5
Total Parallel Resistance ............................................ 12-40
Towing ......................................................................... 1-17
Transformer Losses .................................................... 12-67
Transformers .............................................................. 12-65
Transistors ................................................................ 12-105
Transonic ...................................................................... 5-47
Transparent Plastics ..................................................... 7-29
Transportation Security Administration (TSA) ........... 13-1
Trigonometric Functions .............................................. 3-24
Trimmers .................................................................... 12-53
Troposphere ................................................................. 5-33
True Power ................................................................. 12-64
Turbofan ....................................................................... 1-15
Turbofan Engines ......................................................... 1-15
Turboprop .................................................................... 1-14
Turboprop Engines ....................................................... 1-14
Turnbuckles
Double Wrap ......................................................... 7-79
Single Wrap .......................................................... 7-79
Twist Drills ................................................................ 11-12
Twisting ....................................................................... 9-21
Two-Phase Alternator .............................................. 12-156
Two Resistors in Parallel ........................................... 12-40
Type Certificate Data Sheets (TCDS) ......... 2-30, 6-2, 10-4
Type certificates (TCs) .................................................. 2-3
U
Ultrasonic ................................................................... 10-21
Pulse Echo .............................................................. 10-22
Resonance ............................................................... 10-24
Underwriters Laboratory (UL) ....................................... 1-6
Unified Coarse (UNC) ................................................. 7-37
Unified Fine (UNF) ...................................................... 7-37
Uniform Motion ........................................................... 5-14
Universal Bulkhead Fittings .......................................... 9-8
Universal Numbering System ........................................ 4-3
U.S. Department of Labor Occupational Safety and Health
Administration (OSHA) ................................................. 1-2
Useful Load .................................................................... 6-4
V
Vacuum Tubes ......................................................... 12-110
Valence Electrons ........................................................ 12-2
Valves .......................................................................... 7-77Varactors .................................................................... 12-53
Varmeters ................................................................... 12-75
Velocity ........................................................................ 5-14
Vernier Scale .............................................................. 11-23
Vertical Axis ...................................................... 5-43, 5-52
Vertical Deflection ..................................................... 12-77
Vibrating-Type Regulator ........................................ 12-159
Voltage ..................................................................... 12-164
Voltage Dividers ........................................................ 12-37
Voltage Regulation .................................... 12-164 , 12-171
Voltmeter ................................................................... 12-70
W
Washers .............................................................. 7-46, 7-47
Bolt and Hole Sizes .............................................. 7-47
Installation Practices ............................................. 7-48
Lock ...................................................................... 7-46
Plain ...................................................................... 7-46
Repair of Damaged Internal Threads .................... 7-48
Special Washers .................................................... 7-47
Waterline (WL) .............................................................. 4-9
Wattmeter ................................................................... 12-76
Wave Motion ............................................................... 5-30
Weighing an Aircraft ..................................................... 6-5
Weighing Points ........................................................... 6-14
Weight and Balance ........................................ 6-1, 6-6, 6-7
Weight-Shift Control ................................ 1-11, 5-56, 6-27
Whole Numbers ............................................................. 3-1
Wing Flap ..................................................................... 8-13
Wingtip Vortices .......................................................... 5-40
Wiring ................................................................ 7-77, 7-80
Cotter Pin Safetying .............................................. 7-80
Snap Rings ............................................................ 7-80
Turnbuckles .......................................................... 7-79
Wiring Diagram Manual .............................................. 10-4
Work .............................................................................. 5-4
Wrenches ...................................................................... 11-4
Wrought Aluminum ....................................................... 7-7
Wye Connection (Three-Phase) ............................... 12-156
Z
Zener Diodes ............................................................ 12-103
Zone Numbers ................................................................ 4-8
Fly GACA is an independent educational platform. It is not affiliated with, endorsed by, or operated by the General Authority of Civil Aviation (GACA) or the Government of the Kingdom of Saudi Arabia. The official and authoritative source for all civil aviation regulations, publications, and aeronautical information is always GACA. Always verify against the latest official GACA publication at gaca.gov.sa.