← The Saudi aviation reference, in one place.
Aviation Maintenance Technician Handbook - Powerplant (FAA-H-8083-32B)
Reproduced for study. Always verify against the official GACAR at gaca.gov.sa.
Aviation Maintenance Technician
Handbook–Powerplant
U.S. Department of Transportation
Federal Aviation Administration
Flight Standards Service
iiThe Aviation Maintenance Technician Handbook–Powerplant (FAA-H-8083-32B) is one of a series of three handbooks for
persons preparing for certification as a powerplant mechanic. It is intended that this handbook provide the basic information
on principles, fundamentals, and technical procedures in the subject matter areas relating to the powerplant rating. It is
designed to aid students enrolled in a formal course of instruction, as well as the individual who is studying on his or her
own. Since the knowledge requirements for the airframe and powerplant ratings closely parallel each other in some subject
areas, the chapters which discuss fire protection systems and electrical systems contain some material which is also duplicated
in the Aviation Maintenance Technician Handbook–Airframe (FAA-H-8083-31B).
This handbook contains an explanation of the units that make up each of the systems that bring fuel, air, and ignition together
in an aircraft engine for combustion. It also contains information on engine construction features, lubrication systems, exhaust
systems, cooling systems, cylinder removal and replacement, compression checks, and valve adjustments. Because there
are so many different types of aircraft in use today, it is reasonable to expect that differences exist in airframe components
and systems. To avoid undue repetition, the practice of using representative systems and units is carried out throughout
the handbook. Subject matter treatment is from a generalized point of view and should be supplemented by reference to
manufacturer's manuals or other textbooks if more detail is desired. This handbook is not intended to replace, substitute
for, or supersede official regulations or the manufacturer’s instructions. Occasionally the word “must” or similar language
is used where the desired action is deemed critical. The use of such language is not intended to add to, interpret, or relieve
a duty imposed by Title 14 of the Code of Federal Regulations (14 CFR).
The subject of Human Factors is contained in the Aviation Maintenance Technician Handbook—General (FAA-H-8083-30)
(as revised).
This handbook is available for download, in PDF format, from www.faa.gov .
This handbook is published by the United States 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–Powerplant (FAA-H-8083-32B) was produced by the Federal Aviation
Administration (FAA). The FAA wishes to acknowledge the following contributors:
Mr. Tom Wild for images used throughout this handbook
Free Images Live (www.freeimageslive.co.uk) for image used in Chapter 1
Mr. Stephen Sweet (www.stephensweet.com) for image used in Chapter 1
Mr. Omar Filipovic (www.glasair-owners.com) for image used in Chapter 1
Mr. Warren Lane (Atomic Metalsmith, Inc.) for image used in Chapter 1
Pratt & Whitney for images used in Chapters 2, 3, 6, 7, and 8
Teledyne Continental Motors (www.genuinecontinental.aero) for images used in Chapters 2, 3, and 11
Aircraft Tool Supply Company (www.aircraft-tool.com) for images used in Chapter 4
Chief Aircraft (www.chiefaircraft.com) for images used in Chapter 4
DeltaHawk Engines, Inc. (www.deltahawkengines.com) for image used in Chapter 6
Mr. Felix Gottwald for image used in Chapter 7
Mr. Stephen Christopher (www.schristo.com) for images used in Chapter 8
Mr. Yunjin Lee for images used in Chapter 9
Mr. Marco Leerentveld (www.flightillusion.com ) for image used in Chapter 10
Aeromax Aviation, LLC (www.aeromaxaviation.com) for images used in Chapter 11
Avid Aircraft (www.avidflyeraircraft.com) for image used in Chapter 11
Flight and Safety Design (www.eco1aircraft.com) for image used in Chapter 11
Great Plains Aircraft Supply Co., Inc. (www.greatplainsas.com) for image used in Chapter 11
Lycoming Engines (www.lycoming.textron.com) for image used in Chapter 11
Revmaster LLC Aviation (revmasteraviation.com) for images used in Chapter 11
Rotech Research Canada, Ltd. (www.rotec.com) for images used in Chapter 11
Additional appreciation is extended to Mr. Gary E. Hoyle, Dean of Students, Pittsburgh Institute of Aeronautics; Mr. Tom
Wild, Purdue University; Dr. Ronald Sterkenburg, Associate Professor of the Department of Aviation Technology, Purdue
University; for their technical support and input.
Acknowledgments
iv
Table of Contents
Chapter 1
Aircraft Engines
General Requirements ................................................................................................................................. 1-1
Power & Weight ..................................................................................................................................... 1-2
Fuel Economy ........................................................................................................................................ 1-2
Durability & Reliability ............................................................................................................................ 1-2
Operating Flexibility ............................................................................................................................... 1-3
Compactness ......................................................................................................................................... 1-3
Powerplant Selection ............................................................................................................................. 1-3
Types of Engines .......................................................................................................................................... 1-3
Inline Engines ........................................................................................................................................ 1-3
Opposed or O-Type Engines ................................................................................................................. 1-4
V-Type Engines ...................................................................................................................................... 1-4
Radial Engines ....................................................................................................................................... 1-4
Reciprocating Engines ................................................................................................................................ 1-5
Design & Construction ........................................................................................................................... 1-5
Crankcase Section ................................................................................................................................. 1-5
Accessory Section ................................................................................................................................. 1-7
Accessory Gear Trains .......................................................................................................................... 1-7
Crankshafts .................................................................................................................................................. 1-7
Crankshaft Balance .............................................................................................................................. 1-8
Dynamic Dampers ................................................................................................................................ 1-8
Connecting Rods .......................................................................................................................................... 1-9
Master-and-Articulated Rod Assembly .................................................................................................. 1-9
Knuckle Pins ........................................................................................................................................ 1-10
Plain-Type Connecting Rods ............................................................................................................... 1-11
Fork-and-Blade Rod Assembly ............................................................................................................ 1-11
Pistons ........................................................................................................................................................ 1-11
Piston Construction .............................................................................................................................. 1-12
Piston Pin ............................................................................................................................................. 1-13
Piston Rings ............................................................................................................................................... 1-13
Piston Ring Construction .................................................................................................................... 1-13
Compression Ring ............................................................................................................................... 1-13
Oil Control Rings .................................................................................................................................. 1-13
Oil Scraper Ring .................................................................................................................................. 1-13
Cylinders .................................................................................................................................................... 1-14
Cylinder Heads .................................................................................................................................... 1-14
Cylinder Barrels ................................................................................................................................... 1-15
Cylinder Numbering ............................................................................................................................. 1-15
Firing Order ................................................................................................................................................ 1-16
Single-Row Radial Engines ................................................................................................................. 1-16
Double-Row Radial Engines ................................................................................................................ 1-16
Valves ......................................................................................................................................................... 1-16
Valve Construction ............................................................................................................................... 1-17
Valve Operating Mechanism ...................................................................................................................... 1-18
Cam Rings ........................................................................................................................................... 1-18
vCamshaft ............................................................................................................................................. 1-20
Tappet Assembly .................................................................................................................................. 1-20
Solid Lifters/Tappets ............................................................................................................................ 1-20
Hydraulic Valve Tappets/Lifters ............................................................................................................ 1-20
Push Rod ............................................................................................................................................. 1-21
Rocker Arms ........................................................................................................................................ 1-21
Valve Springs ....................................................................................................................................... 1-21
Bearings ..................................................................................................................................................... 1-22
Plain Bearings ...................................................................................................................................... 1-22
Ball Bearings ........................................................................................................................................ 1-22
Roller Bearings .................................................................................................................................... 1-22
Propeller Reduction Gearing ...................................................................................................................... 1-23
Propeller Shafts .......................................................................................................................................... 1-23
Reciprocating Engine Operating Principles ................................................................................................ 1-25
Operating Cycles ........................................................................................................................................ 1-25
Four-Stroke Cycle ................................................................................................................................ 1-25
Intake Stroke ................................................................................................................................... 1-26
Compression Stroke ....................................................................................................................... 1-27
Power Stroke .................................................................................................................................. 1-27
Exhaust Stroke ................................................................................................................................ 1-27
Two-Stroke Cycle ................................................................................................................................. 1-27
Rotary Cycle ........................................................................................................................................ 1-27
Diesel Cycle ......................................................................................................................................... 1-27
Reciprocating Engine Power & Efficiencies ............................................................................................... 1-27
Work .................................................................................................................................................... 1-27
Horsepower ......................................................................................................................................... 1-28
Piston Displacement ............................................................................................................................ 1-28
Area of a Circle .................................................................................................................................... 1-28
Compression Ratio .............................................................................................................................. 1-29
Indicated Horsepower .......................................................................................................................... 1-30
Brake Horsepower ............................................................................................................................... 1-31
Friction Horsepower ............................................................................................................................. 1-32
Friction & Brake Mean Effective Pressures ......................................................................................... 1-33
Thrust Horsepower .............................................................................................................................. 1-33
Efficiencies ................................................................................................................................................. 1-33
Thermal Efficiency ............................................................................................................................... 1-33
Mechanical Efficiency .......................................................................................................................... 1-35
Volumetric Efficiency ............................................................................................................................ 1-35
Propulsive Efficiency ............................................................................................................................ 1-36
Gas Turbine Engines ................................................................................................................................. 1-36
Types & Construction ........................................................................................................................... 1-36
Air Entrance ............................................................................................................................................... 1-37
Accessory Section ...................................................................................................................................... 1-38
Compressor Section ................................................................................................................................... 1-38
Compressor Types ............................................................................................................................... 1-38
Centrifugal-Flow Compressors ....................................................................................................... 1-38
Axial-Flow Compressor ................................................................................................................... 1-40
Diffuser ....................................................................................................................................................... 1-43
Combustion Section ................................................................................................................................... 1-43
Turbine Section .................................................................................................................................... 1-46
Exhaust Section ................................................................................................................................... 1-51
viGas Turbine Engine Bearings & Seals ....................................................................................................... 1-52
Turboprop Engines ..................................................................................................................................... 1-54
Turboshaft Engines .................................................................................................................................... 1-55
Turbofan Engines ....................................................................................................................................... 1-55
Turbine Engine Operating Principles .......................................................................................................... 1-56
Thrust ......................................................................................................................................................... 1-58
Gas Turbine Engine Performance .............................................................................................................. 1-59
Ram Recovery ..................................................................................................................................... 1-60
Chapter 2
Engine Fuel & Fuel Metering Systems
Fuel System Requirements .......................................................................................................................... 2-1
Vapor Lock ............................................................................................................................................. 2-1
Basic Fuel System ....................................................................................................................................... 2-2
Fuel Metering Devices for Reciprocating Engines ....................................................................................... 2-2
Air-Fuel Mixtures .................................................................................................................................... 2-3
Carburetion Principles .................................................................................................................................. 2-5
Venturi Principles ................................................................................................................................... 2-5
Application of Venturi Principle to Carburetor ........................................................................................ 2-5
Metering & Discharge of Fuel ................................................................................................................ 2-6
Carburetor Systems ..................................................................................................................................... 2-7
Carburetor Types .......................................................................................................................................... 2-8
Carburetor Icing ........................................................................................................................................... 2-8
Float-Type Carburetors ................................................................................................................................ 2-9
Float Chamber Mechanism System ..................................................................................................... 2-10
Main Metering System ......................................................................................................................... 2-10
Idling System ....................................................................................................................................... 2-12
Mixture Control System ....................................................................................................................... 2-12
Accelerating System ............................................................................................................................ 2-13
Economizer System ............................................................................................................................. 2-13
Pressure Injection Carburetors .................................................................................................................. 2-14
Typical Injection Carburetor ................................................................................................................. 2-15
Throttle Body ....................................................................................................................................... 2-15
Regulator Unit ...................................................................................................................................... 2-15
Fuel Control Unit .................................................................................................................................. 2-17
Automatic Mixture Control (AMC) .............................................................................................................. 2-18
Stromberg PS Carburetor .......................................................................................................................... 2-19
Accelerating Pump ............................................................................................................................... 2-21
Manual Mixture Control ........................................................................................................................ 2-21
Fuel-Injection Systems ............................................................................................................................... 2-21
Bendix/Precision Fuel-Injection System .............................................................................................. 2-21
Fuel Injector ......................................................................................................................................... 2-21
Airflow Section ..................................................................................................................................... 2-21
Regulator Section ................................................................................................................................ 2-22
Fuel Metering Section .......................................................................................................................... 2-23
Flow Divider ......................................................................................................................................... 2-23
Fuel Discharge Nozzles ....................................................................................................................... 2-24
Continental/TCM Fuel-Injection System .............................................................................................. 2-24
Fuel-Injection Pump ............................................................................................................................. 2-25
Air-Fuel Control Unit ............................................................................................................................ 2-25
Fuel Control Assembly ......................................................................................................................... 2-26
viiFuel Manifold Valve ............................................................................................................................. 2-26
Fuel Discharge Nozzle ......................................................................................................................... 2-27
Carburetor Maintenance ............................................................................................................................ 2-27
Carburetor Removal ............................................................................................................................ 2-27
Installation of Carburetor ..................................................................................................................... 2-29
Rigging Carburetor Controls ................................................................................................................ 2-29
Adjusting Idle Mixtures ......................................................................................................................... 2-29
Idle Speed Adjustment ......................................................................................................................... 2-30
Fuel System Inspection & Maintenance ..................................................................................................... 2-31
Complete System ................................................................................................................................ 2-31
Fuel Tanks ........................................................................................................................................... 2-32
Lines & Fittings .................................................................................................................................... 2-32
Selector Valves .................................................................................................................................... 2-32
Pumps .................................................................................................................................................. 2-32
Main Line Strainers .............................................................................................................................. 2-32
Fuel Quantity Gauges .......................................................................................................................... 2-32
Fuel Pressure Gauge ........................................................................................................................... 2-32
Pressure Warning Signal ..................................................................................................................... 2-32
Water Injection Systems for Reciprocating Engines ............................................................................ 2-33
Turbine Engine Fuel System—General Requirements .............................................................................. 2-33
Turbine Fuel Controls .......................................................................................................................... 2-33
Hydromechanical Fuel Control ............................................................................................................ 2-34
Hydromechanical/Electronic Fuel Control ............................................................................................ 2-34
FADEC Fuel Control Systems ............................................................................................................. 2-36
FADEC for an Auxiliary Power Unit ...................................................................................................... 2-36
FADEC Fuel Control Propulsion Engine .............................................................................................. 2-36
Fuel System Operation ........................................................................................................................ 2-40
Water Injection System .............................................................................................................................. 2-40
Fuel Control Maintenance .......................................................................................................................... 2-40
Engine Fuel System Components .............................................................................................................. 2-41
Main Fuel Pumps (Engine Driven) ....................................................................................................... 2-41
Fuel Heater .......................................................................................................................................... 2-42
Fuel Filters ........................................................................................................................................... 2-42
Fuel Spray Nozzles & Fuel Manifolds .................................................................................................. 2-43
Simplex Fuel Nozzle ............................................................................................................................ 2-44
Duplex Fuel Nozzle .............................................................................................................................. 2-44
Airblast Nozzles ................................................................................................................................... 2-44
Flow Divider ......................................................................................................................................... 2-45
Fuel Pressurizing & Dump Valves ....................................................................................................... 2-45
Combustion Drain Valves .................................................................................................................... 2-46
Fuel Quantity Indicating Units .................................................................................................................... 2-46
Chapter 3
Induction & Exhaust Systems
Reciprocating Engine Induction Systems ..................................................................................................... 3-1
Basic Carburetor Induction System ....................................................................................................... 3-1
Induction System Icing ...................................................................................................................... 3-3
Induction System Filtering ................................................................................................................. 3-4
Induction System Inspection & Maintenance .................................................................................... 3-4
Extinguishing Engine Fires .............................................................................................................. 3-4
Induction System Troubleshooting .................................................................................................... 3-5
viiiSupercharged Induction Systems .......................................................................................................... 3-5
Internally Driven Superchargers ....................................................................................................... 3-6
Turbosuperchargers ......................................................................................................................... 3-7
Normalizer Turbocharger .................................................................................................................. 3-9
Ground-Boosted Turbosupercharger System ................................................................................... 3-9
A Typical Turbosupercharger System ............................................................................................. 3-10
Turbocharger Controllers & System Descriptions ........................................................................... 3-14
Variable Absolute Pressure Controller (VAPC) .............................................................................. 3-14
Sloped Controller ............................................................................................................................ 3-15
Absolute Pressure Controller ......................................................................................................... 3-15
Turbocharger System Troubleshooting ........................................................................................... 3-15
Turbine Engine Inlet Systems .................................................................................................................... 3-16
Divided-Entrance Duct ......................................................................................................................... 3-17
Variable-Geometry Duct ...................................................................................................................... 3-17
Compressor Inlet Screens ................................................................................................................... 3-18
Bellmouth Compressor Inlets ............................................................................................................... 3-19
Turboprop & Turboshaft Compressor Inlets ......................................................................................... 3-19
Turbofan Engine Inlet Sections ............................................................................................................ 3-19
Reciprocating Engine Exhaust Systems .................................................................................................... 3-21
Radial Engine Exhaust Collector Ring System .................................................................................... 3-22
Manifold & Augmentor Exhaust Assembly ........................................................................................... 3-22
Reciprocating Engine Exhaust System Maintenance Practices .......................................................... 3-22
Exhaust System Inspection ................................................................................................................. 3-22
Muffler & Heat Exchanger Failures ...................................................................................................... 3-23
Exhaust Manifold & Stack Failures ...................................................................................................... 3-24
Internal Muffler Failures ....................................................................................................................... 3-24
Exhaust Systems with Turbocharger .......................................................................................................... 3-24
Augmentor Exhaust System ................................................................................................................ 3-24
Exhaust System Repairs ..................................................................................................................... 3-24
Turbine Engine Exhaust Nozzles ......................................................................................................... 3-25
Convergent Exhaust Nozzle ................................................................................................................ 3-26
Convergent-Divergent Exhaust Nozzle ................................................................................................ 3-26
Thrust Reversers ........................................................................................................................................ 3-27
Afterburning/Thrust Augmentation ............................................................................................................ 3-27
Thrust Vectoring ........................................................................................................................................ 3-29
Engine Noise Suppression ......................................................................................................................... 3-29
Turbine Engine Emissions .......................................................................................................................... 3-31
Chapter 4
Engine Ignition & Electrical Systems
Reciprocating Engine Ignition Systems ........................................................................................................ 4-1
Magneto-Ignition System Operating Principles ............................................................................................ 4-1
High-Tension Magneto System Theory of Operation ............................................................................. 4-2
Magnetic Circuit ................................................................................................................................ 4-2
Primary Electrical Circuit ................................................................................................................... 4-3
Secondary Electrical Circuit .............................................................................................................. 4-6
Magneto & Distributor Venting ............................................................................................................... 4-7
Ignition Harness ..................................................................................................................................... 4-7
Ignition Switches .................................................................................................................................... 4-8
Single & Dual High-Tension System Magnetos ................................................................................... 4-10
ixMagneto Mounting Systems ................................................................................................................ 4-10
High- & Low-Tension Magneto Systems ......................................................................................... 4-11
Types of DC Generators ............................................................................................................................ 4-12
Series Wound DC Generators ............................................................................................................. 4-12
Parallel (Shunt) Wound DC Generators .......................................................................................... 4-12
Compound Wound DC Generators ................................................................................................. 4-13
Generator Ratings ............................................................................................................................... 4-13
DC Generator Maintenance ................................................................................................................. 4-13
FADEC System Description ....................................................................................................................... 4-14
Low-Voltage Harness ........................................................................................................................... 4-14
Electronic Control Unit (ECU) .............................................................................................................. 4-15
PowerLink Ignition System .................................................................................................................. 4-16
Engine Indicating & Crew Alerting System (EICAS) ............................................................................ 4-16
Auxiliary Ignition Units ................................................................................................................................ 4-17
Booster Coil ......................................................................................................................................... 4-18
Impulse Coupling ................................................................................................................................. 4-19
High-Tension Retard Breaker Vibrator ................................................................................................. 4-21
Low-Tension Retard Breaker Vibrator .................................................................................................. 4-23
Spark Plugs ................................................................................................................................................ 4-24
Reciprocating Engine Ignition System Maintenance & Inspection ....................................................... 4-26
Magneto-Ignition Timing Devices ............................................................................................................... 4-26
Built-In Engine Timing Reference Marks ............................................................................................. 4-26
Timing Discs ........................................................................................................................................ 4-27
Piston Position Indicators .................................................................................................................... 4-27
Timing Lights ........................................................................................................................................ 4-28
Checking the Internal Timing of a Magneto ................................................................................................ 4-29
High-Tension Magneto E-Gap Setting (Bench Timing) ........................................................................ 4-29
Timing the High-tension Magneto to the Engine .................................................................................. 4-30
Performing Ignition System Checks ..................................................................................................... 4-31
Ignition Switch Check .......................................................................................................................... 4-31
Maintenance & Inspection of Ignition Leads ........................................................................................ 4-31
Replacement of Ignition Harness ......................................................................................................... 4-33
Checking Ignition Induction Vibrator Systems ..................................................................................... 4-33
Spark Plug Inspection & Maintenance ....................................................................................................... 4-33
Carbon Fouling of Spark Plugs ............................................................................................................ 4-33
Oil Fouling of Spark Plugs ................................................................................................................... 4-34
Lead Fouling of Spark Plugs ................................................................................................................ 4-34
Graphite Fouling of Spark Plugs .......................................................................................................... 4-35
Gap Erosion of Spark Plugs ................................................................................................................ 4-35
Spark Plug Removal ............................................................................................................................ 4-35
Spark Plug Reconditioning Service ..................................................................................................... 4-36
Inspection Prior to Installation .............................................................................................................. 4-37
Spark Plug Installation ......................................................................................................................... 4-38
Spark Plug Lead Installation ................................................................................................................ 4-38
Breaker Point Inspection ...................................................................................................................... 4-39
Dielectric Inspection ............................................................................................................................. 4-41
Ignition Harness Maintenance ............................................................................................................. 4-41
High-Tension Ignition Harness Faults .................................................................................................. 4-41
Harness Testing ................................................................................................................................... 4-42
Turbine Engine Ignition Systems ................................................................................................................ 4-43
Capacitor Discharge Exciter Unit ......................................................................................................... 4-44
xIgniter Plugs ......................................................................................................................................... 4-45
Turbine Ignition System Inspection & Maintenance ................................................................................... 4-45
Inspection ............................................................................................................................................ 4-45
Check System Operation ..................................................................................................................... 4-46
Repair .................................................................................................................................................. 4-46
Removal, Maintenance, & Installation of Ignition System Components ..................................................... 4-46
Ignition System Leads ......................................................................................................................... 4-46
Igniter Plugs ......................................................................................................................................... 4-47
Powerplant Electrical Systems ................................................................................................................... 4-47
Wire Size ............................................................................................................................................. 4-48
Factors Affecting the Selection of Wire Size ........................................................................................ 4-48
Factors Affecting Selection of Conductor Material ............................................................................... 4-50
Voltage Drop in Aircraft Wire & Cable .................................................................................................. 4-50
Conductor Insulation ............................................................................................................................ 4-52
Identifying Wire & Cable ...................................................................................................................... 4-53
Electrical Wiring Installation ................................................................................................................. 4-54
Wire Groups & Bundles ....................................................................................................................... 4-54
Twisting Wires ...................................................................................................................................... 4-54
Spliced Connections in Wire Bundles .................................................................................................. 4-54
Slack in Wiring Bundles ....................................................................................................................... 4-54
Bend Radii ........................................................................................................................................... 4-55
Routing & Installation ........................................................................................................................... 4-55
Protection Against Chafing .................................................................................................................. 4-56
Protection Against High Temperature .................................................................................................. 4-56
Protection Against Solvents & Fluids ................................................................................................... 4-56
Protection of Wires in Wheel Well Area ............................................................................................... 4-57
Routing Precautions ............................................................................................................................ 4-57
Installation of Cable Clamps ................................................................................................................ 4-57
Lacing & Tying Wire Bundles ..................................................................................................................... 4-58
Single-Cord Lacing .............................................................................................................................. 4-58
Double-Cord Lacing ............................................................................................................................. 4-59
Lacing Branch-Offs .............................................................................................................................. 4-59
Tying .................................................................................................................................................... 4-59
Cutting Wire & Cable .................................................................................................................................. 4-60
Stripping Wire & Cable ............................................................................................................................... 4-61
Solderless Terminals & Splices ............................................................................................................ 4-61
Copper Wire Terminals ........................................................................................................................ 4-62
Crimping Tools ..................................................................................................................................... 4-62
Aluminum Wire Terminals .................................................................................................................... 4-63
Splicing Copper Wires Using Preinsulated Wires ................................................................................ 4-63
Emergency Splicing Repairs ...................................................................................................................... 4-63
Splicing with Solder & Potting Compound ........................................................................................... 4-63
Connecting Terminal Lugs to Terminal Blocks ............................................................................................ 4-64
Bonding & Grounding ................................................................................................................................. 4-65
General Bonding & Grounding Procedures ......................................................................................... 4-65
Connectors ................................................................................................................................................. 4-67
Types of Connectors ............................................................................................................................ 4-67
Connector Identification ....................................................................................................................... 4-68
Installation of Connectors .................................................................................................................... 4-69
Conduit ....................................................................................................................................................... 4-69
Electrical Equipment Installation ................................................................................................................ 4-70
xiElectrical Load Limits ........................................................................................................................... 4-70
Controlling or Monitoring the Electrical Load ....................................................................................... 4-70
Circuit Protection Devices .................................................................................................................... 4-70
Switches ............................................................................................................................................. 4-71
Relays .................................................................................................................................................. 4-71
Chapter 5
Engine Starting Systems
Introduction .................................................................................................................................................. 5-1
Reciprocating Engine Starting Systems ....................................................................................................... 5-1
Inertia Starters ....................................................................................................................................... 5-1
Direct Cranking Electric Starter ............................................................................................................. 5-2
Direct Cranking Electric Starting System for Large Reciprocating Engines ...................................... 5-3
Direct Cranking Electric Starting System for Small Aircraft ............................................................... 5-6
Reciprocating Engine Starting System Maintenance Practices ............................................................. 5-6
Troubleshooting Small Aircraft Starting Systems ................................................................................... 5-7
Gas Turbine Engine Starters ........................................................................................................................ 5-8
Electric Starting Systems & Starter Generator Starting System ................................................................ 5-10
Troubleshooting a Starter Generator Starting System ......................................................................... 5-12
Air Turbine Starters .................................................................................................................................... 5-12
Air Turbine Starter Troubleshooting Guide .......................................................................................... 5-16
Chapter 6
Lubrication & Cooling Systems
Principles of Engine Lubrication ................................................................................................................... 6-1
Types of Friction ................................................................................................................................... 6-1
Functions of Engine Oil .......................................................................................................................... 6-1
Requirements & Characteristics of Reciprocating Engine Lubricants .......................................................... 6-2
Viscosity ................................................................................................................................................. 6-2
Viscosity Index ....................................................................................................................................... 6-2
Flash Point & Fire Point ......................................................................................................................... 6-3
Cloud Point & Pour Point ....................................................................................................................... 6-3
Specific Gravity ...................................................................................................................................... 6-3
Reciprocating Engine Lubrication Systems .................................................................................................. 6-4
Combination Splash & Pressure Lubrication ......................................................................................... 6-4
Lubrication System Requirements ......................................................................................................... 6-4
Dry Sump Oil Systems ........................................................................................................................... 6-4
Oil Tanks ................................................................................................................................................ 6-4
Oil Pump ................................................................................................................................................ 6-6
Oil Filters ................................................................................................................................................ 6-7
Oil Pressure Regulating Valve ............................................................................................................... 6-7
Oil Pressure Gauge ............................................................................................................................... 6-8
Oil Temperature Indicator ...................................................................................................................... 6-9
Oil Cooler ............................................................................................................................................... 6-9
Oil Cooler Flow Control Valve ................................................................................................................ 6-9
Surge Protection Valves ...................................................................................................................... 6-10
Airflow Controls .................................................................................................................................... 6-10
Dry Sump Lubrication System Operation ............................................................................................ 6-11
Wet-Sump Lubrication System Operation ........................................................................................... 6-13
Lubrication System Maintenance Practices .............................................................................................. 6-13
Oil Tank ................................................................................................................................................ 6-13
xiiOil Cooler ............................................................................................................................................. 6-14
Oil Temperature Bulbs ......................................................................................................................... 6-15
Pressure & Scavenge Oil Screens ...................................................................................................... 6-15
Oil Pressure Relief Valve ..................................................................................................................... 6-16
Recommendations for Changing Oil .......................................................................................................... 6-17
Draining Oil .......................................................................................................................................... 6-17
Oil & Filter Change & Screen Cleaning .............................................................................................. 6-17
Oil Filter Removal Canister Type Housing ........................................................................................... 6-17
Oil Filter/Screen Content Inspection ................................................................................................... 6-17
Assembly of & Installation of Oil Filters ............................................................................................... 6-18
Troubleshooting Oil Systems ............................................................................................................... 6-18
Requirements for Turbine Engine Lubricants ............................................................................................. 6-18
Turbine Oil Health & Safety Precautions ............................................................................................ 6-19
Spectrometric Oil Analysis Program .................................................................................................... 6-20
Typical Wear Metals & Additives .......................................................................................................... 6-20
Turbine Engine Lubrication Systems .......................................................................................................... 6-20
Turbine Lubrication System Components .................................................................................................. 6-21
Oil Tank ................................................................................................................................................ 6-21
Oil Pump .............................................................................................................................................. 6-22
Turbine Oil Filters ................................................................................................................................. 6-23
Oil Pressure Regulating Valve ............................................................................................................. 6-24
Oil Pressure Relief Valve ..................................................................................................................... 6-24
Oil Jets ................................................................................................................................................. 6-25
Lubrication System Instrumentation .................................................................................................... 6-25
Lubrication System Breather Systems (Vents) .................................................................................... 6-25
Lubrication System Check Valve ......................................................................................................... 6-26
Lubrication System Thermostatic Bypass Valves ................................................................................ 6-27
Air-Oil Coolers ..................................................................................................................................... 6-27
Fuel-Oil Coolers .................................................................................................................................. 6-27
Deoiler ................................................................................................................................................. 6-28
Magnetic Chip Detectors ..................................................................................................................... 6-28
Typical Dry-Sump Pressure Regulated Turbine Lubrication System ........................................................ 6-28
Pressure System ........................................................................................................................................ 6-28
Scavenge System ................................................................................................................................ 6-28
Breather Pressurizing System ............................................................................................................. 6-29
Typical Dry-Sump Variable Pressure Lubrication System .......................................................................... 6-29
Pressure Subsystem ............................................................................................................................ 6-29
Scavenger Subsystem ......................................................................................................................... 6-30
Breather Subsystem ............................................................................................................................ 6-30
Turbine Engine Wet-Sump Lubrication System ......................................................................................... 6-30
Turbine Engine Oil System Maintenance ................................................................................................... 6-31
Engine Cooling Systems ............................................................................................................................ 6-32
Reciprocating Engine Cooling Systems ............................................................................................... 6-32
Reciprocating Engine Cooling System Maintenance ........................................................................... 6-34
Maintenance of Engine Cowling .......................................................................................................... 6-34
Engine Cylinder Cooling Fin Inspection ............................................................................................... 6-36
Cylinder Baffle & Deflector System Inspection .................................................................................... 6-36
Cylinder Temperature Indicating Systems ........................................................................................... 6-37
Exhaust Gas Temperature Indicating Systems .................................................................................... 6-38
Turbine Engine Cooling .............................................................................................................................. 6-38
Accessory Zone Cooling ...................................................................................................................... 6-38
xiiiTurbine Engine Insulation Blankets ..................................................................................................... 6-40
Chapter 7
Propellers
General ........................................................................................................................................................ 7-1
Basic Propeller Principles ............................................................................................................................ 7-1
Propeller Aerodynamic Process ................................................................................................................... 7-2
Aerodynamic Factors ............................................................................................................................. 7-4
Propeller Controls & Instruments ........................................................................................................... 7-5
Propeller Location ........................................................................................................................................ 7-5
Tractor Propeller .................................................................................................................................... 7-5
Pusher Propellers .................................................................................................................................. 7-6
Types of Propellers ...................................................................................................................................... 7-6
Fixed-Pitch Propeller ............................................................................................................................. 7-6
Test Club Propeller ................................................................................................................................ 7-6
Ground-Adjustable Propeller ................................................................................................................. 7-6
Controllable-Pitch Propeller ................................................................................................................... 7-6
Constant-Speed Propellers .................................................................................................................... 7-7
Feathering Propellers ............................................................................................................................ 7-8
Reverse-Pitch Propellers ....................................................................................................................... 7-8
Propeller Governor ....................................................................................................................................... 7-8
Governor Mechanism ............................................................................................................................ 7-9
Underspeed Condition ........................................................................................................................... 7-9
Overspeed Condition ........................................................................................................................... 7-11
On-Speed Condition ............................................................................................................................ 7-11
Governor System Operation ................................................................................................................ 7-11
Propellers Used on General Aviation Aircraft ............................................................................................. 7-12
Fixed-Pitch Wooden Propellers ........................................................................................................... 7-12
Metal Fixed-Pitch Propellers ................................................................................................................ 7-13
Constant-Speed Propellers ........................................................................................................................ 7-14
Hartzell Constant-Speed, Nonfeathering ............................................................................................. 7-14
Constant-Speed Feathering Propeller ................................................................................................. 7-15
Unfeathering ........................................................................................................................................ 7-16
Propeller Auxiliary Systems ........................................................................................................................ 7-17
Ice Control Systems ............................................................................................................................ 7-17
Anti-Icing Systems .......................................................................................................................... 7-17
Deicing Systems ............................................................................................................................. 7-18
Propeller Synchronization & Synchrophasing ...................................................................................... 7-19
Autofeathering System ........................................................................................................................ 7-20
Propeller Inspection & Maintenance .......................................................................................................... 7-20
Wood Propeller Inspection ................................................................................................................... 7-21
Metal Propeller Inspection ................................................................................................................... 7-21
Aluminum Propeller Inspection ............................................................................................................ 7-21
Composite Propeller Inspection ........................................................................................................... 7-21
Propeller Vibration ...................................................................................................................................... 7-22
Blade Tracking ..................................................................................................................................... 7-22
Checking & Adjusting Propeller Blade Angles ..................................................................................... 7-23
Universal Propeller Protractor .............................................................................................................. 7-23
Propeller Balancing .................................................................................................................................... 7-24
Static Balancing ................................................................................................................................... 7-24
Dynamic Balancing .............................................................................................................................. 7-25
xivBalancing Procedure ........................................................................................................................... 7-25
Propeller Removal & Installation ................................................................................................................ 7-26
Removal .............................................................................................................................................. 7-26
Installation ............................................................................................................................................ 7-27
Servicing Propellers ................................................................................................................................... 7-27
Cleaning Propeller Blades ................................................................................................................... 7-27
Charging the Propeller Air Dome ......................................................................................................... 7-27
Propeller Lubrication ............................................................................................................................ 7-27
Propeller Overhaul ..................................................................................................................................... 7-29
The Hub ............................................................................................................................................... 7-29
Prop Reassembly ................................................................................................................................ 7-29
Troubleshooting Propellers ....................................................................................................................... 7-30
Hunting & Surging ................................................................................................................................ 7-30
Engine Speed Varies with Flight Attitude (Airspeed) ........................................................................... 7-30
Failure to Feather or Feathers Slowly ................................................................................................. 7-30
Turboprop Engines & Propeller Control Systems ....................................................................................... 7-30
Reduction Gear Assembly ................................................................................................................... 7-31
Turbo-Propeller Assembly .................................................................................................................... 7-31
Pratt & Whitney PT6 Hartzell Propeller System ......................................................................................... 7-31
Hamilton Standard Hydromatic Propellers ................................................................................................. 7-34
Principles of Operation ........................................................................................................................ 7-36
Feathering Operation ........................................................................................................................... 7-37
Unfeathering Operation ....................................................................................................................... 7-38
Setting the Propeller Governor ............................................................................................................ 7-40
Chapter 8
Engine Removal & Replacement
Introduction .................................................................................................................................................. 8-1
Reasons for Removal of Reciprocating Engines .......................................................................................... 8-1
Engine or Component Lifespan Exceeded ............................................................................................ 8-1
Engine Sudden Stoppage or Propeller Strike ........................................................................................ 8-1
Sudden Reduction in Speed .................................................................................................................. 8-1
Metal Particles in the Oil ........................................................................................................................ 8-2
Spectrometric Oil Analysis Engine Inspection Program ......................................................................... 8-2
Turbine Engine Condition Monitoring Programs .................................................................................... 8-2
Engine Operational Problems ................................................................................................................ 8-2
General Procedures for Engine Removal & Installation ............................................................................... 8-3
Preparation of Engines for Installation ................................................................................................... 8-3
QECA Buildup Method for Changing of Engines ................................................................................... 8-3
Depreservation of an Engine ................................................................................................................. 8-4
Inspection & Depreservation of Accessories ......................................................................................... 8-5
Inspection & Replacement of Powerplant External Units & Systems ........................................................... 8-5
Preparing the Engine for Removal ............................................................................................................... 8-6
Draining the Engine ............................................................................................................................... 8-6
Electrical Disconnects ............................................................................................................................ 8-6
Disconnection of Engine Controls .......................................................................................................... 8-7
Disconnection of Lines ........................................................................................................................... 8-8
Other Disconnections ............................................................................................................................ 8-8
Removing the Engine ................................................................................................................................... 8-8
Hoisting the Engine ................................................................................................................................ 8-9
Hoisting & Mounting the Engine for Installation ........................................................................................... 8-9
xvConnections & Adjustments ................................................................................................................. 8-10
Preparation of Engine for Ground & Flight Testing ..................................................................................... 8-12
Pre-Oiling ............................................................................................................................................. 8-12
Fuel System Bleeding .......................................................................................................................... 8-13
Propeller Check .......................................................................................................................................... 8-13
Checks & Adjustments After Engine Runup & Operation ........................................................................... 8-13
Rigging, Inspections, & Adjustments .......................................................................................................... 8-13
Rigging Power Controls ....................................................................................................................... 8-14
Adjusting the Fuel Control ................................................................................................................... 8-14
Turboprop Powerplant Removal & Installation ........................................................................................... 8-15
Reciprocating Helicopter Engine & QECA ................................................................................................. 8-15
Removal of Helicopter QECA .............................................................................................................. 8-16
Installation, Rigging, & Adjustment of Helicopter QECA ...................................................................... 8-16
Testing the Engine Installation ............................................................................................................. 8-16
Engine Mounts .......................................................................................................................................... 8-16
Mounts for Reciprocating Engines ....................................................................................................... 8-16
Mounts for Turbofan Engines ............................................................................................................... 8-17
Turbine Vibration Isolation Engine Mounts .......................................................................................... 8-17
Preservation & Storage of Engines ............................................................................................................ 8-18
Corrosion-Preventive Materials ........................................................................................................... 8-18
Corrosion-Preventive Compounds ....................................................................................................... 8-18
Dehydrating Agents ............................................................................................................................. 8-18
Engine Preservation & Return to Service ................................................................................................... 8-19
Engine Shipping Containers ....................................................................................................................... 8-21
Inspection of Stored Engines ..................................................................................................................... 8-21
Preservation & Depreservation of Gas Turbine Engines ............................................................................ 8-22
Chapter 9
Engine Fire Protection Systems
Introduction .................................................................................................................................................. 9-1
Components .......................................................................................................................................... 9-1
Engine Fire Detection Systems ............................................................................................................. 9-2
Thermal Switch System .................................................................................................................... 9-2
Thermocouple Systems .................................................................................................................... 9-2
Optical Fire Detection Systems ......................................................................................................... 9-3
Pneumatic Thermal Fire Detection ................................................................................................... 9-3
Continuous-Loop Detector Systems ................................................................................................. 9-3
Fire Zones ............................................................................................................................................. 9-7
Engine Fire Extinguishing System ............................................................................................................... 9-7
Fire Extinguishing Agents ...................................................................................................................... 9-7
Turbine Engine Ground Fire Protection ................................................................................................. 9-8
Containers ............................................................................................................................................ 9-8
Discharge Valves ................................................................................................................................... 9-8
Pressure Indication ................................................................................................................................ 9-8
Two-Way Check Valve ........................................................................................................................... 9-9
Discharge Indicators .............................................................................................................................. 9-9
Thermal Discharge Indicator (Red Disc) ........................................................................................... 9-9
Yellow Disc Discharge Indicator ........................................................................................................ 9-9
Fire Switch ............................................................................................................................................. 9-9
Warning Systems ................................................................................................................................. 9-10
Fire Detection System Maintenance ......................................................................................................... 9-10
xviFire Detection System Troubleshooting ...................................................................................................... 9-11
Fire Extinguisher System Maintenance Practices ...................................................................................... 9-12
Boeing 777 Aircraft Fire Detection & Extinguishing System ....................................................................... 9-13
Overheat Detection .............................................................................................................................. 9-13
Fire Detection ...................................................................................................................................... 9-13
Nacelle Temperature Recording .......................................................................................................... 9-13
Continuous Fault Monitoring ................................................................................................................ 9-13
Single/Dual Loop Operation ................................................................................................................. 9-14
System Test ......................................................................................................................................... 9-14
Boeing 777 Fire Extinguisher System .................................................................................................. 9-14
Fire Extinguisher Containers ........................................................................................................... 9-14
Squib .................................................................................................................................................... 9-16
Engine Fire Switches ........................................................................................................................... 9-16
Engine Fire Operation .......................................................................................................................... 9-17
APU Fire Detection & Extinguishing System .............................................................................................. 9-17
APU Fire Warning ................................................................................................................................ 9-18
Fire Bottle Discharge ........................................................................................................................... 9-18
Chapter 10
Engine Maintenance & Operation
Reciprocating Engine Overhaul ................................................................................................................. 10-1
Top Overhaul ....................................................................................................................................... 10-1
Major Overhaul & Major Repairs ......................................................................................................... 10-1
General Overhaul Procedures ................................................................................................................... 10-1
Receiving Inspection .................................................................................................................................. 10-2
Disassembly ............................................................................................................................................... 10-2
Inspection Process ..................................................................................................................................... 10-2
Visual Inspection ........................................................................................................................................ 10-3
Cylinder Head ...................................................................................................................................... 10-4
Piston, Valve Train, & Piston Pin ........................................................................................................ 10-5
Crankshaft & Connecting Rods ........................................................................................................... 10-5
Cleaning ..................................................................................................................................................... 10-5
Degreasing .......................................................................................................................................... 10-5
Removing Hard Carbon ....................................................................................................................... 10-5
Structural Inspection .................................................................................................................................. 10-6
Dye Penetrant Inspection .................................................................................................................... 10-6
Eddy Current Inspection ...................................................................................................................... 10-7
Ultrasonic Inspection ........................................................................................................................... 10-7
Pulse-Echo ...................................................................................................................................... 10-7
Through Transmission .................................................................................................................... 10-7
Resonance ...................................................................................................................................... 10-7
Magnetic Particle Inspection ................................................................................................................ 10-7
X-ray .................................................................................................................................................... 10-7
Dimensional Inspection .............................................................................................................................. 10-7
Cylinder Barrel ..................................................................................................................................... 10-7
Rocker Arms & Shafts .......................................................................................................................... 10-8
Crankshaft ........................................................................................................................................... 10-9
Checking Alignment ............................................................................................................................. 10-9
Repair & Replacement ....................................................................................................................... 10-10
Cylinder Assembly Reconditioning .................................................................................................... 10-11
Piston & Piston Pins .......................................................................................................................... 10-11
xviiValves & Valve Springs ...................................................................................................................... 10-11
Refacing Valve Seats ......................................................................................................................... 10-13
Valve Reconditioning ......................................................................................................................... 10-14
Valve Lapping & Leak Testing ............................................................................................................ 10-17
Piston Repairs ................................................................................................................................... 10-17
Cylinder Grinding & Honing ............................................................................................................... 10-17
Reassembly ............................................................................................................................................. 10-19
Installation & Testing ................................................................................................................................ 10-19
Testing Reciprocating Engines .......................................................................................................... 10-19
Test Cell Requirements ...................................................................................................................... 10-20
Engine Instruments ............................................................................................................................ 10-20
Carburetor Air Temperature (CAT) Indicator ...................................................................................... 10-21
Fuel Pressure Indicator ...................................................................................................................... 10-21
Oil Pressure Indicator ........................................................................................................................ 10-22
Oil Temperature Indicator .................................................................................................................. 10-22
Fuel-Flow Meter ................................................................................................................................. 10-22
Manifold Pressure Indicator ............................................................................................................... 10-23
Tachometer Indicator ......................................................................................................................... 10-23
Cylinder Head Temperature Indicator ................................................................................................ 10-23
Torquemeter ....................................................................................................................................... 10-24
Warning Systems ............................................................................................................................... 10-24
Reciprocating Engine Operation .............................................................................................................. 10-24
Engine Instruments ............................................................................................................................ 10-24
Engine Starting .................................................................................................................................. 10-25
Pre-Oiling ........................................................................................................................................... 10-25
Hydraulic Lock ................................................................................................................................... 10-25
Engine Warm-Up ............................................................................................................................... 10-25
Ground Check .................................................................................................................................... 10-26
Fuel Pressure & Oil Pressure Check ................................................................................................. 10-27
Propeller Pitch Check ........................................................................................................................ 10-27
Power Check .................................................................................................................................... 10-27
Idle Speed & Idle Mixture Checks ...................................................................................................... 10-28
Engine Stopping ................................................................................................................................ 10-28
Basic Engine Operating Principles ........................................................................................................... 10-29
Combustion Process .......................................................................................................................... 10-29
Detonation ......................................................................................................................................... 10-29
Pre-Ignition ........................................................................................................................................ 10-30
Backfiring ........................................................................................................................................... 10-30
Afterfiring ........................................................................................................................................... 10-31
Factors Affecting Engine Operation .......................................................................................................... 10-31
Compression ...................................................................................................................................... 10-31
Fuel Metering ..................................................................................................................................... 10-31
Idle Mixture ........................................................................................................................................ 10-33
Induction Manifold .............................................................................................................................. 10-33
Operational Effect of Valve Clearance ............................................................................................... 10-33
Engine Troubleshooting ........................................................................................................................... 10-35
Valve Blow-By .................................................................................................................................... 10-39
Cylinder Compression Tests ..................................................................................................................... 10-39
Differential Pressure Tester ................................................................................................................ 10-39
Cylinder Replacement ....................................................................................................................... 10-41
Cylinder Removal ..................................................................................................................................... 10-41
xviiiCylinder Installation .................................................................................................................................. 10-42
Cold Cylinder Check ................................................................................................................................ 10-43
Turbine Engine Maintenance ................................................................................................................... 10-44
Compressor Section .......................................................................................................................... 10-45
Inspection & Cleaning ........................................................................................................................ 10-45
Causes of Blade Damage .................................................................................................................. 10-45
Blending & Replacement ................................................................................................................... 10-47
Combustion Section Inspection ................................................................................................................ 10-47
Marking Materials for Combustion Section Parts ............................................................................... 10-49
Inspection & Repair of Combustion Chambers ................................................................................. 10-49
Fuel Nozzle & Support Assemblies .................................................................................................... 10-50
Turbine Disc Inspection ..................................................................................................................... 10-50
Turbine Blade Inspection ................................................................................................................... 10-50
Turbine Blade Replacement Procedure ............................................................................................. 10-51
Turbine Nozzle Inlet Guide Vane Inspection ...................................................................................... 10-52
Clearances ......................................................................................................................................... 10-52
Exhaust Section ................................................................................................................................. 10-54
Engine Ratings ......................................................................................................................................... 10-54
Turbine Engine Instruments ..................................................................................................................... 10-54
Engine Pressure Ratio Indicator ........................................................................................................ 10-54
Torquemeter (Turboprop Engines) ..................................................................................................... 10-55
Tachometer ....................................................................................................................................... 10-55
Exhaust Gas Temperature Indicator (EGT) ....................................................................................... 10-55
Fuel-Flow Indicator ............................................................................................................................ 10-55
Engine Oil Pressure Indicator ............................................................................................................ 10-55
Engine Oil Temperature Indicator ...................................................................................................... 10-55
Turbine Engine Operation ........................................................................................................................ 10-57
Ground Operation Engine Fire ........................................................................................................... 10-57
Engine Checks ................................................................................................................................... 10-57
Checking Takeoff Thrust .................................................................................................................... 10-57
Ambient Conditions ............................................................................................................................ 10-58
Engine Shutdown ..................................................................................................................................... 10-58
Troubleshooting Turbine Engines ............................................................................................................. 10-59
Turboprop Operation ................................................................................................................................ 10-59
Troubleshooting Procedures for Turboprop Engines ......................................................................... 10-59
Turbine Engine Calibration & Testing ....................................................................................................... 10-59
Turbine Engine Analyzer Uses ........................................................................................................... 10-59
Analyzer Safety Precautions ............................................................................................................. 10-61
Continuity Check of Aircraft EGT Circuit ............................................................................................ 10-63
Functional Check of Aircraft EGT Circuit ........................................................................................... 10-63
EGT Indicator Check ......................................................................................................................... 10-64
Resistance & Insulation Check .......................................................................................................... 10-64
Tachometer Check ............................................................................................................................. 10-64
Troubleshooting EGT System .................................................................................................................. 10-64
One or More Inoperative Thermocouples in Engine Parallel Harness ............................................... 10-64
Engine Thermocouples Out of Calibration ......................................................................................... 10-64
EGT Circuit Error ............................................................................................................................... 10-65
Resistance of Circuit Out of Tolerance .............................................................................................. 10-65
Shorts to Ground/Shorts Between Leads .......................................................................................... 10-65
Troubleshooting Aircraft Tachometer System ........................................................................................... 10-65
xixChapter 11
Light-Sport Aircraft Engines
Engine General Requirements .................................................................................................................... 11-1
Personnel Authorized to Perform Inspection & Maintenance on Light-Sport Engines ................................ 11-2
Authorized Personnel That Meet FAA Regulations .............................................................................. 11-3
Types of Light-Sport & Experimental Engines ............................................................................................. 11-3
Light-Sport Aircraft Engines ................................................................................................................. 11-3
Two-Cycle, Two Cylinder Rotax Engine .............................................................................................. 11-3
Rotax 447 UL Single Capacitor Discharge Ignition (SCDI) & Rotax 503 UL Dual Capacitor Discharge
Ignition (DCDI) .................................................................................................................................... 11-3
Rotax 582 UL DCDI ....................................................................................................................... 11-4
Description of Systems for Two-Stroke Engines .................................................................................. 11-4
Cooling System of Rotax 447 UL SCDI & Rotax 503 UL DCDI ...................................................... 11-4
Cooling System of the Rotax 582 UL DCDI ................................................................................... 11-4
Lubrication Systems ............................................................................................................................ 11-4
Oil Injection Lubrication of Rotax 503 UL DCDE & 582 UL DCDI .................................................. 11-4
Electric System .................................................................................................................................... 11-5
Fuel System ......................................................................................................................................... 11-5
Fuel-Oil Mixing Procedure .............................................................................................................. 11-5
Opposed Light-Sport, Experimental, & Certificated Engines ..................................................................... 11-5
Rotax 912/914 .................................................................................................................................... 11-5
Description of Systems ........................................................................................................................ 11-6
Cooling System ............................................................................................................................... 11-6
Fuel System .................................................................................................................................... 11-6
Lubrication System ......................................................................................................................... 11-7
Electric System ............................................................................................................................... 11-8
Turbocharger & Control System ..................................................................................................... 11-8
HKS 700T Engine ................................................................................................................................ 11-9
Jabiru Light-Sport Engines ................................................................................................................ 11-10
Jabiru 2200 Aircraft Engine ........................................................................................................... 11-11
Aeromax Aviation 100 (IFB) Aircraft Engine .................................................................................. 11-11
Direct Drive VW Engines ........................................................................................................................... 11-12
Revmaster R-2300 Engine ............................................................................................................... 11-12
Great Plains Aircraft Volkswagen (VW)Conversions ......................................................................... 11-14
Teledyne Continental 0-200 Engine ................................................................................................... 11-15
Lycoming 0-233 Series Light-Sport Aircraft Engine ........................................................................... 11-15
General Maintenance Practices on Light-Sport Rotax Engines ............................................................... 11-16
Maintenance Schedule Procedures & Maintenance Checklist .................................................................. 11-16
Carburetor Synchronization ............................................................................................................... 11-17
Pneumatic Synchronization ............................................................................................................... 11-18
Idle Speed Adjustment ....................................................................................................................... 11-19
Optimizing Engine Running ............................................................................................................... 11-19
Checking the Carburetor Actuation .................................................................................................... 11-19
Lubrication System .................................................................................................................................... 11-19
Oil Level Check .................................................................................................................................. 11-19
Oil Change ......................................................................................................................................... 11-20
Cleaning the Oil Tank ......................................................................................................................... 11-20
Inspecting the Magnetic Plug ............................................................................................................. 11-21
Checking the Propeller Gearbox ........................................................................................................ 11-21
Checking the Friction Torque in Free Rotation .................................................................................. 11-21
xxDaily Maintenance Checks ................................................................................................................ 11-21
Pre-flight Checks ....................................................................................................................................... 11-22
Troubleshooting & Abnormal Operation .................................................................................................... 11-22
Troubleshooting ................................................................................................................................. 11-23
Engine Keeps Running With Ignition OFF .................................................................................... 11-23
Knocking Under Load ................................................................................................................... 11-23
Abnormal Operation ........................................................................................................................... 11-23
Exceeding the Maximum Admissible Engine Speed ..................................................................... 11-23
Exceeding Maximum Admissible Cylinder Head Temperature ..................................................... 11-23
Exceeding Maximum Admissible Exhaust Gas Temperature ........................................................ 11-23
Engine Preservation .................................................................................................................................. 11-23
General Maintenance Practices for the Light-Sport Jabiru Engines ......................................................... 11-23
Engine & Engine Compartment Inspection ........................................................................................ 11-23
Lubrication System ............................................................................................................................ 11-24
Carburetor Adjustment & Checks ...................................................................................................... 11-24
Spark Plugs ....................................................................................................................................... 11-24
Exhaust System ................................................................................................................................. 11-24
Head Bolts ......................................................................................................................................... 11-24
Tachometer & Sender ........................................................................................................................ 11-25
Engine Inspection Charts .......................................................................................................................... 11-25
Glossary ....................................................................................................................................................... G-1
Index ............................................................................................................................................................... I-1
General Requirements
Aircraft require thrust to produce enough speed for the wings
to provide lift or enough thrust to overcome the weight of
the aircraft for vertical takeoff. For an aircraft to remain in
level flight, thrust must be provided that is equal to and in
the opposite direction of the aircraft drag. This thrust, or
propulsive force, is provided by a suitable type of aircraft
heat engine. All heat engines have in common the ability to
convert heat energy into mechanical energy by the flow of
some fluid mass (generally air) through the engine. In all
cases, the heat energy is released at a point in the cycle where
the working pressure is high relative to atmospheric pressure.
The propulsive force is obtained by the displacement of
a working fluid (again, atmospheric air). This air is not
necessarily the same air used within the engine. By displacing
air in a direction opposite to that in which the aircraft is
propelled, thrust can be developed. This is an application of
Newton’s third law of motion. It states that for every action
there is an equal and opposite reaction. So, as air is being
displaced to the rear of the aircraft the aircraft is moved forward
by this principle. One misinterpretation of this principle is air
is pushing against the air behind the aircraft making it move
forward. This is not true. Rockets in space have no air to push
against, yet, they can produce thrust by using Newton’s third
law. Atmospheric air is the principal fluid used for propulsion
in every type of aircraft powerplant except the rocket, in which
the total combustion gases are accelerated and displaced. The
rocket must provide all the fuel and oxygen for combustion
and does not depend on atmospheric air. A rocket carries its
own oxidizer rather than using ambient air for combustion. It
discharges the gaseous byproducts of combustion through the
exhaust nozzle at an extremely high velocity (action) and it is
propelled in the other direction (reaction).
The propellers of aircraft powered by reciprocating or
turboprop engines accelerate a large mass of air at a relatively
lower velocity by turning a propeller. The same amount of
thrust can be generated by accelerating a small mass of air
to a very high velocity. The working fluid (air) used for the
propulsive force is a different quantity of air than that used
within the engine to produce the mechanical energy to turn
the propeller.
Aircraft Engines
Chapter 1
Turbojets, ramjets, and pulse jets are examples of engines that
accelerate a smaller quantity of air through a large velocity
change. They use the same working fluid for propulsive force
that is used within the engine. One problem with these types
of engines is the noise made by the high velocity air exiting
the engine. The term turbojet was used to describe any gas
turbine engine, but with the differences in gas turbines used in
aircraft, this term is used to describe a type of gas turbine that
passes all the gases through the core of the engine directly.
Turbojets, ramjets, and pulse jets have very little to no use
in modern aircraft due to noise and fuel consumption. Small
general aviation aircraft use mostly horizontally opposed
reciprocating piston engines. While some aircraft still use
radial reciprocating piston engines, their use is very limited.
Many aircraft use a form of the gas turbine engine to produce
power for thrust. These engines are normally the turboprop,
turboshaft, turbofan, and a few turbojet engines. “Turbojet”
is the former term for any turbine engine. Now that there are
so many different types of turbine engines, the term used to
describe most turbine engines is “gas turbine engine.” All
four of the previously mentioned engines belong to the gas
turbine family.
All aircraft engines must meet certain general requirements of
efficiency, economy, and reliability. Besides being economical
in fuel consumption, an aircraft engine must be economical in
the cost of original procurement and the cost of maintenance;
and it must meet exacting requirements of efficiency and low
weight-to-horsepower ratio. It must be capable of sustained
high-power output with no sacrifice in reliability; it must also
have the durability to operate for long periods of time between
overhauls. It needs to be as compact as possible yet have easy
accessibility for maintenance. It is required to be as vibration
free as possible and be able to cover a wide range of power
output at various speeds and altitudes.
These requirements dictate the use of ignition systems
that deliver the firing impulse to the spark plugs at the
proper time in all kinds of weather and under other adverse
conditions. Engine fuel delivery systems provide metered fuel
at the correct proportion of air-fuel ingested by the engine
regardless of the attitude, altitude, or type of weather in which
the engine is operated. The engine needs a type of oil system
1-2that delivers oil under the proper pressure to lubricate and
cool all of the operating parts of the engine when it is running.
Also, it must have a system of damping units to damp out the
vibrations of the engine when it is operating.
Power & Weight
The useful output of all aircraft powerplants is thrust, the
force which propels the aircraft. Since the reciprocating
engine is rated in brake horsepower (bhp), the gas turbine
engine is rated in thrust horsepower (thp):
The value of 375 mile-pounds per hour is derived from the
basic horsepower formula as follows:
One horsepower equals 33,000 ft-lb per minute or 375 mile-
pounds per hour. Under static conditions, thrust is figured as
equivalent to approximately 2.6 pounds per hour.
If a gas turbine is producing 4,000 pounds of thrust and the
aircraft in which the engine is installed is traveling at 500
mph, the thp is:
It is necessary to calculate the horsepower for each speed of an
aircraft, since the horsepower varies with speed. Therefore, it
is not practical to try to rate or compare the output of a turbine
engine on a horsepower basis. The aircraft engine operates
at a relatively high percentage of its maximum power output
throughout its service life. The aircraft engine is at full power
output whenever a takeoff is made. It may hold this power for
a period of time up to the limits set by the manufacturer. The
engine is seldom held at a maximum power for more than
2 minutes, and usually not that long. Within a few seconds
after lift-off, the power is reduced to a power that is used for
climbing and that can be maintained for longer periods of
time. After the aircraft has climbed to cruising altitude, the
power of the engine(s) is further reduced to a cruise power Thp = thrust x aircraft speed (mph)
375 mile-pounds per hour
= 375 mile-pounds per hour1,980,000
5,280 ft in a mile1 hp = 33,000 ft-lb per minute
33,000 x 60 = 1,980,000 ft-lb per hour
= 5,333.33 thp4,000 x 500
375which can be maintained for the duration of the flight.
If the weight of an engine per brake horsepower (called
the specific weight of the engine) is decreased, the useful
load that an aircraft can carry and the performance of the
aircraft obviously are increased. Every excess pound of
weight carried by an aircraft engine reduces its performance.
Tremendous improvement in reducing the weight of the
aircraft engine through improved design and metallurgy
has resulted in reciprocating engines with a much improved
power-to-weight ratio (specific weight).
Fuel Economy
The basic parameter for describing the fuel economy of
aircraft engines is usually specific fuel consumption. Specific
fuel consumption for gas turbines is the fuel flow measured
in (lb/hr) divided by thrust (lb), and for reciprocating engines
the fuel flow (lb/hr) divided by brake horsepower. These are
called thrust-specific fuel consumption and brake-specific
fuel consumption, respectively. Equivalent specific fuel
consumption is used for the turboprop engine and is the fuel
flow in pounds per hour divided by a turboprop’s equivalent
shaft horsepower. Comparisons can be made between the
various engines on a specific fuel consumption basis. At low
speed, the reciprocating and turboprop engines have better
economy than the pure turbojet or turbofan engines. However,
at high speed, because of losses in propeller efficiency, the
reciprocating or turboprop engine’s efficiency becomes
limited above 400 mph less than that of the turbofan.
Durability & Reliability
Durability and reliability are usually considered identical
factors since it is difficult to mention one without including
the other. Simply put, reliability is measured as the mean time
between failures, while durability is measured as the mean
time between overhauls.
More specifically, an aircraft engine is reliable when it can
perform at the specified ratings in widely varying flight
attitudes and in extreme weather conditions. Standards
of powerplant reliability are agreed upon by the Federal
Aviation Administration (FAA), the engine manufacturer,
and the airframe manufacturer. The engine manufacturer
ensures the reliability of the product by design, research,
and testing. Close control of manufacturing and assembly
procedures is maintained, and each engine is tested before
it leaves the factory.
Durability is the amount of engine life obtained while
maintaining the desired reliability. The fact that an engine
has successfully completed its type or proof test indicates
that it can be operated in a normal manner over a long period
before requiring overhaul. However, no definite time interval
between overhauls is specified or implied in the engine
1-3rating. The time between overhauls (TBO) varies with the
operating conditions, such as engine temperatures, amount
of time the engine is operated at high-power settings, and the
maintenance received. Recommended TBOs are specified by
the engine manufacturer.
Reliability and durability are built into the engine by the
manufacturer, but the continued reliability of the engine is
determined by the maintenance, overhaul, and operating
personnel. Careful maintenance and overhaul methods,
thorough periodical and preflight inspections, and strict
observance of the operating limits established by the engine
manufacturer make engine failure a rare occurrence.
Operating Flexibility
Operating flexibility is the ability of an engine to run
smoothly and give desired performance at all speeds from
idling to full-power output. The aircraft engine must also
function efficiently through all the variations in atmospheric
conditions encountered in widespread operations.
Compactness
To affect proper streamlining and balancing of an aircraft, the
shape and size of the engine must be as compact as possible.
In single-engine aircraft, the shape and size of the engine also
affect the view of the pilot, making a smaller engine better
from this standpoint, in addition to reducing the drag created
by a large frontal area.
Weight limitations, naturally, are closely related to the
compactness requirement. The more elongated and spread
out an engine is, the more difficult it becomes to keep the
specific weight within the allowable limits.
Powerplant Selection
Engine specific weight and specific fuel consumption were
discussed in the previous paragraphs, but for certain design
requirements, the final powerplant selection may be based
on factors other than those that can be discussed from an
analytical point of view. For that reason, a general discussion
of powerplant selection follows.
For aircraft whose cruising speed does not exceed 250 mph,
the reciprocating engine is the usual choice of powerplant.
When economy is required in the low speed range, the
conventional reciprocating engine is chosen because of
its excellent efficiency and relatively low cost. When high
altitude performance is required, the turbo-supercharged
reciprocating engine may be chosen because it is capable
of maintaining rated power to a high altitude (above 30,000
feet). Gas turbine engines operate most economically at
high altitudes. Although in most cases the gas turbine engine
provides superior performance, the cost of gas turbine engines is a limiting factor. In the range of cruising speed of
180 to 350 mph, the turboprop engine performs very well.
It develops more power per pound of weight than does
the reciprocating engine, thus allowing a greater fuel load
or payload for engines of a given power. From 350 mph
up to Mach .8–.9, turbofan engines are generally used for
airline operations. Aircraft intended to operate at Mach 1 or
higher are powered by pure turbojet engines/afterburning
(augmented) engines, or low-bypass turbofan engines.
Types of Engines
Aircraft engines can be classified by several methods. They
can be classed by operating cycles, cylinder arrangement,
or the method of thrust production. All are heat engines that
convert fuel into heat energy that is converted to mechanical
energy to produce thrust. Most of the current aircraft engines
are of the internal combustion type because the combustion
process takes place inside the engine. Aircraft engines
come in many different types, such as gas turbine based,
reciprocating piston, rotary, two or four cycle, spark ignition,
diesel, and air or water cooled. Reciprocating and gas turbine
engines also have subdivisions based on the type of cylinder
arrangement (piston) and speed range (gas turbine).
Many types of reciprocating engines have been designed.
However, manufacturers have developed some designs that
are used more commonly than others and are, therefore,
recognized as conventional. Reciprocating engines may be
classified according to the cylinder arrangement (inline,
V-type, radial, and opposed) or according to the method of
cooling (liquid cooled or air cooled). Actually, all piston
engines are cooled by transferring excess heat to the
surrounding air. In air-cooled engines, this heat transfer is
direct from the cylinders to the air. Therefore, it is necessary to
provide thin metal fins on the cylinders of an air-cooled engine
in order to have increased surface for sufficient heat transfer.
Most reciprocating aircraft engines are air cooled although
a few high powered engines use an efficient liquid-cooling
system. In liquid-cooled engines, the heat is transferred
from the cylinders to the coolant, which is then sent through
tubing and cooled within a radiator placed in the airstream.
The coolant radiator must be large enough to cool the liquid
efficiently. The main problem with liquid cooling is the added
weight of coolant, heat exchanger (radiator), and tubing to
connect the components. Liquid cooled engines do allow high
power to be obtained from the engine safely.
Inline Engines
An inline engine generally has an even number of cylinders,
although some three-cylinder engines have been constructed.
This engine may be either liquid cooled or air cooled and has
only one crank shaft, which is located either above or below
the cylinders. If the engine is designed to operate with the
Figure 1-3. Double row radials.Figure 1-2. Radial engine.
Figure 1-1. A typical four-cylinder opposed engine.cylinders below the crankshaft, it is called an inverted engine.
The inline engine has a small frontal area and is better
adapted to streamlining. When mounted with the cylinders
in an inverted position, it offers the added advantages of a
shorter landing gear and greater pilot visibility. With increase
in engine size, the air cooled, inline type offers additional
problems to provide proper cooling; therefore, this type of
engine is confined to low- and medium-horsepower engines
used in very old light aircraft.
Opposed or O-Type Engines
The opposed-type engine has two banks of cylinders
directly opposite each other with a crankshaft in the center.
[Figure 1-1] The pistons of both cylinder banks are connected
to the single crankshaft. Although the engine can be either
liquid cooled or air cooled, the air-cooled version is used
predominantly in aviation. It is generally mounted with
the cylinders in a horizontal position. The opposed-type
engine has a low weight-to-horsepower ratio, and its narrow
silhouette makes it ideal for horizontal installation on the
aircraft wings (twin engine applications). Another advantage
is its low vibration characteristics.
V-Type Engines
In V-type engines, the cylinders are arranged in two inline
banks generally set 60° apart. Most of the engines have 12
cylinders, which are either liquid cooled or air cooled. The
engines are designated by a V followed by a dash and the
piston displacement in cubic inches. For example, V-1710.
This type of engine was used mostly during the Second World
War and its use is mostly limited to older aircraft.
Radial Engines
The radial engine consists of a row, or rows, of cylinders
arranged radially about a central crankcase. [Figure 1-2] This
type of engine has proven to be very rugged and dependable.
The number of cylinders which make up a row may be three, five, seven, or nine. Some radial engines have two rows of
seven or nine cylinders arranged radially about the crankcase,
one in front of the other. These are called double-row radials.
[Figure 1-3] One type of radial engine has four rows of
cylinders with seven cylinders in each row for a total of 28
cylinders. Radial engines are still used in some older cargo
airplanes, war birds, and crop spray airplanes. Although
many of these engines still exist, their use is limited. The
single-row, nine-cylinder radial engine is of relatively simple
construction, having a one-piece nose and a two-section
main crankcase. The larger twin-row engines are of slightly
more complex construction than the single row engines.
For example, the crankcase of the Wright R-3350 engine
is composed of the crankcase front section, four crankcase
main sections (front main, front center, rear center, and
The cylinder forms a part of the
chamber in which the fuel is
compressed and burned.
The crankshaft and connecting
rod change the straight line
motion of the piston to a rotary
turning motion. The crankshaft
in an aircraft
engine also
absorbs the
power or work
from all the
cylinders and
transfers it to the propeller.An intake valve is needed to let
the fuel/air into the cylinder.
The piston, moving within the
cylinder, forms one of the walls
of the combustin chamber.
The piston has rings which seal
the gases in the
cylinder, preventing
any loss of power
around the sides of
the piston.
The connecting rod forms a link
between the piston and
the crankshaft.Spark plugAn exhaust valve is needed
to let the exhaust gases out.
Crankcase
Figure 1-4. Basic parts of a reciprocating engine. rear main), rear cam and tappet housing, supercharger front
housing, supercharger rear housing, and supercharger rear
housing cover. Pratt and Whitney engines of comparable size
incorporate the same basic sections, although the construction
and the nomenclature differ considerably.
Reciprocating Engines
Design & Construction
The basic major components of a reciprocating engine are
the crankcase, cylinders, pistons, connecting rods, valves,
valve-operating mechanism, and crankshaft. In the head
of each cylinder are the valves and spark plugs. One of the
valves is in a passage leading from the induction system; the
other is in a passage leading to the exhaust system. Inside
each cylinder is a movable piston connected to a crankshaft
by a connecting rod. Figure 1-4 illustrates the basic parts of
a reciprocating engine.
Crankcase Section
The foundation of an engine is the crankcase. It contains
the bearings and bearing supports in which the crankshaft
revolves. Besides supporting itself, the crankcase must
provide a tight enclosure for the lubricating oil and must
support various external and internal mechanisms of the engine. It also provides support for attachment of the
cylinder assemblies, and the powerplant to the aircraft. It
must be sufficiently rigid and strong to prevent misalignment
of the crankshaft and its bearings. Cast or forged aluminum
alloy is generally used for crankcase construction because
it is light and strong. The crankcase is subjected to many
variations of mechanical loads and other forces. Since the
cylinders are fastened to the crankcase, the tremendous
forces placed on the cylinder tend to pull the cylinder
off the crankcase. The unbalanced centrifugal and inertia
forces of the crankshaft acting through the main bearings
subject the crankcase to bending moments which change
continuously in direction and magnitude. The crankcase
must have sufficient stiffness to withstand these bending
moments without major deflections. [Figure 1-5]
If the engine is equipped with a propeller reduction gear, the
front or drive end is subjected to additional forces. In addition
to the thrust forces developed by the propeller under high
power output, there are severe centrifugal and gyroscopic
forces applied to the crankcase due to sudden changes in the
direction of flight, such as those occurring during maneuvers
of the airplane. Gyroscopic forces are particularly severe
when a heavy propeller is installed. To absorb centrifugal
loads, a large centrifugal bearing is used in the nose section.
Figure 1-5. The crankcase.
The shape of the nose or front of the crankcase section
varies considerably. In general, it is either tapered or round.
Depending upon the type of reciprocating engine, the nose or
front area of the crankcase varies somewhat. If the propeller
is driven directly by the crankshaft, less area is needed for
this component of the engine. The crankcases used on engines
having opposed or inline cylinder arrangements vary in form
for the different types of engines, but in general they are
approximately cylindrical. One or more sides are surfaced to
serve as a base to which the cylinders are attached by means
of cap screws, bolts, or studs. These accurately machined
surfaces are frequently referred to as cylinder pads.
If the propeller is driven by reduction gearing (gears that slow
down the speed of the propeller less than the engine), more
area is required to house the reduction gears. A tapered nose
section is used quite frequently on direct-drive, low-powered
engines, because extra space is not required to house the
propeller reduction gears. Crankcase nose sections are usually
cast of either aluminum alloy or magnesium. The crankcase
nose section on engines that develop from 1,000 to 2,500
hp is usually larger to house reduction gears and sometimes
ribbed to get as much strength as possible.
The governor is used to control propeller speed and blade
angle. The mounting of the propeller governor varies. On
some engines, it is located on the rear section, although this
complicates the installation, especially if the propeller is
operated or controlled by oil pressure, because of the distance
between the governor and propeller. Where hydraulically
operated propellers are used, it is good practice to mount
the governor on the nose section as close to the propeller
as possible to reduce the length of the oil passages. The
governor is then driven either from gear teeth on the periphery
of the bell gear or by some other suitable means. This basic arrangement is also used for turboprops.
On some of the larger radial engines, a small chamber is located
on the bottom of the nose section to collect the oil. This is called
the nose section oil sump. Since the nose section transmits
many varied forces to the main crankcase or power section,
it must be secured properly to transmit the loads efficiently.
The machined surfaces on which the cylinders are mounted are
called cylinder pads. They are provided with a suitable means
of retaining or fastening the cylinders to the crankcase. The
general practice in securing the cylinder flange to the pad is
to mount studs in threaded holes in the crankcase. The inner
portion of the cylinder pads are sometimes chamfered or
tapered to permit the installation of a large rubber O-ring around
the cylinder skirt, which effectively seals the joint between the
cylinder and the crankcase pads against oil leakage.
Because oil is thrown about the crankcase, especially on
inverted inline and radial-type engines, the cylinder skirts
extend a considerable distance into the crankcase sections to
reduce the flow of oil into the inverted cylinders. The piston
and ring assemblies must be arranged so that they throw out
the oil splashed directly into them.
Mounting lugs are spaced about the periphery of the rear
of the crankcase or the diffuser section of a radial engine.
These are used to attach the engine assembly to the engine
mount or framework provided for attaching the powerplant
to the fuselage of single-engine aircraft or to the wing nacelle
structure of multiengine aircraft. The mounting lugs may
be either integral with the crankcase or diffuser section or
detachable, as in the case of flexible or dynamic engine mounts.
The mounting arrangement supports the entire powerplant
including the propeller, and therefore is designed to provide
ample strength for rapid maneuvers or other loadings. Because
of the elongation and contraction of the cylinders, the intake
pipes which carry the mixture from the diffuser chamber
through the intake valve ports are arranged to provide a slip
joint which must be leak proof. The atmospheric pressure on
the outside of the case of an un-supercharged engine is higher
than on the inside, especially when the engine is operating at
idling speed. If the engine is equipped with a supercharger
and operated at full throttle, the pressure is considerably
higher on the inside than on the outside of the case. If the
slip joint connection has a slight leakage, the engine may
idle fast due to a slight leaning of the mixture. If the leak is
quite large, it may not idle at all. At open throttle, a small
leak probably would not be noticeable in operation of the
engine, but the slight leaning of the air-fuel mixture might
cause detonation or damage to the valves and valve seats. On
some radial engines, the intake pipe has considerable length
Magneto
Oil pumpAccessory case assemblyProp shaft Generator
Crankcase (left half)
Tachometer
generator
Starter
Oil sumpCamshaftConnecting rod
CylinderCrankcase (right half)
Induction systemCrankshaftMain bearing surface
Parting surface
Transverse websCamshaft bearings
Figure 1-6. Typical opposed engine exploded into component assemblies. and on some inline engines, the intake pipe is at right angles
to the cylinders. In these cases, flexibility of the intake pipe
or its arrangement eliminates the need for a slip joint. In any
case, the engine induction system must be arranged so that it
does not leak air and change the desired air-fuel ratio.
Accessory Section
The accessory (rear) section usually is of cast construction
and the material may be either aluminum alloy, which is used
most widely, or magnesium, which has been used to some
extent. On some engines, it is cast in one piece and provided
with means for mounting the accessories, such as magnetos,
carburetors, fuel, oil, vacuum pumps, starter, generator,
tachometer drive, etc., in the various locations required
to facilitate accessibility. Other adaptations consist of an
aluminum alloy casting and a separate cast magnesium cover
plate on which the accessory mounts are arranged. Accessory
drive shafts are mounted in suitable drive arrangements
that are carried out to the accessory mounting pads. In this
manner, the various gear ratios can be arranged to give the
proper drive speed to magnetos, pumps, and other accessories
to obtain correct timing or functioning. Accessory Gear Trains
Gear trains, containing both spur- and bevel-type gears, are
used in the different types of engines for driving engine
components and accessories. Spur-type gears are generally
used to drive the heavier loaded accessories or those requiring
the least play or backlash in the gear train. Bevel gears permit
angular location of short stub shafts leading to the various
accessory mounting pads. On opposed, reciprocating engines,
the accessory gear trains are usually simple arrangements.
Many of these engines use simple gear trains to drive the
engine’s accessories at the proper speeds.
Crankshafts
The crankshaft is carried in a position parallel to the
longitudinal axis of the crankcase and is generally supported
by a main bearing between each throw. The crankshaft main
bearings must be supported rigidly in the crankcase. This
usually is accomplished by means of transverse webs in the
crankcase, one for each main bearing. The webs form an
integral part of the structure and, in addition to supporting
the main bearings, add to the strength of the entire case.
1-8The crankcase is divided into two sections in a longitudinal
plane. This division may be in the plane of the crankshaft so
that one-half of the main bearing (and sometimes camshaft
bearings) are carried in one section of the case and the other
half in the opposite section. [Figure 1-6] Another method is
to divide the case in such a manner that the main bearings
are secured to only one section of the case on which the
cylinders are attached, thereby providing means of removing
a section of the crankcase for inspection without disturbing
the bearing adjustment.
The crankshaft is the backbone of the reciprocating engine. It
is subjected to most of the forces developed by the engine. Its
main purpose is to transform the reciprocating motion of the
piston and connecting rod into rotary motion for rotation of
the propeller. The crankshaft, as the name implies, is a shaft
composed of one or more cranks located at specified points
along its length. The cranks, or throws, are formed by forging
offsets into a shaft before it is machined. Since crankshafts
must be very strong, they generally are forged from a very
strong alloy, such as chromium-nickel-molybdenum steel.
A crankshaft may be of single-piece or multipiece
construction. Figure 1-7 shows two representative types of
solid crankshafts used in aircraft engines. The four-throw
construction may be used either on four-cylinder horizontal
opposed or four-cylinder inline engines. The six-throw shaft
is used on six-cylinder inline engines, 12-cylinder V-type
engines, and six-cylinder opposed engines. Crankshafts
of radial engines may be the single-throw, two-throw, or
four-throw type, depending on whether the engine is the
single-row, twin-row, or four-row type. A single-throw radial
engine crankshaft is shown in Figure 1-8 . No matter how
many throws it may have, each crankshaft has three main
parts—a journal, crankpin, and crank cheek. Flyweights and
dampers, although not a true part of a crankshaft, are usually
attached to it to reduce engine vibration.
The journal is supported by, and rotates in, a main bearing. It
serves as the center of rotation of the crankshaft. It is surface-
hardened to reduce wear. The crankpin is the section to which
the connecting rod is attached. It is off-center from the main
journals and is often called the throw. Two crank cheeks
and a crankpin make a throw. When a force is applied to the
crankpin in any direction other than parallel or perpendicular
to and through the center line of the crankshaft, it causes the
crankshaft to rotate. The outer surface is hardened by nitriding
to increase its resistance to wear and to provide the required
bearing surface. The crankpin is usually hollow. This reduces
the total weight of the crankshaft and provides a passage for
the transfer of lubricating oil. On early engines, the hollow
crankpin also served as a chamber for collecting sludge,
carbon deposits, and other foreign material. Centrifugal force threw these substances to the outside of the chamber and
kept them from reaching the connecting-rod bearing surface.
Due to the use of ashless dispersant oils, newer engines no
longer use sludge chambers. On some engines, a passage
is drilled in the crank cheek to allow oil from the hollow
crankshaft to be sprayed on the cylinder walls. The crank
cheek connects the crankpin to the main journal. In some
designs, the cheek extends beyond the journal and carries a
flyweight to balance the crankshaft. The crank cheek must be
of sturdy construction to obtain the required rigidity between
the crankpin and the journal.
In all cases, the type of crankshaft and the number of crankpins
must correspond with the cylinder arrangement of the engine.
The position of the cranks on the crankshaft in relation to the
other cranks of the same shaft is expressed in degrees.
The simplest crankshaft is the single-throw or 360° type.
This type is used in a single-row radial engine. It can be
constructed in one or two pieces. Two main bearings (one
on each end) are provided when this type of crankshaft is
used. The double-throw or 180° crankshaft is used on double-
row radial engines. In the radial-type engine, one throw is
provided for each row of cylinders.
Crankshaft Balance
Excessive vibration in an engine not only results in fatigue
failure of the metal structures, but also causes the moving
parts to wear rapidly. In some instances, excessive vibration
is caused by a crankshaft that is not balanced. Crankshafts are
balanced for static balance and dynamic balance. A crankshaft
is statically balanced when the weight of the entire assembly
of crankpins, crank cheeks, and flyweights is balanced around
the axis of rotation. When checked for static balance, it is
placed on two knife edges. If the shaft tends to turn toward
any one position during the test, it is out of static balance.
Any engine to be overhauled completely should receive a
runout check of its crankshaft as a first step. Any question
concerning crankshaft replacement is resolved at this time
since a shaft whose runout is beyond limits must be replaced.
Dynamic Dampers
A crankshaft is dynamically balanced when all the forces
created by crankshaft rotation and power impulses are
balanced within themselves so that little or no vibration is
produced when the engine is operating. To reduce vibration
to a minimum during engine operation, dynamic dampers are
incorporated on the crankshaft. A dynamic damper is merely
a pendulum that is fastened to the crankshaft so that it is free
to move in a small arc. It is incorporated in the flyweight
assembly. Some crankshafts incorporate two or more of these
assemblies, each being attached to a different crank cheek.
The distance the pendulum moves and, thus, its vibrating
180° 180°
120°
120°120°
1 2 5 63 43 4
6Cylinders
Crank arm
Journal
Crankpin
Journal
Crank cheek
Counterweight Damping weights
Figure 1-8. A single-throw radial engine crankshaft.Figure 1-7. Solid types of crankshafts.
frequency corresponds to the frequency of the power impulses
of the engine. When the vibration frequency of the crankshaft
occurs, the pendulum oscillates out of time with the crankshaft
vibration, thus reducing vibration to a minimum.
The construction of the dynamic damper used in one engine
consists of a movable slotted-steel flyweight attached to the crank cheek. Two spool-shaped steel pins extend into the
slot and pass through oversized holes in the flyweight and
crank cheek. The difference in the diameter between the pins
and the holes provides a pendulum effect. An analogy of the
functioning of a dynamic damper is shown in Figure 1-9 .
Connecting Rods
The connecting rod is the link that transmits forces between
the piston and the crankshaft. [Figure 1-10] Connecting rods
must be strong enough to remain rigid under load and yet be
light enough to reduce the inertia forces that are produced
when the rod and piston stop, change direction, and start
again at the end of each stroke.
There are four types of connecting-rod assemblies [Figure 1-11] :
1. Plain.
2. Fork and blade.
3. Master and articulated.
4. Split-type.
Master-and-Articulated Rod Assembly
The master-and-articulated rod assembly is commonly used
in radial engines. In a radial engine, the piston in one cylinder
in each row is connected to the crankshaft by a master rod.
All other pistons in the row are connected to the master
rod by articulated rods. In an 18-cylinder engine, which
has two rows of cylinders, there are two master rods and
16 articulated rods. The articulated rods are constructed of
forged steel alloy in either the I- or H-shape, denoting the
cross-sectional shape. Bronze bushings are pressed into the
Pendulum length
Handpump bellowHandpump bellow
Figure 1-10. A connecting rod between the piston and crankshaft.Figure 1-9. Principles of a dynamic damper.
bores in each end of the articulated rod to provide knuckle-
pin and piston-pin bearings.
The master rod serves as the connecting link between the
piston pin and the crankpin. The crankpin end, or the big end,
contains the crankpin or master rod bearing. Flanges around
the big end provide for the attachment of the articulated rods.
The articulated rods are attached to the master rod by knuckle
pins, which are pressed into holes in the master rod flanges
during assembly. A plain bearing, usually called a piston-pin
bushing, is installed in the piston end of the master rod to
receive the piston pin.
When a crankshaft of the split-spline or split-clamp type is
employed, a one-piece master rod is used. The master and
articulated rods are assembled and then installed on the
crankpin; the crankshaft sections are then joined together.
In engines that use the one-piece type of crankshaft, the big
end of the master rod is split, as is the master rod bearing.
The main part of the master rod is installed on the crankpin;
then the bearing cap is set in place and bolted to the master
rod. The centers of the knuckle pins do not coincide with
the center of the crankpin. Thus, while the crankpin center
describes a true circle for each revolution of the crankshaft,
the centers of the knuckle pins describe an elliptical path.
[Figure 1-12] The elliptical paths are symmetrical about a
center line through the master rod cylinder. It can be seen that
the major diameters of the ellipses are not the same. Thus,
the link rods have varying degrees of angularity relative to
the center of the crank throw.
Because of the varying angularity of the link rods and the
elliptical motion of the knuckle pins, all pistons do not move
an equal amount in each cylinder for a given number of degrees
of crank throw movement. This variation in piston position
between cylinders can have considerable effect on engine
operation. To minimize the effect of these factors on valve and
ignition timing, the knuckle pin holes in the master rod flange
are not equidistant from the center of the crankpin, thereby offsetting to an extent the effect of the link rod angularity.
Another method of minimizing the adverse effects on engine
operation is to use a compensated magneto. In this magneto
the breaker cam has a number of lobes equal to the number
of cylinders on the engine. To compensate for the variation
in piston position due to link rod angularity, the breaker cam
lobes are ground with uneven spacing. This allows the breaker
contacts to open when the piston is in the correct firing position.
This is further outlined during the discussion on ignition timing
in Chapter 4, Engine Ignition & Electrical Systems.
Knuckle Pins
The knuckle pins are of solid construction except for the
oil passages drilled in the pins, which lubricate the knuckle
pin bushings. These pins may be installed by pressing into
holes in the master rod flanges so that they are prevented
from turning in the master rod. Knuckle pins may also be
installed with a loose fit so that they can turn in the master rod
flange holes, and also turn in the articulating rod bushings.
These are called full-floating knuckle pins. In either type of
installation, a lock plate on each side retains the knuckle pin
and prevents a lateral movement.
Fork-and-blade rod Solid-type master rod
Split-type master rod Plain rodBlade rod
Fork rod
Shank
Cap Connecting rod boltsCrimp or pinchBearing shells lined with bearing materialPiston pin end
Bronze bushingArticulating rod
Knuckle pin lock plate
Knuckle pin
Figure 1-11. Connecting rod assemblies.
Figure 1-12. Elliptical travel path of knuckle pins in an articulated
rod assembly. Plain-Type Connecting Rods
Plain-type connecting rods are used in inline and opposed
engines. The end of the rod attached to the crankpin is fitted
with a cap and a two-piece bearing. The bearing cap is held
on the end of the rod by bolts or studs. To maintain proper
fit and balance, connecting rods should always be replaced
in the same cylinder and in the same relative position.
Fork-and-Blade Rod Assembly
The fork-and-blade rod assembly is used primarily in V-type
engines. The forked rod is split at the crankpin end to allow
space for the blade rod to fit between the prongs. A single two-
piece bearing is used on the crankshaft end of the rod. This
type of connecting rod is not used much on modern engines.
Pistons
The piston of a reciprocating engine is a cylindrical member
which moves back and forth within a steel cylinder.
[Figure 1-13] The piston acts as a moving wall within the
combustion chamber. As the piston moves down in the
cylinder, it draws in the air-fuel mixture. As it moves upward,
it compresses the charge, ignition occurs, and the expanding
gases force the piston downward. This force is transmitted
to the crankshaft through the connecting rod. On the return
upward stroke, the piston forces the exhaust gases from the
cylinder and the cycle repeats.
Aluminum plugPiston pin
Compression rings
Oil control ring
Recessed head
Piston
Piston pin boss
Flat head Recessed head Concave head Dome headSlipper type Trunk type
Figure 1-14. Piston assembly and types of pistons. Figure 1-13. A piston.
Piston Construction
The majority of aircraft engine pistons are machined from
aluminum alloy forgings. Grooves are machined in the
outside surface of the piston to receive the piston rings, and
cooling fins are provided on the inside of the piston for greater
heat transfer to the engine oil.
Pistons may be either the trunk type or the slipper type.
[Figure 1-14] Slipper-type pistons are not used in modern,
high-powered engines because they do not provide adequate strength or wear resistance. The top of the piston, or head,
may be flat, convex, or concave. Recesses may be machined
in the piston head to prevent interference with the valves.
Modern engines use cam ground pistons that are a larger
diameter perpendicular to the piston pin. This larger diameter
keeps the piston straight in the cylinder as the engine warms
up from initial startup. As the piston heats up during warm
up, the part of the piston in line with the pin has more mass
and expands more making the piston completely round.
At low temperatures, the piston is oval shaped and, when
it warms to operating temperature, it becomes round. This
process reduces the tendency of the piston to cock or slap in
the cylinder during warm up. When the engine reaches its
normal operating temperature, the piston assumes the correct
dimensions in the cylinder.
As many as six grooves may be machined around the
piston to accommodate the compression rings and oil rings.
[Figure 1-15] The compression rings are installed in the
three uppermost grooves; the oil control rings are installed
immediately above the piston pin. The piston is usually
drilled at the oil control ring grooves to allow surplus oil
scraped from the cylinder walls by the oil control rings to
pass back into the crankcase. An oil scraper ring is installed
at the base of the piston wall or skirt to prevent excessive oil
consumption. The portions of the piston walls that lie between
Figure 1-15. Machined rings around a piston. ring grooves are called the ring lands. In addition to acting
as a guide for the piston head, the piston skirt incorporates
the piston-pin bosses. The piston-pin bosses are of heavy
construction to enable the heavy load on the piston head to
be transferred to the piston pin.
Piston Pin
The piston pin joins the piston to the connecting rod. It is
machined in the form of a tube from a nickel steel alloy
forging, casehardened, and ground. The piston pin is
sometimes called a wristpin because of the similarity between
the relative motions of the piston and the articulated rod and
that of the human arm. The piston pin used in modern aircraft
engines is the full-floating type, so called because the pin is
free to rotate in both the piston and in the connecting rod
piston-pin bearing. The piston pin must be held in place to
prevent the pin ends from scoring the cylinder walls. A plug
of relatively soft aluminum in the pin end provides a good
bearing surface against the cylinder wall.
Piston Rings
The piston rings prevent leakage of gas pressure from the
combustion chamber and reduce to a minimum the seepage
of oil into the combustion chamber. [Figure 1-15] The rings
fit into the piston grooves but spring out to press against the
cylinder walls; when properly lubricated, the rings form an
effective gas seal.
Piston Ring Construction
Most piston rings are made of high-grade cast iron.
[Figure 1-14] After the rings are made, they are ground to the cross-section desired. Then they are split so that they can
be slipped over the outside of the piston and into the ring
grooves that are machined in the piston wall. Since their
purpose is to seal the clearance between the piston and the
cylinder wall, they must fit the cylinder wall snugly enough
to provide a gastight fit. They must exert equal pressure at
all points on the cylinder wall and must make a gastight fit
against the sides of the ring grooves.
Gray cast iron is most often used in making piston rings. In
some engines, chrome-plated mild steel piston rings are used
in the top compression ring groove because these rings can
better withstand the high temperatures present at this point.
Chrome rings must be used with steel cylinder walls. Never
use chrome rings on chrome cylinders.
Compression Ring
The purpose of the compression rings is to prevent the escape
of combustion gases past the piston during engine operation.
They are placed in the ring grooves immediately below the
piston head. The number of compression rings used on each
piston is determined by the type of engine and its design,
although most aircraft engines use two compression rings
plus one or more oil control rings.
The cross-section of the ring is either rectangular or wedge
shaped with a tapered face. The tapered face presents a narrow
bearing edge to the cylinder wall, which helps to reduce
friction and provide better sealing.
Oil Control Rings
Oil control rings are placed in the grooves immediately below
the compression rings and above the piston pin bores. There
may be one or more oil control rings per piston; two rings
may be installed in the same groove, or they may be installed
in separate grooves. Oil control rings regulate the thickness
of the oil film on the cylinder wall. If too much oil enters the
combustion chamber, it burns and leaves a thick coating of
carbon on the combustion chamber walls, the piston head, the
spark plugs, and the valve heads. This carbon can cause the
valves and piston rings to stick if it enters the ring grooves
or valve guides. In addition, the carbon can cause spark plug
misfiring as well as detonation, pre-ignition, or excessive
oil consumption. To allow the surplus oil to return to the
crankcase, holes are drilled in the bottom of the oil control
piston ring grooves or in the lands next to these grooves.
Oil Scraper Ring
The oil scraper ring usually has a beveled face and is installed
in the groove at the bottom of the piston skirt. The ring is
installed with the scraping edge away from the piston head
or in the reverse position, depending upon cylinder position
and the engine series. In the reverse position, the scraper ring
PistonCast aluminum headExhaust valve Intake valve
Connecting rod
Forged steel barrelCombustion chamber
CrankshaftPiston pin
Figure 1-17. Cutaway view of the cylinder assembly. Figure 1-16. An example of an engine cylinder. retains the surplus oil above the ring on the upward piston
stroke, and this oil is returned to the crankcase by the oil
control rings on the downward stroke.
Cylinders
The portion of the engine in which the power is developed
is called the cylinder. [Figure 1-16] The cylinder provides
a combustion chamber where the burning and expansion of
gases take place, and it houses the piston and the connecting
rod. There are four major factors that need to be considered in
the design and construction of the cylinder assembly. It must:
1. Be strong enough to withstand the internal pressures
developed during engine operation.
2. Be constructed of a lightweight metal to keep down
engine weight.
3. Have good heat-conducting properties for efficient
cooling.
4. Be comparatively easy and inexpensive to manufacture,
inspect, and maintain.
The cylinder head of an air cooled engine is generally made of
aluminum alloy because aluminum alloy is a good conductor
of heat and its light weight reduces the overall engine weight.
Cylinder heads are forged or die-cast for greater strength. The
inner shape of a cylinder head is generally semispherical.
The semispherical shape is stronger than conventionalist design and aids in a more rapid and thorough scavenging of
the exhaust gases.
The cylinder used in the air cooled engine is the overhead
valve type. [Figure 1-17] Each cylinder is an assembly of two
major parts: cylinder head and cylinder barrel. At assembly,
the cylinder head is expanded by heating and then screwed
down on the cylinder barrel, which has been chilled. When
the head cools and contracts and the barrel warms up and
expands, a gastight joint results. The majority of the cylinders
used are constructed in this manner using an aluminum head
and a steel barrel. [Figure 1-18]
Cylinder Heads
The purpose of the cylinder head is to provide a place for
combustion of the air-fuel mixture and to give the cylinder
more heat conductivity for adequate cooling. The air-fuel
mixture is ignited by the spark in the combustion chamber
and commences burning as the piston travels toward top
dead center (top of its travel) on the compression stroke. The
ignited charge is rapidly expanding at this time, and pressure
is increasing so that, as the piston travels through the top dead
center position, it is driven downward on the power stroke.
Figure 1-18. The aluminum head and steel barrel of a cylinder.The intake and exhaust valve ports are located in the cylinder
head along with the spark plugs and the intake and exhaust
valve actuating mechanisms.
After the cylinder head is cast, the spark plug bushings, valve
guides, rocker arm bushings, and valve seats are installed in
the cylinder head. Spark plug openings may be fitted with
bronze or steel bushings that are shrunk and screwed into
the openings. Stainless steel Heli-Coil spark plug inserts
are used in many engines currently manufactured. Bronze or
steel valve guides are usually shrunk or screwed into drilled
openings in the cylinder head to provide guides for the valve
stems. These are generally located at an angle to the center
line of the cylinder. The valve seats are circular rings of
hardened metal that protect the relatively soft metal of the
cylinder head from the hammering action of the valves (as
they open and close) and from the exhaust gases.
The cylinder heads of air cooled engines are subjected to
extreme temperatures; it is therefore necessary to provide
adequate cooling fin area and to use metals that conduct heat
rapidly. Cylinder heads of air cooled engines are usually cast
or forged. Aluminum alloy is used in the construction for a
number of reasons. It is well adapted for casting or for the
machining of deep, closely spaced fins, and it is more resistant
than most metals to the corrosive attack of tetraethyl lead in
gasoline. The greatest improvement in air cooling has resulted
from reducing the thickness of the fins and increasing their
depth. In this way, the fin area has been increased in modern
engines. Cooling fins taper from 0.090" at the base to 0.060"
at the tip end. Because of the difference in temperature in
the various sections of the cylinder head, it is necessary to
provide more cooling-fin area on some sections than on
others. The exhaust valve region is the hottest part of the
internal surface; therefore, more fin area is provided around
the outside of the cylinder in this section.Cylinder Barrels
The cylinder barrel in which the piston operates must be
made of a high-strength material, usually steel. It must be
as light as possible yet have the proper characteristics for
operating under high temperatures. It must be made of a
good bearing material and have high tensile strength. The
cylinder barrel is made of a steel alloy forging with the inner
surface hardened to resist wear of the piston and the piston
rings which bear against it. This hardening is usually done
by exposing the steel to ammonia or cyanide gas while the
steel is very hot. The steel soaks up nitrogen from the gas,
which forms iron nitrides on the exposed surface. As a result
of this process, the metal is said to be nitrided. This nitriding
only penetrates into the barrel surface a few thousands of an
inch. As the cylinder barrels wear due to use, they can be
repaired by chroming. This is a process that plates chromium
on the surface of the cylinder barrel and brings it back to new
standard dimensions. Chromium-plated cylinders should use
cast iron rings. Honing the cylinder walls is a process that
brings it to the correct dimensions and provides crosshatch
pattern for seating the piston rings during engine break-in.
Some engine cylinder barrels are choked at the top, or they
are smaller in diameter to allow for heat expansion and wear.
In some instances, the barrel has threads on the outside
surface at one end so that it can be screwed into the cylinder
head. The cooling fins are machined as an integral part of
the barrel and have limits on repair and service.
Cylinder Numbering
Occasionally, it is necessary to refer to the left or right side
of the engine or to a particular cylinder. Therefore, it is
necessary to know the engine directions and how cylinders of
an engine are numbered. The propeller shaft end of the engine
is always the front end, and the accessory end is the rear end,
regardless of how the engine is mounted in an aircraft. When
referring to the right side or left side of an engine, always
assume the view is from the rear or accessory end. As seen
from this position, crankshaft rotation is referred to as either
clockwise or counterclockwise.
Inline and V-type engine cylinders are usually numbered from
the rear. In V-engines, the cylinder banks are known as the
right bank and the left bank, as viewed from the accessory end.
[Figure 1-19] The cylinder numbering of the opposed engine
shown begins with the right rear as No. 1 and the left rear as
No. 2. The one forward of No. 1 is No. 3; the one forward of
No. 2 is No. 4, and so on. The numbering of opposed engine
cylinders is by no means standard. Some manufacturers
number their cylinders from the rear and others from the front
of the engine. Always refer to the appropriate engine manual
to determine the numbering system used by that manufacturer.
1-16 Single-Row Radial Double-Row Radial Opposed
Figure 1-19. Numbering of engine cylinders.Single-row radial engine cylinders are numbered clockwise
when viewed from the rear. Cylinder No. 1 is the top cylinder.
In double-row engines, the same system is used. The No. 1
cylinder is the top one in the rear row. No. 2 cylinder is the
first one clockwise from No. 1, but No. 2 is in the front row.
No. 3 cylinder is the next one clockwise to No. 2 but is in
the rear row. Thus, all odd-numbered cylinders are in the rear
row, and all even-numbered cylinders are in the front row.
Firing Order
The firing order of an engine is the sequence in which the
power event occurs in the different cylinders. The firing order
is designed to provide for balance and to eliminate vibration
to the greatest extent possible. In radial engines, the firing
order must follow a special pattern since the firing impulses
must follow the motion of the crank throw during its rotation.
In inline engines, the firing orders may vary somewhat, yet
most orders are arranged so that the firing of cylinders is
evenly distributed along the crankshaft. Six-cylinder inline
engines generally have a firing order of 1-5-3-6-2-4. Cylinder
firing order in opposed engines can usually be listed in pairs
of cylinders, as each pair fires across the center main bearing.
The firing order of six-cylinder opposed engines is 1-4-5-
2-3-6. The firing order of one model four-cylinder opposed
engine is 1-4-2-3, but on another model, it is 1-3-2-4.
Single-Row Radial Engines
On a single-row radial engine, all the odd-numbered cylinders
fire in numerical succession; then, the even numbered
cylinders fire in numerical succession. On a five-cylinder
radial engine, for example, the firing order is 1-3-5-2-4, and
on a seven-cylinder radial engine it is 1-3-5-7-2-4-6. The firing
order of a nine-cylinder radial engine is 1-3-5-7-9-2-4-6-8.Double-Row Radial Engines
On a double-row radial engine, the firing order is somewhat
complicated. The firing order is arranged with the firing
impulse occurring in a cylinder in one row and then in a
cylinder in the other row; therefore, two cylinders in the
same row never fire in succession.
An easy method for computing the firing order of a
14-cylinder, double-row radial engine is to start with any
number from 1 to 14 and add 9 or subtract 5 (these are called
the firing order numbers), whichever gives an answer between
1 and 14, inclusive. For example, starting with 8, 9 cannot be
added since the answer would then be more than 14; therefore,
subtract 5 from 8 to get 3, add 9 to 3 to get 12, subtract 5 from
12 to get 7, subtract 5 from 7 to get 2, and so on.
The firing order numbers of an 18-cylinder, double-row radial
engine are 11 and 7; that is, begin with any number from 1 to
18 and add 11 or subtract 7. For example, beginning with 1,
add 11 to get 12; 11 cannot be added to 12 because the total
would be more than 18, so subtract 7 to get 5, add 11 to 5 to get
16, subtract 7 from 16 to get 9, subtract 7 from 9 to get 2, add
11 to 2 to get 13, and continue this process for 18 cylinders.
Valves
The air-fuel mixture enters the cylinders through the intake
valve ports, and burned gases are expelled through the
exhaust valve ports. The head of each valve opens and closes
these cylinder ports. The valves used in aircraft engines are
the conventional poppet type. The valves are also typed by
their shape and are called either mushroom or tulip because
of their resemblance to the shape of these plants. Figure 1-20
illustrates various shapes and types of these valves.
Tulip type Semi-tulip type Tulip typeHollow-head
mushroom type Mushroom type Tulip typeLarge stemSodium chamber
Small stemHardened tip
NeckFace
Head
Figure 1-20. Various valve types.
Figure 1-21. View of valve guide installed on a cylinder head.
Figure 1-22. Stem keys forming a lock ring to hold valve spring
retaining washers in place. Valve Construction
The valves in the cylinders of an aircraft engine are subjected
to high temperatures, corrosion, and operating stresses; thus,
the metal alloy in the valves must be able to resist all these
factors. Because intake valves operate at lower temperatures
than exhaust valves, they can be made of chromic-nickel
steel. Exhaust valves are usually made of nichrome,
silchrome, or cobalt-chromium steel because these materials
are much more heat resistant.
The valve head has a ground face that forms a seal against
the ground valve seat in the cylinder head when the valve is
closed. The face of the valve is usually ground to an angle
of either 30° or 45°. In some engines, the intake-valve face
is ground to an angle of 30°, and the exhaust-valve face is
ground to a 45° angle. Valve faces are often made more
durable by the application of a material called stellite. About
1⁄16 inch of this alloy is welded to the valve face and ground
to the correct angle. Stellite is resistant to high-temperature
corrosion and also withstands the shock and wear associated
with valve operation. Some engine manufacturers use a
nichrome facing on the valves. This serves the same purpose
as the stellite material.
The valve stem acts as a pilot for the valve head and rides in the valve guide installed in the cylinder head for this purpose.
[Figure 1-21] The valve stem is surface hardened to resist wear.
The neck is the part that forms the junction between the head
and the stem. The tip of the valve is hardened to withstand the
hammering of the valve rocker arm as it opens the valve. A
machined groove on the stem near the tip receives the split-ring
stem keys. These stem keys form a lock ring to hold the valve
spring retaining washer in place. [Figure 1-22]
Some intake and exhaust valve stems are hollow and
partially filled with metallic sodium. This material is used
because it is an excellent heat conductor. The sodium melts
at approximately 208 °F and the reciprocating motion of
the valve circulates the liquid sodium, allowing it to carry
away heat from the valve head to the valve stem where it
is dissipated through the valve guide to the cylinder head
and the cooling fins. Thus, the operating temperature of the
valve may be reduced as much as 300° to 400 °F. Under
no circumstances should a sodium-filled valve be cut open
Cam rampsCamshaft
Cam trackCam lobeRoller
Cam ringCam rollerPressure oilCaseTappetAdjusting screw Rocker arm
Return oilLock screw
Push rod tube
Tappet guideSeatPortValve guideValve spring
Figure 1-24. Valve-operating mechanism (radial engine).
Figure 1-23. Typical cam lobes.or subjected to treatment which may cause it to rupture.
Exposure of the sodium in these valves to the outside air
results in fire or explosion with possible personal injury.
The most commonly used intake valves have solid stems,
and the head is either flat or tulip shaped. Intake valves for
low-power engines are usually flat headed. In some engines,
the intake valve may be the tulip type and have a smaller stem
than the exhaust valve or it may be similar to the exhaust
valve but have a solid stem and head. Although these valves
are similar, they are not interchangeable since the faces of the
valves are constructed of different material. The intake valve
usually has a flat milled on the tip to identify it.
Valve Operating Mechanism
For a reciprocating engine to operate properly, each valve
must open at the proper time, stay open for the required
length of time, and close at the proper time. Intake valves
are opened just before the piston reaches top dead center,
and exhaust valves remain open after top dead center. At
a particular instant, therefore, both valves are open at the
same time (end of the exhaust stroke and beginning of the
intake stroke). This valve overlap permits better volumetric
efficiency and lowers the cylinder operating temperature.
This timing of the valves is controlled by the valve-operating
mechanism and is referred to as the valve timing.
The valve lift (distance that the valve is lifted off its seat) and
the valve duration (length of time the valve is held open) are
both determined by the shape of the cam lobes. Typical cam
lobes are illustrated in Figure 1-23 . The portion of the lobe
that gently starts the valve operating mechanism moving is
called a ramp, or step. The ramp is machined on each side of
the cam lobe to permit the rocker arm to be eased into contact
with the valve tip and thus reduce the shock load which would
otherwise occur. The valve operating mechanism consists of a
cam ring or camshaft equipped with lobes that work against a
cam roller or a cam follower. [Figures 1-24 and 1-25] The cam
follower pushes a push rod and ball socket, actuating a rocker
arm, which in turn opens the valve. Springs, which slip over
the stem of the valves and are held in place by the valve-spring
retaining washer and stem key, close each valve and push the
valve mechanism in the opposite direction. [Figure 1-26]
Cam Rings
The valve mechanism of a radial engine is operated by one or
two cam rings, depending upon the number of rows of cylinders.
In a single-row radial engine, one ring with a double cam
track is used. One track operates the intake valves, the other
Camshaft
Valve springPush rod Tappet
Figure 1-25. Valve-operating mechanism (opposed engine).
Figure 1-26. A typical set of valve springs used to dampen
oscillations. Multiple springs are used to protect against breakage. operates the exhaust valves. The cam ring is a circular piece of
steel with a series of cams or lobes on the outer surface. The
surface of these lobes and the space between them (on which
the cam rollers ride) is known as the cam track. As the cam ring
revolves, the lobes cause the cam roller to raise the tappet in the
tappet guide, thereby transmitting the force through the push
rod and rocker arm to open the valve. In a single-row radial
engine, the cam ring is usually located between the propeller
reduction gearing and the front end of the power section. In a
twin-row radial engine, a second cam for the operation of the
valves in the rear row is installed between the rear end of the
power section and the supercharger section.
The cam ring is mounted concentrically with the crankshaft
and is driven by the crankshaft at a reduced rate of speed through the cam intermediate drive gear assembly. The cam
ring has two parallel sets of lobes spaced around the outer
periphery, one set (cam track) for the intake valves and the
other for the exhaust valves. The cam rings used may have
four or five lobes on both the intake and the exhaust tracks.
The timing of the valve events is determined by the spacing
of these lobes and the speed and direction at which the cam
rings are driven in relation to the speed and direction of the
crankshaft. The method of driving the cam varies on different
makes of engines. The cam ring can be designed with teeth
on either the inside or outside periphery. If the reduction
gear meshes with the teeth on the outside of the ring, the
cam turns in the direction of rotation of the crankshaft. If the
ring is driven from the inside, the cam turns in the opposite
direction from the crankshaft. [Figure 1-24]
A four-lobe cam may be used on either a seven-cylinder or
nine-cylinder engine. [Figure 1-27] On the seven cylinder,
it rotates in the same direction as the crankshaft, and on the
nine cylinder, opposite the crankshaft rotation. On the nine-
cylinder engine, the spacing between cylinders is 40° and
the firing order is 1-3-5-7-9-2-4-6-8. This means that there
is a space of 80° between firing impulses. The spacing on
the four lobes of the cam ring is 90°, which is greater than
the spacing between impulses. Therefore, to obtain proper
relation of valve operations and firing order, it is necessary
to drive the cam opposite the crankshaft rotation. Using the
four-lobe cam on the seven-cylinder engine, the spacing
between the firing of the cylinders is greater than the spacing
of the cam lobes. Therefore, it is necessary for the cam to
rotate in the same direction as the crankshaft.
1-20Number
of Lobes
Speed
Number
of Lobes
Speed
Number
of Lobes
Speed5 Cylinders
21/6
1/41/8
1/61/10
1/8with crankshaft
opposite crankshaft4
47 Cylinders 9 Cylinders
Direction
of Rotation
Timing gearCrankshaft gearCrankshaftCamshaft lobe Cam gear is twice the
size of the crankshaft
gear and operates at
1/2 speed
Camshaft
Figure 1-29. Cam load on lifter body.
Figure 1-28. Cam drive mechanism opposed-type aircraft engine. Figure 1-27. Radial engines, cam ring table.
Camshaft
The valve mechanism of an opposed engine is operated by a
camshaft. The camshaft is driven by a gear that mates with
another gear attached to the crankshaft. [Figure 1-28] The
camshaft always rotates at one-half the crankshaft speed. As
the camshaft revolves, the lobes cause the tappet assembly
to rise in the tappet guide, transmitting the force through the
push rod and rocker arm to open the valve. [Figure 1-29]
Tappet Assembly
The tappet assembly consists of:
1. A cylindrical tappet, which slides in and out in a tappet
guide installed in one of the crankcase sections around
the cam ring;
2. A tappet roller, which follows the contour of the cam
ring and lobes;
3. A tappet ball socket or push rod socket; and
4. A tappet spring.The function of the tappet assembly is to convert the rotational
movement of the cam lobe into reciprocating motion and to
transmit this motion to the push rod, rocker arm, and then
to the valve tip, opening the valve at the proper time. The
purpose of the tappet spring is to take up the clearance
between the rocker arm and the valve tip to reduce the shock
load when the valve is opened. A hole is drilled through the
tappet to allow engine oil to flow to the hollow push rods to
lubricate the rocker assemblies.
Solid Lifters/Tappets
Solid lifters or cam followers generally require the valve
clearance to be adjusted manually by adjusting a screw and
lock nut. Valve clearance is needed to assure that the valve
has enough clearance in the valve train to close completely.
This adjustment or inspection was a continuous maintenance
item until hydraulic lifters were used.
Hydraulic Valve Tappets/Lifters
Some aircraft engines incorporate hydraulic tappets that
automatically keep the valve clearance at zero, eliminating
the necessity for any valve clearance adjustment mechanism.
A typical hydraulic tappet (zero-lash valve lifter) is shown
in Figure 1-30 .
When the engine valve is closed, the face of the tappet body
(cam follower) is on the base circle or back of the cam.
[Figure 1-30] The light plunger spring lifts the hydraulic
plunger so that its outer end contacts the push rod socket,
exerting a light pressure against it, thus eliminating any
clearance in the valve linkage. As the plunger moves outward,
the ball check valve moves off its seat. Oil from the supply
chamber, which is directly connected with the engine
lubrication system, flows in and fills the pressure chamber. As
the camshaft rotates, the cam pushes the tappet body and the
Cam
Tappet body
CylinderBall check valve
Plunger Plunger springPush rod socket Push rod
Push rod shroud tubeOil pressure chamber Oil supply chamberOil hole
Figure 1-30. Hydraulic valve tappets.
hydraulic lifter cylinder outward. This action forces the ball
check valve onto its seat; thus, the body of oil trapped in the
pressure chamber acts as a cushion. During the interval when
the engine valve is off its seat, a predetermined leakage occurs
between plunger and cylinder bore, which compensates for
any expansion or contraction in the valve train. Immediately
after the engine valve closes, the amount of oil required to
fill the pressure chamber flows in from the supply chamber,
preparing for another cycle of operation.
Hydraulic valve lifters are normally adjusted at the time of
overhaul. They are assembled dry (no lubrication), clearances
checked, and adjustments are usually made by using push
rods of different lengths. A minimum and maximum valve
clearance is established. Any measurement between these
extremes is acceptable, but approximately half way between
the extremes is desired. Hydraulic valve lifters require less
maintenance, are better lubricated, and operate more quietly
than the screw adjustment type.
Push Rod
The push rod, tubular in form, transmits the lifting force from
the valve tappet to the rocker arm. A hardened-steel ball is
pressed over or into each end of the tube. One ball end fits into
the socket of the rocker arm. In some instances, the balls are
on the tappet and rocker arm, and the sockets are on the push
rod. The tubular form is employed because of its lightness and
strength. It permits the engine lubricating oil under pressure
to pass through the hollow rod and the drilled ball ends to
lubricate the ball ends, rocker-arm bearing, and valve-stem
guide. The push rod is enclosed in a tubular housing that
extends from the crankcase to the cylinder head, referred to as push rod tubes.
Rocker Arms
The rocker arms transmit the lifting force from the cams
to the valves. [Figure 1-31] Rocker arm assemblies are
supported by a plain, roller, or ball bearing, or a combination
of these, which serves as a pivot. Generally, one end of the
arm bears against the push rod and the other bears on the
valve stem. One end of the rocker arm is sometimes slotted
to accommodate a steel roller. The opposite end is constructed
with either a threaded split clamp and locking bolt or a tapped
hole. The arm may have an adjusting screw, for adjusting
the clearance between the rocker arm and the valve stem
tip. The screw can be adjusted to the specified clearance to
make certain that the valve closes fully.
Valve Springs
Each valve is closed by two or three helical springs. If a single
spring were used, it would vibrate or surge at certain speeds.
To eliminate this difficulty, two or more springs (one inside
the other) are installed on each valve. Each spring vibrates
at a different engine speed and rapid damping out of all
spring-surge vibrations during engine operation results. Two
or more springs also reduce danger of weakness and possible
failure by breakage due to heat and metal fatigue. The springs
are held in place by split locks installed in the recess of the
valve spring upper retainer or washer, and engage a groove
machined into the valve stem. The functions of the valve
springs are to close the valve and to hold the valve securely
on the valve seat.
Straight roller bearing
Deep-groove ball bearing
controls both radial and
thrust loads No thrust loadsThrust loads
Rotating shaft
Radial loads only
Figure 1-32. Radial and thrust loads. Figure 1-31. Rocker arms.
Bearings
A bearing is any surface which supports, or is supported by,
another surface. A good bearing must be composed of material
that is strong enough to withstand the pressure imposed on it
and should permit the other surface to move with a minimum
of friction and wear. The parts must be held in position within
very close tolerances to provide efficient and quiet operation,
and yet allow freedom of motion. Lubricated bearings of
many types are used to accomplish this, and at the same
time reduce friction of moving parts so that power loss is
not excessive and to prevent high oil temperatures caused
by failed or failing bearings. Bearings are required to take
radial loads, thrust loads, or a combination of the two. An
example of a radial load would be a rotating shaft being held
or contained in one position on a radial plane. Thrust load
would be the rotating shaft being contained from moving
axially along the shafts axis. These radial and thrust loads
are illustrated in Figure 1-32 . There are two ways in which
bearing surfaces move in relation to each other. One is by
the sliding movement of one metal against the other (sliding
friction), and the second is for one surface to roll over the
other (rolling friction). The three different types of bearings
in general use are plain, roller, and ball. [Figure 1-33]Plain Bearings
Plain bearings are generally used for the crankshaft, cam
ring, camshaft, connecting rods, and the accessory drive shaft
bearings. Such bearings are usually subjected to radial loads
only, although some have been designed to take thrust loads.
Plain bearings are usually made of nonferrous (having no
iron) metals, such as silver, bronze, aluminum, and various
alloys of copper, tin, or lead. Master rod or crankpin bearings
in some engines are thin shells of steel, plated with silver on
both the inside and the outside surfaces and with lead-tin
plated over the silver on the inside surface only. Smaller
bearings, such as those used to support various shafts in the
accessory section, are called bushings. Porous Oilite bushings
are widely used in this instance. They are impregnated with
oil so that the heat of friction brings the oil to the bearing
surface during engine operation.
Ball Bearings
A ball bearing assembly consists of grooved inner and outer
races, one or more sets of balls, in bearings designed for
disassembly, and a bearing retainer. They are used for shaft
bearings and rocker arm bearings in some reciprocating
engines. Special deep-groove ball bearings are used to
transmit propeller thrust and radial loads to the engine nose
section of radial engines. Since this type of bearing can accept
both radial and thrust loads, it is used in gas turbine engines
to support one end of a shaft (radial loads) and to keep the
shaft from moving axially (thrust loads).
Roller Bearings
Roller bearings are made in many types and shapes, but the
two types generally used in the aircraft engine are the straight
roller and the tapered roller bearings. Straight roller bearings
are used where the bearing is subjected to radial loads only.
In tapered roller bearings, the inner- and outer-race bearing
surfaces are cone-shaped. Such bearings withstand both
radial and thrust loads. Straight roller bearings are used in
high power reciprocating aircraft engines for the crankshaft
main bearings. They are also used in gas turbine applications
where radial loads are high. Generally, a rotating shaft in a
gas turbine engine is supported by a deep-groove ball bearing
Plain
Roller
Ball
Figure 1-33. Bearings.
(radial and thrust loads) on one end and a straight roller
bearing (radial loads only) on the other end.
Propeller Reduction Gearing
The increased brake horsepower delivered by a high
horsepower engine results partly from increased crankshaft
rpm. It is therefore necessary to provide reduction gears to
limit the propeller rotation speed to a value at which efficient
operation is obtained. Whenever the speed of the blade tips
approaches the speed of sound, the efficiency of the propeller
decreases rapidly. Reduction gearing for engines allows the
engine to operate at a higher rpm, developing more power
while slowing down the propeller rpm. This prevents the
propeller efficiency from decreasing. Since reduction gearing
must withstand extremely high stresses, the gears are machined
from steel forgings. Many types of reduction gearing systems are in use. The three types most commonly used are spur
planetary, bevel planetary, and spur and pinion. [ Figure 1-34 ]
The spur planetary reduction gearing consists of a large
driving gear or sun gear splined (and sometimes shrunk)
to the crankshaft, a large stationary gear, called a bell gear,
and a set of small spur planetary pinion gears mounted on a
carrier ring. The ring is fastened to the propeller shaft and the
planetary gears mesh with both the sun gear and the stationary
bell or ring gear. The stationary gear is bolted or splined to
the front section housing. When the engine is operating, the
sun gear rotates. Because the planetary gears are meshed with
this ring, they also must rotate. Since they also mesh with the
stationary gear, they walk or roll around it as they rotate, and
the ring in which they are mounted rotates the propeller shaft
in the same direction as the crankshaft but at a reduced speed.
In some engines, the bell gear is mounted on the propeller
shaft, and the planetary pinion gear cage is held stationary.
The sun gear is splined to the crankshaft and acts as a
driving gear. In such an arrangement, the propeller travels at
a reduced speed but in opposite direction to the crankshaft.
In the bevel planetary reduction gearing system, the driving
gear is machined with beveled external teeth and is attached to
the crankshaft. A set of mating bevel pinion gears is mounted
in a cage attached to the end of the propeller shaft. The
pinion gears are driven by the drive gear and walk around the
stationary gear, which is bolted or splined to the front section
housing. The thrust of the bevel pinion gears is absorbed by a
thrust ball bearing of special design. The drive and the fixed
gears are generally supported by heavy-duty ball bearings.
This type of planetary reduction assembly is more compact
than the other one described and, therefore, can be used
where a smaller propeller gear step-down is desired. In the
case of gas turbine turboprop engines, more than one stage
of reduction gearing is used due to the high output speeds of
the engine. Several types of lower powered engines can use
the spur and pinion reduction gear arrangement.
Propeller Shafts
Propeller shafts may be of three major types: tapered, splined,
or flanged. Tapered shafts are identified by taper numbers.
Splined and flanged shafts are identified by SAE numbers.
The propeller shaft of most low power output engines is
forged as part of the crankshaft. It is tapered, and a milled
slot is provided so that the propeller hub can be keyed to
the shaft. The keyway and key index of the propeller are in
relation to the No. 1 cylinder top dead center. The end of the
shaft is threaded to receive the propeller retaining nut. Tapered
propeller shafts are common on older and smaller engines.
The propeller shaft of high-output radial engines is generally
Spur planetary
Bevel planetary
Spur and pinion
Pinion
Driven gearBell gear
Sun gear
Drive gear
Crank shaft
Figure 1-34. Reduction gears.
splined. It is threaded on one end for a propeller hub nut.
The thrust bearing, which absorbs propeller thrust, is located
around the shaft and transmits the thrust to the nose section
housing. The shaft is threaded for attaching the thrust-bearing
retaining nut. On the portion protruding from the housing (between the two sets of threads), splines are located to
receive the splined propeller hub. The shaft is generally
machined from a steel-alloy forging throughout its length.
The propeller shaft may be connected by reduction gearing
to the engine crankshaft, but in smaller engines the propeller
TDC
BDCPistonSpark plug
Intake valve Exhaust valve
Connecting rodStroke
Top center
Bottom centerCrankshaftCylinder flangeCombustion chamber
Figure 1-35. Components and terminology of engine operation.shaft is simply an extension of the engine crankshaft. To
turn the propeller shaft, the engine crankshaft must revolve.
Flanged propeller shafts are used on most modern
reciprocating and turboprop engines. One end of the shaft is
flanged with drilled holes to accept the propeller mounting
bolts. The installation may be a short shaft with internal
threading to accept the distributor valve to be used with a
controllable propeller. The flanged propeller shaft is a very
common installation on most propeller driven aircraft.
Reciprocating Engine Operating Principles
The relationships between pressure, volume, and temperature
of gases are the basic principles of engine operation. An
internal combustion engine is a device for converting heat
energy into mechanical energy. Gasoline is vaporized and
mixed with air, forced or drawn into a cylinder, compressed
by a piston, and then ignited by an electric spark. The
conversion of the resultant heat energy into mechanical
energy and then into work is accomplished in the cylinder.
Figure 1-35 illustrates the various engine components
necessary to accomplish this conversion and also presents
the principal terms used to indicate engine operation.
The operating cycle of an internal combustion reciprocating
engine includes the series of events required to induct,
compress, ignite, and burn, causing expansion of the air-
fuel charge in the cylinder and to scavenge or exhaust the
byproducts of the combustion process. When the compressed
mixture is ignited, the resultant gases of combustion expand
very rapidly and force the piston to move away from the
cylinder head. This downward motion of the piston, acting
on the crankshaft through the connecting rod, is converted to
a circular or rotary motion by the crankshaft. A valve in the
top or head of the cylinder opens to allow the burned gases to
escape, and the momentum of the crankshaft and the propeller
forces the piston back up in the cylinder where it is ready for
the next event in the cycle. Another valve in the cylinder head
then opens to let in a fresh charge of the air-fuel mixture. The
valve allowing for the escape of the burning exhaust gases
is called the exhaust valve, and the valve which lets in the
fresh charge of the air-fuel mixture is called the intake valve.
These valves are opened and closed mechanically at the
proper times by the valve-operating mechanism. Therefore,
the order of the five events of a four stroke cycle engine are
intake, compression, ignition, power, and exhaust.
The bore of a cylinder is its inside diameter. The stroke is the
distance the piston moves from one end of the cylinder to
the other, specifically from top dead center (TDC) to bottom
dead center (BDC), or vice versa. [Figure 1-35]Operating Cycles
There are several operating cycles in use:
1. Four stroke.
2. Two stroke.
3. Rotary.
4. Diesel.
Four-Stroke Cycle
The vast majority of certified aircraft reciprocating engines
operate on the four-stroke cycle, sometimes called the Otto
cycle after its originator, a German physicist. The four-stroke
cycle engine has many advantages for use in aircraft. One
advantage is that it lends itself readily to high performance
through supercharging.
In this type of engine, four strokes are required to complete the
required series of events or operating cycle of each cylinder.
[Figure 1-36] Two complete revolutions of the crankshaft
(720°) are required for the four strokes; thus, each cylinder in
an engine of this type fires once in every two revolutions of
the crankshaft. In the following discussion of the four-stroke
cycle engine operation, note that the timing of the ignition and
Intake strokeA Compression strokeB
Power strokeC Exhaust strokeDIntake
openValves
closed
Valves
closedExhaust
openValve overlap 75° both valves openExhaust strokeCompression strokeIntake strokePower strokeIgnition
BTC = before top center
ATC = after top center
ABC = after bottom center
BBC = before bottom centerFinish cycle 25° ATC
Exhaust valve closed
Exhaust valve opens
30° BBCStart cycle 50° BTC
Intake valve opens
Intake valve
closes 30° ABCOne complete actual cycle of a four-stroke cycle reciprocating engine
Figure 1-37. Valve timing chart.
Figure 1-36. Four-stroke cycle.the valve events vary considerably in different engines. Many
factors influence the timing of a specific engine, and it is most
important that the engine manufacturer’s recommendations
in this respect be followed in maintenance and overhaul. The
timing of the valve and ignition events is always specified in
degrees of crankshaft travel. It should be remembered that a
certain amount of crankshaft travel is required to open a valve
fully; therefore, the specified timing represents the start of
opening rather than the full-open position of the valve. An
example valve timing chart can be seen in Figure 1-37 .
Intake Stroke
During the intake stroke, the piston is pulled downward in
the cylinder by the rotation of the crankshaft. This reduces
the pressure in the cylinder and causes air under atmospheric
pressure to flow through the carburetor, which meters the
correct amount of fuel. The air-fuel mixture passes through
the intake pipes and intake valves into the cylinders. The
quantity or weight of the air-fuel charge depends upon the
degree of throttle opening.
The intake valve is opened considerably before the piston
reaches TDC on the exhaust stroke, in order to induce a
greater quantity of the air-fuel charge into the cylinder and
thus increase the horsepower. The distance the valve may be
opened before TDC, however, is limited by several factors,
such as the possibility that hot gases remaining in the cylinder
from the previous cycle may flash back into the intake pipe
and the induction system.
In all high-power aircraft engines, both the intake and the
exhaust valves are off the valve seats at TDC at the start
of the intake stroke. As mentioned above, the intake valve
opens before TDC on the exhaust stroke (valve lead), and the
closing of the exhaust valve is delayed considerably after the
piston has passed TDC and has started the intake stroke (valve
lag). This timing is called valve overlap and is designed
to aid in cooling the cylinder internally by circulating the
cool incoming air-fuel mixture, to increase the amount of
the air-fuel mixture induced into the cylinder, and to aid in
scavenging the byproducts of combustion from the cylinder.
The intake valve is timed to close about 50° to 75° past BDC
on the compression stroke, depending upon the specific
engine, to allow the momentum of the incoming gases
to charge the cylinder more completely. Because of the
comparatively large volume of the cylinder above the piston
when the piston is near BDC, the slight upward travel of the
piston during this time does not have a great effect on the
incoming flow of gases. This late timing can be carried too
far because the gases may be forced back through the intake
valve and defeat the purpose of the late closing.
1-27Compression Stroke
After the intake valve is closed, the continued upward travel
of the piston compresses the air-fuel mixture to obtain the
desired burning and expansion characteristics. The charge is
fired by means of an electric spark as the piston approaches
TDC. The time of ignition varies from 20° to 35° before TDC,
depending upon the requirements of the specific engine to
ensure complete combustion of the charge by the time the
piston is slightly past the TDC position.
Many factors affect ignition timing, and the engine
manufacturer has expended considerable time in research and
testing to determine the best setting. All engines incorporate
devices for adjusting the ignition timing, and it is most
important that the ignition system be timed according to the
engine manufacturer’s recommendations.
Power Stroke
As the piston moves through the TDC position at the end
of the compression stroke and starts down on the power
stroke, it is pushed downward by the rapid expansion of the
burning gases within the cylinder head with a force that can
be greater than 15 tons (30,000 psi) at maximum power output
of the engine. The temperature of these burning gases may
be between 3,000 °F and 4,000 °F. As the piston is forced
downward during the power stroke by the pressure of the
burning gases exerted upon it, the downward movement of
the connecting rod is changed to rotary movement by the
crankshaft. Then, the rotary movement is transmitted to the
propeller shaft to drive the propeller. As the burning gases
are expanded, the temperature drops to within safe limits
before the exhaust gases flow out through the exhaust port.
The timing of the exhaust valve opening is determined by,
among other considerations, the desirability of using as much
of the expansive force as possible and of scavenging the
cylinder as completely and rapidly as possible. The valve is
opened considerably before BDC on the power stroke (on
some engines at 50° and 75° before BDC) while there is still
some pressure in the cylinder. This timing is used so that the
pressure can force the gases out of the exhaust port as soon
as possible. This process frees the cylinder of waste heat
after the desired expansion has been obtained and avoids
overheating the cylinder and the piston. Thorough scavenging
is very important, because any exhaust products remaining
in the cylinder dilute the incoming air-fuel charge at the start
of the next cycle.
Exhaust Stroke
As the piston travels through BDC at the completion of the
power stroke and starts upward on the exhaust stroke, it
begins to push the burned exhaust gases out the exhaust port. The speed of the exhaust gases leaving the cylinder creates
a low pressure in the cylinder. This low or reduced pressure
speeds the flow of the fresh air-fuel charge into the cylinder
as the intake valve is beginning to open. The intake valve
opening is timed to occur at 8° to 55° before TDC on the
exhaust stroke on various engines.
Two-Stroke Cycle
The two-stroke-cycle engine has re-emerged and is being
used in ultra-light, light sport, and many experimental
aircraft. As the name implies, two-stroke cycle engines
require only one upstroke and one down stroke of the piston
to complete the required series of events in the cylinder. Thus,
the engine completes the operating cycle in one revolution
of the crankshaft. The intake and exhaust functions are
accomplished during the same stroke. These engines can
be either air or water cooled and generally require a gear
reduction housing between the engine and propeller.
Rotary Cycle
The rotary cycle has a three-sided rotor that turns inside an
elliptical housing, completing three of the four cycles for each
revolution. These engines can be single rotor or multi-rotor
and can be air or water cooled. They are used mostly with
experimental and light aircraft. Vibration characteristics are
also very low for this type of engine.
Diesel Cycle
The diesel cycle depends on high compression pressures to
provide for the ignition of the air-fuel charge in the cylinder.
As air is drawn in the cylinder, it is compressed by a piston
and, at maximum pressure, fuel is sprayed in the cylinder. At
this point, the high pressure and temperature in the cylinder
causes the fuel to burn increasing the internal pressure of the
cylinder. This drives the piston down, turning or driving the
crankshaft. Water and air cooled engines that can operate on
JET A fuel (kerosene) use a version of the diesel cycle. There
are many types of diesel cycles in use including two-stroke
and four-stroke diesels.
Reciprocating Engine Power & Efficiencies
All aircraft engines are rated according to their ability
to do work and produce power. This section presents an
explanation of work and power and how they are calculated.
Also discussed are the various efficiencies that govern the
power output of a reciprocating engine.
Work
A physicist defines work as force times distance. Work done
by a force acting on a body is equal to the magnitude of the
force multiplied by the distance through which the force acts.
Work (W) = Force (F) x Distance (D)
1-28Work is measured by several standards. The most common
unit is called foot-pound (ft-lb). If a one-pound mass is raised one foot, one ft-lb of work has been performed. The greater the mass is and/or the greater the distance is, the greater the work performed.
Horsepower
The common unit of mechanical power is the horsepower
(hp). Late in the 18
th century, James Watt, the inventor of the
steam engine, found that an English workhorse could work at the rate of 550 ft-lb per second, or 33,000 ft-lb per minute, for a reasonable length of time. From his observations came the unit of horsepower, which is the standard unit of mechanical power in the English system of measurement. To calculate the hp rating of an engine, divide the power developed in ft-lb per minute by 33,000, or the power in ft-lb per second by 550.
As stated above, work is the product of force and distance,
and power is work per unit of time. Consequently, if a
33,000 -lb weight is lifted through a vertical distance of 1 foot
in 1 minute, the power expended is 33,000 ft-lb per minute, or exactly 1 hp.
Work is performed not only when a force is applied for lifting;
force may be applied in any direction. If a 100-lb weight is dragged along the ground, a force is still being applied to perform work, although the direction of the resulting motion is approximately horizontal. The amount of this force would depend upon the roughness of the ground.
If the weight were attached to a spring scale graduated in
pounds, then dragged by pulling on the scale handle, the amount of force required could be measured. Assume that the force required is 90 lb, and the 100-lb weight is dragged 660 feet in 2 minutes. The amount of work performed in the 2 minutes is 59,400 ft-lb or 29,700 ft-lb per minute. Since 1 hp is 33,000 ft-lb per minute, the hp expended in this case is 29,700 divided by 33,000, or 0.9 hp.
Piston Displacement
When other factors remain equal, the greater the piston
displacement, the greater the maximum horsepower an engine is capable of developing. When a piston moves from BDC to TDC, it displaces a specific volume. The volume displaced by the piston is known as piston displacement and is expressed One hp = ft-lb per min
33,000
ft-lb per sec
550or in cubic inches for most American-made engines and cubic
centimeters for others.
The piston displacement of one cylinder may be obtained by multiplying the area of the cross-section of the cylinder by the total distance the piston moves in the cylinder in one stroke. For multicylinder engines, this product is multiplied by the number of cylinders to get the total piston displacement of the engine.
Since the volume (V) of a geometric cylinder equals the area
(A) of the base multiplied by the height (h), it is expressedmathematically as follows:
The area of the base is the area of the cross-section of the cylinder.
Area of a Circle
To find the area of a circle, it is necessary to use a number
called pi (π). This number represents the ratio of the circumference to the diameter of any circle. Pi cannot be stated exactly because it is a never-ending decimal. It is 3.1416 expressed to four decimal places, which is accurate enough for most computations.
The area of a circle, as in a rectangle or triangle, must be
expressed in square units. The distance that is one-half the diameter of a circle is known as the radius. The area of any circle is found by squaring the radius (r) and multiplying by π. The formula is as follows:
The radius of a circle is equal to ½ the diameter:
Example
Compute the piston displacement of the PWA 14 cylinder engine having a cylinder with a 5.5 inch diameter and a 5.5 inch stroke. Formulas required are:
Substitute values into these formulas and complete the
calculation.V = A x h
A = πr2
r =2d
dr = 2
A = πr2
V = A x h
Total V = V x n (number of cylinders)
1-29Rounded off to the next whole number, total piston
displacement equals 1,829 cubic inches.
Another method of calculating the piston displacement uses
the diameter of the piston instead of the radius in the formula for the area of the base.
From this point on, the calculations are identical to the
preceding example.
Compression Ratio
All internal combustion engines must compress the air-fuel
mixture to receive a reasonable amount of work from each power stroke. The air-fuel charge in the cylinder can be compared to a coil spring in that the more it is compressed, the more work it is potentially capable of doing.
The compression ratio of an engine is a comparison of the
volume of space in a cylinder when the piston is at the bottom of the stroke to the volume of space when the piston is at the top of the stroke. [Figure 1-38] This comparison is expressed as a ratio, hence the term compression ratio. Compression ratio is a controlling factor in the maximum horsepower developed by an engine, but it is limited by present day fuel grades and the high engine speeds and manifold pressures required for takeoff. For example, if there are 140 cubic inches of space in the cylinder when the piston is at the bottom and there are 20 cubic inches of space when the piston is at the top of the stroke, the compression ratio would be 140 to 20. If this ratio is expressed in fraction form, it would be 140/20 or 7 to 1, usually represented as 7:1.
The limitations placed on compression ratios, manifold
pressure, and the manifold pressure’s effect on compression r =2d= 5.5 inches (in)
2= 2.75 in
A = πr2 = 3.1416 (2.75 in x 2.75 in)
A = 3.1416 x 7.5625 square inches (in2) = 23.7584 in2
V = A x h = 23.7584 in2 x 5.5 in = 130.6712 cubic inches (in3)
Total V = V x n = 130.6712 in3 x 14
Total V = 1829.3968 in3
A = 0.7854 x 30.25 in2 A = ¼ (π)(d2)
Substituting A = ¼ x 3.1416 x 5.5 in x 5.5 in
A = 23.758 in2 pressures has a major effect on engine operation. Manifold
pressure is the average absolute pressure of the air or air-fuel charge in the intake manifold and is measured in units of inches of mercury ("Hg). Manifold pressure is dependent on engine speed (throttle setting) and the degree supercharging. The operation of the supercharger increases the weight of the charge entering the cylinder. When a true supercharger is used with the aircraft engine, the manifold pressure may be considerably higher than the pressure of the outside atmosphere. The advantage of this condition is that a greater amount of charge is forced into a given cylinder volume, and a greater output of horsepower results.
Compression ratio and manifold pressure determine the pressure in the cylinder in that portion of the operating cycle when both valves are closed. The pressure of the charge before compression is determined by manifold pressure, while the pressure at the height of compression (just prior to ignition) is determined by manifold pressure times the compression ratio. For example, if an engine were operating at a manifold pressure of 30 "Hg with a compression ratio of 7:1, the pressure at the instant before ignition would be approximately 210 "Hg. However, at a manifold pressure of 60 "Hg, the pressure would be 420 "Hg.
Without going into great detail, it has been shown that
the compression event magnifies the effect of varying the manifold pressure, and the magnitude of both affects the pressure of the fuel charge just before the instant of ignition. If the pressure at this time becomes too high, pre-ignition or detonation occur and produce overheating. Pre-ignition is when the fuel air charge starts to burn before the spark plug fires. Detonation occurs when the fuel air charge is ignited by the spark plug, but instead of burning at a controlled rate, it explodes causing cylinder temperatures and pressures to spike very quickly. If this condition exists for very long, the engine can be damaged or destroyed.
One of the reasons for using engines with high compression
ratios is to obtain long-range fuel economy, to convert more heat energy into useful work than is done in engines of low compression ratio. Since more heat of the charge is converted into useful work, less heat is absorbed by the cylinder walls. This factor promotes cooler engine operation, which in turn increases the thermal efficiency. Here again, a compromise is needed between the demand for fuel economy and the demand for maximum horsepower without detonation. Some manufacturers of high compression engines suppress detonation at high manifold pressures by using high octane fuel and limiting maximum manifold pressure.
234567Clearance volume
Total volume
Displacement volume
BDCTDC
Figure 1-38. Compression ratio.Indicated Horsepower
The indicated horsepower produced by an engine is the
horsepower calculated from the indicated mean effective pressure and the other factors which affect the power output of an engine. Indicated horsepower is the power developed in the combustion chambers without reference to friction losses within the engine. This horsepower is calculated as a function of the actual cylinder pressure recorded during engine operation.
To facilitate the indicated horsepower calculations, a
mechanical indicating device, such as is attached to the engine cylinder, captures the actual pressure existing in the cylinder during the complete operating cycle. This pressure variation can be represented by the kind of graph shown in Figure 1-39. Notice that the cylinder pressure rises on the compression stroke, reaches a peak after top center, and decreases as the piston moves down on the power stroke. Since the cylinder pressure varies during the operating cycle, an average pressure (line AB) is computed. This average pressure, if applied steadily during the time of the power stroke, would do the same amount of work as the varying pressure during the same period. This average pressure is known as indicated mean effective pressure and is included in the indicated horsepower calculation with other engine specifications. If the characteristics and the indicated mean effective pressure of an engine are known, it is possible to calculate the indicated horsepower rating.The indicated horsepower for a four-stroke cycle engine can be calculated from the following formula, in which the letter symbols in the numerator are arranged to spell the word “PLANK” to assist in memorizing the formula:
In the formula above, the area of the piston multiplied by the
indicated mean effective pressure gives the force acting on the piston in pounds. This force multiplied by the length of the stroke in feet gives the work performed in one power stroke, which, multiplied by the number of power strokes per minute, gives the number of ft-lb per minute of work produced by one cylinder. Multiplying this result by the number of cylinders in the engine gives the amount of work performed, in ft-lb, by the engine. Since hp is defined as work done at the rate of 33,000 ft-lb per minute, the total number of ft-lb of work performed by the engine is divided by 33,000 to find the indicated horsepower.
P = Indicated mean effective pressure, in psi
L = Length of the stroke, in feet or in fractions of a footA = Area of the piston head or cross-sectional area of the
cylinder, in square inches
N = Number of power strokes per minute: K = Number of cylindersPLANK
33,000Indicated horsepower =
Where:
rpm
Top center BC Bottom center TC BCIndicated mean effective pressure (IMEP)
SparkPressure in cylinderPeak pressureCompression Power Exhaust Intake
A
D CB
EP
I
Figure 1-39. Compression ratio.Example
P = 1.65 lb/in2
L = 0.5 ft.
A = 5.5 inchesN = 1,500K = 12Indicated hp = PLANK
33,000 ft-lb/minGiven:
Find indicated hp.
A is found by using the equation:
A = ¼πD2
A = ¼ x 3.1416 x 5.5 in x 5.5 in
= 23.76 in2
N is found by multiplying the rpm by ½:
N = ½ x 3,000 = 1,500 rpm
Now, substituting in the formula:
Indicated hp =1.65 lb/in2 x 0.5 in x 23.76 in2 x 1,500 rpm x 12
33,000 ft-lb/min
Indicated hp = 1,069.123
Brake Horsepower
The indicated horsepower calculation discussed in the
preceding paragraph is the theoretical power of a frictionless engine. The total horsepower lost in overcoming friction must be subtracted from the indicated horsepower to arrive at the actual horsepower delivered to the propeller. The power delivered to the propeller for useful work is known as brake horsepower (bhp). The difference between indicated and brake horsepower is known as friction horsepower, which is the horsepower required to overcome mechanical losses, such as the pumping action of the pistons, the friction of the pistons, and the friction of all other moving parts.
The measurement of an engine’s bhp involves the measurement
of a quantity known as torque or twisting moment. Torque is the product of a force and the distance of the force from
the axis about which it acts, or
Torque = force x distance
(at right angles to the force)
Torque is a measure of load and is properly expressed in
pound-inches (lb-in) or po und-feet (lb-ft). Torque should not
be confused with work, which is expressed in inch-pounds
(in-lb) or foot-pounds (ft-lb).
There are numerous devices for measuring torque, such as a
dynamometer or a torque meter. One very simple type of
device that can be used to demonstrate torque calculations
is the Prony brake. [Figure 1-40] All of these torq ue-
measuring devices are usable to calculate power output of
an engine on a test stand. It consists essentially of a hinged
collar, or brake, which can be clamped to a drum splined to
the propeller shaft. The collar an d drum form a friction brake,
which can be adjusted by a wheel. An arm of a known length
is rigidly attached to or is a part of the hinged collar and
terminates at a point that rests on a set of scales. As the
propeller shaft rotates, it tends to carry the hinged collar of
the brake with it and is prevented from doing so only by the
arm that rests on the scale. The scale indicates the force
necessary to arrest the motion of the arm. If the resulting
force registered on the scale is multiplied by the length of
the arm, the resulting product is the torque exerted by the
rotating shaft. For example, if the scale registers 200 pounds
and the length of the arm is 3.18 feet, the torque exerted by
the shaft is:
200 lb x 3.18 ft = 636 lb-ft
Once the torque is known, the work done per revolution of
the propeller shaft can be computed without difficulty by the
equation:
Work per revolution = 2π x torque
Length of arm 3.18 ftCollarFriction adjusting wheel
Arm
Hinge
DrumPropeller shaftScale 200 lb
Figure 1-40. Typical Prony brake.If work per revolution is multiplied by the rpm, the result is
work per minute, or power. If the work is expressed in ft-lb per minute, this quantity is divided by 33,000. The result is the brake horsepower of the shaft.
Power = Work per revolution x rpm
Work per revolution x rpmbhp = 33,000
2πr x force on scales (lb) x length of arm (ft) x rpm
33,000
Example
As long as the friction between the brake collar and propeller shaft drum is great enough to impose an appreciable load on Force on scales = 200 lb
= 3.18 ft Length of arm
= 3,000 rpm
= 3.1416 π
6.2832 x 200 x 3.18 x 3,000bhp = 33,000Find bhp substituting in equation:
= 363.2
= 363Given:the engine, but is not great enough to stop the engine, it is
not necessary to know the amount of friction between the collar and drum to compute the bhp. If there were no load imposed, there would be no torque to measure, and the engine would “run away.” If the imposed load is so great that the engine stalls, there may be considerable torque to measure, but there is no rpm. In either case, it is impossible to measure the bhp of the engine. However, if a reasonable amount of friction exists between the brake drum and the collar and the load is then increased, the tendency of the propeller shaft to carry the collar and arm about with it becomes greater, thus imposing a greater force upon the scales. As long as the torque increase is proportional to the rpm decrease, the horsepower delivered at the shaft remains unchanged. This can be seen from the equation in which 2πr and 33,000 are constants and torque and rpm are variables. If the change in rpm is inversely proportional to the change in torque, their product remains unchanged, and bhp remains unchanged. This is important. It shows that horsepower is the function of both torque and rpm, and can be changed by changing either torque, rpm, or both.
Friction Horsepower
Friction horsepower is the indicated horsepower minus
brake horsepower. It is the horsepower used by an engine in overcoming the friction of moving parts, drawing in fuel, expelling exhaust, driving oil and fuel pumps, and other engine accessories. On modern aircraft engines, this power loss through friction may be as high as 10 to 15 percent of
1-33the indicated horsepower.
Friction & Brake Mean Effective Pressures
The indicated mean effective pressure (IMEP), discussed
previously, is the average pressure produced in the combustion chamber during the operating cycle and is an expression of the theoretical, frictionless power known as indicated horsepower. In addition to completely disregarding power lost to friction, indicated horsepower gives no indication of how much actual power is delivered to the propeller shaft for doing useful work. However, it is related to actual pressures that occur in the cylinder and can be used as a measure of these pressures.
To compute the friction loss and net power output, the
indicated horsepower of a cylinder may be thought of as two separate powers, each producing a different effect. The first power overcomes internal friction, and the horsepower thus consumed is known as friction horsepower. The second power, known as brake horsepower, produces useful work at the propeller. That portion of IMEP that produces brake horsepower is called brake mean effective pressure (BMEP). The remaining pressure used to overcome internal friction is called friction mean effective pressure (FMEP). [Figure 1-41] IMEP is a useful expression of total cylinder power output, but is not a real physical quantity; likewise, FMEP and BMEP are theoretical but useful expressions of friction losses and net power output.
Although BMEP and FMEP have no real existence in the
cylinder, they provide a convenient means of representing pressure limits or rating engine performance throughout its entire operating range. There is an operating relationship between IMEP, BMEP, and FMEP.
One of the basic limitations placed on engine operation is the pressure developed in the cylinder during combustion. In the discussion of compression ratios and indicated mean effective pressure, it was found that, within limits, increased pressure resulted in increased power. It was also noted that if the cylinder pressure were not controlled within close limits, it would impose dangerous internal loads that might result in engine failure. Therefore, it is important to have a means of determining these cylinder pressures as a protective measure and for efficient application of power.
If the bhp is known, the BMEP can be computed by means
of the following equation:bhp = 1,000
Stroke = 6 inBore = 5.5 inrpm = 3,000
1,000 bhp x 33,000 ft-lb/min BMEP =
0.5 ft x 23.76 in2 x 1,500 strokes/min x 12Find number of power strokes per min:
Then, substituting in the equation:Find BMEP:
= 154.32 lb/in2Given:bhp x 33,000BMEP = LANK
Number of cycles = 12
Find length of stroke (in ft):
L = 6 in = 0.5 ft
Find area of cylinder bore:
A = ¼πrD2
A = ¼ x 3.1416 x 5.5 in x 5.5 in
A = 23.76 in2
N = ½ x rpm
N = ½ x 3,000
N = 1,500
Thrust Horsepower
Thrust horsepower can be considered the result of the engine and the propeller working together. If a propeller could be designed to be 100 percent efficient, the thrust and the bph would be the same. However, the efficiency of the propeller varies with the engine speed, attitude, altitude, temperature, and airspeed. Thus, the ratio of the thrust horsepower and the bhp delivered to the propeller shaft will never be equal. For example, if an engine develops 1,000 bhp, and it is used with a
propeller having 85 percent efficiency, the thrust horsepower
of that engine-propeller combination is 85 percent of 1,000 or 850 thrust hp. Of the four types of horsepower discussed, it is the thrust horsepower that determines the performance of the engine-propeller combination.
Efficiencies
Thermal Efficiency
Any study of engines and power involves consideration of
heat as the source of power. The heat produced by the burning of gasoline in the cylinders causes a rapid expansion of the gases in the cylinder, and this, in turn, moves the pistons
FMEP
BMEP
TC BCIMEPPower stroke
Actual
pressure
Friction
ImpellerAccessories
BHPIMEP
FHP
Figure 1-41. Power and pressure.
and creates mechanical energy. It has long been known that
mechanical work can be converted into heat and that a given amount of heat contains the energy equivalent of a certain amount of mechanical work. Heat and work are theoretically interchangeable and bear a fixed relation to each other. Heat can therefore be measured in work units (for example, ft-lb) as well as in heat units. The British thermal unit (BTU) of heat is the quantity of heat required to raise the temperature of 1 pound of water by 1 °F. It is equivalent to 778 ft-lb of mechanical work. A pound of petroleum fuel, when burned with enough air to consume it completely, gives up about 20,000 BTU, the equivalent of 15,560,000 ft-lb of mechanical work. These quantities express the heat energy of the fuel in heat and work units, respectively.
The ratio of useful work done by an engine to the heat energy of the fuel it uses, expressed in work or heat units,
is called the thermal efficiency of the engine. If two similar engines use equal amounts of fuel, the engine that converts into work the greater part of the energy in the fuel (higher thermal efficiency) delivers the greater amount of power. Furthermore, the engine that has the higher thermal efficiency has less waste heat to dispose of to the valves, cylinders, pistons, and cooling system of the engine. A high thermal efficiency also means low specific fuel consumption and, therefore, less fuel for a flight of a given distance at a given power. Thus, the practical importance of a high thermal efficiency is threefold, and it constitutes one of the most desirable features in the performance of an aircraft engine.
Of the total heat produced, 25 to 30 percent is utilized
for power output, 15 to 20 percent is lost in cooling (heat radiated from cylinder head fins), 5 to 10 percent is lost in overcoming friction of moving parts; and 40 to 45 percent is lost through the exhaust. Anything that increases the heat content going into mechanical work on the piston, which reduces the friction and pumping losses, or which reduces the quantity of unburned fuel or the heat lost to the engine parts, increases the thermal efficiency.
The portion of the total heat of combustion that is turned
into mechanical work depends to a great extent upon the compression ratio. The compression ratio is the ratio of the piston displacement plus combustion chamber space to the combustion chamber space, as mentioned earlier. Other things being equal, the higher the compression ratio is, the larger is the proportion of the heat energy of combustion turned into useful work at the crankshaft. On the other hand, increasing the compression ratio increases the cylinder head temperature. This is a limiting factor because the extremely high temperature created by high compression ratios causes the material in the cylinder to deteriorate rapidly and the fuel to detonate instead of burning at a controlled rate.
The thermal efficiency of an engine may be based on either
bhp or indicated horsepower (ihp) and is represented by the following formula:
The formula for brake thermal efficiency is the same as
shown above, except the value for bhp is inserted instead of the value for ihp.
Example
An engine delivers 85 bhp for a period of 1 hour and during that time consumes 50 pounds of fuel. Assuming the fuel has
Indicated thermal efficiency =
ihp x 33,000
weight of fuel burned/min. x heat value x 778
Heat released by combustion
25–30% is converted into useful power40–45% is carried out with exhaust
5–10% is removed by the oil15–20% is removed by fins
Figure 1-42. Thermal distribution in an engine. a heat content of 18,800 BTU per pound, find the thermal
efficiency of the engine:
Reciprocating engines are only about 34 percent thermally
efficient; that is, they transform only about 34 percent of
the total heat potential of the burning fuel into mechanical
energy. The remainder of the heat is lost through the exhaust
gases, the cooling system, and the friction within the engine.
Thermal distribution in a reciprocating engine is illustrated
in Figure 1-42 .
Mechanical Efficiency
Mechanical efficiency is the ratio that shows how much of
the power developed by the expanding gases in the cylinder
is actually delivered to the output shaft. It is a comparison
between the bhp and the ihp. It can be expressed by the
following formula:
Brake horsepower is the useful power delivered to the
propeller shaft. Indicated horsepower is the total hp
developed in the cylinders. The difference between the two
is friction horsepower (fhp), the power lost in overcoming
friction. The factor that has the greatest effect on mechanical
efficiency is the friction within the engine itself. The friction
between moving parts in an engine remains practically
constant throughout an engine’s speed range. Therefore, the
mechanical efficiency of an engine is highest when the engine
is running at the rpm at which maximum bhp is developed.
Mechanical efficiency of the average aircraft reciprocating
engine approaches 90 percent.
8.5 ihp x 33,000
0.833 x 18,800 BTU x 7782,805,000
12,184,569=
Brake thermal efficiency = 0.23 or 23 percent
Mechanical efficiency = bhp
ihpVolumetric Efficiency
V olumetric efficiency is a ratio expressed in terms of
percentages. It is a comparison of the volume of air-fuel
charge (corrected for temperature and pressure) inducted
into the cylinders to the total piston displacement of the
engine. Various factors cause departure from a 100 percent
volumetric efficiency. The pistons of a naturally aspirated
engine displace the same volume each time they travel from
top center to bottom center of the cylinders. The amount of
charge that fills this volume on the intake stroke depends
on the existing pressure and temperature of the surrounding
atmosphere. Therefore, to find the volumetric efficiency of an
engine, standards for atmospheric pressure and temperature
had to be established. The U.S. standard atmosphere was
established in 1958, and provides the necessary pressure
and temperature values to calculate volumetric efficiency.
The standard sea level temperature is 59 °F, or 15 °C. At this
temperature, the pressure of one atmosphere is 14.69 lb/in2,
and this pressure supports a column of mercury (Hg) 29.92
inches high, or 29.92 "Hg. These standard sea level conditions
determine a standard density, and if the engine draws in a
volume of charge of this density exactly equal to its piston
displacement, it is said to be operating at 100 percent
volumetric efficiency. An engine drawing in less volume than
this has a volumetric efficiency lower than 100 percent. An
engine equipped with true supercharging (boost above 30.00
"Hg) may have a volumetric efficiency greater than 100
percent. The equation for volumetric efficiency is as follows :
Many factors decrease volumetric efficiency, including:
• Part-throttle operation;
• Long intake pipes of small diameter;
• Sharp bends in the induction system;V olumetric
efficiencyV olume of charge (corrected for
temperature and pressure)
Piston displacement=
1-36• Carburetor air temperature too high;
• Cylinder-head temperature too high;
• Incomplete scavenging; and
• Improper valve timing.
Propulsive Efficiency
A propeller is used with an engine to provide thrust. The
engine supplies bhp through a rotating shaft, and the
propeller absorbs the bhp and converts it into thrust hp. In this
conversion, some power is wasted. Since the efficiency of any
machine is the ratio of useful power output to the power input,
propulsive efficiency (in this case, propeller efficiency) is the
ratio of thrust hp to bhp. On the average, thrust hp constitutes
approximately 80 percent of the bhp. The other 20 percent
is lost in friction and slippage. Controlling the blade angle
of the propeller is the best method of obtaining maximum
propulsive efficiency for all conditions encountered in flight.
During takeoff, when the aircraft is moving at low speeds
and when maximum power and thrust are required, a low
propeller blade angle gives maximum thrust. For high-
speed flying or diving, the blade angle is increased to obtain
maximum thrust and efficiency. The constant-speed propeller
is used to give required thrust at maximum efficiency for all
flight conditions.
Gas Turbine Engines
Types & Construction
In a reciprocating engine, the functions of intake, compression,
combustion, and exhaust all take place in the same combustion
chamber. Consequently, each must have exclusive occupancy
of the chamber during its respective part of the combustion
cycle. A significant feature of the gas turbine engine is
that separate sections are devoted to each function, and all
functions are performed simultaneously without interruption.
A typical gas turbine engine consists of:
1. An air inlet,
2. Compressor section,
3. Combustion section,
4. Turbine section,
5. Exhaust section,
6. Accessory section, and
7. The systems necessary for starting, lubrication, fuel
supply, and auxiliary purposes, such as anti-icing,
cooling, and pressurization.
The major components of all gas turbine engines are basically
the same; however, the nomenclature of the component parts of various engines currently in use varies slightly due
to the difference in each manufacturer’s terminology. These
differences are reflected in the applicable maintenance
manuals. One of the greatest single factors influencing the
construction features of any gas turbine engine is the type of
compressor or compressors for which the engine is designed.
Four types of gas turbine engines are used to propel and
power aircraft. They are the turbofan, turboprop, turboshaft,
and turbojet. The term “ turbojet” was used to describe any
gas turbine engine used in aircraft. As gas turbine technology
evolved, these other engine types were developed to take
the place of the pure turbojet engine. The turbojet engine
has problems with noise and fuel consumption in the speed
range that airliners fly (.8 Mach). Due to these problems,
use of pure turbojet engines is very limited. So, almost all
airliner-type aircraft use a turbofan engine. It was developed
to turn a large fan or set of fans at the front of the engine and
produces about 80 percent of the thrust from the engine. This
engine was quieter and had better fuel consumption in this
speed range. Turbofan engines have more than one shaft in the
engine; many are two-shaft engines. This means that there are
two sets of compressors and turbines that drive them. These
two-shafted engines use two spools (a spool is a compressor
and a shaft and turbines that drive that compressor). In a
two-spool engine, there is a high-pressure spool and a low-
pressure spool. The low-pressure spool generally contains
the fan(s) and the turbine stages it takes to drive them. The
high-pressure spool is the high-pressure compressor, shaft,
and turbines. This spool makes up the core of the engine, and
this is where the combustion section is located.
Turbofan engines can be low bypass or high bypass. The
amount of air that is bypassed around the core of the engine
determines the bypass ratio. As can be seen in Figure 1-43 ,
the air generally driven by the fan does not pass through the
internal working core of the engine. The amount of air flow
in lb/sec from the fan bypass to the core flow of the engine
is the bypass ratio.
Some low-bypass turbofan engines are used in speed
ranges above .8 Mach (military aircraft). These engines use
augmenters or afterburners to increase thrust. By adding
more fuel nozzles and a flame holder in the exhaust system
extra fuel can be sprayed and burned which can give large
increases in thrust for short amounts of time.
The turboprop engine is a gas turbine engine that turns a
propeller through a speed reduction gear box. This type of
engine is most efficient in the 300 to 400 mph speed range and Bypass ratio =100 lb/sec flow fan
20 lb/sec flowcore= 5:1 bypass ratio
Figure 1-43. Turbofan engine with separate nozzles, fan, and core. can use shorter runways that other aircraft. Approximately
80 to 85 percent of the energy developed by the gas turbine
engine is used to drive the propeller. The rest of the available
energy exits the exhaust as thrust. By adding the horsepower
developed by the engine shaft and the horsepower in the
exiting thrust, the answer is equivalent shaft horsepower.
With regard to aircraft, the turboshaft engine is a gas turbine
engine made to transfer horsepower to a shaft that turns a
helicopter transmission or is an onboard auxiliary power
unit (APU). An APU is used on turbine-powered aircraft to
provide electrical power and bleed air on the ground and a
backup generator in flight. Turboshaft engines can come in
many different styles, shapes, and horsepower ranges.
Air Entrance
The air entrance is designed to conduct incoming air to the
compressor with a minimum energy loss resulting from drag
or ram pressure loss; that is, the flow of air into the compressor
should be free of turbulence to achieve maximum operating
efficiency. Proper inlet design contributes materially to
aircraft performance by increasing the ratio of compressor
discharge pressure to duct inlet pressure.
This is also referred to as the compressor pressure ratio. This
ratio is the outlet pressure divided by the inlet pressure. The
amount of air passing through the engine is dependent upon
three factors:
1. The compressor speed (rpm).
2. The forward speed of the aircraft.
3. The density of the ambient (surrounding) air.Turbine inlet type is dictated by the type of gas turbine engine.
A high-bypass turbofan engine inlet is completely different
from a turboprop or turboshaft inlet. Large gas turbine-
powered aircraft almost always have a turbofan engine. The
inlet on this type of engine is bolted to the front (A flange)
of the engine. These engines are mounted on the wings, or
nacelles, on the aft fuselage, and a few are in the vertical fin.
A typical turbofan inlet can be seen in Figure 1-44 . Since on
most modern turbofan engines the huge fan is the first part of
the aircraft the incoming air comes into contact with, icing
protection must be provided. This prevents chunks of ice
from forming on the leading edge of the inlet, breaking loose,
and damaging the fan. Warm air is bled from the engine’s
compressor and is ducted through the inlet to prevent ice
from forming. If inlet guide vanes are used to straighten the
air flow, then they also have anti-icing air flowing through
them. The inlet also contains some sound-reducing materials
that absorb the fan noise and make the engine quieter.
Turboprops and turboshafts can use an inlet screen to help
filter out ice or debris from entering the engine. A deflector
vane and a heated inlet lip are used to prevent ice from
forming and allowing large chunks to enter the engine.
On military aircraft, the divided entrance permits the use of very
short ducts with a resultant small pressure drop through skin
friction. Military aircraft can fly at speeds above Mach 1, but
the airflow through the engine must always stay below Mach
1. Supersonic air flow in the engine would destroy the engine.
By using convergent and divergent shaped ducts, the air flow is
controlled and dropped to subsonic speeds before entering the
engine. Supersonic inlets are used to slow the incoming engine
air to less than Mach 1 before it enters the engine.
1-38Accessory Section
The accessory section of the gas turbine engine has various
functions. The primary function is to provide space for
the mounting of accessories necessary for operation and
control of the engine. Generally, it also includes accessories
concerned with the aircraft, such as electric generators and
hydraulic pumps. Secondary functions include acting as an
oil reservoir and/or oil sump and housing the accessory drive
gears and reduction gears.
The arrangement and driving of accessories has always been
a major problem on gas turbine engines. Driven accessories
on turbofans are usually mounted on the accessory gear box,
which is on the bottom of the engine. The location of the
accessory gear box varies somewhat, but most turboprops
and turboshafts have the accessory cases mounted to the
back section of the engine.
The components of the accessory section of all gas turbine
engines have essentially the same purpose, even though
they often differ quite extensively in construction details
and nomenclature.
The basic elements of the accessory section are:
1. The accessory case, which has machined mounting
pads for the engine-driven accessories, and
2. The gear train, which is housed within the accessory
case.
The accessory case may be designed to act as an oil reservoir. If
an oil tank is utilized, a sump is usually provided below the front
bearing support for the drainage and scavenging of oil used to
lubricate bearings and drive gears. The accessory case is also
provided with adequate tubing or cored passages for spraying
lubricating oil on the gear train and supporting bearings.
The gear train is driven by the engine high-pressure
compressor through an accessory drive shaft (tower shaft)
gear coupling, which splines with a gear box gear and the
high-pressure compressor. The reduction gearing within the
case provides suitable drive speeds for each engine accessory
or component. Because the rotor operating rpm is so high,
the accessory reduction gear ratios are relatively high. The
accessory drives are supported by ball bearings assembled in
the mounting pad bores of the accessory case. [Figure 1-45]
Compressor Section
The compressor section of the gas turbine engine has many
functions. Its primary function is to supply air in sufficient
quantity to satisfy the requirements of the combustion
burners. Specifically, to fulfill its purpose, the compressor
must increase the pressure of the mass of air received from the air inlet duct, and then discharge it to the burners in the
quantity and at the pressures required.
A secondary function of the compressor is to supply bleed-
air for various purposes in the engine and aircraft. The
bleed-air is taken from any of the various pressure stages of
the compressor. The exact location of the bleed ports is, of
course, dependent on the pressure or temperature required
for a particular job. The ports are small openings in the
compressor case adjacent to the particular stage from which
the air is to be bled; thus, varying degrees of pressure are
available simply by tapping into the appropriate stage. Air is
often bled from the final or highest pressure stage since, at
this point, pressure and air temperature are at a maximum.
At times it may be necessary to cool this high-pressure air. If
it is used for cabin pressurization or other purposes to which
excess heat would be uncomfortable or detrimental, the air is
sent through an air conditioning unit before it enters the cabin.
Bleed air is utilized in a wide variety of ways. Some of the
current applications of bleed air are:
1. Cabin pressurization, heating, and cooling;
2. Deicing and anti-icing equipment;
3. Pneumatic starting of engines; and
4. Auxiliary drive units (ADU).
Another function of the compressor bleed air is for the
operation of vortex dissipaters. The vortex dissipater supplies
a high-velocity stream of compressor bleed air blown from a
nozzle into an area where vortices are likely to form. V ortex
dissipaters destroy the vortices that would otherwise suck
debris from the ground into engines mounted in pods that
are low to the ground.
Compressor Types
The two principal types of compressors currently being
used in gas turbine aircraft engines are centrifugal flow and
axial flow. The centrifugal-flow compressor achieves its
purpose by picking up the entering air and accelerating it
outwardly by centrifugal action. The axial-flow compressor
compresses air while the air continues in its original direction
of flow, thus avoiding the energy loss caused by turns. The
components of each of these two types of compressor have
their individual functions in the compression of air for the
combustion section. A stage in a compressor is considered
to be a rise in pressure.
Centrifugal-Flow Compressors
The centrifugal-flow compressor consists of an impeller
(rotor), a diffuser (stator), and a compressor manifold.
[Figure 1-46] Centrifugal compressors have a high pressure
rise per stage that can be around 8:1. Generally centrifugal
Figure 1-44. Typical turbofan inlet.
Figure 1-45. Typical turboprop accessory case.compressors are limited to two stages due to efficiency
concerns. The two main functional elements are the impeller
and the diffuser. Although the diffuser is a separate unit and is
placed inside and bolted to the manifold, the entire assembly
(diffuser and manifold) is often referred to as the diffuser.
For clarification during compressor familiarization, the units
are treated individually. The impeller is usually made from
forged aluminum alloy, heat treated, machined, and smoothed
for minimum flow restriction and turbulence.
In most types, the impeller is fabricated from a single
forging. This type impeller is shown in Figure 1-46 . The
impeller, whose function is to pick up and accelerate the air
outwardly to the diffuser, may be either of two types—single
entry or double entry. The principal differences between the
two types of impellers are size and ducting arrangement.
The double-entry type has a smaller diameter but is usually
operated at a higher rotational speed to assure sufficient
airflow. The single-entry impeller, shown in Figure 1-47 ,
permits convenient ducting directly to the impeller eye
(inducer vanes) as opposed to the more complicated ducting
necessary to reach the rear side of the double-entry type.
Although slightly more efficient in receiving air, the single-
entry impeller must be large in diameter to deliver the same
quantity of air as the double-entry type. This, of course,
increases the overall diameter of the engine.
Included in the ducting for double-entry compressor engines
is the plenum chamber. This chamber is necessary for a
double-entry compressor because the air must enter the
engine at almost right angles to the engine axis. Therefore,
in order to give a positive flow, the air must surround the
engine compressor at a positive pressure before entering the compressor. Included in some installations as necessary
parts of the plenum chamber are the auxiliary air-intake doors
(blow-in doors). These blow-in doors admit air to the engine
compartment during ground operation, when air requirements
for the engine are in excess of the airflow through the inlet
ducts. The doors are held closed by spring action when the
engine is not operating. During operation, however, the doors
open automatically whenever engine compartment pressure
drops below atmospheric pressure. During takeoff and flight,
ram air pressure in the engine compartment aids the springs
in holding the doors closed.
The diffuser is an annular chamber provided with a number
of vanes forming a series of divergent passages into the
manifold. The diffuser vanes direct the flow of air from the
impeller to the manifold at an angle designed to retain the
maximum amount of energy imparted by the impeller. They
also deliver the air to the manifold at a velocity and pressure
satisfactory for use in the combustion chambers. Refer to
Figure 1-46A and note the arrow indicating the path of airflow
through the diffuser, then through the manifold.
The compressor manifold shown in Figure 1-46A diverts
the flow of air from the diffuser, which is an integral part of
the manifold, into the combustion chambers. The manifold
has one outlet port for each chamber so that the air is evenly
divided. A compressor outlet elbow is bolted to each of the
outlet ports. These air outlets are constructed in the form
of ducts and are known by a variety of names, such as air
outlet ducts, outlet elbows, or combustion chamber inlet
ducts. Regardless of the terminology used, these outlet ducts
perform a very important part of the diffusion process; that
is, they change the radial direction of the airflow to an axial
direction, in which the diffusion process is completed after the
turn. To help the elbows perform this function in an efficient
manner, turning vanes (cascade vanes) are sometimes fitted
inside the elbows. These vanes reduce air pressure losses by
Compressor manifold Impeller DiffuserAir outlet elbow with turning vanes
for reducing air pressure lossesA B
Compressor shaftImpeller
CouplingC Rear inducer Front inducer
Compressor Front bearing Center bearing
Figure 1-46. (A) Components of a centrifugal-flow compressor; (B) Air outlet elbow with turning vanes for reducing air pressure losses;
(C) Components of a double-entry centrifugal-flow compressor.
Figure 1-47. Single-entry impeller.rotor increases the compression of the air at each stage and
accelerates it rearward through several stages. With this
increased velocity, energy is transferred from the compressor
to the air in the form of velocity energy. The stator blades act
as diffusers at each stage, partially converting high velocity
to pressure. Each consecutive pair of rotor and stator blades
constitutes a pressure stage. The number of rows of blades
(stages) is determined by the amount of air and total pressure
rise required. Compressor pressure ratio increases with the
number of compression stages. Most engines utilize up to
16 stages and more.
The stator has rows of vanes, which are in turn attached inside
an enclosing case. The stator vanes, which are stationary,
project radially toward the rotor axis and fit closely on
either side of each stage of the rotor blades. In some cases,
the compressor case, into which the stator vanes are fitted,
is horizontally divided into halves. Either the upper or lower
half may be removed for inspection or maintenance of rotor
and stator blades.
The function of the stator vanes is to receive air from the
air inlet duct or from each preceding stage and increase
the pressure of the air and deliver it to the next stage at the
correct velocity and pressure. They also control the direction
of air to each rotor stage to obtain the maximum possible
compressor blade efficiency. Shown in Figure 1-48 are the
rotor and stator elements of a typical axial-flow compressor.
The first stage rotor blades can be preceded by an inlet guide
vane assembly that can be fixed or variable.
The guide vanes direct the airflow into the first stage rotor
blades at the proper angle and impart a swirling motion to
the air entering the compressor. This preswirl, in the direction
of engine rotation, improves the aerodynamic characteristics
of the compressor by reducing drag on the first stage rotor
blades. The inlet guide vanes are curved steel vanes usually
welded to steel inner and outer shrouds. presenting a smooth, turning surface. [Figure 1-46B]
Axial-Flow Compressor
The axial flow compressor is a combination of an engine
compressor and high-pressure turbine that drives it using a
connecting drive shaft. In a single-spool engine, the high-
pressure turbine drives the entire compressor. In a dual-spool
engine, the compressor and high-pressure turbine are both
split into two segments. Each compressor segment is driven
by its corresponding turbine using two separate drive shafts,
with one inside the other. The first stage turbine drives the
N2 compressor.
The axial-flow compressor has two main elements: a
rotor and a stator. The rotor has blades fixed on a spindle.
These blades impel air rearward in the same manner as a
propeller because of their angle and airfoil contour. The
rotor, turning at high speed, takes in air at the compressor
inlet and impels it through a series of stages. From inlet to
exit, the air flows along an axial path and is compressed at
a ratio of approximately 1.25:1 per stage. The action of the
Front compressor stator casing
Rear compressor stator casing
Compressor rotor Compressor rear frame
Front compressor stator casingRear compressor stator casing
Figure 1-48. Rotor and stator elements of a typical axial-flow
compressor. At the discharge end of the compressor, the stator vanes are
constructed to straighten the airflow to eliminate turbulence.
These vanes are called straightening vanes or the outlet vane
assembly. The casings of axial-flow compressors not only
support the stator vanes and provide the outer wall of the
axial path the air follows, but they also provide the means
for extracting compressor air for various purposes. The stator
vanes are usually made of steel with corrosion- and erosion-
resistant qualities. Quite frequently, they are shrouded
(enclosed) by a band of suitable material to simplify the
fastening problem. The vanes are welded into the shrouds,
and the outer shroud is secured to the compressor housing
inner wall by radial retaining screws.
The rotor blades are usually made of stainless steel with
the latter stages being made of titanium. The design of
blade attachment to the rotor disc rims varies, but they are
commonly fitted into discs by either bulb-type or fir-tree
methods. [Figure 1-49] The blades are then locked into place
by differing methods. Compressor blade tips are reduced in thickness by cutouts, referred to as blade profiles. These
profiles prevent serious damage to the blade or housing
should the blades contact the compressor housing. This
condition can occur if rotor blades become excessively loose
or if rotor support is reduced by a malfunctioning bearing.
Even though blade profiles greatly reduce such possibilities,
occasionally a blade may break under stress of rubbing and
cause considerable damage to compressor blades and stator
vane assemblies. The blades vary in length from entry to
discharge because the annular working space (drum to casing)
is reduced progressively toward the rear by the decrease in
the casing diameter. [Figure 1-50] This feature provides for a
fairly constant velocity through the compressor, which helps
to keep the flow of air constant.
The rotor features either drum-type or disc-type construction.
The drum-type rotor consists of rings that are flanged to fit
one against the other, wherein the entire assembly can then
be held together by through bolts. This type of construction
is satisfactory for low-speed compressors where centrifugal
stresses are low. The disc-type rotor consists of a series of
discs machined from aluminum forgings, shrunk over a steel
shaft, with rotor blades dovetailed into the disc rims. Another
method of rotor construction is to machine the discs and
shaft from a single aluminum forging, and then to bolt steel
stub shafts on the front and rear of the assembly to provide
bearing support surfaces and splines for joining the turbine
shaft. The drum-type and disc-type rotors are illustrated in
Figures 1-50 and 1-51, respectively.
The combination of the compressor stages and turbine stages
on a common shaft is an engine referred to as an engine
spool. The common shaft is provided by joining the turbine
and compressor shafts by a suitable method. The engine’s
spool is supported by bearings, which are seated in suitable
bearing housings.
As mentioned earlier, there are two configurations of the
axial compressor currently in use: the single rotor/spool and
the dual rotor/spool, sometimes referred to as solid spool and
split spool (two spool, dual spool).
One version of the solid-spool (one spool) compressor uses
variable inlet guide vanes. Also, the first few rows of stator
vanes are variable. The main difference between variable inlet
guide vane (VIGV) and a variable stator vane (VSV) is their
position with regard to the rotor blades. VIGV are in front of
the rotor blades, and VSV are behind the rotor blades. The
angles of the inlet guide vanes and the first several stages of
the stator vanes are can be variable. During operation, air enters
the front of the engine and is directed into the compressor at
the proper angle by the variable inlet guide and directed by the
VSV . The air is compressed and forced into the combustion
Bulb root Fir-tree root
Air flow
Air flow
Stator blade Rotor blade
Figure 1-51. Disc-type compressor rotor. Figure 1-50. Drum-type compressor rotor. Figure 1-49. Common designs of compressor blade attachment to the rotor disc.
section. A fuel nozzle that extends into each combustion
liner atomizes the fuel for combustion. These variables are
controlled in direct relation to the amount of power the engine
is required to produce by the power lever position.
Most turbofan engines are of the split-spool compressor type.
Most large turbofan engines use a large fan with a few stages
of compression called the low-pressure spool. These turbofans
incorporate two compressors with their respective turbines and
interconnecting shafts, which form two physically independent rotor systems. Many dual rotor systems have rotors turning
in opposite directions and with no mechanical connection to
each other. The second spool, referred to as the high-pressure
spool and is the compressor for the gas generator and core of
the engine, supplies air to the combustion section of the engine.
The advantages and disadvantages of both types of
compressors are included in the following list. Even though
each type has advantages and disadvantages, each has its use
by type and size of engine.
The centrifugal-flow compressor’s advantages are:
• High pressure rise per stage,
• Efficiency over wide rotational speed range,
• Simplicity of manufacture and low cost,
1-43• Low weight, and
• Low starting power requirements.
The centrifugal-flow compressor’s disadvantages are:
• Its large frontal area for a given airflow and
• Losses in turns between stages.
The axial-flow compressor’s advantages are:
• High peak efficiencies;
• Small frontal area for given airflow;
• Straight-through flow, allowing high ram efficiency;
and
• Increased pressure rise by increasing number of stages,
with negligible losses.
The axial-flow compressor’s disadvantages are:
• Good efficiencies over only narrow rotational speed
range,
• Difficulty of manufacture and high cost,
• Relatively high weight, and
• High starting power requirements (partially overcome
by split compressors).
Diffuser
The diffuser is the divergent section of the engine after the
compressor and before the combustion section. It has the
all-important function of reducing high-velocity compressor
discharge air to increased pressure at a slower velocity. This
prepares the air for entry into the flame burning area of the
combustion section at a lower velocity so that the flame of
combustion can burn continuously. If the air passed through
the flame area at a high velocity, it could extinguish the flame.
Combustion Section
The combustion section houses the combustion process,
which raises the temperature of the air passing through the
engine. This process releases energy contained in the air-
fuel mixture. The major part of this energy is required at the
turbine or turbine stages to drive the compressor. About ²⁄ ³ of
the energy is used to drive the gas generator compressor. The
remaining energy passes through the remaining turbine stages
that absorb more of the energy to drive the fan, output shaft,
or propeller. Only the pure turbojet allows the air to create
all the thrust or propulsion by exiting the rear of the engine
in the form of a high-velocity jet. These other engine types
have some jet velocity out the rear of the engine but most
of the thrust or power is generated by the additional turbine
stages driving a large fan, propeller, or helicopter rotor blades.The primary function of the combustion section is, of course,
to burn the air-fuel mixture, thereby adding heat energy to
the air. To do this efficiently, the combustion chamber must:
• Provide the means for proper mixing of the fuel and
air to assure good combustion,
• Burn this mixture efficiently,
• Cool the hot combustion products to a temperature
that the turbine inlet guide vanes/blades can withstand
under operating conditions, and
• Deliver the hot gases to the turbine section.
The location of the combustion section is directly between
the compressor and the turbine sections. The combustion
chambers are always arranged coaxially with the compressor
and turbine regardless of type, since the chambers must be in a
through-flow position to function efficiently. All combustion
chambers contain the same basic elements:
1. Casing.
2. Perforated inner liner.
3. Fuel injection system.
4. Some means for initial ignition.
5. Fuel drainage system to drain off unburned fuel after
engine shutdown.
There are currently four basic types of combustion chambers,
variations within type being in detail only. These types are:
1. Can-type.
2. Can-annular type.
3. Annular type.
4. Reverse-flow type.
The can-type combustion chamber is typical of the type
used on turboshaft and APUs. [Figure 1-52] Each of the
can-type combustion chambers consists of an outer case or
housing, within which there is a perforated stainless steel
(highly heat resistant) combustion chamber liner or inner
liner. [Figure 1-53] The outer case is removed to facilitate
liner replacement.
Older engines with several combustion cans had each can
with interconnector (flame propagation) tube, which was a
necessary part of the can-type combustion chambers. Since
each can is a separate burner operating independently of the
other cans, there must be some way to spread combustion
during the initial starting operation. This is accomplished
by interconnecting all the chambers. As the flame is started
by the spark igniter plugs in two of the lower chambers, it
passes through the tubes and ignites the combustible mixture
in the adjacent chamber and continues until all the chambers
Figure 1-53. Inside view of a combustion chamber liner. Figure 1-52. Can-type combustion chamber.
are burning.
The flame tubes vary in construction details from one
engine to another, although the basic components are almost
identical. [Figure 1-54] The spark igniters previously
mentioned are normally two in number and are located in
two of the can-type combustion chambers.
Another very important requirement in the construction of
combustion chambers is providing the means for draining
unburned fuel. This drainage prevents gum deposits in the fuel
manifold, nozzles, and combustion chambers. These deposits are caused by the residue left when the fuel evaporates.
Probably most important is the danger of afterfire if the fuel
is allowed to accumulate after shutdown. If the fuel is not
drained, a great possibility exists that, at the next starting
attempt, the excess fuel in the combustion chamber will ignite
and exhaust gas temperature will exceed safe operating limits.
The liners of the can-type combustors have perforations
of various sizes and shapes, each hole having a specific
purpose and effect on flame propagation within the liner.
[Figure 1-52] The air entering the combustion chamber
is divided by the proper holes, louvers, and slots into two
main streams—primary and secondary air. The primary or
combustion air is directed inside the liner at the front end,
where it mixes with the fuel and is burned. Secondary or
cooling air passes between the outer casing and the liner and
joins the combustion gases through larger holes toward the
rear of the liner, cooling the combustion gases from about
3,500 °F to near 1,500 °F. To aid in atomization of the fuel,
holes are provided around the fuel nozzle in the dome or
inlet end of the can-type combustor liner. Louvers are also
provided along the axial length of the liners to direct a cooling
layer of air along the inside wall of the liner. This layer of air
also tends to control the flame pattern by keeping it centered
in the liner, thereby preventing burning of the liner walls.
Figure 1-55 illustrates the annular combustion chamber liner.
Some provision is always made in the combustion chamber
case for installation of a fuel nozzle. The fuel nozzle delivers
the fuel into the liner in a finely atomized spray. The more the
spray is atomized, the more rapid and efficient the burning
process is.
Two types of fuel nozzle currently being used in the various
types of combustion chambers are the simplex nozzle and the
duplex nozzle. The construction features of these nozzles are
covered in greater detail in Chapter 2, Engine Fuel & Fuel
Metering Systems.
The spark igniter plugs of the annular combustion chamber
are the same basic type used in the can-type combustion
chambers, although construction details may vary. There are
usually two igniters mounted on the boss provided on each
of the chamber housings. The igniters must be long enough
to protrude from the housing into the combustion chamber.
The burners are interconnected by projecting flame
tubes which facilitate the engine-starting process as
mentioned previously in the can-type combustion chamber
familiarization. The flame tubes function identically to those
previously discussed, differing only in construction details.
The can-annular combustion chamber is not used in modern
Elbow Elbow Ferrule Seal Seal Interconnecter tube Extension tubeInterconnecter tube
Figure 1-54. Interconnecting flame tubes for can-type combustion chambers.
Figure 1-55. Annular combustion chamber liner.
Rotor shaft
Fuel nozzle cluster Spark igniterFlame tubesOutlet ducts
Figure 1-56. Can-annular combustion chamber components and
arrangement. engines. The forward face of each chamber presents six
apertures, which align with the six fuel nozzles of the
corresponding fuel nozzle cluster. [Figure 1-56] These
nozzles are the dual-orifice (duplex) type requiring the use
of a flow-divider (pressurizing valve), as mentioned in the
can-type combustion chamber discussion. Around each
nozzle are preswirl vanes for imparting a swirling motion to
the fuel spray, which results in better atomization of the fuel,
better burning, and efficiency. The swirl vanes function to
provide two effects imperative to proper flame propagation:
1. High flame speed—better mixing of air and fuel,
ensuring spontaneous burning.
2. Low air velocity axially—swirling eliminates overly
rapid flame movement axially.
The swirl vanes greatly aid flame propagation, since a high
degree of turbulence in the early combustion and cooling
stages is desirable. The vigorous mechanical mixing of the
fuel vapor with the primary air is necessary, since mixing by
diffusion alone is too slow. This same mechanical mixing
is also established by other means, such as placing coarse screens in the diffuser outlet, as is the case in most axial-
flow engines.
The can-annular combustion chambers also must have the
required fuel drain valves located in two or more of the
bottom chambers, assuring proper drainage and elimination
of residual fuel burning at the next start.
The flow of air through the holes and louvers of the can-
annular chambers, is almost identical with the flow through
other types of burners. [Figure 1-56] Special baffling is used
to swirl the combustion airflow and to give it turbulence.
Figure 1-57 shows the flow of combustion air, metal cooling
air, and the diluent or gas cooling air. The air flow direction
Combustion air
Metal cooling air
Gas cooling air
Figure 1-58. Annular combustion chamber with ceramic coating.Figure 1-57. Airflow through a can-annular combustion chamber.
Figure 1-59. Combustion chamber louvers and holes.
Figure 1-60. Turbine inlet guide vanes.is indicated by the arrows.
The basic components of an annular combustion chamber
are a housing and a liner, as in the can type. The liner
consists of an undivided circular shroud extending all the
way around the outside of the turbine shaft housing. The
chamber is constructed of heat-resistant materials, which
are sometimes coated with thermal barrier materials, such
as ceramic materials. The annular combustion chamber is
illustrated in Figure 1-58 . Modern turbine engines usually
have an annular combustion chamber. As can be seen in
Figure 1-59 , the annular combustion chamber also uses
louvers and holes to prevent the flame from contacting the
side of the combustion chamber.
A reverse-flow combustor is a type of combustor in which
the air from the compressor enters the combustor outer case
and reverses its direction as it flows into the inner liner.
It again reverses its direction before it flows through the
turbine. Reverse-flow combustors are used where engine
length is critical.
Turbine Section
The turbine transforms a portion of the kinetic (velocity)
energy of the exhaust gases into mechanical energy to drive the
gas generator compressor and accessories. The sole purpose
of the gas generator turbine is to absorb approximately 60
to 70 percent of the total pressure energy from the exhaust
gases. The exact amount of energy absorption at the turbine is
determined by the load the turbine is driving (i.e., compressor
size and type, number of accessories, and the load applied by
the other turbine stages). These turbine stages can be used to
drive a low-pressure compressor (fan), propeller, and shaft.
The turbine section of a gas turbine engine is located aft, or
downstream, of the combustion chamber. Specifically, it is
directly behind the combustion chamber outlet.
The turbine assembly consists of two basic elements: turbine
inlet guide vanes and turbine disc. [Figures 1-60 and 1-61]
The stator element is known by a variety of names, of which
turbine inlet nozzle vanes, turbine inlet guide vanes, and
nozzle diaphragm are three of the most commonly used. The turbine inlet nozzle vanes are located directly aft of the
combustion chambers and immediately forward of the turbine
wheel. This is the highest or hottest temperature that comes
in contact with metal components in the engine. The turbine
inlet temperature must be controlled, or damage will occur
to the turbine inlet vanes.
After the combustion chamber has introduced the heat energy
into the mass airflow and delivered it evenly to the turbine
Figure 1-61. Turbine disc.
inlet nozzles, the nozzles must prepare the mass air flow to
drive the turbine rotor. The stationary vanes of the turbine
inlet nozzles are contoured and set at such an angle that they
form a number of small nozzles discharging gas at extremely
high speed; thus, the nozzle converts a varying portion of the
heat and pressure energy to velocity energy that can then be
converted to mechanical energy through the turbine blades.
There are three types of turbine blades: the impulse turbine
blade, reaction turbine blade, and the reaction-impulse
turbine blade. The impulse turbine blade is also referred to
as a bucket. This is because as the stream of air strikes the
center of the blade it changes the direction of the energy as it
causes the blades to rotate the disc and rotor shaft. The turbine
nozzle guide vanes can usually be adjusted during engine
overhaul and assembly in order to increase the efficiency of
the air stream striking the blades or buckets of the turbine.
[Figure 1-62]
Reaction turbine blades cause the disc to rotate by the
aerodynamic action of the airstream directed to flow past
the blade at a particular angle in order to develop the most
efficient power from the turbine engine. [Figure 1-62]
The reaction-impulse turbine blade combines the action of
both the impulse and reaction blades designs. The blade has
more of the bucket shape of the impulse blade at the blade root
and it also has more of an airfoil shape of the reaction blade on
the second half of the blade toward the outer end of the blade.
The second purpose of the turbine inlet nozzle is to deflect
the gases to a specific angle in the direction of turbine wheel
rotation. Since the gas flow from the nozzle must enter the
turbine blade passageway while it is still rotating, it is essential
to aim the gas in the general direction of turbine rotation.
The turbine inlet nozzle assembly consists of an inner shroud and an outer shroud between which the nozzle vanes
are fixed. The number and size of inlet vanes employed
vary with different types and sizes of engines. Figure 1-63
illustrates typical turbine inlet nozzles featuring loose and
welded vanes. The vanes of the turbine inlet nozzle may be
assembled between the outer and inner shrouds or rings in
a variety of ways. Although the actual elements may vary
slightly in configuration and construction features, there is
one characteristic peculiar to all turbine inlet nozzles: the
nozzle vanes must be constructed to allow thermal expansion.
Otherwise, there would be severe distortion or warping of
the metal components because of rapid temperature changes.
The thermal expansion of turbine nozzles is accomplished
by one of several methods. One method necessitates loose
assembly of the supporting inner and outer vane shrouds.
[Figure 1-63A]
Each vane fits into a contoured slot in the shrouds, which
conforms to the airfoil shape of the vane. These slots are
slightly larger than the vanes to give a loose fit. For further
support, the inner and outer shrouds are encased by inner
and outer support rings, which provide increased strength
and rigidity. These support rings also facilitate removal of
the nozzle vanes as a unit. Without the rings, the vanes could
fall out as the shrouds were removed.
Another method of thermal expansion construction is to fit the
vanes into inner and outer shrouds; however, in this method
the vanes are welded or riveted into position. [Figure 1-63B]
Some means must be provided to allow thermal expansion;
therefore, either the inner or the outer shroud ring is cut
into segments. The saw cuts separating the segments allow
sufficient expansion to prevent stress and warping of the vanes.
The rotor element of the turbine section consists essentially
of a shaft and a wheel. [Figure 1-64] The turbine wheel is
a dynamically balanced unit consisting of blades attached
to a rotating disc. The disc, in turn, is attached to the main
power-transmitting shaft of the engine. The exhaust gases
leaving the turbine inlet nozzle vanes act on the blades of
the turbine wheel, causing the assembly to rotate at a very
high rate of speed. The high rotational speed imposes severe
centrifugal loads on the turbine wheel, and at the same
time the elevated temperatures result in a lowering of the
strength of the material. Consequently, the engine speed and
temperature must be controlled to keep turbine operation
within safe limits.
The turbine disc is referred to as such without blades. When
the turbine blades are installed, the disc then becomes the
turbine wheel. The disc acts as an anchoring component for
the turbine blades. Since the disc is bolted or welded to the
Impulse Turbine
Reaction TurbineRotor
RotorStator
StatorRotor
RotorMoving buckets
Fixed nozzle
Rotating nozzleSteam pressure
Steam pressureSteam velocity
Steam velocity
Figure 1-62. Impulse and reaction turbine blades.
shaft, the blades can transmit to the rotor shaft the energy
they extract from the exhaust gases.
The disc rim is exposed to the hot gases passing through the
blades and absorbs considerable heat from these gases. In
addition, the rim also absorbs heat from the turbine blades
by conduction. Hence, disc rim temperatures are normally
high and well above the temperatures of the more remote
inner portion of the disc. As a result of these temperature
gradients, thermal stresses are added to the rotational stresses. Additionally, turbine blades are generally more susceptible to
operating damage than compressor blades due to the exposure
of high temperatures. There are various methods to relieve, at
least partially, the aforementioned stresses. One such method
is to bleed cooling air back onto the face of the disc.
Another method of relieving the thermal stresses of the disc
is incidental to blade installation. A series of grooves or
notches, conforming to the blade root design, are broached
in the rim of the disc. These grooves allow attachment of the
B. Turbine nozzle vane assembly with welded vanesA. Turbine nozzle vane assembly with loose-fitting vanes
Figure 1-63. Typical turbine nozzle vane assemblies.
Figure 1-64. Rotor elements of the turbine assembly. turbine blades to the disc; at the same time, space is provided
by the notches for thermal expansion of the disc. Sufficient
clearance exists between the blade root and the notch to
permit movement of the turbine blade when the disc is cold.
During engine operation, expansion of the disc decreases
the clearance. This causes the blade root to fit tightly in the
disc rim.
The turbine shaft is usually fabricated from alloy steel.
[Figure 1-64] It must be capable of absorbing the high torque
loads that are exerted on it.
The methods of connecting the shaft to the turbine disc vary.
In one method, the shaft is welded to the disc, which has a
butt or protrusion provided for the joint. Another method is
by bolting. This method requires that the shaft have a hub
that fits a machined surface on the disc face. Then, the bolts
are inserted through holes in the shaft hub and anchored in
tapped holes in the disc. Of the two connection methods,
bolting is more common.
The turbine shaft must have some means for attachment to
the compressor rotor hub. This is usually accomplished by
a spline cut on the forward end of the shaft. The spline fits
into a coupling device between the compressor and turbine
shafts. If a coupling is not used, the splined end of the turbine
shaft may fit into a splined recess in the compressor rotor hub.
This splined coupling arrangement is used almost exclusively
with centrifugal compressor engines, while axial compressor
engines may use either of these described methods.There are various ways of attaching turbine blades, some
similar to compressor blade attachment. The most satisfactory
method utilizes the fir-tree design. [Figure 1-65]
The blades are retained in their respective grooves by a
variety of methods, the more common of which are peening,
welding, lock tabs, and riveting. Figure 1-66 shows a typical
turbine wheel using rivets for blade retention.
The peening method of blade retention is used frequently
in various ways. One of the most common applications of
peening requires a small notch to be ground in the edge of
the blade fir-tree root prior to the blade installation. After
the blade is inserted into the disc, the notch is filled by the
disc metal, which is “flowed” into it by a small punch-mark
made in the disc adjacent to the notch. The tool used for this
job is similar to a center punch.
Another method of blade retention is to construct the root
of the blade so that it contains all the elements necessary for
its retention. This method uses the blade root as a stop made
on one end of the root so that the blade can be inserted and
removed in one direction only, while on the opposite end is
a tang. This tang is bent to secure the blade in the disc.
Turbine blades may be either forged or cast, depending on the
composition of the alloys. Most blades are precision cast and
finish ground to the desired shape. Many turbine blades are
cast as a single crystal, which gives the blades better strength
and heat properties. Heat barrier coating, such as ceramic
coating, and air flow cooling help keep the turbine blades and
inlet nozzles cooler. This allows the exhaust temperature to
be raised, increasing the efficiency of the engine. Figure 1-67
Figure 1-67. Turbine blade with cooling holes.
Figure 1-66. Rivet method of turbine blade retention.
Figure 1-65. Turbine blade with fir-tree design and lock-tab method
of blade retention. shows a turbine blade with air holes for cooling purposes.
Most turbines are open at the outer perimeter of the blades;
however, a second type called the shrouded turbine is
sometimes used. The shrouded turbine blades, in effect, form
a band around the outer perimeter of the turbine wheel. This
improves efficiency and vibration characteristics and permits
lighter stage weights. On the other hand, it limits turbine
speed and requires more blades. [Figure 1-68]
In turbine rotor construction, it occasionally becomes
necessary to utilize turbines of more than one stage. A
single turbine wheel often cannot absorb enough power
from the exhaust gases to drive the components dependent
on the turbine for rotative power; thus, it is necessary to add
additional turbine stages.
A turbine stage consists of a row of stationary vanes or
nozzles, followed by a row of rotating blades. In some
models of turboprop engine, as many as five turbine stages
have been utilized successfully. It should be remembered
that, regardless of the number of wheels necessary for
driving engine components, there is always a turbine nozzle
preceding each wheel.
As was brought out in the preceding discussion of turbine
stages, the occasional use of more than one turbine wheel is
warranted in cases of heavy rotational loads. It should also
be pointed out that the same loads that necessitate multistage
turbines often make it advantageous to incorporate multiple
compressor rotors.
In the single-stage rotor turbine, the power is developed by
one turbine rotor, and all engine-driven parts are driven by this single wheel. [Figure 1-69] This arrangement is used
on engines where the need for low weight and compactness
predominates. This is the simplest version of the pure turbojet
engine.
A multistage turbine is shown in Figure 1-70 . In multiple
spool engines, each spool has its own set of turbine stages.
Each set of turbine stages turns the compressor attached to
it. Most turbofan engines have two spools: low pressure (fan
shaft a few stages of compression and the turbine to drive it)
and high pressure (high pressure compressor shaft and high
pressure turbine). [Figure 1-71]
The remaining element to be discussed concerning turbine
familiarization is the turbine casing or housing. The turbine
casing encloses the turbine wheel and the nozzle vane
assembly, and at the same time gives either direct or indirect
support to the stator elements of the turbine section. It always
has flanges provided front and rear for bolting the assembly
to the combustion chamber housing and the exhaust cone
assembly, respectively. A turbine casing is illustrated in
Figure 1-68. Shrouded turbine blades.
Figure 1-72. Turbine casing assembly.Figure 1-71. Dual-rotor turbine for split-spool compressor.
Figure 1-70. Multirotor turbine.Figure 1-69. Single-stage rotor turbine.
Figure 1-72 .
Exhaust Section
The exhaust section of the gas turbine engine consists
of several components. Although the components have
individual purposes, they also have one common function:
they must direct the flow of hot gases rearward in such a
manner as to prevent turbulence and, at the same time, impart
a high final or exit velocity to the gases. In performing the
various functions, each of the components affects the flow
of gases in different ways. The exhaust section is located
directly behind the turbine section and ends when the gases
are ejected at the rear in the form of high-velocity exhaust
gases. The components of the exhaust section include the
exhaust cone, tailpipe (if required), and the exhaust nozzle.
The exhaust cone collects the exhaust gases discharged from
the turbine section and gradually converts them into a solid
flow of gases. In performing this, the velocity of the gases is
decreased slightly and the pressure increased. This is due to
the diverging passage between the outer duct and the inner
cone; that is, the annular area between the two units increases
rearward. The exhaust cone assembly consists of an outer
shell or duct, an inner cone, three or four radial hollow struts
or fins, and the necessary number of tie rods to aid the struts
in supporting the inner cone from the outer duct.
The outer shell or duct is usually made of stainless steel and
is attached to the rear flange of the turbine case. This element
collects the exhaust gases and delivers them directly to the
exhaust nozzle. The duct must be constructed to include such
features as a predetermined number of thermocouple bosses
for installing exhaust temperature thermocouples, and there
must also be insertion holes for the supporting tie rods. In
some cases, tie rods are not used for supporting the inner
cone. If such is the case, the hollow struts provide the sole
support of the inner cone, the struts being spot-welded in
position to the inside surface of the duct and to the inner cone,
respectively. [Figure 1-73] The radial struts actually have a
twofold function. They not only support the inner cone in the exhaust duct, but they also perform the important function of
straightening the swirling exhaust gases that would otherwise
leave the turbine at an angle of approximately 45°.
The centrally located inner cone fits rather closely against
the rear face of the turbine disc, preventing turbulence of the
gases as they leave the turbine wheel. The cone is supported
by the radial struts. In some configurations, a small hole is
located in the exit tip of the cone. This hole allows cooling
Outer duct
Inner cone Struts
Figure 1-73. Exhaust collector with welded support struts.
Induction air Ram airCooling air ejectorInsulation blanket
Figure 1-74. Exhaust system insulation blanket.air to be circulated from the aft end of the cone, where the
pressure of the gases is relatively high, into the interior of the
cone and consequently against the face of the turbine wheel.
The flow of air is positive, since the air pressure at the turbine
wheel is relatively low due to rotation of the wheel; thus, air
circulation is assured. The gases used for cooling the turbine
wheel return to the main path of flow by passing through the
clearance between the turbine disc and the inner cone. The
exhaust cone assembly is the terminating component of the
basic engine. The remaining component (the exhaust nozzle)
is usually considered an airframe component.
The tailpipe is usually constructed so that it is semiflexible.
On some tailpipes, a bellows arrangement is incorporated
in its construction, allowing movement in installation,
maintenance, and in thermal expansion. This eliminates stress
and warping which would otherwise be present.
The heat radiation from the exhaust cone and tailpipe could
damage the airframe components surrounding these units.
For this reason, some means of insulation had to be devised.
There are several suitable methods of protecting the fuselage
structure; two of the most common are insulation blankets
and shrouds.
The insulation blanket, illustrated in Figures 1-74 and 1-75,
consists of several layers of aluminum foil, each separated by
a layer of fiberglass or some other suitable material. Although
these blankets protect the fuselage from heat radiation, they
are used primarily to reduce heat loss from the exhaust system.
The reduction of heat loss improves engine performance. There are two types of exhaust nozzle designs: the converging
design for subsonic gas velocities and the converging-
diverging design for supersonic gas velocities. These exhaust
nozzle designs are discussed in greater detail in Chapter 3,
Induction & Exhaust Systems.
The exhaust nozzle opening may be of either fixed or variable
area. The fixed-area type is the simpler of the two exhaust
nozzles since there are no moving parts. The outlet area of the
fixed exhaust nozzle is very critical to engine performance.
If the nozzle area is too large, thrust is wasted; if the area is
too small, the engine could choke or stall. A variable-area
exhaust nozzle is used when an augmenter or afterburner is
used due to the increased mass of flow when the afterburner is
activated. It must increase its open area when the afterburner
is selected. When the afterburner is off, the exhaust nozzle
closes to a smaller area of opening.
Gas Turbine Engine Bearings & Seals
The main bearings have the critical function of supporting
the main engine rotor. The number of bearings necessary
for proper engine support is, for the most part, determined
by the length and weight of the engine rotor. The length and
weight are directly affected by the type of compressor used
in the engine. Naturally, a two-spool compressor requires
more bearing support. The minimum number of bearings
required to support one shaft is one deep groove ball bearing
(thrust and radial loads) and one straight roller bearing (radial
load only). Sometimes, it is necessary to use more than one
roller bearing if the shaft is subject to vibration or its length
is excessive. The gas turbine rotors are supported by ball and
roller bearings, which are antifriction bearings. [Figure 1-76]
Many newer engines use hydraulic bearings, in which the
outside race is surrounded by a thin film of oil. This reduces
vibrations transmitted to the engine.
In general, antifriction bearings are preferred largely because
they:
Stainless steel shroud – 350 ° F
Fiberglass
Fiberglass
Silver foilAluminum foil
Jet tailpipe 90 0 ° F
Exhaust gasCooling air 120 ° F
1000 ° F
Figure 1-75. Insulation blanket with the temperatures obtained at the various locations shown.
for this purpose. A typical ball or roller bearing assembly
includes a bearing support housing, which must be strongly
constructed and supported in order to carry the radial and
axial loads of the rapidly rotating rotor. The bearing housing
usually contains oil seals to prevent the oil leaking from its
normal path of flow. It also delivers the oil to the bearing for
its lubrication, usually through spray nozzles. The oil seals
may be the labyrinth or thread (helical) type. These seals
also may be pressurized to minimize oil leaking along the
compressor shaft. The labyrinth seal is usually pressurized,
but the helical seal depends solely on reverse threading to
stop oil leakage. These two types of seals are very similar,
differing only in thread size and the fact that the labyrinth
seal is pressurized.
Another type of oil seal used on some of the later engines is
the carbon seal. These seals are usually spring loaded and
are similar in material and application to the carbon brushes • Offer little rotational resistance,
• Facilitate precision alignment of rotating elements,
• Are relatively inexpensive,
• Are easily replaced,
• Withstand high momentary overloads,
• Are simple to cool, lubricate, and maintain,
• Accommodate both radial and axial loads, and
• Are relatively resistant to elevated temperatures.
The main disadvantages are their vulnerability to foreign
matter and tendency to fail without appreciable warning.
Usually the ball bearings are positioned on the compressor or
turbine shaft so that they can absorb any axial (thrust) loads
or radial loads. Because the roller bearings present a larger
working surface, they are better equipped to support radial
loads than thrust loads. Therefore, they are used primarily
Ball bearing Roller bearing
Preformed sleeve bearing Slipper bearing
Figure 1-76. Types of main bearings used for gas turbine rotor support.
used in electrical motors. Carbon seals rest against a surface
provided to create a sealed bearing cavity or void; thus, the
oil is prevented from leaking out along the shaft into the
compressor airflow or the turbine section. [Figure 1-77]
The ball or roller bearing is fitted into the bearing housing and
may have a self-aligning feature. If a bearing is self-aligning,
it is usually seated in a spherical ring. This allows the shaft
a certain amount of radial movement without transmitting
stress to the bearing inner race.
The bearing surface is usually provided by a machined journal
on the appropriate shaft. The bearing is usually locked in
position by a steel snap ring or other suitable locking device.
The rotor shaft also provides the matching surface for the oil
seals in the bearing housing. These machined surfaces are
called lands and fit in rather close to the oil seal.
Turboprop Engines
The turbopropeller (turboprop) engine is a combination of
a gas turbine engine, reduction gear box, and a propeller.
[Figure 1-78] Turboprops are basically gas turbine engines
that have a compressor, combustion chamber(s), turbine,
and an exhaust nozzle (gas generator), all of which operate
in the same manner as any other gas engine. However, the
difference is that the turbine in the turboprop engine usually
has extra stages to extract energy to drive the propeller. In addition to operating the compressor and accessories, the
turboprop turbine transmits increased power forward through
a shaft and a gear train to drive the propeller. The increased
power is generated by the exhaust gases passing through
additional stages of the turbine.
Some engines use a multirotor turbine with coaxial shafts
for independent driving of the compressor and propeller.
Although there are three turbines utilized in this illustration,
as many as five turbine stages have been used for driving the
two rotor elements, propeller, and accessories.
The exhaust gases also contribute to engine power output
through thrust production, although the amount of energy
available for thrust is considerably reduced. Two basic types
of turboprop engine are in use: fixed turbine and free turbine.
The fixed turbine has a mechanical connection from the gas
generator (gas-turbine engine) to the reduction gear box
and propeller. The free turbine has only an air link from gas
generator to the power turbines. There is no mechanical link
from the propeller to the gas turbine engine (gas generator).
There are advantages and disadvantages of each system, with
the airframe generally dictating the system used.
Since the basic components of normal gas-turbine and
turboprop engines differ slightly only in design features, it
should be fairly simple to apply acquired knowledge of the
Spacers
Seal housing
Seals Bearing inner race
Figure 1-77. Carbon oil seal.
Figure 1-78. PT6 turboprop engine. basic gas turbine to the turboprop.
The typical turboprop engine can be broken down into
assemblies as follows:
1. The power section assembly—contains the usual major
components of a gas turbine engine (i.e., compressor,
combustion chamber, turbine, and exhaust sections).
2. The reduction gear or gearbox assembly—contains those sections unique to turboprop configurations.
3. The torquemeter assembly—transmits the torque from
the engine to the gearbox of the reduction section.
4. The accessory drive housing assembly—mounted
on the bottom of the compressor air inlet housing.
It includes the necessary gear trains for driving all
power section driven accessories at their proper rpm
in relation to engine rpm.
Turboshaft Engines
A gas-turbine engine that delivers power through a shaft to
operate something other than a propeller is referred to as
a turboshaft engine. [Figure 1-79] The output shaft may
be coupled directly to the engine turbine, or the shaft may
be driven by a turbine of its own (free turbine) located in
the exhaust stream. As mentioned with the turboprop, the
free turbine rotates independently. This principle is used
extensively in current production of turboshaft engines. The
turboshaft engine’s output is measured in horsepower instead
of thrust because the power output is a turning shaft.
Turbofan Engines
The turbofan gas turbine engine is, in principle, the same as
a turboprop, except that the propeller is replaced by a duct-
enclosed axial-flow fan. [Figure 1-80] The fan can be a part
of the first-stage compressor blades or can be mounted as a
separate set of fan blades. The blades are mounted forward
of the compressor.
The general principle of the fan engine is to convert more
of the fuel energy into pressure. With more of the energy
Figure 1-79. Turboshaft engine.
converted to pressure, a greater product of pressure times
area can be achieved. One of the major advantages is turbofan
production of this additional thrust without increasing fuel
flow. The end result is fuel economy with the consequent
increase in range. Because more of the fuel energy is turned
into pressure in the turbofan engine, additional stages must
be added in the turbine section to provide the power to drive
the fan. This means there is less energy left over and less
thrust from the core exhaust gases. Also, in a mixed-exhaust
nozzle (where fan air and core air mix in a common nozzle
before entering ambient conditions) the exhaust nozzle must
be larger in area. The result is that the fan develops most of
the thrust. The thrust produced by the fan more than makes
up for the decrease in thrust of the core (gas generator) of
the engine. Depending on the fan design and bypass ratio,
it produces 80 percent of the turbofan engine’s total thrust.
Two different exhaust nozzle designs are used with turbofan
engines. The air leaving the fan can be ducted overboard by
a separate fan nozzle [Figure 1-43] , or it can be ducted along
the outer case of the basic engine to be discharged through
the mixed nozzle (core and fan exhaust together). The fan air
is either mixed with the exhaust gases before it is discharged
(mixed or common nozzle), or it passes directly to the
atmosphere without prior mixing (separate nozzle). Turbofans
are the most widely used gas turbine engine for air transport
aircraft. The turbofan is a compromise between the good operating efficiency and high thrust capability of a turboprop
and the high speed, high altitude capability of a turbojet.
Turbine Engine Operating Principles
The principle used by a gas turbine engine as it provides
force to move an airplane is based on Newton’s Third Law.
This law states that for every action there is an equal and
opposite reaction; therefore, if the engine accelerates a mass
of air (action), it applies a force on the aircraft (reaction).
The turbofan generates thrust by giving a relatively slower
acceleration to a large quantity of air. The old pure turbojet
engine achieves thrust by imparting greater acceleration to a
smaller quantity of air. This was its main problem with fuel
consumption and noise.
The mass of air is accelerated within the engine by the use of
a continuous-flow cycle. Ambient air enters the inlet diffuser
where it is subjected to changes in temperature, pressure, and
velocity due to ram effect. The compressor then increases
pressure and temperature of the air mechanically. The air
continues at constant pressure to the burner section where its
temperature is increased by combustion of fuel. The energy
is taken from the hot gas by expanding through a turbine
which drives the compressor, and by expanding through an
exhaust nozzle designed to discharge the exhaust gas at high
velocity to produce thrust.
Figure 1-80. Turbofan engine.
The high velocity gases from the engine may be considered
continuous, imparting this force against the aircraft in which
it is installed, thereby producing thrust. The formula for thrust
can be derived from Newton’s second law, which states that
force is proportional to the product of mass and acceleration.
This law is expressed in the following formula:
In the above formula, mass is similar to weight, but it is
actually a different quantity. Mass refers to the quantity
of matter, while weight refers to the pull of gravity on that
quantity of matter. At sea level under standard conditions,
1 pound of mass has a weight of 1 pound. To calculate the
acceleration of a given mass, the gravitational constant is
used as a unit of comparison. The force of gravity is 32.2
feet per second squared (ft/sec2). This means that a free
falling 1 pound object accelerates at the rate of 32.2 feet per
second each second that gravity acts on it. Since the object
mass weighs 1 pound, which is also the actual force imparted
to it by gravity, it can be assumed that a force of 1 pound
accelerates a 1 pound object at the rate of 32.2 ft/sec2.F = M x A
where:
M = mass in pounds per secondF = force in pounds
A = acceleration in feet per secondAlso, a force of 10 pound accelerates a mass of 10 pound at
the rate of 32.2 ft/sec2. This is assuming there is no friction
or other resistance to overcome. It is now apparent that the
ratio of the force (in pounds) is to the mass (in pounds) as
the acceleration in ft/sec2 is to 32.2. Using M to represent
the mass in pounds, the formula may be expressed thus:
In any formula involving work, the time factor must be
considered. It is convenient to have all time factors in
equivalent units (i.e., seconds, minutes, or hours). In
calculating jet thrust, the term “pounds of air per second” is
convenient, since the second is the same unit of time used
for the force of gravity. F = or
where:
M = massF = force
A = acceleration F
M A
G = MA
G
G = gravity
1-58Thrust
Using the following formula, compute the force necessary to
accelerate a mass of 50 pounds by 100 ft/sec2.
F = MA
G
F = 50 lb x 100 ft/sec2
32.2 ft/sec2
F = 5,000 lb-ft/sec2
32.2 ft/sec2
F = 155 lb
This illustrates that if the velocity mass per second is
increased by 100, the resulting thrust is 155 pounds.
Since the turbojet engine accelerates air, the following
formula can be used to determine jet thrust:
As an example, to use the formula for changing the velocity
of 100 pounds of mass airflow per second from 600 ft/sec to
800 ft/sec, the formula can be applied as follows:
As shown by the formula, if the mass airflow per second and
the difference in the velocity of the air from the intake to the
exhaust are known, it is easy to compute the force necessary
to produce the change in the velocity. Therefore, the thrust of
the engine must be equal to the force required to accelerate
the air mass through the engine. Then, by using the symbol
“Fn” for thrust pounds, the formula becomes:F = Ms (V2 - V1)
G
F = force in pounds
Ms = mass flow in lb/sec
V1 = inlet velocity
V2 = jet velocity (exhaust)
G = acceleration of gravity or 32.2 ft/sec2where:
change in velocity; difference between
inlet velocity and jet velocityV2 - V1 =
F = 100 lb/sec (800 ft/sec - 600 ft/sec)
32.2 ft/sec2
F = 621 lbF = 20,000 lb/sec
32.2 ft/sec2Fn = Ms (V2 - V1)
G
Thrust of a gas turbine engine can be increased by two
methods: increasing the mass flow of air through the engine
or increasing the gas velocity. If the velocity of the turbojet
engine remains constant with respect to the aircraft, the thrust
decreases if the speed of the aircraft is increased. This is
because V 1 increases in value. This does not present a serious
problem, however, because as the aircraft speed increases,
more air enters the engine, and jet velocity increases. The
resultant net thrust is almost constant with increased airspeed.
The Brayton cycle is the name given to the thermodynamic
cycle of a gas turbine engine to produce thrust. This is a
variable volume constant-pressure cycle of events and is
commonly called the constant-pressure cycle. A more recent
term is “continuous combustion cycle.” The four continuous
and constant events are intake, compression, expansion
(includes power), and exhaust. These cycles are discussed
as they apply to a gas-turbine engine. In the intake cycle, air
enters at ambient pressure and a constant volume. It leaves
the intake at an increased pressure and a decrease in volume.
At the compressor section, air is received from the intake at
an increased pressure, slightly above ambient, and a slight
decrease in volume. Air enters the compressor where it is
compressed. It leaves the compressor with a large increase in
pressure and decrease in volume, created by the mechanical
action of the compressor. The next step, expansion, takes
place in the combustion chamber by burning fuel, which
expands the air by heating it. The pressure remains relatively
constant, but a marked increase in volume takes place. The
expanding gases move rearward through the turbine assembly
and are converted from velocity energy to mechanical energy
by the turbine. The exhaust section, which is a convergent
duct, converts the expanding volume and decreasing pressure
of the gases to a final high velocity. The force created inside
the engine to keep this cycle continuous has an equal and
opposite reaction (thrust) to move the aircraft forward.
Bernoulli’s principle (whenever a stream of any fluid has its
velocity increased at a given point, the pressure of the stream
at that point is less than the rest of the stream) is applied to
gas turbine engines through the design of convergent and
divergent air ducts. The convergent duct increases velocity
and decreases pressure. The divergent duct decreases
velocity and increases pressure. The convergent principle is
usually used for the exhaust nozzle. The divergent principle
is used in the compressor and diffuser where the air is
slowing and pressurizing.
0 1 2 3 4 5 6 7 8
Compression ratio
Thermal efficiency (in percent)
Temperature
1600 °F 1400 °F1200 °F 1000 °F
Figure 1-81. The effect of compression ratio on thermal efficiency.
0 1 2 3 4 5 6 7 8
Compression ratio
Thermal efficiency (in percent)
Efficiency of C&T
75% 85% 80% 90%
Figure 1-82. Turbine and compressor efficiency vs. thermal
efficiency. Gas Turbine Engine Performance
Thermal efficiency is a prime factor in gas turbine performance.
It is the ratio of net work produced by the engine to the
chemical energy supplied in the form of fuel. The three most
important factors affecting the thermal efficiency are turbine
inlet temperature, compression ratio, and the component
efficiencies of the compressor and turbine. Other factors that
affect thermal efficiency are compressor inlet temperature
and combustion efficiency. Figure 1-81 shows the effect that
changing compression ratio (compressor pressure ratio) has on
thermal efficiency when compressor inlet temperature and the
component efficiencies of the compressor and turbine remain
constant. The effects that compressor and turbine component
efficiencies have on thermal efficiency when turbine and
compressor inlet temperatures remain constant are shown in
Figure 1-82 . In actual operation, the turbine engine exhaust
temperature varies directly with turbine inlet temperature at
a constant compression ratio.
Rpm is a direct measure of compression ratio; therefore, at
constant rpm, maximum thermal efficiency can be obtained
by maintaining the highest possible exhaust temperature.
Since engine life is greatly reduced at high turbine inlet
temperatures, the operator should not exceed the exhaust
temperatures specified for continuous operation. Figure 1-83
illustrates the effect of turbine inlet temperature on turbine
blade life. In the previous discussion, it was assumed that
the state of the air at the inlet to the compressor remains
constant. Since this is a practical application of a turbine
engine, it becomes necessary to analyze the effect of varying
inlet conditions on the thrust or power produced. The three
principal variables that affect inlet conditions are the speed
of the aircraft, the altitude of the aircraft, and the ambient
temperature. To make the analysis simpler, the combination
of these three variables can be represented by a single variable
called stagnation density.
The power produced by a turbine engine is proportional to
the stagnation density at the inlet. The next three illustrations
show how changing the density by varying altitude, airspeed,
and outside air temperature affects the power level of the
engine. Figure 1-84 shows that the thrust output improves
rapidly with a reduction in outside air temperature (OAT) at
constant altitude, rpm, and airspeed. This increase occurs
partly because the energy required per pound of airflow to
drive the compressor varies directly with the temperature,
leaving more energy to develop thrust. In addition, the thrust
output increases since the air at reduced temperature has an
increased density. The increase in density causes the mass
flow through the engine to increase. The altitude effect on
thrust, as shown in Figure 1-85 , can also be discussed as a
density and temperature effect. In this case, an increase in
altitude causes a decrease in pressure and temperature.Since the temperature lapse rate is lower than the pressure
lapse rate as altitude is increased, the density is decreased.
Although the decreased temperature increases thrust, the
effect of decreased density more than offsets the effect of
the colder temperature. The net result of increased altitude
is a reduction in the thrust output.
The effect of airspeed on the thrust of a gas-turbine engine
is shown in Figure 1-86 . To explain the airspeed effect, it
is necessary to understand first the effect of airspeed on the
factors that combine to produce net thrust: specific thrust
and engine airflow. Specific thrust is the net thrust in pounds
developed per pound of airflow per second. It is the remainder
of specific gross thrust minus specific ram drag. As airspeed
is increased, ram drag increases rapidly. The exhaust velocity
remains relatively constant; thus, the effect of the increase in
1-600 100 200 300 400 500
Turbine bucket life (in hours)
Turbine inlet temperature
Increase
Dec STD INC
Outside air temperature (OAT)
Net thrust
60%100%Constant altitude, rpm, and airspeed
Figure 1-84. Effect of OAT on thrust output. Figure 1-83. Effect of turbine inlet temperature on turbine bucket
life.
0 10 20 30 40 50
Altitude—thousand of ft
Thrust—percent sea level rated
0Constant airspeed and rpm
TAS
Net thrust
Increase100
0Constant rpm, altitude, and OAT
With ram
Without ram
Figure 1-86. Effect of airspeed on net thrust. Figure 1-85. Effect of altitude on thrust output. airspeed results in decreased specific thrust. [Figure 1-86] In
the low-speed range, the specific thrust decreases faster than
the airflow increases and causes a decrease in net thrust. As the
airspeed increases into the higher range, the airflow increases
faster than the specific thrust decreases and causes the net
thrust to increase until sonic velocity is reached. The effect
of the combination on net thrust is illustrated in Figure 1-87 .
Ram Recovery
A rise in pressure above existing outside atmospheric pressure
at the engine inlet, as a result of the forward velocity of an
aircraft, is referred to as ram pressure. Since any ram effect
causes an increase in compressor entrance pressure over
atmospheric, the resulting pressure rise causes an increase
in the mass airflow and gas velocity, both of which tend to
increase thrust. Although ram effect increases engine thrust,
the thrust being produced by the engine decreases for a given
throttle setting as the aircraft gains airspeed. Therefore, two
opposing trends occur when an aircraft’s speed is increased.
What actually takes place is the net result of these two
different effects.
An engine’s thrust output temporarily decreases as aircraft
speed increases from static, but soon ceases to decrease.
Moving toward higher speeds, thrust output begins to increase
again due to the increased pressure of ram recovery.
TASIncreaseAirflow (in lb/sec)Specific thrustTotal airflow (wa)Constant rpm, altitude, and OAT
Figure 1-87. Effect of airspeed on specific thrust and total engine
airflow.
2-1Fuel System Requirements
The engine fuel system must supply fuel to the engine’s
fuel metering device under all conditions of ground and air
operation. It must function properly at constantly changing
altitudes and in any climate. The most common fuels are
A VGAS for reciprocating engines and Jet A for turbine
engines. A VGAS is generally either 80 (red) or 100LL (blue)
octane. The LL stands for low lead although it contains four
times the lead of 80 octane A VGAS. Jet A is a kerosene-based
fuel that is clear to straw in color.
Electronic engine controls have allowed great increases in
controlling the metered fuel flow to the engine. Engine fuel
systems have become very accurate at providing the correct
mixture of fuel and air to the engines. Gas turbine fuel controls
have also greatly improved the ability to schedule (meter)
the fuel correctly during all flight regimes. Improvements in
electronics and the use of digital computers have enabled the
aircraft and engines to be electronically interfaced together.
By the use of electronic sensors and computer logic built
in to electronic controls, the engines can be controlled with
much more accuracy. Fuel cost and availability have also
become factors in providing engines with fuel systems
that are efficient and very precise in scheduling fuel flow
to the engine. Many engines use an interactive system that
senses engine parameters and feeds the information to the
onboard computer (electronic engine control). The computer
determines the amount of fuel needed and then sends a signal
to the metering device. This signal sent to the metering device
determines the correct amount of fuel needed by the engine.
Electronic controls have become quite common with gas
turbines and have increased the capabilities of the fuel system,
making it less complicated for the technician and decreasing
maintenance problems.
Engine fuel systems can be fairly complicated, yet some are
quite simple, such as on small aircraft with a simple gravity-
feed fuel system. This system, consisting of a tank to supply
fuel to the engine, is often installed in the overhead wing and
feeds a small float-type carburetor. On multiengine aircraft,
complex systems are necessary so that fuel can be pumped
from any combination of tanks to any combination of engines
through a crossfeed system. Provisions for transferring fuel
from one tank to another may also be included on large aircraft.
Engine Fuel & Fuel Metering Systems
Chapter 2
Vapor Lock
All fuel systems should be designed so that vapor lock cannot
take place. Older gravity-feed systems were more prone to
vapor lock. The fuel system should be free of tendency to
vapor lock, which can result from changes in ground and
in-flight climatic conditions. Normally, the fuel remains in
a liquid state until it is discharged into the air stream and
then instantly changes to a vapor. Under certain conditions,
the fuel may vaporize in the lines, pumps, or other units.
The vapor pockets formed by this premature vaporization
restrict the fuel flow through units which are designed to
handle liquids rather than gases. The resulting partial or
complete interruption of the fuel flow is called vapor lock.
The three general causes of vapor lock are the lowering of the
pressure on the fuel, high fuel temperatures, and excessive
fuel turbulence.
At high altitudes, the pressure on the fuel in the tank is low.
This lowers the boiling point of the fuel and causes vapor
bubbles to form. This vapor trapped in the fuel may cause
vapor lock in the fuel system.
Transfer of heat from the engine tends to cause boiling of the
fuel in the lines and the pump. This tendency is increased if
the fuel in the tank is warm. High fuel temperatures often
combine with low pressure to increase vapor formation. This
is most apt to occur during a rapid climb on a hot day. As the
aircraft climbs, the outside temperature drops, but the fuel
does not lose temperature rapidly. If the fuel is warm enough
at takeoff, it retains enough heat to boil easily at high altitude.
The chief causes of fuel turbulence are sloshing of the fuel in
the tanks, the mechanical action of the engine-driven pump,
and sharp bends or rises in the fuel lines. Sloshing in the tank
tends to mix air with the fuel. As this mixture passes through
the lines, the trapped air separates from the fuel and forms
vapor pockets at any point where there are abrupt changes
in direction or steep rises. Turbulence in the fuel pump often
combines with the low pressure at the pump inlet to form a
vapor lock at this point.
Vapor lock can become serious enough to block the fuel flow
completely and stop the engine. Even small amounts of vapor
in the inlet line restrict the flow to the engine-driven pump
2-2and reduce its output pressure. To reduce the possibility of
vapor lock, fuel lines are kept away from sources of heat;
also, sharp bends and steep rises are avoided. In addition,
the volatility of the fuel is controlled in manufacture so that
it does not vaporize too readily. The major improvement in
reducing vapor lock, however, is the incorporation of booster
pumps in the fuel system. These booster pumps, which are
used widely in most modern aircraft, keep the fuel in the
lines to the engine-driven pump under pressure. The pressure
on the fuel reduces vapor formation and aids in moving a
vapor pocket along. The boost pump also releases vapor from
the fuel as it passes through the pump. The vapor moves
upward through the fuel in the tank and out the tank vents.
To prevent the small amount of vapor that remains in the
fuel from upsetting its metering action, vapor eliminators are
installed in some fuel systems ahead of the metering device
or are built into this unit.
Basic Fuel System
The basic parts of a fuel system include tanks, boost pumps,
lines, selector valves, strainers, engine-driven pumps, and
pressure gauges. A review of fuel systems in the Aviation
Maintenance Technician—General Handbook provides some
information concerning these components.
Generally, there are several tanks, even in a simple system, to
store the required amount of fuel. The location of these tanks
depends on both the fuel system design and the structural
design of the aircraft. From each tank, a line leads to the
selector valve. This valve is set from the flight deck to select
the tank from which fuel is to be delivered to the engine. The
boost pump forces fuel through the selector valve to the main
line strainer. This filtering unit, located in the lowest part of
the system, removes water and dirt from the fuel. During
starting, the boost pump forces fuel through a bypass in the
engine-driven pump to the metering device. Once the engine-
driven pump is rotating at sufficient speed, it takes over and
delivers fuel to the metering device at the specified pressure.
The airframe fuel system begins with the fuel tank and ends
at the engine fuel system. The engine fuel system usually
includes the engine-driven pumps and the fuel metering
systems. In aircraft powered with a reciprocating engine, the
engine-driven fuel pump and metering system consists of the
main components from the point at which the fuel enters the
first control unit until the fuel is injected into the intake pipe
or cylinder. For example, the engine fuel system of a typical
engine has an engine-driven fuel pump, the air-fuel control
unit (metering device), the fuel manifold valve, and the fuel
discharge nozzles. The fuel metering system on current
reciprocating engines meters the fuel at a predetermined
ratio to airflow. The airflow to the engine is controlled by
the carburetor or air-fuel control unit.The fuel metering system of the typical gas turbine engine
consists of an engine-driven pump, fuel flow transmitter,
fuel control with an electronic engine control, a distribution
system or manifold, flow divider, and fuel discharge nozzles.
On some turboprop engines, a fuel heater and a start control is
a part of the engine fuel system. The rate of fuel delivery can
be a function of air mass flow, compressor inlet temperature,
compressor discharge pressure, compressor revolutions per
minute (rpm), exhaust gas temperature, and combustion
chamber pressure.
Fuel Metering Devices for Reciprocating
Engines
Basic principles of operation are discussed here with no attempt
being made to give detailed maintenance instructions. For the
specific information needed to inspect or maintain a particular
installation or unit, consult the manufacturer’s instructions.
The basic requirement of a reciprocating fuel metering system
is the same, regardless of the type of system used or the model
engine on which the equipment is installed. It must meter fuel
proportionately to air to establish the proper air-fuel mixture
ratio for the engine at all speeds and altitudes at which the
engine may be operated. In the air-fuel mixture curves shown
in Figure 2-1, note that the basic best power and best economy
air-fuel mixture requirements for reciprocating engines are
approximately the same. The fuel metering system must
atomize and distribute the fuel from the carburetor into the
mass airflow. This must be accomplished so that the air-fuel
charges going to all cylinders holds equal amounts of fuel.
Each one of the engine’s cylinders should receive the same
quantity of air-fuel mixture and at the same air-fuel ratio.
Due to the drop in atmospheric pressure as altitude is
increased, the density of the air also decreases. A normally-
aspirated engine has a fixed amount or volume of air that
it can draw in during the intake stroke, therefore less air is
drawn into the engine as altitude increases. Less air tends to
make carburetors run richer at altitude than at ground level,
because of the decreased density of the airflow through
the carburetor throat for a given volume of air. Thus, it is
necessary that a mixture control be provided to lean the
mixture and compensate for this natural enrichment. Some
aircraft use carburetors in which the mixture control is
operated manually. Other aircraft employ carburetors which
automatically lean the carburetor mixture at altitude to
maintain the proper air-fuel mixture.
The rich mixture requirements for an aircraft engine are
established by running a power curve to determine the air-
fuel mixture for obtaining maximum usable power. This
curve is plotted at 100 rpm intervals from idle speed to
2-3Typical Air-Fuel Mixture Curve—Float-Type Carburetor
Rich
F/A
LeanTakeoff
Maximum cruise
Minimum cruiseAuto rich
Auto leanClimbIdleTypical Air-Fuel Mixture Curve—Pressure Injection Carburetor
Airflow in lb/hr Low HighRich
F/A
LeanIdle
Takeoff
Maximum cruise
Minimum cruise
Rich
Manual
LeanClimb
Airflow in lb/hr Low High
Figure 2-1. Air-fuel mixture curves.
Air-Fuel Mixture Lean Rich
PowerHigh
Brake
HP
LowIdle
TakeoffAuto leanAuto rich
Figure 2-2. Power versus air-fuel mixture curve.
Air-Fuel Mixture Lean Rich
Specific fuel consumptionHigh
LowAuto leanAuto rich
Figure 2-3. Specific fuel consumption curve. takeoff speed. [Figure 2-2] Since it is necessary in the power
range to add fuel to the basic air-fuel mixture requirements
to keep cylinder-head temperatures in a safe range, the fuel
mixture must become gradually richer as powers above
cruise are used. [Figure 2-1] In the power range, the engine
runs on a much leaner mixture, as indicated in the curves.
However, on the leaner mixture, cylinder-head temperature
would exceed the maximum permissible temperatures and
detonation would occur.
The best economy setting is established by running a series
of curves through the cruise range, as shown in the graph in
Figure 2-3, the low point (auto-lean) in the curve being the
air-fuel mixture where the minimum fuel per horsepower is
used. In this range the engine operates normally on slightly
leaner mixtures and obviously operates on richer mixtures
than the low-point mixture. If a mixture leaner than that
specified for the engine is used, the leanest cylinder of the
engine is apt to backfire because the slower burning rate of
the lean mixture results in a continued burning in the cylinder
when the next intake stroke starts.Air-Fuel Mixtures
Gasoline and other liquid fuels do not burn at all unless they
are mixed with air. If the mixture is to burn properly within
the engine cylinder, the ratio of air to fuel must be kept
within a certain range. It would be more accurate to state that
the fuel is burned with the oxygen in the air. Seventy-eight
percent of air by volume is nitrogen, which is inert and does
not participate in the combustion process, and 21 percent is
oxygen. Heat is generated by burning the mixture of gasoline
and oxygen. Nitrogen and gaseous byproducts of combustion
absorb this heat energy and turn it into power by expansion.
The mixture proportion of fuel and air by weight is of extreme
importance to engine performance. The characteristics of a
given mixture can be measured in terms of flame speed and
combustion temperature.
The composition of the air-fuel mixture is described by the
mixture ratio. For example, a mixture with a ratio of 12 to 1
2-4(12:1) is made up of 12 pounds of air and 1 pound of fuel. The
ratio is expressed in weight because the volume of air varies
greatly with temperature and pressure. The mixture ratio can
also be expressed as a decimal. Thus, an air-fuel ratio of 12:1
and an air-fuel ratio of 0.083 describe the same mixture ratio.
Mixtures of air and gasoline as rich as 8:1 and as lean as 16:1
will burn in an engine cylinder, but beyond these mixtures,
either lean or rich blow out could occur. The engine develops
maximum power with a mixture of approximately 12 parts
of air and 1 part of gasoline by weight.
From a chemist’s point of view, the perfect mixture for
combustion of fuel and air would be 0.067 pounds of fuel
to 1 pound of air (mixture ratio of 15:1). The scientist calls
this chemically correct combination a stoichiometric mixture
(pronounced stoy-key-o-metric). With this mixture (given
sufficient time and turbulence), all the fuel and all the oxygen
in the air is completely used in the combustion process. The
stoichiometric mixture produces the highest combustion
temperatures because the proportion of heat released to a
mass of charge (fuel and air) is the greatest. If more fuel is
added to the same quantity of air charge than the amount
giving a chemically perfect mixture, changes of power
and temperature occur. The combustion gas temperature is
lowered as the mixture is enriched, and the power increases
until the air-fuel ratio is approximately 0.0725. For mixtures
from 0.0725 air-fuel ratio to 0.080 air-fuel ratio, the power
remains essentially constant even though the combustion
temperature continues downward. Mixtures from 0.0725
air-fuel ratio to 0.080 air-fuel ratio are called best power
mixtures, since their use results in the greatest power for
a given airflow or manifold pressure. In this air-fuel ratio
range, there is no increase in the total heat released, but the
weight of nitrogen and combustion products is augmented
by the vapor formed with the excess fuel. Thus, the working
mass of the charge is increased. In addition, the extra fuel in
the charge (over the stoichiometric mixture) speeds up the
combustion process, which provides a favorable time factor
in converting fuel energy into power.
If the air-fuel ratio is enriched above 0.080, there is loss of
power and a reduction in temperature. The cooling effects
of excess fuel overtake the favorable factor of increased
mass. This reduced temperature and slower rate of burning
lead to an increasing loss of combustion efficiency. If, with
constant airflow, the mixture is leaned below 0.067, air-fuel
ratio power and temperature decrease together. This time, the
loss of power is not a liability but an asset. The purpose in
leaning is to save fuel. Air is free and available in limitless
quantities. The object is to obtain the required power with
the least fuel flow. A measure of the economical use of fuel
is called specific fuel consumption (SFC), which is the fuel
weight in pounds per hour per horsepower.By using this ratio, the engine’s use of fuel at various power
settings can be compared. When leaning below 0.067 air-
fuel ratio with constant airflow, even though the power
diminishes, the cost in fuel to support each horsepower
hour (SFC) also is lowered. While the mixture charge is
becoming weaker, this loss of strength occurs at a rate
lower than that of the reduction of fuel flow. This favorable
tendency continues until a mixture strength known as best
economy is reached. With this air-fuel ratio, the required
hp is developed with the least fuel flow or, to put it another
way, the greatest power produced by a given fuel flow. The
best economy air-fuel ratio varies somewhat with rpm and
other conditions, but for cruise powers on most reciprocating
engines, it is sufficiently accurate to define this range of
operation as being from 0.060 to 0.065 air-fuel ratios on
aircraft where manual leaning is practiced.
Below the best economical mixture strength, power and
temperature continue to fall with constant airflow while
the SFC increases. As the air-fuel ratio is reduced further,
combustion becomes so cool and slow that power for a
given manifold pressure gets so low as to be uneconomical.
The cooling effect of rich or lean mixtures results from the
excess fuel or air over that needed for combustion. Internal
cylinder cooling is obtained from unused fuel when air-fuel
ratios above 0.067 are used. The same function is performed
by excess air when air-fuel ratios below 0.067 are used.
Varying the mixture strength of the charge produces
changes in the engine operating condition affecting power,
temperature, and spark-timing requirements. The best power
air-fuel ratio is desirable when the greatest power from a
given airflow is required. The best economy mixture results
from obtaining the given power output with the least fuel
flow. The air-fuel ratio which gives most efficient operation
varies with engine speed and power output.
In the graph showing this variation in air-fuel ratio, note that
the mixture is rich at both idling and high-speed operation
and is lean through the cruising range. [Figure 2-1] At idling
speed, some air or exhaust gas is drawn into the cylinder
through the exhaust port during valve overlap. The mixture
that enters the cylinder through the intake port must be
rich enough to compensate for this gas or additional air. At
cruising power, lean mixtures save fuel and increase the range
of the airplane. An engine running near full power requires a
rich mixture to prevent overheating and detonation. Since the
engine is operated at full power for only short periods, the SFC = pounds fuel/hour
horsepower
Venturi throatAs air passes through the
throat of the venturi, there
is an increase in velocity
and a drop in pressure.86 4
0 I0VELOCITY
0 I0PRESSURE
0 I0VELOCITY
0 I0PRESSURE
0 I0VELOCITY
0 I0PRESSURE
Figure 2-4. Simple venturi.
Figure 2-5. Side draft horizontal flow carburetor. high fuel consumption is not a serious matter. If an engine
is operating on a mixture that is too lean, and adjustments
are made to increase the amount of fuel, the power output
of the engine increases rapidly at first, then gradually until
maximum power is reached. With a further increase in the
amount of fuel, the power output drops gradually at first, then
more rapidly as the mixture is further enriched.
There are specific instructions concerning mixture ratios
for each type of engine under various operating conditions.
Failure to follow these instructions results in poor
performance and often in damage to the engine. Excessively
rich mixtures result in loss of power and waste of fuel. With
the engine operating near its maximum output, very lean
mixtures cause a loss of power and, under certain conditions,
serious overheating. When the engine is operated on a
lean mixture, the cylinder head temperature gauge should
be watched closely. If the mixture is excessively lean, the
engine may backfire through the induction system or stop
completely. Backfire results from slow burning of the lean
mixture. If the charge is still burning when the intake valve
opens, it ignites the fresh mixture and the flame travels back
through the combustible mixture in the induction system.
Carburetion Principles
Venturi Principles
The carburetor must measure the airflow through the
induction system and use this measurement to regulate
the amount of fuel discharged into the airstream. The air
measuring unit is the venturi, which makes use of a basic law
of physics: as the velocity of a gas or liquid increases, the
pressure decreases. As shown in Figure 2-4, simple venturi
is a passageway or tube in which there is a narrow portion
called the throat. As the velocity of the air increases to get
through the narrow portion, its pressure drops. Note that the
pressure in the throat is lower than that in any other part of
the venturi. This pressure drop is proportional to the velocity and is, therefore, a measure of the airflow. The basic operating
principle of most carburetors depends on the differential
pressure between the inlet and the venturi throat.
Application of Venturi Principle to Carburetor
The carburetor is mounted on the engine so that air to the
cylinders passes through the barrel, the part of the carburetor
which contains the venturi. The size and shape of the venturi
depends on the requirements of the engine for which the
carburetor is designed. A carburetor for a high-powered
engine may have one large venturi or several small ones. The
air may flow either up or down the venturi, depending on the
design of the engine and the carburetor. Those in which the
air passes downward are known as downdraft carburetors,
and those in which the air passes upward are called updraft
carburetors. Some carburetors are made to use a side draft
or horizontal air entry into the engine induction system, as
shown in Figure 2-5 .
Air flows through the induction system covered in Chapter
3. When a piston moves toward the crankshaft (down) on
the intake stroke, the pressure in the cylinder is lowered.
Airflow
Open position
Closed position
Fuel
Fuel level is maintained
by a float-type device.Float chamber
Fuel is received into
the carburetor through
the fuel inlet.Fuel inlet
The mixture needle
controls fuel to the
discharge nozzle.
Mixture needle position
can be adjusted using
the mixture control.Mixture needle
Air enters the carburetor
through the air inlet.Air inlet
The blend of fuel and
air is routed to the
combustion chambers
to be burned. Air-fuel mixture
The flow of the air-fuel
mixture is controlled by
the throttle valve. The
throttle valve is adjusted
from the flight deck by
the throttle.Throttle valve
Fuel is forced through
the discharge nozzle
into the venturi by
greater atmospheric
pressure in the float
chamber.Discharge nozzle
The shape of the venturi
creates an area of low
pressure.Venturi
The air bleed allows air to be mixed
with fuel being drawn out of the
discharge nozzle to decrease fuel
density and promote fuel vaporization.Air bleedFigure 2-6. Wide open throttle position.
Figure 2-7. Fuel discharge. Air rushes through the carburetor and intake manifold to
the cylinder to replace the air displaced by the piston as it
moved down on the intake stroke. Due to this low pressure
area caused by the piston moving down, the higher pressure
air in the atmosphere flows in to fill the low pressure area.
As it does, the airflow must pass through the carburetor
venturi. The throttle valve is located between the venturi
and the engine. Mechanical linkage connects this valve with
the throttle lever in the flight deck. By means of the throttle,
airflow to the cylinders is regulated and controls the power output of the engine. Actually, more air is admitted to the
engine, and the carburetor automatically supplies enough
additional gasoline to maintain the correct air-fuel ratio. This
is because as the volume of airflow increases, the velocity
in the venturi increases, lowering the pressure and allowing
more fuel to be forced into the airstream. The throttle valve
obstructs the passage of air very little when it is parallel with
the flow, in the wide open throttle position. Throttle action
is illustrated in Figure 2-6 . Note how it restricts the airflow
more and more as it rotates toward the closed position.
Metering & Discharge of Fuel
In Figure 2-7, showing the discharge of fuel into the airstream,
locate the inlet through which fuel enters the carburetor from
the engine-driven pump. The float-operated needle valve
regulates the flow through the inlet, which maintains the
correct level in the fuel float chamber. [Figures 2-8 and 2-9]
This level must be slightly below the outlet of the discharge
nozzle to prevent overflow when the engine is not running.
The discharge nozzle is located in the throat of the venturi
at the point where the lowest drop in pressure occurs as
air passes through the carburetor to the engine cylinders.
There are two different pressures acting on the fuel in the
carburetor—a low pressure at the discharge nozzle and a
higher (atmospheric) pressure in the float chamber. The
higher pressure in the float chamber forces the fuel through
Figure 2-8. Needle valve and seat.
Figure 2-9. Float chamber discharge nozzle and float.
the discharge nozzle into the airstream. If the throttle is
opened wider to increase the airflow to the engine, there is
a greater drop in pressure at the venturi throat. Because of
the higher differential pressure, the fuel discharge increases
in proportion to the increase in airflow. If the throttle is
moved toward the “closed” position, the airflow and fuel
flow decrease.
The fuel must pass through the metering jet to reach the
discharge nozzle. [Figure 2-7] A metering jet is really a
certain size hole that the fuel passes through. The size of this
jet determines the rate of fuel discharge at each differential
pressure. If the jet is replaced with a larger one, the fuel flow
increases, resulting in a richer mixture. If a smaller jet is
installed, there is a decrease in fuel flow and a leaner mixture.
Carburetor Systems
To provide for engine operation under various loads and at
different engine speeds, each carburetor has six systems:
1. Main metering,
2. Idling,
3. Accelerating,
4. Mixture control,5. Idle cutoff, and
6. Power enrichment or economizer.
Each of these systems has a definite function. It may act alone
or with one or more of the others.
The main metering system supplies fuel to the engine at all
speeds above idling. The fuel discharged by this system is
determined by the drop in pressure in the venturi throat.
A separate system is necessary for idling because the main
metering system can be erratic at very low engine speeds.
At low speeds the throttle is nearly closed. As a result, the
velocity of the air through the venturi is low and there is little
drop in pressure. Consequently, the differential pressure is not
sufficient to operate the main metering system, and no fuel
is discharged from this system. Therefore, most carburetors
have an idling system to supply fuel to the engine at low
engine speeds.
The accelerating system supplies extra fuel during sudden
increases in engine power. When the throttle is opened, the
airflow through the carburetor increases to obtain more
power from the engine. The main metering system then
increases the fuel discharge. During sudden acceleration,
however, the increase in airflow is so rapid that there is
a slight time lag before the increase in fuel discharge is
sufficient to provide the correct mixture ratio with the new
airflow. By supplying extra fuel during this period, the
accelerating system prevents a temporary leaning out of the
mixture and gives smooth acceleration.
The mixture control system determines the ratio of fuel to
air in the mixture. By means of a flight deck control, the
manual mixture control can select the mixture ratio to suit
operating conditions. In addition to these manual controls,
many carburetors have automatic mixture controls so that
the air-fuel ratio, once it is selected, does not change with
variations in air density. This is necessary because as the
airplane climbs and the atmospheric pressure decreases,
there is a corresponding decrease in the weight of air passing
through the induction system. The volume, however,
remains constant. Since it is the volume of airflow that
determines the pressure drop at the throat of the venturi,
the carburetor tends to meter the same amount of fuel to
this thin air as to the dense air at sea level. Thus, the natural
tendency is for the mixture to become richer as the airplane
gains altitude. The automatic mixture control prevents this
by decreasing the rate of fuel discharge to compensate for
the decrease in air density.
The carburetor has an idle cutoff system so that the fuel can
be shut off to stop the engine. This system, incorporated in
2-8the manual mixture control, stops the fuel discharge from the
carburetor completely when the mixture control lever is set
to the “idle cutoff” position. An aircraft engine is stopped by
shutting off the fuel rather than by turning off the ignition. If
the ignition is turned off with the carburetor still supplying
fuel, fresh air-fuel mixture continues to pass through the
induction system to the cylinders. As the engine is coasting
to a stop and if it is excessively hot, this combustible mixture
may be ignited by local hot spots within the combustion
chambers. This can cause the engine to continue running
or kick backward. Also, the mixture may pass through
the cylinders unburned, but be ignited in the hot exhaust
manifold. Or, the engine comes to an apparently normal stop,
but a combustible mixture remains in the induction passages,
the cylinders, and the exhaust system. This is an unsafe
condition since the engine may kick over after it has been
stopped and seriously injure anyone near the propeller. When
the engine is shut down by means of the idle cutoff system,
the spark plugs continue to ignite the air-fuel mixture until the
fuel discharge from the carburetor ceases. This alone should
prevent the engine from coming to a stop with a combustible
mixture in the cylinders. Some engine manufacturers suggest
that just before the propeller stops turning, the throttle be
opened wide so that the pistons can pump fresh air through
the induction system, the cylinders, and the exhaust system
as an added precaution against accidental kick-over. After
the engine has come to a complete stop, the ignition switch
is turned to the “off” position.
The power enrichment system automatically increases the
richness of the mixture during high power operation. It makes
possible the variation in air-fuel ratio necessary to fit different
operating conditions. Remember that at cruising speeds, a
lean mixture is desirable for economy reasons, while at high
power output, the mixture must be rich to obtain maximum
power and to aid in cooling the engine cylinders. The power
enrichment system automatically brings about the necessary
change in the air-fuel ratio. Essentially, it is a valve that is
closed at cruising speeds and opened to supply extra fuel
to the mixture during high power operation. Although it
increases the fuel flow at high power, the power enrichment
system is actually a fuel saving device. Without this system,
it would be necessary to operate the engine on a rich mixture
over the complete power range. The mixture would then
be richer than necessary at cruising speed to ensure safe
operation at maximum power. The power enrichment system
is sometimes called an economizer or a power compensator.
Although the various systems have been discussed separately,
the carburetor functions as a unit. The fact that one system
is in operation does not necessarily prevent another from
functioning. At the same time that the main metering system
is discharging fuel in proportion to the airflow, the mixture control system determines whether the resultant mixture
is rich or lean. If the throttle is suddenly opened wide, the
accelerating and power enrichment systems act to add fuel to
that already being discharged by the main metering system.
Carburetor Types
There are two types of carburetor used on aircraft with
reciprocating engines—the float-type and the pressure-type.
The float-type carburetor, the most common of the two,
has several distinct disadvantages. The effect that abrupt
maneuvers have on the float action and the fact that its fuel
must be discharged at low pressure leads to incomplete
vaporization and difficulty in discharging fuel into some
types of supercharged systems. The chief disadvantage of the
float carburetor, however, is its icing tendency. Since the float
carburetor must discharge fuel at a point of low pressure, the
discharge nozzle must be located at the venturi throat, and
the throttle valve must be on the engine side of the discharge
nozzle. This means that the drop in temperature due to fuel
vaporization takes place within the venturi. As a result, ice
readily forms in the venturi and on the throttle valve.
A pressure-type carburetor discharges fuel into the airstream
at a pressure well above atmospheric. This results in better
vaporization and permits the discharge of fuel into the
airstream on the engine side of the throttle valve. With the
discharge nozzle located at this point, the drop in temperature
due to fuel vaporization takes place after the air has passed
the throttle valve and at a point where engine heat tends
to offset it. Thus, the danger of fuel vaporization icing is
practically eliminated. The effects of rapid maneuvers and
rough air on the pressure-type carburetors are negligible
since its fuel chambers remain filled under all operating
conditions. Pressure carburetors have been replaced mostly
by fuel injection systems and have limited use on modern
aircraft engines.
Carburetor Icing
There are three general classifications of carburetor icing:
1. Fuel evaporation ice,
2. Throttle ice, and
3. Impact ice.
Fuel evaporation ice or refrigeration ice is formed because of
the decrease in air temperature resulting from the evaporation
of fuel after it is introduced into the airstream. As the fuel
evaporates, the temperature is lowered in the area where
the evaporation takes place. Any moisture in the incoming
air can form ice in this area. It frequently occurs in those
systems in which fuel is injected into the air upstream
from the carburetor throttle, as in the case of float-type
carburetors. It occurs less frequently in systems in which the
From fuel pumpThrottle
Metering jetDischarge nozzle
Main air bleedFloat chamber
Figure 2-10. A float-type carburetor.fuel is injected into the air downstream from the carburetor.
Refrigeration ice can be formed at carburetor air temperatures
as high as 100 °F over a wide range of atmospheric humidity
conditions, even at relative humidity well below 100 percent.
Generally, fuel evaporation ice tends to accumulate on the
fuel distribution nozzle in the carburetor. This type of ice can
lower manifold pressure, interfere with fuel flow, and affect
mixture distribution.
Throttle ice is formed on the rear side of the throttle, usually
when the throttle is in a partially “closed” position. The
rush of air across and around the throttle valve causes a low
pressure on the rear side; this sets up a pressure differential
across the throttle, which has a cooling effect on the air-
fuel charge. Moisture freezes in this low pressure area and
collects as ice on the low pressure side. Throttle ice tends
to accumulate in a restricted passage. The occurrence of a
small amount of ice may cause a relatively large reduction in
airflow and manifold pressure. A large accumulation of ice
may jam the throttles and cause them to become inoperable.
Throttle ice seldom occurs at temperatures above 38 °F.
Impact ice is formed either from water present in the
atmosphere as snow, sleet, or from liquid water which
impinges on surfaces that are at temperatures below 32 °F.
Because of inertia effects, impact ice collects on or near a
surface that changes the direction of the airflow. This type of ice may build up on the carburetor elbow, as well as the carburetor
screen and metering elements. The most dangerous impact ice
is that which collects on the carburetor screen and causes a
very rapid reduction of airflow and power. In general, danger
from impact ice normally exists only when ice forms on the
leading edges of the aircraft structure. Under some conditions,
ice may enter the carburetor in a comparatively dry state and
will not adhere to the inlet screen or walls or affect engine
airflow or manifold pressure. This ice may enter the carburetor
and gradually build up internally in the carburetor air metering
passages and affect carburetor metering characteristics.
Float-Type Carburetors
A float-type carburetor consists essentially of six subsystems
that control the quantity of fuel discharged in relation to the
flow of air delivered to the engine cylinders. These systems
work together to provide the engine with the correct fuel flow
during all engine operating ranges.
The essential subsystems of a float-type carburetor are
illustrated in Figure 2-10 . These systems are:
1. Float chamber mechanism system,
2. Main metering system,
3. Idling system,
4. Mixture control system,
p.s.i.Throttle valve
Main metering jetFuel discharge nozzle
Main air bleedVenturi
Idle feed passage
Airflow
Figure 2-11. Float chamber (bowl) with float removed. Figure 2-12. Main metering system. 5. Accelerating system, and
6. Economizer system.
Float Chamber Mechanism System
A float chamber is provided between the fuel supply and the
main metering system of the carburetor. The float chamber,
or bowl, serves as a reservoir for fuel in the carburetor.
[Figure 2-11] This chamber provides a nearly constant level
of fuel to the main discharge nozzle which is usually about
1⁄8" below the holes in the main discharge nozzle. The fuel
level must be maintained slightly below the discharge nozzle
outlet holes to provide the correct amount of fuel flow and
to prevent fuel leakage from the nozzle when the engine is
not operating.
The level of fuel in the float chamber is kept nearly constant by
means of a float-operated needle valve and a seat. The needle
seat is usually made of bronze. The needle valve is constructed
of hardened steel, or it may have a synthetic rubber section
which fits the seat. With no fuel in the float chamber, the float
drops toward the bottom of the chamber and allows the needle
valve to open wide. As fuel is admitted from the supply line,
the float rises (floats in the fuel) and closes the needle valve
when the fuel reaches a predetermined level. When the engine
is running, and fuel is being drawn out of the float chamber,
the valve assumes an intermediate position so that the valve
opening is just sufficient to supply the required amount of
fuel and keep the level constant. [Figure 2-10] If fuel is found
leaking from the discharge nozzle of the carburetor when the
engine is not running, the most likely cause is that the float
needle valve and seat is leaking and needs to be replaced.
With the fuel at the correct level (float chamber), the
discharge rate is controlled accurately by the air velocity
through the carburetor venturi where a pressure drop at the
discharge nozzle causes fuel to flow into the intake airstream.
Atmospheric pressure on top of the fuel in the float chamber forces the fuel out the discharge nozzle. A vent or small
opening in the top of the float chamber allows air to enter or
leave the chamber as the level of fuel rises or falls.
Main Metering System
The main metering system supplies fuel to the engine at all
speeds above idling and consists of:
1. Venturi,
2. Main metering jet,
3. Main discharge nozzle,
4. Passage leading to the idling system, and
5. Throttle valve.
Since the throttle valve controls the mass airflow through
the carburetor venturi, it must be considered a major unit
in the main metering system as well as in other carburetor
systems. A typical main metering system is illustrated in
Figure 2-12 . The venturi performs three functions:
1. Proportions the air-fuel mixture,
2. Decreases the pressure at the discharge nozzle, and
3. Limits the airflow at full throttle.
The fuel discharge nozzle is located in the carburetor barrel
so that its open end is in the throat or narrowest part of the
A
Suction
1 inch
B
Pin hole
3/4 inch
C
Air
Restriction
Figure 2-13. Air bleed principle.venturi. A main metering orifice, or jet, is placed in the fuel
passage between the float chamber and the discharge nozzle
to limit the fuel flow when the throttle valve is wide open.
When the engine crankshaft is revolved with the carburetor
throttle open, the low pressure created in the intake manifold
acts on the air passing through the carburetor barrel. Due
to the difference in pressure between the atmosphere and
the intake manifold, air flows from the air intake through
the carburetor barrel into the intake manifold. The volume
of airflow depends upon the degree of throttle opening. As
the air flows through the venturi, its velocity increases. This
velocity increase creates a low pressure area in the venturi
throat. The fuel discharge nozzle is exposed to this low
pressure. Since the float chamber is vented to atmospheric
pressure, a pressure drop across the discharge nozzle is
created. It is this pressure difference, or metering force, that
causes fuel to flow from the discharge nozzle. The fuel comes
out of the nozzle in a fine spray, and the tiny particles of fuel
in the spray quickly vaporize in the air.
The metering force (pressure differential) in most carburetors
increases as the throttle opening is increased. The fuel must be
raised in the discharge nozzle to a level at which it discharges
into the airstream. To accomplish this, a pressure differential of
0.5 "Hg is required. When the metering force is considerably
reduced at low engine speeds, the fuel delivery from the
discharge nozzle decreases if an air bleed (air metering jet) is
not incorporated in the carburetor. The decrease in fuel flow
in relation to airflow is due to two factors:
1. The fuel tends to adhere to the walls of the discharge
nozzle and break off intermittently in large drops
instead of forming a fine spray.
2. A part of the metering force is required to raise the
fuel level from the float chamber level to the discharge nozzle outlet.
The basic principle of the air bleed can be explained by
simple diagrams, as shown in Figure 2-13 . In each case, the
same degree of suction is applied to a vertical tube placed in
the container of liquid. As shown in A, the suction applied
on the upper end of the tube is sufficient to lift the liquid a
distance of about 1 inch above the surface. If a small hole is
made in the side of the tube above the surface of the liquid,
as in B, and suction is applied, bubbles of air enter the tube
and the liquid is drawn up in a continuous series of small
slugs or drops. Thus, air “bleeds” into the tube and partially
reduces the forces tending to retard the flow of liquid through
the tube. However, the large opening at the bottom of the
tube effectively prevents any great amount of suction from
being exerted on the air bleed hole or vent. Similarly, an air
bleed hole that is too large in proportion to the size of the
tube would reduce the suction available to lift the liquid. If
the system is modified by placing a metering orifice in the
bottom of the tube and air is taken in below the fuel level by
means of an air bleed tube, a finely divided mixture of air
and liquid is formed in the tube, as shown in C.
In a carburetor, a small air bleed is bled into the fuel nozzle
slightly below the fuel level. The open end of the air bleed is
in the space behind the venturi wall where the air is relatively
motionless and at approximately atmospheric pressure. The
low pressure at the tip of the nozzle not only draws fuel from
the float chamber but also draws air from behind the venturi.
Air bled into the main metering fuel system decreases the
fuel density and destroys surface tension. This results in
better vaporization and control of fuel discharge, especially
at lower engine speeds. The throttle, or butterfly valve, is
located in the carburetor barrel near one end of the venturi.
It provides a means of controlling engine speed or power
output by regulating the airflow to the engine. This valve is
p.s.i.Idle mixture
adjustmentIdling jet
Idle air bleed13.5 p.s.i.11 p.s.i.
M
Full richFigure 2-14. Throttle action in idle position.
Figure 2-15. Idling system.
Figure 2-16. Needle-type mixture control system. a disc that can rotate on an axis, so that it can be turned to
open or close the carburetor air passage.
Idling System
With the throttle valve closed at idling speeds, air velocity
through the venturi is so low that it cannot draw enough fuel
from the main discharge nozzle; in fact, the spray of fuel
may stop altogether. However, low pressure (piston suction)
exists on the engine side of the throttle valve. In order to
allow the engine to idle, a fuel passageway is incorporated to
discharge fuel from an opening in the low pressure area near
the edge of the throttle valve. [Figure 2-14] This opening is
called the idling jet. With the throttle open enough so that the
main discharge nozzle is operating, fuel does not flow out
of the idling jet. As soon as the throttle is closed far enough
to stop the spray from the main discharge nozzle, fuel flows
out the idling jet. A separate air bleed, known as the idle air
bleed, is included as part of the idling system. It functions
in the same manner as the main air bleed. An idle mixture
adjusting device is also incorporated. A typical idling system
is illustrated in Figure 2-15 .
Mixture Control System
As altitude increases, the air becomes less dense. At an
altitude of 18,000 feet, the air is only half as dense as it is
at sea level. This means that a cubic foot of space contains
only half as much air at 18,000 feet as at sea level. An engine
cylinder full of air at 18,000 feet contains only half as much
oxygen as a cylinder full of air at sea level.
The low pressure area created by the venturi is dependent upon
air velocity rather than air density. The action of the venturi
draws the same volume of fuel through the discharge nozzle
at a high altitude as it does at a low altitude. Therefore, the
fuel mixture becomes richer as altitude increases. This can be
overcome either by a manual or an automatic mixture control.On float-type carburetors, two types of purely manual
or flight deck controllable devices are in general use for
controlling air-fuel mixtures, the needle type and the back-
suction type. [Figures 2-16 and 2-17]
With the needle-type system, manual control is provided by a
Full rich
positionBack
suction
lineIdle cutoffIdle cutoff line
Figure 2-17. Back-suction-type mixture control system.
Economizer openingForce
downward
Figure 2-18. Accelerating system.needle valve in the base of the float chamber. [Figure 2-16] This
can be raised or lowered by adjusting a control in the flight
deck. Moving the control to “rich,” opens the needle
valve wide, which permits the fuel to flow unrestricted
to the nozzle. Moving the control to “lean,” partially closes
the valve and restricts the flow of fuel to the nozzle.
The back-suction-type mixture control system is the most
widely used. [Figure 2-17] In this system, a certain amount
of venturi low pressure acts upon the fuel in the float
chamber so that it opposes the low pressure existing at the
main discharge nozzle. An atmospheric line, incorporating
an adjustable valve, opens into the float chamber. When the
valve is completely closed, pressures on the fuel in the float
chamber and at the discharge nozzle are almost equal, and fuel
flow is reduced to maximum lean. With the valve wide open,
pressure on the fuel in the float chamber is greatest and fuel
mixture is richest. Adjusting the valve to positions between
these two extremes controls the mixture. The quadrant in
the flight deck is usually marked “lean” near the back end
and “rich” at the forward end. The extreme back position is
marked “idle cutoff” and is used when stopping the engine.
On float carburetors equipped with needle-type mixture
control, placing the mixture control in idle cutoff seats the
needle valve, thus shutting off fuel flow completely. On
carburetors equipped with back-suction mixture controls, a
separate idle cutoff line, leading to the extreme low pressure
on the engine side of the throttle valve, is incorporated. (See
the dotted line in Figure 2-17. ) The mixture control is so
linked that when it is placed in the “idle cutoff” position,
it opens another passage that leads to piston suction. When
placed in other positions, the valve opens a passage leading to the atmosphere. To stop the engine with such a system, close
the throttle and place the mixture in the “idle cutoff” position.
Leave the throttle in the closed position until the engine has
stopped running and then open the throttle completely.
Accelerating System
When the throttle valve is opened quickly, a large volume
of air rushes through the air passage of the carburetor; the
amount of fuel that is mixed with the air is less than normal
due to the slow response rate of the main metering system.
As a result, after a quick opening of the throttle, the air-fuel
mixture leans out momentarily. This can cause the engine to
accelerate slowly or stumble as it tries to accelerate.
To overcome this tendency, the carburetor is equipped with a
small fuel pump called an accelerating pump. A common type
of accelerating system used in float carburetors is illustrated
in Figure 2-18 . It consists of a simple piston pump operated
through linkage by the throttle control and a passageway
opening into the main metering system or the carburetor
barrel near the venturi. When the throttle is closed, the piston
moves back, and fuel fills the cylinder. If the piston is pushed
forward slowly, the fuel seeps past it back into the float
chamber; if pushed rapidly, it sprays fuel in the venturi and
enriches the mixture. An example of a cutaway accelerator
pump is shown in Figure 2-19 .
Economizer System
For an engine to develop maximum power at full throttle, the
fuel mixture must be richer than for cruise. The additional
fuel is used for cooling the engine combustion chambers to
prevent detonation. An economizer is essentially a valve
that is closed at throttle settings below approximately 60–70
Economizer metering jet
Main air bleedEconomizer
needle
Main discharge nozzleMain metering jet
Fulcrum
Economizer opening automatically
controlled by supercharger pressureTo manifold
pressure
Dashpot
Evacuated
bellows (force
downward)Compression
spring (force
upward)Figure 2-19. Accelerating pump shown in cutaway.
Figure 2-21. A pressure operated economizer system. Figure 2-20. A needle-valve type economizer system. percent of rated power. This system, like the accelerating
system, is operated by the throttle control.
A typical economizer system consists of a needle valve
which begins to open when the throttle valve reaches
a predetermined point near the wide-open position.
[Figure 2-20] As the throttle continues to open, the needle
valve is opened further and additional fuel flows through
it. This additional fuel supplements the flow from the main
metering jet direct to the main discharge nozzle.
A pressure-operated economizer system is shown in
Figure 2-21 . This type has a sealed bellows located in an
enclosed compartment. The compartment is vented to engine
manifold pressure. When the manifold pressure reaches a certain value, the bellows is compressed and opens a valve in
a carburetor fuel passage, supplementing the normal quantity
of fuel being discharged through the main nozzle.
Another type of economizer is the back-suction system.
[Figure 2-22] Fuel economy in cruising is provided by
reducing the effective pressure acting on the fuel level in
the float compartment. With the throttle valve in cruising
position, suction is applied to the float chamber through an
economizer hole and back-suction economizer channel and
jet. The suction applied to the float chamber opposes the
nozzle suction applied by the venturi. Fuel flow is reduced,
leaning the mixture for cruising economy.
Another type of mixture control system uses a metering valve
that is free to rotate in a stationary metering sleeve. Fuel
enters the main and idling systems through a slot cut in the
mixture sleeve. Fuel metering is accomplished by the relative
position between one edge of the slot in the hollow metering
valve and one edge of the slot in the metering sleeve. Moving
the mixture control to reduce the size of the slot provides a
leaner mixture for altitude compensation.
Pressure Injection Carburetors
Pressure injection carburetors are distinctly different from
float-type carburetors as they do not incorporate a vented
float chamber or suction pickup from a discharge nozzle
located in the venturi tube. Instead, they provide a pressurized
fuel system that is closed from the engine fuel pump to the
discharge nozzle. The venturi serves only to create pressure
differentials for controlling the quantity of fuel to the
metering jet in proportion to airflow to the engine.
Back suction
economizer
channel and jet
Main discharge nozzleVent
Main metering jetAirMain air bleed
Figure 2-22. Float-type carburetor.
Discharge nozzle Fuel intake lineFuel valveDiaphragm
Figure 2-23. Pressure-type carburetor.
Typical Injection Carburetor
The injection carburetor is a hydromechanical device
employing a closed feed system from the fuel pump to the
discharge nozzle. It meters fuel through fixed jets according
to the mass airflow through the throttle body and discharges
it under a positive pressure.
The illustration in Figure 2-23 represents a pressure-type
carburetor simplified so that only the basic parts are shown.
Note the two small passages, one leading from the carburetor
air inlet to the left side of the flexible diaphragm and the other
from the venturi throat to the right side of the diaphragm.
When air passes through the carburetor to the engine, the
pressure on the right of the diaphragm is lowered because
of the drop in pressure at the venturi throat. As a result, the
diaphragm moves to the right, opening the fuel valve. Pressure
from the engine-driven pump then forces fuel through the
open valve to the discharge nozzle, where it sprays into the
airstream. The distance the fuel valve opens is determined
by the difference between the two pressures acting on the
diaphragm. This difference in pressure is proportional to the
airflow through the carburetor. Thus, the volume of airflow
determines the rate of fuel discharge.
The pressure injection carburetor is an assembly of the
following units:
1. Throttle body,
2. Automatic mixture control,
3. Regulator unit, and4. Fuel control unit (some are equipped with an adapter).
Throttle Body
The throttle body contains the throttle valves, main venturi,
boost venturi, and the impact tubes. All air entering the
cylinders must flow through the throttle body; therefore, it is
the air control and measuring device. The airflow is measured
by volume and by weight so that the proper amount of fuel can
be added to meet the engine demands under all conditions.
As air flows through the venturi, its velocity is increased,
and its pressure is decreased (Bernoulli’s principle). This
low pressure is vented to the low pressure side of the
air diaphragm [Figure 2-24 chamber B] in the regulator
assembly. The impact tubes sense carburetor inlet air pressure
and direct it to the automatic mixture control, which measures
the air density. From the automatic mixture control, the air
is directed to the high pressure side of the air diaphragm
(chamber A). The pressure differential of the two chambers
acting upon the air diaphragm is known as the air metering
force which opens the fuel poppet valve.
The throttle body controls the airflow with the throttle valves.
The throttle valves may be either rectangular or disc shaped,
depending on the design of the carburetor. The valves are
mounted on a shaft, which is connected by linkage to the
idle valve and to the throttle control in the flight deck. A
throttle stop limits the travel of the throttle valve and has an
adjustment which sets engine idle speed.
Regulator Unit
The regulator is a diaphragm-controlled unit divided into
five chambers and contains two regulating diaphragms
and a poppet valve assembly. [Figure 2-24] Chamber A is
Idle
spring
Air
diaphragm
Fuel pressure
gauge connectionFuel
strainerVent chamber
Vapor
separator
Balance
diaphragm
Sealing
diaphragm
Fuel inletPoppet
valve and
diaphram
Duel diaphragmMixture control bleedsABCD
E
Figure 2-24. Regulator unit.
regulated air-inlet pressure from the air intake. Chamber B
is boost venturi pressure. Chamber C contains metered fuel
pressure controlled by the discharge nozzle or fuel feed valve.
Chamber D contains unmetered fuel pressure controlled by
the opening of the poppet valve. Chamber E is fuel pump
pressure controlled by the fuel pump pressure relief valve.
The poppet valve assembly is connected by a stem to the two
main control diaphragms. The purpose of the regulator unit is
to regulate the fuel pressure to the inlet side of the metering
jets in the fuel control unit. This pressure is automatically
regulated according to the mass airflow to the engine.
The carburetor fuel strainer (also called the gascolator),
located in the inlet to chamber E, is a fine mesh screen through
which all the fuel must pass as it enters chamber D. The
strainer must be removed and cleaned at scheduled intervals.
Referring to Figure 2-24 , assume that for a given airflow
in lb/hr through the throttle body and venturi, a negative
pressure of 1⁄4 psi is established in chamber B. This tends
to move the diaphragm assembly and the poppet valve in a
direction to open the poppet valve permitting more fuel to
enter chamber D. The pressure in chamber C is held constant
at 5 psi (10 psi on some installations) by the discharge
nozzle or impeller fuel feed valve. Therefore, the diaphragm
assembly and poppet valve moves in the open direction until
the pressure in chamber D is 51⁄4 psi. Under these pressures,
there is a balanced condition of the diaphragm assembly with
a pressure drop of 1⁄4 psi across the jets in the fuel control unit
(auto-rich or auto-lean).If nozzle pressure (chamber C pressure) rises to 51⁄2 psi, the
diaphragm assembly balance is upset, and the diaphragm
assembly moves to open the poppet valve to establish
the necessary 53⁄4 psi pressure in chamber D. Thus, the 1⁄4
psi differential between chamber C and chamber D is re-
established, and the pressure drop across the metering jets
remains the same.
If the fuel inlet pressure is increased or decreased, the fuel
flow into chamber D tends to increase or decrease with
the pressure change causing the chamber D pressure to do
likewise. This upsets the balanced condition previously
established, and the poppet valve and diaphragm assembly
respond by moving to increase or decrease the flow to re-
establish the pressure at the 1⁄4 psi differential.
The fuel flow changes when the mixture control plates
are moved from auto-lean to auto-rich, thereby selecting
a different set of jets or cutting one or two in or out of the
system. When the mixture position is altered, the diaphragm
and poppet valve assembly repositions to maintain the
established pressure differential of 1⁄4 psi between chambers
C and D, maintaining the established differential across the
jets. Under low power settings (low airflows), the difference
in pressure created by the boost venturi is not sufficient to
accomplish consistent regulation of the fuel. Therefore, an idle
spring, shown in Figure 2-24 , is incorporated in the regulator.
As the poppet valve moves toward the closed position, it
contacts the idle spring. The spring holds the poppet valve
off its seat far enough to provide more fuel than is needed
for idling. This potentially overrich mixture is regulated by
the idle valve. At idling speed, the idle valve restricts the fuel
flow to the proper amount. At higher speeds, it is withdrawn
from the fuel passage and has no metering effect.
Vapor vent systems are provided in these carburetors to
eliminate fuel vapor created by the fuel pump, heat in the
engine compartment, and the pressure drop across the poppet
valve. The vapor vent is located in the fuel inlet (chamber E)
or, on some models of carburetors, in both chambers D and E.
The vapor vent system operates in the following way. When
air enters the chamber in which the vapor vent is installed,
the air rises to the top of the chamber, displacing the fuel
and lowering its level. When the fuel level has reached a
predetermined position, the float (which floats in the fuel)
pulls the vapor vent valve off its seat, permitting the vapor
in the chamber to escape through the vapor vent seat, its
connecting line, and back to the fuel tank.
If the vapor vent valve sticks in a closed position or the vent
line from the vapor vent to the fuel tank becomes clogged, the
vapor-eliminating action is stopped. This causes the vapor to
Auto lean jet
Power enrichment valveAuto rich jet
Power enrichment jetManual mixture control
Regulator fill valveIdle needle valve
Figure 2-25. Fuel control unit.build up within the carburetor to the extent that vapor passes
through the metering jets with the fuel. With a given size
carburetor metering jet, the metering of vapor reduces the
quantity of fuel metered. This causes the air-fuel mixture to
lean out, usually intermittently.
If the vapor vent valve sticks open or the vapor vent float
becomes filled with fuel and sinks, a continuous flow of
fuel and vapor occurs through the vent line. It is important
to detect this condition, as the fuel flow from the carburetor
to the fuel supply tank may cause an overflowing tank with
resultant increased fuel consumption.
To check the vent system, disconnect the vapor vent line
where it attaches to the carburetor, and turn the fuel booster
pump on while observing the vapor vent connection at the
carburetor. Move the carburetor mixture control to auto-rich;
then return it to idle cutoff. When the fuel booster pump is
turned on, there should be an initial ejection of fuel and air
followed by a cutoff with not more than a steady drip from
the vent connection. Installations with a fixed bleed from the
D chamber connected to the vapor vent in the fuel inlet by a
short external line should show an initial ejection of fuel and
air followed by a continuing small stream of fuel. If there
is no flow, the valve is sticking closed; if there is a steady
flow, it is sticking open.
Fuel Control Unit
The fuel control unit is attached to the regulator assembly
and contains all metering jets and valves. [Figure 2-25] The
idle and power enrichment valves, together with the mixture
control plates, select the jet combinations for the various
settings (i.e., auto-rich, auto-lean, and idle cutoff).
The purpose of the fuel control unit is to meter and control
the fuel flow to the discharge nozzle. The basic unit consists
of three jets and four valves arranged in series, parallel, and
series-parallel hookups. [Figure 2-25] These jets and valves
receive fuel under pressure from the regulator unit and then
meter the fuel as it flows to the discharge nozzle. The manual
mixture control valve controls the fuel flow. By using proper
size jets and regulating the pressure differential across the
jets, the right amount of fuel is delivered to the discharge
nozzle, giving the desired air-fuel ratio in the various power
settings. It should be remembered that the inlet pressure to the
jets is regulated by the regulator unit and the outlet pressure
is controlled by the discharge nozzle.
The jets in the basic fuel control unit are the auto-lean jet, the
auto-rich jet, and power enrichment jet. The basic fuel flow
is the fuel required to run the engine with a lean mixture and
is metered by the auto-lean jet. The auto-rich jet adds enough
fuel to the basic flow to give a slightly richer mixture than best power mixture when the manual mixture control is in
the auto-rich position.
The four valves in the basic fuel control unit are:
1. Idle needle valve,
2. Power enrichment valve,
3. Regulator fill valve, and
4. Manual mixture control.
The functions of these valves are:
1. The idle needle valve meters the fuel in the idle
range only. It is a round, contoured needle valve, or a
cylinder valve placed in series with all other metering
devices of the basic fuel control unit. The idle needle
valve is connected by linkage to the throttle shaft so
that it restricts the fuel flowing at low power settings
(idle range).
2. The manual mixture control is a rotary disc valve
consisting of a round stationary disc with ports leading
from the auto-lean jet, the auto-rich jet, and two
smaller ventholes. Another rotating part, resembling
a cloverleaf, is held against the stationary disc by
spring tension and rotated over the ports in that disc
by the manual mixture control lever. All ports and
vents are closed in the idle cutoff position. In the auto-
lean position, the ports from the auto-lean jet and the
two ventholes are open. The port from the auto-rich
jet remains closed in this position. In the auto-rich
position, all ports are open. The valve plate positions
Full rich Auto-lean Auto-rich Idle cutoffVent channel from chamber D
Vent for space above valveChannel from auto-lean jetChannel from auto-rich jet
Figure 2-26. Manual mixture control valve plate positions.
are illustrated in Figure 2-26 . The three positions of
the manual mixture control lever make it possible to
select a lean mixture a rich mixture, or to stop fuel flow
entirely. The idle cutoff position is used for starting or
stopping the engine. During starting, fuel is supplied
by the primer.
3. The regulator fill valve is a small poppet-type valve
located in a fuel passage which supplies chamber C
of the regulator unit with metered fuel pressure. In
idle cutoff, the flat portion of the cam lines up with
the valve stem, and a spring closes the valve. This
provides a means of shutting off the fuel flow to
chamber C and thus provides for a positive idle cutoff.
4. The power enrichment valve is another poppet-type
valve. It is in parallel with the auto-lean and auto-rich
jets, but it is in series with the power enrichment jet.
This valve starts to open at the beginning of the power
range. It is opened by the unmetered fuel pressure
overcoming metered fuel pressure and spring tension.
The power enrichment valve continues to open wider
during the power range until the combined flow
through the valve and the auto-rich jet exceeds that
of the power enrichment jet. At this point the power
enrichment jet takes over the metering and meters fuel
throughout the power range.
5. Carburetors equipped for water injection are modified
by the addition of a derichment valve and a derichment
jet. The derichment valve and derichment jet are in
series with each other and parallel with the power
enrichment jet.
The carburetor controls fuel flow by varying two basic
factors. The fuel control unit, acting as a pressure-reducing
valve, determines the metering pressure in response to the metering forces. The regulator unit, in effect, varies the size
of the orifice through which the metering pressure forces
the fuel. It is a basic law of hydraulics that the amount of
fluid that passes through an orifice varies with the size of the
orifice and the pressure drop across it. The internal automatic
devices and mixture control act together to determine the
effective size of the metering passage through which the fuel
passes. The internal devices, fixed jets, and variable power
enrichment valve are not subject to direct external control.
Automatic Mixture Control (AMC)
The automatic mixture control unit consists of a bellows
assembly, calibrated needle, and seat. [Figure 2-27] The
purpose of the automatic mixture control is to compensate for
changes in air density due to temperature and altitude changes.
The automatic mixture control contains a metallic bellows,
which is sealed at 28 "Hg absolute pressure. The bellows
responds to changes in pressure and temperature. In the
illustration, the automatic mixture control is located at the
carburetor air inlet. As the density of the air changes, the
expansion and contraction of the bellows moves the tapered
needle in the atmospheric line. At sea level, the bellows
is contracted, and the needle is not in the atmospheric
passage. As the aircraft climbs and the atmospheric pressure
decreases, the bellows expands, inserting the tapered
needle farther and farther into the atmospheric passage
and restricting the flow of air to chamber A of the regulator
unit. [Figure 2-24] At the same time, air leaks slowly from
chamber A to chamber B through the small bleed (often
referred to as the back-suction bleed or mixture control
bleed). The rate at which air leaks through this bleed is
about the same at high altitude as it is at sea level. As the
tapered needle restricts the flow of air into chamber A, the
pressure on the left side of the air diaphragm decreases. As
Throttle valveBoost venturi
Impact tubes
Automatic mixture control
Main venturiIntake air Intake air
Figure 2-27. Automatic mixture control and throttle body.
Throttle valve
VenturiDischarge air bleed
Impact air (chamber A) Unmetered fuel Metered fuel
Inlet fuel pressure Venturi suction (chamber B) Pressure above throttleIntake airVapor vent
Poppet valve
Fuel pressure
connection
Idle cutoff cam
Manual mixture control needle valveFuel inlet
Idle cutoff plunger
Impact airVenturi
suctionFuel strainer
Venturi drainDischarge nozzleDischarge nozzle
needle valve
Main metering jet
Accelerating pump
suction channel
Accelerating pump
Relief valve
Manual idle control rod
Idle needle valve
Vacuum channel reducerAA B
DC
Figure 2-28. Schematic of the PS series carburetor.a result, the poppet valve moves toward its seat, reducing
the fuel flow to compensate for the decrease in air density. The automatic mixture control can be removed and cleaned
if the lead seal at the point of adjustment is not disturbed.
Stromberg PS Carburetor
The PS series carburetor is a low-pressure, single-barrel,
injection-type carburetor. The carburetor consists basically
of the air section, the fuel section, and the discharge nozzle
mounted together to form a complete fuel metering system.
This carburetor is similar to the pressure-injection carburetor;
therefore, its operating principles are the same.
In this type carburetor, metering is accomplished on a mass
airflow basis. [Figure 2-28] Air flowing through the main
venturi creates suction at the throat of the venturi, which
is transmitted to the B chamber in the main regulating part
of the carburetor and to the vent side of the fuel discharge
nozzle diaphragm. The incoming air pressure is transmitted
to a chamber A of the regulating part of the carburetor and
to the main discharge bleed in the main fuel discharge jet.
The discharge nozzle consists of a spring-loaded diaphragm
connected to the discharge nozzle valve, which controls the
Enrichment valve and seat assembly
Metering jet
Unmetered fuel chamber D pressureMetered fuel chamber C pressure
Power enrichment valve adjustment screw
Venturi suction chamber B pressure
Figure 2-29. Airflow power enrichment valve.flow of fuel injected into the main discharge jet. Here, it is
mixed with air to accomplish distribution and atomization
into the airstream entering the engine.
In the PS series carburetor, as in the pressure-injection
carburetor, the regulator spring has a fixed tension, which
tends to hold the poppet valve open during idling speeds
or until the D chamber pressure equals approximately 4
psi. The discharge nozzle spring has a variable adjustment
which, when tailored to maintain 4 psi, results in a balanced
pressure condition of 4 psi in chamber C of the discharge
nozzle assembly and 4 psi in chamber D. This produces a
zero drop across the main jets at zero fuel flow.
At a given airflow, if the suction created by the venturi is
equivalent to 1⁄4 pound, the pressure decrease is transmitted
to chamber B and to the vent side of the discharge nozzle.
Since the area of the air diaphragm between chambers A
and B is twice as great as that between chambers B and D,
the 1⁄4 pound decrease in pressure in chamber B moves the
diaphragm assembly to the right to open the poppet valve.
Meanwhile, the decreased pressure on the vent side of the
discharge nozzle assembly causes a lowering of the total
pressure from 4 pounds to 33⁄4 pounds. The greater pressure
of the metered fuel (41⁄4 pounds) results in a differential across
the metering head of 1⁄4 pound (for the 1⁄4 pound pressure
differential created by the venturi).
The same ratio of pressure drop across the jet to venturi
suction applies throughout the range. Any increase or
decrease in fuel inlet pressure tends to upset the balance in
the various chambers in the manner already described. When
this occurs, the main fuel regulator diaphragm assembly
repositions to restore the balance.The mixture control, whether operated manually or
automatically, compensates for enrichment at altitude
by bleeding impact air pressure into chamber B, thereby
increasing the pressure (decreasing the suction) in chamber
B. Increasing the pressure in chamber B tends to move the
diaphragm and poppet valve more toward the closed position,
restricting fuel flow to correspond proportionately to the
decrease in air density at altitude.
The idle valve and economizer jet can be combined in one
assembly. The unit is controlled manually by the movement
of the valve assembly. At low airflow positions, the tapered
section of the valve becomes the predominant jet in the
system, controlling the fuel flow for the idle range. As the
valve moves to the cruise position, a straight section on the
valve establishes a fixed orifice effect which controls the
cruise mixture. When the valve is pulled full-open by the
throttle valve, the jet is pulled completely out of the seat, and
the seat side becomes the controlling jet. This jet is calibrated
for takeoff power mixtures.
An airflow-controlled power enrichment valve can also be
used with this carburetor. It consists of a spring-loaded,
diaphragm-operated metering valve. Refer to Figure 2-29
for a schematic view of an airflow power enrichment valve.
One side of the diaphragm is exposed to unmetered fuel
pressure and the other side to venturi suction plus spring
tension. When the pressure differential across the diaphragm
establishes a force strong enough to compress the spring, the
valve opens and supplies an additional amount of fuel to the
metered fuel circuit in addition to the fuel supplied by the
main metering jet.
Figure 2-30. Cutaway view of airflow measuring section.Accelerating Pump
The accelerating pump of the Stromberg PS carburetor is a
spring-loaded diaphragm assembly located in the metered
fuel channel with the opposite side of the diaphragm vented
to the engine side of the throttle valve. With this arrangement,
opening the throttle results in a rapid decrease in suction. This
decrease in suction permits the spring to extend and move
the accelerating pump diaphragm. The diaphragm and spring
action displace the fuel in the accelerating pump and force it
out the discharge nozzle.
Vapor is eliminated from the top of the main fuel chamber
D through a bleed hole, then through a vent line back to the
main fuel tank in the aircraft.
Manual Mixture Control
A manual mixture control provides a means of correcting
for enrichment at altitude. It consists of a needle valve and
seat that form an adjustable bleed between chamber A and
chamber B. The valve can be adjusted to bleed off the venturi
suction to maintain the correct air-fuel ratio as the aircraft
gains altitude.
When the mixture control lever is moved to the idle cutoff
position, a cam on the linkage actuates a rocker arm which
moves the idle cutoff plunger inward against the release
lever in chamber A. The lever compresses the regulator
diaphragm spring to relieve all tension on the diaphragm
between chambers A and B. This permits fuel pressure plus
poppet valve spring force to close the poppet valve, stopping
the fuel flow. Placing the mixture control lever in idle cutoff
also positions the mixture control needle valve off its seat and
allows metering suction within the carburetor to bleed off.
Fuel-Injection Systems
The fuel-injection system has many advantages over a
conventional carburetor system. There is less danger of
induction system icing, since the drop in temperature due
to fuel vaporization takes place in or near the cylinder.
Acceleration is also improved because of the positive action
of the injection system. In addition, fuel injection improves
fuel distribution. This reduces the overheating of individual
cylinders often caused by variation in mixture due to uneven
distribution. The fuel-injection system also gives better fuel
economy than a system in which the mixture to most cylinders
must be richer than necessary so that the cylinder with the
leanest mixture operates properly.
Fuel-injection systems vary in their details of construction,
arrangement, and operation. The Bendix and Continental
fuel-injection systems are discussed in this section. They
are described to provide an understanding of the operating
principles involved. For the specific details of any one system, consult the manufacturer’s instructions for the
equipment involved.
Bendix/Precision Fuel-Injection System
The Bendix inline stem-type regulator injection system
(RSA) series consists of an injector, flow divider, and fuel
discharge nozzle. It is a continuous-flow system which
measures engine air consumption and uses airflow forces to
control fuel flow to the engine. The fuel distribution system
to the individual cylinders is obtained by the use of a fuel
flow divider and air bleed nozzles.
Fuel Injector
The fuel injector assembly consists of:
1. An airflow section,
2. A regulator section, and
3. A fuel metering section. Some fuel injectors are
equipped with an automatic mixture control unit.
Airflow Section
The airflow consumption of the engine is measured by
sensing impact pressure and venturi throat pressure in the
throttle body. These pressures are vented to the two sides of
an air diaphragm. A cutaway view of the airflow measuring
section is shown in Figure 2-30 . Movement of the throttle
valve causes a change in engine air consumption. This
results in a change in the air velocity in the venturi. When
airflow through the engine increases, the pressure on the left
of the diaphragm is lowered due to the drop in pressure at
the venturi throat. [Figure 2-31] As a result, the diaphragm
moves to the left, opening the ball valve. Contributing to this
force is the impact pressure that is picked up by the impact
tubes. [Figure 2-32] This pressure differential is referred
to as the “air metering force.” This force is accomplished
by channeling the impact and venturi suction pressures to
opposite sides of a diaphragm. The difference between these
Air inletConstant head idle spring
Ball valve
Air diaphragmFuel diaphragm
Impact tubeVenturi suction
Inlet air pressure
Fuel inlet pressure
Metered fuel pressure
Line to flow dividerThrottle valveMetering fuel pressure Unmetered fuel pressure
Venturi
Figure 2-31. Airflow section of a fuel injector.
Figure 2-32. Impact tubes for inlet air pressure. Figure 2-33. Fuel diaphragm with ball valve attached.two pressures becomes a usable force that is equal to the area
of the diaphragm times the pressure difference.
Regulator Section
The regulator section consists of a fuel diaphragm that
opposes the air metering force. Fuel inlet pressure is applied
to one side of the fuel diaphragm and metered fuel pressure
is applied to the other side. The differential pressure across
the fuel diaphragm is called the fuel metering force. The fuel
pressure shown on the ball side of the fuel diaphragm is the
pressure after the fuel has passed through the fuel strainer
and the manual mixture control rotary plate and is referred to
as metered fuel pressure. Fuel inlet pressure is applied to the opposite side of the fuel diaphragm. The ball valve attached
to the fuel diaphragm controls the orifice opening and fuel
flow through the forces placed on it. [Figure 2-33]
The distance the ball valve opens is determined by the
difference between the pressures acting on the diaphragms.
This difference in pressure is proportional to the airflow
through the injector. Thus, the volume of airflow determines
the rate of fuel flow.
Under low power settings, the difference in pressure created
by the venturi is insufficient to accomplish consistent
regulation of the fuel. A constant-head idle spring is
Metered fuel pressure
Unmetered fuel pressure
Idle valve lever connected to throttle lever linkage Manual mixture control and idle cut off leverFuel inlet pressure
Metered fuel pressure
Fuel inletFuel strainer
Metering jet
Figure 2-34. Fuel metering section of the injector.
Figure 2-35. Fuel inlet and metering.incorporated to provide a constant fuel differential pressure.
This allows an adequate final flow in the idle range.
Fuel Metering Section
The fuel metering section is attached to the air metering
section and contains an inlet fuel strainer, a manual mixture
control valve, an idle valve, and the main metering jet.
[Figure 2-34] The idle valve is connected to the throttle valve
by means of an external adjustable link. In some injector
models, a power enrichment jet is also located in this section.
The purpose of the fuel metering section is to meter and
control the fuel flow to the flow divider. [Figure 2-35] The
manual mixture control valve produces full rich condition
when the lever is against the rich stop, and a progressively
leaner mixture as the lever is moved toward idle cutoff. Both
idle speed and idle mixture may be adjusted externally to
meet individual engine requirements.
Flow Divider
The metered fuel is delivered from the fuel control unit to a
pressurized flow divider. This unit keeps metered fuel under
pressure, divides fuel to the various cylinders at all engine
speeds, and shuts off the individual nozzle lines when the
control is placed in idle cutoff.
Referring to the diagram in Figure 2-36 , metered fuel pressure
enters the flow divider through a channel that permits fuel to
pass through the inside diameter of the flow divider needle.
At idle speed, the fuel pressure from the regulator must build
up to overcome the spring force applied to the diaphragm and valve assembly. This moves the valve upward until fuel can
pass out through the annulus of the valve to the fuel nozzle.
[Figure 2-37] Since the regulator meters and delivers a fixed
amount of fuel to the flow divider, the valve opens only as
far as necessary to pass this amount to the nozzles. At idle,
the opening required is very small; the fuel for the individual
cylinders is divided at idle by the flow divider.
As fuel flow through the regulator is increased above idle
requirements, fuel pressure builds up in the nozzle lines.
This pressure fully opens the flow divider valve, and
fuel distribution to the engine becomes a function of the
discharge nozzles.
Nozzle pressure or lb/hr fuel flow (gauge)Fuel nozzle (one per cylinder)Flow divider
Nozzle discharge pressure
Metered fuel pressureAmbient air pressure
Figure 2-38. Fuel nozzle assembly.Figure 2-36. Flow divider.
Figure 2-37. Flow divider cutaway.A fuel pressure gauge, calibrated in pounds per hour fuel
flow, can be used as a fuel flow meter with the Bendix RSA
injection system. This gauge is connected to the flow divider and senses the pressure being applied to the discharge nozzle. This pressure is in direct proportion to the fuel flow and indicates the engine power output and fuel consumption.
Fuel Discharge Nozzles
The fuel discharge nozzles are of the air bleed configuration.
There is one nozzle for each cylinder located in the cylinder head. [Figure 2-38] The nozzle outlet is directed into the
intake port. Each nozzle incorporates a calibrated jet. The jet size is determined by the available fuel inlet pressure and the maximum fuel flow required by the engine. The fuel is discharged through this jet into an ambient air pressure chamber within the nozzle assembly. Before entering the individual intake valve chambers, the fuel is mixed with air to aid in atomizing the fuel. Fuel pressure, before the individual nozzles, is in direct proportion to fuel flow; therefore, a simple pressure gauge can be calibrated in fuel flow in gallons
per hour and be employed as a flow meter. Engines modified with turbosuperchargers must use shrouded nozzles. By the use of an air manifold, these nozzles are vented to the injector air inlet pressure.
Continental/TCM Fuel-Injection System
The Continental fuel-injection system injects fuel into the
intake valve port in each cylinder head. [Figure 2-39] The
40 Psi
0 50Psi
Fuel control
Fuel injectors
Unmetered fuel pressureThrottle bodyIdle speed stop screwMetered fuel pressureManifold valve assembly
Idle mixture adjustTo aircraft gauge
Fuel pump assemblyVapor return
Fuel returnFuel inlet
from fuel tank
DrainAdjustable orificeLow pressure
relief valve
BypassInlet fuel from ACFT
Unmetered fuel pressure
Metered fuel pressure
Return fuel from fuel control
Nozzle pressure
Vapor return
Figure 2-39. Continental/TCM Fuel-Injection System. means that changes in engine speed affect total pump flow
proportionally. Since the pump provides greater capacity than
is required by the engine, a recirculation path is required. By
arranging a calibrated orifice and relief valve in this path, the
pump delivery pressure is also maintained in proportion to
engine speed. These provisions assure proper pump pressure
and fuel delivery for all engine operating speeds.
A check valve is provided so that boost pump pressure to the
system can bypass the engine-driven pump for starting. This
feature also suppresses vapor formation under high ambient
temperatures of the fuel and permits use of the auxiliary
pump as a source of fuel pressure in the event of engine-
driven pump failure.
Air-Fuel Control Unit
The function of the air-fuel control assembly is to control
engine air intake and to set the metered fuel pressure for
proper air-fuel ratio. The air throttle is mounted at the
manifold inlet and its butterfly valve, positioned by the
throttle control in the aircraft, controls the flow of air to the
engine. [Figure 2-42]system consists of a fuel injector pump, a control unit, a fuel
manifold, and a fuel discharge nozzle. It is a continuous-flow
type, which controls fuel flow to match engine airflow. The
continuous-flow system permits the use of a rotary vane pump
which does not require timing to the engine.
Fuel-Injection Pump
The fuel pump is a positive-displacement, rotary-vane type with
a splined shaft for connection to the accessory drive system
of the engine. [Figure 2-40] A spring-loaded, diaphragm-type
relief valve is provided. The relief valve diaphragm chamber
is vented to atmospheric pressure. A sectional view of a fuel-
injection pump is shown in Figure 2-41 .
Fuel enters at the swirl well of the vapor separator. Here,
vapor is separated by a swirling motion so that only liquid
fuel is delivered to the pump. The vapor is drawn from the
top center of the swirl well by a small pressure jet of fuel
and is directed into the vapor return line. This line carries
the vapor back to the fuel tank.
Ignoring the effect of altitude or ambient air conditions,
the use of a positive-displacement, engine-driven pump
Fuel pump inletVapor return
Low pressure relief valve
Mixture control
Adjustable orificeEngine mount flange
Dry bay inspection drain
Fuel pump outlet
Drive shaft
Vapor separatorInlet
OutletVapor ejector
Pump assembly
Orifice
Relief valve
assemblyFigure 2-40. Fuel pump.
Figure 2-41. Fuel injection pump. The air throttle assembly is an aluminum casting which
contains the shaft and butterfly-valve assembly. The casting
bore size is tailored to the engine size, and no venturi or other
restriction is used.
Fuel Control Assembly
The fuel control body is made of bronze for best bearing
action with the stainless steel valves. Its central bore contains
a metering valve at one end and a mixture control valve at
the other end. Each stainless steel rotary valve includes a
groove which forms a fuel chamber.Fuel enters the control unit through a strainer and passes to the
metering valve. [Figure 2-43] This rotary valve has a cam-
shaped edge on the outer part of the end face. The position
of the cam at the fuel delivery port controls the fuel passed
to the manifold valve and the nozzles. The fuel return port
connects to the return passage of the center metering plug.
The alignment of the mixture control valve with this passage
determines the amount of fuel returned to the fuel pump.
By connecting the metering valve to the air throttle, the fuel
flow is properly proportioned to airflow for the correct air-
fuel ratio. A control level is mounted on the mixture control
valve shaft and connected to the flight deck mixture control.
Fuel Manifold Valve
The fuel manifold valve contains a fuel inlet, a diaphragm
chamber, and outlet ports for the lines to the individual
nozzles. [Figure 2-44] The spring-loaded diaphragm operates
a valve in the central bore of the body. Fuel pressure provides
the force for moving the diaphragm. The diaphragm is
enclosed by a cover that retains the diaphragm loading spring.
When the valve is down against the lapped seat in the body,
the fuel lines to the cylinders are closed off. The valve is
drilled for passage of fuel from the diaphragm chamber to
its base, and a ball valve is installed within the valve. All
incoming fuel must pass through a fine screen installed in
the diaphragm chamber.
Intake air
Intake air
Part throttle position
Full throttle position
Figure 2-42. Fuel air control unit.
Fuel inlet filter screenFuel metering plug
Throttle metering shaft Mixture control shaftFuel inlet from fuel pump
To fuel manifold valve Fuel return to fuel pump
Figure 2-43. Dual fuel control assembly.From the fuel-injection control valve, fuel is delivered to
the fuel manifold valve, which provides a central point for
dividing fuel flow to the individual cylinders. In the fuel
manifold valve, a diaphragm raises or lowers a plunger
valve to open or close the individual cylinder fuel supply
ports simultaneously.Fuel Discharge Nozzle
The fuel discharge nozzle is located in the cylinder head
with its outlet directed into the intake port. The nozzle body
contains a drilled central passage with a counterbore at each
end. [Figure 2-45] The lower end is used as a chamber for
air-fuel mixing before the spray leaves the nozzle. The upper
bore contains a removable orifice for calibrating the nozzles.
Nozzles are calibrated in several ranges, and all nozzles
furnished for one engine are of the same range and are
identified by a letter stamped on the hex of the nozzle body.
Drilled radial holes connect the upper counterbore with the
outside of the nozzle body. These holes enter the counterbore
above the orifice and draw air through a cylindrical screen
fitted over the nozzle body. A shield is press-fitted on the
nozzle body and extends over the greater part of the filter
screen, leaving an opening near the bottom. This provides
both mechanical protection and an abrupt change in the
direction of airflow which keeps dirt and foreign material
out of the nozzle interior.
Carburetor Maintenance
Carburetor Removal
The removal procedures vary with both the type of
carburetor concerned and the type of engine on which it
is used. Always refer to the applicable manufacturer’s
technical instructions for a particular installation. Generally,
the procedures are much the same, regardless of the type
of carburetor concerned.
Before removing a carburetor, make sure the fuel shutoff (or
selector) valve is closed. Disconnect the throttle and mixture
control linkages, and lockwire the throttle valve in the closed
position. Disconnect the fuel inlet line and all vapor return,
gauge, and primer lines. If the same carburetor is to be re-
installed, do not alter the rigging of the throttle and mixture
controls. Remove the airscoop or airscoop adapter. Remove
Cover & vent
Fuel valve assembly
Fuel filter screenLow pressure
High pressure Fuel manifold body
Standard nozzle 550 series crossflow Crossflow IO-360 series Turbocharge nozzleAir screenProtective shroud
Air inlet holes
Cylinder #1 12B nozzle sizeFigure 2-44. Fuel manifold valve assembly.
Figure 2-45. Fuel discharge nozzles.
2-29the air screens and gaskets from the carburetor. Remove
the nuts and washers securing the carburetor to the engine.
When removing a downdraft carburetor, use extreme care
to ensure that nothing is dropped into the engine. Remove
the carburetor. Immediately install a protective cover on the
carburetor mounting flange of the engine to prevent small
parts or foreign material from falling into the engine. When
there is danger of foreign material entering open fuel lines
during removal or installation of the carburetor, plug them
using the proper cover fittings.
Installation of Carburetor
Check the carburetor for proper lockwiring before installation
on an engine. Be sure that all shipping plugs have been
removed from the carburetor openings.
Remove the protective cover from the carburetor mounting
flange on the engine. Place the carburetor mounting flange
gasket in position. On some engines, bleed passages are
incorporated in the mounting pad. The gasket must be
installed so that the bleed hole in the gasket is aligned with
the passage in the mounting flange.
Inspect the induction passages for the presence of any
foreign material before installing the carburetor. As soon
as the carburetor is placed in position on the engine, close
and lockwire the throttle valves in the closed position until
the remainder of the installation is completed. Place the
carburetor deck screen, when feasible, in position to further
eliminate the possibility of foreign objects entering the
induction system.
When installing a carburetor that uses diaphragms for
controlling fuel flow, connect the fuel lines and fill the
carburetor with fuel. To do this, turn on the fuel boost pump
and move the mixture control from the idle cutoff position
to rich position. Continue the flow until oil-free fuel flows
from the drain valve. This indicates that the preservative oil
has been flushed from the carburetor. Turn off the fuel flow,
plug the fuel inlet and vapor vent outlet, and then allow
the carburetor, filled with fuel, to stand for a minimum of
8 hours. This is necessary in order to soak the diaphragms
and render them pliable to the same degree as when the unit
was originally calibrated. Tighten the carburetor mounting
bolts to the value specified in the table of torque limits in the
applicable maintenance manual. Tighten and safety any other
nuts and bolts incidental to the installation of the carburetor
before connecting the throttle and mixture-control levers.
After the carburetor has been bolted to the engine, check the
throttle and mixture-control lever on the unit for freedom of
movement before connecting the control cables or linkage.
Check the vapor vent lines or return lines from the carburetor
to the aircraft fuel tank for restriction.Rigging Carburetor Controls
Connect and adjust carburetor or fuel metering equipment
throttle controls so that full movement of the throttle is
obtained from corresponding full movement of the control
in the flight deck. In addition, check and adjust the throttle
control linkages so that springback on the throttle quadrant
in the aircraft is equal in both the full-open and full-closed
positions. Correct any excess play or looseness of control
linkage or cables. Controls should be checked so that they
go stop-to-stop on the carburetor. Check for complete and
full travel of each control.
When installing carburetors or fuel metering equipment
incorporating manual-type mixture controls that do not have
marked positions, adjust the mixture control mechanism to
provide an equal amount of springback at both the rich and
lean ends of the control quadrant in the flight deck when
the mixture control on the carburetor or fuel metering
equipment is moved through the full range. Where mixture
controls with detents are used, rig the control mechanism
so that the designated positions on the control quadrant in
the aircraft agree with the corresponding positions on the
carburetor or fuel metering equipment. Controls should
move freely and smoothly without binding throughout
their total travel. In all cases, check the controls for proper
positioning in both the advance and retard positions. Correct
excess play or looseness of control linkage or cables. Safety
all controls properly to eliminate the possibility of loosening
from vibration during operation.
Adjusting Idle Mixtures
Excessively rich or lean idle mixtures result in incomplete
combustion within the engine cylinder, with resultant
formation of carbon deposits on the spark plugs and
subsequent spark plug fouling. In addition, excessively rich
or lean idle mixtures make it necessary to taxi at high idle
speeds with resultant fast taxi speeds and excessive brake
wear. Each engine must have the carburetor idle mixture
tailored for the particular engine and installation if best
operation is to be obtained.
Engines that are properly adjusted, insofar as valve operation,
cylinder compression, ignition, and carburetor idle mixture
are concerned, idle at the prescribed rpm for indefinite periods
without loading up, overheating, or spark plug fouling. If an
engine does not respond to idle mixture adjustment with the
resultant stable idling characteristics previously outlined,
some other phase of engine operation is not correct. In such
cases, determine and correct the cause of the difficulty. A
general guide to check and adjust the idle mixture and speed
on many types of reciprocating engine is discussed in the
following paragraphs. Always refer to the appropriate manual
Figure 2-46. Idle mixture adjustment for carburetor. for specific information.
Before checking the idle mixture on any engine, warm
up the engine until oil and cylinder head temperatures are
normal. Keep the propeller control in the increase rpm setting
throughout the entire process of warming up the engine.
Always make idle mixture adjustments with cylinder head
temperatures at normal values. The idle mixture adjustment is
made on the idle mixture fuel control valve. [Figure 2-46] It
should not be confused with the adjustment of the idle speed
stop. The importance of idle mixture adjustment cannot be
overstressed. Optimum engine operation at low speeds can be
obtained only when proper air-fuel mixtures are delivered to
every cylinder of the engine. Excessively rich idle mixtures
and the resultant incomplete combustion are responsible
for more spark plug fouling than any other single cause.
Excessively lean idle mixtures result in faulty acceleration.
Furthermore, the idle mixture adjustment affects the air-fuel
mixture and engine operation well up into the cruise range.
On an engine with a conventional carburetor, the idle mixture
is checked by manually leaning the mixture with the flight
deck mixture control. Move the carburetor mixture control
slowly and smoothly toward the idle cutoff position. On
installations that do not use a manifold pressure gauge, it is
necessary to observe the tachometer for an indication of a
rpm change. With most installations, the idle mixture should
be adjusted to provide an rpm rise prior to decreasing as the
engine ceases to fire. This rpm increase varies from 10 to 50
rpm, depending on the installation. Following the momentary
increase in rpm, the engine speed starts to drop. Immediately
move the mixture control back to rich to prevent the engine
from stopping completely.
On RSA fuel-injection engines, the optimum idle setting is
one that is rich enough to provide a satisfactory acceleration
under all conditions and lean enough to prevent spark plug fouling or rough operation. A rise of 25–50 rpm as the mixture
control is moved to the idle cutoff position usually satisfies
both of these conditions. The actual idle mixture adjustment is
made by the lengthening or shortening of the linkage between
the throttle lever and the idle lever. [Figure 2-47]
If the check of the idle mixture reveals it to be too lean or too
rich, increase or decrease the idle fuel flow as required. Then,
repeat the check. Continue checking and adjusting the idle
mixture until it checks out properly. During this process, it
may be desirable to move the idle speed stop completely out
of the way and to hold the engine speed at the desired rpm by
means of the throttle. This eliminates the need for frequent
readjustments of the idle stop as the idle mixture is improved
and the idle speed picks up. After each adjustment, clear the
engine by briefly running it at higher rpm. This prevents
fouling of the plugs which might otherwise be caused by
incorrect idle mixture. After adjusting the idle mixture,
recheck it several times to determine definitively that the
mixture is correct and remains constant on repeated changes
from high power back to idle. Correct any inconsistency in
engine idling before releasing the aircraft for service.
Setting the idle mixture on the continental TCM fuel injection
system consists of a conventional spring loaded screw located
in the air throttle lever. [Figure 2-48] The fuel pump pressure
is part of the basic calibration and requires servicing to make
sure the pump pressure are set correctly before making idle
adjustments. The idle mixture adjustment is the locknut at
the metering valve end of the linkage between the metering
valve and the air throttle levers. Tightening the nut to shorten
the linkage provides a richer mixture. A leaner mixture is
obtained by backing off the nut to lengthen the linkage.
Adjust to obtain a slight and momentary gain in idle speed
as the mixture control is slowly moved toward idle cut off.
If the idle mixture is set too lean, the idle speed drops with
no gain in speed.
Idle Speed Adjustment
After adjusting the idle mixture, reset the idle stop to the
idle rpm specified in the aircraft maintenance manual. The
engine must be warmed up thoroughly and checked for
ignition system malfunctioning. Throughout any carburetor
adjustment procedure, periodically run the engine up to
approximately half of normal rated speed to clear the engine.
Some carburetors are equipped with an eccentric screw to
adjust idle rpm. Others use a spring-loaded screw to limit
the throttle valve closing. In either case, adjust the screw
as required to increase or decrease rpm with the throttle
retarded against the stop. Open the throttle to clear the engine;
close the throttle and allow the rpm to stabilize. Repeat this
operation until the desired idling speed is obtained.
Idle speed adjustment
Idle mixture adjustmentFuel inlet and strainerInlet fuel pressure tap
Idle valveMetered fuel outlet
Impact air
Figure 2-47. Bendix adjustment of idle mixture linkage.
Idle speed adjustment Idle mixture adjustment
Figure 2-48. TCM adjustment points.Fuel System Inspection & Maintenance
The inspection of a fuel system installation consists basically
of an examination of the system for conformity to design
requirements together with functional tests to prove correct
operation. Since there are considerable variations in the fuel
systems used on different aircraft, no attempt has been made to describe any particular system in detail. It is important that
the manufacturer’s instructions for the aircraft concerned
be followed when performing inspection or maintenance
functions.
Complete System
Inspect the entire system for wear, damage, or leaks. Make
sure that all units are securely attached and properly safetied.
The drain plugs or valves in the fuel system should be opened
to check for the presence of sediment or water. The filter and
sump should also be checked for sediment, water, or slime.
The filters or screens, including those provided for flow
meters and auxiliary pumps, must be clean and free from
corrosion. The controls should be checked for freedom of
movement, security of locking, and freedom from damage
due to chafing. The fuel vents should be checked for correct
positioning and freedom from obstruction; otherwise, fuel
flow or pressure fueling may be affected. Filler neck drains
should be checked for freedom from obstruction.
If booster pumps are installed, the system should be checked
for leaks by operating the pumps. During this check, the
ammeter or load meter should be read and the readings of
all the pumps, where applicable, should be approximately
the same.
2-32Fuel Tanks
All applicable panels in the aircraft skin or structure should
be removed and the tanks inspected for corrosion on the
external surfaces, for security of attachment, and for correct
adjustment of straps and slings. Check the fittings and
connections for leaks or failures.
Some fuel tanks manufactured of light alloy materials are
provided with inhibitor cartridges to reduce the corrosive
effects of combined leaded fuel and water. Where
applicable, the cartridge should be inspected and renewed
at the specified periods.
Lines & Fittings
Be sure that the lines are properly supported and that the nuts
and clamps are securely tightened. To tighten hose clamps
to the proper torque, use a hose-clamp torque wrench. If
this wrench is not available, tighten the clamp finger-tight
plus the number of turns specified for the hose and clamp.
If the clamps do not seal at the specified torque, replace the
clamps, the hose, or both. After installing a new hose, check
the clamps daily and tighten if necessary. When this daily
check indicates that cold flow has ceased, inspect the clamps
at less frequent intervals.
Replace the hose if the plys have separated, if there is
excessive cold flow, or if the hose is hard and inflexible.
Permanent impressions from the clamp and cracks in the tube
or cover stock indicate excessive cold flow. Replace any hose
that has collapsed at the bends or as a result of misaligned
fittings or lines. Some hoses tend to flare at the ends beyond
the clamps. This is not an unsatisfactory condition unless
leakage is present.
Blisters may form on the outer synthetic rubber cover of the
hose. These blisters do not necessarily affect the serviceability
of the hose. When a blister is discovered on a hose, remove
the hose from the aircraft and puncture the blister with a
pin. The blister should then collapse. If fluid (oil, fuel, or
hydraulic) emerges from the pinhole in the blister, reject the
hose. If only air emerges, then test the hose pressure at 11⁄2
times the working pressure. If no fluid leakage occurs, the
hose can be regarded as serviceable.
Puncturing the outer cover of the hose may permit the entry
of corrosive elements, such as water, which could attack
the wire braiding and ultimately result in failure. For this
reason, puncturing the outer covering of hoses exposed to
the elements should be avoided.
The external surface of hose may develop fine cracks, usually
short in length, which are caused by surface aging. The hose assembly may be regarded as serviceable, provided these
cracks do not penetrate to the first braid.
Selector Valves
Rotate selector valves and check for free operation, excessive
backlash, and accurate pointer indication. If the backlash
is excessive, check the entire operating mechanism for
worn joints, loose pins, and broken drive lugs. Replace any
defective parts. Inspect cable control systems for worn or
frayed cables, damaged pulleys, or worn pulley bearings.
Pumps
During an inspection of booster pumps, check for the
following conditions:
1. Proper operation;
2. Leaks and condition of fuel and electrical connections;
and
3. Wear of motor brushes.
Be sure the drain lines are free of traps, bends, or restrictions.
Check the engine-driven pump for leaks and security of
mounting. Check the vent and drain lines for obstructions.
Main Line Strainers
Drain water and sediment from the main line strainer at
each preflight inspection. Remove and clean the screen at
the periods specified in the airplane maintenance manual.
Examine the sediment removed from the housing. Particles of
rubber are often early warnings of hose deterioration. Check
for leaks and damaged gaskets.
Fuel Quantity Gauges
If a sight gauge is used, be sure that the glass is clear and that
there are no leaks at the connections. Check the lines leading to
it for leaks and security of attachment. Check the mechanical
gauges for free movement of the float arm and for proper
synchronization of the pointer with the position of the float.
On the electrical and electronic gauges, be sure that both the
indicator and the tank units are securely mounted and that
their electrical connections are tight.
Fuel Pressure Gauge
Check the pointer for zero tolerance and excessive oscillation.
Check the cover glass for looseness and for proper range
markings. Check the lines and connections for leaks. Be sure
that there is no obstruction in the vent. Replace the instrument
if it is defective.
Pressure Warning Signal
Inspect the entire installation for security of mounting
and condition of the electrical, fuel, and air connections.
2-33Check the lamp by pressing the test switch to see that it
lights. Check the operation by turning the battery switch on,
building up pressure with the booster pump, and observing
the pressure at which the light goes out. If necessary, adjust
the contact mechanism.
Water Injection Systems for Reciprocating Engines
These systems have very limited use in modern aircraft
engines. Water injection was used mostly on large radial
engines. The water injection system enabled more power to
be obtained from the engine at takeoff than is possible without
water injection. The carburetor (operating at high power
settings) delivers more fuel to the engine than it actually
needs. A leaner mixture would produce more power; however,
the additional fuel is necessary to prevent overheating and
detonation. With the injection of the antidetonant fluid, the
mixture can be leaned out to that which produces maximum
power, and the vaporization of the water-alcohol mixture then
provides the cooling formerly supplied by the excess fuel.
Turbine Engine Fuel System—General
Requirements
The fuel system is one of the more complex aspects of the gas
turbine engine. It must be possible to increase or decrease the
power at will to obtain the thrust required for any operating
condition. In turbine-powered aircraft, this control is provided
by varying the flow of fuel to the combustion chambers.
However, some turboprop aircraft also use variable-pitch
propellers; thus, the selection of thrust is shared by two
controllable variables, fuel flow and propeller blade angle.
The quantity of fuel supplied must be adjusted automatically
to correct for changes in ambient temperature or pressure.
If the quantity of fuel becomes excessive in relation to mass
airflow through the engine, the limiting temperature of the
turbine blades can be exceeded, or it will produce compressor
stall and a condition referred to as rich blowout. Rich blowout
occurs when the amount of oxygen in the air supply is
insufficient to support combustion and when the mixture is
cooled below the combustion temperature by the excess fuel.
The other extreme, lean flameout, occurs if the fuel quantity
is reduced proportionally below the air quantity. The engine
must operate through acceleration and deceleration without
any fuel-control-related problems.
The fuel system must deliver fuel to the combustion chambers
not only in the right quantity, but also in the right condition
for satisfactory combustion. The fuel nozzles form part of
the fuel system and atomize or vaporize the fuel so that
it ignites and burns efficiently. The fuel system must also
supply fuel so that the engine can be easily started on the
ground and in the air. This means that the fuel must be
injected into the combustion chambers in a combustible condition during engine starting, and that combustion must
be sustained while the engine is accelerating to its normal
idling speed. Another critical condition to which the fuel
system must respond occurs during a rapid acceleration.
When the engine is accelerated, energy must be furnished to
the turbine in excess of that necessary to maintain a constant
rpm. However, if the fuel flow increases too rapidly, an over
rich mixture can be produced, with the possibility of a rich
blowout or compressor stall.
Turbofan, turbojet, turboshaft, and turboprop engines are
equipped with a fuel control unit which automatically
satisfies the requirements of the engine. Although the basic
requirements apply generally to all gas turbine engines, the
way in which individual fuel controls meet these needs cannot
be conveniently generalized.
Turbine Fuel Controls
Gas turbine engine fuel controls can be divided into three
basic groups:
1. Hydromechanical,
2. Hydromechanical/electronic, and
3. Full Authority Digital Engine (or Electronics) Control
(FADEC).
The hydromechanical/electronic fuel control is a hybrid
of the two types of fuel control but can function solely
as a hydromechanical control. In the dual mode, inputs
and outputs are electronic, and fuel flow is set by servo
motors. The third type, FADEC, uses electronic sensors for
its inputs and controls fuel flow with electronic outputs.
The FADEC-type control gives the electronic controller
(computer) complete control. The computing section of the
FADEC system depends completely on sensor inputs to the
electronic engine control (EEC) to meter the fuel flow. The
fuel metering device meters the fuel using only outputs from
the EEC. Most turbine fuel controls are quickly going to the
FADEC type of control. This electronically controlled fuel
control is very accurate in scheduling fuel by sensing many
of the engine parameters.
Regardless of the type, all fuel controls accomplish essentially
the same function. That function is to schedule the fuel flow
to match the power required by the pilot. Some sense more
engine variables than others. The fuel control can sense many
different inputs, such as power lever position, engine rpm for
each spool, compressor inlet pressure and temperature, burner
pressure, compressor discharge pressure, and many more
parameters as needed by the specific engine. These variables
affect the amount of thrust that an engine produces for a given
fuel flow. By sensing these parameters, the fuel control has
a clear picture of what is happening in the engine and can
2-34adjust fuel flow as needed. Each type of turbine engine has
its own specific needs for fuel delivery and control.
Hydromechanical Fuel Control
Hydromechanical fuel controls were used and are still used
on many engines, but their use is becoming limited giving
way to electronic based controls. Fuel controls have two
sections, computing and metering, to provide the correct fuel
flow for the engine. A pure hydromechanical fuel control has
no electronic interface assisting in computing or metering the
fuel flow. It also is generally driven by the gas generator gear
train of the engine to sense engine speed. Other mechanical
engine parameters that are sensed are compressor discharge
pressure, burner pressure, exhaust temperature, and inlet
air temperature and pressure. Once the computing section
determines the correct amount of fuel flow, the metering
section through cams and servo valves delivers the fuel to
the engine fuel system. Actual operating procedures for a
hydromechanical fuel control is very complicated and still
the fuel metering is not as accurate as with an electronic
type of interface or control. Electronic controls can receive
more inputs with greater accuracy than hydromechanical
controls. Early electronic controls used a hydromechanical
control with an electronic system added on the system to fine
tune the metering of the fuel. This arrangement also used the
hydromechanical system as a backup if the electronic system
failed. [Figure 2-49]
Hydromechanical/Electronic Fuel Control
The addition of the electronic control to the basic
hydromechanical fuel control was the next step in the
development of turbine engine fuel controls. Generally, this
type of system used a remotely located EEC to adjust the
fuel flow. A description of a typical system is explained in
the following information. The basic function of the engine
fuel system is to pressurize the fuel, meter fuel flow, and
deliver atomized fuel to the combustion section of the engine.
Fuel flow is controlled by a hydromechanical fuel control
assembly, which contains a fuel shutoff section and a fuel
metering section.
This fuel control unit is sometimes mounted on the vane
fuel pump assembly. It provides the power lever connection
and the fuel shutoff function. The unit provides mechanical
overspeed protection for the gas generator spool during
normal (automatic mode) engine operation. In automatic
mode, the EEC is in control of metering the fuel. In manual
mode, the hydromechanical control takes over.
During normal engine operation, a remotely mounted
electronic fuel control unit (EFCU) (same as an EEC)
performs the functions of thrust setting, speed governing and
acceleration, and deceleration limiting through EFCU outputs to the fuel control assembly in response to power lever inputs.
In the event of electrical or EFCU failure, or at the option of
the pilot, the fuel control assembly functions in manual mode
to allow engine operation at reduced power under control of
the hydromechanical portion of the controller only.
The total engine fuel and control system consists of the
following components and provides the functions as
indicated:
1. The vane fuel pump assembly is a fixed displacement
fuel pump that provides high pressure fuel to the
engine fuel control system. [Figure 2-50]
2. The filter bypass valve in the fuel pump allows fuel to
bypass the fuel filter when the pressure drop across the
fuel filter is excessive. An integral differential pressure
indicator visually flags an excessive differential
pressure condition before bypassing occurs, by
extending a pin from the fuel filter bowl. Fuel pump
discharge flow in excess of that required by the fuel
control assembly is returned from the control to the
pump interstage.
3. The hydromechanical fuel control assembly provides
the fuel metering function of the EFCU.
Fuel is supplied to the fuel control through a
200-micron inlet filter screen and is metered to the
engine by the servo-operated metering valve. It is
a fuel flow/compressor discharge pressure (Wf/
P3) ratio device that positions the metering valve in
response to engine compressor discharge pressure
(P3). Fuel pressure differential across the servo
valve is maintained by the servo-operated bypass
valve in response to commands from the EFCU.
[Figure 2-49] The manual mode solenoid valve is
energized in the automatic mode. The automatic mode
restricts operation of the mechanical speed governor.
It is restricted to a single overspeed governor setting
above the speed range controlled electronically.
Deenergizing the manual mode valve enables the
mechanical speed governor to function as an all speed
governor in response to power lever angle (PLA). The
fuel control system includes a low power sensitive
torque motor which may be activated to increase or
decrease fuel flow in the automatic mode (EFCU
mode). The torque motor provides an interface to an
electronic control unit that senses various engine and
ambient parameters and activates the torque motor
to meter fuel flow accordingly. This torque motor
provides electromechanical conversion of an electrical
signal from the EFCU. The torque motor current is
zero in the manual mode, which establishes a fixed
Wf/P3 ratio.
2-35Pressurizing valve Start fuel enrichment solenoidFuel outlet
(to fuel oil
cooler)Fuel shutoff valveTorque engine
Metering head
sensorEvacuated bellows
Servo pressure regulatorServo metering valve assembly
Fuel bypass
valve
Manual
mode
camIdle set stop
Manual mode solenoid (shown auto mode)
Speed
set
Overspeed stopP3 Inlet
Vane fuel pumpFuel return
Fuel inlet
Mechanical speed governorP3 Air
Pressure fuel
Return/bypass fuel
Figure 2-49. Fuel control assembly schematic hydromechanical/electronic.
Figure 2-50. Fuel pump and filter.
This fixed Wf/P3 ratio is such that the engine operates
surge free and is capable of producing a minimum of
90 percent thrust up to 30,000 feet for this example
system. All speed governing of the high-pressure spool
(gas generator) is achieved by the flyweight governor.
The flyweight governor modulates a pneumatic servo,
consistent with the speed set point as determined by
the power lever angle (PLA) setting. The pneumatic servo accomplishes Wf/P3 ratio modulation to govern
the gas generator speed by bleeding down the P3
acting on the metering valve servo. The P3 limiter
valve bleeds down the P3 pressure acting in the
metering valve servo when engine structural limits
are encountered in either control mode. The start
fuel enrichment solenoid valve provides additional
fuel flow in parallel with the metering valve when
required for engine cold starting or altitude restarts.
The valve is energized by the EFCU when enrichment
is required. It is always deenergized in the manual
mode to prevent high altitude sub-idle operation.
Located downstream of the metering valve are the
manual shutoff and pressurizing valves. The shutoff
valve is a rotary unit connected to the power lever. It
allows the pilot to direct fuel to the engine manually.
The pressurizing valve acts as a discharge restrictor to
the hydromechanical control. It functions to maintain
minimum operating pressures throughout the control.
The pressurizing valve also provides a positive leak-
tight fuel shutoff to the engine fuel nozzles when the
manual valve is closed.
4. The flow divider and drain valve assembly proportions
fuel to the engine primary and secondary fuel
nozzles. It drains the nozzles and manifolds at engine
shutdown. It also incorporates an integral solenoid for
2-36modifying the fuel flow for cold-starting conditions.
During an engine start, the flow divider directs all
flow through the primary nozzles. After start, as the
engine fuel demand increases, the flow divider valve
opens to allow the secondary nozzles to function.
During all steady-state engine operation, both primary
and secondary nozzles are flowing fuel. A 74-micron,
self-bypassing screen is located under the fuel inlet
fitting and provides last chance filtration of the fuel
prior to the fuel nozzles.
5. The fuel manifold assembly is a matched set consisting
of both primary and secondary manifolds and the fuel
nozzle assemblies.
Twelve fuel nozzles direct primary and secondary fuel
through the nozzles causing the fuel to swirl and form a finely
atomized spray. The manifold assembly provides fuel routing
and atomizing to ensure proper combustion.
The EEC system consists of the hydromechanical fuel
control, EFCU, and aircraft mounted power lever angle
potentiometer. Aircraft-generated control signals include inlet
pressure, airstream differential pressure, and inlet temperature
plus pilot selection of either manual or auto mode for the
EFCU operation. Engine-generated control signals include
fan spool speed, gas generator spool speed, inner turbine
temperature, fan discharge temperature, and compressor
discharge pressure. Aircraft- and engine-generated control
signals are directed to the EFCU where these signals are
interpreted. The PLA potentiometer is mounted in the throttle
quadrant. The PLA potentiometer transmits an electrical
signal to the EFCU, which represents engine thrust demand
in relation to throttle position. If the EFCU determines a
power change is required, it commands the torque motor
to modulate differential pressure at the head sensor. This
change in differential pressure causes the metering valve
to move, varying fuel flow to the engine as required. The
EFCU receives electrical signals which represent engine
operating variables. It also receives a pilot-initiated signal
(by power-lever position) representing engine thrust demand.
The EFCU computes electrical output signals for use by the
engine fuel control for scheduling engine operation within
predetermined limits. The EFCU is programmed to recognize
predetermined engine operating limits and to compute output
signals such that these operating limits are not exceeded.
The EFCU is remotely located and airframe mounted. An
interface between the EFCU and aircraft/engine is provided
through the branched wiring harness assembly. [Figure 2-51]
FADEC Fuel Control Systems
A full authority digital electronic control (FADEC) has been
developed to control fuel flow on most new turbine engine models. A true FADEC system has no hydromechanical fuel
control backup system. The system uses electronic sensors
that feed engine parameter information into the EEC. The
EEC gathers the needed information to determine the amount
of fuel flow and transmits it to a fuel metering valve. The
fuel metering valve simply reacts to the commands from the
EEC. The EEC is a computer that is the computing section
of the fuel delivery system and the metering valve meters
the fuel flow. FADEC systems are used on many types of
turbine engines from APUs to the largest propulsion engines.
FADEC for an Auxiliary Power Unit
An APU engine uses the aircraft fuel system to supply fuel
to the fuel control. An electric boost pump may be used to
supply fuel under pressure to the control. The fuel usually
passes through an aircraft shutoff valve that is tied to the
fire detecting/extinguishing system. An aircraft furnished
inline fuel filter may also be used. Fuel entering the fuel
control unit first passes through a 10-micron filter. If the filter
becomes contaminated, the resulting pressure drop opens the
filter bypass valve and unfiltered fuel then is supplied to the
APU. Shown in Figure 2-52 is a pump with an inlet pressure
access plug so that a fuel pressure gauge might be installed
for troubleshooting purposes. Fuel then enters a positive
displacement, gear-type pump. Upon discharge from the
pump, the fuel passes through a 70-micron screen. The screen
is installed at this point to filter any wear debris that might
be discharged from the pump element. From the screen, fuel
branches to the metering valve, differential pressure valve,
and the ultimate relief valve. Also shown at this point is a
pump discharge pressure access plug, another point where a
pressure gauge might be installed.
The differential pressure valve maintains a constant pressure
drop across the metering valve by bypassing fuel to the pump
inlet so that metered flow is proportional to metering valve
area. The metering valve area is modulated by the torque
motor, which receives variable current from the engine
control unit (ECU). The ultimate relief valve opens to bypass
excess fuel back to the pump inlet whenever system pressure
exceeds a predetermined pressure. This occurs during each
shutdown since all flow is stopped by the shutoff valve and
the differential pressure valve, is unable to bypass full pump
capacity. Fuel flows from the metering valve out of the fuel
control unit (FCU), through the solenoid shutoff valve and
on to the atomizer. Initial flow is through the primary nozzle
tip only. The flow divider opens at higher pressure and adds
flow through the secondary path.
FADEC Fuel Control Propulsion Engine
Many large high-bypass turbofan engines use the FADEC
type of fuel control system. The EEC is the primary
component of the FADEC engine fuel control system. The
2-37TT4.5 Bleed Valve
P0NL T13
PT0
TM
[WF/P3]
Engine Control SystemEFCU cross-linkFault Status IND
Control Mode INDEngine monitoring systemFlight data recorderPotentiometerIGN Power
ContMode
Sel ECU
PWRNH
BackupNHMechanical LinkageP3
28V PWRIgniter
IGN ContPMGFuel
ControlStarter
Gen
Control
Solenoids
and RelaysNL
Trim
and
Engine
ID
EFCU
Mode Sel SWTT0
400 HzPLA
SYMBOLS
Core spool speed
Fan spool speed
Free stream total pressure
Power lever angle
Total temperature
Interturbine temperature
Fan exit air temperature
Indicated airspeedNH
NL
PT0
PLA
TT0
T4.5
T13
P
Figure 2-51. Engine control system.
EEC is a computer that controls the operation of the engine.
The EEC housing contains two electronic channels (two
separate computers) that are physically separated internally
and is naturally cooled by convection. The EEC is generally
placed in an area of the engine nacelle that is cool during
engine operation. It attaches to the lower-left fan case with shock mounts. [Figure 2-53]
The EEC computer uses data it receives from many engine
sensors and airplane systems to control the engine operation.
It receives electronic signals from the flight deck to set engine
power or thrust. The throttle lever angle resolver supplies the
ECU
Torque motorMetering valve Pump discharge port
Pump inlet pressure
Metered outlet port
Inlet
P ValveFilter bypass valveHigh pressure pump
Metered pressure70 Micron screen
Ultimate relief valve10 Micron inlet filter
Fuel shutoff solenoid
Ignitor plugAutomizer
LEGEND
Pump discharge pressure
Metered pressure
Inlet pressure
Figure 2-52. APU fuel system schematic.
ENGINE
EECChannel BElectrical connection (TYP)Programming plug Lanyard
Shock mount (TYP)
Test connectionChannel A
Figure 2-53. EEC and programming plug.
EEC with a signal in proportion to the thrust lever position.
The EEC controls most engine components and receives
feedback from them. Many components supply the EEC with
data for engine operation.
Power for the EEC comes from the aircraft electrical system
or the permanent magnet alternator (PMA). When the engine
is running, the PMA supplies power to the EEC directly. The
EEC is a two channel computer that controls every aspect
of engine operation. Each channel, which is an independent
computer, can completely control the operation of the engine.
The processor does all of the control calculations and supplies
all the data for the control signals for the torque motors and solenoids. The cross-talk logic compares data from channels
A and B and uses the cross-talk logic to find which EEC
channel is the best to control the output driver for a torque
motor or solenoid bank. The primary channel controls all of
the output drivers. If the cross-talk logic finds that the other
channel is better for control of a specific bank, the EEC
changes control of that one bank to the other channel. The
EEC has output driver banks that supply the control signals
to engine components. Each channel of the EEC supplies
the driver banks with control signals. The EEC has both
volatile and nonvolatile memory to store performance and
maintenance data.
The EEC can control the engine thrust in two modes, which
can be selected by use of a mode selection switch. In the
normal mode, engine thrust is set with engine pressure ratio
(EPR); in the alternate mode, thrust is set by N1. When the
fuel control switch is moved from run to cutoff, the EEC
resets. During this reset, all fault data is recorded in the
nonvolatile memory. The EEC controls the metering valve
in the fuel metering unit to supply fuel flow for combustion.
[Figure 2-54] The fuel metering unit is mounted on the
front face of the gearbox and is attached to the front of
the fuel pump. [Figure 2-55] The EEC also sends a signal
to the minimum pressure and shutoff valve in the fuel
metering unit to start or stop fuel flow. The EEC receives
position feedback for several engine components by using
Figure 2-54. Fuel metering unit.
Figure 2-55. Fuel pump.
PROG PLUG
FOC bypass
valve
ENG AOC
valve
IDG AOC
valve
BU GEN
AOC valve
FMU2.5 bleed
valveTAC
valve
PMA VSV TCC valves
Sensors2.9 bleed
valveNAC zone
vent valve
Figure 2-56. Systems controlled by EEC. rotary differential transformer, linear variable differential
transformer, and thermocouples. These sensors feed engine
parameter information from several systems back to the
EEC. The fuel control run cutoff switch controls the high
pressure fuel shut off valve that allows or cuts off fuel flow.
The fuel temperature sensor thermocouple attaches to the
fuel outlet line on the rear of the fuel/oil cooler and sends
this information to the EEC. The EEC uses a torque motor
driver to control the position of the metering valve in the fuel
metering unit. The EEC uses solenoid drivers to control the
other functions of the fuel metering unit (FMU). The EEC
also controls several other subsystems of the engine, as
shown in Figure 2-56 , through torque motors and solenoids,
such as fuel and air oil coolers, bleed valves, variable stator
vanes, turbine cooling air valves, and the turbine case cooling
system. Each channel of the EEC has seven electrical connections,
three on each side and one on the bottom. Both channels
share the inputs of the two connections on the top of the
EEC. These are the programming plug and test connector.
The programming plug selects the proper software in the
EEC for the thrust rating of the engine. The plug attaches
to the engine fan case with a lanyard. When removing the
EEC, the plug remains with the engine. Each channel of the
EEC has three pneumatic connections on the bottom of the
EEC. Transducers inside the EEC supply the related and
opposite EEC channel with a signal in proportion to the
pressure. The pressures that are read by the EEC are ambient
pressure, burner pressure, low pressure compressor (LPC)
exit pressure, and fan inlet pressure. Each channel has its own
wire color that connects the EEC to its sensors. Channel A
wiring is blue and channel B sensor signals are green. The
non-EEC circuit wire is gray while the thermocouple signals
are yellow. This color coding helps simplify which sensors
are used with each channel.
Figure 2-57. Fuel flow transmitter.
Figure 2-58. Fuel distribution valve.
Figure 2-59. Fuel manifolds. Fuel System Operation
The fuel pump receives fuel from the airplane fuel system.
The low pressure boost stage of the pump pressurizes the fuel
and sends it to the fuel/oil cooler (FOC). The fuel flows from
the FOC, through the fuel pump filter element, and then to
the high pressure main stage of the pump. The high pressure
main stage increases the fuel pressure and sends it to the fuel
metering unit (FMU). It also supplies servo fuel to the servo
fuel heater and engine components. Fuel for combustion
(metered fuel) goes through the fuel flow transmitter to
the distribution valve. [Figure 2-57] The fuel distribution
valve supplies metered fuel to the fuel supply manifolds.
[Figure 2-58] The fuel injectors get the metered fuel from the
fuel supply manifolds and spray the fuel into the engine for
combustion. [Figure 2-59] The fuel pump housing contains
a disposable fuel filter element. The fuel filter differential
pressure switch supplies a signal to the EEC that indicates
an almost clogged filter condition. Unfiltered fuel can then
bypass the filter element if the element becomes clogged.
Water Injection System
On warm days, thrust is reduced because of the decrease in
air density. This can be compensated for by injecting water
at the compressor inlet or diffuser case. This lowers the air
temperature and increases air density. A microswitch in the
fuel control is actuated by the control shaft when the power
lever is moved toward the maximum power position.
A water injection speed reset servo resets the speed adjustment
to a higher value during water injection. Without this
adjustment, the fuel control would decrease rpm so that no
additional thrust would be realized during water injection. The
servo is a shuttle valve that is acted upon by water pressure
during water injection. Movement of the servo displaces a
lever on the cam-operated lever linkage to the speed governor
speeder spring, increasing the force of the speeder spring
and increasing the set speed. Because the resulting rpm is
usually higher while water is flowing, increased thrust during
water injection is ensured. If the water injection system is
not armed in the flight deck or if there is no water available,
nothing happens when the water injection switch in the fuel
control unit is actuated. When water is available, a portion of
it is directed to the water injection speed re-set servo. Water
injection systems are not normally used on high-bypass
turbofan engines.
Fuel Control Maintenance
The field repair of the turbine engine fuel control is very
limited. The only repairs permitted in the field are the
replacement of the control and adjustments afterwards. These
adjustments are limited to the idle rpm and the maximum
speed adjustment, commonly called trimming the engine.
Both adjustments are made in the normal range of operation. During engine trimming, the fuel control is checked for idle
rpm, maximum rpm, acceleration, and deceleration. The
procedures used to check the fuel control vary depending
on the aircraft and engine installation.
The engine is trimmed in accordance with the procedures in
2-41the maintenance or overhaul manual for a particular engine.
In general, the procedure consists of obtaining the ambient air
temperature and the field barometric pressure (not sea level)
immediately preceding the trimming of the engine. Care must
be taken to obtain a true temperature reading comparable to
that of the air that enters the engine. Using these readings, the
desired turbine discharge pressure or EPR (engine pressure
ratio) reading is computed from charts published in the
maintenance manual.
The engine is operated at full throttle (or at the part power
control trim stop) for a sufficient period of time to ensure
that it has completely stabilized. Five minutes is the usual
recommended stabilization period. A check should be made
to ensure that the compressor air-bleed valves have fully
closed and that all accessory drive air bleed for which the
trim curve has not been corrected (such as a cabin air-
conditioning unit) has been turned off. When the engine
has stabilized, a comparison is made of the observed and
the computed turbine discharge pressure Pt7 (or EPR) to
determine the approximate amount of trimming required. If
a trim is necessary, the engine fuel control is then adjusted
to obtain the target turbine discharge pressure Pt7 or EPR on
the gauge. Immediately following the fuel control adjustment,
the tachometer reading is observed and recorded. Fuel flow
and exhaust gas temperature readings should also be taken.
On Pratt and Whitney engines, using a dual-spool compressor,
the observed N2 tachometer reading is next corrected for
speed bias by means of temperature/rpm curve. The observed
tachometer reading is divided by the percent trim speed
obtained from the curve. The result is the new engine trim
speed in percent, corrected to standard day (59 °F or 15 °C)
temperature. The new trim speed in rpm may be calculated
when the rpm at which the tachometer reads 100 percent is
known. This value may be obtained from the appropriate
engine manual. If all these procedures have been performed
satisfactorily, the engine has been properly trimmed.
Engine trimming should always be carried out under precisely
controlled conditions with the aircraft headed into the wind.
Precise control is necessary to ensure maintenance of a
minimum thrust level upon which the aircraft performance
is based. In addition, precise control of engine trimming
contributes to better engine life in terms of both maximum
time between overhaul and minimum out-of-commission
time due to engine maintenance requirements. Engines should
never be trimmed if icing conditions exist.
Most electronic control fuel control systems do not require
trimming or mechanical adjustments. Changes to the EEC
in the FADEC system is normally accomplished through
software changes or changing the EEC.Engine Fuel System Components
Main Fuel Pumps (Engine Driven)
Main fuel pumps deliver a continuous supply of fuel at
the proper pressure and at all times during operation of the
aircraft engine. The engine-driven fuel pump must be capable
of delivering the maximum needed flow at appropriate
pressure to obtain satisfactory nozzle spray and accurate
fuel regulation.
These engine driven fuel pumps may be divided into two
distinct system categories:
1. Nonconstant displacement and
2. Nonpositive displacement.
Their use depends on where in the engine fuel system they
are used. A nonpositive-displacement pump produces a
continuous flow. However, because it does not provide
a positive internal seal against slippage, its output varies
considerably as pressure varies. Centrifugal and propeller
pumps are examples of nonpositive-displacement pumps.
If the output port of a nonpositive-displacement pump
was blocked off, the pressure would rise and output would
decrease to zero. Although the pumping element would
continue moving, flow would stop because of slippage
inside the pump. In a positive displacement pump, slippage
is negligible compared to the pump’s volumetric output flow.
If the output port were plugged, pressure would increase
instantaneously to the point that the pump pressure relief
valve opens. Generally, a nonpositive-displacement is used at
the inlet of the engine-driven pump to provide positive flow
to the second stage of the pump. The output of a centrifugal
pump can be varied as needed and is sometimes referred to
as a boost stage of the engine-driven pump.
The second or main stage of the engine-driven fuel pump
for turbine engines is generally a positive displacement
type of pump. The term “positive displacement” means that
the gear supplies a fixed quantity of fuel to the engine for
every revolution of the pump gears. Gear-type pumps have
approximately straight line flow characteristics, whereas fuel
requirements fluctuate with flight or ambient air conditions.
Hence, a pump of adequate capacity at all engine operating
conditions has excess capacity over most of the range of
operation. This is the characteristic that requires the use
of a pressure relief valve for bypassing excess fuel back to
the inlet. A typical two-stage turbine engine driven pump is
illustrated in Figure 2-60 . The impeller, which is driven at a
greater speed than the high pressure elements, increases the
fuel pressure depending upon engine speed.
The fuel is discharged from the boost element (impeller)
to the two high-pressure gear elements. A relief valve is
Figure 2-60. Dual element fuel pump.
incorporated in the discharge port of the pump. This valve
opens at a predetermined pressure and is capable of bypassing
the total fuel flow. This allows fuel in excess of that required
for engine operation at the time to be recirculated. The bypass
fuel is routed to the inlet side of the second stage pump.
Fuel flows from the pump to the fuel metering unit or fuel
control. The fuel control is often attached to the fuel pump.
The fuel pump is also lubricated by the fuel passing through
the pump, and it should never be turned without fuel flow
supplied to the inlet of the pump. As the engine coasts down
at shutdown, the fuel pump should be provided with fuel until
it comes to a stop.
Fuel Heater
Gas turbine engine fuel systems are very susceptible to the
formation of ice in the fuel filters. When the fuel in the aircraft
fuel tanks cools to 32 °F or below, residual water in the fuel
tends to freeze, forming ice crystals. When these ice crystals
in the fuel become trapped in the filter, they block fuel flow to
the engine, which causes a very serious problem. To prevent
this problem, the fuel is kept at a temperature above freezing.
Warmer fuel also can improve combustion, so some means
of regulating the fuel temperature is needed.
The method of regulating fuel temperature is to use a fuel
heater which operates as a heat exchanger to warm the fuel.
The heater can use engine bleed air or engine lubricating oil
as a source of heat. The bleed air type is called an air-to-liquid
exchanger and the oil type is known as a liquid-to-liquid
heat exchanger. The function of a fuel heater is to protect the
engine fuel system from ice formation. However, should ice
form in the filter, the heater can also be used to thaw ice on
the fuel screen to allow fuel to flow freely again. On most
installations, the fuel filter is fitted with a pressure-drop
warning switch, which illuminates a warning light on the flight deck instrument panel. If ice begins to collect on the
filter surface, the pressure across the filter slowly decreases.
When the pressure reaches a predetermined value, the
warning light alerts the flight deck personnel.
Fuel deicing systems are designed to be used intermittently.
The control of the system may be manual, by a switch in
the flight deck, or automatic, using a thermostatic sensing
element in the fuel heater to open or close the air or oil shutoff
valve. A fuel heater system is shown in Figure 2-61 . In a
FADEC system, the computer controls the fuel temperature
by sensing the fuel temperature and heating it as needed.
Fuel Filters
A low-pressure filter is installed between the supply tanks and
the engine fuel system to protect the engine-driven fuel pump
and various control devices. An additional high-pressure fuel
filter is installed between the fuel pump and the fuel control
to protect the fuel control from contaminants that could come
from the low pressure pump.
The three most common types of filters in use are the micron
filter, the wafer screen filter, and the plain screen mesh filter.
The individual use of each of these filters is dictated by the
filtering treatment required at a particular location. The
micron filter has the greatest filtering action of any present-
day filter type and, as the name implies, is rated in microns.
[Figure 2-62] (A micron is one thousandth of 1 millimeter.)
The porous cellulose material frequently used in construction
of the filter cartridges is capable of removing foreign matter
measuring from 10–25 microns. The minute openings make
this type of filter susceptible to clogging; therefore, a bypass
valve is a necessary safety factor.
Since the micron filter does such a thorough job of removing
foreign matter, it is especially valuable between the fuel
tank and engine. The cellulose material also absorbs water,
preventing it from passing through the pumps. If water does
seep through the filter, which happens occasionally when
filter elements become saturated with water, the water can and
does quickly damage the working elements of the fuel pump
and control units, since these elements depend solely on the
fuel for their lubrication. To reduce water damage to pumps
and control units, periodic servicing and replacement of filter
elements is imperative. Daily draining of fuel tank sumps and
low-pressure filters eliminates much filter trouble and undue
maintenance of pumps and fuel control units.
The most widely used fuel filters are the 200-mesh and the
35-mesh micron filters. They are used in fuel pumps, fuel
controls, and between the fuel pump and fuel control where
removal of micronic particles is needed. These filters, usually
made of fine-mesh steel wire, are a series of layers of wire.
Cooling finsFuel inletAir baffle Fuel tubes
Fuel temperature sensor Air inletAir outletFuel outlet
Air shutoff valve
Figure 2-61. Fuel heater.
Filter element
OutletBaffleRelief valve
Drain cockInlet
Figure 2-62. Aircraft fuel filter.
The wafer screen type of filter has a replaceable element,
which is made of layers of screen discs of bronze, brass,
steel, or similar material. [Figure 2-63] This type of filter
is capable of removing micronic particles. It also has the
strength to withstand high pressure.Fuel Spray Nozzles & Fuel Manifolds
Although fuel spray nozzles are an integral part of the
fuel system, their design is closely related to the type of
combustion chamber in which they are installed. The fuel
nozzles inject fuel into the combustion area in a highly
atomized, precisely patterned spray so that burning is
completed evenly, in the shortest possible time, and in the
smallest possible space. It is very important that the fuel be
evenly distributed and well centered in the flame area within
the liners. This is to preclude the formation of any hot spots
or hot streaking in the combustion chambers and to prevent
the flame burning through the liner.
Fuel nozzle types vary considerably between engines,
although for the most part fuel is sprayed into the combustion
area under pressure through small orifices in the nozzles. The
two types of fuel nozzles generally used are the simplex and
the duplex configurations. The duplex nozzle usually requires
a dual manifold and a pressurizing valve or flow divider for
dividing primary and secondary (main) fuel flow, but the
simplex nozzle requires only a single manifold for proper
fuel delivery.
The fuel nozzles can be constructed to be installed in various
ways. The two methods used quite frequently are:
1. External mounting wherein a mounting pad is provided
for attachment of the nozzles to the case or the inlet
air elbow, with the nozzle near the dome; or
Filter headFilter sumpSpacer
O-ring
Filter diskHuman hair is about 100 microns in diameter
25,400 microns = 1 inch
Figure 2-63. Wafer screen filter.
Figure 2-64. Simplex airblast nozzle cutaway.2. Internal mounting at the liner dome, in which the
chamber cover must be removed for replacement or
maintenance of the nozzle.
The nozzles used in a specific engine should be matched so
that they flow equal amounts of fuel. Even fuel distribution
is important to efficient combustion in the burner section.
The fuel nozzle must present a fine spray with the correct
pattern and optimum atomization.
Simplex Fuel Nozzle
The simplex fuel nozzle was the first nozzle type used in
turbine engines and was replaced in most installations with
the duplex nozzle, which gave better atomization at starting
and idling speeds. The simplex nozzle is still being used
in several installations. [Figure 2-64] Each of the simplex
nozzles consists of a nozzle tip, an insert, and a strainer made
up of fine-mesh screen and a support.
Duplex Fuel Nozzle
The duplex fuel nozzle is widely used in present day gas
turbine engines and produces two different spray patterns.
As mentioned previously, its use requires a flow divider,
but at the same time it offers a desirable spray pattern
for combustion over a wide range of operating pressures.
[Figure 2-65] A nozzle typical of this type is illustrated in
Figure 2-66 .
Airblast Nozzles
Airblast nozzles are used to provide improved mixing of the
fuel and airflow to provide an optimum spray for combustion.
As can be seen in Figure 2-64 , swirl vanes are used to mix the
Primary only
Primary and main
Figure 2-65. Duplex nozzle spray pattern.
Flow divider valve
Fuel discharge orificeScreen
Spin chamberSmall slot
Large slot
Compressor discharge airFuel inlet port
Figure 2-66. Duplex fuel nozzle. air and fuel at the nozzle opening. By using a proportion of
the primary combustion airflow in the fuel spray, locally rich
fuel concentrations can be reduced. This type of fuel nozzle
can be either simplex or duplex, depending upon the engine.
This nozzle type can operate at lower working pressures than
other nozzles which allows for lighter pumps. This airblast
nozzle also helps in reducing the tendency of the nozzle to
carbon up which can disturb the flow pattern.Flow Divider
A flow divider creates primary and secondary fuel supplies
that are discharged through separate manifolds, providing two
separate fuel flows. [Figure 2-67] Metered fuel from the fuel
control enters the inlet of the flow divider and passes through
an orifice and then on to the primary nozzles. A passage in the
flow divider directs fuel flow from both sides of the orifice
to a chamber. This chamber contains a differential pressure
bellows, a viscosity compensated restrictor (VCR), and a
surge dampener. During engine start, fuel pressure is applied
to the inlet port and across the VCR, surge dampener, and
on to the primary side of the nozzles. Fuel is also applied
under pressure to the outside of the flow divider bellows and
through the surge dampener to the inside of the flow divider
bellows. This unequal pressure causes the flow divider valve
to remain closed. When fuel flow increases, the differential
pressure on the bellows also increases. At a predetermined
pressure, the bellows compresses, allowing the flow divider
valve to open. This action starts fuel flow to the secondary
manifold, which increases the fuel flow to the engine. This
fuel flows out of the secondary opening in the nozzles.
Fuel Pressurizing & Dump Valves
The fuel pressurizing valve is usually required on engines
incorporating duplex fuel nozzles to divide the flow into
primary and secondary manifolds. As the fuel required
for starting and altitude idling flows, it passes through the
primary line. As the fuel flow increases, the valve begins to
open the main line until at maximum flow the secondary line
is passing approximately 90 percent of the fuel.
Fuel pressurizing valves usually trap fuel forward of the
Figure 2-67. Flow divider.
manifold, giving a positive cutoff. This cutoff prevents
fuel from dribbling into the manifold and through the fuel
nozzles, limiting afterfires and carbonization of the fuel
nozzles. Carbonization occurs because combustion chamber
temperatures are lowered, and the fuel is not completely
burned.
A flow divider performs essentially the same function as a
pressurizing valve. It is used, as the name implies, to divide
flow to the duplex fuel nozzles. It is not unusual for units
performing identical functions to have different nomenclature
between engine manufacturers.
Combustion Drain Valves
The drain valves are units used for draining fuel from the
various components of the engine where accumulated fuel
is most likely to present operating problems. The possibility
of combustion chamber accumulation with the resultant fire
hazard is one problem. A residual problem is leaving gum
deposits, after evaporation, in such places as fuel manifolds
and fuel nozzles.
In some instances, the fuel manifolds are drained by an
individual unit known as a drip or dump valve. This type
of valve may operate by pressure differential, or it may be
solenoid operated.
The combustion chamber drain valve drains fuel that
accumulates in the combustion chamber after each shutdown
and fuel that may have accumulated during a false start. If the
combustion chambers are the can type, fuel drains by gravity
down through the flame tubes or interconnector tubes until
it gathers in the lower chambers, which are fitted with drain
lines to the drain valve. If the combustion chamber is of the
basket or annular type, the fuel merely drains through the air
holes in the liner and accumulates in a trap in the bottom of the chamber housing, which is connected to the drain line.
After the fuel accumulates in the bottom of the combustion
chamber or drain lines, the drain valve allows the fuel to
be drained whenever pressure within the manifold or the
burner(s) has been reduced to near atmospheric pressure.
A small spring holds the valve off its seat until pressure in
the combustion chamber during operation overcomes the
spring and closes the valve. The valve is closed during engine
operation. It is imperative that this valve be in good working
condition to drain accumulated fuel after each shutdown.
Otherwise, a hot start during the next starting attempt or an
afterfire after shutdown is likely to occur.
Fuel Quantity Indicating Units
Fuel quantity units vary from one installation to the next. A
fuel counter or indicator, mounted on the instrument panel,
is electrically connected to a flow meter installed in the fuel
line to the engine.
The fuel counter, or totalizer, is used to keep record of fuel
use. When the aircraft is serviced with fuel, the counter is
manually set to the total number of pounds of fuel in all
tanks. As fuel passes through the measuring element of the
flow meter, it sends electrical impulses to the fuel counter.
These impulses actuate the fuel counter mechanism so that
the number of pounds passing to the engine is subtracted
from the original reading. Thus, the fuel counter continually
shows the total quantity of fuel, in pounds, remaining in the
aircraft. However, there are certain conditions that cause the
fuel counter indication to be inaccurate. Any jettisoned fuel
is indicated on the fuel counter as fuel still available for use.
Any fuel that leaks from a tank or a fuel line upstream of the
flow meter is not counted.
Figure 3-1. Inlet scoop in engine cowling. Reciprocating Engine Induction Systems
The basic induction system of an aircraft reciprocating engine
consists of an air scoop used to collect the inlet air and ducting
that transfers the air to the inlet filter. The air filter is generally
housed in the carburetor heat box or other housing close by
that is attached to the carburetor or fuel injection controller.
The engine used in light aircraft is usually equipped with
either a carburetor or a fuel-injection system. After air passes
through the fuel metering device, an intake manifold with
long curved pipes or passages is used to send the air-fuel
mixture to the cylinders. An induction air scoop is shown in
Figure 3-1. The air scoop is located on the engine cowling to
allow maximum airflow into the engine’s induction system.
The air filter, shown in Figure 3-2, prevents dirt and other
foreign matter from entering the engine. Filtered air enters
the fuel metering device (carburetor/fuel injector) where the
throttle plate controls the amount of air flowing to the engine.
The air coming out of the throttle is referred to as manifold
pressure. This pressure is measured in inches of mercury
("Hg) and controls engine power output.
Induction systems can consist of several different
arrangements. Two that are used are the updraft and downdraft
induction systems. An updraft induction system consists of
Induction & Exhaust Systems
Chapter 3
two runners and a balance tube with intake pipes for each
cylinder to deliver induction air to each cylinder’s intake
port. [Figure 3-3] The balance tube is used to reduce pressure
imbalances between the two side induction runners. With
carbureted engines, it is important to maintain a constant and
even pressure in the induction system so that each cylinder
receives equal amounts of fuel. On fuel-injected engines,
the fuel is injected at the intake port just before the intake
valve. It is important with this system to keep the pressure
consistent at each intake port.
A downdraft balanced induction system provides optimum
airflow to each of the individual cylinders throughout a
wide operational range. [Figure 3-4] Better matched air-fuel
ratios provide a much smoother and more efficient engine
operation. Air from the induction manifold flows into the
intake ports where it is mixed with fuel from the fuel nozzles
and then enters the cylinders as a combustible mixture as the
intake valve opens.
Basic Carburetor Induction System
Figure 3-2 is a diagram of an induction system used in an
engine equipped with a carburetor. In this induction system,
carburetor normal flow air is admitted at the lower front nose
cowling below the propeller spinner and is passed through an
air filter into air ducts leading to the carburetor. A carburetor
heat air valve is located below the carburetor for selecting
an alternate warm air source (carburetor heat) to prevent
carburetor icing. [Figure 3-5] Carburetor icing occurs when
the temperature is lowered in the throat of the carburetor and
enough moisture is present to freeze and block the flow of air
to the engine. The carburetor heat valve admits air from the
outside air scoop for normal operation, and it admits warm
air from the engine compartment for operation during icing
conditions. The carburetor heat is operated by a push-pull
control in the flight deck. When the carburetor heat air door
is closed, warm ducted air from around the exhaust is directed
into the carburetor. This raises the intake air temperature.
An alternate air door can be opened by engine suction if the
normal route of airflow should be blocked by something.
The valve is spring loaded closed and is sucked open by the
engine if needed.
The carburetor air filter, shown in Figure 3-6, is installed in
the air scoop in front of the carburetor air duct. Its purpose is
Drain lineCarburetor air valve
Air intake duct
Cold air actuator leverCarburetor
Temperature bulbAir filter
Warm air
Cold air
Hose clamp
Induction elbowInduction risersThrottle body
HoseBalance crossover tubeInduction riser
Intake manifold tube
Intake manifold tubeThrottle body
Induction manifold assemblyHose
Intake gasket
Figure 3-2. Nonsupercharged induction system using a carburetor.
Figure 3-3. Updraft induction system.
Figure 3-4. Downdraft balanced induction system.Figure 3-5. Location of a carburetor heat air valve.
to stop dust and other foreign matter from entering the engine
through the carburetor. The screen consists of an aluminum
alloy frame and a deeply crimped screen, arranged to present
maximum screen area to the airstream. There are several types
of air filters in use including paper, foam, and other types of
filters. Most air filters require servicing at regular intervals
and the specific instructions for the type of filter must be
followed. [Figure 3-6]
The carburetor air ducts consist of a fixed duct riveted to the
nose cowling and a flexible duct between the fixed duct and
the carburetor air valve housing. The carburetor air ducts
normally provide a passage for outside air to the carburetor.
Applying carburetor heat to an operating engine decreases the
density of the air, which leans the air-fuel mixture. Air enters
the system through the ram-air intake. The intake opening is
located in the slipstream so the air is forced into the induction
system giving a ram effect to the incoming airflow. The air
passes through the air ducts to the carburetor. The carburetor
meters the fuel in proportion to the air and mixes the air with
Figure 3-6. Location of air filter.
Throttle ice
Fuel evaporation iceImpact ice
Figure 3-7. Location of a carburetor heat air valve. the correct amount of fuel. The throttle plate of the carburetor
can be controlled from the flight deck to regulate the flow
of air (manifold pressure), and in this way, power output of
the engine can be controlled.
Although many newer aircraft are not so-equipped, some
engines are equipped with carburetor air temperature
indicating systems which shows the temperature of the air
at the carburetor inlet. If the bulb is located at the engine
side of the carburetor, the system measures the temperature
of the air-fuel mixture.
Induction System Icing
A short discussion concerning the formation and location of
induction system ice is helpful, even though a technician is
not normally concerned with operations that occur when the
aircraft is in flight. [Figure 3-7] Technicians should know
something about induction system icing because of its effect
on engine performance and troubleshooting. Even when an
inspection shows that everything is in proper working order
and the engine performs perfectly on the ground, induction
system ice can cause an engine to act erratically and lose
power in the air. Many engine troubles commonly attributed
to other sources are actually caused by induction system icing.
Induction system icing is an operating hazard because it can
cut off the flow of the air-fuel charge or vary the air-fuel
ratio. Ice can form in the induction system while an aircraft
is flying in clouds, fog, rain, sleet, snow, or even clear air that
has high moisture content (high humidity). Induction system
icing is generally classified in three types:
• Impact ice,
• Fuel evaporation ice, and
• Throttle ice.
Induction system ice can be prevented or eliminated by raising
the temperature of the air that passes through the system,
using a carburetor heat system located upstream near the
induction system inlet and well ahead of the dangerous icing zones. This air is collected by a duct surrounding the exhaust
manifold. Heat is usually obtained through a control valve
that opens
the induction system to the warm air circulating in the engine
compartment and around the exhaust manifold.
Improper or careless use of carburetor heat can be just as
dangerous as the most advanced stage of induction system
ice. Increasing the temperature of the air causes it to expand
and decrease in density. This action reduces the weight of the
charge delivered to the cylinder and causes a noticeable loss in
power because of decreased volumetric efficiency. If icing is
not present when carburetor heat or induction system anti-icing
is applied and the throttle setting does not change, the mixture
will become richer. In addition, high intake air temperature
may cause detonation and engine failure, especially during
takeoff and high power operation. Therefore, during all phases
of engine operation, the carburetor temperature must afford
the greatest protection against icing and detonation.
When there is danger of induction system icing, the flight deck
carburetor heat control is moved to the hot position. Throttle
ice or any ice that restricts airflow or reduces manifold
pressure can best be removed by using full carburetor heat.
3-4If the heat from the engine compartment is sufficient and the
application has not been delayed, it is only a matter of a few
minutes until the ice is cleared.
When there is no danger of icing, the heat control is normally
kept in the “cold” position. It is best to leave the control in
this position if there are particles of dry snow or ice in the air.
The use of heat may melt the ice or snow, and the resulting
moisture may collect and freeze on the walls of the induction
system. To prevent damage to the heater valves in the case of
backfire, carburetor heat should not be used while starting the
engine. Also, during ground operation only enough carburetor
heat should be used to give smooth engine operation.
Part-throttle operation can lead to icing in the throttle area.
When the throttle is placed in a partly closed position, it, in
effect, limits the amount of air available to the engine. When
the aircraft is in a glide, a fixed-pitch propeller windmills,
causing the engine to consume more air than it normally
would at this same throttle setting, thus adding to the lack of
air behind the throttle. The partly closed throttle, under these
circumstances, establishes a much higher than normal air
velocity past the throttle, and an extremely low-pressure area is
produced. The low-pressure area lowers the temperature of the
air surrounding the throttle valve. If the temperature in this air
falls below freezing and moisture is present, ice forms on the
throttles and nearby units restricting the airflow to the engine
causing it to quit. Throttle ice may be minimized on engines
equipped with controllable-pitch propellers by the use of a
higher than normal brake mean effective pressure (BMEP) at
this low power. The high BMEP decreases the icing tendency
because a large throttle opening at low engine revolutions
per minute (rpm) partially removes the temperature-reducing
obstruction that part-throttle operation offers.
Induction System Filtering
Dust and dirt can be a serious source of trouble to an aircraft
engine. Dust consists of small particles of hard, abrasive
material that can be carried by the air and drawn into the
engine cylinders. It can also collect on the fuel-metering
elements of the carburetor, upsetting the proper relation
between airflow and fuel flow at all engine power settings. It
acts on the cylinder walls by grinding down these surfaces and
the piston rings. Then, it contaminates the oil and is carried
through the engine, causing further wear on the bearings and
gears. In extreme cases, an accumulation may clog an oil
passage and cause oil starvation. Although dust conditions
are most critical at ground level, continued operation under
such conditions without engine protection results in extreme
engine wear and can produce excessive oil consumption.
When operation in a dusty atmosphere is necessary, the
engine can be protected by an alternate induction system air
inlet which incorporates a dust filter. This type of air filter system normally consists of a filter element, a door, and
an electrically operated actuator. When the filter system is
operating, air is drawn through a louvered access panel that
does not face directly into the airstream. With this entrance
location, considerable dust is removed as the air is forced
to turn and enter the duct. Since the dust particles are solid,
they tend to continue in a straight line, and most of them are
separated at this point. Those that are drawn into the louvers
are easily removed by the filter.
In flight, with air filters operating, consideration must be
given to possible icing conditions which may occur from
actual surface icing or from freezing of the filter element
after it becomes rain soaked. Some installations have a
spring-loaded filter door which automatically opens when
the filter is excessively restricted. This prevents the airflow
from being cut off when the filter is clogged with ice or dirt.
Other systems use an ice guard in the filtered-air entrance.
The ice guard consists of a coarse-mesh screen located a short
distance from the filtered-air entrance. In this location, the
screen is directly in the path of incoming air so that the air
must pass through or around the screen. When ice forms on
the screen, the air, which has lost its heavy moisture particles,
passes around the iced screen and into the filter element.
The efficiency of any filter system depends upon proper
maintenance and servicing. Periodic removal and cleaning of
the filter element is essential to satisfactory engine protection.
Induction System Inspection & Maintenance
The induction system should be checked for cracks and leaks
during all regularly scheduled engine inspections. The units
of the system should be checked for security of mounting.
The system should be kept clean at all times, since pieces of
rags or paper can restrict the airflow if allowed to enter the
air intakes or ducts. Loose bolts and nuts can cause serious
damage if they pass into the engine.
On systems equipped with a carburetor air filter, the filter
should be checked regularly. If it is dirty or does not have
the proper oil film, the filter element should be removed and
cleaned. After it has dried, it is usually immersed in a mixture
of oil and rust-preventive compound. The excess fluid should
be allowed to drain off before the filter element is reinstalled.
Paper-type filters should be inspected and replaced as needed.
The efficiency of any filter system depends upon proper
maintenance and servicing. Periodic removal and cleaning of
the filter element is essential to satisfactory engine protection.
If the induction system air filter becomes excessively dirty, it
will cause a loss of power or the engine will not start.
Extinguishing Engine Fires
In all cases, a fireguard should stand by with a CO 2 fire
3-5Probable Cause Isolation Procedure Correction
Engine fails to start 1
Engine runs rough 2
Low power 3
Engine idles improperly 4aInduction system obstructed
bAir leaks
aLoose air ducts
bLeaking intake pipes
cEngine valves sticking
aRestricted intake duct
bBroken door in carburetor air valve
cDirty air filter
aShrunken intake packing
bHole in intake pipe
cLoose carburetor mountingdBent or worn valve push rodsInspect air scoop and air ducts
Inspect carburetor mounting and intake pipes
Inspect air ducts
Inspect intake pipe packing nuts
Remove rocker arm cover and check valve action
Examine intake duct
Inspect air valve
Inspect air filter
Inspect packing for proper fit
Inspect intake pipe
Inspect mount boltsInspect push rodsRemove obstructions
Tighten carburetor and repair or replace intake pipe
Tighten air ducts
Tighten nuts
Lubricate and free sticking valves
Remove restrictions
Replace air valve
Clean air filter
Replace packing
Replace defective intake pipes
Tighten mount boltsReplace worn or damaged push rods
Figure 3-8. Common problems for troubleshooting induction systems.
Figure 3-9. An example of a naturally aspirated reciprocating
engine.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,
continue cranking to start the engine and blow out 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.
Induction System Troubleshooting
Figure 3-8 provides a general guide to the most common
induction system troubles.
Supercharged Induction Systems
Since aircraft operate at altitudes where the air pressure is
lower, it is useful to provide a system for compressing the
air-fuel mixture. Some systems are used to normalize the air
pressure entering the engine. These systems are used to regain
the air pressure lost by the increase in altitude. This type of
system is not a ground boost system and it is not used to ever
boost the manifold pressure above 30 inches of mercury. A
true supercharged engine, called ground boosted engines, can boost the manifold pressure above 30 inches of mercury. In
other words, a true supercharger boosts the manifold pressure
above ambient pressure.
Since many engines installed in light aircraft do not
use any type of compressor or supercharging device,
induction systems for reciprocating engines can be broadly
classified as supercharged or nonsupercharged. [Figure 3-9]
Supercharging systems used in reciprocating engine induction
systems are normally classified as either internally driven or
externally driven (turbosupercharged). Internally driven
superchargers compress the air-fuel mixture after it leaves
the carburetor, while externally driven superchargers
(turbochargers) compress the air before it is mixed with the
IN Hg
ALg.MANIFOLD
PRESS35
CARB. TEMP.
C°
Carburetor air temperature bulbIntake pipeIntake air duct
Carburetor heat valveManifold pressure
Throttle valveCarburetor
Supercharger
Figure 3-10. Internally-driven supercharger induction system. metered fuel from the carburetor.
Internally Driven Superchargers
Internally-driven superchargers were used almost exclusively
in high horsepower radial reciprocating engines and are
engine driven through a mechanical connection. Although
their use is very limited, some are still used in cargo
carriers and spray planes. Except for the construction and
arrangement of the various types of superchargers, all
induction systems with internally driven superchargers were
very similar. Aircraft engines require the same air temperature
control to produce good combustion in the engine cylinders.
For example, the charge must be warm enough to ensure
complete fuel vaporization and, thus, even distribution. At
the same time, it must not be so hot that it reduces volumetric
efficiency or causes detonation. All reciprocating engines
must guard against intake air that is too hot. As with any
type of supercharging (compressing intake air), the air gains
heat as it is compressed. Sometimes this air requires cooling
before it is routed to the engine’s intake ports. With these
requirements, most induction systems that use internally
driven superchargers must include pressure and temperature-
sensing devices and the necessary units required to warm or
cool the air.
The simple internally driven supercharger induction system is
used to explain the location of units and the path of the air and
air-fuel mixture. [ Figure 3-10] Air enters the system through
the ram air intake. The intake opening is located so that the
air is forced into the induction system, giving a ram effect
caused by the aircraft moving through the air. The air passes
through ducts to the carburetor. The carburetor meters the
fuel in proportion to the air and mixes the air with the correct amount of fuel. The carburetor can be controlled from the
flight deck to regulate the flow of air. In this way, the power
output of the engine can be controlled. The manifold pressure
gauge measures the pressure of the air-fuel mixture before it
enters the cylinders. It is an indication of the performance that
can be expected of the engine. The carburetor air temperature
indicator measures either the temperature of the inlet air or
of the air-fuel mixture. Either the air inlet or the mixture
temperature indicator serves as a guide so that the temperature
of the incoming charge may be kept within safe limits. If
the temperature of the incoming air at the entrance to the
carburetor scoop is 100 °F, there is approximately a 50 °F
drop in temperature because of the partial vaporization of the
fuel at the carburetor discharge nozzle. Partial vaporization
takes place and the air temperature falls due to absorption
of the heat by vaporization. The final vaporization takes
place as the mixture enters the cylinders where higher
temperatures exist. The fuel, as atomized into the airstream
that flows in the induction system, is in a globular form.
The problem, then, becomes one of uniformly breaking up
and distributing the fuel, remaining in globular form to the
various cylinders. On engines equipped with a large number
of cylinders, the uniform distribution of the mixture becomes
a greater problem, especially at high engine speeds when full
advantage is taken of large air capacity.
One method used mainly on radial reciprocating engines of
improving fuel distribution is shown in Figure 3-11. This
device is known as a distribution impeller. The impeller is
attached directly to the end of the rear shank of the crankshaft
by bolts or studs. Since the impeller is attached to the end
of the crankshaft and operates at the same speed, it does
not materially boost or increase the pressure on the mixture
IN Hg
ALg.MANIFOLD
PRESS35
Diffuser chamberCrankshaft
CounterweightPrimer nozzle
Large diameter impellerManifold pressure
Slip joint
Gland nutRubber gasketManifold ringInletTail shaftRing type oil seal
Figure 3-11. Radial engine distribution impeller.
flowing into the cylinders. But, the fuel remaining in the
globular form is broken up into finer particles as it strikes the
impeller, thereby coming in contact with more air. This creates
a more homogeneous mixture with a consequent improvement
in distribution to the various cylinders, especially on
acceleration of the engine or when low temperatures prevail.
To obtain greater pressure of the air-fuel mixture within the
cylinders, the diffuser or blower section contains a high speed
impeller. Unlike the distribution impeller, which is connected
directly to the crankshaft, the supercharger, or blower
impeller, is driven through a gear train from the crankshaft.
Turbosuperchargers
Externally driven superchargers (turbosuperchargers)
are designed to deliver compressed air to the inlet of the
carburetor or air-fuel control unit of an engine. Externally
driven superchargers derive their power from the energy of
engine exhaust gases directed against a turbine that drives an impeller that compresses the incoming air. For this
reason, they are commonly called turbosuperchargers or
turbochargers. To be a true supercharger, it must boost the
manifold pressure above 30 "Hg.
The typical turbosupercharger, shown in Figure 3-12, is
composed of three main parts:
1. Compressor assembly,
2. Turbine wheel assembly, and
3. A full floating shaft bearing assembly.
Detail examples of a turbosupercharger are shown in
Figure 3-13. In addition to the major assemblies, there is a
baffle between the compressor casing and the exhaust-gas
turbine that directs cooling air to the pump and bearing casing,
and also shields the compressor from the heat radiated by the
turbine. In installations where cooling air is limited, the baffle
is replaced by a regular cooling shroud that receives its air
directly from the induction system.
The compressor assembly is made up of an impeller, a
diffuser, and a casing. The air for the induction system enters
through a circular opening in the center of the compressor
casing, where it is picked up by the blades of the impeller,
which gives it high velocity as it travels outward toward the
diffuser. The diffuser vanes direct the airflow as it leaves
the impeller and also converts the high velocity of the air
to high-pressure.
Motive power for the impeller is furnished through the
impeller’s attachment to the turbine wheel shaft of the
exhaust-gas turbine. This complete assembly is referred to as
the rotor. (The rotor revolves on the oil feed bearings. ) The
exhaust gas turbine assembly consists of the turbocharger and
waste gate valve. [Figure 3-14] The turbine wheel, driven by
exhaust gases, drives the impeller. The turbo housing collects
and directs the exhaust gases onto the turbine wheel, and the
waste gate regulates the amount of exhaust gases directed to
the turbine. The waste gate controls the volume of the exhaust
gas that is directed onto the turbine and thereby regulates
the speed of the rotor (turbine and impeller). [Figure 3-15]
If the waste gate is completely closed, all the exhaust gases
are “backed up” and forced through the turbine wheel. If the
waste gate is partially closed, a corresponding amount of
exhaust gas is directed to the turbine. The exhaust gasses, thus
directed, strike the turbine blades, arranged radially around
the outer edge of the turbine, and cause the rotor (turbine and
impeller) to rotate. The gases, having exhausted most of their
energy, are then exhausted overboard. When the waste gate
is fully open, nearly all of the exhaust gases pass overboard
providing little or no boost.
Turbine housing
Compressor HousingCenter housing
Full floating shaft bearing assemblyCompressor wheel assemblyTurbine wheel assembly
Exhaust Inlet
Exhaust Outlet
Compressor Inlet
Compressor Outlet
Compressor housing
Compressor impeller
Center housing
Turbine
Turbine housing Turbocharger componentsOil feed pressure bearing
Figure 3-13. Detail examples of the main components of a turbosupercharger.Figure 3-12. A typical turbosupercharger and its main parts.
Exhaust risers
Turbocharger 1-3-5Exhaust risers
Exhaust waste gate valveController
Tailpipe 1-3-5 Tailpipe 2-4-6Turbocharger 2-4-6
Figure 3-14. Exhaust gas turbine assembly.
Oil outlet port
controller
ButterflyPiston
Power cylinder drain portOil inlet port
Seal
Figure 3-15. Waste gate control of exhaust.
Normalizer Turbocharger
Some engines used in light aircraft are equipped with an
externally driven normalizing system. These systems are
powered by the energy of exhaust gases and are usually referred
to as “normalizing turbocharger” systems. These systems were
not designed to be used as a true supercharger (boost manifold
pressure over 30 "Hg). They compensate for the power lost due
to the pressure drop resulting from increased altitude. On many
small aircraft engines, the turbocharger (normalizing) system
is designed to be operated only above a certain altitude, 5,000
feet for example, since maximum power without normalizing is available below that altitude. The location of the air induction
and exhaust systems of a typical normalizing turbocharger
system for a small aircraft is shown in Figure 3-16.
Ground-Boosted Turbosupercharger System
Some ground-boosted (sea level) turbosupercharged systems
are designed to operate from sea level up to their critical altitude.
These engines, sometimes referred to as sea level-boosted
engines, can develop more power at sea level than an engine
without turbosupercharging. As was mentioned earlier, an
engine must be boosted above 30 "Hg to truly be supercharged.
This type of turbocharger accomplishes this by increasing the
manifold pressure above 30 "Hg to around 40 "Hg.
The turbosupercharger air induction system consists of a
filtered ram-air intake located on the side of the nacelle.
[Figure 3-17] An alternate air door within the nacelle permits
compressor suction automatically to admit alternate air
(heated engine compartment air) if the induction air filter
becomes clogged. In many cases, the alternate air door can
be operated manually in the event of filter clogging.
Almost all turbocharger systems use engine oil as the control
fluid for controlling the amount of boost (extra manifold
pressure) provided to the engine. The waste-gate actuator and
controllers use pressurized engine oil for their power supply.
The turbocharger is controlled by the waste gate and waste
gate actuator. The waste gate actuator, which is physically
connected to the waste gate by mechanical linkage, controls
Intake manifoldThrottle and mixture control
Air intake Alternate air intake TurbochargerControl system
Exhaust system
Induction system
Figure 3-16. Typical location of the air induction and exhaust systems of a normalizing turbocharger system.
the position of the waste gate butterfly valve. The waste gate
bypasses the engine exhaust gases around the turbocharger
turbine inlet. By controlling the amount of exhaust gases
that pass through the turbine of the turbocharger, the speed
of the compressor and the amount of intake boost (upper
deck pressure) is controlled. Engine oil is also used to cool
and lubricate the bearings that support the compressor and
turbine in the turbocharger. Turbocharger lubricating oil is
engine oil supplied through the engine oil system. An oil
supply hose from the rear of the oil cooler directs oil to the
turbocharger center housings and bearings. Oil hoses return
oil from the turbochargers to the oil scavenge pump located
on the rear of the engine. The one-way check valve in the oil
supply line prevents oil from draining into the turbocharger
while the engine is not operating. Piston ring-like oil seals are
used on the compressor wheel shaft to prevent the lubricating
oil from entering the turbine and compressor housings from
the center housing.
The position of the waste gate is controlled by adjusting the
oil pressure in the waste gate actuator. Several different types
of controllers are used to provide the correct pressure in the
waste gate actuator. This is done either by restricting the oil
flow or by allowing the oil to return to the engine. The more
the oil is restricted, the more pressure is in the waste gate actuator and the more closed the waste gate is. This causes
the exhaust gases to pass through the turbine, increasing the
speed of the compressor raising the inlet pressure. The reverse
happens if the oil is not restricted by the controllers and boost
is reduced. The pressure from the outlet of the compressor of
the turbocharger to the throttle is referred to as deck pressure
or upper deck pressure.
A Typical Turbosupercharger System
Figure 3-18 is a schematic of a sea level booster
turbosupercharger system. This system used widely is
automatically regulated by three components:
• Exhaust bypass valve assembly,
• Density controller, and
• Differential pressure controller.
By regulating the waste gate position and the “fully open”
and “closed” positions, a constant power output can be
maintained. When the waste gate is fully open, all the exhaust
gases are directed overboard to the atmosphere, and no air is
compressed and delivered to the engine air inlet. Conversely,
when the waste gate is fully closed, a maximum volume
of exhaust gases flows into the turbocharger turbine, and
maximum supercharging is accomplished. Between these
Filtered Ram-Air
Heated air
Throttle body
Alternate air door (Normally closed)
FilterInduction system balance tube
Induction manifold
Left hand bankInduction manifold
Right hand bank
Turbocharger
Waste gate controller
Waste gate actuator
Waste gate
Throttle body
Alternate air door (open)
FilterInduction system balance tube
Induction manifold
Left hand bankInduction manifold
Right hand bank
Turbocharger
Waste gate controller
Waste gate actuator
Waste gate
CODERam air
Ram filtered air
Compressed air
Heated air
Mechanical linkage
Oil pressure
Oil return
Figure 3-17. A turbocharger air induction system.
Fuel injector
Density controllerOil return to
crankcaseDifferential pressure controlExhaust manifold
Oil return
to crankcase
Filter Air inletTurbocharger
Exhaust gas dischargeEngine oil
pressure inlet
Exhaust bypass
valve assembly
Exhaust gases
Compressor
discharge pressure
Inlet manifold
pressure
Oil
Air Oil Exhaust gas
Figure 3-18. Sea level booster turbosupercharger system.
two extremes of waste gate position, constant power output
can be achieved below the maximum altitude at which the
system is designed to operate. An engine with a critical
altitude of 16,000 feet cannot produce 100 percent of its rated
manifold pressure above 16,000 feet. Critical altitude means
the maximum altitude at which, in standard atmosphere, it
is possible to maintain, at a specified rotational speed, a
specified power or a specified manifold pressure.
A critical altitude exists for every possible power setting
below the maximum operating ceiling. If the aircraft is
flown above this altitude without a corresponding change in
the power setting, the waste gate is automatically driven to
the fully closed position in an effort to maintain a constant
power output. Thus, the waste gate is almost fully open at sea
level and continues to move toward the closed position as the
aircraft climbs, in order to maintain the preselected manifold
pressure setting. When the waste gate is fully closed (leaving
only a small clearance to prevent sticking), the manifold
pressure begins to drop if the aircraft continues to climb. If
a higher power setting cannot be selected, the turbocharger’s
critical altitude has been reached. Beyond this altitude, the power output continues to decrease. If a turbocharger waste
gate will not close fully, then the aircraft will not be able to
reach its critical altitude.
The position of the waste gate valve, which determines power
output, is controlled by oil pressure. Engine oil pressure acts
on a piston in the waste gate assembly, which is connected
by linkage to the waste gate valve. When oil pressure is
increased on the piston, the waste gate valve moves toward
the closed position, and engine output power increases.
Conversely, when the oil pressure is decreased, the waste gate
valve moves toward the open position, and output power is
decreased as described earlier.
The position of the piston attached to the waste gate valve is
dependent on bleed oil, which controls the engine oil pressure
applied to the top of the piston. Oil is returned to the engine
crankcase through two control devices, the density controller
and the differential pressure controller. These two controllers,
acting independently, determine how much oil is bled back to
the crankcase and establishes the oil pressure on the piston.
Sump tank
Engine oil coolerInduction “Y”
Cylinder 5 3 1
Fuel pumpEngine intake
Alternate air source
—pull to openAir filter
CompressorTurbine
Scavenge pump
Actuator
One-way
check valveTo engine
case6 4 2Injector nozzle shrouds
Engine exhaustAmbient
air intake
To engine sump
Tank ventThrottle control
Waste gateVent
overboardExhaust bypassExhaust
tailpipe
BellowsThrottle
connected
variable
controllerTo fuel pressure
gauge vent port Check valve
Figure 3-19. Components of a turbocharger system engine.
The density controller is designed to limit the manifold
pressure below the turbocharger’s critical altitude and regulates
bleed oil only at the full throttle position. The pressure- and
temperature-sensing bellows of the density controller react to
pressure and temperature changes between the fuel injector
inlet and the turbocharger compressor. The bellows, filled
with dry nitrogen, maintain a constant density by allowing the
pressure to increase as the temperature increases. Movement
of the bellows repositions the bleed valve, causing a change
in the quantity of bleed oil, which changes the oil pressure on
top of the waste gate piston. [Figure 3-18]
The differential pressure controller functions during all
positions of the waste gate valve other than the fully open
position, which is controlled by the density controller. One
side of the diaphragm in the differential pressure controller
senses air pressure upstream from the throttle; the other side
samples pressure on the cylinder side of the throttle valve.
[Figure 3-18] At the “wide open” throttle position when the
density controller controls the waste gate, the pressure across
the differential pressure controller diaphragm is at a minimum
and the controller spring holds the bleed valve closed. At “part throttle” position, the air differential is increased, opening
the bleed valve to bleed oil to the engine crankcase and
reposition the waste gate piston. Thus, the two controllers
operate independently to control turbocharger operation at all
positions of the throttle. Without the overriding function of the
differential pressure controller during part-throttle operation,
the density controller would position the waste gate valve for
maximum power. The differential pressure controller reduces
injector entrance pressure and continually repositions the
valve over the whole operating range of the engine.
The differential pressure controller reduces the unstable
condition known as “bootstrapping” during part-throttle
operation. Bootstrapping is an indication of unregulated power
change that results in the continual drift of manifold pressure.
This condition can be illustrated by considering the operation
of a system when the waste gate is fully closed. During this
time, the differential pressure controller is not modulating the
waste gate valve position. Any slight change in power caused
by a change in temperature or rpm fluctuation is magnified
and results in manifold pressure change since the slight change
causes a change in the amount of exhaust gas flowing to the
Sloped controller
TurbochargerAftercoolerAir reference lines
Scavenge oil returnInduction Y
Figure 3-20. An aftercooler installation. turbine. Any change in exhaust gas flow to the turbine causes
a change in power output and is reflected in manifold pressure
indications. Bootstrapping, then, is an undesirable cycle of
turbocharging events causing the manifold pressure to drift
in an attempt to reach a state of equilibrium.
Bootstrapping is sometimes confused with the condition
known as overboost, but bootstrapping is not a condition that
is detrimental to engine life. An overboost condition is one in
which manifold pressure exceeds the limits prescribed for a
particular engine and can cause serious damage. A pressure
relief valve when used in some systems, set slightly in excess
of maximum deck pressure, is provided to prevent damaging
over boost in the event of a system malfunction.
The differential pressure controller is essential to smooth
functioning of the automatically controlled turbocharger,
since it reduces bootstrapping by reducing the time required
to bring a system into equilibrium. There is still extra
throttle sensitivity with a turbocharged engine than with a
naturally aspirated engine. Rapid movement of the throttle
can cause a certain amount of manifold pressure drift in a
turbocharged engine. Less severe than bootstrapping, this
condition is called overshoot. While overshoot is not a
dangerous condition, it can be a source of concern to the
pilot or operator who selects a particular manifold pressure
setting only to find it has changed in a few seconds and must
be reset. Since the automatic controls cannot respond rapidly
enough to abrupt changes in throttle settings to eliminate the
inertia of turbocharger speed changes, overshoot must be
controlled by the operator. This can best be accomplished by
slowly making changes in throttle setting, accompanied by a
few seconds’ wait for the system to reach a new equilibrium.
Such a procedure is effective with turbocharged engines,
regardless of the degree of throttle sensitivity.
Turbocharger Controllers & System Descriptions
Turbocharger system engines contain many of the same
components mentioned with the previous systems.
[Figure 3-19] Some systems use special lines and fittings that
are connected to the upper-deck pressure for air reference to
the fuel injection system and in some cases for pressurizing
the magnetos. Basic system operation is similar to other
turbocharger systems with the main differences being in the
controllers. The controller monitors deck pressure by sensing
the output of the compressor. The controller controls the oil
flow through the waste gate actuator, which opens or closes
the exhaust bypass valve. If a malfunction occurs causing
the waste gate or controller to not be stable, it will cause the
engine to surge due to an erratic manifold pressure. When
deck pressure is insufficient, the controller restricts oil flow
thereby increasing oil pressure at the waste gate actuator. This
pressure acts on the piston to close off the waste gate valve, forcing more exhaust gas pulses to turn the turbine faster and
cause an increase in compressor output. When deck pressure
is too great, the opposite occurs. The exhaust waste gate fully
opens and bypasses some of the exhaust gases to decrease
exhaust flow across the turbine. An aftercooler is installed
in the induction air path between the compressor stage and
the air throttle inlet. [Figure 3-20]
Most turbochargers are capable of compressing the induction
air to the point at which it can raise the air temperature by a
factor of five. This means that full power takeoff on a 100 °F
day could produce induction air temperatures exiting the
compressor at up to 500 °F. This would exceed the allowable
throttle air inlet temperature on all reciprocating engine
models. Typically, the maximum air throttle inlet temperature
ranges from a low 230 °F to a high of 300 °F. Exceeding
these maximums can place the combustion chambers closer
to detonation. The function of the aftercooler is to cool the
compressed air, which decreases the likelihood of detonation
and increases the charge air density, which improves the
turbocharger performance for that engine design. On engine
start, the controller senses insufficient compressor discharge
pressure (deck pressure) and restricts the flow of oil from the
waste gate actuator to the engine. This causes the waste gate
butterfly valve to close. As the throttle is advanced, exhaust
gas flows across the turbine increases, thereby increasing
turbine/compressor shaft speed and compressor discharge
pressure. The controller senses the difference between
upper deck and manifold pressure. If either deck pressure
or throttle differential pressure rises, the controller poppet
valve opens, relieving oil pressure to the waste gate actuator.
This decreases turbocharger compressor discharge pressure
(deck pressure).
Variable Absolute Pressure Controller (VAPC)
The V APC contains an oil control valve similar to the other
Aneroid bellows assembly
Oil restrictor valve
Throttle controlled camEngine oil
from
wastegate
actuator
Oil return to
engineUpper
deck
pressure
Figure 3-21. A diagram of a variable absolute pressure controller
(VAPC).
Aneroid bellows assembly
Poppet
Poppet seat
Adjustment screwOil drain portOil inlet portDock
pressure
sensing
portManifold
pressure
sensing
port
Diaphragm
Figure 3-22. A diagram of a sloped controller used to maintain the
rated compressor discharge pressure at wide-open throttle.
controllers that were discussed. [Figure 3-21] The oil
restrictor is actuated by an aneroid bellows that is referenced
to upper deck pressure. A cam connected to the throttle
mechanism applies pressure to the restrictor valve and
aneroid. As the throttle is opened to greater values, the cam
applies a greater pressure to the aneroid. This increases the
amount of upper deck pressure necessary to compress the
aneroid and thereby open the oil restrictor valve. This means
that the scheduled absolute value of upper deck pressure that
is required to overcome the aneroid is variable by throttle
position. As the throttle is opened wide, the manifold pressure
and upper deck pressure requirements greatly increase.
Sloped Controller
The sloped controller is designed to maintain the rated
compressor discharge pressure at wide-open throttle and to
reduce this pressure at part throttle settings. [Figure 3-22]
A diaphragm, coupled with a spring-supported bellows for
absolute pressure reference, is exposed to deck pressure and
intake manifold pressure through ports located before and
after the throttle, respectively. This arrangement constantly
monitors deck pressure and the pressure differential between
the deck and manifold pressure due to a partially closed
throttle. If either deck pressure or throttle differential pressure rises, the controller poppet opens and decreases turbocharger
discharge (deck) pressure. The sloped controller is more
sensitive to the throttle differential pressure than to deck
pressure, thereby accomplishing deck pressure reduction as
the throttle is closed.
Absolute Pressure Controller
One device used to control the speed and output of the
turbocharger, but controls the system only at maximum
output, is the absolute pressure controller. The absolute
pressure controller contains an aneroid bellows that is
referenced to upper deck pressure. It operates the waste gate,
which diverts, more or less, exhaust gas over the turbine.
As an absolute pressure setting is reached, it bypasses
oil, and relieves the pressure on the waste gate actuator.
This allows the absolute pressure controller to control the
maximum turbocharger compressor discharge pressure. The
turbocharger is completely automatic, requiring no pilot
action up to the critical altitude.
Turbocharger System Troubleshooting
Figure 3-23 includes some of the most common turbocharger
system malfunctions together with their cause and repair.
These troubleshooting procedures are presented as a
guide only and should not be substituted for applicable
Trouble Probable Cause Remedy
Aircraft fails to
reach critical
altitude
Engine surges
Wastegate will
not close fully
Wastegate will
not open
Differential
controller
malfunctions
Density controller
malfunctionsDamaged compressor or turbine wheel
Exhaust system leaks
Faulty turbocharger bearings
Wastegate will not close fully
Malfunctioning controller
Bootstrapping
Wastegate malfunction
Controller malfunction
Wastegate bypass valve bearing tight
Oil inlet orifice blocked
Differential controller malfunction
Broken wastegate linkage
Oil outlet obstructed
Broken wastegate linkage
Controller malfunction
Seals leaking
Diaphragm broken
Controller valve stuck
Seals leaking
Bellows damaged
Valve stuckReplace turbocharger
Repair leaks
Replace turbocharge
Refer to wastegate in the trouble column
Refer to differential controller in the trouble column
Ensure engine is operated in proper range
Refer to wastegate in the trouble column
Refer to differential controller in the trouble column
Replace bypass valve
Clean orifice
Refer to controller in the trouble column
Replace linkage and adjust waste gate for proper opening and closing
Clean and reconnect oil return line
Replace linkage and adjust waste gate opening and closing
Refer to controller in the trouble column
Replace controller
Replace controller
Replace controller
Replace controller
Replace controller
Replace controller
Figure 3-23. Common issues when troubleshooting turbocharger systems.
Figure 3-24. An example of a turbine engine inlet.manufacturer’s instructions or troubleshooting procedures.
Turbine Engine Inlet Systems
The engine inlet of a turbine engine is designed to provide a
relatively distortion-free flow of air, in the required quantity,
to the inlet of the compressor. [Figure 3-24] Many engines
use inlet guide vanes (IGV) to help straighten the airflow and
direct it into the first stages of the compressor. A uniform and
steady airflow is necessary to avoid compressor stall (airflow
tends to stop or reverse direction of flow) and excessive
internal engine temperatures in the turbine section. Normally,
the air-inlet duct is considered an airframe part and not a
part of the engine. However, the duct is very important to
the engine’s overall performance and the engine’s ability to
produce an optimum amount of thrust.
A gas turbine engine consumes considerable more airflow
than a reciprocating engine. The air entrance passage is
correspondingly larger. Furthermore, it is more critical in
determining engine and aircraft performance, especially
at high airspeeds. Inefficiencies of the inlet duct result in
successively magnified losses through other components of the engine. The inlet varies according to the type of turbine
engine. Small turboprop and turboshaft engines have a
lower airflow than large turbofan engines which require a
completely different type of inlet. Many turboprop, auxiliary
power units, and turboshaft engines use screens that cover
the inlet to prevent foreign object damage (FOD).
Figure 3-25. An example of a divided-entrance duct.
Figure 3-26. An inlet duct acts as a diffuser to decrease the airflow
velocity and to increase the static pressure of air. As aircraft speed increases, thrust tends to decrease
somewhat; as the aircraft speed reaches a certain point, ram
recovery compensates for the losses caused by the increases
in speed. The inlet must be able to recover as much of the total
pressure of the free airstream as possible. As air molecules
are trapped and begin to be compressed in the inlet, much
of the pressure loss is recovered. This added pressure at the
inlet of the engine increases the pressure and airflow to the
engine. This is known as “ram recovery” or “total pressure
recovery.” The inlet duct must uniformly deliver air to
the compressor inlet with as little turbulence and pressure
variation as possible. The engine inlet duct must also hold
the drag effect on the aircraft to a minimum.
Air pressure drop in the engine inlet is caused by the friction
of the air along both sides of the duct and by the bends in the
duct system. Smooth flow depends upon keeping the amount
of turbulence to a minimum as the air enters the duct. On
engines with low flow rates, turning the airflow allows the
engine nacelle to be smaller and have less drag. On turbofan
engines, the duct must have a sufficiently straight section to
ensure smooth, even airflow because of the high airflows.
The choice of configuration of the entrance to the duct is
dictated by the location of the engine within the aircraft
and the airspeed, altitude, and attitude at which the aircraft
is designed to operate. To accomplish this, inlet ducts are
designed to function as diffusers with a divergent shape,
decreasing the velocity and increasing the static pressure of
the air passing through them.
Divided-Entrance Duct
The requirements of high-speed, single- or twin-engine
military aircraft, in which the pilot sits low in the fuselage and
close to the nose, render it difficult to employ the older type
single-entrance duct, which is not used on modern aircraft.
Some form of a divided duct, which takes air from either side
of the fuselage, has become fairly widely used. This divided
duct can be either a wing-root inlet or a scoop at each side of
the fuselage. [Figure 3-25] Either type of duct presents more
problems to the aircraft designer than a single-entrance duct
because of the difficulty of obtaining sufficient airscoop area
without imposing prohibitive amounts of drag. Internally,
the problem is the same as that encountered with the single-
entrance duct: to construct a duct of reasonable length with
as few bends as possible. Scoops at the sides of the fuselage
are often used. These side scoops are placed as far forward
as possible to permit a gradual bend toward the compressor
inlet, making the airflow characteristics approach those of a
single-entrance duct. A series of turning vanes is sometimes
placed in the side-scoop inlet to assist in straightening the
incoming airflow and to prevent turbulence.Variable-Geometry Duct
The main function of an inlet duct is to furnish the proper
amount of air to the engine inlet. In a typical military aircraft
using a turbojet or low bypass turbofan engine, the maximum
airflow requirements are such that the Mach number of the
airflow directly ahead of the face of the engine is less than
Mach 1. Airflow through the engine must be less than Mach 1
at all times. Therefore, under all flight conditions, the velocity
of the airflow as it enters the air-inlet duct must be reduced
through the duct before the airflow is ready to enter the
compressor. To accomplish this, inlet ducts are designed to
function as diffusers, decreasing the velocity and increasing the
static pressure of the air passing through them. [Figure 3-26]
As with military supersonic aircraft, a diffuser progressively
decreases in area in the downstream direction. Therefore, a
supersonic inlet duct follows this general configuration until
the velocity of the incoming air is reduced to Mach 1. The
Figure 3-29. An example of an inlet screen on an APU.
Figure 3-28. An example of a turboprop engine that incorporates
inlet screens.
Convergent
supersonic
sectionDivergent
subsonic
section
Figure 3-27. The aft section of an inlet duct acting as a subsonic
diffuser. aft section of the duct then increases in area, since this part
must act as a subsonic diffuser. [Figure 3-27] In practice, inlet
ducts for supersonic aircraft follows this general design only
as much as practical, depending upon the design features of
the aircraft. For very high speed aircraft, the inside area of
configuration of the duct is changed by a mechanical device
as the speed of the aircraft increases or decreases. A duct of
this type is usually known as a variable-geometry inlet duct.
Military aircraft use the three methods described above to
diffuse the inlet air and slow the inlet airflow at supersonic
flight speeds. One is to vary the area, or geometry, of the inlet
duct either by using a movable restriction, such as a ramp
or wedge, inside the duct. Another system is some sort of a
variable airflow bypass arrangement, which extracts part of
the inlet airflow from the duct ahead of the engine. In some
cases, a combination of both systems is used.
The third method is the use of a shock wave in the airstream.
A shock wave is a thin region of discontinuity in a flow of
air or gas, during which the speed, pressure, density, and
temperature of the air or gas undergo a sudden change.
Stronger shock waves produce larger changes in the
properties of the air or gas. A shock wave is willfully set up
in the supersonic flow of the air entering the duct, by means
of some restriction or small obstruction which automatically
protrudes into the duct at high flight Mach numbers. The
shock wave results in diffusion of the airflow, which, in turn,
decreases the velocity of the airflow. In at least one aircraft
installation, both the shock method and the variable-geometry
method of causing diffusion are used in combination. The
same device that changes the area of the duct also sets up a
shock wave that further reduces the speed of the incoming
air within the duct. The amount of change in duct area and
the magnitude of the shock are varied automatically with the
airspeed of the aircraft.Compressor Inlet Screens
To prevent the engine from readily ingesting any items that
can be drawn in the intake, a compressor inlet screen is
sometimes placed across the engine air inlet at some location
along the inlet duct. Engines that incorporate inlet screens,
such as turboprops [Figure 3-28] and APUs [Figure 3-29] are
not as vulnerable to FOD. The advantages and disadvantages
of a screen vary. If the engine is readily subjected to internal
damage, as would be the case for an engine having an axial
compressor fitted with aluminum compressor blades, an
inlet screen is almost a necessity. Screens, however, add
appreciably to inlet duct pressure loss and are very susceptible
to icing. Failure due to fatigue is also a problem. A failed
screen can sometimes cause more damage than no screen
at all. In some instances, inlet screens are made retractable
and may be withdrawn from the airstream after takeoff or
whenever icing conditions prevail. Such screens are subject
to mechanical failure and add both weight and bulk to
the installation. In large turbofan engines having steel or
titanium compressor (fan) blades, which do not damage
Figure 3-30. A bellmouth inlet used during system tests.
Figure 3-31. Probes within a bellmouth inlet used to measure intake
temperature and pressure.
Figure 3-32. An example of a ducted arrangement on a turboprop
engine.easily, the disadvantages of compressor screens outweigh
the advantages, so they are not generally used.
Bellmouth Compressor Inlets
A bellmouth inlet is usually installed on an engine undergoing
testing in a test cell. For this reason, the bellmouth inlet duct
is most often used on helicopter airframes. [Figure 3-30]
It is generally equipped with probes that, with the use of
instruments, can measure intake temperature and pressure
(total and static). [Figure 3-31] During testing, it is important
that the outside static air is allowed to flow into the engine
with as little resistance as possible. The bellmouth is attached
to the movable part of the test stand and moves with the
engine. The thrust stand is made up of two components,
one nonmoving and one moving. This is so the moving
component can push against a load cell and measure thrust
during the testing of the engine. The bellmouth is designed
with the single objective of obtaining very high aerodynamic
efficiency. Essentially, the inlet is a bell-shaped funnel
having carefully rounded shoulders which offer practically
no air resistance. [Figure 3-30] Duct loss is so slight that it is considered zero. The engine can, therefore, be operated
without the complications resulting from losses common
to an installed aircraft inlet duct. Engine performance data,
such as rated thrust and thrust specific fuel consumption, are
obtained while using a bellmouth inlet. Usually, the inlets are
fitted with protective screening. In this case, the efficiency
lost as the air passes through the screen must be taken into
account when very accurate engine data are necessary.
Turboprop & Turboshaft Compressor Inlets
The air inlet on a turboprop is more of a problem than some
other gas turbine engines because the propeller drive shaft,
the hub, and the spinner must be considered in addition to
other inlet design factors. The ducted arrangement is generally
considered the best inlet design of the turboprop engine as
far as airflow and aerodynamic characteristics are concerned.
[Figure 3-32] The inlet for many types of turboprops are
anti-iced by using electrical elements in the lip opening of
the intake. Ducting either part of the engine or nacelle directs
the airflow to the intake of the engine. Deflector doors are
sometimes used to deflect ice or dirt away from the intake.
[Figure 3-33] The air then passes through a screen and into
the engine on some models. A conical spinner, which does
not allow ice to build up on the surface, is sometimes used
with turboprop and turbofan engines. In either event, the
arrangement of the spinner and the inlet duct plays an important
function in the operation and performance of the engine.
Turbofan Engine Inlet Sections
High-bypass turbofan engines are usually constructed with
the fan at the forward end of the compressor. A typical
turbofan intake section is shown in Figure 3-34. Sometimes,
the inlet cowl is bolted to the front of the engine and provides
the airflow path into the engine. In dual compressor (dual
spool) engines, the fan is integral with the relatively slow-
turning, low-pressure compressor, which allows the fan
Figure 3-35. Rubber stripping inside a turbofan engine inlet allows
for friction for short periods of time during changes in the flightpath.
Figure 3-36. The air that passes through the inner part of the fan
blades becomes the primary airstream.
Figure 3-34. A typical turbofan intake section.
Figure 3-33. Deflector doors used to deflect ice or dirt away from
the intake.
blades to rotate at low tip speed for best fan efficiency. The
fan permits the use of a conventional air inlet duct, resulting
in low inlet duct loss. The fan reduces engine damage from
ingested foreign material because much of any material
that may be ingested is thrown radially outward and passes
through the fan discharge rather than through the core of
the engine. Warm bleed air is drawn from the engine and
circulated on the inside of the inlet lip for anti-icing. The
fan hub or spinner is either heated by warm air or is conical
as mentioned earlier. Inside the inlet by the fan blade tips is
an abraidable rub strip that allows the fan blades to rub for
short times due to flightpath changes. [Figure 3-35] Also,
inside the inlet are sound-reducing materials to lower the
noise generated by the fan.
The fan on high-bypass engines consists of one stage
of rotating blades and stationary vanes that can range in
diameter from less than 84 inches to more than 112 inches.
[Figure 3-36] The fan blades are either hollow titanium or
composite materials. The air accelerated by the outer part of the fan blades forms a secondary airstream, which is ducted
overboard without passing through the main engine. This
secondary air (fan flow) produces 80 percent of the thrust in
high-bypass engines. The air that passes through the inner
part of the fan blades becomes the primary airstream (core
flow) through the engine itself. [Figure 3-36]
The air from the fan exhaust, which is ducted overboard, may
be discharged in either of two ways:
1. To the outside air through short ducts (dual exhaust
nozzles) directly behind the fan. [Figure 3-37]
2. Ducted fan, which uses closed ducts all the way to the
rear of the engine, where it is exhausted to the outside
air through a mixed exhaust nozzle. This type engine
is called a ducted fan and the core airflow and fan
airflow mix in a common exhaust nozzle.
Fan discharge
Figure 3-37. Air from the fan exhaust can be discharged overboard
through short ducts directly behind the fan.
Upper sheet jacket
Lower sheet jacketHeat exchanger collector tubeFigure 3-38. Location of a typical collector exhaust system.
Figure 3-39. A cabin heater exhaust shroud.Reciprocating Engine Exhaust Systems
The reciprocating engine exhaust system is fundamentally
a scavenging system that collects and disposes of the high
temperature, noxious gases being discharged by the engine.
Its main function is to dispose of the gases with complete
safety to the airframe and the occupants of the aircraft. The
exhaust system can perform many useful functions, but its
first duty is to provide protection against the potentially
destructive action of the exhaust gases. Modern exhaust
systems, though comparatively light, adequately resist high
temperatures, corrosion, and vibration to provide long,
trouble-free operation with minimum maintenance.
There are two general types of exhaust systems in use on
reciprocating aircraft engines: the short stack (open) system
and the collector system. The short stack system is generally
used on nonsupercharged engines and low-powered engines
where noise level is not too objectionable. The collector system
is used on most large nonsupercharged engines and on all
turbosupercharged engines and installations on which it would
improve nacelle streamlining or provide easier maintenance in
the nacelle area. On turbosupercharged engines, the exhaust
gases must be collected to drive the turbine compressor of the
supercharger. Such systems have individual exhaust headers
that empty into a common collector ring with only one outlet.
From this outlet, the hot exhaust gas is routed via a tailpipe
to the turbosupercharger that drives the turbine. Although the
collector system raises the back pressure of the exhaust system,
the gain in horsepower from turbosupercharging more than
offsets the loss in horsepower that results from increased back
pressure. The short stack system is relatively simple, and its
removal and installation consists essentially of removing and
installing the hold-down nuts and clamps. Short stack systems
have limited use on most modern aircraft.
In Figure 3-38, the location of typical collector exhaust system components of a horizontally opposed engine is
shown in a side view. The exhaust system in this installation
consists of a down-stack from each cylinder, an exhaust
collector tube on each side of the engine, and an exhaust
ejector assembly protruding aft and down from each side of
the firewall. The down-stacks are connected to the cylinders
with high temperature locknuts and secured to the exhaust
collector tube by ring clamps. A cabin heater exhaust shroud
is installed around each collector tube. [Figure 3-39]
The collector tubes terminate at the exhaust ejector openings
at the firewall and are tapered to deliver the exhaust gases
at the proper velocity to induce airflow through the exhaust
Figure 3-41. A radial engine exhaust collector ring is graduated in
size from the inboard side to the outboard side.
A
C
B DE F
Clamp assembly
Telescoping flange
Main exhaust segment
Engine diaphragm
Clamp assembly
Clevis pin & washerA
B
C
D
E
F
Figure 3-40. Elements of an exhaust collector ring installed on a
radial engine.ejectors. The exhaust ejectors consist of a throat-and-duct
assembly that utilizes the pumping action of the exhaust
gases to induce a flow of cooling air through all parts of the
engine compartment (augmenter tube action).
Radial Engine Exhaust Collector Ring System
Figure 3-40 shows the exhaust collector ring installed on
a 14-cylinder radial engine. The collector ring is a welded
corrosion-resistant steel assembly manufactured in seven
sections, with each section collecting the exhaust from two
cylinders. The sections are graduated in size. [Figure 3-41]
The small sections are on the inboard side, and the largest
sections are on the outboard side at the point where the tailpipe
connects to the collector ring. Each section of the collector
ring is bolted to a bracket on the blower section of the engine
and is partly supported by a sleeve connection between the
collector ring ports and the short stack on the engine exhaust
ports. The exhaust tailpipe is joined to the collector ring by a
telescoping expansion joint, which allows enough slack for the
removal of segments of the collector ring without removing
the tailpipe. The exhaust tailpipe is a welded, corrosion-
resistant steel assembly consisting of the exhaust tailpipe and,
on some aircraft, a muff-type heat exchanger.
Manifold & Augmentor Exhaust Assembly
Some radial engines are equipped with a combination
exhaust manifold and augmentor assembly. On a typical
18-cylinder engine, two exhaust assemblies and two
augmentor assemblies are used. Each manifold assembly
collects exhaust gases from nine cylinders and discharges the
gases into the forward end of the augmentor assembly. The
exhaust gases are directed into the augmentor bellmouths.
The augmentors are designed to produce a venturi effect to
draw an increased airflow over the engine to augment engine cooling. An augmentor vane is located in each tailpipe. When
the vane is fully closed, the cross-sectional area of the tailpipe
is reduced by approximately 45 percent. The augmentor
vanes are operated by an electrical actuator, and indicators
adjacent to the augmentor vane switches in the flight deck
show vane positions. The vanes may be moved toward the
“closed” position to decrease the velocity of flow through
the augmentor to raise the engine temperature. This system is
only used with older aircraft that generally use radial engines.
Reciprocating Engine Exhaust System Maintenance
Practices
Any exhaust system failure should be regarded as a severe
hazard. Depending on the location and type of failure,
an exhaust system failure can result in carbon monoxide
poisoning of crew and passengers, partial or complete loss
of engine power, or an aircraft fire. Cracks in components,
leaking gaskets, or complete failure can cause serious
problems in flight. Often, these failures can be detected
before complete failure. Black soot around an exhaust gasket
shows the gasket has failed. The exhaust system should be
inspected very thoroughly.
Exhaust System Inspection
While the type and location of exhaust system components
vary somewhat with the type of aircraft, the inspection
requirements for most reciprocating engine exhaust systems
A
B
C
Figure 3-42. Primary inspection areas of three types of exhaust
systems.are very similar. The following paragraphs include a
discussion of the most common exhaust system inspection
items and procedures for all reciprocating engines.
Figure 3-42 shows the primary inspection areas of three
types of exhaust systems.
When performing maintenance on exhaust systems, never use
galvanized or zinc-plated tools on the exhaust system. Exhaust
system parts should never be marked with a lead pencil. The
lead, zinc, or galvanized mark is absorbed by the metal of the
exhaust system when heated, creating a distinct change in its
molecular structure. This change softens the metal in the area
of the mark, causing cracks and eventual failure.
After the installation of a complete exhaust system and all
pieces of engine cowl are installed and secured, the engine
should be operated to allow the exhaust system to heat up
to normal operating temperatures. The engine is then shut down and the cowling removed to expose the exhaust system.
Each clamped connection and each exhaust port connection
should be inspected for evidence of exhaust gas leakage.
An exhaust leak is indicated by a flat gray or a sooty black
streak on the pipes in the area of the leak. An exhaust leak
is usually the result of poor alignment of two mated exhaust
system members. When a leaking exhaust connection is
discovered, the clamps should be loosened, and the leaking
units repositioned to ensure a gas-tight fit.
After repositioning, the system nuts should be retightened
enough to eliminate any looseness without exceeding the
specified torque. If tightening to the specified torque does
not eliminate looseness, the bolts and nuts should be replaced
since they have probably stretched. After tightening to the
specified torque, all nuts should be safetied. With the cowling
removed, all necessary cleaning operations can be performed.
Some exhaust units are manufactured with a plain sandblast
finish. Others may have a ceramic-coated finish. Ceramic-
coated stacks should be cleaned by degreasing only. They
should never be cleaned with sandblast or alkali cleaners.
During the inspection of an exhaust system, close attention
should be given to all external surfaces of the exhaust system
for cracks, dents, or missing parts. This also applies to welds,
clamps, supports, support attachment lugs, bracing, slip joints,
stack flanges, gaskets, and flexible couplings. Each bend
should be examined, as well as areas adjacent to welds. Any
dented areas or low spots in the system should be inspected
for thinning and pitting due to internal erosion by combustion
products or accumulated moisture. An ice pick or similar
pointed instrument is useful in probing suspected areas.
The system should be disassembled as necessary to inspect
internal baffles or diffusers. If a component of the exhaust
system is inaccessible for a thorough visual inspection or is
hidden by nonremovable parts, it should be removed and
checked for possible leaks. This can often be accomplished
best by plugging the openings of the component, applying
a suitable internal pressure (approximately 2 psi), and
submerging it in water. Any leaks cause bubbles that
can readily be detected. The procedures required for an
installation inspection are also performed during most regular
inspections. Daily inspection of the exhaust system usually
consists of checking the exposed exhaust system for cracks,
scaling, excessive leakage, and loose clamps.
Muffler & Heat Exchanger Failures
Approximately half of all muffler and heat exchanger failures
can be traced to cracks or ruptures in the heat exchanger
surfaces used for cabin and carburetor heat sources. Failures
in the heat exchanger surface (usually in the outer wall) allow
Figure 3-43. An example of internal muffler failure. Muffler failure
can be caused by erosion and carbonization, which in turn can lead
to breakage blocking exhaust flow.exhaust gases to escape directly into the cabin heat system.
These failures, in most cases, are caused by thermal and
vibration fatigue cracking in areas of stress concentration.
Failure of the spot-welds, which attach the heat transfer pins,
can result in exhaust gas leakage. In addition to a carbon
monoxide hazard, failure of heat exchanger surfaces can
permit exhaust gases to be drawn into the engine induction
system, causing engine overheating and power loss.
Exhaust Manifold & Stack Failures
Exhaust manifold and stack failures are usually fatigue
failures at welded or clamped points (e.g., stack-to-flange,
stack-to-manifold, and crossover pipe or muffler connections).
Although these failures are primarily fire hazards, they also
present carbon monoxide problems. Exhaust gases can
enter the cabin via defective or inadequate seals at firewall
openings, wing strut fittings, doors, and wing root openings.
Internal Muffler Failures
Internal failures (baffles, diffusers, etc.) can cause partial or
complete engine power loss by restricting the flow of the
exhaust gases. If pieces of the internal baffling breaks loose
and partially or totally blocks the flow of exhaust gases,
engine failure can occur. [Figure 3-43] As opposed to other
failures, erosion and carburization caused by the extreme
thermal conditions are the primary causes of internal failures.
Engine backfiring and combustion of unburned fuel within the
exhaust system are probable contributing factors. In addition,
local hot-spot areas caused by uneven exhaust gas flow can
result in burning, bulging, or rupture of the outer muffler wall.Exhaust Systems with Turbocharger
When a turbocharger or a turbosupercharger system is
included, the engine exhaust system operates under greatly
increased pressure and temperature conditions. Extra
precautions should be taken in exhaust system care and
maintenance. During high-pressure altitude operation, the
exhaust system pressure is maintained at or near sea level
values. Due to the pressure differential, any leaks in the system
allow the exhaust gases to escape with torch-like intensity
that can severely damage adjacent structures. A common
cause of malfunction is coke deposits (carbon buildup) in the
waste gate unit causing erratic system operation. Excessive
deposit buildups may cause the waste gate valve to stick in
the “closed” position, causing an overboost condition. Coke
deposit buildup in the turbo itself causes a gradual loss of
power in flight and low manifold pressure reading prior to
takeoff. Experience has shown that periodic de-coking, or
removal of carbon deposits, is necessary to maintain peak
efficiency. Clean, repair, overhaul, and adjust the system
components and controls in accordance with the applicable
manufacturer’s instructions.
Augmentor Exhaust System
On exhaust systems equipped with augmentor tubes, the
augmentor tubes should be inspected at regular intervals for
proper alignment, security of attachment, and general overall
condition. Even where augmentor tubes do not contain heat
exchanger surfaces, they should be inspected for cracks
along with the remainder of the exhaust system. Cracks
in augmentor tubes can present a fire or carbon monoxide
hazard by allowing exhaust gases to enter the nacelle, wing,
or cabin areas.
Exhaust System Repairs
It is generally recommended that exhaust stacks, mufflers,
tailpipes, etc., be replaced with new or reconditioned
components rather than repaired. Welded repairs to exhaust
systems are complicated by the difficulty of accurately
identifying the base metal so that the proper repair materials
can be selected. Changes in composition and grain structure
of the original base metal further complicate the repair.
However, when welded repairs are necessary, the original
contours should be retained; the exhaust system alignment
must not be warped or otherwise affected. Repairs or sloppy
weld beads that protrude internally are not acceptable as they
cause local hot spots and may restrict exhaust gas flow. The
proper hardware and clamps should always be used when
repairing or replacing exhaust system components. Steel or
low temperature, self-locking nuts should not be substituted
for brass or special high temperature locknuts used by the
manufacturer. Old gaskets should never be re-used. When
disassembly is necessary, gaskets should be replaced with
new ones of the same type provided by the manufacturer.
Fan airflow
Fan airflow
Core airflow
Core airflow
Figure 3-44. Path of both core exhaust flow and fan flow from the engine to separate nozzles.
Figure 3-45. Fan air is directed through the fan blades and exit
vanes.Turbine Engine Exhaust Nozzles
Turbine engines have several different types of exhaust
nozzles depending upon the type of engine. Turboshaft
engines in helicopters can have an exhaust nozzle that forms
a divergent duct. This type of nozzle would not provide any
thrust, all engine power going to rotate the rotors, improving
helicopter hovering abilities. Turbofan engines tend to fall
into either ducted fan of unducted fan engines. Ducted fan
engines take the fan airflow and direct it through closed
ducts along the engine. Then, it flows into a common exhaust
nozzle. The core exhaust flow and the fan flow mix and flow
from the engine through this mixed nozzle. The unducted fan
has two nozzles, one for the fan airflow and one for the core
airflow. These both flow to ambient air separate from each
other and have separate nozzles. [Figure 3-44]
The unducted engine or the separate nozzle engine handles
high amounts of airflow. The fan air which creates most of the
thrust (80–85 percent total thrust) must be directed through
the fan blades and exit vanes with little turbulence as possible.
[Figure 3-45] The core airflow needs to be straightened as
it comes from the turbine. Through the use of a converging
nozzle, the exhaust gases increase in velocity before they are
discharged from the exhaust nozzle. Increasing the velocity of
the gases increases their momentum and increases the thrust
produced (20–15 percent total thrust). Most of the energy of
the gases have been absorbed to drive the fan through the
low-pressure turbine stages.
Turboprop exhaust nozzles provide small amounts of thrust
(10–15 percent) but are mainly used to discharge the exhaust
gases from the aircraft. Most of the energy has been transferred
to the propeller. On some turboprop aircraft, an exhaust duct
is often referred to as a tailpipe, although the duct itself is essentially a simple, stainless steel, conical or cylindrical pipe.
The assembly also includes an engine tail cone and the struts
inside the duct. The tail cone and the struts add strength to
the duct, impart an axial direction to the gas flow, and smooth
the gas flow. In a typical installation, the tailpipe assembly is
mounted in the nacelle and attached at its forward end to the
firewall. The forward section of the tailpipe is funnel shaped
and surrounds but does not contact the turbine exhaust section.
This arrangement forms an annular gap that serves as an air
ejector for the air surrounding the engine hot section. As the
high-velocity exhaust gases enter the tailpipe, a low-pressure
effect is produced which causes the air around the engine hot
section to flow through the annular gap into the tailpipe. The
rear section of the tailpipe is secured to the airframe by two
support arms, one on each side of the tailpipe. The support
arms are attached to the upper surface of the wing in such
Gas attains sonic velocityExhaust nozzleSubsonic convergent
sectionSupersonic divergent
section
Tail cone Exhaust nozzle
Figure 3-46. Exhaust gases exit the rear of the engine through the
exhaust nozzle.Figure 3-47. A convergent-divergent nozzle can be used to help
produce more thrust when exhaust gas velocities are greater than
Mach 1.a way that allow movement fore and aft to compensate for
expansion. The tailpipe assembly is wrapped in an insulating
blanket to shield the surrounding area from the high heat
produced by the exhaust gases. Such blankets may be made of
a stainless steel laminated sheet on the outside and fiberglass
on the inside. This is used when the engine exhaust is located
some distance from the edge of the wing or aircraft structure.
Immediately aft of the turbine outlet, and usually just forward
of the flange to which the exhaust duct is attached, the engine
is instrumented for turbine discharge pressure. One or more
pressure probes are inserted into the exhaust duct to provide
adequate sampling of the exhaust gases. In large engines,
it is not practical to measure the internal temperature at the
turbine inlet, so the engine is often also instrumented for
exhaust gas temperature at the turbine outlet.
Convergent Exhaust Nozzle
As the exhaust gases exit the rear of the engine, they flow
into the exhaust nozzle. [Figure 3-46] The very first part of
the exhaust nozzle and the exhaust plug form a divergent duct
to reduce turbulence in the airflow, then the exhaust gases
flow into the convergent component of the exhaust nozzle
where the flow is restricted by a smaller outlet opening. Since
this forms a convergent duct, the gas velocity is increased
providing increased thrust. The restriction of the opening of
the outlet of the exhaust nozzle is limited by two factors. If
the nozzle opening is too big, thrust is being wasted. If it is
too little, the flow is choked in the other components of the
engine. In other words, the exhaust nozzle acts as an orifice,
the size of which determines the density and velocity of
the gases as they emerge from the engine. This is critical to
thrust performance. Adjusting the area of the exhaust nozzle
changes both the engine performance and the exhaust gas
temperature. When the velocity of the exhaust gases at the nozzle opening becomes Mach 1, the flow passes only at this
speed—it does not increase or decrease. Sufficient flow to
maintain Mach 1 at the nozzle opening and have extra flow
(flow that is being restricted by the opening) creates what
is called a choked nozzle. The extra flow builds up pressure
in the nozzle, which is sometimes called pressure thrust. A
differential in pressure exists between the inside of the nozzle
and the ambient air. By multiplying this difference in pressure
times the area of the nozzle opening, pressure thrust can be
calculated. Many engines cannot develop pressure thrust
because most of the energy is used to drive turbines that turn
propellers, large fans, or helicopter rotors.
Convergent-Divergent Exhaust Nozzle
Whenever the engine pressure ratio is high enough to produce
exhaust gas velocities which might exceed Mach 1 at the
engine exhaust nozzle, more thrust can be gained by using
a convergent-divergent type of nozzle. [Figure 3-47] The
advantage of a convergent-divergent nozzle is greatest at
high Mach numbers because of the resulting higher pressure
ratio across the engine exhaust nozzle.
To ensure that a constant weight or volume of a gas flows past
any given point after sonic velocity is reached, the rear part
of a supersonic exhaust duct is enlarged to accommodate the
additional weight or volume of a gas that flows at supersonic
rates. If this is not done, the nozzle does not operate efficiently.
This is the divergent section of the exhaust duct.
When a divergent duct is used in combination with a
conventional exhaust duct, it is called a convergent-divergent
exhaust duct. In the convergent-divergent, or C-D nozzle, the
convergent section is designed to handle the gases while they
remain subsonic, and to deliver the gases to the throat of the
nozzle just as they attain sonic velocity. The divergent section
Forward thrust
Reverse thrust
Figure 3-48. Engine exhaust gases are blocked and diverted in a
reserve direction during thrust reversal.handles the gases, further increasing their velocity, after they
emerge from the throat and become supersonic. As the gas
flows from the throat of the nozzle, it becomes supersonic
(Mach 1 and above) and then passes into the divergent section
of the nozzle. Since it is supersonic, it continues to increase
in velocity. This type of nozzle is generally used on very high
speed aerospace vehicles.
Thrust Reversers
As aircraft have increased in gross weights with higher
landing airspeeds, the problem of stopping an aircraft after
landing has greatly increased. In many instances, the aircraft
brakes can no longer be relied upon solely to slow the aircraft
within a reasonable distance, immediately after touchdown.
Most thrust reverser systems can be divided into two
categories: mechanical-blockage and aerodynamic-blockage.
Mechanical blockage is accomplished by placing a removable
obstruction in the exhaust gas stream, usually somewhat to the
rear of the nozzle. The engine exhaust gases are mechanically
blocked and diverted at a suitable angle in the reverse
direction by an inverted cone, half-sphere, or clam shell.
[Figure 3-48] This is placed in position to reverse the flow of
exhaust gases. This type is generally used with ducted turbofan
engines, where the fan and core flow mix in a common nozzle
before exiting the engine. The clamshell-type or mechanical-
blockage reverser operates to form a barrier in the path of
escaping exhaust gases, which nullifies and reverses the
forward thrust of the engine. The reverser system must be
able to withstand high temperatures, be mechanically strong,
relatively light in weight, reliable, and “fail-safe.” When not
in use, it must be streamlined into the configuration of the
engine nacelle. When the reverser is not in use, the clamshell
doors retract and nest neatly around the engine exhaust duct,
usually forming the rear section of the engine nacelle.
In the aerodynamic blockage type of thrust reverser, used
mainly with unducted turbofan engines, only fan air is used
to slow the aircraft. A modern aerodynamic thrust reverser
system consists of a translating cowl, blocker doors, and
cascade vanes that redirect the fan airflow to slow the
aircraft. [Figure 3-49] If the thrust levers are at idle position
and the aircraft has weight on the wheels, moving the thrust
levers aft activates the translating cowl to open, closing the
blocker doors. This action stops the fan airflow from going
aft and redirects it through the cascade vanes, which direct
the airflow forward to slow the aircraft. Since the fan can
produce approximately 80 percent of the engine’s thrust,
the fan is the best source for reverse thrust. By returning the
thrust levers (power levers) to the idle position, the blocker
doors open and the translating cowl closes.
A thrust reverser must not have any adverse effect on engine
operation either deployed or stowed. Generally, there is an indication in the flight deck with regard to the status of
the reverser system. The thrust reverser system consists of
several components that move either the clam shell doors
or the blocker door and translating cowl. Actuating power
is generally pneumatic or hydraulic and uses gearboxes,
flexdrives, screwjacks, control valves, and air or hydraulic
motors to deploy or stow the thrust reverser systems. The
systems are locked in the stowed position until commanded to
deploy by the flight deck. Since there are several moving parts,
maintenance and inspection requirements are very important.
While performing any type of maintenance, the reverser
system must be mechanically locked out from deploying while
personnel are in the area of the reverser system.
Afterburning/Thrust Augmentation
The terms afterburning and thrust augmentation generally
pertain to military engine applications. The terms are used to
describe the same system. Normally, this is used to increase
the thrust of the engine up to double the original thrust. The
required additions to the exhaust nozzle for this system are
a flame stabilizer, fuel manifold, flame holder, igniter, and a
Forward thrust positionBlocker doors
Lock indicator light switch
Air motorVentReverse thrust
select lever
(forward thrust)
ScrewjackTranslating cowl Cascade vanes
Power lever
Flexible drive
GearboxDirection and speed control valve
Flexible driveFlexible drive
Lock and sequence valve
ExhaustSelector valve
Air motor unitPressure regulator and SOV
Blocker doors (folded)Feedback gearboxFuel regulatorReverse thrust
select lever
(reverse thrust)
Reverse thrust position
Figure 3-49. Components of a thrust reverser system.
Figure 3-50. An example of a variable area exhaust nozzle used to
increase or decrease exhaust flow during afterburn. Figure 3-51. A pilot can direct thrust via the vectoring nozzle 20°
up or down to increase flight maneuverability. variable area exhaust nozzle. [Figure 3-50] After the engine
has reached full power under normal operation, the power
lever can be advanced to activate the afterburner. This allows
more fuel to flow into the exhaust nozzle where it is ignited
and burned. As energy and mass are added to the gas flow,
the exhaust nozzle must open wider to allow greater flow.
As the power lever is moved back out of the afterburner, the
exhaust nozzle closes down again. Some low-bypass turbofan
engines used in military aircraft use bypass (fan air) to flow
into the exhaust nozzle. Just as in a ducted fan, this air is
used in the afterburner. It contains more oxygen and assists
combustion in the afterburner. Since fuel is being burned in
the exhaust nozzle, the heat buildup around the nozzle is a
problem. A special type of liner is used around the nozzle to
allow cooler air to circulate around the nozzle. This operates
somewhat like a single burner can combustion chamber.
Operation in the afterburner mode is somewhat limited by
high fuel consumption, which can be almost double normal
consumption.
Thrust Vectoring
Thrust vectoring is the ability of an aircraft’s main engines to
direct thrust other than parallel to the vehicle’s longitudinal
axis, allowing the exhaust nozzle to move or change position
to direct the thrust in varied directions. Vertical takeoff
aircraft use thrust vectoring as takeoff thrust and then change
direction to propel the aircraft in horizontal flight. Military
aircraft use thrust vectoring for maneuvering in flight to
change direction. Thrust vectoring is generally accomplished
by relocating the direction of the exhaust nozzle to direct the
thrust to move the aircraft in the desired path. At the rear of
a gas turbine engine, a nozzle directs the flow of hot exhaust
gases out of the engine and afterburner. Usually, the nozzle
points straight out of the engine. The pilot can move, or
vector, the vectoring nozzle up and down by 20°. This makes
the aircraft much more maneuverable in flight. [Figure 3-51]Engine Noise Suppression
Aircraft powered by gas turbine engines sometimes require
noise suppression for the engine exhaust gases when
operating from airports located in or near highly populated
areas. Several types of noise suppressors are used. A common
type of noise suppressor is an integral, airborne part of the
aircraft engine installation or engine exhaust nozzle. Engine
noise comes from several sources on the engine, the fan, or
compressor and the air discharge from the core of the engine.
There are three sources of noise involved in the operation
of a gas turbine engine. The engine air intake and vibration
from engine housing are sources of some noise, but the
noise generated does not compare in magnitude with that
produced by the engine exhaust. [Figure 3-52] The noise
produced by the engine exhaust is caused by the high degree
of turbulence of a high-velocity jet stream moving through
a relatively quiet atmosphere. For a distance of a few nozzle
diameters downstream behind the engine, the velocity of the
jet stream is high, and there is little mixing of the atmosphere
with the jet stream. In this region, the turbulence within
the high speed jet stream is very fine grain turbulence and
produces relatively high-frequency noise. This noise is caused
by violent, turbulent mixing of the exhaust gases with the
atmosphere and is influenced by the shearing action caused by
the relative speeds between the velocity and the atmosphere.
Farther downstream, as the velocity of the jet stream
slows down, the jet stream mixes with the atmosphere and
turbulence of a coarser type begins. Compared with noise
from other portions of the jet stream, noise from this portion
has a much lower frequency. As the energy of the jet stream
finally is dissipated in large turbulent swirls, a greater portion
of the energy is converted into noise. The noise generated as
the exhaust gases dissipate is at a frequency near the low end
of the audible range. The lower the frequency of the noise,
30 D
10 DD Most of the noise radiates from this low frequency
turbulence region
D = Nozzle diameter10°
Figure 3-52. Engine noise from engine exhaust is created by the turbulence of a high velocity jet stream moving through the relatively
quiet atmosphere.
Tail
cone
Figure 3-53. Noise suppressors currently in use are corrugated-
perimeter type, or multi-tube type. the greater the distance the noise travels. This means that the
low-frequency noises reach an individual on the ground in
greater volume than the high-frequency noises, and hence
are more objectionable. High-frequency noise is weakened
more rapidly than low-frequency noise, both by distance
and the interference of buildings, terrain, and atmospheric
disturbances. A deep-voiced, low-frequency foghorn, for
example, may be heard much farther than a shrill, high-
frequency whistle, even though both may have the same
overall volume (decibels) at their source.
Noise levels vary with engine thrust and are proportional
to the amount of work done by the engine on the air that
passes through it. An engine having relatively low airflow
but high thrust due to high turbine discharge (exhaust gas)
temperature, pressure, and/or afterburning produces a gas
stream of high velocity and, therefore, high noise levels. A
larger engine, handling more air, is quieter at the same thrust
and large engines operating at partial thrust are less noisy than
smaller engines operating at full thrust. Thus, the noise level
can be reduced considerably by operating the engine at lower
power settings. Compared with a turbojet, a turbofan version
of the same engine is quieter during takeoff. The noise level
produced by a fan-type engine is lower, principally because
the exhaust gas velocities ejected at the engine tailpipe are
slower than those for a turbojet of comparative size.
Fan engines require a larger turbine to provide additional
power to drive the fan. The large turbine, which usually has
an additional turbine stage, reduces the velocity of the gas
and, therefore, reduces the noise produced because exhaust
gas noise is proportional to exhaust gas velocity. The exhaust
from the fan is at a relatively low velocity and, therefore, does not create a noise problem. Because of the characteristic of
low-frequency noise to linger at a relatively high volume,
effective noise reduction for a turbojet aircraft must be
achieved by revising the noise pattern or by changing the
frequency of the noise emitted by the jet nozzle.
The noise suppressors in current use are either of the corrugated-
perimeter type, or the multi-tube type. [Figure 3-53] Both
types of suppressors break up the single, main jet exhaust
stream into a number of smaller jet streams. This increases
the total perimeter of the nozzle area and reduces the size
of the air stream eddies created as the gases are discharged
into the open air. Although the total noise-energy remains
unchanged, the frequency is raised considerably. The size
of the air stream eddies scales down at a linear rate with
the size of the exhaust stream. This has two effects: 1) the
change in frequency may put some of the noise above the
audibility range of the human ear, and 2) high frequencies
3-31within the audible range, while perhaps more annoying, are
more highly attenuated by atmospheric absorption than are
low frequencies. Thus, the falloff in intensity is greater and
the noise level is less at any given distance from the aircraft.
In the engine nacelle, the area between the engine and the
cowl has acoustic linings surrounding the engine. This
noise-absorbing lining material converts acoustic energy
into heat. These linings normally consist of a porous skin
supported by a honeycomb backing and provide a separation
between the face sheet and the engine duct. For optimum
suppression, the acoustic properties of the skin and the liner
are carefully matched.
Turbine Engine Emissions
Engineers are introducing new combustion technology that
has dramatically reduced emissions from gas turbine engines.
Lowering exhaust emissions from gas turbine, especially
oxides of nitrogen (NO X), continue to require improvement.
Most of the research has centered around the combustion
section of the engine. New technology with unique combustor
design has greatly reduced emissions. One manufacturer
has a design called the Twin Annular, Pre-mixing Swirler
(TAPS) combustor. Most advanced designs rely on a method
of pre-mixing the air-fuel before it enters the combustion
burner area. In the TAPS design, air from the high-pressure
compressor is directed into the combustor through two high-
energy swirlers adjacent to the fuel nozzles. This swirl creates
a more thorough and leaner mix of fuel and air, which burns
at lower temperatures than in previous gas turbine engine
designs. Most of the NO X is formed by the reaction of oxygen
and nitrogen at high temperatures. The NO X levels are higher
if the burning air-fuel mixture stays at high temperatures for a
longer time. Newly designed combustors also produce lower
levels of carbon monoxide and unburned hydrocarbons. The
increases in gas turbine engine component efficiencies have
resulted in fewer emissions from gas turbine engines.
Figure 4-1. Single and dual magnetos.
Battery
S PIgnition coil
CondenserDistributor
Mechanical linkage
Cylinder
& pistonCam
Breaker
contact
points
Figure 4-2. Battery-ignition system.Reciprocating Engine Ignition Systems
The basic requirements for reciprocating engine ignition
systems are similar, regardless of the type of engine. All
ignition systems must deliver a high-tension spark across the
electrodes of each spark plug in each cylinder of the engine
in the correct firing order. At a predetermined number of
degrees ahead of the top dead center position of the piston,
as measured by crankshaft travel in degrees of rotation, the
spark occurs in the cylinder. The potential output voltage of
the system must be adequate to arc the gap in the spark plug
electrodes under all operating conditions. The spark plug is
threaded into the cylinder head with the electrodes exposed
to the combustion area of the engine’s cylinder.
Ignition systems can be divided into two classifications:
magneto-ignition systems or electronic Full Authority Digital
Engine Control (FADEC) systems for reciprocating engines.
Ignition systems can also be subclassified as either single or
dual magneto-ignition systems. The single magneto-ignition
system, usually consisting of one magneto and the necessary
wiring, was used with another single magneto on the same
engine. Dual magnetos generally use one rotating magnet
that feeds two complete magnetos in one magneto housing.
An example of each type is shown in Figure 4-1 .
Aircraft magneto-ignition systems can be classified as either
high-tension or low-tension. The low-tension magneto
system, covered in a later section of this chapter, generates a
low-voltage that is distributed to a transformer coil near each
spark plug. This system eliminates some problems inherent in
the high-tension system that was containing the high-voltage
until it passed through the spark plug. The materials that were
used for ignition leads could not withstand the high-voltage
Chapter 4
Engine Ignition & Electrical Systems
and were prone to leak to ground before the spark would get
to the cylinder. As new materials evolved and shielding was
developed, the problems with high-tension magnetos were
overcome. The high-tension magneto system is still the most
widely used aircraft ignition system.
Some very old antique aircraft used a battery-ignition system.
In this system, the source of energy is a battery or generator,
rather than a magneto. This system was similar to that used in
most automobiles at the time. Figure 4-2 shows a simplified
schematic of a battery-ignition system.
Magneto-Ignition System Operating
Principles
The magneto, a special type of engine-driven alternating
current (AC) generator, uses a permanent magnet as a source
of energy. By the use of a permanent magnet (basic magnetic
field), coil of wire (concentrated lengths of conductor), and
relative movement of the magnetic field, current is generated
in the wire. At first, the magneto generates electrical power
by the engine rotating the permanent magnet and inducing
a current to flow in the coil windings. As current flows
through the coil windings, it generates its own magnetic
field that surrounds the coil windings. At the correct time,
this current flow is stopped and the magnetic field collapses
across a second set of windings in the coil and a high-voltage
is generated. This is the voltage used to arc across the spark
plug gap. In both cases, the three basic things needed to
generate electrical power are present to develop the high-
voltage that forces a spark to jump across the spark plug gap
4-2A B C Flux to right No flux Flux to left
N S
S N
N
S S
N
S N
N S
0° 45° 90°Coil core
Pole shoe
Figure 4-3. Magnetic flux at three positions of the rotating magnet. in each cylinder. Magneto operation is timed to the engine
so that a spark occurs only when the piston is on the proper
stroke at a specified number of crankshaft degrees before the
top dead center piston position.
High-Tension Magneto System Theory of Operation
The high-tension magneto system can be divided, for
purposes of discussion, into three distinct circuits: magnetic,
primary electrical, and secondary electrical circuits.
Magnetic Circuit
The magnetic circuit consists of a permanent multi-pole
rotating magnet, a soft iron core, and pole shoes. [Figure 4-3]
The magnet is geared to the aircraft engine and rotates in the
gap between two pole shoes to furnish the magnetic lines of
force (flux) necessary to produce an electrical voltage. The
poles of the magnet are arranged in alternate polarity so that
the flux can pass out of the north pole through the coil core
and back to the south pole of the magnet. When the magnet is
in the position shown in Figure 4-3A , the number of magnetic
lines of force through the coil core is maximum because two
magnetically opposite poles are perfectly aligned with the
pole shoes.
This position of the rotating magnet is called the full register
position and produces a maximum number of magnetic lines
of force, flux flow clockwise through the magnetic circuit
and from left to right through the coil core. When the magnet
is moved away from the full register position, the amount of
flux passing through the coil core begins to decrease. This
occurs because the magnet’s poles are moving away from
the pole shoes, allowing some lines of flux to take a shorter
path through the ends of the pole shoes.As the magnet moves farther from the full register position,
more lines of flux are short circuited through the pole shoe
ends. Finally, at the neutral position 45° from the full register
position, all flux lines are short circuited, and no flux flows
through the coil core. [Figure 4-3B] As the magnet moves
from full register to the neutral position, the number of flux
lines through the coil core decreases in the same manner
as the gradual collapse of flux in the magnetic field of an
ordinary electromagnet.
The neutral position of the magnet is where one of the poles
of the magnet is centered between the pole shoes of the
magnetic circuit. As the magnet is moved clockwise from
this position, the lines of flux that had been short circuited
through the pole shoe ends begin to flow through the coil
core again. But this time, the flux lines flow through the coil
core in the opposite direction. [Figure 4-3C] The flux flow
reverses as the magnet moves out of the neutral position
because the north pole of the rotating permanent magnet is
opposite the right pole shoe instead of the left. [Figure 4-3A]
When the magnet is again moved a total of 90°, another full
register position is reached with a maximum flux flow in
the opposite direction. The 90° of magnet travel is shown in
Figure 4-4, where a curve shows how the flux density in the
coil core, without a primary coil around the core, changes
as the magnet is rotated.
Figure 4-4 shows that as the magnet moves from the full
register position 0°, flux flow decreases and reaches a zero
value as it moves into the neutral position 45°. While the
magnet moves through the neutral position, flux flow reverses
and begins to increase as indicated by the curve below the
Flux density45° 90° 360° 315°
Full register0° 135° 225°Neutral
Figure 4-4. Change in flux density as magnet rotates.
Primary coil
Condenser
Breaker contact points
Figure 4-5. Primary electrical circuit of a high-tension magneto.horizontal line. At the 90° position, another position of
maximum flux is reached. Thus, for one revolution 360° of
the four pole magnet, there are four positions of maximum
flux, four positions of zero flux, and four flux reversals.
This discussion of the magnetic circuit demonstrates how the
coil core is affected by the rotating magnet. It is subjected to
an increasing and decreasing magnetic field and a change in
polarity each 90° of magnet travel.
When a coil of wire as part of the magneto’s primary electrical
circuit is wound around the coil core, it is also affected by
the varying magnetic field.
Primary Electrical Circuit
The primary electrical circuit consists of a set of breaker
contact points, a condenser, and an insulated coil. [Figure 4-5]
The coil is made up of a few turns of heavy copper wire,
one end is grounded to the coil core and the other end to
the ungrounded side of the breaker points. [Figure 4-5] The
primary circuit is complete only when the ungrounded
breaker point contacts the grounded breaker point. The
third unit in the circuit, the condenser (capacitor), is wired
in parallel with the breaker points. The condenser prevents
arcing at the points when the circuit is opened and hastens
the collapse of the magnetic field about the primary coil.The primary breaker closes at approximately full register
position. When the breaker points are closed, the primary
electrical circuit is completed and the rotating magnet
induces current flow in the primary circuit. This current
flow generates its own magnetic field, which is in such a
direction that it opposes any change in the magnetic flux of
the permanent magnet’s circuit.
While the induced current is flowing in the primary circuit,
it opposes any decrease in the magnetic flux in the core. This
is in accordance with Lenz’s Law that states: “An induced
current always flows in such a direction that its magnetism
opposes the motion or the change that induced it.” (For a
review of Lenz’s Law, refer to the Aviation Maintenance
Technician—General Handbook, FAA-H-8083-30). Thus,
the current flowing in the primary circuit holds the flux in
the core at a high value in one direction until the rotating
magnet has time to rotate through the neutral position to a
point a few degrees beyond neutral. This position is called
the E-gap position (E stands for efficiency).
There are three basic events required to fire a spark plug
when its piston is in the prescribed position:
1. the magneto must be in the E-gap position,
2. the breaker contact points must be open, and
3. the distributor must be aligned correctly.
With the magnetic rotor in E-gap position and the primary
coil holding the magnetic field of the magnetic circuit in
the opposite polarity, a very high rate of flux change can be
obtained by opening the primary breaker points. Opening
the breaker points stops the flow of current in the primary
circuit and allows the magnetic rotor to quickly reverse the
field through the coil core. This sudden flux reversal produces
a high rate of flux change in the core, that cuts across the
secondary coil of the magneto (wound over and insulated
from the primary coil), inducing the pulse of high-voltage
electricity in the secondary needed to fire a spark plug. As
the rotor continues to rotate to approximately full register
position, the primary breaker points close again and the
cycle is repeated to fire the next spark plug in firing order.
The sequence of events can now be reviewed in greater detail
to explain how the state of extreme magnetic stress occurs.
With the breaker points, cam, and condenser connected in
the circuit as shown in Figure 4-6 , the action that takes place
as the magnetic rotor turns is depicted by the graph curve
in Figure 4-7 . At the top (A) of Figure 4-7 , the original
static flux curve of the magnets is shown. Shown below the
static flux curve is the sequence of opening and closing the
magneto breaker points. Note that opening and closing the
breaker points is timed by the breaker cam. The points close
CondenserCoil (about 180 turns no. 18 wire)
Pole shoeCoil core
Magnet
Contact breaker
Breaker cam
Flux densityOPEN CLOSED OPEN CLOSED OPEN CLOSED OPEN CLOSED OPEN CLOSEDNeutral NeutralNeutral Neutral0° 90° 180° 270° 360°
Static fluxResultant fluxStatic flux curveA
Primary currentC
Resultant fluxDBreaker timing point
opening before neutral
closing interval point
closing E-gapB
Figure 4-7. Magneto flux curves. Figure 4-6. Components of a high-tension magneto circuit.when a maximum amount of flux is passing through the coil
core and open at a position after neutral. Since there are four
lobes on this particular cam (there are some magnetos with
cams that have only two lobes), the breaker points close and
open in the same relation to each of the four neutral positions
of the rotor magnet. Also, the point opening and point closing
intervals are approximately equal.
Starting at the maximum flux position marked 0° at the
top of Figure 4-7 , the sequence of events in the following
paragraphs occurs.
As the magnet rotor is turned toward the neutral position,
the amount of flux through the core starts to decrease.
[Figure 4-7D] This change in flux linkages induces a current
in the primary winding. [Figure 4-7C] This induced current
creates a magnetic field of its own that opposes the change of
flux linkages inducing the current. Without current flowing in
the primary coil, the flux in the coil core decreases to zero as
the magnet rotor turns to neutral and starts to increase in the
opposite direction (dotted static flux curve in Figure 4-7D ).
But, the electromagnetic action of the primary current
prevents the flux from changing and temporarily holds the
field instead of allowing it to change (resultant flux line in
Figure 4-7D ).
Figure 4-8. Pivotless type breaker assembly and cam.
Two-lobe cam Four-lobe cam Eight-lobe cam Compensated 14-lobe cam
Figure 4-9. Typical breaker assemblies.As a result of the holding process, there is a very high stress in
the magnetic circuit by the time the magnet rotor has reached
the position where the breaker points are about to open. The
breaker points, when opened, function with the condenser to
interrupt the flow of current in the primary coil, causing an
extremely rapid change in flux linkages. The high-voltage in
the secondary winding discharges across the gap in the spark
plug to ignite the air-fuel mixture in the engine cylinder. Each
spark actually consists of one peak discharge, after which a
series of small oscillations takes place.
They continue to occur until the voltage becomes too low
to maintain the discharge. Current flows in the secondary
winding during the time that it takes for the spark to
completely discharge. The energy or stress in the magnetic
circuit is completely dissipated by the time the contacts close
for the production of the next spark. Breaker assemblies, used
in high-tension magneto-ignition systems, automatically open
and close the primary circuit at the proper time in relation
to piston position in the cylinder to which an ignition spark
is being furnished. The interruption of the primary current flow is accomplished through a pair of breaker contact points
made of an alloy that resists pitting and burning.
Most breaker points used in aircraft ignition systems are
of the pivotless type in which one of the breaker points is
movable and the other stationary. [Figure 4-8] The movable
breaker point attached to the leaf spring is insulated from
the magneto housing and is connected to the primary coil.
[Figure 4-8] The stationary breaker point is grounded to the
magneto housing to complete the primary circuit when the
points are closed and can be adjusted so that the points can
open at the proper time.
Another part of the breaker assembly is the cam follower,
which is spring-loaded against the cam by the metal leaf
spring. The cam follower is a Micarta block or similar material
that rides the cam and moves upward to force the movable
breaker contact away from the stationary breaker contact
each time a lobe of the cam passes beneath the follower. A
felt oiler pad is located on the underside of the metal spring
leaf to lubricate and prevent corrosion of the cam.
The breaker-actuating cam may be directly driven by the
magneto rotor shaft or through a gear train from the rotor
shaft. Most large radial engines use a compensated cam that
is designed to operate with a specific engine and has one lobe
for each cylinder to be fired by the magneto. The cam lobes
are machine ground at unequal intervals to compensate for
the elliptical path of the articulated connecting rods. This
path causes the pistons top dead center position to vary
from cylinder to cylinder with regard to crankshaft rotation.
A compensated 14-lobe cam, together with a two-, four-,
and eight-lobe uncompensated cam, is shown in Figure 4-9 .
The unequal spacing of the compensated cam lobes, although
it provides the same relative piston position for ignition
to occur, causes a slight variation of the E-gap position of
the rotating magnet and thus a slight variation in the high-
voltage impulses generated by the magneto. Since the spacing
between each lobe is tailored to a particular cylinder of a
particular engine, compensated cams are marked to show
Engine
CoilDistributor
Figure 4-10. Relation between distributor terminal numbers and
cylinder numbers. the series of the engine, the location of the master rods, the
lobe used for magneto timing, the direction of cam rotation,
and the E-gap specification in degrees past neutral of magnet
rotation. In addition to these markings, a step is cut across
the face of the cam, that, when aligned with scribed marks
on the magneto housing, places the rotating magnet in the
E-gap position for the timing cylinder. Since the breaker
points should begin to open when the rotating magnet moves
into the E-gap position, alignment of the step on the cam with
marks in the housing provides a quick and easy method of
establishing the exact E-gap position to check and adjust the
breaker points.
Secondary Electrical Circuit
The secondary circuit contains the secondary windings of the
coil, distributor rotor, distributor cap, ignition lead, and spark
plug. The secondary coil is made up of a winding containing
approximately 13,000 turns of fine, insulated wire; one end
of which is electrically grounded to the primary coil or to
the coil core and the other end connected to the distributor
rotor. The primary and secondary coils are encased in a non-
conducting material. The whole assembly is then fastened to
the pole shoes with screws and clamps.
When the primary circuit is closed, the current flow through
the primary coil produces magnetic lines of force that cut
across the secondary windings, inducing an electromotive
force. When the primary circuit current flow is stopped, the
magnetic field surrounding the primary windings collapses,
causing the secondary windings to be cut by the lines of
force. The strength of the voltage induced in the secondary
windings, when all other factors are constant, is determined
by the number of turns of wire. Since most high-tension
magnetos have many thousands of turns of wire in the
secondary coil windings, a very high-voltage, often as high
as 20,000 volts, is generated in the secondary circuit. The
high-voltage induced in the secondary coil is directed to
the distributor, which consists of two parts: revolving and
stationary. The revolving part is called a distributor rotor and
the stationary part is called a distributor block. The rotating
part, which may take the shape of a disc, drum, or finger,
is made of a non-conducting material with an embedded
conductor. The stationary part consists of a block also made of
non-conducting material that contains terminals and terminal
receptacles into which the ignition lead wiring that connects
the distributor to the spark plug is attached. This high-voltage
is used to jump the air gap of electrodes of the spark plug in
the cylinder to ignite the air-fuel mixture.
As the magnet moves into the E-gap position for the No.
1 cylinder and the breaker points just separate or open, the
distributor rotor aligns itself with the No. 1 electrode in the
distributor block. The secondary voltage induced as the breaker points open enters the rotor where it arcs a small air
gap to the No. 1 electrode in the block.
Since the distributor rotates at one-half crankshaft speed
on all four-stroke cycle engines, the distributor block has
as many electrodes as there are engine cylinders, or as
many electrodes as cylinders served by the magneto. The
electrodes are located circumferentially around the distributor
block so that, as the rotor turns, a circuit is completed to a
different cylinder and spark plug each time there is alignment
between the rotor finger and an electrode in the distributor
block. The electrodes of the distributor block are numbered
consecutively in the direction of distributor rotor travel.
[Figure 4-10]
The distributor numbers represent the magneto sparking
order rather than the engine cylinder numbers. The distributor
electrode marked “1” is connected to the spark plug in the
No. 1 cylinder; distributor electrode marked “2” to the second
Figure 4-11. A high-tension ignition harness. cylinder to be fired; distributor electrode marked “3” to the
third cylinder to be fired, and so forth.
In Figure 4-10 , the distributor rotor finger is aligned with
the distributor electrode marked “3,” which fires the No. 5
cylinder of a nine-cylinder radial engine. Since the firing
order of a nine-cylinder radial engine is 1-3-5-7-9-2-4-6-8,
the third electrode in the magneto sparking order serves the
No. 5 cylinder.
Magneto & Distributor Venting
Since magneto and distributor assemblies are subjected to
sudden changes in temperature, the problems of condensation
and moisture are considered in the design of these units.
Moisture in any form is a good conductor of electricity. If
absorbed by the nonconducting material in the magneto, such
as distributor blocks, distributor fingers, and coil cases, it can
create a stray electrical conducting path. The high-voltage
current that normally arcs across the air gaps of the distributor
can flash across a wet insulating surface to ground, or the
high-voltage current can be misdirected to some spark plug
other than the one that should be fired. This condition is called
flashover and usually results in cylinder misfiring. This can
cause a serious engine condition called pre-ignition, which
can damage the engine. For this reason, coils, condensers,
distributors, and distributor rotors are waxed so that moisture
on such units stand in separate beads and do not form a
complete circuit for flashover.
Flashover can lead to carbon tracking, which appears as a fine
pencil-like line on the unit across which flashover occurs.
The carbon trail results from the electric spark burning dirt
particles that contain hydrocarbon materials. The water in the
hydrocarbon material is evaporated during flashover, leaving
carbon to form a conducting path for current. When moisture
is no longer present, the spark continues to follow the carbon
track to the ground. This prevents the spark from getting to
the spark plug, so the cylinder does not fire.
Magnetos cannot be hermetically sealed to prevent moisture
from entering a unit, because the magneto is subject to
pressure and temperature changes in altitude. Thus, adequate
drains and proper ventilation reduce the tendency of flashover
and carbon tracking. Good magneto circulation also ensures
that corrosive gases produced by normal arcing across the
distributor air gap, such as ozone, are carried away. In some
installations, pressurization of the internal components of
the magnetos and other various parts of the ignition system
is essential to maintain a higher absolute pressure inside the
magneto and to eliminate flashover due to high altitude flight.
This type of magneto is used with turbocharged engines
that operate at higher altitudes. Flashover becomes more
likely at high altitudes because of the lower air pressure, which makes it easier for the electricity to jump air gaps.
By pressurizing the interior of the magneto, the normal air
pressure is maintained and the electricity or the spark is
held within the proper areas of the magneto even though the
ambient pressure is very low.
Even in a pressurized magneto, the air is allowed to flow
through and out of the magneto housing. By providing
more air and allowing small amounts of air to bleed out for
ventilation, the magneto remains pressurized. Regardless of
the method of venting employed, the vent bleeds or valves
must be kept free of obstructions. Further, the air circulating
through the components of the ignition system must be free of
oil since even minute amounts of oil on ignition parts result
in flashover and carbon tracking.
Ignition Harness
The ignition lead directs the electrical energy from the
magneto to the spark plug. The ignition harness contains an
insulated wire for each cylinder that the magneto serves in the
engine. [Figure 4-11] One end of each wire is connected to
the magneto distributor block and the other end is connected
to the proper spark plug. The ignition harness leads serve a
dual purpose. It provides the conductor path for the high-
tension voltage to the spark plug. It also serves as a shield
for stray magnetic fields that surround the wires as they
momentarily carry high-voltage current. By conducting these
magnetic lines of force to the ground, the ignition harness
cuts down electrical interference with the aircraft radio and
other electrically sensitive equipment.
A magneto is a high frequency radiation emanating (radio
wave) device during its operation. The wave oscillations
produced in the magneto are uncontrolled and cover a wide
range of frequencies and must be shielded. If the magneto and
ignition leads were not shielded, they would form antennas
and pick up the random frequencies from the ignition system.
InsulatorBlue coating
ConductorSheiding
Figure 4-12. Ignition lead.Figure 4-13. Ignition lead spark plug end. The lead shielding is a metal mesh braid that surrounds the
entire length of the lead. The lead shielding prevents the
radiation of the energy into the surrounding area.
Capacitance is the ability to store an electrostatic charge
between two conducting plates separated by a dielectric. Lead
insulation is called a dielectric, meaning it can store electrical
energy as an electrostatic charge. An example of electrostatic
energy storage in a dielectric is the static electricity stored
in a plastic hair comb. When shielding is placed around the
ignition lead, capacitance increases by bringing the two
plates closer together. Electrically, the ignition lead acts as
a capacitor and has the ability to absorb and store electrical
energy. The magneto must produce enough energy to charge
the capacitance caused by the ignition lead and have enough
energy left over to fire the plug.
Ignition lead capacitance increases the electrical energy
required to provide a spark across the plug gap. More magneto
primary current is needed to fire the plug with the shielded
lead. This capacitance energy is discharged as fire across the
plug gap after each firing of the plug. Reversing the polarity
during servicing by rotating the plugs to new locations, the
plug wear is equalized across the electrodes. The very center
of the ignition lead is the high-voltage carrier surrounded by
a silicone insulator material that is surrounded by a metal
mesh, or shielding, covered with a thin silicone rubber coating
that prevents damage by engine heat, vibration, or weather.
A sectional view of the typical ignition lead is shown in
Figure 4-12 . Ignition leads must be routed and clamped
correctly to avoid hot spots on the exhaust and vibration
points as the leads are routed from the magneto to the
individual cylinders. Ignition leads are normally of the
all-weather type and are hard connected at the magneto
distributor and affixed to the spark plug by threads. The
shielded ignition lead spark plug terminal is available in all-
weather ¾ inch diameter and 5⁄8 inch diameter barrel ignition
lead nut. [Figure 4-13] The 5⁄8 – 24 plug takes a ¾ wrench
on the lead nut and the ¾ – 20 plug takes a 7⁄8 wrench on the
lead nut. The ¾ inch all-weather design utilizes a terminal
seal that results in greater terminal well insulation. This is recommended because the lead end of the spark plug is
completely sealed from moisture.
An older radial engine type of ignition harness is a manifold
formed to fit around the crankcase of the engine with flexible
extensions terminating at each spark plug. A typical high-
tension ignition harness is shown in Figure 4-14 . Many older
single-row radial engine aircraft ignition systems employ a
dual-magneto system, in which the right magneto supplies
the electric spark for the front plugs in each cylinder, and the
left magneto fires the rear plugs.
Ignition Switches
All units in an aircraft ignition system are controlled by
an ignition switch. The type of switch used varies with the
number of engines on the aircraft and the type of magnetos
used. All switches, however, turn the system off and on in
much the same manner. The ignition switch is different in at
least one respect from all other types of switches: when the
ignition switch is in the off position, a circuit is completed
through the switch to ground. In other electrical switches, the
off position normally breaks or opens the circuit.
Right magneto
Switch BoosterLeft magneto
Figure 4-14. Accessory-mounted nine cylinder engine ignition
harness.
Coil
Off
CondenserIgnition switch
Figure 4-15. Typical ignition switch in off position.
Coil
On
CondenserIgnition switch
Figure 4-16. Typical ignition switch in on position. The ignition switch has one terminal connected to the primary
electrical circuit between the coil and the breaker contact
points. The other terminal of the switch is connected to the
aircraft ground structure. As shown in Figure 4-15 , two ways
to complete the primary circuit are:
1. Through the closed breaker points to ground and
2. Through the closed ignition switch to ground.
Figure 4-15 shows that the primary current is not interrupted
when the breaker contacts open since there is still a path to
ground through the closed, or off, ignition switch. Since
primary current is not stopped when the contact points open,
there can be no sudden collapse of the primary coil flux field
and no high-voltage induced in the secondary coil to fire the
spark plug.
As the magnet rotates past the electrical gap (E-gap) position,
a gradual breakdown of the primary flux field occurs. But that
breakdown occurs so slowly that the induced voltage is too
low to fire the spark plug. Thus, when the ignition switch is
in the off position with the switch closed, the contact points
are as completely short-circuited as if they were removed
from the circuit, and the magneto is inoperative.
When the ignition switch is placed in the on position, switch
open, the interruption of primary current and the rapid
collapse of the primary coil flux field is once again controlled
or triggered by the opening of the breaker contact points.
[Figure 4-16] When the ignition switch is in the on position,
the switch has absolutely no effect on the primary circuit.The ignition/starter switch, or magneto switch, controls the
magnetos on or off and can also connect the starter solenoid
for turning the starter. When a starting vibrator, a box that
emits pulsating direct current (DC), is used on the engine,
the ignition/starter switch is used to control the vibrator
and retard points. This system is explained in detail later in
this chapter. Some ignition starter switches have a push to
prime feature during the starting cycle. This system allows
additional fuel to spray into the intake port of the cylinder
during the starting cycle.
MagnetImpulse coupling
Pinion gear
Ball bearing
CamDistributor block
CapacitorHigh output coil
Distributor gear
Figure 4-17. Magneto cutaway.Figure 4-18. A dual magneto with two distributors.Single & Dual High-Tension System Magnetos
High-tension system magnetos used on aircraft engines are
either single or dual type magnetos. The single magneto
design incorporates the distributor in the housing with
the magneto breaker assembly, rotating magnet, and coil.
[Figure 4-17] The dual magneto incorporates two magnetos
contained in a single housing. One rotating magnet and a
cam are common to two sets of breaker points and coils.
Two separate distributor units are mounted in the magneto.
[Figure 4-18]
Magneto Mounting Systems
Flange-mounted magnetos are attached to the engine by
a flange around the driven end of the rotating shaft of the
magneto. [Figure 4-19] Elongated slots in the mounting
flange permit adjustment through a limited range to aid in
timing the magneto to the engine. Some magnetos mount by
the flange and use clamps on each side to secure the magneto
to the engine. This design also allows for timing adjustments.
Base mounted magnetos are only used on very old or antique
aircraft engines.
4-11Mounting flange
Adjustment slot
Figure 4-19. Magneto mounting flange.
ST1
L1L2
L3C2
C3C1R1R2R3
V1
R4OffS3OnL
R
BothOff
Retard
breaker
Main
breakerMain breaker
Spark plug
T2
Spark plugL4
L5
Starter solenoidStarting vibrator
Magneto switch
Left magneto
Right magneto
Figure 4-20. Simplified low-tension ignition system schematic. High- & Low-Tension Magneto Systems
High-tension ignition systems have undergone many
refinements and improvements in design. This includes new
electronic systems that control more than just providing
ignition to the cylinders. High-tension voltage presents
certain problems with carrying the high-voltage from the
magneto internally and externally to the spark plugs. In early
years, it was difficult to provide insulators that could contain
the high-voltage, especially at high altitudes when the air
pressures were reduced. Another requirement of high-tension
systems was that all weather and radio-equipped aircraft have ignition wires enclosed in shielding to prevent radio noise
due to high-voltages. Many aircraft were turbosupercharged
and operated at increased high altitudes. The low pressure at
these altitudes would allow the high-voltage to leak out even
more. To meet these problems, low-tension ignition systems
were developed.
Electronically, the low-tension system is different from the
high-tension system. In the low-tension system, low-voltage
is generated in the magneto and flows to the primary winding
of a transformer coil located near the spark plug. There,
the voltage is increased to high by transformer action and
conducted to the spark plug by very short high-tension leads.
[Figure 4-20]
The low-tension system virtually eliminates flashover in both
the distributor and the harness because the air gaps within the
distributor have been eliminated by the use of a brush-type
distributor, and high-voltage is present only in short leads
between the transformer and spark plug.
Although a certain amount of electrical leakage is characteristic
of all ignition systems, it is more pronounced on radio-
shielded installations because the metal conduit is at ground
potential and close to the ignition wires throughout their
entire length. In low-tension systems, however, this leakage is
reduced considerably because the current throughout most of
the system is transmitted at a low-voltage potential. Although
N S
+−Field rheostat
Field winding
Load
Figure 4-21. Diagram of a series wound generator. Figure 4-22. Shunt wound generator.
A
BN S
+−Field
rheostatField
winding
Load
Shunt circuit Main circuitArmature winding
Armature Field coilsthe leads between the transformer coils and the spark plugs of
a low-tension ignition system are short, they are high-tension
high-voltage conductor, and are subject to the same failures
that occur in high-tension systems. Low-tension ignition
systems have limited use in modern aircraft because of the
excellent materials and shielding available to construct high-
tension ignition leads and the added cost of a coil for each
spark plug with the low-tension system.
Types of DC Generators
There are three types of DC generators: series wound, parallel
(shunt) wound, and series-parallel (compound) wound. The
appropriate generator is determined by the connections to
the armature and field circuits with respect to the external
circuit. The external circuit is the electrical load powered
by the generator. In general, the external circuit is used
for charging the aircraft battery and supplying power to
all electrical equipment being used by the aircraft. As their
names imply, windings in series have characteristics different
from windings in parallel.
Series Wound DC Generators
The series generator contains a field winding connected
in series with the external circuit. [Figure 4-21] Series
generators have very poor voltage regulation under changing
load, since the greater the current is through the field coils
to the external circuit, the greater the induced EMFs and the
greater the output voltage is. When the aircraft electrical
load is increased, the voltage increases; when the load is
decreased, the voltage decreases.
Since the series wound generator has such poor voltage
and current regulation, it is never employed as an airplane
generator. Generators in airplanes have field windings, that are connected either in shunt or in compound formats.
Parallel (Shunt) Wound DC Generators
A generator having a field winding connected in parallel with
the external circuit is called a shunt generator. [Figure 4-22] It
should be noted that, in electrical terms, shunt means parallel.
Therefore, this type of generator could be called either a shunt
generator or a parallel generator.
In a shunt generator, any increase in load causes a decrease
in the output voltage, and any decrease in load causes an
increase output voltage. This occurs since the field winding
is connected in parallel to the load and armature, and all the
current flowing in the external circuit passes only through
the armature winding (not the field).
As shown in Figure 4-22A , the output voltage of a shunt
generator can be controlled by means of a rheostat inserted
in series with the field windings. As the resistance of the field
circuit is increased, the field current is reduced; consequently,
the generated voltage is also reduced. As the field resistance
is decreased, the field current increases and the generator
output increases. In the actual aircraft, the field rheostat
would be replaced with an automatic control device, such
A
BN S
+−Series
field
windingParallel
field
winding
LoadArmature winding
Series field coil
To load
ArmatureShunt
field
coil
Compound wound
Figure 4-23. Compound wound generator. as a voltage regulator.
Compound Wound DC Generators
A compound wound generator employs two field windings one
in series and another in parallel with the load. [Figure 4-23]
This arrangement takes advantage of both the series and
parallel characteristics described earlier. The output of a
compound wound generator is relatively constant, even with
changes in the load.
Generator Ratings
A DC generator is typically rated for its voltage and power
output. Each generator is designed to operate at a specified
voltage, approximately 14 or 28 volts. It should be noted that
aircraft electrical systems are designed to operate at one of
these two voltage values. The aircraft’s voltage depends on
which battery is selected for that aircraft. Batteries are either
12 or 24 volts when fully charged. The generator selected
must have a voltage output slightly higher than the battery
voltage. Hence, the 14- or 28-volt rating is required for
aircraft DC generators.
The power output of any generator is given as the maximum
number of amperes the generator can safely supply. 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 ratings.
The rotation of generators is termed either clockwise or
counterclockwise, as viewed from the driven end. The
direction of rotation may also be stamped on the data plate. It
is important that a generator with the correct rotation be used;
otherwise, the polarity of the output 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 turn 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. Called the
“coming in” speed, it is usually about 1,500 rpm.
DC Generator Maintenance
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. 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 seals. Blow out any dirt with compressed air.
4. Condition of generator brushes.
5. Generator operation.
6. V oltage regulator operation.
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 and possible replacement of various
components. [Figure 4-24]
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 extra-fine 000 (triple aught) grit,
or finer, sandpaper under the brush, rough side towards the
carbon brush. [Figure 4-25]
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
Figure 4-24. Wear areas of commutator and brushes.
Unseated brush
000 sandpaper (sand side next to brush)
Properly seated brush1/32" to 1/16"
Figure 4-25. Seating brushes with sandpaper.
on the sandpaper. Sand the brush only in the direction of
rotation. Carbon dust resulting from brush sanding should
be thoroughly cleaned from all parts of the generators after
a sanding operation.
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. 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. It is important that the brush spring pressure be correct.
Excessive pressure causes rapid wear of brushes. Too little
pressure, however, allows bouncing of the brushes, resulting
in burned and pitted surfaces. The pressure recommended by
the manufacturer should be checked by the use of a spring
scale graduated in ounces. Brush spring tension on some
generators can be adjusted. A spring scale is used to measure
the pressure that a brush exerts on the commutator.
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
from the armature, through the brushes, to the external circuit
of the generator.
FADEC System Description
A FADEC is a solid-state digital electronic ignition and
electronic sequential port fuel injection system with only
one moving part that consists of the opening and closing
of the fuel injector. FADEC continuously monitors and
controls ignition, timing, and fuel mixture/delivery/injection,
and spark ignition as an integrated control system. FADEC
monitors engine operating conditions (crankshaft speed, top
dead center position, the induction manifold pressure, and
the induction air temperature) and then automatically adjusts
the fuel-to-air ratio mixture and ignition timing accordingly
for any given power setting to attain optimum engine
performance. As a result, engines equipped with FADEC
require neither magnetos nor manual mixture control.
This microprocessor-based system controls ignition timing
for engine starting and varies timing with respect to engine
speed and manifold pressure. [Figure 4-26]
PowerLink provides control in both specified operating
conditions and fault conditions. The system is designed to
prevent adverse changes in power or thrust. In the event of
loss of primary aircraft-supplied power, the engine controls
continue to operate using a secondary power source (SPS).
As a control device, the system performs self-diagnostics to
determine overall system status and conveys this information
to the pilot by various indicators on the health status
annunciator (HSA) panel. PowerLink is able to withstand
storage temperature extremes and operate at the same
capacity as a non-FADEC-equipped engine in extreme heat,
cold, and high humidity environments.
Low-Voltage Harness
The low-voltage harness connects all essential components
of the FADEC System. [Figure 4-26] This harness acts as
a signal transfer bus interconnecting the electronic control
Low voltage harnessHigh voltage harness
Electronic control unit (ECU)
Figure 4-26. PowerLink system components.
units (ECUs) with aircraft power sources, the ignition switch,
speed sensor assembly (SSA), temperature and pressure
sensors. The fuel injector coils and all sensors, except the
SSA and fuel pressure and manifold pressure sensors, are
hardwired to the low-voltage harness. This harness transmits
sensor inputs to the ECUs through a 50-pin connector. The
harness connects to the engine-mounted pressure sensors
via cannon plug connectors. The 25-jpin connectors connect
the harness to the speed sensor signal conditioning unit.
The low-voltage harness attaches to the cabin harness by a
firewall-mounted data port through the same cabin harness/
bulkhead connector assembly. The bulkhead connectors also
supply the aircraft electrical power required to run the system.
The ECU is at the heart of the system, providing both ignition
and fuel injection control to operate the engine with the
maximum efficiency realizable. Each ECU contains two
microprocessors, referred to as a computer, that control two
cylinders. Each computer controls its own assigned cylinder
and is capable of providing redundant control for the other
computer’s cylinder.
The computer constantly monitors the engine speed and
timing pulses developed from the camshaft gear as they are
detected by the SSA. Knowing the exact engine speed and
the timing sequence of the engine, the computers monitor
the manifold air pressure and manifold air temperature to calculate air density and determine the mass air flow into the
cylinder during the intake stroke. The computers calculate
the percentage of engine power based on engine revolutions
per minute (rpm) and manifold air pressure.
From this information, the computer can then determine the
fuel required for the combustion cycle for either best power
or best economy mode of operation. The computer precisely
times the injection event, and the duration of the injector
should be on time for the correct fuel-to-air ratio. Then, the
computer sets the spark ignition event and ignition timing,
again based on percentage of power calculation. Exhaust
gas temperature is measured after the burn to verify that the
fuel-to-air ratio calculations were correct for that combustion
event. This process is repeated by each computer for its own
assigned cylinder on every combustion/power cycle.
The computers can also vary the amount of fuel to control
the fuel-to-air ratio for each individual cylinder to control
both cylinder head temperature (CHT) and exhaust gas
temperature (EGT).
Electronic Control Unit (ECU)
An ECU is assigned to a pair of engine cylinders. [Figure 4-27]
The ECUs control the fuel mixture and spark timing for
their respective engine cylinders; ECU 1 controls opposing
cylinders 1 and 2, ECU 2 controls cylinders 3 and 4, and ECU
Figure 4-27. Electronic control unit.
3 controls cylinders 5 and 6. Each ECU is divided into upper
and lower portions. The lower portion contains an electronic
circuit board, while the upper portion houses the ignition
coils. Each electronic control board contains two independent
microprocessor controllers that serve as control channels.
During engine operation, one control channel is assigned to
operate a single engine cylinder. Therefore, one ECU can
control two engine cylinders, one control channel per cylinder.
The control channels are independent, and there are no shared
electronic components within one ECU. They also operate
on independent and separate power supplies. However, if
one control channel fails, the other control channel in the
pair within the same ECU is capable of operating both its
assigned cylinder and the other opposing engine cylinder as
backup control for fuel injection and ignition timing. Each
control channel on the ECU monitors the current operating
conditions and operates its cylinder to attain engine operation
within specified parameters. The following transmit inputs to
the control channels across the low-voltage harness:
1. Speed sensor that monitors engine speed and crank
position,
2. Fuel pressure sensors,
3. Manifold pressure sensors,
4. Manifold air temperature (MAT) sensors,
5. CHT sensors, and
6. EGT sensors.
All critical sensors are dually redundant with one sensor
from each type of pair connected to control channels in
different ECUs. Synthetic software default values are also
used in the unlikely event that both sensors of a redundant
pair fail. The control channel continuously monitors changes
in engine speed, manifold pressure, manifold temperature,
and fuel pressure based on sensor input relative to operating
conditions to determine how much fuel to inject into the
intake port of the cylinder. PowerLink Ignition System
The ignition system consists of the high-voltage coils atop the
ECU, the high-voltage harness, and spark plugs. Since there
are two spark plugs per cylinder on all engines, a six-cylinder
engine has 12 leads and 12 spark plugs. One end of each lead
on the high-voltage harness attaches to a spark plug, and the
other end of the lead wire attaches to the spark plug towers on
each ECU. The spark tower pair is connected to opposite ends
of one of the ECU’s coil packs. Two coil packs are located
in the upper portion of the ECU. Each coil pack generates
a high-voltage pulse for two spark plug towers. One tower
fires a positive polarity pulse and the other of the same coil
fires a negative polarity pulse. Each ECU controls the ignition
spark for two engine cylinders. The control channel within
each ECU commands one of the two coil packs to control
the ignition spark for the engine cylinders. [Figure 4-28]
The high-voltage harness carries energy from the ECU spark
towers to the spark plugs on the engine.
For both spark plugs in a given cylinder to fire on the
compression stroke, both control channels must fire their
coil packs. Each coil pack has a spark plug from each of the
two cylinders controlled by that ECU unit.
The ignition spark is timed to the engine’s crankshaft position.
The timing is variable throughout the engine’s operating
range and is dependent upon the engine load conditions. The
spark energy is also varied with respect to the engine load.
Note: Engine ignition timing is established by the ECUs and
cannot be manually adjusted.
Engine Indicating & Crew Alerting System
(EICAS)
An engine indicating and crew alerting system (EICAS)
performs many of the same functions as an ECAM system.
The objective is still to monitor the aircraft systems for
the pilot. All EICAS display engine, as well as airframe,
parameters. Traditional gauges are not utilized, other than
a standby combination engine gauge in case of total system
failure.
EICAS is also a two-monitor, two-computer system with a
display select panel. Both monitors receive information from
the same computer. The second computer serves as a standby.
Digital and analog inputs from the engine and airframe
systems are continuously monitored. Caution and warning
lights, as well as aural tones, are incorporated. [Figure 4-29]
EICAS provides full time primary engine parameters (EPR,
N1, EGT) on the top, primary monitor. Advisories and
warnings are also shown there. Secondary engine parameters
Top spark plug
Top spark plugBottom spark plug
Bottom spark plugCylinder
No. 1
Cylinder
No. 2Coil Pack 1
Coil Pack 2CH
A
NN
E
L 1
CH
A
NN
E
L 2Electronic Control Unit 1
Figure 4-28. Ignition control.
and nonengine system status are displayed on the bottom
screen. The lower screen is also used for maintenance
diagnosis when the aircraft is on the ground. Color coding
is used, as well as message prioritizing.
The display select panel allows the pilot to choose which
computer is actively supplying information. It also controls
the display of secondary engine information and system status
displays on the lower monitor. EICAS has a unique feature
that automatically records the parameters of a failure event
to be regarded afterwards by maintenance personnel. Pilots
that suspect a problem may be occurring during flight can
press the event record button on the display select panel. This
also records the parameters for that flight period to be studied
later by maintenance. Hydraulic, electrical, environmental,
performance, and APU data are examples of what may be
recorded.
EICAS uses BITE for systems and components. A
maintenance panel is included for technicians. From this
panel, when the aircraft is on the ground, push-button
switches display information pertinent to various systems
for analysis. [Figure 4-30]
Auxiliary Ignition Units
During engine starting, the output of a magneto is low
because the cranking speed of the engine is low. This is
understandable when the factors that determine the amount of voltage induced in a circuit are considered.
To increase the value of an induced voltage, the strength
of the magnetic field must be increased by using a stronger
magnet, by increasing the number of turns in the coil, or by
increasing the rate of relative motion between the magnet
and the conductor.
Since the strength of the rotating magnet and the number
of turns in the coil are constant factors in magneto ignition
systems, the voltage produced depends upon the speed at
which the rotating magnet is turned. When the engine is
being cranked for starting, the magnet is rotated at about
80 rpm. Since the value of the induced voltage is so low, a
spark may not jump the spark plug gap. To facilitate engine
starting, an auxiliary device is connected to the magneto to
provide a high ignition voltage.
Ordinarily, such auxiliary ignition units are energized by the
battery and connected to the left magneto. Reciprocating
engine starting systems normally include one of the following
types of auxiliary starting systems: booster coil (older style),
starting vibrator (sometimes called shower of sparks),
impulse coupling, or electronic ignition systems.
During the starting cycle, the engine is turning very slowly
compared to normal speed. The ignition must be retarded
or moved back to prevent kickback of the piston trying to
Standy engine
indicatorsAural warningUpper DU
Lower DU
Warning & cautions
Engine primary displays
Engine secondary
or
status display
or
maintenance display
Discrete caution
& warning lights
R computer
L computer
Display switching
Maintenance panel
Display select panel
Other system discretes
FCC MCDP
TMC interface
FEC interface
FMC interface
RAD Altitude interface
ADC interfaceEngine sensors
N1 Oil press
N2 Oil quantity
N3 Oil temperature
EPR Vibration
EGT
FFSystem sensors
Hydraulic quantity & press
ADC hydraulic system temperature
Control surface positions
Electical system: volts amps freq
Generator drive temperature
ECS temps
APU EGT. RPM
Brake temperatureData buses
Figure 4-29. Schematic of an engine indicating and crew alerting system (EICAS).
rotate opposite normal rotation. Each starting system has a
method of retarding the spark during starting of the engine.
Booster Coil
Used mainly with older radial engine ignition systems,
the booster coil assembly consists of two coils wound on
a soft iron core, a set of contact points, and a condenser.
[Figure 4-31] The booster coil is separate from the magneto
and can generate a series of sparks on its own. During the
start cycle, these sparks are routed to the trailing finger on
the distributor rotor and then to the appropriate cylinder
ignition lead. The primary winding has one end grounded at
the internal grounding strip and its other end connected to the
moving contact point. The stationary contact is fitted with a
terminal to which battery voltage is applied when the magneto
switch is placed in the start position, or automatically applied
when the starter is engaged. The secondary winding, which contains several times as many turns as the primary coil,
has one end grounded at the internal grounding strip and the
other terminated at a high-tension terminal. The high-tension
terminal is connected to an electrode in the distributor by an
ignition cable.
Since the regular distributor terminal is grounded through
the primary or secondary coil of a high-tension magneto,
the high-voltage furnished by the booster coil must be
distributed by a separate circuit in the distributor rotor. This is
accomplished by using two electrodes in one distributor rotor.
The main electrode, or finger, carries the magneto output
voltage; the auxiliary electrode or trailing finger, distributes
only the output of the booster coil. The auxiliary electrode
is always located so that it trails the main electrode, thus
retarding the spark during the starting period.
ECS
MSGELEC
HYDPERF
APU
CONF
MCDPENG
EXCDAUTO MANUAL
REC ERASE
TESTDESPLAY SELECTEICAS MAINT EVENT
READEnvironmental control systems and maintenance message formats
Engine exceedances
BITE test switch for self-test routine
Configuration and maintenance control/display panelElectrical and hydraulic systems formal
Performance and auxiliary power unit formatsSelects data from auto or manual event in memory
Erases stored data currently displayed
Records real-time data currently displayed (in manual event)
Figure 4-30. The EICAS maintenance control panel is for the exclusive use of technicians.
Figure 4-32 illustrates, in schematic form, the booster coil
components shown in Figure 4-31 . In operation, battery
voltage is applied to the positive (+) terminal of the booster
coil through the start switch. This causes current to flow
through the closed contact points to the primary coil and
ground. [Figure 4-32] Current flow through the primary coil
sets up a magnetic field about the coil that magnetizes the
coil core. As the core is magnetized, it attracts the movable
contact point, which is normally held against the stationary
contact point by a spring.
As the movable contact point is pulled toward the iron
core, the primary circuit is broken, collapsing the magnetic
field that extended about the coil core. Since the coil core
acts as an electromagnet only when current flows in the
primary coil, it loses its magnetism as soon as the primary
coil circuit is broken. This permits the action of the spring
to close the contact points and again complete the primary
coil circuit. This remagnetizes the coil core, and again
attracts the movable contact point, which again opens the
primary coil circuit. This action causes the movable contact
point to vibrate rapidly, as long as the start switch is held
in the closed, or on, position. The result of this action is a continuously expanding and collapsing magnetic field that
links the secondary coil of the booster coil. With several
times as many turns in the secondary as in the primary, the
induced voltage that results from lines of force linking the
secondary is high enough to furnish ignition for the engine.
The condenser, which is connected across the contact points,
has an important function in this circuit. [Figure 4-32] As
current flow in the primary coil is interrupted by the opening
of the contact points, the high self-induced voltage that
accompanies each collapse of the primary magnetic field
surges into the condenser. Without a condenser, an arc would
jump across the points with each collapse of the magnetic
field. This would burn and pit the contact points and greatly
reduce the voltage output of the booster coil. The booster
coil generates a pulsating DC in the primary winding that
induces a high-voltage spark in the secondary windings of
the booster coil.
Impulse Coupling
Many opposed reciprocating engines are equipped with
an impulse coupling as the auxiliary starting system. An
impulse coupling gives one of the magnetos attached to the
Access screw
Wire coreCoverTo battery negative
To booster terminal of magnetoContact springTo starter side of start switch
Internal grounding stripPrimary winding
CondenserVibrator moving contactVibrator stationary contactExternal ground
through mounting bolt
Coil core
Primary windingStationary
contact point
Moving
contact point+
Secondary
winding
Internal grounding stripMovable contact springCondenserFigure 4-31. Booster coil.
Figure 4-32. Booster coil schematic.
engine, generally the left, a brief acceleration, that produces
an intense spark for starting. This device consists of a cam
and flyweight assembly, spring, and a body assembly.
[Figure 4-33] The assembled impulse coupling is shown
installed on a typical magneto in Figure 4-34 .
The magneto is flexibly connected through the impulse
coupling by means of the spring so that at low speed the
magneto is temporarily held. [Figure 4-35] The flyweight,
because of slow rotation, catches on a stud or stop pins,
and the magneto spring is wound as the engine continues to
turn. The engine continues to rotate until the piston of the cylinder to be fired reaches approximately a top dead center
position. At this point, the magneto flyweight contacts the
body of the impulse coupling and is released. The spring
kicks back to its original position, resulting in a quick twist
of the rotating magnet of the magneto. [Figure 4-36] This,
being equivalent to high-speed magneto rotation, produces
a spark that jumps the gap at the spark plug electrodes. The
impulse coupling has two functions: rotating the magneto fast
enough to produce a good spark and retarding the timing of
the spark during the start cycle. After the engine is started and
the magneto reaches a speed at which it furnishes sufficient
current, the flyweights in the impulse coupling fly outward
due to centrifugal force or rapid rotation. This action prevents
the two flyweight coupling members from contact with the
stop pin. That makes it a solid unit, returning the magneto to
a normal timing position relative to the engine. The presence
of an impulse coupling is identified by a sharp clicking noise
as the crankshaft is turned at starter cranking speed past top
center on each cylinder.
A problem that can arise from impulse couplings is that
the flyweights can become magnetized and not engage the
stop pins. Congealed oil or sludge on the flyweights during
cold weather may produce the same results. This prevents
the flyweight weights from engaging the stop pins, which
results in no starting spark being produced. Wear can cause
problems with impulse couplings. They should be inspected
and any maintenance should be performed as set forth by the
manufacturer. Another disadvantage of the impulse coupling
is that it can produce only one spark for each firing cycle
of the cylinder. This is a disadvantage, especially during
Body
Counterweights
Cam
Spring
Figure 4-33. Parts of an impulse coupling.
Stop pin
Impulse coupling
Figure 4-34. Impulse coupling on a magneto.
Engages stop pin
Figure 4-35. Flyweights engage stop pins.
adverse starting conditions. Even with these disadvantages,
the impulse coupling is still in wide use.
High-Tension Retard Breaker Vibrator
To provide for more spark power during the starting cycle,
the shower of sparks system was developed, which provides
several sparks at the spark plug electrodes during starting.
The starting vibrator, or shower of sparks, consists essentially
of an electrically operated vibrator, a condenser, and a relay.
[Figure 4-37] These units are mounted on a base plate and enclosed in a metal case.
The starting vibrator, unlike the booster coil, does not
produce the high ignition voltage within itself. The function
of this starting vibrator is to change the DC of the battery
into a pulsating DC and deliver it to the primary coil of the
magneto. Closing the ignition switch energizes the starter
solenoid and causes the engine to rotate. At the same time,
current also flows through the vibrator coil and its contact
points. Current flow in the vibrator coil sets up a magnetic
field that attracts and opens the vibrator points. When the
vibrator points open, current flow in the coil stops, and the
magnetic field that attracted the movable vibrator contact
point disappears. This allows the vibrator points to close and
Body hits flyweight
ST1
T2L1L2
L3C2
C3C1R1R2R3
R4OffS1OnL
R
BothOff
L
RGrdL
R
L
InSwitch
Retard
SwitchRetard
breaker
Main
breakerMain
breakerStarting vibrator
Magneto switch
Left magneto
Right magnetoV1Figure 4-36. Flyweight contacts body, releasing impulse coupling
to spin.
Figure 4-37. High-tension retard breaker magneto and starting vibrator circuit. again permits battery current to flow in the vibrator coil. This
completes a cycle of operation. The cycle, however, occurs
many times per second, so rapidly that the vibrator points
produce an audible buzz.
Each time the vibrator points close, current flows to the
magneto as a pulsating DC. Since this current is being
interrupted many times per second, the resulting magnetic
field is building and collapsing across the primary and secondary coils of the magneto many times per second.
The rapid successions of separate voltages induced in the
secondary coil produces a shower of sparks across the
selected spark plug air gap.
The retard breaker magneto and starting vibrator system is
used as part of the high-tension starting system on many
types of aircraft. Designed for four- and six-cylinder ignition
systems, the retard breaker magneto eliminates the need for
the impulse coupling in light aircraft. This system uses an
additional breaker to obtain retarded sparks for starting. The
starting vibrator is also adaptable to many helicopter ignition
systems. A schematic diagram of an ignition system using
the retard breaker magneto and starting vibrator concept is
shown in Figure 4-37 .
With the magneto switch in the both position and the starter
switch S1 in the on position, starter solenoid L3 and coil L1
are energized, closing relay contacts R4, R1, R2, and R3. R3
connects the right magneto to ground, keeping it inoperative
during starting operation. Electrical current flows from the
battery through R1, vibrator points V1, coil L2, through both
the retard breaker points, through R2, and the main breaker
points of the left magneto to ground.
The energized coil L2 opens vibrator points V1, interrupting
the current flow through L2. The magnetic field about L2
collapses, and vibrator points V1 close again. Once more,
4-23ST1
L1L2
L3C2
C3C1R1R2R3
V1
R4OffS3OnL
R
BothOff
Retard
breaker
Main
breakerMain breaker
Spark plug
T2
Spark plugL4
L5
Starter solenoidStarting vibrator
Magneto switch
Left magneto
Right magneto
Figure 4-38. Low-tension retard breaker magneto and starting vibrator circuit.current flows through L2, and again V1 vibrator points open.
This process is repeated continuously, and the interrupted
battery current flows to ground through the main and retard
breaker points of the left magneto.
Since relay R4 is closed, the starter is energized and the engine
crankshaft is rotated. When the engine reaches its normal
advance firing position, the main breaker points of the left
magneto begin to open. The interrupted surges of current from
the vibrator can still find a path to ground through the retard
breaker points, which do not open until the retarded firing
position of the engine is reached. At this point in crankshaft
travel, the retard points open. Since the main breaker points
are still open, the magneto primary coil is no longer shorted,
and current produces a magnetic field around T1.
Each time the vibrator points V1 open, current flow through
V1 is interrupted. The collapsing field about T1 cuts through
the magneto coil secondary and induces a high-voltage surge
of energy used to fire the spark plug. Since the V1 points are
opening and closing rapidly and continuously, a shower of
sparks is furnished to the cylinders when both the main and
retard breaker points are open.
After the engine begins to accelerate, the manual starter switch
is released, causing L1 and L3 to become deenergized. This
action causes both the vibrator and retard breaker circuits to become inoperative. It also opens relay contact R3, which
removes the ground from the right magneto. Both magnetos
now fire at the normal running advanced degree position of
crankshaft rotation before top dead center piston position.
Low-Tension Retard Breaker Vibrator
This system, which is in limited use, is designed for light
aircraft reciprocating engines. A typical system consists of a
retard breaker magneto, a single breaker magneto, a starting
vibrator, transformer coils, and a starter and ignition switch.
[Figure 4-38]
To operate the system, place the starter switch S3 in the
on position. This energizes starter solenoid L3 and coil
L1, closing relay contacts R1, R2, R3, and R4. With the
magneto switch in the L position, current flows through R1,
the vibrator points, L2, R2, and through the main breaker
points to ground. Current also flows through R3 and the
retard breaker points to ground. Current through L2 builds
up a magnetic field that opens the vibrator points. Then, the
current stops flowing through L2, reclosing the points. These
surges of current flow through both the retard and main
breaker points to ground.
Since the starter switch is closed, the engine crankshaft is
turning. When it has turned to the normal advance or running
ignition position, the main breaker points of the magneto
Outer shell
ElectrodesInsulator5/8"-24 Shielding
barrel with connector3/4"-20 Shielding
barrel with connector
Figure 4-39. Spark plug cutaway. open. However, current still flows to ground through the
closed retard breaker points. As the engine continues to turn,
the retard ignition position is reached, and the retard breaker
points open. Since the main breaker points are still open,
current must flow to ground through coil L4, producing a
magnetic field around the coil L4.
As the engine continues to turn, the vibrator breaker points
open, collapsing the L4 magnetic field through T1 primary,
inducing a high-voltage in the secondary of T1 to fire the
spark plug.
When the engine fires, the starter switch is released, de-
energizing L1 and L3. This opens the vibrator circuit and
retard breaker points circuit. The ignition switch is then
turned to the both position, permitting the right magneto to
operate in time with the left magneto.
Spark Plugs
The function of the spark plug in an ignition system is to
conduct a short impulse of high-voltage current through
the wall of the combustion chamber. Inside the combustion
chamber, it provides an air gap across which the impulse
can produce an electric spark to ignite the air-fuel charge.
While the aircraft spark plug is simple in construction and
operation, it can be the cause of malfunctions in aircraft
engines. Despite this fact, spark plugs provide a great deal of
trouble-free operation when properly maintained and when
correct engine operating procedures are practiced.
Spark plugs operate at extreme temperatures, electrical
pressures, and very high cylinder pressures. A cylinder of an
engine operating at 2,100 rpm must produce approximately
17 separate and distinct high-voltage sparks that bridge the
air gap of a single spark plug each second. This would appear
as a continuous spark across the spark plug electrodes at
temperatures of over 3,000 °F. At the same time, the spark
plug is subjected to gas pressures as high as 2,000 pounds
per square inch (psi) and electrical pressure as high as 20,000
volts. Given the extremes that spark plugs must operate under,
and the fact that the engine loses power if one spark does
not occur correctly, proper function of a spark plug in the
operation of the engine is imperative.
The three main components of a spark plug are the electrode,
insulator, and outer shell. [Figure 4-39] The outer shell,
threaded to fit into the cylinder, is usually made of finely
machined steel and is often plated to prevent corrosion from
engine gases and possible thread seizure. Close-tolerance
screw threads and a copper gasket prevent cylinder gas
pressure from escaping around the plug. Pressure that might
escape through the plug is retained by inner seals between the
outer metal shell and the insulator, and between the insulator and the center electrode assembly. The other end is threaded
to receive the ignition lead from the magneto. All-weather
plugs form a seal between the lead and the plug that is water
proof to prevent moisture from entering this connection.
The insulator provides a protective core around the electrode.
In addition to affording electrical insulation, the ceramic
insulator core also transfers heat from the ceramic tip, or nose,
to the cylinder. The insulator is made from aluminum oxide
ceramic having excellent dielectric strength, high mechanical
strength, and thermal conductivity. The types of spark plugs
used in different engines vary in respect to heat range, reach,
massive electrode, fine wire electrode (Iridium/platinum),
or other characteristics of the installation requirements for
different engines.
The electrodes can be of several designs from massive
electrodes or Nickel-base alloy to fine wire electrodes.
[Figure 4-39 and 4-40] The massive electrode material has
a lower melting point and is more susceptible to corrosion.
The main differences include cost and length of service.
Fine wire iridium and platinum electrodes have a very high
melting point and are considered precious metals. Therefore,
the cost of this type of spark plug is higher, but they have
Figure 4-40. Fine wire electrodes.
Hot Cold
Figure 4-41. Hot and cold spark plugs.
Reach
Figure 4-42. Spark plug reach.a longer service life with increased performance. Fine wire
spark plugs are more effective than massive electrode plugs
because the size shields its own spark from some of the fuel
air mixture. Less than efficient combustion occurs due to
uneven ignition. The iridium electrode allows for a larger
spark gap, which creates a more intense spark that increases
performance. The spark gap of any electrode is vulnerable
to erosion and the melting point of the electrode material.
The heat range of a spark plug is a measure of its ability to
transfer the heat of combustion to the cylinder head. The
plug must operate hot enough to burn off carbon deposits,
which can cause fouling, a condition where the plug no
longer produces a spark across the electrodes, yet remain
cool enough to prevent a pre-ignition condition. Spark plug
pre-ignition is caused by plug electrodes glowing red hot as
a glow plug, setting off the air-fuel mixture before the normal
firing position. The length of the nose core is the principal
factor in establishing the plug’s heat range. [Figure 4-41]
Hot plugs have a long insulator nose that creates a long heat
transfer path; cold plugs have a relatively short insulator
to provide a rapid transfer of heat to the cylinder head.
[Figure 4-41]
If an engine were operated at only one speed, spark plug
design would be greatly simplified. Because flight demands
impose different loads on the engine, spark plugs must be
designed to operate as hot as possible at slow speeds and light
loads, and as cool as possible at cruise and takeoff power.
The choice of spark plugs to be used in a specific aircraft
engine is determined by the engine manufacturer after
extensive tests. When an engine is certificated to use hot or cold spark plugs, the plug used is determined by the
compression ratio, the degree of supercharging, and how the
engine is to be operated. High-compression engines tend to
use colder range plugs while low-compression engines tend
to use hot range plugs.
A spark plug with the proper reach ensures that the electrode
end inside the cylinder is in the best position to achieve
ignition. The spark plug reach is the length of the threaded
portion that is inserted in the spark plug bushing of the
cylinder. [Figure 4-42] Spark plug seizure and/or improper
combustion within the cylinder can occur if a plug with the
wrong reach is used. In extreme cases, if the reach is too
long, the plug may contact a piston or valve and damage
the engine. If the plug threads are too long, they extend into
the combustion chamber and carbon adheres to the threads
making it almost impossible to remove the plug. This can
also be a source of pre-ignition. Heat of combustion can make
some of the carbon a source for ignition, which can ignite the
air-fuel mixture prematurely. It is very important to select
the approved spark plugs for the engine.
4-26Reciprocating Engine Ignition System
Maintenance & Inspection
An aircraft’s ignition system is the result of careful design and
thorough testing. The ignition system usually provides good,
dependable service, provided it is maintained and inspected
properly. However, difficulties can occur with normal wear,
which affects ignition system performance, especially
with magneto systems. Breakdown and deterioration of
insulating materials, breaker point wear, corrosion, bearing
and oil seal wear, and electrical connection problems are all
possible defects that can be associated with magneto-ignition
systems. The ignition timing requires precise adjustment and
painstaking care so that the following four conditions occur
at the same instant:
1. The piston in the No. 1 cylinder must be in a position
a prescribed number of degrees before top dead center
on the compression stroke.
2. The rotating magnet of the magneto must be in the
E-gap position.
3. The breaker points must be just opening on the No. 1
cam lobe.
4. The distributor finger must be aligned with the
electrode serving the No. 1 cylinder.
If one of these conditions is out of synchronization with any
of the others, the ignition system is out of time. If the spark
is out of time, it is not delivered to the cylinder at the correct
time and engine performance decreases.
When ignition in the cylinder occurs before the optimum
crankshaft position is reached, the timing is said to be early.
If ignition occurs too early, the piston rising in the cylinder
is opposed by the full force of combustion. This condition
results in a loss of engine power, overheating, and possible
detonation and pre-ignition.
If ignition occurs at a time after the optimum crankshaft
position is reached, the ignition timing is said to be late. If it
occurs too late, not enough time is allowed to consume the
air-fuel charge, and combustion is incomplete. As a result,
the engine loses power and requires a greater throttle opening
to carry a given propeller load.
Moisture forming on different parts of the ignition system
causes more common irregularities. Moisture can enter
ignition system units through cracks or loose covers, or it
can result from condensation. Breathing, a situation that
occurs during the readjustment of the system from low to
high atmospheric pressure, can result in drawing in moisture-
laden air. Ordinarily, the heat of the engine is sufficient
to evaporate this moisture, but occasionally the moist air
condenses as the engine cools. The result is an appreciable moisture accumulation which causes the insulation materials
to lose electrical resistance. A slight amount of moisture
contamination may cause reduction in magneto output by
short-circuiting to ground a part of the high-voltage current
intended for the spark plug. If the moisture accumulation is
appreciable, the entire magneto output may be dissipated to
ground by way of flashover and carbon tracking. Moisture
accumulation during flight is extremely rare because the high
operating temperature of the system is effective in preventing
condensation. Difficulties from moisture accumulation are
probably more evident during starting and ground operation.
Spark plugs are often diagnosed as being faulty when the real
malfunction exists in a different system. Malfunctioning of
the carburetor, poor fuel distribution, too much valve overlap,
leaking primer system, or poor idle speed and mixture settings
show symptoms that are the same as those for faulty ignition.
Unfortunately, many of these conditions can be temporarily
improved by a spark plug change, but the trouble recurs in a
short time because the real cause of the malfunction has not
been eliminated. A thorough understanding of the various
engine systems, along with meticulous inspection and good
maintenance methods, can substantially reduce such errors.
Magneto-Ignition Timing Devices
Built-In Engine Timing Reference Marks
Most reciprocating engines have timing reference marks
built into the engine. The timing reference marks vary by
manufacturer. [Figure 4-43] When the starter gear hub is
installed correctly, the timing marks are marked on it that
line up with the mark on the starter. On an engine that has no
starter gear hub, the timing mark is normally on the propeller
flange edge. [Figure 4-44] The top center (TC) mark stamped
on the edge aligns with the crankcase split line below the
crankshaft when the No. 1 piston is at top dead center. Other
flange marks indicate degrees before top center.
Some engines have degree markings on the propeller
reduction drive gear. To time these engines, the plug
provided on the exterior of the reduction gear housing must
be removed to view the timing marks. On other engines, the
timing marks are on a crankshaft flange and can be viewed
by removing a plug from the crankcase. In every case, the
engine manufacturer’s instructions give the location of built-
in timing reference marks.
In using built-in timing marks to position the crankshaft, be
sure to sight straight across the stationary pointer or mark on
the nose section, the propeller shaft, crankshaft flange, or bell
gear. [Figure 4-45] Sighting at an angle results in an error
in positioning the crankshaft. Normally, the No. 1 cylinder
is used to time or check the timing of the magnetos. When
installing magnetos, the timing marks must be lined up and
the No. 1 cylinder must be on the compression stroke.
Index mark is a
dot on the starter
Number of degrees of crank-
shaft rotation is marked on
the propeller hub
Figure 4-43. Lycoming timing marks.
T
C
TC 24 32Timing marks
Split line
Figure 4-44. Propeller flange timing marks.
20Indexing groove
Figure 4-45. Typical built-in timing mark on propeller reduction
gear.
The amount of gear backlash in any system of gears varies
between installations because there is clearance between
the gear teeth. Always take timing when reading, or stop
movement of the engine for timing set up, in the direction
of rotation. Another unfavorable aspect in the use of timing
marks on the reduction gear is the small error that exists when
sighting down the reference mark to the timing mark inside
the housing on the reduction gear. This can occur because
there is depth between the two reference marks.Timing Discs
Most timing disc devices are mounted to the crankshaft
flange and use a timing plate. [Figure 4-46] The markings
vary according to the specifications of the engine. This plate
is temporarily installed on the crankshaft flange with a scale
numbered in crankshaft degrees and the pointer attached to
the timing disc.
Piston Position Indicators
Any given piston position, whether it is to be used for
ignition, valve, or injection pump timing, is referenced to a
piston position called top dead center. This piston position is
not to be confused with a piston position called top center. A
piston in top center has little value from a timing standpoint
because the corresponding crankshaft position may vary
from 1° to 5° for this piston position. This is illustrated in
Figure 4-47, which is exaggerated to emphasize the no-travel
zone of the piston. Notice that the piston does not move
while the crankshaft describes the small arc from position A
to position B. This no-travel zone occurs between the time
the crankshaft and connecting rod stop pushing the piston
upward, and continues until the crankshaft has swung the
lower end of the connecting rod into a position where the
crankshaft can start pulling the piston downward. Top dead
center is a piston and crankshaft position from which all other
piston and crankshaft locations are referenced. When a piston
is in the top dead center position of the crankshaft, it is also
in the center of the no-travel zone. The piston is in a position
where a straight line can be drawn through the center of the
crankshaft journal, the crankpin, and the piston pin. This is
A B
Crankshaft moves about 8° without moving pistonTop center Top dead center
TC25°
Figure 4-46. A timing plate and pointer.
Figure 4-47. Difference between top center and top dead center.
Figure 4-49. Timing light.
Figure 4-48. E50 Magneto Synchronizer.shown on the right of Figure 4-47 . With such an alignment,
a force applied to the piston could not move the crankshaft.
Timing Lights
Timing lights are used to help determine the exact instant
that the magneto points open. There are two general types
of timing lights in common use. Both have two lights and
three external wire connections. Although both have internal
circuits that are somewhat different, their function is very
much the same. [Figures 4-48 and 4-49]
Three wires plug into the light box. [Figure 4-49] There are
two lights on the front face of the unit, one green and one
red, and a switch to turn the unit on and off. To use the timing
light, the center lead, which is black, marked “ground lead” is connected to the case of the magneto being tested. The
other leads are connected to the primary leads of the breaker
point assembly of the magnetos being timed. The color of the
lead corresponds to the color of the light on the timing light.
With the leads connected in this manner, it can be easily
determined whether the points are open or closed by turning
on the switch and observing the two lights. If the points are
closed, most of the current flows through the breaker points
and not through the transformers, and the lights do not come
on. If the points are open, the current flows through the
transformer and the lights glow. Some models of timing lights
operate in the reverse manner (i.e., the light goes out when
the points open). Each of the two lights is operated separately
by the set of breaker points to which it is connected. This
makes it possible to observe the time, or point in reference to
magneto rotor rotation, that each set of points opens.
Most timing lights use batteries that must be replaced after
long use. Attempts to use a timing light with weak batteries
may result in erroneous readings because of low current flow
in the circuits.
Timing markChamfered tooth
Figure 4-50. Timing marks indicate the number one firing position
of a magneto.
Red marks visible through vent hole must align
Timing pin
Figure 4-51. Checking magneto E-gap.Checking the Internal Timing of a Magneto
When replacing or preparing a magneto for installation,
the first concern is with the internal timing of the magneto.
For each magneto model, the manufacturer determines how
many degrees beyond the neutral position a pole of the rotor
magnet should be to obtain the strongest spark at the instant
of breaker point separation. This angular displacement from
the neutral position, known as the E-gap angle, varies with
different magneto models. On one model, a step is cut on
the end of the breaker cam to check internal timing of the
magneto. When a straightedge is laid along this step and it
coincides with the timing marks on the rim of the breaker
housing, the magneto rotor is then in the E-gap position, and
the breaker contact points should just begin to open.
Another method for checking E-gap is to align a timing
mark with a pointed chamfered tooth. [Figure 4-50] The
breaker points should be just starting to open when these
marks line up.
In a third method, the E-gap is correct when a timing pin is
in place and red marks visible through a vent hole in the side
of the magneto case are aligned. [Figure 4-51] The contact
points should be just opening when the rotor is in the position
just described.
Bench timing the magneto, or setting the E-gap, involves
positioning the magneto rotor at the E-gap position and
setting the breaker points to open when the timing lines or
marks provided for that purpose are perfectly aligned.High-Tension Magneto E-Gap Setting (Bench
Timing)
The following steps are taken to check and adjust the timing
of the breaker points for the S-200 magneto, which does not
have timing marks in the breaker compartment:
1. Remove the timing inspection plug from the top of
the magneto. Turn the rotating magnet in its normal
direction of rotation until the painted, chamfered tooth
on the distributor gear is approximately in the center
of the inspection window. Then, turn the magnet back
a few degrees until it is in its neutral position. Because
of its magnetism, the rotating magnet holds itself in
the neutral position.
2. Install the timing kit and place the pointer in the zero
position. [Figure 4-52]
3. Connect a suitable timing light across the main breaker
points and turn the magnet in its normal direction of
rotation 10° as indicated by the pointer. This is the
E-gap position. The main breaker points should be
adjusted to open at this point.
4. Turn the rotating magnet until the cam follower is
at the highpoint on the cam lobe, and measure the
clearance between the breaker points. This clearance
must be 0.018 inch ± 0.006 inch [0.46 millimeter
(mm) ± 0.15 mm]. If the breaker point clearance is
not within these limits, the points must be adjusted
for correct setting. It is then necessary to recheck and
readjust the timing for breaker opening. If the breaker
points cannot be adjusted to open at the correct time,
they should be replaced.
Figure 4-52. Installing timing kit.Figure 4-53. Timing marks aligned.
Timing the High-tension Magneto to the Engine
When replacing magnetos on aircraft engines, two factors
must be considered:
1. The internal timing of the magneto, including breaker
point adjustment, which must be correct to obtain
maximum potential voltage from the magneto.
2. The engine crankshaft position where the spark occurs.
The engine is usually timed by using the No. 1 cylinder
on the compression stroke.
The magneto must be timed by first adjusting or checking the
internal timing with the magneto off the engine. This is done
by checking and adjusting the ignition points to open at the
E-gap position. The chamfered tooth should line up (reference
timing mark for the magneto) in the middle of the timing
window. The magneto is set to fire the No. 1 cylinder. Remove
the most accessible spark plug from the No. 1 cylinder. Pull
the propeller through in the direction of rotation until the No.
1 piston is coming up on the compression stroke. This can be
determined by holding a thumb over the spark plug hole until
the compression air is felt. Set the engine crankshaft at the
prescribed number of degrees ahead of true top dead center
as specified in the applicable manufacturer’s instruction,
usually using the timing marks on the engine. With the engine
set at a prescribed number of degrees ahead of true top dead
center on the compression stroke and with final movement
of the engine stopped in the direction of normal rotation,
the magneto can be installed on the engine. [Figure 4-53]
While holding the magneto drive in the firing position for the
No. 1 cylinder as indicated by the alignment of the reference
marks for the magneto, install the magneto drive into the engine drive. It should be installed in the middle of its slotted
flange to allow for fine timing of the magneto to the engine.
Attach a timing light to both magnetos. With the engine
still in the firing position, the magnetos should be timed by
moving them in the flange slots until the breaker points in the
magneto just open. If the slots in the mounting flange of the
magneto do not permit sufficient movement to effect breaker
point opening for the No. 1 cylinder, move the magneto out
of position far enough to permit turning the magneto drive
shaft. Then, install the magneto in position again and repeat
the previous check for point opening.
Install the magneto attaching nuts on the studs and tighten
slightly. The nuts must not be tight enough to prevent the
movement of the magneto assembly when the magneto
mounting flange is tapped with a mallet. Reconnect the timing
light to the magneto and breaker points. With the light and
ignition switch turned on, rotate the magneto assembly first
in the direction of rotation and then in the opposite direction.
This is done to determine that the points just opened. After
completing this adjustment, tighten the mounting nuts. Move
the propeller one blade opposite the direction of rotation and
then, while observing the timing light, move the propeller
in the direction of rotation until the prescribed number of
degrees ahead of top dead center is reached. Be sure that the
lights for both sets of points come on points open, within the
prescribed timing position.
Both right and left sets of breaker points should open at the
same instant, proper magneto-to-engine timing exists, and all
phases of magneto operation are synchronized. Some early
engines had what was referred to as staggered timing where
one magneto would fire at a different number of degrees
before top dead center on the compression stroke. In this
case, each magneto had to be timed separately. If staggered
ignition timing is used, the spark plug nearest the exhaust
valve will fire first.
4-31In the following example, a timing light is used for timing the
magneto to the engine. The timing light is designed in such
a way that one of two lights come on when the points open.
The timing light incorporates two lights. When connecting
the timing light to the magneto, the leads should be connected
so that the light on the right side of the box represents the
breaker points on the right magneto, and the light on the left
side represents the left magneto breaker points. The black lead
or ground lead must be attached to the engine or an effective
ground. When using the timing light to check a magneto in a
complete ignition system installed on the aircraft, the ignition
switch for the engine must be turned to both. Otherwise, the
lights do not indicate breaker point opening.
Performing Ignition System Checks
The ignition system has checks performed on it during the
aircraft engine run-up, which is the engine check before
each flight. The magneto check, as it is usually referred to,
is performed during the engine run-up check list.
One other check is accomplished prior to engine shutdown.
The ignition system check is used to check the individual
magnetos, harnesses, and spark plugs. After reaching the
engine rpm specified for the ignition system check, allow the
rpm to stabilize. Place the ignition switch in the right position
and note the rpm drop on the tachometer. Return the switch
to the both position. Allow the switch to remain in the both
position for a few seconds so that the rpm stabilizes again.
Place the ignition switch in the left position and again note the
rpm drop. Return the ignition switch to the both position. Note
the amount of total rpm drop that occurs for each magneto
position. The magneto drop should be even for both magnetos
and is generally in the area of a 25–75 rpm drop for each
magneto. Always refer to the aircraft operating manual for
specific information. This rpm drop is because operating on
one magneto combustion is not as efficient as it is with two
magnetos providing sparks in the cylinder.
Remember, this tests not only the magnetos but also the
ignition leads and spark plugs. If either magneto has
excessive rpm drop while operating by itself, the ignition
system needs to be checked for problems. If only one
magneto has a high magneto drop, the problem can be
isolated and corrected by operating on that magneto. This
ignition system check is usually performed at the beginning
of the engine run-up because rpm drops not within the
prescribed limits affect later checks.
Ignition Switch Check
The ignition switch check is performed to see that all
magneto ground leads are electrically grounded. The
ignition switch check is usually made at 700 rpm. On those aircraft engine installations that do not idle at this low rpm,
set the engine speed to the lowest possible to perform this
check. When the speed to perform this check is obtained,
momentarily turn the ignition switch to the off position.
The engine should completely quit firing. After a drop of
200–300 rpm is observed, return the switch to the both
position as rapidly as possible. Do this quickly to eliminate
the possibility of afterfire and backfire when the ignition
switch is returned to both.
If the ignition switch is not returned quickly enough, the
engine rpm drops off completely and the engine stops. In
this case, leave the ignition switch in the off position and
place the mixture control in the idle-cutoff position to avoid
overloading the cylinders and exhaust system with raw fuel.
When the engine has completely stopped, allow it to remain
inoperative for a short time before restarting.
If the engine does not cease firing in the off position, the
magneto ground lead, more commonly referred to as the P
lead, is open, and the trouble must be corrected. This means
that one or more of the magnetos are not being shut off even
when the ignition switch is in the off position. Turning the
propeller of this engine can result in personnel injury or
death. If the propeller is turned in this condition, the engine
can start with personnel in the propeller arch.
Maintenance & Inspection of Ignition Leads
Inspection of ignition leads should include both a visual
and an electrical test. During the visual test, the lead cover
should be inspected for cracks or other damage, abrasions,
mutilated braid, or other physical damage. Inspect leads for
overheating if routed close to exhaust stacks. Disconnect the
harness coupling nuts from the top of the spark plugs and
remove the leads from the spark plug lead well. Inspect the
contact springs and compression springs for any damage
or distortion and the sleeves for cracks or carbon tracking.
The coupling nut that connects to the spark plug should be
inspected for damaged threads or other defects.
Each lead should be checked for continuity using a high-
tension lead tester by connecting the black lead to the contact
spring and the red lead to the eyelet of the same lead in the
cover. The continuity lamp on the tester should illuminate
when tested. The insulation resistance test of each lead is
accomplished using the high-tension lead tester by attaching
the red, or high-voltage, lead to the spring of the harness
lead. Then, attach the black lead to the ferrule of the same
lead. Depress the press-to-test push button switch on the lead
tester. Observe that the indictor lamp flashes and gap fires
simultaneously as long as the press-to-test switch is held in
the depressed position.
Elbow
Seal
Terminal
sleeve
assembly
Protector cap5mm ferrule
7mm sleeve 5mm ignition cable 5mm ferrule5mm ignition cable
For distributor blocks with 5mm wells
For distributor blocks with 7mm wells
Figure 4-54. Replacement procedure for ignition lead terminals. If the indicator lamp flashes and the gap fails to fire, the lead
under test is defective and must be replaced. The indicator
lamp flashes to show that a high-voltage impulse was sent
out. If it fails to pass through the tester, then the electrical
pulse leaked through the wire showing it to be defective.
When defective leads are revealed by an ignition harness test,
continue the test to determine whether the leads or distributor
block are defective. If the difficulty is in an individual ignition
lead, determine whether the electrical leak is at the spark plug
elbow or elsewhere. Remove the elbow, pull the ignition lead
out of the manifold a slight amount, and repeat the harness
test on the defective lead. If this stops the leakage, cut away
the defective portion of the lead and reinstall the elbow
assembly, integral seal, and terminal (sometimes referred to
as cigarette). [Figure 4-54] When installing the leads, you
should avoid bends because weak points may develop in
the insulation through which high-tension current can leak.
If the lead is too short to repair in the manner described, or the
electrical leak is inside the harness, replace the defective lead.
Single ignition lead replacement procedures are as follows:
1. Disassemble the magneto or distributor so that the
distributor block is accessible.
2. Loosen the piercing screw in the distributor block for
the lead to be replaced, and remove the lead from the
distributor block.
3. Remove approximately 1 inch of insulation from the distributor block end of the defective lead and
approximately 1 inch of insulation from the end of
the replacement cable. Splice this end to the end of
the lead to be replaced and solder the splice.
4. Remove the elbow adapter from the spark plug end
of the defective lead, then pull the old lead out and
put the new lead into the harness. While pulling the
leads through the harness, have someone push the
replacement lead into the ignition manifold at the
distributor end to reduce the force required to pull the
lead through the ignition manifold.
5. When the replacement lead has been pulled completely
through the manifold, force the ignition lead up into
the manifold from the distributor block end to provide
extra length for future repairs, which may be necessary
because of chafing at the spark plug elbow.
6. Remove approximately 3⁄8 inch of insulation from
the distributor block end. Bend the ends of the wire
back and prepare the ends of the cable for installation
into the distributor block well. Insert the lead in the
distributor and tighten the piercing screw.
7. Remove approximately ¼ inch of insulation from
the spark plug end of the lead and install the elbow,
integral seal, and cigarette. [Figure 4-54]
8. Install a marker on the distributor end of the cable to
identify its cylinder number. If a new marker is not
available, use the marker removed from the defective
cable.
Figure 4-55. Carbon fouled spark plug. Replacement of Ignition Harness
Replace a complete ignition harness only when the
shielding of the manifold is damaged or when the number
of defective leads makes it more practical to replace the
harness than to replace the individual leads. Replace a cast-
filled harness only when leakage in the cast-filled portion
is indicated. Before replacing any harness to correct engine
malfunctioning, make extensive ignition harness tests.
Typical procedures for installing an ignition harness are
detailed in the following paragraphs.
Install the ignition harness on the engine. Tighten and safety
the hold down nuts and bolts and install and tighten the
individual lead brackets according to instructions. The ignition
harness is then ready for connection of the individual leads
to the distributor block. A band is attached to each lead at the
distributor end of the harness to identify the cylinder for the
lead. However, each lead should be checked individually with
a continuity or timing light prior to connecting it.
Check for continuity by grounding the lead at the cylinder
and then checking at the distributor block end to establish that
the lead grounded is as designated on the band for the lead.
After checking all leads for proper identification, cut them
to the proper length for installation into the distributor
block. Before cutting the leads, however, force them back
into the manifold as far as possible to provide surplus wire
in the ignition manifold. This extra wire may be needed at
a later date in the event that chafing of a lead at the spark
plug elbow necessitates cutting a short section of wire from
the spark plug end of the harness. After cutting each lead to
length, remove approximately 3⁄8 inch of insulation from the
end and prepare the lead for insertion into the distributor
block. Before installing the lead, back out the set screw in
the distributor block far enough to permit slipping the end of
the wire into the hole without force. Insert the lead into the
block and tighten the set screw. Connect the wires in firing
order (the first cylinder to fire No. 1 location on the block,
the second in the firing order to No. 2 location, etc).
After connecting each lead, check continuity between the
lead and its distributor block electrode with continuity light
or timing light. To perform one test lead, touch the other test
lead to the proper distributor block electrode. If the light does
not indicate a complete circuit, the set screw is not making
contact with the ignition wire or the lead is connected to the
wrong block location. Correct any faulty connections before
installing the distributor block.
Checking Ignition Induction Vibrator Systems
To check the induction vibrator, ensure that the manual
mixture control is in idle cutoff, the fuel shutoff valve and
booster pump for that engine are in the off position, and the battery switch is on. Since the induction vibrator buzzes
whether the ignition switch is on or off, leave the switch off
during the check. If the engine is equipped with an inertia or
combination starter, make the check by closing the engage
mesh switch; if the engine is equipped with a direct-cranking
starter, see that the propeller is clear and close the start switch.
An assistant stationed close to the induction vibrator should
listen for an audible buzzing sound. If the unit buzzes when
the starter is engaged or cranked, the induction vibrator is
operating properly.
Spark Plug Inspection & Maintenance
Spark plug operation can often be a major source of engine
malfunctions because of lead, oil, graphite, carbon fouling,
and spark plug gap erosion. Most of these failures, which
usually accompany normal spark plug operation, can be
minimized by good operational and maintenance practices.
A spark plug is considered fouled if it has stopped allowing
the spark to bridge the gap either completely or intermittently.
Carbon Fouling of Spark Plugs
Carbon fouling from fuel is associated with mixtures that
are too rich to burn or mixtures that are so lean they cause
intermittent firing. [Figure 4-55] Each time a spark plug does
not fire, raw fuel and oil collect on the nonfiring electrodes
and nose insulator. These difficulties are almost invariably
associated with an improper idle mixture adjustment, a
leaking primer, or carburetor malfunctions that cause too rich
a mixture in the idle range. A rich air-fuel mixture is detected
by soot or black smoke coming from the exhaust and by an
increase in rpm when the idling air-fuel mixture is leaned
to best power. The soot that forms as a result of overly rich
idle air-fuel mixtures settles on the inside of the combustion
chamber because the heat of the engine and the turbulence
Figure 4-56. Lead fouled spark plug. in the combustion chamber are slight. At higher engine
speeds and powers, however, the soot is swept out and does
not condense out of the charge in the combustion chamber.
Oil Fouling of Spark Plugs
Even though the idling air-fuel mixture is correct, there is a
tendency for oil to be drawn into the cylinder past the piston
rings, valve guides, and impeller shaft oil seal rings. At low
engine speeds, the oil combines with the soot in the cylinder
to form a solid that is capable of shorting out the spark plug.
Spark plugs that are wet or covered with lubricating oil are
usually grounded out during the engine start. In some cases,
these plugs may clear up and operate properly after a short
period of engine operation.
Engine oil that has been in service for any length of time
holds in suspension minute carbon particles that are capable
of conducting an electric current. Thus, a spark plug will not
arc the gap between the electrodes when the plug is full of
oil. Instead, the high-voltage impulse flows through the oil
from one electrode to the other without a spark as though
a wire conductor were placed between the two electrodes.
Combustion in the affected cylinder does not occur until, at
a higher rpm, increased airflow has carried away the excess
oil. Then, when intermittent firing starts, combustion assists
in emitting the remaining oil. In a few seconds, the engine is
running clean with white fumes of evaporating and burning
oil coming from the exhaust.
Lead Fouling of Spark Plugs
Lead fouling of aviation spark plugs is a condition likely
to occur in any engine using leaded fuels. Lead is added
to aviation fuel to improve its anti-knock qualities. The
lead, however, has the undesirable effect of forming lead
oxide during combustion. This lead oxide forms as a solid
with varying degrees of hardness and consistency. Lead
deposits on combustion chamber surfaces are good electrical
conductors at high temperatures and cause misfiring. At low
temperatures, the same deposits may be good insulators. In
either case, lead formations on aircraft spark plugs prevent
their normal operation. [Figure 4-56] To minimize the
formation of lead deposits, ethylene dibromide is added to
the fuel as a scavenging agent that combines with the lead
during combustion.
Lead fouling may occur at any power setting, but perhaps the
power setting most conducive to lead fouling is cruising with
lean mixtures. At this power, the cylinder head temperature
is relatively low and there is more oxygen than needed to
consume all the fuel in the air-fuel mixture. Oxygen, when
hot, is very active and aggressive. When all the fuel has
been consumed, some of the excess oxygen unites with
some of the lead and some of the scavenger agent to form oxygen compounds of lead or bromine or both. Some of
these undesirable lead compounds solidify and build up
in layers as they contact the relatively cool cylinder walls
and spark plugs. Although lead fouling may occur at any
power setting, experience indicates that the lead buildup
is generally confined to a specific combustion temperature
range. Combustion temperatures outside this specific range
minimize the lead fouling tendency.
If lead fouling is detected before the spark plugs become
completely fouled, the lead can usually be eliminated
or reduced by either a sharp rise or a sharp decrease in
combustion temperature. This imposes a thermal shock on
cylinder parts, causing them to expand or contract. Since
there is a different rate of expansion between deposits and
metal parts on which they form, the deposits chip off or are
loosened and then scavenged from the combustion chamber
by the exhaust or are burned in the combustion process.
Several methods of producing thermal shock to cylinder
parts are used. The method used depends on the accessory
equipment installed on the engine. A sharp rise in combustion
temperatures can be obtained on all engines by operating
them at full takeoff power for approximately 1 minute.
When using this method to eliminate fouling, the propeller
control must be placed in low pitch, or high rpm, and the
throttle advanced slowly to produce takeoff rpm and manifold
pressure. Slow movement of the throttle control provides
reasonable freedom from backfiring in the affected cylinders
during the application of power.
Figure 4-57. Spark plug gap erosion. Another method of producing thermal shock is the use of
excessively rich air-fuel mixtures. This method suddenly
cools the combustion chamber because the excess fuel does
not contribute to combustion; instead, it absorbs heat from
the combustion area. Some carburetor installations use two-
position manual mixture controls that provide a lean mixture
setting for cruising economy and a richer mixture setting for
all powers above cruising. Neither manual mixture control
setting in this type of configuration is capable of producing
an excessively rich air-fuel mixture. Even when the engine
is operated in auto-rich at powers where an auto-lean setting
would be entirely satisfactory, the mixture is not rich enough.
Graphite Fouling of Spark Plugs
As a result of careless and excessive application of thread
lubricant, called antiseize compound, to the spark plug, the
lubricant flows over the electrodes and causes shorting.
Shorting occurs because graphite is a good electrical
conductor. The elimination of service difficulties caused
by graphite is up to the aircraft technician. Use care when
applying the lubricant to make certain that smeared fingers,
shop towels, or brushes do not contact the electrodes or any
part of the ignition system except the spark plug threads.
Never apply to the first set of threads.
Gap Erosion of Spark Plugs
Erosion of the electrodes takes place in all aircraft spark
plugs as the spark jumps the air gap between the electrodes.
[Figure 4-57]
The spark carries with it a portion of the electrode, part of
which is deposited on the other electrode. The remainder
is blown off in the combustion chamber. As the airgap
is enlarged by erosion, the resistance that the spark must overcome in jumping the air gap also increases. This means
that the magneto must produce a higher voltage to overcome
the higher resistance. With higher voltages in the ignition
system, a greater tendency exists for the spark to discharge
at some weak insulation point in the ignition system. Since
the resistance of an air gap also increases as the pressure
in the engine cylinder increases, a double danger exists at
takeoff and during sudden acceleration with enlarged airgaps.
Insulation breakdown, premature flashover, and carbon
tracking result in misfiring of the spark plug and go hand in
hand with excessive spark plug gap. Wide gap settings also
raise the coming in speed of a magneto and therefore cause
hard starting.
Spark plug manufacturers have partially overcome the
problem of gap erosion by using a hermetically sealed resistor
in the center electrode of spark plugs. This added resistance
in the high-tension circuit reduces the peak current at the
instant of firing. This reduced current flow helps prevent
metal disintegration in the electrodes. Also, due to the high
erosion rate of steel or any of its known alloys, spark plug
manufacturers are using tungsten or an alloy of nickel for
their massive electrode plugs and iridium/platinum plating
for their fine wire electrode plugs.
Spark Plug Removal
Spark plugs should be removed for inspection and servicing
at the intervals recommended by the manufacturer. Since the
rate of gap erosion varies with different operating conditions,
engine models, and type of spark plug, engine malfunction
traceable to faulty spark plugs may occur before the regular
servicing interval is reached. Normally, in such cases, only
the faulty plugs are replaced.
Since spark plugs can be easily damaged, careful handling
of the used and replacement plugs during installation
and removal of spark plugs from an engine cannot be
overemphasized. To prevent damage, spark plugs should
always be handled individually and new and reconditioned
plugs should be stored in separate cartons. A common method
of storage is illustrated in Figure 4-58 . This is a drilled tray,
which prevents the plugs from bumping against one another
and damaging the fragile insulators and threads. Additionally,
the tray helps identify where the spark plug came from and
its proper rotation. If a plug is dropped on the floor or other
hard surface, it should not be installed in an engine, since
the shock of impact usually causes small, invisible cracks
in the insulators. A dropped spark plug should be discarded.
Before a spark plug can be removed, the ignition harness lead
must be disconnected. Using the special spark plug coupling
elbow wrench, loosen and remove the spark plug to elbow
coupling nut from the spark plug. Take care to pull the lead
Damage to
shielding barrel
insulator at this
point
Figure 4-58. Spark plug tray.
Figure 4-59. Improper lead removal technique.Figure 4-60. Proper spark plug removal technique.
straight out and in line with the centerline of the plug barrel.
If a side load is applied, damage to the barrel insulator and
the ceramic lead terminal may result. [Figure 4-59] If the
lead cannot be removed easily in this manner, the neoprene
collar may be stuck to the shielding barrel. Break loose the
neoprene collar by twisting the collar as though it were a nut
being unscrewed from a bolt.After the lead has been disconnected, select the proper size
deep socket for spark plug removal. Apply steady pressure
with one hand on the hinge handle, holding the socket in
alignment with the other hand. Failure to hold the socket in
correct alignment causes the socket to tilt to one side and
damage the spark plug. [Figure 4-60]
In the course of engine operation, carbon and other products of
combustion are deposited across the spark plug and cylinder,
and some carbon may even penetrate the lower threads of the
shell. As a result, a high torque is generally required to break
the spark plug loose. This factor imposes a shearing load on
the shell section of the plug. After removing the plugs, they
should be placed in a spark plug tray. [Figure 4-58]
Spark Plug Reconditioning Service
A visual inspection should be the first step in servicing
spark plugs. The threads on the shielding barrel and on the
shell that screws into the cylinder should be inspected for
damaged or nicked threads. Inspect the lead shielding barrel
for corrosion, nicks, and cracks. The firing end should be
checked for insulator cracks, chips, and excessive electrode
wear. The shell hex or wrench hex should be checked to see
if it is rounded off or mutilated. If the spark plug passes the
visual check, then it should be degreased using petroleum
solvent. Take care to keep solvent out of the shielding barrel.
Never soak the plugs in solvent. After drying the firing end
of the plugs, remove the lead compound deposits using a
vibrator cleaner. [Figure 4-61] The firing end can now be
cleaned by using an abrasive blaster. This is usually done
using a spark plug cleaner tester. [Figure 4-62] As the
firing end is subjected to the abrasive blast, the plug should
be rotated so all the area of the firing end is cleaned. After
the abrasive blast, the firing end gets a thorough air blast to
remove the abrasive material. The shielding barrel insulators
may be cleaned with a cotton cloth or felt swab saturated with
solvent, wood alcohol, or other approved cleaner. The firing
end should be inspected using a light and a magnifying glass.
If the plug passes the firing end visual and cleaning checks,
then the spark gap should be set using a round thickness
Figure 4-61. Spark plug vibrator cleaner.
Figure 4-62. Spark plug cleaner tester.
gauge. The spark plug should be tested by using a tester as
shown in Figure 4-62 , which passes a high-voltage through
the spark plug and fire the gap. As this test takes place, the
firing end of the plug is subjected to air pressure to simulate
the pressure in the engine’s cylinder. If the firing pattern
is good, the plug should be returned to its holder ready for
installation in the engine.
Inspection Prior to Installation
Before installing new or reconditioned spark plugs in the
engine cylinders, clean the spark plug bushings or Heli-
Coil inserts.
Brass or stainless steel spark plug bushings are usually
cleaned with a spark plug bushing cleanout tap. Before
inserting the cleanout tap in the spark plug hole, fill the flutes
of the tap, or channels between threads, with clean grease to
prevent hard carbon or other material removed by the tap from
dropping into the inside of the cylinder. Align the tap with the bushing threads by sight where possible, and start the tap by
hand until there is no possibility of it being cross-threaded in
the bushing. To start the tap on installations where the spark
plug hole is located deeper than can be reached by a clenched
hand, it may be necessary to use a short length of hose slipped
over the square end of the tap to act as an extension. When
screwing the tap into the bushing, be sure that the full tap
cutting thread reaches the bottom thread of the bushing. This
removes carbon deposits from the bushing threads without
removing bushing metal, unless the pitch diameter of the
threads has contracted as the result of shrinkage or some
other unusual condition. Replace the cylinder if, during the
thread-cleaning process, the bushing is found to be loose,
loosened in the cylinder, or the threads are cross-threaded
or otherwise seriously damaged.
Spark plug Heli-Coil inserts are cleaned with a round wire
brush, preferably one having a diameter slightly larger than
the diameter of the spark plug hole. A brush considerably
larger than the hole may cause removal of material from
the Heli-Coil proper or from the cylinder head surrounding
the insert. Also, the brush should not disintegrate with use,
allowing wire bristles to fall into the cylinder. Clean the insert
by carefully rotating the wire brush with a power tool. When
using the power brush, be careful that no material is removed
from the spark plug gasket seating surface, since this may
cause a change in the spark plug’s heat range, combustion
leakage, and eventual cylinder damage. Never clean the Heli-
Coil inserts with a cleaning tap, since permanent damage to
the insert results. If a Heli-Coil insert is damaged as a result
of normal operation or while cleaning it, replace it according
to the applicable manufacturer’s instructions.
Using a lint-free rag and cleaning solvent, wipe the spark
plug gasket seating surface of the cylinder to eliminate the
possibility of dirt or grease being accidentally deposited on
the spark plug electrodes at the time of installation.
Before the new or reconditioned plugs are installed, they must
be inspected for each of the following conditions:
1. Ensure that the plug is of the approved type,
as indicated by the applicable manufacturer’s
instructions.
2. Check for evidence of rust-preventive compound
on the spark plug exterior and core insulator and on
the inside of the shielding barrel. Rust-preventive
compound accumulations are removed by washing
the plug with a brush and cleaning solvent. It must
then be dried with a dry air blast.
3. Check both ends of the plug for nicked or cracked
threads and any indication of cracks in the nose
insulator.
Figure 4-63. Wire gap gauge. Installation with Normal Torque Installation with Excess Torque
Stretching occurs over
this length
Seal for combustion gas intact
Seal is opened at this point
Figure 4-64. Effect of excessive torque in installing a spark plug. 4. Inspect the inside of the shielding barrel for cracks in
the barrel insulator, and the center electrode contact
for rust and foreign material that might cause poor
electrical contact.
5. Install a new spark plug gasket. When the thermocouple
gasket is used, do not use an additional gasket.
The gap setting should be checked with a round wire-
thickness gauge. [Figure 4-63] A flat-type gauge gives an
incorrect clearance indication because the massive ground
electrodes are contoured to the shape of the round center
electrode. When using the wire thickness gauge, insert the
gauge in each gap parallel to the centerline of the center
electrode. If the gauge is tilted slightly, the indication is
incorrect. Do not install a plug that does not have an air gap
within the specified clearance range.
Spark Plug Installation
Prior to spark plug installation, carefully coat the first two
or three threads from the electrode end of the shell with a
graphite base antiseize compound. Prior to application, stir
the antiseize compound to ensure thorough mixing. When
applying the antiseize compound to the threads, be extremely
careful that none of the compound gets on the ground,
center electrodes, or on the nose of the plug, where it can
spread to the ground or center electrode during installation.
This precaution is mentioned because the graphite in the
compound is an excellent electrical conductor and could
cause permanent fouling.
To install a spark plug, start it into the cylinder without using
a wrench of any kind, and turn it until the spark plug is seated
on the gasket. If you can screw the plug into the cylinder with
comparative ease using your fingers, this indicates good,
clean threads. In this case, only a small amount of additional
tightening torque is needed to compress the gasket to form
a gastight seal. If a high torque is needed to install the plug,
dirty or damaged threads on either the plug or plug bushing
are indicated. The use of excessive torque might compress the gasket out of shape and distort and stretch the plug shell
to a point where breakage would result during the next
removal or installation. Shell stretching occurs as excessive
torque continues to screw the lower end of the shell into the
cylinder after the upper end has been stopped by the gasket
shoulder. As the shell stretches, the seal between the shell
and core insulator is opened, creating a loss of gas tightness
or damage to the core insulator. After a spark plug has been
seated with the fingers, use a torque wrench and tighten to
the specified torque. [Figure 4-64]
Spark Plug Lead Installation
Before installing the spark plug lead, carefully wipe the
terminal sleeve and the integral seal with a cloth moistened
with acetone or an approved solvent. After the plug lead is
cleaned, inspect it for cracks and scratches. If the terminal
sleeve is damaged or heavily stained, replace it.
Application of a light coating of an insulating material to the
outer surface of the terminal sleeve, as well as filling the space
occupied by the contact spring, is sometimes recommended.
By occupying the space in the electrical contact area of the
shielding barrel, the insulating material prevents moisture
from entering the contact area and shorting the spark plug.
One drop here
And one drop here
Figure 4-65. Felt lubricator.Some manufacturers recommend the use of such insulating
compounds only when moisture in the ignition system
becomes a problem, and others have discontinued the use
of such materials.
After inspection of the spark plug lead, slip the lead into
the shielding barrel of the plug with care. Then, tighten the
spark plug coupling elbow nut with the proper tool. Most
manufacturers’ instructions specify the use of a tool designed
to help prevent an overtorque condition. After the coupling
nut is tightened, avoid checking for tightness by twisting the
body of the elbow.
After all plugs have been installed, torqued, and the leads
properly installed, start the engine and perform a complete
ignition system operational check.
Breaker Point Inspection
Inspection of the magneto consists essentially of a periodic
breaker point and dielectric inspection. After the magneto
has been inspected for security of mounting, remove the
magneto cover, or breaker cover, and check the cam for
proper lubrication. Under normal conditions, there is usually
ample oil in the felt oiler pad of the cam follower to keep
the cam lubricated between overhaul periods. However,
during the regular routine inspection, examine the felt pad
on the cam follower to be sure it contains sufficient oil for
cam lubrication. Make this check by pressing the thumbnail
against the oiler pad. If oil appears on the thumbnail, the
pad contains sufficient oil for cam lubrication. If there is no
evidence of oil on the fingernail, apply one drop of a light
aircraft engine oil to the bottom felt pad and one drop to the
upper felt pad of the follower assembly. [Figure 4-65]
After application, allow at least 15 minutes for the felt to
absorb the oil. At the end of 15 minutes, blot off any excess
oil with a clean, lint-free cloth. During this operation, or any
time the magneto cover is off, use extreme care to keep the
breaker compartment free of oil, grease, or engine cleaning
solvents, since each of these have an adhesiveness that
collects dirt and grime that could foul an otherwise good set
of breaker contact points.After the felt oiler pad has been inspected, serviced, and
found to be satisfactory, visually inspect the breaker contacts
for any condition that may interfere with proper operation
of the magneto. If the inspection reveals an oily or gummy
substance on the sides of the contacts, swab the contacts with
a flexible wiper, such as a pipe cleaner dipped in acetone
or other approved solvent. By forming a hook on the end
of the wiper, ready access can be gained to the back side of
the contacts.
To clean the contact mating surfaces, force open the breaker
points enough to admit a small swab. Whether spreading the
points for purposes of cleaning or checking the surfaces for
condition, always apply the opening force at the outer end of
the mainspring and never spread the contacts more than 1⁄16
inch. If the contacts are spread wider than recommended, the
mainspring, the spring carrying the movable contact point,
is likely to take a permanent set. If the mainspring takes a
permanent set, the movable contact point loses some of its
closing tension and the points then either bounce or float,
preventing the normal induction buildup of the magneto.
A swab can be made by wrapping a piece of linen tape or
a small piece of lint-free cloth over one of the leaves of
a clearance gauge and dipping the swab in an approved
solvent. Pass the swab between the carefully separated
contact surfaces until the surfaces are clean. During this
entire operation, take care that drops of solvent do not fall
on lubricated parts, such as the cam, follower block, or felt
oiler pad.
To inspect the breaker contact surfaces, it is necessary to
know what a normal operating set of contacts looks like,
what surface condition is considered as permissible wear, and
what surface condition is cause for dressing or replacement.
The probable cause of an abnormal surface condition can be
determined from the contact appearance. The normal contact
surface has a dull gray, sandblasted, almost rough appearance
over the area where electrical contact is made. [Figure 4-66]
This gray, sandblasted appearance indicates that the points
have worn in and have mated to each other and are providing
the best possible electrical contact. This does not imply that
this is the only acceptable contact surface condition. Slight,
smooth-surfaced irregularities, without deep pits or high
peaks, such as shown in Figure 4-67 , are considered normal
wear and are not cause for replacement.
However, when wear advances to a point where the slight,
smooth irregularities develop into well-defined peaks
extending noticeably above the surrounding surface, the
breaker contacts must be replaced. [Figure 4-68]
Unfortunately, when a peak forms on one contact, the mating
Figure 4-66. Normal contact surface.
Figure 4-67. Points with normal irregularities.
Figure 4-68. Points with well-defined peaks.
contact has a corresponding pit or hole. This pit is more
troublesome than the peak because it penetrates the platinum
pad of the contact surface. It is sometimes difficult to judge
whether a contact surface is pitted deeply enough to require
replacement because this depends on how much of the
original platinum is left on the contact surface. The danger
arises from the possibility that the platinum pad may already
be thin as a result of long service life and previous dressings.
At overhaul facilities, a gauge is used to measure the
remaining thickness of the pad, and no difficulty in
determining the condition of the pad exists. But at line
maintenance activities, this gauge is generally unavailable. Therefore, if the peak is quite high or the pit quite deep,
remove and replace them with a new assembly. A comparison
between Figures 4-67 and 4-68 will help to draw the line
between minor irregularities and well-defined peaks.
Some examples of possible breaker contact surface conditions
are illustrated in Figure 4-69 . Item A illustrates an example
of erosion or wear called frosting. This condition results from
an open-circuited condenser and is easily recognized by the
coarse, crystalline surface and the black “sooty” appearance
of the sides of the points. The lack of effective condenser
action results in an arc of intense heat being formed each
time the points open. This, together with the oxygen in the
air, rapidly oxidizes and erodes the platinum surface of the
points, producing the coarse, crystalline, or frosted appearance.
Properly operating points have a fine-grained, frosted, or
silvery appearance and should not be confused with the coarse-
grained and sooty point caused by faulty condenser action.
Figure 4-69B and C illustrate badly pitted points. In the early
stage, these points are identified by a fairly even contact edge
and minute pits or pocks in or near the center of the contact
surface with an overall smoky appearance. In more advanced
stages, the pit may develop into a large, jagged crater, and
eventually the entire contact surface takes on a burned, black,
and crumpled appearance. Pitted points, as a general rule,
are caused by dirt and impurities on the contact surfaces. If
points are excessively pitted, a new breaker assembly must
be installed.
Figure 4-69E illustrates a built-up point that can be
recognized by the mound of metal that has been transferred
from one point to another. Buildup, like the other conditions
mentioned, results primarily from the transfer of contact
material by means of the arc as the points separate. But, unlike
the others, there is no burning or oxidation in the process
because of the closeness of the pit of one point and the buildup
of the other. This condition may result from excessive breaker
point spring tension that retards the opening of the points or
causes a slow, lazy break. It can also be caused by a poor
primary condenser or a loose connection at the primary coil.
If excessive buildup has occurred, a new breaker assembly
must be installed.
Figure 4-69F illustrates oily points that can be recognized
by their smoked and smudged appearance and by the lack of
any of the previously mentioned irregularities. This condition
may be the result of excessive cam lubrication or of oil
vapors that may come from within or outside the magneto.
A smoking or fuming engine, for example, could produce the
oil vapors. These vapors then enter the magneto through the
magneto ventilator and pass between and around the points.
These conductive vapors produce arcing and burning on the
A B
C
D
E
F
Figure 4-69. Examples of contact surface conditions.
contact surfaces. The vapors also adhere to the other surfaces
of the breaker assembly and form the sooty deposit. If so,
install new breaker assembly.
Dielectric Inspection
Another phase of magneto inspection is the dielectric
inspection. This inspection is a visual check for cleanliness
and cracks. If inspection reveals that the coil cases,
condensers, distributor rotor, or blocks are oily or dirty or
have any trace of carbon tracking, they require cleaning and
possibly waxing to restore their dielectric qualities.
Clean all accessible condensers and coil cases that contain
condensers by wiping them with a lint-free cloth moistened
with acetone. Many parts of this type have a protective
coating. This protective coating is not affected by acetone, but
it may be damaged by scraping or by the use of other cleaning
fluids. Never use unapproved cleaning solvents or improper
cleaning methods. Also, when cleaning condensers or parts
that contain condensers, do not dip, submerge, or saturate
the parts in any solution because the solution used may seep
inside the condenser and short out the plates.
Coil cases, distributor blocks, distributor rotors, and other
dielectric parts of the ignition system are treated with a wax
coating when they are new and again at overhaul. The waxing
of dielectrics aids their resistance to moisture absorption,
carbon tracking, and acid deposits. When these parts become
dirty or oily, some of the original protection is lost, and carbon
tracking may result.
If any hairline carbon tracks or acid deposits are present on the
surface of the dielectric, immerse the part in approved cleaning
solvent and scrub it vigorously with a stiff bristle brush. When
the carbon track or acid deposits have been removed, wipe the part with a clean, dry cloth to remove all traces of the solvent
used for cleaning. Then, coat the part with a special ignition-
treating wax. After wax treating the part, remove excess wax
deposits and reinstall the part in the magneto.
Ignition Harness Maintenance
Although the ignition harness is simple, it is a vital link
between the magneto and spark plug. Because the harness
is mounted on the engine and exposed to the atmosphere, it
is vulnerable to heat, moisture, and the effects of changing
altitude. These factors, plus aging insulation and normal
gap erosion, work against efficient engine operation. The
insulation may break down on a wire inside the harness and
allow the high-voltage to leak through the insulation to the
harness shielding instead of going to the spark plug. Open
circuits may result from broken wires or poor connections.
A bare wire may be in physical contact with the shielding,
or two wires may be shorted together.
Any serious defect in an individual lead prevents the high-
tension impulse from reaching the spark plug to which the
lead is connected. As a result, this plug will not fire. When
only one spark plug is firing in a cylinder, the charge is
not consumed as quickly as it would be if both plugs were
firing. This factor causes the peak pressure of combustion to
occur later on in the power stroke. If the peak pressure in the
cylinder occurs later, a loss of power in that cylinder results.
However, the power loss from a single cylinder becomes a
minor factor when the effects of a longer burning time is
considered. A longer burning time overheats the affected
cylinder, causing detonation, possible pre-ignition, and
perhaps permanent damage to the cylinder.
High-Tension Ignition Harness Faults
Perhaps the most common and most difficult high-tension
ignition system faults to detect are high-voltage leaks. This
is leakage from the core conductor through insulation to the
ground of the shielded manifold. A certain small amount of
leakage exists even in brand new ignition cable during normal
operation. Various factors combine to produce first a high rate
of leakage and then complete breakdown. Of these factors,
moisture in any form is probably the worst.
Under high-voltage stress, an arc forms and burns a path
across the insulator where the moisture exists. If there is
gasoline, oil, or grease present, it breaks down and forms
carbon. The burned path is called a carbon track, since it
is actually a path of carbon particles. With some types of
insulation, it may be possible to remove the carbon track
and restore the insulator to its former useful condition. This
is generally true of porcelain, ceramics, and some of the
plastics because these materials are not hydrocarbons and
any carbon track forming on them is the result of a dirt film
that can be wiped away.
A
B
CD
Figure 4-70. Cross section of an ignition harness.
CABLE TESTER
MODEL E5
PUSH TO TEST
12 Volt
24 Volt
Figure 4-71. Harness tester. Differences in location and amount of leakage produce
different indications of malfunction during engine operation.
Indications are generally misfiring or crossfiring. The
indication may be intermittent, changing with manifold
pressure or with climate conditions. An increase in manifold
pressure increases the compression pressure and the resistance
of the air across the air gap of the spark plugs. An increase in
the resistance at the air gap opposes the spark discharge and
produces a tendency for the spark to discharge at some weak
point in the insulation. A weak spot in the harness may be
aggravated by moisture collecting in the harness manifold.
With moisture present, continued engine operation causes the
intermittent faults to become permanent carbon tracks. Thus,
the first indication of ignition harness unserviceability may
be engine misfiring or roughness caused by partial leakage
of the ignition voltage.
Figure 4-70 demonstrates four faults that may occur. Fault A
shows a short from one cable conductor to another. This fault
usually causes misfiring, since the spark is short circuited to
a plug in a cylinder where the cylinder pressure is low. Fault
B illustrates a cable with a portion of its insulation scuffed
away. Although the insulation is not completely broken
down, more than normal leakage exists, and the spark plug
to which this cable is connected may be lost during takeoff
when the manifold pressure is quite high. Fault C is the
result of condensation collecting in the lowest portion of
the ignition manifold. This condensation may completely
evaporate during engine operation, but the carbon track that
is formed by the initial flashover remains to allow continued
flashover whenever high manifold pressure exists. Fault
D may be caused by a flaw in the insulation or the result
of a weak spot in the insulation that is aggravated by the
presence of moisture. However, since the carbon track is in direct contact with the metal shielding, it probably results in
flashover under all operating conditions.
Harness Testing
The electrical test of the ignition harness checks the condition
or effectiveness of the insulation around each cable in the
harness. [Figure 4-71] This test involves application of a
definite voltage to each lead, and then measurement with
a very sensitive meter of the amount of current leakage
between the lead and the grounded harness manifold. This
reading, when compared with known specifications, becomes
a guide to the condition or serviceability of the cable. As
mentioned earlier, there is a gradual deterioration of flexible
insulating material. When new, the insulation has a low rate
of conductivity; so low that, under several thousand volts
of electrical pressure, the current leakage is only a very few
millionths of an ampere. Natural aging causes an extremely
slow, but certain, change in the resistance of insulating
material, allowing an ever-increasing rate of current leakage.
The procedures for testing ignition harness and leads were
discussed earlier in this chapter.
Ignition exciters Exciter-to-igniter plug cable (2 locations)
Igniter plug (2 locations)
Figure 4-72. Turbine ignition system components. Turbine Engine Ignition Systems
Since turbine ignition systems are operated mostly for a
brief period during the engine-starting cycle, they are, as a
rule, more trouble-free than the typical reciprocating engine
ignition system. The turbine engine ignition system does
not need to be timed to spark during an exact point in the
operational cycle. It is used to ignite the fuel in the combustor
and then it is switched off. Other modes of turbine ignition
system operation, such as continuous ignition that is used
at a lower voltage and energy level, are used for certain
flight conditions.
Continuous ignition is used in case the engine were to flame
out. This ignition could relight the fuel and keep the engine
from stopping. Examples of critical flight modes that use
continuous ignition are takeoff, landing, and some abnormal
and emergency situations.
Most gas turbine engines are equipped with a high-energy,
capacitor-type ignition system and are air cooled by fan
airflow. Fan air is ducted to the exciter box, and then flows
around the igniter lead and surrounds the igniter before
flowing back into the nacelle area. Cooling is important when
continuous ignition is used for some extended period of time.
Gas turbine engines may be equipped with an electronic-type
ignition system, which is a variation of the simpler capacitor-
type system.
The typical turbine engine is equipped with a capacitor-type,
or capacitor discharge, ignition system consisting of two
identical independent ignition units operating from a common
low-voltage (DC) electrical power source: the aircraft battery,
115AC, or its permanent magnet generator. The generator
is turned directly by the engine through the accessory gear
box and produces power any time the engine is turning.
The fuel in turbine engines can be ignited readily in ideal
atmospheric conditions, but since they often operate in the
low temperatures of high altitudes, it is imperative that the
system be capable of supplying a high heat intensity spark.
Thus, a high-voltage is supplied to arc across a wide igniter
spark gap, providing the ignition system with a high degree
of reliability under widely varying conditions of altitude,
atmospheric pressure, temperature, fuel vaporization, and
input voltage.
A typical ignition system includes two exciter units, two
transformers, two intermediate ignition leads, and two high-
tension leads. Thus, as a safety factor, the ignition system
is actually a dual system designed to fire two igniter plugs.
[Figure 4-72]
Figure 4-73 is a functional schematic diagram of a typical
older style capacitor-type turbine ignition system. A 24-volt DC input voltage is supplied to the input receptacle of the
exciter unit. Before the electrical energy reaches the exciter
unit, it passes through a filter that prevents noise voltage
from being induced into the aircraft electrical system. The
low-voltage input power operates a DC motor that drives
one multilobe cam and one single-lobe cam. At the same
time, input power is supplied to a set of breaker points that
are actuated by the multilobe cam.
From the breaker points, a rapidly interrupted current is
delivered to an auto transformer. When the breaker closes,
the flow of current through the primary winding of the
transformer establishes a magnetic field. When the breaker
opens, the flow of current stops, and the collapse of the field
induces a voltage in the secondary of the transformer. This
voltage causes a pulse of current to flow into the storage
capacitor through the rectifier, which limits the flow to a
single direction. With repeated pulses, the storage capacitor
assumes a charge, up to a maximum of approximately 4
joules. (Note: 1 joule per second equals 1 watt.) The storage
capacitor is connected to the spark igniter through the
triggering transformer and a contactor, normally open.
When the charge on the capacitor has built up, the contactor
is closed by the mechanical action of the single-lobe cam.
A portion of the charge flows through the primary of the
triggering transformer and the capacitor connected with it.
This current induces a high-voltage in the secondary, which
ionizes the gap at the spark igniter.
When the spark igniter is made conductive, the storage
capacitor discharges the remainder of its accumulated energy
along with the charge from the capacitor in series with the
primary of the triggering transformer. The spark rate at the
4-44Auto transformer Input leadPower source
Noise filterBreaker
Multilobe cam Single-lobe cam MotorContactorRectifierStorage capacitor
To other exciter
Trigger capacitor
Triggering transformer Spark igniterBleeder resistor
Figure 4-73. Capacitor-type ignition system schematic.
spark igniter varies in proportion to the voltage of the DC
power supply that affects the rpm of the motor. However,
since both cams are geared to the same shaft, the storage
capacitor always accumulates its store of energy from the
same number of pulses before discharge. The employment
of the high-frequency triggering transformer, with a low-
reactance secondary winding, holds the time duration of the
discharge to a minimum. This concentration of maximum
energy in minimum time achieves an optimum spark for
ignition purposes, capable of blasting carbon deposits and
vaporizing globules of fuel.
All high-voltage in the triggering circuits is completely
isolated from the primary circuits. The complete exciter is
hermetically sealed, protecting all components from adverse
operating conditions, eliminating the possibility of flashover
at altitude due to pressure change. This also ensures shielding against leakage of high-frequency voltage interfering with
the radio reception of the aircraft.
Capacitor Discharge Exciter Unit
This capacity-type system provides ignition for turbine
engines. Like other turbine ignition systems, it is required
only for starting the engine; once combustion has begun, the
flame is continuous. [Figure 4-74]
The energy is stored in capacitors. Each discharge circuit
incorporates two storage capacitors; both are located in the
exciter unit. The voltage across these capacitors is stepped
up by transformer units. At the instant of igniter plug firing,
the resistance of the gap is lowered sufficiently to permit the
larger capacitor to discharge across the gap. The discharge
of the second capacitor is of low-voltage, but of very high
WARNINGIDENT
ENG IGN
A (AIRBUS)
1(BOEING)
A(DAC)
TOP
IGNITER
PLUG
ENG IGN
A (AIRBUS)
1(BOEING)
A(DAC)
TOP
IGNITER
PLUGExciter-to-ignitor plug cable connector115 VAC input
Bracket
115 VAC inputAnti-shock mount
Exciter-to-ignitor plug cable connectorCooling air
Cooling airCooling air
Fan cooling air
from cables
Fan cooling air
from cables
Figure 4-74. Fan air-cooled exciter.
energy. The result is a spark of great heat intensity, capable
of not only igniting abnormal fuel mixtures but also burning
away any foreign deposits on the plug electrodes.
The exciter is a dual unit that produces sparks at each of the
two igniter plugs. A continuous series of sparks is produced
until the engine starts. The power is then cut off, and the
plugs do not fire while the engine is operating other than on
continuous ignition for certain flight conditions. This is why
the exciters are air cooled to prevent overheating during long
use of continuous ignition.
Igniter Plugs
The igniter plug of a turbine engine ignition system differs
considerably from the spark plug of a reciprocating engine
ignition system. [Figure 4-75] Its electrode must be
capable of withstanding a current of much higher energy
than the electrode of a conventional spark plug. This high
energy current can quickly cause electrode erosion, but the
short periods of operation minimize this aspect of igniter
maintenance. The electrode gap of the typical igniter plug
is designed much larger than that of a spark plug since the
operating pressures are much lower and the spark can arc
more easily than in a spark plug. Finally, electrode fouling, common to the spark plug, is minimized by the heat of the
high-intensity spark.
Figure 4-76 is a cutaway illustration of a typical annular-gap
igniter plug, sometimes referred to as a long reach igniter
because it projects slightly into the combustion chamber liner
to produce a more effective spark.
Another type of igniter plug, the constrained-gap plug, is used
in some types of turbine engines. [Figure 4-77] It operates
at a much cooler temperature because it does not project into
the combustion-chamber liner. This is possible because the
spark does not remain close to the plug, but arcs beyond the
face of the combustion chamber liner.
Turbine Ignition System Inspection &
Maintenance
Maintenance of the typical turbine engine ignition system
consists primarily of inspection, test, troubleshooting,
removal, and installation.
Inspection
Inspection of the ignition system normally includes the
following:
High-voltage air surface gap
High-voltage surface gap
High-voltage recessed surface gap
Low-voltage shunted surface gap
InsulatorTerminal screwUpper shellGasket
Lower shell
Gasket
Coupling nutSealing wireCementCenter electrode
Figure 4-75. Igniter plugs.
Figure 4-76. Typical annular gap igniter plug.• Ignition lead terminal inspection; ceramic terminal
should be free of arcing, carbon tracking and cracks.
• The grommet seal should be free of flashover and
carbon tracking. [Figure 4-78]
• The wire insulation should remain flexible with no
evidence of arcing through the insulation.
• Inspect the complete system for security of component
mounting, shorts or high-voltage arcing, and loose
connections.
Check System Operation
The igniter can be checked by listening for a snapping noise
as the engine begins to turn, driven by the starter. Though
the following procedure is not common practice and should
only be used when the maintenance manual suggests it as
an alternative method, the igniter can also be checked by
removing it and activating the start cycle, noting the spark
across the igniter.
Caution: The high energy level and voltage associated
with turbine ignition systems can cause injury or death to
personnel coming into contact with the activated system.Repair
Tighten and secure as required and replace faulty components
and wiring. Secure, tighten, and safety as required.
Removal, Maintenance, & Installation of
Ignition System Components
The following instructions are typical procedures suggested
by many gas turbine manufacturers. These instructions are
applicable to the engine ignition components. Always consult
the applicable manufacturer’s instructions before performing
any ignition system maintenance.
Ignition System Leads
1. Remove clamps securing ignition leads to engine.
2. Remove safety wire and disconnect electrical
connectors from exciter units.
Figure 4-77. Constrained gap igniter plug.
Evidence of arcing
Evidence of flashover
Figure 4-78. Flashover inspection.
3. Remove safety wire and disconnect lead from igniter
plug.
4. Discharge any electrical charge stored in the system
by grounding and remove ignition leads from engine.
5. Clean leads with approved dry cleaning solvent.
6. Inspect connectors for damaged threads, corrosion,
cracked insulators, and bent or broken connector pins.
7. Inspect leads for worn or burned areas, deep cuts,
fraying, and general deterioration.
8. Perform continuity check of ignition leads.
9. Reinstall leads, reversing the removal procedure.
Igniter Plugs
1. Disconnect ignition leads from igniter plugs. A good
procedure to perform before disconnecting the ignition
lead is to disconnect the low-voltage primary lead from the ignition exciter unit and wait at least one
minute to permit the stored energy to dissipate before
disconnecting the high-voltage cable from the igniter.
2. Remove igniter plugs from mounts.
3. Inspect igniter plug gap surface material. Before
inspection, remove residue from the shell exterior
using a dry cloth. Do not remove any deposits or
residue from the firing end of the low-voltage igniters.
High-voltage igniters can have the firing end cleaned
to aid in inspection. [Figure 4-79]
4. Inspect for fretting of igniter plug shank.
5. Replace an igniter plug whose surface is granular,
chipped, or otherwise damaged.
6. Replace dirty or carbonized igniter plugs.
7. Install igniter plugs in mounting pads.
8. Check for proper clearance between chamber liner
and igniter plug.
9. Tighten igniter plugs to manufacturer’s specified
torque.
10. Safety wire igniter plugs.
Powerplant Electrical Systems
The satisfactory performance of any modern aircraft depends
to a great degree on the continuing reliability of electrical
systems and subsystems. Improperly or carelessly installed
or maintained wiring can be a source of both immediate
and potential danger. The continued proper performance of
electrical systems depends upon the knowledge and technique
of the mechanic who installs, inspects, and maintains the
electrical wire and cable of the electrical systems.
The procedures and practices outlined in this section are
general recommendations and are not intended to replace the
manufacturer’s instructions in approved practices.
4-48High-voltage
air surface gapYes
Yes
Yes
NoHigh-voltage
surface gap
High-voltage
recessed
surface gap
Low-voltage
shunted
surface gap
Gap
DescriptionTypical Firing
End ConfigurationClean
Firing
End
Conductors Wire single solid conductor
Solid conductor Standed conductor
A B
Figure 4-79. Firing end cleaning.Figure 4-80. Two types of aircraft wire.
and the smallest is number 40. Larger and smaller sizes are
manufactured but are not commonly used.
Wire size may be determined by using a wire gauge.
[Figure 4-82] This type of gauge measures wires ranging
in size from number 0 (zero) to number 36. The wire to be
measured is inserted in the smallest slot that just accommodates
the bare wire. The gauge number corresponding to that slot
indicates the wire size. The slot has parallel sides and should
not be confused with the semicircular opening at the end of
the slot. The opening simply permits the free movement of
the wire all the way through the slot.
Gauge numbers are useful in comparing the diameter of
wires, but not all types of wire or cable can be accurately
measured with a gauge. Large wires are usually stranded to
increase their flexibility. In such cases, the total area can be
determined by multiplying the area of one strand, usually
computed in circular mils (commonly used as a reference to
wire size) when diameter or gauge number is known by the
number of strands in the wire or cable.
Factors Affecting the Selection of Wire Size
Several factors must be considered in selecting the size of
wire for transmitting and distributing electric power. One
factor is the allowable power loss (PR loss) in the line. This
loss represents electrical energy converted into heat. The
use of large conductors reduces the resistance and therefore
the PR loss. However, large conductors are more expensive
initially than small ones; they are heavier and require more
substantial supports.
A second factor is the permissible voltage drop (IR drop)
in the line. If the source maintains a constant voltage at For the purpose of this discussion, a wire is described as a
single solid conductor, or a stranded conductor, covered with
an insulating material. [Figure 4-80] The term “cable,” as
used in aircraft electrical installations, includes the following:
1. Multiconductor cable—two or more separately
insulated conductors in the same jacket.
2. Twisted pair—two or more separately insulated
conductors twisted together.
3. Shielded cable—one or more insulated conductors,
covered with a metallic braided shield.
4. Radio frequency cable—a single, insulated center
conductor with a metallic braided outer conductor.
The concentricity of the center conductor and the outer
conductor is carefully controlled during manufacture
to ensure that they are coaxial.
Wire Size
Wire is manufactured in sizes according to a standard known
as the American wire gauge (AWG). The wire diameters
become smaller as the gauge numbers become larger. The
largest wire size shown in Figure 4-81 is number 0000,
3.1212,000.0
168,000.0
133,000.0
106,000.0
83,700.0
66,400.0
52,600.0
41,700.0
33,100.0
26,300.0
20,800.0
16,500.0
13,100.0
10,400.0
8,230.0
6,530.0
5,180.0
4,110.0
3,260.0
2,580.0
2,050.0
1,620.0
1,290.0
1,020.0
0.00000780.0500
1.070.00 0.0577
1,230.00 Gauge Number Diameter (mils) Circular (mils) Square inches 25 °C (77 °F) 65 °C (149 °F)Cross Section Ohms per 1,000 ft
Figure 4-81. American wire gauge for standard annealed solid copper wire.
232425 26272829303132
Wire
Figure 4-82. Wire gauge.
the input to the line, any variation in the load on the line
causes a variation in line current and a consequent variation
in the IR drop in the line. A wide variation in the IR drop
in the line causes poor voltage regulation at the load. The
obvious remedy is to reduce either current or resistance. A
reduction in load current lowers the amount of power being
transmitted, whereas a reduction in line resistance increases
the size and weight of conductors required. A compromise is
generally reached whereby the voltage variation at the load
is within tolerable limits and the weight of line conductors
is not excessive.
A third factor is the current carrying ability of the conductor.
When current is drawn through the conductor, heat is
generated. The temperature of the wire rises until the
heat radiated, or otherwise dissipated, is equal to the heat
generated by the passage of current through the line. If the
conductor is insulated, the heat generated in the conductor is
not so readily removed as it would be if the conductor were
not insulated. Thus, to protect the insulation from too much
heat, the current through the conductor must be maintained
below a certain value.
When electrical conductors are installed in locations where
the ambient temperature is relatively high, the heat generated
by external sources constitutes an appreciable part of the total
conductor heating. Allowance must be made for the influence
of external heating on the allowable conductor current, and
each case has its own specific limitations. The maximum
allowable operating temperature of insulated conductors
varies with the type of conductor insulation being used.
Tables are available that list the safe current ratings for
various sizes and types of conductors covered with various types of insulation. The chart in Figure 4-83 shows the current
carrying capacity and resistance of copper wire continuous
duty wire in bundles at various temperature ratings.
Factors Affecting Selection of Conductor Material
Although silver is the best conductor, its cost limits its use to
special circuits where a substance with high conductivity is
needed. The two most generally used conductors are copper
and aluminum. Each has characteristics that make its use
advantageous under certain circumstances; also, each has
certain disadvantages.
Copper has a higher conductivity; it is more ductile, can be
drawn out, has relatively high tensile strength, and can be easily
soldered. It is more expensive and heavier than aluminum.
Although aluminum has only about 60 percent of the
conductivity of copper, it is used extensively. Its light weight
makes possible long spans, and its relatively large diameter
for a given conductivity reduces corona, the discharge of
electricity from the wire when it has a high potential. The
discharge is greater when smaller diameter wire is used than
when larger diameter wire is used. Some bus bars are made of
aluminum which has a greater radiating surface than copper
for the same conductance. The characteristics of copper and
aluminum are compared in Figure 4-84 .
Voltage Drop in Aircraft Wire & Cable
The voltage drop in the main power cables from the aircraft
generation source or the battery to the bus should not exceed
2 percent of the regulated voltage when the generator is
carrying rated current or the battery is being discharged at a
5-minute rate. The 5-minute rate in this case means that the
battery should last a minimum of 5 minutes in an emergency,
with all battery operated equipment running. Figure 4-85
shows the recommended maximum voltage drop in the load
circuits between the bus and the utilization equipment.
The resistance of the current return path through the aircraft
structure is always considered negligible. However, this is
based on the assumption that adequate bonding of the structure
or a special electric current return path has been provided
that is capable of carrying the required electric current with
a negligible voltage drop. A resistance measurement of 0.005
ohms from ground point of the generator or battery to ground
terminal of any electrical device is considered satisfactory.
Another satisfactory method of determining circuit resistance
is to check the voltage drop across the circuit. If the voltage
drop does not exceed the limit established by the aircraft or
product manufacturer, the resistance value for the circuit is
considered satisfactory. When using the voltage drop method
of checking a circuit, the input voltage must be maintained
at a constant value.
Continuous Duty Current (Amps)-Wires in Bundles,
Groups, Harnesses, or Conduits (See Note #1)
h t i w , r e g r a l d n a 8 e z i s r o f s e r i w 9 d n a , 0 1 h g u o r h t 4 2 s e z i s r o f e l d n u b e h t n i s e r i w e r o m r o 3 3 , t n e i b m a C ° 0 7 r o f s i g n i t a R : 1 e t o N
no more than 20 percent of harness current carrying capacity being used, at an operating altitude of 60,000 feet.
Note 2: For resistance of silver or nickel-plated conductors, see wire specifications.Wire
Size
105 °C 150 °C 200 °C
28533539528.4016.20
0.090.070.06475755
1,216
1,900
2,426
3,831
5,8749,354
16,98326,818
42,615
66,50081,700
104,500133,000166,500210,900Wire Conductor Temperature RatingMax. Resistance
ohms/1,000 ft@20 °C
Tin Plated Conductor
(See Note #2)Nominal
Conductor
Area
(circ.mils)
Figure 4-83. Current-carrying capacity and resistance of copper wire.
Copper Aluminum Characteristic
25,000
40,000
1755,00055,000
10.6Tensile strength (lb/in2)
Tensile strength for same conductivity (lb)
Weight for same conductivity (lb)
Cross section for same conductivity (CM)
Specific resistance (Ω/mil ft)
Figure 4-84. Characteristics of copper and aluminum.
Allowable Voltage Drop
Continuous
OperationIntermittent
OperationNominal
System
Voltage
Figure 4-85. Recommended voltage drop in load circuits.
The graph in Figure 4-86 applies to copper conductors
carrying direct current. To select the correct size of conductor,
two major requirements must be met. First, the size must be sufficient to prevent an excessive voltage drop while carrying the required current over the required distance. Second, the size must be sufficient to prevent overheating of the cable while carrying the required current. The graphs in Figures 4-86 and 4-87 can simplify these determinations.
To use this graph to select the proper size of conductor, the following must be known:
1.The conductor length in feet;
2.The number of amperes of current to be carried;
3.The amount of voltage drop permitted;
4.Whether the current to be carried is intermittent or
continuous;
5.T he estimated or measured temperature of the
conductor;
6.Whether the wire to be installed is in a conduit or in
a bundle; and
7.Whether it is a single conductor in free air .
Suppose that you want to install a 50-foot conductor from the aircraft bus to the equipment in a 28-volt system. For this length, a 1-volt drop is permissible for continuous operation with a conductor temperature of 20 ºC or less. By referring to the chart in Figure 4-86, the maximum number of feet a conductor may be run carrying a specified current with a 1-volt drop can be determined. In this example, the number50 is selected.
4-52NOTE
Length (LI) is based on conductor temperatures of
20 °C to determine length (L2) at a higher conductor
temperature use formula where T2 = estimated
conductor temperature °CWIRE SIZE
Voltage drop chart
continuous flow at 20 °C
tin-plated MIL-W-27759
conductor EXAMPLE 1
EXAMPLE 2
No. 8 wire at 20 amps
No. 12 wire at 20 amps
No. 14 wire at 20 ampsVOLTAGE DROPCONTINUOUS
CIRCUIT
VOLTAGE
1 Wire length (ft)
24 22 20 18 16 14 12 10 8 6 4 2 1 1/0 2/0 3/0 4/01 1.5 2 3 4 5 7 10 15 20 30 50 70
L2 =(254.5) (L1)
(234.5) + (T2)}AMPERES
Figure 4-86. Conductor graph—continuous flow.
Assuming the current required by the equipment is 20
amperes, the line indicating the value of 20 amperes should
be selected from the diagonal lines. Follow this diagonal
line downward until it intersects the horizontal line number
50. From this point, drop straight down to the bottom of the
graph to find that a conductor between size No. 8 and No.
10 is required to prevent a greater drop than 1 volt. Since the
indicated value is between two numbers, the larger size, No.
8, should be selected. This is the smallest size that should be
used to avoid an excessive voltage drop.
If the installation is for equipment having only an intermittent
(maximum 2 minutes) requirement for power, the graph in
Figure 4-87 is used in the same manner.
Conductor Insulation
Two fundamental properties of insulation materials (e.g.,
rubber, glass, asbestos, and plastic) are insulation resistance
and dielectric strength. These are entirely different and
distinct properties.Insulation resistance is the resistance to current leakage
through and over the surface of insulation materials.
Insulation resistance can be measured with a megger without
damaging the insulation. This serves as a useful guide in
determining the general condition of insulation. However, the
data obtained in this manner may not give a true picture of
the condition of the insulation. Clean, dry insulation having
cracks or other faults may show a high value of insulation
resistance but would not be suitable for use.
Dielectric strength is the ability of the insulator to withstand
potential difference and is usually expressed in terms of
the voltage at which the insulation fails due to electrostatic
stress. Maximum dielectric strength values can be measured
by raising the voltage of a test sample until the insulation
breaks down.
Because of the expense of insulation, its stiffening effect,
and the great variety of physical and electrical conditions
under which the conductors are operated, only the necessary
minimum insulation is applied for any particular type of cable
4-53NOTE
Length (LI) is based on conductor temperatures of
20 °C to determine length (L2) at a higher conductor
temperature use formula where T2 = estimated
conductor temperature °CWIRE SIZE
Voltage drop chart
intermittent flow at 20°
tin-plated MIL-W-27759
conductor VOLTAGE DROPCIRCUIT
VOLTAGE
2 Wire length (ft)
24 22 20 18 16 14 12 10 8 6 4 2 1 1/0 2/0 3/0 4/01 1.5 2 3 4 5 7 10 15 20 30 50 70
L2 =(254.5) (L1)
(234.5) + (T2)AMPERES
Figure 4-87. Conductor graph—intermittent flow.
H215A20 H215A20 H215A203 in
max
6 ft
6 ft3 in
max
Figure 4-88. Spacing of printed identification marks. designed to do a specific job.
The type of conductor insulation material varies with the type
of installation. Rubber, silk, and paper insulation are no longer
used extensively in aircraft systems. More common today are
such materials as vinyl, cotton, nylon, Teflon, and Rockbestos.
Identifying Wire & Cable
To aid in testing and repair operations, many maintenance
activities mark wire or cable with a combination of letters
and numbers that identify the wire, the circuit it belongs to,
the gauge number, and other information necessary to relate
the wire or cable to a wiring diagram. Such markings are the
identification code.
There is no standard procedure for marking and identifying
wiring; each manufacturer normally develops its own
identification code. Figure 4-88 illustrates one identification
system and shows the usual spacing in marking a wire.
Some system components, especially plugs and jacks, are
identified by a letter or group of letters and numbers added
to the basic identification number. These letters and numbers may indicate the location of the component in the system.
Interconnected cables are also marked in some systems to
indicate location, proper termination, and use. In any system,
the marking should be legible, and the stamping color should
contrast with the color of the wire insulation. For example,
use black stamping with light-colored backgrounds, or white
stamping on dark-colored backgrounds.
Most manufacturers mark the wires at intervals of not more
than 15 inches lengthwise and within 3 inches of each
junction or terminating point. [Figure 4-89]
Coaxial cable and wires at terminal blocks and junction
boxes are often identified by marking or stamping a wiring
sleeve rather than the wire itself. For general purpose wiring,
flexible vinyl sleeving, either clear or white opaque, is
4 3 2 1
4 3 2 1
TB - 1H215B20
H216B20
H217B20
H218B20H215A20
H216A20
H217A20
H218A20
Figure 4-89. Wire identification at a terminal block.
XYZ 15
XYZ15Sleeve marker tied in placePressure-sensitive tape
Figure 4-90. Alternate methods of identifying wire bundles.
commonly used. For high-temperature applications, silicone
rubber or silicone fiberglass sleeving is recommended.
Where resistance to synthetic hydraulic fluids or other
solvents is necessary, either clear or white opaque nylon
sleeving can be used.
While the preferred method is to stamp the identification
marking directly on the wire or on sleeving, other methods
are often employed. One method uses a marked sleeve tied in
place. The other uses a pressure-sensitive tape. [Figure 4-90]
Electrical Wiring Installation
The following recommended procedures for installing aircraft
electrical wiring are typical of those used on most types
of aircraft. For purposes of this discussion, the following
definitions are applicable:
1. Open wiring—any wire, wire group, or wire bundle
not enclosed in conduit.
2. Wire group—two or more wires in the same location,
tied together to identity the group.
3. Wire bundle—two or more wire groups tied together
because they are going in the same direction at the
point where the tie is located. The bundle facilitates
maintenance.
4. Electrically protected wiring—wires that include in
the circuit protections against overloading, such as
fuses, circuit breakers, or other limiting devices.
5. Electrically unprotected wiring—wires, generally
from generators to main bus distribution points, that do not have protection, such as fuses, circuit breakers,
or other current-limiting devices.
Wire Groups & Bundles
Grouping or bundling certain wires, such as electrically
unprotected power wiring and wiring to duplicate vital
equipment, should be avoided. Wire bundles should generally
be limited in size to a bundle of 75 wires, or 2 inches in
diameter where practicable. When several wires are grouped
at junction boxes, terminal blocks, panels, etc., the identity
of the group within a bundle can be retained. [Figure 4-91]
Twisting Wires
When specified on the engineering drawing, parallel wires
must be twisted. The most common examples are:
1. Wiring in the vicinity of magnetic compass or flux
valve,
2. Three-phase distribution wiring, and
3. Certain other wires (usually radio wiring).
Twist the wires so that they lie snugly against each other,
making approximately the number of twists per foot as listed
in Figure 4-92. Always check wire insulation for damage after
twisting. If the insulation is torn or frayed, replace the wire.
Spliced Connections in Wire Bundles
Spliced connections in wire groups or bundles should be
located so that they can be easily inspected. Splices should
also be staggered so that the bundle does not become
excessively enlarged. [Figure 4-93] All noninsulated splices
should be covered with plastic, securely tied at both ends.
Slack in Wiring Bundles
Single wires or wire bundles should not be installed with
excessive slack. Slack between supports should normally not
exceed ½ inch. This is the maximum it should be possible
Bundle tie Bundle tie Group tie
Figure 4-91. Group and bundle ties.
#22 #20 #18 #16 #14 #12 #10 #8 #6 #4
10 10 9 8 7 1/2 7 6 1/2 6 5 4
10 10 8 1/2 7 6 1/2 6 5 1/2 5 4 32 Wires
3 Wires
Figure 4-92. Recommended number of twists per foot.
1/2" maximum with normal hand pressureFigure 4-93. Staggered splices in wire bundle.
Figure 4-94. Slack in wire bundle between supports. to deflect the wire with normal hand force. However, this
may be exceeded if the wire bundle is thin and the clamps
are far apart. But the slack should never be so great that
the wire bundle can abrade against any surface it touches.
[Figure 4-94] A sufficient amount of slack should be allowed
near each end of a bundle to:
1. Permit easy maintenance;
2. Allow replacement of terminals;
3. Relieve mechanical strain on the wires, wire junctions,
or supports;
4. Permit free movement of shock and vibration-mounted
equipment; and
5. Permit shifting of equipment for purposes of
maintenance.Bend Radii
Bends in wire groups or bundles should not be less than
ten times the outside diameter of the wire group or bundle.
However, at terminal strips, where wire is suitably supported
at each end of the bend, a minimum radius of three times
the outside diameter of the wire, or wire bundle, is usually
acceptable. There are exceptions to these guidelines in the
case of certain types of cable; for example, coaxial cable
should never be bent to a smaller radius than six times the
outside diameter.
Routing & Installation
All wiring should be installed so that it is mechanically
and electrically sound and neat in appearance. Whenever
practicable, wires and bundles should be routed parallel with,
or at right angles to, the stringers or ribs of the area involved.
An exception to this general rule is the coaxial cables, which
are routed as directly as possible.
The wiring must be adequately supported throughout its
length. A sufficient number of supports must be provided
to prevent undue vibration of the unsupported lengths. All
wires and wire groups should be routed and installed to
protect them from:
1. Chafing or abrasion;
2. High temperature;
3. Being used as handholds, or as support for personal
belongings and equipment;
4. Damage by personnel moving within the aircraft;
5. Damage from cargo stowage or shifting;
6. Damage from battery acid fumes, spray, or spillage;
Cable clamp1/4" minimum clearance
Angle bracket with two point fastening
Figure 4-95. Cable clamp at bulkhead hole.
Approved grommetWires less than 1/4" from hole edge
Figure 4-96. Cable clamp and grommet at bulkhead hole. and
7. Damage from solvents and fluids.
Protection Against Chafing
Wires and wire groups should be installed so that they are
protected against chafing or abrasion in those locations where
contact with sharp surfaces or other wires would damage the
insulation. Damage to the insulation can cause short circuits,
malfunctions, or inadvertent operation of equipment. Cable
clamps should be used to support wire bundles at each hole
through a bulkhead. [Figure 4-95] If wires come closer than
¼ inch to the edge of the hole, a suitable grommet is used in
the hole. [Figure 4-96]
Sometimes, it is necessary to cut nylon or rubber grommets
to facilitate installation. In these instances, after insertion,
the grommet can be secured in place with general purpose
cement. The slot should be at the top of the hole, and the
cut should be made at an angle of 45° to the axis of the wire
bundle hole.
Protection Against High Temperature
To prevent insulation deterioration, wires should be kept
separate from high-temperature equipment, such as resistors,
exhaust stacks, heating ducts. The amount of separation is
usually specified by engineering drawings. Some wires must
be run through hot areas. These wires must be insulated with
high-temperature rated material, such as asbestos, fiberglass,
or Teflon. Additional protection is also often required in the
form of conduits. A low-temperature insulated wire should
never be used to replace a high-temperature insulated wire.Many coaxial cables have soft plastic insulation, such as
polyethylene, that is especially subject to deformation and
deterioration at elevated temperatures. All high-temperature
areas should be avoided when installing these cables.
Additional abrasion protection should be given to asbestos
wires enclosed in conduit. Either conduit with a high
temperature rubber liner should be used or asbestos wires
can be enclosed individually in high-temperature plastic
tubes before being installed in the conduit.
Protection Against Solvents & Fluids
Avoid installing wires in areas where they are subjected to
damage from fluids. Wires should not be placed in the lowest
four inches of the aircraft fuselage, except those that must
terminate in that area. If there is a possibility that wiring
without a protective nylon outer jacket may be soaked with
fluids, plastic tubing should be used to protect it. This tubing
should extend past the exposure area in both directions
and should be tied at each end. If the wire has a low point
between the tubing ends, provide a 1⁄8-inch drainage hole.
[Figure 4-97] This hole should be punched into the tubing
after the installation is complete and the low point definitely
established by using a hole punch to cut a half circle. Care
should be taken not to damage any wires inside the tubing
when using the punch.
Wire should never be routed below a battery. All wires in the
vicinity of a battery should be inspected frequently. Wires
discolored by battery fumes should be replaced.
Figure 4-97. Drainage hole in low point of tubing.
Figure 4-98. Separation of wires from plumbing lines.
Dangerous angles
Safe angles
45° max45° max
Figure 4-99. Proper mounting angle for cable clamps.Protection of Wires in Wheel Well Area
Wires located in wheel wells are subject to many additional
hazards, such as exposure to fluids, pinching, and severe
flexing in service. All wire bundles should be protected by
sleeves of flexible tubing securely held at each end. There
should be no relative movement at points where flexible
tubing is secured. These wires and the insulating tubing
should be inspected carefully at very frequent intervals, and
wires or tubing should be replaced at the first sign of wear.
There should be no strain on attachments when parts are fully
extended, but slack should not be excessive.
Routing Precautions
When wiring must be routed parallel to combustible fluid
or oxygen lines for short distances, as much separation as
possible should be maintained. The wires should be on a level
with, or above, the plumbing lines. Clamps should be spaced
so that if a wire is broken at a clamp, it will not contact the
line. Where a 6-inch separation is not possible, both the wire
bundle and the plumbing line can be clamped to the same
structure to prevent any relative motion. If the separation is
less than 2 inches but more than 1⁄2 inch, two cable clamps
back to back can be used to maintain a rigid separation only
and not for support of the bundle. [Figure 4-98] No wire
should be routed so that it is located nearer than ½ inch to a
plumbing line, nor should a wire or wire bundle be supported
from a plumbing line that carries flammable fluids or oxygen.
Wiring should be routed to maintain a minimum clearance
of at least 3 inches from control cables. If this cannot be
accomplished, mechanical guards should be installed to
prevent contact between wiring and control cables.Installation of Cable Clamps
Cable clamps should be installed with regard to the proper
mounting angle. [Figure 4-99] The mounting screw should
be above the wire bundle. It is also desirable that the back
of the cable clamp rest against a structural member where
practicable. Figure 4-100 shows some typical mounting
hardware used in installing cable clamps. Be sure that wires
are not pinched in cable clamps. Where possible, mount them
directly to structural members. [Figure 4-101]
Clamps can be used with rubber cushions to secure wire
bundles to tubular structures. [Figure 4-102] Such clamps
must fit tightly but should not be deformed when locked in
place.
Lockwasher external teeth Lockwasher splitSelf-locking nut NutPlain washers Plain washerScrewCable clamps
Figure 4-100. Typical mounting hardware for cable clamps.
Wire is pinched in clamp“Z” member “Angle” memberCable clamps
Angle bracket
Incorrect
Correct
Figure 4-101. Mounting cable clamps to structure.
ClampRubber cushion
Cable clampTubular structure
Figure 4-102. Installing cable clamps to tubular structure. The material used for lacing and tying is either cotton or
nylon cord. Nylon cord is moisture- and fungus-resistant,
but cotton cord must be waxed before using to give it these
necessary protective characteristics.
Single-Cord Lacing
Figure 4-103 shows the steps in lacing a wire bundle with a
single cord. The lacing procedure is started at the thick end
of the wire group or bundle with a knot consisting of a clove
hitch with an extra loop. The lacing is then continued at regular
intervals with half hitches along the wire group or bundle and
at each point where a wire or wire group branches off. The half
hitches should be spaced so that the bundle is neat and secure. Lacing & Tying Wire Bundles
Wire groups and bundles are laced or tied with cord to provide
ease of installation, maintenance, and inspection. This
section describes and illustrates recommended procedures for
lacing and tying wires with knots that hold tightly under all
conditions. For the purposes of this discussion, the following
terms are defined:
1. Tying is the securing together of a group or bundle
of wires by individual pieces of cord tied around the
group or bundle at regular intervals.
2. Lacing is the securing together of a group or bundle
of wires by a continuous piece of cord forming loops
at regular intervals around the group or bundle.
Cord crosses under loop
Starting knot tightened
Trim to 3/8" minimumPull here until tight before finishing knot
Final knot
First part of final knot tightenedStep C—part IStep C—part IIStep A—Starting knot
Step B—Intermediate half hitches
Figure 4-103. Single cord lacing. The lacing is ended by tying a knot consisting of a clove hitch
with an extra loop. After the knot is tied, the free ends of the
lacing cord should be trimmed to approximately 3⁄8 inch.
Double-Cord Lacing
Figure 4-104 illustrates the procedure for double-cord lacing.
The lacing is started at the thick end of the wire group or
bundle with a bowline-on-a-bight knot. [Figure 4-104A] At
regular intervals along the wire group or bundle, and at each
point where a wire branches off, the lacing is continued using
half hitches, with both cords held firmly together. The half
hitches should be spaced so that the group or bundle is neat
and secure. The lacing is ended with a knot consisting of a
half hitch, continuing one of the cords clockwise and the other
counterclockwise and then tying the cord ends with a square
knot. The free ends of the lacing cord should be trimmed to
approximately 3⁄8 inch.
Lacing Branch-Offs
Figure 4-105 illustrates a recommended procedure for lacing
a wire group that branches off the main wire bundle. The
branch-off lacing is started with a knot located on the main
bundle just past the branch-off point. Continue the lacing
along the branched-off wire group using regularly spaced
half hitches. If a double cord is used, both cords should be
held snugly together. The half hitches should be spaced to
lace the bundle neatly and securely. End the lacing with the
regular terminal knot used in single- or double-cord lacing,
as applicable, and trim the free ends of the lacing cord neatly.Tying
All wire groups or bundles should be tied where supports
are more than 12 inches apart. Ties are made using
waxed cotton cord, nylon cord, or fiberglass cord. Some
manufacturers permit the use of pressure-sensitive vinyl
electrical tape. When permitted, the tape should be wrapped
three turns around the bundle and the ends heat sealed to
prevent unwinding of the tape. Figure 4-106 illustrates a
recommended procedure for tying a wire group or bundle.
The tie is started by wrapping the cord around the wire group
to tie a clove-hitch knot. Then, a square knot with an extra
loop is tied and the free ends of the cord trimmed.
Temporary ties are sometimes used in making up and
installing wire groups and bundles. Colored cord is normally
used to make temporary ties, since they are removed when
the installation is complete.
Whether lacing or tying, bundles should be secured tightly
enough to prevent slipping, but not so tightly that the cord
cuts into or deforms the insulation. This applies especially to
coaxial cable, which has a soft dielectric insulation between
the inner and outer conductor. Coaxial cables have been
damaged by the use of lacing materials or by methods of
lacing or tying wire bundles that cause a concentrated force
on the cable insulation. Elastic lacing materials, small-
diameter lacing cord, and excessive tightening deform the
interconductor insulation and result in short circuits or
impedance changes. Flat nylon braided waxed lacing tape
Starting knot tightenedStep A—Bowline-on-a-bight as starting knot
Step B—Intermediate half hitches
Step C—Final knot
Figure 4-104. Double cord lacing.
Starting knot for branch off
Half-hitch required at branch-off pointBranch-off lacing
Main bundle lacing
Figure 4-105. Lacing a branch off.
Wrap cord twice over bundle Clove hitch and square knot
Figure 4-106. Tying a wire group of bundle. should be used for lacing or tying any wire bundles containing
coaxial cables.
The part of a wire group or bundle located inside a conduit
is not tied or laced; however, wire groups or bundles inside
enclosures, such as junction boxes, should be laced only.
Cutting Wire & Cable
To make installation, maintenance, and repair easier, runs of
wire and cable in aircraft are broken at specified locations
by junctions, such as connectors, terminal blocks, or buses.
4-61Stripper Wire Size Insulations
#26–#4 All except asbestos
#26–#4 All
#20–#6 All
#26–#8 All
#2–#0000 AllHot blade
Rotary, electric
Bench
Hand pliers
Knife
Figure 4-107. Wire strippers for copper wire.
Figure 4-108. Light duty hand wire strippers. Before assembly to these junctions, wires and cables must
be cut to length.
All wires and cables should be cut to the lengths specified on
drawings and wiring diagrams. The cut should be made clean
and square, and the wire or cable should not be deformed.
If necessary, large diameter wire should be reshaped after
cutting. Good cuts can be made only if the blades of cutting
tools are sharp and free from nicks. A dull blade deforms and
extrudes wire ends.
Stripping Wire & Cable
Nearly all wire and cable used as electrical conductors are
covered with some type of insulation. In order to make
electrical connections with the wire, a part of this insulation
must be removed to expose the bare conductor. Copper wire
can be stripped in a number of ways depending on the size and
insulation. Figure 4-107 lists some types of stripping tools
recommended for various wire sizes and types of insulation.
Aluminum wire must be stripped using extreme care, since
individual strands break very easily after being nicked.
The following general precautions are recommended when
stripping any type of wire:
1. When using any type of wire stripper, hold the wire
so that it is perpendicular to the cutting blades.
2. Adjust automatic stripping tools carefully; follow the
manufacturer’s instructions to avoid nicking, cutting,
or otherwise damaging strands. This is especially
important for aluminum wires and for copper wires
smaller than No. 10. Examine stripped wires for
damage. Cut off and restrip, if length is sufficient,
or reject and replace any wires with more than the
allowable number of nicked or broken strands listed
in the manufacturer’s instructions.
3. Make sure insulation is clean cut with no frayed or
ragged edges. Trim, if necessary.
4. Make sure all insulation is removed from stripped area.
Some types of wires are supplied with a transparent
layer of insulation between the conductor and the
primary insulation. If this is present, remove it.
5. When using hand wire strippers to remove lengths of insulation longer than 3⁄4 inch, it is easier to accomplish
in two or more operations.
6. Retwist copper strands by hand or with pliers, if
necessary, to restore natural lay and tightness of strands.
A pair of hand wire strippers is shown in Figure 4-108 .
This tool is commonly used to strip most types of wire. The
following general procedures describe the steps for stripping
wire with a hand stripper. [Figure 4-109]
1. Insert wire into exact center of correct cutting slot
for wire size to be stripped. Each slot is marked with
wire size.
2. Close handles together as far as they will go.
3. Release handles allowing wire holder to return to the
open position.
4. Remove stripped wire.
Solderless Terminals & Splices
Splicing of electrical cable should be kept to a minimum and
avoided entirely in locations subject to extreme vibrations.
Individual wires in a group or bundle can usually be spliced
if the completed splice is located where it can be inspected
periodically. The splices should be staggered so that the
bundle does not become excessively enlarged. Many types of
aircraft splice connectors are available for splicing individual
wires. Self-insulated splice connectors are usually preferred;
however, a noninsulated splice connector can be used if the
splice is covered with plastic sleeving secured at both ends.
Solder splices may be used, but they are particularly brittle
and not recommended.
Electric wires are terminated with solderless terminal lugs
to permit easy and efficient connection to and disconnection
from terminal blocks, bus bars, or other electrical equipment.
Solderless splices join electric wires to form permanent
continuous runs. Solderless terminal lugs and splices
are made of copper or aluminum and are preinsulated or
uninsulated, depending on the desired application.
Figure 4-109. Stripping wire with hand strippers.
Barrel
Wire insulation Stripped wireTongue
Color-coded insulationInsulation grip
Figure 4-110. Preinsulated terminal lug.Terminal lugs are generally available in three types for use
in different space conditions. These are the flag, straight,
and right-angle lugs. Terminal lugs are crimped, sometimes
called staked or swaged, to the wires by means of hand or
power crimping tools.Copper Wire Terminals
Copper wires are terminated with solderless, preinsulated
straight copper terminal lugs. The insulation is part of the
terminal lug and extends beyond its barrel so that it covers a
portion of the wire insulation, making the use of an insulation
sleeve unnecessary. [Figure 4-110]
In addition, preinsulated terminal lugs contain an insulation
grip (a metal reinforcing sleeve) beneath the insulation for
extra gripping strength on the wire insulation. Preinsulated
terminals accommodate more than one size of wire; the
insulation is usually color coded to identify the wire sizes
that can be terminated with each of the terminal lug sizes.
Crimping Tools
Hand, portable power, and stationary power tools are
available for crimping terminal lugs. These tools crimp the
barrel of the terminal lug to the conductor and simultaneously
crimp the insulation grip to the wire insulation.
Hand crimping tools all have a self-locking ratchet that
prevents opening the tool until the crimp is complete. Some
hand crimping tools are equipped with a nest of various size
inserts to fit different size terminal lugs. Others are used on
one terminal lug size only. All types of hand crimping tools
are checked by gauges for proper adjustment of crimping jaws.
Figure 4-111 shows a terminal lug inserted into a hand tool.
The following general guidelines outline the crimping
procedure:
1. Strip the wire insulation to proper length.
2. Insert the terminal lug, tongue first, into the hand tool
barrel crimping jaws until the terminal lug barrel butts
flush against the tool stop.
3. Insert the stripped wire into the terminal lug barrel
until the wire insulation butts flush against the end of
the barrel.
Stop
Barrel butts flush against tool stopTongue is horizontalLocator
Figure 4-111. Inserting terminal lug into hand tool.
1" (approximate)
Tight or shrunk sleeve
Loose sleeveLacing cord
Figure 4-112. Insulating sleeves.4. Squeeze the tool handles until the ratchet releases.
5. Remove the completed assembly and examine it for
proper crimp.
Some types of uninsulated terminal lugs are insulated after
assembly to a wire by means of pieces of transparent flexible
tubing called sleeves. The sleeve provides electrical and
mechanical protection at the connection. When the size of
the sleeves used is such that it fits tightly over the terminal
lug, the sleeves need not be tied; otherwise, it should be tied
with lacing cord [Figure 4-112]
Aluminum Wire Terminals
Aluminum wire is being used increasingly in aircraft
systems because of its weight advantage over copper.
However, bending aluminum causes “work hardening”
of the metal, making it brittle. This results in failure or
breakage of strands much sooner than in a similar case with
copper wire. Aluminum also forms a high-resistant oxide film immediately upon exposure to air. To compensate for
these disadvantages, it is important to use the most reliable
installation procedures. Only aluminum terminal lugs are
used to terminate aluminum wires.
All aluminum terminals incorporate an inspection hole that
permits checking the depth of wire insertion. [Figure 4-113]
The barrel of aluminum terminal lugs is filled with a
petrolatum-zinc dust compound. This compound removes the
oxide film from the aluminum by a grinding process during
the crimping operation. The compound also minimizes later
oxidation of the completed connection by excluding moisture
and air. The compound is retained inside the terminal lug
barrel by a plastic or foil seal at the end of the barrel.
Splicing Copper Wires Using Preinsulated Wires
Preinsulated permanent copper splices join small wires of
sizes 22 through 10. Each splice size can be used for more
than one wire size. Splices are usually color coded in the
same manner as preinsulated, small copper terminal lugs.
Some splices are insulated with white plastic. Splices are
also used to reduce wire sizes [Figure 4-114]
Crimping tools are used to accomplish this type of splice. The
crimping procedures are the same as those used for terminal
lugs, except that the crimping operation must be done twice,
one for each end of the splice.
Emergency Splicing Repairs
Broken wires can be repaired by means of crimped splices,
by using terminal lugs from which the tongue has been cut
off, or by soldering together and potting broken strands.
These repairs are applicable to copper wire. Damaged
aluminum wire must not be temporarily spliced. These
repairs are for temporary emergency use only and should
be replaced as soon as possible with permanent repairs.
Since some manufacturers prohibit splicing, the applicable
manufacturer’s instructions should always be consulted.
Splicing with Solder & Potting Compound
When neither a permanent splice nor a terminal lug
is available, a broken wire can be repaired as follows
[Figure 4-115] :
1. Install a piece of plastic sleeving about 3 inches long
and of the proper diameter to fit loosely over the
insulation on one piece of the broken wire.
2. Strip approximately 1½ inches from each broken end
of the wire.
3. Lay the stripped ends side by side and twist one wire
around the other with approximately four turns.
4. Twist the free end of the second wire around the first
Petrolatum compound
Check to see if wire is properly inserted.Cover hole to prevent forcing
petroleum compound out.Protective cover
Removing cover
Half full of compound
Stripped
wire
Figure 4-113. Inserting aluminum wire into aluminum terminal lugs.
Thinner wire doubled over
Cover with vinyl tube tied at both endsHeavy wire
Figure 4-114. Reducing wire size with a permanent splice.
wire with approximately four turns. Solder the wire
turns together using 60⁄40 tin-lead resin-core solder.
5. When solder is cool, draw the sleeve over the soldered
wires and tie at one end. If potting compound is
available, fill the sleeve with potting material and
tie securely.
6. Allow the potting compound to set without touching
for 4 hours. Full cure and electrical characteristics are
achieved in 24 hours.Connecting Terminal Lugs to Terminal Blocks
Terminal lugs should be installed on terminal blocks in such a
manner that they are locked against movement in the direction
of loosening. [Figure 4-116]
Terminal blocks are normally supplied with studs secured
in place by a plain washer, an external tooth lockwasher,
and a nut. In connecting terminals, a recommended practice
is to place copper terminal jugs directly on top of the nut,
followed with a plain washer and elastic stop nut, or with a
plain washer, split steel lockwasher, and plain nut.
Aluminum terminal lugs should be placed over a plated
brass plain washer, followed with another plated brass plain
washer, split steel lockwasher, and plain nut or elastic stop
nut. The plated brass washer should have a diameter equal to
the tongue width of the aluminum terminal lug. Consult the
manufacturer’s instructions for recommended dimensions of
these plated brass washers. Do not place any washer in the
current path between two aluminum terminal lugs or between
two copper terminal lugs. Also, do not place a lockwasher
directly against the tongue or pad of the aluminum terminal.
To join a copper terminal lug to an aluminum terminal lug,
place a plated brass plain washer over the nut that holds the
stud in place; follow with the aluminum terminal lug, a plated
1½" (approx.) 1½" (approx.)
Step 1
Step 2
Step 4
Step 5Step 3
Figure 4-115. Repairing broken wire by soldering and potting.
Figure 4-116. Connecting terminals to terminal block.
brass plain washer, the copper terminal lug, plain washer, split
steel lockwasher and plain nut or self-locking, all metal nut.
As a general rule, use a torque wrench to tighten nuts to ensure
sufficient contact pressure. Manufacturer’s instructions
provide installation torques for all types of terminals.
Bonding & Grounding
Bonding is the electrical connecting of two or more
conducting objects not otherwise connected adequately.
Grounding is the electrical connecting of a conducting
object to the primary structure for return of current. Primary
structure is the main frame, fuselage, or wing structure of
the aircraft. Bonding and grounding connections are made
in aircraft electrical systems to:
1. Protect aircraft and personnel against hazards from
lightning discharge,
2. Provide current return paths,3. Prevent development of radio-frequency potentials,
4. Protect personnel from shock hazard,
5. Provide stability of radio transmission and reception,
and
6. Prevent accumulation of static charge.
General Bonding & Grounding Procedures
The following general procedures and precautions are
recommended when making bonding or grounding connections.
1. Bond or ground parts to the primary aircraft structure,
where practicable.
2. Make bonding or grounding connections in such
a manner that no part of the aircraft structure is
weakened.
3. Bond parts individually, if possible.
4. Install bonding or grounding connections against
smooth, clean surfaces.
5. Install bonding or grounding connections so that
vibration, expansion or contraction, or relative
movement in normal service does not break or loosen
the connection.
6. Install bonding and grounding connections in
protected areas whenever possible.
Bonding jumpers should be kept as short as practicable, and
installed so that the resistance of each connection does not
exceed 0.003 ohm. The jumper should not interfere with
the operation of movable aircraft elements, such as surface
controls; normal movement of these elements should not
result in damage to the bonding jumper.
To be sure a low resistance connection has been made,
nonconducting finishes, such as paint and anodizing films,
should be removed from the surface to be contacted by the
bonding terminal.
Electrolytic action can rapidly corrode a bonding connection
if suitable precautions are not observed. Aluminum alloy
jumpers are recommended for most cases; however,
copper jumpers can be used to bond together parts made
of stainless steel, cadmium-plated steel, copper, brass, or
bronze. Where contact between dissimilar metals cannot
be avoided, the choice of jumper and hardware should be
such that corrosion is minimized, and the part most likely to
corrode is the jumper or associated hardware. Parts A and B
of Figure 4-117 illustrate some proper hardware combinations
for making bonding connections. At locations where finishes
are removed, a protective finish should be applied to the
completed connection to prevent corrosion.
Cadmium plated steel locknutCylindrical surfaceAluminum washersCadmium plated steel screw
Cadmium plated steel lockwasherCopper terminal Clamp
Cadmium plated steel locknutCylindrical surfaceAluminum washersCadmium plated steel screw
Cadmium plated steel lockwasherCopper terminalClampCadmium plated locknut
Aluminum alloy or corrosion-resisting steel conduitAluminum washerCadmium plated steel screw
Cadmium plated steel lockwasherAluminum washer Clamp
Structure
Aluminum washerA. Copper jumper connection to tubular structure
B. Bonding conduit to structure
C. Aluminum jumper connection to tubular structure
Figure 4-117. Hardware combinations used in making bonding
connections.
LockwasherTerminal (limit to 4)
LocknutScrew or bolt Washer
Lockwasher
Washer
Plain nut
WasherStructureWasher
Rivet or weldTerminal (limited to 4)
Nut plateScrew or bolt Lockwasher
WashersFigure 4-118. Stud bonding or grounding to a flat surface.
Figure 4-119. Nut plate bonding or grounding to flat surface.The use of solder to attach bonding jumpers should be avoided.
Tubular members should be bonded by means of clamps to
which the jumper is attached. The proper choice of clamp
material minimizes the probability of corrosion. When bonding jumpers carry a substantial amount of ground return current, the
current rating of the jumper should be adequate, and it should
be determined that a negligible voltage drop is produced.
Bonding and grounding connections are normally made to
flat surfaces by means of through-bolts or screws where there
is easy access for installation. The general types of bolted
connections are:
1. In making a stud connection, a bolt or screw is
locked securely to the structure becoming a stud.
[Figure 4-118] Grounding or bonding jumpers can
be removed or added to the shank of the stud without
removing the stud from the structure.
2. Nutplates are used where access to the nut for repairs
is difficult. Nutplates are riveted or welded to a clean
area of the structure. [Figure 4-119]
Bonding and grounding connections are also made to a tab
riveted to a structure. [Figure 4-120] In such cases, it is
important to clean the bonding or grounding surface and
make the connection as though the connection were being
made to the structure. If it is necessary to remove the tab for
any reason, the rivets should be replaced with rivets one size
larger, and the mating surfaces of the structure and the tab
should be clean and free of anodic film.
Bonding or grounding connections can be made to aluminum
alloy, magnesium, or corrosion-resistant steel tubular
Bonding or grounding
area—clean before
installing connection
This area of structure and back
of tab must be cleaned before
riveting tab to structure.
Figure 4-120. Bonding or grounding tab riveted to structure.
Clamp Steel screw
Cadmium-plated steel locknut Cadmium-plated steel lockwasherPlain aluminum washer Cylindrical SurfacePlain aluminum washer
Aluminum terminal
Figure 4-121. Bonding or grounding connections to a cylindrical structure.structure. Figure 4-121 shows the arrangement of hardware
for bonding with an aluminum jumper. Because of the ease
with which aluminum is deformed, it is necessary to distribute
screw and nut pressure by means of plain washers.
Hardware used to make bonding or grounding connections
should be selected on the basis of mechanical strength,
current to be carried, and ease of installation. If connection
is made by aluminum or copper jumpers to the structure of
a dissimilar material, a washer of suitable material should be
installed between the dissimilar metals so that any corrosion
occurs on the washer.
Hardware material and finish should be selected on the basis of the material of the structure to which attachment is made
and on the material of the jumper and terminal specified for
the bonding or grounding connection. Either a screw or bolt
of the proper size for the specified jumper terminal should
be used. When repairing or replacing existing bonding or
grounding connections, the same type of hardware used in
the original connection should always be used.
Connectors
Connectors (plugs and receptacles) facilitate maintenance
when frequent disconnection is required. Since the cable
is soldered to the connector inserts, the joints should be
individually installed and the cable bundle firmly supported to
avoid damage by vibration. Connectors have been particularly
vulnerable to corrosion in the past, due to condensation within
the shell. Special connectors with waterproof features have
been developed that may replace nonwaterproof plugs in
areas where moisture causes a problem. A connector of the
same basic type and design should be used when replacing
a connector. Connectors that are susceptible to corrosion
difficulties may be treated with a chemically inert waterproof
jelly. When replacing connector assemblies, the socket-type
insert should be used on the half that is “ live” or “hot” after the
connector is disconnected to prevent unintentional grounding.
Types of Connectors
Connectors are identified by Air Force-Navy (AN) numbers
and are divided into classes with the manufacturer’s
variations in each class. The manufacturer’s variations are
differences in appearance and in the method of meeting a
specification. Some commonly used connectors are shown in
Figure 4-122 . There are five basic classes of AN connectors
used in most aircraft. Each class of connector has slightly
different construction characteristics. Classes A, B, C, and D
are made of aluminum, and class K is made of steel.
1. Class A—solid, one-piece back shell general-purpose
connector.
AN3102 Box receptacle AN3101 Cable receptacle AN3102 Box receptacle
AN3107 MCK disconnect plug AN3106 Straight plug AN3106 Straight plug
AN3108 Angle plug AN3106 Angle plug
Figure 4-122. AN connectors.
2. Class B—connector back shell separates into two parts
lengthwise. Used primarily where it is important that
the soldered connectors are readily accessible. The
back shell is held together by a threaded ring or by
screws.
3. Class C—a pressurized connector with inserts that
are not removable. Similar to a class A connector
in appearance, but the inside sealing arrangement is
sometimes different. It is used on walls or bulkheads
of pressurized equipment.
4. Class D—moisture and vibration resistant connector
that has a sealing grommet in the back shell. Wires are
threaded through tight fitting holes in the grommet,
sealing against moisture.5. Class K—a fireproof connector used in areas where it
is vital that the electric current is not interrupted, even
though the connector may be exposed to continuous
open flame. Wires are crimped to the pin or socket
contacts and the shells are made of steel. This class of
connector is normally longer than other connectors.
Connector Identification
Code letters and numbers are marked on the coupling ring
or shell to identify a connector. This code provides all the
information necessary to obtain the correct replacement for
a defective or damaged part. [Figure 4-123]
Many special-purpose connectors have been designed for
D
A C
BStandard
ClassType (straight plug)
Insert rotationContact style (socket)Contact arrangement numberSize (in 1/16")AN 3106 A 18 -4 S i
Figure 4-123. AN connector markings.use in aircraft applications. These include subminiature and
rectangular shell connectors, and connectors with short body
shells, or of split-shell construction.
Installation of Connectors
The following procedures outline one recommended method
of assembling connectors to receptacles:
1. Locate the proper position of the plug in relation to
the receptacle by aligning the key of one part with the
groove or keyway of the other part.
2. Start the plug into the receptacle with a slight forward
pressure and engage the threads of the coupling ring
and receptacle.
3. Alternately push in the plug and tighten the coupling
ring until the plug is completely seated.
4. Use connector pliers to tighten coupling rings one-
sixteenth to one-eighth turn beyond finger tight if
space around the connector is too small to obtain a
good finger grip.
5. Never use force to mate connectors to receptacles.
Do not hammer a plug into its receptacle and never use a
torque wrench or pliers to lock coupling rings.
A connector is generally disassembled from a receptacle in
the following manner:
1. Use connector pliers to loosen coupling rings that are
too tight to be loosened by hand.
2. Alternately pull on the plug body and unscrew the
coupling ring until the connector is separated.
3. Protect disconnected plugs and receptacles with
caps or plastic bags to keep debris from entering and
causing faults.
4. Do not use excessive force and do not pull on attached
wires.Conduit
Conduit is used in aircraft installations for the mechanical
protection of wires and cables. It is available in metallic and
nonmetallic materials and in both rigid and flexible form.
When selecting conduit size for a specific cable bundle
application, it is common practice to allow for ease in
maintenance and possible future circuit expansion by
specifying the conduit inner diameter about 25 percent larger
than the maximum diameter of the conductor bundle. The
nominal diameter of a rigid metallic conduit is the outside
diameter. Therefore, to obtain the inside diameter, subtract
twice the tube wall thickness.
From the abrasion standpoint, the conductor is vulnerable
at the ends of the conduit. Suitable fittings are affixed to
conduit ends in such a manner that a smooth surface comes in
contact with the conductor within the conduit. When fittings
are not used, the conduit end should be flared to prevent wire
insulation damage. The conduit is supported by clamps along
the conduit run.
Many of the common conduit installation problems can be
avoided by proper attention to the following details:
1. Do not locate conduit where it can be used as a
handhold or footstep.
2. Provide drain holes at the lowest point in a conduit
run. Drilling burrs should be carefully removed from
the drain holes.
3. Support the conduit to prevent chafing against the
structure and to avoid stressing its end fittings.
Damaged conduit sections should be repaired to prevent
injury to the wires or wire bundle. The minimum acceptable
tube bend radii for rigid conduit as prescribed by the
manufacturer’s instructions should be carefully followed.
Kinked or wrinkled bends in a rigid conduit are normally
not considered acceptable.
Flexible aluminum conduit is widely available in two types:
bare flexible and rubber-covered conduit. Flexible brass
conduit is normally used instead of flexible aluminum where
it is necessary to minimize radio interference. Flexible
conduit may be used where it is impractical to use rigid
conduit, such as areas that have motion between conduit ends
or where complex bends are necessary. Transparent adhesive
tape is recommended when cutting flexible conduit with a
hacksaw to minimize fraying of the braid.
4-70Electrical Equipment Installation
This section provides general procedures and safety
precautions for installation of commonly used aircraft
electrical equipment and components. Electrical load limits,
acceptable means of controlling or monitoring electrical
loads, and circuit protection devices are subjects with which
mechanics must be familiar to properly install and maintain
aircraft electrical systems.
Electrical Load Limits
When installing additional electrical equipment that
consumes electrical power in an aircraft, the total electrical
load must be safely controlled or managed within the rated
limits of the affected components of the aircraft’s power-
supply system. Regulation of the field current strength is
used to control DC generator voltage.
Before any aircraft electrical load is increased, the associated
wires, cables, and circuit-protection devices, such as fuses
or circuit breakers, should be checked to determine that the
new electrical load—previous maximum load plus added
load—does not exceed the rated limits of the existing wires,
cables, or protection devices.
The generator or alternator output ratings prescribed by the
manufacturer should be compared with the electrical loads
that can be imposed on the affected generator or alternator
by installed equipment. When the comparison shows that the
probable total connected electrical load can exceed the output
load limits of the generator(s) or alternator(s), the load should
be reduced so that an overload cannot occur. When a storage
battery is part of the electrical power system, ensure that the
battery is continuously charged in flight, except when short
intermittent loads are connected, such as a radio transmitter,
a landing gear motor, or other similar devices that may place
short-time demand loads on the battery.
Controlling or Monitoring the Electrical Load
Placards are recommended to inform crewmembers of an
aircraft about the combinations of loads that can safely be
connected to the power source.
In installations where the ammeter is in the battery lead
and the regulator system limits the maximum current that
the generator or alternator can deliver, a voltmeter can be
installed on the system bus. As long as the ammeter does
not read discharge, except for short intermittent loads such
as operating the gear and flaps, and the voltmeter remains at
system voltage, the generator or alternator is not overloaded.
The ammeter can be redlined at 100 percent of the generator
or alternator rating in installations with the ammeter in the
generator or alternator lead, and the regulator system does not limit the maximum current that the generator or alternator
can deliver. If the ammeter reading is never allowed to exceed
the red line, except for short, intermittent loads, the generator
or alternator is not overloaded.
Where the use of placards or monitoring devices is not
practical or desired, and where assurance is needed that the
battery in a typical small aircraft generator or battery power
source is charged in flight, the total continuous connected
electrical load may be held to approximately 80 percent of
the total rated generator output capacity. When more than
one generator is used in parallel, the total rated output is the
combined output of the installed generators.
Means must be provided for quickly coping with the sudden
overloads that can be caused by generator or engine failure if
two or more generators are operated in parallel and the total
connected system load can exceed the rated output of one
generator. A quick load-reduction system can be employed
or a specified procedure where the total load is reduced to
a quantity that is within the rated capacity of the remaining
operable generator or generators.
Electrical loads should be connected to inverters, alternators,
or similar aircraft electrical power sources in such a manner
that the rated limits of the power source are not exceeded,
unless some type of effective monitoring means is provided
to keep the load within prescribed limits.
Circuit Protection Devices
Conductors should be protected with circuit breakers or fuses
located as close as possible to the electrical power source
bus. Normally, the manufacturer of the electrical equipment
specifies the fuse or circuit breaker to be used when installing
the equipment.
The circuit breaker or fuse should open the circuit before the
conductor emits smoke. To accomplish this, the time/current
characteristic of the protection device must fall below that
of the associated conductor. Circuit protector characteristics
should be matched to obtain the maximum utilization of the
connected equipment.
Figure 4-124 shows an example of the table used in selecting
the circuit breaker and fuse protection for copper conductors.
This limited table is applicable to a specific set of ambient
temperatures and wire bundle sizes and is presented as a
typical example only. It is important to consult such guides
before selecting a conductor for a specific purpose. For
example, a wire run individually in the open air may be
protected by the circuit breaker of the next higher rating to
that shown in the table.
0Wire AN
Gauge CopperCircuit
Breaker AmperageFuse
Amperage
Figure 4-124. Wire and circuit protector table.
Lamp
Inductive (Relay-Solenoid)
Resistive (Heater)
Motor
Lamp
Inductive (Relay-Solenoid)
Resistive (Heater)
Motor8
224 VDC
24 VDC
24 VDC
24 VDC
12 VDC
12 VDC
12 VDC
12 VDCNominal System
VoltageType of
LoadDerating
Factor
Figure 4-125. Switch derating factors.All resettable circuit breakers should open the circuit in which
they are installed, regardless of the position of the operating
control when an overload or circuit fault exists. Such circuit
breakers are referred to as trip-free. Automatic reset circuit
breakers automatically reset themselves periodically. They
should not be used as circuit protection devices in aircraft.
Switches
A specifically designed switch should be used in all circuits
in which a switch malfunction would be hazardous. Such
switches are of rugged construction and have sufficient
contact capacity to break, make, and carry continuously
the connected load current. Snap-action design is generally
preferred to obtain rapid opening and closing of contacts
regardless of the speed of the operating toggle or plunger,
thereby minimizing contact arcing.
The nominal current rating of the conventional aircraft
switch is usually stamped on the switch housing. This rating
represents the continuous current rating with the contacts
closed. Switches should be derated from their nominal current
rating for the following types of circuits:
1. High rush-in circuits—circuits containing incandescent
lamps can draw an initial current that is 15 times
greater than the continuous current. Contact burning
or welding may occur when the switch is closed.
2. Inductive circuits—magnetic energy stored in solenoid
coils or relays is released and appears as an arc as the
control switch is opened.
3. Motors—direct current motors draw several times
their rated current during starting, and magnetic energy
stored in their armature and field coils is released when
the control switch is opened.Figure 4-125 is typical of those tables available for selecting
the proper nominal switch rating when the continuous load
current is known. This selection is essentially a derating to
obtain reasonable switch efficiency and service life.
Hazardous errors in switch operation can be avoided by
logical and consistent installation. Two position on-off
switches should be mounted so that the on position is reached
by an upward or forward movement of the toggle. When the
switch controls movable aircraft elements, such as landing
gear or flaps, the toggle should move in the same direction
as the desired motion. Inadvertent operation of a switch can
be prevented by mounting a suitable guard over the switch.
Relays
Relays are used as switching devices in which a weight
reduction can be achieved or electrical controls can be
simplified. A relay is an electrically operated switch and
is therefore subject to dropout under low system voltage
conditions. The previous discussion of switch ratings is
generally applicable to relay contact ratings.
Hand crank adapter
Hard steel insertCentrifugal clutchFlywheelTorque overload release clutch
Starter driving jaw
Figure 5-1. Combination hand and electric inertia starter.Introduction
Most aircraft engines, reciprocating or turbine, require help
during the starting process. Hence, this device is termed the
starter. A starter is an electromechanical mechanism capable
of developing large amounts of mechanical energy that can
be applied to an engine, causing it to rotate. Reciprocating
engines need only to be turned through at a relatively slow
speed until the engine starts and turns on its own. Once
the reciprocating engine has fired and started, the starter is
disengaged and has no further function until the next start. In
the case of a turbine engine, the starter must turn the engine
up to a speed that provides enough airflow through the engine
for fuel to be ignited. Then, the starter must continue to help
the engine accelerate to a self-sustaining speed. Turbine
engine starters have a critical role in starting of the engine.
If the starter turns the turbine engine up to a self-sustaining
speed, the engine start process will be successful. There are
only a few types or methods used to turn the engine. Almost
all reciprocating engines use a form of electric motor geared
to the engine. Modern turbine engines use electric motors,
starter/generators (electric motor and a generator in the
same housing), and air turbine starters. Air turbine starters
are driven by compressed air through a turbine wheel that
is mechanically connected through reduction gears to one
of the engine’s compressors, generally the highest pressure
compressor.
Reciprocating Engine Starting Systems
In the early stages of aircraft development, relatively low
powered reciprocating engines were started by pulling the
Engine Starting Systems
propeller through a part of a revolution by hand. Difficulty
was often experienced in cold weather starting when
lubricating oil temperatures were near the congealing point.
In addition, the magneto systems delivered a weak starting
spark at the very low cranking speeds. This was often
compensated for by providing a hot spark using such ignition
system devices as the booster coil, induction vibrator, or
impulse coupling.
Some small, low-powered aircraft which use hand-cranking
of the propeller, or propping, for starting are still being
operated. For general instructions on starting this type of
aircraft, refer to the Aviation Maintenance Technician—
General Handbook, Chapter 1, Safety, Ground Operations,
and Servicing. Throughout the development of the aircraft
reciprocating engine from the earliest use of starting systems
to the present, a number of different starter systems have
been used. Most reciprocating engine starters are the direct
cranking electric type. A few older model aircraft are still
equipped with inertia starters. Thus, only a brief description
of these starting systems is included in this section.
Inertia Starters
There are three general types of inertia starters: hand, electric,
and combination hand and electric. The operation of all types
of inertia starters depends on the kinetic energy stored in a
rapidly rotating flywheel for cranking ability. Kinetic energy
is energy possessed by a body by virtue of its state of motion,
which may be movement along a line or spinning action.
In the inertia starter, energy is stored slowly during an
energizing process by a manual hand crank or electrically
Chapter 5
5-2Starter
inertia
+
BusEner.
MeshBattery relayBattery control switch
Starter control switchStarting solenoid
Engaging solenoid
Crank socketEngaging levelFlywheelBarrel
Mounting flangeSprings
Starter
driving
jaw
Disks
Starter+Bus
To auxiliary
igniter deviceStarter solenoid
Starter switchBattery switchBattery solenoidFigure 5-2. Electric inertia starting circuit.
Figure 5-3. Torque overload release clutch.
Figure 5-4. Typical starting circuit using a direct cranking electric starter. with a small motor. The flywheel and movable gears of
a combination hand electric inertia starter are shown in
Figure 5-1 . The electrical circuit for an electric inertia
starter is shown in Figure 5-2 . During the energizing of the
starter, all movable parts within it, including the flywheel,
are set in motion. After the starter has been fully energized,
it is engaged to the crankshaft of the engine by a cable pulled manually or by a meshing solenoid that is energized
electrically. When the starter is engaged, or meshed, flywheel
energy is transferred to the engine through sets of reduction
gears and a torque overload release clutch. [Figure 5-3]
Direct Cranking Electric Starter
The most widely used starting system on all types of
reciprocating engines utilizes the direct cranking electric
starter. This type of starter provides instant and continual
cranking when energized. The direct cranking electric starter
consists basically of an electric motor, reduction gears, and
an automatic engaging and disengaging mechanism that is
operated through an adjustable torque overload release clutch.
A typical circuit for a direct cranking electric starter is shown
in Figure 5-4 . The engine is cranked directly when the starter
solenoid is closed. As shown in Figure 5-4 , the main cables
leading from the starter to the battery are heavy duty to carry
the high current flow, which may be in a range from as high
as 350 amperes to 100 amperes (amps), depending on the
starting torque required. The use of solenoids and heavy
wiring with a remote control switch reduces overall cable
weight and total circuit voltage drop.
+ 60 - 60-30 +300
A M P
Heavy current to starter
24 Volts
DC input
24 Volts
DC inputStarter
vibrator
Ground through switch actuates battery solenoid
Auxiliary voltage inputHeavy current to starterSolenoid actuating voltage
Solenoid actuating voltage
External power receptacleLeft magnetos
Circuit breaker
Starter switch left
Left engine starter
Left starter solenoid
Battery solenoid
Battery switchRight engine starterRight starter solenoidAmmeter shunt BUSStarter switch rightRight magnetos
Figure 5-5. Engine starting schematic for a light twin-engine aircraft.
The typical starter motor is a 12- or 24-volt, series-wound
motor that develops high starting torque. The torque of
the motor is transmitted through reduction gears to the
overload release clutch. Typically, this action actuates a
helically-splined shaft moving the starter jaw outward
to engage the engine cranking jaw before the starter jaw
begins to rotate. After the engine reaches a predetermined
speed, the starter automatically disengages. The schematic
in Figure 5-5 provides a pictorial arrangement of an entire
starting system for a light twin-engine aircraft.
Direct Cranking Electric Starting System for Large
Reciprocating Engines
In a typical high horsepower reciprocating engine starting
system, the direct cranking electric starter consists of two
basic components: a motor assembly and a gear section. The
gear section is bolted to the drive end of the motor to form
a complete unit.
The motor assembly consists of the armature and motor pinion assembly, the end bell assembly, and the motor
housing assembly. The motor housing also acts as the
magnetic yoke for the field structure.
The starter motor is a nonreversible, series interpole motor. Its
speed varies directly with the applied voltage and inversely
with the load. The starter gear section consists of an external
housing with an integral mounting flange, planetary gear
reduction, a sun and integral gear assembly, a torque-limiting
clutch, and a jaw and cone assembly. [Figure 5-6] When the
starter circuit is closed, the torque developed in the starter
motor is transmitted to the starter jaw through the reduction
gear train and clutch. The starter gear train converts the high
speed low torque of the motor to the low speed high torque
required to crank the engine. In the gear section, the motor
pinion engages the gear on the intermediate countershaft.
[Figure 5-6] The pinion of the countershaft engages the
internal gear. The internal gear is an integral part of the sun
gear assembly and is rigidly attached to the sun gear shaft.
The sun gear drives three planet gears that are part of the
Jaw springBronze clutch plates
Planetary gear
Planetary carrying arm
Jaw engaging spline
Sun gear shaft extension
and jaw stop retainer nutReturn spring
Starter jaw
Conical clutch suface
Traveling nut
Sun gearSteel clutch Clutch spring retainer
Internal gear
Motor pinion
Motor shaft
Intermediate countershaft
Countershaft pinion
Figure 5-6. Starter gear section.
planetary gear assembly. The individual planet gear shafts
are supported by the planetary carrying arm, a barrel-like
part shown in Figure 5-6 .
The carrying arm transmits torque from the planet gears to
the starter jaw as follows:
1. The cylindrical portion of the carrying arm is splined
longitudinally around the inner surface.
2. Mating splines are cut on the exterior surface of the
cylindrical part of the starter jaw.
3. The jaw slides fore and aft inside the carrying arm to
engage and disengage with the engine.
The three planet gears also engage the surrounding internal
teeth on the six steel clutch plates. [Figure 5-6] These plates
are interleaved with externally splined bronze clutch plates
that engage the sides of the housing, preventing them from
turning. The proper pressure is maintained upon the clutch
pack by a clutch spring retainer assembly. A cylindrical
traveling nut inside the starter jaw extends and retracts the jaw. Spiral jaw-engaging splines around the inner wall of the
nut mate with similar splines cut on an extension of the sun
gear shaft. [Figure 5-6]
Being splined in this fashion, rotation of the shaft forces
the nut out and the nut carries the jaw with it. A jaw spring
around the traveling nut carries the jaw with the nut and
tends to keep a conical clutch surface around the inner wall
of the jaw head seated against a similar surface around the
underside of the nut head. A return spring is installed on the
sun gear shaft extension between a shoulder, formed by the
splines around the inner wall of the traveling nut, and a jaw
stop retaining nut on the end of the shaft. Because the conical
clutch surfaces of the traveling nut and the starter jaw are
engaged by jaw spring pressure, the two parts tend to rotate
at the same speed. However, the sun gear shaft extension
turns six times faster than the jaw. The spiral splines on it
are cut left hand, and the sun gear shaft extension, turning to
the right in relation to the jaw, forces the traveling nut and
the jaw out from the starter its full travel (about 5⁄16 inches)
in approximately 12° of rotation of the jaw.
5-5Off
4Bus
Starter bus
Starter bus cableMaster busIgnition boost switch
Primer switchTo induction vibrator
To primer
Starter relay
To prop deicing relay
To feathering pump relay
Firewall junction boxEngine selector switchStart switch safely switchStarter primer and
induction vibrator
Starter motor (engine No. 1)
Figure 5-7. Starter control circuit.The jaw moves out until it is stopped either by engagement
with the engine or by the jaw stop retaining nut. The travel nut
continues to move slightly beyond the limit of jaw travel, just
enough to relieve some of the spring pressure on the conical
clutch surfaces. As long as the starter continues to rotate,
there is just enough pressure on the conical clutch surfaces
to provide torque on the spiral splines that balance most of
the pressure of the jaw spring. If the engine fails to start,
the starter jaw does not retract since the starter mechanism
provides no retracting force. However, when the engine
fires and the engine jaw overruns the starter jaw, the sloping
ramps of the jaw teeth force the starter jaw into the starter
against the jaw spring pressure. This disengages the conical
clutch surfaces entirely, and the jaw spring pressure forces
the traveling nut to slide in along the spiral splines until the
conical clutch surfaces are again in contact.
When the starter and engine are both running, there is an
engaging force keeping the jaws in contact that continue
until the starter is de-energized. However, the rapidly moving
engine jaw teeth, striking the slowly moving starter jaw teeth,
hold the starter jaw disengaged. As soon as the starter comes to rest, the engaging force is removed, and the small return
spring throws the starter jaw into its fully retracted position
where it remains until the next start. When the starter jaw first
engages the engine jaw, the motor armature has had time to
reach considerable speed because of its high starting torque.
The sudden engagement of the moving starter jaw with the
stationary engine jaw would develop forces sufficiently high
enough to severely damage the engine or the starter were it
not for the plates in the clutch pack that slip when the engine
torque exceeds the clutch-slipping torque.
In normal direct cranking action, the internal steel gear clutch
plates are held stationary by the friction of the bronze plates
with which they are interleaved. When the torque imposed by
the engine exceeds the clutch setting, however, the internal
gear clutch plates rotate against the clutch friction, allowing
the planet gears to rotate while the planetary carrying arm
and the jaw remain stationary. When the engine reaches the
speed that the starter is trying to achieve, the torque drops
off to a value less than the clutch setting, the internal gear
clutch plates are again held stationary, and the jaw rotates at
the speed that the motor is attempting to drive it. The starter
control switches are shown schematically in Figure 5-7 .
Adjusting stud
Starter switch
Starter drive pinionReturn spring
Starter leverFlexible starter control rod
9/16”
1/16" clearance
Figure 5-8. Starter level controls and adjustment.The engine selector switch must be positioned and the starter
switch and the safety switch—wired in series—must be
closed before the starter can be energized. Current is supplied
to the starter control circuit through a circuit breaker labeled
“Starter, Primer, and Induction Vibrator.” [Figure 5-7] When
the engine selector switch is in position for the engine start,
closing the starter energizes the starter relay located in the
engine nacelle area. Energizing the starter relay completes the
power circuit to the starter motor. The current necessary for
this heavy load is taken directly from the master bus through
the starter bus cable.
All starting systems have operating time limits because of the
high energy used during cranking or rotation of the engine.
These limits are referred to as starter limits and must be
observed or overheating and damage of the starter occurs.
After energizing the starter for 1 minute, it should be allowed
to cool for at least 1 minute. After a second or subsequent
cranking period of 1 minute, it should cool for 5 minutes.
Direct Cranking Electric Starting System for Small
Aircraft
Most small, reciprocating engine aircraft employ a direct
cranking electric starting system. Some of these systems
are automatically engaged starting systems, while others are
manually engaged.
Manually engaged starting systems used on many older, small
aircraft employ a manually operated overrunning clutch drive
pinion to transmit power from an electric starter motor to a
crankshaft starter drive gear. [Figure 5-8] A knob or handle
on the instrument panel is connected by a flexible control to
a lever on the starter. This lever shifts the starter drive pinion
into the engaged position and closes the starter switch contacts
when the starter knob or handle is pulled. The starter lever
is attached to a return spring that returns the lever and the
flexible control to the off position. When the engine starts,
the overrunning action of the clutch protects the starter drive
pinion until the shift lever can be released to disengage the
pinion. For the typical unit, there is a specified length of travel
for the starter gear pinion. [Figure 5-8] It is important that the
starter lever move the starter pinion gear this proper distance
before the adjustable lever stud contacts the starter switch.
The automatic, or remote solenoid engaged, starting systems
employ an electric starter mounted on an engine adapter. A
starter solenoid is activated by either a push button or turning
the ignition key on the instrument panel. When the solenoid
is activated, its contacts close, and electrical energy energizes
the starter motor. Initial rotation of the starter motor engages
the starter through an overrunning clutch in the starter
adapter, which incorporates worm reduction gears. Some engines incorporate an automatic starting system that
employs an electric starter motor mounted on a right angle
drive adapter. As the starter motor is electrically energized,
the adapter worm shaft and gear engage the starter shaft gear
by means of a spring and clutch assembly. The shaft gear, in
turn, rotates the crankshaft. When the engine begins to turn
on its own power, the clutch spring disengages from the shaft
gear. The starter adapter uses a worm drive gear shaft and
worm gear to transfer torque from the starter motor to the
clutch assembly. [Figure 5-9] As the worm gear rotates the
worm wheel and clutch spring, the clutch spring is tightened
around the drum of the starter shaft gear. As the shaft gear
turns, torque is transmitted directly to the crankshaft gear.
Other engines use a starter that drives a ring gear mounted to
the propeller hub. [Figure 5-10] It uses an electric motor and
a drive gear that engages as the motor is energized and spins
the gear, which moves out and engages the ring gear on the
propeller hub cranking the engine for start. [Figure 5-11] As
the engine starts, the starter drive gear is spun back by the
engine turning, which disengages the drive gear. [Figure 5-12]
The starter motors on small aircraft also have operational
limits with cool down times that should be observed.
Reciprocating Engine Starting System Maintenance
Practices
Most starting system maintenance practices include replacing
the starter motor brushes and brush springs, cleaning dirty
commutators, and turning down burned or out-of-round
Starter motor
Starter adapter housingBearing
Worm gear
Starter shaft gear
Worm wheel
Clutch spring
Figure 5-9. Starter adapter.
PropellerStarter ring gear mounted on propeller hub plate
Figure 5-10. Starter ring gear mounted on the propeller hub.
Electrical connectionStarter drive gearMounting bolts and alignment pin
Figure 5-11. Starter drive gear mounting holes and electrical
connector.
starter commutators. As a rule, starter brushes should be
replaced when worn down to approximately one-half the
original length. Brush spring tension should be sufficient to
give brushes a good firm contact with the commutator. Brush
leads should be unbroken and lead terminal screws tight.
A glazed or dirty starter commutator can be cleaned by
holding a strip of double-0 sandpaper or a brush seating stone
against the commutator as it is turned. The sandpaper or stone
should be moved back and forth across the commutator to
avoid wearing a groove. Emery paper or carborundum should
never be used for this purpose because of their possible
shorting action. Roughness, out-of-roundness, or high-mica conditions are
reasons for turning down the commutator. In the case of a
high-mica condition, the mica should be undercut after the
turning operation is accomplished. Refer to FAA-H-8083-30,
Aviation Maintenance Technician—General for a review of
high-mica commutators in motors.
The drive gear should be checked for wear along with the
ring gear. The electrical connections should be checked for
looseness and corrosion. Also, check the security of the
mounting of the housing of the starter.
Troubleshooting Small Aircraft Starting Systems
The troubleshooting procedures listed in Figure 5-13 are
typical of those used to isolate malfunctions in small aircraft
starting systems.
Starter ring gear mounted on propeller hub
Starter gear
Figure 5-12. Engine starter mounted on the engine.
Small Aircraft Troubleshooting ProceduresStarter motor
runs but does not
turn crankshaftStarter dragsStarter
excessively
noisyProbable Cause Isolation Procedure RemedyStarter will
not operate• Repair circuit.
• Replace switch or wires.
• Adjust starter lever in accordance
with manufacturer’s instructions.
• Remove and repair or replace
starter.
• Adjust starter lever in accordance
with manufacturer’s instructions.
• Replace defective parts.
• Replace defective parts.
• Charge or replace battery.
• Replace with serviceable unit.
• Repair or replace starter.
• Turn down commutator.
• Replace starter drive.
• Replace crankshaft gear.• Check master circuit.
• Check switch circuit continuity.
• Check starter lever adjustment.
• Check through items above. If
another cause is not apparent,
starter is defective.
• Check starter lever adjustment.
• Remove starter and check starter
drive and overrunning clutch.
• Remove and check pinion gear and
crankshaft gear.
• Check battery.
• Check contacts.
• Check starter brushes, brush spring
tension for solder thrown on brush
cover.
• Clean and check visually.
• Remove and examine pinion.
• Remove starter and turn over engine
by hand to examine crankshaft gear.• Defective master switch or circuit.
• Defective starter switch or switch
circuit.
• Starter lever does not activate switch.
• Defective starter.
• Starter lever adjusted to activate
switch without engaging pinion with
crankshaft gear.
• Defective overrunning clutch or drive.
• Damaged starter pinion gear or
crankshaft gear.
• Low battery.
• Starter switch or relay contacts
burned or dirty.
• Defective starter.
• Dirty, worn commutator.
• Worn starter pinion.
• Worn or broken teeth on crankshaft
gears.
Figure 5-13. Small aircraft troubleshooting procedures.Gas Turbine Engine Starters
Gas turbine engines are started by rotating the high-pressure
compressor. On dual-spool, axial flow engines, the high-
pressure compressor and N1 turbine system is only rotated
by the starter. To start a gas turbine engine, it is necessary
to accelerate the compressor to provide sufficient air to
support combustion in the combustion section, or burners.
Once ignition and fuel have been introduced and the lite-off
has occurred, the starter must continue to assist the engine
until the engine reaches a self-sustaining speed. The torque supplied by the starter must be in excess of the torque required
to overcome compressor inertia and the friction loads of the
engine’s compressor.
Figure 5-14 illustrates a typical starting sequence for a gas
turbine engine, regardless of the type of starter employed. As
soon as the starter has accelerated the compressor sufficiently
to establish airflow through the engine, the ignition is turned
on followed by the fuel. The exact sequence of the starting
procedure is important since there must be sufficient airflow
through the engine to support combustion before the air-fuel
mixture is ignited. At low engine cranking speeds, the fuel
flow rate is not sufficient to enable the engine to accelerate;
for this reason, the starter continues to crank the engine until
after self-accelerating speed has been attained. If assistance
from the starter were cut off below the self-accelerating
speed, the engine would either fail to accelerate to idle
speed or might even decelerate because it could not produce
sufficient energy to sustain rotation or to accelerate during the
initial phase of the starting cycle. The starter must continue
to assist the engine considerably above the self-accelerating
speed to avoid a delay in the starting cycle, which would
result in a hot or hung false start or a combination of both.
At the proper points in the sequence, the starter and ignition
Time (seconds)
Compressor rpm
Exhaust gas temperature (°C)
Idle EGTPeak starting ECT
Exhaust gas
temperature
versus timeCompressor rpm
versus time
Starter cuts out
Ignition off
Engine attains self-accelerating speedIgnition onFuel onEngine lights up
EGT rise indicates light upEngine attains idle rpm
Figure 5-14. Typical gas turbine engine starting sequence.
Relief valve
Turbine exhaust ring Overrunning sprag clutchHot gas nozzles
Turbine rotorGear shaft
Compressed air inletSpline shaft
Exhaust from turbine and fanBreech capBreech handleSwitchFlyweight
Cartridge
Switch
actuating rod
Figure 5-15. Cartridge/pneumatic starter schematic. are automatically cut off. The basic types of starters that
are in current use for gas turbine engines are direct current
(DC) electric motor, starter/generators, and the air turbine
type of starters.
Many types of turbine starters have included several different
methods for turning the engine for starting. Several methods have been used but most of these have given way to electric
or air turbine starters. An air impingement starting system,
which is sometimes used on small engines, consists of jets of
compressed air piped to the inside of the compressor or turbine
case so that the jet air blast is directed onto the compressor or
turbine rotor blades, causing them to rotate.
A typical cartridge/pneumatic turbine engine starter may be
operated as an ordinary air turbine starter from a ground-
operated air supply or an engine cross-bleed source. It may
also be operated as a cartridge starter. [Figure 5-15] To
accomplish a cartridge start, a cartridge is first placed in the
breech cap. The breech is then closed on the breech chamber
by means of the breech handle and then rotated a partial turn to
engage the lugs between the two breech sections. The cartridge
is ignited by applying voltage through the connector at the
end of the breech handle. Upon ignition, the cartridge begins
to generate gas. The gas is forced out of the breech to the hot
gas nozzles that are directed toward the buckets on the turbine
rotor, and rotation is produced via the overboard exhaust
collector. Before reaching the nozzle, the hot gas passes an
outlet leading to the relief valve. This valve directs hot gas
to the turbine, bypassing the hot gas nozzle, as the pressure
rises above the preset maximum. Thus, the pressure of the gas
within the hot gas circuit is maintained at the optimum level.
The air-fuel combustion starter was used to start gas turbine
Gear ratio approximately 435 to 1 Cooling air
To voltage regulator
OutputTo generator
paralleling and
protective
circuitryShunt
Series
Figure 5-16. Typical starter generator.
D B +
C + A + ECommutating
Compensated field
Shunt field
C field
Figure 5-17. Starter generator internal circuit. engines by using the combustion energy of jet A fuel and
compressed air. The starter consists of a turbine-driven power
unit and auxiliary fuel, air, and ignition systems. Operation
of this type starter is, in most installations, fully automatic;
actuation of a single switch causes the starter to fire and
accelerate the engine from rest to starter cutoff speed.
Hydraulic pumps and motors have also been used for some
smaller engines. Many of these systems are not often used
on modern commercial aircraft because of the high power
demands required to turn the large turbofan engines during
the starting cycle on transport aircraft.
Electric Starting Systems & Starter Generator
Starting System
Electric starting systems for gas turbine aircraft are of
two general types: direct cranking electrical systems and
starter generator systems. Direct cranking electric starting
systems are used mostly on small turbine engines, such as
Auxiliary Power Units (APUs), and some small turboshaft
engines. Many gas turbine aircraft are equipped with starter
generator systems. Starter generator starting systems are
also similar to direct cranking electrical systems except that
after functioning as a starter, they contain a second series of
windings that allow it to switch to a generator after the engine
has reached a self-sustaining speed. This saves weight and
space on the engine.
The starter generator is permanently engaged with the engine
shaft through the necessary drive gears, while the direct
cranking starter must employ some means of disengaging
the starter from the shaft after the engine has started. The
starter generator unit is basically a shunt generator with an
additional heavy series winding. [Figure 5-16] This series
winding is electrically connected to produce a strong field
and a resulting high torque for starting. Starter generator
units are desirable from an economic standpoint, since one
unit performs the functions of both starter and generator.
Additionally, the total weight of starting system components
is reduced and fewer spare parts are required.
The starter generator internal circuit has four field windings:
a series field (C field), a shunt field, a compensating field,
and an interpole or commutating winding. [Figure 5-17]
During starting, the C field, compensating, and commutating
windings are used. The unit is similar to a direct cranking
starter since all of the windings used during starting are in
series with the source. While acting as a starter, the unit makes
no practical use of its shunt field. A source of 24 volts and
1,500 peak amperes is usually required for starting.
When operating as a generator, the shunt, compensating,
and commutating windings are used. The C field is used only for starting purposes. The shunt field is connected in
the conventional voltage control circuit for the generator.
Compensating and commutating or interpole windings
provide almost sparkless commutation from no load to full
load. Figure 5-18 illustrates the external circuit of a starter
generator with an undercurrent controller. This unit controls
the starter generator when it is used as a starter. Its purpose is
5-11To external power receptacle
B
G
C M
CBA
E
Advance
RetardTo fuel
pumpsOn
OffTo fuel valve
Battery onStartEmergency stop
Engine master switch
Battery and start switch
Battery cutout relay
Throttle switchThrottle relayMotor relay Under current relay
Throttle relay lockIgnition relay
Ignition vibratorStarter
generatorB-4 relay
Figure 5-18. Starter generator circuit.
starter motor would damage the battery. Closing the motor
relay allows a very high current to flow to the motor. Since
this current flows through the coil of the undercurrent relay, it
closes. Closing the undercurrent relay completes a circuit from
the positive bus to the motor relay coil, ignition relay coil, and
battery cutout relay coil. The start switch is allowed to return
to its normal off position, and all units continue to operate.
As the motor builds up speed, the current draw of the motor
begins to decrease. As it decreases to less than 200 amps,
the undercurrent relay opens. This action breaks the circuit
from the positive bus to the coils of the motor, ignition, and
battery cutout relays. The de-energizing of these relay coils
halts the start operation.
After these procedures are completed, the engine should be
operating efficiently, and ignition should be self-sustaining.
If, however, the engine fails to reach sufficient speed to halt
the starter operation, the stop switch may be used to break
the circuit from the positive bus to the main contacts of the
undercurrent relay.to assure positive action of the starter and to keep it operating
until the engine is rotating fast enough to sustain combustion.
The control block of the undercurrent controller contains
two relays. One is the motor relay that controls the input
to the starter; the other, the undercurrent relay, controls the
operation of the motor relay.
The sequence of operation for the starting system is discussed
in the following paragraphs. [Figure 5-18] To start an engine
equipped with an undercurrent relay, it is first necessary to
close the engine master switch. This completes the circuit
from the aircraft’s bus to the start switch, to the fuel valves,
and to the throttle relay. Energizing the throttle relay starts
the fuel pumps and completing the fuel valve circuit gives
the necessary fuel pressure for starting the engine. As the
battery and start switch is turned on, three relays close: the
motor relay, ignition relay, and battery cutout relay. The motor
relay closes the circuit from the power source to the starter
motor; the ignition relay closes the circuit to the ignition units;
the battery cutout relay disconnects the battery. Opening the
battery circuit is necessary because the heavy drain of the
Starter Generator Starting System Troubleshooting Procedures
Engine starts but does not accelerate to idle
Engine fails to start when throttle is placed in idleProbable Cause Isolation Procedure Remedy
Engine does not rotate during start attempt
• Adjust voltage of the external power
source or charge batteries.
• Replace switch.
• Replace switch.
• Place switch in OFF position.
• Replace relay if no voltage is present.
• Replace relay if no voltage is present.
• Replace the starter if voltage is present.
• Replace relay if voltage is not present.
• Replace the engine.
• Use larger capacity ground power unit
or charge batteries.
• Clean or replace spark igniters, or
replace exciters or leads to igniters.• Check voltage of the battery or external
power source.
• Check switch for continuity.
• Check switch for continuity.
• Check position of generator control
switch.
• With start circuit energized, check for 48
volts DC across series relay coil.
• With start circuit energized, check for 48
volts DC across starter relay coil.
• With start circuit energized, check for
proper voltage at the starter.
• With start circuit energized, check for 28
volts DC across the relay coil.
• Listen for sounds of starter rotation
during an attempted start. If the starter
rotates but the engine does not, the
drive shaft is sheared.
• Check starter terminal voltage.
• Turn on system and listen for spark-
igniter operation.• Low supply voltage to the starter.
• Power switch is defective.
• Ignition switch in throttle quadrant.
• Start-lockout relay is defective.
• Battery series relay is defective.
• Starter relay is defective.
• Defective starter.
• Start lock-in relay defective.
• Starter drive shaft in component drive
gearbox is sheared.
• Insufficient starter voltage.
• Defective ignition system.
Figure 5-19. Starter generator starting system troubleshooting procedures.Troubleshooting a Starter Generator Starting
System
The procedures listed in Figure 5-19 are typical of those
used to repair malfunctions in a starter generator starting
system similar to the system described in this section. These
procedures are presented as a guide only. The appropriate
manufacturer’s instructions and approved maintenance
directives should always be consulted for the aircraft involved.
Air Turbine Starters
Air turbine starters are designed to provide high starting
torque from a small, lightweight source. The typical air
turbine starter weighs from one-fourth to one-half as much
as an electric starter capable of starting the same engine. It
is capable of developing considerable more torque than the
electric starter.
The typical air turbine starter consists of an axial flow turbine
that turns a drive coupling through a reduction gear train and
a starter clutch mechanism. The air to operate an air turbine
starter is supplied from either a ground-operated air cart, the
APU, or a cross-bleed start from an engine already operating. [Figure 5-20] Only one source of around 30–50 pounds per
square inch (psi) is used at a time to start the engines. The
pressure in the ducts must be high enough to provide for a
complete start with a normal limit minimum of about 30 psi.
When starting engines with an air turbine starter, always
check the duct pressure prior to the start attempt.
Figure 5-21 is a cutaway view of an air turbine starter. The
starter is operated by introducing air of sufficient volume and
pressure into the starter inlet. The air passes into the starter
turbine housing where it is directed against the rotor blades by
the nozzle vanes causing the turbine rotor to turn. As the rotor
turns, it drives the reduction gear train and clutch arrangement,
which includes the rotor pinion, planet gears and carrier, sprag
clutch assembly, output shaft assembly, and drive coupling.
The sprag clutch assembly engages automatically as soon
as the rotor starts to turn but disengages as soon as the drive
coupling turns more rapidly than the rotor side. When the
starter reaches this overrun speed, the action of the sprag
clutch allows the gear train to coast to a halt. The output
shaft assembly and drive coupling continue to turn as long
as the engine is running. A rotor switch actuator, mounted in
the turbine rotor hub, is set to open the turbine switch when
Air control valveAirframe pylon
Engine air starterExternal gearbox
High pressure airExhaust airCross feed from running engine Auxiliary power unit (APU)
Ground start supply
Figure 5-20. Air turbine starters are supplied by ground cart, APU, or another operating onboard engine.
the starter reaches cutout speed. Opening the turbine switch
interrupts an electrical signal to the start valve. This closes
the valve and shuts off the air supply to the starter.
The turbine housing contains the turbine rotor, the rotor
switch actuator, and the nozzle components that direct
the inlet air against the rotor blades. The turbine housing
incorporates a turbine rotor containment ring designed to
dissipate the energy of blade fragments and direct their
discharge at low energy through the exhaust duct in the event
of rotor failure due to excessive turbine overspeed.
The transmission housing contains the reduction gears, the
clutch components, and the drive coupling. The transmission
housing also provides a reservoir for the lubricating oil.
[Figure 5-22] Normal maintenance for air turbine starters
includes checking the oil level, inspecting the magnetic chip
detector for metal particles, and checking for leaks. Oil can
be added to the transmission housing sump through a port in the starter. This port is closed by a vent plug containing a ball
valve that allows the sump to be vented to the atmosphere
during normal flight. The housing also incorporates a sight
gauge that is used to check the oil quantity. A magnetic drain
plug in the transmission drain opening attracts any ferrous
particles that may be in the oil. The starter uses turbine oil,
the same as the engine, but this oil does not circulate through
the engine.
The ring gear housing, which is internal, contains the rotor
assembly. The switch housing contains the turbine switch
and bracket assembly. To facilitate starter installation and
removal, a mounting adapter is bolted to the mounting pad
on the engine. Quick-detach clamps join the starter to the
mounting adapter and inlet duct. [Figure 5-22] Thus, the
starter is easily removed for maintenance or overhaul by
disconnecting the electrical line, loosening the clamps, and
carefully disengaging the drive coupling from the engine
starter drive as the starter is withdrawn.
Engine drive shaftTurbine rotor
Reduction gearClutch
Air inlet
FWDAir outlet
Oil level overflowQAD adapter
Magnetic oil drain plugQAD coupling clamp
Sight glassOil fill plug
Air inlet
FWDExhaustFigure 5-21. Cutaway view of an air turbine starter.
Figure 5-22. Air turbine starter.
The air path is directed through a combination pressure-
regulating and shutoff valve, or bleed valve, that controls all
duct pressure flowing to the starter inlet ducting. This valve
regulates the pressure of the starter operating air and shuts off
the air supply to the engine when selected off. Downstream from the bleed valve is the start valve, which is used to control
air flow into the starter. [Figure 5-23]
The pressure-regulating and shutoff valve consists of two
subassemblies: pressure-regulating valve and pressure-
Butterfly air control valve
Pneumatic valve operating mechanism
Figure 5-23. Regulating and shutoff bleed valve.
Solenoid
To pressure sensing port
(downstream pressure)From air supply (upstream)
To starter (downstream)RestrictionFilter
On
Control crank
Control rod BellowsPilot valve cap Stationary camRotating cam
Inlet pressure
Regulating valveValve shaftServo-piston
Pilot valve rod
Open
Figure 5-24. Pressure-regulating and shutoff valve in on position. regulating valve control. [Figure 5-24] The regulating valve
assembly consists of a valve housing containing a butterfly-
type valve. [Figure 5-24] The shaft of the butterfly valve is
connected through a cam arrangement to a servo piston. When
the piston is actuated, its motion on the cam causes rotation
of the butterfly valve. The slope of the cam track is designed
to provide small initial travel and high initial torque when the
starter is actuated. The cam track slope also provides more
stable action by increasing the opening time of the valve.
The control assembly is mounted on the regulating valve
housing and consists of a control housing in which a
solenoid is used to stop the action of the control crank in
the off position. [Figure 5-24] The control crank links a
pilot valve that meters pressure to the servo piston, with
the bellows connected by an air line to the pressure-sensing
port on the starter.Turning on the starter switch energizes the regulating valve
solenoid. The solenoid retracts and allows the control crank
to rotate to the open position. The control crank is rotated
by the control rod spring moving the control rod against the
closed end of the bellows. Since the regulating valve is closed
and downstream pressure is negligible, the bellows can be
fully extended by the bellows spring.
As the control crank rotates to the open position, it causes
the pilot valve rod to open the pilot valve, allowing upstream
air, which is supplied to the pilot valve through a suitable
filter and a restriction in the housing, to flow into the servo
piston chamber. The drain side of the pilot valve, which
bleeds the servo chamber to the atmosphere, is now closed
by the pilot valve rod and the servo piston moves inboard.
[Figure 5-24] This linear motion of the servo piston is
translated to rotary motion of the valve shaft by the rotating
cam, thus opening the regulating valve. As the valve opens,
downstream pressure increases. This pressure is bled
back to the bellows through the pressure-sensing line and
compresses the bellows. This action moves the control rod,
thereby turning the control crank, and moving the pilot valve
rod gradually away from the servo chamber to vent to the
atmosphere. [Figure 5-24] When downstream (regulated)
pressure reaches a preset value, the amount of air flowing
into the servo through the restriction equals the amount of
air being bled to the atmosphere through the servo bleed; the
system is in a state of equilibrium.
When the bleed valve and the start valve are open, the
regulated air passing through the inlet housing of the starter
Air Turbine Starter System Troubleshooting Procedures
Trouble Probable Cause Remedy
• Check air supply.
• Check switch continuity. If no continuity, remove
starter and adjust or replace switch.
• Remove starter and replace drive coupling.
• Remove and replace starter.
• Check air source pressure.
• Adjust rotor switch actuator.
• Replace valve.
• Remove and replace starter.
• Check air supply.
• Adjust switch actuator assembly.
• Replace switch and bracket assembly.
• Drain oil and re-service properly.
• Tighten magnetic plug to proper torque.
• Tighten vent and oil filler plugs as necessary
and lock wire. Tighten clamp band assembly to
higher torque.
• Remove starter and replace the drive coupling.
If couplings persist in breaking in unusually short
periods of time, remove and replace starter.
• Check installation and/or indexing for conformance
with manufacturer’s installation instructions and
the proper index position of the turbine housing
specified for the aircraft.
• No remedial action required.
• Remove and replace starter.
• Remove and replace starter and check air
supply filter.
• Check installed position for levelness of oil
plugs and correct as required in accordance
with manufacturer’s installation instructions.
• Remove and replace starter.• No air supply.
• Electrical open in cutout switch.
• Sheared starter drive coupling.
• Internal starter discrepancy.
• Low starter air supply.
• Starter cutout switch set improperly.
• Valve pressure regulated too low.
• Internal starter malfunction.
• Low air supply.
• Rotor switch actuator set too high.
• Starter cutout switch shorted.
• Oil level too high.
• Loose vent, oil filler, or magnetic plugs.
• Loose clamp band assembly.
• Sheared drive coupling.
• Improper installation of starter on engine,
or improper indexing of turbine housing
on starter.
• Small fuzzy particles indicate normal wear.
• Particles coarser than fuzzy (chips,
slivers, etc.) indicate internal difficulty.
• Large foreign particles in air supply.
• Improper starter installation position.
• Leaking rear seal assembly.• Starter does not operate (no rotation).
• Starter will not accelerate to normal cutoff
speed.
• Starter will not cut off.
• External oil leakage.
• Starter runs, but engine does not turn over.
• Starter inlet will not line up with supply
ducting.
• Metallic particles on magnetic drain plug.
• Broken nozzle vanes.
• Oil leakage from vent plug assembly.
• Oil leakage at drive coupling.
Figure 5-25. Air turbine starter system troubleshooting procedures.impinges on the turbine causing it to turn. As the turbine
turns, the gear train is activated and the inboard clutch gear,
which is threaded onto a helical screw, moves forward as
it rotates; its jaw teeth engage those of the outboard clutch
gear to drive the output shaft of the starter. The clutch is
an overrunning type to facilitate positive engagement and
minimize chatter. When starter cut-out speed is reached, the
start valve is closed. When the air to the starter is terminated,
the outboard clutch gear, driven by the engine, begins to turn
faster than the inboard clutch gear; the inboard clutch gear,
actuated by the return spring, disengages the outboard clutch
gear allowing the rotor to coast to a halt. The outboard clutch
shaft continues to turn with the engine.Air Turbine Starter Troubleshooting Guide
The troubleshooting procedures listed in Figure 5-25 are
applicable to air turbine starting systems equipped with a
combination pressure-regulating and shutoff valve. These
procedures should be used as a guide only and are not
intended to replace the manufacturer’s instructions.
Friction from metal-to-metal contact
Oil film between parts preventing metal-
to-metal contact, reducing friction
Figure 6-1. Two moving surfaces in direct contact create excessive
friction.
Figure 6-2. Oil film acts as a cushion between two moving surfaces.Principles of Engine Lubrication
The primary purpose of a lubricant is to reduce friction
between moving parts. Because liquid lubricants or oils
can be circulated readily, they are used universally in
aircraft engines. In theory, fluid lubrication is based on the
actual separation of the surfaces so that no metal-to-metal
contact occurs. As long as the oil film remains unbroken,
metallic friction is replaced by the internal fluid friction of
the lubricant. Under ideal conditions, friction and wear are
held to a minimum. Oil is generally pumped throughout the
engine to all areas that require lubrication. Overcoming the
friction of the moving parts of the engine consumes energy
and creates unwanted heat. The reduction of friction during
engine operation increases the overall potential power output.
Engines are subjected to several types of friction.
Types of Friction
Friction may be defined as the rubbing of one object or
surface against another. One surface sliding over another
surface causes sliding friction, as found in the use of plain
bearings. The surfaces are not completely flat or smooth and
have microscopic defects that cause friction between the two
moving surfaces. [Figure 6-1] Rolling friction is created
when a roller or sphere rolls over another surface, such as
with ball or roller bearings, also referred to as antifriction
bearings. The amount of friction created by rolling friction is
less than that created by sliding friction and this bearing uses
an outer race and an inner race with balls, or steel spheres,
rolling between the moving parts or races. Another type of
friction is wiping friction, which occurs between gear teeth.
With this type of friction, pressure can vary widely and loads
applied to the gears can be extreme, so the lubricant must be
able to withstand the loads.
Functions of Engine Oil
In addition to reducing friction, the oil film acts as a cushion
between metal parts. [Figure 6-2] This cushioning effect is
particularly important for such parts as reciprocating engine
crankshafts and connecting rods, which are subject to shock-
loading. As the piston is pushed down on the power stroke,
it applies loads between the connecting rod bearing and
the crankshaft journal. The load-bearing qualities of the oil
must prevent the oil film from being squeezed out, causing
metal-to-metal contact in the bearing. Also, as oil circulates
through the engine, it absorbs heat from the pistons and
cylinder walls. In reciprocating engines, these components
are especially dependent on the oil for cooling.
Lubrication & Cooling Systems
Chapter 6
Oil cooling can account for up to 50 percent of the total
engine cooling and is an excellent medium to transfer the
heat from the engine to the oil cooler. The oil also aids in
forming a seal between the piston and the cylinder wall to
prevent leakage of the gases from the combustion chamber.
Oils clean the engine by reducing abrasive wear by picking
up foreign particles and carrying them to a filter where they
are removed. The dispersant, an additive, in the oil holds the
particles in suspension and allows the filter to trap them as the
oil passes through the filter. The oil also prevents corrosion
on the interior of the engine by leaving a coating of oil on
parts when the engine is shut down. This is one of the reasons
why the engine should not be shut down for long periods of
time. The coating of oil preventing corrosion will not last on
6-2Commercial
Aviation No.Commercial
SAE No.Army and Navy
Specification No.
Figure 6-3. Grade designations for aviation oils. the parts, allowing them to rust or corrode.
The engine’s oil is the life blood of the engine and it is very
important for the engine to perform its function and to extend
the length between overhauls.
Requirements & Characteristics of
Reciprocating Engine Lubricants
While there are several important properties that satisfactory
reciprocating engine oil must possess, its viscosity is most
important in engine operation. The resistance of an oil to flow
is known as its viscosity. Oil that flows slowly is viscous or
has a high viscosity; if it flows freely, it has a low viscosity.
Unfortunately, the viscosity of oil is affected by temperature.
It was not uncommon for earlier grades of oil to become
practically solid in cold weather, increasing drag and making
circulation almost impossible. Other oils may become so thin at
high temperatures that the oil film is broken, causing a low load
carrying ability, resulting in rapid wear of the moving parts.
The oil selected for aircraft engine lubrication must be light
enough to circulate freely at cold temperatures, yet heavy
enough to provide the proper oil film at engine operating
temperatures. Since lubricants vary in properties and since
no one oil is satisfactory for all engines and all operating
conditions, it is extremely important that only the approved
grade or Society of Automotive Engineers (SAE) rating be
used.
Several factors must be considered in determining the proper
grade of oil to use in a particular engine, the most important of
which are the operating load, rotational speeds, and operating
temperatures. The grade of the lubricating oil to be used
is determined by the operating conditions to be met in the
various types of engines. The oil used in aircraft reciprocating
engines has a relatively high viscosity required by:
1. Large engine operating clearances due to the relatively
large size of the moving parts, the different materials
used, and the different rates of expansion of the various
materials;
2. High operating temperatures; and
3. High bearing pressures.
Viscosity
Generally, commercial aviation oils are classified by a
number, (such as 80, 100, 140, etc.) that is an approximation
of the viscosity as measured by a testing instrument called
the Saybolt Universal Viscosimeter. In this instrument, a tube
holds a specific quantity of the oil to be tested. The oil is
brought to an exact temperature by a liquid bath surrounding
the tube. The time in seconds required for exactly 60 cubic
centimeters of oil to flow through an accurately calibrated orifice is recorded as a measure of the oil’s viscosity. If actual
Saybolt values were used to designate the viscosity of oil,
there would probably be several hundred grades of oil.
To simplify the selection of oils, they are often classified
under an SAE system that divides all oils into seven groups
(SAE 10 to 70) according to viscosity at either 130 °F or
210 °F. SAE ratings are purely arbitrary and bear no direct
relationship to the Saybolt or other ratings.
The letter W occasionally is included in the SAE number
giving a designation, such as SAE 20W. This W indicates
that the oil, in addition to meeting the viscosity requirements
at the testing temperature specifications, is satisfactory oil
for winter use in cold climates. This should not be confused
with the W used in front of the grade or weight number that
indicates the oil is of the ashless dispersant type.
Although the SAE scale has eliminated some confusion in
the designation of lubricating oils, it must not be assumed
that this specification covers all the important viscosity
requirements. An SAE number indicates only the viscosity
grade or relative viscosity; it does not indicate quality or
other essential characteristics. It is well known that there are
good oils and inferior oils that have the same viscosities at a
given temperature and, therefore, are subject to classification
in the same grade.
The SAE letters on an oil container are not an endorsement
or recommendation of the oil by the SAE. Although each
grade of oil is rated by an SAE number, depending on its
specific use, it may be rated with a commercial aviation
grade number or an Army and Navy specification number.
The correlation between these grade numbering systems is
shown in Figure 6-3 .
Viscosity Index
The viscosity index is a number that indicates the effect of
temperature changes on the viscosity with the oil. When oil
has a low viscosity index, it signifies a relatively large change
of viscosity of increased temperature. The oil becomes thin
at high temperatures and thick at low temperatures. Oils with
a high viscosity index have small changes in viscosity over
a wide temperature range.
6-3The best oil for most purposes is one that maintains a constant
viscosity throughout temperature changes. Oil having a high
viscosity index resists excessive thickening when the engine
is subjected to cold temperatures. This allows for rapid
cranking speeds during starting and prompt oil circulation
during initial startup. This oil resists excessive thinning
when the engine is at operating temperature and provides
full lubrication and bearing load protection.
Flash Point & Fire Point
Flash point and fire point are determined by laboratory tests
that show the temperature at which a liquid begins to give
off ignitable vapors, flash, and the temperature at which
there are sufficient vapors to support a fire. These points
are established for engine oils to determine that they can
withstand the high temperatures encountered in an engine.
Cloud Point & Pour Point
Cloud point and pour point also help to indicate suitability.
The cloud point of oil is the temperature at which its wax
content, normally held in solution, begins to solidify and
separate into tiny crystals, causing the oil to appear cloudy
or hazy. The pour point of oil is the lowest temperature at
which it flows or can be poured.
Specific Gravity
Specific gravity is a comparison of the weight of the
substance to the weight of an equal volume of distilled water
at a specified temperature. As an example, water weighs
approximately 8 pounds to the gallon; oil with a specific
gravity of 0.9 would weigh 7.2 pounds to the gallon.
In the early years, the performance of aircraft piston engines
was such that they could be lubricated satisfactorily by means
of straight mineral oils, blended from specially selected
petroleum base stocks. Oil grades 65, 80, 100, and 120 are
straight mineral oils blended from selected high-viscosity
index base oils. These oils do not contain any additives except
for very small amounts of pour point depressant, which helps
improve fluidity at very low temperatures, and an antioxidant.
This type of oil is used during the break-in period of a new
aviation piston engine or those recently overhauled.
Demand for oils with higher degrees of thermal and oxidation
stability necessitated fortifying them with the addition of
small quantities of nonpetroleum materials. The first additives
incorporated in straight mineral piston engine oils were based
on the metallic salts of barium and calcium. In most engines,
the performance of these oils with respect to oxidation and
thermal stability was excellent, but the combustion chambers
of the majority of engines could not tolerate the presence of the
ash deposits derived from these metal-containing additives. To overcome the disadvantages of harmful combustion chamber
deposits, a nonmetallic (i.e., non-ash forming, polymeric)
additive was developed that was incorporated in blends of
selected mineral oil base stocks. W oils are of the ashless
type and are still in use. The ashless dispersant grades contain
additives, one of which has a viscosity stabilizing effect
that removes the tendency of the oil to thin out at high oil
temperatures and thicken at low oil temperatures.
The additives in these oils extend operating temperature
range and improve cold engine starting and lubrication of
the engine during the critical warm-up period permitting
flight through wider ranges of climatic changes without the
necessity of changing oil.
Semi-synthetic multigrade SAE W15 W50 oil for piston
engines has been in use for some time. Oils W80, W100,
and W120 are ashless dispersant oils specifically developed
for aviation piston engines. They combine nonmetallic
additives with selected high viscosity index base oils to
give exceptional stability, dispersancy, and antifoaming
performance. Dispersancy is the ability of the oil to hold
particles in suspension until they can either be trapped by
the filter or drained at the next oil change. The dispersancy
additive is not a detergent and does not clean previously
formed deposits from the interior of the engine.
Some multigrade oil is a blend of synthetic and mineral-
based oil semisynthetic, plus a highly effective additive
package, that is added due to concern that fully synthetic oil
may not have the solvency to handle the lead deposits that
result from the use of leaded fuel. As multigrade oil, it offers
the flexibility to lubricate effectively over a wider range of
temperatures than monograde oils. Compared to monograde
oil, multigrade oil provides better cold-start protection and a
stronger lubricant film (higher viscosity) at typical operating
temperatures. The combination of nonmetallic, antiwear
additives and selected high viscosity index mineral and
synthetic base oils give exceptional stability, dispersancy,
and antifoaming performance. Startup can contribute up to
80 percent of normal engine wear due to lack of lubrication
during the start-up cycle. The more easily the oil flows to
the engine’s components at start up, the less wear occurs.
The ashless dispersant grades are recommended for aircraft
engines subjected to wide variations of ambient temperature,
particularly the turbocharged series engines that require oil
to activate the various turbo controllers. At temperatures
below 20 °F, preheating of the engine and oil supply tank is
normally required regardless of the type of oil used.
Premium, semisynthetic multigrade ashless dispersant oil is
a special blend of a high-quality mineral oil and synthetic
6-4hydrocarbons with an advanced additive package that has
been specifically formulated for multigrade applications.
The ashless antiwear additive provides exceptional wear
protection for wearing surfaces.
Many aircraft manufacturers add approved preservative
lubricating oil to protect new engines from rust and corrosion
at the time the aircraft leaves the factory. This preservative oil
should be removed at end of the first 25 hours of operation.
When adding oil during the period when preservative oil is
in the engine, use only aviation grade straight mineral oil or
ashless dispersant oil, as required, of the viscosity desired.
If ashless dispersant oil is used in a new engine, or a newly
overhauled engine, high oil consumption might possibly be
experienced. The additives in some of these ashless dispersant
oils may retard the break in of the piston rings and cylinder
walls. This condition can be avoided by the use of mineral
oil until normal oil consumption is obtained, then change to
the ashless dispersant oil. Mineral oil should also be used
following the replacement of one or more cylinders or until
the oil consumption has stabilized.
In all cases, refer to the manufacturers’ information when oil
type or time in service is being considered.
Reciprocating Engine Lubrication Systems
Aircraft reciprocating engine pressure lubrication systems
can be divided into two basic classifications: wet sump and
dry sump. The main difference is that the wet sump system
stores oil in a reservoir inside the engine. After the oil is
circulated through the engine, it is returned to this crankcase-
based reservoir. A dry sump engine pumps the oil from the
engine’s crankcase to an external tank that stores the oil.
The dry sump system uses a scavenge pump, some external
tubing, and an external tank to store the oil.
Other than this difference, the systems use similar types of
components. Because the dry sump system contains all the
components of the wet sump system, the dry sump system
is explained as an example system.
Combination Splash & Pressure Lubrication
The lubricating oil is distributed to the various moving parts
of a typical internal combustion engine by one of the three
following methods: pressure, splash, or a combination of
pressure and splash.
The pressure lubrication system is the principal method of
lubricating aircraft engines. Splash lubrication may be used
in addition to pressure lubrication on aircraft engines, but it
is never used by itself; aircraft-engine lubrication systems are
always either the pressure type or the combination pressure and splash type, usually the latter.
The advantages of pressure lubrication are:
1. Positive introduction of oil to the bearings.
2. Cooling effect caused by the large quantities of oil
that can be pumped, or circulated, through a bearing.
3. Satisfactory lubrication in various attitudes of flight.
Lubrication System Requirements
The lubrication system of the engine must be designed and
constructed so that it functions properly within all flight
attitudes and atmospheric conditions that the aircraft is
expected to operate. In wet sump engines, this requirement
must be met when only half of the maximum lubricant supply
is in the engine. The lubrication system of the engine must
be designed and constructed to allow installing a means of
cooling the lubricant. The crankcase must also be vented to the
atmosphere to preclude leakage of oil from excessive pressure.
Dry Sump Oil Systems
Many reciprocating and turbine aircraft engines have pressure
dry sump lubrication systems. The oil supply in this type of
system is carried in a tank. A pressure pump circulates the
oil through the engine. Scavenger pumps then return it to the
tank as quickly as it accumulates in the engine sumps. The
need for a separate supply tank is apparent when considering
the complications that would result if large quantities of oil
were carried in the engine crankcase. On multiengine aircraft,
each engine is supplied with oil from its own complete and
independent system.
Although the arrangement of the oil systems in different
aircraft varies widely and the units of which they are composed
differ in construction details, the functions of all such systems
are the same. A study of one system clarifies the general
operation and maintenance requirements of other systems.
The principal units in a typical reciprocating engine dry
sump oil system include an oil supply tank, an engine-driven
pressure oil pump, a scavenge pump, an oil cooler with an
oil cooler control valve, oil tank vent, necessary tubing, and
pressure and temperature indicators. [Figure 6-4]
Oil Tanks
Oil tanks are generally associated with a dry sump lubrication
system, while a wet sump system uses the crankcase of the
engine to store the oil. Oil tanks are usually constructed of
aluminum alloy and must withstand any vibration, inertia,
and fluid loads expected in operation.
Each oil tank used with a reciprocating engine must have
expansion space of not less than the greater of 10 percent
Oil tank
Supply
Pressure
Vent
Return
Drain
Oil
Temp
Oil
TempOil
Press.
Oil
Press.
Scupper drainOil tank ventOil coolerScavenger pump
Oil pressure pumpEngine breather
Oil tank drain valveFlexible weighted internal hose assemblyOil pressure gauge
Oil temperature gauge
Figure 6-4. Oil system schematic.
Hopper tank
Baffles
Figure 6-5. Oil tank with hopper. of the tank capacity or 0.5 gallons. Each filler cap of an oil
tank that is used with an engine must provide an oil-tight
seal. The oil tank usually is placed close to the engine and
high enough above the oil pump inlet to ensure gravity feed.
Oil tank capacity varies with the different types of aircraft, but
it is usually sufficient to ensure an adequate supply of oil for the
total fuel supply. The tank filler neck is positioned to provide
sufficient room for oil expansion and for foam to collect.
The filler cap or cover is marked with the word OIL. A drain
in the filler cap well disposes of any overflow caused by the
filling operation. Oil tank vent lines are provided to ensure
proper tank ventilation in all attitudes of flight. These lines
are usually connected to the engine crankcase to prevent the
loss of oil through the vents. This indirectly vents the tanks
to the atmosphere through the crankcase breather.
Early large radial engines had many gallons of oil in their
tank. To help with engine warm up, some oil tanks had a built-
in hopper or temperature accelerating well. [Figure 6-5] This
well extended from the oil return fitting on top of the oil tank
to the outlet fitting in the sump in the bottom of the tank. In
some systems, the hopper tank is open to the main oil supply
at the lower end. Other systems have flapper-type valves that separate the main oil supply from the oil in the hopper.
The opening at the bottom of the hopper in one type and
the flapper valve-controlled openings in the other allow oil
from the main tank to enter the hopper and replace the oil
consumed by the engine. Whenever the hopper tank includes
Relief valve
Oil filter
Gear-type oil pumpBypass valve
Oil valveOil pressure passage to engine
Figure 6-6. Engine oil pump and associated units. gauge that indicates the quantity of oil.
Oil Pump
Oil entering the engine is pressurized, filtered, and regulated
by units within the engine. They are discussed along with
the external oil system to provide a concept of the complete
oil system.
As oil enters the engine, it is pressurized by a gear-type
pump. [Figure 6-6] This pump is a positive displacement
pump that consists of two meshed gears that revolve inside
the housing. The clearance between the teeth and housing is
small. The pump inlet is located on the left and the discharge
port is connected to the engine’s system pressure line. One
gear is attached to a splined drive shaft that extends from
the pump housing to an accessory drive shaft on the engine.
Seals are used to prevent leakage around the drive shaft. As
the lower gear is rotated counterclockwise, the driven idler
gear turns clockwise.
As oil enters the gear chamber, it is picked up by the gear
teeth, trapped between them and the sides of the gear
chamber, then it is carried around the outside of the gears and
discharged from the pressure port into the oil screen passage.
The pressurized oil flows to the oil filter, where any solid
particles suspended in the oil are separated from it, preventing
possible damage to moving parts of the engine.
Oil under pressure then opens the oil filter check valve
mounted in the top of the filter. This valve is used mostly the flapper controlled openings, the valves are operated by
differential oil pressure. By separating the circulating oil from
the surrounding oil in the tank, less oil is circulated. This
hastens the warming of the oil when the engine was started.
Very few of these types of tanks are still in use and most are
associated with radial engine installations.
Generally, the return line in the top of the tank is positioned
to discharge the returned oil against the wall of the tank in a
swirling motion. This method considerably reduces foaming
that occurs when oil mixes with air. Baffles in the bottom
of the oil tank break up this swirling action to prevent air
from being drawn into the inlet line of the oil pressure pump.
Foaming oil increases in volume and reduces its ability to
provide proper lubrication. In the case of oil-controlled
propellers, the main outlet from the tank may be in the form
of a standpipe so that there is always a reserve supply of
oil for propeller feathering in case of engine failure. An oil
tank sump, attached to the undersurface of the tank, acts as a
trap for moisture and sediment. [Figure 6-4] The water and
sludge can be drained by manually opening the drain valve
in the bottom of the sump.
Most aircraft oil systems are equipped with the dipstick-type
quantity gauge, often called a bayonet gauge. Some larger
aircraft systems also have an oil quantity indicating system
that shows the quantity of oil during flight. One type system
consists essentially of an arm and float mechanism that rides
the level of the oil and actuates an electric transmitter on
top of the tank. The transmitter is connected to a flight deck
Hex head screw
Copper gasket
Filter element
Rubber gasket
Cover plate
Rubber gasket
Nylon nutCase housing or canister
Figure 6-7. Housing filter element type oil filter. with dry sump radial engines and is closed by a light spring
loading of 1 to 3 pounds per square inch (psi) when the engine
is not operating to prevent gravity-fed oil from entering
the engine and settling in the lower cylinders or sump area
of the engine. If oil were allowed to gradually seep by the
rings of the piston and fill the combustion chamber, it could
cause a liquid lock. This could happen if the valves on the
cylinder were both closed, and the engine was cranked for
start. Damage could occur to the engine.
The oil filter bypass valve, located between the pressure side
of the oil pump and the oil filter, permits unfiltered oil to
bypass the filter and enter the engine if the oil filter is clogged
or during cold weather if congealed oil is blocking the filter
during engine start. The spring loading on the bypass valve
allows the valve to open before the oil pressure collapses
the filter; in the case of cold, congealed oil, it provides a
low-resistance path around the filter. Dirty oil in an engine
is better than no lubrication.
Oil Filters
The oil filter used on an aircraft engine is usually one of four
types: screen, Cuno, canister, or spin-on. A screen-type filter
with its double-walled construction provides a large filtering
area in a compact unit. [Figure 6-6] As oil passes through the
fine-mesh screen, dirt, sediment, and other foreign matter are
removed and settle to the bottom of the housing. At regular
intervals, the cover is removed, and the screen and housing
cleaned with a solvent. Oil screen filters are used mostly as
suction filters on the inlet of the oil pump.
The Cuno oil filter has a cartridge made of discs and
spacers. A cleaner blade fits between each pair of discs. The
cleaner blades are stationary, but the discs rotate when the
shaft is turned. Oil from the pump enters the cartridge well
that surrounds the cartridge and passes through the spaces
between the closely spaced discs of the cartridge, then
through the hollow center, and on to the engine. Any foreign
particles in the oil are deposited on the outer surface of the
cartridge. When the cartridge is rotated, the cleaner blades
comb the foreign matter from the discs. The cartridge of the
manually operated Cuno filter is turned by an external handle.
Automatic Cuno filters have a hydraulic motor built into
the filter head. This motor, operated by engine oil pressure,
rotates the cartridge whenever the engine is running. There is
a manual turning nut on the automatic Cuno filter for rotating
the cartridge manually during inspections. This filter is not
often used on modern aircraft.
A canister housing filter has a replaceable filter element
that is replaced with the rest of the components other than
seals and gaskets being reused. [Figure 6-7] The filter
element is designed with a corrugated, strong steel center tube supporting each convoluted pleat of the filter media,
resulting in a higher collapse pressure rating. The filter
provides excellent filtration, because the oil flows through
many layers of locked-in-fibers.
Full flow spin-on filters are the most widely used oil filters
for reciprocating engines. [Figure 6-8] Full flow means that
all the oil is normally passed through the filter. In a full flow
system, the filter is positioned between the oil pump and the
engine bearings, which filters the oil of any contaminants
before they pass through the engine bearing surfaces. The
filter also contains an antidrain back valve and a pressure
relief valve, all sealed in a disposable housing. The relief
valve is used in case the filter becomes clogged. It would open
to allow the oil to bypass, preventing the engine components
from oil starvation. A cutaway of the micronic filter element
shows the resin-impregnated cellulosic full-pleat media that
is used to trap harmful particles, keeping them from entering
the engine. [Figure 6-9]
Oil Pressure Regulating Valve
An oil pressure regulating valve limits oil pressure to
a predetermined value, depending on the installation.
[Figure 6-6] This valve is sometimes referred to as a relief
valve, but its real function is to regulate the oil pressure at a
preset pressure level. The oil pressure must be sufficiently
high to ensure adequate lubrication of the engine and its
Safety wire tabs conveniently located
on hex nut for easy access
Resin-impregnated,
cellulosic full-pleat
media for uniform
flow and collapse
resistanceCorrugated center support tube for
maximum resistance to collapse
Figure 6-8. Full flow spin-on filter.
Figure 6-10. Oil pressure adjustment screw. Figure 6-9. Cutaway view of a filter.
accessories at high speeds and powers. This pressure helps
ensure that the oil film between the crankshaft journal and
bearing is maintained. However, the pressure must not be too
high, as leakage and damage to the oil system may result. The
oil pressure is generally adjusted by loosening the locknut
and turning the adjusting screw. [Figure 6-10] On most
aircraft engines, turning the screw clockwise increases the
tension of the spring that holds the relief valve on its seat
and increases the oil pressure; turning the adjusting screw
counterclockwise decreases the spring tension and lowers
the pressure. Some engines use washers under the spring that
are either removed or added to adjust the regulating valve
and pressure. The oil pressure should be adjusted only after
the engine’s oil is at operating temperature and the correct viscosity is verified. The exact procedure for adjusting the
oil pressure and the factors that vary an oil pressure setting
are included in applicable manufacturer’s instructions.
Oil Pressure Gauge
Usually, the oil pressure gauge indicates the pressure that oil
enters the engine from the pump. This gauge warns of possible
engine failure caused by an exhausted oil supply, failure of
the oil pump, burned-out bearings, ruptured oil lines, or other
causes that may be indicated by a loss of oil pressure.
One type of oil pressure gauge uses a Bourdon-tube
mechanism that measures the difference between oil
pressure and cabin, or atmospheric, pressure. This gauge is
constructed similarly to other Bourdon-type gauges, except
that it has a small restriction built into the instrument case,
or into the nipple connection leading to the Bourdon tube.
This restriction prevents the surging action of the oil pump
from damaging the gauge or causing the pointer to oscillate
too violently with each pressure pulsation. The oil pressure
gauge has a scale ranging from 0–200 psi, or from 0–300 psi.
Operation range markings are placed on the cover glass, or
the face of the gauge, to indicate the safe range of oil pressure
for a given installation.
A dual-type oil pressure gauge is available for use on
Bypass jacket CoreBafflesInlet from engine
Outlet from coreOutlet from bypass jacket
Figure 6-11. Oil cooler.
the annular or bypass jacket. Two paths are open to the flow
of oil through a cooler. From the inlet, it can flow halfway
around the bypass jacket, enter the core from the bottom,
and then pass through the spaces between the tubes and out
to the oil tank. This is the path the oil follows when it is hot
enough to require cooling. As the oil flows through the core,
it is guided by baffles that force the oil to travel back and
forth several times before it reaches the core outlet. The oil
can also pass from the inlet completely around the bypass
jacket to the outlet without passing through the core. Oil
follows this bypass route when the oil is cold or when the
core is blocked with thick, congealed oil.
Oil Cooler Flow Control Valve
As discussed previously, the viscosity of the oil varies with
its temperature. Since the viscosity affects its lubricating
properties, the temperature at which the oil enters an engine
must be held within close limits. Generally, the oil leaving
an engine must be cooled before it is recirculated. Obviously,
the amount of cooling must be controlled if the oil is to return
to the engine at the correct temperature. The oil cooler flow
control valve determines which of the two possible paths the
oil takes through the oil cooler. [Figure 6-12]
There are two openings in a flow control valve that fit over
the corresponding outlets at the top of the cooler. When the oil
is cold, a bellows within the flow control contracts and lifts
a valve from its seat. Under this condition, oil entering the
cooler has a choice of two outlets and two paths. Following
the path of least resistance, the oil flows around the jacket
and out past the thermostatic valve to the tank. This allows
the oil to warm up quickly and, at the same time, heats the multiengine aircraft. The dual indicator contains two Bourdon
tubes, housed in a standard instrument case; one tube being
used for each engine. The connections extend from the back
of the case to each engine. There is one common movement
assembly, but the moving parts function independently. In
some installations, the line leading from the engine to the
pressure gauge is filled with light oil. Since the viscosity of this
oil does not vary much with changes in temperature, the gauge
responds better to changes in oil pressure. In time, engine oil
mixes with some of the light oil in the line to the transmitter;
during cold weather, the thicker mixture causes sluggish
instrument readings. To correct this condition, the gauge line
must be disconnected, drained, and refilled with light oil.
The current trend is toward electrical transmitters and
indicators for oil and fuel pressure-indicating systems in all
aircraft. In this type of indicating system, the oil pressure
being measured is applied to the inlet port of the electrical
transmitter where it is conducted to a diaphragm assembly
by a capillary tube. The motion produced by the diaphragm’s
expansion and contraction is amplified through a lever and
gear arrangement. The gear varies the electrical value of the
indicating circuit, which in turn, is reflected on the indicator
in the flight deck. This type of indicating system replaces
long fluid-filled tubing lines with an almost weightless piece
of wire.
Oil Temperature Indicator
In dry-sump lubricating systems, the oil temperature bulb
may be anywhere in the oil inlet line between the supply
tank and the engine. Oil systems for wet-sump engines have
the temperature bulb located where it senses oil temperature
after the oil passes through the oil cooler. In either system, the
bulb is located so that it measures the temperature of the oil
before it enters the engine’s hot sections. An oil temperature
gauge in the flight deck is connected to the oil temperature
bulb by electrical leads. The oil temperature is indicated on
the gauge. Any malfunction of the oil cooling system appears
as an abnormal reading.
Oil Cooler
The cooler, either cylindrical or elliptical shaped, consists
of a core enclosed in a double-walled shell. The core is built
of copper or aluminum tubes with the tube ends formed to
a hexagonal shape and joined together in the honeycomb
effect. [Figure 6-11] The ends of the copper tubes of the
core are soldered, whereas aluminum tubes are brazed or
mechanically joined. The tubes touch only at the ends so
that a space exists between them along most of their lengths.
This allows oil to flow through the spaces between the tubes
while the cooling air passes through the tubes.
The space between the inner and outer shells is known as
Control valve outletA
Check valveBSurge valveC
Control valve inletDPoppet valveE
Bypass jacketFCore outletG
Bypass jacket outletHSurge condition Cold oil flow Hot oil flow
A
B
C
D
E
FH G
Figure 6-12. Control valve with surge protection.
oil in the core. As the oil warms up and reaches its operating
temperature, the bellows of the thermostat expand and closes
the outlet from the bypass jacket. The oil cooler flow control
valve, located on the oil cooler, must now flow oil through the
core of the oil cooler. No matter which path it takes through
the cooler, the oil always flows over the bellows of the
thermostatic valve. As the name implies, this unit regulates
the temperature by either cooling the oil or passing it on to
the tank without cooling, depending on the temperature at
which it leaves the engine.
Surge Protection Valves
When oil in the system is congealed, the scavenger pump
may build up a very high pressure in the oil return line. To
prevent this high pressure from bursting the oil cooler or
blowing off the hose connections, some aircraft have surge
protection valves in the engine lubrication systems. One type
of surge valve is incorporated in the oil cooler flow control
valve; another type is a separate unit in the oil return line.
[Figure 6-12]
The surge protection valve incorporated in a flow control
valve is the more common type. Although this flow control
valve differs from the one just described, it is essentially
the same except for the surge protection feature. The high-
pressure operation condition is shown in Figure 6-12 , in
which the high oil pressure at the control valve inlet has
forced the surge valve (C) upward. Note how this movement
has opened the surge valve and, at the same time, seated
the poppet valve (E). The closed poppet valve prevents oil from entering the cooler proper; therefore, the scavenge oil
passes directly to the tank through outlet (A) without passing
through either the cooler bypass jacket or the core. When the
pressure drops to a safe value, the spring forces the surge and
poppet valves downward, closing the surge valve (C) and
opening the poppet valve (E). Oil then passes from the control
valve inlet (D), through the open poppet valve, and into the
bypass jacket (F). The thermostatic valve, according to oil
temperature, determines oil flow either through the bypass
jacket to port (H) or through the core to port (G). The check
valve (B) opens to allow the oil to reach the tank return line.
Airflow Controls
By regulating the airflow through the cooler, the temperature
of the oil can be controlled to fit various operating conditions.
For example, the oil reaches operating temperature more
quickly if the airflow is cut off during engine warm-up. There
are two methods in general use: shutters installed on the rear
of the oil cooler, and a flap on the air-exit duct. In some cases,
the oil cooler air-exit flap is opened manually and closed by
a linkage attached to a flight deck lever. More often, the flap
is opened and closed by an electric motor.
One of the most widely used automatic oil temperature
control devices is the floating control thermostat that provides
manual and automatic control of the oil inlet temperatures.
With this type of control, the oil cooler air-exit door is
opened and closed automatically by an electrically operated
actuator. Automatic operation of the actuator is determined
by electrical impulses received from a controlling thermostat
A B C D E
GC FShaftA
Bimetal elementB
Grounded center contact armC
Electric motorD
Gear trainE
CamF
Floating contact armGTop view
Side view
Figure 6-13. Floating control thermostat. inserted in the oil pipe leading from the oil cooler to the oil
supply tank. The actuator may be operated manually by an
oil cooler air-exit door control switch. Placing this switch in
the “open” or “closed” position produces a corresponding
movement of the cooler door. Placing the switch in the “auto”
position puts the actuator under the automatic control of the
floating control thermostat. [Figure 6-13] The thermostat
shown in Figure 6-13 is adjusted to maintain a normal oil
temperature so that it does not vary more than approximately
5° to 8 °C, depending on the installation.
During operation, the temperature of the engine oil flowing
over the bimetal element causes it to wind or unwind slightly.
[Figure 6-13B] This movement rotates the shaft (A) and the
grounded center contact arm (C). As the grounded contact
arm is rotated, it is moved toward either the open or closed
floating contact arm (G). The two floating contact arms are
oscillated by the cam (F), which is continuously rotated by
an electric motor (D) through a gear train (E). When the
grounded center contact arm is positioned by the bimetal
element so that it touches one of the floating contact arms,
an electric circuit to the oil cooler exit-flap actuator motor is
completed, causing the actuator to operate and position the
oil cooler air-exit flap. Newer systems use electronic control
systems, but the function or the overall operation is basically
the same regarding control of the oil temperature through
control of the air flow through the cooler.
In some lubrication systems, dual oil coolers are used. If the
typical oil system previously described is adapted to two oil
coolers, the system is modified to include a flow divider, two
identical coolers and flow regulators, dual air-exit doors, a
two-door actuating mechanism, and a Y-fitting. [Figure 6-14] Oil is returned from the engine through a single tube to the
flow divider (E), where the return oil flow is divided equally
into two tubes (C), one for each cooler. The coolers and
regulators have the same construction and operations as the
cooler and flow regulator just described. Oil from the coolers is
routed through two tubes (D) to a Y-fitting, where the floating
control thermostat (A) samples oil temperature and positions
the two oil cooler air-exit doors through the use of a two-door
actuating mechanism. From the Y-fitting, the lubricating oil is
returned to the tank where it completes its circuit.
Dry Sump Lubrication System Operation
The following lubrication system is typical of those on small,
single-engine aircraft. The oil system and components are
those used to lubricate a 225 horsepower (hp) six-cylinder,
horizontally opposed, air-cooled engine. In a typical dry sump
pressure-lubrication system, a mechanical pump supplies
oil under pressure to the bearings throughout the engine.
[Figure 6-4] The oil flows into the inlet or suction side of
the oil pump through a suction screen and a line connected to
the external tank at a point higher than the bottom of the oil
sump. This prevents sediment that falls into the sump from
being drawn into the pump. The tank outlet is higher than
the pump inlet, so gravity can assist the flow into the pump.
The engine-driven, positive-displacement, gear-type pump
forces the oil into the full flow filter. [Figure 6-6] The oil
either passes through the filter under normal conditions or,
if the filter were to become clogged, the filter bypass valve
would open as mentioned earlier. In the bypass position, the
oil would not be filtered. As seen in Figure 6-6 , the regulating
(relief) valve senses when system pressure is reached and
opens enough to bypass oil to the inlet side of the oil pump.
Then, the oil flows into a manifold that distributes the oil
A B
E
DC
Floating control thermostatA
Y-fittingBOutlet from cooler tubesD
Flow dividerE
Inlet to cooler tubesC
Oil sump pick-up
Oil temperature control valveOil filterOil pressure
relief valve
by-pass to
sump
Oil coolerOil pressure gauge connectionHydraulic liftersGovernor padTo propellerCamshaft bearing
Crankshaft bearing
Starter bushingIdler shaft bushing
Accessory drive bushingFigure 6-14. Dual oil cooler system.
Figure 6-15. Oil circulation through the engine. through drilled passages to the crankshaft bearings and other
bearings throughout the engine. Oil flows from the main
bearings through holes drilled in the crankshaft to the lower
connecting rod bearings. [Figure 6-15]
Oil reaches a hollow camshaft (in an inline or opposed
engine), or a cam plate or cam drum (in a radial engine),
through a connection with the end bearing or the main oil
manifold; it then flows out to the various camshaft, cam drum,
or cam plate bearings and the cams.
The engine cylinder surfaces receive oil sprayed from the
crankshaft and also from the crankpin bearings. Since oil
seeps slowly through the small crankpin clearances before it
is sprayed on the cylinder walls, considerable time is required
for enough oil to reach the cylinder walls, especially on a
cold day when the oil flow is more sluggish. This is one of
the chief reasons for using modern multiviscosity oils that
flow well at low temperatures.
Figure 6-16. Basic wet-sump oil system.
Figure 6-17. Wet-sump system’s sump with intake tube running
through it.When the circulating oil has performed its function of
lubricating and cooling the moving parts of the engine, it
drains into the sumps in the lowest parts of the engine. Oil
collected in these sumps is picked up by gear or gerotor-type
scavenger pumps as quickly as it accumulates. These pumps
have a greater capacity than the pressure pump. This is needed
because the volume of the oil has generally increased due
to foaming (mixing with air). On dry sump engines, this oil
leaves the engine, passes through the oil cooler, and returns
to the supply tank.
A thermostat attached to the oil cooler controls oil temperature
by allowing part of the oil to flow through the cooler and part
to flow directly into the oil supply tank. This arrangement
allows hot engine oil with a temperature still below 65 °C
(150 °F) to mix with the cold uncirculated oil in the tank. This
raises the complete engine oil supply to operating temperature
in a shorter period of time.
Wet-Sump Lubrication System Operation
A simple form of a wet-sump system is shown in Figure 6-16 .
The system consists of a sump or pan in which the oil supply
is contained. The oil supply is limited by the sump (oil pan)
capacity. The level (quantity) of oil is indicated or measured
by a vertical rod that protrudes into the oil from an elevated
hole on top of the crankcase. In the bottom of the sump (oil
pan) is a screen strainer having a suitable mesh, or series of
openings, to strain undesirable particles from the oil and yet
pass sufficient quantity to the inlet or (suction) side of the oil
pressure pump. Figure 6-17 shows a typical oil sump that has
the intake tube running through it. This preheats the air-fuel
mixture before it enters the cylinders.
The rotation of the pump, which is driven by the engine,
causes the oil to pass around the outside of the gears.
[Figure 6-6] This develops a pressure in the crankshaft
oiling system (drilled passage holes). The variation in the speed of the pump from idling to full-throttle operating range
of the engine and the fluctuation of oil viscosity because of
temperature changes are compensated by the tension on the
relief valve spring. The pump is designed to create a greater
pressure than required to compensate for wear of the bearings
or thinning out of oil. The parts oiled by pressure throw a
lubricating spray into the cylinder and piston assemblies.
After lubricating the various units it sprays, the oil drains
back into the sump and the cycle is repeated. The system is
not readily adaptable to inverted flying since the entire oil
supply floods the engine.
Lubrication System Maintenance Practices
Oil Tank
The oil tank, constructed of welded aluminum, is serviced
(filled) through a filler neck located on the tank and equipped
with a spring-loaded locking cap. Inside the tank, a weighted,
flexible rubber oil hose is mounted so that it is repositioned
automatically to ensure oil pickup during all maneuvers. A
dipstick guard is welded inside the tank for the protection
of the flexible oil hose assembly. During normal flight,
the oil tank is vented to the engine crankcase by a flexible
line at the top of the tank. The location of the oil system
components in relation to each other and to the engine is
shown in Figure 6-18 .
Repair of an oil tank usually requires that the tank be
removed. The removal and installation procedures normally
remain the same regardless of whether the engine is removed
or not. First, the oil must be drained. Most light aircraft
provide an oil drain similar to that shown in Figure 6-19 . On
some aircraft, the normal ground attitude of the aircraft may
prevent the oil tank from draining completely. If the amount
of undrained oil is excessive, the aft portion of the tank can
be raised slightly after the tank straps have been loosened to
complete the drainage.
Engine breather1
Oil outlet2
Oil temperature gauge4
Oil pressure gauge5Oil inlet3
Oil tank6
Oil filter7
Oil tank vent line9
Engine oil pressure line10Oil tank drain81
Supply Pressure Return Drain Vent2
Figure 6-19. Oil tank drain. Figure 6-18. Oil system perspective.
After disconnecting the oil inlet and vent lines, the scupper
drain hose and bonding wire can be removed. [Figure 6-20]
The securing straps fitted around the tank can now be
removed. [Figure 6-21] Any safety wire securing the clamp
must be removed before the clamp can be loosened and the strap disconnected. The tank can now be lifted out of the
aircraft. The tank is reinstalled by reversing the sequence used
in the tank removal. After installation, the oil tank should be
filled to capacity. [Figure 6-22]
After the oil tank has been filled, the engine should be run for
at least two minutes. Then, the oil level should be checked
and, if necessary, sufficient oil should be added to bring the
oil up to the proper level on the dipstick. [Figure 6-23]
Oil Cooler
The oil cooler used with this aircraft’s opposed-type engine
is the honeycomb type. [Figure 6-24] With the engine
operating and an oil temperature below 65 °C (150 °F), oil
cooler bypass valve opens allowing oil to bypass the core.
This valve begins to close when the oil temperature reaches
approximately 65 °C (150 °F). When the oil temperature
reaches 85 °C (185 °F), ±2 °C, the valve is closed completely,
diverting all oil flow through the cooler core.
Figure 6-20. Disconnect oil lines.
Figure 6-21. Removal of securing straps.
Figure 6-22. Filling an oil tank.Figure 6-23. Checking oil level with dipstick.
Figure 6-24. Oil cooler.
Oil Temperature Bulbs
Most oil temperature bulbs are mounted in the pressure oil
screen housing. They relay an indication of engine oil inlet
temperature to the oil temperature indicators mounted on
the instrument panel. Temperature bulbs can be replaced by
removing the safety wire and disconnecting the wire leads
from the temperature bulbs, then removing the temperature
bulbs using the proper wrench. [Figure 6-25]Pressure & Scavenge Oil Screens
Sludge accumulates on the pressure and scavenges oil screens
during engine operation. [Figure 6-26] These screens must
be removed, inspected, and cleaned at the intervals specified
by the manufacturer.
Typical removal procedures include removing the safety
devices and loosening the oil screen housing or cover plate.
A suitable container should be provided to collect the oil that
drains from the filter housing or cavity. The container must
be clean so that the oil collected in it can be examined for
foreign particles. Any contamination already present in the
container gives a false indication of the engine condition.
This could result in a premature engine removal.
A BFigure 6-25. Removing oil temperature bulb.
Figure 6-26. Oil pressure screen (A) and scavenge oil screen assembly (B).to the torque value specified in the applicable maintenance
manual. Filters should be safetied as required.
Oil Pressure Relief Valve
An oil pressure regulating (relief) valve limits oil pressure to
the value specified by the engine manufacturer. Oil pressure
settings can vary from around 35 psi minimum to around 90
psi maximum, depending on the installation. The oil pressure
must be high enough to ensure adequate lubrication of the
engine and accessories at high speeds and power settings.
On the other hand, the pressure must not be too high, since
leakage and damage to the oil system may result. Before any
attempt is made to adjust the oil pressure, the engine must be
at the correct operating temperature and a check should be
made to assure that the correct viscosity oil is being used in
the engine. One example of adjusting the oil pressure is done
by removing a cover nut, loosening a locknut, and turning
the adjusting screw. [Figure 6-27] Turn the adjusting screw
clockwise to increase the pressure, or counterclockwise
to decrease the pressure. Make the pressure adjustments
while the engine is idling and tighten the adjustment screw
lock-nut after each adjustment. Check the oil pressure
reading while the engine is running at the rpm specified in
the manufacturer’s maintenance manual. This may be from
around 1,900 rpm to 2,300 rpm. The oil pressure reading
should be between the limits prescribed by the manufacturer
at all throttle settings.After the screens are removed, they should be inspected
for contamination and for the presence of metal particles
that may indicate possible engine internal wear, damage,
or in extreme cases, engine failure. The screen must be
cleaned prior to reinstalling in the engine. In some cases,
it is necessary to disassemble the filter for inspection and
cleaning. The manufacturer’s procedures should be followed
when disassembling and reassembling an oil screen assembly.
When reinstalling a filter or screen, use new O-rings and
gaskets and tighten the filter housing or cover retaining nuts
6-17Figure 6-27. Oil pressure relief valve adjustment.
IncreaseDecrease
Recommendations for Changing Oil
Draining Oil
Oil, in service, is constantly exposed to many harmful
substances that reduce its ability to protect moving parts.
The main contaminants are:
• Gasoline,
• Moisture,
• Acids,
• Dirt,
• Carbon, and
• Metallic particles.
Because of the accumulation of these harmful substances,
common practice is to drain the entire lubrication system at
regular intervals and refill with new oil. The time between
oil changes varies with each make and model aircraft and
engine combination.
In engines that have been operating on straight mineral oil
for several hundred hours, a change to ashless dispersant
oil should be made with a degree of caution as the cleaning
action of some ashless dispersant oils tends to loosen sludge
deposits and cause plugged oil passages. When an engine has
been operating on straight mineral oil, and is known to be in
excessively dirty condition, the switch to ashless dispersant
oil should be deferred until after the engine is overhauled.
When changing from straight mineral oil to ashless dispersant
oil, the following precautionary steps should be taken:
1. Do not add ashless dispersant oil to straight mineral
oil. Drain the straight mineral oil from the engine and
fill with ashless dispersant oil.
2. Do not operate the engine longer than 5 hours before
the first oil change. 3. Check all oil filters and screens for evidence of sludge
or plugging. Change oil every 10 hours if sludge
conditions are evident. Repeat 10-hour checks until
clean screen is noted, then change oil at recommended
time intervals.
4. All turbocharged engines must be broken in and
operated with ashless dispersant oil.
Oil & Filter Change & Screen Cleaning
One manufacturer recommends that for new, remanufactured;
or newly overhauled engines and for engines with any newly
installed cylinders, the oil should be changed after the first
replacement/screen cleaning at 25 hours. The oil should be
changed, filter replaced, or pressure screen cleaned, and oil
sump suction screen cleaned and inspected. A typical interval
for oil change is 25 hours, along with a pressure screen
cleaning and oil sump suction screen check for all engines
employing a pressure screen system. Typical 50-hour interval
oil changes generally include the oil filter replacement and
suction screen check for all engines using full-flow filtration
systems. A time maximum of 4 months between servicing is
also recommended for oil system service.
Oil Filter Removal Canister Type Housing
Remove the filter housing from the engine by removing the
safety wire and loosening the hex head screw and housing
by turning counterclockwise and removing the filter from
the engine. [Figure 6-7] Remove the nylon nut that holds
the cover plate on the engine side of the filter. Remove the
cover plate, hex head screw from the housing. To remove the
spin-on type of filter, cut the safety wire and use the wrench
pad on the rear of the filter to turn the filter counterclockwise,
and remove filter. Inspect the filter element as described in
the following paragraph. Discard old gaskets and replace
with new replacement kit gaskets.
Oil Filter/Screen Content Inspection
Check for premature or excessive engine component wear
that is indicated by the presence of metal particles, shavings,
or flakes in the oil filter element or screens. The oil filter
can be inspected by opening the filter paper element. Check
the condition of the oil from the filter for signs of metal
contamination. Then, remove the paper element from the
filter and carefully unfold the paper element; examine the
material trapped in the filter. If the engine employs a pressure
screen system, check the screen for metal particles. After
draining the oil, remove the suction screen from the oil sump
and check for metal particles. [Figure 6-28] If examination of
the used oil filter or pressure screen and the oil sump suction
screen indicates abnormal metal content, additional service
may be required to determine the source and possible need for
corrective maintenance. To inspect the spin on filter the can
must be cut open to remove the filter element for inspection.
A B C
Figure 6-28. Oil sump screen.
Figure 6-29. Cutting open a spin-on type oil filter using a special filter cutter. Using the special filter cutting tool, slightly tighten the
cutter blade against filter and rotate 360º until the mounting
plate separates from the can. [Figure 6-29] Using a clean
plastic bucket containing varsol, move the filter to remove
contaminants. Use a clean magnet and check for any ferrous
metal particles in the filter or varsol solution. Then, take the
remaining varsol and pour it through a clean filter or shop
towel. Using a bright light, inspect for any nonferrous metals.
Assembly of & Installation of Oil Filters
After cleaning the parts, installation of the canister or filter
element type filter is accomplished by lightly oiling the new
rubber gaskets and installing a new copper gasket on the
hex head screw. Assemble the hex head screw into the filter
case using the new copper gasket. Install the filter element
and place the cover over the case, then manually thread on
the nylon nut by hand. Install the housing on the engine by
turning it clockwise, then torque and safety it. Spin-on filters
generally have installation instructions on the filter. Place a
coating of engine oil on the rubber gasket, install the filter,
torque and safety it. Always follow the manufacturer’s current
instructions to perform any maintenance.
Troubleshooting Oil Systems
The outline of malfunctions and their remedies listed in
Figure 6-30 can expedite troubleshooting of the lubrication system. The purpose of this section is to present typical
troubles. It is not intended to imply that any of the troubles
are exactly as they may be in a particular aircraft.
Requirements for Turbine Engine Lubricants
There are many requirements for turbine engine lubricating
oils. Due to the absence of reciprocating motion and the
presence of ball and roller bearings (antifriction bearings),
the turbine engine uses a less viscous lubricant. Gas turbine
engine oil must have a high viscosity for good load-carrying
ability but must also be of sufficiently low viscosity to
provide good flowability. It must also be of low volatility
to prevent loss by evaporation at the high altitudes at which
the engines operate. In addition, the oil should not foam and
should be essentially nondestructive to natural or synthetic
rubber seals in the lubricating system. Also, with high-speed
antifriction bearings, the formation of carbons or varnishes
must be held to a minimum. Synthetic oil for turbine engines
are usually supplied in sealed one-quart cans.
The many requirements for lubricating oils are met in the
synthetic oils developed specifically for turbine engines.
Synthetic oil has two principal advantages over petroleum oil.
It has a lower tendency to deposit lacquer and coke (solids
left after solvents have been evaporated) because it does not
evaporate the solvents from the oil at high temperature. Oil
grades used in some turbine engines normally contain thermal
and oxidation preventives, load-carrying additives, and
substances that lower the pour point in addition to synthetic
chemical-base materials. MIL-L-7808, which is a military
specification for turbine oil, is a type I turbine oil. Turbine
synthetic oil has a viscosity of around 5 to 5.5 centistokes
at 210º F that is approved against the military specification
MIL-PRF-23699F. This oil is referred to as type II turbine
oil. Most turbine oils meet this type II specification and are
made with the following characteristics:
1. Vapor phase deposits—carbon deposits formed from
oil mist and vapor contact with hot engine surfaces.
2. Load-carrying ability—provides for heavy loads on
Trouble Isolation Procedure Remedy
Oil line leakage. Check external lines for evidence of oil leakage. Replace or repair defective lines.
Accessory seal leakage. Check for leak at accessories immediately after Replace accessory and/or defective
engine operation. accessory oil seal.
Low grade of oil. Fill tank with proper grade oil.
Failing or failed bearing. Check sump and oil pressure pump screen for Replace engine if metal particles are found.
material particles.
Defective pressure gauge. Check indicator. Replace indicator if defective.
Improper operation of oil Erratic pressure indications either excessively Remove, clean, and inspect relief valve
pressure. high or low. accessory oil seal.
Inadequate oil supply. Check oil quantity. Fill oil tank.
Diluted or contaminated oil. Drain engine and tank; refill tank.
Clogged oil screen. Remove and clean oil screen.
Oil viscosity incorrect. Make sure correct oil is being used. Drain engine and tank; refill tank.
Oil pump pressure relief Check pressure relief valve adjustment. Make correct adjustment on oil pump
valve adjustment incorrect. pressure relief valve.
Defective temperature gauge. Check indicator. Replace indicator if defective.
Inadequate oil supply. Check oil quantity. Fill oil tank.
Diluted or contaminated oil. Drain engine and tank; refill tank.
Obstruction in oil tank. Check tank. Drain oil and remove obstruction.
Clogged oil screen. Remove and clean oil screens.
Obstruction in oil cooler Check cooler for blocked or deformed passages. Replace oil cooler if defective.
passages.
Diluted or contaminated oil. Drain engine and tank; refill tank.
Oil level in tank too high. Check oil quantity. Drain excess oil from tank.Excessive oil consumption1
High or low indicated oil pressure2
High or low indicated oil temperature3
Oil foaming4
Figure 6-30. Oil system troubleshooting procedures.
the bearing systems of turbine engines.
3. Cleanliness—minimum formation of sludge deposits
during severe operation.
4. Bulk stability—resistance to physical or chemical
change resulting from oxidation. Permits long periods
of service operation without significant increase
in viscosity or total acidity, the main indicators of
oxidation.
5. Compatibility—most turbine oil is compatible with
other oils that meet the same military specification.
But, most engine manufacturers do not recommend
the indiscriminate mixing of approved oil brands and this is not a generally accepted practice.
6. Seal Wear—essential for the life of engines with
carbon seals that lubricant properties prevent wear of
the carbon at the carbon seal face.
Turbine Oil Health & Safety Precautions
Under normal conditions, the use of turbine oil presents a
low health risk for humans. Although each person reacts
somewhat differently to exposure, contact with liquids,
vapors, and mist of turbine oil should be minimized.
Information on established limits on exposure to turbine
oil can generally be found in the material safety data
sheets (MSDS). Prolonged breathing of hydrocarbon vapor
6-20concentrations in excess of the prescribed limits may result
in lightheadedness, dizziness, and nausea. If turbine oil is
ingested, call a doctor immediately; identify the product and
how much was ingested. Because of the risk of ingestion,
petroleum products should never be siphoned by mouth.
Prolonged or repeated contact of turbine oil with the skin can
cause irritation and dermatitis. In case of skin contact, wash
the skin thoroughly with soap and warm water. Promptly
remove oil-soaked clothing and wash. If turbine oil contacts
the eyes, flush the eyes with fresh water until the irritation
subsides. Protective clothing, gloves, and eye protection
should be used when handling turbine oil.
During operation, it is possible for the oil to be subjected to
very high temperatures that can break it down and produce a
product of unknown toxicity. If this happens, all precautions
to avoid explosives should be taken. It can also have a
tendency to blister, discolor, or remove paint whenever it
is spilled. Painted surfaces should be wiped clean with a
petroleum solvent after spillage.
Spectrometric Oil Analysis Program
The Spectrometric Oil Analysis Program allows an oil
sample to be analyzed and searched for the presence of
minute metallic elements. Due to oil circulation throughout
an aircraft engine, every lubricant that is in service contains
microscopic particles of metallic elements called wear metals.
As the engine operates over time, the oil picks up very
small particles that stay suspended in the oil. Oil analysis
programs identify and measure these particles in parts per
million (PPM) by weight. The analyzed elements are grouped
into categories, such as wear metals and additives, and their
measurement in PPM provides data that expert analysts can
use as one of many tools to determine the engine’s condition.
An increase in PPM of certain materials can be a sign of
component wear or impending failure of the engine. When
you take a sample, note and record the amount of wear metals.
If the amount of wear metals increases beyond a normal
rate, then the operator can be notified quickly so repair or
a recommendation of a specific maintenance procedure or
inspection can be ordered.
Oil analysis increases safety by identifying an engine problem
before engine failure. It also saves money by finding engine
problems before they become large problems or complete
engine failure. This procedure can be used for both turbine
and reciprocating engines.
Typical Wear Metals & Additives
The following examples of wear metals are associated
with areas of the engine that could be lead to their source.
Identifying the metal can help identify the engine components that are wearing or failing.
• Iron—wear from rings, shafts, gears, valve train,
cylinder walls, and pistons in some engines.
• Chromium—primary sources are chromed parts (such
as rings, liners, etc.) and some coolant additives.
• Nickel—secondary indicator of wear from certain
types of bearings, shafts, valves, and valve guides.
• Aluminum—indicates wear of pistons, rod bearings,
and certain types of bushings.
• Lead—mostly from tetraethyl lead contamination.
• Copper—wear from bearings, rocker arm bushings,
wrist pin bushings, thrust washers, and other bronze
or brass parts, and oil additive or antiseize compound.
• Tin—wear from bearings.
• Silver—wear of bearings that contain silver and, in
some instances, a secondary indicator of oil cooler
problems.
• Titanium—alloy in high-quality steel for gears and
bearings.
• Molybdenum—gear or ring wear and used as an
additive in some oils.
• Phosphorous—antirust agents, spark plugs, and
combustion chamber deposits.
Turbine Engine Lubrication Systems
Both wet- and dry-sump lubrication systems are used in
gas turbine engines. Wet-sump engines store the lubricating
oil in the engine proper, while dry-sump engines utilize an
external tank mounted on the engine or somewhere in the
aircraft structure near the engine, similar to reciprocating
piston engines mentioned earlier.
Turbine engine’s oil systems can also be classified as a
pressure relief system that maintains a somewhat constant
pressure: the full flow type of system, in which the pressure
varies with engine speed, and the total loss system, used in
engines that are for short duration operation (target drones,
missiles, etc.). The most widely used system is the pressure
relief system with the full flow used mostly on large fan-
type engines. One of the main functions of the oil system in
turbine engines is cooling the bearings by carrying the heat
away from the bearing by circulating oil around the bearing.
The exhaust turbine bearing is the most critical lubricating
point in a gas turbine engine because of the high temperature
normally present. In some engines, air cooling is used in
addition to oil cooling the bearing, which supports the turbine.
Air cooling, referred to as secondary air flow, is cooling air
provided by bleed air from the early stages of the compressor.
Return inlet assemblyBaffle
Vent assembly
Vent tubes
Flapper door Swivel assemblyOil outletStrainer assembly
Figure 6-31. Oil tank. This internal air flow has many uses on the inside of the
engine. It is used to cool turbine disc, vanes, and blades.
Also, some turbine wheels may have bleed air flowing over
the turbine disc, which reduces heat radiation to the bearing
surface. Bearing cavities sometimes use compressor air to aid
in cooling the turbine bearing. This bleed air, as it is called,
is usually bled off a compressor stage at a point where air
has enough pressure but has not yet become too warm (as
the air is compressed, it becomes heated).
The use of cooling air substantially reduces the quantity of
oil necessary to provide adequate cooling of the bearings.
Since cooling is a major function of the oil in turbine engines,
the lubricating oil for bearing cooling normally requires an
oil cooler. When an oil cooler is required, usually a greater
quantity of oil is necessary to provide for circulation between
the cooler and engine. To ensure proper temperature, oil
is routed through either air-cooled and/or fuel-cooled oil
coolers. This system is used to also heat (regulate) the fuel
to prevent ice in the fuel.
Turbine Lubrication System Components
The following component descriptions include most found
in the various turbine lubrication systems. However, since
engine oil systems vary somewhat according to engine model
and manufacturer, not all of these components are necessarily
found in any one system.Oil Tank
Although the dry-sump systems use an oil tank that contains
most of the oil supply, a small sump is usually included on the
engine to hold a small supply of oil. It usually contains the
oil pump, the scavenge and pressure inlet strainers, scavenge
return connection, pressure outlet ports, an oil filter, and
mounting bosses for the oil pressure gauge and temperature
bulb connections.
A view of a typical oil tank is shown in Figure 6-31 . It is
designed to furnish a constant supply of oil to the engine
during any aircraft attitude. This is done by a swivel outlet
assembly mounted inside the tank, a horizontal baffle
mounted in the center of the tank, two flapper check valves
mounted on the baffle, and a positive vent system.
The swivel outlet fitting is controlled by a weighted end
that is free to swing below the baffle. The flapper valves
in the baffle are normally open; they close only when the
oil in the bottom of the tank tends to rush to the top of the
tank during decelerations. This traps the oil in the bottom of
the tank where it is picked up by the swivel fitting. A sump
drain is located in the bottom of the tank. The vent system
inside the tank is so arranged that the airspace is vented at
all times even though oil may be forced to the top of the tank
by deceleration of the aircraft.
All oil tanks are provided with expansion space. This allows
Scavenge elementShear sectionRelief valve
Pressure element
From main bearings and coupling assemblyTo accessories caseTo oil filter
From accessories case
From supply Pressure oil Scavenge oil
Figure 6-32. Cutaway view of gear oil pump. expansion of the oil after heat is absorbed from the bearings
and gears and after the oil foams as a result of circulating
through the system. Some tanks also incorporate a deaerator
tray for separating air from the oil returned to the top of the
tank by the scavenger system. Usually these deaerators are
the can type in which oil enters at a tangent. The air released
is carried out through the vent system in the top of the tank.
In most oil tanks, a pressure buildup is desired within the
tank to ensure a positive flow of oil to the oil pump inlet.
This pressure buildup is made possible by running the vent
line through an adjustable check relief valve. The check
relief valve is usually set to relieve at about 4 psi, keeping
positive pressure on the oil pump inlet. If the air temperature
is abnormally low, the oil may be changed to a lighter
grade. Some engines may provide for the installation of an
immersion-type oil heater.
Oil Pump
The oil pump is designed to supply oil under pressure to the
parts of the engine that require lubrication, then circulate the
oil through coolers as needed, and return the oil to the oil
tank. Many oil pumps consist of not only a pressure supply element, but also scavenge elements, such as in a dry-sump
system. However, there are some oil pumps that serve a single
function; that is, they either supply or scavenge the oil. These
pump elements can be located separate from each other and
driven by different shafts from the engine. The numbers
of pumping elements (two gears that pump oil), pressure
and scavenge, depend largely on the type and model of the
engine. Several scavenge oil pump elements can be used
to accommodate the larger capacity of oil and air mix. The
scavenge elements have a greater pumping capacity than the
pressure element to prevent oil from collecting in the bearing
sumps of the engine.
The pumps may be one of several types, each type having
certain advantages and limitations. The two most common
oil pumps are the gear and gerotor, with the gear-type being
the most commonly used. Each of these pumps has several
possible configurations.
The gear-type oil pump has only two elements: one for
pressure oil and one for scavenging. [Figure 6-32] However,
some types of pumps may have several elements: one or more
Figure 6-33. Typical gerotor pumping elements.
Figure 6-34. Turbine oil filter element.
Filter element
Figure 6-35. Turbine oil filter paper element. elements for pressure and two or more for scavenging. The
clearances between the gear teeth and the sides of the pump
wall and plate are critical to maintain the correct output of
the pump.
A regulating (relief) valve in the discharge side of the pump
limits the output pressure of the pump by bypassing oil to the
pump inlet when the outlet pressure exceeds a predetermined
limit. [Figure 6-32] The regulating valve can be adjusted, if
needed, to bring the oil pressure within limits. Also shown
is the shaft shear section that causes the shaft to shear if the
pump gears should seize up and not turn.
The gerotor pump, like the gear pump, usually contains
a single element for oil pressure and several elements for
scavenging oil. Each of the elements, pressure and scavenge,
is almost identical in shape; however, the capacity of
the elements can be controlled by varying the size of the
gerotor elements. For example, the pressure element may
have a pumping capacity of 3.1 gallons per minute (gpm) as
compared to 4.25 gpm capacity for the scavenge elements.
Consequently, the pressure element is smaller since the
elements are all driven by a common shaft. The pressure is
determined by engine rpm with a minimum pressure at idling
speed and maximum pressure at intermediate and maximum
engine speeds.
A typical set of gerotor pumping elements is shown in
Figure 6-33 . Each set of gerotors is separated by a steel plate,
making each set an individual pumping unit consisting of
an inner and an outer element. The small star-shaped inner
element has external lobes that fit within and are matched
with the outer element that has internal lobes. The small
element fits on and is keyed to the pump shaft and acts as a
drive for the outer free-turning element. The outer element fits
within a steel plate having an eccentric bore. In one engine
model, the oil pump has four elements: one for oil feed and
three for scavenge. In some other models, pumps have six
elements: one for feed and five for scavenge. In each case,
the oil flows as long as the engine shaft is turning.Turbine Oil Filters
Filters are an important part of the lubrication system
because they remove foreign particles that may be in the
oil. This is particularly important in gas turbines as very
high engine speeds are attained; the antifriction types of ball
and roller bearings would become damaged quite rapidly
if lubricated with contaminated oil. Also, there are usually
numerous drilled or core passages leading to various points
of lubrication. Since these passages are usually rather small,
they are easily clogged.
There are several types and locations of filters used for
filtering the turbine lubricating oil. The filtering elements
come in a variety of configurations and mesh sizes. Mesh
sizes are measured in microns, which is a linear measurement
equal to one millionth of a meter (a very small opening).
Pressure regulating valve
Figure 6-36. Last-chance filter before spray nozzle. Figure 6-37. Pressure regulating valve. A main oil strainer filter element is shown in Figure 6-34 .
The filtering element interior is made of varying materials
including paper and metal mesh. [Figure 6-35] Oil normally
flows through the filter element from the outside into the
filter body. One type of oil filter uses a replaceable laminated
paper element, while others use a very fine stainless steel
metal mesh of about 25–35 microns.
Most filters are located close to the pressure pump and consist
of a filter body or housing, filter element, a bypass valve, and
a check valve. The filter bypass valve prevents the oil flow
from being stopped if the filter element becomes clogged. The
bypass valve opens whenever a certain pressure is reached.
If this occurs, the filtering action is lost, allowing unfiltered
oil to be pumped to the bearings. However, this prevents the
bearings from receiving no oil at all. In the bypass mode,
many engines have a mechanical indicator that pops out to
indicate the filter is in the bypass mode. This indication is
visual and can only be seen by inspecting the engine directly.
An antidrain check valve is incorporated into the assembly to
prevent the oil in the tank from draining down into the engine
sumps when the engine is not operating. This check valve is
normally spring loaded closed with 4 to 6 psi needed to open it.
The filters generally discussed are used as main oil filters;
that is, they strain the oil as it leaves the pump before being
piped to the various points of lubrication. In addition to
the main oil filters, there are also secondary filters located
throughout the system for various purposes. For instance,
there may be a finger screen filter that is sometimes used
for straining scavenged oil. These screens tend to be large
mesh screens that trap larger contaminants. Also, there are
fine-mesh screens called last chance filters for straining
the oil just before it passes from the spray nozzles onto the
bearing surfaces. [Figure 6-36] These filters are located at
each bearing and help screen out contaminants that could
plug the oil spray nozzle.Oil Pressure Regulating Valve
Most turbine engine oil systems are pressure regulating type
systems that keep the pressure fairly constant. An oil pressure
regulating valve is included in the oil system on the pressure
side of the pressure pump. A regulating valve system controls
the systems pressure to a limited pressure within the system.
It is more of a regulating valve than a relief valve because it
keeps the pressure in the system within certain limits other
than only opening when the absolute maximum pressure of
the system is exceeded.
The regulating valve Figure 6-37 has a valve held against a
seat by a spring. By adjusting the tension (increase) on the
spring, you change the pressure at which the valve opens, and
you also increase the system pressure. A screw pressing on the
spring adjusts the tension on the valve and the system pressure.
Oil Pressure Relief Valve
Some large turbofan oil systems do not have a regulating valve.
The system pressure varies with engine rpm and pump speed.
There is a wide range of pressure in this system. A relief valve
is used to relieve pressure only if it exceeds the maximum limit
for the system. [Figure 6-38] This true relief valve system
is preset to relieve pressure and bypass the oil back to the
inlet side of the oil pump whenever the pressure exceeds the
maximum preset system limit. This relief valve is especially
important when oil coolers are incorporated in the system
since the coolers are easily ruptured because of their thin-wall
construction. Under normal operation, it should never open.
BearingOil nozzle
Oil seal
Figure 6-39. Oil nozzles spray lubricate on bearings. Figure 6-38. Pressure relief valve.
Pressure relief valve
Oil Jets
Oil jets (or nozzles) are located in the pressure lines adjacent
to, or within, the bearing compartments and rotor shaft
couplings. [Figure 6-39] The oil from these nozzles is
delivered in the form of an atomized spray. Some engines use
an air-oil mist spray that is produced by tapping high-pressure
bleed air from the compressor to the oil nozzle outlet. This
method is considered adequate for ball and roller bearings;
however, the solid oil spray method is considered the better
of the two methods.The oil jets are easily clogged because of the small orifice in
their tips; consequently, the oil must be free of any foreign
particles. If the last-chance filters in the oil jets should become
clogged, bearing failure usually results since nozzles are not
accessible for cleaning except during engine maintenance.
To prevent damage from clogged oil jets, main oil filters are
checked frequently for contamination.
Lubrication System Instrumentation
Gauge connection provisions are incorporated in the oil
system for oil pressure, oil quantity, low oil pressure, oil
filter differential pressure switch, and oil temperature. The
oil pressure gauge measures the pressure of the lubricant as
it leaves the pump and enters the pressure system. The oil
pressure transmitter connection is located in the pressure
line between the pump and the various points of lubrication.
An electronic sensor is placed to send a signal to the Full
Authority Digital Engine Control (FADEC) control unit and
through the Engine Indicating and Crew Alerting System
(EICAS) computers, and on to the displays in the flight deck.
[Figure 6-40] The tank quantity transmitter information is
sent to the EICAS computers. The low oil pressure switch
alerts the crew if the oil pressure falls below a certain pressure
during engine operation. The differential oil pressure switch
alerts the flight crew of an impending oil filter bypass because
of a clogged filter. A message is sent to the display in the
upper EICAS display in the flight deck as can be seen in
Figure 6-40 . Oil temperature can be sensed at one or more
points in the engine’s oil flow path. The signal is sent to the
FADEC/EICAS computer and is displayed on the lower
EICAS display.
Lubrication System Breather Systems (Vents)
Breather subsystems are used to remove excess air from the
bearing cavities and return the air to the oil tank where it is
separated from any oil mixed in the vapor of air and oil by
the deaerator. Then, the air is vented overboard and back to
the atmosphere. All engine bearing compartments, oil tanks,
and accessory cases are vented together so the pressure in
the system remains the same.
The vent in an oil tank keeps the pressure within the tank
from rising above or falling below that of the outside
atmosphere. However, the vent may be routed through
a check relief valve that is preset to maintain a slight
(approximately 4 psi) pressure on the oil to assure a positive
flow to the oil pump inlet.
In the accessory case, the vent (or breather) is a screen-
protected opening that allows accumulated air pressure
within the accessory case to escape to the atmosphere. The
scavenged oil carries air into the accessory case and this air
49.7 33.8N1
N2ITT
L oil pressure R oil pressure
L oil filter R oil filterUPPER EICAS DISPLAY
11.0 1.0LOWER EICAS DISPLAY
108 118 49.7 49.7 15 15
L ENG
Oil pressureR ENG
Oil pressure
EICAS
Computers
Oil filter differential pressure switchOil tankOil pressure transmitterLow oil pressure switchFADEC/EEC oil
temperature probe
No. 3 bearing scavenge
oil temperature probe
No. 1, 11/2, and 2
bearing breatherEngine oil pressure
Oil filter in pressureOil filter out pressureOil quantity transmitter
Figure 6-40. Oil indicating system.
Figure 6-41. Typical thermostatic bypass valve. must be vented. Otherwise, the pressure buildup within the
accessory case would stop the flow of oil draining from the
bearing, forcing this oil past the bearing oil seals and into
the compressor housing. If in enough quantity, oil leakage
could cause burning and seal and bearing malfunction. The
screened breathers are usually located in the front center
of the accessory case to prevent oil leakage through the
breather when the aircraft is in unusual flight attitudes.
Some breathers may have a baffle to prevent oil leakage
during flight maneuvers. A vent that leads directly to the
bearing compartment may be used in some engines. This
vent equalizes pressure around the bearing surface so that the
lower pressure at the first compressor stage does not cause oil
to be forced past the bearing rear oil seal into the compressor.
Lubrication System Check Valve
Check valves are sometimes installed in the oil supply lines
of dry-sump oil systems to prevent reservoir oil from seeping
(by gravity) through the oil pump elements and high-pressure
lines into the engine after shutdown. Check valves, by
stopping flow in an opposite direction, prevent accumulations
of undue amounts of oil in the accessory gearbox, compressor
rear housing, and combustion chamber. Such accumulations could cause excessive loading of the accessory drive gears
during starts, contamination of the cabin pressurization
air, or internal oil fires. The check valves are usually the
spring-loaded ball-and-socket type constructed for free flow
of pressure oil. The pressure required to open these valves
Figure 6-42. Air oil cooler.
Oil temperature control valveOil inletFuel outlet
Fuel inletOil return
Figure 6-43. Fuel-oil heat exchanger cooler. varies, but the valves generally require from 2 to 5 psi to
permit oil to flow to the bearings.
Lubrication System Thermostatic Bypass Valves
Thermostatic bypass valves are included in oil systems
using an oil cooler. Although these valves may be called
different names, their purpose is always to maintain proper
oil temperature by varying the proportion of the total oil
flow passing through the oil cooler. A cutaway view of a
typical thermostatic bypass valve is shown in Figure 6-41 .
This valve consists of a valve body, having two inlet ports
and one outlet port, and a spring-loaded thermostatic element
valve. The valve is spring loaded because the pressure drop
through the oil cooler could become too great due to denting
or clogging of the cooler tubing. In such a case, the valve opens, bypassing the oil around the cooler.
Air-Oil Coolers
Two basic types of oil coolers in general use are the air-cooled
and the fuel-cooled. Air-oil coolers are used in the lubricating
systems of some turbine engines to reduce the temperature
of the oil to a degree suitable for recirculation through the
system. The air-cooled oil cooler is normally installed at
the forward end of the engine. It is similar in construction
and operation to the air-cooled cooler used on reciprocating
engines. An air-oil cooler is usually included in a dry-sump
oil system. [Figure 6-42] This cooler may be air-cooled or
fuel-cooled and many engines use both. Dry-sump lubrication
systems require coolers for several reasons. First, air cooling
of bearings by using compressor bleed-air is not sufficient to
cool the turbine bearing cavities because of the heat present
in area of the turbine bearings. Second, the large turbofan
engines normally require a greater number of bearings, which
means that more heat is transferred to the oil. Consequently,
the oil coolers are the only means of dissipating the oil heat.
Fuel-Oil Coolers
The fuel-cooled oil cooler acts as a fuel-oil heat exchanger
in that the fuel cools the hot oil and the oil heats the fuel
for combustion. [Figure 6-43] Fuel flowing to the engine
must pass through the heat exchanger; however, there is a
thermostatic valve that controls the oil flow, and the oil may
bypass the cooler if no cooling is needed. The fuel-oil heat
Return oil
Permanent magnetSelf-sealing housingChip detector
Figure 6-44. Chip detector. exchanger consists of a series of joined tubes with an inlet
and outlet port. The oil enters the inlet port, moves around
the fuel tubes, and goes out the oil outlet port.
Deoiler
The deoiler removes the oil from the breather air. The breather
air goes into an impeller that turns in the deoiler housing.
Centrifugal force drives the oil towards the outer wall of the
impeller. Then, the oil drains from the deoiler into a sump or
oil tank. Because the air is much lighter than the oil, it goes
through the center of the impeller and is vented overboard.
Magnetic Chip Detectors
Magnetic chip detectors are used in the oil system to detect
and catch ferrous (magnetic) particles present in the oil.
[Figure 6-44] Scavenge oil generally flows past chip detectors
so any magnetic particles are attracted and stick to the chip
detector. Chip detectors are placed in several locations but
generally are in the scavenge lines for each scavenge pump,
oil tank, and in the oil sumps. Some engines have several
detectors to one detector. During maintenance, the chip
detectors are removed from the engine and inspected for
metal; if none is found, the detector is cleaned, replaced,
and safety wired. If metal is found on a chip detector, an
investigation should be made to find the source of the metal
on the chip.
Typical Dry-Sump Pressure Regulated
Turbine Lubrication System
The turbine lubrication system is representative of turbine
engines using a dry-sump system. [Figure 6-45] The
lubrication system is a pressure regulated, high-pressure
design. It consists of the pressure, scavenge, and breather
subsystems.
The pressure system supplies oil to the main engine bearings
and to the accessory drives. The scavenger system returns
the oil to the engine oil tank that is usually mounted on the compressor case. It is connected to the inlet side of the
pressure oil pump and completes the oil flow cycle. A breather
system connecting the individual bearing compartments and
the oil tank with the breather pressurizing valve completes
the engine lubrication system. In a turbine pressure relief
dry-sump lubrication system, the oil supply is carried in a
tank mounted on the engine. With this type of system, a larger
oil supply can be carried, and the temperature of the oil can
be readily controlled.
Pressure System
The oil pressure branch of the engine lubrication system
is pressurized by a gear-type pressure pump located in the
oil pump and accessory drive housing. [Figure 6-45] The
pressure pump receives engine oil at its lower (inlet) side
and discharges pressurized oil to an oil filter located on the
housing. From the oil filter, which is equipped with a bypass
valve for operation in case the filter clogs, the pressurized
oil is transmitted to a cored passage running through to the
pressure regulating (relief) valve that maintains system
pressure. The pressure regulating (relief) valve is located
downstream of the pump. It is adjusted to maintain a proper
pressure to the oil metering jets in the engine. The pressure
regulating (relief) valve is usually easily accessible for
adjustment. Then, the oil flows through the fuel-oil cooler and
on to the bearing cavities through last-chance filters and out
spray nozzles to the bearings. Pressurized oil distributed to
the engine main bearings is sprayed on the bearings through
fixed orifice nozzles providing a relatively constant oil flow
at all engine operating speeds.
Scavenge System
The scavenge system scavenges the main bearing
compartments and circulates the scavenged oil back to
the tank. The scavenge oil system includes five gear-type
pumps. [Figure 6-45] The No.1 bearing oil scavenge
pump scavenges accumulated oil from the front bearing
case. It directs the oil through an external line to a
central collecting point in the main accessory gearbox.
The oil return from No. 2 and 3 bearings is through
internal passages to a central collecting point in the main
accessory case. The accessory gearbox oil suction pump,
located in the main accessory gearbox, scavenges oil from
the gearbox housing to the oil tank. Oil from the No. 4, No.
41⁄2 and No. 5 bearing accumulates in the bearing cavity and
is scavenged to the accessory gearbox.
The turbine rear bearing oil suction pump scavenges oil from
the No. 6 bearing compartment and directs the scavenged oil
through a passage in the turbine case strut. From there, it is
directed to the bearing cavity for the 4, 41⁄2, and 5 bearing
cavities where it joins the oil and is returned to the oil tank.
The scavenge oil passes through the deaerator as it enters the
S
CNo. 4No. 4 1/2No. 5 No. 6 No. 2 No. 1 No. 3
C
SS S S
C
CScavenge
pumps
NO. 4 Bearing seal check valveScavenge pumpsTower
shaft
housing
STo low oil pressure switch To oil pressure transmitter
Cooler
bypass
valve
Supply oil
Pressure oil
Sense oil
Bypass oil
Scavenge oil
Breather air
Seal drain
Chip detector
Last-chance oil strainer
Positive-displacement
gear pumpTest portTo oil temperature indicator
Fuel-oil cooler
Deaerator
Oil tank
To oil quantity indicatorPressure
stage
Scavenge stage Oil pump
Filter bypass valveDeoiler
Oil filter
LP5LP6Breather
vent port
To fuel pump
driver spline LP3
Accessory gearbox
Oil pressure
regulating valve
To oil filter P switch
Figure 6-45. Typical turbine dry-sump pressure regulated lubrication system.
oil tank, which separates the air mixed in the return oil. The
oil stays in the tank while the air flows into the accessory
gearbox and enters the deoiler.
Breather Pressurizing System
The breather pressurizing system ensures a proper oil
spray pattern from the main bearing oil jets and furnishes a
pressure head to the scavenge system. Breather tubes in the
compressor inlet case, the oil tank, the diffuser case, and the
turbine exhaust case are connected to external tubing at the
top of the engine. By means of this tubing, the vapor-laden
atmospheres of the various bearing compartments and the
oil tank are brought together in the deoiler in the accessory
gearbox. The deoiler separates out the oil from the air-oil
mist and vents the air back to the atmosphere.
Typical Dry-Sump Variable Pressure
Lubrication System
The dry-sump variable-pressure lubrication system uses
the same basic subsystems that the regulated systems use
(pressure Scavenge breather). [Figure 6-46] The main
difference is that the pressure in this system is not regulated
by a regulating bypass valve. Most large turbofan engine
pressure systems are variable-pressure systems in which the pump outlet pressure (oil pressure) depends on the engine
rpm. In other words, the pump output pressure is proportional
to the engine speed. Since the resistance to flow in the system
does not vary much during operation and the pump has only
the variable of turning faster or slower, the pressure is a
function of engine speed. As an example, oil pressure can
vary widely in this type of system, from 100 psi to over 260
psi, with the relief valve opening at about 540 psi.
Pressure Subsystem
The oil flows from the oil tank down to the pressure stage
of the oil pump. A slight pressure in the tank assures that the
flow of oil into the pressure pump is continuous. After being
pressurized, it moves on to the oil filter where it is filtered.
If the filter is clogged, the bypass valve sends the oil around
the filter. There is no regulating valve but there is a relief
valve to prevent the system pressure from exceeding the
maximum limits. This valve is usually set to open well above
the system’s operating pressure. The oil flows from the filter
housing to the engine air-oil cooler. The oil either bypasses
the cooler (cold) or passes through the cooler (hot) and then
on to the fuel-oil cooler. Through the use of the coolers, the
fuel temperature is adjusted to meet the requirements needed
for the engine. Some of the oil passes through the classified
SS S
S
C CCCC
C
SNo. 3
Bearing
compartmentNo. 1, 1.5 and 2
Bearing
compartmentNo. 4
Bearing
compartment
Angle
gearbox
CSCheck valveNo. 4
bearing
damper oil
Oil quantity
transmitter
Pressure dampenerFiller neck and
flapper valve
Deaerator
Breather air
overboard
Lubrication and
scavenge oil
pumpOil tank
Baffle
Pressure stage
Main gearboxLayshaftDeoiler
Oil temperature portsNo. 3 bearing breather
restrictor valveNo. 2 bearing damper
oil shutoff valveNo. 1, 1.5 and
2 breather pre-
ssure tap
Breather restrictorOil pressure metering nipple
Fuel-oil cooler
outlet manifold
Oil pressure port
IDGS oil to IDG
Pressure
fuel
return
Bypass fuel
from pump
Fuel from pump
boost stage
IDGS oil from IDGs
air-oil heat exchanger
Engine air-oil heat
exchanger and valve
Fan air
or 2.5 airTower
shaftFuel to
fuel
pump
filterServo
return
to oil
tankFuel-oil cooler
and valveSecondary filter
Primary filter
Oil filter switch
Primary
filter
bypass
valve
Pressure
relief
valve
Supply oil
Pressure oil
Return oil
Scavenge oilLast-chance oil filter
Chip detector provisionBreather air
Scavenge and breather
IDGS oil
Interstage fuel
Figure 6-46. Typical turbine dry-sump variable pressure lubrication system.
oil pressure trim orifice that helps adjust oil pressure at low
speeds. The oil now flows through the last-chance oil filters
(strainers) that remove particles from the oil if the oil filter has
been bypassed. The engine oil passes through the nozzles to
lubricate the bearings, gearboxes, seals, and accessory drive
splines. After performing its functions of lubricating, cleaning,
and cooling the bearings, the oil needs to be returned to the
old tank by the scavenge system.
Scavenger Subsystem
The scavenger oil pump has several stages that pull oil from
the bearing compartments and gearboxes and sends the oil
to the tank. At the tank, the oil enters the deaerator, which
separates the air from the scavenge oil. The oil returns to the
tank and the air is vented through a check valve overboard.
Each stage of the scavenge pump has a magnetic chip detector
that can be removed for inspection.
Breather Subsystem
The purpose of the breather system is to remove air from the
bearing compartments, separate breather air from oil, and
vent the air overboard. The breather air from the bearing
compartments is drawn to the gearbox by the deoiler.
The deoiler is turned at high speed and causes the oil to separate from the air. The air is then vented with air from
the deaerator overboard. By referring to Figure 6-46 , notice
that the deaerator is in the oil tank and the deoiler is in the
main gearbox.
Turbine Engine Wet-Sump Lubrication
System
In some engines, the lubrication system is the wet-sump type.
There are relatively few engines using a wet-sump type of oil
system. The components of a wet-sump system are similar to
those of a dry-sump system. The major difference between the
two systems is the location of the oil reservoir. The reservoir
for the wet-sump oil system may be the accessory gear case
or it may be a sump mounted on the bottom of the accessory
case. Regardless of configuration, reservoirs for wet-sump
systems are an integral part of the engine and contain the
bulk of the engine oil supply. [Figure 6-47]
Included in the wet-sump reservoir are the following
components:
1. A sight gauge indicates the oil level in the sump.
2. A vent or breather equalizes pressure within the
accessory casing.
Wet-sumpOil fill capSight gauge
Figure 6-47. Typical turbine wet-sump system.
Oil filter housing
Figure 6-48. Oil filter housing.
3. A magnetic drain plug may be provided to drain the
oil and also to trap any ferrous metal particles in the
oil. This plug should always be examined closely
during inspections. The presence of metal particles
may indicate gear or bearing failure.
4. Provision may also be made for a temperature bulb
and an oil pressure fitting.
This system is typical of all engines using a wet-sump
lubrication system. The bearing and drive gears in the
accessory drive casing are lubricated by a splash system. The
oil for the remaining points of lubrication leaves the pump
under pressure and passes through a filter to jet nozzles that
direct the oil into the rotor bearings and couplings. The oil
is returned to the reservoir (sump) by gravity. Oil from the
compressor bearing and the accessories drive coupling shaft
drains directly into the reservoir. Turbine oil drains into a
sump where the oil was originally pumped.
Turbine Engine Oil System Maintenance
Maintenance of gas turbine lubrication systems consists
mainly of adjusting, removing, cleaning, and replacing
various components. Oil filter maintenance and oil change
intervals for turbine engines vary widely from model to
model, depending on the severity of the oil temperature
conditions imposed by the specific airframe installation
and engine configuration. The applicable manufacturer’s
instructions should be followed. The oil filter should
be removed at every regular inspection. It should be
disassembled, cleaned, and any worn or damaged filter
elements replaced. The following steps illustrate typical oil
filter removal cleaning and replacement procedures:
1. Provide a suitable container for collecting the drained
oil, if needed.2. Remove the filter housing and withdraw the filter
assembly. [Figure 6-48] Discard the old seals.
3. Immerse the screen or filter in an approved carbon
remover at room temperature for a few minutes. Rinse
them in a degreaser fluid or cleaning solvent. Then,
blow them dry with an air jet.
4. Then, install the filter in the filter housing assembly.
Place a new seal and tighten it to the torque prescribed
in the manufacturer’s instructions.
5. Secure with lock wire.
To adjust the oil pressure, first remove the adjusting screw
acorn cap on the oil pressure relief valve. Then, loosen the
locknut and turn the adjusting screw clockwise to increase,
or counterclockwise to decrease, the oil pressure. In a typical
turbojet lubrication system, the adjusting screw is adjusted
to provide an oil pressure of 45, ±5 psi, at approximately
75 percent of normal rated thrust. The adjustment should
be made while the engine is idling; it may be necessary to
perform several adjustments before the desired pressure
is obtained. When the proper pressure setting is achieved,
tighten the adjusting screw locknut, and install the acorn cap
with a new gasket, then tighten and secure with lock wire.
Checking or servicing aircraft engine oil is an important
maintenance function. Before servicing any aircraft engine,
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. Turbine engines
must be checked just after shutdown.
Figure 6-49. Diesel liquid-cooled aircraft engine. Figure 6-50. Cylinder baffle and deflector system.
Deflector
Air baffleMaintenance of scavenge and breather systems at regular
inspections includes checks for oil leaks and security of
mounting of system components. Also, check chip detectors
for particles of ferrous material and clean last-chance filters;
install and safety.
Engine Cooling Systems
Excessive heat is always undesirable in both reciprocating
and turbine aircraft engines. If means were not available
for its control or elimination, major damage or complete
engine failure would occur. Although the vast majority of
reciprocating engines are air cooled, some diesel liquid-
cooled engines are being made available for light aircraft.
[Figure 6-49] In a liquid-cooled engine, around the cylinder
are water jackets, in which liquid coolant is circulated and
the coolant takes away the excess heat. The excess heat is
then dissipated by a heat exchanger or radiator using air flow.
Turbine engines use secondary airflow to cool the inside
components and many of the exterior components.
Reciprocating Engine Cooling Systems
An internal-combustion engine is a heat machine that
converts chemical energy in the fuel into mechanical energy
at the crankshaft. It does not do this without some loss of
energy, however, and even the most efficient aircraft engines
may waste 60 to 70 percent of the original energy in the
fuel. Unless most of this waste heat is rapidly removed,
the cylinders may become hot enough to cause complete
engine failure. Excessive heat is undesirable in any internal-
combustion engine for three principal reasons:
1. It affects the behavior of the combustion of the air-fuel
charge.
2. It weakens and shortens the life of engine parts.
3. It impairs lubrication.
If the temperature inside the engine cylinder is too great,
the air-fuel mixture is preheated, and combustion occurs
before the desired time. Since premature combustion causes detonation, knocking, and other undesirable conditions, there
must be a way to eliminate heat before it causes damage.
One gallon of aviation gasoline has enough heat value to
boil 75 gallons of water; thus, it is easy to see that an engine
that burns 4 gallons of fuel per minute releases a tremendous
amount of heat. About one-fourth of the heat released is
changed into useful power. The remainder of the heat must
be dissipated so that it is not destructive to the engine. In a
typical aircraft powerplant, half of the heat goes out with the
exhaust and the other is absorbed by the engine. Circulating
oil picks up part of this soaked-in heat and transfers it to the
airstream through the oil cooler. The engine cooling system
takes care of the rest. Cooling is a matter of transferring the
excess heat from the cylinders to the air, but there is more
to such a job than just placing the cylinders in the airstream.
A cylinder on a large engine is roughly the size of a gallon
jug. Its outer surface, however, is increased by the use of
cooling fins so that it presents a barrel-sized exterior to the
cooling air. Such an arrangement increases the heat transfer
by convection. If too much of the cooling fin area is broken
off, the cylinder cannot cool properly, and a hotspot develops.
Therefore, cylinders are normally replaced if a specified
number of square inches of fins are missing.
Cowling and baffles are designed to force air over the
cylinder cooling fins. [Figure 6-50] The baffles direct the
air close around the cylinders and prevent it from forming
hot pools of stagnant air while the main streams rush by
unused. Blast tubes are built into the baffles to direct jets of
cooling air onto the rear spark plug elbows of each cylinder
to prevent overheating of ignition leads. Blast tubes also
provide cooling to engine accessories such as alternators,
generators, and starters.
Figure 6-51. Regulating the cooling airflow.
AugmentorExhaust gases Cooling air Heated air
Exhaust gas and cooling air mixture
Figure 6-52. Augmentor. An engine can have an operating temperature that is too low.
For the same reasons that an engine is warmed up before
takeoff, it is kept warm during flight. Fuel evaporation and
distribution and oil circulation depend on an engine being
kept at its optimum operating temperature. The aircraft engine
has temperature controls that regulate air circulation over
the engine. Unless some controls are provided, the engine
could overheat on takeoff and get too cold in high altitude,
high-speed and low-power letdowns.
The most common means of controlling cooling is the use of
cowl flaps. [Figure 6-51] These flaps are opened and closed
by electric motor-driven jackscrews, by hydraulic actuators,
or manually in some light aircraft. When extended for
increased cooling, the cowl flaps produce drag and sacrifice
streamlining for the added cooling. On takeoff, the cowl flaps
are opened only enough to keep the engine below the red-line
temperature. Heating above the normal range is allowed so
that drag is as low as possible. During ground operations, the
cowl flaps should be opened wide since drag does not matter
and cooling needs to be set at maximum. Cowl flaps are used
mostly with older aircraft and radial engine installations.Some aircraft use augmentors to provide additional cooling
airflow. [Figure 6-52] Each nacelle has two pairs of tubes
running from the engine compartment to the rear of the
nacelle. The exhaust collectors feed exhaust gas into the inner
augmentor tubes. The exhaust gas mixes with air that has
passed over the engine and heats it to form a high-temperature,
low-pressure, jet-like exhaust. This low-pressure area in the
augmentors draws additional cooling air over the engine. Air
entering the outer shells of the augmentors is heated through
contact with the augmentor tubes but is not contaminated with
exhaust gases. The heated air from the shell goes to the cabin
heating, defrosting, and anti-icing system.
Augmentors use exhaust gas velocity to cause airflow over
the engine so that cooling is not entirely dependent on the
prop wash. Vanes installed in the augmentors control the
volume of air. These vanes are usually left in the trail position
to permit maximum flow. They can be closed to increase the
heat for cabin or anti-icing use or to prevent the engine from
cooling too much during descent from altitude. In addition to
augmentors, some aircraft have residual heat doors or nacelle
flaps that are used mainly to let the retained heat escape after
engine shutdown. The nacelle flaps can be opened for more
cooling than that provided by the augmentors. A modified
form of the previously described augmentor cooling system
is used on some light aircraft. [Figure 6-53] Augmentor
systems are not used much on modern aircraft.
As shown in Figure 6-53 , the engine is pressure cooled by
air taken in through two openings in the nose cowling, one
on each side of the propeller spinner. A pressure chamber is
sealed off on the top side of the engine with baffles properly
directing the flow of cooling air to all parts of the engine
compartment. Warm air is drawn from the lower part of the
engine compartment by the pumping action of the exhaust
Exhaust stack
Ambient air
Exhaust
Figure 6-53. Engine cooling and exhaust system.
gases through the exhaust ejectors. This type of cooling
system eliminates the use of controllable cowl flaps and
assures adequate engine cooling at all operating speeds.
Reciprocating Engine Cooling System Maintenance
The engine cooling system of most reciprocating engines
usually consists of the engine cowling, cylinder baffles,
cylinder fins, and some use a type of cowl flaps. In addition to
these major units, there are also some temperature-indicating
systems, such as cylinder head temperature, oil temperature,
and exhaust gas temperature.
The cowling performs two functions:
1. It streamlines the bulky engine to reduce drag.
2. It forms an envelope around the engine that forces air
to pass around and between the cylinders, absorbing
the heat dissipated by the cylinder fins.
The cylinder bases are metal shields, designed and arranged
to direct the flow of air evenly around all cylinders. This even
distribution of air aids in preventing one or more cylinders
from being excessively hotter than the others. The cylinder
fins radiate heat from the cylinder walls and heads. As the
air passes over the fins, it absorbs this heat, carries it away from the cylinder, and is exhausted overboard through the
bottom rear of the cowl.
The controllable cowl flaps provide a means of decreasing
or increasing the exit area at the rear of the engine cowling.
[Figure 6-54] Closing the cowl flaps decreases the exit
area, which effectively decreases the amount of air that can
circulate over the cylinder fins. The decreased airflow cannot
carry away as much heat; therefore, it has a tendency for the
engine temperature to increase. Opening the cowl flaps makes
the exit area larger. The flow of cooling air over the cylinders
increases, absorbing more heat and the engine temperature
tends to decrease. Good inspection and maintenance in the
care of the engine cooling system aids in overall efficient
and economical engine operation.
Maintenance of Engine Cowling
Of the total ram airflow approaching the airborne engine
nacelle, only about 15 to 30 percent enters the cowling to
provide engine cooling. The remaining air flows over the
outside of the cowling. Therefore, the external shape of the
cowl must be faired in a manner that permits the air to flow
smoothly over the cowl with a minimum loss of energy.
45Cowl flap control level1
Position bracket2
Cowl flaps4
Right cowl flap control5Left cowl flap control3
Figure 6-54. Small aircraft cowl flaps.
Aircraft supplied cooling baffles
Low pressure areaHigh pressure area
Ram air
Cowl flapsCooling air exit
Figure 6-55. Differential air cooling.
Figure 6-56. A cylinder head and fins. The engine cowling discussed in this section is typical of
that used on many radial or horizontally opposed engines.
All cooling systems function in the same manner, with minor
engineering changes designed for specific installations.
The cowl is manufactured in removable sections, the number
varies with the aircraft make and model. The installation
shown in Figure 6-55 contains two sections that are locked
together when installed.
The cowl panels, made from sheet aluminum or composite
material, have a smooth external surface to permit
undisturbed airflow over the cowl. The internal construction
is designed to give strength to the panel and, in addition, to
provide receptacles for the toggle latches, cowl support, and
engine air seal.
An air seal is constructed of rubber material, bolted to a
metal rib riveted to the cowl panel. [Figure 6-55] This seal,
as the name implies, seals the air in the engine section,
preventing the air from escaping along the inner surface
of the panel without circulating around the cylinders. The
engine air seal must be used on engines that have a complete
cylinder baffling system that covers the cylinder heads. Its
purpose is to force the air to circulate around and through the
baffle system. Inspect the cowl panels during each regular
engine and aircraft inspection. Removing the cowling for
maintenance provides an opportunity for a more detailed
inspection of the cowling.
Inspect the cowling panels for scratches, dents, and tears
in the panels. This type of damage causes weakness of the
panel structure, increases drag by disrupting airflow, and
contributes to the starting of corrosion. The cowling panel
latches should be inspected for pulled rivets and loose or
damaged handles. The internal construction of the panel
should be examined to see that the reinforcing ribs are not
cracked and that the air seal is not damaged. The cowl flap
hinges, if equipped, and cowl flap hinge bondings should be
checked for security of mounting and for breaks or cracks.
These inspections are visual checks and should be performed
frequently to ensure that the cowling is serviceable and is
contributing to efficient engine cooling.
Engine Cylinder Cooling Fin Inspection
The cooling fins are of the utmost importance to the
cooling system, since they provide a means of transferring
the cylinder heat to the air. Their condition can mean the
difference between adequate or inadequate cylinder cooling.
The fins are inspected at each regular inspection. Fin area
is the total area (both sides of the fin) exposed to the air.
During the inspection, the fins should be examined for
cracks and breaks. [Figure 6-56] Small cracks are not a reason for cylinder removal. These cracks can be filled or
even sometimes stop-drilled to prevent any further cracking.
Rough or sharp corners on fins can be smoothed out by
filing, and this action eliminates a possible source of new
cracks. However, before reprofiling cylinder cooling fins,
consult the manufacturer’s service or overhaul manual for
the allowable limits.
The definition of fin area becomes important in the
examination of fins for broken areas. It is a determining
factor for cylinder acceptance or removal. For example, on
a certain engine, if more than 12 inches in length of any one
fin, as measured at its base, is completely broken off, or if
the total fins broken on any one cylinder head exceed 83
square inches of area, the cylinder is removed and replaced.
The reason for removal in this case is that an area of that size
would cause a hot spot on the cylinder; since very little heat
transfer could occur.
Where adjacent fins are broken in the same area, the total
length of breakage permissible is six inches on any two
adjacent fins, four inches on any three adjacent fins, two
inches on any four adjacent fins, and one inch on any five
adjacent fins. If the breakage length in adjacent fins exceeds
this prescribed amount, the cylinder should be removed and
replaced. These breakage specifications are applicable only
to the engine used in this discussion as a typical example.
In each specific case, applicable manufacturer’s instructions
should be consulted.
Cylinder Baffle & Deflector System Inspection
Reciprocating engines use some type of intercylinder and
cylinder head baffles to force the cooling air into close
contact with all parts of the cylinders. Figure 6-50 shows a
baffle and deflector system around a cylinder. The air baffle
DeflectorAir baffleCowl panel air seal
Figure 6-57. Cylinder head baffle and deflector system.
The nut locks the probe in the cylinder head socketThis part of the probe presses against
the cylinder head when installed
Figure 6-58. Bayonet type CHT probe.
Gasket type cylinder head temperature probe
Figure 6-59. Gasket type CHT probe. blocks the flow of air and forces it to circulate between the
cylinder and the deflectors. Figure 6-57 illustrates a baffle
and deflector arrangement designed to cool the cylinder
head. The air baffle prevents the air from passing away
from the cylinder head and forces it to go between the head
and deflector. Although the resistance offered by baffles to
the passage of the cooling air demands that an appreciable
pressure differential be maintained across the engine to obtain
the necessary airflow, the volume of cooling air required is
greatly reduced by employing properly designed and located
cylinder deflectors.
As shown in Figure 6-55 , the airflow approaches the nacelle
and piles up at the top of the engine, creating a high pressure
in the top of the cylinders. This piling up of the air reduces
the air velocity. The outlet at the bottom rear of the cowling
produces a low-pressure area. As the air nears the cowl exit, it
is speeded up again and merges smoothly with the airstream.
The pressure differential between the top and the bottom of the
engine forces the air past the cylinders through the passages
formed by the deflectors. The baffles and deflectors normally
are inspected during the regular engine inspection, but they
should be checked whenever the cowling is removed for any
purpose. Checks should be made for cracks, dents, or loose
hold down studs. Cracks or dents, if severe enough, would
necessitate repair or removal and replacement of these units.
However, a crack that has just started can be stop-drilled, and
dents can be straightened, permitting further service from
these baffles and deflectors.
Cylinder Temperature Indicating Systems
This system usually consists of an indicator, electrical wiring,
and a thermocouple. The wiring is between the instrument
and the nacelle firewall. At the firewall, one end of the
thermocouple leads connects to the electrical wiring, and the
other end of the thermocouple leads connects to the cylinder.
The thermocouple consists of two dissimilar metals, generally
constantan and iron, connected by wiring to an indicating
system. If the temperature of the junction is different from
the temperature where the dissimilar metals are connected
to wires, a voltage is produced. This voltage sends a current
through wires to the indicator, a current-measuring instrument graduated in degrees.
The thermocouple end that connects to the cylinder is either
the bayonet or gasket type. To install the bayonet type, the
knurled nut is pushed down and turned clockwise until it is
snug. [Figure 6-58] In removing this type, the nut is pushed
down and turned counterclockwise until released. The gasket
type fits under the spark plug and replaces the normal spark
plug gasket. [Figure 6-59] When installing a thermocouple
lead, remember not to cut off the lead because it is too long,
but coil and tie up the excess length. The thermocouple is
designed to produce a given amount of resistance. If the
length of the lead is reduced, an incorrect temperature reading
results. The bayonet or gasket of the thermocouple is inserted
or installed on the hottest cylinder of the engine, as determined
in the block test. When the thermocouple is installed and the
wiring connected to the instrument, the indicated reading is the
cylinder temperature. Prior to operating the engine, provided
it is at ambient temperature, the cylinder head temperature
indicator indicates the free outside air temperature; that is one
test for determining that the instrument is working correctly.
The cover glass of the cylinder head temperature indicator
EGT probe mounted in exhaust stack
Figure 6-60. EGT probe in exhaust stack. should be checked regularly to see that it has not slipped or
cracked. The cover glass should be checked for indications
of missing or damaged decals that indicate temperature
limitations. If the thermocouple leads were excessive in length
and had to be coiled and tied down, the tie should be inspected
for security or chafing of the wire. The bayonet or gasket
should be inspected for cleanness and security of mounting.
When operating the engine, all of the electrical connections
should be checked if the cylinder head temperature pointer
fluctuates.
Exhaust Gas Temperature Indicating Systems
The exhaust gas temperature indicator consists of a
thermocouple placed in the exhaust stream just after the
cylinder port. [Figure 6-60] It is then connected to the
instrument in the instrument panel. This allows for the
adjustment of the mixture, which has a large effect on engine
temperature. By using this instrument to set the mixture, the
engine temperature can be controlled and monitored.
Turbine Engine Cooling
The intense heat generated when fuel and air are burned
necessitates that some means of cooling be provided for
all internal combustion engines. Reciprocating engines are
cooled either by passing air over fins attached to the cylinders
or by passing a liquid coolant through jackets that surround
the cylinders. The cooling problem is made easier because
combustion occurs only during every fourth stroke of a four-
stroke-cycle engine.
The burning process in a gas turbine engine is continuous, and
nearly all of the cooling air must be passed through the inside
of the engine. If only enough air were admitted to the engine
to provide an ideal air-fuel ratio of 15:1, internal temperatures
would increase to more than 4,000 °F. In practice, a large
amount of air in excess of the ideal ratio is admitted to the
engine. The large surplus of air cools the hot sections of the
engine to acceptable temperatures ranging from 1,500° to 2,100 °F. Because of the effect of cooling, the temperatures
of the outside of the case are considerably less than those
encountered within the engine. The hottest area occurs in and
around the turbines. Although the gases have begun to cool
a little at this point, the conductivity of the metal in the case
carries the heat directly to the outside skin.
The secondary air passing through the engine cools the
combustion-chamber liners. The liners are constructed to
induce a thin, fast-moving film of air over both the inner
and outer surfaces of the liner. Can-annular-type burners
frequently are provided with a center tube to lead cooling
air into the center of the burner to promote high combustion-
efficiency and rapid dilution of the hot combustion gases
while minimizing pressure losses. In all types of gas turbines,
large amounts of relatively cool air join and mix with the
burned gases aft of the burners to cool the hot gases just
before they enter the turbines.
Cooling-air inlets are frequently provided around the exterior
of the engine to permit the entrance of air to cool the turbine
case, the bearings, and the turbine nozzle. Internal air is
bled from the engine compressor section and is vented to
the bearings and other parts of the engine. Air vented into
or from the engine is ejected into the exhaust stream. When
located on the side of the engine, the case is cooled by outside
air flowing around it. The engine exterior and the engine
nacelle are cooled by passing fan air around the engine and
the nacelle. The engine compartment frequently is divided
into two sections. The forward section is referred to as the
cold section and the aft section (turbine) is referred to as the
hot section. Case drains drain potential leaks overboard to
prevent fluids from building up in the nacelle.
Accessory Zone Cooling
Turbine powerplants can be divided into primary zones
that are isolated from each other by fireproof bulkheads
and seals. The zones are the fan case compartment,
intermediate compressor case compartment, and the core
engine compartment. [Figure 6-61] Calibrated airflows are
supplied to the zones to keep the temperatures around the
engine at levels that are acceptable. The airflow provides for
proper ventilation to prevent a buildup of any harmful vapors.
Zone 1, for example, is around the fan case that contains
the accessory case and the electronic engine control (EEC).
This area is vented by using ram air through an inlet in the
nose cowl and is exhausted through a louvered vent in the
right fan cowling.
If the pressure exceeds a certain limit, a pressure relief door
opens and relieves the pressure. Zone 2 is cooled by fan air
from the upper part of the fan duct and is exhausted at the
lower end back into the fan air stream. This area has both
Turbine case cooling
Zone 2 drainage pipe
Zone 1 ventilation exitRam air
Zone 2 ventilation exitZone 1 pressure relief doorPrecoolerFancase compartment (zone 1) Thrust reverser seal leaks (zone 3 inlet)
Zone 1 ventilation inletZone 2 ventilation inlet IP compressor compartment (zone 2)
Figure 6-61. Accessory zone cooling.
Outer engine compartment
Stainless steel shroud 350° F
Fiberglass
Aluminum foil
Fiberglass
Silver foil
Exhaust duct 900° FExhaust gas 1,000° FCooling air 120° F
Figure 6-62. Typical engine insulation blanket.fuel and oil lines, so removing any unwanted vapors would
be important.
Zone 3 is the area around the high-pressure compressor to
the turbine cases. This zone also contains fuel and oil lines
and other accessories. Air enters from the exhaust of the pre-cooler and other areas and is exhausted from the zone
through the aft edge of the thrust reverser inner wall and the
turbine exhaust sleeve.
6-40Turbine Engine Insulation Blankets
To reduce the temperature of the structure in the vicinity
of the exhaust duct or thrust augmentor (afterburner) and
to eliminate the possibility of fuel or oil coming in contact
with the hot parts of the engine, it is sometimes necessary
to provide insulation on the exhaust duct of gas turbine
engines. The exhaust duct surface temperature runs quite
high. A typical insulation blanket and the temperatures
obtained at various locations are shown in Figure 6-62 . This
blanket contains fiberglass as the low conductance material
and aluminum foil as the radiation shield. The blanket is
suitably covered so that it does not become oil soaked.
Insulation blankets have been used rather extensively on
many installations in which long exhaust is needed. Some
auxiliary power units (APU) mounted in the tail cone of
transport aircraft have air that surrounds the exhaust tail pipe
that provides cooling and protects the surrounding structure.
Leading edgeTrailing edge
Blade tipHub
Propeller informationBack of blade
Figure 7-1. Basic nomenclature of propellers.
Blade back
Blade faceAngle of attack
Relative windChord line
Figure 7-2. Cross-sectional area of a propeller blade airfoil. General
The propeller, the unit that must absorb the power output of
the engine, has passed through many stages of development.
Although most propellers are two-bladed, great increases
in power output have resulted in the development of four-
and six-bladed propellers of large diameters. However, all
propeller-driven aircraft are limited by the revolutions per
minute (rpm) at which propellers can be turned.
There are several forces acting on the propeller as it turns; a
major one is centrifugal force. This force at high rpm tends
to pull the blades out of the hub, so blade weight is very
important to the design of a propeller. Excessive blade tip
speed (rotating the propeller too fast) may result not only in
poor blade efficiency, but also in fluttering and vibration.
Since the propeller speed is limited, the aircraft speed of a
propeller driven aircraft is also limited—to approximately
400 miles per hour (mph). As aircraft speeds increased,
turbofan engines were used for higher speed aircraft.
Propeller-driven aircraft have several advantages and are
widely used for applications in turboprops and reciprocating
engine installations. Takeoff and landing can be shorter and
less expensive. New blade materials and manufacturing
techniques have increased the efficiency of propellers.
Many smaller aircraft will continue to use propellers well
into the future.
The basic nomenclature of the parts of a propeller is shown
in Figure 7-1 for a simple fixed-pitch, two-bladed wood
propeller. The aerodynamic cross-section of a blade in
Figure 7-2 includes terminology to describe certain areas
shown.
Many different types of propeller systems have been
developed for specific aircraft installation, speed, and
mission. Propeller development has encouraged many
Propellers
Chapter 7
changes as propulsion systems have evolved. The first
propellers were fabric-covered sticks made to force air
in a rearward direction. Propellers started as simple two-
bladed wood propellers and have advanced to the complex
propulsion systems of turboprop aircraft that involve more
than just the propeller. As an outgrowth of operating large,
more complex propellers, a variable-pitch, constant-speed
feathering and reversing propeller system was developed.
This system allows the engine rpm to be varied only slightly
during different flight conditions and, therefore, increases
flying efficiency. A basic constant-speed system consists of a
flyweight-equipped governor unit that controls the pitch angle
of the blades so that the engine speed remains constant. The
governor can be regulated by controls in the flight deck so that
any desired blade angle setting and engine operating speed
can be obtained. A low-pitch, high-rpm setting, for example,
can be utilized for takeoff. Then, after the aircraft is airborne,
a higher pitch and lower rpm setting can be used. Figure 7-3
shows normal propeller movement with the positions of low
pitch, high pitch, feather (used if the engine quits to reduce
drag), and zero pitch into negative pitch, or reverse pitch.
Basic Propeller Principles
The aircraft propeller consists of two or more blades and a
central hub to which the blades are attached. Each blade of
Typical propeller blade positions from feather position through the reverse position. Normal sequence of blade
travel is feather, high pitch, low pitch, locks/ground idle, reverse pitch, and then back following the same path. Fixed turboprop engines are shut down on the locks to prevent load on the engine during restart.Reverse Locks Flight idle Ground idle Power Feather
−2 to −8° 1 to 2° 10 to 12° 0° 20 to 35° 85 to 90°
Slip
Effective
pitch
Geometric pitchFigure 7-3. Propeller range positions.
Figure 7-4. Effective pitch and geometric pitch. an aircraft propeller is essentially a rotating wing. As a result
of their construction, the propeller blades produce forces that create thrust to pull or push the aircraft through the air.The power needed to rotate the propeller blades is furnished by the engine. The propeller is mounted on a shaft, which may be an extension of the crankshaft on low-horsepower engines; on high-horsepower engines, it is mounted on a propeller shaft that is geared to the engine crankshaft. In either case, the engine rotates the airfoils of the blades through the air at high speeds, and the propeller transforms the rotary power of the engine into thrust.
Propeller Aerodynamic Process
An aircraft moving through the air creates a drag force opposing its forward motion. If an aircraft is to fly on a level path, there must be a force applied to it that is equal to the drag but acting forward. This force is called thrust. The work done by thrust is equal to the thrust times the distance it moves the aircraft.
Work = Thrust x Distance
The power expended by thrust is equal to the thrust times the
velocity at which it moves the aircraft.
Power = Thrust x Velocity
If the power is measured in horsepower units, the power
expended by the thrust is termed thrust horsepower.
The engine supplies brake horsepower through a rotating
shaft, and the propeller converts it into thrust horsepower. In this conversion, some power is wasted. For maximum efficiency, the propeller must be designed to keep this waste as small as possible. Since the efficiency of any machine is the ratio of the useful power output to the power input, propeller efficiency is the ratio of thrust horsepower to brake horsepower. The usual symbol for propeller efficiency is the Greek letter η (eta). Propeller efficiency varies from 50 percent
to 87 percent, depending on ho w much the propeller slips.
Pitch is not the same as blad e angle, but becau se pitch is
largely determined by blade angle, the two terms are often
used interchangeably. An increase o r decrease in one is usually
associated with an increase or decrease in the other. Propeller
slip is the difference between the geometr ic pitch of the
propeller and its effectiv e pitch. [Figure 7-4] Geometric pitch
is the distance a propeller sh ould advance in one revo lution
with no slippage; effective pitch is the distance it actually
advances. Thus, geometric o r theoretical pitch is based on
no slippage. Actual, or effective, pitch recognizes propeller
slippage in the air. The relatio nship can be shown as:
Geometric pitch - Effective pitch = slip
Geometric pitch is usually ex pressed in pitch inches and
calculated by using the following formula:
GP = 2 x π x R x tangent of blade angle at 75 percent station
R = Radius at the 75 percent blade station
π = 3.14
Although blade angle and propeller pitch are closely related,
blade angle is the angle between the face o r chord of a
blade section and the plane in which the propeller rotates.
7-3Angle of relative windChordlineThrust
Forward velocity
Rotational velocity
blade angle Pitch or
of attack Angle
Relative wind
Figure 7-5. Propeller aerodynamic factors.
Tip section
6"6"
6"
42"6"
6"
6"
6"
6"
6"42" Station
18" Station
12" Station
Blade shank
Blade butt Center of hub
Figure 7-6. Typical propeller blade elements.
Camber of upper surface
Camber of lower surface
Leading edge ChordlineMean camber lineTrailing edge
Figure 7-7. Cross-section of a propeller blade.[Figure 7-5] Blade angle, usually measured in degrees, is
the angle between the chordline of the blade and the plane of
rotation. The chordline of the propeller blade is determined
in about the same manner as the chordline of an airfoil. In
fact, a propeller blade can be considered as being composed
of an infinite number of thin blade elements, each of which
is a miniature airfoil section whose chord is the width of the
propeller blade at that section. Because most propellers have
a flat blade face, the chordline is often drawn along the face
of the propeller blade.
The typical propeller blade can be described as a twisted
airfoil of irregular planform. Two views of a propeller blade
are shown in Figure 7-6 . For purposes of analysis, a blade can
be divided into segments that are located by station numbers
in inches from the center of the blade hub. The cross-sections
of each 6-inch blade segment are shown as airfoils in the
right side of Figure 7-6 . Also identified in Figure 7-6 are the
blade shank and the blade butt. The blade shank is the thick,
rounded portion of the propeller blade near the hub and is
designed to give strength to the blade. The blade butt, also
called the blade base or root, is the end of the blade that fits
in the propeller hub. The blade tip is that part of the propeller
blade farthest from the hub, generally defined as the last 6
inches of the blade.
A cross-section of a typical propeller blade is shown in
Figure 7-7 . This section or blade element is an airfoil
comparable to a cross-section of an aircraft wing. The blade
back is the cambered or curved side of the blade, similar to
the upper surface of an aircraft wing. The blade face is the
flat side of the propeller blade. The chordline is an imaginary
line drawn through the blade from the leading edge to the
trailing edge. The leading edge is the thick edge of the blade
that meets the air as the propeller rotates.
A rotating propeller is acted upon by centrifugal twisting,
aerodynamic twisting, torque bending, and thrust bending
forces. The principal forces acting on a rotating propeller are illustrated in Figure 7-8 .
Centrifugal force is a physical force that tends to
throw the rotating propeller blades away from the hub.
[Figure 7-8A] This is the most dominant force on the
propeller. Torque bending force, in the form of air resistance,
tends to bend the propeller blades in the direction opposite
that of rotation. [Figure 7-8B] Thrust bending force is the
thrust load that tends to bend propeller blades forward as the
aircraft is pulled through the air. [Figure 7-8C] Aerodynamic
twisting force tends to turn the blades to a high blade angle.
[Figure 7-8D] Centrifugal twisting force, being greater than
the aerodynamic twisting force, tends to force the blades
toward a low blade angle.
At least two of these forces acting on the propellers blades
are used to move the blades on a controllable pitch propeller.
Centrifugal twisting force is sometimes used to move the
blades to the low pitch position, while aerodynamic twisting
force is used to move the blades into high pitch. These forces
can be the primary or secondary forces that move the blades
to the new pitch position.
Centrifugal
forceATorque bending
forceBThrust bending
forceCAerodynamic
twisting forceDCentrifugal
twisting forceE
Center of rotation
Center of pressure
Forward velocity
Linear velocity
Thrust
Blade pathChord of sectionB
APropeller
Relative wind
of the bladeBA= angle of attack (angle formed by relative wind and chord)
= pitch or blade angleFigure 7-8. Forces acting on a rotating propeller.
Figure 7-9. Propeller forces. A propeller must be capable of withstanding severe stresses,
which are greater near the hub, caused by centrifugal force and
thrust. The stresses increase in proportion to the rpm. The blade
face is also subjected to tension from the centrifugal force and
additional tension from the bending. For these reasons, nicks or
scratches on the blade may cause very serious consequences.
These could lead to cracks and failure of the blade and are
addressed in the repair section later in this chapter.
A propeller must also be rigid enough to prevent fluttering,
a type of vibration in which the ends of the blade twist back
and forth at high frequency around an axis perpendicular
to the engine crankshaft. Fluttering is accompanied by a
distinctive noise, often mistaken for exhaust noise. The
constant vibration tends to weaken the blade and eventually
causes failure.
Aerodynamic Factors
To understand the action of a propeller, consider first its
motion, which is both rotational and forward. Thus, as shown
by the vectors of propeller forces in Figure 7-9, a section
of a propeller blade moves downward and forward. As far
as the forces are concerned, the result is the same as if the
blade were stationary and the air coming at it from a direction
opposite its path. The angle at which this air (relative wind) strikes the propeller blade is called angle of attack (AOA).
The air deflection produced by this angle causes the dynamic
pressure at the engine side of the propeller blade to be greater
than atmospheric pressure, creating thrust.
The shape of the blade also creates thrust because it is
shaped like a wing. As the air flows past the propeller, the
pressure on one side is less than that on the other. As in a
wing, this difference in pressure produces a reaction force
in the direction of the lesser pressure. The area above a wing
has less pressure, and the force (lift) is upward. The area of
decreased pressure is in front of a propeller which is mounted
in a vertical instead of a horizontal position, and the force
(thrust) is in a forward direction. Aerodynamically, thrust is
the result of the propeller shape and the AOA of the blade.
Another way to consider thrust is in terms of the mass of
air handled. In these terms, thrust is equal to the mass of
air handled multiplied by the slipstream velocity minus
the velocity of the aircraft. Thus, the power expended in
producing thrust depends on the mass of air moved per
second. On the average, thrust constitutes approximately
80 percent of the torque (total horsepower absorbed by
the propeller). The other 20 percent is lost in friction and
slippage. For any speed of rotation, the horsepower absorbed
by the propeller balances the horsepower delivered by
the engine. For any single revolution of the propeller, the
amount of air displaced (moved) depends on the blade angle,
which determines the quantity or amount of mass of air the
propeller moves. Thus, the blade angle is an excellent means
of adjusting the load on the propeller to control the engine
rpm. If the blade angle is increased, more load is placed on the
engine, tending to slow it down unless more power is applied.
As an airfoil is moved through the air, it produces two forces:
lift and drag. Increasing propeller blade angle increases the
AOA and produces more lift and drag; this action increases
the horsepower required to turn the propeller at a given rpm.
Since the engine is still producing the same horsepower, the
propeller slows down. If the blade angle is decreased, the
propeller speeds up. Thus, the engine rpm can be controlled
by increasing or decreasing the blade angle.
Figure 7-10. Turboprop propeller controls. The blade angle is also an excellent method of adjusting the
AOA of the propeller. On constant-speed propellers, the blade
angle must be adjusted to provide the most efficient AOA
at all engine and aircraft speeds. Lift versus drag curves,
which are drawn for propellers as well as wings, indicate
that the most efficient AOA is a small one varying from 2°
to 4° positive. The actual blade angle necessary to maintain
this small AOA varies with the forward speed of the aircraft.
This is due to a change in the relative wind direction, which
varies with aircraft speed.
Fixed-pitch and ground-adjustable propellers are designed
for best efficiency at one rotation and forward speed. In other
words, they are designed to fit a given aircraft and engine
combination. A propeller may be used that provides the
maximum propeller efficiency for takeoff, climb, cruising,
or high speeds. Any change in these conditions results in
lowering the efficiency of both the propeller and the engine.
A constant-speed propeller, however, keeps the blade angle
adjusted for maximum efficiency for most conditions
encountered in flight. During takeoff, when maximum power
and thrust are required, the constant-speed propeller is at a
low propeller blade angle or pitch. The low blade angle keeps
the AOA small and efficient with respect to the relative wind.
At the same time, it allows the propeller to handle a smaller
mass of air per revolution. This light load allows the engine
to turn at high rpm and to convert the maximum amount
of fuel into heat energy in a given time. The high rpm also
creates maximum thrust. Although the mass of air handled
per revolution is small, the engine rpm is high, the slipstream
velocity (air coming off the propeller) is high, and, with the
low aircraft speed, the thrust is maximum.
After liftoff, as the speed of the aircraft increases, the
constant-speed propeller changes to a higher angle (or pitch).
Again, the higher blade angle keeps the AOA small and
efficient with respect to the relative wind. The higher blade
angle increases the mass of air handled per revolution. This
decreases the engine rpm, reducing fuel consumption and
engine wear, and keeps thrust at a maximum.
For climb after takeoff, the power output of the engine is
reduced to climb power by decreasing the manifold pressure
and increasing the blade angle to lower the rpm. Thus, the
torque (horsepower absorbed by the propeller) is reduced to
match the reduced power of the engine. The AOA is again
kept small by the increase in blade angle. The greater mass
of air handled per second, in this case, is more than offset by
the lower slipstream velocity and the increase in airspeed.
At cruising altitude, when the aircraft is in level flight and
less power is required than is used in takeoff or climb, engine power is again reduced by lowering the manifold pressure
and increasing the blade angle to decrease the rpm. Again,
this reduces torque to match the reduced engine power; for,
although the mass of air handled per revolution is greater, it
is more than offset by a decrease in slipstream velocity and
an increase in airspeed. The AOA is still small because the
blade angle has been increased with an increase in airspeed.
Pitch distribution is the twist in the blade from the shank to
the blade tip, due to the variation in speeds that each section
of the blade is traveling. The tip of the blade is traveling much
faster than the inner portion of the blade.
Propeller Controls & Instruments
Fixed pitch propellers have no controls and require no
adjustments in flight. The constant-speed propeller has a
propeller control in the center pedestal between the throttle
and the mixture control. [Figure 7-10] The two positions for
the control are increase rpm (full forward) and decrease rpm
(pulled aft). This control is directly connected to the propeller
governor and, by moving the control, adjusts the tension on
the governor speeder spring. This control can also be used to
feather the propeller in some aircraft by moving the control
to the full decrease rpm position. The two main instruments
used with the constant-speed propeller are the engine
tachometer and the manifold pressure gauge. Rotations per
minute (rpm) is controlled by the propeller control and the
manifold pressure is adjusted by the throttle.
Propeller Location
Tractor Propeller
Tractor propellers are those mounted on the upstream end of
a drive shaft in front of the supporting structure. Most aircraft
are equipped with this type of propeller. The tractor type of
propeller comes in all types of propellers. A major advantage
of the tractor propeller is that lower stresses are induced in
the propeller as it rotates in relatively undisturbed air.
Figure 7-11. Fixed-pitch propeller. Pusher Propellers
Pusher propellers are those mounted on the downstream
end of a drive shaft behind the supporting structure. Pusher
propellers are constructed as fixed- or variable-pitch
propellers. Seaplanes and amphibious aircraft have used a
greater percentage of pusher propellers than other kinds of
aircraft. On land aircraft, where propeller-to-ground clearance
usually is less than propeller-to-water clearance of watercraft,
pusher propellers are subject to more damage than tractor
propellers. Rocks, gravel, and small objects dislodged by
the wheels are quite often thrown or drawn into a pusher
propeller. Similarly, aircraft with pusher propellers are apt
to encounter propeller damage from water spray thrown up
by the hull during landing or takeoff airspeed. Consequently,
the pusher propeller is mounted above and behind the wings
to prevent such damage.
Types of Propellers
There are various types or classes of propellers, the simplest
of which are the fixed-pitch and ground-adjustable propellers.
The complexity of propeller systems increases from these
simpler forms to controllable-pitch and complex constant-
speed systems (automatic systems). Various characteristics
of several propeller types are discussed in the following
paragraphs, but no attempt is made to cover all types of
propellers.
Fixed-Pitch Propeller
As the name implies, a fixed-pitch propeller has the blade
pitch, or blade angle, built into the propeller. [Figure 7-11]
The blade angle cannot be changed after the propeller is
built. Generally, this type of propeller is one piece and is
constructed of wood or aluminum alloy.
Fixed-pitch propellers are designed for best efficiency at
one rotational and forward speed. They are designed to fit a
set of conditions of both aircraft and engine speeds and any
change in these conditions reduces the efficiency of both the
propeller and the engine. The fixed-pitch propeller is used on
aircraft of low power, speed, range, or altitude. Many single-
engine aircraft use fixed-pitch propellers and the advantages
of these are less expense and their simple operation. This
type of propeller does not require any control inputs from
the pilot in flight.
Test Club Propeller
A test club is used to test and break in reciprocating engines.
[Figure 7-12] They are made to provide the correct amount
of load on the engine during the test break-in period. The
multi-blade design also provides extra cooling air flow
during testing.Ground-Adjustable Propeller
The ground-adjustable propeller operates as a fixed-pitch
propeller. The pitch, or blade angle, can be changed only
when the propeller is not turning. This is done by loosening
the clamping mechanism that holds the blades in place. After
the clamping mechanism has been tightened, the pitch of the
blades cannot be changed in flight to meet variable flight
requirements. The ground-adjustable propeller is not often
used on present-day aircraft.
Controllable-Pitch Propeller
The controllable-pitch propeller permits a change of blade
pitch, or angle, while the propeller is rotating. This allows
Figure 7-12. Test club.
Blade bearing support area for each blade
Figure 7-13. Blade bearing areas in hub. the propeller to assume a blade angle that gives the best
performance for particular flight conditions. The number
of pitch positions may be limited, as with a two-position
controllable propeller, or the pitch may be adjusted to any
angle between the minimum and maximum pitch settings of
a given propeller. The use of controllable-pitch propellers
also makes it possible to attain the desired engine rpm for a
particular flight condition.
This type of propeller is not to be confused with a constant-
speed propeller. With the controllable-pitch type, the blade
angle can be changed in flight, but the pilot must change the
propeller blade angle directly. The blade angle will not change
again until the pilot changes it. The use of a governor is the
next step in the evolution of propeller development, making
way for constant-speed propellers with governor systems. An
example of a two-position propeller is a Hamilton Standard
flyweight two-position propeller. These types of propeller
are not in wide use today.
Constant-Speed Propellers
The propeller has a natural tendency to slow down as the
aircraft climbs and to speed up as the aircraft dives because
the load on the engine varies. To provide an efficient propeller,
the speed is kept as constant as possible. By using propeller
governors to increase or decrease propeller pitch, the engine
speed is held constant. When the aircraft goes into a climb, the
blade angle of the propeller decreases just enough to prevent
the engine speed from decreasing. The engine can maintain its
power output if the throttle setting is not changed. When the
aircraft goes into a dive, the blade angle increases sufficiently
to prevent overspeeding and, with the same throttle setting,
the power output remains unchanged. If the throttle setting
is changed instead of changing the speed of the aircraft by
climbing or diving, the blade angle increases or decreases
as required to maintain a constant engine rpm. The power output (not the rpm) changes in accordance with changes in
the throttle setting. The governor-controlled, constant-speed
propeller changes the blade angle automatically, keeping
engine rpm constant.
One type of pitch-changing mechanism is operated by oil
pressure (hydraulically) and uses a piston-and-cylinder
arrangement. The piston may move in the cylinder, or the
cylinder may move over a stationary piston. The linear
motion of the piston is converted by several different types
of mechanical linkage into the rotary motion necessary to
change the blade angle. The mechanical connection may be
through gears, the pitch-changing mechanism that turns the
butt of each blade. Each blade is mounted with a bearing
that allows the blade to rotate to change pitch. [Figure 7-13]
In most cases, the oil pressure for operating the different types
of hydraulic pitch-changing mechanisms comes directly from
the engine lubricating system. When the engine lubricating
system is used, the engine oil pressure is usually boosted
by a pump that is integral with the governor to operate the
propeller. The higher oil pressure (approximately 300 pounds
per square inch (psi)) provides a quicker blade-angle change.
The governors direct the pressurized oil for operation of the
hydraulic pitch-changing mechanisms.
The governors used to control hydraulic pitch-changing
mechanisms are geared to the engine crankshaft and are
sensitive to changes in rpm. When rpm increases above the
value for which a governor is set, the governor causes the
propeller pitch-changing mechanism to turn the blades to a
higher angle. This angle increases the load on the engine,
and rpm decreases. When rpm decreases below the value
for which a governor is set, the governor causes the pitch-
changing mechanism to turn the blades to a lower angle; the
load on the engine is decreased, and rpm increases. Thus, a
propeller governor tends to keep engine rpm constant.
Start lock latch
Figure 7-14. Feathering latches. In constant-speed propeller systems, the control system adjusts
pitch through the use of a governor, without attention by the
pilot, to maintain a specific preset engine rpm within the set
range of the propeller. For example, if engine speed increases,
an overspeed condition occurs and the propeller needs to slow
down. The controls automatically increase the blade angle
until desired rpm has been reestablished. A good constant-
speed control system responds to such small variations of rpm
that for all practical purposes, a constant rpm is maintained.
Each constant-speed propeller has an opposing force
that operates against the oil pressure from the governor.
Flyweights mounted to the blades move the blades in the high
pitch direction as the propeller turns. [Figure 7-13] Other
forces used to move the blades toward the high pitch direction
include air pressure (contained in the front dome), springs,
and aerodynamic twisting moment.
Feathering Propellers
Feathering propellers must be used on multi-engine aircraft to
reduce propeller drag to a minimum under one or more engine
failure conditions. A feathering propeller is a constant-speed
propeller used on multi-engine aircraft that has a mechanism
to change the pitch to an angle of approximately 90°. A
propeller is usually feathered when the engine fails to develop
power to turn the propeller. By rotating the propeller blade
angle parallel to the line of flight, the drag on the aircraft is
greatly reduced. With the blades parallel to the airstream, the
propeller stops turning and minimum windmilling, if any,
occurs. The blades are held in feather by aerodynamic forces.
Almost all small feathering propellers use oil pressure to take
the propeller to low pitch and blade flyweights, springs, and
compressed air to take the blades to high pitch. Since the
blades would go to the feather position during shutdown,
latches lock the propeller in the low pitch position as the
propeller slows down at shutdown. [Figure 7-14] These can
be internal or external and are contained within the propeller
hub. In flight, the latches are prevented from stopping the
blades from feathering because they are held off their seat by
centrifugal force. Latches are needed to prevent excess load
on the engine at start up. If the blade were in the feathered
position during engine start, the engine would be placed
under an undue load during a time when the engine is already
subject to wear.
Reverse-Pitch Propellers
Additional refinements, such as reverse-pitch propellers
(mainly used on turbo props), are included in some propellers
to improve their operational characteristics. Almost all
reverse-pitch propellers are of the feathering type. A reverse-
pitch propeller is a controllable propeller in which the blade
angles can be changed to a negative value during operation. The purpose of the reversible pitch feature is to produce a
negative blade angle that produces thrust opposite the normal
forward direction. Normally, when the landing gear is in
contact with the runway after landing, the propellers blades
can be moved to negative pitch (reversed), which creates thrust
opposite of the aircraft direction and slows the aircraft. As the
propeller blades move into negative pitch, engine power is
applied to increase the negative thrust. This aerodynamically
brakes the aircraft and reduces ground roll after landing.
Reversing the propellers also reduces aircraft speed quickly
on the runway just after touchdown and minimizes aircraft
brake wear.
Propeller Governor
A governor is an engine rpm-sensing device and high-
pressure oil pump. In a constant-speed propeller system, the
governor responds to a change in engine rpm by directing
oil under pressure to the propeller hydraulic cylinder or by
releasing oil from the hydraulic cylinder. The change in oil
volume in the hydraulic cylinder changes the blade angle and
maintains the propeller system rpm. The governor is set for
a specific rpm via the flight deck propeller control, which
compresses or releases the governor speeder spring.
A propeller governor is used to sense propeller and engine
speed and normally provides oil to the propeller for low pitch
position. [Figure 7-15] There are a couple of nonfeathering
propellers that operate opposite to this; they are discussed
later in this chapter. Fundamental forces, some already
discussed, are used to control blade angle variations required
for constant-speed propeller operation. These forces are:
1. Centrifugal twisting moment—a component of the
Drive gear from engineGovernor oil pumpGovernor pilot valveFlyweightSpeeder springAdjusting rack connected to the pilot control
Figure 7-15. Parts of a governor.
centrifugal force acting on a rotating blade that tends
at all times to move the blade into low pitch.
2. Propeller-governor oil on the propeller piston side—
balances the propeller blade flyweights, which moves
the blades toward high pitch.
3. Propeller blade flyweights—always move the blades
toward high pitch.
4. Air pressure against the propeller piston—pushes
toward high pitch.
5. Large springs—push in the direction of high pitch and
feather.
6. Centrifugal twisting force—moves the blades toward
low pitch.
7. Aerodynamic twisting force—moves the blades
toward high pitch.
All of the forces listed are not equal in strength. The most
powerful force is the governor oil pressure acting on the
propeller piston. This piston is connected mechanically to
the blades; as the piston moves, the blades are rotated in
proportion. By removing the oil pressure from the governor,
the other forces can force the oil from the piston chamber and
move the propeller blades in the other direction.
Governor Mechanism
The engine-driven single-acting propeller governor
(constant-speed control) receives oil from the lubricating
system and boosts its pressure to that required to operate
the pitch-changing mechanism. [Figure 7-16] It consists
of a gear pump to increase the pressure of the engine oil,
a pilot valve controlled by flyweights in the governor to control the flow of oil through the governor to and away
from the propeller, and a relief valve system that regulates
the operating oil pressures in the governor. A spring called
the speeder spring opposes the governor flyweight’s ability
to fly outward when turning. The tension on this spring can
be adjusted by the propeller control on the control quadrant.
The tension of the speeder spring sets the maximum rpm of
the engine in the governor mode. As the engine and propeller
rpm is increased at the maximum set point (maximum speed)
of the governor, the governor flyweights overcome the
tension of the speeder spring and move outward. This action
moves the pilot valve in the governor to release oil from the
propeller piston and allows the blade flyweights to increase
blade pitch, which increases the load on the engine, slowing
it down or maintaining the set speed.
In addition to boosting the engine oil pressure to produce one
of the fundamental control forces, the governor maintains the
required balance between control forces by metering to, or
draining from, the propeller piston the exact quantity of oil
necessary to maintain the proper blade angle for constant-
speed operation. The position of the pilot valve, with
respect to the propeller-governor metering port, regulates
the quantity of oil that flows through this port to or from
the propeller.
A speeder spring above the rack opposes the action of the
governor flyweights, which sense propeller speed. If the
flyweights turn faster than the tension on the speeder spring,
they fly out; this is an overspeed condition. To slow the engine
propeller combination down, the blade angle (pitch) must be
increased. Oil is allowed to flow away from the propeller
piston and the flyweights increase the pitch or blade angle
slowing the propeller until it reaches an on-speed condition
where the force on the governor flyweights and the tension
on the speeder spring are balanced. This balance of forces
can be disturbed by the aircraft changing attitude (climb or
dive) or the pilot changing the tension on the speeder spring
with the propeller control on the instrument panel (i.e., if the
pilot selects a different rpm).
Underspeed Condition
When the engine is operating below the rpm set by the pilot
using the flight deck control, the governor is operating in an
underspeed condition. [Figure 7-17] In this condition, the
flyweights tilt inward because there is not enough centrifugal
force on the flyweights to overcome the force of the speeder
spring. The pilot valve, forced down by the speeder spring,
meters oil flow to decrease propeller pitch and raise engine
rpm. If the nose of the aircraft is raised or the blades are moved
to a higher blade angle, this increases the load on the engine
and the propeller tries to slow down. To maintain a constant
speed, the governor senses the decrease in speed and increases
Underspeed On speedOverspeed
and feathering
Drain
PressureRelief valve (set 275 psi)Adjusting worm
Pilot valve plungerMinimum rpm adjustment (feathering valve adjustment)
Lift rodSpeed adjusting control lever
Drive gear shaft
Bypass plug A (part 187824)
Engine oil inletPassage C
Propeller control lineBypass plug BFlyweight headToeFlyweightSpeeder spring
Oil supply tap for unfeathering and reversing
Figure 7-16. Typical governor.
oil flow to the propeller, moving the blades to a lower pitch and
allowing them to maintain the same speed. When the engine
speed starts to drop below the rpm for which the governor is
set, the resulting decrease in centrifugal force exerted by the
flyweights permits the speeder spring to lower the pilot valve (flyweights inward), thereby opening the propeller-governor
metering port. The oil then flows through the valve port and
into the propeller piston causing the blades to move to a lower
pitch (a decrease in load).
Speeder spring
Pilot valvePilot control
Flyweight Flyweight
Speeder spring
Pilot valveFlyweight FlyweightPilot control
Speeder spring
Pilot valvePilot control
Flyweight Flyweight
Figure 7-17. Underspeed condition. Figure 7-18. Overspeed condition.
Figure 7-19. On-speed condition. Overspeed Condition
When the engine is operating above the rpm set by the pilot
using the flight deck control, the governor is operating in
an overspeed condition. [Figure 7-18] In an overspeed
condition, the centrifugal force acting on the flyweights
is greater than the speeder spring force. The flyweights
tilt outward and raise the pilot valve. The pilot valve then
meters oil flow to increase propeller pitch and lower engine
rpm. When the engine speed increases above the rpm for
which the governor is set, note that the flyweights move
outward against the force of the speeder spring, raising the
pilot valve. This opens the propeller-governor metering port,
allowing governor oil flow from the propeller piston allowing
flyweights on the blades to increase pitch and slow the engine.
On-Speed Condition
When the engine is operating at the rpm set by the pilot using
the flight deck control, the governor is operating on speed.
[Figure 7-19] In an on-speed condition, the centrifugal force
acting on the flyweights is balanced by the speeder spring, and
the pilot valve is neither directing oil to nor from the propeller
hydraulic cylinder. In the on-speed condition, the forces of
the governor flyweights and the tension on the speeder spring
are equal; the propeller blades are not moving or changing
pitch. If something happens to unbalance these forces, such
as if the aircraft dives or climbs, or the pilot selects a new
rpm range through the propeller control (changes tension
on the speeder spring), then these forces are unequal and an
underspeed or overspeed condition would result. A change in
rpm comes about in the governing mode by pilot selection
of a new position of the propeller control, which changes
the tension of the governor speeder spring or by the aircraft
changing attitude. The governor, as a speed-sensing device,
causes the propeller to maintain a set rpm regardless of the
aircraft attitude. The speeder spring propeller governing range
is limited to about 200 rpm. Beyond this rpm, the governor
cannot maintain the correct rpm.Governor System Operation
If the engine speed drops below the rpm for which the
governor is set, the rotational force on the engine-driven
governor flyweights becomes less. [Figure 7-17] This allows
the speeder spring to move the pilot valve downward. With
the pilot valve in the downward position, oil from the gear
type pump flows through a passage to the propeller and moves
the cylinder outward. This in turn decreases the blade angle
and permits the engine to return to the on-speed setting. If the
engine speed increases above the rpm for which the governor
is set, the flyweights move against the force of the speeder
spring and raise the pilot valve. This permits the oil in the
propeller to drain out through the governor drive shaft. As
the oil leaves the propeller, the centrifugal force acting on the
flyweights turns the blades to a higher angle, which decreases
the engine rpm. When the engine is exactly at the rpm set
by the governor, the centrifugal reaction of the flyweights
balances the force of the speeder spring, positioning the pilot
valve so that oil is neither supplied to nor drained from the
propeller. With this condition, propeller blade angle does
Hub assembly Laminated wood blade Fabric sheathing Metal tipping
Notch to prevent buckling when forming
Notch to prevent buckling when formingSolder
SolderDrainholes
DrainholesRivets or screws
Rivets or screws
Figure 7-21. Installation of metal sheath and tipping.
Figure 7-20. Fix-pitch wooden propeller assembly. not change. Note that the rpm setting is made by varying the
amount of compression in the speeder spring. Positioning
of the speeder rack is the only action controlled manually.
All others are controlled automatically within the governor.
Propellers Used on General Aviation Aircraft
An increasing number of light aircraft are designed for
operation with governor-regulated, constant-speed propellers.
Significant segments of general aviation aircraft are still
operated with fixed-pitch propellers. Light-sport aircraft
(LSA) use multiblade fixed-pitch composite propellers on up
to medium size turbo prop aircraft with reversing propeller
systems. Larger transport and cargo turbo prop aircraft use
propeller systems with dual or double-acting governors
and differential oil pressure to change pitch. Some types
of propeller systems are beyond the scope of this text, but
several propellers and their systems are described.
Fixed-Pitch Wooden Propellers
Although many of the wood propellers were used on older
aircraft, some are still in use. The construction of a fixed-
pitch, wooden propeller is such that its blade pitch cannot be
changed after manufacture. [Figure 7-20] The choice of the
blade angle is decided by the normal use of the propeller on
an aircraft during level flight when the engine performs at
maximum efficiency. The impossibility of changing the blade
pitch on the fixed-pitch propeller restricts its use to small
aircraft with low horsepower engines in which maximum
engine efficiency during all flight conditions is of lesser
importance than in larger aircraft. The wooden, fixed-pitch
propeller is well suited for such small aircraft because of its
light weight, rigidity, economy of production, simplicity of
construction, and ease of replacement.
A wooden propeller is not constructed from a solid block
but is built up of a number of separate layers of carefully
selected and well-seasoned hardwoods. Many woods, such
as mahogany, cherry, black walnut, and oak, are used to
some extent, but birch is the most widely used. Five to nine
separate layers are used, each about 3⁄4 inch thick. The several
layers are glued together with a waterproof, resinous glue and allowed to set. The blank is then roughed to the approximate
shape and size of the finished product. The roughed-out
propeller is then allowed to dry for approximately one week to
permit the moisture content of the layers to become equalized.
This additional period of seasoning prevents warping and
cracking that might occur if the blank were immediately
carved. Following this period, the propeller is carefully
constructed. Templates and bench protractors are used to
assure the proper contour and blade angle at all stations.
After the propeller blades are finished, a fabric covering is
cemented to the outer 12 or 15 inches of each finished blade.
A metal tipping is fastened to most of the leading edge and tip
of each blade to protect the propeller from damage caused by
flying particles in the air during landing, taxiing, or takeoff.
[Figure 7-21] Metal tipping may be of terneplate, Monel
metal, or brass. Stainless steel has been used to some extent.
It is secured to the leading edge of the blade by countersunk
wood screws and rivets. The heads of the screws are soldered
to the tipping to prevent loosening, and the solder is filed
Flange plate Faceplate
Figure 7-22. Hub assembly. to make a smooth surface. Since moisture condenses on
the tipping between the metal and the wood, the tipping is
provided with small holes near the blade tip to allow this
moisture to drain away or be thrown out by centrifugal force.
It is important that these drain holes be kept open at all times.
Since wood is subject to swelling, shrinking, and warping
because of changes of moisture content, a protective coating
is applied to the finished propeller to prevent a rapid change
of moisture content. The finish most commonly used is
a number of coats of water-repellent, clear varnish. After
these processes are completed, the propeller is mounted on
a spindle and very carefully balanced.
Several types of hubs are used to mount wooden propellers
on the engine crankshaft. The propeller may have a forged
steel hub that fits a splined crankshaft; it may be connected
to a tapered crankshaft by a tapered, forged steel hub; or it
may be bolted to a steel flange forged on the crankshaft. In
any case, several attaching parts are required to mount the
propeller on the shaft properly.
Hubs fitting a tapered shaft are usually held in place by a
retaining nut that screws onto the end of the shaft. On one
model, a locknut is used to safety the retaining nut and to
provide a puller for removing the propeller from the shaft.
This nut screws into the hub and against the retaining nut.
The locknut and the retaining nut are safetied together with
lock-wire or a cotter pin.
Front and rear cones may be used to seat the propeller
properly on a splined shaft. The rear cone is a one-piece
bronze cone that fits around the shaft and against the thrust
nut (or spacer) and seats in the rear-cone seat of the hub.
The front cone is a two-piece, split-type steel cone that has
a groove around its inner circumference so that it can be
fitted over a flange of the propeller retaining nut. Then, the
retaining nut is threaded into place and the front cone seats
in the front cone hub. A snap ring is fitted into a groove in
the hub in front of the front cone so that when the retaining
nut is unscrewed from the propeller shaft, the front cone acts
against the snap ring and pulls the propeller from the shaft.
One type of hub incorporates a bronze bushing instead of a
front cone. When this type of hub is used, it may be necessary
to use a puller to start the propeller from the shaft. A rear-
cone spacer is sometimes provided with the splined-shaft
propeller assembly to prevent the propeller from interfering
with the engine cowling. The wide flange on the rear face of
some types of hubs eliminates the use of a rear-cone spacer.
One type of hub assembly for the fixed-pitch, wooden
propeller is a steel fitting inserted in the propeller to mount it
on the propeller shaft. It has two main parts: the faceplate and the flange plate. [Figure 7-22] The faceplate is a steel disc that
forms the forward face of the hub. The flange plate is a steel
flange with an internal bore splined to receive the propeller
shaft. The end of the flange plate opposite the flange disc is
externally splined to receive the faceplate; the faceplate bore
has splines to match these external splines. Both faceplate and
flange plates have a corresponding series of holes drilled on
the disc surface concentric with the hub center. The bore of
the flange plate has a 15° cone seat on the rear end and a 30°
cone seat on the forward end to center the hub accurately on
the propeller shaft.
Metal Fixed-Pitch Propellers
Metal fixed-pitch propellers are similar in general appearance
to a wooden propeller, except that the sections are usually
thinner. The metal fixed-pitch propeller is widely used on
many models of light aircraft and LSA. Many of the earliest
metal propellers were manufactured in one piece of forged
Duralumin. Compared to wooden propellers, they were
lighter in weight because of elimination of blade-clamping
devices, offered a lower maintenance cost because they were
made in one piece, provided more efficient cooling because
of the effective pitch nearer the hub, and, because there was
no joint between the blades and the hub, the propeller pitch
could be changed, within limits, by twisting the blade slightly
by a propeller repair station.
Propellers of this type are now manufactured as one-piece
anodized aluminum alloy. They are identified by stamping the
propeller hub with the serial number, model number, Federal
Aviation Administration (FAA) type certificate number,
production certificate number, and the number of times the
propeller has been reconditioned. The complete model number
of the propeller is a combination of the basic model number
and suffix numbers to indicate the propeller diameter and
pitch. An explanation of a complete model number, using the
McCauley 1B90/CM propeller, is provided in Figure 7-23.
7-14Basic model number
1B90/CM 71 44
Inches pitch at 0.75 radius.
Propeller diameter, inches.
CF denotes installation on SAE
No. 1 flanged shaft; elliptical blade tips.
CH denotes assembly with Continental
Motors Corp. hub; elliptical tips.
CM denotes installation on SAE
No. 1 flanged shaft; square blade tips.
LF denotes installation on SAE
No. 2 flanged shaft with McCauley
C-1210 adapter; elliptical blade tips.
LM denotes installation on SAE
No. 2 flanged shaft with McCauley
C-1210 adapter; square blade tips.
Basic design number (planform, etc.).
Mounting stud (K or R flange shown)
Shaft O-ringEngine flange
OilSpinner adapter (if required)
Hub
Low pitch stop
Pitch change rodSpinner domeStarter gear (Lycoming engine)
Balance weightsBlade
Lubrication fittingForkBlade retension bearing
Spring Piston
Cylinder (oil
pressure to increase
blade angle and
reduce rpm)Figure 7-23. Complete propeller model numbers.
Figure 7-24. Constant speed non-feathering propeller. Constant-Speed Propellers
Hartzell Constant-Speed, Nonfeathering
Hartzell propellers can be divided by Aluminum hub
(compact) and steel hub. Hartzell compact aluminum
propellers represent new concepts in basic design. They
combine low weight and simplicity in design and rugged
construction. In order to achieve these ends, the hub is made
as compact as possible, utilizing aluminum alloy forgings for
most of the parts. The hub shell is made in two halves, bolted
together along the plane of rotation. This hub shell carries the pitch change mechanism and blade roots internally. The
hydraulic cylinder, which provides power for changing the
pitch, is mounted at the front of the hub. The propeller can be
installed only on engines with flanged mounting provisions.
One model of nonfeathering aluminum hub constant-speed
propeller utilizes oil pressure from a governor to move the
blades into high pitch (reduced rpm). The centrifugal twisting
moment of the blades tends to move them into low pitch
(high rpm) in the absence of governor oil pressure. This is an
exception to most of the aluminum hub models and feathering
models. Most of the Hartzell propeller aluminum and steel
hub models use centrifugal force acting on blade flyweights to
increase blade pitch and governor oil pressure for low pitch.
Many types of light aircraft use governor-regulated, constant-
speed propellers in two-bladed and up to six-bladed versions.
These propellers may be the nonfeathering type, or they may
be capable of feathering and reversing. The steel hub contains
a central “spider,” that supports aluminum blades with a
tube extending inside the blade roots. Blade clamps connect
the blade shanks with blade retention bearings. A hydraulic
cylinder is mounted on the rotational axis connected to the
blade clamps for pitch actuation. [Figure 7-24]
The basic hub and blade retention is common to all models
described. The blades are mounted on the hub spider for
angular adjustment. The centrifugal force of the blades,
amounting to as much as 25 tons, is transmitted to the hub
Mounting nut (F flange shown)Shaft O-ringEngine flangeOilSpinner bulkheadHub
Low pitch stop
Removable spinner cap
Spinner domePitch change rod
Lubrication fitting
Balance weightsBlade
FlyweightForkBlade retentention bearing
SpringStart lock latch
Piston
Air charge
Cylinder (oil
pressure to reduce
blade angle and
increase RPM)
Figure 7-25. Constant-speed feathering propeller. spider through blade clamps and then through ball bearings.
The propeller thrust and engine torque is transmitted from the
blades to the hub spider through a bushing inside the blade
shank. In order to control the pitch of the blades, a hydraulic
piston-cylinder element is mounted on the front of the hub
spider. The piston is attached to the blade clamps by means
of a sliding rod and fork system for nonfeathering models
and a link system for the feathering models. The piston is
actuated in the forward direction by means of oil pressure
supplied by a governor, which overcomes the opposing force
created by the flyweights. Hartzell and McCauley propellers
for light aircraft are similar in operation. The manufacturer’s
specifications and instructions must be consulted for
information on specific models.
Constant-Speed Feathering Propeller
The feathering propeller utilizes a single oil supply from a
governing device to hydraulically actuate a change in blade
angle. [Figure 7-25] This propeller has five blades and is
used primarily on Pratt & Whitney turbine engines. A two
piece aluminum hub retains each propeller blade on a thrust
bearing. A cylinder is attached to the hub and contains a
feathering spring and piston. The hydraulically actuated
piston transmits linear motion through a pitch change rod and
fork to each blade to result in blade angle change.
While the propeller is operating, the following forces are
constantly present: 1) spring force, 2) flyweight force, 3) centrifugal twisting moment of each blade, and 4) blade
aerodynamic twisting forces. The spring and flyweight forces
attempt to rotate the blades to higher blade angle, while
the centrifugal twisting moment of each blade is generally
toward lower blade angle. Blade aerodynamic twisting force
is usually very small in relation to the other forces and can
attempt to increase or decrease blade angle. The summation
of the propeller forces is toward higher pitch (low rpm) and
is opposed by a variable force toward lower pitch (high rpm).
The variable force is oil under pressure from a governor with
an internal pump that is mounted on and driven by the engine.
The oil from the governor is supplied to the propeller and
hydraulic piston through a hollow engine shaft. Increasing
the volume of oil within the piston and cylinder decreases
the blade angle and increases propeller rpm. If governor-
supplied oil is lost during operation, the propeller increases
pitch and feather. Feathering occurs because the summation
of internal propeller forces causes the oil to drain out of the
propeller until the feather stop position is reached. Normal
in-flight feathering is accomplished when the pilot retards the
propeller condition lever past the feather detent. This permits
control oil to drain from the propeller and return to the engine
sump. Engine shutdown is normally accomplished during
the feathering process. Normal in-flight unfeathering is
accomplished when the pilot positions the propeller condition
lever into the normal flight (governing) range and restarts the
engine. As engine speed increases, the governor supplies oil
7-16to the propeller and the blade angle decreases. Decreasing the
volume of oil increases blade angle and decreases propeller
rpm. By changing blade angle, the governor can vary the
load on the engine and maintain constant engine rpm (within
limits), independent of where the power lever is set. The
governor uses engine speed sensing mechanisms that permit
it to supply or drain oil as necessary to maintain constant
engine speed (rpm). Most of the steel hub Hartzell propellers
and many of the aluminum hub are full feathering. These
feathering propellers operate similarly to the nonfeathering
ones except the feathering spring assists the flyweights to
increase the pitch. This propeller is normally placed in the full
high pitch position before the engine is shut down to prevent
exposure and corrosion of the pitch changing mechanism.
Feathering is accomplished by releasing the governor oil
pressure, allowing the flyweights and feathering spring to
feather the blades. This is done by pulling the condition lever
(pitch control) back to the limit of its travel, which opens up
a port in the governor allowing the oil from the propeller
to drain back into the engine. Feathering occurs because
the summation of internal propeller forces causes the oil to
drain out of the propeller until the feather stop position is
reached. The time necessary to feather depends upon the size
of the oil passage from the propeller to the engine, and the
force exerted by the spring and flyweights. The larger the
passage is through the governor and the heavier the spring,
the quicker the feathering action is. An elapsed time for
feathering of between 3 and 10 seconds is usual with this
system. Engine shutdown is normally accomplished during
the feathering process.
In order to prevent the feathering spring and flyweights from
feathering the propeller when the engine is shut down and the
engine stopped, automatically removable high-pitch stops
were incorporated in the design. These consist of spring-
loaded latches fastened to the stationary hub that engage
high-pitch stop plates bolted to the movable blade clamps.
When the propeller is in rotation at speeds over 600–800 rpm,
centrifugal force acts to disengage the latches from the high-
pitch stop plates so that the propeller pitch may be increased
to the feathering position. At lower rpm, or when the engine
is stopped, the latch springs engage the latches with the high-
pitch stops, preventing the pitch from increasing further due to
the action of the feathering spring. As mentioned earlier, the
engine load would be excessive, especially on fixed-turbine
turboprop engines. One safety feature inherent in this method
of feathering is that the propeller feathers if the governor oil
pressure drops to zero for any reason. As the governor obtains
its supply of oil from the engine lubricating system, it follows
that if the engine runs out of oil or if oil pressure fails due
to breakage of a part of the engine, the propeller feathers
automatically. This action may save the engine from further damage in case the pilot is not aware of trouble.
Unfeathering
Unfeathering can be accomplished by any of several methods,
as follows:
1. Start the engine, so the governor can pump oil back
into the propeller to reduce pitch. In most light twins,
this procedure is considered adequate since the
feathering of the propeller would happen infrequently.
Vibration can occur when the engine starts and the
propeller starts to come out of feather.
2. Provide an accumulator connected to the governor
with a valve to trap an air-oil charge when the propeller
is feathered but released to the propeller when the rpm
control is returned to normal position. This system is
used with training aircraft because it unfeathers the
propeller in a very short time and starts the engine
windmilling.
3. Provide an unfeathering pump that provides pressure
to force the propeller back to low pitch quickly using
engine oil.
Normal in-flight unfeathering is accomplished when the pilot
positions the propeller condition lever into the normal flight
(governing) range. [Figure 7-26] This causes the governor to
disconnect the propeller oil supply from drain and reconnects
it to the governed oil supply line from the governor. At that
point, there is no oil available from the engine oil pump to
the governor; therefore, no governed oil is available from the
governor for controlling the propeller blade angle and rpm.
As the engine is started, its speed increases, the governor
supplies oil to the propeller, and the blade angle decreases.
As soon as the engine is operating, the governor starts to
unfeather the blades. Soon, windmilling takes place, which
speeds up the process of unfeathering.
In general, restarting and unfeathering of propellers can
be classified as reciprocating engine restart unfeathering,
turboprop engine restart unfeathering, and accumulator
unfeathering. When reciprocating unfeathering is used, the
engine takes a little longer to start turning enough to provide
oil pressure to the governor and then to the propeller. This
delay can cause vibration as the propeller is unfeathered.
Many aircraft can use an accumulator to provide stored
pressure to unfeather the propeller much quicker.
Special unfeathering systems are available for certain aircraft
where restarting the engine is difficult or for training purposes.
The system consists of an oil accumulator connected to the
governor through a valve. [Figure 7-26] The air or nitrogen
pressure in one side of the accumulator pushes a piston to
force oil from the other side of the accumulator through
Oil drain
Governor to accumulator connectionPropeller control oil Engine oil
PropellerChange valveAccumulator
Air or nitrogen
Piston
Oil
ENGINE
Governor
Figure 7-26. Unfeathering system.
the governor to the propeller piston to move the propeller
blades from feather to a lower blade angle. The propeller
then begins to windmill and permits the engine to start. When
the unfeathering pump is used, it is an additional pump that,
once the propeller control is in the correct position, the (full
increase rpm) pump is actuated and the oil pressure from the
pump unfeathers the propeller.
Propeller Auxiliary Systems
Ice Control Systems
Ice formation on a propeller blade, in effect, produces a
distorted blade airfoil section that causes a loss in propeller
efficiency. Generally, ice collects asymmetrically on a
propeller blade and produces propeller unbalance and
destructive vibration and increases the weight of the blades.
Anti-Icing Systems
A typical fluid system includes a tank to hold a supply of
anti-icing fluid. [Figure 7-27] This fluid is forced to each
propeller by a pump. The control system permits variation
in the pumping rate so that the quantity of fluid delivered to
a propeller can be varied, depending on the severity of icing. Fluid is transferred from a stationary nozzle on the engine
nose case into a circular U-shaped channel (slinger ring)
mounted on the rear of the propeller assembly. The fluid
under pressure of centrifugal force is transferred through
nozzles to each blade shank.
Because airflow around a blade shank tends to disperse
anti-icing fluids to areas where ice does not collect in large
quantities, feed shoes, or boots, are installed on the blade
leading edge. These feed shoes are a narrow strip of rubber
extending from the blade shank to a blade station that is
approximately 75 percent of the propeller radius. The feed
shoes are molded with several parallel open channels in
which fluid flows from the blade shank toward the blade
tip by centrifugal force. The fluid flows laterally from the
channels over the leading edge of the blade.
Isopropyl alcohol is used in some anti-icing systems because
of its availability and low cost. Phosphate compounds are
comparable to isopropyl alcohol in anti-icing performance
and have the advantage of reduced flammability. However,
phosphate compounds are comparatively expensive
and, consequently, are not widely used. This system has
RheostatFluid pumpVent lineFluid tank
Slinger ring
Fluid feed shoe
Control switchFuselage busLoadmeter
Control panel
Control circuit breakerNacelle busShunt
Power relayPower relay
Timer or
cycling unitTo other propellersTo other pair of bladesDeicing power lines
Internal or external heating elementsDeicing power circuit breaker or limiterFigure 7-27. Typical propeller fluid anti-icing system.
Figure 7-28. Typical electrical deicing system. disadvantages in that it requires several components that add
weight to the aircraft, and the time of anti-ice available is
limited to the amount of fluid on board. This system is not used
on modern aircraft, giving way to the electric deicing systems.
Deicing Systems
An electric propeller-icing control system consists of an
electrical energy source, a resistance heating element, system
controls, and necessary wiring. [Figure 7-28] The heating
elements are mounted internally or externally on the propeller
spinner and blades. Electrical power from the aircraft system
is transferred to the propeller hub through electrical leads, which terminate in slip rings and brushes. Flexible connectors
are used to transfer power from the hub to the blade elements.
A deice system consists of one or more on-off switches.
The pilot controls the operation of the deice system by
turning on one or more switches. All deice systems have a
master switch and may have another toggle switch for each
propeller. Some systems may also have a selector switch
to adjust for light or heavy icing conditions or automatic
switching for icing conditions.
The timer or cycling unit determines the sequence of which
blades (or portion thereof) are currently being deiced, and for
what length of time. The cycling unit applies power to each
deice boot, or boot segment, in a sequence or all on order.
A brush block, which is normally mounted on the engine
just behind the propeller, is used to transfer electricity to
the slip ring. A slip ring and brush block assembly is shown
in Figure 7-29 . The slip ring rotates with the propeller and
provides a current path to the blade deice boots. A slip ring
wire harness is used on some hub installations to electrically
connect the slip ring to the terminal strip connection screw.
A deice wire harness is used to electrically connect the deice
boot to the slip ring assembly.
A deice boot contains internal heating elements or dual
elements. [Figure 7-30] The boot is securely attached to the
leading edge of each blade with adhesive.
BrushesBrush block
Slip rings
Figure 7-29. Deicing brush block and slip ring assembly.
Deice boot
Figure 7-30. Electric deice boot.
DC bus
Syncrophasing controller
DC power input to controller
Output to left governor
Left engine input to controller
Right engine input to controller
Output to right governorPropeller spinner
Propeller governor
Propeller synchronization
OFFRPM sensor RPM sensor
Synchroscope
Figure 7-31. Synchrophasing system. Icing control is accomplished by converting electrical energy
to heat energy in the heating element. Balanced ice removal
from all blades must be obtained as nearly as possible if
excessive vibration is to be avoided. To obtain balanced ice
removal, variation of heating current in the blade elements
is controlled so that similar heating effects are obtained in
opposite blades.
Electric deicing systems are usually designed for intermittent
application of power to the heating elements to remove ice
after formation but before excessive accumulation. Proper
control of heating intervals aids in preventing runback,
since heat is applied just long enough to melt the ice face in
contact with the blade. If heat supplied to an icing surface
is more than that required for melting just the inner ice face,
but insufficient to evaporate all the water formed, water will
run back over the unheated surface and freeze. Runback of
this nature causes ice formation on uncontrolled icing areas
of the blade or surface.Cycling timers are used to energize the heating element
circuits for periods of 15 to 30 seconds, with a complete cycle
time of 2 minutes. A cycling timer is an electric motor driven
contactor that controls power contactors in separate sections
of the circuit. Controls for propeller electrical deicing systems
include on-off switches, ammeters or loadmeters to indicate
current in the circuits, and protective devices, such as current
limiters or circuit breakers. The ammeters or loadmeters
permit monitoring of individual circuit currents and reflect
operation of the timer. To prevent element overheating, the
propeller deicing system is used only when the propellers are
rotating and for short test periods of time during the takeoff
check list or system inspection.
Propeller Synchronization & Synchrophasing
Most multi-engine aircraft are equipped with propeller
synchronization systems. Synchronization systems provide
a means of controlling and synchronizing engine rpm.
Synchronization reduces vibration and eliminates the
unpleasant beat produced by unsynchronized propeller
operation. The synchrophasing system is designed to
maintain a preset angular relationship between the designated
master propeller and the slave propellers.
A typical synchrophasing system is an electronic system.
[Figure 7-31] It functions to match the rpm of both engines
and establish a blade phase relationship between the left
and right propellers to reduce cabin noise. The system is
controlled by a two-position switch located forward of the
throttle quadrant. Turning the control switch on supplies
direct current (DC) power to the electronic control box.
Input signals representing propeller rpm are received from
magnetic pickup on each propeller. The computed input
signals are corrected to a command signal and sent to a rpm
trimming coil located on the propeller governor of the slow
engine. Its rpm is adjusted to that of the other propeller.
7-20Autofeathering System
An autofeather system is used normally only during takeoff,
approach, and landing. It is used to feather the propeller
automatically if power is lost from either engine. The system
uses a solenoid valve to dump oil pressure from the propeller
cylinder (this allows the prop to feather) if two torque
switches sense low torque from the engine. This system has
a test-off-arm switch that is used to arm the system.
The autofeather system automatically energizes the holding
coil (pulling in the feather button) when engine power loss
results in a propeller thrust drop to a preset value. This system
is switch-armed for use during takeoff and can function only
when the power lever is near or in the “takeoff” position.
The NTS device mechanically moves the NTS plunger,
which actuates a linkage in the propeller control when a
predetermined negative torque value is sensed (when the
propeller drives the engine). This plunger, working through
control linkage, shifts the feather valve plunger, sending the
blades toward feather.
As the blade angle increases, negative torque decreases until
the NTS signal is removed, closing the feather valve. If the
predetermined negative torque value is again exceeded, the
NTS plunger again causes the feather valve plunger to shift.
The normal effect of the NTS is a cycling of rpm slightly
below the rpm at which the negative torque was sensed.
Unfeathering is initiated by pulling the feather button to
the “unfeather” position. This action supplies voltage to the
auxiliary motor to drive the auxiliary pump. Because the
propeller governor is in an underspeed position with the
propeller feathered, the blades will move in a decreased pitch
direction under auxiliary pump pressure.
The pitch lock operates in the event of a loss of propeller
oil pressure or an overspeed. The ratchets of the assembly
become engaged when the oil pressure, which keeps them
apart, is dissipated through a flyweight-actuated valve, which
operates at an rpm slightly higher than the 100% rpm The
ratchets become disengaged when high pressure and rpm
settings are restored.
At the “flight idle” power lever position, the control beta
follow-up low-pitch stop on the beta set cam (on the alpha
shaft) is set about 2' below the flight low-pitch stop setting,
acting as a secondary low-pitch stop. At the “takeoff” power
lever position, this secondary low-pitch stop sets a higher
blade angle stop than the mechanical flight low-pitch stop.
This provides for control of overspeed after rapid power lever
advance, as well as a secondary low-pitch stop.Propeller Inspection & Maintenance
Propellers must be inspected regularly. The exact time
interval for particular propeller inspections is usually
specified by the propeller manufacturer. The regular daily
inspection of propellers varies little from one type to another.
Typically, it is a visual inspection of propeller blades, hubs,
controls, and accessories for security, safety, and general
condition. Visual inspection of the blades does not mean
a careless or casual observation. The inspection should be
meticulous enough to detect any flaw or defect that may exist.
Inspections performed at greater intervals of time (e.g., 25,
50, or 100 hours) usually include a visual check of:
1. Blades, spinners, and other external surfaces for
excessive oil or grease deposits.
2. Weld and braze sections of blades and hubs for
evidence of failure.
3. Blade, spinner, and hubs for nicks, scratches, or other
flaws. Use a magnifying glass if necessary.
4. Spinner or dome shell attaching screws for tightness.
5. The lubricating requirements and oil levels, when
applicable.
If a propeller is involved in an accident, and a possibility
exists that internal damage may have occurred, or if a
propeller has had a ground strike or sudden stoppage, the
recommendations of the engine and propeller manufacturer's
maintenance manual need to be adhered to. The propeller
should be disassembled and inspected. Whenever a propeller
is removed from a shaft, the hub cone seats, cones, and other
contact parts should be examined to detect undue wear,
galling, or corrosion.
It is also vitally important to keep up-to-date airworthiness
directives (ADs) or service bulletins (SBs) for a propeller.
Compliance with ADs is required to make the aircraft legally
airworthy, but it is also important to follow the SBs. All work
performed on the propeller, including AD and SB compliance,
should be noted in the propeller logbook.
The propeller inspection requirements and maintenance
procedures discussed in this section are representative of
those in widespread use on most of the propellers described
in this chapter. No attempt has been made to include detailed
maintenance procedures for a particular propeller, and
all pressures, figures, and sizes are solely for the purpose
of illustration and do not have specific application. For
maintenance information on a specific propeller, always refer
to applicable manufacturer instructions.
Lightning conductor earthed to metal rootCarbon/glass fiber envelope
All over polyurethane spray coat
Carbon fiber spars
Polyurethane foam core Nickel leading-edge protection
Figure 7-32. Composite blade construction. Wood Propeller Inspection
Wood propellers should be inspected frequently to ensure
airworthiness. Inspect for defects, such as cracks, dents,
warpage, glue failure, delamination defects in the finish, and
charring of the wood between the propeller and the flange
due to loose propeller mounting bolts. Examine the wood
close to the metal sleeve of wood blades for cracks extending
outward on the blade. These cracks sometimes occur at the
threaded ends of the lag screws and may be an indication of
internal cracking of the wood. Check the tightness of the lag
screws, which attach the metal sleeve to the wood blade, in
accordance with the manufacturer’s instructions. In-flight tip
failures may be avoided by frequent inspections of the metal
cap, leading edge strip, and surrounding areas. Inspect for
such defects as looseness or slipping, separation of soldered
joints, loose screws, loose rivets, breaks, cracks, eroded
sections, and corrosion. Inspect for separation between the
metal leading edge and the cap, which would indicate the cap
is moving outward in the direction of centrifugal force. This
condition is often accompanied by discoloration and loose
rivets. Inspect the tip for cracks by grasping it with your hand
and slightly twisting about the longitudinal blade centerline
and by slightly bending the tip backward and forward. If the
leading edge and the cap have separated, carefully inspect
for cracks at this point. Cracks usually start at the leading
edge of the blade. Inspect moisture holes to ensure that they
are open. A fine line appearing in the fabric or plastic may
indicate a crack in the wood. Check the trailing edge of the
propeller blades for bonding, separation, or damage.
Metal Propeller Inspection
Metal propellers and blades are generally susceptible to
fatigue failure resulting from the concentration of stresses
at the bottoms of sharp nicks, cuts, and scratches. It is
necessary, therefore, to frequently and carefully inspect them
for such defects and make repairs promptly. The inspection
of steel blades may be accomplished by either visual,
fluorescent penetrant or magnetic particle inspection. The
visual inspection is easier if the steel blades are covered with
engine oil or rust-preventive compound. The full length of the
leading edge (especially near the tip), the full length of the
trailing edge, the grooves and shoulders on the shank, and all
dents and scars should be examined with a magnifying glass
to decide whether defects are scratches or cracks.
Tachometer inspection is a very important part of the overall
propeller inspection. Operation with an inaccurate tachometer
may result in restricted rpm operation and damaging high
stresses. This could shorten blade life and could result in
catastrophic failure. If the tachometer is inaccurate, then
the propeller could be turning much faster than it is rated
to turn, providing extra stress. Accuracy of the engine
tachometer should be verified at 100-hour intervals or at annual inspection, whichever occurs first. Hartzell Propeller
recommends using a tachometer that is accurate within ± 10
rpm and has an appropriate calibration schedule.
Aluminum Propeller Inspection
Carefully inspect aluminum propellers and blades for cracks
and other flaws. A transverse crack or flaw of any size is
cause for rejection. No repairs are permitted to the shanks
(roots or hub ends) of aluminum-alloy, adjustable-pitch
blades. The shanks must be within manufacturer’s limits.
Multiple deep nicks and gouges on the leading edge and
face of the blade is cause for rejection. Use dye penetrant or
fluorescent dye penetrant to confirm suspected cracks found
in the propeller. Refer any unusual condition or appearance
revealed by these inspections to the manufacturer.
Composite Propeller Inspection
Composite blades need to be visually inspected for nicks,
gouges, loose material, erosion, cracks and debonds,
and lightning strike. [Figure 7-32] Composite blades are
inspected for delaminations and debonds by tapping the
blade or cuff (if applicable) with a metal coin. If an audible
change is apparent, sounding hollow or dead, a debond or
delamination is likely. [Figure 7-33] Blades that incorporate
a “cuff” have a different tone when coin tapped in the cuff
area. To avoid confusing the sounds, coin tap the cuff area and
the transition area between the cuff and the blade separately
from the blade area. Additional nondestructive testing (NDT)
techniques for composite materials, such as phased array
inspections, and ultrasound inspections, are available for
more detailed inspections.
Repairs to propellers are often limited to minor type repairs.
Certificated mechanics are not allowed to perform major
repairs on propellers. Major repairs need to be accomplished
by a certificated propeller repair station.
LIBERTY
LIBERTY
2008Coin tap along the entire
surface of the erosion shield
to check for debonds.
Coin tap the
composite blade
suface to check
for delaminations.
Wood block± 1/16"
Cowling fixtureFigure 7-33. Coin-tap test to check for debonds and delaminations.
Figure 7-34. Propeller blade tracking. Propeller Vibration
Although vibration can be caused by the propeller, there are
numerous other possible sources of vibration that can make
troubleshooting difficult. If a propeller vibrates, whether
due to balance, angle, or track problems, it typically vibrates
throughout the entire rpm range, although the intensity of the
vibration may vary with the rpm. If a vibration occurs only at
one particular rpm or within a limited rpm range (e.g., 2200–
2350 rpm), the vibration is not normally a propeller problem
but a problem of a poor engine-propeller match. If a propeller
vibration is suspected but cannot be positively determined,
the ideal troubleshooting method is to temporarily replace the
propeller with one known to be airworthy and then test fly the
aircraft if possible. Blade shake is not the source of vibration
problems. Once the engine is running, centrifugal force holds
the blades firmly (approximately 30,000–40,000 pounds)
against blade bearings. Cabin vibration can sometimes be
improved by reindexing the propeller to the crankshaft. The
propeller can be removed, rotated 180°, and reinstalled. The
propeller spinner can be a contributing factor to an out-of-
balance condition. An indication of this would be a noticeable
spinner wobble while the engine is running. This condition is
usually caused by inadequate shimming of the spinner front
support or a cracked or deformed spinner.
When powerplant vibration is encountered, it is sometimes
difficult to determine whether it is the result of engine
vibration or propeller vibration. In most cases, the cause of
the vibration can be determined by observing the propeller
hub, dome, or spinner while the engine is running within a
1,200- to 1,500-rpm range and determining whether or not
the propeller hub rotates on an absolutely horizontal plane.
If the propeller hub appears to swing in a slight orbit, the
vibration is usually caused by the propeller. If the propeller
hub does not appear to rotate in an orbit, the difficulty is
probably caused by engine vibration.
When propeller vibration is the reason for excessive vibration,
the difficulty is usually caused by propeller blade imbalance, propeller blades not tracking, or variation in propeller blade
angle settings. Check the propeller blade tracking and then
the low-pitch blade angle setting to determine if either is
the cause of the vibration. If both propeller tracking and
low blade angle setting are correct, the propeller is statically
or dynamically unbalanced and should be replaced, or re-
balanced if permitted by the manufacturer.
Blade Tracking
Blade tracking is the process of determining the positions of
the tips of the propeller blades relative to each other (blades
rotating in the same plane of rotation). Tracking shows only
the relative position of the blades, not their actual path. The
blades should all track one another as closely as possible. The
difference in track at like points must not exceed the tolerance
specified by the propeller manufacturer. The design and
manufacture of propellers is such that the tips of the blades
give a good indication of tracking. The following method for
checking tracking is normally used:
1. Chock the aircraft so it cannot be moved.
2. Remove one spark plug from each cylinder. This
makes the propeller easier and safer to turn.
3. Rotate one of the blades so it is pointing down.
4. Place a solid object (e.g., a heavy wooden block that is
at least a couple of inches higher off the ground than the
distance between the propeller tip and the ground) next
to the propeller tip so that it just touches or attaches a
pointer/indicator to the cowling itself. [Figure 7-34]
5. Rotate the propeller slowly to determine if the next
blade tracks through the same point (touches the block/
pointer). Each blade track should be within 1⁄16 inch
7-23Protractor TE97Reference blade radius location
Bench top protractor TE96
Figure 7-35. Blade angle measurement. Figure 7-36. Bench top protractor.
30 20 10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 170 160 150 140 130 120 110 100 90 80 70 60 50 10 0 10
Degree scale on discVernier scale on ring
Disc-to-ring lockRing adjusterCenter spirit level
Disc adjuster Ring
Ring-to-frame lock Corner spirit levelDisc
Edge used for checking
against the propeller bladeEdge used for checking against the propeller blade
Edge used for checking against the propeller blade
Nut—loosen to rotate face of level
Figure 7-37. Universal propeller protractor. (plus or minus) from the opposite blade’s track.
6. An out-of-track propeller, may be due to one or more
propeller blades being bent, a bent propeller flange,
or propeller mounting bolts that are either over- or
undertorqued. An out-of-track propeller causes
vibration and stress to the airframe and engine and
may cause premature propeller failure.
Checking & Adjusting Propeller Blade Angles
When you find an improper blade angle setting during
installation or when indicated by engine performance,
follow basic maintenance guidelines. From the applicable
manufacturer’s instructions, obtain the blade angle setting and
the station at which the blade angle is checked. Do not use
metal scribes or other sharply pointed instruments to mark the
location of blade stations or make reference lines on propeller
blades, since such surface scratches can induce failure (stress
concentrator), eventually resulting in blade failure. Use a
bench-top protractor if the propeller is removed from the
aircraft. [Figure 7-35] Use a handheld protractor (a digital
protractor provides an easy measurement) to check blade angle
if the propeller is installed on the aircraft or is placed on the
knife-edge balancing stand. [Figure 7-36]
Universal Propeller Protractor
The universal propeller protractor can be used to check
propeller blade angles when the propeller is on a balancing
stand or installed on the aircraft engine. Figure 7-37 shows
the parts and adjustments of a universal propeller protractor.
The following instructions for using the protractor apply to a
propeller installed on the engine. Turn the propeller until the
first blade to be checked is horizontal with the leading edge
up. Place the corner spirit level at right angles to the face of
the protractor. Align degree and vernier scales by turning
the disc adjuster before the disc is locked to the ring. The
locking device is a pin that is held in the engaged position
by a spring. The pin can be released by pulling it outward
and turning it 90°.Release the ring-to-frame lock (a right-hand screw with
thumb nut) and turn the ring until both ring and disc zeros
are at the top of the protractor.
Check the blade angle by determining how much the flat
side of the block slants from the plane of rotation. First,
locate a point to represent the plane of rotation by placing
the protractor vertically against the end of the hub nut or any
convenient surface known to lie in the plane of propeller
7-24rotation. Keep the protractor vertical by the corner spirit
level and turn the ring adjuster until the center spirit level is
horizontal. This sets the zero of the vernier scale at a point
representing the plane of propeller rotation. Then, lock the
ring to the frame.
While holding the protractor by the handle with the curved
edge up, release the disc-to-ring lock. Place the forward
vertical edge (the edge opposite the one first used) against
the blade at the station specified in the manufacturer’s
instructions. Keep the protractor vertical by the corner spirit
level and turn the disc adjuster until the center spirit level
is horizontal. The number of degrees and tenths of a degree
between the two zeros indicates the blade angle.
In determining the blade angle, remember that ten points on the
vernier scale are equal to nine points on the degree scale. The
graduations on the vernier scale represent tenths of a degree,
but those of the degree scale represent whole degrees. The
number of tenths of a degree in the blade angle is given by the
number of vernier scale spaces between the zero of the vernier
scale and the vernier scale graduation line nearest to perfect
alignment with a degree scale graduation line. This reading
should always be made on the vernier scale. The vernier
scale increases in the same direction that the protractor scale
increases. This is opposite to the direction of rotation of the
moving element of the protractor. After making any necessary
adjustment of the blade, lock it in position and repeat the same
operations for the remaining blades of the propeller.
Propeller Balancing
Propeller unbalance, which is a source of vibration in an
aircraft, may be either static or dynamic. Propeller static
imbalance occurs when the center of gravity (CG) of the
propeller does not coincide with the axis of rotation. Dynamic
unbalance results when the CG of similar propeller elements,
such as blades or flyweights, does not follow in the same
plane of rotation. Since the length of the propeller assembly
along the engine crankshaft is short in comparison to its
diameter, and since the blades are secured to the hub so they
lie in the same plane perpendicular to the running axis, the
dynamic unbalance resulting from improper mass distribution
is negligible, provided the track tolerance requirements are
met. Another type of propeller unbalance, aerodynamic
unbalance, results when the thrust (or pull) of the blades is
unequal. This type of unbalance can be largely eliminated by
checking blade contour and blade angle setting.
Static Balancing
The knife-edge test stand has two hardened steel edges
mounted to allow the free rotation of an assembled propeller
between them. [Figure 7-38] The knife-edge test stand must
be located in a room or area that is free from any air motion, and preferably removed from any source of heavy vibration.
The standard method of checking propeller assembly balance
involves the following sequence of operations:
1. Insert a bushing in the engine shaft hole of the
propeller.
2. Insert a mandrel or arbor through the bushing.
3. Place the propeller assembly so that the ends of the
arbor are supported upon the balance stand knife-
edges. The propeller must be free to rotate.
If the propeller is properly balanced statically, it remains at
any position in which it is placed. Check two-bladed propeller
assemblies for balance: first with the blades in a vertical
position and then with the blades in a horizontal position.
Repeat the vertical position check with the blade positions
reversed; that is, with the blade that was checked in the
downward position placed in the upward position.
Check a three-bladed propeller assembly with each blade
placed in a downward vertical position. [Figure 7-39]
During a propeller static balance check, all blades must be at
the same blade angle. Before conducting the balance check,
inspect to see that each blade has been set at the same blade
angle.
Unless otherwise specified by the manufacturer, an
acceptable balance check requires that the propeller assembly
have no tendency to rotate in any of the positions previously
described. If the propeller balances perfectly in all described
positions, it should also balance perfectly in all intermediate
positions. When necessary, check for balance in intermediate
positions to verify the check in the originally described
positions. [Figure 7-40]
When a propeller assembly is checked for static balance and
there is a definite tendency of the assembly to rotate, certain
corrections to remove the unbalance are allowed.
1. The addition of permanent fixed weights at acceptable
locations when the total weight of the propeller
assembly or parts is under the allowable limit.
2. The removal of weight at acceptable locations when
the total weight of the propeller assembly or parts is
equal to the allowable limit.
The location for removal or addition of weight for propeller
unbalance correction has been determined by the propeller
manufacturer. The method and point of application of
unbalance corrections must be checked to see that they are
according to applicable drawings.
A. Vertical balance check B. Horizontal balance check
13Figure 7-38. Positions of two-bladed propeller during a balance check.
Figure 7-39. Positions of three-bladed propeller during balance check.
Dynamic Balancing
Propellers can also be dynamically balanced (spin balanced)
with an analyzer kit to reduce the vibration levels of the
propeller and spinner assembly. Some aircraft have the
system hardwired in the aircraft and on other aircraft the
sensors and cables need to be installed before the balancing
run. Balancing the propulsion assembly can provide
substantial reductions in transmitted vibration and noise to
the cabin and also reduces excessive damage to other aircraft
and engine components. The dynamic imbalance could be
caused by mass imbalance or any aerodynamic imbalance.
Dynamic balancing only improves the vibration caused by
mass unbalance of the externally rotating components of the
propulsion system. Balancing does not reduce the vibration level if the engine or aircraft is in poor mechanical condition.
Defective, worn, or loose parts will make balancing
impossible. Several manufacturers make dynamic propeller
balancing equipment, and their equipment operation could
differ. The typical dynamic balancing system consists of a
vibration sensor that is attached to the engine close to the
propeller, and an analyzer unit that calculates the weight and
location of balancing weights.
Balancing Procedure
Face the aircraft directly into the wind (maximum 20 knots)
and place chocks at the wheels. When you have installed the
analyzing equipment, run the engine up at low cruise rpm; the
Figure 7-40. Static propeller balancing.
dynamic analyzer calculates the balancing weight required at
each blade position. After installing the balancing weights,
run the engine up again to check if the vibration levels have
diminished. This process may have to be repeated several
times before satisfactory results are achieved.
A dynamic balancing example procedure is listed here, but
always refer to the aircraft and propeller manuals when
performing any balancing procedure. Dynamic balance is
accomplished by using an accurate means of measuring the
amount and location of the dynamic imbalance. The number
of balance weights installed must not exceed the limits
specified by the propeller manufacturer. Follow the dynamic
balance equipment manufacturer’s instructions for dynamic
balance in addition to the specifications of the propeller.
Most equipment use an optical pickup that senses reflective
tape for rpm reading. Also, there is an accelerometer mounted
to the engine that senses vibration in inches per second (ips).
Visually inspect the propeller assembly before dynamic
balancing. The first runup of a new or overhauled propeller
assembly may leave a small amount of grease on the blades
and inner surface of the spinner dome. Use Stoddard solvent
(or equivalent) to completely remove any grease on the blades or inner surface of the spinner dome. Visually examine each
propeller blade assembly for evidence of grease leakage.
Visually examine the inner surface of the spinner dome for
evidence of grease leakage. If there is no evidence of grease
leakage, lubricate the propeller in accordance with the
maintenance manual. If grease leakage is evident, determine
the location of the leak and correct before relubricating the
propeller and dynamic balancing. Before dynamic balance,
record the number and location of all balance weights. Static
balance is accomplished at a propeller overhaul facility when
an overhaul or major repair is performed. Twelve equally
spaced locations are recommended for weight attachment.
Install the balancing weights using aircraft quality 10-32 or
AN-3 type screws or bolts. Balance weight screws attached to
the spinner bulkhead must protrude through the self-locking
nuts or nut plates a minimum of one thread and a maximum
of four threads. Unless otherwise specified by the engine
or airframe manufacturer, Hartzell recommends that the
propeller be dynamically balanced to a reading of 0.2 ips, or
less. If reflective tape is used for dynamic balancing, remove
the tape immediately after balancing is completed. Make a
record in the propeller logbook of the number and location
of dynamic balance weights, and static balance weights if
they have been reconfigured.
Propeller Removal & Installation
Removal
The following procedure is for demonstration purposes only.
Always use the current manufacturer’s information when
removing and installing any propeller.
1. Remove the spinner dome in accordance with the
spinner removal procedures. Cut and remove the safety
wire (if installed) on the propeller mounting studs.
2. Support the propeller assembly with a sling. If the
propeller is reinstalled and has been dynamically
balanced, make an identifying mark (with a felt-tipped
pen only) on the propeller hub and a matching mark on
the engine flange to make sure of proper orientation
during reinstallation to prevent dynamic imbalance.
3. Unscrew the four mounting bolts from the engine
bushings. Unscrew the two mounting nuts and the
attached studs from the engine bushings. If the
propeller is removed between overhaul intervals,
mounting studs, nuts, and washers may be reused if
they are not damaged or corroded.
Caution: Remove the propeller from the mounting
flange with care to prevent damaging the propeller
mounting studs. Using the support sling, remove the
propeller from the mounting flange.
4. Place the propeller on a cart for transport.
7-27Installation
A flange propeller has six studs configured in a four-inch
circle. Two special studs that also function as dowel pins
are provided to transfer torque and index the propeller with
respect to the engine crankshaft. The dowel pin locations
used on a particular propeller installation are indicated in the
propeller model stamped on the hub. Perform the applicable
steps under Spinner Pre-Installation and clean the engine
flange and propeller flange with quick dry Stoddard solvent
or methyl ethyl ketone (MEK). Install the O-ring in the O-ring
groove in the hub bore. Note: When the propeller is received
from the factory, the O-ring has usually been installed. With a
suitable support, such as a crane hoist or similar equipment,
carefully move the propeller assembly to the aircraft engine
mounting flange in preparation for installation.
Install the propeller on the engine flange. Make certain
to align the dowel studs in the propeller flange with the
corresponding holes in the engine mounting flange. The
propeller may be installed on the engine flange in a given
position, or 180° from that position. Check the engine and
airframe manuals to determine if either manual specifies a
propeller mounting position.
Caution: Mounting hardware must be clean and dry to
prevent excessive preload of the mounting flange.
Caution: Tighten nuts evenly to avoid hub damage.
Install the propeller mounting nuts (dry) with spacers. Torque
the propeller mounting nuts (dry) in accordance with the
proper specifications and safety wire the studs in pairs (if
required by the aircraft maintenance manual) at the rear of
the propeller mounting flange.
Servicing Propellers
Propeller servicing includes cleaning, lubricating, and
replenishing operating lubrication supplies.
Cleaning Propeller Blades
Aluminum and steel propeller blades and hubs are usually
cleaned by washing the blades with a suitable cleaning
solvent, using a brush or cloth but current manufacturer's
information should always be used. Do not use acid or caustic
materials. Power buffers, steel wool, steel brushes, or any
other tool or substance that may scratch or mar the blade
should be avoided. If a high polish is desired, a number of
good grades of commercial metal polish are available. After
completing the polishing operation, immediately remove all
traces of polish. When the blades are clean, coat them with a
clean film of engine oil or suitable equivalent.
To clean wooden propellers, use warm water and a mild soap, together with brushes or cloth. If a propeller has been
subjected to salt water, flush it with fresh water until all traces
of salt have been removed. This should be accomplished
as soon as possible after the salt water has splashed on
the propeller, regardless of whether the propeller parts are
aluminum alloy, steel, or wood. After flushing, thoroughly
dry all parts, and coat metal parts with clean engine oil or a
suitable equivalent.
To remove grease or oil from propeller surfaces, apply
Stoddard solvent or equivalent to a clean cloth and wipe
the part clean. Using a noncorrosive soap solution, wash
the propeller. Thoroughly rinse with water. Permit to dry.
Aluminum and steel propeller blades and hubs usually are
cleaned by washing the blades with a suitable cleaning
solvent, using a brush or cloth. Do not use acid or caustic
materials. Avoid power buffers, steel wool, steel brushes,
or any other tool or substance that may scratch or mar the
blade. If a high polish is desired, a number of good grades of
commercial metal polish are available. After completing the
polishing operation, immediately remove all traces of polish.
When the blades are clean, coat them with a clean film of
engine oil or suitable equivalent.
Charging the Propeller Air Dome
These instructions are general in nature and do not represent
any aircraft procedure. Always check the correct manual
before servicing any propeller system. Examine the propeller
to make sure that it is positioned on the start locks and using
the proper control, then charge the cylinder with dry air or
nitrogen. The air charge valve is located on the cylinder as
indicated in Figure 7-41 . Nitrogen is the preferred charging
medium. The correct charge pressure is identified by checking
the correct table shown. The temperature is used to find the
correct pressure to charge the hub air pressure.
Propeller Lubrication
Hydromatic propellers operated with engine oil and some
sealed propellers do not require lubrication. Electric propellers
require oils and greases for hub lubricants and pitch change
drive mechanisms. Proper propeller lubrication procedures,
with oil and grease specifications, are usually published in the
manufacturer’s instructions. Experience indicates that water
sometimes gets into the propeller blade bearing assembly
on some models of propellers. For this reason, the propeller
manufacturer’s greasing schedule must be followed to ensure
proper lubrication of moving parts and protection from
corrosion. Observe overhaul periods because most defects
in propellers are not external but unseen internal corrosion.
Dissimilar metals in the prop and hub create an environment
ripe for corrosion, and the only way to properly inspect many
of these areas is through a teardown. Extensive corrosion can
dramatically reduce the strength of the blades or hub. Even
Air charge valveAir charge temperature/
pressure chart
Pump grease in hereRemove grease fittingLubricating bearing in this areaFigure 7-41. Servicing air charge in propeller.
Figure 7-42. Lubricating propeller bearings. seemingly minor corrosion may cause a blade or hub to fail
an inspection. Because of the safety implications (blade loss),
this is clearly an area in which close monitoring is needed.
One example of the lubrication requirements and procedures
is detailed here for illustration purposes only. Lubrication
intervals are important to adhere to because of corrosion
implications. The propeller must be lubricated at intervals
not to exceed 100 hours or at 12 calendar months, whichever
occurs first. If annual operation is significantly less than
100 hours, calendar lubrication intervals should be reduced
to 6 months. If the aircraft is operated or stored under
adverse atmospheric conditions, such as high humidity, salt
air, calendar lubrication intervals should be reduced to 6 months. Hartzell recommends that new or newly overhauled
propellers be lubricated after the first 1 or 2 hours of operation
because centrifugal loads pack and redistribute grease,
which may result in a propeller imbalance. Redistribution
of grease may also result in voids in the blade bearing area
where moisture can collect. Remove the lubrication fitting
from the cylinder-side hub half installed in the engine-side
hub half. [Figure 7-42] Pump 1 fluid ounce (30 milliliters
(ml)) grease into the fitting located nearest the leading edge
of the blade on a tractor installation, or nearest the trailing
edge on a pusher installation, until grease emerges from the
hole where the fitting was removed, whichever occurs first.
Note: 1 fluid ounce (30 ml) is approximately six pumps with
7-29a hand-operated grease gun. Reinstall the removed lubrication
fittings. Tighten the fittings until snug. Make sure that the
ball of each lubrication fitting is properly seated. Reinstall
a lubrication fitting cap on each lubrication fitting. Perform
grease replacement through attached pressure fittings (zerks)
in accordance with the manufacturer’s instructions.
Propeller Overhaul
Propeller overhaul should be accomplished at the maximum
hours or calendar time limit, whichever occurs first. Upon
receipt for overhaul, prepare a document that tracks the
propeller components throughout the overhaul process.
Research all applicable ADs, current specifications, and
manufacturers’ SBs for incorporation during the overhaul
process. Double check the serial number and make notes on
the work order regarding the general condition in which the
propeller was received. As you disassemble and clean the
unit, perform a preliminary inspection on all related parts.
Record those revealing discrepancies requiring rework or
replacement in the overhaul record by part number, along
with the reason for the required action. Discard all threaded
fasteners during disassembly and, with a few exceptions
permitted by the manufacturer, replace with new components.
Many specialized tools and fixtures are required in the
disassembly and proper reassembly of propellers. These
tools are generally model specific and range from massive
15-foot torque adapter bars and 100-ton presses down to
tiny dowel pin alignment devices. Dimensionally inspect
components that are subject to wear to the manufacturer’s
specifications. After passing inspection, anodize aluminum
parts and cadmium plate steel parts for maximum protection
against corrosion.
The Hub
Strip nonferrous hubs and components of paint and anodize
and inspect for cracks using a liquid penetrant inspection
(LPI) procedure. Etch, rinse, dry, and then immerse the
parts in a fluorescent penetrant solution. After soaking in
the penetrant, rinse them again and blow dry. Then, apply
developer, which draws any penetrant caught in cracks or
defects to the surface. Under an ultraviolet inspection lamp,
the penetrant clearly identifies the flaw. Certain models of
hubs are also eddy-current inspected around critical, high-
stress areas. Eddy-current testing passes an electrical current
through a conductive material that, when disturbed by a
crack or other flaw, causes a fluctuation on a meter or CRT
display. This method of inspection can detect flaws that are
below the surface of the material and not exposed to the eye.
Magnetic particle inspection (MPI) is used to locate flaws in
steel parts. The steel parts of the propeller are magnetized by
passing a strong electrical current through them. A suspension
of fluorescent iron oxide powder and solvent is spread over
the parts. While magnetized, the particles within the fluid on the parts surface immediately align themselves with the
discontinuity. When examined under black light, the crack
or fault shows as a bright fluorescent line.
The first step in blade overhaul is the precise measurement
of blade width, thickness, face alignment, blade angles,
and length. Then, record the measurements on each blade’s
inspection record and check against the minimum acceptable
overhaul specifications established by the manufacturer.
Blade overhaul involves surface grinding and repitching,
if necessary. Occasionally, blade straightening is also
required. The manufacturer’s specification dictates certain
allowable limits within which a damaged blade may be cold
straightened and returned to airworthy condition. Specialized
tooling and precision measuring equipment permit pitch
changes or corrections of less than one-tenth of one degree.
To ensure accuracy, take frequent face alignment and angle
measurements during the repair process. Precision hand grind
the blade airfoil to remove all corrosion, scratches, and surface
flaws. After completely removing all stress risers and faults,
take final blade measurements and record on each blade’s
inspection record. Balance and match the propeller blades and
anodize and paint them for long-term corrosion protection.
Prop Reassembly
When both the hubs and the blades have completed the
overhaul process, the propeller is ready for final assembly.
Recheck part numbers with the manufacturer’s specifications.
Lubricate and install the parts per each unit’s particular
overhaul manual. After final assembly, check both high-
and low-pitch blade angles on constant-speed propellers
for proper operation and leaks by cycling the propeller with
air pressure through its blade angle range. Then, check the
assembled propeller for static balance. If necessary, place
weights on the hub areas of each “light” blade socket to
bring about its proper balance. These weights should be
considered part of the basic hub assembly and should not be
moved during subsequent dynamic balancing to the engine.
As with most aircraft components, all of the hardware on
the propeller assembly must be safety wired, unless secured
by self-locking devices. Then, the final inspector fills out
and signs maintenance release tags reflecting the work
accomplished, applicable ADs, and all incorporated service
documents. These documents certify that the major repairs
and/or alterations that have been made meet established
standards and that the propeller is approved for return to
service. All minor repairs and minor alterations on propellers
must be accomplished by a certified repair station, an airframe
and powerplant technician (A&P), or a person working under
the direct supervision of such a technician or an appropriately
rated air carrier. Major repairs or alterations, including the
overhaul of controllable pitch propellers, must be done by an
appropriately rated repair station, manufacturer, or air carrier.
Figure 7-43. Turboprop commuter.
Figure 7-44. Pratt & Whitney PT6 engine.
Figure 7-45. Pratt & Whitney 150 turboprop engine. Troubleshooting Propellers
Some brief examples of troubleshooting problems and
possible causes are provided in the following subsections.
Always refer to the correct manual for actual information
on troubleshooting.
Hunting & Surging
Hunting is characterized by a cyclic variation in engine speed
above and below desired speed. Surging is characterized
by a large increase/decrease in engine speed, followed by a
return to set speed after one or two occurrences. If propeller is
hunting, an appropriately licensed repair facility should check:
1. Governor,
2. Fuel control, and
3. Synchrophaser or synchronizer.
Engine Speed Varies with Flight Attitude (Airspeed)
Small variances in engine speed are normal and are no cause
for concern. An increase in engine speed while descending or
increasing airspeed with a nonfeathering propeller could be:
1. The governor not increasing oil volume in the
propeller,
2. Engine transfer bearing leaking excessively, or
3. Excessive friction in blade bearings or pitch changing
mechanism.
Failure to Feather or Feathers Slowly
Failure to feather or slow feathering of the propeller requires
the FAA-certificated A&P technician to:
1. Refer to the air charge section in the maintenance
manual if the air charge is lost or low.
2. Check for proper function and rigging of propeller/
governor control linkage.
3. Check the governor drain function.
4. Check the propeller for misadjustment or internal
corrosion (usually in blade bearings or pitch change
mechanism) that results in excessive friction. This
must be performed at an appropriately licensed
propeller repair facility.Turboprop Engines & Propeller Control
Systems
Turboprop engines are used for many single, twin, and
commuter aircraft. [Figure 7-43] Smaller turboprop engines,
such as the PT-6, are used on single and twin engine designs;
the power ranges from 500 to 2,000 shaft horsepower.
[Figure 7-44] Large commuter aircraft use turboprop engines,
such as the P&W 150 and AE2100 that can deliver up to
5,000 shaft horsepower to power mid-sized to large turboprop
aircraft. [Figure 7-45] The turboprop propeller is operated
by a gas turbine engine through a reduction-gear assembly.
It has proved to be an extremely efficient power source.
The combination of propeller, reduction-gear assembly, and
turbine engine is referred to as a turboprop powerplant.
The turbofan engine produces thrust directly; the turboprop
engine produces thrust indirectly because the compressor and
turbine assembly furnishes torque to a propeller, producing
Figure 7-46. Reduction gearbox. the major portion of the propulsive force that drives the
aircraft. The turboprop fuel control and the propeller governor
are connected and operate in coordination with each other.
The power lever directs a signal from the flight deck to the
fuel control for a specific amount of power from the engine.
The fuel control and the propeller governor together establish
the correct combination of rpm, fuel flow, and propeller
blade angle to create sufficient propeller thrust to provide
the desired power.
The propeller control system is divided into two types of
control: one for flight and one for ground operation. For
flight, the propeller blade angle and fuel flow for any given
power lever setting are governed automatically according to a
predetermined schedule. Below the “flight idle” power lever
position, the coordinated rpm blade angle schedule becomes
incapable of handling the engine efficiently. Here, the ground
handling range, referred to as the beta range, is encountered. In
the beta range of the throttle quadrant, the propeller blade angle
is not governed by the propeller governor but is controlled by
the power lever position. When the power lever is moved below
the start position, the propeller pitch is reversed to provide
reverse thrust for rapid deceleration of the aircraft after landing.
A characteristic of the turboprop is that changes in power are
not related to engine speed, but to turbine inlet temperature.
During flight, the propeller maintains a constant engine
speed. This speed is known as the 100 percent rated speed
of the engine, and it is the design speed at which most power
and best overall efficiency can be obtained. Power changes
are affected by changing the fuel flow. An increase in fuel
flow causes an increase in turbine inlet temperature and a
corresponding increase in energy available at the turbine. The
turbine absorbs more energy and transmits it to the propeller
in the form of torque. The propeller, in order to absorb the
increased torque, increases blade angle, thus maintaining constant engine rpm with added thrust.
Reduction Gear Assembly
The function of the reduction gear assembly is to reduce
the high rpm from the engine to a propeller rpm that can be
maintained without exceeding the maximum propeller tip
speed (speed of sound). Most reduction gear assemblies use
a planetary gear reduction. [Figure 7-46] Additional power
takeoffs are available for propeller governor, oil pump, and
other accessories. A propeller brake is often incorporated into
the gearbox. The propeller brake is designed to prevent the
propeller from windmilling when it is feathered in flight, and
to decrease the time for the propeller to come to a complete
stop after engine shutdown.
Turbo-Propeller Assembly
The turbo-propeller provides an efficient and flexible
means of using the power of the engine at any condition
in flight (alpha range). [Figure 7-47] For ground handling
and reversing (beta range), the propeller can be operated
to provide either zero or negative thrust. The major
subassemblies of the propeller assembly are the barrel,
dome, low-pitch stop assembly, overspeed governor, pitch
control unit, auxiliary pump, feather and unfeather valves,
torque motor, spinner, deice timer, beta feedback assembly,
and propeller electronic control. Modern turboprop engines
use dual Full Authority Digital Engine Control (FADEC) to
control both engine and propeller. The spinner assembly is a
cone-shaped configuration that mounts on the propeller and
encloses the dome and barrel to reduce drag.
Propeller operation is controlled by a mechanical linkage
from the flight deck-mounted power lever and the emergency
engine shutdown handle (if the aircraft is provided with one)
to the coordinator, which, in turn, is linked to the propeller
control input lever. Newer designs use electronic throttle
control that is linked to the FADEC controller.
Turbo-propeller control assemblies have a feathering system
that feather the propeller when the engine is shut down in
flight. The propeller can also be unfeathered during flight, if
the engine needs to be started again. Propeller control systems
for large turboprop engines differ from smaller engines
because they are dual acting, which means that hydraulic
pressure is used to increase and decrease propeller blade
angle. [Figure 7-48]
Pratt & Whitney PT6 Hartzell Propeller System
The PT6 Hartzell propeller system incorporates three-, four -,
or six-bladed propellers made of aluminum or composite
materials. It is a constant-speed, feathering, reversing
propeller system using a single-acting governor. Oil from
the propeller governor feeds into the propeller shaft and to
Outer bearing assembly
Counterweight assemblyInner bearing assembly
Deicing slip ringCrossheadBackplate
Flange attachment
Operating pin assembly
Hub
Hub assembly boltsCylinderPistonBlade assembly
Blade grease seal
Beta tubes
Figure 7-47. Turboprop propeller.
the servo piston via the oil transfer sleeve mounted on the
propeller shaft. [Figure 7-49] As oil pressure increases,
the servo piston is pushed forward, and the feather spring
is compressed. Servo piston movement is transmitted to
the propeller blade collars via a system of levers. When oil
pressure is decreased, the return spring and flyweights force
the oil out of the servo piston and change the blade pitch to
a high pitch position. An increase in oil pressure drives the
blades towards low pitch.
Engine oil is supplied to the governor from the engine oil
supply. A gear pump, mounted at the base of the governor,
increases the flow of oil going to the constant speed unit
(CSU) relief valve. When the oil pressure reaches the desired
level, the relief valve opens to maintain the governor oil
pressure. When the speed selected by the pilot is reached, the
flyweight force equals the spring tension of the speeder spring.
The governor flyweights are then on speed. When the engine
output power is increased, the power turbines tend to increase
speed. The flyweights in the CSU sense this acceleration and
the flyweights go into an overspeed condition because of
the increase centrifugal force. This force causes the control
valve to move up and restrict oil flow to the propeller dome.
[Figure 7-50] The feathering spring increases the propeller pitch to maintain the selected speed. Reducing power causes
an under-speed of the flyweights, downward movement of
the control valve, more oil in propeller dome, resulting in a
lower pitch to control propeller speed. The propeller governor
houses an electro-magnetic coil, which is used to match the
rpm of both propellers during cruise. An aircraft supplied
synchrophaser unit controls this function.
At low power, the propeller and governor flyweights
do not turn fast enough to compress the speeder spring.
[Figure 7-51] In this condition, the control valve moves
down, and high pressure oil pushes the dome forward moving
the blades towards low pitch. Any further movement pulls
the beta rod and slip ring forward. The forward motion of
the slip ring is transmitted to the beta valve via the beta lever
and the carbon block. Forward movement of the beta valve
stops the oil supply to the propeller. This prevents the blade
angles from going any lower. This is the primary blade angle
(PBA) and is the minimum blade angle allowed for flight
operation. From this point, the propeller is in the beta mode.
If the engine power is reduced when the propeller is at the
primary blade angle, the propeller speed decreases since the
blade angle does not change.
CODEOil from PGB pump
HP and OSG oil
Drain oil
Pitch change oil
Auxiliary feather pump oil
Pressure switch P
Propeller gearbox
HPP/OSG
Beta feedback
Torque motor
Pitch control unitDrain DrainAFP
Drain
Drain
DrainGearbox oil supply180-210
psi 1100 psiGovernor
DrainHP pump
Relief valve Drain
R1
R2
R3R4
Feather
valve
NRV ANRV B
Pitch
change
pressPilot press
FLT fine Ground fine
Coarse
Coarse FineP(Energized)(Deenergized)Servo valveUnfeather solenoid
Unfeather
valve
GBEV solenoid
Ground beta
enable valve
Note: AFP (Auxiliary feather pump)
SOLSOL
Blade
PropFlyweight
Figure 7-48. Propeller control system schematic.
The lock-pitch solenoid valve prevents the propeller from
going into reverse or below the primary blade angle in the
event of a beta system malfunction in flight. The solenoid
is energized by a switch (airframe supplied) mechanically
connected to the propeller slip-ring linkage via a second
carbon block. As oil pressure leaks off around the propeller
shaft oil transfer sleeve, the blade angle slowly drifts back
toward high pitch. This deactivates the low pitch solenoid
valve and restores the oil supply to the propeller servo. The
low pitch solenoid valve cycles (close/open) as backup to
the beta valve function. Moving the power lever backwards
causes the reversing cam and cable to move the beta valve
backward, allowing more oil to flow into the propeller
dome, and causing the blades to go towards reverse pitch.
[Figure 7-52]
As the blades move to reverse, the dome pulls the slip ring
forward and moves the beta valve outward, restricting the
oil flow. This stops the blade movement toward reverse. To
obtain more reverse thrust, move the power lever back more
to reset the beta valve inward, and repeat the process. Move
the reset arm on the CSU rearward by the interconnecting
rod at the same time the blade angle moves toward reverse.
This causes the reset lever and reset post to move down in
the CSU, bringing the reset lever closer to the speeder spring
cup. As propeller speed increases due to the increase in engine power, the governor flyweights begin to move outwards.
Since the reset lever is closer to the speeder spring cup, the
cup contacts the reset lever before the flyweights would
normally reach the on-speed position (95 percent propeller
speed instead of 100 percent). As the reset lever is pushed up
by the flyweights/speeder spring cup, the Py air bleeds from
the fuel control unit (FCU) which lowers the fuel flow, engine
power, and thus propeller speed. In reverse, propeller speed
remains 5 percent below the selected propeller speed so that
the control valve remains fully open, and only the beta valve
controls the oil flow to the propeller dome.
In this mode, the propeller speed is no longer controlled by
changing the blade angle. It is now controlled by limiting
engine power. Bringing the propeller lever to the feather
position causes the speed selection lever on the CSU to push
the feathering valve plunger and allows propeller servo oil
to dump into the reduction gearbox sump. The pressure loss
in the propeller hub causes the feathering spring and the
propeller flyweights to feather the propeller. In the event
of a propeller overspeed not controlled by the propeller
overspeed governor (oil governor), the flyweights in the
propeller governor move outward until the speeder spring cup
contacts the reset lever. [Figure 7-53] The movement of the
reset lever around its pivot point opens the Py air passage. Py
bleeds into the reduction gearbox limiting the fuel supply to
Spider hubOil transfer sleevePropeller governor (CSU)Propeller
Moves forward from PBA to REV
Servo piston (dome)
Blade actuating leverFeathering spring
Figure 7-49. Pitch change mechanism.
the engine. This prevents the propeller/power turbines from
accelerating beyond 106 percent rpm.
The oil overspeed governor houses a set of flyweights
connected to a control valve that is driven by a beveled gear
mounted on the propeller shaft. [Figure 7-54] The flyweight’s
centrifugal force is acting against two springs: a speeder spring
and a reset spring. When the propeller speed reaches a specified
limit (4 percent over maximum propeller speed), the governor
flyweights lift the control valve and bleed off propeller servo
oil into the reduction gearbox sump, causing the blade angle
to increase. An increase in blade pitch puts more load on the
engine and slows down the propeller. To test the unit, the speed
reset solenoid is activated, and servo oil pressure pushes against
the reset piston to cancel the effect of the reset spring. With less
spring tension acting on the flyweights, the overspeed governor
can be tested at speeds lower than maximum.
On twin installation, a second solenoid valve is mounted on
the overspeed governor and is used in conjunction with the
aircraft autofeather system. The system is switched on for
takeoff and, in the event of an engine malfunction, energizes
the solenoid valve to dump propeller servo oil into the
reduction gearbox sump. The feathering spring and propeller flyweights move the blade quickly to feather.
Hamilton Standard Hydromatic Propellers
Many of the hydromatic propellers are used with older type
aircraft involved in cargo operations. A hydromatic propeller
has a double-acting governor that uses oil pressure on both
sides of the propeller piston. Many larger turboprop systems
also use this type of system. The governors are similar in
construction and principle of operation in normal constant-
speed systems. The major difference is in the pitch-changing
mechanism. In the hydromatic propeller, no flyweights are
used, and the moving parts of the mechanism are completely
enclosed. Oil pressure and the centrifugal twisting moment
of the blades are used together to turn the blades to a lower
angle. The main advantages of the hydromatic propeller are
the large blade angle range and the feathering and reversing
features.
This propeller system is a double-acting hydraulic propeller
system in which the hydraulic pressure (engine oil pressure)
on one piston dome is used against governor oil pressure on
the other side of the piston. These two opposing hydraulic
forces are used to control and change blade angle or pitch.
Although hydromatic propeller systems are very old, some
CODESupply pressure
Return to pump
Propeller servo pressurePyGovernor spring
Hydraulic low pitch adjustmentCarbon blockReset postPropeller speed control lever
Minimum govenor adjustmentBeta valvePressure relief valve
Pilot valve
Engine oil CSU pump
Beta rodPT6A-67B
with fixed propeller speedno movement
Reset armno movement
no movement
Figure 7-50. Governing mode.
are still used on radial engines. Larger new turboprop systems
also use this opposing hydraulic force and double-acting
governor systems.
The distributor valve or engine-shaft-extension assembly
provides oil passages for governor or auxiliary oil to the
inboard side of the piston and for engine oil to the outboard
side. During unfeathering operation, the distributor shifts
under auxiliary pressure and reverses these passages so that
oil from the auxiliary pump flows to the outboard side of the
piston and oil on the inboard side flows back to the engine.
The engine-shaft-extension assembly is used with propellers
that do not have feathering capabilities.
The hydromatic propeller [Figure 7-55] is composed of four
major components:
1. The hub assembly,
2. The dome assembly,
3. The distributor valve assembly (for feathering on single-acting propellers) or engine-shaft-extension
assembly (for nonfeathering or double-acting
propellers), and
4. The anti-icing assembly.
The hub assembly is the basic propeller mechanism. It
contains both the blades and the mechanical means for
holding them in position. The blades are supported by the
spider and retained by the barrel. Each blade is free to turn
about its axis under the control of the dome assembly.
The dome assembly contains the pitch-changing mechanism
for the blades. Its major components are the:
1. rotating cam,
2. fixed cam,
3. piston, and
4. dome shell.
When the dome assembly is installed in the propeller hub,
CODESupply pressure
Return to pump
Propeller servo pressurePyGovernor spring
Hydraulic low pitch adjustmentCarbon blockReset postPropeller speed control lever
Minimum govenor adjustmentBeta valvePilot valve
Engine oil CSU pump
Beta rodno movement
Reset arm
Beta feedback ring moves
forward as prop reaches ≈10°Beta valve lever
Shut-off (lock pitch) solenoid valve
Moves in response to beta feedback ring
Figure 7-51. Beta mode forward operation.
the fixed cam remains stationary with respect to the hub. The
rotating cam, which can turn inside the fixed cam, meshes
with gear segments on the blades. The piston operates inside
the dome shell and is the mechanism that converts engine and
governor oil pressure into forces that act through the cams
to turn propeller blades.
Principles of Operation
The pitch-changing mechanism of hydromatic propellers is
a mechanical-hydraulic system in which hydraulic forces
acting on a piston are transformed into mechanical twisting
forces acting on the blades. Linear movement of the piston
is converted to rotary motion by a cylindrical cam. A bevel
gear on the base of the cam mates with bevel gear segments
attached to the butt ends of the blades, thereby turning the
blades. This blade pitch-changing action can be understood
by studying the schematic in Figure 7-56 .
The centrifugal force acting on a rotating blade includes a
component force that tends to move the blade toward low
pitch. As shown in Figure 7-56 , a second force, engine oil pressure, is supplied to the outboard side of the propeller
piston to assist in moving the blade toward low pitch.
Propeller governor oil, taken from the engine oil supply and
boosted in pressure by the engine-driven propeller governor,
is directed against the inboard side of the propeller piston. It
acts as the counterforce, which can move the blades toward
higher pitch. By metering this high-pressure oil to, or draining
it from, the inboard side of the propeller piston by means of
the constant-speed control unit, the force toward high pitch
can balance and control the two forces toward low pitch. In
this way, the propeller blade angle is regulated to maintain
a selected rpm.
The basic propeller control forces acting on the Hamilton
Standard propeller are centrifugal twisting force and high
pressure oil from the governor. The centrifugal force acting
on each blade of a rotating propeller includes a component
force that results in a twisting moment about the blade center
line that tends, at all times, to move the blade toward low
pitch. Governor pump output oil is directed by the governor
CODESupply pressure
Return to pump
Propeller servo pressurePy
FCU ArmGovernor spring
Hydraulic low pitch adjustmentCarbon blockReset postPropeller speed control lever
Minimum govenor adjustmentBeta valve Pilot valve
Engine oil CSU pump
Beta rodRearward movement
Reset armBeta valve lever
Shut-off (lock pitch) solenoid valve
Rearward movement
Full forward positionPush-pull control
Reversing cam
Figure 7-52. Beta mode reverse operation.
to either side of the propeller piston. The oil on the side of
the piston opposite this high-pressure oil returns to the intake
side of the governor pump and is used over again. Engine oil
at engine supply pressure does not enter the propeller directly
but is supplied only to the governor. During constant-speed
operations, the double-acting governor mechanism sends oil
to one side or the other of the piston as needed to keep the
speed at a specified setting.
Feathering Operation
A typical hydromatic propeller feathering installation is
shown in Figure 7-57 . When the feathering push-button
switch is depressed, the low current circuit is established
from the battery through the push-button holding coil and
from the battery through the solenoid relay. As long as the
circuit remains closed, the holding coil keeps the push button
in the depressed position. Closing the solenoid establishes
the high current circuit from the battery to the feathering
motor pump unit. The feathering pump picks up engine oil
from the oil supply tank, boosts its pressure, if necessary, to the relief valve setting of the pump, and supplies it to the
governor high-pressure transfer valve connection. Auxiliary
oil entering the high-pressure transfer valve connection shifts
the governor transfer valve, which hydraulically disconnects
the governor from the propeller and at the same time opens
the propeller governor oil line to auxiliary oil. The oil flows
through the engine transfer rings, through the propeller shaft
governor oil passage, through the distributor valve port,
between lands, and finally to the inboard piston end by way
of the valve inboard outlet.
The distributor valve does not shift during the feathering
operation. It merely provides an oil passageway to the inboard
piston end for auxiliary oil and the outboard piston end for
engine oil. The same conditions described for underspeed
operation exist in the distributor valve, except that oil at
auxiliary pressure replaces drain oil at the inboard end of
the land and between lands. The distributor-valve spring is
backed up by engine oil pressure, which means that at all
times the pressure differential required to move the piston is
CODESupply from FCU
Oil to reduction gearbox
Propeller servo oil
Py
To sumpFlyweightsSpeeder spring cupReset lever
Py bleed to atmosphere
Air bleed orificePy seal
Pivot
Figure 7-53. Nf overspeed governor.
identical with that applied to the distributor valve.
The propeller piston moves outboard under the auxiliary oil
pressure at a speed proportional to the rate the oil is supplied.
This piston motion is transmitted through the piston rollers
operating in the oppositely inclined cam tracks of the fixed
cam and the rotating cam and is converted by the bevel gears
into the blade-twisting moment. Only during feathering or
unfeathering is the low mechanical advantage portion of the
cam tracks used. (The low mechanical advantage portion lies
between the break and the outboard end of the track profile.)
Oil at engine pressure, displaced from the outboard piston
end, flows through the distributor valve outboard inlet, past
the outboard end of the valve land, through the valve port, into
the propeller shaft engine oil passage, and is finally delivered
into the engine lubricating system. Thus, the blades move
toward the full high-pitch (or feathered) angle.
Having reached the full-feathered position, further movement
of the mechanism is prevented by contact between the high-
angle stop ring in the base of the fixed cam and the stop
lugs set in the teeth of the rotating cam. The pressure in the
inboard piston end now increases rapidly, and upon reaching
a set pressure, the electric cutout switch automatically opens. This cutout pressure is less than that required to shift the
distributor valve.
Opening the switch deenergizes the holding coil and releases
the feathering push-button control switch. Release of this
switch breaks the solenoid relay circuit, which shuts off the
feathering pump motor. The pressures in both the inboard
and outboard ends of the piston drop to zero, and, since all
the forces are balanced, the propeller blades remain in the
feathered position. Meanwhile, the governor high-pressure
transfer valve has shifted to its normal position as soon as
the pressure in the propeller governor line drops below that
required to hold the valve open.
Unfeathering Operation
To unfeather a hydromatic propeller, depress and hold in the
feathering switch push-button control switch. As in the case
of feathering a propeller, the low-current control circuits
from the battery through the holding coil and from the
battery through the solenoid are completed when the solenoid
closes. The high-current circuit from the battery starts the
motor-pump unit, and oil is supplied at a high pressure to
the governor transfer valve.
Reset spring
Reset piston
Feathering solenoid valveFlyweightsSpeed adjusting screw
Governor spline driveSpeed reset solenoidSpeeder spring
From CSUReduces spring tension = lower O/S
Speed reset
oil activation
Figure 7-54. Propeller overspeed governor.
Auxiliary oil entering through the high-pressure transfer
valve connection shifts the governor transfer valve and
disconnects the governor from the propeller line; in the same
operation, auxiliary oil is admitted. The oil flows through the
engine oil transfer rings, through the propeller shaft governor
oil passage, and into the distributor valve assembly.
When the unfeathering operation begins, the piston is in the
extreme outboard position. The oil enters the inboard piston
end of the cylinder by way of the distributor valve inboard
outlet. As the pressure on the inboard end of the piston
increases, the pressure against the distributor valve land builds
up. When the pressure becomes greater than the combined
opposing force of the distributor valve spring and the oil
pressure behind this spring, the valve shifts. Once the valve
shifts, the passages through the distributor valve assembly to
the propeller are reversed. A passage is opened between lands
and through a port to the outboard piston end by way of the
distributor valve outlet. As the piston moves inboard under the
auxiliary pump oil pressure, oil is displaced from the inboard piston end through the inlet ports between the valve lands,
into the propeller shaft engine oil lands, and into the propeller
shaft engine oil passage where it is discharged into the engine
lubricating system. At the same time, the pressure at the cutout
switch increases and the switch opens. However, the circuit
to the feathering pump and motor unit remains complete as
long as the feathering switch is held in.
With the inboard end of the propeller piston connected to
drain and auxiliary pressure flowing to the outboard end of
the piston, the piston moves inboard, unfeathering the blades.
As the blades are unfeathered, they begin to windmill and
assist the unfeathering operation by the added force toward
low pitch brought about by the centrifugal twisting moment.
When the engine speed has increased to approximately 1,000
rpm, the operator shuts off the feathering pump motor. The
pressure in the distributor valve and at the governor transfer
valve decreases, allowing the distributor valve to shift under
the action of the governor high-pressure transfer valve spring.
This action reconnects the governor with the propeller and
Preload shim (use as required)
Governor oil pressure
Centrifugal twisting moment
Low pitch
Engine oil pressure
1. Centrifugal twisting moment
2. Engine oil pressure
3. Governor oil pressureFigure 7-55. Typical hydromatic propeller installation.
Figure 7-56. Diagram of hydromatic propeller operational forces. establishes the same oil passages through the distributor valve
that are used during constant-speed and feathering operations.
Setting the Propeller Governor
The propeller governor incorporates an adjustable stop that
limits the maximum speed at which the engine can run. As
soon as the takeoff rpm is reached, the propeller moves off the
low-pitch stop. The larger propeller blade angle increases the
load on the engine, thus maintaining the prescribed maximum
engine speed. At the time of propeller, propeller governor, or
engine installation, the following steps are normally taken
to ensure that the powerplant obtains takeoff rpm. During
ground runup, move the throttle to takeoff position and note
the resultant rpm and manifold pressure. If the rpm obtained
is higher or lower than the takeoff rpm prescribed in the
Battery
Solenoid relayOil tankPressure cutoff switch Holding coilFeathering button
Oil pump
Figure 7-57. Typical feathering installation. manufacturer’s instructions, reset the adjustable stop on the
governor until the prescribed rpm is obtained.
8-1Introduction
Procedures for removing or installing an aircraft engine
usually vary widely with the type of aircraft and the type of
engine. Thus, no single list of instructions can be provided
as a guideline for all engines. Because of the many types
of engine and aircraft installations and the large number of
design variations within each type or category, representative
examples have been selected to illustrate the most typical
installation procedures for reciprocating, turboprop, and
turbofan engines. There are some common tasks that
must be accomplished when replacing an aircraft engine.
Most engines require disconnecting and reconnecting
electrical, hydraulic, fuel supply, intake and exhaust air
path components, engine controls, and engine mounting
connections to the airframe.
Reciprocating engines and gas turbine engines are used in
this chapter to describe and represent general and typical
procedures for engine buildup, removal, preservation,
storage, and installation. Although these two types of engines
have some common areas, each procedure has been included
to ensure adequate coverage of the engines used in both
heavy and light aircraft. It should be emphasized that while
procedures for specific engines and aircraft are included
in this chapter, many pertinent or mandatory references
have been omitted because of their irrelevance to a general
discussion. For this reason, always reference the applicable
manufacturer’s instructions before performing any phase of
engine removal or installation.
Reasons for Removal of Reciprocating
Engines
The following paragraphs outline the most common reasons
for removing and replacing an engine. Information to aid
in determining engine conditions that require removal
is included; however, in every case, consult applicable
manufacturer’s instructions as the final authority in
establishing the basis for engine replacement.
Engine or Component Lifespan Exceeded
Engine life is dependent upon such factors as operational
use, the quality of manufacture or overhaul, the type of
aircraft in which the engine is installed, the kind of operation
being carried out, and the degree to which maintenance is
accomplished. Thus, the manufacturer sets engine removal
times. Based on service experience, it is possible to establish
Engine Removal & Replacement
Chapter 8
a maximum expected time before overhaul (TBO) or span of
time within which an engine needs to be overhauled. Certain
critical components of turbine engines such as turbine blades,
turbine discs or combustion liners may have shorter life
limits established by the manufacturer due to the stresses
imposed on those parts during operation. The FAA requires
that the manufacture identify and establish which parts have
mandatory replacement times.
Engine Sudden Stoppage or Propeller Strike
Engine sudden stoppage causes a very rapid and complete
engine stoppage. Propeller strikes can cause either reduction
in speed or a complete engine stoppage. Either can be caused
by engine seizure or by one or more of the propeller blades
striking an object in such a way that revolutions per minute
(rpm) goes to zero in less than one complete revolution
of the propeller. Sudden stoppage may occur under such
conditions as complete and rapid collapse of the landing
gear, nosing over of the aircraft, or crash landing. Sudden
stoppage can cause internal damage, such as cracked
propeller gear teeth, gear train damage, crankshaft flyweights
becoming detuned or misalignment, or damaged propeller
bearings. When sudden stoppage occurs, the engine usually
requires replacement or disassembly and inspection as per
manufacturer’s instructions.
Sudden Reduction in Speed
Sudden reduction in engine speed can occur when one or
more of the propeller blades strike an object at a low engine
rpm. After impact, the foreign object is cleared and the engine
recovers rpm and continues to run unless stopped to prevent
further damage. While taxiing an aircraft, sudden reduction in
speed can occur when the propeller strikes a foreign object,
such as a raised section in the runway, a tool box, or a portion of
another airplane. When the accident occurs at high engine rpm,
shocks are much more severe. When sudden reduction in rpm
occurs, the following procedure can be used as a general rule,
but you must comply with the manufacturer’s information.
Make a thorough external inspection of the engine mount,
crankcase, and nose section to determine whether any parts
have been damaged. If damage is found that cannot be
corrected by line maintenance, remove the engine. Internal
components can be damaged, especially counter weights on
the crankshaft.
Remove the engine oil screens or filters. Inspect them for
8-2the presence of metal particles. Remove the engine sump
plugs, drain the oil into a clean container, strain it through a
clean cloth, and check the cloth and the strained oil for metal
particles. Heavy metal particles in the oil indicate a definite
engine failure, and the engine must be removed. However, if
the metal particles present are similar to fine filings, continue
the inspection of the engine to determine its serviceability.
If there are no heavy metal particles in the engine oil, check
again for metal in the oil system after operating the engine.
Metal in the screens is a sign that the bearings have been
compromised and are in the process of failing.
Remove the propeller and check the crankshaft, or the
propeller drive shaft on reduction-gear engines, for
misalignment. Clamp a test indicator to the nose section of the
engine. Use the dial-indicator that has 1⁄1,000-inch graduations.
Remove the spark plugs from all the cylinders. Then, turn the
crankshaft, and observe if the crankshaft, propeller shaft, or
flange turns straight without any bending taking place. If there
is an excessive runout (bend in the crankshaft or propeller
flange) reading at the crankshaft or propeller-drive shaft at
the front seat location, the engine should be removed. Consult
the applicable manufacturer’s instructions for permissible
limits. If the crankshaft or propeller drive shaft runout does
not exceed these limits, install a serviceable propeller. Make
an additional check by tracking the propeller at the tip in the
same plane, perpendicular to the axis of rotation, to assure
that blade track tolerance is within the prescribed limits.
Start the engine to see if operation is smooth, without vibration,
and the power output adequate. If the engine operates properly
during this ground check, shut the engine down and repeat the
inspection for metal particles in the oil system.
Metal Particles in the Oil
Metal particles in the engine oil screens or the magnetic chip
detectors are generally an indication of partial internal failure
of the engine. Carbon tends to break loose from the interior
of the engine in rock-like pieces that have the appearance
of metal. It is necessary to consider these possibilities when
foreign particles are found on the engine oil screens or
magnetic chip detectors.
Before removing an engine for suspected internal failure, as
indicated by foreign material on the oil screens or oil sump
plugs, determine if the foreign particles are ferrous metal by
placing them close to a magnet to see if they are magnetic. If
the material is not magnetic, it is not attracted by the magnet.
Any ferrous metal in the oil screens is cause for concern.
Very small amounts of nonferrous metal, especially after
major engine maintenance, can sometimes be normal. If the
particles are metal, determine the probable extent of internal
damage. For example, if only small particles are found that are similar in nature to filings, drain the oil system, and refill
it. Then, ground-run the engine and reinspect the oil screens
and magnetic chip detectors. If no further evidence of foreign
material is found, continue the engine in service or per the
manufacturer’s instructions. However, engine performance
should be closely observed for any indication of difficulty
or internal failure.
Spectrometric Oil Analysis Engine Inspection
Program
Spectrometric oil analysis program allows an oil sample
to be analyzed and searched for the presence of minute
metallic elements. Due to oil circulation throughout an
aircraft engine, every lubricant that is in service contains
microscopic particles of metallic elements called wear
metals. As the engine operates over a certain amount of time,
the oil picks up very small particles that stay suspended in
the oil. Oil analysis programs identify and measure these
particles in parts per million (PPM) by weight. The analyzed
elements are grouped into categories, such as wear metals
and additives, and their measurement in PPM provides the
data that expert analysts can use as one of many tools to
determine the engine’s condition. If certain metals have an
increase in PPM, it can be a signal of component wear or
pending failure of the engine. The amount of wear metals
is recorded and noted each time a sample is taken. If the
amount of wear metals increases beyond a normal rate, then
the operator can be notified quickly so repair, a recommended
specific maintenance procedure, or inspection can be ordered.
The advantage of oil analysis is an increase in safety by
noticing an engine problem before engine failure. It also saves
money by finding engine problems before they become large
problems or complete engine failure. This procedure can be
used for both turbine and reciprocating engines. Oil analysis
can be used to diagnose impending engine failure and would
be a reason for removing the engine from the aircraft and
sending it to overhaul.
Turbine Engine Condition Monitoring Programs
Many turbine engines are monitored by an engine condition
program that helps determine the health of the engine in
service. This can also be called trend analysis performance
monitoring, but it consists mainly of monitoring certain
engine parameters daily and watching for trend shifts or
changes in the engine parameters. A shift in key parameters
(change over time) could be a warning that the engine has
serious internal deterioration and should be overhauled.
Engine Operational Problems
Engines are usually removed when there are consistent
engine operational problems. Engine operational problems
generally include, but are not limited to, one or more of the
Figure 8-1. Open cowling view of a typical power package.
Figure 8-2. Typical firewall with components mounted on it.following conditions:
1. Excessive engine vibration; this is especially true with
turbine engines.
2. Backfiring, or misfiring, either consistent or
intermittent due to valve train or other mechanical
defect in reciprocating engines.
3. Turbine engines that exceed normal operating
parameters or life limited components exceeding
maximum time in service or cycles.
4. Low power output, generally caused by low
compression, with reciprocating engines and internal
engine deterioration or damage with turbines.
General Procedures for Engine Removal &
Installation
Preparation of Engines for Installation
After the decision has been made to remove an engine, the
preparation of the replacement engine must be considered.
The maintenance procedures and methods used vary widely.
Commercial operators, whose maintenance operations
require the most efficient and expeditious replacement of
aircraft engines, usually rely on a system that utilizes the
quick-engine-change assembly (QECA), also sometimes
referred to as the engine power package. The QECA is
essentially a powerplant and the necessary accessories
installed in the engine.
Other operators of aircraft equipped with reciprocating
engines sometimes use a different replacement method in
these repair facilities because engine changes often occur
at random intervals. Such replacement engines may be
partially or wholly built up with the necessary accessories
and subassemblies, or they may be stored as received from
the manufacturer in packing boxes, cases, or cans and are
uncrated and built up for installation only when needed to
replace an engine.
QECA Buildup Method for Changing of Engines
Because the QECA system is most commonly used with large
turbine engines used in the airlines, such engines are used to
describe QECA buildup and installation procedures. Many
of these procedures are applicable to all other methods of
engine buildup and installation.
The following study of QECA buildup is not designed to
outline procedures to follow in a practical application;
always use those recommended by the manufacturer.
The procedures included in this chapter provide a logical
sequence in following a QECA and its components through
the stages of a typical buildup to gain a better understanding
of units and systems interconnection. The components of a QECA are illustrated in Figure 8-1. As shown, the QECA
consists of several units. On many aircraft, the engines are
mounted in streamlined housings called nacelles that extend
from the wings. These nacelles are divided into two main
sections: wing nacelle and engine nacelle. The wing nacelle
is that portion of the nacelle that is attached to the wing
structure. The engine nacelle is that portion of the nacelle
that is constructed separately from the wing. Also, the wing
nacelles normally contain lines and units of the oil, fuel, and
hydraulic systems, as well as linkages and other controls for
the operation of the engine.
The firewall is usually the foremost bulkhead of the engine
nacelle and differs from most other aircraft bulkheads in that
it is constructed of stainless steel or some other fire-resistant
material. [Figure 8-2] The primary purpose of the firewall is to
Figure 8-3. Draining corrosion preventive compound.confine any engine fire to the engine nacelle. It also provides
a mounting surface for units within the engine nacelle and a
point of disconnect for lines, linkages, and electrical wiring
that are routed between the engine and the aircraft. Without
this firewall, an engine fire would have ready access to the
interior of the aircraft. Since the consequences of an engine
fire are obvious, the necessity of sealing all unused openings
in the firewall cannot be overstressed.
An aircraft engine and its accessories that have been in
storage must undergo careful depreservation and inspection
before they may be installed in an aircraft. This involves
more than removing an engine from its container and bolting
it to the aircraft. If the engine is stored in a pressurized metal
container, the air valve should be opened to bleed off the
air pressure. Depending upon the size of the valve, the air
pressure should bleed off in somewhat less than 30 minutes.
Prepare the container for opening by removing the bolts that
hold the two sections together. Then, attach a hoist to the
“hoisting points” and lift the top section clear of the container
and place it away from the work area. If the engine is installed
in a wooden shipping case, it is necessary to carefully break
the seal of the protective envelope and fold it down around
the engine. Remove the dehydrating agent or desiccant bags
and the humidity indicator from the outside of the engine.
Also, remove and set safely aside any accessories that are
not installed on the engine but are mounted on a special
stand or otherwise installed inside the protective envelope
with the engine.
Depreservation of an Engine
After the engine has been secured to an engine stand, all
covers must be removed from the points where the engine was
sealed or closed with ventilatory covers, such as the engine
breathers, exhaust outlets, and accessory mounting-pad cover
plates. As each cover is removed, inspect the uncovered part
of the engine for signs of corrosion. Also, as the dehydrator
plugs are removed from each cylinder, make a very careful
check of the walls of any cylinder for which the dehydrator
plug color indicates an unsafe condition. Care is emphasized
in the inspection of the cylinders, even if it is necessary to
remove a cylinder.
On radial engines, the inside of the lower cylinders and
intake pipes should be carefully checked for the presence of
excessive corrosion-preventive compound that has drained
from throughout the interior of the engine and settled at these
low points. This excessive compound could cause the engine
to become damaged from a hydraulic lock (also referred to
as liquid-lock) when a starting attempt is made.
The check for excessive corrosion-preventive compound in
the cylinders of reciprocating engines can be made as the dehydrator plugs are removed from each cylinder. Much of
the compound drains from the spark plug holes of the lower
cylinders of a radial engine when the dehydrator plugs are
removed. But some of the mixture remains in the cylinder head
below the level of the spark plug hole and can be removed
with a hand pump. [Figure 8-3] A more positive method is
to remove the lower intake pipes and open the intake valve
of the cylinder by rotating the crankshaft. This latter method
allows the compound to drain from the cylinder through the
open intake valve. If excessive compound is present in an
upper cylinder, it can be removed with a hand pump.
The oil screens should be removed from the engine and
thoroughly washed in an approved solvent to remove all
accumulations that could restrict the oil circulation and cause
engine failure. After the screens are cleaned, immerse them
in clean oil and then reinstall them in the engine.
When the cover has been removed from the intake area, the
silica gel desiccant bags (used to remove moisture from the
engine in storage) must be removed from the engine area. If
the engine uses a propeller, remove the protective covering
from the propeller shaft and wash all corrosion-preventive
compounds from both the inside and outside surfaces of the
shaft. Then, coat the propeller shaft lightly with engine oil.
Turbine engines require the removal of several covers on
many external areas on the engine.
As a final check, see that the exterior of the engine is clean.
Usually a quantity of compound runs out of the engine when
the dehydrator plugs and oil screens are removed. To clean
the engine, spray it with an approved commercial solvent.
8-5Inspection & Depreservation of Accessories
An engine’s performance is no better than that of its
accessories. Though the engine has been completely
overhauled and is in top condition, any oversight or error
in installing the accessories can result in improper engine
operation or even irreparable damage to it.
Before depreserving any of the accessories enclosed with
the engine, consult the storage data usually stenciled on the
outside of the engine container or the records enclosed with
the engine to determine how long the engine and accessories
were in storage. Certain accessories that normally accompany
an engine from overhaul are considered unsafe for use if their
time in storage has exceeded a specified period. This time
varies according to the limits prescribed by the manufacturer.
Any accessory that has been removed from an old engine that
can be installed on the new one must be given a thorough
inspection to determine its condition. This inspection
includes a check for general condition, cleanliness, absence
of corrosion, and absence of wear as evidenced by excessive
play in the moving parts.
Some accessories must be replaced, regardless of their
operating time, if the engine is being changed because
of internal failure. Such accessories may have been
contaminated by metal particles carried into their operating
mechanisms by the engine oil that lubricates them.
Before installing any replacement accessory, check it
visually for signs of corrosion and for freedom of operation.
Always wipe the mounting pad, flange, and coupling clean
before mounting the accessory, and install the proper gasket
between the mounting pad and the accessory mounting
flange. Lubricate the accessory drive shaft if so indicated in
the manufacturer’s instructions.
Inspection & Replacement of Powerplant
External Units & Systems
The engine nacelle must be cleaned thoroughly before it is
inspected. The design of an engine nacelle varies with different
aircraft. Basically, it is a framework covered with removable
cowling, in which the engine is mounted. This assembly is
attached to the aircraft and incorporates an insulating firewall
between the engine and the airframe. The interconnecting
wiring, tubing, and linkages between the engine and its various
systems and controls pass through the firewall.
Inspect the complete engine nacelle for condition of the
framework and the sheet-metal cowling and riveted plates
that cover the nacelle. The engine mounting frame assembly
should be checked for any distortion of the steel tubing, such
as bends, dents, flat spots, corrosion, or cracks. Use the dye penetrant inspection method to reveal a crack, porous area,
or other defects.
The engine mounting bolts are usually checked for condition
by magnetic particle inspection or other approved processes.
While the bolts are removed, the bolt holes should be checked
for elongation caused by the movement of an improperly
tightened bolt.
Check the outer surface of all exposed electrical wiring for
breaks, chafing, or other damage. Also, check the security
of crimped or soldered cable ends. In addition, carefully
inspect connector plugs for overall condition. Any item that
is damaged must be repaired or replaced, depending on the
extent of the damage.
Before installing an engine, inspect all tubing in the nacelle
for dents, nicks, scratches, chafing, or corrosion. Check all
tubing carefully for indications of fatigue or excessive flatness
caused by improper or accidental bending. Thoroughly
inspect all hoses used in various engine systems. Weather
checking (a cracking of the outside covering of the hose)
sometimes penetrates to the hose reinforcement. Replace any
length of hose that shows indications of the cover peeling or
flaking or has exposed fabric reinforcement. Replace a hose
that shows indications of excessive cold flow. Cold flow is
a term used to describe the deep and permanent impressions
or cracks caused by hose clamp pressure.
Always replace a control rod if it is nicked or corroded
deeply enough to affect its strength. If the corrosion cannot
be removed by rubbing with steel wool, the pitting is too
deep for safety.
On older aircraft, check the pulleys in the control system
for freedom of movement. It is easy to spot a pulley that is
not turning freely, for both it and the cable are worn from
the cable sliding over the pulley instead of rolling free. The
bearings of a pulley may be checked by inspecting the pulley
for excessive play or wobble with the tension removed from
the cable. The cable must also be inspected for corrosion
and broken strands. Locate any broken strands by wiping
the cable with a cloth.
Check bonding for fraying, loose attachment, and cleanness
of terminal ends. The electrical resistance of the complete
bond must not exceed the resistance values specified in the
applicable manufacturer’s instructions.
Inspect the exhaust stacks, collector ring, and tailpipe assembly
for security, cracks, or excessive corrosion. Depending on the
installation, these units, or parts of them, may be mounted on
the engine before it is installed in the aircraft.
Drains
Figure 8-4. Oil system drain points.Check all air ducts for dents and for the condition of the
fabric or rubber anti-chafing strips at the points where
sections of duct are joined. The dents may be pounded out;
the anti-chafing strips should be replaced if they are pulled
loose from the duct or are worn to the point at which they
no longer form a tight seal at the joint.
Thoroughly inspect the engine oil system and perform any
required special maintenance upon it before installing a
replacement engine. If an engine is being changed at the end
of its normal time in service, it is usually necessary only to
flush the oil system; however, if an engine has been removed
for internal failure, usually some units of the oil system must
be replaced and others thoroughly cleaned and inspected.
If the engine has been removed because of internal failure,
the oil tank is generally removed to permit thorough cleaning.
Also, the oil cooler and temperature regulator must be
removed and sent to a repair facility for overhaul. The vacuum
pump pressure line and the oil separator in the vacuum system
must also be removed, cleaned, and inspected. Internal failure
also requires that the propeller governor and feathering pump
mechanism be replaced if these units are operated by engine
oil pressure.
Preparing the Engine for Removal
Before starting to work on the aircraft or reciprocating engine,
always be sure that the magneto switch is in the off position.
Aircraft engines can be started accidentally by turning the
propeller if the magneto switch is on.
Check to see that all fuel selectors or solenoid-operated fuel
shutoff valves are closed. The fuel selector valves are either
manually or solenoid operated. If solenoid-operated fuel
shutoff valves are installed, it may be necessary to turn the
battery switch on before the valves can be closed, since the
solenoid depends on electricity for operation. These valves
close the fuel line at the firewall between the engine and the
aircraft. After ensuring that all fuel to the engine is shut off,
disconnect the battery to eliminate the possibility of a hot
wire starting a fire. If it is anticipated that the aircraft will
be out of service for more than 6 days, the battery is usually
removed and taken to the battery shop and placed on charge.
Also, a few other preparations should be made before starting
to work on the engine removal. First, make sure that there are
enough fire extinguishers near at hand to meet any possible
emergency. Check the seals on these extinguishers to be sure
the extinguishers have not been discharged. Then, check
the wheel chocks. If these are not in place, the aircraft can,
and probably will, inch forward or back during some crucial
operation. Also, if the aircraft has a tricycle landing gear, be sure that the tail is supported so that the aircraft cannot tip back
when the weight of the engine is removed from the forward
end. It is not necessary to support the tail on some multiengine
aircraft if only one engine is to be removed. In addition, the
landing gear shock struts can be deflated to prevent them from
extending as the engine weight is removed from the aircraft.
After taking these necessary precautions, begin removing the
cowling from around the engine. As it is removed, clean it and
check for cracks so that the necessary repairs can be made
while the engine change is in progress. Place all cowling that
does not need repair on a rack where it can be readily found
when the time comes to reinstall it on the new engine. After
removing the cowling, the propeller should be removed for
inspection or repair.
Draining the Engine
Place a large metal pan (drip pan) on the floor under the
engine to catch any spilled mixture or oil. Next, secure a clean
container in which to drain the oil or corrosion-preventive
mixture. Place the container beneath the engine, open the
drain valve, and allow the oil to drain. Figure 8-4 shows the
points at which a typical aircraft engine oil system is drained.
Other points at which the oil system is drained can typically
include the oil cooler, oil return line, and engine sumps.
All valves, drains, and lines must remain open until the oil
system has been completely drained. After draining the oil,
reinstall all drain plugs and close all drain valves. Then, wipe
all excess oil from around the drain points.
Electrical Disconnects
Electrical disconnections are usually made at the engine
firewall. When the basic engine is being removed, the
electrical leads to such accessories as the starter and
generators are disconnected at the units themselves. When
Figure 8-5. Electrical connections.
Figure 8-6. Engine control cable and turnbuckle assembly. disconnecting electrical leads, it is a good safety habit to
disconnect the magnetos first and immediately ground them
at some point on the engine or the assembly being removed.
Most firewall disconnections of electrical conduit and cable
are simplified by use of (Army/Navy) AN or (Military
Standard) MS connectors. Each connector consists of two
parts: a plug assembly and a receptacle assembly. To prevent
accidental disconnection during airplane operation, the outlet
is threaded to permit a knurled sleeve nut to be screwed to
the outlet and then fastened with safety wire, if necessary.
A typical plug fitting assembly is shown in Figure 8-5. It
also shows a typical junction box assembly, which is used as
a disconnect on some aircraft engine installations. After the
safety wire is broken, remove all of it from the sleeve nuts that
hold the conduit to the junction boxes, as well as from the nuts
on the connectors. Wrap moisture proof tape over the exposed
ends of connectors to protect them from dirt and moisture.
Also, do not leave long electrical cables or conduits hanging
loose, since they may become entangled with some part of the
aircraft while the engine is being hoisted. It is a good practice
to coil all lengths of cable or flexible conduit neatly, and tie or tape them to some portion of the assembly being removed.
Disconnection of Engine Controls
The engine control rods and cables connect such units as
the carburetor or fuel control throttle valve and the mixture
control valve with their manually actuated control in the flight
deck. The controls are sometimes disconnected by removing
the turnbuckle that joins the cable ends. [Figure 8-6] A typical
reciprocating engine control linkage consisting of a control
rod attached to a bell crank is illustrated in Figure 8-7.
The control rod in the linkage shown has two rod-end
assemblies, a clevis, and an eye screwed onto opposite ends.
These rod-end assemblies determine the length of the control
rod by the distance they are screwed onto it, and they are
locked into position by check nuts. An antifriction bearing is
usually mounted in the eye end of a rod. This eye is slipped
over a bolt in the bell crank arm and is held in position by
a castle nut safety with a cotter pin. The clevis rod end is
slipped over the end of a bell crank arm, which also usually
contains an antifriction bearing. A bolt is passed through the
clevis and the bell crank eye, fastened with a castle nut, and
safetied with a cotter pin. Most control rod ends are provided
with a test hole in the shank for inspection. If safety wire can
be inserted through the test hole, the terminal is not being
held by the required number of threads.
Sometimes linkage assemblies do not include the antifriction
bearings and are held in position only by a washer and cotter
pin in the end of a clevis pin that passes through the bell
crank and rod end. After the engine control linkages have
been disconnected, the nuts and bolts should be replaced in
the rod ends or bell crank arms to prevent their being lost.
All control rods should be removed completely or tied back
to prevent them from being bent or broken if they are struck
by the replacement engine or QECA as it is being hoisted.
Clevis Control rod
ChecknutsBellcrankEye
Sleeve and nut fitting
Threaded fitting and receptacle
Hose clamp and fitting
Interconnecting hoseFigure 8-7. Engine control linkage assembly.
Figure 8-8. Types of line disconnects.
Disconnection of Lines
The lines between units within the aircraft and the engine
are either flexible hose or aluminum-alloy tubes joined by
lengths of hose clamped to them. Lines that must withstand
high pressure, such as hydraulic lines, are often made of
stainless steel tubing.
Figure 8-8 shows the basic types of line disconnects. Most
lines leading from a QECA are secured to a threaded fitting
at the firewall by a sleeve nut around the tubing. Hoses are
sometimes secured in this manner but may also be secured
by a threaded fitting on the unit to which they lead, or by a hose clamp. The firewall fittings for some lines have a quick-
disconnect fitting that contains a check valve to prevent the
system from losing fluid when the line is disconnected. Metal
tubing on some installations may also be disconnected at a
point at which two lengths of it are joined together by a length
of rubber hose. Such a disconnection is made by loosening
the hose clamps and sliding the length of rubber hose over
the length of tubing that remains on the aircraft. There may
be some further variations in these types of disconnections,
but they follow the same basic pattern.
Some type of a container should be used to collect any fuel,
oil, or other fluid that may drain from the disconnected lines.
After the lines have drained, they should be immediately
plugged or covered with moisture-proof tape to prevent
foreign matter from entering them, as well as to prevent any
accumulated fluid from dripping out.
Other Disconnections
The points at which the various air ducts are disconnected
depend upon the engine and the aircraft in which it is
installed. Usually, the air intake ducts and the exhaust system
must be disconnected so the basic engine or the QECA can
be removed. After the engine connections are free (except
the engine mounts) and all the disconnections are entirely
clear so they do not bind or become entangled, the engine
can be prepared for hoisting.
Removing the Engine
If a QECA is being removed, the engine mount accompanies
the engine. The mount remains on the aircraft if only the
engine is being removed. Before the engine can be freed
from its attachment points, a sling must be installed so the
engine’s weight can be supported with a hoist when the
mounting bolts are removed.
Aircraft engines, or QECAs, have marked points for attaching
Engine lifting point
Engine lifting point
Figure 8-9. Hoisting sling attached to engine.
Figure 8-10. Hoist and frame assembly used for engine removal. a hoisting sling. The location of these attaching points varies
according to the size and weight distribution of the engine.
Figure 8-9 shows a sling supporting an engine that has two
attaching points. As a matter of safety, carefully inspect the
sling for condition before installing it on the engine.
Before attaching the sling to the hoist, be sure that the hoist
has sufficient capacity to lift the engine safely. The engine’s
center of gravity (CG) should also be taken into account as
the engine is hoisted. A manually operated hoist mounted in a portable frame is shown in Figure 8-10. This hoist assembly
is specifically manufactured for the purpose of removing
engines and other large assemblies from aircraft. Some
frames are fitted with power-operated hoists. These should
be used with care, since considerable damage can be done
if an inexperienced operator allows a power-operated hoist
to overrun. The hoist and frame should also be checked for
condition before being used to lift the engine.
Hoisting the Engine
Before the hoist is hooked onto the engine sling, recheck the
aircraft tail supports and the wheel chocks. Fasten lines to the
engine, at points on the sides or rear, so that the engine can
be controlled as it is being hoisted. Hook the hoist onto the
sling and hoist the engine just enough to relieve the engine
weight from the mount attachments. Remove the nuts from
the mount attachments in the order recommended in the
manufacturer’s instructions for the aircraft. As the last nuts
are being removed, pull back on the lines fastened to the
engine (or force it back by other means if lines are not being
used), thus steadying the engine. If bolts must be removed
from the mount attachments, be sure the engine is under
control before doing so. If the bolts are to remain in the mount
attachments, the hoist can be gently maneuvered upward or
downward as necessary after all the nuts have been removed.
Meanwhile, gently relax the backward force on the engine
just enough to allow the engine gradual forward movement
when it is free from the mount attachments. When the hoist
has removed all engine weight from the mount attachments,
the engine should be eased gently forward, away from the
aircraft. If the engine binds at any point, maneuver it with
the hoist until it slips free.
The procedure just discussed applies to removal of most
reciprocating and turbine aircraft engines. Any variation
in details is outlined in the manufacturer’s instructions.
Before attempting any engine removal, always consult these
instructions for the aircraft concerned. When the engine has
been removed, it can be carefully lowered onto a stand. The
engine should be fastened to the stand and prepared for the
removal of accessories.
Hoisting & Mounting the Engine for Installation
When the new or overhauled engine is ready to be hoisted
for installation, move the engine stand as close as possible
to the nacelle in which the replacement is to be installed.
Then, attach the sling to the engine and hook the hoist to the
sling. Take up the slack until the hoist is supporting most of
the engine weight. Next, remove the engine attaching bolts
from the stand and hoist the engine clear.
The engine stand may be moved and the hoist frame
positioned so the engine can be hoisted easily into the nacelle.
8-10To prevent injury to the crew or damage to the aircraft or
engine, be sure that the engine is steadied when moving the
hoist frame.
Engine nacelles are rarely designed for the engine to be
fitted and bolted into place as though it were being mounted
on a bare wall. The engine must be guided into position and
mated with its various connections, such as the mounting
bolt holes and the exhaust tailpipe. This must be done despite
such obstacles as the nacelle framework, ducts, or firewall
connections and without leaving a trail of broken and bent
parts, scratched paint, or crushed fingers.
When the engine has been aligned correctly in the nacelle,
insert the mounting bolts into their holes and start all
of the nuts on them. Always use the type of bolt and
nut recommended by the manufacturer. Never use an
unauthorized substitution of a different type or specification
of nut and bolt than that prescribed.
The nuts on the engine mount bolts must be tightened to the
torque recommended by the aircraft manufacturer. While the
nuts are being tightened, the hoist should support the engine
weight sufficiently to allow alignment of the mounting bolts.
If the engine is permitted to exert upward or downward
pressure on the bolts, it is necessary for the nuts to pull the
engine into proper alignment. This results in nuts being
tightened to the proper torque value without actually holding
the engine securely to the aircraft.
The applicable manufacturer’s instructions outline the
sequence for tightening the mounting bolts to ensure security
of fastening. After the nuts are safetied and the engine sling
and hoist are removed, bonding strips should be connected
across each engine mount to provide an electrical path from
the mount to the airframe.
Mounting the engine in the nacelle is, of course, only the
beginning. All the ducts, electrical leads, controls, tubes, and
conduits must be connected before the engine can be operated.
Connections & Adjustments
There are no hard-and-fast rules that direct the order in which
units or systems should be connected to the engine. Each
maintenance organization normally supplies a worksheet
or checklist to be followed during this procedure. This list
is based upon past engine installations on each particular
aircraft. If this is followed carefully, it serves as a guide for
an efficient installation. The following instructions are not a
sequence of procedures but a discussion of correct methods
for completing an engine installation.
The system of ducts for routing air to the engine varies with all types of aircraft. In connecting them, the goal is to fit the
ducts closely at all points of disconnect so that the air they
route does not escape its intended path. The duct systems of
some aircraft must be pressure checked for leaks. This is done
by blocking the system at one end, supplying compressed air
at a specified pressure at the other end, and then checking
the rate of leakage.
The filters in the air induction system must be cleaned to
ensure an unrestricted flow of clean air to the engine and
its units. Because methods for cleaning air filters vary with
the materials used in the filtering element, clean them in
accordance with the technical instructions for the aircraft
being serviced.
The exhaust system should also be carefully connected
to prevent the escape of hot gases into the nacelle. When
assembling the exhaust system, check all clamps, nuts,
and bolts, and replace any in doubtful condition. During
assembly, the nuts should be gradually and progressively
tightened to the correct torque. The clamps should be tapped
with a rawhide mallet as they are being tightened to prevent
binding at any point. On some systems, a ball joint connects
the stationary portion of the exhaust system to the portion
that is attached to the engine. This ball joint absorbs the
normal engine movement caused by the unbalanced forces
of the engine operation. Ball joints must be installed with
the specified clearance to prevent binding when expanded
by hot exhaust gases.
Hoses used inside low-pressure systems are generally
fastened into place with clamps. Before using a hose clamp,
inspect it for security of welding or riveting and for smooth
operation of the adjusting screw. A clamp that is badly
distorted or materially defective should be rejected. Material
defects include extremely brittle or soft areas that may easily
break or stretch when the clamp is tightened. After a hose is
installed in a system, it should be supported with rubber-lined
supporting clamps at regular intervals.
Before installing metal tubing with threaded fittings, ensure
the threads are clean and in good condition. Apply sealing
compound, of the correct specification for the system, to
the threads of the fittings before installing them. While
connecting metal tubing, follow the same careful procedure
for connecting hose fittings to prevent cross-threading and
to ensure correct torque.
When connecting the starter, generator, or various other
electrical units within the nacelle, make sure that all lead
connections are clean and properly secured. On leads that
are fastened to a threaded terminal with a nut, a lock washer
is usually inserted under the nut to prevent the lead from
ABC
DE
F
GHJ
K
A
B
CD
E
FG
H
JK Serrated throttle control arm
Control rod
Adjustable rod endsCable drum
Tensiometer
Cushion movementQuadrant locking pin
Cable turnbuckle
Cable drum locking pinThrottle stop
Figure 8-11. Schematic drawing of throttle control system. working loose. When required, connector plugs can be
safetied with steel wire to hold the knurled nut in the full-
tight position.
Electrical leads within the engine nacelle are usually passed
through either flexible or rigid conduit. The conduit must be
anchored, as necessary, to provide a secure installation and
bonded when required.
All engine controls must be accurately adjusted to ensure
instantaneous response to the control setting. For flexibility,
the engine controls are usually a combination of rods and
cables. Since these controls are tailored to the model of
aircraft in which they are installed, their adjustment must
follow exactly the step-by-step procedure outlined in the
manufacturer’s instructions for each particular model
of aircraft.
Figure 8-11 illustrates a simplified schematic drawing of a
throttle control system for a reciprocating aircraft engine.
Follow a general procedure for adjusting throttle controls.
First, loosen the serrated throttle control arm at the carburetor
and back off the throttle stop until the throttle valve is in the fully closed position. After locking the cable drum into
position with the locking pin, adjust the control rod to a
specified length. Then, attach one end of the control rod to
the locked cable drum, and reinstall the throttle control arm
on the carburetor in the serrations that allow the other end of
the control rod to be attached to it. This correctly connects
the control arm to the cable drum.
Now, loosen the cable turnbuckles until the throttle control
can be locked at the quadrant with the locking pin. Then, with
both locking pins in place, adjust the cables to the correct
tension as measured with a tensiometer. Remove the locking
pins from the cable drum and quadrant.
Next, adjust the throttle control so that it has a slight cushion
action at two positions on the throttle quadrant: one when
the carburetor throttle valve is in the full-open position and
the other when it is closed to the idle position (stop to stop).
Adjust the cushion by turning the cable turnbuckles equally
in opposite directions until the throttle control cushion is
correct at the full-open position of the throttle valve. Then,
when the throttle arm stop is adjusted to the correct idle speed
Figure 8-12. Pre-oiler tank. setting, the amount of cushion should be within tolerance at
the idle speed position of the throttle valve. The presence of
this cushion ensures that the travel of the throttle valve is not
limited by the stops on the throttle control quadrant, but that
they are opening fully and closing to the correct idle speed
as determined by the throttle arm stop.
Adjustment of the engine controls is basically the same on all
aircraft regarding the linkage adjustment to a predetermined
length for a specific setting of the unit to be controlled. Then
adjust cables, if used in the control system, to a specific
tension with the control system locked. Finally, the full
travel of the unit to be controlled is ensured by establishing
the correct cushion in the controls. In general, the same basic
procedure is used to connect the linkage of the remaining
engine controls. After rigging the engine controls, safety the
turnbuckles and castle nuts, and make certain the jam nuts
on all control rods are tightened.
On multiengine aircraft, the amount of cushion of all engine
controls on each quadrant must be equal so that all are
aligned at any specific setting chosen. This eliminates the
necessity of setting each control individually to synchronize
engine operations.
After the engine has been installed, it is necessary to adjust
the cowl flaps, if installed, so that the passage of the cooling
air over the engine can be regulated accurately. Operate the
system and recheck for opening and closing to the specified
limits. Also, check the cowl flap position indicators, if
installed, to ensure that they indicate the true position of the
cowl flaps. Cowl flaps are doors at the bottom of the rear
cowling that are used to control air flow through the cowling.
The oil cooler doors are adjusted in a manner similar to that
used to adjust the cowl flaps. In some cases, the procedure
is reversed in so far as the door is first adjusted to retract to
a specified point, and the limit switch on the motor is set to
cut out at this point. Then, the jackscrew is adjusted to permit
the door to open only a specified distance, and the open limit
switch is set to stop the motor when this point is reached.
After the engine has been completely installed and connected,
install the propeller on the aircraft. Before doing so, the thrust
bearing retaining nut should be checked for correct torque. If
required, the propeller shaft must be coated with light engine
oil before the propeller is installed; the propeller governor and
anti-icing system must be connected according to applicable
manufacturer’s instructions.Preparation of Engine for Ground & Flight
Testing
Pre-Oiling
Before the new engine is flight tested, it must undergo a
thorough ground check. Before this ground check can be
made, several operations are usually performed on the engine.
To prevent failure of the engine bearings during the initial
start, the engine should be pre-oiled. When an engine
has been idle for an extended period of time, its internal
bearing surfaces are likely to become dry at points where
the corrosion-preventive mixture has dried out or drained
away from the bearings. Hence, it is necessary to force oil
throughout the entire engine oil system. If the bearings are dry
when the engine is started, the friction at high rpm destroys
the bearings before lubricating oil from the engine-driven oil
pump can reach them.
There are several methods of pre-oiling an engine. The
method selected should provide an expeditious and adequate
pre-oiling service. Before using any pre-oiling method,
remove one spark plug from each cylinder to allow the engine
to be turned over more easily with the starter. Also, connect
an external source of electrical power (auxiliary power unit)
to the aircraft electrical system to prevent an excessive drain
on the aircraft battery.
In using some types of pre-oilers, such as that shown in
Figure 8-12, the oil line from the inlet side of the engine-
driven oil pump must be disconnected to permit the pre-oiler
tank to be connected at this point. Then, a line must be
disconnected, or an opening made in the oil system at the
nose of the engine, to allow oil to flow out of the engine.
Oil flowing out of the engine indicates the completion of the
pre-oiling operation, since the oil has now passed through
the entire system.
8-13In order to force oil from the pre-oiler tank through the
engine, apply air pressure to the oil in the tank while the
engine is being turned through with the starter. When this
action has forced oil through the disconnection at the nose
of the engine, stop cranking the engine and disconnect the
pre-oiler tank. A motor-driven oil pump can also be used to
pump oil through the engine during the pre-oiling operation.
When no external means of pre-oiling an engine are available,
the engine oil pump may be used. Fill the engine oil tank,
or crankcase, to the proper level. Then, with the mixture
in the idle cutoff position (reciprocating engine), the fuel
shutoff valve and ignition switches in the off position, and
the throttles fully open, crank the engine with the starter
until the oil pressure gauge mounted on the instrument panel
indicates oil pressure.
After the engine has been pre-oiled, replace the spark plugs
and connect the oil system. Generally, the engine should be
operated within 4 hours of being pre-oiled; otherwise, the
pre-oiling procedure normally must be repeated.
Fuel System Bleeding
To purge the fuel system of air locks, and to aid in flushing
any traces of preservative oil from a pressure carburetor,
fuel injector unit, or fuel control (turbine), remove the drain
plug in the fuel unit chamber that is farthest from the fuel
inlet to the fuel unit. In its place, screw a threaded fitting to
a length of hose leading to a suitable container. Then, set
the fuel control unit to flow fuel so that fuel is permitted to
flow through the system. After ensuring the fuel shutoff and
main fuel tank valves are open, turn on the fuel boost pump
until there are no traces of preservative oil in the fuel being
pumped through the system. The passage of air is indicated
by the absence of air mixed in the fuel emerging from the end
of the hose submerged in the container of fuel. Air trapped in
the system should not be confused with the numerous small
air bubbles that may appear as a result of the velocity of the
fuel being ejected from the engine’s fuel unit. Usually, after
approximately a gallon of fuel has been bled off, the system
can be considered safe for operation. After completing the
bleeding operation, return all switches and controls to their
normal, or off, position, and replace and safety all fuel unit
connections disturbed.
Propeller Check
The propeller, if equipped, must be checked before, during,
and after the engine has been ground operated. The propeller
should be checked for proper torque on the mounting bolts,
leaks, vibration, and for correct safety.
A propeller whose pitch-changing mechanism is electrically actuated may be checked before the engine is operated.
Propellers whose pitch-changing mechanisms are oil actuated
must be checked during engine operation after the normal
operating oil temperature has been reached. In addition to
checking the increase or decrease in rpm, the feathering cycle
of the propeller should also be checked.
Checks & Adjustments After Engine Runup &
Operation
After the engine has been ground operated, and again after
flight test, operational factors must be adjusted, as necessary,
and the entire installation given a thorough visual inspection.
These adjustments often include fuel pressure and oil pressure,
as well as rechecks of such factors as ignition timing, valve
clearances, and idle speed and mixture. If these rechecks are
indicated by the manner in which the engine performs.
After both the initial ground runup and the test flight, remove
the oil sump plugs and screens and inspect for metal particles.
Clean the screens before reinstalling them.
Check all lines for leakage and security of attachment.
Especially, check the oil system hose clamps for security
as evidenced by oil leakage at the hose connections. Also,
inspect the cylinder holddown nuts or cap screws for security
and safety. This check should also be performed after the
flight immediately succeeding the test flight.
Rigging, Inspections, & Adjustments
The following instructions cover some of the basic
inspections and procedures for rigging and adjusting fuel
controls, fuel selectors, and fuel shutoff valves.
1. Inspect all bellcranks for looseness, cracks, or
corrosion.
2. Inspect rod ends for damaged threads and the number
of threads remaining after final adjustment.
3. Inspect cable drums for wear and cableguards for
proper position and tension.
While rigging the fuel selector, power controls, and shutoff
valve linkages, follow the manufacturer’s step-by-step
procedure for the particular aircraft model being rigged.
The cables should be rigged with the proper tension with
the rigging pins installed. The pins should be free to be
removed without any binding; if they are hard to remove, the
cables are not rigged properly and should be rechecked. The
power lever should have the proper cushion at the idle and
full-power positions. The pointers, or indicators, on the fuel
control should be within limits. The fuel selectors must be
rigged so that they have the proper travel and do not restrict
the fuel flow to the engines.
Pylon and nacelle control
Power levers
Control rodsWing front span
Center console
Figure 8-13. Power lever control system. Rigging Power Controls
Many older conventional turbofan engines use various power
lever control systems. One of the common types is the cable
and rod system. This system uses bellcranks, push-pull rods,
drums, fairleads, flexible cables, and pulleys. All of these
components make up the control system and must be adjusted
or rigged from time to time. On single-engine aircraft, the
rigging of the power lever controls is not very difficult. The
basic requirement is to have the desired travel on the power
lever and correct travel at the fuel control. On multiengine
turbojet aircraft, the power levers must be rigged so that they
are aligned at all power settings.
Most computer controlled engines have an electronic
connection from the flight deck to the engine. This eliminates
the need for any type of cable or linkages. In the computer
controlled system, the computer sends electronic information
through wires or buses to the fuel control to command it to
follow pilot inputs from the flight deck.
On older style aircraft the power lever control cables and
push-pull rods in the airframe system to the pylon and nacelle
are not usually disturbed at engine change time and usually
no rigging is required, except when some component has
been changed. The control system from the pylon to the
engine must be rigged after each engine change and fuel
control change. Figure 8-13 shows the control system from
the bellcrank in the upper pylon to the fuel control.
Before adjusting the power controls at the engine, be sure
that the power lever is free from binding and the controls
have full throw on the console. If they do not have full throw
or are binding, the airframe system should be checked, and
the discrepancies repaired. After all adjustments have been made, move the power levers through their complete range,
carefully inspecting for adequate clearance between the
various push-pull rods and tubes. Secure all locknuts, cotter
pins, and safety as required.
Adjusting the Fuel Control
The fuel control unit of the typical turbofan on older aircraft
can be a hydromechanical device that schedules the quantity
of fuel flowing to the engine so that the desired amount of
thrust can be obtained. The amount of thrust is dictated by the
position of the power lever in the flight deck and the particular
operation of the engine. Thus, the thrust of the engine and
the consequent rpm of its turbine are scheduled by fuel flow.
The fuel control unit of the engine is adjusted to trim the
engine to obtain maximum thrust output of the engine when
desired. The engine must be retrimmed after a fuel control
unit is replaced, the engine does not develop maximum thrust,
engine change, or excessive throttle stagger.
After trimming the engine, the idle rpm can be adjusted.
The idle rpm is adjusted by turning the INC. IDLE screw an
eighth of a turn at a time, allowing sufficient time for the rpm
to stabilize between adjustments. Retard the power lever to
idle and recheck the idle rpm.
If wind velocity is a factor, the aircraft should be headed
into the wind while trimming or checking the trim on an
engine. Since trimming accuracy decreases as windspeed
and moisture content increase, the most accurate trimming is
obtained under conditions of no wind and clear, moisture-free
air. Do not trim when there is a tailwind because hot exhaust
gases may be reingested. As a practical matter, the engine
should never be trimmed when icing conditions exist because
of the adverse effects on trimming accuracy. To obtain the
most accurate results, the aircraft should always be headed
into the wind while the engine is being trimmed.
With the aircraft headed into the wind, verify that the exhaust
area is clear. Install an engine trim gauge to the T-fitting in the
turbine discharge pressure line. Start the engine and allow it
to stabilize for 5 minutes before attempting to adjust the fuel
control. Refer to the applicable manufacturer’s instructions
for correct trim values. Compensate for temperature and
pressure during the trimming process. If a hydromechanical
fuel control is not within limits, turn the INC. MAX screw
[Figure 8-14] about one-eighth turn in the appropriate
direction. Repeat, if necessary, until the desired value is
attained. If the aircraft is equipped with a pressure ratio gauge,
set it to the correct value.
An example of a trim check using an electronic controlled
fuel control must take into account temperature and pressure
IDLE MAX
INC INC
RPM adjustment0.125 hex socket0.1875 hex socket
Figure 8-14. Typical fuel control adjustments.
for each parameter measured. The parameters checked can
include:
1. Minimum idle (percent N 2).
2. Approach idle (percent N 2).
3. 2.5 bleed open (percent N 1).
4. 2.5 bleed closed (percent N 1).
5. Takeoff engine pressure ratio (EPR).
6. 95 percent takeoff thrust (EPR).
7. 90 percent thrust change decal (EPR).
The actual trim check would be done based on a temperature
and pressure, such as the example in Figure 8-15. For these
given temperature and pressures, the target parameter values
can be derived from a chart in the manufacturer’s manual.
The engine is run up, and these values are checked against
the tolerances given in the manual.
Turboprop Powerplant Removal & Installation
Since most turboprop powerplant removal and installation
instructions are developed for QECA, the following procedures
reflect those used for a typical QECA. The procedures for
turboprop engine removal and installation are similar to those
presented in the section of this chapter for turbojet engines,
except for those systems related to the turboprop propeller.
Open the engine side panels and remove the nacelle access
panels. Disconnect the engine thermocouple leads at the
terminal board. Before disconnecting any lines, make sure
that all fuel, oil, and hydraulic fluid valves are closed. Plug all lines as they are disconnected to prevent entrance of
foreign material.
Remove the clamps securing the bleed-air ducts at the firewall.
Then, disconnect the electrical connector plugs, engine
breather and vent lines, and fuel, oil, and hydraulic lines.
Disconnect the engine power lever and propeller control rods
or cables. Remove the covers from the QECA lift points,
attach the QECA sling, and remove slack from the cables
using a suitable hoist. The sling must be adjusted to position
the hoisting eye over the QECA CG. Failure to do so may
result in engine damage.
Remove the engine mount bolts. The QECA is then ready to
be removed. Recheck all of the disconnect points to make
certain they are all disconnected prior to moving the engine.
Move the engine forward, out of the nacelle structure, until
it clears the aircraft. Lower the QECA into position on the
QECA stand and secure it prior to removing the engine sling.
The installation procedures are essentially the reverse of
the removal procedures. Move the QECA straight back into
the nacelle structure and align the mount bolt holes and the
firewall. Start all the bolts before torqueing. With all the
bolts started, and using the correct torque wrench adapter,
tighten the mount bolts to their proper torque. Remove the
sling and install the access covers at the lift point. Using
the reverse of the removal procedures, connect the various
lines and connectors. New O-ring seals should be used.
The manufacturer’s instructions should be consulted for the
proper torque limits for the various clamps and bolts.
After installation, an engine runup should be made. In
general, the runup consists of checking proper operation of
the powerplant and related systems. Several functional tests
are performed to evaluate each phase of engine operation.
The tests and procedures outlined by the engine or airframe
manufacturers should be followed.
Reciprocating Helicopter Engine & QECA
The engine is installed facing aft with the propeller shaft
approximately 39° above horizontal. The engine is supported
by the engine mount, which is bolted to the fuselage structure.
The installation of the engine provides for ease of maintenance
by allowing easy access to all accessories and components
when the engine access doors are opened. The QECA contains
the engine, engine mount, engine accessories, engine controls,
fuel system, lubrication system, ignition system, cooling
system, and hydromechanical clutch and fan assembly.
Barometer (inches of mercury)
DAT °F (°C) Trim targets
12 (−11)
16 (−9)
20 (−7)MIN IDLE (%N2)
APP IDLE (%N2)
2.5 BLEED OPEN INC (%N1)
2.5 BLEED CLOSED INC (%N1)
TAKEOFF (EPR)
95% TAKEOFF THRUST (EPR)
90% THRUST CHANGE DECEL (EPR)
MIN IDLE (%N2)
APP IDLE (%N2)
2.5 BLEED OPEN INC (%N1)
2.5 BLEED CLOSED INC (%N1)
TAKEOFF (EPR)
95% TAKEOFF THRUST (EPR)
90% THRUST CHANGE DECEL (EPR)
MIN IDLE (%N2)
APP IDLE (%N2)
2.5 BLEED OPEN INC (%N1)
2.5 BLEED CLOSED INC (%N1)
TAKEOFF (EPR)
95% TAKEOFF THRUST (EPR)
90% THRUST CHANGE DECEL (EPR)31.0 30.0 29.0 28.0 27.0 26.0 25.0 24.0 23.0 22.0
1.04ASSUME
1. Ambient temperature: OAT = 12 °F
2. Barometric pressure = 29.0 inches of mercury
SELECT TRIM TARGETS FOR THE FOLLOWING
1. Minimum idle (%N2) is 57.7 (+1.5/−0.5)%N2
2. Approach idle (%N2) is 67.6 ±0.5 %N2
3. 2.5 bleed open inc (%N1) 60.4 ±0.5 %N1
4. 2.5 bleed closed inc (%N1) is 63.5 ±0.5 %N1 5. Takeoff (EPR) is 1.50 +0.01/−0.00 EPR
6. 95% takeoff thrust (EPR) is 1.46 EPR
7. 90% thrust change deceleration (EPR)
is 1.04 EPR
Figure 8-15. Trim check data (Boeing).
Removal of Helicopter QECA
Prior to removing the helicopter QECA, the engine should
be preserved if it is possible to do so. Then, shut off the
fuel supply to the engine and drain the oil. Make the
disconnections necessary to remove the QECA, and then
perform the following steps:
1. Attach the engine lifting sling to a hoist of at least a
two-ton capacity.
2. Raise the hoist to apply a slight lift to the QECA.
Loosen both engine mount lower attachment bolt nuts
before leaving the upper attachment bolts.
3. Remove the bolts from the sway braces and remove
both engine upper attachment bolts. Then, remove
both engine mount lower attachment bolts and remove
the QECA from the helicopter. Mount the power
package in a suitable workstand and remove the sling.
Installation, Rigging, & Adjustment of Helicopter
QECA
The installation of a new or an overhauled engine is in reverse
of the removal procedure. The manufacturer’s instructions for the helicopter must be consulted to ascertain the correct
interchange of parts from the old engine to the new engine.
The applicable maintenance instructions should be followed.
Refer to the Maintenance Instructions Manual and associated
technical publications for detailed information concerning
rigging the throttle, mixture control, cable tensions, and
related data.
Testing the Engine Installation
Normal engine run-in procedures must be followed in
accordance with the manufacturer’s instructions. A flight
test is usually performed after the engine has been installed
and the engine controls have been adjusted.
Engine Mounts
Mounts for Reciprocating Engines
Most aircraft equipped with reciprocating engines use an
engine mount structure made of welded steel tubing. The
mount is constructed in one or more sections that incorporate
the engine mount ring, bracing members (V-struts), and
fittings for attaching the mount to the wing nacelle.
Figure 8-16. Engine mounting ring.
Figure 8-17. Turbine engine front mount.The engine mounts are usually secured to the aircraft by
special heat-treated steel bolts. The importance of using only
these special bolts can be readily appreciated, since they
alone support the entire weight of, and withstand all, the
stresses imposed by the engine and propeller in flight. The
upper bolts support the weight of the engine while the aircraft
is on the ground, but when the aircraft is airborne another
stress is added. This stress is torsional and affects all bolts,
not just the top bolts. A typical engine mount ring shown in
Figure 8-16 discloses fittings and attachment points located
at four positions on the engine mount structure. Each fitting
houses a dynamic engine mount.
The section of an engine mount where the engine is attached
is known as the engine mount ring. It is usually constructed
of steel tubing having a larger diameter than the rest of the
mount structure. It is circular in shape so that it can surround
the engine, which is near the point of balance for the engine.
The engine is usually attached to the mount by dynafocal
mounts, attached to the engine at the point of balance forward
of the mount ring. Other types of mounting devices are also
used to secure the different engines to their mount rings.
As aircraft engines became larger and produced more power,
some method was needed to absorb their vibration. This
demand led to the development of the rubber and steel engine-
suspension units called shock mounts. This combination
permits restricted engine movement in all directions. These
vibration isolators are commonly known as flexible, or elastic,
shock mounts. An interesting feature common to most shock
mounts is that the rubber and metal parts are arranged so that, under normal conditions, rubber alone supports the engine.
Of course, if the engine is subjected to abnormal shocks or
loads, the metal snubbers limit excessive movement of the
engine. Dynafocal engine mounts, or vibration isolators, are
units that give directional support to the engines. Dynafocal
engine mounts have the mounting pad angled to point to the
CG of the engines mass. [Figure 8-16]
Mounts for Turbofan Engines
The engine mounts on most turbofan engines perform the
same basic functions of supporting the engine and transmitting
the loads imposed by the engine to the aircraft structure. Most
turbine engine mounts are made of stainless steel and are
typically located as illustrated in Figure 8-17. Some engine
mounting systems use two mounts to support the forward end
of the engine and a single mount at the rear end.
Turbine Vibration Isolation Engine Mounts
The vibration isolator engine mounts support the power
plants and isolate the airplane structure from adverse
engine vibrations. Each power plant is generally supported
by forward vibration isolator mounts and an aft vibration
isolator mount.
The forward vibration isolator engine mounts carry vertical,
side, and axial (thrust) loads and allow engine growth due
to thermal expansion. The aft mounts take only vertical and
side loads; however, they will also accommodate thermal
expansion of the engine without applying axial loads to the
engine flanges.
8-18The vibration isolators consist of a resilient material
permanently enclosed in a metal case. As an engine vibrates,
the resilient material deforms slightly, thereby dampening
the vibrations before they reach the airplane structure. If
complete failure or loss of the resilient material occurs, the
isolators will continue to support the engine.
Preservation & Storage of Engines
An engine awaiting overhaul or return to service after
overhaul must be given careful attention. It does not receive
the daily care and attention necessary to detect and correct
early stages of corrosion. For this reason, some definite
action must be taken to prevent corrosion from affecting the
engine. Engines that are not flown regularly may not achieve
normal service life because of corrosion in and around the
cylinders. The normal combustion process creates moisture
and corrosive by-products that attack the unprotected surfaces
of the cylinder walls, valves, and any other exposed areas
that are unprotected. In engines that have accumulated 50
hours or more time in service in a short period, the cylinder
walls have acquired a varnish that tends to protect them
from corrosive action; engines under favorable atmospheric
conditions can remain inactive for several weeks without
evidence of damage by corrosion. This is the best-case
scenario, but aircraft that operate close to oceans, lakes,
rivers, and humid regions have a greater need for engine
preservation than engines operated in dry low humid areas.
Corrosion-Preventive Materials
An engine in service is in a sense self-purging of moisture,
since the heat of combustion evaporates the moisture in and
around the engine, and the lubricating oil circulated through
the engine temporarily forms a protective coating on the
metal it contacts. If the operation of an engine in service
is limited or suspended for a period of time, the engine is
preserved to a varying extent, depending upon how long it
is to be inoperative. There are three types of engine storage:
active engine, temporary, and indefinite. An engine in active
storage is defined as having at least one continuous hour of
operation with an oil temperature of at least 165 °F to 200 °F
and storage time not to exceed 30 days. Temporary storage
describes an aircraft and engine that is not flown for 30 to
90 days, and indefinite storage is for an aircraft not to be
flown for over 90 days or is removed from the aircraft for
extended time.
Corrosion-Preventive Compounds
The preservation materials discussed are used for all types
of engine storage. Corrosion-preventive compounds are
petroleum-based products that form a wax-like film over the
metal to which they are applied. Several types of corrosion-
preventive compounds are manufactured according to different specifications to fit the various aviation needs. The
type mixed with engine oil to form a corrosion-preventive
mixture is a relatively light compound that readily blends
with engine oil when the mixture is heated to the proper
temperature.
The light mixture is available in three forms: MIL-C-6529C
type I, type II, or type III. Type I is a concentrate and must be
blended with three parts of MIL-L-22851 or MIL-L-6082C
(SAE J1966) grade 1100 oil to one part of concentrate. Type
II is a ready-mixed material with MIL-L-22851 or grade 1100
oil and does not require dilution. Type III is a ready-mixed
material with grade 1010 oil for use in turbine engines only.
The light mixture is intended for use when a preserved engine
is to remain inactive for less than 30 days. It is also used to
spray cylinders and other designated areas.
The desired proportions of lubricating oil, and either heavy or
light corrosion-preventive compound, must not be obtained
by adding the compound to the oil already in the engine. The
mixture must be prepared separately before applying to the
engine or placing in an oil tank.
A heavy compound is used for the dip treating of metal
parts and surfaces. It must be heated to a high temperature
to be sufficiently liquid to effectively coat the objects to be
preserved. A commercial solvent, or kerosene spray, is used
to remove corrosion-preventive compounds from the engine
or parts when they are being prepared for return to service.
Although corrosion-preventive compounds act as an insulator
from moisture, in the presence of excessive moisture, they
eventually break down and corrosion begins. Also, the
compounds eventually dry because their oil base gradually
evaporates. This allows moisture to contact the engine’s metal
and aids in corroding it. Therefore, when an engine is stored
in a shipping case or container, some dehydrating (moisture
removing) agent must be used to remove the moisture from
the air in and around the engine.
Dehydrating Agents
There are a number of substances (referred to as desiccants)
that can absorb moisture from the atmosphere in sufficient
quantities to be useful as dehydrators. One of these is silica
gel. This gel is an ideal dehydrating agent since it does not
dissolve when saturated.
As a corrosion preventive, bags of silica gel are placed around
and inside various accessible parts of a stored engine. It is
also used in clear plastic plugs, called dehydrator plugs,
that can be screwed into engine openings, such as the
spark plug holes. Cobalt chloride is added to the silica gel
used in dehydrator plugs. This additive makes it possible
Figure 8-18. Dehydrator plug “pink” showing high humidity
(Sacramento Sky Ranch).
Figure 8-19. Dehydrator plug “blue” showing low humidity
(Sacramento Sky Ranch).
for the plugs to indicate the moisture content, or relative
humidity, of the air surrounding the engine. The cobalt-
chloride-treated silica gel remains a bright blue color with
low relative humidity; as the relative humidity increases, the
shade of the blue becomes progressively lighter, becoming
lavender at 30 percent relative humidity and fading through
the various shades of pink [Figure 8-18], until at 60 percent
relative humidity it is a natural or white color. Some types of dehydrator plugs can be dried by removing the silica gel
and heating the gel to dry it out, returning it to its original
blue color. [Figure 8-19] When the relative humidity is less
than 30 percent, corrosion does not normally take place.
Therefore, if the dehydrator plugs are bright blue, the air in
the engine has so little moisture that internal corrosion is held
to a minimum. This same cobalt-chloride-treated silica gel is
used in humidity indicator envelopes. These envelopes can
be fastened to the stored engine so that they can be inspected
through a small window in the shipping case or metal engine
container. All desiccants are sealed in containers to prevent
their becoming saturated with moisture before they are used.
Care should be taken never to leave the container open or
improperly closed.
Engine Preservation & Return to Service
Before an engine is placed in temporary or indefinite storage,
it should be operated and filled with a corrosion-preventive oil
mixture added in the oil system to retard corrosion by coating
the engine’s internal parts. Drain the normal lubricating oil
from the sump or system and replace with a preservative oil
mixture according to the manufacturer’s instructions. Operate
the engine until normal operating temperatures are obtained
for at least one hour.
Always take the appropriate precautions when turning or
working around a propeller. After the flight, remove all the
spark plug leads and the top spark plugs.
To prevent corrosion, spray each cylinder interior with
corrosion-preventive mixture to prevent moisture and oxygen
from contacting the deposits left by combustion. Spray the
cylinders by inserting the nozzle of the spray gun into each
spark plug hole and playing the gun to cover as much area
as possible. Before spraying, each cylinder to be treated
should be at the bottom center position and the oil at room
temperature. This allows the entire inside of the cylinder
to become coated with corrosion-preventive mixture. After
spraying each engine cylinder at bottom center, respray each
cylinder while the crankshaft is stationary with none of the
cylinder’s pistons at top dead center.
The crankshaft must not be moved after this final spraying,
or the seal of corrosion-preventive mixture between the
pistons and cylinder walls are broken. Air can then enter past
the pistons into the engine. Also, the coating of corrosion-
preventive mixture on the cylinder walls is scraped away,
exposing the bare metal to possible corrosion. The engine
should have a sign attached similar to the following: “DO
NOT TURN CRANKSHAFT—ENGINE PRESERVED
PRESERV ATION DATE ____________.”
When preparing the engine for storage, dehydrator plugs are
Ventilator plug
Figure 8-20. Ignition harness lead support installation.
screwed into the spark plug opening of each cylinder. If the
engine is to be stored in a wooden shipping case, the ignition
harness leads are attached to the dehydrator plugs with lead
supports. [Figure 8-20] Special ventilatory plugs are installed
in the spark plug holes of an engine stored horizontally in a
storage container. Any engine being prepared for storage must
receive thorough treatment around the exhaust ports. Because
the residue of exhaust gases is potentially very corrosive,
a corrosion-preventive mixture must be sprayed into each
exhaust port, including the exhaust valve. After the exhaust
ports have been thoroughly coated, a moisture-proof and
oil-proof gasket backed by a metal or wooden plate should
be secured over the exhaust ports using the exhaust stack
mounting studs and nuts. These covers form a seal to prevent
moisture from entering the interior of the engine through the
exhaust ports. Engines stored in metal containers usually
have special ventilatory covers. Another point at which the
engine must be sealed is the intake manifold. If the carburetor
is to remain on the engine during storage, the throttle valve
should be wired open and a seal installed over the air inlet.
But, if the carburetor is removed and stored separately, the
seal is made at the carburetor mounting pad. The seal used in
either instance can be an oil-proof and moisture-proof gasket,
backed by a wooden or metal plate securely bolted into place.
Silica gel should be placed in the intake manifold to absorb
moisture. The silica gel bags are usually suspended from
the cover plate. This eliminates the possibility of forgetting
to remove the silica gel bags when the engine is eventually
removed from storage. A ventilatory cover, without silica
gel bags attached, can be used when the engine is stored in
a metal container.
After the following details have been taken care of, the
engine is ready to be packed into its container. If the engine
has not been spray coated with corrosion-preventive mixture,
the propeller shaft and propeller shaft thrust bearing must be coated with the compound. Then, a plastic sleeve, or
moisture-proof paper, is secured around the shaft, and a
threaded protector cap is screwed onto the propeller retaining
nut threads.
All engine openings into which dehydrator plugs (or
ventilatory plugs if the engine is stored in a metal container)
have not been fitted must be sealed. At points where
corrosion-preventive mixture can seep from the interior of
the engine, such as the oil inlet and outlet, oil-proof and
moisture-proof gasket material backed by a metal or wooden
plate should be used. At other points moisture-proof tape can
be used if it is carefully installed.
Before its installation in a shipping container, the engine
should be carefully inspected to determine if the following
accessories, which are not a part of the basic engine, have
been removed: spark plugs and spark plug thermocouples,
remote fuel pump adapters (if applicable), propeller hub
attaching bolts (if applicable), starters, generators, vacuum
pumps, hydraulic pumps, propeller governors, and engine-
driven fuel pumps.
1. Remove seals and all desiccant bags.
2. Remove cylinder dehydrators and plugs or spark plugs
from upper and lower spark plug holes.
3. Remove oil sump drain plug and drain the corrosion
preventive mixture. Replace drain plug, torque and
safety. Remove oil filter. Install new oil filter, torque
and safety. Service the engine with oil in accordance
with the manufacturer’s instructions.
Warning: To prevent serious bodily injury or death,
accomplish the following before moving the propeller:
a. Disconnect all spark plug leads.
b. Verify that magneto switches are connected to
magnetos and that they are in the off position and
P-leads are grounded.
c. Throttle position CLOSED.
d. Mixture control IDLE-CUT-OFF.
e. Set brakes and block aircraft wheels. Ensure that
aircraft tiedowns are installed and verify that the
cabin door latch is open.
f. Do not stand within the arc of the propeller blades
while turning the propeller.
4. Rotate propeller by hand several revolutions to remove
preservative oil.
5. Service and install spark plugs and ignition leads in
accordance with the manufacturer’s instructions.
6. Service engine and aircraft in accordance with the
manufacturer’s instruction.
Engine attachments
Cover
Base
Figure 8-21. Turbine engine shipping container.7. Thoroughly clean the aircraft and engine. Perform
visual inspection.
8. Correct any discrepancies.
9. Conduct a normal engine start.
10. Perform operational test in accordance with operational
inspection of the applicable Maintenance Manual.
11. Correct any discrepancies.
12 Perform a test flight in accordance with airframe
manufacturer’s instructions.
13. Correct any discrepancies prior to returning aircraft
to service.
14. Change oil and filter after 25 hours of operation.
Engine Shipping Containers
For protection, engines are sealed in plastic or foil envelopes
and can be packed in a wooden shipping case or in pressurized
metal containers.
The engine is lowered into the shipping container so that the
mounting plate can be bolted into position. The protective
envelope is attached directly to the base of the shipping
case. Then, the engine is lowered vertically onto the base
and bolted directly to it. A carburetor not mounted on its
reciprocating engine (or no provision is made to seal it in a
small container to be placed inside the shipping case) can, in
some cases, be fastened to a specially constructed platform
bolted to the engine.
Before the protective envelope is sealed, silica gel should be
placed around the engine to dehydrate the air sealed into the
envelope. The amount of silica gel used is determined by the
size of the engine. The protective envelope is then carefully
gathered around the engine and partially sealed, leaving an
opening at one end from which as much air as possible is
exhausted. A vacuum applied to the container is very useful
for this purpose and is also an aid in detecting any leaks in
the envelope. The envelope is then completely sealed, usually
by pressing the edges together and fusing them with heat.
Before lowering the shipping case cover over the engine,
a quick inventory should be made. Be sure the humidity
indicator card is placed so that it can be seen through the inspection window and that everything required is enclosed
in the container. While lowering the wooden shipping case
cover into position, be careful that it does not twist and tear
the protective envelope. Secure the cover and stencil or mark
the date of preservation on the case. Also, indicate whether
the engine is repairable or serviceable.
There are several types of shipping containers in use.
[Figure 8-21] Another type allows horizontal installation
of an engine, thus eliminating the need for an extra hoist.
The engine is simply lowered onto the base portion of the
container and secured. Then, silica gel bags are packed into
the container, usually in a special section. The amount of
silica gel required in a metal container is generally greater
than that needed in a wooden shipping case, since the volume
of air in the metal container is much greater than that in the
protective envelope installed around an engine in a wooden
shipping case. Also, in the metal container the silica gel bags
must dehydrate the interior of the engine, since ventilatory
plugs are normally installed in the engine openings in place
of dehydrator plugs. All records of the engine should be
enclosed inside the shipping container or on the outside for
accessibility. A humidity indicator should be fastened inside
the containers with an inspection window provided. Then,
the rubber seal between the base and the top of the container
must be carefully inspected. This seal is usually suitable for
re-use several times. After the top of the container has been
lowered into position and fastened to the base of the container,
dehydrated air at approximately 5 pounds per square inch
(psi) pressure is forced into the container. The container
should be checked for leaks by occasional rechecks of the
air pressure, since radical changes in temperature affect the
air pressure in the container.
Inspection of Stored Engines
Most maintenance shops provide a scheduled inspection
system for engines in storage. Normally, the humidity
indicators on engines stored in shipping cases are inspected
every 30 days. When the protective envelope must be opened
to inspect the humidity indicator, the inspection period may
be extended to once every 90 days, if local conditions permit.
The humidity indicator of a metal container is inspected every
180 days under normal conditions.
If the humidity indicator in a wooden shipping case shows
by its color that more than 30 percent relative humidity is
present in the air around the engine, all desiccants should
be replaced. If more than half the dehydrator plugs installed
in the spark plug holes indicate the presence of excessive
moisture, the interior of the cylinders should be resprayed.
If the humidity indicator in a metal container gives a safe
blue indication, but air pressure has dropped below 1 psi, the
container needs only to be brought to the proper pressure with
8-22dehydrated air. However, if the humidity indicator shows an
unsafe (pink) condition, the engine should be represerved.
Preservation & Depreservation of Gas
Turbine Engines
The procedures for preserving and depreserving gas turbine
engines vary depending upon the length of inactivity, the
type of preservative used, and whether or not the engine
may be rotated during the inactive period. Much of the basic
information on corrosion control presented in the section on
reciprocating engines is applicable to gas turbine engines.
However, the requirements for the types of preservatives and
their use are normally different.
The lubrication system is usually drained and may or may
not be flushed with preservative oil. The engine fuel system
is generally filled with preservative oil, including the fuel
control. Before the engine can be returned to service,
the preservative oil must be completely flushed from the
fuel system by motoring the engine and bleeding the fuel
system. Always follow the manufacturer’s instructions
when performing any preservation or depreservation of gas
turbine engines.
9-1Introduction
Because fire is one of the most dangerous threats to an
aircraft, the potential fire zones of all multiengine aircraft
currently produced are protected by a fixed fire protection
system. A “fire zone” is an area or region of an aircraft
designated by the manufacturer to require fire detection
and/or fire extinguishing equipment and a high degree
of inherent fire resistance. The term “fixed” describes a
permanently installed system in contrast to any type of
portable fire extinguishing equipment, such as a hand-held
fire extinguisher.
In accordance with Title 14 of the Code of Federal
Regulations (14 CFR) parts 23 and 25, engine fire protection
systems are mandatory on: multiengine turbine-powered
aircraft, multiengine reciprocating engine-powered aircraft
incorporating turbochargers, aircraft with engine(s) located
where they are not readily visible from the flight deck, all
commuter and transport category aircraft, and the auxiliary
power unit (APU) compartment of any aircraft incorporating
an APU. Fire protection systems are not mandatory for many
single and twin reciprocating engine general aviation (GA)
aircraft.
Several general failures or hazards can result in overheat
conditions or fires peculiar to turbine engine aircraft because
of their operating characteristics. The two major types of
turbine failure can be classified as 1) thermodynamic and
2) mechanical.
Thermodynamic causes upset the proportion of air used to
cool combustion temperatures to the levels that the turbine
materials can tolerate. When the cooling cycle is upset,
turbine blades can melt, causing a sudden loss of thrust. The
rapid buildup of ice on inlet screens or inlet guide vanes can
result in severe overheating, causing the turbine blades to
melt or to be severed and thrown outward. Such failure can
result in a severed tail cone and possible penetration of the
aircraft structure, tanks, or equipment near the turbine wheel.
In general, most thermodynamic failures are caused by ice,
excess air bleed or leakage, or faulty controls that permit
compressor stall or excess fuel.
Mechanical failures, such as fractured or thrown blades,
can also lead to overheat conditions or fires. Thrown blades
can puncture the tail cone, creating an overheat condition.
Failure of forward stages of multi-stage turbines is usually
Engine Fire Protection Systems
Chapter 9
much more severe. Penetration of the turbine case by failed
blades is a possible fire hazard, as is the penetration of lines
and components containing flammable fluids.
A high flow of fuel through an improperly adjusted fuel
nozzle can cause burn-through of the tail cone in some
engines. Engine fires can be caused by burning fluid that
occasionally runs out through the exhaust pipe.
Components
A complete fire protection system includes both a fire
detection and a fire extinguishing system. To detect fires
or overheat conditions, detectors are placed in the various
zones to be monitored. Fires are detected in aircraft by using
one or more of the following: overheat detectors, rate-of-
temperature-rise detectors, and flame detectors. In addition
to these methods, other types of detectors are used in aircraft
fire protection systems but are not used to detect engine fires.
For example, smoke detectors are better suited to monitor
areas such as baggage compartments or lavatories, where
materials burn slowly or smolder. Other types of detectors
in this category include carbon monoxide detectors.
Fire protection systems on current-production aircraft do not
rely on observation by crewmembers as a primary method
of fire detection. An ideal fire detector system includes as
many of the following features as possible:
1. A system that does not cause false warnings under any
flight or ground condition.
2. Rapid indication of a fire and accurate location of the
fire.
3. Accurate indication that a fire is out.
4. Indication that a fire has reignited.
5. Continuous indication for duration of a fire.
6. Means for electrically testing the detector system from
the aircraft flight deck.
7. Detectors that resist damage from exposure to oil,
water, vibration, extreme temperatures, or handling.
8. Detectors that are light in weight and easily adaptable
to any mounting position.
9. Detector circuitry that operates directly from the
aircraft power system without inverters.
10. Minimum electrical current requirements when not
indicating a fire.
Indicator light
R
Dimming relayResister
Test switch
Thermal switches
Test relay
Detector
circuit
Alarm
circuit
Test circuitThermocouples Sensitive relay Slave relay
HeaterTest switch
Figure 9-1. Thermal switch fire circuit. Figure 9-2. Thermocouple fire warning circuit. 11. Each detector system should turn on a flight deck light,
indicating the location of the fire, and have an audible
alarm system.
12. A separate detector system for each engine.
Engine Fire Detection Systems
Several different types of fire detection systems are installed
in aircraft to detect engine fires. Two common types used are
spot detectors and continuously loop systems. Spot detector
systems use individual sensors to monitor a fire zone.
Examples of spot detector systems are the thermal switch
system, the thermocouple system, the optical fire detection
system, and the pneumatic-based thermal fire detection
system. Continuous loop systems are typically installed
on transport type aircraft and provide more complete fire
detection coverage by using several loop-type sensors.
Thermal Switch System
A number of detectors or sensing devices are available.
Many older model aircraft still operating have some type of
thermal switch system or thermocouple system. A thermal
switch system has one or more lights energized by the aircraft
power system and thermal switches that control operation of
the light(s). These thermal switches are heat-sensitive units
that complete electrical circuits at a certain temperature.
They are connected in parallel with each other, but in series
with the indicator lights [Figure 9-1] . If the temperature rises
above a set value in any one section of the circuit, the thermal
switch closes, completing the light circuit to indicate a fire
or overheat condition.
No set number of thermal switches is required; the exact
number usually is determined by the aircraft manufacturer.
On some installations, all the thermal detectors are connected
to one light; others may have a separate thermal switch for
each indicator light.Some warning lights are push-to-test lights. The bulb is tested
by pushing it in to check an auxiliary test circuit. The circuit
shown in Figure 9-1 includes a test relay. With the relay
contact in the position shown, there are two possible paths
for current flow from the switches to the light. This is an
additional safety feature. Energizing the test relay completes
a series circuit and checks all the wiring and the light bulb.
Also included in the circuit shown in Figure 9-1 is a dimming
relay. By energizing the dimming relay, the circuit is altered to
include a resistor in series with the light. In some installations,
several circuits are wired through the dimming relay, and all
the warning lights may be dimmed at the same time.
Thermocouple Systems
The thermocouple fire warning system operates on an
entirely different principle than the thermal switch system.
A thermocouple depends on the rate of temperature rise and
does not give a warning when an engine slowly overheats
or a short circuit develops. The system consists of a relay
box, warning lights, and thermocouples. The wiring system
of these units may be divided into the following circuits:
(1) the detector circuit, (2) the alarm circuit, and (3) the test
circuit. These circuits are shown in Figure 9-2 .
The relay box contains two relays, the sensitive relay and the
slave relay, and the thermal test unit. Such a box may contain
from one to eight identical circuits, depending on the number
of potential fire zones. The relays control the warning lights.
In turn, the thermocouples control the operation of the relays.
The circuit consists of several thermocouples in series with
each other and with the sensitive relay.
Thermocouple leads are made from a variety of metals,
depending on the maximum temperature to which they are
exposed. Iron and constantan, or copper and constantan,
are common for CHT measurement. Chromel and alumel
are used for turbine EGT thermocouples. The point where
these metals are joined and exposed to the heat of a fire
is called a hot junction. There is also a reference junction
enclosed in a dead air space between two insulation blocks.
9-3A metal cage surrounds the thermocouple to give mechanical
protection without hindering the free movement of air to the
hot junction. Thermocouple leads are designed for a specific
installation and may not be altered, if they are altered they
will no longer be calibrated specific probe and instrument
combination.
If the temperature rises rapidly, the thermocouple produces
a voltage because of the temperature difference between the
reference junction and the hot junction. If both junctions
are heated at the same rate, no voltage results. In the engine
compartment, there is a normal, gradual rise in temperature
from engine operation; because it is gradual, both junctions
heat at the same rate and no warning signal is given.
If there is a fire, however, the hot junction heats more rapidly
than the reference junction. The reference junction is also
commonly referred to as the cold junction. The ensuing
voltage causes a current to flow within the detector circuit.
Any time the current is greater than 4 milliamperes (0.004
ampere), the sensitive relay closes. This completes a circuit
from the aircraft power system to the coil of the slave relay.
The slave relay then closes and completes the circuit to the
warning light to give a visual fire warning.
The total number of thermocouples used in individual
detector circuits depends on the size of the fire zones and the
total circuit resistance, which usually does not exceed 5 ohms.
As shown in Figure 9-2 , the circuit has two resistors. The
resistor connected across the slave relay terminals absorbs
the coil’s self-induced voltage to prevent arcing across the
points of the sensitive relay. The contacts of the sensitive relay
are so fragile that they burn or weld if arcing is permitted.
When the sensitive relay opens, the circuit to the slave
relay is interrupted and the magnetic field around its coil
collapses. When this happens, the coil gets a voltage through
self-induction, but with the resistor across the coil terminals,
there is a path for any current flow as a result of this voltage.
Thus, arcing at the sensitive relay contacts is eliminated.
Optical Fire Detection Systems
Optical sensors, often referred to as “flame detectors,”
are designed to alarm when they detect the presence of
prominent, specific radiation emissions from hydrocarbon
flames. The two types of optical sensors available are infrared
(IR) and ultraviolet, based on the specific emission wave
lengths they are designed to detect.
Infrared Optical Fire Protection
IR-based optical flame detectors are used primarily on light
turboprop aircraft and helicopter engines. These sensors
have proven to be very dependable and economical for the relatively benign environments of these applications.
Principle of Operation
Radiation emitted by the fire crosses the airspace between
the fire and the detector and impinges on the detector front
face and window. The window allows a broad spectrum
of radiation to pass into the detector where it impinges
on the face of the sensing device filter. The filter allows
only radiation in a tight waveband centered around 4.3
micrometers in the IR to pass on to the radiation-sensitive
surface of the sensing device. The radiation striking the
sensing device minutely raises its temperature causing small
thermoelectric voltages to be generated. These voltages are
fed to an amplifier whose output is connected to various
analytical electronic processing circuits. The processing
electronics is tailored exactly to the time signature of all
known hydrocarbon flame sources and ignores false alarm
sources, such as incandescent lights and sunlight. Alarm
sensitivity level is accurately controlled by a digital circuit.
A typical warning system is illustrated in Figure 9-3.
Pneumatic Thermal Fire Detection
Pneumatic detectors are based on the principles of gas laws.
The sensing element consists of a closed helium-filled tube
connected at one end to a responder assembly. As the element
is heated, the gas pressure inside the tube increases until the
alarm threshold is reached. At this point, an internal switch
closes and reports an alarm to the flight deck. The pneumatic
detector integrity pressure switch opens and triggers the
fault alarm if the pneumatic detector losses pressure, as in
the case of a leak.
Continuous-Loop Detector Systems
Large commercial aircraft almost exclusively use continuous
thermal sensing elements for powerplant protection, since
these systems offer superior detection performance and
coverage, and they have the proven ruggedness to survive in
the harsh environment of modern turbofan engines.
A continuous-loop detector, or sensing system, permits
more complete coverage of a fire hazard area than any
of the spot-type temperature detectors. Continuous-loop
systems are versions of the thermal switch system. They are
overheat systems, heat-sensitive units that complete electrical
circuits at a certain temperature. There is no rate-of-heat-
rise sensitivity in a continuous-loop system. Two widely
used types of continuous-loop systems are the Kidde and
the Fenwal systems. This text briefly discusses the Fenwal
system, while the Kidde system is discussed more in-depth.
Fenwal Continuous-Loop System
The Fenwal system uses a slender inconel tube packed with
thermally sensitive eutectic salt and a nickel wire center
F
FVoltage
regulator
Duration
discriminator
Micrometer
sensorReset
counter
clock
Flicker rate
discriminator
SIGNAL2.5812V+15V
+15VGNDGND
THRESHOLDFunctional Schematic
Figure 9-3. Optical fire detection system circuit.
Figure 9-4. Fenwal sensing element.conductor. [Figure 9-4] Lengths of these sensing elements
are connected in series to a control unit. The elements may
be of equal or varying length and of the same or different
temperature settings. The control unit, operating directly from
the power source, impresses a small voltage on the sensing
elements. When an overheat condition occurs at any point
along the element length, the resistance of the eutectic salt
within the sensing element drops sharply, causing current to
flow between the outer sheath and the center conductor. This current flow is sensed by the control unit, which produces a
signal to actuate the output relay.
When the fire has been extinguished or the critical temperature
lowered, the Fenwal system automatically returns to standby
alert, ready to detect any subsequent fire or overheat
condition. The Fenwal system may be wired to employ a
“loop” circuit. In this case, should an open circuit occur, the
system still signals fire or overheat. If multiple open circuits
occur, only that section between breaks becomes inoperative.
Kidde Continuous-Loop System
In the Kidde continuous-loop system, two wires are imbedded
in an inconel tube filled with a thermistor core material.
[Figure 9-5] Two electrical conductors go through the length
of the core. One conductor has a ground connection to the
tube and the other conductor connects to the fire detection
control unit.
As the temperature of the core increases, electrical resistance
to ground decreases. The fire detection control unit monitors
this resistance. If the resistance decreases to the overheat
set point, an overheat indication occurs in the flight deck.
Typically, a 10-second time delay is incorporated for the
Thermistor core
Inconel tubeNo. 10 screw terminal
Electrical conductors
SheathAircraft wiring (ref)No. 8 screw terminal
Retaining nut
End bracket
Support tube
Bushing
Element loop 2Element loop 1Quick release clamp
ThermistorWires
Figure 9-5. Kidde continuous-loop system.
overheat indication. If the resistance decreases more to the
fire set point, a fire warning occurs. When the fire or overheat
condition is gone, the resistance of the core material increases
to the reset point and the flight deck indications go away.
The rate of change of resistance identifies an electrical short
or a fire. The resistance decreases more quickly with an
electrical short than with a fire. In addition to fire and overheat
detection, the Kidde continuous-loop system can supply
nacelle temperature data to the aircraft condition monitoring
function of the Aircraft In-Flight Monitoring System (AIMS).
Sensing Element
The sensing element consists, essentially, of an infinite
number of unit thermistors electrically in parallel along its
length. The resistance of the sensing element is a function of
the length heated, as well as the temperature-heating of less
than the full length of element, which requires that portion to
be heated to a higher temperature to achieve the same total
resistance change. As a result, the system responds not to a
fixed alarm temperature but to the sum of the resistances (in
parallel) that reflects a nonarithmetic “average.” The sensing
element may be routed close to nonhazardous hot spots that
may have a normal temperature well above the overall alarm
temperature, without danger of causing a false alarm. This
feature permits the alarm point to be set close to the maximum
general ambient temperature, giving greater sensitivity to a
general overheat or fire without being subject to false alarms
from localized nonhazardous hot spots.Combination Fire & Overheat Warning
The analog signal from the thermistor sensing element permits
the control circuits to be arranged to give a two-level response
from the same sensing element loop. The first is an overheat
warning at a temperature level below the fire warning,
indicating a general engine compartment temperature
rise, which could be caused by leakage of hot bleed air or
combustion gas into the engine compartment. It could be an
early warning of fire and would alert the crew to appropriate
action to reduce the engine compartment temperature. The
second-level response would be at a level above that attainable
by the leaking hot gas and would be the fire warning.
Temperature Trend Indication
The analog signal produced by the sensing element loop as
its temperature changes can readily be converted to signals
suitable for meter or cathode ray tube (CRT) display to
indicate engine bay temperature increases from normal.
A comparison of the readings from each loop system also
provides a check on the condition of the fire detection system,
because the two loops should normally read alike.
System Test
The integrity of the continuous-loop fire detection system
may be tested by actuating a test switch in the flight deck,
which switches one end of the sensing element loop from
its control circuit to a test circuit, built into the control unit,
that simulates the sensing element resistance change due to
fire. [Figure 9-6] If the sensing element loop is unbroken,
the resistance detected “seen” by the control circuit is now
Monitor circuitTest circuitTest switchSensing element loop
Control unit
Figure 9-6. Continuous-loop fire detection system test circuit.
that of the simulated fire and so the alarm is signaled. This
demonstrates, in addition to the continuity of the sensing
element loop, the integrity of the alarm indicator circuit and
the proper functioning of the control circuits. The thermistor
properties of the sensing element remain unchanged for the
life of the element (no chemical or physical changes take
place on heating), so that it functions properly as long as it
is electrically connected to the control unit.
Fault Indication
Provision can be made in the control unit to send a fault signal
to activate a fault indicator whenever the short discriminator
circuit detects a short in the sensing element loop. While this
is a requirement in 14 CFR for transport category aircraft
because such a short disables the fire detection system, it is
offered as an option for other aircraft types in which it may
not be a requirement.
Dual-Loop Systems
Dual-loop systems are, in essence, two complete basic fire
detection systems with their output signals connected so that
both must signal to result in a fire warning. This arrangement,
called “AND” logic, results in greatly increased reliability
against false fire warnings from any cause. Should one of
the two loops be found inoperative at the preflight integrity
test, a flight deck selector switch disconnects that loop and
allows the signal from the other loop alone to activate the
fire warning. Since the single operative loop meets all fire
detector requirements, the aircraft can be safely dispatched,
and maintenance deferred to a more convenient time.
However, should one of the two loops become inoperative
in flight and a fire subsequently occur, the fire signaling
loop activates a flight deck fault signal that alerts the flight
crew to select single-loop operation to confirm the possible
occurrence of fire.Automatic Self-Interrogation
Dual-loop systems automatically perform the loop switching
and decision-making function required of the flight crew
upon appearance of the fault indication in the flight deck.
Automatic self-interrogation eliminates the fault indication
and assures the immediate appearance of the fire indication
should fire occur while at least one loop of the dual-loop
system is operative. Should the control circuit from a single
loop signal “fire,” the self-interrogation circuit automatically
tests the functioning of the other loop. If it tests operative,
the circuit suppresses the fire signal (because the operative
loop would have signaled if a fire existed). If, however, the
other loop tests inoperative, the circuit outputs a fire signal.
The interrogation and decision takes place in milliseconds,
so that no delay occurs if a fire actually exists.
Support Tube-Mounted Sensing Elements
When you want to mount the sensing elements on the engine,
and in some cases, on the aircraft structure, the support tube-
mounted element solves the problem of providing sufficient
element support points, and greatly facilitates the removal and
reinstallation of the sensing elements for engine or system
maintenance.
Most modern installations use the support tube concept of
mounting sensing elements for better maintainability as well
as increased reliability. The sensing element is attached to
a prebent stainless steel tube by closely spaced clamps and
bushings, where it is supported from vibration damage and
protected from pinching and excessive bending. The support
tube-mounted elements can be furnished with either single- or
dual-sensing elements.
Being prebent to the designed configuration assures its
installation in the aircraft precisely in its designed location,
where it has the necessary clearance to be free from the
possibility of the elements chafing against engine or aircraft
structure. The assembly requires only a few attachment
points, and removal for engine maintenance is quick and
easy. Should the assembly require repair or maintenance, it
is easily replaced with another assembly, leaving the repair
for the shop. A damaged sensing element is easily replaced
in the assembly. The assembly is rugged, easy to handle, and
unlikely to suffer damage during handling for installation
or removal.
Fire Detection Control Unit (Fire Detection Card)
The control unit for the simplest type of system typically
contains the necessary electronic resistance monitoring
and alarm output circuits, housed in a hermetically sealed
aluminum case and filled with a mounting bracket and
circular electrical connector. For more sophisticated systems,
control modules may be employed that contain removable
Engine fire detector (lower FWD)Engine fire detector (upper)
Engine section view (looking forward) Engine section side viewEngine fire detector (lower)
Figure 9-7. Large turbofan engine fire zones.control cards having circuitry for individual hazard areas, and/
or unique functions. In the most advanced applications, the
detection system circuitry controls all aircraft fire protection
functions, including fire detection and extinguishing for
engines, APUs, cargo bays, and bleed air systems.
Fire Zones
The powerplant installation has several designated fire
zones: (1) the engine power section; (2) the engine accessory
section; (3) except for reciprocating engines, any complete
powerplant compartment in which no isolation is provided
between the engine power section and the engine accessory
section; (4) any APU compartment; (5) any fuel-burning
heater and other combustion equipment installation; (6)
the compressor and accessory sections of turbine engines;
and (7) combustor, turbine, and tailpipe sections of turbine
engine installations that contain lines or components carrying
flammable fluids or gases. Figure 9-7 shows fire protection
for a large turbo fan engine.
In addition to the engine and nacelle area zones, other areas
on multiengine aircraft are provided with fire detection
and protection systems. These areas include baggage
compartments, lavatories, APU, combustion heater
installations, and other hazardous areas. Discussion of fire
protection for these areas is not included in this section, which
is limited to engine fire protection.
Engine Fire Extinguishing System
Commuter aircraft certificated under 14 CFR part 23 are
required to have, at a minimum, a one-shot fire extinguishing
system. All transport category aircraft certificated under 14
CFR part 25 are required to have two discharges, each of
which produces adequate agent concentration. An individual
one-shot system may be used for APUs, fuel burning heaters,
and other combustion equipment. For each “other” designated
fire zone, two discharges (two-shot system) must be provided,
each of which produces adequate agent concentration.
[Figure 9-8]Fire Extinguishing Agents
The fixed fire extinguisher systems used in most engine fire
protection systems are designed to dilute the atmosphere with
an inert agent that does not support combustion. Many systems
use perforated tubing or discharge nozzles to distribute the
extinguishing agent. High rate of discharge (HRD) systems
use open-end tubes to deliver a quantity of extinguishing
agent in 1 to 2 seconds. The most common extinguishing
agent still used today is Halon 1301 because of its effective
firefighting capability and relatively low toxicity (U.L.
classification Group 61). Noncorrosive, Halon 1301 does not
affect the material it contacts and requires no clean-up when
discharged. Halon 1301 is the current extinguishing agent for
commercial aircraft, but a replacement is under development.
Because Halon 1301 depletes the ozone layer only recycled
Halon 1301 is currently available. Halon 1301 is used until
a suitable replacement is developed. Some military aircraft
use HCL-125, which the Federal Aviation Administration
(FAA) is testing for use in commercial aircraft.
Carbon dioxide (CO 2) is an effective extinguishing agent. It
is most often used in fire extinguishers that are available on
the ramp to fight fires on the exterior of the aircraft, such as
engine or APU fires. CO 2 has been used for many years to
extinguish flammable fluid fires and fires involving electrical
equipment. It is noncombustible and does not react with most
substances. It provides its own pressure for discharge from
the storage vessel, except in extremely cold climates where
a booster charge of nitrogen may be added to winterize the
system. Normally, CO 2 is a gas, but it is easily liquefied by
compression and cooling. After liquefaction, CO 2 remains in
a closed container as both liquid and gas. When CO 2 is then
discharged to the atmosphere, most of the liquid expands to
gas. Heat absorbed by the gas during vaporization cools the
remaining liquid to –110 °F, and it becomes a finely divided
white solid, dry ice snow.
9-8Thermal relief outlet
No. 1 engineAPUNo. 2 engine
Directional control valveForward bottleAFT bottle
Figure 9-8. Typical fire extinguishing system.
Turbine Engine Ground Fire Protection
On many aircraft, means are usually provided for rapid access
to the compressor, tailpipe, or burner compartments. Many
aircraft systems are equipped with spring-loaded or pop-out
access doors in the skin of the various compartments. Internal
engine tailpipe fires that take place during engine shutdown
or false starts can be blown out by motoring the engine with
the starter. A running engine can be accelerated to rated
speed to achieve the same result. If such a fire persists, a fire
extinguishing agent can be directed into the tailpipe. It should
be remembered that excessive use of CO 2, or other agents
that have a cooling effect, can shrink the turbine housing on
the turbine and cause the engine to disintegrate.
Containers
Fire extinguisher containers (HRD bottles) store a liquid
halogenated extinguishing agent and pressurized gas
(typically nitrogen) normally manufactured from stainless
steel. Depending upon design considerations, alternate
materials are available including titanium. Containers are
also available in a wide range of capacities and are produced
under Department of Transportation (DOT) specifications or
exemptions. Most aircraft containers are spherical in design,
which provides the lightest weight possible. However,
cylindrical shapes are available where space limitations are
a factor. Each container incorporates a temperature/pressure
sensitive safety relief diaphragm that prevents container
pressure from exceeding container test pressure in the event
of exposure to excessive temperatures. [Figure 9-9]Discharge Valves
Discharge valves are installed on the containers. A cartridge
(squib) and frangible disc type valve are installed in the
outlet of the discharge valve assembly. Special assemblies
having solenoid-operated or manually-operated seat type
valves are also available. Two types of cartridge disc-release
techniques are used. Standard release type uses a slug driven
by explosive energy to rupture a segmented closure disc.
For high temperature or hermetically sealed units, a direct
explosive impact type cartridge is used, which applies
fragmentation impact to rupture a prestressed corrosion-
resistant steel diaphragm. Most containers use conventional
metallic gasket seals that facilitate refurbishment following
discharge. [Figure 9-10]
Pressure Indication
A wide range of diagnostics are utilized to verify the fire
extinguisher agent charge status. A simple visually indicated
gauge is available, typically a vibration-resistant helical
bourdon-type indicator. [see Figure 9-9]
A combination gauge switch visually indicates actual
container pressure and also provides an electrical signal if
container pressure is lost, precluding the need for discharge
indicators. A ground checkable diaphragm-type low-pressure
switch is commonly used on hermetically sealed containers.
The Kidde system also has a temperature compensated
pressure switch that tracks the container pressure variations
with temperatures by using a hermetically sealed reference
chamber.
CAUTIONMounting lug
Data plateWeldment
Caution plate
Discharge outlet
Cartridge
Caution tagNameplate
Fill fitting
Pressure device (pressure switch and gauge guard)
Figure 9-9. Fire extinguisher containers (HRD bottles).
Figure 9-10. Discharge valve (left) and cartridge (squib) (right).
Two-Way Check Valve
A complete line of two-way check valves is available,
manufactured from either lightweight aluminum or steel.
These valves are required in a two-shot system to prevent
the agent in a reserve container from backing up into the
previous emptied main container. Valves are supplied with
either MS-33514 or MS-33656 fitting configurations.
Discharge Indicators
Discharge indicators provide immediate visual evidence
of container discharge on fire extinguishing systems. Two
kinds of indicators can be furnished: thermal and discharge.
Both types are designed for aircraft and skin mounting.
[Figure 9-11]
Thermal Discharge Indicator (Red Disc)
The thermal discharge indicator is connected to the fire
container relief fitting and ejects a red disc to show when
container contents have dumped overboard due to excessive
heat. The agent discharges through the opening created when
the disc blows out. This gives the flight and maintenance
crews an indication that the fire extinguisher container needs to be replaced before the next flight.
Yellow Disc Discharge Indicator
If the flight crew activates the fire extinguisher system, a
yellow disc is ejected from the skin of the aircraft fuselage.
This is an indication for the maintenance crew that the fire
extinguishing system was activated by the flight crew, and
that the fire extinguishing container needs to be replaced
before the next flight.
Fire Switch
Fire switches are typically installed on the center overhead
panel or center console in the flight deck. [Figure 9-12]
When the fire switch is activated, the following happens: the
engine stops because the fuel control shuts off, the engine is
isolated from the aircraft systems, and the fire extinguishing
system is activated. Some aircraft use fire switches that need
to be pulled and turned to activate the system, while others
use a push-type switch with a guard. To prevent accidental
activation of the fire switch, a lock is installed that releases the
fire switch only when a fire has been detected. This lock can
be manually released by the flight crew if the fire detection
Figure 9-11. Discharge indicators.
Figure 9-12. Engine fire switches.
system malfunctions. [Figure 9-13]
Warning Systems
Visible and audible warning systems are installed in the
flight deck to alert the flight crew. A horn sounds and one
or several warning lights illuminate to alert the flight crew
that an engine fire has been detected. These indications stop
when the fire is extinguished.Fire Detection System Maintenance
Fire detector sensing elements are located in many high-
activity areas around aircraft engines. Their location, together
with their small size, increases the chance of damage to
the sensing elements during maintenance. An inspection
and maintenance program for all types of continuous-loop
systems should include the following visual checks. These
procedures are examples and should not be used to replace
the applicable manufacturer’s instructions.
Sensing elements of a continuous-loop system should be
inspected for the following:
1. Cracked or broken sections caused by crushing or
squeezing between inspection plates, cowl panels, or
engine components.
2. Abrasion caused by rubbing of the element on cowling,
accessories, or structural members.
3. Pieces of safety wire or other metal particles that may
short the spot-detector terminals.
4. Condition of rubber grommets in mounting clamps,
which may be softened from exposure to oils or
hardened from excessive heat.
5. Dents and kinks in sensing element sections. Limits on
the element diameter, acceptable dents and kinks, and
degree of smoothness of tubing contour are specified
by the manufacturer. No attempt should be made to
straighten any acceptable dent or kink, since stresses
may be set up that could cause tubing failure. (See
illustration of kinked tubing in Figure 9-14 .)
6. Nuts at the end of the sensing elements [Figure 9-15]
should be inspected for tightness and the presence of
a safety wire. Loose nuts should be retorqued to the
value specified by the manufacturer’s instructions.
Some types of sensing element connection joints
require the use of copper crush gaskets, which should
be replaced any time a connection is separated.
7. If shielded flexible leads are used, they should be
inspected for fraying of the outer braid. The braided
sheath is composed of many fine metal strands woven
into a protective covering surrounding the inner
insulated wire. Continuous bending of the cable or
rough treatment can break these fine wires, especially
those near the connectors.
8. Sensing element routing and clamping should be
inspected carefully. [Figure 9-14] Long, unsupported
sections may permit excessive vibration that can cause
breakage. The distance between clamps on straight
runs, usually about 8–10 inches, is specified by the
manufacturer. At end connectors, the first support
clamp is usually located about four to six inches from
9-11Engine fire switch (P8) Switch positions
LEFTDisch
L
E
F
T1 2DISCH
Switch locked
Switch released electrically
Switch pulled
Switch released manuallyElectrical connectorPush-pull contacts (internal)Engine fire override switch
Solenoid energizedEngine fire warning lightSpring return
(switch open)
Rotary contacts (internal)Discharge 2
(switch closed)Spring return
(switch open)
Discharge 2
(switch closed)Center
Engine fire
override
switch
pushed
Figure 9-13. Engine fire switch operation.
Long unsupported loop KinkSharp bendCrushed
section
Figure 9-14. Sensing element defects.
Heat-sensing element
Figure 9-15. Connector joint fitting attached to the structure.
the end connector fittings. In most cases, a straight run
of one inch is maintained from all connectors before a
bend is started, and an optimum bend radius of three
inches is normally applied.
9. Interference between a cowl brace and a sensing
element can cause rubbing. [Figure 9-16] This
interference may cause wear and short the sensing
element.
10. Grommets should be installed on the sensing element
so that both ends are centered on its clamp. The split
end of the grommet should face the outside of the
nearest bend. Clamps and grommets should fit the
element snugly. [Figure 9-17]Fire Detection System Troubleshooting
The following troubleshooting procedures represent the
most common difficulties encountered in engine fire
detection systems:
1. Intermittent alarms are most often caused by an
intermittent short in the detector system wiring. Such
shorts may be caused by a loose wire that occasionally
touches a nearby terminal, a frayed wire brushing
against a structure, or a sensing element rubbing
against a structural member long enough to wear
through the insulation. Intermittent faults often can
be located by moving wires to re-create the short.
Rub pointLoose clamp
Loose clamp
Heat-sensing elementBracket
Clamp hingeGrommetClamp screw
Temperature (°F)
Pressure (psig)
−40 −30 −20 −10 0 10 20 30 40 50 60 70 80900
0Minimum gauge readingMaximum gauge readingFigure 9-16. Rubbing interference.
Figure 9-18. Fire extinguisher container pressure-temperature chart.Figure 9-17. Inspection of fire detector loop clamp.
2. Fire alarms and warning lights can occur when no
engine fire or overheat condition exists. Such false
alarms can be most easily located by disconnecting
the engine sensing loop connections from the control
unit. If the false alarm ceases when the engine sensing
loop is disconnected, the fault is in the disconnected
sensing loop, which should be examined for areas
that have been bent into contact with hot parts of the
engine. If no bent element can be found, the shorted
section can be located by isolating the connecting
elements consecutively around the entire loop.
3. Kinks and sharp bends in the sensing element can
cause an internal wire to short intermittently to the
outer tubing. The fault can be located by checking the
sensing element with an ohm meter while tapping the
element in the suspected areas to produce the short.
4. Moisture in the detection system seldom causes a
false fire alarm. If, however, moisture does cause an
alarm, the warning persists until the contamination is
removed or boils away, and the resistance of the loop
returns to its normal value.
5. Failure to obtain an alarm signal when the test switch
is actuated may be caused by a defective test switch or
control unit, the lack of electrical power, inoperative indicator light, or an opening in the sensing element
or connecting wiring. When the test switch fails to
provide an alarm, the continuity of a two-wire sensing
loop can be determined by opening the loop and
measuring the resistance. In a single-wire, continuous-
loop system, the center conductor should be grounded.
Fire Extinguisher System Maintenance
Practices
Regular maintenance of fire extinguisher systems typically
includes such items as the inspection and servicing of fire
extinguisher bottles (containers), removal and reinstallation
of cartridge and discharge valves, testing of discharge tubing
for leakage, and electrical wiring continuity tests. The
following paragraphs contain details of some of the most
typical maintenance procedures.
Fire extinguisher containers are checked periodically to
determine that the pressure is between the prescribed
minimum and maximum limits. Changes of pressure with
ambient temperatures must also fall within prescribed
limits. The graph shown in Figure 9-18 is typical of the
pressure temperature curve graphs that provide maximum
and minimum gauge readings. If the pressure does not fall
within the graph limits, the extinguisher container is replaced.
The service life of fire extinguisher discharge cartridges is
calculated from the manufacturer’s date stamp, which is
usually placed on the face of the cartridge. The cartridge
service life recommended by the manufacturer is usually in
terms of years. Cartridges are available with a service life
of 5 years or more. To determine the unexpired service life
of a discharge cartridge, it is usually necessary to remove
the electrical leads and discharge line from the plug body,
which can then be removed from the extinguisher container.
Mounting lugs (4)
NutPressure gauge
Bottle
Packing ring
Cartridge
Strainer
Contact assembly
Gasket
Discharge plug (or valve) bodySwivel nutBottle outlet
Figure 9-19. Components of fire extinguisher container.Be careful when replacing cartridge and discharge valves.
Most new extinguisher containers are supplied with
their cartridge and discharge valve disassembled. Before
installation on the aircraft, properly assemble the cartridge
in the discharge valve and connect the valve to the container,
usually by means of a swivel nut that tightens against a
packing ring gasket. [Figure 9-19]
If a cartridge is removed from a discharge valve for any
reason, it should not be used in another discharge valve
assembly, since the distance the contact point protrudes may
vary with each unit. Thus, continuity might not exist if a used
plug that had been indented with a long contact point were
installed in a discharge valve with a shorter contact point.
The preceding material in this chapter is general in nature,
addressing the principles involved and general procedures to
be followed. When actually performing maintenance, always
refer to the applicable maintenance manuals and other related
publications pertaining to a particular aircraft.
Boeing 777 Aircraft Fire Detection &
Extinguishing System
The following section discusses the fire detection and
extinguishing system of the B777 aircraft. The information
is included only for familiarization purposes.
Each engine has two fire detection loops: loop 1 and loop
2. A fire detection card in the system card file monitors the
loops for fires, overheat conditions, and faults. There is a fire
detection card for each engine.
Overheat Detection
If the fire detection loops detect an overheat condition, the
fire detection card sends a signal to the AIMS and to the
warning electronics unit. The following indications occur
in the flight deck:
• The master caution lights come on.
• The caution aural operates.
• An engine overheat caution message shows.
Fire Detection
If an engine fire occurs, the fire detection card sends a
signal to the AIMS and to the warning electronics unit, and
a warning message illuminates. The following indications
occur in the flight deck:
• The master warning lights come on.
• The fire warning aural operates.
• An engine fire warning message shows.
• The engine fire warning light comes on.• The fuel control switch fire warning light comes on.
Nacelle Temperature Recording
The fire detection card measures the average temperature of
the loops. This data goes to the AIMS through the system’s
ARINC 629 buses and is recorded by the aircraft condition
monitoring function.
Continuous Fault Monitoring
The fire detection card monitors the two loops and their
wiring for defects. In normal (dual loop) operation, both
loops must have a fire or overheat condition to cause the
flight deck indications.
If a failure occurs in a loop, the fire detection card sends the
data to the AIMS. A status message shows, and the system
changes to single-loop operation. In this mode, fire/overheat
indications occur when one loop is defective and the other
has a fire or overheat condition.
CAUTION
WARNINGAIMS
DISCH
L
E
FTDISCH
R
IG
H
TEngine fire
detectors
Loop 1Master caution light (2)
Master warning light (2)
P10 control standL FUEL CONTROL R
ENGINE BOTTLE
1 DISCHARGE ENGINE BOTTLE
2 DISCHARGEFIRE/
OVERHEAT
TESTRUN
CUTOFFL&R Systems
ARINC 629
buses
P8 engine fire panelSpeaker (2)
Loop 2
Fire
detection
card - ENG
Pressure switch cargo
fire/engine control panel
Figure 9-20. Engine fire detection system.Single/Dual Loop Operation
The fire detection card monitors the loops for faults. In
normal (dual loop) operation, both loops must have a fire or
overheat condition to cause the flight deck indications. If one
detection loop fails, the card sends data about the failure to
the AIMS, and a status message shows. The card changes to
single-loop operation, if necessary. If both detection loops
fail, an advisory message and status messages are displayed,
and the fire detection system does not operate.
System Test
Built-in test equipment (BITE) performs a test of the engine
fire detection system for these conditions:
• When the system first gets power.
• After a power interrupt.
• Every 5 minutes of operation. [Figures 9-20 and 9-21]
Boeing 777 Fire Extinguisher System
Fire Extinguisher Containers
The B777 aircraft has two fire extinguishing bottles that
contain Halon fire extinguishing agent pressurized with
nitrogen. The engine fire switches in the flight deck are pulled
and rotated to release the Halon. Halon from each bottle can
be discharged to the right or left engine. Engine indicating
and crew alerting system (EICAS) messages, status messages, and indicator lights show when the bottle pressure is low. The
two engine fire extinguishing bottles are located behind the
right sidewall lining of the forward cargo compartment, aft
of the cargo door. [Figure 9-22]
The two engine fire extinguishing bottles are identical. Each
bottle has these following components:
• A safety relief and fill port,
• A handle for removal and installation,
• A pressure switch,
• Two discharge assemblies,
• An identification plate, and
• Four mounting lugs. [Figures 9-23, 9-24, 9-25]
The bottles contain Halon fire extinguishing agent pressurized
with nitrogen. If the pressure in the bottle becomes too high,
the safety relief and fill port opens so that the bottle does not
explode. The discharge assembly has an explosive squib. An
electric current from the fire extinguishing circuit fires the
squib. This releases the Halon through the discharge port.
The pressure switch gives flight deck indications when bottle
pressure decreases. The switch monitors the pressure inside
the bottle and is normally open. When the pressure decreases
because of a leak or bottle discharge, the switch closes an
indicating circuit.
L
E
F
T1 2DISCH
R
I
G
H
T1 2DISCHENG BTL
1 DISCHENG BTL
2 DISCH
ENGINE FIRE PANEL (P8)
Discharge nozzlesEngine bottle discharge lights
Engine fire switches
Front spar (ref)
Engine fire extinguishing bottles
Discharge manifold
Discharge nozzles
Figure 9-22. Boeing 777 fire extinguisher container location.
CAUTIONWARNING
Loop 1
Master warning/
caution light (2)Left engine fire switchRight engine fire
switch APU fire switch
Unlocking
SolendidP310 standby power mgmt panel
P105 left integration panel
LR
LRWEU (2)28V DC
BAT. BUS
FIRE HANDLE
UNLOCK
ASG card (2)AIMS cabinet (2)
Systems arine 629 busesLeft engine fire
detection card
Left fuel control switch
Speaker (2)Loop 2
• Fire alarm
• OVHT alarm
• Temp data
• Fault data}Fire alarm 1MD&TFuel spar
ignition relay
MD&T
Fire alarm 2Fire warning switch
Figure 9-21. Engine fire detection system functional description.
Left engine Right engineBottle 1
Bottle 2L
E
F
T1 2DISCH
R
I
G
H
T1 2DISCHENG BTL
1 DISCHENG BTL
2 DISCH
ENGINE FIRE PANEL (P8)Discharge assembly
To discharge nozzles
Check valveDischarge manifoldTo discharge nozzles
Pressure switch
Identification plate
Pressure switch test buttonSafety relief and fill port
Handle
Mounting lug (4)
Discharge port (2)Discharge assembly (2)Pressure
switch
Squib (2) Pressure switch
electrical
connector
Figure 9-24. Location of fire extinguishing bottles.Figure 9-23. Fire extinguishing bottle.
Squib
The squib is installed in the discharge assembly at the bottom
of the fire container. A fire container has two squibs, one for
each engine. The squib is an electrically operated explosive
device. When the squib is activated, it fires a slug through
the breakable disc, and nitrogen pressure inside the bottle
pushes the Halon through the discharge port. The squib fires
when the fire switch is pulled and turned to the DISCH 1 or
DISCH 2 position. [Figure 9-22]
Engine Fire Switches
The engine fire panel is in the flight deck on the P8 aisle stand.
The engine fire panel has a fire switch for each engine and a
discharge light for each fire bottle. [Figure 9-26]The engine fire switch has four functions:
• Gives an indication of an engine fire,
• Stops the engine,
• Isolates the engine from the aircraft systems, and
• Controls the engine fire extinguishing system.
The fire switch assembly incorporates a solenoid that locks
the fire switch so that the flight crew cannot pull it accidently.
If an engine has a fire, the fire warning light comes on and the
solenoid energizes to release the switch. When the solenoid
is energized, the fire switch can be pulled.
When the fire detection system malfunctions or the solenoid
is defective, and the flight crew wants to extinguish an engine
fire, someone must push the fire override switch. The fire
override switch allows the fire switch to be pulled when the
solenoid is not energized. When the fire switch is pulled, the
push-pull switch contacts operate electrical circuits that stop
the engine and isolate it from the aircraft systems. With the
switch pulled, it can be rotated to left or right to a mechanical
stop at the discharge position. The rotary switch contacts
close and operate the fire extinguishing system.
When the fire switch is pulled, the switch isolates the
following aircraft systems from the engine:
• Closes the fuel spar valve.
• Deenergizes the engine fuel metering unit (FMU)
cutoff solenoid.
• Closes the engine hydraulic pump shutoff valve.
• Depressurizes the engine driven hydraulic pump valve.
• Closes the pressure regulator and shutoff valve.
• Removes power from thrust reverser isolation valve.
• Trips the generator field.
• Trips the backup generator field. [Figure 9-27]
Electrical connectorDiaphragm
Gland nutBottle (REF)
Discharge port
Ground lug SquibDischarge assembly
Figure 9-25. Squib or cartridge.
LEFTDisch
LEFTDisch
ENG BTL
1 DISCHENG BTL
2 DISCH
Engine fire warning lightQuick release fastener (4)
Engine fire switchEngine fire override switchElectrical connectorEngine bottle discharge lights
Figure 9-26. Fire switch.
Engine Fire Operation
If an engine has a fire, the engine fire detection system gives a
fire warning in the flight deck. The engine fire warning lights
come on to identify which fire switch to use to extinguish the
fire. The solenoid in the fire switch energizes and releases the
switch so that the fire switch can be pulled. If the solenoid
does not energize, push the fire override switch to release
the fire switch manually. When the fire switch is pulled, it
stops the engine, and the fire switch isolates the engine from
the aircraft systems.
If the fire warnings do not go away when the switch is pulled, position the switch to the DISCH 1 or DISCH 2 position, and
hold the switch against the stop for one second. This fires
the squib in the fire extinguisher container and releases the
fire extinguishing agent into the engine nacelle. Ensure that
the engine bottle discharge light comes on. If the first bottle
does not extinguish the fire, the switch must be placed to the
other DISCH position. This fires the squib for the other bottle.
APU Fire Detection & Extinguishing System
The APU fire protection system is similar in design to engine
fire protection systems, but there are some differences.
The APU is often operated with no personnel in the flight
P310 standby power
management panelENG fuel spar/
ignition relaySolenoid
Engine fire switch28V DC
BAT. BUS
RUN
CUTOFFFIRE HANDLE
UNLOCK}
}}
}
}
}
}
}
}
}
}AIMS
Fuel spar valve circuit
Engine FMU cutoff solenoid
EDP shutoff valve circuit
EDP depressurization circuit
Pressure regulator and shutoff valve circuit
Thrust reverser isolation valve circuit
Generator field circuit
Backup generator field circuit
Engine fire extinguishing circuitEngine fire detection circuit
Figure 9-27. Engine fire switch circuit.
deck and; the APU fire protection system can operate in
an unattended mode on the ground with the engines not
running. If there is an APU fire in the unattended mode, the
fire extinguisher discharges automatically. The APU operates
in the attended mode when at least one engine is running. If
there is an APU fire in this mode, the crew discharges the
bottle manually. Fire switches are located on the cargo fire/
engine control panel and the service and APU shutdown
panel located outside the aircraft on the nose landing gear.
[Figure 9-28]
APU Fire Warning
If there is an APU fire, the APU fire detection system gives
fire warnings and automatically stops the APU. The APU fire
warning light comes on to identify the correct fire switch to
use to extinguish the fire. The fire switch solenoid releases
the switch so that it can be pulled up. If the APU is running,
it stops when the fire switch is pulled. The fire switch isolates
the APU from the aircraft systems.
Fire Bottle Discharge
If the fire warnings do not go away with the switch out,
put the switch to the left or right DISCH position. Hold the switch against the discharge stop for one second. This fires
the bottle squib and releases the fire extinguishing agent into
the APU compartment. Verify that the APU bottle discharge
light comes on. [Figure 9-29]
Bottle discharged lightP40 service and APU shutdown panel
Fire bottle armed light
APU fire shutdown switch
APU bottle discharge switch
Figure 9-28. P40 service and APU shutdown panel.
AA
A
APressure switchTest
button
APU bottle dischargeMaster
Dim &
Test
APU fire switch
disch
disch In
Out
Fire/Overheat testAPU fire extinguishing bottleCargo fire/engine control panel (P5)
APU bottle discharge
APU fire shutdown
Bottle dischargedFire bottle armed
AIMS cabinet (2)
P40 service and APU shutdown panel P310 standby power management panel10 seconds delay
APU EXT shutdownAPU
remote
warning
Fire alarm 1
Unattended
fire alarm
APU fuel
system & ELMSFire EXT APU28V DC hot battery bus
APU RMT fire indicator/auto discharge arm28V DC hot
battery bus
Figure 9-29. APU fire extinguishing circuit.
10-1Reciprocating Engine Overhaul
Both maintenance and complete engine overhauls are
performed normally at specified intervals. This interval is
usually governed by the number of hours the powerplant
has been in operation. The actual overhaul period for a
specific engine is generally determined by the manufacturer’s
recommendations. Each engine manufacturer sets a total
time in service when the engine should be removed from
service and overhauled. Depending upon how the engine
is used in service, the overhaul time can be mandatory. The
overhaul time is listed in hours and is referred to as time
before overhaul (TBO). For example, if an engine had a life
of 2,000 hours and had operated 500 hours, it would have a
TBO of 1,500 hours. Tests and experience have shown that
operation beyond this period of time could result in certain
parts being worn beyond their safe limits. For an overhauled
engine to be as airworthy as a new one, worn parts, as well
as damaged parts, must be detected and replaced during
overhaul. The only way to detect all unairworthy parts is to
perform a thorough and complete overhaul process while the
engine is disassembled. The major purpose of overhaul is to
inspect, repair, and replace worn engine parts.
A complete overhaul process includes the following ten
steps: receiving inspection; disassembly; visual inspection;
cleaning; structural inspection; non-destructive testing (NDT)
inspection; dimensional inspection; repair and replacement;
reassembly; and testing and break in. The inspection phases
are the most precise and the most important phases of the
overhaul. Inspection cannot be slighted or performed in a
careless or incomplete manner. It is always recommended
that complete records be made of the inspection process and
kept with the engine records.
Each engine manufacturer provides very specific tolerances
to which the engine parts must conform and provides general
instructions to aid in determining the airworthiness of the
part. However, in many cases, the final determination must
be made by the technician. Although the determination must
be made if the part is serviceable, repairable, or should be
rejected, the technician should follow the manufacturer’s
manuals and information. When dimensional tolerances
are concerned, the manufacturer publishes a new minimum
and serviceable dimension for all critical component parts.
Knowledge of the operating principles, strength, and stresses
Engine Maintenance & Operation
Chapter 10
applied to a part is essential in making decisions regarding
visible wear. When the powerplant technician signs the release
for the return to service for an overhauled engine, this certifies
that the complete overhaul process has been performed using
methods, techniques, and practices acceptable to the Federal
Aviation Administration (FAA) Administrator.
Top Overhaul
Reciprocating piston aircraft engines can be repaired by a
top overhaul. This means an overhaul of those parts on top of
the crankcase, without completely dismantling the engine. It
includes removal of the units (i.e., exhaust collectors, ignition
harness, intake pipes) necessary to remove the cylinders. The
actual top overhaul consists of reconditioning the engine’s
cylinders by replacing or reconditioning the piston and
piston rings, and reconditioning or plating the cylinder wall
and valve-operating mechanism, including valve guides if
needed. A top overhaul is a little misleading, because it is
really an engine repair procedure and not a real overhaul
as described earlier. Usually at this time, the accessories
require no attention other than that normally required during
ordinary maintenance functions. This repair is generally due
to valves or piston rings wearing prematurely. Many stress
that if an engine requires this much dismantling, it should
be completely disassembled and receive a major overhaul.
Major Overhaul & Major Repairs
Major overhaul consists of the complete reconditioning of
the powerplant. A reciprocating engine would require that the
crankcase be disassembled per the FAA; a major overhaul
is not generally a major repair. A certified powerplant-rated
technician can perform or supervise a major overhaul of an
engine if it is not equipped with an internal supercharger
or has a propeller reduction system other than spur-type
gears. At regular intervals, an engine should be completely
dismantled, thoroughly cleaned, and inspected. Each part
should be overhauled in accordance with the manufacturer’s
instructions and tolerances for the engine involved. At this
time all accessories are removed, overhauled, and tested.
Again, instructions from the manufacturer of the accessory
concerned should be followed.
General Overhaul Procedures
Because of the continued changes and the many different
types of engines in use, it is not possible to treat the specific
Figure 10-1. Engine mounted on an overhaul stand. overhaul of each engine in this text. However, there are
various overhaul practices and instructions of a nonspecific
nature that apply to all makes and models of engines.
Any engine to be overhauled completely should receive a
runout check of its crankshaft or propeller shaft as a first
step. Any question concerning crankshaft or propeller shaft
replacement is resolved at this time, since a shaft whose
runout is beyond limits must be replaced.
Throughout the life of a product (whether type-certificated
or not), manufacturing defects, changes in service, or
design improvements often occur. When that happens, the
OEM frequently uses a safety bulletin (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 airworthiness
directive (AD) note or if compliance is required as a part of
the authorized inspection program. Refer to 14 CFR part 39,
section 39.27.
Receiving Inspection
The receiving inspection consists of determining the
general condition of the total engine as received, along with
an inventory of the engine’s components. The accessory
information should be recorded, such as model and serial
numbers, and the accessories should be sent to overhaul
if needed. The overhaul records should be organized, and
the appropriate manuals obtained and reviewed along with
a review of the engine’s history (log books). The engine’s
service bulletins, airworthiness directives, and type certificate
compliance should be checked. The exterior of the engine
should be cleaned after mounting it on an overhaul stand.
[Figure 10-1]
Disassembly
As visual inspection immediately follows disassembly, all
individual parts should be laid out in an orderly manner on
a workbench as they are removed. To guard against damage
and to prevent loss, suitable containers should be available
in which to place small parts (nuts, bolts, etc.) during the
disassembly operation.
Other practices to observe during disassembly include:
1. Drain the engine oil sumps and remove the oil filter.
Drain the oil into a suitable container; strain it through
a clean cloth. Check the oil and the cloth for metal
particles.
2. Dispose of all safety devices (safety wire, cotter pins,
etc.) as they are removed. Never use them a second
time. Always replace with new safety devices.3. All loose studs, and loose or damaged fittings, should
be carefully tagged to prevent being overlooked during
inspection.
4. Always use the proper tool for the job. Use sockets
and box end wrenches wherever possible. If special
tools are required, use them rather than improvising.
Inspection Process
The inspection of engine parts during overhaul is divided
into three categories:
1. Visual.
2. Structural.
3. Dimensional.
Many defects on the engine components can be detected
visually, and a determination of airworthiness can be made
at this time. If, by visual inspection, the component is
determined to be unairworthy, the part is rejected, and no
further inspection or repair is required. Structural failures can
be determined by several different methods. Magnetic parts
can readily be examined by the magnetic particle method.
Other methods, such as dye penetrate, eddy current, ultra
sound, and X-ray, can also be used. The first two methods
are aimed at determining structural failures in the parts,
while the last method deals with the size and shape of each
part. By using very accurate measuring equipment, each
engine component can be dimensionally evaluated and
compared to service limits and standards (tolerances) set by
the manufacturer.
10-3Visual Inspection
Visual inspection should precede all other inspection
procedures. Parts should not be cleaned before a preliminary
visual inspection, since indications of a failure may often be
detected from the residual deposits of metallic particles in
some recesses in the engine.
Several terms are used to describe defects detected in engine
parts during inspection. Some of the more common terms
and definitions are:
1. Abrasion—an area of roughened scratches or marks
usually caused by foreign matter between moving
parts or surfaces.
2. Brinelling—one or more indentations on bearing races,
usually caused by high static loads or application of
force during installation or removal. Indentations are
rounded or spherical due to the impression left by the
contacting balls or rollers of the bearing.
3. Burning—surface damage due to excessive heat. It is
usually caused by improper fit, defective lubrication,
or over-temperature operation.
4. Burnishing—polishing of one surface by sliding
contact with a smooth, harder surface. Usually no
displacement nor removal of metal.
5. Burr—a sharp or roughened projection of metal
usually resulting from machine processing.
6. Chafing—a condition caused by a rubbing action
between two parts under light pressure that results in
wear.
7. Chipping—breaking away of pieces of material, that
is usually caused by excessive stress concentration or
careless handling.
8. Corrosion—loss of metal by a chemical or
electrochemical action. The corrosion products are
easily removed by mechanical means. Iron rust is an
example of corrosion.
9. Crack—a partial separation of material usually caused
by vibration, overloading, internal stresses, defective
assembly, or fatigue. Depth may be a few thousandths,
to the full thickness of the piece.
10. Cut—loss of metal, usually to an appreciable depth
over a relatively long and narrow area, by mechanical
means, as would occur with the use of a saw blade,
chisel, or sharp-edged stone striking a glancing blow.
11. Dent—a small, rounded depression in a surface usually
caused by the part being struck with a rounded object.
12. Erosion—loss of metal from the surface by mechanical
action of foreign objects, such as grit or fine sand. The eroded area is rough and may be lined in the direction
that the foreign material moved relative to the surface.
13. Flaking—the breaking loose of small pieces of metal
or coated surfaces, that is usually caused by defective
plating or excessive loading.
14. Fretting—a condition of surface erosion caused by
minute movement between two parts usually clamped
together with considerable unit pressure.
15. Galling—a severe condition of chafing or fretting in
which a transfer of metal from one part to another
occurs. It is usually caused by a slight movement of
mated parts having limited relative motion and under
high loads.
16. Gouging—a furrowing condition in which a
displacement of metal has occurred (a torn effect).
It is usually caused by a piece of metal, or foreign
material, between close moving parts.
17. Grooving—a recess, or channel, with rounded and
smooth edges usually caused by faulty alignment
of parts.
18. Inclusion—presence of foreign or extraneous material
entirely within a portion of metal. Such material is
introduced during the manufacture of rod, bar, or
tubing by rolling or forging.
19. Nick—a sharp-sided gouge or depression with a
V-shaped bottom, that is generally the result of careless
handling of tools and parts.
20. Peening—a series of blunt depressions in a surface.
21. Pick up or scuffing—a buildup or rolling of metal
from one area to another, that is usually caused by
insufficient lubrication, clearances, or foreign matter.
22. Pitting—small hollows of irregular shape in the
surface, usually caused by corrosion or minute
mechanical chipping of surfaces.
23. Scoring—a series of deep scratches caused by foreign
particles between moving parts or careless assembly
or disassembly techniques.
24. Scratches—shallow, thin lines or marks, varying in
degree of depth and width, caused by presence of fine
foreign particles during operation or contact with other
parts during handling.
25. Stain—a change in color, locally, causing a noticeably
different appearance from the surrounding area.
26. Upsetting—a displacement of material beyond the
normal contour or surface (a local bulge or bump).
Usually indicates no metal loss.
Examine all gears for evidence of pitting or excessive wear.
10-4Check for
depression with
straightedge and
thickness gageCheck piston for
cracks at base of
ring landsStraightedge
Junction of wall and
head
Figure 10-2. Checking a piston head for flatness. These conditions are of particular importance when they
occur on the teeth; deep pit marks in this area are sufficient
cause to reject the gear. Bearing surfaces of all gears should be
free from deep scratches. However, minor abrasions usually
can be dressed out with a fine abrasive cloth.
All bearing surfaces should be examined for scores, galling,
and wear. Considerable scratching and light scoring of
aluminum bearing surfaces in the engine do no harm and
should not be considered a reason for rejecting the part,
provided it falls within the clearances set forth in the table of
limits in the engine manufacturer’s overhaul manual. Even
though the part comes within the specific clearance limits, it is
not satisfactory for re-assembly in the engine unless inspection
shows the part to be free from other serious defects.
Ball bearings should be inspected visually and by feel for
roughness, flat spots on balls, flaking or pitting of races, or
scoring on the outside of races. All journals should be checked
for galling, scores, misalignment, or out-of-round condition.
Shafts, pins, etc., should be checked for straightness. This
may be done, in most cases, by using V-blocks and a dial
indicator.
Pitted surfaces in highly stressed areas, resulting from
corrosion, can cause ultimate failure of the part. The
following areas should be examined carefully for evidence
of such corrosion:
1. Interior surfaces of piston pins.
2. The fillets at the edges of crankshaft main and crankpin
journal surfaces.
3. Thrust bearing races.
If pitting exists on any of the surfaces mentioned, to the extent
that it cannot be removed by polishing with crocus cloth or
other mild abrasive, the part usually must be rejected.
Parts, such as threaded fasteners or plugs, should be inspected
to determine the condition of the threads. Badly worn or
mutilated threads cannot be tolerated; the parts should be
rejected. However, small defects, such as slight nicks or burrs,
may be dressed out with a small file, fine abrasive cloth,
or stone. If the part appears to be distorted, badly galled,
mutilated by overtightening, or from the use of improper
tools, replace it with a new one.
Cylinder Head
Inspect the cylinder head for internal and external cracks.
Use a bright light to inspect for cracks and investigate any
suspicious areas with a magnifying glass or microscope.
Carbon deposits must be cleaned from the inside of the
head, and paint must be removed from the outside for this
inspection. Exterior cracks show up on the head fins where they have
been damaged by tools or contact with other parts because
of careless handling. Cracks near the edge of the fins are not
dangerous, if the portion of the fin is removed and contoured
properly. Cracks at the base of the fin are a reason for rejecting
the cylinder. Cracks may also occur on the rocker box or in
the rocker bosses.
Interior cracks almost always radiate from the valve seat
bosses or the spark plug bushing boss. These cracks are
usually caused by improper installation of the seats or
bushings. They may extend completely from one boss to
the other.
Inspect all the studs on the cylinder head for looseness,
straightness, damaged threads, and proper length. Slightly
damaged threads may be chased with the proper die. The
length of the stud should be correct within ±1⁄32 (0.03125)
inch to allow for proper installation of safety devices.
Be sure the valve guides are clean before inspection. Often,
carbon covers pits inside the guide. If a guide in this condition
is put back in service, carbon again collects in the pits and
valve sticking results. Besides pits, scores, and burned areas
inside the valve guide, inspect them for wear or looseness.
Inspection of valve seat inserts before they are re-faced is
mostly a matter of determining if there is enough of the seat
left to correct any pitting, burning, scoring, or out-of-trueness.
Inspect the rocker shaft bosses for scoring, cracks, oversize,
or out-of-roundness. Scoring is generally caused by the rocker
shaft turning in the bosses, which means either the shaft was
too loose in the bosses or a rocker arm was too tight on the
shaft. Out-of-roundness is usually caused by a stuck valve.
If a valve sticks, the rocker shaft tends to work up and down
when the valve offers excessive resistance to opening. Inspect
for out-of-roundness and oversize using a telescopic gauge
and a micrometer.
Figure 10-3. Typical solvent degreasing tank. Piston, Valve Train, & Piston Pin
When applicable, check for flatness of the piston head
using a straightedge and thickness gauge. [Figure 10-2] If
a depression is found, check for cracks on the inside of the
piston. A depression in the top of the piston usually means
that detonation has occurred within the cylinder.
Inspect the exterior of the piston for scores and scratches.
Scores on the top ring land are not cause for rejection, unless
they are excessively deep. Deep scores on the side of the
piston are usually a reason for rejection.
Examine the piston for cracked skirts, broken ring lands,
and scored piston-pin holes. Do not mistake casting marks
or laps for a crack. During major overhaul, most pistons are
generally replaced, as it requires more labor to clean and
inspect the piston than it costs to replace it.
Crankshaft & Connecting Rods
Carefully inspect all surfaces of the crankshaft for cracks.
Check the bearing surfaces for evidence of galling, scoring,
or other damage. When a shaft is equipped with oil transfer
tubes, check them for tightness.
Visual inspection of connecting rods should be done with the
aid of a magnifying glass or bench microscope. A rod that is
obviously bent or twisted should be rejected without further
inspection. Inspect all surfaces of the connecting rods for
cracks, corrosion, pitting, galling, or other damage. Galling
is caused by a slight amount of movement between the
surfaces of the bearing insert and the connecting rod during
periods of high loading, such as that produced during over-
speed or excessive manifold pressure operation. The visual
evidence produced by galling appears as if particles from
one contacting surface had welded to the other. Evidence
of any galling is sufficient reason for rejecting the complete
rod assembly. Galling is a distortion in the metal and is
comparable to corrosion in the manner in which it weakens
the metallic structure of the connecting rod.
Cleaning
After visually inspecting engine recesses for deposits of metal
particles, it is important to clean all engine parts thoroughly
to facilitate further inspection. Two processes for cleaning
engine parts are:
1. Degreasing to remove dirt and sludge (soft carbon).
2. The removal of hard carbon deposits by decarbonizing,
brushing or scraping, and grit-blasting.
Degreasing
Degreasing can be done by immersing or spraying the part in a
suitable commercial solvent. [Figure 10-3] Extreme care must
be used if any water-mixed degreasing solutions containing caustic compounds or soap are used. Such compounds, in
addition to being potentially corrosive to aluminum and
magnesium, may become impregnated in the pores of the
metal and cause oil foaming when the engine is returned to
service. Therefore, when using water-mixed solutions, it is
imperative that the parts be rinsed thoroughly and completely
in clear boiling water after degreasing. Regardless of the
method and type of solution used, coat or spray all parts with
lubricating oil immediately after cleaning to prevent corrosion.
Removing Hard Carbon
While the degreasing solution removes dirt, grease, and soft
carbon, deposits of hard carbon almost invariably remain
on many interior surfaces. To remove these deposits, they
must first be loosened by immersion in a tank containing a
decarbonizing solution (usually heated). A great variety of
commercial decarbonizing agents are available. Decarbonizers,
like the degreasing solutions previously mentioned, fall
generally into two categories, water-soluble and hydrocarbons.
The same caution concerning the use of water-soluble
degreasers is applicable to water-soluble decarbonizers.
Caution: When using a decarbonizing solution on magnesium
castings, avoid immersing steel and magnesium parts in the
same decarbonizing tank, as this practice often results in
damage to the magnesium parts from corrosion.
Decarbonizing will usually loosen most of the hard carbon
deposits remaining after degreasing. However, the complete
removal of all hard carbon generally requires brushing,
scraping, or grit-blasting. In all of these operations, be careful
to avoid damaging the machined surfaces. In particular, wire
brushes and metal scrapers must never be used on any bearing
or contact surface.
Follow the manufacturer's recommendations when grit-
Figure 10-4. Grit-blasting machine. blasting parts for the abrasive material being used. Sand,
rice, baked wheat, plastic pellets, glass beads, or crushed
walnut shells are examples of abrasive substances that are
used for grit-blasting parts. A grit-blasting machine is shown
in Figure 10-4 .
All machined surfaces must be masked properly and
adequately, and all openings tightly plugged before blasting.
The one exception to this is the valve seats, which may be
left unprotected when blasting the cylinder head combustion
chamber. It is often advantageous to grit-blast the seats, since
this will cut the glaze which tends to form (particularly on
the exhaust valve seat), thus facilitating subsequent valve
seat reconditioning. Piston ring grooves may be grit-blasted
if necessary; however, extreme caution must be used to
avoid the removal of metal from the bottom and sides of
the grooves. When grit-blasting housings, plug all drilled
oil passages with rubber plugs or other suitable material to
prevent the entrance of foreign matter.
The decarbonizing solution will generally remove most of the
enamel on exterior surfaces. All remaining enamel should be
removed by grit-blasting, particularly in the crevices between
cylinder cooling fins.
At the conclusion of cleaning operations, rinse the part in
petroleum solvent, dry and remove any loose particles of
carbon or other foreign matter by air-blasting, and apply a
liberal coating of preservative oil to all surfaces.
Magnesium parts should be cleaned thoroughly with a
dichromate treatment prior to painting. This treatment
consists of cleaning all traces of grease and oil from the
part by using a neutral, noncorrosive degreasing medium
followed by a rinse, after which the part is immersed for at
least 45 minutes in a hot dichromate solution (three-fourths of
a pound of sodium dichromate to 1 gallon of water at 180 °F to 200 °F). Then the part should be washed thoroughly in
cold running water, dipped in hot water, and dried in an air
blast. Immediately thereafter, the part should be painted with
a prime coat and engine enamel in the same manner as that
suggested for aluminum parts.
Some older engines used sludge chambers in the crankshafts,
which were manufactured with hollow crankpins that serve as
sludge removers. The sludge chambers require inspection and
cleaning at overhaul. Sludge chambers are formed by means
of spool-shaped tubes pressed into the hollow crankpins, or
by plugs pressed into each end of the crankpin. If an engine
has a sludge chamber or tubes, they must be removed for
cleaning at overhaul. If these are not removed, accumulated
sludge loosened during cleaning may clog the crankshaft
oil passages and cause subsequent bearing failures. If the
sludge chambers are formed by means of tubes pressed into
the hollow crankpins, make certain they are re-installed
correctly to avoid covering the ends of the oil passages. Due
to improved oils, sludge chambers are no longer used with
modern engines.
Structural Inspection
One of the best methods to double check your visual
inspection findings is to supplement them with one of the
forms of nondestructive testing, such as magnetic particle,
dye penetrate, eddy current, ultrasound, and x-ray inspections.
Defects in nonmagnetic parts (aluminum parts) can be found
by all these methods except for magnetic particle inspection,
which is used for magnetic or ferrous materials (steel).
Dye Penetrant Inspection
Dye 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. Dye 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 of two
types of dye: visible or fluorescent. When using a fluorescent
dye, the inspection is accomplished using an ultraviolet (UV)
light source (black light).
The steps for performing a dye penetrant inspection are:
1. Thorough cleaning of the metal surface.
2. Applying penetrant.
3. Removing penetrant with remover emulsifier or
cleaner.
10-74. Drying the part.
5. Applying the developer.
6. Inspecting and interpreting results.
Eddy Current Inspection
Eddy currents are composed of free electrons under the
influence of an induced electromagnetic field, that are made
to drift through metal. 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. 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.
Ultrasonic Inspection
Ultrasonic detection equipment makes it possible to locate
defects in all types of materials. There are three basic
ultrasonic inspection methods:
1. Pulse-echo.
2. Through transmission.
3. Resonance.
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.
Through Transmission
Through transmission inspection uses two transducers, one to
generate the pulse and another placed on the opposite surface
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-echo method, in that
the frequency of transmission may be continuously varied.
The resonance method is principally used for thickness
measurements when the two sides of the material being tested
are smooth and parallel, and the backside is inaccessible. The
point at which the frequency matches the resonance point of
the material being tested is the thickness determining factor.
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. 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 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.
X-ray
X-rays can penetrate material and disclose discontinuities
through the metal or non-metal components, making it an
excellent inspection process when needed to determine the
structural integrity of an engine component. 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, as a portable unit,
provides a fast and reliable means for checking the integrity
of engine components.
Additional and more thorough information on NDT inspection
is covered in detail in the Aviation Maintenance Technician
Handbook - General (FAA-H-8083-30, as amended).
Dimensional Inspection
The dimensional inspection is used to assure that the engine’s
component parts and clearances meet the manufacturer’s
specifications. These specs are listed in a table of limits,
which lists serviceable limits and the manufacturer’s new part
maximum and minimum dimensions. Many measuring tools
are used to perform the dimensional inspection of the engine.
Some examples of these devices are discussed as the procedure
for measuring the engine’s components for dimensional
inspection is explained in the following paragraphs.
Cylinder Barrel
Inspect the cylinder barrel for wear, using a cylinder bore
gauge [Figure 10-5] , a telescopic gauge, and micrometer or
an inside micrometer. Dimensional inspection of the barrel
consists of the following measurements:
1. Maximum taper of cylinder walls.
2. Maximum out-of-roundness.
Figure 10-5. A cylinder bore gauge.
3. Bore diameter.
4. Step.
5. Fit between piston skirt and cylinder.
All measurements involving cylinder barrel diameters must
be taken at a minimum of two positions 90° apart in the
particular plane being measured. It may be necessary to take
more than two measurements to determine the maximum
wear.
Taper of the cylinder walls is the difference between the
diameter of the cylinder barrel at the bottom and the diameter
at the top. The cylinder is usually worn larger at the top than
at the bottom. This taper is caused by the natural wear pattern.
At the top of the stroke, the piston is subjected to greater heat
and pressure and more erosive environment than at the bottom
of the stroke. Also, there is greater freedom of movement
at the top of the stroke. Under these conditions, the piston
wears the cylinder wall more at the top of the cylinder. In
most cases, the taper ends with a ridge, that must be removed
during overhaul. [Figure 10-6]
Where cylinders are built with an intentional choke,
measurement of taper becomes more complicated. Cylinder
choke is where the top of the cylinder has been made with
the very top diameter of the cylinder smaller, to compensate
for wear and expansion during operation. It is necessary to
know exactly how the size indicates wear or taper. Taper
can be measured in any cylinder by a cylinder dial gauge
as long as there is not a sharp step. The dial gauge tends to
ride up on the step and causes inaccurate readings at the top
of the cylinder.
The measurement for out-of-roundness is usually taken at the
top of the cylinder. However, a reading should also be taken at the skirt of the cylinder to detect dents or bends caused by
careless handling. A step, or ridge, is formed in the cylinder
by the wearing action of the piston rings. [Figure 10-6] The
greatest wear is at the top of the ring travel limit. The ridge
that results is likely to cause damage to the rings or piston. If
the step exceeds tolerances, it should be removed by grinding
the cylinder oversize, or it should be blended by hand-stoning
to break the sharp edge.
A step also may be found where the bottom ring reaches the
lowest travel. This step is rarely found to be excessive, but
it should be checked.
Inspect the cylinder walls for rust, pitting, or scores. Mild
damage of this sort can be removed when the cylinders are
deglazed. With more extensive damage, the cylinder has to be
reground or honed. If the damage is too deep to be removed
by either of these methods, the cylinder usually will have to
be rejected. Most engine manufacturers, or engine overhaul
repair stations, have an exchange service on cylinders with
damaged barrels.
Check the cylinder flange for warpage by placing the cylinder
on a suitable jig. Check to see that the flange contacts the jig
all the way around. The amount of warpage can be checked by
using a thickness gauge. A cylinder whose flange is warped
beyond the limits should be rejected.
Rocker Arms & Shafts
Inspect the valve rockers for cracks and worn, pitted, or scored
tips. See that all oil passages are free from obstructions.
Inspect the shaft’s diameter for correct size with a micrometer.
Rocker shafts are often found to be scored and burned because
of excessive turning in the cylinder head. Also, there may be
some pickup on the shaft (bronze from the rocker bushing
transferred to the steel shaft). Generally, this is caused
by overheating and too little clearance between shaft and
bushing. The clearance between the shaft and the bushing
is most important.
Inspect the rocker arm bushing for correct size. Check for
proper clearance between the shaft and the bushing. Very
often the bushings are scored because of mishandling during
disassembly. Check to see that the oil holes line up. .At least
50% of the hole in the bushing should align with the hole in
the rocker arm.
On engines that use a bearing, rather than a bushing, inspect
the bearing to make certain it has not been turning in the
rocker arm boss. Also inspect the bearing to determine its
serviceability.
10-9Ridge removed by grindingRidge worn at top of ring travel
Original cylinder wall
Ridge removed by hand-stoning
Figure 10-6. Ridge or step formed in an engine cylinder.
Figure 10-7. Checking crankshaft runout.
Figure 10-8. A micrometer. Crankshaft
Use extreme care in inspecting and checking the crankshaft
for straightness. Place the crankshaft in V-blocks, supported
at the locations specified in the applicable engine overhaul
manual as in Figure 10-7. Using a surface plate and a dial
indicator, measure the shaft runout. If the total indicator
reading exceeds the dimensions given in the manufacturer’s
table of limits, the shaft must not be re-used. A bent crankshaft
should not be straightened. Any attempt to do so results
in rupture of the nitrided surface of the bearing journals,
a condition that causes eventual failure of the crankshaft.
Measure the outside diameter of the crankshaft main and rod-
bearing journals using a micrometer. [Figure 10-8] Internal
measurements can be made by using telescoping gauges, and
then measuring the telescoping gauge with a micrometer.
[Figure 10-9] Compare the resulting measurements with
those in the table of limits.
Checking Alignment
Check bushings that have been replaced to determine if the
bushing and rod bores are square and parallel to each other.
The alignment of a connecting rod can be checked several
ways. One method requires a push fit arbor for each end of
the connecting rod, a surface plate, and two parallel blocks
of equal height.
To measure rod squareness, or twist, insert the arbors into
the rod bores. [Figure 10-10] Place the parallel blocks on
Figure 10-9. Telescoping gauges and micrometer combination.
Parallel blocks
Figure 10-10. Checking connecting rod squareness.
a surface plate. Place the ends of the arbors on the parallel
blocks. Using a thickness gauge, check the clearance at the
points where the arbors rest on the blocks. This clearance,
divided by the separation of the blocks in inches, gives the
twist per inch of length.
To determine bushing or bearing parallelism (convergence),
insert the arbors in the rod bores. Measure the distance
between the arbors on each side of the connecting rod at
points that are equidistant from the rod centerline. For exact
parallelism, the distances checked on both sides should be
the same. Consult the manufacturer’s table of limits for the
amount of misalignment permitted.
The preceding operations are typical of those used for most
reciprocating engines and are included to introduce some
of the operations involved in engine overhaul. It would be impractical to list all the steps involved in the overhaul
of an engine. It should be understood that there are other
operations and inspections that must be performed. For exact
information regarding a specific engine model, consult the
manufacturer’s overhaul manual.
Repair & Replacement
The engine components that have failed inspection, or are
unrepairable, should have been discarded. The component
parts that need repair and replacement are now given the
attention required. The replacement components (new parts)
are organized and laid out for reassembly.
Minor damage to engine parts, such as burrs, nicks, scratches,
scoring, or galling, should be removed with a fine oil stone,
crocus cloth, or any similar abrasive substance. Following
any repairs of this type, the part should be cleaned carefully
to be certain that all abrasive has been removed, and then
checked with its mating part to assure that the clearances
are not excessive. Flanged surfaces that are bent, warped,
or nicked can be repaired by lapping to a true surface on a
surface plate. Again, the part should be cleaned to be certain
that all abrasive has been removed. Defective threads can
sometimes be repaired with a suitable die or tap. Small
nicks can be satisfactorily removed with Swiss pattern files
or small, edged stones. Pipe threads should not be tapped
deeper to clean them, because this practice results in an
oversized tapped hole. If galling or scratches are removed
from a bearing surface of a journal, it should be buffed to a
high polished finish.
In general, welding of highly-stressed engine parts can be
accomplished only when approved by the manufacturer.
However, welding may be accomplished using methods that
are approved by the engine manufacturer, and if it can be
reasonably expected that the welded repair will not adversely
affect the airworthiness of the engine.
Many minor parts not subjected to high stresses may be
safely repaired by welding. Mounting lugs, cowl lugs,
cylinder fins, rocker box covers, and many parts originally
fabricated by welding are in this category. The welded part
should be suitably stress-relieved after welding. However,
before welding any engine part, consult the manufacturer’s
instructions for the engine concerned to see if it is approved
for repair by welding.
Parts requiring use of paint for protection or appearance
should be repainted according to the engine manufacturer’s
recommendations. Aluminum alloy parts should have
original, exterior painted surfaces rubbed smooth to provide
a proper paint base. See that surfaces to be painted are
thoroughly cleaned. Care must be taken to avoid painting
mating surfaces. Exterior aluminum parts should be primed
Figure 10-11. Checking a piston pin for bends. checked for side clearance with a feeler gauge to determine
the amount of wear in the grooves. Examine the piston pin
for scoring, cracks, excessive wear, and pitting. Check the
clearance between the piston pin and the bore of the piston
pin bosses using a telescopic gauge and a micrometer. Use
the magnetic particle method to inspect the pin for cracks.
Since the pins are often case hardened, cracks show up inside
the pin more often than they do on the outside. Check the pin
for bends using V-blocks and a dial indicator on a surface
plate. [Figure 10-11] Measure the fit of the plugs in the
pin. In many cases, the pistons and piston pins are routinely
replaced at overhaul.
Valves & Valve Springs
The locations for checking runout and edge thickness of the
valves are shown in Figure 10-12 . Measure the edge thickness
of valve heads. If, after re-facing, the edge thickness is less
than the limit specified by the manufacturer, the valve must
not be re-used. The edge thickness can be measured with
sufficient accuracy by a dial indicator and a surface plate.
Using a magnifying glass, examine the valve in the stem area
and the tip for evidence of cracks, nicks, or other indications
of damage. This type of damage seriously weakens the
valve, making it susceptible to failure. If superficial nicks
and scratches on the valve indicate that it might be cracked,
inspect it using a structural inspection method described later.
Examine the valve springs for cracks, rust, broken ends, and
compression. Cracks can be located by visual inspection or
the magnetic particle method.first with a thin coat of zinc chromate primer. After the primer
is dry, parts should be painted with engine enamel, that should
be air dried until hard, or baked for 1⁄2 hour at 82 °C (180 °F).
Aluminum parts from which the paint has not been removed
may be repainted without the use of a priming coat, provided
no bare aluminum is exposed.
Any studs that are bent, broken, damaged, or loose must be
replaced. After a stud has been removed, the tapped stud hole
should be examined for size and condition of threads. If it
is necessary to re-tap the stud hole, it also is necessary to
use a suitable oversize stud. Studs that have been broken off
flush with the case must be drilled and removed with suitable
stud remover. Be careful not to damage any threads. When
replacing studs, coat the coarse threads of the stud with an
anti-seize compound.
Cylinder Assembly Reconditioning
Cylinder and piston assemblies are inspected according
to the procedures contained in the engine manufacturer’s
manuals, charts, and service bulletins. A general procedure
for inspecting and reconditioning cylinders is discussed in
the following section to provide an understanding of the
operations involved.
Visually inspect the head fins for other damage besides cracks.
Dents or bends in the fins should be left alone unless there
is danger of cracking. Where pieces of fin are missing, the
sharp edges should be filed to a smooth contour. Fin breakage
in a concentrated area causes dangerous local hot spots. Fin
breakage near the spark plug bushings or on the exhaust side
of the cylinder is obviously more dangerous than in other
areas. When removing or re-profiling a cylinder fin, follow
the instructions and the limits in the manufacturer’s manual.
Inspect spark plug inserts for the condition of the threads
and for looseness. Run a tap of the proper size through the
bushing. Very often, the inside threads of the bushing are
burned. If more than one thread is missing, the bushing
should be rejected. Tighten a plug in the bushing to check
for looseness.
Piston & Piston Pins
If the old piston is to be reused, or a new piston is to be used,
measure the outside of the piston by means of a micrometer.
Measurements must be taken in several directions and on
the skirt, as well as on the lands section. Check these sizes
against the cylinder size. Most engines use cam ground
pistons to compensate for the greater expansion parallel to
the pin during engine operation. The diameter of these pistons
measures several thousandths of an inch larger at an angle to
the piston pin hole, than parallel to the pin hole. Inspect the
ring grooves for evidence of wear. The groove needs to be
Valve face
Figure 10-13. Valve face surface.
Figure 10-14. Checking valve stretch with a manufacturer’ s gauge.
Figure 10-15. Valve spring compression tester.
Face of valve must run true with stem within 0.0015-inch
Valve face. Do not include
when measuring thicknessA
A
Figure 10-12. Valve showing locations for checking runout and
section for measuring edge thickness.
Critical areas of the valve include the face and tip
[Figure 10-13] , both of which should be examined for
pitting and excessive wear. Minor pitting on valve faces can
sometimes be removed by grinding.
Inspect the valve for stretch and wear using a micrometer
or a valve radius gauge. [Figure 10-14] If a micrometer is
used, stretch is found as a smaller diameter of the valve stem
near the neck of the valve. Measure the diameter of the valve
stem and check the fit of the valve in its guide.
Examine the valve visually for physical damage and damage
from burning or corrosion. Do not re-use valves that indicate
damage of this nature.
Compression is tested with a valve spring compression tester.
[Figure 10-15] The spring is compressed until its total height
is that specified by the manufacturer. The dial on the tester
should indicate the pressure, in pounds, required to compress
10-13Figure 10-16. Valve seat grinding equipment.
Pilot
Expander screwStoneGrinding
Fluid hose
Stone
holder
Spacing washers
Figure 10-17. Valve seat grinding setup. the spring to the specified height. This must be within the
pressure limits established by the manufacturer.
Refacing Valve Seats
The valve seat inserts of aircraft engine cylinders usually
are in need of refacing at every overhaul. They are refaced
to provide a true, clean, and correct size seat for the valve.
When valve guides or valve seats are replaced in a cylinder,
the seats must be made concentric with the valve guide.
Low power engines can use either bronze or steel seats. Bronze
seats, although not widely used on current engines, are made
of aluminum bronze or phosphor bronze alloys. Steel seats
are commonly used for valve seats on higher powered engines
and are made of heat-resistant steel with a layer of stellite steel
alloy on the valve contact surface. Stellite seats can require a
special stone to grind this very hard material.
Steel valve seats are refaced by grinding equipment.
[Figure 10-16] Bronze seats are refaced preferably by the
use of cutters or reamers, but they may be ground when this
equipment is not available. The only disadvantage of using
a stone on bronze is that the soft metal loads the stone to
such an extent that much time is consumed in redressing the
stone to keep it clean.
The equipment used on steel seats can be either wet or
dry valve seat grinding equipment. The wet grinder uses a
mixture of soluble oil and water to wash away the chips and
to keep the stone and seat cool; this produces a smoother,
more accurate job than the dry grinder. The stones may be
either silicon carbide or aluminum oxide.
Before refacing the seat, make sure that the valve guide is in
good condition, clean, and does not have to be replaced. Mount
the cylinder firmly in the hold down fixture. An expanding pilot is inserted in the valve guide from the inside of the
cylinder, and an expander screw is inserted in the pilot from
the top of the guide. [Figure 10-17] The pilot must be tight in
the guide, because any movement can cause a poor grind. The
fluid hose is inserted through one of the spark plug inserts.
The three grades of stones available for use are classified as
rough, finishing, and polishing stones. The rough stone is
designed to true and clean the seat. The finishing stone must
follow the rough to remove grinding marks and produce a
smooth finish. The polishing stone does just as the name
implies and is used only where a highly polished seat is desired.
The stones are installed on special stone holders. The face
of the stone is trued by a diamond dresser. The stone should
be refaced whenever it is grooved or loaded, and when the
stone is first installed on the stone holder. The diamond
dresser also may be used to cut down the diameter of the
stone. Dressing of the stone should be kept to a minimum
as a matter of conservation; therefore, it is desirable to have
sufficient stone holders for all the stones to be used on the job.
In the actual grinding job, considerable skill is required
in handling the grinding gun. The gun must be centered
accurately on the stone holder. If the gun is tilted off-
center, chattering of the stone results, and a rough grind is
produced. It is very important that the stone be rotated at a
speed that permits grinding instead of rubbing. This speed is
approximately 8,000 to 10,000 revolutions per minute (rpm).
Excessive pressure on the stone can slow it down. It is not a
good technique to let the stone grind at slow speed by putting
pressure on the stone when starting or stopping the gun. The
maximum pressure used on the stone at any time should be
no more than that exerted by the weight of the gun.
10-14Seat out of alignment with guide
Needs further rough grindingExcessive pitting
True and clean ready for finish grind
Figure 10-18. Valve seat grinding. Another practice, conducive to good grinding, is to ease off
on the stone every second or so to let the coolant wash away
the chips on the seat. This rhythmic grinding action also
helps keep the stone up to its correct speed. Since it is quite
a job to replace a seat, remove as little material as possible
during the grinding. Inspect the job frequently to prevent
unnecessary grinding.
The rough stone is used until the seat is true to the valve
guide and until all pits, scores, or burned areas are removed.
[Figure 10-18] After refacing, the seat should be smooth
and true. The finishing stone is used only until the seat has a
smooth, polished appearance. Extreme caution should be used
when grinding with the finishing stone to prevent chattering.
The size and trueness of the seat can be checked by several
methods. Runout of the seat is checked with a special dial
indicator and should not exceed 0.002 inch. The size of the
seat may be determined by using Prussian blue. Prussian
blue is used to check for contact transfer from one surface
to the other. To check the fit of the seat, spread a thin coat
of Prussian blue evenly on the seat. Press the valve onto the
seat. The blue transferred to the valve indicates the contact
surface. The contact surface should be one-third to two-thirds
the width of the valve face and in the middle of the face. In
some cases, a-go and no-go gauge is used in place of the valve
when making the Prussian blue check. If Prussian blue is
not used, the same check may be made by lapping the valve
lightly to the seat. Lapping is accomplished by using a small
amount of lapping compound placed between the valve face
and seat. The valve is then moved in a rotary motion back
and forth until the lapping compound grinds slightly into the
surface. After cleaning the lapping contact compound off, a
contact area can be seen. Examples of test results are shown
in Figure 10-19 .
If the seat contacts the upper third of the valve face, grind off
the top corner of the valve seat. [Figure 10-20] Such grinding
is called narrowing grinding. This permits the seat to contact
the center third of the valve face without touching the upper portion of the valve face.
If the seat contacts the bottom third of the valve face, grind
off the inner corner of the valve seat. [Figure 10-21] The
seat is narrowed by a stone other than the standard angle. It
is common practice to use a 15° angle and 45° angle cutting
stone on a 30° angle valve seat, and a 30° angle and 75° angle
stone on a 45° angle valve seat. [Figure 10-22]
If the valve seat has been cut or ground too much, the valve
contacts the seat too far up into the cylinder head, and the
valve clearance, spring tension, and the fit of the valve to
the seat is affected. To check the height of a valve, insert the
valve into the guide, and hold it against the seat. Check the
height of the valve stem above the rocker box or some other
fixed position.
Before refacing a valve seat, consult the overhaul manual
for the particular model engine. Each manufacturer specifies
the desired angle for grinding and narrowing the valve seat.
Valve Reconditioning
One of the most common jobs during engine overhaul is
grinding the valves. The equipment used should preferably
be a wet valve grinder. With this type of machine, a mixture
of soluble oil and water is used to keep the valve cool and
carry away the grinding chips.
Like many machine jobs, valve grinding is mostly a matter
of setting up the machine. The following points should be
checked or accomplished before starting a grind. True the
stone by means of a diamond nib. The machine is turned on,
and the diamond is drawn across the stone, cutting just deep
enough to true and clean the stone. Determine the face angle
of the valve being ground and set the movable head of the
machine to correspond to this valve angle. Usually, valves
are ground to the standard angles of 30° or 45°. However, in
some instances, an interference fit of 0.5° or 1.5° less than
the standard angle may be ground on the valve face.
Valve and seat ground to 45° Grind seat to 45°SeatSeat45° stone Valve
Top edge reduced to 30° Reduce seat 1/3 with 30° stone30° stone
SeatSeatValve
Lower edge reduced to 75° Reduce seat 1/3 with 75° stone75° stone
SeatSeatValveFigure 10-19. Fitting the valve and seat.
Figure 10-20. Grinding top surface of the valve seat.
Figure 10-21. Grinding the inner corner of the valve seat.
Figure 10-22. Valve seat angles. 45°
75°30°
30°
45°15°
Inner
cornerInner
cornerUpper corner
Upper corner
Seat
Seat Inner
cornerInner
corner
Line contact45°
44°
Figure 10-23. Interference fit of valve and valve seat. The interference fit is used to obtain a more positive seal by
means of a narrow contact surface. [Figure 10-23] Theoretically,
there is a line contact between the valve and seat. With this
line contact, the load that the valve exerts against the seat
is concentrated in a very small area, thereby increasing the
unit load at any one spot. The interference fit is especially
beneficial during the first few hours of operation after an
overhaul. The positive seal reduces the possibility of a burned
valve or seat that a leaking valve might produce. After the
first few hours of running, these angles tend to pound down
and become identical.
Notice that the interference angle is ground into the valve, not
the seat. It is easier to change the angle of the valve grinder
work head than to change the angle of a valve seat grinder
stone. Do not use an interference fit unless the manufacturer
approves it.Install the valve into the chuck and adjust the chuck so that
the valve face is approximately 2 inches from the chuck.
[Figure 10-24] If the valve is chucked any further out,
there is danger of excessive wobble and also a possibility of
grinding into the stem.
There are various types of valve grinding machines. In one
type, the stone is moved across the valve face; in another, the
valve is moved across the stone. Whichever type is used, the
following procedures are typical of those performed when
refacing a valve.
10-16Travel adjusted by stopsValve adjusted correctly
“In” stop prevents
grinding the stem
Out InWorkhead table2"
Feather edgeNormal marginFigure 10-24. Valve installed in grinding machine.
Figure 10-25. Valve in chuck ready to grind. Figure 10-26. Engine valves showing normal margin and a feather
edge. Check the travel of the valve face across the stone. The valve
should completely pass the stone on both sides, yet not travel
far enough to grind the stem. There are stops on the machine
that can be set to control this travel.
With the valve set correctly in place, turn on the machine and
the grinding fluid so that it splashes on the valve face. Back
the grinding wheel off all the way. Place the valve directly
in front of the stone. [Figure 10-25] Slowly bring the wheel
forward until a light cut is made on the valve. The intensity
of the grind is measured by sound more than anything else.
Slowly draw the valve back and forth across the stone without
increasing the cut. Move the work head table back and forth
using the full face of the stone, but always keep the valve
face on the stone. When the sound of the grind diminishes,
indicating that some valve material has been removed, move
the workhead table to the extreme left to stop rotation of the
valve. Inspect the valve to determine if further grinding is
necessary. If another cut must be made, bring the valve in front of the stone, then advance the stone out to the valve.
Do not increase the cut without having the valve directly in
front of the stone.
An important precaution in valve grinding, as in any kind
of grinding, is to make light cuts only. Heavy cuts cause
chattering, that may make the valve surface so rough that
much time is lost in obtaining the desired finish.
After grinding, check the valve margin to be sure that the
valve edge has not been ground too thin. A thin edge is called
a feather edge and can lead to pre-ignition; the valve edge
would burn away in a short period of time, and the cylinder
would have to be overhauled again. Figure 10-26 shows a
valve with a normal margin and one with a feather edge.
The valve tip may be resurfaced on the valve grinder. The
tip is ground to remove cupping or wear, and also to adjust
valve clearances on some engines.
10-17Figure 10-27. Grinding a valve tip. StoneShield
Smooth, even
gray bandSharply
defined edges
Figure 10-28. A correctly lapped valve.
Piston Repairs
Piston repairs are not required as often as cylinder repairs
since most of the wear is between the piston ring and cylinder
wall, valve stem and guide, and valve face and seat. A lesser
amount of wear is encountered between the piston skirt and
cylinder, ring and ring groove, or piston pin and bosses.
The most common repair is the removal of scores. Usually,
these may be removed only on the piston skirt if they
are very light. On engines where the entire rotating and
reciprocating assembly is balanced, the pistons must weigh
within one-fourth ounce of each other. When a new piston is
installed, it must be within the same weight tolerance as the
one removed. It is not enough to have the pistons matched
alone; they must be matched to the crankshaft, connecting
rods, piston pins, etc. To make weight adjustments on new
pistons, the manufacturer provides a heavy section at the
base of the skirt. To decrease weight, file metal evenly off
the inside of this heavy section. The piston weight can be
decreased easily, but welding, metalizing, or plating cannot
be done to increase the piston weight.
If ring grooves are worn or stepped, the pistons are normally
replaced. Small nicks on the edge of the piston pin boss may be
sanded down. Deep scores inside the boss, or anywhere around
the boss, are definite reasons for rejection. It has become more
economical to replace pistons rather than reconditioning and
reusing old ones, especially during overhaul.
Cylinder Grinding & Honing
If a cylinder has excessive taper, out-of-roundness, step, or
its maximum size is beyond limits, it can be reground to
the next allowable oversize. If the cylinder walls are lightly
rusted, scored, or pitted, the damage may be removed by
honing or lapping.
Regrinding a cylinder is a specialized job that the powerplant
mechanic is not usually expected to be able to do. However, The valve is held by a clamp on the side of the stone.
[Figure 10-27] With the machine and grinding fluid turned on,
the valve is pushed lightly against the stone and swung back
and forth. Do not swing the valve stem off either edge of the
stone. Because of the tendency for the valve to overheat during
this grinding, be sure plenty of grinding fluid covers the tip.
Grinding of the valve tip may remove, or partially remove,
the bevel on the edge of the valve. To restore this bevel,
mount a V-way approximately 45° to the grinding stone.
Hold the valve onto the V-way and twist the valve tip onto
the stone. With a light touch, grind all the way around the
tip. This bevel prevents scratching the valve guide when the
valve is installed.
Valve Lapping & Leak Testing
After the grinding procedure is finished, it is sometimes
necessary that the valve be lapped to the seat. This is done by
applying a small amount of lapping compound to the valve
face, inserting the valve into the guide, and rotating the valve
with a lapping tool until a smooth, gray finish appears at the
contact area. The appearance of a correctly lapped valve is
shown in Figure 10-28 .
After the lapping process is finished, be sure that all lapping
compound is removed from the valve face, seat, and adjacent
areas. The final step is to check the mating surface for leaks
to see if it is sealing properly. This is done by installing the
valve in the cylinder, holding the valve by the stem with the
fingers, and pouring kerosene or solvent into the valve port.
While holding finger pressure on the valve stem, check to see
if the kerosene is leaking past the valve into the combustion
chamber. If it is not, the valve re-seating operation is
finished. If kerosene is leaking past the valve, continue the
lapping operation until the leakage is stopped. The incorrect
indications are of value in diagnosing improper valve and
valve seat grinding. Incorrect indications, their cause, and
remedy are shown in Figure 10-29 .
Indication Fuzzy edge
Cause Rough grind
Remedy Regrind valve or
continue lappingToo narrow contact
Unintentional
interference fit
Grind both valve and setToo lap bands
Improper narrowing of seat
Renarrow seat
Figure 10-29. Incorrectly lapped valves.
the mechanic must be able to recognize when a cylinder
needs regrinding, and they must know what constitutes a
good or bad job.
Generally, standard aircraft cylinder oversizes are 0.010 inch,
0.015 inches, 0.020 inch, or 0.030 inch. Aircraft cylinders
have relatively thin walls and may have a nitrided surface,
that must not be ground away. Nitriding is a surface hardening
process that hardens the steel surface to a depth of several
thousandths of an inch. Any one manufacturer usually does
not allow all of the above oversizes. Some manufacturers do
not allow regrinding to an oversize at all. The manufacturer’s
overhaul manual, or parts catalog, usually lists the oversizes
allowed for a particular make and model engine.
To determine the regrind size, the standard bore size
must be known. This usually can be determined from the
manufacturer’s specifications or manuals. The regrind size is
figured from the standard bore. For example, a certain cylinder
has a standard bore of 3.875 inches. To have a cylinder ground
to 0.015 inches oversize, it is necessary to grind to a bore
diameter of 3.890 inch (3.875 + 0.015). A tolerance of ±0.0005
inches is usually accepted for cylinder grinding.
Another factor to consider when determining the size to which
a cylinder must be reground is the maximum wear that has
occurred. If there are spots in the cylinder wall that are worn
larger than the first oversize, then obviously it is necessary
to grind to the next oversize to clean up the entire cylinder.
The type of finish desired in the cylinder is an important
consideration when ordering a regrind. Some engine
manufacturers specify a fairly rough finish on the cylinder
walls, that allows the rings to seat even if they are not lapped
to the cylinder. Other manufacturers desire a smooth finish
to which a lapped ring seats without much change in ring
or cylinder dimensions. The latter type of finish is more
expensive to produce.The standard used when measuring the finish of a cylinder
wall is known as micro-inch root mean square (micro-inch
RMS). In a finish where the depth of the grinding scratches
are one-millionth (0.000001) of an inch deep, it is specified
as 1 micro-inch RMS. Most aircraft cylinders are ground to
a finish of 15 to 20 micro-inch RMS. Several low-powered
engines have cylinders that are ground to a relatively
rough 20- to 30-micro-inch RMS finish. On the other end
of the scale, some manufacturers require a superfinish of
approximately 4- to 6-micro-inch RMS.
Cylinder grinding is accomplished by a firmly mounted stone
that revolves around the cylinder bore, as well as up and down
the length of the cylinder barrel. [Figure 10-30] The cylinder,
the stone, or both may move to get this relative movement.
The size of the grind is determined by the distance the stone
is set away from the centerline of the cylinder. Some cylinder
bore grinding machines produce a perfectly straight bore,
while others are designed to grind a choked bore. A choked
bore grind refers to the manufacturing process in which
the cylinder walls arc ground to produce a smaller internal
diameter at the top than at the bottom. The purpose of this
type grind or taper is to maintain a straight cylinder wall
during operation. As a cylinder heats up during operation,
the head and top of the cylinder are subjected to more heat
than the bottom. This causes greater expansion at the top than
at the bottom, thereby maintaining the desired straight wall.
After grinding a cylinder, it may be necessary to hone the
cylinder bore to produce the desired finish. In this case,
specify the cylinder regrind size to allow for some metal
removal during honing. The usual allowance for honing is
0.001 inch. If a final cylinder bore size of 3.890 inches is
desired, specify the regrind size of 3.889 inches, and then
hone to 3.890 inches.
There are several different makes and models of cylinder
hones. The burnishing hone is used only to produce the
desired finish on the cylinder wall. The more elaborate
Stone
GrinderCylinder
Figure 10-30. Cylinder bore grinding.
CylinderStone
Figure 10-31. Cylinder honing.
Figure 10-32. Cross-hatch pattern on cylinder wall.micromatic hone can also be used to straighten out the
cylinder walls. A burnishing hone should not be used in an
attempt to straighten cylinder walls. [Figure 10-31] Since
the stones are only spring loaded, they follow the contour
of the cylinder wall and may aggravate a tapered condition.
Deglazing the cylinder walls is accomplished with the use
of a deglazing hone. A cross-hatch pattern must be placed
on the cylinder wall to allow for piston ring break-in. This
is accomplished by a deglazing hone turned by a drill being
moved in and out of the cylinder rapidly. [Figure 10-32]
After the cylinders have been reground or deglazed, or both,
check the size and wall finish, and check for evidence of
overheating or grinding cracks before installing on an engine.
Reassembly
Before starting reassembly, all serviceable and new engine
components need to be cleaned, organized, and laid out in the
order they are to be assembled. A popular method of engine
assembly is for the engine to be assembled at one work station
with the same technicians completing the total assembly of
the engine. It is also important to refer to the parts catalog to
ensure that the correct hardware is used during the assembly
of the engine. The engine overhaul manual should be referred
to for information on the use of safety wire, self-locking
nuts, and torque values. During assembly, the components
should be pre-lubricated as the overhaul manual sets forth. It
is important to follow the manufacturer’s overhaul assembly
procedures completely and perform all checks and procedures
that are called for in the manual. Installation & Testing
Testing Reciprocating Engines
The procedures and equipment used in determining that
an engine is ready for airworthy service and is in excellent
mechanical condition, normally requires the use of a
test stand, or test cell, although the aircraft can be used.
Figure 10-33. Test stand. Figure 10-34. Test club. [Figure 10-33] The method of engine testing or run-in that
takes place during overhaul prior to delivery of the engine
is critical to the airworthiness of the engine. It must be
emphasized that engine run-in is as vital as any other phase
of engine overhaul, for it is the means by which the quality
of a new or newly overhauled engine is checked, and it is
the final step in the preparation of an engine for service.
Thus, the reliability and potential service life of an engine
is in question until it has satisfactorily passed the cell test.
The test serves a dual purpose. First, it accomplishes piston
ring run-in and bearing burnishing. Second, it provides
valuable information that it used to evaluate engine
performance and determine engine condition. To provide
proper oil flow to the upper portion of the cylinder barrel
walls with a minimum loss of oil, it is important that piston
rings be properly seated in the cylinder in which they are
installed. The process is called piston ring run-in (break-
in) and is accomplished chiefly by controlled operation of
the engine in the high-speed range. Improper piston ring
conditioning, or run-in, may result in unsatisfactory engine
operation with high oil consumption. A process called bearing
burnishing creates a highly polished surface on new bearings
and bushings installed during overhaul. The burnishing is
usually accomplished during the first periods of the engine
run-in at comparatively slow engine speeds.
The failure of any part during engine testing or run-in
requires that the engine be returned, repaired, and completely
retested. After an engine has successfully completed test
requirements, it is then specially treated to prevent corrosion,
if it is shipped or stored before being installed in an aircraft.
During the final run-in period during testing, the engines
are operated on the proper grade of fuel prescribed for the
particular kind of engine. The oil system is serviced with a
mixture of corrosion-preventive compound and engine oil.
The temperature of this mixture is maintained at 105 °C to 121 °C. Near the end of final run-in, corrosion-preventive
mixture (CPM) is used as the engine lubricant. The engine
induction passages and combustion chambers are also
treated with CPM by an aspiration method. CPM is drawn
or breathed into the engine.
Test Cell Requirements
The test cell requires an area to mount and hold the engine for
testing. The cell needs to have the controls, instruments, and
any special equipment to evaluate the total performance of the
engine. A test club should be used for testing instead of a flight
propeller. [Figure 10-34] A test club provides more cooling
air flow and the correct amount of load. The operational tests
and test procedures vary with individual engines, but the basic
requirements are generally closely related.
Engine Instruments
The test cell control room contains the controls used to
operate the engine and the instruments used to measure
various temperatures and pressures, fuel flow, and other
factors. These devices are necessary in providing an accurate
check and an evaluation of the operating engine. The control
room is separate from, but adjacent to, the space (test cell)
that houses the engine being tested. The safe, economical, and
reliable testing of modern aircraft engines depends largely
upon the use of instruments. In engine run-in procedures, the
same basic engine instruments are used as when the engine
is installed in the aircraft, plus some additional connections
to these instruments, and some indicating and measuring
devices that cannot be practically installed in the aircraft.
Instruments used in the testing procedures are inspected
and calibrated periodically, as are instruments installed in
the aircraft; thus, accurate information concerning engine
operation is ensured.
Engine instruments can operate using different methods,
some mechanically, some electrically, and some by sensing
10-21Yellow
Green
Red
C°50
Figure 10-35. Carburetor air temperature gauge. the direct pressure of air or liquid. Some of the basic
instruments are:
1. Carburetor air temperature gauge.
2. Fuel pressure gauge.
3. Fuel-flow meter.
4. Manifold pressure gauge.
5. Oil temperature gauge.
6. Oil pressure gauge.
7. Tachometer.
8. Exhaust gas temperature gauge.
9. Cylinder head temperature gauge.
10. Torquemeter.
Instrument markings, ranges of operation, minimum and
maximum limits, and the interpretation of these markings
are general to all the instruments. Generally, the instrument
marking system consists of three colors: red, yellow, and
green. A red line, or mark, indicates a point beyond which
a dangerous operating condition exists. A red arc indicates
a dangerous operating range due generally to an engine
propeller vibration range. This arc can be passed through,
but the engine cannot be operated in this area. Of the two,
the red mark is used more commonly and is located radially
on the cover glass or dial face. The yellow arc covers a given
range of operation and is an indication of caution. Generally,
the yellow arc is located on the outer circumference of the
instrument cover glass or dial face. The green arc shows
a normal and safe range of operation. When the markings
appear on the cover glass, a white line is used as an index
mark, often called a slippage mark. The white radial mark
indicates any movement between the cover glass and the
case, a condition that would cause mislocation of the other
range and limit markings.
Carburetor Air Temperature (CAT) Indicator
Measured at the carburetor entrance, carburetor air temperature
(CAT) is regarded by many as an indication of induction system
ice formation. Although it serves this purpose, it also provides
many other important items of information.
The powerplant is a heat machine, and the temperature of
its components, or the fluids flowing through it, affects
the combustion process either directly or indirectly. The
temperature level of the induction air affects not only the
charge density, but also the vaporization of the fuel. CAT
is also useful for checking induction system condition.
Backfiring is indicated as a momentary rise on the gauge,
provided it is of sufficient severity for the heat to be sensed
at the carburetor air-measuring point. A sustained induction
system fire shows a continuous increase of CAT.The CAT should be noted before starting and just after
shutdown. The temperature before starting is the best
indication of the temperature of the fuel in the carburetor
body and tells whether vaporization is sufficient for the initial
firing, or whether the mixture must be augmented by priming.
If an engine has been shut down for only a short time, the
residual heat in the carburetor may make it possible to rely
on the vaporizing heat in the fuel and powerplant. Priming
would then be unnecessary.
After shutdown, a high CAT is a warning that the fuel
trapped in the carburetor will expand, producing high internal
pressure. When a high temperature is present at this time,
the fuel line and manifold valves should be open so that the
pressure can be relieved by allowing fuel passage back to
the tank. The CAT gauge indicates the temperature of the air
before it enters the carburetor. The temperature reading is
sensed by a bulb or electric sensor. In the test cell, the sensor
is located in the air intake passage to the engine and, in an
aircraft, it is located in the ram-air intake duct. The CAT
gauge is calibrated in the centigrade scale. [Figure 10-35]
This gauge, like many other multi-engine aircraft instruments,
is a dual gauge; two gauges, each with a separate pointer and
scale, are used in the same case.
Notice the range markings used. The yellow arc indicates a
range from –10 °C to +15 °C, since the danger of icing occurs
between these temperatures. The green range indicates the
normal operating range from +15 °C to +40 °C. The red line
indicates the maximum operating temperature of 40 °C; any
operation at a temperature over this value places the engine
in danger of detonation.
Fuel Pressure Indicator
The fuel pressure gauge is calibrated in pounds per square
inch (psi) of pressure. It is used during the test run-in to
measure engine fuel pressure at the carburetor inlet, the fuel
Figure 10-36. Engine instrument clusters. feed valve discharge nozzle, and the main fuel supply line.
Fuel gauges are located in the operator’s control room and
are connected by flexible lines to the different points at which
pressure readings are desired during the testing procedures.
In some aircraft installations, the fuel pressure is sensed at the
carburetor or fuel injection unit inlet of each engine, and the
pressure is indicated on individual gauges on the instrument
panel. [Figure 10-36] The dial is calibrated in graduations
and is extended and numbered. The numbers range from
0 to 10 in this example. The red line on the dial at the 2
pounds psi graduation shows the minimum fuel pressure
allowed during flight. The green arc shows the desired range
of operation, which is 2 to 9 psi. The red line at the 9 psi
graduation indicates the maximum allowable fuel pressure.
Fuel pressures vary with the type of fuel system installation
and the size of the engine. When fuel injection systems are
used, the fuel pressure range is much higher; the minimum
allowable pressure is approximately 10 psi, and the maximum
is generally 25 psi.
Oil Pressure Indicator
The main oil pressure reading is taken at the pressure side of
the oil pump. Generally, there is only one oil pressure gauge
for each aircraft engine. The oil pressure gauge dial does
not show the pressure range or limits for all installations.
[Figure 10-36] The actual markings for specific aircraft may
be found in the aircraft specifications or Type Certificate Data
Sheets. The lower red line at 25 psi indicates the minimum oil
pressure permissible in flight. The green arc between 60 to 85
psi illustrates the desired operating oil pressure range. The red
line at 100 psi indicates maximum permissible oil pressure.The oil pressure gauge indicates the pressure, in psi, that
the oil of the lubricating system is being supplied to the
moving parts of the engine. The engine should be shut down
immediately if the gauge fails to register pressure when the
engine is operating. Excessive oscillation of the gauge pointer
indicates that there is air in the lines leading to the gauge, or
that some unit of the oil system is functioning improperly.
Oil Temperature Indicator
During engine run-in in the test cell, engine oil temperature
readings are taken at the oil inlet and outlet. From these
readings, it can be determined if the engine heat transferred
to the oil is low, normal, or excessive. This information is of
extreme importance during the breaking-in process of large
reciprocating engines. The oil temperature gauge line in the
aircraft is connected at the oil inlet to the engine.
Three range markings are used on the oil temperature
gauge. The green arc in Figure 10-36 , on the dial, shows the
minimum oil temperature permissible for ground operational
checks or during flight. The green mark between 25 °F
and below 245 °F shows the desired oil temperature for
continuous engine operation. The red mark at 245 °F indicates
the maximum permissible oil temperature.
Fuel-Flow Meter
The fuel-flow meter measures the amount of fuel delivered to
the engine. During engine testing procedures, the fuel-flow
to the engine can be measured by three different methods: a
direct flow meter, a pressure-based flow meter, or a turbine
sensor-based flow meter. The direct reading flow meter uses
a series of calibrated tubes located in the control room. The
tubes are of various sizes to indicate different volumes of
fuel-flow. Each tube contains a float that can be seen by the
operator, and as the fuel-flow through the tube varies, the
float is either raised or lowered, indicating the amount of
fuel-flow. From these indications, the operator can determine
whether an engine is operating at the correct air-fuel mixture
for a given power setting. Reciprocating engines on light
aircraft usually use a fuel pressure gauge that is also used for
the flow meter. This is because the fuel-flow is proportional
to the fuel pressure in this system. Fuel-flow is normally
measured in gallons per hour.
In most turbine aircraft installations, the fuel-flow indicating
system consists of a transmitter and an indicator for each
engine. The fuel-flow transmitter is conveniently mounted
in the engine’s accessory section and measures the fuel-flow
between the engine-driven fuel pump and the fuel control
device. The transmitter is an electrical device that contains a
turbine that turns faster as the flow increases, which increases
the electrical signal to the indicator. The fuel-flow transmitter
10-23Green
Red
MAN.
IN. HG. ABS.
65 70751520253035
Figure 10-37. Manifold pressure gauge.
Figure 10-38. Tachometer. is connected electrically to the indicator located on the aircraft
flight deck, or on the test cell operator’s panel. The reading
on the indicator on turbine aircraft is calibrated to record the
amount of fuel-flow in pounds of fuel per hour.
Manifold Pressure Indicator
The preferred type of instrument for measuring the manifold
pressure on reciprocating engines is a gauge that records the
pressure as an absolute pressure reading. Absolute pressure
takes into account the atmospheric pressure plus the pressure
in the intake manifold. To read the manifold pressure of the
engines, a specially designed manifold pressure gauge that
indicates absolute manifold pressure in inches of mercury
("Hg) is used. The red line indicates the maximum manifold
pressure permissible during takeoff.
The manifold pressure gauge range markings and indications
vary with different kinds of engines and installations.
Figure 10-37 illustrates the dial of a typical manifold pressure
gauge and shows how the range markings are positioned.
The green arc starts at 35 "Hg and continues to the 44 "Hg.
The red line on the gauge, at 49 "Hg shows the manifold
pressure recommended for takeoff. This pressure should
not be exceeded.
Tachometer Indicator
The tachometer for reciprocating engines shows the engine
crankshaft rpm. The system used for testing the engine is the
same as the system in the aircraft installation. The tachometer,
often referred to as TACH, is calibrated in hundreds with
graduations at every 100-rpm interval. The dial shown in
Figure 10-38 starts at 0 rpm and goes to 35 (3,500 rpm).
The green arc indicates the rpm range within operation that
is permissible. The red line indicates the maximum rpm
permissible during takeoff; any rpm beyond this value is an
overspeed condition.
Turbine engines use percent rpm indicators due to the
high rpm that the engines generally operate. Each rotating
assembly in an engine has its own percent rpm indicator.
The tachometer indicates speed of the compressor section.
The 100 percent position on the indicator is the highest rpm
at which the engine can operate. Red lines and green arcs
operate the same as with reciprocating engines. Rotorcraft
use two synchronous tachometers.
Cylinder Head Temperature Indicator
During the engine test procedures, the cylinder head
temperatures of various cylinders on the reciprocating
engine are normally tested. Thermocouples are connected
to several cylinders and, by a selector switch, any cylinder
head temperature can be indicated on the indicators.
When installed in the aircraft, there is sometimes only one thermocouple lead and indicator for each engine installed
in an aircraft.
Cylinder head temperatures are indicated by a gauge
connected to a thermocouple attached to the cylinder, that
tests show to be the hottest on an engine in a particular
installation. The thermocouple may be placed in a special
gasket located under a rear spark plug, or in a special well
in the top or rear of the cylinder head.
The temperature recorded at either of these points is merely
a reference or control temperature; but as long as it is
kept within the prescribed limits, the temperatures inside
10-24Blue
Green
Red
CYL. TEMP.
HEAD
C°300
1520253035404550
TORQUE
FTLB
x100
Figure 10-39. Cylinder head temperature gauge. Figure 10-40. Torquemeter readout. the cylinder dome, exhaust valve, and piston is within a
satisfactory range. Since the thermocouple is attached to only
one cylinder, it can do no more than give evidence of general
engine temperature. While normally it can be assumed that
the remaining cylinder temperatures are lower, conditions
such as detonation are not indicated unless they occur in the
cylinder that has the thermocouple attached.
The cylinder head temperature gauge range marking is
similar to that of the manifold pressure and tachometer
indicator. The cylinder head temperature gauge is a dual
gauge that incorporates two separate temperature scales.
[Figure 10-39] The scales are calibrated in increments of 10°,
with numerals at the 0°, 100°, 200°, and 300° graduations. The
space between any two graduation marks represents 10 °C.
The blue arc on the gauge indicates the range within which
operation is permitted in auto-lean. The bottom of this arc,
100 °C, indicates the minimum desired temperature to ensure
efficient engine operation during flight. The top of the blue
arc, 230 °C, indicates the temperature at which the mixture
control must be moved to the “auto-rich” position. The green
arc describes the range within which operation must be in
auto-rich. The top of this arc, 248 °C, indicates maximum
continuous power; all operation above this temperature is
limited in time (usually 5 to 15 minutes). The red line indicates
maximum permissible temperature, 260 °C.
Torquemeter
Most torque systems use an oil pressure output from a torque
valve to indicate actual engine power output at various power
settings. The torquemeter indicates the amount of torque being
produced at the propeller shaft. A helical gear moves back and
forth as the torque on the propeller shaft varies. This gear,
acting on a piston, positions a valve that meters the oil pressure
proportionally to the torque being produced. A change in
pressure from the valve that is connected to a transducer is
then converted to an electrical signal and is transmitted to
the flight deck. The torquemeter can read out in foot-pounds of torque, percent of horsepower, or horsepower. The earlier
systems read out in psi, and the flight engineer converted this
to the correct power setting. [Figure 10-40] Some systems use
strain gauges to attach to the ring gear to provide an electrical
signal directly to the readout.
Warning Systems
Many of the miscellaneous gauges and devices indicate only
that a system is functioning or has failed to function. On some
aircraft, a warning light illuminates when the fuel pressure
or oil pressure is low.
Reciprocating Engine Operation
The operation of the powerplant is controlled from the
flight deck. Some installations have numerous control
handles and levers connected to the engine by rods, cables,
bellcranks, pulleys, etc. In most cases, the control handles
are conveniently mounted on quadrants in the flight deck.
Placards, or markings, are placed on the quadrant to indicate
the functions and positions of the levers. In some installations,
friction clutches are installed to hold the controls in place.
Engine Instruments
The term engine instruments usually includes all instruments
required to measure and indicate the functioning of the
powerplant. The engine instruments are generally installed on
the instrument panel so that all of them can easily be observed
at one time. Manifold pressure, rpm, engine temperature, oil
temperature, CAT, and the air-fuel ratio can be controlled
by manipulating the flight deck controls. Coordinating the
movement of the controls with the instrument readings
protects against exceeding operating limits.
Engine operation is usually limited by specified operating
ranges of the following:
1. Crankshaft speed (rpm).
2. Manifold pressure.
Draining engine oil Partially filled intake pipe
Figure 10-41. Initial step in developing a hydraulic lock. 3. Cylinder head temperature.
4. CAT.
5. Oil temperature.
6. Oil pressure.
7. Fuel pressure.
8. Fuel-flow meter.
9. Air-fuel mixture setting.
The procedures, pressures, temperatures, and rpm used
throughout this section are solely for the purpose of
illustration and do not have general application. The operating
procedures and limits used on individual makes and models
of aircraft engines vary considerably from the values shown
here. For exact information regarding a specific engine
model, consult the applicable instructions.
Engine Starting
Before starting the engine, observe the manifold pressure
gauge that should read approximate atmospheric (barometric)
pressure when the engine is not running. At sea level, this
is approximately 30 "Hg, and at fields above sea level, the
atmospheric pressure is less, depending on the height above
sea level. Also, observe all engine gauges for the correct
reading for engine off settings.
Correct starting technique is an important part of engine
operation. Improper procedures often are used, because
some of the basic principles involved in engine operation
are misunderstood. Read more about typical procedures for
starting reciprocating engines in the Aviation Maintenance
Technician Handbook - General.
Pre-Oiling
Engines that have undergone overhaul or major maintenance
can have air trapped in some of the oil passages that must
be removed before the first start. This is done by pre-oiling
the engine by cranking, with the spark plugs removed, the
engine with the starter or by hand (turning) until oil pressure
is indicated. A second method is to pump oil under pressure
through the oil system using an external pump until oil comes
out of the oil outlet of the engine.
Hydraulic Lock
Whenever a radial engine remains shut down for any
length of time beyond a few minutes, oil or fuel may drain
into the combustion chambers of the lower cylinders or
accumulate in the lower intake pipes ready to be drawn into
the cylinders when the engine starts. [Figure 10-41] As the
piston approaches top center of the compression stroke (both
valves closed), this liquid being incompressible, stops piston
movement. If the crankshaft continues to rotate, something must give. Therefore, starting or attempting to start an engine
with a hydraulic lock of this nature may cause the affected
cylinder to blow out or, more likely, may result in a bent or
broken connecting rod.
To eliminate a lock, remove either the front or rear spark plug
of the lower cylinders and pull the propeller through in the
direction of rotation. The piston expels any liquid that may
be present. If the hydraulic lock occurs as a result of over-
priming prior to initial engine start, eliminate the lock in the
same manner (i.e., remove one of the spark plugs from the
cylinder and rotate the crankshaft through two turns). 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
with the possibility of a complete or partial lock occurring
on the subsequent start.
Engine Warm-Up
Proper engine warm-up is important, particularly when the
condition of the engine is unknown. Improperly adjusted idle
mixture, intermittently firing spark plugs, and improperly
adjusted engine valves all have an overlapping effect on
engine stability. Therefore, the warm-up should be made at
the engine speed where maximum engine stability is obtained.
Experience has shown that the optimum warm-up speed is
from 1,000 to 1,600 rpm. The actual speed selected should be
10-26the speed at which engine operation is the smoothest, since
the smoothest operation is an indication that all phases of
engine operation are the most stable.
Some engines incorporate temperature-compensated oil
pressure relief valves. This type of relief valve results in high
engine oil pressures immediately after the engine starts, if
oil temperatures are very low. Consequently, start the warm-
up of these engines at approximately 1,000 rpm and then
move to the higher, more stable engine speed as soon as oil
temperature reaches a warmer level.
During warm-up, watch the instruments associated with
engine operation. This aids in making sure that all phases
of engine operation are normal. For example, engine oil
pressure should be indicated within 30 seconds after the start.
Furthermore, if the oil pressure is not up to or above normal
within 1 minute after the engine starts, the engine should be
shut down. Cylinder head or coolant temperatures should
be observed continually to see that they do not exceed the
maximum allowable limit.
A lean mixture should not be used to hasten the warm-up.
Actually, at the warm-up rpm, there is very little difference
in the mixture supplied to the engine, whether the mixture is
in a rich or lean position, since metering in this power range
is governed by throttle position.
Carburetor heat can be used as required under conditions
leading to ice formation. For engines equipped with a float-type
carburetor, it is desirable to raise the CAT during warm-up to
prevent ice formation and to ensure smooth operation.
The magneto safety check can be performed during warm-
up. Its purpose is to ensure that all ignition connections are
secure, and that the ignition system permits operation at the
higher power settings used during later phases of the ground
check. The time required for proper warm-up gives ample
opportunity to perform this simple check, which may disclose
a condition that would make it inadvisable to continue
operation until after corrections have been made.
The magneto safety check is conducted with the propeller
in the high rpm (low pitch) position, at approximately 1,000
rpm. Move the ignition switch from “both” to “right” and
return to “both;” from “both” to “left” and return to “both;”
from “both” to “off” momentarily and return to “both.”
While switching from “both” to a single magneto position, a
slight but noticeable drop in rpm should occur. This indicates
that the opposite magneto has been properly grounded out.
Complete cutting out of the engine when switching from
“both” to “off” indicates that both magnetos are grounded properly. While in the single magneto position, failure to
obtain any rpm drop, or failure of the engine to cut out while
switching to off, indicates that one or both ground connections
are faulty. This indicates a safety problem; the magnetos are
not secured at shut down and may fire if the propeller is turned.
Ground Check
The ground check is performed to evaluate the functioning
of the engine by comparing power input, as measured by
manifold pressure, with power output, as measured by rpm
or torque.
The engine may be capable of producing a prescribed power,
even rated takeoff, and not be functioning properly. Only by
comparing the manifold pressure required during the check
against a known standard is an unsuitable condition disclosed.
The magneto check can also fail to show shortcomings, since
the allowable rpm dropoff is only a measure of an improperly
functioning ignition system and is not necessarily affected
by other factors. Conversely, it is possible for the magneto
check to prove satisfactory when an unsatisfactory condition
is present elsewhere in the engine.
The ground check is made after the engine is thoroughly
warm. It consists of checking the operation of the powerplant
and accessory equipment by ear, by visual inspection, and
by proper interpretation of instrument readings, control
movements, and switch reactions. During the ground check,
the aircraft should be headed into the wind, if possible, to take
advantage of the cooling airflow. A ground check procedure
is outlined below:
1. Control position check.
2. Cowl flaps (if equipped)—open.
3. Mixture—rich.
4. Propeller—high rpm.
5. Carburetor heat—cold.
6. Check propeller according to propeller manufacturer’s
instruction.
7. Open throttle to the run-up rpm setting as per
manufacturer’s instructions (specified RPM and
manifold pressure).
8. Ignition system operational check.
In performing the ignition system operational check (magneto
check), the power-absorbing characteristics of the propeller
in the low fixed-pitch position are utilized. In switching to
individual magnetos, cutting out the opposite plugs results
in a slower rate of combustion, which gives the same effect
as retarding the spark advance. The drop in engine speed is
a measure of the power loss at this slower combustion rate.
10-27When the magneto check is performed, a drop in torquemeter
pressure indication is a good supplement to the variation
in rpm. In cases where the tachometer scale is graduated
coarsely, the torquemeter variation may give more positive
evidence of the power change when switching to the
individual magneto condition. A loss in torquemeter pressure
not to exceed 10 percent can be expected when operating on
a single magneto. By comparing the rpm drop with a known
standard, the following are determined:
1. Proper timing of each magneto.
2. General engine performance as evidenced by smooth
operation.
3. Additional check of the proper connection of the
ignition leads.
Any unusual roughness on either magneto is an indication of
faulty ignition caused by plug fouling or by malfunctioning
of the ignition system. The operator should be very sensitive
to engine roughness during this check. Lack of dropoff in
rpm may be an indication of faulty grounding of one side of
the ignition system. Complete cutting out when switching to
one magneto is definite evidence that its side of the ignition
system is not functioning. Excessive difference in rpm drop
off between the left and right switch positions can indicate
a difference in time between the left and right magnetos.
Sufficient time should be given to the check on each single
switch position to permit complete stabilization of engine
speed and manifold pressure. There is a tendency to perform
this check too rapidly with resultant wrong indications.
Operation as long as 1 minute on a single ignition system
is not excessive.
Another point that must be emphasized is the danger of
sticking tachometer. The tachometer should he tapped lightly
to make sure the indicator needle moves freely. In some cases
using older mechanical tachometers, sticking has caused
errors in indication to the extent of 100 rpm. Under such
conditions, the ignition system could have had as much as
a 200 rpm drop with only a 100 rpm drop indicated on the
instrument. In most cases, tapping the instrument eliminates
the sticking and results in accurate readings.
In recording the results of time ignition system check,
record the amount of the total rpm drop that occurs rapidly
and the amount that occurs slowly. This breakdown in rpm
drop provides a means of pinpointing certain troubles in
the ignition system. This can reduce unnecessary work by
confining maintenance to the specific part of the ignition
system that is responsible for the trouble.
Fast rpm drop is usually the result of either faulty spark plugs or faulty ignition harness. This is true because faulty
plugs or leads, take effect at once. The cylinder goes dead
or starts firing intermittently the instant the switch is moved
from “both” to the “right” or “left” position.
Slow rpm drop usually is caused by incorrect ignition timing
or faulty valve adjustment. With late ignition timing, the
charge is fired too late (in relation to piston travel) for the
combustion pressures to build up to the maximum at the
proper time. The result is a power loss greater than normal for
single ignition because of the lower peak pressures obtained
in the cylinder. However, this power loss does not occur as
rapidly as that which accompanies a dead spark plug. This
explains the slow rpm drop as compared to the instantaneous
drop with a dead plug or defective lead. Incorrect valve
clearances, through their effect on valve overlap, can cause
the mixture to be too rich or too lean. The too rich or too lean
mixture may affect one plug more than another, because of the
plug location and show up as a slow rpm drop on the ignition
check. Switch from “both” to “right” and return to “both.”
Switch from “both” to “left” and return to “both.” Observe
the rpm drop while operating on the right and left positions.
The maximum drop should not exceed that specified by the
engine manufacturer.
Fuel Pressure & Oil Pressure Check
Fuel pressure and oil pressure must be within the established
tolerance (green arc) for the engine.
Propeller Pitch Check
The propeller is checked to ensure proper operation of the
pitch control and the pitch-change mechanism. The operation
of a controllable pitch propeller is checked by the indications
of the tachometer and manifold pressure gauge when the
propeller governor control is moved from one position to
another. Because each type of propeller requires a different
procedure, the applicable manufacturer’s instructions should
be followed.
Power Check
Specific rpm and manifold pressure relationship should be
checked during each ground check. This can be done at the
time the engine is run-up to make the magneto check. The
purpose of this check is to measure the performance of the
engine against an established standard. Calibration tests have
determined that the engine is capable of delivering a given
power at a given rpm and manifold pressure. The original
calibration, or measurement of power, is made by means of a
dynamometer in a test cell. During the ground check, power
is measured with the propeller. With constant conditions of
air density, the propeller, at any fixed-pitch position, always
requires the same rpm to absorb the same horsepower from
the engine. This characteristic is used in determining the
condition of the engine.
10-28With the governor control set for full low pitch, the propeller
operates as a fixed-pitch propeller, because the engine is
static. Under these conditions, the manifold pressure for any
specific engine, with the mixture control in rich, indicates
whether all the cylinders are operating properly. With one
or more dead or intermittently firing cylinders, the operating
cylinders must provide more power for a given rpm.
Consequently, the carburetor throttle must be opened further,
resulting in higher manifold pressure. Different engines of
the same model using the same propeller installation, and at
the same barometer and temperature readings, should require
the same manifold pressure to within 1 "Hg. A higher than
normal manifold pressure usually indicates a dead cylinder
or late ignition timing. An excessively low manifold pressure
for a particular rpm usually indicates that the ignition timing
is early. Early ignition can cause detonation and loss of power
at takeoff power settings.
The accuracy of the power check may be affected by the
following variables:
1. Wind—any appreciable air movement (5 mph or more)
changes the air load on the propeller blade when it is
in the fixed-pitch position. A head wind increases the
rpm obtainable with a given manifold pressure. A tail
wind decreases the rpm.
2. Atmospheric temperatures—the effects of variations
in atmospheric temperature tend to cancel each other.
Higher carburetor intake and cylinder temperatures
tend to lower the rpm, but the propeller load is
lightened because of the less dense air.
3. Engine and induction system temperature—if the
cylinder and carburetor temperatures are high because
of factors other than atmospheric temperature, a low
rpm results since the power is lowered without a
compensating lowering of the propeller load.
4. Oil temperature—cold oil tends to hold down the rpm,
since the higher viscosity results in increased friction
horsepower losses.
Idle Speed & Idle Mixture Checks
Plug fouling difficulty is the inevitable result of failure to
provide a proper idle mixture setting. The tendency seems
to be to adjust the idle mixture on the extremely rich side
and to compensate for this by adjusting the throttle stop to
a relatively high rpm for minimum idling. With a properly
adjusted idle mixture setting, it is possible to run the engine at
idle rpm for long periods. Such a setting results in a minimum
of plug fouling and exhaust smoking, and it pays dividends
from the savings on the aircraft brakes after landing and
while taxiing.If the wind is not too strong, the idle mixture setting can be
checked easily during the ground check as follows:
1. Close throttle.
2. Move the mixture control to the idle cutoff position
and observe the change in rpm. Return the mixture
control back to the rich position before engine cutoff.
As the mixture control lever is moved into idle cutoff,
and before normal dropoff, one of two things may occur
momentarily:
1. The engine speed may increase. An increase in rpm,
but less than that recommended by the manufacturer
(usually 20 rpm), indicates proper mixture strength. A
greater increase indicates that the mixture is too rich.
2. The engine speed may not increase or may drop
immediately. This indicates that the idle mixture is too
lean. The idle mixture should be set to give a mixture
slightly richer than best power, resulting in a 10- to
20-rpm rise after idle cutoff.
Engine Stopping
With each type of engine installation, specific procedures are
used in stopping the engine. The general procedure, outlined
in the following paragraphs, reduces the time required for
stopping, minimizes backfiring tendencies, and prevents
overheating of tightly baffled air-cooled engine during
operation on the ground.
In stopping any aircraft engine, the controls are set as follows,
irrespective of the type or fuel system installation.
1. Cowl flaps and any other shutters or doors are always
placed in the full open position to avoid overheating
the engine and are left in that position after the engine
is stopped to prevent engine residual heat from
deteriorating the ignition system.
2. Carburetor air-heater control is left in the cold position
to prevent damage that may occur from backfire.
3. Constant speed propeller is usually stopped with the
control set in the high pitch (decrease rpm) position to
prevent exposure and corrosion of the pitch changing
mechanism.
No mention is made of the throttle, mixture control, fuel
selector valve, and ignition switches in the preceding set
of directions because the operation of these controls varies
with the type of carburetor used with the engine. An engine
equipped with a carburetor incorporating an idle cutoff
mixture control is stopped as follows:
1. Idle the engine by setting the throttle for 800 to 1,000
rpm.
10-29Figure 10-42. Normal combustion within a cylinder.
Intake valve Exhaust valve
Piston
Connecting rodCrankcase
Intake valve Exhaust valve
Piston
Crankcase
Connecting rod
Figure 10-43. Detonation within a cylinder. 2. Move the mixture control to the idle cutoff position.
In a float-type carburetor, it equalizes the pressure in
the float chamber and at the discharge nozzle.
3. After the propeller has stopped rotating, place the
ignition switch in the off position.
In addition to the operations outlined previously, check the
functioning of various items of aircraft equipment, such as
generator systems, hydraulic systems, etc.
Basic Engine Operating Principles
Combustion Process
Normal combustion occurs when the air-fuel mixture ignites
in the cylinder and burns progressively at a fairly uniform rate
across the combustion chamber. When ignition is properly
timed, maximum pressure is built up just after the piston has
passed top dead center at the end of the compression stroke.
The flame fronts start at each spark plug and burn in more
or less wavelike forms. [Figure 10-42] The velocity of the
flame travel is influenced by the type of fuel, the ratio of the
air-fuel mixture, and the pressure and temperature of the fuel
mixture. With normal combustion, the flame travel is about
100 feet/second. The temperature and pressure within the
cylinder rises at a normal rate as the air-fuel mixture burns.
Detonation
There is a limit, however, to the amount of compression and
the degree of temperature rise that can be tolerated within an
engine cylinder and still permit normal combustion. All fuels have critical limits of temperature and compression. Beyond
this limit, they ignite spontaneously and burn with explosive
violence. This instantaneous and explosive burning of the
air-fuel mixture or, more accurately, of the latter portion of
the charge is called detonation.
Detonation is the spontaneous combustion of the unburned
charge ahead of the flame fronts after ignition of the charge.
[Figure 10-43] During normal combustion, the flame fronts
progress from the point of ignition across the cylinder. These
flame fronts compress the gases ahead of them. At the same
time, the gases are being compressed by the upward movement
of the piston. If the total compression on the remaining
unburned gases exceeds the critical point, detonation occurs.
The explosive burning during detonation results in an
extremely rapid pressure rise. This rapid pressure rise and
the high instantaneous temperature, combined with the
high turbulence generated, cause a scrubbing action on the
cylinder and the piston. This can burn a hole completely
through the piston.
The critical point of detonation varies with the ratio of fuel to
air in the mixture. Therefore, the detonation characteristic of
the mixture can be controlled by varying the air-fuel ratio. At
high power output, combustion pressures and temperatures
are higher than they are at low or medium power. Therefore,
at high power, the air-fuel ratio is made richer than is needed
for good combustion at medium or low power output. This
is done because, in general, a rich mixture does not detonate
as readily as a lean mixture.
10-30Unless detonation is heavy, there is no flight deck evidence
of its presence. Light to medium detonation does not cause
noticeable roughness, temperature increase, or loss of power.
As a result, it can be present during takeoff and high-power
climb without being known to the flight crew.
In fact, the effects of detonation are often not discovered until
after teardown of the engine. When the engine is overhauled,
however, the presence of severe detonation during its
operation is indicated by dished piston heads, collapsed
valve heads, broken ring lands, or eroded portions of valves,
pistons, or cylinder heads.
The basic protection from detonation is provided in the design
of the engine carburetor setting, which automatically supplies
the rich mixtures required for detonation suppression at high
power; the rating limitations, which include the maximum
operating temperatures; and selection of the correct grade of
fuel. The design factors, cylinder cooling, magneto timing,
mixture distribution, degree of supercharging, and carburetor
setting are taken care of in the design and development of the
engine and its method of installation in the aircraft.
The remaining responsibility for prevention of detonation
rests squarely in the hands of the ground and flight crews.
They are responsible for observance of rpm and manifold
pressure limits. Proper use of supercharger and fuel mixture,
and maintenance of suitable cylinder head and carburetor-
air-temperature (CAT) must be adhered to.
Pre-Ignition
Pre-ignition, as the name implies, means that combustion
takes place within the cylinder before the timed spark jumps
across the spark plug terminals. This condition can often be
traced to excessive carbon or other deposits that cause local
hot spots. Detonation often leads to pre-ignition. However,
pre-ignition may also be caused by high-power operation on
excessively lean mixtures. Pre-ignition is usually indicated
in the flight deck by engine roughness, backfiring, and by a
sudden increase in cylinder head temperature.
Any area within the combustion chamber that becomes
incandescent serves as an igniter in advance of normal timed
ignition and causes combustion earlier than desired. Pre-
ignition may be caused by an area roughened and heated by
detonation erosion. A cracked valve or piston, or a broken
spark plug insulator, may furnish a hot point, that serves as
a glow plug.
The hot spot can be caused by deposits on the chamber
surfaces resulting from the use of leaded fuels. Normal
carbon deposits can also cause pre-ignition. Specifically, pre-
ignition is a condition similar to early timing of the spark. The
charge in the cylinder is ignited before the required time for normal engine firing. However, do not confuse pre-ignition
with the spark that occurs too early in the cycle. Pre-ignition
is caused by a hot spot in the combustion chamber, not by
incorrect ignition timing. The hot spot may be due to either
an overheated cylinder or a defect within the cylinder.
The most obvious method of correcting pre-ignition is to
reduce the cylinder temperature. The immediate step is to
retard the throttle. This reduces the amount of fuel charge
and the amount of heat generated. If a supercharger is in use,
reduce manifold pressure as much as possible to reduce the
charge temperature. Following this, the mixture should be
enriched, if possible, to lower combustion temperature. If the
engine is at high power when pre-ignition occurs, retarding
the throttle for a few seconds may provide enough cooling
to chip off some of the lead, or other deposit, within the
combustion chamber. These chipped-off particles pass out
through the exhaust.
Backfiring
When an air-fuel mixture does not contain enough fuel
to consume all the oxygen, it is called a lean mixture.
Conversely, a charge that contains more fuel than required
is called a rich mixture. An extremely lean mixture either
does not burn at all or burns so slowly that combustion is
not complete at the end of the exhaust stroke. The flame
lingers in the cylinder and then ignites the contents in the
intake manifold or the induction system when the intake
valve opens. This causes an explosion known as backfiring,
which can damage the carburetor and other parts of the
induction system.
Incorrect ignition timing, or faulty ignition wires, can cause
the cylinder to fire at the wrong time, allowing the cylinder
to fire when the intake valve is open, which can cause
backfiring. A point worth stressing is that backfiring rarely
involves the whole engine. Therefore, it is seldom the fault of
the carburetor. In practically all cases, backfiring is limited to
one or two cylinders. Usually, it is the result of faulty valve
clearance setting, defective fuel injector nozzles, or other
conditions that cause these cylinders to operate leaner than
the engine as a whole. There can be no permanent cure until
these defects are discovered and corrected. Because these
backfiring cylinders fire intermittently and, therefore, run
cool, they can be detected by the cold cylinder check. The
cold cylinder check is discussed later in this chapter.
In some instances, an engine backfires in the idle range but
operates satisfactorily at medium and high power settings.
The most likely cause, in this case, is an excessively lean
idle mixture. Proper adjustment of the idle air-fuel mixture
usually corrects this problem.
Air-Fuel Mixture Lean Rich
PowerHigh
Brake
HP
LowAuto leanAuto rich
Figure 10-44. Power versus air-fuel mixture curve. Afterfiring
Afterfiring, sometimes called afterburning, often results
when the air-fuel mixture is too rich. Overly rich mixtures
are also slow burning; therefore, charges of unburned fuel are
present in the exhausted gases. Air from outside the exhaust
stacks mixes with this unburned fuel that ignites. This causes
an explosion in the exhaust system. Afterfiring is perhaps
more common where long exhaust ducting retains greater
amounts of unburned charges. As in the case of backfiring,
the correction for afterfiring is the proper adjustment of the
air-fuel mixture.
Afterfiring can also be caused by cylinders that are not
firing because of faulty spark plugs, defective fuel-injection
nozzles. or incorrect valve clearance. The unburned mixture
from these dead cylinders passes into the exhaust system,
where it ignites and burns. Unfortunately, the resultant
torching or afterburning can easily be mistaken for evidence
of a rich carburetor. Cylinders that are firing intermittently
can cause a similar effect. Again, the malfunction can be
remedied only by discovering the real cause and correcting
the defect. Dead or intermittent cylinders can be located by
the cold cylinder check.
Factors Affecting Engine Operation
Compression
To prevent loss of power, all openings to the cylinder must
close and seal completely on the compression and power
strokes. In this respect, there are three items in the proper
operation of the cylinder that must be operating correctly for
maximum efficiency. First, the piston rings must be in good
condition to provide maximum sealing during the stroke of
the piston. There must be no leakage between the piston and
the walls of the combustion chamber. Second, the intake and
exhaust valves must close tightly so that there is no loss of
compression at these points. Third, and very important, the
timing of the valves (opening and closing) must be such that
highest efficiency is obtained when the engine is operating at
its normal rated rpm. A failure at any of these points results
in greatly reduced engine efficiency.
Fuel Metering
The induction system is the distribution and fuel-metering
part of the engine. Obviously, any defect in the induction
system seriously affects engine operation. For best operation,
each cylinder of the engine must be provided with the proper
air-fuel mixture, usually metered by the carburetor. On some
fuel-injection engines, fuel is metered by the fuel injector
flow divider and fuel-injection nozzles.
The relation between air-fuel ratio and power is illustrated
in Figure 10-44 . As the fuel mixture is varied from lean to
rich, the power output of the engine increases until it reaches a maximum. Beyond this point, the power output falls off
as the mixture is further enriched. This is because the fuel
mixture is now too rich to provide perfect combustion. Note
that maximum engine power can be obtained by setting the
carburetor for one point on the curve.
In establishing the carburetor settings for an aircraft engine,
the design engineers run a series of curves similar to the one
shown. A curve is run for each of several engine speeds. If,
for example, the idle speed is 600 rpm, the first curve might
be run at this speed. Another curve might be run at 700 rpm,
another at 800 rpm, and so on, in 100-rpm increments, up to
takeoff rpm. The points of maximum power on the curves are
then joined to obtain the best power curve of the engine for
all speeds. This best power curve establishes the rich setting
of the carburetor.
In establishing the detailed engine requirements regarding
carburetor setting, the fact that the cylinder head temperature
varies with air-fuel ratio must be considered. This variation is
illustrated in the curve shown in Figure 10-45 . Note that the
cylinder head temperature is lower with the auto-lean setting
than it is with the auto-rich mixture. This is exactly opposite
common belief, but it is true. Furthermore, knowledge
of this fact can be used to advantage by flight crews. If,
during cruise, it becomes difficult to keep the cylinder head
temperature within limits, the air-fuel mixture may be leaned
out to get cooler operation. The desired cooling can then be
obtained without going to auto-rich with its costly waste of
fuel. The curve shows only the variation in cylinder head
temperature. For a given rpm, the power output of the engine
is less with the best-economy setting (auto-lean) than with
the best-power mixture.
The decrease in cylinder head temperature with a leaner
mixture holds true only through the normal cruise range.
At higher power settings, cylinder temperatures are higher
Low High Airflow in lb/hrHigh
Air-fuel
mixture
LowNormal A-F mixture
auto-rich positionDry takeoff power
A-F mixture with water injection
Wet takeoff power
Low High Airflow in lb/hrRich
Air-fuel
LeanIdle
Climb
Minimum cruiseTakeoff
Maximum cruise
Rich
Manual leanFigure 10-46. Air-fuel curve for a water-injection engine.
Figure 10-47. Typical air-fuel mixture curve for a float-type
carburetor.
Air-Fuel Mixture Lean Rich
PowerHigh
Brake
HP
LowAuto leanAuto rich
Figure 10-45. Variation in head temperature with air-fuel mixture
(cruise power).
with the leaner mixtures. The reason for this reversal hinges
on the cooling ability of the engine. As higher powers are
approached, a point is reached where the airflow around the
cylinders do not provide sufficient cooling. At this point, a
secondary cooling method must be used. This secondary
cooling is done by enriching the air-fuel mixture beyond
the best-power point. Although enriching the mixture to this
extent results in a power loss, both power and economy must
be sacrificed for engine cooling purposes.
Many older, large, high-powered radial engines were
influenced by the cooling requirements on air-fuel mixture,
by effects of water injection. Figure 10-46 shows an air-fuel
curve for a water-injection engine. The dotted portion of the
curve shows how the air-fuel mixture is leaned out during
water injection. This leaning is possible because water, rather
than extra fuel, is used as a cylinder coolant. These types of
systems are not used on modern aircraft.
This permits leaning out to approximately best-power mixture
without danger of overheating or detonation. This leaning
out gives an increase in power. The water does not alter the
combustion characteristics of the mixture. Fuel added to the
auto-rich mixture in the power range during dry operation is
solely for cooling. A leaner mixture would give more power.
Actually, water or, more accurately, the antidetonant (water-
alcohol) mixture is a better coolant than extra fuel. Therefore,
water injection permits higher manifold pressures and a still
further increase in power.
In establishing the final curve for engine operation, the
engine’s ability to cool itself at various power settings is, of
course, taken into account. Sometimes the mixture must be
altered for a given installation to compensate for the effect
of cowl design, cooling airflow, or other factors on engine cooling. The final air-fuel mixture curves take into account
economy, power, engine cooling, idling characteristics, and
all other factors that affect combustion.
Figure 10-47 shows a typical final curve for a float-type
carburetor. Note that the air-fuel mixture at idle is the same
in rich and in manual lean. The mixture remains the same
until the low cruise range is reached. At this point, the curves
separate and then remain parallel through the cruise and
power ranges.
Note the spread between the rich and lean setting in the
cruise range of both curves. Because of this spread, there
is a decrease in power when the mixture control is moved
from auto-rich to auto-lean with the engine operating in the
cruise range. This is true because the auto-rich setting in
the cruise range is very near the best power mixture ratio.
10-33Figure 10-48. Idle mixture curve.
Low High Airflow in lb/hrRich
Air-fuel
LeanIdle
Rated power
Minimum cruiseTakeoff power
Maximum cruiseTherefore, any leaning out gives a mixture that is leaner
than best power.
Idle Mixture
The idle mixture curve shows how the mixture changes when
the idle mixture adjustment is changed. [Figure 10-48] Note
that the greatest effect is at idling speeds. However, there
is some effect on the mixture at airflows above idling. The
airflow at which the idle adjustment effect cancels out varies
from minimum cruise to maximum cruise. The exact point
depends on the type of carburetor and the carburetor setting.
In general, the idle adjustment affects the air-fuel mixture up
to low cruise on engines equipped with float-type carburetors.
This means that incorrect idle mixture adjustments can easily
give faulty cruise performance, as well as poor idling.
There are variations in mixture requirements between
one engine and another because of the fuel distribution
within the engine and the ability of the engine to cool.
Remember, a carburetor setting must be rich enough to
supply a combustible mixture for the leanest cylinder. If fuel
distribution is poor, the overall mixture must be richer than
would be required for the same engine if distribution were
good. The engine’s ability to cool depends on such factors
as cylinder design (including the design of the cooling fins),
compression ratio, accessories on the front of the engine that
cause individual cylinders to run hot, and the design of the
baffling used to deflect airflow around the cylinder. At takeoff
power, the mixture must be rich enough to supply sufficient
fuel to keep the hottest cylinder cool.
Induction Manifold
The induction manifold provides the means of distributing air,
or the air-fuel mixture, to the cylinders. Whether the manifold
handles an air-fuel mixture or air alone depends on the type
of fuel metering system used. On an engine equipped with a carburetor, the induction manifold distributes an air-fuel
mixture from the carburetor to the cylinders. On a fuel-
injection engine, the fuel is delivered to injection nozzles,
one in each cylinder, that provide the proper spray pattern for
efficient burning. Thus, the mixing of fuel and air takes place
at the inlet port to the cylinder. On a fuel-injection engine
the induction manifold handles only air.
The induction manifold is an important item because of the
effect it can have on the air-fuel mixture that finally reaches the
cylinder. Fuel is introduced into the airstream by the carburetor
in a liquid form. To become combustible, the fuel must be
vaporized in the air. This vaporization takes place in the
induction manifold, which includes the internal supercharger,
if one is used. Any fuel that does not vaporize clings to the
walls of the intake pipes. Obviously, this affects the effective
air-fuel ratio of the mixture that finally reaches the cylinder
in vapor form. This explains the reason for the apparently
rich mixture required to start a cold engine. In a cold engine,
some of the fuel in the airstream condenses out and clings to
the walls of the manifold. This is in addition to that fuel that
never vaporized in the first place. As the engine warms up,
less fuel is required because less fuel is condensed out of the
airstream and more of the fuel is vaporized, thus giving the
cylinder the required air-fuel mixture for normal combustion.
Any leak in the induction system has an effect on the mixture
reaching the cylinders. This is particularly true of a leak at the
cylinder end of an intake pipe. At manifold pressures below
atmospheric pressure, such a leak leans out the mixture. This
occurs because additional air is drawn in from the atmosphere
at the leaky point. The affected cylinder may overheat, fire
intermittently, or even cut out altogether.
Operational Effect of Valve Clearance
While considering the operational effect of valve clearance,
keep in mind that all aircraft reciprocating engines of current
design use valve overlap. Valve overlap is when the intake
and exhaust valves are open at the same time. This takes
advantage of the momentum of the entering and exiting gases
to improve the efficiency of getting air-fuel in and exhaust
gases out. Figure 10-49 shows the pressures at the intake and
exhaust ports under two different sets of operating conditions.
In one case, the engine is operating at a manifold pressure
of 35 "Hg. Barometric pressure (exhaust back pressure) is
29 "Hg. This gives a pressure acting in the direction indicated
by the arrow of differential of 6 "Hg (3 psi).
During the valve overlap period, this pressure differential
forces the air-fuel mixture across the combustion chamber
toward the open exhaust. This flow of air-fuel mixture forces
ahead of it the exhaust gases remaining in the cylinder,
resulting in complete scavenging of the combustion chamber.
Intake valve Exhaust valve
ImpellerManifold pressure
MAP Pressure differential Barometric
pressure
35" Hg. 6" Hg. 29" Hg.
20" Hg. 9" Hg. 29" Hg.
Low (idle) (Takeoff) High AirflowRich
Air-fuel
mixture
LeanOpen stack
Collector ringFigure 10-49. Effect of valve overlap.
Figure 10-50. Comparison of air-fuel mixture curves for open-stack
and collector-ring installations. This, in turn, permits complete filling of the cylinder with
a fresh charge on the following intake event. This is the
situation in which valve overlap gives increased power.
There is a pressure differential in the opposite direction
of 9 "Hg (4.5 psi) when the manifold pressure is below
atmospheric pressure, for example, 20 "Hg. These cause air
or exhaust gases to be drawn into the cylinder through the
exhaust port during valve overlap.
In engines with collector rings, this inflow through the
exhaust port at low power settings consists of burned exhaust
gases. These gases are pulled back into the cylinder and mix
with the incoming air-fuel mixture. However, these exhaust
gases are inert; they do not contain oxygen. Therefore, the
air-fuel mixture ratio is not affected much. With open exhaust
stacks, the situation is entirely different. Here, fresh air
containing oxygen is pulled into the cylinders through the
exhaust. This leans out the mixture. Therefore, the carburetor
must be set to deliver an excessively rich idle mixture so that,
when this mixture is combined with the fresh air drawn in
through the exhaust port, the effective mixture in the cylinder
will be at the desired ratio.
At first thought, it does not appear possible that the effect
of valve overlap on air-fuel mixture is sufficient to cause
concern. However, the effect of valve overlap becomes
apparent when considering idle air-fuel mixtures. These
mixtures must be enriched 20 to 30 percent when open stacks,
instead of collector rings (radial engines) are used on the
same engine. [Figure 10-50] Note the spread at idle between an open stack and an exhaust collector ring installation for
engines that are otherwise identical. The mixture variation
decreases as the engine speed or airflow is increased from
idle into the cruise range.
Engine, airplane, and equipment manufacturers provide a
powerplant installation that gives satisfactory performance.
Cams are designed to give best valve operation and correct
overlap. But valve operation is correct only if valve clearances
are set and remain at the value recommended by the engine
manufacturer. If valve clearances are set wrong, the valve
overlap period is longer or shorter than the manufacturer
intended. The same is true if clearances get out of adjustment
during operation.
Where there is too much valve clearance, the valves do not
open as wide or remain open as long as they should. This
reduces the overlap period. At idling speed, it affects the
air-fuel mixture, since a less-than-normal amount of air or
exhaust gases is drawn back into the cylinder during the
shortened overlap period. As a result, the idle mixture tends
to be too rich.
When valve clearance is less than it should be, the valve
overlap period is lengthened. A greater than normal amount
of air, or exhaust gases, is drawn back into the cylinder at
idling speeds. As a result, the idle mixture is leaned out at the
cylinder. The carburetor is adjusted with the expectation that
a certain amount of air or exhaust gases is drawn back into
the cylinder at idling. If more or less air, or exhaust gases,
are drawn into the cylinder during the valve overlap period,
the mixture is too lean or too rich.
When valve clearances are wrong, it is unlikely that they are
all wrong in the same direction. Instead, there is too much
clearance on some cylinders and too little on others. Naturally,
10-351 2 3 4 5 6 7 8 9 10 11 12 13 14Lengthened
Valve overlap as
affected by valve
clearance
ReducedLean
Effect on air-fuel
mixture or combustible
charge
Rich
Figure 10-51. Effect of variation in valve overlap on air-fuel mixture between cylinders. this gives a variation in valve overlap between cylinders.
This results in a variation in air-fuel ratio at idling and lower-
power settings, since the carburetor delivers the same mixture
to all cylinders. The carburetor cannot tailor the mixture to
each cylinder to compensate for variation in valve overlap.
The effect of variation in valve clearance and valve overlap
on the air-fuel mixture between cylinders is illustrated in
Figure 10-51 . Note how the cylinders with too little clearance
run rich, and those with too much clearance run lean. Note
also the extreme mixture variation between cylinders.
Valve clearance also effects volumetric efficiency. Any
variations in air-fuel into, and exhaust gases out of, the
cylinder affects the volumetric efficiency of the cylinder. With
the use of hydraulic valve lifters that set the valve clearance
automatically, engine operation has been greatly improved.
Hydraulic lifters do have a limited range in which they can
control the valve clearance, or they can become stuck in one
position that can cause them to be a source of engine trouble.
Normally engines equipped with hydraulic lifters require
little to no maintenance.
Engine Troubleshooting
Troubleshooting is a systematic analysis of the symptoms
that indicate engine malfunction. It would be impractical to
list all the malfunctions that could occur in a reciprocating
engine, so only the most common malfunctions are discussed.
A thorough knowledge of the engine systems, applied with
logical reasoning, solves most problems that may occur.Figure 10-52 lists general conditions or troubles that may
be encountered on reciprocating engines, such as engine
fails to start. They are further divided into the probable
causes contributing to such conditions. Corrective actions
are indicated in the remedy column. The items are presented
with consideration given to frequency of occurrence, ease
of accessibility, and complexity of the corrective action
indicated.
The need for troubleshooting normally is dictated by poor
operation of the complete powerplant. Power settings for the
type of operation at which any difficulty is encountered, in
many cases, indicate that part of the powerplant that is the
basic cause of difficulty.
The cylinders of an engine, along with any type of
supercharging, form an air pump. Furthermore, the power
developed in the cylinders varies directly with the rate that
air can be consumed by the engine. Therefore, a measure
of air consumption or airflow into the engine is a measure
of power input. Ignoring for the moment such factors as
humidity and exhaust back pressure, the manifold pressure
gauge and the engine tachometer provide a measure of
engine air consumption. Thus, for a given rpm, any change
in power input is reflected by a corresponding change in
manifold pressure.
The power output of an engine is the power absorbed by the
propeller. Therefore, propeller load is a measure of power
output. Propeller load, in turn, depends on the propeller
rpm, blade angle, and air density. For a given angle and air
10-36• Lack of fuel.
• Underpriming.
• Overpriming.
• Incorrect throttle setting.
• Defective spark plugs.
• Defective ignition wire.
• Defective or weak battery.
• Improper operation of magneto or breaker
points.
• Water in carburetor.
• Internal failure.
• Magnetized impulse coupling, if installed.
• Frozen spark plug electrodes.
• Mixture control in idle cutoff.
• Shorted ignition switch or loose ground.
• Incorrect carburetor idle speed adjustment.
• Incorrect idle mixture.
• Leak in the induction system.
• Low cylinder compression.
• Faulty ignition system.
• Open or leaking primer.
• Improper spark plug setting for altitude.
• Dirty air filter.
• Mixture too rich; indicated by sluggish engine
operation, red exhaust flame, and black smoke.
• Mixture too lean; indicated by overheating or
backfiring.
• Leaks in induction system.
• Defective spark plugs.
• Improper grade of fuel.
• Magneto breaker points not working properly.
• Defective ignition wire.
• Defective spark plug terminal connectors.
• Incorrect valve clearance.Engine fails to start.
Engine fails to idle properly.
Low power and engine
running uneven.Trouble Probable causes Remedy
• Check fuel system for leaks fill fuel tank.
• Clean dirty lines, strainers, or fuel valves.
• Use correct priming procedure.
• Open throttle and “unload” engine by
rotating the propeller.
• Open throttle to one-tenth of its range.
• Clean and re-gap or replace spark plugs.
• Test and replace any defective wires.
• Replace with charged battery.
• Check internal timing of magnetos.
• Drain carburetor and fuel lines.
• Check oil sump strainer for metal particles.
• Demagnetize impulse coupling.
• Replace spark plugs or dry out plugs.
• Open mixture control.
• Check and replace or obtain correct idle.
• Adjust throttle stop to obtain correct idle.
• Adjust mixture. (Refer to engine manufacturer’s
handbook for proper procedure.)
• Tighten all connections in the induction
system. Replace any defective parts.
• Check cylinder compression.
• Check entire ignition system.
• Lock or repair primer.
• Check spark plug gap.
• Clean or replace.
• Check primer. Re-adjust carburetor mixture.
• Check fuel lines for dirt or other restrictions.
Check fuel supply.
• Tighten all connections. Replace defective
parts.
• Clean or replace spark plugs.
• Fill tank with recommended grade.
• Clean points. Check internal timing of magneto.
• Test and replace any defective wires.
• Replace connectors on spark plug wire.
• Adjust valve clearance.
• Check and replace or repair.
Figure 10-52. Troubleshooting opposed engines.
density, propeller load (power output) is directly proportional
to engine speed. The basic power of an engine is related to
manifold pressure, fuel flow, and rpm. Because the rpm of
the engine and the throttle opening directly control manifold
pressure, the primary engine power controls are the throttle
and the rpm control. An engine equipped with a fixed-pitch propeller has only a throttle control. In this case, the throttle
setting controls both manifold pressure and engine rpm.
With proper precautions, manifold pressure can be taken as a
measure of power input, and rpm can be taken as a measure
of power output. However, the following factors must be
10-37• Restriction in exhaust system.
• Improper ignition timing.
• Throttle lever out of adjustment.
• Leak in induction system.
• Restriction in carburetor airscoop.
• Improper fuel.
• Propeller governor out of adjustment.
• Faulty ignition.
• Cracked engine mount(s).
• Unbalanced propeller.
• Defective mounting bushings.
• Lead deposit on spark plugs.
• Primer unlocked.
• Insufficient oil.
• Dirty oil strainers.
• Defective pressure gauge.
• Air lock or dirt in relief valve.
• Leak in suction line or pressure line.
• High oil temperature.
• Stoppage in oil pump intake passage.
• Worn or scored bearings.
• Insufficient air cooling.
• Insufficient oil supply.
• Clogged oil lines or strainers.
• Failing or failed bearings.
• Defective thermostats.
• Defective temperature gauge.
• Excessive blow-by.
• Failing or failed bearing.
• Worn or broken piston rings.
• Incorrect installation of piston rings.
• External oil leakage.
• Leakage through engine fuel pump vent.
• Engine breather or vacuum pump breather.• Remove restriction.
• Check magnetos for timing and synchronization.
• Adjust throttle lever.
• Tighten all connections and replace defective
parts.
• Examine airscoop and remove restriction.
• Fill tank with recommended fuel.
• Adjust governor.
• Tighten all connections. Check system. Check
ignition timing.
• Repair or replace engine mount(s).
• Remove propeller and have it checked for
balance.
• Install new mounting bushings.
• Clean or replace plugs.
• Lock primer.
• Check oil supply.
• Remove and clean oil strainers.
• Replace gauge.
• Remove and clean oil pressure relief valve.
• Check gasket between accessory housing
crankcase.
• See “high oil temperature” in trouble column.
• Check line for obstruction. Clean suction
strainer.
• Overhaul engine.
• Check air inlet and outlet for deformation or
obstruction.
• Fill oil tank to proper level.
• Remove and clean oil line or strainers.
• Examine sump for metal particles and, if found,
overhaul engine.
• Replace thermostats.
• Usually caused by weak or stuck rings.
• Overhaul engine.
• Check sump for metal particles and if found,
an overhaul of engine is indicated.
• Install new rings.
• Install new rings.
• Check engine carefully for leaking gaskets or
O-rings.
• Replace fuel pump seal.
• Check engine, and overhaul or replace vacuum
pump.Low power and engine
running uneven.
Engine fails to develop full
power.
Rough running engine.
Low oil pressure.
High oil temperature.
Excessive oil consumption.Trouble Probable causes Remedy
Figure 10-52. Troubleshooting opposed engines (continued).
10-38considered:
1. Atmospheric pressure and air temperature must be
considered, since they affect air density.
2. These measures of power input and power output
should be used only for comparing the performance
of an engine with its previous performance, or for
comparing identical powerplants.
3. With a controllable propeller, the blades must be
against their low-pitch stops, since this is the only
blade position in which the blade angle is known and
does not vary. Once the blades are off their low-pitch
stops, the propeller governor takes over and maintains
a constant rpm, regardless of power input or engine
condition. This precaution means that the propeller
control must be set to maximum or takeoff rpm, and
the checks made at engine speeds below this setting.
Having relative measures of power input and power output,
the condition of an engine can be determined by comparing
input and output. This is done by comparing the manifold
pressure required to produce a given rpm with the manifold
pressure required to produce the same rpm at a time when
the engine (or an identical powerplant) was known to be in
top operating condition.
An example shows the practical application of this method of
determining engine condition. With the propeller control set for
takeoff rpm (full low blade angle), an engine may require 32
inches of manifold pressure to turn 2,200 rpm for the ignition
check. On previous checks, this engine required only 30 inches
of manifold pressure to turn 2,200 rpm at the same station
(altitude) and under similar atmospheric conditions. Obviously,
something is wrong; a higher power input (manifold pressure)
is now required for the same power output (rpm). There is a
good chance that one cylinder has a malfunction.
There are several standards against which engine performance
can be compared. The performance of a particular engine can
be compared with its past performance, provided adequate
records are kept. Engine performance can be compared with
that of other engines on the same aircraft or aircraft having
identical installations.
If a fault does exist, it may be assumed that the trouble lies
in one of the following systems:
1. Ignition system.
2. Fuel-metering system.
3. Induction system.
4. Power section (valves, cylinders, etc.).
5. Instrumentation.If a logical approach to the problem is taken and the
instrument readings properly utilized, the malfunctioning
system can be pinpointed, and the specific problem in the
defective system can be singled out.
The more information available about any particular problem,
the better the opportunity for a rapid repair. Information that
is of value in locating a malfunction includes:
1. Was any roughness noted? Under what conditions of
operation?
2. What is the time on the engine and spark plugs? How
long since last inspection?
3. Was the ignition system operational check and power
check normal?
4. When did the trouble first appear?
5. Was backfiring or afterfiring present?
6. Was the full throttle performance normal?
From a different point of view, the powerplant is, in reality,
a number of small engines turning a common crankshaft
and being operated by two common phases: fuel metering
and ignition. When backfiring, low power output or other
powerplant difficulty is encountered, first find out which
system, fuel metering or ignition, is involved and then
determine whether the entire engine or only one cylinder
is at fault. For example, backfiring normally is caused by:
1. Valves holding open or sticking open in one or more
of the cylinders.
2. Lean mixture.
3. Intake pipe leakage.
4. An error in valve adjustment that causes individual
cylinders to receive too small a charge or one too large,
even though the mixture to the cylinders has the same
air-fuel ratio.
Ignition system reasons for backfiring might be a cracked
distributor block or a high-tension leak between two ignition
leads. Either of these conditions could cause the charge in
the cylinder to be ignited during the intake stroke. Ignition
system troubles involving backfiring normally are not
centered in the basic magneto, since a failure of the basic
magneto would result in the engine not running, or it would
run well at low speeds but cut out at high speeds. On the other
hand, replacement of the magneto would correct a difficulty
caused by a cracked distributor where the distributor is a part
of the magneto.
If the fuel system, ignition system, and induction system are
10-39functioning properly, the engine should produce the correct
bhp unless some fault exists in the basic power section.
Valve Blow-By
Valve blow-by is indicated by a hissing or whistle when
pulling the propeller through prior to starting the engine,
when turning the engine with the starter, or when running
and blow-by past the intake valve is audible through the
carburetor.
Correct valve blow-by immediately to prevent valve failure
and possible engine failure by taking the following steps:
1. Perform a cylinder compression test to locate the faulty
cylinder.
2. Check the valve clearance on the affected cylinder.
If the valve clearance is incorrect, the valve may be
sticking in the valve guide. To release the sticking
valve, place a fiber drift on the rocker arm immediately
over the valve stem and strike the drift several times
with a mallet. Sufficient hand pressure should be
exerted on the fiber drift to remove any space between
the rocker arm and the valve stem prior to hitting the
drift.
3. If the valve is not sticking and the valve clearance is
incorrect, adjust it as necessary.
4. Determine whether blow-by has been eliminated by
again pulling the engine through by hand or turning
it with the starter. If blow-by is still present, it may be
necessary to replace the cylinder.
Cylinder Compression Tests
The cylinder compression test determines if the valves, piston
rings, and pistons are adequately sealing the combustion
chamber. If pressure leakage is excessive, the cylinder
cannot develop its full power. The purpose of testing cylinder
compression is to determine whether cylinder replacement
is necessary. The detection and replacement of defective
cylinders prevents a complete engine change because of
cylinder failure. It is essential that cylinder compression tests
be made periodically. Low compression, for the most part,
can be traced to leaky valves.
Conditions that affect engine compression are:
1. Incorrect valve clearances.
2. Worn, scuffed, or damaged piston.
3. Excessive wear of piston rings and cylinder walls.
4. Burned or warped valves.
5. Carbon particles between the face and the seat of the
valve or valves.6. Early or late valve timing.
Perform a compression test as soon as possible after the
engine is shut down so that piston rings, cylinder walls,
and other parts are still freshly lubricated. However, it is
not necessary to operate the engine prior to accomplishing
compression tests during engine buildup or on individually
replaced cylinders. In such cases, before making the test,
spray a small quantity of lubricating oil into the cylinder(s),
and turn the engine over several times to seal the piston and
rings in the cylinder barrel.
Be sure that the ignition switch is in the OFF position so
that there is no accidental firing of the engine. Remove
necessary cowling and the most accessible spark plug from
each cylinder. When removing the spark plugs, identify
them to coincide with the cylinder. Close examination of
the plugs aid in diagnosing problems within the cylinder.
Review the maintenance records of the engine being tested.
Records of previous compression checks help in determining
progressive wear conditions and in establishing the necessary
maintenance actions.
Differential Pressure Tester
The differential pressure tester checks the compression
of aircraft engines by measuring the leakage through the
cylinders. The design of this compression tester is such that
minute valve leakages can be detected, making possible the
replacement of cylinders where valve burning is starting. The
operation of the compression tester is based on the principle
that, for any given airflow through a fixed orifice, a constant
pressure drop across the orifice results.
As the airflow and pressure changes, pressure varies
accordingly in the same direction. If air is supplied under
pressure to the cylinder with both intake and exhaust valves
closed, the amount of air that leaks by the valves or piston
rings indicates their condition; the perfect cylinder would
have no leakage. The differential pressure tester requires the
application of air pressure to the cylinder being tested with
the piston at top-center compression stroke. [Figure 10-53]
Guidelines for performing a differential compression test are:
1. Perform the compression test as soon as possible after
engine shutdown to provide uniform lubrication of
cylinder walls and rings.
2. Remove the most accessible spark plug from the
cylinder, or cylinders, and install a spark plug adapter
in the spark plug insert.
3. Connect the compression tester assembly to a 100 to
150 psi compressed air supply. [Figure 10-54] With
the shutoff valve on the compression tester closed,
65 7075101520253035
65 7075Regulated pressure gauge
Shutoff valve
To air compressor
Piston on true top dead centerCylinder pressure gauge
Metering orifice
Figure 10-53. Differential compression tester diagrams.
Figure 10-54. Compression tester and adapter. adjust the regulator of the regulated pressure gauge
compression tester to obtain 80 psi.
4. Open the shutoff valve and attach the air hose quick-
connect fitting to the spark plug adapter. The shutoff
valve, when open, automatically maintains a pressure
in the cylinder of 15 to 20 psi when both the intake
and exhaust valves are closed.
5. By hand, turn the engine over in the direction of rotation
until the piston in the cylinder being tested comes up
on the compression stroke against the 15 psi. Continue
turning the propeller slowly in the direction of rotation
until the piston reaches top dead center. Top dead center
can be detected by a decrease in force required to move
the propeller. If the engine is rotated past top dead
center, the 15 to 20 psi tends to move the propeller in the
direction of rotation. If this occurs, back the propeller up
at least one blade prior to turning the propeller again in
the direction of rotation. This backing up is necessary to
eliminate the effect of backlash in the valve-operating mechanism and to keep the piston rings seated on the
lower ring lands.
6. Close the shutoff valve in the compression tester and
re-check the regulated pressure to see that it is 80 psi
with air flowing into the cylinder. If the regulated
pressure is more or less than 80 psi, readjust the
regulator in the test unit to obtain 80 psi. When closing
the shutoff valve, make sure that the propeller path is
clear of all objects. There is sufficient air pressure in
the combustion chamber to rotate the propeller if the
piston is not on top dead center.
7. With regulated pressure adjusted to 80 psi, if the
cylinder pressure reading indicated on the cylinder
pressure gauge is below the minimum specified for the
engine being tested, move the propeller in the direction
of rotation to seat the piston rings in the grooves.
Check all the cylinders and record the readings.
If low compression is obtained on any cylinder, turn the
engine through with the starter, or re-start, and run the engine
to takeoff power and re-check the cylinder, or cylinders,
having low compression.
If the low compression is not corrected, remove the rocker-
box cover and check the valve clearance to determine if the
difficulty is caused by inadequate valve clearance. If the low
compression is not caused by inadequate valve clearance,
place a fiber drift on the rocker arm immediately over the
valve stem and tap the drift several times with a 1 to 2 pound
hammer to dislodge any foreign material that may be lodged
between the valve and valve seat.
After staking the valve in this manner, rotate the engine with
the starter and re-check the compression. Do not make a
compression check after staking a valve until the crankshaft
has been rotated either with the starter or by hand to re-seat
the valve in normal manner. The higher seating velocity
obtained when staking the valve will indicate valve seating,
even though valve seats are slightly egged or eccentric.
This procedure should only be performed if approved by
the manufacturer.
Cylinders having compression below the minimum specified
should be further checked to determine whether leakage is
past the exhaust valve, intake valve, or piston. Excessive
leakage can be detected (during the compression check):
1. At the exhaust valve by listening for air leakage at the
exhaust outlet;
2. At the intake valve by escaping air at the air intake;
and
3. Past the piston rings by escaping air at the engine
10-41breather outlets.
Next to valve blow-by, the most frequent cause of
compression leakage is excessive leakage past the piston.
This leakage may occur because of lack of oil. To check this
possibility, apply engine oil into the cylinder and around the
piston. Then, re-check the compression. If this procedure
raises compression to or above the minimum required,
continue the cylinder in service. If the cylinder pressure
readings still do not meet the minimum requirement, replace
the cylinder. When it is necessary to replace a cylinder as a
result of low compression, record the cylinder number and
the compression value of the newly installed cylinder on the
compression check sheet.
Cylinder Replacement
Reciprocating engine cylinders are designed to operate for
a specified time before normal wear requires their overhaul.
If the engine is operated as recommended and proficient
maintenance is performed, the cylinders normally last until
the engine has reached its TBO. It is known from experience
that materials fail, and engines are abused through incorrect
operation; this has a serious effect on cylinder life. Another
reason for premature cylinder change is poor maintenance.
Therefore, exert special care to ensure that all the correct
maintenance procedures are adhered to when working on the
engine. Some of the reasons for cylinder replacement are:
1. Low compression.
2. High oil consumption in one or more cylinders.
3. Excessive valve guide clearance.
4. Loose intake pipe flanges.
5. Loose or defective spark plug inserts.
6. External damage, such as cracks.
The cylinder is always replaced as a complete assembly,
which includes piston, rings, valves, and valve springs.
Obtain the cylinder by ordering the cylinder assembly under
the part number specified in the engine parts catalog. Parts,
such as valve springs, rocker arms, and rocker box covers,
may be replaced individually.
Normally, all the cylinders in an engine are similar, all are
standard size or all a certain oversize, and all are steel bore or
all are chrome-plated. The size of the cylinder is indicated by
a color code around the barrel between the attaching flange
and the lower barrel cooling fin. In some instances, air-cooled
engines are equipped with chrome-plated cylinders. Chrome-
plated cylinders are usually identified by a paint band around
the barrel between the attaching flange and the lower barrel
cooling fin. This color band is usually international orange.
When installing a chrome-plated cylinder, do not use chrome-plated piston rings. The matched assembly includes
the correct piston rings. However, if a piston ring is broken
during cylinder installation, check the cylinder marking to
determine what ring, chrome-plated or otherwise, is correct
for replacement. Similar precautions must be taken to be sure
that the correct size rings are installed.
Correct procedures and care are important when replacing
cylinders. Careless work or the use of incorrect tools can
damage the replacement cylinder or its parts. Incorrect
procedures in installing rocker-box covers may result in
troublesome oil leaks. Improper torque on cylinder hold down
nuts or cap-screws can easily result in a cylinder malfunction
and subsequent engine failure.
Cylinder Removal
Since these instructions are meant to cover all air-cooled
engines, they are of a very general nature. The applicable
manufacturer’s maintenance manual should be consulted for
torque values and special precautions applying to a particular
aircraft and engine. However, always practice neatness
and cleanliness, and always protect openings so that nuts,
washers, tools, and miscellaneous items do not enter the
engine’s internal sections.
Assuming that all obstructing cowling and brackets have
been removed, first remove the intake pipe and exhaust pipes.
Plug or cover openings in the intake or diffuser section. Then,
remove cylinder deflectors and any attaching brackets that
would obstruct cylinder removal. Loosen the spark plugs and
remove the spark plug lead clamps. Do not remove the spark
plugs until ready to pull the cylinder off. Remove the rocker
box covers. First, remove the nuts and then tap the cover
lightly with a rawhide mallet or plastic hammer. Never pry
the cover off with a screwdriver or similar tool.
Loosen the pushrod packing gland nuts or hose clamps, top
and bottom. Pushrods are removed by depressing the rocker
arms with a special tool, or by removing the rocker arm.
Before removing the pushrods, turn the crankshaft until the
piston is at top dead center on the compression stroke. This
relieves the pressure on both intake and exhaust rocker arms.
It is also wise to back off the adjusting nut as far as possible,
because this allows maximum clearance for pushrod removal
when the rocker arms are depressed.
On some model engines, or if the engine is rotated, tappets
and springs of lower cylinders can fall out. Provision must be
made to catch them as the pushrod and housing are removed.
After removing the pushrods, examine them for markings or
mark them so that they may be replaced in the same location
as they were before removal. The ball ends are usually
worn to fit the sockets in which they have been operating.
Furthermore, on some engines, pushrods are not all of the
10-42same length. A good procedure is to mark the pushrods near
the valve tappet ends No. 1 IN, No. 1 EX, No. 2 IN, No. 2 EX.,
etc. On fuel injection engines, disconnect the fuel injection
line and any line clamps that interfere with cylinder removal.
The next step in removing the cylinder is to cut the lock wire
or remove the cotter pin, and pry off the locking device from
the cylinder-attaching cap-screws or nuts. Remove all the
screws or nuts except two located 180° apart. Use the wrench
specified for this purpose in the special tools section of the
applicable manual.
Finally, while supporting the cylinder, remove the two
remaining nuts and gently pull the cylinder away from the
crankcase. Two technicians working together during this
step, as well as during the remaining procedure for cylinder
replacement, helps prevent damage or dropping of the
cylinder. After the cylinder skirt has cleared the crankcase,
but before the piston protrudes from the skirt, provide some
means (usually a shop cloth) for preventing pieces of broken
rings from falling into the crankcase. After the piston has
been removed, remove the cloths and carefully check that
all pieces were prevented from falling into the crankcase.
Place a support on the cylinder mounting pad and secure it
with two cap-screws or nuts. Then, remove the piston and
ring assembly from the connecting rod. A pin pusher or puller
tool can be used when varnish makes it hard to remove the
pin. If the special tool is not available and a drift is used to
remove the piston pin, the connecting rod should be supported
so that it does not have to take the shock of the blows. If this
is not done, the rod may be damaged.
After the removal of a cylinder and piston, the connecting
rod must be supported to prevent damage to the rod and
crankcase. This can be done by supporting each connecting
rod with the removed cylinder base oil seal ring looped
around the rod and cylinder base studs.
Using a wire brush, clean the studs or cap-screws and
examine them for cracks, damaged threads, or any other
visible defects. If one cap-screw is found loose or broken
at the time of cylinder removal, all the cap-screws for the
cylinder should be discarded, since the remaining cap-screws
may have been seriously weakened. A cylinder hold down
stud failure places the adjacent studs under a greater operating
pressure, and they are likely to be stretched beyond their
elastic limit. The engine manufacturer’s instruction must be
followed for the number of studs that have to be replaced
after a stud failure. When removing a broken stud, take proper
precautions to prevent metal chips from entering the engine
crankcase section. In all cases, both faces of the washers and
the seating faces of stud nuts or cap-screws must be cleaned and any roughness or burrs removed.
Cylinder Installation
See that all preservative oil accumulation on the cylinder and
piston assembly is washed off with solvent and thoroughly
dried with compressed air. Install the piston and ring assembly
on the connecting rod. Be sure that the piston faces in the
right direction. The piston number stamped on the bottom of
the piston head should face toward the front of the engine.
Lubricate the piston pin before inserting it. It should fit with
a push fit. If a drift must be used, follow the same precaution
that was taken during pin removal.
Oil the exterior of the piston assembly generously, forcing oil
around the piston rings and in the space between the rings and
grooves. Stagger the ring gaps around the piston and check
to see that rings are in the correct grooves, and whether they
are positioned correctly, as some are used as oil scrapers,
others as pumper rings. The number, type, and arrangement
of the compression and oil-control rings vary with the make
and model of engine.
Perform any and all visual, structural, and dimensional
inspection checks before installing the cylinder. Check the
flange to see that the mating surface is smooth and clean.
Coat the inside of the cylinder barrel generously with oil. Be
sure that the cylinder oil-seal ring is in place and that only
one seal ring is used.
Using a ring compressor, compress the rings to a diameter
equal to that of the piston. With the piston at TDC, start the
cylinder assembly down over the piston, making certain
that the cylinder and piston plane remain the same. Ease
the cylinder over the piston with a straight, even movement
that moves the ring compressor as the cylinder slips on. Do
not rock the cylinder while slipping it on the piston, since
any rocking is apt to release a piston ring or a part of a ring
from the ring compressor prior to the ring’s entrance into the
cylinder bore. A ring released in this manner expands and
prevents the piston from entering the cylinder. Any attempt
to force the cylinder onto the piston is apt to cause cracking
or chipping of the ring or damage to the ring lands.
After the cylinder has slipped on the piston, so that all piston
rings are in the cylinder bore, remove the ring compressor
and the connecting rod guide. Then, slide the cylinder into
place on the mounting pad. If cap-screws are used, rotate the
cylinder to align the holes. While still supporting the cylinder,
install two cap-screws or stud nuts 180° apart.
Install the remaining nuts or cap-screws and tighten them
until they are snug. The hold down nuts, or cap-screws, must
now be torqued to the value specified in the table of torque
OIL5
Figure 10-55. Cold cylinder indicator. values in the engine manufacturer’s service or overhaul
manual. Apply the torque with a slow, steady motion until
the prescribed value is reached. Hold the tension on the
wrench for a sufficient length of time to ensure that the nut
or cap-screw tightens no more at the prescribed torque value.
In many cases, additional turning of the cap-screw, or nut,
as much as one-quarter turn can be done by maintaining
the prescribed torque on the nut for a short period of time.
After the stud nuts, or cap-screws, have been torqued to the
prescribed value, safety them in the manner recommended
in the engine manufacturer’s service manual.
Reinstall the push rods, push rod housings, rocker arms, barrel
deflectors, intake pipes, ignition harness lead clamps and
brackets, fuel injection line clamps and fuel injection nozzles
(if removed), exhaust stack, cylinder head deflectors, and
spark plugs. Remember that the push rods must be installed
in their original locations and must not be turned end to end.
Make sure that the push rod ball end seats properly in the
tappet. If it rests on the edge or shoulder of the tappet during
valve clearance adjustment and later drops into place, valve
clearance is off.
Furthermore, rotating the crankshaft with the push rod
resting on the edge of the tappet may bend the push rod.
After installing the push rods and rocker arms, set the valve
clearance. Before installing the rocker-box covers, lubricate
the rocker arm bearings and valve stems. Check the rocker-
box covers for flatness; re-surface them if necessary. After
installing the gaskets and covers, tighten the rocker-box cover
nuts to the specified torque. Always follow the recommended
safety procedures.
Cold Cylinder Check
The cold cylinder check determines the operating
characteristics of each cylinder of an air-cooled engine.
The tendency for any cylinder, or cylinders, to be cold,
or to be only slightly warm, indicates lack of combustion
or incomplete combustion within the cylinder. This must
be corrected if best operation and power conditions are to
be obtained. The cold cylinder check is made with a cold
cylinder indicator.
Engine difficulties that can be analyzed by use of the cold
cylinder indicator are [Figure 10-55] :
1. Rough engine operation.
2. Excessive rpm drop during the ignition system check.
3. High manifold pressure for a given engine rpm during
the ground check when the propeller is in the full low-
pitch position.
4. Faulty mixture ratios caused by improper valve
clearance.In preparation for the cold cylinder check, head the aircraft
into the wind to minimize irregular cooling of the individual
cylinders and to ensure even propeller loading during
engine operation.
Operate the engine on its roughest magneto at a speed between
1,200 and 1,600 rpm until the cylinder head temperature
reading is stabilized. If engine roughness is encountered
at more than one speed, or if there is an indication that a
cylinder ceases operating at idle or higher speeds, run the
engine at each of these speeds, and perform a cold cylinder
check to pick out all the dead or intermittently operating
cylinders. When low power output or engine vibration is
encountered at speeds above 1,600 rpm when operating
with the ignition switch on both, run the engine at the speed
where the difficulty is encountered until the cylinder head
temperatures have stabilized.
When cylinder head temperatures have reached the stabilized
values, stop the engine by moving the mixture control to
Cylinder No.
Right magnetoTemperature readings
Left magneto
Figure 10-56. Readings taken during a cold cylinder check. the idle cutoff or full lean position. When the engine ceases
firing, turn off both ignition and master switches. Record the
cylinder head temperature reading registered on the flight
deck gauge. As soon as the propeller has ceased rotating,
apply the instrument to each cylinder head, and record the
relative temperature of each cylinder. Start with number one
and proceed in numerical order around the engine, as rapidly
as possible. To obtain comparative temperature values, a firm
contact must be made at the same relative location on each
cylinder. Note any outstandingly low (cold) values. Compare
the temperature readings to determine which cylinders are
dead (cold cylinders) or are operating intermittently.
Difficulties that may cause a cylinder to be inoperative
(dead) when isolated to one magneto, either the right or left
positions, are:
1. Defective spark plugs.
2. Incorrect valve clearances.
3. Leaking intake pipes.
4. Lack of compression.
5. Defective spark plug lead.
6. Defective fuel-injection nozzle.
Repeat the cold cylinder test for the other magneto positions
on the ignition switch, if necessary. Cooling the engine
between tests is unnecessary. The airflow created by the
propeller, and the cooling effect of the incoming air-fuel
mixture is sufficient to cool any cylinders that are functioning
on one test and not functioning on the next.
In interpreting the results of a cold cylinder check,
remember that the temperatures are relative. A cylinder
temperature taken alone means little, but when compared
with the temperatures of other cylinders on the same engine,
it provides valuable diagnostic information. The readings
shown in Figure 10-56 illustrate this point. On this check,
the cylinder head temperature gauge reading at the time the
engine was shut down was 160 °C on both tests.
A review of these temperature readings reveals that, on the
right magneto, cylinder number 3 runs cool and cylinders
5 and 6 run cold. This indicates that cylinder 3 is firing
intermittently, and cylinders 5 and 6 are dead during engine
operation on the plugs fired by the right magneto. Cylinders
4 and 6 are dead during operation on the plugs fired by the
left magneto. Cylinder 6 is completely dead. An ignition
system operational check would not disclose this dead
cylinder, since the cylinder is inoperative on both right and
left switch positions.
A dead cylinder can be detected during run-up, since an
engine with a dead cylinder requires a higher than normal manifold pressure to produce any given rpm below the cut-in
speed of the propeller governor. A dead cylinder could also
be detected by comparing power input and power output with
the aid of a torquemeter.
Defects within the ignition system that can cause a cylinder
to go completely dead are:
1. Both spark plugs inoperative.
2. Both ignition leads grounded, leaking, or open.
3. A combination of inoperative spark plugs and
defective ignition leads.
4. Faulty fuel-injection nozzles, incorrect valve
clearances, and other defects outside the ignition
system.
In interpreting the readings obtained on a cold cylinder
check, the amount the engine cools during the check must be
considered. To determine the extent to which this factor should
be considered in evaluating the readings, re-check some of
the first cylinders tested, and compare the final readings with
those made at the start of the check. Another factor to be
considered is the normal variation in temperature between
cylinders and between rows. This variation results from those
design features that affect the airflow past the cylinders.
Turbine Engine Maintenance
Turbine powerplant maintenance procedures vary widely
according to the design and construction of the particular
engine being serviced. The detailed procedures recommended
by the engine manufacturer should be followed when
performing inspections or maintenance. Maintenance
information presented in this section is not intended to specify
the exact manner in which maintenance operations are to be
performed but is included to convey a general idea of the
procedures involved. For inspection purposes, the turbine
engine is divided into two main sections: the cold and hot.
10-45Compressor Section
Maintenance of the compressor, or cold section, is one of
concern because damage to blades can cause engine failure.
Much of the damage to the blades arises from foreign
matter being drawn into the turbine engine air intakes. The
atmosphere near the ground is filled with tiny particles of
dirt, oil, soot, and other foreign matter. A large volume of
air is introduced into the compressor, and centrifugal force
throws the dirt particles outward so that they build up to form
a coating on the casing, the vanes, and the compressor blades.
Accumulation of dirt on the compressor blades reduces
the aerodynamic efficiency of the blades with resultant
deterioration in engine performance. The efficiency of the
blades is impaired by dirt deposits in a manner similar to that
of an aircraft wing under icing conditions. Unsatisfactory
acceleration and high exhaust gas temperature can result from
foreign deposits on compressor components.
An end result of foreign particles, if allowed to accumulate
in sufficient quantity, would be inefficiency. The condition
can be remedied by periodic inspection, cleaning, and repair
of compressor components.
Inspection & Cleaning
Minor damage to axial-flow engine compressor blades may
be repaired if the damage can be removed without exceeding
the allowable limits established by the manufacturer. Typical
compressor blade repair limits are shown in Figure 10-57 .
Well-rounded damage to leading and trailing edges that is
evident on the opposite side of the blade is usually acceptable
without re-work, provided the damage is in the outer half of
the blade only, and the indentation does not exceed values
specified in the engine manufacturer’s service and overhaul
instruction manuals. When working on the inner half of the
blade, damage must be treated with extreme caution. Repaired
compressor blades are inspected by either magnetic particle
or fluorescent penetrant inspection methods to ensure that all
traces of the damage have been removed. All repairs must be
well blended so that surfaces are smooth. [Figure 10-58] No
cracks of any extent are tolerated in any area.
Whenever possible, stoning and local re-work of the blade
should be performed parallel to the length of the blade. Re-
work must be accomplished by hand, using stones, files, or
emery cloth. Do not use a power tool to buff the entire area
of the blade. The surface finish in the repaired area must
be comparable to that of a new blade. On centrifugal flow
engines, it is difficult to inspect the compressor inducers
without first removing the air-inlet screen. After removing
the screen, clean the compressor inducer and inspect it with
a strong light. Check each vane for cracks by slowly turning
the compressor. Look for cracks in the leading edges. A crack
is usually cause for component rejection. The compressor inducers are normally the parts that are damaged by the
impingement of foreign material during engine operation.
Compressor inducers are repaired by stoning out and blending
the nicks and dents in the critical band (11⁄2 to 21⁄2 inches
from the outside edge), if the depth of such nicks or dents
does not exceed that specified in the engine manufacturer’s
service or overhaul instruction manuals. Repair nicks by
stoning out material beyond the depth of damage to remove
the resulting cold-worked metal. A generous radius must be
applied at the edges of the blend. After blending the nick, it
should be smoothed over with a crocus cloth. Pitting nicks or
corrosion found on the sides of the inducer vanes are similarly
removed by blending.
Causes of Blade Damage
Loose objects often enter an engine either accidentally or
through carelessness. Foreign object damage (FOD), such as
pencils, tools, and flashlights, are often drawn into the engine
and can cause damage to the fan blades. [Figure 10-59]
Do not carry any objects in pockets when working around
operational turbine engines.
A compressor rotor can be damaged beyond repair by tools
that are left in the air intake, where they are drawn into the
engine on subsequent starts. A simple solution to the problem
is to check the tools against a tool checklist. Prior to starting
a turbine engine, make a minute inspection of engine inlet
ducts to assure that items, such as nuts, bolts, lock wire, or
tools, were not left there after work had been performed.
Figure 10-60 shows some examples of blade damage to an
axial-flow engine. The descriptions and possible causes of
blade damage are given in Figure 10-61 . Corrosion pitting
is not considered serious on the compressor stator vanes
of axial-flow engines if the pitting is within the allowed
tolerance. Do not attempt to repair any vane by straightening,
brazing, welding, or soldering. Crocus cloth, fine files, and
stones are used to blend out damage by removing a minimum
of material and leaving a surface finish comparable to that
of a new part. The purpose of this blending is to minimize
stresses that concentrate at dents, scratches, or cracks.
The inspection and repair of air intake guide vanes, swirl
vanes, and screens on centrifugal-flow engines necessitates
the use of a strong light. Inspect screen assemblies for breaks,
rips, or holes. Screens may be tin-dipped to tighten the wire
mesh, provided the wires are not worn too thin. If the frame
strip or lugs have separated from the screen frames, re-brazing
may be necessary.
Inspect the guide and swirl vanes for looseness. Inspect the
outer edges of the guide vanes, paying particular attention to
10-46These dimensions
controlled by depth limit
Area EThese dimensions
controlled by depth limit
Area C
The limits referred to in this figure in areas C
and E pertain to local, isolated, damaged
areas only and must not be interpreted as authority for removal
of material all across the tip and leading or trailing edges
as might be done in a single machining cut.CAUTIONMaximum allowable repair limits-inches
5/16 R
1/32 D
5/32 D
.008 D
1/32 D1/4 R
1/32 D
1/8 D
.005 D
1/32 D5/16 R
1/32 D
5/32 D
NONE
1/32 D1/4 R
1/32 D
1/8 D
NONE
1/32 DSteel blades Titanium blades
Stages Stages
1 through 4 5 through 9 1 through 4 5 through 9Blade
area
A
B
C
D
E
R—radius D—depth
Area B
Area B
Area B
Area BArea AArea C
Area C
Area C
Area E Area E
Area D Fillet areaFillet areaConcave and convex surfaceConcave and convex surfaceApproximate centerlineApproximate centerlineRadiusRadius
Concave and
convex surfaceConcave and
convex surface
¼" ¼"
Damaged blade Damaged blade after blending Damaged blade Damaged blade after blending
Figure 10-57. Typical compressor blade repair limits.
Figure 10-58. Examples of repairs to damaged blades.
10-47Figure 10-59. Fan blade damage.
the point of contact between the guides and swirl vanes for
cracks and dents due to the impingement of foreign particles.
Inspect the edges of the swirl vanes. Inspect the downstream
edge of the guide vanes very closely, because cracks are
generally more prevalent in this area. Cracks that branch or
fork out so that a piece of metal could break free and fall into
the compressor are cause for vane rejection.
Blending & Replacement
Because of the thin-sheet construction of hollow vanes,
blending on the concave and convex surfaces, including the
leading edge, is limited. Small, shallow dents are acceptable
if the damage is of a rounded or gradual contour type and
not a sharp or V-type, and if no cracking or tearing of vane
material is evident in the damaged area.
Trailing edge damage may be blended, if one-third of the weld
seam remains after repair. [Figure 10-62] Concave surfaces
of rubber-filled vanes may have allowable cracks extending
inward from the outer airfoil, provided there is no suggestion
of pieces breaking away. Using a light and mirror, inspect
each guide vane trailing edge and vane body for cracks or
damage caused by foreign objects.
Any inspection and repair of turbine compressor section
components require that the technician always use the
specific manufacturer’s current information for evaluation
and limits of repairs.
Combustion Section Inspection
One of the controlling factors in the service life of the turbine
engine is the inspection and cleaning of the hot section.
Emphasis must be placed on the importance of careful
inspection and repair of this section.
The following are general procedures for performing a hot
section (turbine and combustion section) inspection. It is not intended to imply that these procedures are to be followed
when performing repairs or inspections on turbine engines.
However, the various practices are typical of those used on
many turbine engines. Where a clearance or tolerance is
shown, it is for illustrative purposes only. Always follow
the instructions contained in the applicable manufacturer's
maintenance and overhaul manuals.
The entire external combustion case should be inspected for
evidence of hot spots, exhaust leaks, and distortions before the
case is opened. After the combustion case has been opened,
the combustion chambers can be inspected for localized
overheating, cracks, or excessive wear. [Figure 10-63]
Inspect the first stage turbine blades and nozzle guide vanes
for cracks, warping, or FOD. Also inspect the combustion
chamber outlet ducts and turbine nozzle for cracks and for
evidence of FOD.
One of the most frequent discrepancies that are detected while
inspecting the hot section of a turbine engine is cracking.
These cracks may occur in many forms, and the only way
to determine that they are within acceptable limits or if
they are allowed at all, is to refer to the applicable engine
manufacturer’s service and overhaul manuals.
Cleaning the hot section is not usually necessary for a repair
in the field, but in areas of high salt water or other chemicals
a turbine rinse should be accomplished.
Engine parts can be degreased by using the emulsion-type
cleaners or chlorinated solvents. The emulsion-type cleaners
are safe for all metals, since they are neutral and noncorrosive.
Cleaning parts by the chlorinated solvent method leaves the
parts absolutely dry. If they are not to be subjected to further
cleaning operations, they should be sprayed with a corrosion-
preventive solution to protect them against rust or corrosion.
The hot section, which generally includes the combustion
section and turbine sections, normally require inspections at
regular intervals. The extent of disassembly of the engine to
accomplish this inspection varies from different engine types.
Most engines require that the combustion case be open for
the inspection of the hot section. However, in performing this
disassembly, numerous associated parts are readily accessible
for inspection. The importance of properly supporting the
engine and the parts being removed cannot be overstressed.
The alignment of components being removed and installed
is also of the utmost importance. After all the inspections
and repairs are made, the manufacturer’s detailed assembly
instructions should be followed. These instructions are
important in efficient engine maintenance, and the ultimate
life and performance of the engine. Extreme care must be
Dent
Calling
ScratchesScore
Cracks
BurrBurnDamage repair (blend)Corrosion (putting)
Term Appearance Usual Causes
• Smooth repair of ragged edge or surface into
the contour of surrounding area.
• Bent blade.
• Damage to surfaces evidenced by discoloration
or, in severe cases, by flow of material.
• A ragged or turned out edge.
• Breakdown of the surface; pitted appearance.
• A partial fracture (separation).
• Small, smoothly rounded hollow.
• A transfer of metal from one surface to another.
• Displacement of material from a surface; a
cutting or tearing effect.
• Elongation of blade.
• (See corrosion).
• Contour of a blade or surface.
• Deep scratches.
• Narrow shallow marks.• Blend
• Bow
• Burning
• Burr
• Corrosion (pits)
• Cracks
• Dent
• Gall
• Gouging
• Growth
• Pit
• Profile
• Score
• Scratch• Foreign objects.
• Excessive heat.
• Grinding or cutting operation.
• Corrosive agents—moisture, etc.
• Excessive stress due to shock, overloading, or
faulty processing; defective materials;
overheating.
• Striking of a part with a dull object.
• Severe rubbing.
• Presence of a comparatively large foreign body
between moving parts.
• Continued and/or excessive heat and
centrifugal force.
• Presence of chips between surfaces.
• Sand or fine foreign particles; careless
handling.Figure 10-60. Compressor blade damage.
Figure 10-61. Blade maintenance terms.
10-49Figure 10-62. Guide vane trailing edge damage.
Before blending
After blending
Figure 10-63. Combustion case inspection.
taken during assembly to prevent dirt, dust, cotter pins, lock
wire, nuts, washers, or other foreign material from entering
the engine.
Marking Materials for Combustion Section Parts
Certain materials may be used for temporary marking during
assembly and disassembly. Always refer to manufacturer’s
information for marking parts. Layout dye (lightly applied),
a felt tip marker, or chalk may be used to mark parts that are directly exposed to the engine’s gas path, such as turbine
blades and discs, turbine vanes, and combustion chamber
liners. A wax marking pencil may be used for parts that are
not directly exposed to the gas path. Do not use a wax marking
pencil on a liner surface or a turbine rotor. The use of carbon
alloy or metallic pencils is not recommended because of the
possibility of causing intergranular corrosion attack, that
could result in a reduction in material strength and cracking.
Inspection & Repair of Combustion Chambers
Inspect the combustion chambers and covers for cracks by
using visible dye or fluorescent penetrant inspection method.
Any cracks, nicks, or dents are usually cause for rejecting the
component. Inspect the covers, noting particularly the area
around the fuel drain bosses for any pits or corrosion. When
repairing the combustion chamber liner, the procedures given
in the appropriate engine manufacturer’s overhaul instruction
manual should be followed. If there is doubt that the liner is
serviceable, it should be replaced.
Combustion chambers should be replaced or repaired if two
cracks are progressing from a free edge so that their meeting
is imminent and could allow a piece of metal that could
cause turbine damage to break loose. Separate cracks in
the baffle are acceptable. Cracks in the cone are rare but, at
any location on this component, is cause for rejection of the
liner. Cracks in the swirl vanes are cause for rejection of the
liner. Loose swirl vanes may be repaired by silver brazing.
Cracks in the front liner emanating from the air holes are
acceptable, provided they do not exceed allowable limits.
If such cracks fork or link with others, the liner must be
repaired. If two cracks originating from the same air hole are
diametrically opposite, the liner is acceptable. Radial cracks
extending from the interconnector and spark igniter boss
are acceptable, if they do not exceed allowable limits and if
such cracks do not fork or link with others. Circumferential
10-50Figure 10-64. Stress rupture cracks.
cracks around the boss pads should be repaired prior to re-use
of the liner. Baffle cracks connecting more than two holes
should be repaired.
After long periods of engine operation, the external surfaces
of the combustion chamber liner location pads often show
signs of fretting. This is acceptable, provided no resultant
cracks or perforation of the metal is apparent. Any cover
or chamber inadvertently dropped on a hard surface or
mishandled should be thoroughly inspected for minute
cracks that may elongate over a period of time and then open,
creating a hazard.
Parts may be found where localized areas have been heated
to an extent to buckle small portions of the chamber. Such
parts are considered acceptable if the burning of the part
has not progressed into an adjacent welded area, or to such
an extent as to weaken the structure of the liner weldment.
Buckling of the combustion chamber liner can be corrected
by straightening the liner. Moderate buckling and associated
cracks are acceptable in the row of cooling holes. More severe
buckling that produces a pronounced shortening or tilting
of the liner is cause for rejection. Upon completion of the
repairs by welding, the liner should be restored as closely as
possible to its original shape.
Fuel Nozzle & Support Assemblies
Clean all carbon deposits from the nozzles by washing with
a cleaning fluid approved by the engine manufacturer and
remove the softened deposits with a soft bristle brush. It
is desirable to have filtered air passing through the nozzle
during the cleaning operation to carry away deposits as they
are loosened. Make sure all parts are clean. Dry the assemblies
with clean, filtered air. Because the spray characteristics of
the nozzle may become impaired, no attempt should be made
to clean the nozzles by scraping with a hard implement or
by rubbing with a wire brush. Inspect each component part
of the fuel nozzle assembly for nicks and burrs. Many fuel
nozzles can be checked by flowing fluid through the nozzle
under pressure and closely checking the flow pattern coming
for the nozzle.
Turbine Disc Inspection
The inspection for cracks is very important because cracks are
not normally allowed. Crack detection, when dealing with the
turbine disc and blades, is mostly visual, although structural
inspection techniques can be used, such as penetrant
methods and others to aid in the inspection. Cracks on the
disc necessitate the rejection of the disc and replacement of
the turbine rotor. Slight pitting caused by the impingement
of foreign matter may be blended by stoning and polishing.Turbine Blade Inspection
Turbine blades are usually inspected and cleaned in the
same manner as compressor blades. However, because of
the extreme heat under which the turbine blades operate,
they are more susceptible to damage. Using a strong light
and a magnifying glass, inspect the turbine blades for
stress rupture cracks and deformation of the leading edge.
[Figures 10-64 and 10-65]
Stress rupture cracks usually appear as minute hairline
cracks on or across the leading or trailing edge at a right
angle to the edge length. Visible cracks may range in length
from one-sixteenth inch upward. Deformation, caused by
over-temperature, may appear as waviness and/or areas of
varying airfoil thickness along the leading edge. The leading
edge must be straight and of uniform thickness along its
entire length, except for areas repaired by blending. Do not
confuse stress rupture cracks or deformation of the leading
edge with foreign material impingement damage or with
10-51Figure 10-65. Turbine blade waviness.
½"½"
¼"Nicks at tip edge not critical
blend raised edge only
Examine carefully for indication
of cracks at edges of fir-tree
serrations. Reject for any
indication of cracks.
Tip curling permissible in
these areas, if under certain
circumstances.Tip curling permissible in
these areas, if under certain
circumstances.Nicks over .008-inch deep
are cause for rejection
except that if nicks are
slightly deeper than .006-inch
but do not exceed .012-inch
in depth and are well away
from the lead or trail edge
the blade is acceptable for
continued use.Nicks on the convex surface away from the leading
and the trailing edges need not be completely
blended out. Same for the concave surface.
Nicks on the trailing edge must be
completely removed. If too much
material must be removed, reject blade.Entire trailing edge is critical area.
Nicks at outer tip end not as critical as
nicks nearer the root end, since the
nearer the root end, the greater the
moment-weight tending to produce a
crack.Nicks that “come through” to underside
of blade are cause for rejection.
Nicks on the leading edge
must be completely blended
out. If too much material must
be removed, reject blade.
Figure 10-66. Typical turbine blade inspection. blending repairs to the blade. When any stress rupture cracks
or deformation of the leading edges of the first-stage turbine
blades are found, an over-temperature condition must be
suspected. Check the individual blades for stretch and the
turbine disc for hardness and stretch. Blades removed for
a detailed inspection or for a check of turbine disc stretch
must be re-installed in the same slots from which they were
removed. Number the blades prior to removal.
The turbine blade outer shroud should be inspected for air seal
wear. If shroud wear is found, measure the thickness of the
shroud at the worn area. Use a micrometer or another suitable
and accurate measuring device that ensures a good reading
in the bottom of the comparatively narrow wear groove. If
the remaining radial thickness of the shroud is less than that
specified, the stretched blade must be replaced. Typical blade
inspection requirements are indicated in Figure 10-66 . Blade
tip curling within a one-half inch square area on the leading
edge of the blade tip is usually acceptable if the curling is not
sharp. Curling is acceptable on the trailing edge if it does not
extend beyond the allowable area. Any sharp bends that may
result in cracking or a piece breaking out of the turbine blade
is cause for rejection, even though the curl may be within
the allowable limits. Each turbine blade should be inspected
for cracks.
Turbine Blade Replacement Procedure
Turbine blades are generally replaceable, subject to moment-
weight limitations. These limitations are contained in the
DK
1DV
54DU
53 DU
52 DU
51DU50
DU49
DU48
DT47
DT46
DT45
DS44
DR43
DR42DP41DP40DN39DN38DM37DM36DM
35DL
34DL
33DL
32DL
31DL
30DK
29DK
28DV
27DU
26 DU
25 DU
24DU23
DU22
DU21
DT20
DT19
DT18
DS17
DR16
DR15DP14DP13DN12DN11DM10DM
9DM
8DL
7DL
6DL
5DL
4DL
3DK
Spherical indent on face of discs
Clockwise
Viewed from rearViewed from rear
Figure 10-67. Typical turbine rotor blade moment-weight
distribution. Figure 10-68. Turbine blades.
for inspection. The blade limits specified in the engine
manufacturer’s overhaul and service instruction manual
should he adhered to. Figure 10-69 shows where cracks
usually occur on a turbine nozzle assembly. Slight nicks and
dents are permissible if the depth of damage is within limits.
Inspect the nozzle vanes for nicks or cracks. Small nicks
are not cause for vane rejection, provided such nicks blend
out smoothly.
Inspect the nozzle vane supports for defects caused by the
impingement of foreign particles. Use a stone to blend any
doubtful nicks to a smooth radius. Like turbine blades, it is
possible to replace a maximum number of turbine nozzle
vanes in some engines. If more than the maximum vanes
are damaged, a new turbine nozzle vane assembly must be
installed. With the tailpipe (exhaust nozzle) removed, the rear
turbine stage can be inspected for any cracks or evidence of
blade stretch. Additional nozzle stages can also be inspected
with a strong light by looking through the rear-stage turbine.
Clearances
Checking the clearances is one of the procedures in the
maintenance of the turbine section of a turbine engine.
The manufacturer’s service and overhaul manual gives the
procedures and tolerances for checking the turbine. Turbine
clearances being measured at various locations are shown
in Figures 10-70 and 10-71 . To obtain accurate readings,
special tools provided by each manufacturer must be used engine manufacturer’s applicable technical instructions. If
visual inspection of the turbine assembly discloses several
broken, cracked, or eroded blades, replacing the entire
turbine assembly may be more economical than replacing
the damaged blades. [Figure 10-67]
In the initial buildup of the turbine, a complete set of 54
blades made in coded pairs (two blades having the same
code letters) is laid out on a bench in the order of diminishing
moment-weight. The code letters, indicating the moment-
weight balance in ounces, are marked on the rear face of the
fir-tree section of the blade (viewing the blade as installed
at final assembly of the engine). The pair of blades having
the heaviest moment-weight is numbered 1 and 28; the
next heaviest pair of blades is numbered 2 and 29; the third
heaviest pair is numbered 3 and 30. This is continued until
all the blades have been numbered. Mark a number 1 on the
face of the hub on the turbine disc. The number 1 blade is then
installed adjacent to the number 1 on the disc. [Figure 10-68]
The remaining blades are then installed consecutively in a
clockwise direction, viewed from the rear face of the turbine
disc. If there are several pairs of blades having the same
code letters, they are installed consecutively before going
to the next code letters. If a blade requires replacement, the
diametrically opposite blade must also be replaced. Computer
programs generally determine the location for turbine blades
for turbine wheels on modern engines.
Turbine Nozzle Inlet Guide Vane Inspection
After removing the required components, the first stage
turbine blades and turbine nozzle vanes are accessible
Turbine nozzle assembly
Spot weld cracks on inner ductCracked area along spot weld line on inner ductTurbine nozzle assembly at junction of combustion
chamber outlet duct and turbine nozzle outer case
Figure 10-69. Turbine nozzle assembly defects.
Figure 10-70. Measuring the turbine blades to shroud (tip)
clearances.
Figure 10-71. Measuring turbine wheel to exhaust cone clearance. as described in the service instructions for specific engines.
Exhaust Section
The exhaust section of the turbine engine is susceptible to
heat cracking. This section must be thoroughly inspected
along with the inspection the combustion section and turbine
section of the engine. Inspect the exhaust cone and exhaust
nozzle for cracks, warping, buckling, or hot spots. Hot spots
on the tail cone are a good indication of a malfunctioning
fuel nozzle or combustion chamber.
The inspection and repair procedures for the hot section
of any one gas turbine engine share similarities to those
of other gas turbine engines. One usual difference is the
nomenclature applied to the various parts of the hot section
by the different manufacturers. Other differences include the
manner of disassembly, the tooling necessary, and the repair
methods and limits.
Engine Ratings
The flat rating of a turbine engine is the thrust performance
that is guaranteed by the manufacturer for a new engine under
specific operating conditions, such as takeoff, maximum
continuous climb, and cruise power settings. The turbine inlet
temperature is proportional to the energy available to turn
the turbine. This means that the hotter the gases are that are
entering the turbine section of the engine, the more power is
available to turn the turbine wheel. The exhaust temperature
is proportional to the turbine inlet temperature. Regardless of
how or where the exhaust temperature is taken on the engine
for the flight deck reading, this temperature is proportional to
the temperature of the exhaust gases entering the first stage
of inlet guide vanes. A higher EGT corresponds to a larger
amount of energy to the turbine so it can turn the compressor
faster. This works fine until the temperature reaches a point
when the turbine inlet guide vanes start to be damaged. EGT
must be held constant or lowered as the result of a prolonged
hot section life and, at the same time, provide the thrust to
meet the certification requirements.
Before high bypass turbofan engines, some older types of
engines used water injection to increase thrust for takeoff
(wet). This is the maximum allowable thrust for takeoff. The
rating is obtained by actuating the water-injection system and
setting the computed wet thrust with the throttle, in terms of a
predetermined turbine discharge pressure or engine pressure
ratio for the prevailing ambient conditions. The rating is
restricted to takeoff, is time-limited, and has an altitude
limitation. Water injection is not used very much on turbine
engines any more.Turbine Engine Instruments
Engine Pressure Ratio Indicator
Engine pressure ratio (EPR) is an indication of the thrust
being developed by a turbofan engine and is used to set power
for takeoff on many types of aircraft. It is instrumented by
total pressure pickups in the engine inlet (Pt2) and in the
turbine exhaust (Pt7). The reading is displayed in the flight
deck by the EPR gauge, which is used in making engine
power settings. [Figure 10-72]
PRESSURE
RATIO34
Figure 10-72. Engine pressure ratio indications. Torquemeter (Turboprop Engines)
Only 10 to 15 percent of the thrust produced by a turboprop
engine is from propulsive force derived from the jet thrust
exiting the exhaust. Engine pressure ratio is not used as an
indicator of the power produced by a turboprop engine.
Turboprops are usually fitted with a torquemeter that
measures torque applied to a shaft turned by the gas generator
and power turbines of the turbine engine. The torquemeter can
be operated by engine oil pressure metered through a valve
that is controlled by a helical ring gear that moves in response
to the applied torque. [Figure 10-73] This gear moves against
a piston that controls the opening of a valve, which controls
the oil pressure flow. This action makes the oil pressure
proportional to torque being applied at the propeller shaft.
Generally, transducer is used to transfer the oil pressure into
an electrical signal to be read by the flight deck instrument.
The read out in the flight deck is normally in lb/ft of torque,
or percent horsepower. The torquemeter is very important
as it is used to set power settings. This instrument must be
calibrated at intervals to assure its accuracy.
Tachometer
Gas turbine engine speeds are measured by the engines
rpm, which are also the compressor/turbine combination
rpm of each rotating spool. Most turbofan engines have
two or more spools, compressor, and turbine sections that
turn independently at different speeds. Tachometers are
usually calibrated in percent rpm so that various types of
engines can be operated on the same basis of comparison.
[Figure 10-73] Also, turbine speeds are generally very high,
and the large numbers of rpm would make it very confusing.
Turbofan engines with two spools or separate shafts, high pressure and low pressure spools, are generally referred to as
N1 and N2, with each having their own indicator. The main
purpose of the tachometer is to be able to monitor rpm under
normal conditions, during an engine start, and to indicate an
overspeed condition, if one occurs.
Exhaust Gas Temperature Indicator (EGT)
Exhaust gas temperature (EGT), turbine inlet temperature,
(TIT), turbine gas temperature (TGT), interstage turbine
temperature (ITT), and turbine outlet temperature (TOT) are
all relative temperatures used to monitor the temperature of
the exhaust gases entering the first stage turbine inlet guide
vanes. Even though these temperatures are taken at different
locations on the engine (each engine having one location),
they are all relative to the temperature of the gases entering
the first stage turbine inlet guide vanes.
Temperature is an engine operating limit and is used to
monitor the mechanical integrity of the turbines, as well as to
check engine operating conditions. Actually, the temperature
of the gases entering the first stage turbine inlet guide vanes
is the important consideration, since it is the most critical
of all the engine variables. However, it is impractical to
measure turbine inlet temperature in most engines, especially
large engines. Consequently, temperature thermocouples
are inserted at the turbine discharge, where the temperature
provides a relative indication of that at the inlet. Although
the temperature at this point is much lower than at the inlet,
it provides surveillance over the engine’s internal operating
conditions. Several thermocouples are usually used, that
are spaced at intervals around the perimeter of the engine
exhaust duct near the turbine exit. The EGT indicator in the
flight deck shows the average temperature measured by the
individual thermocouples. [Figure 10-73]
Fuel-Flow Indicator
Fuel-flow instruments indicate the fuel flow in pounds per
hour (lb/hr) from the engine fuel control. Fuel flow in turbine
aircraft is measured in lb/hr instead of gallons, because
the fuel weight is a major factor in the aerodynamics of
large turbine aircraft. Fuel flow is of interest in monitoring
fuel consumption and checking engine performance.
[Figure 10-73]
Engine Oil Pressure Indicator
To guard against engine failure resulting from inadequate
lubrication and cooling of the various engine parts, the
oil supply to critical areas must be monitored. The oil
pressure indicator usually shows the engine oil pump
discharge pressure.
Engine Oil Temperature Indicator
The ability of the engine oil to lubricate and cool depends
on the temperature of the oil, as well as the amount of
OIL PRESS
PSI80
FUEL FLOW
EXT GAS
TEMP
PSI
TORQUEMETER
PERCENT
RPM
8050403020100
°C
OIL5
4150-50100050
Figure 10-73. Typical turbine engine instruments.
10-57oil supplied to the critical areas. An oil inlet temperature
indicator frequently is provided to show the temperature
of the oil as it enters the oil pressure pump. Oil inlet
temperature is also an indication of proper operation of the
engine oil cooler.
Turbine Engine Operation
The engine operating procedures presented here apply
generally to turbofan, turboprop, turboshaft, and auxiliary
power units (APU). The procedures, pressures, temperatures,
and rpm that follow are intended primarily to serve as a
guide. It should be understood that they do not have general
application. The manufacturer’s operating instructions
should be consulted before attempting to start and operate
any turbine engine.
A turbofan engine has only one power control lever. Adjusting
the power lever, or throttle lever, sets up a thrust condition
for which the fuel control meters fuel to the engine. Engines
equipped with thrust reversers go into reverse thrust at throttle
positions below idle. A separate fuel shutoff lever is usually
provided on engines equipped with thrust reversers.
Prior to start, particular attention should be paid to the
engine air inlet, the visual condition and free movement of
the compressor and turbine assembly, and the parking ramp
area fore and aft of the aircraft. The engine is started by using
an external air power source, APU, or an already operating
engine. Starter types and the engine starting cycle have been
discussed previously. On multi-engine aircraft, the engines
are usually started by an onboard APU that supplies the air
pressure for a pneumatic starter on each engine. Air bled from
the APU is used as a source of power for starting the engines.
During the start, it is necessary to monitor the tachometer,
the oil pressure, and the exhaust gas temperature. The normal
starting sequence is:
1. Rotate the compressor with the starter;
2. Turn the ignition on; and
3. Open the engine fuel valve, either by moving the
throttle to idle or by moving a fuel shutoff lever or
turning a switch.
Adherence to the procedure prescribed for a particular engine
is necessary as a safety measure and to avoid a hot or hung
start. A successful start is noted first by a rise in exhaust gas
temperature. If the engine does not light up, meaning that fuel
starts to burn inside of the engine within a prescribed period
of time, or if the exhaust gas starting temperature limit is
exceeded, a hot start, the starting procedure should be aborted.
Hot starts are not common, but when they do occur, they can
usually be stopped in time to avoid excessive temperature by observing the exhaust gas temperature constantly during the
start. When necessary, the engine is cleared of trapped fuel or
gases by continuing to rotate the compressor with the starter,
but with the ignition and fuel turned off. If the engine did not
light off during start after the allotted time, about 10 seconds
although this time varies from engine to engine, the fuel must
be shut off as the engine is being filled with unburned fuel. A
hung start is when the engine lights off, but the engine will
not accelerate to idle rpm.
Ground Operation Engine Fire
Move the fuel shutoff lever to the off position if an engine
fire occurs, or if the fire warning light is illuminated during
the starting cycle. Continue cranking or motoring the engine
until the fire has been expelled from the engine. If the fire
persists, CO 2 can be discharged into the inlet duct while it is
being cranked. Do not discharge CO 2 directly into the engine
exhaust, because it may damage the engine. If the fire cannot
be extinguished, secure all switches and leave the aircraft. If
the fire is on the ground under the engine overboard drain,
discharge the CO 2 on the ground rather than on the engine.
This also is true if the fire is at the tailpipe and the fuel is
dripping to the ground and burning.
Engine Checks
Checking turbofan engines for proper operation consists
primarily of simply reading the engine instruments and then
comparing the observed values with those known to be correct
for any given engine operating condition. After the engine
has started, idle rpm has been attained, and the instrument
readings have stabilized, the engine should be checked
for satisfactory operation at idling speed. The oil pressure
indicator, tachometer, and the exhaust gas temperature
readings should be compared with the allowable ranges.
Checking Takeoff Thrust
Takeoff thrust is checked by adjusting the throttle to obtain
a single, predicted reading on the engine pressure ratio
indicator in the aircraft. The value for engine pressure ratio,
which represents takeoff thrust for the prevailing ambient
atmospheric conditions, is calculated from a takeoff thrust
setting curve or, on newer aircraft, is a function of the onboard
computer. This curve has been computed for static conditions.
[Figure 10-74] Therefore, for all precise thrust checking,
the aircraft should be stationary, and stable engine operation
should be established. If it is needed for calculating thrust
during an engine trim check, turbine discharge pressure (Pt7)
is also shown on these curves. Appropriate manuals should
be consulted for the charts for a specific make and model
engine. Engine trimming procedure is also covered in Chapter
2, Engine Fuel & Fuel Metering Systems. The engine pressure
ratio computed from the thrust setting curve represents thrust
or a lower thrust call part power thrust used for testing. The
10-5828.00−40 −20 0 +20 +40
31.92Ambient temperature −°C
Engine pressure ratio − P†7/P†2
Turbine discharge pressure − P†7 −IN. H g Field barometric pressure "Hg
Interpolate to locate these
points
Example:
Ambient temperature = 20°C
Field barometric
pressure = 30.50 "Hg
Adjust throttle to obtain EPR
determined at point A or the R†7
valve determined at point BBurner pressure limit24.0025.0026.0027.0028.0029.00
A
B
Figure 10-74. Typical takeoff thrust setting curve for static conditions. aircraft throttle is advanced to obtain this predicted reading
on the engine pressure ratio indicator, or the part power stop
is engaged in the aircraft. If an engine develops the predicted
thrust and if all the other engine instruments are reading
within their proper ranges, engine operation is considered
satisfactory. Full authority digital engine controls (FADEC)
(computer controls) also have means of checking the engine
with the results displayed on the flight deck.
Ambient Conditions
The sensitivity of gas turbine engines to compressor inlet air
temperature and pressure necessitates that considerable care
be taken to obtain correct values for the prevailing ambient
air conditions when computing takeoff thrust. Some things
to remember are:
1. The engine senses the air temperature and pressure at
the compressor inlet. This is the actual air temperature
just above the runway surface. When the aircraft is
stationary, the pressure at the compressor inlet is
the static field or true barometric pressure, and not
the barometric pressure corrected to sea level that
is normally reported by airport control towers as the
altimeter setting. On FADEC engines, the computer
reads this information and sends it to the engine
controls.2. Temperature sensed is the total air temperature (TAT)
that is used by several onboard computers. The engine
controls set the engine computers according to the
TAT.
3. Relative humidity, which affects reciprocating engine
power appreciably, has a negligible effect on turbine
engine thrust, fuel flow, and rpm. Therefore, relative
humidity is not usually considered when computing
thrust for takeoff or determining fuel flow and rpm
for routine operation.
Engine Shutdown
On turbine engines that have a thrust reverser, retarding the
aircraft throttle to idle or power lever to OFF cuts the fuel
supply to the engine and shuts down the engine. On engines
equipped with thrust reversers, this is accomplished by means
of a separate fuel shutoff lever or switch. When an engine
has been operated at high power levels for extended periods
of time, a cool down time should be allowed before shutting
down. It is recommended the engine be operated at below a
low power setting, preferably at idle for a period of 5 minutes
to prevent possible seizure of the rotors. This applies, in
particular, to prolonged operation at high rpm on the ground,
such as during engine trimming. The turbine case and the
turbine wheels operate at approximately the same temperature
10-59when the engine is running. However, the turbine wheels
are relatively massive, compared with the case, and are not
cooled so readily. The turbine case is exposed to cooling
air from both inside and outside the engine. Consequently,
the case and the wheels lose their residual heat at different
rates after the engine has been shut down. The case, cooling
faster, tends to shrink upon the wheels, that are still rotating.
Under extreme conditions, the turbine blades may squeal or
seize; thus, a cooling period is required if the engine has been
operating at prolonged high speed. Should the turbine wheels
seize, no harm normally results, provided no attempt is made
to turn the engine over until it has cooled sufficiently to free
the wheels. In spite of this, every effort should be made to
avoid seizure.
To ensure that fuel remains in the lines and that the engine-
driven fuel pumps are not starved for fuel that lubricates the
pumps, the aircraft fuel boost pump must be turned off after,
not before, the throttle or the fuel shutoff lever is placed in
the OFF position.
Generally, an engine should not be shut down by the fuel
shutoff lever until after the aircraft throttle has been retarded
to idle. Because the fuel shutoff valve is located on the fuel
control discharge, a shutdown from high thrust settings results
in high fuel pressures within the control that can harm the
fuel system parts.
When an accurate reading of the oil level in the oil tank is
needed following an engine shutdown, the engine should
be operated and shut down with the oil check taking place
within not more than 30 minutes after shutdown. Check the
engine manuals for the specific procedure.
Troubleshooting Turbine Engines
Included in this section are typical guidelines for locating
engine malfunctions on most turbine engines. Since it would
be impractical to list all the malfunctions that could occur,
only the most common malfunctions are covered. A thorough
knowledge of the engine systems, applied with logical
reasoning, solves most problems that may occur.
Figure 10-75 enumerates some malfunctions that may be
encountered. Possible causes and suggested actions are given
in the adjacent columns. The malfunctions presented herein
are solely for the purpose of illustration and should not be
construed to have general application. For exact information
about a specific engine model, consult the applicable
manufacturer’s instructions.
Turboprop Operation
Turboprop engine operation is quite similar to that of a
turbojet engine, except for the added feature of a propeller. The starting procedure and the various operational features
are very much alike. The turboprop chiefly requires attention
to engine operating limits, the throttle or power lever setting,
and the torquemeter pressure gauge. Although torquemeters
indicate only the power being supplied to the propeller and
not the equivalent shaft horsepower, torquemeter pressure
is approximately proportional to the total power output
and, thus, is used as a measure of engine performance. The
torquemeter pressure gauge reading during the takeoff engine
check is an important value. It is usually necessary to compute
the takeoff power in the same manner as is done for a turbojet
engine. This computation is to determine the maximum
allowable exhaust gas temperature and the torquemeter
pressure that a normally functioning engine should produce
for the outside, or ambient, air temperature and barometric
pressure prevailing at the time.
Troubleshooting Procedures for Turboprop Engines
All test run-ups, inspections, and troubleshooting
should be performed in accordance with the applicable
engine manufacturer’s instructions. In Figure 10-76 , the
troubleshooting procedure for the turboprop reduction gear,
torquemeter, and power section are combined because of their
inter-relationships. The table includes the principal troubles,
together with their probable causes and remedies.
Turbine Engine Calibration & Testing
Some of the most important factors affecting turbine engine
life are EGT, engine cycles (a cycle is generally a takeoff
and landing) and engine speed. Excess EGT of a few degrees
reduces turbine component life. Low EGT materially reduces
turbine engine efficiency and thrust. So, to make the engine
highly efficient, the exhaust temperatures need to be as high
as possible, while maintaining an EGT operating temperature
that does not damage the turbine section of the engine. If the
engine is operated at excess exhaust temperatures, engine
deterioration occurs. Since the EGT temperature is set by the
EGT temperature gauge, it is imperative that it is accurate.
Excessive engine speed can cause premature engine wear
and, if extreme, can cause engine failure.
One older type of calibration test unit used to analyze the
turbine engine is the jetcal analyzer. [Figure 10-77] A jetcal
analyzer is a portable instrument made of aluminum, stainless
steel, and plastic. The major components of the analyzer
are the thermocouple, rpm, EGT indicator, resistance, and
insulation check circuits, as well as the potentiometer,
temperature regulators, meters, switches, and all the necessary
cables, probes, and adapters for performing all tests.
Turbine Engine Analyzer Uses
Many different types of analyzers are used each with its own
function, including onboard systems that use computers to
10-60Indicated Malfunction Probable Causes Suggested Action
• Engine pressure ratio indication has high
reading error.
• Engine pressure ratio indication has low
reading error due to:
- Misaligned or cracked turbine discharge
probe.
- Leak in turbine discharge pressure line from
probe to transmitter.
- Inaccurate engine pressure ratio transmitter
or indicator.
- Carbon particles collected in turbine
discharge pressure line or restrictor orifices.
• Possible turbine damage and/or loss of turbine
efficiency.
• If only exhaust gas temperature is high, other
parameters normal, the problem may be
thermocouple leads or instrument.
• Turbine damage.
• Damage in compressor section.
• Engine-mounted accessory such as
constant-speed drive, generator, hydraulic
pump, etc.
• Engine main bearings.
• Engine bleed-air valve malfunction.
• Turbine discharge pressure probe or line to
transmitter leaking.
• Engine out of trim. Engine has low rpm, exhaust
gas temperature, and fuel
flow when set to expected
engine pressure ratio.
Engine has high rpm, exhaust
gas temperature, and fuel
flow when set to expect
engine pressure ratio.
Engine has high exhaust gas
temperature, low rpm, and
high fuel flow at all engine
pressure ratio settings.
Note: Engines with damage
in turbine section may have
tendency to hang up during
starting.
Engine vibrates throughout
rpm range, but indicated
amplitude reduces as rpm is
reduced.
Engine vibrates at high rpm
and fuel flow when compared
to constant engine pressure
ratio.
Engine vibrates throughout
rpm range, but is more
pronounced in cruise or idle
rpm range.
No change in power setting
parameters, but oil
temperature high.
Engine has higher than
normal exhaust gas
temperature during takeoff,
climb, and cruise. Rpm and
fuel flow higher than normal.
Engine has high exhaust gas
temperature at target engine
pressure ratio for takeoff.• Check inlet pressure line from probe to
transmitter for leaks.
• Check engine pressure ratio transmitter and
indicator for accuracy.
• Check probe condition.
• Pressure-test turbine discharge pressure line
for leaks.
• Check engine pressure ratio transmitter and
indicator for accuracy.
• Confirm indication of turbine damage by:
- Checking engine coast-down for abnormal
noise and reduced time.
- Visually inspect turbine area with strong light.
• Re-calibrate exhaust gas temperature
instrumentation.
• Check turbine as outlined in preceding item.
• Check compressor section for damage.
• Check each component in turn.
• Check scavenge oil filters and magnetic plugs.
• Check operation of bleed valve.
• Check condition of probe and pressure line to
transmitter.
• Check engine with jetcal. Re-trim as desired.
Figure 10-75. Troubleshooting turbojet engines.
10-61Indicated Malfunction Probable Causes Suggested Action
• Pressurizing and drain valve malfunction.
• Cracked air duct.
• Fuel control malfunction.
• Subzero ambient temperatures.
• Compressor section damage.
• Turbine section damage.
• Scavenge pump failure.
• Fuel heater malfunction.
• Scavenge pump failure.
• High sump pressure.
• Gearbox seal leakage.
• Can be caused by high airflow through the
tank, foaming oil, or unusual amounts of oil
returned to the tank through the vent system. Engine rumbles during
starting and at low power
cruise conditions.
Engine rpm hangs up during
starting.
High oil temperature.
High oil consumption.
Overboard oil loss.• Replace pressurizing and drain valves.
• Repair or replace duct.
• Replace fuel control.
• If hang-up is due to low ambient temperature,
engine usually can be started by turning on
fuel booster pump or by positioning start lever
to run earlier in the starting cycle.
• Check compressor for damage.
• Inspect turbine for damage.
• Check lubricating system and scavenge
pumps.
• Replace fuel heater.
• Check scavenge pumps.
• Check sump pressure as outlined in
manufacturer’s maintenance manual.
• Check gearbox seal by pressurizing overboard
vent.
• Check oil for foaming.
• Vacuum-check sumps.
• Check scavenge pumps.
Figure 10-75. Troubleshooting turbojet engines (continued).
test aircraft systems. Depending upon the specific analyzer
used, procedures vary somewhat, but the basic test are
outlined here. Always refer to the specific instructions
associated with the analyzer being used.
Most analyzers may be used to:
1. Functionally check the aircraft EGT system for error,
without running the engine or disconnecting the
wiring.
2. Check individual thermocouples before placement in
a parallel harness.
3. Check each engine thermocouple in a parallel harness
for continuity.
4. Check the thermocouples and parallel harness for
accuracy.
5. Check the resistance of the EGT circuit.
6. Check the insulation of the EGT circuit for shorts to
ground, or for shorts between leads.
7. Check EGT indicators , either in or out of the aircraft,
for error.
8. Determine engine rpm accuracy during engine testing.
Added to this is the checking and troubleshooting of
the aircraft tachometer system.9. Establish the proper relationship between the EGT
and engine rpm during engine run-up.
Analyzer Safety Precautions
Observe the following safety precautions while operating the
engine analyzer or other types of test equipment:
1. Never use a voltammeter to check the potentiometer
for continuity. If a voltammeter is used, damage to the
galvanometer and standard battery cell results.
2. Check the thermocouple harness before engine run-up.
This must be done because the circuit must be correct
before the thermocouples can be used for true EGT
pickup.
3. For safety, ground the jetcal analyzer when using an
AC power supply. Any electrical equipment operated
on AC power and utilizing wire-wound coils, such as
the probes with the jetcal analyzer, has an induced
voltage on the case that can be discharged if the
equipment is not grounded. This condition is not
apparent during dry weather, but on damp days the
operator can be shocked slightly. Therefore, for the
operator’s protection, the jetcal analyzer should be
grounded using the pigtail lead in the power inlet
cable.
4. Use heater probes designed for use on the engine
10-62Trouble Probable causes Remedy
• No air to starter.
• Propeller brake locked.
• Starter speed low because of inadequate air
supply to starter.
• If fuel is not observed leaving the exhaust pipe
during start, fuel selector valve may be
inoperative because of low power supply or
may be locked in “OFF.”
• Fuel pump inoperative.
• Aircraft fuel filter dirty.
• Fuel control cutoff valve closed.
• Insufficient fuel supply to control unit.
• Fuel control main metering valve sticking.
• Fuel control bypass valve sticking open.
• Drain valve stuck open. Starting fuel
enrichment pressure switch setting too high.
• Fuel control bypass valve sticking closed.
• Fuel control acceleration cam incorrectly
adjusted.
• Defective fuel nozzle.
• Fuel control thermostat failure.
• Acceleration cam of fuel control incorrectly
adjusted.
• Unstable fuel control governor operation.
• Oil supply low.
• Oil pressure transmitter or indicator giving false
indication.
• Seal failure.
• Faulty propeller governor.
• Faulty fuel control or air sensing tip.
• Vibration pickup or vibration meter malfunction. Power unit fails to turn over
during attempted start.
Power unit fails to start.
Engine fires, but will not
accelerate to correct speed.
Acceleration temperature too
high during starting.
Acceleration temperature
during starting too low.
Engine speed cycles after
start.
Power unit oil pressure drops
off severely.
Oil leakage at accessory
drive seals.
Engine unable to reach
maximum controlled speed of
100 percent.
Vibration indication high.• Check started air valve solenoid and air supply.
• Unlock brake by turning propeller by hand in
direction of normal rotation.
• Check starter air valve solenoid and air supply.
• Check power supply or electrically operated
valves. Replace valves if defective.
• Check pump for sheared drives or internal
damage. Check for air leaks at outlet.
• Clean filter and replace filtering elements if
necessary.
• Check electrical circuit to ensure that actuator
is being energized. Replace actuator or control.
• Check fuel system to ensure all valves are
open and pumps are operative.
• Flush system. Replace control.
• Flush system. Replace control.
• Replace drain valve. Replace pressure switch.
• Flush system. Replace control.
• Replace control.
• Replace nozzle with a known satisfactory unit.
• Replace control.
• Replace control.
• Continue engine operation to allow control to
condition itself.
• Check oil supply and refill as necessary.
• Check transmitter or indicator and repair or
replace if necessary.
• Replace seal or seals.
• Replace propeller control assembly.
• Replace faulty control. If dirty, use air pressure
in reverse direction of normal flow through
internal engine passage and sensing tip.
• Calibrate vibration meter.
• Start engine and increase power gradually.
• Observe vibration indicator. If indications prove
pickup to be at fault, replace it. If high vibration
remains as originally observed, remove power
unit for overhaul.
Figure 10-76. Troubleshooting turboprop engines.
GALVO-2
GALVO-3RPM
SW-9MECH ZEROMECH ZERO
MECH ZERO
OFF RANGE
MECH ZEROSELECTOR
SWITCH
RANGE ELEC
ZERG
LEG
ZEROR - 3 R 10
R 1R 100110V 60-400RPM SW-4
REG INSUL
HEATER CABLE CHECK CABLESW-3EGT IND CHECK
RESISTANCE CHECK
THERMOCOUPLE CHECKADJ RHEOINPUT
P-3
P-3
S-1 S-2R-1
2 AMP 20 AMPR 1000
R-2
SW-9SW-1EGT
T/C RES
OFF INSUL
TEMP REGULATOR
BATTERY
OPERATIONRPM CHECK
POTENTIOMETER INSULATION
CHECKRPM check (takeal unit)
Heater cable receptacle (S-1) Check cable receptacle (S-2)
Potentiometer Insulation check meter operating knobPower input receptacle (P-1)Insulation check meterTemperature regulatorTemperature regulator
Fuses
Heater cable switch (SW-2)Resistance
insulation check
input receptacle
(P-2)Resistance check
selector switchTachometer
-EGT
indicator
check input
receptacle
(P-3)EGT indicator selector switch (SW-4) EGT indicator adjusting rheostat
Selector switch (SW-1)
Figure 10-77. Jetcal analyzer instrument compartment.
thermocouples to be tested. Temperature gradients are
very critical in the design of heater probes. Each type
of aircraft thermocouple has its own specially designed
probe. Never attempt to modify heater probes to test
other types of thermocouples.
5. Do not leave heater probe assemblies in the exhaust
nozzle during engine run-up.
6. Never allow the heater probes to go over 900 °C (1,652
°F). Exceeding these temperatures results in damage
to the jetcal analyzer and heater probe assemblies.
Continuity Check of Aircraft EGT Circuit
To eliminate any error caused by one or more inoperative
aircraft thermocouples, a continuity check is performed.
The check is made by heating one heater probe to between
500 and 700 °C and placing the hot probe over each of the
aircraft thermocouples, one at a time. The EGT indicator must
show a temperature rise as each thermocouple is checked.
When large numbers (eight or more) of thermocouples are
used in the harness, it is difficult to see a rise on the aircraft
instrument because of the electrical characteristics of a
parallel circuit. Therefore, the temperature indication of the
aircraft thermocouples is read on the potentiometer of the
analyzer by using the check cable and necessary adapter.Functional Check of Aircraft EGT Circuit
During the EGT system functional test and the thermocouple
harness checks, the analyzer has a specific degree of accuracy at
the test temperature, which is usually the maximum operating
temperature of the turbine engine. [Figure 10-78] Each engine
has its own maximum operating temperature, that can be
found in applicable technical instructions.
The test is made by heating the engine thermocouples in
the exhaust nozzle or turbine section to the engine test
temperature. The heat is supplied by heater probes through
the necessary cables. With the engine thermocouples hot,
their temperature is registered on the aircraft EGT indicator.
At the same time, the thermocouples embedded in the heater
probes, which are completely isolated from the aircraft
system, are picking up and registering the same temperature
on the test analyzer.
The temperature registered on the aircraft EGT indicator
should be within the specified tolerance of the aircraft system
and the temperature reading on the temperature analyzer.
When the temperature difference exceeds the allowable
tolerance, troubleshoot the aircraft system.
Figure 10-78. EGT analyzer.
EGT Indicator Check
The EGT indicator is tested after being removed from the
aircraft instrument panel and disconnected from the aircraft
EGT circuit leads. Attach the instrument cable and EGT
indicator adapter leads to the indicator terminals and place
the indicator in its normal operating position. Adjust the
analyzer switches to the proper settings. The indicator reading
should correspond to the readings of the analyzer within the
allowable limits of the EGT indicator.
Correction for ambient temperature is not required for this
test, as both the EGT indicator and analyzer are temperature
compensated. The temperature registered on the aircraft
EGT indicator should be within the specified tolerance
of the aircraft system and the temperature reading on the
analyzer readout. When the temperature difference exceeds
the allowable tolerance, troubleshoot the aircraft system.
Resistance & Insulation Check
The thermocouple harness continuity is checked while the
EGT system is being checked functionally. The resistance
of the thermocouple harness is held to very close tolerances,
since a change in resistance changes the amount of current
flow in the circuit. A change of resistance gives erroneous
temperature readings. The resistance and insulation check
circuits make it possible to analyze and isolate any error in
the aircraft system. How the resistance and insulation circuits
are used is discussed with troubleshooting procedures.
Tachometer Check
To read engine speed with an accuracy of ±0.1 percent during
engine run, the frequency of the tachometer-generator (older
style) is measured by the rpm check analyzer. The scale of the
rpm check circuit is calibrated in percent rpm to correspond to
the aircraft tachometer indicator, which also reads in percent
rpm. The aircraft tachometer and the rpm check circuit are connected in parallel, and both are indicating during engine
run-up. The rpm check circuit readings can be compared
with the readings of the aircraft tachometer to determine the
accuracy of the aircraft instrument.
Many newer engines use a magnetic pickup that counts
passing gear teeth edges, which are seen electrically as
pulses of electrical power as they pass by the pickup.
[Figure 10-79] By counting the amount of pulses, the rpm
of the shaft is obtained. This type of system requires little
maintenance, other than setting the clearance between the
gear teeth and the magnetic pickup.
Troubleshooting EGT System
An appropriate analyzer is used to test and troubleshoot the
aircraft thermocouple system at the first indication of trouble,
or during periodic maintenance checks.
The test circuits of the analyzer make it possible to isolate
the troubles listed below. Following the list is a discussion
of each trouble mentioned.
1. One or more inoperative thermocouples in engine
parallel harness.
2. Engine thermocouples out of calibration.
3. EGT indicator error.
4. Resistance of circuit out of tolerance.
5. Shorts to ground.
6. Shorts between leads.
One or More Inoperative Thermocouples in
Engine Parallel Harness
This error is found in the regular testing of aircraft
thermocouples with a hot heater probe and is a broken lead
wire in the parallel harness, or a short to ground in the harness.
In the latter case, the current from the grounded thermocouple
can leak off and never be shown on the indicator. However,
this grounded condition can be found by using the insulation
resistance check.
Engine Thermocouples Out of Calibration
When thermocouples are subjected for a period of time
to oxidizing atmospheres, such as encountered in turbine
engines, they drift appreciably from their original calibration.
On engine parallel harnesses, when individual thermocouples
can be removed, these thermocouples can be bench-checked,
using one heater probe. The temperature reading obtained
from the thermocouples should be within manufacturer’s
tolerances.
10-65Figure 10-79. Magnetic pickup and gear.
EGT Circuit Error
This error is found by using the EGT and comparing the
reading of the aircraft EGT indicator with the analyzer
temperature reading. [Figure 10-78] The analyzer and aircraft
temperature readings are then compared.
Resistance of Circuit Out of Tolerance
The engine thermocouple circuit resistance is a very
important adjustment since a high-resistance condition gives
a low indication on the aircraft EGT indicator. This condition
is dangerous, because the engine is operating with excess
temperature, but the high resistance makes the indicator
read low. It is important to check and correct this condition.
Shorts to Ground/Shorts Between Leads
These errors are found by doing the insulation check using
an ohmmeter. Resistance values from zero to 550,000 ohms
can be read on the insulation check ohmmeter by selecting
the proper range.
Troubleshooting Aircraft Tachometer System
A function of the rpm check is troubleshooting the aircraft
tachometer system. The rpm check circuit in the analyzer
is used to read engine speed during engine run-up with an
accuracy of ±0.1 percent. The connections for the rpm check
are the instrument cable and aircraft tachometer system lead
to the tachometer indicator. After the connections have been
made between the analyzer rpm check circuit and the aircraft
tachometer circuit, the two circuits, now classed as one, are
a parallel circuit. The engine is then run-up as prescribed in
applicable technical instructions. Both systems can be read
simultaneously.
If the difference between the readings of the aircraft
tachometer indicator and the analyzer rpm check circuit
exceeds the tolerance prescribed in applicable technical
instructions, the engine must be stopped, and the trouble located and corrected.
11-1Engine General Requirements
Engines used for light-sport aircraft and other types of
aircraft, such as some experimental aircraft, ultralight
aircraft, and powered parachutes, must be very light for
the power they develop. Each aircraft requires thrust to
provide enough forward speed for the wings to provide lift
to overcome the weight of the aircraft. An aircraft that meets
the requirements of the light-sport categories must meet the
following requirements.
Note: All of the following requirements and regulations are
subject to change. Always refer to the latest Federal Aviation
Regulations for current information.
A light-sport aircraft means an aircraft, other than a rotorcraft
or powered-lift, since its original certification, has continued
to meet the following:
1. A maximum takeoff weight of not more than 1,320
pounds (lb) (600 kilograms (kg)) for aircraft not
intended for operation on water; or 1,430 lb (650 kg)
for an aircraft intended for operation on water.
2. A maximum airspeed in level flight with maximum
continuous power (V H) of not more than 120 knots
calibrated airspeed (CAS) under standard atmospheric
conditions at sea level.
3. A maximum never-exceed speed (V NE) of not more
than 120 knots CAS for a glider.
4. A maximum stalling speed or minimum steady flight
speed without the use of lift-enhancing devices
(VS1) of not more than 45 knots CAS at the aircraft’s
maximum certificated takeoff weight and most critical
center of gravity.
5. A maximum seating capacity of no more than two
persons, including the pilot.
6. A single, reciprocating engine, if powered.
7. A fixed or ground-adjustable propeller, if a powered
aircraft other than a powered glider.
8. A fixed or auto-feathering propeller system, if a
powered glider.
9. A fixed-pitch, semirigid, teetering, two-blade rotor
system, if a gyroplane.
10. A non-pressurized cabin, if equipped with a cabin.
11. Fixed landing gear, except for an aircraft intended for
Light-Sport Aircraft Engines
Chapter 11
operation on water or a glider.
12. Fixed or retractable landing gear, or a hull, for an
aircraft intended for operation on water.
13. Fixed or retractable landing gear for a glider.
Powered parachute means a powered aircraft comprised of
a flexible or semirigid wing connected to a fuselage so that
the wing is not in position for flight until the aircraft is in
motion. The fuselage of a powered parachute contains the
aircraft engine, a seat for each occupant, and is attached to
the aircraft’s landing gear.
Weight shift control aircraft means a powered aircraft with
a framed pivoting wing and a fuselage controllable only in
pitch and roll by the pilot’s ability to change the aircraft’s
center of gravity with respect to the wing. Flight control of
the aircraft depends on the wing’s ability to flexibly deform
rather than the use of control surfaces.
As the weight of an engine is decreased, the useful load that
an aircraft can carry and the performance of the aircraft are
obviously increased. Every excess pound of weight carried
by an aircraft engine reduces its performance. Since light-
sport aircraft have a narrow margin of useful load, engine
weight is a very important concern with all of the light, low
airspeed aircraft. Tremendous gains in reducing the weight of
the aircraft engine through improvements in design, operating
cycles, and metallurgy have resulted in engines with a much
improved power to weight ratio.
A light-sport aircraft engine is reliable when it can perform
at the specified ratings in widely varying flight attitudes and
in extreme weather conditions. The engine manufacturer
ensures the reliability and durability of the product by design,
research, and testing. Although most of these engines are not
certificated by the Federal Aviation Administration (FAA),
close control of manufacturing and assembly procedures is
generally maintained, and normally each engine is tested
before it leaves the factory and meets certain American
Society for Testing and Materials (ASTM) standards.
Some engines used on light-sport aircraft are certificated
by the FAA and these engines are maintained as per the
manufacturer’s instructions and Title 14 of the Code of
Federal Regulations (14 CFR).
Most light-sport engines require a definite time interval
11-21,000 hours or 10 years,
whichever comes first
1,200 hours or 12 years,
whichever comes first
1,000 hours or 10 years,
whichever comes first
1,200 hours or 12 years,
whichever comes firstSB-914-027
1,000 hours to 1,200 hours or 12 years,
whichever comes first
None
NoneSB-914-027
1,000 hours to 1,200 hours or 12 years,
whichever comes firstto 4,420.313
from 4,420.314
to 4,418.103
from 4,418.104914 F
914 F
914 UL
914 ULDesignation of Engine Type For Engine S/N Time Between Overhaul (TBO) SB To Be Carried Out To Increase TBO
Figure 11-1. Examples of TBO and calendar life for engines.between overhauls. This is specified or implied by the engine
manufacturer. The time between overhauls (TBO) varies
with the type of engine (cycle), operating conditions, such as
engine temperatures, amount of time the engine is operated
at high-power settings, and the maintenance received. After
reaching the time limit, the engine has to be overhauled.
Sometimes this requires the engine to be shipped to an
authorized manufacturer’s overhaul facility. [Figure 11-1]
One consideration when selecting a light-sport engine is the
shape, size, and number of cylinders of the engine. Since
these engines range from single cylinder to multicylinder
engines, the mounting in the airframe is important to maintain
the view of the pilot, aircraft center of gravity, and to reduce
aircraft drag.
Personnel Authorized to Perform Inspection
& Maintenance on Light-Sport Engines
Given they meet all applicable regulations, the holder of
a powerplant certificate can perform maintenance and
inspections on light-sport engines. 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
under the provisions of 14 CFR part 43, section 43.3 (g). All
maintenance must be performed in accordance with 14 CFR
part 65, section 65.81, which describes specific experience
requirements and current instructions for performing
maintenance.
The following is used to determine eligibility for a repairman
certificate (light-sport aircraft) and appropriate rating. To
be eligible for a repairman certificate (light-sport aircraft),
you must:
• Be at least 18 years old.
• Be able to read, speak, write, and understand English.
If for medical reasons you cannot meet one of these requirements, the FAA may place limits on the
repairman certificate necessary to safely perform the
actions authorized by the certificate and rating.
• Demonstrate the requisite skill to determine whether a
light-sport aircraft is in a condition for safe operation.
• Be a citizen of the United States, or a citizen of a
foreign country who has been lawfully admitted for
permanent residence in the United States.
• To be eligible for a repairman certificate (light-sport
aircraft) with an inspection rating, the applicant must:
o Meet the requirements stated above for a
repairman’s certificate.
o Complete a 16-hour training course acceptable
to the FAA on inspecting the particular class of
experimental light-sport aircraft for which these
privileges are intended to be exercised.
• To be eligible for a repairman certificate (light-sport
aircraft) with a maintenance rating, the applicant must:
o Meet the requirements stated above for a
repairman’s certificate.
o Complete a training course acceptable to the FAA
on maintaining the particular class of light-sport
aircraft upon which the privileges are intended
to be exercised. The training course must, at a
minimum provide the following number of hours
of instruction:
- For airplane class privileges: 120 hours.
- Weight-shift control aircraft class privileges:
104 hours.
- Powered parachute class privileges: 104
hours.
- Lighter-than-air class privileges: 80 hours.
- Glider class privileges: 80 hours.
11-3The holder of a repairman certificate (light-sport aircraft)
with an inspection rating may perform the annual condition
inspection on a light-sport aircraft that is owned by the
holder, has been issued an experimental certificate for
operating a light-sport aircraft under 14 CFR part 21, section
21.191(i), and is in the same class of light-sport aircraft for
which the holder has completed the training specified in the
above paragraphs.
The holder of a repairman certificate (light-sport aircraft)
with a maintenance rating may approve and return to service
an aircraft that has been issued a special Airworthiness
Certificate in the light-sport category under 14 CFR part
21, section 21.190, or any part thereof, after performing
or inspecting maintenance (to include the annual condition
inspection and the 100-hour inspection required by 14 CFR
part 91, section 91.327), preventive maintenance, or an
alteration (excluding a major repair or a major alteration on a
product produced under an FAA approval). They may perform
the annual condition inspection on a light-sport aircraft that
has been issued an experimental certificate for operating a
light-sport aircraft under 14 CFR part 21, section 21.191(i).
However, they may only perform maintenance, preventive
maintenance, and an alteration on a light-sport aircraft for
which the holder has completed the training specified in the
preceding paragraphs. Before performing a major repair, the
holder must complete additional training acceptable to the
FAA and appropriate to the repair performed.
The holder of a repairman certificate (light-sport aircraft) with
a maintenance rating may not approve for return to service
any aircraft or part thereof unless that person has previously
performed the work concerned satisfactorily. If that person
has not previously performed that work, the person may
show the ability to do the work by performing it under
the direct supervision of a certificated and appropriately
rated mechanic, or a certificated repairman who has had
previous experience in the specific operation concerned. The
repairman may not exercise the privileges of the certificate
unless the repairman understands the current instructions
of the manufacturer and the maintenance manuals for the
specific operation concerned.
Authorized Personnel That Meet FAA Regulations
All applicable aviation regulatory authority regarding
maintenance procedures must be met. Maintenance
organizations and personnel are encouraged to contact the
manufacturer for more information and guidance on any of
the maintenance procedures.
It is a requirement that every individual or maintenance
provider possess the required special tooling, training,
or experience to perform all tasks outlined. Maintenance providers that meet the following conditions outlined below
may perform engine maintenance providing they meet all of
the following FAA requirements:
• Knowledge of the specific task as a result of receiving
authorized training from a training provider.
• Previous experience in performing the task and formal
instruction from a manufacturer’s authorized training
facility or “on-the-job” instruction by a manufacturer’s
representative.
• A suitable work environment to prevent contamination
or damage to engine parts or modules is needed.
• Suitable tools and fixtures as outlined in the
manufacturers’ Maintenance Manual should be used
while performing maintenance requiring such tooling.
• Reasonable and prudent maintenance practices should
be utilized.
Types of Light-Sport & Experimental Engines
Note: All information in this text is for educational
illustrational purposes and is not to be used for actual aircraft
maintenance. This information is not revised at the same
rate as the maintenance manual; always refer to the current
maintenance information when performing maintenance on
any engine.
Light-Sport Aircraft Engines
Light-sport/ultralight aircraft engines can be classified
by several methods, such as by operating cycles, cylinder
arrangement, and air- or water-cooled. An inline engine
generally has two cylinders, is two-cycle, and is available
in several horsepower ranges. These engines may be either
liquid-cooled, air-cooled, or a combination of both. They
have only one crankshaft that drives the reduction gearbox or
propeller directly. Most of the other cylinder configurations
used are horizontally opposed, ranging from two to six
cylinders from several manufacturers. These engines are
either gear reduction or direct drive.
Two-Cycle, Two Cylinder Rotax Engine
Rotax 447 UL Single Capacitor Discharge Ignition
(SCDI) & Rotax 503 UL Dual Capacitor Discharge
Ignition (DCDI)
The Rotax inline cylinder arrangement has a small frontal
area and provides improved streamlining. [Figure 11-2] The
two cylinder, inline two-stroke engine, which is piston ported
with air-cooled cylinder heads and cylinders, is available in
a fan or free air-cooled version. Being a two-stroke cycle
engine, the oil and fuel must be mixed in the fuel tank on
some models. Other models use a lubrication system, such
as the 503 oil injection lubrication system. This system does
not mix the fuel and oil as the oil is stored in a separate tank.
Figure 11-3. Rotax 582 engine. Figure 11-2. Rotax inline cylinder arrangement.
As the engine needs lubrication, the oil is injected directly
from this tank. The typical ignition system is a breakerless
ignition system with a dual ignition system used on the 503,
and a single ignition system used on the 447 engine series.
Both systems are of a magneto capacitor discharge design.
The engine is equipped with a carburetion system with one
or two piston-type carburetors. One pneumatic driven fuel
pump delivers the fuel to the carburetors. The propeller is
driven via a flange connected gearbox with an incorporated
shock absorber. The exhaust system collects the exhaust
gases and directs them overboard. These engines come with
an integrated alternating current (AC) generator (12V 170W)
with external rectifier-regulator as an optional extra.
Rotax 582 UL DCDI
The Rotax 582 is a two-stroke engine, two cylinder inline
with rotary valve inlet, has liquid-cooled cylinder heads, and
cylinders that use an integrated water pump. [Figure 11-3]
The lubrication system can be a fuel-oil mixture or oil
injection lubrication. The ignition system is a dual ignition
using a breakerless magneto capacitor discharge design. Dual
piston type carburetors and a pneumatic fuel pump deliver
the fuel to the cylinders. The propeller is driven via the prop
flange connected gearbox with an incorporated torsional
vibration shock absorber. This engine also uses a standard
version exhaust system with an electric starter or manual
rewind starter.
Description of Systems for Two-Stroke Engines
Cooling System of Rotax 447 UL SCDI & Rotax 503
UL DCDI
Two versions of air-cooling are available for these engines.
The first method is free air-cooling, which is a process of
engine cooling by an air-stream generated by aircraft speed and propeller. The second is fan cooling, which is cooling
by an air-stream generated by a fan permanently driven from
the crankshaft via a V-belt.
Cooling System of the Rotax 582 UL DCDI
Engine cooling for the Rotax 582 is accomplished by liquid
cooled cylinders and cylinder heads. [Figure 11-4] The
cooling system is in a two circuit arrangement. The cooling
liquid is supplied by an integrated pump in the engine
through the cylinders and the cylinder head to the radiator.
The cooling system has to be installed, so that vapor coming
from the cylinders and the cylinder head can escape to the
top via a hose, either into the water tank of the radiator or
to an expansion chamber. The expansion tank is closed by a
pressure cap (with excess pressure valve and return valve).
As the temperature of the coolant rises, the excess pressure
valve opens, and the coolant flows via a hose at atmospheric
pressure to the transparent overflow bottle. When cooling
down, the coolant is sucked back into the cooling circuit.
Lubrication Systems
Oil Injection Lubrication of Rotax 503 UL DCDE &
582 UL DCDI
Generally, the smaller two cycle engines are designed to run
on a mixture of gasoline and 2 percent oil that is premixed
in the fuel tank. The engines are planned to run on an oil-
gasoline mixture of 1:50. Other engines use oil injection
systems that use an oil pump driven by the crankshaft via the
pump gear that feeds the engine with the correct amount of
fresh oil. The oil pump is a piston type pump with a metering
system. Diffuser jets in the intake inject pump supplied two-
stroke oil with the exact proportioned quantity needed. The
oil quantity is defined by the engine rotations per minute and
the oil pump lever position. This lever is actuated via a cable
connected to the throttle cable. The oil comes to the pump
TemperatureC°
1413Cooling liquid
Crankcase
Cylinder
Cylinder head
Water pump
Radiator
Hose from radiator to water pump
Hose from cylinder head to radiator
Radiator screw cap, with excess pressure valve and return valve
Temperature gauge for cooling water
Overflow hose
Overflow bottle
Bottle venting
Expansion tank
Cylinder head venting hose
Figure 11-4. Rotax 582 cooling system.
from an oil tank by gravity.
Note: In engines that use oil injection, the carburetors are
fed with pure gasoline (no oil-gasoline mixture). The oil
quantity in the oil tank must be checked before putting the
engine into service as the oil is consumed during operation
and needs to be replenished.
Electric System
The 503 UL DCDI, 582 UL DCDI engine types are equipped
with a breakerless, single capacitor discharge ignition unit
with an integrated generator. [Figure 11-5] The 447 UL
SCDI engine is equipped with a breakerless, single capacitor
discharge ignition unit with integrated generator. The ignition
unit is completely free of maintenance and needs no external
power supply. Two charging coils fitted on the generator
stator, independent from each other, each feed one ignition
circuit. The energy supplied is stored in the ignition capacitor.
At the moment of ignition, the external triggers supply an
impulse to the control circuits and the ignition capacitors
are discharged via the primary winding of the ignition coil.
The secondary winding supplies the high voltage for the
ignition spark.
Fuel System
Due to higher lead content in aviation gas (A VGAS),
operation can cause wear and deposits in the combustion
chamber to increase. Therefore, A VGAS should only be used
if problems are encountered with vapor lock or if the other
fuel types are not available. Caution must be exercised to use
only fuel suitable for the relevant climatic conditions, such
as using winter fuel for summer operation. Fuel-Oil Mixing Procedure
The following describes the process for fuel-oil mixing. Use a
clean approved container of known volume. To help predilute
the oil, pour a small amount of fuel into the container. Fill
known amount of oil (two-stroke oil ASTM/Coordinating
European Council (CEC) standards, API-TC classification
(e.g., Castrol TTS) mixing ratio 1:50 (2 percent)), into
container. Oil must be approved for air-cooled engines at
50:1 mixing ratio. Agitate slightly to dilute oil with gasoline.
Add gasoline to obtain desired mixture ratio; use fine mesh
screen. Replace the container cap and shake the container
thoroughly. Then, using a funnel with a fine mesh screen
to prevent the entry of water and foreign particles, transfer
mixture from container into the fuel tank.
Warning: To avoid electrostatic charging at refueling, use
only metal containers and ground the aircraft in accordance
with the grounding specifications.
Opposed Light-Sport, Experimental, &
Certificated Engines
Many certificated engines are used with light-sport and
experimental aircraft. Generally, cost is a big factor when
considering this type of powerplant. The certificated engines
tend to be much more costly than the non-certificated engines,
and are not ASTM approved.
Rotax 912/914
Figure 11-6 shows a typical four cylinder, four-stroke Rotax
horizontally opposed engine. The opposed-type engine has
11-6LALC1LC2Spark plug
Spark plug
Spark plug
Spark plugK
DCElectronic box 1
Electronic box 2S
H1H2
C
S
H1H2DKGEN
P1YellowP1
P2Gray
Black/yellow
Green
Red/white
White
Black/yellow
Green
Red/white
WhiteBlack/yellow
Figure 11-5 Rotax 503 and 582 electrical system.
Figure 11-6. Typical four-cylinder, four-stroke horizontally opposed
engine.
two banks of cylinders directly opposite each other with a
crankshaft in the center. The pistons of both cylinder banks
are connected to the single crankshaft. The engine cylinder
heads are both liquid-cooled and air-cooled; the air-cooling
is mostly used on the cylinder. It is generally mounted with
the cylinders in a horizontal position. The opposed-type
engine has a low weight to horsepower ratio, and its narrow
silhouette makes it ideal for horizontal installation on the
aircraft wings (twin-engine applications). Another advantage
is its low vibration characteristics. It is an ideal replacement
for the Rotax 582 two-cylinder, two-stroke engine, which
powers many of the existing light aircraft, as it is the same
weight as the Rotax 582. These engines are ASTM approved
for installation into light-sport category aircraft, with some
models being FAA-certificated engines. Description of Systems
Cooling System
The cooling system of the Rotax 914, shown in Figure 11-7 ,
is designed for liquid cooling of the cylinder heads and
ram-air cooling of the cylinders. The cooling system of the
cylinder heads is a closed circuit with an expansion tank.
[Figure 11-8] The coolant flow is forced by a water pump
driven from the camshaft, from the radiator, to the cylinder
heads. From the top of the cylinder heads, the coolant passes
on to the expansion tank (1). Since the standard location of
the radiator (2) is below engine level, the expansion tank
located on top of the engine allows for coolant expansion.
The expansion tank is closed by a pressure cap (3) (with
excess pressure valve and return valve). As the temperature
of the coolant rises, the excess pressure valve opens and
the coolant flows via a hose at atmospheric pressure to the
transparent overflow bottle (4). When cooling down, the
coolant is sucked back into the cooling circuit. Coolant
temperatures are measured by means of temperature probes
installed in the cylinder heads 2 and 3. The readings are taken
on measuring the hottest point of cylinder head depending
on engine installation. [Figure 11-7]
Fuel System
The fuel flows from the tank (1) via a coarse filter-water trap
(2) to the two electric fuel pumps (3) connected in series.
[Figure 11-9] From the pumps, fuel passes on via the fuel
pressure control (4) to the two carburetors (5). Parallel to
each fuel pump is a separate check valve (6) installed via
the return line (7) that allows surplus fuel to flow back to the
fuel tank. Inspection for possible constriction of diameter or
AUX
MAIN1
CODE1
4Fuel from tank
Filter/water trap
Electric fuel pumps
Fuel pressure control
Two carburetors
Check valve
Return line5
Overflow bottle
RadiatorExpansion tank
4Pressure cap
CODE1
4Cooling liquid
Expansion tank
Radiator
Expansion cap
Overflow bottle
Figure 11-7. Rotax 914 cooling system.
Figure 11-8. Water-cooled heads.
Figure 11-9. Fuel system components.obstruction must be accomplished to avoid overflowing of
fuel from the carburetors. The return line must not have any
resistance to flow. The fuel pressure control ensures that the
fuel pressure is always maintained approximately 0.25 bar
(3.63 pounds per square inch (psi)) above the variable boost
pressure in the airbox and thus, ensures proper operation of
the carburetors.
Lubrication System
The Rotax 914 engine is provided with a dry, sump-forced
lubrication system with a main oil pump with integrated
pressure regulator and an additional suction pump.
[Figure 11-10] The oil pumps are driven by the camshaft.
The main oil pump draws oil from the oil tank (1) via the
oil cooler (2) and forces it through the oil filter to the points
of lubrication. It also lubricates the plain bearings of the turbocharger and the propeller governor. The surplus oil
emerging from the points of lubrication accumulates on the
bottom of crankcase and is forced back to the oil tank by the
blow-by gases. The turbocharger is lubricated via a separate
oil line (from the main oil pump). The oil emerging from
the lower placed turbocharger collects in the oil sump by a
separate pump and is pumped back to the oil tank via the oil
line (3). The oil circuit is vented via bore (5) in the oil tank.
There is an oil temperature sensor in the oil pump flange for
reading of the oil inlet temperature.
CODE1
4Oil lubricant
Oil tank
Oil cooler
Oil line
Pressure line
Oil circuit vent bore5
Figure 11-10. Lubrication system.
Electric System
The Rotax 914 engine is equipped with a dual ignition unit
that uses a breakerless, capacitor discharge design with an
integrated generator. [Figure 11-11] The ignition unit is
completely free of maintenance and needs no external power
supply. Two independent charging coils (1) located on the
generator stator supply one ignition circuit each. The energy
is stored in capacitors of the electronic modules (2). At the
moment of ignition, two each of the four external trigger coils
(3) actuate the discharge of the capacitors via the primary
circuit of the dual ignition coils (4). The firing order is as
follows: 1-4-2-3. The fifth trigger coil (5) is used to provide
the revolution counter signal.
Turbocharger & Control System
The Rotax 914 engine is equipped with an exhaust gas
turbocharger making use of the energy in the exhaust gas for
compression of the intake air or for providing boost pressure
to the induction system. The boost pressure in the induction
system (airbox) is controlled by means of an electronically
controlled valve (wastegate) in the exhaust gas turbine.
The wastegate regulates the speed of the turbocharger and
consequently the boost pressure in the induction system. The
required nominal boost pressure in the induction system is
determined by the throttle position sensor mounted on the
carburetor 2/4. The sensor’s transmitted position is linear
from 0 to 115 percent, corresponding to a throttle position
from idle to full power. [Figure 11-12] For correlation
between throttle position and nominal boost pressure in the
induction, refer to Figure 11-13. As shown in the diagram, with the throttle position at 108–110 percent results in a rapid
rise of nominal boost pressure.
To avoid unstable boost, the throttle should be moved
smoothly through this area either to full power (115 percent)
or at a reduced power setting to maximum continuous
power. In this range (108–110 percent throttle position),
small changes in throttle position have a big effect on engine
performance and speed. These changes are not apparent to
the pilot from the throttle lever position. The exact setting for
a specific performance is virtually impossible in this range
and has to be prevented, as it might cause control fluctuations
or surging. Besides the throttle position, overspeeding of
the engine and too high intake air temperature have an
effect on the nominal boost pressure. If one of the stated
factors exceeds the specified limits, the boost pressure is
automatically reduced, thus protecting the engine against
over boost and detonation.
The turbo control unit (TCU) is furnished with output
connections for an external red boost lamp and an orange
caution lamp for indications of the functioning of the TCU.
When switching on the voltage supply, the two lamps are
automatically subject to a function test. Both lamps illuminate
for one to two seconds, then they extinguish. If they do not,
a check per the engine maintenance manual is necessary. If
the orange caution lamp is not illuminated, then this signals
that TCU is ready for operation. If the lamp is blinking,
this indicates a malfunction of the TCU or its periphery
systems. Exceeding the admissible boost pressure activates
4B3/4
A1/2
A3/4
B1/2
Ignition Circult BIgnition Circult A
CODE1
4Charging coils
Capacitors
Four external trigger coils
Dual ignition coils
Fifth trigger coil5
Figure 11-11. Electric system.
1,500
1,400
1,300
1,200
1,100
1,000
0 10 20 30 40 50 60 70 80 90 100 110 115%46
28hPa"Hg
0%100%
115%
Figure 11-13. Correlation between throttle position and nominal
boost pressure.
Figure 11-12. Turbocharger control system throttle range and
position.
and illuminates the red boost lamp continuously. The TCU
registers the time of full throttle operation (boost pressure).
Full throttle operation for longer than 5 minutes, with the
red boost light illuminated, makes the red boost lamp start
blinking. The red boost lamp helps the pilot to avoid full
power operation for longer than 5 minutes or the engine could
be subject to thermal and mechanical overstress.HKS 700T Engine
The HKS 700T engine is a four-stroke, two cylinder
turbocharged engine equipped with an intercooler.
[Figure 11-14] The horizontally opposed cylinders house
four valves per cylinder, with a piston displacement of 709
cc. It uses an electronic control fuel injection system. A
reduction gearbox is used to drive the propeller flange at
a speed reduction ratio of 2.13 to 1. The engine is rated at
77 horsepower continuous and 80 horsepower takeoff (3
minutes) at 4,900 rpm and 5,300 rpm, respectively. A total
engine weight of 126 pounds provides a good power to weight
ratio. The 700T has a TBO of 500 hours.
Figure 11-14. HKS 700T engine.
Figure 11-15. Jabiru engines.Jabiru Light-Sport Engines
Jabiru engines are designed to be manufactured using
the latest manufacturing techniques. [ Figure 11-15 ] All
Jabiru engines are manufactured, assembled, and ran on a
Dynometer, then calibrated before delivery. The crankcase
halves, cylinder heads, crankshaft, starter motor housings,
gearbox cover (the gearbox powers the distributor rotors),
together with many smaller components are machined from
solid material. The sump (oil pan) is the only casting. The
cylinders are machined from bar 4140 chrome molybdenum
alloy steel, with the pistons running directly in the steel
bores. The crankshaft is also machined from 4140 chrome
molybdenum alloy steel, the journals of which are precision
ground prior to being Magnaflux inspected. The camshaft
is manufactured from 4140 chrome molybdenum alloy steel
with nitrided journals and cams.The propeller is direct crankshaft driven and does not use
a reduction gearbox. This facilitates its lightweight design
and keeps maintenance costs to a minimum. The crankshaft
features a removable propeller flange that enables the easy
replacement of the front crankshaft seal and provides for a
propeller shaft extension to be fitted, should this be required
for particular applications. Cylinder heads are machined from
a solid aluminum billet that is purchased directly from one
company, thereby providing a substantive quality control trail
to the material source. Connecting rods are machined from
4140 alloy steel and the 45 millimeters big end bearings are
of the automotive slipper type. The ignition coils are sourced
from outside suppliers and are modified by Jabiru for their
own particular application.
An integral alternator provides AC rectification for battery
charging and electrical accessories. The alternator is attached
to the flywheel and is driven directly by the crankshaft. The
ignition system is a transistorized electronic system; two
fixed coils mounted adjacent to the flywheel are energized
by magnets attached to the flywheel. The passing of the
coils by the magnets creates the high voltage current, that is
transmitted by high tension leads to the center post of two
automotive type distributors, which are simply rotors and
caps, before distribution to automotive spark plugs (two in the
top of each cylinder head). The ignition system is fixed timing
and, therefore, removes the need for timing adjustment. It is
suppressed to prevent radio interference.
The ignition system is fully redundant, self-generating, and
does not depend on battery power. The crankshaft is designed
with a double bearing at the propeller flange end and a main
bearing between each big end. Thrust bearings are located
fore and aft of the front double bearing, allowing either tractor
or pusher installation. Pistons are remachined to include a
piston pin, circlip, and groove. They are all fitted with three
rings, the top rings being cast iron to complement the chrome
molybdenum cylinder bores. Valves are 7mm (stem diameter)
and are manufactured specifically for the Jabiru engine. The
valve drive train includes pushrods from the camshaft from
the camshaft followers to valve rockers. The valves are
Computer Numerical Control (CNC) machined from steel
billet, induction hardened, polished on contact surfaces,
and mounted on a shaft through Teflon coated bronze-steel
bush. Valve guides are manufactured from aluminum/
bronze. Replaceable valve seats are of nickel steel and are
shrunk into the aluminum cylinder heads. The valve train is
lubricated from the oil gallery. Engines use hydraulic lifters
that automatically adjust valve clearance. An internal gear
pump is driven directly by the camshaft and provides engine
lubrication via an oil circuit that includes an automotive spin-
on filter, oil cooler and built-in relief valve.
11-11The standard engines are supplied with two ram-air cooling
ducts, that have been developed by Jabiru to facilitate the
cooling of the engine by directing air from the propeller
to the critical areas of the engine, particularly the cylinder
heads and barrels. The use of these ducts remove the need
to design and manufacture baffles and the establishment of a
plenum chamber, which is the traditional method of cooling
air-cooled, aircraft engines. The fact that these baffles and
plenum chamber are not required also ensures a cleaner
engine installation, which in turn facilitates maintenance and
inspection of the engine and engine components.
The engine is fitted with a 1.5 kilowatt starter motor that
is also manufactured by Jabiru and provides very effective
starting. The engine has very low vibration level; however,
it is also supported by four large rubber shock mounts
attached to the engine mounts at the rear of the engine.
The fuel induction system uses a pressure compensating
carburetor. Following the carburetor, the air-fuel mixture is
drawn through a swept plenum chamber bolted to the sump
casting, in which the mixture is warmed prior to entering
short induction tubes attached to the cylinder heads.
An effective stainless steel exhaust and muffler system is
fitted as standard equipment ensuring very quiet operations.
For owners wanting to fit vacuum instruments to their
aircraft, the Jabiru engines are designed with a vacuum
pump drive direct mounted through a coupling on the rear
of the crankshaft.
Jabiru 2200 Aircraft Engine
The Jabiru 2,200 cc aircraft engine is a four-cylinder, four-
stroke horizontally opposed air-cooled engine. At 132 pounds
(60 kgs) installed weight, it is one of the lightest four-cylinder,
four-stroke aircraft engines. Small overall dimensions give it
a small frontal area width (23.46 in, 596 mm) that makes it
a good engine for tractor applications. The Jabiru engine is
designed for either tractor or pusher installation. The Jabiru
engine specifications are listed in Figure 11-16 .
The Jabiru 3300 (120 hp) engine features [Figure 11-17] :
• 4-stroke,
• 3,300 cc engine (200 cubic inches),
• 6-cylinder horizontally opposed,
• 1 central camshaft,
• Fully machined aluminum alloy crankcase,
• Overhead valves (OHV) - push rod operated,
• Ram air-cooled,
• Wet sump lubrication - 4 liter capacity,• Direct propeller drive,
• Dual transistorized magneto ignition,
• Integrated AC generator,
• Electric starter,
• Mechanical fuel pump, and
• Naturally aspirated - 1 pressure compensation
carburetor.
Aeromax Aviation 100 (IFB) Aircraft Engine
Aeromax Aviation produces a version of a 100 hp engine
called the Integral Front Bearing. The engine features a
special made integral front bearing. [Figure 11-18] The
engine uses an integral permanent magnet 35 amp alternator,
lightweight starter, and dual ignition. The compact alternator
and starter allow for a streamlined and aerodynamic cowl
which improves the fuel efficiency of an experimental
aircraft. The Aeromax aircraft engine is an opposed six-
cylinder, air-cooled, and direct drive. Being a six-cylinder
engine, it has smooth operation. The Aeromax engines are
known for their heat dissipation qualities, provided the proper
amount of cooling air is provided.
It features a crank extension supported by a massive integral
front bearing (IFB) and bearing housing. These engines start
out as a GM Corvair automobile core engine. These basic
core engines are disassembled and each component that is
reused is refurbished and remanufactured. The crankshaft in
the Areomax 100 IFB aircraft engine is thoroughly inspected,
including a magnaflux inspection. After ensuring the crank
is free of any defects, it is extended by mounting the crank
extension hub on its front. Then, the crank is ground true,
with all five bearings’ surfaces (four original and the new
extended crank’s front bearing), being true to each other
and perpendicular to the crank’s prop flange. [Figure 11-19]
All radiuses are smooth with no sharp corners where stress
could concentrate. Every crankshaft is nitrated, which is a
heat-chemical process that hardens the crank surfaces. The
crank reinforcement coupled with the IFB is required to
counter the additional dynamic and bending loads introduced
on the crank in an aircraft application. The engine case is
totally refurbished and checked for wear. Any studs or bolts
that show wear are replaced. The engine heads are machined
to proper specifications and all new valves, guides, and valve
train components are installed. A three-angle valve grind and
lapping ensure a good valve seal.
Once the engine is assembled, it is installed on a test
stand, pre-lubricated, and inspected. The engine is, then,
run several times for a total of two hours. The engine is
carefully inspected after each run to ensure it is in excellent
11-12Displacement
Bore
Stroke
Compression Ratio
Directional Rotation of Prop Shaft
Ramp Weight
Ignition Timing
Firing Order
Power Rating
Fuel Consumption at 75% power
Fuel
Oil
Oil Capacity
Spark Plugs Engine Features
Opposed
(OHV)
Magneto Ignition
Generator 20 amp
Pressure Compensating Carburetor Four-stroke
Four-cylinder horizontally opposed
One central camshaft
Push rods
Overhead valves (OHV)
Ram air-cooled
Wet sump lubrication
Direct propeller drive
Dual transistorized magneto ignition
Integrated AC generator 20 amp
Electric starter
Mechanical fuel pump
Naturally aspirated - 1 pressure compensating carburetor
Six bearing crankshaft
2,200 cc (134 cu. in.)
97.5 mm
74 mm
8:1
Clockwise - pilot’s view tractor applications
132 lb complete including exhaust, carburetor, starter motor,
alternator, and ignition system
25˚ BTDC
85 hp @ 3,300 rpm
4 US gal/hr
AVGAS 100 LL or auto gas 91 octane minimum
Aeroshell W100 or equivalent
2.3 quarts
NGK D9EA - automotive Specifications: Jabiru 2200cc 85 HP Aircraft Engine
Figure 11-16. Jabiru 2200cc specifications.
operating condition. At the end of test running the engine,
the oil filter is removed and cut for inspection. Its internal
condition is recorded. This process is documented and
kept on file for each individual engine. Once the engine’s
proper performance is assured, it is removed and packaged
in a custom built crate for shipping. Each engine is shipped
with its engine service and operations manual. This manual
contains information pertaining to installation, break–in,
testing, tune-up, troubleshooting, repair, and inspection procedures. The specifications for the Aeromax 100 engine
are outlined in Figure 11-20 .
Direct Drive VW Engines
Revmaster R-2300 Engine
The Revmaster R-2300 engine maintains Revmaster’s
systems and parts, including its RM-049 heads that feature
large fins and a hemispherical combustion chamber.
[Figure 11-21] It maintains the earlier R-2200 engine’s top
11-13Displacement
Bore
Stroke
Aircraft Engine
Compression Ratio
Directional Rotation of Prop Shaft
Ramp Weight
Ignition Timing
Firing order
Power Rating
Fuel Consumption at 75% power
Fuel
Oil
Oil Capacity
Spark Plugs 3,300 cc (202 cu.in.)
97.5 mm (3.838")
74 mm (2.913")
Jabiru 3,300 cc 120 hp
8:1
Clockwise - Pilot's view tractor applications
178 lbs (81 kg) complete including exhaust, carburetor, starter motor,
alternator and ignition system
25˚ BTDC fixed timing
120 hp @ 3,300 rpm
26 l/hr (6.87 US gal/hr)
AVGAS 100 LL or auto gas 91 octane minimum
Aeroshell W100 or equivalent
3.51 (3.69 quarts)
NGK D9EA - automotive Jabiru 3300cc Aircraft Engine
Figure 11-17. Jabiru 3300cc aircraft engine.
Figure 11-18. Aeromax direct drive, air-cooled, six-cylinder engine.Figure 11-19. Front-end bearing on the 1000 IFB engine.
horsepower (82) at 2,950 rpm continuous. [Figure 11-22]
Takeoff power is rated at 85 at 3,350 rpm. The additional
power comes from a bore of 94 mm plus lengthening of the
R-2200’s connecting rods, plus increasing the stroke from 78
to 84 mm. The longer stroke results in more displacement,
and longer connecting rods yield better vibration and power
characteristics. The lower cruise rpm allows the use of longer
propellers, and the higher peak horsepower can be felt in
shorter takeoffs and steeper climbs.
The Revmaster’s four main bearing crankshaft runs on a 60
mm center main bearing, is forged from 4340 steel, and uses nitrided journals. Thrust is handled by the 55 mm #3 bearing
at the propeller end of the crank. Fully utilizing its robust #4
main bearing, the Revmaster crank has built in oil-controlled
propeller capability, a feature unique in this horsepower
range; non-wood props are usable with these engines.
Moving from the crankcase and main bearings, the cylinders
are made by using centrifugally cast chilled iron. The pistons
are forged out of high quality aluminum alloy, machined and
balanced in a set of four. There are two sizes of pistons, 92
mm and 94 mm, designed to be compatible with a 78 mm
to 82 mm stroke crankshafts. The cylinder set also contains
Power Output: 100 hp continuous at 3,200 rpm
Displacement: 2.7 L
Compression: 9:1
Weight: 210 lb
Direct Drive
Rear Light weight. Starter and 45 amp alternator
Counterclockwise rotation
Harmonic balancer
Remanufactured case
Remanufactured heads with new guides, valves, valve train, intake
Remanufactured cylinders
New light weight aluminum cylinder - optional
New high torque cam
New CNC prop hub and safety shaft
New Aeromax top cover and data plateAir-cooled
Six cylinders
Dual ignition–single plug
Normally aspirated
CHT max: 475° F
New forged pistons
Balanced and nitrated crank shaft
New hydraulic lifters
New main/rod bearings
New all replaceable parts
New spark plug wiring harness
Remanufactured dual ignition distributor with
new points set and electronic module
New oil pump
New oil pan
Engine service manualAeromax 100 Engine Specifications
Figure 11-20. Aeromax 100 engine specifications.
Figure 11-21. Revmaster R-2300 engine. Figure 11-22. Hemispherical combustion chamber within the
Revmaster R-2300 Heads.
piston rings, wrist pins, and locks. The direct-drive R-2300
uses a dual CDI ignition with eight coil spark to eight spark
plugs, dual 20-amp alternators, oil cooler, and its proprietary
Rev-Flo carburetor, while introducing the longer cylinders
that do not require spacers. The automotive-based bearings,
valves, valve springs, and piston rings (among others) make
rebuilds easy and inexpensive. Great Plains Aircraft Volkswagen (VW)
Conversions
Great Plains Aircraft is one company that offers several
configurations of the V olkswagen (VW) aircraft engine
conversion. One very popular model is the front drive long
block kits that offer a four-cycle, four-cylinder opposed
engine with horsepower ranges from approximately 60-100.
[Figure 11-23] The long block engine kits, which are the
complete engine kits that are assembled, in the field or can
be shipped completely assembled, are available from 1,600
cc up through 2,276 cc. All the engine kits are built from
Figure 11-23. Great Plain’ s Volkswagen conversion.
Figure 11-24. 0-200 Continental Engine.
Figure 11-25. Lycoming 0-233 engine.proven time tested components and are shipped with a Type
One VW Engine Assembly Manual. This manual was written
by the manufacturer, specifically for the assembly of their
engine kits. Also included are how to determine service and
maintenance procedures and many tips on how to set up and
operate the engine correctly. The crankshaft used in the 2,180
cc to 2,276 cc engines is a 82 mm crankshaft made from a
forged billet of E4340 steel, machined and magnafluxed twice.
The end of the crankshaft features a ½-inch fine thread versus
a 20 mm thread found on the standard automotive crank.
Teledyne Continental 0-200 Engine
The 0-200 Series engine has become a popular engine for
use in light-sport aircraft. The 0-200-A/B is a four-cylinder,
carbureted engine producing 100 brake hp and has a
crankshaft speed of 2,750 rpm. [Figure 11-24] The engine
has horizontally opposed air-cooled cylinders. The engine
cylinders have an overhead valve design with updraft intake
inlets and downdraft exhaust outlets mounted on the bottom
of the cylinder. The 0-200-A/B engines have a 201 cubic inch
displacement achieved by using a cylinder design with a 4.06-
inch diameter bore and a 3.88-inch stroke. The dry weight of
the engine is 170.18 pounds without accessories. The weight
of the engine with installed accessories is approximately
215 pounds. Developed specifically for light aircraft, the
0-200-D engine has a dry weight with installed accessories
of approximately 199 pounds. The engine is provided with
four integral rear engine mounts. A crankcase breather port
is located on the 1-3 side of the crankcase forward of the
number 3 cylinder.
The engine lubrication system is a wet sump, high-pressure
oil system. The engine lubrication system includes the internal
engine-driven pressure oil pump, oil pressure relief valve,
pressure oil screen mounted on the rear of the accessory
case, and pressure instrumentation. A fitting is provided at
the 1-3 side of the crankcase for oil pressure measurement.
The oil sump capacity is six quarts maximum. The 0-200-A/B
induction system consists of an updraft intake manifold with
the air intake and throttle mounted below the engine. Engine manifold pressure is measured at a port located on the 2-4 side
of the intake air manifold. The 0-200-A/B is equipped with
a carburetor that meters fuel flow as the flight deck throttle
and mixture controls are changed.
Lycoming 0-233 Series Light-Sport Aircraft Engine
Lycoming Engines, a Textron Inc. company, produces an
experimental non-certificated version of its 233 series light-
sport aircraft engine. [Figure 11-25] The engine is light and
capable of running on unleaded automotive fuels, as well
as A VGAS. The engine features dual CDI spark ignition,
an optimized oil sump, a streamlined accessory housing,
hydraulically adjusted tappets, a lightweight starter, and a
lightweight alternator with integral voltage regulator. It has
a dry weight of 213 pounds (including the fuel pump) and
offers continuous power ratings up to 115 hp at 2,800 rpm.
In addition to its multi-gasoline fuel capability, it has proven
to be very reliable with a TBO of 2,400 hours. The initial
standard version of the engine is carbureted, but fuel injected
configurations of the engine are also available.
11-16General Maintenance Practices on Light-
Sport Rotax Engines
Some specific maintenance practices that differ from
conventional certificated engines is covered for background
and educational acquaintance purposes only. Always refer
to the current manufacturer’s information when performing
maintenance on any engine.
Safety regulations must be adhered to ensure maintenance
personnel safety when performing maintenance and service
work on any engine installation. The following information
should be followed while performing maintenance.
The ignition should be off and the ignition system grounded
with the battery disconnected. Secure the engine against
unintentional operation. During maintenance work that
requires ignition on and battery connected, secure the propeller
against unintentional turning by hand, and secure and observe
a propeller safety zone. This precautionary measure serves
to avoid any injuries in case of an unintentional start of the
engine, which can result in injuries or death. Remember, as
long as the ground-cable (plead) is not properly connected
to ground, the ignition is switched ON (hot).
Prevent contamination, such as metal chips, foreign material,
and/or dirt, from entering the cooling, lubricating, and fuel
system during maintenance. Severe burns and scalds may
result if the engine is not allowed to cool down to outside
air temperature before starting any work. Before reusing
disassembled parts, clean with a suitable cleaning agent,
check, and refit per instructions. Before every re-assembly,
check for missing components. Only use adhesives,
lubricants, cleaning agents, and solvents listed for use in the
maintenance instructions. Observe the tightening torques for
screws and nuts; overtorque or too loose connection could
cause serious engine damage or failure.
The following are some general maintenance practices that
provide for safety and good technique:
• Work only in a non-smoking area and not close to
sparks or open flames.
• Always use the specified tools.
• During disassembling/reassembling the removal of
any safety items (e.g., safety wiring, self-locking
fastener) each part must be followed with the
replacement of a new one.
• Once loosened, always replace self-securing (locking)
nuts.
• Use clean screws and nuts only and inspect face of
nuts and thread for damage.
• Check the contact faces and threads for damage and replace if any damaged is detected.
• At reassembly of the engine, replace all sealing rings,
gaskets, securing elements, O-rings, and oil seals.
• At disassembly of the engine, mark the engine’s
components as necessary to provide for locating the
original position of the part.
• Parts should be replaced in the same position upon
reassembly.
• Any used components have wear patterns that should
be replaced or matched if reused. Ensure that these
marks are not erased or washed off.
To perform maintenance, the technician must follow the
manufacturer’s instructions. Obtain, read, and understand
the information pertaining to servicing of the light-sport or
experimental engine.
Maintenance Schedule Procedures &
Maintenance Checklist
All stated checks are visual inspections for damage and
wear, unless otherwise stated. All listed work must be carried
out within the specified period. For the intervals between
maintenance work, a tolerance of + or – 0 hour is permissible,
but these tolerances must not be exceeded. This means that
if a 100 hour check is actually carried out at 110 hour, the
next check is due at 200 hour + or – 10 hour and not at 210
hour + or – 10 hour. If maintenance is performed before the
prescribed interval, the next maintenance check is to be done
at the same interval (e.g., if first 100-hour check is done
after 87 hours of operation, the next 100-hour check must
be carried out after 187 hours of operation).
Checks are carried out per the maintenance checklists,
where type and volume of maintenance work is outlined in
key words. The lists must be photocopied and filled out for
each maintenance check. The respective check (e.g., 100-
hour check) must be noted on the top of each page of the
maintenance checklist. All the maintenance work carried
out must be initialed in the signature area by the aircraft
mechanic performing the task. After maintenance, the
completed checklists must be entered in the maintenance
records. The maintenance must be confirmed in the log book.
All discrepancies and remedial action must be recorded in
a report of findings to be generated and maintained by the
company authorized to carry out maintenance work. It is the
responsibility of the aircraft operator to store and keep the
records. Replacement of equipment (e.g., carburetor, fuel
pump, governor) and execution of Service Bulletins must be
entered in the log book, stating required information.
X
CODE1
3Throttle lever
Adjustment screw
Carburetor idle stop4Cable fixation
Return spring
Idle adjustment5
CODE2
7Compensating tube
Resonator hose
Hex screw
Intake manifold
Figure 11-26. Resonator hose and compensating tube.Figure 11-27. Carburetor throttle lever.
CODE6
8Mixture screw
Mixture screw
Figure 11-28. Idle mixture screw.Carburetor Synchronization
For smooth idling, synchronization of the throttle valves is
necessary. When synchronizing, slacken both Bowden cables,
and detach the resonator hose (3) of the compensating tube
(2) to separate the two air intake systems. [Figure 11-26] In
this condition, no significant difference in the engine running
should be noticeable. If adjustment is needed for synchronous
basic throttle adjustments (mechanical synchronization),
proceed as follows. [Figures 11-27 and 11-28]
Adjust the two Bowden cables for simultaneous opening
of the throttle valves. Remove the cable fixation (4) on the
throttle lever (1). Next, release the return spring (5) from its
attachment on the throttle lever (1), and return the throttle
lever (1) to its idle stop position (3) by hand. There should be
no resistance during this procedure. Unscrew the idle speed
adjustment screw (2) until it is free of the stop. Insert a 0.1
mm (0.004 in) feeler gauge (gap X) between the idle speed
adjustment screw (2) and the carburetor idle stop (3), then
gently turn the idle screw clockwise until contact is made
with the 0.1 mm (0.004 in) feeler gauge. Pull out the feeler
gauge and turn each idle speed adjustment screw (2) 1.5 turns
in clockwise direction. Gently turn each idle mixture screw
(6) clockwise until it is fully inserted and, then, open by 1½
turns counterclockwise. Hook the return spring (5) back up
to the throttle lever (1) in its original position. Check that the
throttle valve opens fully, automatically. Carry out the above
procedure on both carburetors.
Note: The mechanical carburetor synchronization is
sufficiently exact.
At this point, place the throttle lever in the flight deck to
the idle stop position. Ensure that the throttle lever remains
in this position during the next steps of the synchronization
process. With the throttle lever in the idle stop position, move
the throttle lever (1) to the carburetor idle stop position,
using the cable fixation (4), and secure the Bowden cable
accordingly. As soon as the two carburetor Bowden cables
are installed (throttle lever idle position), check that the idle
speed adjustment screw (2) rests fully on the idle stop (3)
without pressure.
Caution: An idle speed that is too low results in gearbox
damage, and if an idle speed is too high, the engine is harder
7bar01
7bar01
CODE1
5Flow meter/vacuum gauge
Compensating tube hose
Tension clamp
Push-on connection
Flexible rubber hose8
Figure 11-29. Gauges attached to the engine.to start. Start the engine and verify the idle speed. If the idle
speed is too high or too low, adjust accordingly with idle speed
adjustment screw (2). Check the operational idle mixture of
the engine. If necessary, adjust with the idle mixture screw (6).
Pneumatic Synchronization
Mechanical synchronization should have already been
accomplished. The two carburetors are adjusted to equal
flow rate at idling by use of a suitable flow meter or vacuum
gauges (1).
There are two possible methods to connect test equipment.
One option is to remove the hex screw (6) M6 x 6 from
the intake manifold (7) and connect the vacuum gauge(s).
[Figure 11-26 and Figure 11- 29] Remove the compensating
tube (2) with attached hoses (12) (connection between intake
manifolds) and plug the connections in the intake manifolds.
The other hook up option is to remove the compensating tube
hose (2) from the push-on connection (5) after removing the
tension clamp (4). Using the push-on connection (5), install
a flexible rubber hose (8) leading to the vacuum gauge (1),
using the balance tube (4). Install the other flexible rubber
hose leading to the vacuum gauge. [Figure 11-29] Before
proceeding any further, secure the aircraft on the ground using wheel chocks and ropes.
Warning: Secure and observe the propeller zone during
engine operation.
Start the engine, verify the idle speed, and make any
necessary corrections. If a setting correction of more than ½
turn is required, repeat mechanical synchronization to prevent
too high a load on the idle stops. If the idle speed is too high,
the maximum the idle screw can be unscrewed is a complete
turn. If no satisfactory result can be achieved, inspect the idle
jets for contamination and clean if necessary.
Caution: Also check for translucent, jelly-like contamination.
Inspect for free flow.
Once the proper idling speed has been established, it is
necessary to check the operating range above the idle speed.
First, establish that the engine is developing full takeoff
performance or takeoff rpm when selected in the flight deck.
Then, the setting of the operating range (idle to full throttle)
can be checked or adjusted.
Start and warm up engine as per the operator’s manual.
~1 mm
CODE1
4Throttle valve connection
Choke actuator connection
Return springs
Figure 11-30. Bowden cable routing.Select full power and check that both pressure gauges are
registering the same readings. If the same reading is not
made on both gauges, shut down the engine and check
that carburetor actuation has full travel and that the chokes
are in the full off-position. If necessary, fit/modify the
carburetor actuation as required to achieve full power on
both carburetors. Once full power has been established
on both carburetors, retard the throttle and observe the
pressure gauge settings. The pressure gauges should show
the same reading for both carburetors. Discrepancies must
be compensated for by adjusting the off idle adjustment (7).
[Figure 11-27] The carburetor with the lower indication
must be advanced to match the higher one. This is done by
shutting down the engine and loosening the locknut on the
Bowden cable and screwing the off idle adjustment in by ½
turn, then tightening the locknut and retesting the engine.
Final idle speed adjustment may be required by resetting
the idle speed adjustment screws (2). [Figure 11-27] Equal
adjustment must be made on both carburetors.
Any major adjustments require retesting to verify all
parameters mentioned in this procedure are within limits.
Install compensation tube assembly on the engine in reverse
sequence of removal. Any minor differences in balance at idle
speed is compensated for. Always follow the instructions of
the instrument manufacturer.
Idle Speed Adjustment
If satisfactory idle speed adjustment cannot be achieved,
inspection of the idle jet or additional pneumatic
synchronization is necessary. Always carry out idle speed
adjustment when the engine is warm. Basic adjustment of
the idle speed is first accomplished by using the idle speed
adjustment screw (2) of the throttle valve. [Figure 11-27]
Optimizing Engine Running
Optimizing the engine run is necessary only if not
accomplished at carburetor synchronization. Close the idle
mixture screw (6) by turning clockwise to screw in fully
and, then, opening again by 1½ turns counterclockwise.
[Figures 11-27 and 11-28] Starting from this basic adjustment,
the idle mixture screw (6) is turned until the highest motor
speed is reached. The optimum setting is the middle
between the two positions, at which an rpm drop is noticed.
Readjustment of the idle speed is carried out using the idle
speed adjustment screw (2) and, if necessary, by slightly
turning the idle mixture screw again. Turning the idle mixture
control screw in a clockwise direction results in a leaner
mixture and turning counterclockwise in a richer mixture.
Checking the Carburetor Actuation
The Bowden cables should be routed in such a way that
carburetor actuation is not influenced by any movement of the engine or airframe, thus possibly falsifying idle speed
setting and synchronization. [Figure 11-30] Each carburetor
is actuated by two Bowden cables. At position 1, connection
for throttle valve and at position 2, make the connection for
the choke actuator. The Bowden cables must be adjusted so
that the throttle valve and the choke actuation of the starting
carburetor can be fully opened and closed. Bowden cables
and lever must operate freely and not jam.
Warning: With carburetor actuation not connected, the
throttle valve is fully open. The initial position of the
carburetor is full throttle. Never start the engine with the
actuation disconnected. Inspect Bowden cables and levers
for free movement. Cables must allow for full travel of lever
from stop to stop. Adjust throttle cables to a clearance of 1
mm (0.04 in). Inspect and lubricate linkage on carburetor
and carburetor joints with engine oil. Inspect return springs
(3) and engagement holes for wear.
Lubrication System
Oil Level Check
Always allow engine to cool down to ambient temperature
before starting any work on the lubrication system. Severe
burns and scalds may result from hot oil coming into contact
with the skin. Switch off ignition and remove ignition key. To
assure that the engine does not turn by the starter, disconnect
the negative terminal of aircraft battery. Before checking
the oil level, make sure that there is not excess residue oil
in the crankcase. Prior to oil level check, turn the propeller
Max
Min
CODE1
4Oil drain screw
Profile clamp
Oil tank cover
O-ringBaffle insert
Partition
Sealing ring
Oil tank5
Figure 11-31. Oil dipstick minimum and maximum marks.Figure 11-32. Oil tank. several times by hand in the direction of engine rotation to
pump all the oil from the engine to the oil tank. This process
is completed when air flows back to the oil tank. This air
flow can be perceived as a gurgling noise when the cap of
the oil tank is removed. The oil level in the oil tank should
be between the two marks (maximum/minimum) on the
oil dipstick, but must never fall below the minimum mark.
[Figure 11-31] Replenish oil as required, but for longer
flights, replenish oil to maximum mark to provide for more
of an oil reserve. During standard engine operation, the
oil level should be mid-way between the maximum and
minimum marks a higher oil level (over servicing). Oil can
escape through the venting (breather) passage.
Oil Change
It is advisable to check the oil level prior to an oil change, as
it provides information about oil consumption. Run engine
to warm the oil before beginning the procedure. Taking
proper precautions, crank the engine by hand to transfer the
oil from the crankcase. Remove the safety wire and oil drain
screw (1) from the oil tank, drain the used oil, and dispose
of as per environmental regulations. [Figure 11-32] Remove
and replace oil filter at each oil change. It is not necessary
to remove oil lines and other oil connections. Draining the
suction lines, oil cooler, and return line is not necessary
and must be avoided, as it results in air entering the oil
system. Replacement of the oil filter and the oil change
should be accomplished quickly and without interruption
to prevent a draining of the oil system and the hydraulic
tappets. Compressed air must not be used to blow through the oil system (or oil lines, oil pump housing, oil bores in
the housing). Replace the oil drain screw torque and safety
wire. Only use the appropriate oil in accordance with the
latest operator’s manual and service instruction. The engine
must not be cranked when the oil system is open. After the
oil change is accomplished, the engine should be cranked by
hand in the direction of engine rotation (approximately 20
turns) to completely refill the entire oil circuit.
Cleaning the Oil Tank
Cleaning the oil is optional and requires venting of the oil
system. It is only necessary to clean the oil tank and the inner
parts if there is heavy oil contamination. The procedure for
cleaning the oil tank is shown in Figure 11-32. Detach the
profile clamp (2) and remove the oil tank cover (3), together
with the O-ring (4) and the oil lines. Remove the inner parts of
the oil tank, such as the baffle insert (5) and the partition (6).
Clean oil tank (8) and inner parts (5, 6), and check for damage.
Be aware that incorrect assembly of the oil tank components
can cause engine faults or engine damage. Replace the drain
screw with a new sealing ring (7) and tighten to 25 Newton
meters (Nm) (18.5 ft/lb) and safety wire. Reassemble the oil
tank by following the same steps in reverse order.
11-21Acceptable Not acceptable
Figure 11-33. Inspecting the magnetic plug.Example of minimum torque: F x L = 20N x 0.76m = 15 Nm
Example of maximum torque: F x L = 59 N x 0.76m = 45 Nm
14Length (L) in m
Force (F) in N
Figure 11-34. Checking propeller gearbox. Inspecting the Magnetic Plug
Remove the magnetic plug and inspect it for accumulation
of chips. [Figure 11-33] The magnetic plug (torx screw) is
located on the crankcase between cylinder 2 and the gearbox.
This inspection is important because it allows conclusions
to be drawn on the internal condition of the gearbox and
engine, and reveals information about possible damage. If
a significant amount of metal chips are detected, the engine
must be inspected, repaired, or overhauled. Steel chips in low
numbers can be tolerated if the accumulation is below 3 mm
(0.125 in). [Figure 11-33] In the case of unclear findings,
flush the oil circuit and fit a new oil filter. Afterwards, conduct
an engine test run and inspect the oil filter once more. If there
are larger accumulations of metal chips on the magnetic plug,
the engine must be repaired or overhauled in accordance with
the manufacturer’s instructions for continued airworthiness.
A detailed inspection of affected engine components must
be performed. If the oil circuit is contaminated, replace the
oil cooler and flush the oil circuit, then trace the cause and
remedy the situation. If the magnetic chip is found to have no
metal, then clean and reinstall. Tighten the plug to a torque
of 25 Nm (18.5 ft/lb). Safety wire the plug and inspect all
systems for correct function.
Checking the Propeller Gearbox
The following free rotation check and friction torque check
are necessary only on certificated engines and on engines with
the overload clutch as an optional extra. Engines without the
overload clutch (slipper clutch) still incorporate the torsional
shock absorption. This design is similar to the system with
overload clutch, but without free rotation. For this reason, the
friction torque method cannot be applied on engines without
the overload clutch.
Checking the Friction Torque in Free Rotation
Fit the crankshaft with a locking pin. [Figure 11-34] With
the crankshaft locked, the propeller can be turned by hand
15 or 30 degrees, depending on the profile of the dog gears
installed. This is the maximum amount of movement allowed by the dog gears in the torsional shock absorption unit.
Warning: Ignition OFF and system grounded. Disconnect
negative terminal of aircraft battery.
Turn the propeller by hand back and forth between ramps,
taking into consideration the friction torque. No odd noises or
irregular resistance must be noticeable during this movement.
Attach a calibrated spring scale to the propeller at a certain
distance (L) from the center of the propeller. Measure the
force required to pull the propeller through the 15 or 30
degree range of free rotation. Calculate friction torque Nm by
multiplying the force Newton’s (N) or pounds (lb) obtained
on the spring scale by the distance the scale is attached from
the center of the propeller (L). The distance measurement and
torque measurement must be in the same units either standard
or metric and cannot be mixed up. The friction torque must
be between a minimum of 25 Nm and maximum of 60 Nm
(18.5 to 44.3 ft/lb). A calculation example is as follows:
Remove crankshaft locking pin and reinstall plug with new
gasket. Reconnect negative terminal of aircraft battery.
If the above mentioned friction torque is not achieved,
inspect, repair, or overhaul the gearbox in accordance with
the manufacturer’s instructions for continued airworthiness.
Testing the propeller flange is not normal maintenance but
can be carried out if defects or cracks are suspected.
Daily Maintenance Checks
The following checklist should be used for daily maintenance
checks. Repair, as necessary, all discrepancies before flight.
1. Verify ignition OFF.Friction Torque (FT) = Length (meters) x Torque (Newtons)
FT = 30 NmFT = 0.5 meters x 60 Newtons
11-222. Drain water from fuel tank sump and/or water trap
(if fitted).
3. Inspect carburetor rubber socket or flange for cracks
and verify secure attachment.
4. Inspect carburetor float chamber for water and dirt.
5. Verify security and condition of intake silencer and
air filter.
6. Verify security of radiator mounting. Inspect radiators
for damage and leaks.
7. Verify coolant level in overflow bottle and security of
cap.
8. Verify coolant hoses for security, and inspect for leaks
and chafing.
9. Inspect engine for coolant leaks (cylinder head,
cylinder base, and water pump).
10. Verify oil content for rotary valve gear lubrication and
security of oil cap.
11. Verify oil hoses for security, and inspect for leaks and
chafing (rotary valve gear lubrication system and oil
injection system).
12. Verify ignition coils/electronic boxes for secure
mounting, and check ignition leads and all electrical
wiring for secure connections and chafing.
13. Verify electric starter for secure mounting, and inspect
cover for cracks.
14. Verify engine to airframe mounting for security and
inspect cracks.
15. Verify fuel pump mounting for security, and inspect
all fuel hose connections (filters, primer bulbs, and
taps for security, leakage, chafing and kinks).
16. Verify fuel pump impulse hose for secure connections,
and inspect for chafing and kinks.
17. Verify safety wiring of gearbox drain and level plugs.
18. Inspect rubber coupling for damage and aging (C type
gearbox only).
19. Rotate engine by hand and listen for unusual noises
(first, double verify ignition OFF).
20. Check propeller shaft bearing for clearance by rocking
propeller.
21. Inspect throttle choke and oil pump lever cables for
damage (end fittings, outer casing, and kinks).
Pre-flight Checks
The following checklist should be performed for all pre-
flight checks. Repair, as necessary, all discrepancies and
shortcoming before flight.1. Verify ignition OFF.
2. Check fuel content.
3. Inspect for coolant leaks.
4. Verify oil tank content (oil injection engines).
5. Verify spark plug connectors for security.
6. Inspect engine and gearbox for oil leaks.
7. Inspect engine and gearbox for loose or missing nuts,
bolts, and screws, and verify security of gearbox to
engine mounting.
8. Inspect propeller for splits and chips. If any damage,
repair and/or rebalance before use.
9. Verify security of propeller mounting.
10. Check throttle, oil injection pump, and choke actuation
for free and full movement.
11. Verify that cooling fan turns when engine is rotated
(air-cooled engines).
12. Inspect exhaust for cracks, security of mounting,
springs, and hooks for breakage and wear, and verify
safety wiring of springs.
13. Start engine after assuring that area is clear of
bystanders.
14. Single ignition engines: check operation of ignition
switch (flick ignition off and on again at idling).
15. Dual ignition engines: check operation of both
ignition circuits.
16. Check operation of all engine instruments during
warm up.
17. If possible, visually check engine and exhaust for
excessive vibration during warm up (indicates
propeller out of balance).
18. Verify that engine reaches full power rpm during
takeoff roll.
Troubleshooting & Abnormal Operation
The information in this section is for training purposes
and should never be used for maintenance on the actual
aircraft. Only qualified personnel (experienced two-stroke
technicians) trained on this particular type of engine are
allowed to carry out maintenance and repair work. If
the following information regarding the remedy of the
malfunction does not solve the malfunction, contact an
authorized facility. The engine must not be returned to service
until the malfunction is rectified. As described earlier in the
text, engines require basically two essentials to run: spark
and correct air-fuel mixture. The majority of problems quite
often are a simple lack of one or the other.
11-23Troubleshooting
Follow an organized method of troubleshooting. This
facilitates the identification of discrepancies or malfunctions.
• Fuel—start by checking the supply (tank), fittings
(loose), filter (plugged), and float chamber (fouled).
• Spark—check for spark at the spark plugs.
Problems of a more complex nature are best left to an engine
technician. The following are examples of engine troubles
and potential fixes.
Engine Keeps Running With Ignition OFF
Possible cause: Overheating of engine.
Remedy: Let engine cool down at idling at approximately
2,000 engine rpm.
Knocking Under Load
Possible cause: Octane rating of fuel too low.
Remedy: Use fuel with higher octane rating.
Possible cause: Fuel starvation, lean mixture.
Remedy: Check fuel supply.
Abnormal Operation
Exceeding the Maximum Admissible Engine Speed
Reduce engine speed. Any overage of the maximum
admissible engine speed must be entered by the pilot into the
logbook, stating duration and extent of over-speed.
Exceeding Maximum Admissible Cylinder Head
Temperature
Reduce engine power, setting to the minimum necessary,
and carry out precautionary landing. Any exceeding of the
maximum admissible cylinder head temperature must be
entered by the pilot into the logbook, stating duration and
extent of excess-temperature condition.
Exceeding Maximum Admissible Exhaust Gas
Temperature
Reduce engine power, setting to the minimum necessary,
and carry out precautionary landing. Any exceedence of
the maximum admissible exhaust gas temperature must be
entered by the pilot into the logbook, stating duration and
extent of excess-temperature condition.
Engine Preservation
If the engine is not going to be used for an extended period
of time, certain measures must be taken to protect engine
against heat, direct sun light, corrosion, and formation of
residues. In particular, the water bonded by the alcohol in
the fuel causes increased corrosion problems during storage.After each flight, activate choke for a moment before stopping
engine. Close all engine openings like exhaust pipe, venting
tube, and air filter to prevent entry of contamination and
humidity. For engine storage of one to four weeks, proceed
with preservation prior to engine stop or on the engine at
operating temperature. Let the engine run at increased idle
speed. Shut the engine down and secure against inadvertent
engine start. Remove air filters and inject approximately
3 cubic cm of preservation oil or equivalent oil into the
air intake of each carburetor. Restart the engine and run at
increased idle speed for 10–15 seconds. Shut engine down
and secure against inadvertent engine start. Close all engine
openings, such as exhaust pipe, venting tube, and air filter,
to prevent entry of contamination and humidity.
For engine storage of engine for longer than four weeks and
up to one year, proceed with preservation prior to engine stop
and on the engine at operating temperature. Let the engine
run at increased idle speed. Remove air filters and inject
approximately 6 cubic cm of preservative oil or equivalent
oil into the air intake of each carburetor. Stop the engine.
Remove spark plugs and inject approximately 6 cubic cm
preservation oil or equivalent oil into each cylinder and
slowly turn crankshaft 2 to 3 turns by hand to lubricate top
end parts. Replace and re-torque the spark plugs. Drain
gasoline from float chambers, fuel tank, and fuel lines. Drain
coolant on liquid cooled engines to prevent any damage by
freezing. Lubricate all carburetor linkages using the proper
lubricates. Close all openings of the engine, such as exhaust
pipe openings, venting tube, and air intake, to prevent entry
of any foreign material and humidity. Protect all external
steel parts by spraying with engine oil.
General Maintenance Practices for the Light-
Sport Jabiru Engines
Note: Some specific maintenance practices that differ from
conventional certificated engines is covered for background
and educational acquaintance purposes only. Always refer
to the current manufacturer’s information when performing
maintenance on any engine.
Engine & Engine Compartment Inspection
Check for oil, fuel exhaust, and induction leaks and clean
the entire engine and compartment before inspection. Check
flywheel screw tensions to 24 foot pounds. Check the
carburetor air filter and clean it by removing it from the intake
housing and blowing compressed air against the direction of
the intake flow. For operation in heavy dust conditions, clean
air filter at shorter intervals than recommended for normal
conditions. A clogged filter reduces engine performance,
as well as promotes premature engine wear. The engine
baffles and air ducts should be checked for condition and
functionality.
11-24Two methods can be used to check the cylinders compression.
The compression gauge method is used to measure
compression using a compression tracer. Readings are taken
with a fully open throttle valve at engine oil temperature
between 30 °C and 70 °C (90 °F to 160 °F). If readings are
below 6 bar (90 psi) a check of the pistons, cylinders, valves,
and cylinder heads must be undertaken.
The second method uses the pressure differential test. Check
cylinder compression for a maximum allowable pressure
loss of 25 percent. As an alternative to a compression test, a
pressure differential test (leak down) can be accomplished.
This is a much better test of the condition of rings, bore, head
sealing, and valves. This is the normal test used in aviation
and requires specific equipment. The test is carried out with
the engine in warm to hot condition. Input pressure is best
set at 80 psi; a second gauge reads the differential. This is
done with the piston on TDC on the firing stroke.
Note: The propeller needs to be restrained. A differential
of lower than 80/60 (generally a 25 percent loss) indicates
a problem.
Problems can be better identified by observing where air is
escaping from the cylinder, blow-by. Some examples are as
follows:
1. Blow-by through the crankcase vent indicates worn
rings or bore.
2. Leaking from carburetor indicates a poor intake valve
seal.
3. Leaking from exhaust indicates a poor exhaust valve
seal.
4. Head leak indicates poor head to cylinder seal.
With the problem identified, the malfunction can then
be corrected. Poor compression can be an indication of a
serious problem. For example, continued operation with
poor compression due to a poorly sealing valve can lead
to eventual valve failure and heavy damage to the piston,
connecting-rod, barrel, and head.
Lubrication System
The oil should be changed as required by the manufacturer.
When changed, the oil filter should also be changed. Change
the oil filter at every 50 hourly inspection. Drain the oil while
engine is still warm and visually check for leaks. Fill the
engine with oil (approximately 2.3 liters) and check oil level.
Never exceed the maximum mark. Use only registered brand
oils meeting the correct specifications. Do not drain the oil
cooler during a normal oil change. The cooler holds only a
small amount of old oil that has negligible effect on the new
oil. Taking the hoses on and off the cooler can prematurely age the oil lines and lead to hoses slipping off the cooler.
Carburetor Adjustment & Checks
To adjust the engine’s idle speed, adjust the idle stop screw
(7 mm screw) against throttle lever. Standard idle mixture
screw position is 1¼ turns out from the seated position. Fine
adjustment may be necessary to give a smooth idle.
The mixture is set by selecting jet sizes. As supplied, the
engine has jets to suit a majority of installations; however,
the mixture may be affected by operation with a propeller
that does not meet the requirements listed in the installation
manual or by ambient temperature extremes. If an engine is
to be used in these situations, an exhaust gas temperature
(EGT) gauge should be fitted and monitored against the
limits specified above. Do not change carburetor settings if
EGT readings fall outside the range given without consulting
with Jabiru Aircraft or the local authorized representative.
The carburetor automatically adjusts the mixture to account
for altitude. Visual inspection should include checks for
carburetor joint degradation and carburetor linkage for full
and free movement, correct positioning of stops and security.
Spark Plugs
When plugs are removed from a warm engine, the inspection
of the tip of the spark plug can be used to indicate the health
of the engine. If the tip of the plug is a light brown color,
the plug is operating correctly. A black velvet, sooty looking
plug tip generally is an indication of an overly rich mixture
(check the choke, air filter, and intake). If the firing end tip
is covered with oil, it is an indication of too much oil in
the combustion chamber (check for worn piston rings and
cylinder walls). When servicing the spark plugs, do not
use steel or brass brushes for cleaning, and never sandblast
plugs. Clean the spark plugs with a plastic brush in a solvent.
Check electrode gap and, if necessary, adjust to 0.55–0.6mm
(0.022 in–0.024 in) by carefully bending the electrode. Use
the recommended Plugs (NGK D9EA) and place a suitable
anti-seize compound on threads of the plug before installing
them in the engine. Tighten spark plugs when the engine is
cold and adjust engine to the correct torque value. Reconnect
the ignition lead.
Exhaust System
Visually check the exhaust system for security of mounting,
damage, rubbing, leaks, and general condition. Check nuts
and bolts for tightness and condition; re-torque and replace
if necessary.
Head Bolts
Check the head bolt torque after five hours of operation, and
again after ten hours of operation. The bolts should, thereafter,
be checked annually. Head bolts torque when cold to 20 ft/lb.
11-25Spinner * *
Spinner flange * *
Spinner screws * *
Propeller * *
Propeller bolts/nuts - Tension *
Spinner/prop tracking * *
Annual Inspection**
Each 100 Hours*Check flywheel screw tensions to 24 foot pounds*
Carburetor air filter * *
Engine baffles and air ducts *
Cylinders *
Crankcase & front crankcase seal *
Hoses, lines and fittings * *
Intake and exhaust systems *
Ignition harness, distributor caps & rotors *
NOTE: Check for oil, fuel exhaust and induction leaks, then
clean entire engine and compartment before inspection.Propeller Engine and Engine Compartment
Figure 11-35. Engine inspection charts.Tachometer & Sender
Many apparent engine problems can be caused through
inaccurate tachometers. Where engine performance is
observed to be outside limits, the tachometer should be
checked against a calibrated instrument. Tachometer sender
gap is 0.4mm (0.016 inches). The sender must have at least
60 percent covered by the tags fitted to the gearbox side of
the flywheel. Ensure both tags are equal distance from sender.
Engine Inspection Charts
Note: Read all inspection requirement paragraphs prior to
using these charts. [Figure 11-35]
G-1
Glossary
A
Abradable strip. A strip of material in the compressor
housing of some axial-flow gas turbine engines. The tip of
the compressor blade touches the abradable strip, and wears,
or abrades a groove in it. This groove ensures the minimum
tip clearance.
Abradable tip (compressor blade tip). The tip of some
axial-flow compressor blades constructed so that it will
abrade, or wear away, upon contact with the compressor
housing, which ensures the minimum tip clearance between
the blade and the housing.
Absolute pressure. Pressure referenced from zero pressure
or a vacuum.
AC. Alternating current. Electrical current in which
the electrons continually change their rate of flow and
periodically reverse their direction.
ACC. Active clearance control. A system for controlling the
clearance between tips of the compressor and turbine blades
and the case of high-performance turbofan engines. When
the engine is operating at maximum power, the blade tip
clearance should be minimum, and the ACC system sprays
cool fan discharge air over the outside of the engine case. This
causes the case to shrink enough to decrease the tip clearance.
For flight conditions that do not require such close clearance,
the cooling air is turned off, and the case expands to its normal
dimensions. The control of the ACC system is done by the
FADEC, or full-authority digital electronic control.
Acceleration. The amount the velocity of an object is
increased by a force during each second it is acted upon by
that force. Acceleration is usually measured and expressed
in terms of feet per second, per second (fps²).
Accessory end. The end of a reciprocating engine on which
many of the accessories are mounted. Also, called the anti-
propeller end.
Accumulator. A hydraulic component that stores a
non-compressible fluid, such as oil, under pressure. An
accumulator has two compartments separated by a flexible
or movable partition with one compartment containing
compressed air. When oil is pumped into the other compartment, the partition moves over, further compressing
the air which holds pressure on the oil.
AD (ashless dispersant) oil. A mineral-based lubricating oil
used in reciprocating engines. This oil does not contain any
metallic ash-forming additives, but has additives that disperse
the contaminants and hold them in suspension until they can
be removed by filters.
ADC. Air data computer. An electronic computer in an
aircraft that senses pitot pressure, static pressure, and total air
temperature. It produces an indication of altitude, indicated
airspeed, true airspeed, and Mach number. The output of ADC
is usable by any of the engine or flight control computers.
ADI (antidetonation injection) system. A system used with
some large reciprocating engines in which a mixture of water
and alcohol is sprayed into the engine with the fuel when
operating at extremely high power. The air/fuel mixture is
leaned to allow the engine to develop its maximum power,
and the ADI fluid absorbs excessive heat when it vaporizes.
Adiabatic change. A physical change that takes place within
a material in which heat energy is neither added to the
material, nor taken away. If a container of gas is compressed,
with no heat energy added to or taken from it, the gas will
become hotter; its temperature will rise.
Aeromatic propeller. A patented variable-pitch propeller
that has flyweights around the blade shanks and the
blades angled back from the hub to increase the effects of
aerodynamic and centrifugal twisting forces. This propeller
automatically maintains a relatively constant rpm for any
throttle setting.
Aft-fan engine. A turbofan engine with the fan mounted
behind the compressor section. The blades of an aft-fan are
normally extensions of the free turbine blades.
Afterburner. A component in the exhaust system of a
turbojet or turbofan engine used to increase the thrust for
takeoff and for special flight conditions. Since much of the air
passing through a gas turbine engine is used only for cooling,
it still contains a great deal of oxygen. Fuel is sprayed into
the hot, oxygen-rich exhaust in the afterburner, where it is
ignited and burned to produce additional thrust.
G-2Air bleed (carburetor component). A small hole in the fuel
passage between the float bowl and the discharge nozzle of
a float carburetor. Air drawn into the liquid fuel through the
air bleed breaks the fuel up into an emulsion, making it easy
to atomize and vaporize.
Air cooling. The removal of unwanted heat from an aircraft
engine by transferring the heat directly into the air flowing
over the engine components.
Air/fuel mixture ratio. The ratio of the weight of the air
to that of the fuel in the mixture fed into the cylinders of
an engine.
Air impingement starter. A turbine engine starter that
basically consists of a nozzle that blows a stream of
compressed air against the turbine blades to rotate the
compressor for starting the engine.
Air-oil separator. A component in a turbine engine
lubrication system that removes the air from the scavenged
oil before it is returned to the oil tank.
Airworthiness Directive. Airworthiness Directives (ADs)
are legally enforceable regulations issued by the FAA
in accordance with 14 CFR part 39 to correct an unsafe
condition in a product. Part 39 defines a product as an aircraft,
engine, propeller, or appliance.
All-weather spark plug. A shielded spark plug designed for
high altitude operation. The ceramic insulator is recessed into
the shell to allow a resilient grommet on the ignition harness
to provide a watertight seal. All weather spark plugs, also
called high-altitude spark plugs, are identified by their 3/4-
20 shielding threads.
Alpha control range (alpha mode). The flight operating
mode from takeoff through landing for a turbo-prop engine.
Alpha mode includes operations from 95% to 100% of the
engine’s rated rpm.
Altitude engine. An aircraft reciprocating engine equipped
with a supercharger that allows it to maintain its rated sea-
level horsepower to an established higher altitude.
Amateur-built aircraft. Aircraft built by individuals as
a hobby rather than by factories as commercial products.
Amateur-built or home-built aircraft do not fall under the
stringent requirements imposed by the FAA on commercially
built aircraft.
Ambient air pressure. The pressure of the air that surrounds
an object.Analog indicator. An indicator that shows the value of
the parameter being measured by a number marked on a
graduated dial aligned with a movable pointer.
Angle of attack. The acute angle between the chordline of
a propeller blade and the relative wind. The angle of attack
is affected by both the engine rpm and the forward speed of
the aircraft.
Annual inspection. A complete inspection of the airframe and
powerplant required for FAA-certificated aircraft operating
under 14 CFR part 91 General Operating and Flight Rules,
and not on one of the authorized special inspection programs.
An annual inspection must be conducted every 12 calendar
months, and it must be conducted by an aviation maintenance
technician who holds an Airframe and Powerplant rating
and an Inspection Authorization. The scope of an annual
inspection is the same as that of a 100-hour inspection.
Annular duct. A duct, or passage, that surrounds an object.
The annular fan-discharge duct surrounds the core engine.
Annular orifice. A ring-shaped orifice, normally one that
surrounds another orifice.
Annulus. A ring or groove around the outside of a circular
body or shaft, or around the inside of a cylindrical hole.
Annunciator panel. A panel of warning lights visible to the
flight crew. The lights are identified by the name of the system
they represent and are often covered with colored lenses. Red
lights indicate a dangerous condition and green indicate a
safe condition.
Anodizing. A hard, airtight, unbroken oxide film
electrolytically deposited on an aluminum alloy surface to
protect it from corrosion.
Anti-icing. Prevention of the formation of ice on a surface.
Anti-propeller end. The end of a reciprocating engine that
does not attach to the propeller. Also called the accessory end.
APC. Absolute pressure controller.
APU. Auxiliary power unit. A small turbine- or reciprocating-
engine-powered generator, hydraulic pump, and air pump.
APUs are installed in the aircraft and are used to supply
electrical power, air, and hydraulic pressure when the main
engines are not running.
Aramid fiber. Fiber made from an organic compound of
G-3carbon, hydrogen, oxygen, and nitrogen. It has high strength
and low density. It is flexible under load and is able to
withstand impact, shock, and vibration. Kevlar is a well-
known aramid fiber.
Aromatic compound. A chemical compound such as
toluene, xylene, and benzene that is blended with gasoline
to improve its anti-detonation characteristics.
Articulating rod. See link rod.
Aspect ratio. The ratio of the length of an airfoil, such as a
compressor blade, to its width.
Asymmetrical loading. The loading of a propeller disc that
causes one side to produce more thrust than the other side.
ATF. Aerodynamic twisting force. The aerodynamic force
that acts on a rotating propeller blade to increase its blade
angle. The axis of rotation of a blade is near the center of its
chordline, and the center of pressure is between the axis and
the leading edge. Aerodynamic lift acting through the center
of pressure tries to rotate the blade to a higher pitch angle.
Atomize. The process of breaking a liquid down into tiny
droplets or a fine spray. Atomized liquids vaporize easily.
Augmentor tube. A long, specially shaped stainless steel
tube mounted around the exhaust tail pipe of a reciprocating
engine. As exhaust gases flow through the augmentor tube,
they produce a low pressure in the engine compartment that
draws in cooling air through the cylinder fins.
Automatic intake valve. An intake valve opened by low
pressure created inside the cylinder as the piston moves down.
There is no mechanical means of opening it.
Automatic mixture control (AMC). The device in a fuel
metering system, such as a carburetor or fuel injection system,
that keeps the air/fuel mixture ratio constant as the density
of air changes with altitude.
Autosyn system. The registered trade name of a remote
indicating instrument system. An Autosyn system uses an
electromagnet rotor, excited with 400-hertz AC, and a three-
phase distributed-pole stator.
Axial bearing load. The load on a bearing parallel to the
shaft on which the bearing is mounted. Thrust produces an
axial load on a bearing.
Axial turbine. A turbine that is turned by a fluid flowing
through it in a direction that is approximately parallel to the shaft on which the turbine wheel is mounted.
Axial-flow compressor. A type of compressor used in
gas turbine engines. Air passes through the compressor
in essentially a straight line, parallel to the axis of the
compressor. The compressor is made of a number of stages
of rotating compressor blades between stages of stationary
stator vanes.
Axis of rotation. The center line about which a propeller
rotates.
B
Babbitt. A soft silvery metal used for main bearing inserts
in aircraft reciprocating engines. Babbitt is made of tin with
small amounts of copper and antimony.
Back (propeller nomenclature). The curved surface of a
propeller blade. The back of a propeller blade corresponds
to the upper surface of an airplane wing.
Back-suction mixture control. A type of mixture control
used in some float carburetors that regulates the air/fuel
mixture by varying the air pressure above the fuel in the
float bowl.
Baffle. A thin sheet metal shroud or bulkhead used to direct
the flow of cooling air between and around the cylinder fins
of an air-cooled reciprocating engine.
Bayonet stack. An exhaust stack with an elongated and
flattened end. The gases leave the stack through a slot
perpendicular to its length. Bayonet stacks decrease both
exhaust back pressure and noise.
BDC. Bottom dead center. The position of a piston in a
reciprocating engine when the piston is at the bottom of
its stroke, and the wrist pin, crankpin, and center of the
crankshaft are all in line.
Bell mouth. The shape of the inlet of an augmentor tube
that forms a smooth converging duct. The bell mouth shape
allows the maximum amount of air to be drawn into the tube.
Bell mouth inlet duct. A form of convergent inlet-air duct
used to direct air into the compressor of a gas turbine engine.
It is extremely efficient, and is used where there is little ram
pressure available to force air into the engine. Bell mouth
ducts are used in engine test cells and on engines installed
in helicopters.
Benzene. A colorless, volatile, flammable, aromatic
G-4hydrocarbon liquid which has the chemical formula C 6H6.
Benzene, which is sometimes called benzoil, is used as a
solvent, a cleaning fluid, and a fuel for some special types
of reciprocating engines.
Bernoulli’s principle. A physical principle that explains
the relationship between kinetic and potential energy in a
stream of moving fluid. When energy is neither added to
nor taken from the fluid, any increase in its velocity (kinetic
energy) will result in a corresponding decrease in its pressure
(potential energy).
Beta control range (Beta mode). The range of operation
of a turboprop powerplant used for in-flight approach and
ground handling of the engine and aircraft. Typically, the
Beta mode includes operations from 65% to 95% of the
engine’s rated rpm.
Beta tube. A tube in a Garrett TPE331 turboprop powerplant
that extends into the propeller pitch control to act as a follow-
up device. It provides movement of the propeller blades in
proportion to movement of the power lever.
Bezel. The rim which holds the glass cover in the case of an
aircraft instrument.
BHP. Brake horsepower. The actual horsepower delivered
to the propeller shaft of a reciprocating or turboprop engine.
Bidirectional fibers. Fibers in a piece of composite material
arranged to sustain loads in two directions.
Bimetallic hairspring. A flat, spiral-wound spring made of
two strips of metal laid side-by-side and welded together.
The two metals have different coefficients of expansion,
and as the temperature changes, the spiral either tightens or
loosens. A bimetallic hair spring is used in a thermocouple
temperature changes at the reference junction.
Bimetallic strip. A metal strip made of two different types
of metal fastened together side by side. When heated, the two
metals expand different amounts and the strip warps or bends.
BITE. Built-in test equipment. A troubleshooting system
installed in many modern electronic equipment. BITE
equipment monitors engine and airframe systems, and when a
fault is found, isolates it and provides maintenance personnel
with a code that identifies the LRU (line replaceable unit)
that contains the fault.
Blade. The component of a propeller that converts the
rotation of the propeller shaft into thrust. The blade of a
propeller corresponds to the wing of an airplane.Blending. A method of repairing damaged compressor
and turbine blades. The damage is removed and the area is
cleaned out with a fine file to form a shallow depression with
generous radii. The file marks are then removed with a fine
abrasive stone so the surface of the repaired area will match
the surface of the rest of the blade.
Blisk. A turbine wheel machined from a single slab of steel.
The disc and blades are an integral unit.
Blow-in doors. Spring-loaded doors in the inlet duct of some
turbojet or turbofan engine installations that are opened by
differential air pressure when inlet air pressure drops below
that of the ambient air. Air flowing through the doors adds
to the normal inlet air passing through the engine and helps
prevent compressor stall.
BMEP. Brake mean effective pressure. The average pressure
inside the cylinder of a reciprocating engine during the power
stroke. BMEP, measured in pounds per square inch, relates
to the torque produced by the engine and can be calculated
when you know the brake horsepower.
Boost. A term for manifold pressure that has been increased
above the ambient atmospheric pressure by a supercharger.
Bootstrapping. An action that is self-initiating or self-
sustaining. In a turbocharger system, bootstrapping
describes a transient increase in engine power that causes the
turbocharger to speed up, which in turn causes the engine to
produce more power.
Bore. The diameter of a reciprocating engine cylinder.
Borescope. An inspection tool for viewing the inside of
a turbine engine without disassembling it. The instrument
consists of a light, mirror, and magnifying lens mounted
inside a small-diameter tube that is inserted into a turbine
engine through borescope inspection ports.
Boss. An enlarged area in a casting or machined part. A
boss provides additional strength to the part where holes for
mounting or attaching parts are drilled.
Bottom. (verb) A condition in the installation of a propeller
on a splined shaft when either the front or rear cone contacts
an obstruction that prevents the cone from properly seating
inside the propeller hub.
Bourdon tube. The major component in a gage-pressure
measuring instrument. It is a thin-wall metal tube that has
an elliptical cross section and is formed into a curve. One
end of the tube is sealed and connected to an arm that moves
G-5the pointer across the instrument dial, and the open end is
anchored to the instrument case. The pressure to be measured
is directed into the open end, which causes the elliptical cross
section to become more circular. As the cross section changes,
the curve straightens and moves the pointer over the dial by
an amount proportional to the amount of pressure.
Brayton cycle. The constant-pressure cycle of energy
transformation used by gas turbine engines. Fuel is sprayed
into the air passing through the engine and burned. Heat from
the burning air/fuel mixture expands the air and accelerates it
as it moves through the engine. The Brayton cycle is an open
cycle in that the intake, compression, combustion, expansion,
and exhaust events all take place at the same time, but in
different locations within the engine.
British thermal unit (Btu). The basic unit of heat energy
in the English system. One Btu is the amount of heat energy
needed to raise the temperature of one pound of pure water
from 60 ˚F to 61˚F.
BSFC. Brake specific fuel consumption. A measure of the
amount of fuel used for a given amount of power produced
by a heat engine. BSFC is expressed in pounds of fuel burned
per hour for each brake horse-power the engine is producing.
Buckets. The portions of aft-fan blades that are in the exhaust
of the core engine. Buckets drive the fan from energy received
from hot gases leaving the core engine.
Bungee cord. An elastic cord made of small strips of rubber
encased in a loosely braided cloth tube that holds and protects
the rubber, yet allows it to stretch. The energy in a stretched
bungee cord may be used to crank a large aircraft engine.
Burner. See combustor.
Burnish. To smooth the surface of a metal part that has been
damaged by a deep scratch or gouge. Metal piled at the edge
of the damage is pushed back into the damage with a smooth,
hard steel burnishing tool.
Butterfly valve. A flat, disc-shaped valve used to control
the flow of fluid in a round pipe or tube. When the butterfly
valve is across the tube, the flow is shut off, and when it is
parallel with the tube, the obstruction caused by the valve
is minimum, and the flow is at its greatest. Butterfly-type
throttle valves are used to control the airflow through the
fuel metering system.
Bypass engine. Another name for a turbofan engine. See
turbofan engine.Bypass ratio. The ratio of the mass of air moved by the fan
to the mass of air moved by the core engine.
C
Calendar month. The measurement of time used by the FAA
for inspection and certification purposes. One calendar month
from a given date extends from that date until midnight of
the last day of that month.
Cam. An eccentric, or lobe, on a rotating shaft that changes
rotary motion into linear motion. A cam is mounted on the
magnet shaft in a magneto to push upward on the insulated
breaker point to separate, or open, the points when the magnet
is in a particular location.
Cam engine. A reciprocating engine with axial cylinders
arranged around a central shaft. Rollers on the pistons in
the cylinders press against a sinusoidal cam mounted on the
shaft to produce rotation of the shaft.
Cam-ground piston. A reciprocating engine piston that is
not round, but is ground so that its diameter parallel to the
wrist pin is slightly smaller than its diameter perpendicular
to the pin. The mass of metal used in the wrist pin boss,
the enlarged area around the wrist pin hole, expands when
heated, and when the piston is at its operating temperature,
it is perfectly round.
Can-annular combustor. A type of combustor used in some
large turbojet and turbofan engines. It consists of individual
cans into which fuel is sprayed and ignited. These cans mount
on an annular duct which collects the hot gases and directs
them uniformly into the turbine.
Capacitance afterfiring. The continuation of the spark
across the gap in a shielded spark plug after the air/fuel
mixture in the cylinder is ignited. Afterfiring is caused by
the return of electrical energy stored in the capacitance of the
shielded ignition leads. Capacitance afterfiring is eliminated
by the use of a resistor in the spark plug.
Capacitor. An electrical component, formerly called a
condenser, that consists of two large-area conductors, called
plates, separated by an insulator. Electrons stored on one
of the plates produces an electrostatic pressure difference
between the plates.
Capillary tube. A glass or metal tube with a tiny inside
diameter. Capillary action causes the fluid to move within
the tube.
Carbon pile voltage regulator. A voltage regulator for a high
G-6output DC generator that uses a stack of pure carbon discs for
the variable resistance element. A spring holds pressure on the
stack to reduce its resistance when the generator output voltage
is low. This allows maximum field current to flow. The field
from an electro-magnet, whose strength varies directly with
the generator voltage, opposes the spring to loosen the stack
and increase its resistance when the generator voltage needs
to be decreased. The increased resistance decreases the field
current and reduces the output voltage.
Carbon track. A trail of carbon deposited by an arc across a
high-voltage component such as a distributor block. Carbon
tracks have a relatively low resistance to the high voltage and
can cause misfiring and loss of engine power.
Cartridge starter. A self-contained starter used on some
military aircraft. A cartridge similar in size to a shotgun
shell is ignited in the starter breech. The expanding gases
drive a piston attached to a helical spline that converts the
linear movement of the piston into rotary motion to rotate
the crankshaft.
Cascade effect. The cumulative effect that occurs when the
output of one series of components serves as the input to
the next series.
Catalyst. A substance used to change the speed, or rate, of
a chemical action without being chemically changed itself.
Cavitating. The creation of low pressure in an oil pump
when the inlet system is not able to supply all of the oil
the pump requires. Prolonged cavitation can damage pump
components.
Center of pressure. The point on the chordline of an airfoil
where all aerodynamic forces are concentrated.
Center-line thrust airplane. A twin-engine airplane with
both engines mounted in the fuselage. One is installed as a
tractor in the front of the cabin. The empennage is mounted
on booms.
Centrifugal compressor. A type of compressor that uses a
vaned plate like impeller. Air is taken into the center, or eye,
of the impeller and slung outward by centrifugal force into
a diffuser where its velocity is decreased and its pressure
increased.
Ceramic. Any of several hard, brittle, heat-resistant,
noncorrosive materials made by shaping and then firing a
mineral, such as clay, at a high temperature.
Channel-chromed cylinders. Reciprocating engine cylinders with hard chromium-plated walls. The surface of
this chrome plating forms a spider web of tiny stress cracks.
Deplating current enlarges the cracks and forms channels
that hold lubricating oil on the cylinder wall.
Cheek (crankshaft). The offset portion of a crankshaft that
connects the crankpin to the main bearing journals.
Chip detector. A component in a lubrication system that
attracts and holds ferrous metal chips circulating with the
engine oil. Some chip detectors are part of an electrical
circuit. When metal particles short across the two contacts
in the detector, the circuit is completed, and an annunciator
light is turned on to inform the flight crew that metal particles
are loose in the lubrication system.
Choke of a cylinder. The difference in the bore diameter of
a reciprocating engine cylinder in the area of the head and
in the center of the barrel.
Choke-ground cylinder. A cylinder of a reciprocating engine
that is ground so that its diameter at the top of the barrel is
slightly smaller than the diameter in the center of the stroke.
The large mass of metal in the cylinder head absorbs enough
heat to cause the top end of the barrel to expand more than
the rest of the barrel. At normal operating temperature, the
diameter of a choke-ground cylinder is uniform throughout.
Choke nozzle. A nozzle in a gas turbine engine that limits
the speed of gases flowing through it. The gases accelerate
until they reach the speed of sound, and a normal shock wave
forms that prevents further acceleration.
Chordline. An imaginary line, passing through a propeller
blade, joining the leading and trailing edges.
Cigarette. A commonly used name for a spark plug terminal
connector used with a shielded spark plug.
Circular magnetism. A method of magnetizing a part for
magnetic particle inspection. Current is passed through the
part, and the lines of magnetic flux surround it. Circular
magnetism makes it possible to detect faults that extend
lengthwise through the part.
Circumferential coil spring (garter spring). A coil spring
formed into a ring. This type of spring is used to hold segmented
ring-type carbon seals tightly against a rotating shaft.
Claret red. A dark purplish pink to a dark gray purplish
red color.
Class A fire. A fire with solid combustible materials such as
G-7wood, paper, and cloth as its fuel.
Class B fire. A fire that has combustible liquids as its fuel.
Class C fire. A fire which involves energized electrical
equipment.
Class D fire. A fire in which a metal such as magnesium
burns.
Closed-loop control. A type of control in which part of
the output is fed back to the input. This allows the input to
continually compare the command signals with the output
to determine the extent to which the commands have been
complied with.
Coke. The solid carbon residue left when all volatile parts
of a mineral oil have been evaporated by heat.
Cold-cranking simulation. A method used for specifying
the characteristics of a lubricating oil at low temperature. Oils
rated by this test have the letter W (standing for Winter) in
their designation. For example, SAE 15W50.
Cold section. The portion of a gas turbine engine ahead of
the combustion section. The cold section includes the inlet,
compressor, and diffuser.
Cold-tank lubrication system. A turbine engine lubricating
system in which the oil cooler is in the scavenge subsystem.
Collector ring. A ring made of thin corrosion-resistant steel
tubing that encircles a radial engine and collects exhaust
gases from each cylinder. The ring ends with a connection
to the exhaust tail pipe.
Combustor (combustion chamber). The section of a gas
turbine engine in which fuel is injected. This fuel mixes with
air from the compressor and burns. The intense heat from the
combustion expands the air flowing through the combustor
and directs it our through the turbine. Combustors are also
called burners.
Commutator. A mechanical rectifier mounted on the
armature shaft of a DC generator or motor. It consists of a
cylindrical arrangement of insulated copper bars connected
to the armature coils. Carbon brushes ride on the copper bars
to carry current into or out of the commutator, providing a
unidirectional current from a generator or a reversal of current
in the motor coils.
Compensating winding. A series winding in a compound-
wound DC generator. The compensating windings are embedded in the faces of the field poles and their varying
magnetic field works with the fields from the interpoles to
effectively cancel the field distortion caused by armature
current.
Composite propeller blade. A propeller blade made from
several materials such as metal, graphite, glass or aramid
fibers, and foam.
Compression ratio (reciprocating engine). The ratio of
the volume of a cylinder of a reciprocating engine with the
piston at the bottom of its stroke engine to the volume of the
cylinder with the piston at the top of its stroke.
Compression ratio (turbine engine). The ratio of the
pressure of the air at the discharge of a turbine engine
compressor to the pressure of the air at its inlet.
Compressor (air conditioning system component). The
component in a vapor-cycle cooling system in which the low-
pressure refrigerant vapors, after they leave the evaporator, are
compressed to increase both their temperature and pressure
before they pass into the condenser. Some compressors are
driven by electric motors, others by hydraulic motors and,
in the case of most light airplanes, are belt driven from the
engine.
Compressor bleed air. Air that is tapped off from a turbine
engine compressor. Compressor bleed air is used for anti-
icing the inlet ducts and for cooling the turbine inlet guide
vanes and first stage turbine blades. Bleed air is also used for
certain airframe functions. See customer bleed air.
Compressor pressure ratio. See compression ratio (turbine
engine).
Compressor stall. A condition in a turbine engine axial-flow
compressor in which the angle of attack of one or more blades
is excessive and the smooth airflow through the compressor
is disrupted.
Compressor surge. A stall that affects the entire compressor
and seriously restricts the airflow through the engine.
Con-di ducts. The British name for a convergent-divergent
duct. See convergent-divergent duct.
Condenser. See capacitor.
“Contact.” The term used between a person hand-propping
an aircraft engine and the person in the flightdeck. When the
person is ready to spin the propeller, he calls “contact”. The
person in the flightdeck turns on the fuel, slightly opens the
G-8throttle, applies the brakes, and replies “contact”, and then
turns the ignition switch to BOTH. The propeller is then
pulled through to start the engine.
Constant-displacement pump. A fluid pump that moves a
specific volume of fluid each time it rotates.
Constant-pressure cycle of energy release. The cycle of
energy transformation of a gas turbine engine. See Brayton
cycle.
Constant-volume cycle of energy release. The cycle of
energy transformation of a reciprocating engine. See Otto
cycle.
Continuous magnetic particle inspection. A method of
magnetic particle inspection in which the part is inspected
by flowing a fluid containing particles of iron oxide over the
part while the magnetizing current is flowing.
Contrarotating. Rotating in opposite directions. Turbine
rotors are contrarotating when the different stages have a
common center, but turn in opposite directions.
Convergent-divergent duct. A duct that has a decreasing
cross section in the direction of flow (convergent) until a
minimum area is reached. After this point, the cross section
increases (divergent). Convergent-divergent ducts are called
CD ducts or con-di ducts.
Convergent duct. A duct that has a decreasing cross section
in the direction of flow.
Core engine. The gas generator portion of a turboshaft,
turboprop, or turbofan engine. The core engine consists of the
portion of the compressor used to supply air for the engine
operation, diffuser, combustors, and turbine(s) used to drive
the compressor. The core engine provides the high-velocity
gas to drive the fan and/or any free turbines that provide
power for propellers, rotors, pumps, or generators.
Cowling. The removable cover that encloses an aircraft
engine.
Crankcase. The housing that encloses the crankshaft,
camshaft, and many of the accessory drive gears of a
reciprocating engine. The cylinders are mounted on the
crankcase, and the engine attaches to the airframe by the
crankcase.
Crankshaft. The central component of a reciprocating
engine. This high-strength alloy steel shaft has hardened
and polished bearing surfaces that ride in bearings in the crankcase. Offset throws, formed on the crankshaft, have
ground and polished surfaces on which the connecting rods
ride. The connecting rods change the in-and-out motion of
the pistons into rotation of the crankshaft.
Creep. The deformation of a metal part that is continually
exposed to high centrifugal loads and temperatures.
Critical altitude. The altitude above which a reciprocating
engine will no longer produce its rated horsepower with its
throttle wide open.
Critical engine. The engine of a twin-engine airplane whose
loss would cause the greatest yawing effect.
Critical Match number. The flight match number at which
there is the first indication of air flowing over any part of the
structure at a speed of Mach one, the local speed of sound.
CRT. Cathode ray tube. An electronic display tube in which
a stream of electrons is attracted to the charged inner surface
of the tube face. Acceleration grids and inner surface of the
tube face. Acceleration grids and focusing grids speed the
movement of the electrons and shape the beam to a pin-
point size. Electrostatic or electromagnetic forces caused by
deflection plates or coils move the beam over the face of the
tube. The inside of the tube face is treated with a phosphor
material that emits light when the electrons strike it.
Cryogenic fluid. A liquid which boils at a temperature
lower than about 110 ºK (-163 ºC) under normal atmospheric
pressure.
CSD. Constant-speed drive. A component used with either
aircraft gas turbine or reciprocating engines to drive AC
generators. The speed of the output shaft of the CSD is held
constant while the speed of its input shaft varies. The CSD
holds the speed of the generator, and the frequency of the
AC constant as the engine speed varies through its normal
operating range.
CTF. Centrifugal twisting force. The force acting about
the longitudinal axis of a propeller blade, and which tries to
rotate the blade to a low-pitch angle. As the propeller rotates,
centrifugal force tries to flatten the blade so all of its mass
rotates in the same plane.
Curtiss Jenny (Curtiss JN4-D). A World War I training
airplane powered by a Curtiss OX-5 engine. It was widely
available after the war and helped introduce aviation to the
general public.
Customer bleed air. Air that is tapped off a turbine engine
G-9compressor and used for such airframe functions as the
operation of air conditioning and pressurization systems.
Cylinder. The component of a reciprocating engine which
houses the piston, valves, and spark plugs and forms the
combustion chamber.
D
Data. The input for computer processing in the form of
numerical information that represents characters or analog
quantities.
Dataplate specifications. Specification of each gas turbine
engine determined in the manufacturer’s test cell when the
engine was calibrated. This data includes the engine serial
number with the EPR that produced a specific RPM. The
technician refers to this information when trimming the engine.
Dataplate performance. The performance specifications
of a turbine engine observed and recorded by the engine
manufacturer or overhauler and recorded on the engine
dataplate. This data includes the engine speed at which a
specified EPR is attained. When trimming the engine, the
technician uses this data as the goal.
DC. Direct current. Electrical current in which the electrons
always flow in the same direction.
Deaerator. A component in a turbine engine lubrication
system that removes air from the scavenged oil before it is
returned to the tank.
Deceleration. The amount the velocity of an object,
measured in feet per second, is decreased by a force during
each second it is acted upon by that force. Deceleration is
usually expressed in terms of feet per second, per second
(fps²).
DeHaviland DH-4. An English designed observation
airplane built in large quantities in the united States during
World War I. After the war, surplus DH-4s were used for
carrying the U.S. Mail.
Deicing. The removal of ice that has formed on a surface.
Density altitude. The altitude in standard air at which the
density is the same as that of the existing air.
Detergent oil. A type of mineral oil with metallic-ash-
forming additives that protects the inside of an engine from
sludge and varnish buildup. Used in automotive engines, it
has proven unsuitable for use in aircraft engines.Detonation. An uncontrolled explosion inside the cylinder of
a reciprocating engine. Detonation occurs when the pressure
and temperature of the fuel inside the cylinder exceeds the
critical pressure and temperature of the fuel. Detonation may
be caused by using fuel that has a lower octane rating or
performance number than is specified for the engine.
Dewar bottle. A special container used to store liquid oxygen
and liquid nitrogen. A Dewar bottle has an inner and an outer
container, and the space between them forms a vacuum. The
two surfaces within the vacuum are silvered to reflect heat
away from the container walls.
Differential pressure. A single pressure that is the difference
between two opposing pressures.
Diffuser. A component in a gas turbine engine that decreases
the velocity of air flowing through it and increases its pressure.
Digitized image. A modified image picked up by the
miniature TV camera in the end of a fiber-optic probe.
This image is converted into a digital electronic signal that
eliminates unwanted portions of the viewed area and allows
the desired image to be enhanced for a clearer view of the
inside of a turbine engine.
Dipstick. A gage, in the form of a thin metal rod, used to
measure the level of liquid in a reservoir. The dipstick is
pushed into the reservoir until it contacts a built-in stop; then
it is removed and visually inspected. The level of liquid in
the reservoir is indicated by the amount of the dipstick wet
by the liquid.
Dirigible. A large, cigar shaped, lighter-than-air flying
machine. Dirigibles differ from balloons in that they are
powered and can be steered.
Distributed pole stator winding. Alternator stator windings
wound in a series of slots in the stator frame. A distributed
pole stator is distinguished from a salient pole stator whose
coils are wound around separate pole shoes that project
inward from the field frame toward the rotor.
Distributor. A high-voltage selector switch that is gear-
driven from the shaft of the rotating magnet in a magneto.
The distributor rotor picks up the high voltage from the
secondary winding of the coil and directs it to high-voltage
terminals. From here, it is carried by high-tension ignition
leads to the spark plugs.
Divergent duct. A duct that has an increased cross-sectional
area in the direction of flow.
G-10Downdraft carburetor. A carburetor that mounts on the
top of a reciprocating engine. Air entering the engine flows
downward through the carburetor.
∆P (delta P). Differential pressure.
Droop. A progressive decrease in RPOM with load in a gas
turbine engine whose speed is governed with a fly-weight-
type governor in the fuel control. As the load increases, the
pilot valve drops down to meter more fuel. The lower position
of the valve decreases the compression of the speeder spring
and allows the flyweights to assume an on-speed position at
a lower rpm.
Dry-sump engine. An engine that carries its lubricating oil
supply in a tank external to the engine.
Dual ignition. An ignition system of an aircraft reciprocating
engine that has two of every critical unit, including two spark
plugs in each cylinder. Dual ignition provides safety in the
event of one system malfunctioning, but more important,
igniting the air/fuel mixture inside the cylinder at two
locations provides more efficient combustion of the air/fuel
mixture in the cylinder.
Dual-spool gas turbine engine. An axial-flow turbine engine
that has two compressors, each driven by its own stage or
stages of turbines.
Duct heater. A thrust augmentation system, similar to an
afterburner, where fuel is added to the fan-discharge air and
burned.
Duct losses. A decrease in pressure of the air flowing into a
gas turbine engine caused by friction.
Durability. A measure of engine life. Durability is usually
measured in TBO hours.
Duty cycle. A schedule that allows a device to operate for
a given period of time, followed by a cooling down period
before the device can be operated again.
Dwell chamber. A chamber in a turbine engine into which
the scavenged oil is returned. Entrained air separates from
the oil in the dwell chamber before it is picked up by the
pressure pump.
Dynamometer. A device used to measure the amount of
torque being produced by an engine. The drive shaft of the
engine is loaded with either an electric generator or a fluid
pump, and the output of the generator or pump is measured and converted into units of torque. Torque at a specific rpm
can be converted into brake horsepower.
Dyne. The unit of force that imparts an acceleration of one
centimeter per second, per second to a mass of one gram.
One dyne is equal to 2.248 ∙ 10-6 pounds.
E
Eddy current. Current induced into a conductor due to a
mobbing or non-uniform magnetic field.
EEC. Electronic engine control. An electronic fuel control for
a gas turbine engine. The EEC senses the power-lever angle
(PLA), engine RPM, bleed valve, and variable stator vane
position, and the various engine pressures and temperatures.
It meters the correct amount of fuel to the nozzles for all
flight conditions, to prevent turbine over-speed and over-
temperature.
Effective pitch. The actual distance a propeller advances in
one revolution through the air.
E-gap angle. The position of the rotating magnet in a
magneto when the breaker points are timed to open. The
E-gap (efficiency gap) angle is several degrees of magnet
rotation beyond the magnet’s neutral position. At this point,
the magnetic field stress is the greatest, and the change in
flux is the greatest, inducing the maximum voltage in the
secondary winding.
EGT. Exhaust gas temperature. The temperature of the gases
as they leave the cylinder of a reciprocating engine or the
turbine of a gas turbine engine.
EICAS. Engine indicating and crew alerting system. An
electronic instrumentation system that monitors airframe and
engine parameters and displays the essential information on a
video display on the instrument panel. Only vital information
is continually displayed, but when any sensed parameters
fall outside of their allowable range of operation, they are
automatically displayed.
Elastic limit. The maximum amount of tensile load, in
pounds per square inch, that a material is able to withstand
without permanent deformation.
Electrical potential. The electrical force caused by a
deficiency of electrons in one location and an excess of
electrons in another. Electrical potential is measured in volts.
Electrical steel. A low-carbon iron alloy that contains some
silicon It is used as the core for transformers, field frames
G-11for generators and alternators, and the magnetic circuit of
magnetos.
Electromagnet. A magnet produced by an electrical current
flowing through a coil of wire. The coil is normally wound
around a core of soft iron which has an extremely low
retentivity, allowing it to lose its magnetism as soon as the
current stops flowing.
Electromagnetic radiation. A method of transmitting energy
from one location to another. Current caused by high voltage
in the secondary winding of a magneto produces electric
and magnetic fields which oscillate back and forth at a high
frequency and extend out into space in the form of waves.
These waves of electromagnetic radiation are received as
interference by the radio receivers in the aircraft.
Electromotive force. A force that causes electrons to move
from one atom to another within an electrical circuit. An
electromotive force, or EMF, is the difference in the electrical
pressure, or potential, that exists between two points. An
EMF may be produced by converting mechanical movement,
pressure, chemical, light, or heat energy into electrical energy.
The basic unit of EMF is the volt.
Emulsion. A suspension of small globules of one material
in another when the two materials will not mix. Oil and
water will not mix, but they can be formed into an emulsion.
An emulsion will separate into its components when it is
allowed to sit.
Engine trimming. A maintenance procedure in which the
fuel control on a gas turbine engine is adjusted to cause the
engine to produce the required EGT or EPR at a specified rpm.
Entrained water. Water suspended in jet fuel. The amount
of entrained water that can be held in the fuel is determined
by the temperature of the fuel. When the fuel becomes cold,
the water precipitates out and forms ice crystals on the fuel
filter element.
Epicyclic reduction gears. A gear train in which a series
of small planetary gears rotate around a central gear. More
commonly called a planetary gear train.
EPR. Engine pressure ratio. The ratio of the turbine discharge
total pressure to the compressor inlet total pressure. EPR is
normally used as the parameter to determine the amount of
thrust an axial-flow turbojet or turbofan engine is producing.
ESHP. Equivalent shaft horsepower. A measure of the
power produced by a turboprop engine. ESHP takes
into consideration both the shaft horsepower delivered to the propeller and the thrust developed at the engine
exhaust. Under static conditions, one shaft horsepower is
approximately equal to 2.5 pounds of thrust.
Ethanol. Alcohol made from cereal grains such as corn.
Ether. A volatile, highly flammable liquid that may be used
to prime the cylinders of an aircraft engine when starting
under extremely cold conditions.
Ethylene dibromide. A colorless, poisonous liquid
BrCH 2CH 2Br that is blended with leaded gasoline to help
scavenge lead oxides.
Ethylene glycol. A form of alcohol used as a coolant for
liquid-cooled aircraft engines. It is also used in automobile
engines as a permanent antifreeze.
Eutectic. An alloy or solution that has the lowest possible
constant melting point.
Evaporative cooling. See steam cooling.
Exceedance condition. A condition in which a parameter
sensed by the EICAS exceeds the limits for which it is
programmed.
Exhaust cone. The fixed conical fairing centered in the
turbine wheel. The exhaust cone straightens the flow and
prevents the hot gases from circulating over the rear face of
the turbine wheel.
Exhaust nozzle. The opening at the rear of the exhaust pipe.
Expansion wave. The change in pressure and velocity of
supersonic air as it passes over a surface that drops away from
the flow. As the surface drops away, the air tries to follow it,
and in changing its direction, the air speeds up to a higher
supersonic speed, and its static pressure decreases. There is
no change in the total amount of energy as air passes through
an expansion wave.
External-combustion engine. A form of heat engine in
which the fuel releases its energy outside of the engine.
This released heat expands air which is used to perform
useful work. Steam engines are a popular type of external
combustion engine.
Extreme pressure (EP) lubricant. A lubricant that reacts
with iron to form iron chlorides, sulfides, or phosphides on
the surface of a steel part. These compounds reduce wear and
damage to surfaces in heavy rubbing contact. EP lubricants
are specially suited for lubricating gear trains.
G-12F
FAA Form 337. The Major Repair and Alteration form
that must be completed when an FAA-certificated aircraft
or engine has been given a major repair or major alteration.
Face (propeller nomenclature). The flat surface of a
propeller that strikes the air as the propeller rotates. The face
of a propeller corresponds to the bottom of an airplane wing.
FADEC. Full-authority digital electronic control. A digital
electronic fuel control for a gas turbine engine that is
functioning during all engine operations, hence full authority.
It includes the EEC (see EEC) and functions with the flight
management computer. FADEC schedules the fuel to the
nozzles in such a way that prevents overshooting power
changes and over-temperature conditions. FADEC furnishes
information to the EICAS (engine indication and crew
alerting system).
Fan pressure ratio. The ratio of the fan-discharge pressure
to the fan inlet pressure.
Feathering propeller. A controllable-pitch propeller whose
blades can be moved into a high pitch angle of approximately
90º. Feathering the propeller of an inoperative engine
prevents it from wind-milling and greatly decreases drag.
Feeler gages. A type of measuring tool consisting of strips
of precision-ground steel of accurately measured thickness.
Feeler gages are used to measure the distance between close-
fitting parts, such as the clearances of a mechanical system or
the distance by which moving contacts are separated.
FHP. Friction horsepower. The amount of horsepower used
to turn the crankshaft, pistons, gears, and accessories in
a reciprocating engine and to compress the air inside the
cylinders.
Fiber optics. The technique of transmitting light or images
through long, thin, flexible fibers of plastic or glass. Bundles
of fibers are used to transmit complete images.
Fire sleeve. A covering of fire-resistant fabric used to protect
flexible fluid lines that are routed through areas subject to
high temperature.
Flame tubes. Small-diameter metal tubes that connect can-
type combustors in a turbine engine to carry the ignition
flame to all of the combustion chambers. The British call
combustion liners flame tubes.
Flameout. A condition of turbine engine operation when the fire unintentionally goes out. Improper air/fuel mixture
or interruption of the air flow through the engine can cause
a flameout.
Flash point. The temperature to which a liquid must be
raised for it to ignite, but not continue to burn when a flame
is passed above it.
Flashing the field. A maintenance procedure for a DC
generator that restores residual magnetism to the field frame,
A pulse of current from a battery is sent through the field
coils in the direction in which current normally flows. The
magnetic field produced by this current magnetizes the steel
frame of the generator.
Flashover. An ignition system malfunction in which the
high voltage in the magneto distributor jumps to the wrong
terminal. Flashover causes the wrong spark plug to fire.
This reduces the engine power and produces vibration and
excessive heat.
Flat-rated engine. A turboprop engine whose allowable
output power is less than the engine is physically capable
of producing.
Float carburetor. A fuel metering device that uses a float-
actuated needle valve to maintain fuel level slightly below
the edge of the discharge nozzle.
Flock. Pulverized wood or cotton fibers mixed with an
adhesive. Flock, attached to a wire screen, acts as an effective
induction air filter for small reciprocating engines.
Flow divider (reciprocating engine). The valve in an RSA
fuel injection system that divides the fuel from the fuel control
unit and distributes it to all of the cylinders. It compares with
the manifold valve in a Teledyne-Continental fuel injection
system.
Flow divider (turbine engine). A component in a turbine
engine fuel system that routes all of the fuel to the primary
nozzles or primary orifices when starting the engine or when
the rpm is low. When the engine speed builds up, the flow
divider shifts and opens a passage to send the majority of the
fuel to the secondary nozzles or orifices.
FMC. Flight management computer. An electronic flight
instrumentation system that enables the flight crew to initiate
and implement a given flight plan and monitor its execution.
FOD. Foreign object damage. Damage to components in
the gas path of a turbine engine, caused by ingested objects.
Debris from the runway or ramp cause FOD on the ground.
G-13Ice and birds cause most in-flight FOD.
Four-stroke cycle. A constant-volume cycle of energy
transformation that has separate strokes for intake,
compression, power, and exhaust.
Fractional distillation. Procedure used for separating
various components from a physical mixture of liquids. Crude
oil is a mixture of many different types of hydrocarbon fuels
which can be separated by carefully raising its temperature.
The first products to be released, those having the lowest
boiling points, are some of the gaseous fuels; next are
gasoline, kerosene, diesel fuel, heavy fuel oils, lubricating
oils, and finally, tar and asphalt.
Frangible. Capable of being broken.
Free-turbine engine. A gas turbine engine with a turbine
stage on a shaft independent of the shaft used to drive the
compressor. Free turbines are used to drive the propeller
reduction gear in a turboprop engine and the rotor
transmission in a helicopter.
Freezing point. The temperature at which solids, such as
wax crystals, separate from a hydrocarbon fuel as it is cooled.
Full-register position. The position of a magnet in a
magneto when its poles are aligned with the pole shoes and
the maximum amount of magnetic flux is flowing through
the magnetic circuit.
G
Gauge pressure. Pressure referenced from existing
atmospheric pressure.
Gas generator. The basic gas turbine engine. It consists of
the compressor, diffuser, combustor, and turbine. The gas
generator is also called the core engine.
Gas turbine engine. An internal combustion engine that
burns its fuel in a constant-pressure cycle and uses the
expansion of the air to drive a turbine which, in turn, rotates
a compressor. Energy beyond that needed to rotate the
compressor is used to produce torque or thrust.
General Aviation Airworthiness Alerts. While these
documents are no longer published, they are still available at
www.faa.gov. These are used to alert technicians of problems
that have been found in specific models of aircraft, and
reported on Malfunction and Defect Reports. Airworthiness
Alerts suggest corrective action, but compliance with the
suggestion is not mandatory.General aviation. A term used to describe the total field of
aviation operation except the military and airlines.
Geometric pitch. The distance a propeller would advance
in one revolution if it were rotating in a solid.
Geopotential of the tropopause. The point in the standard
atmosphere where the temperature stops dropping and
becomes constant. This is the tropopause, or the dividing line
between the troposphere and the stratosphere.
Gerotor pump. A form of constant-displacement pump
that uses an external-tooth drive gear that meshes with and
drives an internal-tooth gear that has one more space for a
tooth than there are teeth on the drive gear. Both gears turn
inside a close-tolerance housing. As the gears rotate, fluid
flows between the teeth that are beginning to un-mesh, and
is carried around the pump as the space continues to open
up. On the discharge side of the pump, the teeth becomes
smaller, fluid is forced out of the pump.
Glass flight deck. An aircraft instrument system that uses a
few color cathode-ray-tube displays to replace a large number
of mechanically actuated instruments.
Governor. A control used to automatically change the pitch
of a constant speed propeller to maintain a constant engine
rpm as air loads vary in flight.
GPU. Ground power unit. A service component used to
supply electrical power and compressed air to an aircraft
when it is operating on the ground.
Gross thrust. The thrust produced by a turbojet or turbofan
engine when the engine is static or not moving. The air is
considered to have no inlet velocity, and the velocity of the
gas leaving the engine is considered to be the acceleration
factor.
Ground-boosted engine. An aircraft reciprocating engine
with a built-in supercharger that boosts the sea-level rated
horsepower of the engine.
Gudgeon pin. The British name for a wrist pin, or piston
pin. See wrist pin.
H
Half-wave rectifier. An electrical rectifier circuit that
converts AC into pulsating DC. Only one alternation of each
cycle is present in the output.
G-14Halogenated hydrocarbon. A hydrocarbon compound in
which one or more hydrogen atoms have been replaced
with atoms of one of the halogen elements such as fluorine,
chlorine, or bromine.
Head of pressure. Pressure exerted by a column of fluid and
created by the height of the column.
Heat engine. A mechanical device that converts the chemical
energy in a fuel into heat energy. The heat energy is then
converted into mechanical energy and useful work.
Heli-Coil insert. The registered trade name of a special
helical insert used to restore threads stripped from a bolt
hole, or to reinforce the threads in an aluminum casting. The
damaged threads are drilled out and new threads are cut with
a special oversize tap. A coil of stainless steel wire, with a
cross section in the shape of a diamond, is screwed into the
hole and serves as the new threads. Heli-Coil inserts are
also used to provide durable threads in soft metal castings.
Some spark plug holes in aluminum alloy cylinder heads are
fitted with Heli-Coil inserts to minimize the wear caused by
repeated removal and installation of the spark plugs.
Helical spline. A spline that twists, or winds, around the
periphery of a shaft. Helical splines are used to change
linear motion into rotary motion of the shaft on which the
splines are cut.
Helical spring. A spring wound in the form of a helix, or coil.
Helix. A spiral.
Heptanes. An organic compound, CH 3(CH 2)5CH 3, that is
used as the low reference fuel for rating the antidetonation
characteristics of aviation gasoline.
Hermetically sealed. A complete seal, especially against the
escape or entry of air.
Hertz. A unit of frequency equal to one cycle per second.
High-bypass ratio engine. A turbofan engine whose bypass
ratio is 4:1 or greater.
High-pressure compressor. The second-stage compressor
in a dual-spool gas turbine engine. The high pressure
compressor is called the N2 compressor and is the one that
is rotated by the starter for starting, and the one whose rpm
is controlled by the fuel control.
High unmetered fuel pressure. Pressure in a Teledyne-
Continental fuel injector pump that is adjusted by the variable orifice.
Home-built aircraft. See amateur-built aircraft.
Honing (cylinder wall treatment). Scratching the surface
of the cylinder wall with an abrasive to produce a series of
grooves of microscopic depth and uniform pattern. The honed
pattern holds oil to lubricate the cylinder walls.
Horsepower. The most commonly used unit of mechanical
power. One horsepower is equal to 33,000 foot-pounds of
work done in one second.
Hot section. The portion of a gas turbine engine that
operates at a high temperature. The hot section includes the
combustion, turbine, and exhaust sections.
Hot-tank lubricating system. A turbine engine lubricating
system in which the oil cooler is located in the pressure
subsystem. The oil is returned to the tank without being
cooled.
HRD fire extinguisher. A fire extinguisher that carries the
extinguishing agent in a sealed sphere or cylinder. When the
agent-discharged switch is closed, an ignited powder charge
drives a cutter through a frangible disc which releases the
agent. The entire contents of the container is emptied in much
less than a second.
Hub (propeller component). The high-strength component
inside a propeller that attaches the blades to the engine
propeller shaft.
Hybrid compressor engine. A gas turbine engine that has
both centrifugal and axial-flow compressors.
Hybrid spark plug. A fine-wire spark plug that has a
platinum center electrode and iridium ground electrodes.
Hydraulic lock. A condition in which oil drains into the
lower cylinders of a reciprocating engine and leaks past the
piston rings to fill the combustion chamber. If the oil is not
removed before the engine is started, it can cause serious
damage.
Hydromechanical. Any device that combines fluid pressures
with mechanical actions to achieve a desired result. In a
hydromechanical fuel control used for a turbine engine,
hydraulic servos are used in conjunction with the mechanical
linkages.
G-15I
Ice bridging. A spark plug failure that occurs when starting
a reciprocating engine in extremely cold weather. When a
cylinder fires, the air/fuel mixture is converted into carbon
dioxide and water vapor. The water vapor condenses on the
spark plug electrodes and forms ice that bridges the electrode
gap and prevents the plug firing until the ice is melted. This
normally requires removing the spark plugs from the engine.
IDG. Integrated drive generator. An AC generator installed
on turbine engines. An IDG incorporates a brushless, three-
phase AC generator and a constant-speed drive in a single
component.
Igniter. The component in a turbine-engine ignition system
that provides a high-energy spark for igniting the air/fuel
mixture in the combustion chamber for starting.
IHP. Indicated horsepower. The theoretical horse-power a
reciprocating engine develops.
IMEP. Indicated mean effective pressure. The average
pressure existing inside the cylinder of a reciprocating engine
during its power stroke.
Impulse coupling. A spring-loaded coupling between a
magneto shaft and the drive gear inside the engine. When the
engine is rotated for starting, the impulse coupling locks the
magnet so it cannot turn. The spring in the coupling winds
up as the crankshaft continues to turn, and when the piston is
near top center, the coupling releases and spins the magnet,
producing a hot and retarded spark.
Inline engine. A reciprocating engine with all of the cylinders
arranged in a straight line.
Incandescent. Glowing because of intense heat.
Inconel. The registered trade name for an alloy of chromium,
iron, and nickel. Inconel is similar to stainless steel, but
cannot be hardened by heat treatment.
Inductive reactance. An opposition to the flow of AC or
changing DC caused by inductance in the circuit. Inductive
reactance, whose symbol is XL, causes a voltage drop, but
it does not use power nor produce heat.
Inertia. The tendency of a body to resist acceleration. A body
at rest will remain at rest or a body in motion will stay in
motion in a straight line unless acted on by an outside force.
Inertia starter. A starter for a large reciprocating engine that uses energy stored in a rapidly spinning flywheel to
turn the crankshaft.
Inlet guide vanes. A set of stator vanes in front of the first
stage of compression in a gas turbine engine. The inlet guide
vanes deflect the air entering the compressor in the correct
direction for optimum operation. Inlet guide vanes may be
fixed, or their angle may be controlled hydraulically by fuel
from the fuel control.
Integral fuel tank. An aircraft fuel tank made by sealing off
part of the structure so fuel can be carried in the structure itself.
Intercooler. An air-to-air heat exchanger installed between
a turbosupercharger and the carburetor. Intercoolers decrease
the temperature of compressed air to prevent detonation.
Interference angle (poppet valve dimension). The
difference between the valve seat and the valve face angles.
Normally, the valve seats are ground with between 0.5º and 1º
greater angle than the valve face. This allows the face to touch
the seat with a line contact that provides the best sealing.
Interference fit. A type of fit used when assembling certain
mechanical devices. The hole is made smaller than the part
that fits into it. The material containing the hole is heated to
expand the hole, and the part that fits into the hole is chilled
to shrink it. The parts are assembled, and when they reach
the same temperature their fit is so tight they will not loosen
in service.
Internal-combustion engine. A form of heat engine in which
the fuel and air mixture is burned inside the engine to heat
and expand the air so it can perform useful work.
Internal timing. The adjustment of the breaker points of a
magneto so they will begin to open at the time the magnet
is in its E-gap position.
Interpole. A field pole in a compound-wound DC generator
used to minimize armature reaction. Interpoles are located
between each of the regular field poles, and their coils are
in series with the armature winding so all of the armature
current flows through them. The magnetic field produced
by the interpole coils cancels the distortion caused by the
armature field and allows the brushed to remain in the neutral
plane where there is no potential difference between the
commutator segments. Keeping the brushes in the neutral
plane minimizes sparking.
Inverted engine. An inline or V-engine in which the
cylinders are mounted below the crankshaft.
G-16Iridium. A very hard, brittle, highly corrosion-resistant,
whitish-yellow, metallic chemical element. Iridium is used
for the fine-wire electrodes in spark plugs that must operate
in engines using fuel with an exceptionally high lead content.
Iso-octane. An organic compound used as the high reference
fuel for rating the antidetonation characteristics of aviation
gasoline (CH 3)2CHCH 2C(CH 3)3.
Isothermal change. A physical change that takes place
within a material in which heat energy is added to or taken
from the material as needed to keep its temperature constant.
J
Jet fuel. Fuel designed and produced to be used in aircraft
gas turbine engines.
Jet propulsion. A method of propulsion by accelerating a
relatively small mass of air through a large change in velocity.
Jeweler’s file. A small, fine-cut, metalworking file used by
jewelry manufacturers.
Joule. A measure of energy. In terms of electrical energy,
one joule is equal to one watt-second.
Journal (bearing). A hardened and polished surface on a
rotating shaft that rides in a plain bearing.
K
Kerosene. A light, almost colorless, hydrocarbon liquid
obtained from crude oil through the fractional distillation
process. Kerosene is the base for turbine engine fuel.
Kevlar. The registered trade name by DuPont for a patented
aramid fiber.
Kinematic viscosity. The ratio of the absolute viscosity
of a fluid to its density. Kinematic viscosity is measured in
centistokes.
L
Labyrinth seal. A type of air and/or seal used around the
main –shaft bearings in a gas turbine engine. The seal consists
of a series of rotating blades that almost contact the seal land.
A small amount of air flows between the seal and the land to
prevent oil flowing past the seal.
Land (piston) The portion of a piston between the ring
grooves.Land (splined shaft). The portion of a splined shaft between
the grooves.
Laser tachometer. A highly accurate tachometer that shines
a laser beam on a rotating element that has reflective tape
or a contrasting mark. The reflected laser beam is converted
into electrical pulses which are counted and displayed on a
monitoring instrument.
Last-chance oil filter. A small filter installed in the oil line
to the bearing jet in a gas turbine engine. This filter traps any
contaminants that have passed the main filter and holds them
until the engine is disassembled for overhaul.
LCD. Liquid crystal display. A digital display that consists
of two sheets of glass separated by a sealed-in, normally
transparent liquid crystal material. The outer surface of each
glass sheet has a transparent conductive coating with the
viewing side etched into character-forming segments with
leads going to the edges of the display. A voltage applied
between the front and back coatings disrupts the orderly
arrangement of molecules and causes the liquid to darken
so that light cannot pass through it. The segment to which
the voltage is applied appears as black against a reflected
background.
Leading edge. The thick edge at the front of a propeller blade.
Lean die-out. A condition in which the fire in a gas turbine
engine goes out because the air/fuel mixture ratio is too lean
to sustain combustion.
Lean mixture. A air/fuel mixture that contains more than
15 parts of air to 1 part of fuel, by weight.
Line boring. A method of assuring concentricity of bored
holes. A boring bar extends through all of the holes and cuts
the inside diameters so they all have the same center.
Link rod. The rod in a radial engine that connects one of
the piston wrist pins to a knuckle pin on the master rod. Also
called articulating rods.
Liquid cooling. The removal of unwanted heat from an
aircraft engine by transferring the heat into a liquid and
then passing the heated liquid through a liquid-to-air heat
exchanger (radiator) to transfer the heat into the ambient air.
Longitudinal magnetism. A method of magnetizing through a
solenoid, or coil, that encircles the part so the lines of magnetic
flux pass lengthwise through the part. Longitudinal magnetism
makes it possible to detect faults that extend across the part.
G-17Low bypass ratio engine. A turbofan engine whose bypass
ratio is less than 2:1.
Low-pressure compressor. The first-stage compressor in a
dual-spool gas turbine engine. The low-pressure compressor
is called the N1 compressor and its speed is not governed.
It seeks its own best speed as the atmospheric conditions
change so it can furnish a relatively constant mass of air to
the inlet of the second-stage compressor.
Low unmetered fuel pressure. Pressure in a Teledyne-
Continental fuel injector pump that is adjusted by the relief
valve.
LRU. Line replaceable unit. Aircraft components designed
to be replaced as a unit while the aircraft is on the flight line.
M
M&D (Malfunction and Defect) report. A small postcard-
like form (FAA Form 8330) used by repair stations,
maintenance shops, and technicians to report an unacceptable
condition to the FAA. Information on these forms provides
the basis for the General Airworthiness Alerts and subsequent
Airworthiness Directives.
Mach number. The ratio of the speed of an object through
the air to the speed of sound under the same atmospheric
conditions. An object traveling at the speed of sound is
traveling at Mach one (M1.0).
Magnesyn system. The registered trade name of a remote
indicating instrument system. A Magnesyn system uses a
permanent magnet as its rotor and a toroidal coil excited by
400-hertz AC as its stator. A small magnet in the center of
the indicator coil follows the movement of a larger magnet
in the transmitter coil.
Magnetic field. The invisible, but measurable, force
surrounding a permanent magnet or current-carrying
conductor. This field is produced when the orbital axes of
the electrons of the atoms in the material are all in alignment.
Magnetic flux. Lines of magnetic force that are assumed to
leave a magnet at its north end and return to its south end.
Lines of flux tend to be as short as possible and cannot cross
each other.
Magnetic particle inspection. A method of non-destructive
inspection for ferrous metal components. The part being
inspected is magnetized and then flooded with a solution of
iron oxide suspended in a light oil, much like kerosene. Any flaw, either on the surface or just below the surface, forms a
north and south pole, and the iron oxide attracted to these poles
helps locate the flaw. The iron oxide is normally treated with a
fluorescent dye, and the inspection is conducted in a darkened
booth. When an ultraviolet light (black light) is shone on the
part, the treated iron oxide shows up as a brilliant line.
Major alteration. An alteration not listed in the aircraft, aircraft
engine, or propeller specifications that might appreciably affect
weight, balance, structural strength, powerplant operation,
flight characteristics, or other qualities affecting airworthiness;
an alteration not done according to accepted practices, or one
that cannot be done by elementary operations.
Major overhaul. The disassembly, cleaning, and inspection
of an engine and the repair and replacement of all parts that
do not meet the manufacturer’s specification.
Major repair. A repair to a component that if improperly
done might appreciably affect weight, balance, structural
strength, performance, powerplant operation, flight
characteristics, or other qualities affecting airworthiness; a
repair not done according to accepted practices, or one that
cannot be done by elementary operations.
Mandrel. A precision steel bar on which a propeller is
mounted for balancing. The mandrel is placed across two
perfectly level knife-edge plates, and the propeller is allowed
to rotate until it stops with its heavy point at the bottom.
Manifold pressure. The absolute pressure of the air inside
the induction system of a reciprocating engine.
Manifold valve. See flow divider (reciprocating engine).
MAP. Manifold absolute pressure. The absolute pressure
that exists within the induction system of a reciprocating
engine. It is the MAP that forces air into the cylinders of the
engine. MAP is commonly called manifold pressure.
Mass. A measure of the amount of matter in an object. For
the purpose of measuring the mass of air flowing through a
turbine engine, the weight of the air, in pounds per second,
is divided by the acceleration due to gravity (32.3 feet per
second).
Matrix (advanced composites). The material that bonds
the fibers together in an advanced composite structure. The
matrix carries the stresses into the fibers.
Matter. Something that has mass, takes up space, and exists
as a solid, liquid, or gas.
G-18Medium-bypass ratio engine. A turbofan engine whose
bypass ratio is between 2:1 and 4:1.
MEK. Methul ethyl ketone. A volatile, water soluble, organic
chemical compound that is used as a solvent to remove oily
contaminants from ignition system components.
Methanol. Alcohol made from wood.
MFD. Multifunction display. A liquid crystal or CRT display
that shows a number of parameters and replaces several
analog-type indicators.
Microinches rms. A measure used for cylinder wall surface
roughness. Twenty microinches rms means that the highest
and lowest deviation from the average surface is 20 millionths
of an inch.
Micron. A measurement used to identify the size of particles
trapped by filters. One micron is a micro meter, or one
millionth of a meter. It is 0.000039 inch.
Microprocessor. A single silicon chip that contains the
arithmetic and logic functions of a computer.
Milliammeter. An instrument that measures electrical
current in units of thousandths of an ampere.
Millibar. A unit of pressure in the metric system. One bar is
a pressure of 14.5 psi, or 29.52 in. Hg. One millibar is one
thousandth of a bar, or 0.01469 psi, or 0.02952 in. Hg.
Minor alteration. Any alteration that does not fit the
definition of a major repair. See major repair.
Module (modular engine construction). The method of
construction for mast modern gas turbine engine. The engine
is made of several modules, or units, that can be removed and
replaced or serviced independent of the rest of the engine.
Momentum. A force caused by the inertia of a moving body
as it tries to keep the object moving in the same direction,
at the same speed.
Motor. (verb) The act of rotating a turbine engine using the
starter, with the ignition system deactivated. An engine is
motored to force air through it to purge fuel fumes.
Multiple-can combustor. A combustor used in a gas turbine
engine that consists of a series of individual burner cans,
each made of an inner liner and an outer case. The individual
cans are arranged around the periphery of a centrifugal
compressor. Hot gases flow directly from the cans into the turbine.
N
N1. A symbol representing the rotational speed of the low-
pressure compressor in a dual-spool gas turbine engine.
N2. A symbol representing the rotational speed of the high-
pressure compressor in a dual-spool gas turbine engine.
NACA. National Advisory Committee for Aeronautics.
This organization, dedicated to the technical development
of aviation, has been superseded by NASA.
NACA cowling. A long-chord cowling used over a
radial engine. The forward portion of this cowling has an
aerodynamic shape that produces a forward pull, and the
rear portion extends back to fair in with the fuselage. There
is a narrow peripheral gap between the rear of the cowling
and the fuselage for the cooling air to escape. Some NACA
cowlings have controllable flaps over this opening to control
the amount of cooling air that flows through the engine.
Nacelle. An enclosed compartment, normally in the leading
edge of the wing, in which an aircraft engine is mounted.
Naphtha. A volatile, flammable liquid distilled from
petroleum. It is used as a cleaning agent and solvent, and is
present in some blended turbine-engine fuels.
NASA. National Aeronautics and Space Administration.
Naturally aspirated engine. A reciprocating engine that
depends upon atmospheric pressure to force the air/fuel
mixture into the cylinders. Naturally aspirated engines are
neither supercharged nor turbocharged.
Net thrust. The thrust produced by a turbojet or turbofan
engine in which the acceleration factor is the difference
between the velocity of the incoming air and the velocity of
the exhaust gases leaving the engine.
Neutral position. The position of the magnet in a magneto
when its poles are between the pole shoes and no lines of
flux are flowing through the magnetic circuit.
Newton. The unit of force needed to accelerate a mass of
one kilogram one meter per second per second. One newton
is equal to 1000,000 dynes, or 2.248 × 10-1 pound.
Nichrome. The registered trade name for an alloy of nickel and
chromium. Nichrome wire is used for making electrical heater
elements and precision wire-wound resistors. Nichrome’s
G-19resistance is approximately 65 times that of copper.
Nitriding. A method of case hardening steel. Steel is placed
in a retort (a sealed, high-temperature furnace), and heated to
a specified temperature while surrounded by ammonia gas
(NH 3). The ammonia breaks down into nitrogen and hydrogen,
and the nitrogen unites with some of the alloying elements in
the steel to form an extremely hard surface. Nitriding hardens
crankshaft bearing surfaces and cylinder walls in reciprocating
engines. It takes place at a lower temperature than other forms
of case hardening, and does not cause warping.
Normal category airplane. An aircraft that is certificated
under 14 CFR part 23 that is not certificated under the
acrobatic, utility, or commuter category.
Normal shock wave. A type of pressure wave that forms at
right angles to a surface when air moves at the speed of sound.
Notch sensitivity. A measure of the loss of strength of a
material caused by the presence of a notch, or a V-shaped cut.
Nozzle guide vanes. See turbine inlet guide vanes.
O
Oblique shock wave. A pressure wave that forms on a sharp-
pointed object when air flows past it at a supersonic speed.
Octane rating. A system used to rate the antidetonation
characteristics of a reciprocating engine fuel. Fuel with an
octane rating of 80 performs in a laboratory test engine the
same as the fuel made of a mixture of 80% iso-octane and
20% heptanes.
Odometer. The portion of an automobile speedometer that
indicates the distance traveled.
Offset throw (crankshaft design). Crank arms on a
reciprocating engine crankshaft. The arms, or throws, to
which the connecting rods and pistons are attached are offset
from the center of the crankshaft to move the pistons in and
out of the cylinder. The amount of the offset determines the
stroke of the engine.
Oil analysis. A method of measuring the contents in parts
per million of various chemical elements in oil. A sample
of the oil is burned in an electric arc, and the resulting
light is analyzed with a spectroscope which identifies the
chemical elements in the oil and gives an indication of the
amount of each element. This type of oil analysis is called a
spectrometric oil analysis program, or SOAP.Oil dilution. A method of temporarily decreasing the
viscosity of the lubricating oil to make it possible to start
a reciprocating engine when the temperature is very low.
Before shutting the engine down, enough gasoline from the
fuel system is mixed with the lubricating oil in the engine to
dilute it so the starter can turn the engine over when the oil is
cold and viscous. When the engine starts and the oil warms
up, the gasoline evaporates.
Oil-damped bearing. A type of roller bearing installation
in a gas turbine engine in which the outer race is installed
in an oil damper compartment whose inside diameter is a
few thousandths of an inch larger than the outside diameter
of the outer race. Oil under pressure fills the oil damper
compartment and allows the bearing to compensate for sight
misalignment and to absorb vibrations of the shaft.
On-condition maintenance. A maintenance program that
closely monitors the operating condition of an engine and
allows major repairs or replacements to be made when engine
performance deteriorates to a specific level.
On-speed condition. The speed condition in which the engine
is turning at the rpm for which the propeller governor is set.
One-hundred-hour inspection. An inspection required by
14 CFR part 91, section 91.409 for FAA-certificated aircraft
operated for hire or used for flight instruction for hire. A
100-hour inspection is identical in content to an annual
inspection, but can be conducted by an aviation maintenance
technician who holds an Airframe and Powerplant rating, but
does not have an Inspection Authorization. See 14 CFR part
43, Appendix D for list of the items that must be included in
an annual or 100-hour inspection.
Operating cycle. One complete series of events in the
operation of a turbine engine that consists of starting the
engine, taking off, landing, and shutting the engine down.
Optoelectronic device. An electronic device that produces,
modulates, or senses electromagnetic radiation in the
ultraviolet, visible light, or infrared portions of the energy
spectrum.
Otto cycle. The constant-volume cycle of energy
transformation used by reciprocating engines. A mixture of
fuel and air is drawn into the cylinder as the piston moves
to the bottom of its stroke. The mixture is compressed as the
piston moves upward in the cylinder, and when the piston is
near the top of its stroke, the mixture is electrically ignited
and burns. The burning mixture heats and expands the air
inside the cylinder and forces the piston down, performing
useful work. The piston then moves back up, forcing the
G-20burned gases out of the cylinder.
Overboost. A condition of excessive manifold pressure
in a reciprocating engine. Overboosting occurs when the
supercharger is operated at too high a speed.
Overrunning clutch. A type of clutch that couples an input
shaft with an output shaft. When the input shaft is driven, the
output shaft rotates with it. When the output shaft is driven,
the output shaft rotates with it. But when the output shaft is
driven, the input shaft does not turn.
Overspeed condition. A speed condition in which the engine
is turning at an rpm higher than that for which the propeller
governor is set.
P
P-lead. Primary lead. The wire that connects the primary
winding of a magneto to the ignition switch. The magneto
is turned off by grounding its P-lead.
Pascal. The unit of pressure produced when one newton
of force acts uniformly over an area of one square meter.
One pascal is equal to 14.503 ∙ 10-5 (0.00014503) psi. The
kilopascal (kPa) is easier to manipulate. 1 kPa = 1,000 Pa
= 0.14503 psi.
PCB. Plenum chamber burning. A method of thrust
augmentation used on engines with vectored nozzles. Fuel
injected into the fan-discharge air is burned to increase thrust.
Peak voltage. The voltage of AC electricity that is measured
from zero voltage to the peak of either alternation.
Penetrant dwell time. The length of time a part is left in the
penetrant when preparing it for inspection by the fluorescent
or dye penetrant method. The hotter the part and the longer
the penetrant dwell time, the smaller the fault that will be
detected.
Performance number. The rating of antidetonation
characteristics of a reciprocating engine fuel that is better
than the high rating reference fuel, iso-octane. Performance
numbers are greater than 100.
Permanent magnet. A piece of hardened steel that has been
exposed to a strong magnetizing force which has aligned the
spin axes of the electrons surrounding its atoms. The high
retentivity of the material causes the electrons to retain their
magnetic orientation.
Permanent-mold casting. A casting made in a reusable metal mold. The walls of permanent-mold castings can be made
thinner than similar walls made by sand casting.
Permeability. A measure of the ease with which lines of
magnetic flux can pass through a material.
Phase sequence, or phase rotation. The sequence with
which the output phases of a three-phase generator are
connected to the load. Reversing the phase sequence of a
generator from A-B-C to A-C-B prevents the generator from
being synchronized with the others on the bus.
Pi (π) filter. An electronic filter used to prevent radio
frequency energy produced in the ignition exciter from
feeding back into the aircraft electrical system. The filter is
made of an inductor with a capacitor on its input and output.
The name is derived from the resemblance of the three
components on a schematic diagram to the Greek letter pi (π).
Pinion. A small gear that meshes with and drives a larger gear.
Piston (reciprocating engine component). The movable
plug inside the cylinder of a reciprocating engine. The piston
moves in and out to compress the air/fuel mixture and to
transmit the force from the expanding gas in the cylinder to
the crankshaft.
Piston pin. See wrist pin.
Pitch angle. The angle between the chordline of a propeller
blade and the plane of rotation. See blade angle.
Pitch distribution. The gradual change in pitch angle of a
propeller blade from the root to the tip.
Plane of rotation. The plane in which a propeller blade
rotates. The plane of rotation is perpendicular to the propeller
shaft.
Planetary gears. A type of large-ratio reduction gearing.
A series of small planetary gears are mounted on a spider
attached to the output shaft. The planetary gears rotate
between a fixed sun gear and a driven ring gear.
Plenum chamber. An enclosed chamber in which air can be
held at a pressure slightly higher than that of the surrounding
air. Plenum chambers are used to stabilize the pressure of the
air before it enters a double entry centrifugal compressor.
POH. Pilot’s Operating Handbook. A document published
by the airframe manufacturer and approved by the FAA that
lists the operating conditions for a particular model of aircraft.
Engine operating parameters are included in the POH.
G-21Pole shoe. Inward extensions from the field frame of a
generator around which the field coils are wound.
Poppet valve. A T-shaped valve with a circular head. Poppet
valves are used to cover the intake and exhaust openings in
the cylinder head of a reciprocating engine. The valves are
held closed by one or more coil springs and are opened by a
cam lobe or a rocker arm pushing on the end of the valve stem.
Porcelain. A hard, white, translucent ceramic material that
was used as the insulator in some of the early aircraft spark
plugs.
Positive-displacement pump. A fluid pump that moves
a specific volume of fluid each time it rotates. Spur-gear
pumps, gerotor pumps, and vane pumps are all positive-
displacement pumps.
Power. The time rate of doing work. Power is found by
dividing the amount of work done, measured in floor-pounds,
by the time in seconds or minutes used to do the work. Power
may be expressed in foot-pounds of work per minute or in
horsepower. One horsepower is 33,000 foot-pounds of work
done in one minute, or 550 foot pounds of work done in one
second.
Power-assurance check. A test run made of a gas turbine
engine to determine how its performance compares with its
precious performance as new or freshly overhauled.
Powerplant. The complete installation of an aircraft engine,
propeller, and all accessories needed for its proper function.
Pre-ignition. Ignition of the air/fuel mixture inside the
cylinder of an engine before the time for normal ignition.
Pre-ignition is often caused by incandescent objects inside
the cylinder.
Prepreg. Preimpregnated fabric. A type of composite
material in which the reinforcing fibers are encapsulated in
an uncured resin. Prepreg materials are cut to size and shape
and laid up with the correct ply orientation, and the entire
component is cured with heat and pressure.
Pressure. A measure of force applied uniformly over a given
unit of surface area.
Pressure altitude. The altitude in standard atmosphere at
which the pressure is the same as the existing pressure.
Pressure carburetor. A carburetor installed on some aircraft
reciprocating engines that uses the pressure difference between air inside the venture and ram air entering the
carburetor to produce a fuel-metering force. Pressure
carburetors have generally been replaced with continuous-
flow fuel injection systems.
Pressure cooling. A method of air cooling a reciprocating
engine in which the cylinders are enclosed in tight-fitting
shrouds. The cowling is divided into two compartments
by baffles and seals, with half of each cylinder in each
compartment. Ram air is directed into one compartment, and
the pressure in the other is decreased by air flowing over a
flared exit or adjustable cowl flaps. The pressure difference
across the cylinders causes cooling air to be drawn through
the fins to remove the unwanted heat.
Pressure-injection carburetor. A multibarrel pressure
carburetor used on large radial and V-engines. Fuel is
metered on the basis of air mass flowing into the engine
and is sprayed under pressure into the eye, or center, of the
internal supercharger impeller.
Prevailing torque. The torque required to turn a threaded
fastener before it contacts the surface it is intended to hold.
Primary winding. The winding in a magneto or ignition coil
that is between the source of voltage and the breaker points.
The primary winding is normally made of comparatively
large diameter wire, and has a small number of turns, typically
about 200.
Profile tip (compressor blade tip). The tip of an axial-flow
compressor bladed whose thickness is reduced to give it a
higher resonant frequency so it will not be subject to the
vibrations that would affect a blade with a squared tip. The
profile tip also provides a more aerodynamically efficient
shape for the high velocity air that is moved by the blade.
Profile tips often touch the housing and make a squealing
noise as the engine is shut down. For this reason profile tips
are often called squealer tips.
Profilometer. A precision measuring instrument used to
measure the depth of the hone marks in the surface of a
cylinder wall.
Prony brake. An instrument used to measure the amount of
horsepower an engine is delivering to its output shaft. The
engine is operated at a specific rpm, and a brake is applied
to its output shaft. The amount of torque applied to the brake
is measured, and this, with the rpm, is converted into brake
horsepower.
Propeller. A device for propelling an aircraft that has blades
on an engine-driven shaft and that, when rotated, produces by
G-22its action on the air, a thrust approximately perpendicular to
its plane of rotation. It includes control components normally
supplied by its manufacturer, but does not include main and
auxiliary rotors or rotating airfoils of engines.
Propeller end. The end of a reciprocating engine to which
the propeller is attached.
PropFan engine. The registered trade name by Hamilton
Standard of an ultra-high-bypass turbine engine. See UHB
engine.
Propulsive efficiency. A measure of the effectiveness with
which an aircraft engine converts the fuel it burns into useful
thrust. It is the ratio of the thrust horsepower produced by a
propeller to the torque horsepower of the shaft turning the
propeller. The nearer the speed of the aircraft is to the speed of
the exhaust jet or propeller wake, the less kinetic energy is lost
in the jet or wake, and the higher the propulsive efficiency.
PRT. Power recovery turbine. A turbine driven by exhaust
gases from several cylinders of a reciprocating engine.
Energy extracted from exhaust gases by the turbine is
coupled, through a fluid clutch, to the engine crankshaft.
Pulsating DC. Direct current whose voltage periodically
changes, but whose electrons flow in the same direction all
of the time.
Pulse-jet engine. A type of air-breathing reaction engine used
during World War II to power jet-propelled missiles. Fuel
is sprayed into the combustion chamber and ignited. As the
heated air expands, it closes the one-way shutter valve in the
front of the engine and exits the engine through the nozzle at
the rear. As soon as the pressure inside the combustion chamber
decreases, air enters through the shutter valve and more fuel is
ignited. The thrust is produced in a series of pulses.
Push fit. A fit between pieces in a mechanical assembly that
is close enough to require the parts to be pushed together. A
push fit is looser than a press fit, but closer than a free fit.
Pusher engine. An engine installed with the propeller
facing the rear of the aircraft. Thrust produced by the
propeller mounted on a pusher engine pushes rather than
pulls the aircraft.
Pusher propeller. A propeller installed on an aircraft engine
so that it faces the rear of the aircraft. Thrust from the
propeller pushes rather than pulls the aircraft.
PV diagram. A diagram showing the relationship between
the volume of a cylinder and the pressure during a cycle of engine operation.
Q
Quill shaft. A type of shaft used to couple parts of an engine
that are subject to torsional loads. A quill shaft is a long,
hardened steel shaft with splines on each end. One end splines
into the drive shaft and the other end splines into the device
being driven. Torsional vibrations are absorbed by the quill
shaft twisting.
R
Radial bearing load. The load on a bearing perpendicular
to the shaft on which the bearing is mounted. Centrifugal
loads are radial loads.
Radial engine (static radial). A form of reciprocating
engine in which the cylinders radiate out from a small
central crankcase. The pistons in the cylinders drive a central
crankshaft which in turn drives the propeller.
Radial-inflow turbine. A turbine, similar in appearance to a
centrifugal compressor rotor. Radial-inflow turbines are used
to drive the compressor in reciprocating engine turbochargers
and some of the smaller APU turbine engines. Hot gases flow
into the turbine from its outside rim, then radially inward
through the vanes and out of the turbine at its center.
Radiation. See electromagnetic radiation.
Ram air. Air whose pressure has been increased by the
forward motion of the aircraft. Ram air pressure is the same
as pitot pressure.
Ram drag. The loss of thrust produced by a turbojet or
turbofan engine caused by the increase of velocity of air
entering the engine. Ram drag is the difference between gross
thrust and net thrust.
Ram pressure. Pressure produced when a moving fluid is
stopped.
Ram-recovery speed. The speed of an aircraft at which the
ram effect caused by the forward movement increases the
air pressure at the compressor inlet so that it is the same as
that of the ambient air.
Ramjet engine. The simplest type of air-breathing reaction
engine. Air entering the front of the engine at a high velocity
has fuel sprayed into it and ignited. A barrier formed by the
incoming air forces the expanding gases to leave through
the nozzle at the rear. The energy added by the burning fuel
G-23accelerates the air and produces a forward thrust. Ramjet
engines are used in some military unmanned aircraft that
are initially boosted to a speed high enough for the engine
to function.
Ratiometer indicator. An analog temperature measuring
instrument in which the pointer deflection is proportional to
the ratio between the current flowing in an internal reference
circuit and that flowing through the temperature-sensing
probe.
Reach (spark plug specification). The length of the threads
on the shell of a spark plug.
Reaction engine. A form of heat engine that produces thrust
by heating a mass of air inside the engine and discharging
it at a high velocity through a specially shaped nozzle. The
amount of thrust is determined by the mass of the air and the
amount it is accelerated.
Reactive power. Wattless power in an AC circuit. It is the
power consumed in the inductive and capacitive reactances.
Reactive power is expressed in volt-amps reactive (var) or
in kilovolt-amps reactive (kvar).
Reamed fir. The fit of a shaft in a hole in which the hole
is drilled undersize and cut with a reamer to the correct
diameter. Reamed holes have smooth walls and a consistent
diameter.
Rebuilt engine. A used engine that has been completely
disassembled, inspected, repaired as necessary, and
reassembled, tested, and approved in the same manner and to
the same tolerances and limits as a new engine, using either
new or used parts. However, all parts used must conform to
all production drawings, tolerances, and limits for new parts,
or be of approved oversize or undersize dimensions for a
new engine. According to 14 CFR part 91, section 91.421,
a rebuilt engine is considered to have no precious operating
history and may be issued a zero-time logbook. Only the
engine manufacturer can rebuild an engine and issue a zero-
time record.
Reciprocating engine. A type of heat engine that changes
the reciprocating (back-and-forth) motion of pistons inside
the cylinders into rotary motion of a crank-shaft.
Rectifier. A device that allows electrons to flow in one
direction while preventing their flow in the opposite direction.
Rectifiers are used to change AC into DC.
Reheat system. The British name for an afterburner. See
afterburner.Reid vapor pressure. The amount of pressure that must be
exerted on a liquid to keep it from vaporizing. Reid vapor
pressure is measured at 100 ºF.
Reliability. The ability of an aircraft engine to perform its
designed functions under widely varying operating conditions.
Residual magnetic particle inspection. A form of magnetic
particle inspection for small steel parts that have a high degree
of retentivity. The part is magnetized, removed, and inspected
away from the magnetizing machine.
Residual magnetism. The magnetism that remains in the
field frame of a generator when no current is flowing in the
field coils.
Residual voltage. The voltage produced in a generator
armature when the armature is rotated in the residual
magnetism.
Resistor spark plug. A shielded spark plug with a resistor
between the ignition lead terminal and the center electrode.
The resistor stops the flow of secondary current when its
voltage drops to a specified value. The resistor prevents
capacitive afterfiring.
Retarded sparks. The timing of the firing of the spark plugs
used to start a reciprocating engine. The sparks for starting
occur later in terms of crankshaft rotation than those used for
normal operation. Retarding the sparks prevent the engine
from kicking back when it is being started.
Retentivity. The ability of a magnetizable material to retain
the alignment of the magnetic domains after the magnetizing
force has been removed. Hard steel normally has a high
retentivity, while soft iron and electrical steel both have very
low retentivity.
Reverse-flow combustor. A type of combustor in which
the air from the compressor enters the combustor outer case
and reverses its direction as it flows into the inner liner. It
again reverses its direction as it flows into the inner liner.
It again reverses its direction before it flows through the
turbine. Reverse-flow combustors are used where engine
length is critical.
RF energy. Electromagnetic energy with a frequency high
enough to radiate from any conductor through which it is
flowing.
Rich blowout. A condition in which the fire in a gas turbine
engine goes out because the air/fuel mixture ratio is too rich
G-24to sustain combustion.
Rich mixture. A air/fuel mixture that contains less than 15
parts of air to 1 part of fuel, by weight.
Riffle file. A hand file with its teeth formed on a curved
surface that resembles a spoon.
Rms. Root mean square. A dimension that is the square root
of the average of an infinite number of varying values. An rms
dimension is used to indicate the allowable surface roughness
of a reciprocating engine cylinder wall.
Rocker arm. A pivoted arm on the cylinder head of a
reciprocating engine. The pushrod forces one end of the
rocker arm up, and as the other end moves down, it forces
the poppet valve off of its seat.
Rocker box. The enclosed part of a reciprocating engine
cylinder that houses the rocker arm and valve mechanism.
Rocket engine. A form of reaction engine whose fuel and
oxidizer contain all of the oxygen needed for the release of
heat energy. The released heat expands the gases which are
ejected at a high velocity from a nozzle at the rear of the
rocket. Because rocket engines carry their own oxygen, they
can operate in outer space where there is no atmosphere.
Rotary radial engine. A form of reciprocating engine used
in some early aircraft. The crankshaft is rigidly attached to
the airframe, and the propeller, crankcase, and cylinders all
revolve as a unit.
Rotating combustion (RC) engine. A form of internal
combustion engine in which a rounded, triangular-shaped
rotor with sliding seals at the apexes forms the combustion
space inside an hourglass-shaped chamber. Expanding gases
from the burning air/fuel mixture push the rotor around and
turn a geared drive shaft in its center. The RC engine was
conceived in Germany by Felix Wankel in 1955.
RPM. Revolutions per minute. A measure of rotational speed.
One rpm is one revolution made in one minute.
Run in. A time of controlled operation of a new or freshly
overhauled engine that allows the moving parts to wear
together.
Run up. A procedure in which an aircraft engine is operated
on the ground to determine its condition and performance.
Runout. A measure of the amount a shaft, flange, or disc is
bent or fails to run true. Runout is normally measured with a dial indicator.
S
SAE. Society of Automotive Engineers. A professional
organization that has formulated standards for the automotive
and aviation industries.
Safety gap. A location in a magneto that allows a spark to
jump to ground from the secondary circuit before the voltage
rises high enough to damage the secondary insulation.
Sand casting. A method of molding metal parts in a mold
made of sand. A pattern that duplicates the part to be molded
is made of wood and is covered with a special casting sand
that contains a resin to bind it. The mold is separated along
a special parting line, and the pattern is removed. The mold
is put back together, and molten metal is poured into the
cavity. When the metal cools, the sand is broken away
from the molded part. Sand casting is less expensive than
permanent-mold casting.
Saybolt Seconds Universal (SSU) viscosity. A measurement
of viscosity (resistance to flow)of a lubricating oil. The
number of seconds needed for 60 milliliters of oil at a
specified temperature to flow through a calibrated orifice.
The viscosity number used for commercial aviation engine
lubricating oil relates closely to the SSU viscosity of the oil
at 210 ºF.
Scavenge subsystem. The subsystem in the lubrication
system of a gas turbine engine that collects oil after it has
lubricated the bearings and gears and returns it to the oil tank.
Scimitar shape. The shape of the blades of the propellers
mounted on UHB engines. The name is derived from the
shape of a curved Asian sword that has its edge on the convex
side. See UHB engine.
Scramjet. Supersonic combustion ramjet. A special type of
ramjet engine whose fuel can be ignited while the vehicle is
mobbing at a supersonic speed.
Scuffing. Severe damage to moving parts caused when one
metal part moves across another without sufficient lubricant
between them. Enough heat is generated by friction to cause
the high points of the surfaces to weld together; continued
movement tears, or scuffs, the metal.
Sea-level boosted engine. A reciprocating engine that has had
its sea-level rated horsepower increased by supercharging.
This is the same as a ground-boosted engine.
G-25Secondary winding. The winding in a magneto or ignition
coil that connects to the distributor rotor. The secondary
winding is normally made of very small diameter wire and
has a large number of turns, typically about 20,000.
Self-accelerating speed. The speed attained by a gas turbine
engine during start-up that allows it to accelerate to its normal
idling speed without assistance from the starter.
Semiconductor transducer. A piezoelectric crystal that
converts input energy of one form, such as pressure, into
output energy of another, such as an electrical signal.
Series-wound motor. An electric motor with field coils
connected in series with the armature.
Serviceable limits. Limits included in a reciprocating engine
overhaul manual. If a part measures outside of the new-parts
limits, but within the serviceable limits, it will not likely
wear to the point of causing engine failure within the next
TBO interval.
Servo system. A type of automatic control system in which
part of the output is fed back into the input.
Shaft horsepower. The horsepower actually available at a
rotating shaft.
Shielding. The electrically conductive covering placed
around an electrical component to intercept and conduct to
ground any electromagnetic energy radiated from the device.
Short circuit. A low-resistance connection between two
points in an electric circuit.
Shower of Sparks ignition system. A patented ignition
system for reciprocating engines. An induction vibrator
sends pulsating DC into a set of retard breaker points on one
of the magnetos. This provides a hot and retarded spark for
starting the engine.
Single-shaft turbine engine. A turboprop engine in which
the propeller reduction gears are driven by the same shaft
that drives the compressor for the gas generator.
Single-spool gas-turbine engine. A type of axial-flow-
compressor gas turbine engine that has only one rotating
element.
Skin radiator. A type of radiator used on some early liquid-
cooled racing airplanes. The radiator was made of two thin
sheets of brass, slightly separated so the heated coolant could
flow between them. Skin radiators were mounted on the surface of the wing, on the sides of the fuselage, or on the
floats of seaplanes. Air flowing over the smooth surface of
the radiator removed heat from the coolant.
Slip (propeller specification). The difference between the
geometric and effective pitch of a propeller.
Slip ring. A smooth, continuous ring of brass or copper
mounted on the rotor shaft of an electrical generator or
alternator. Brushes riding on the smooth surface of the slip
ring carry current into and out of the rotor coil.
Slow-blow fuse. A special type of electrical circuit protection
device that allows a momentary flow of excess current, but
opens the circuit if the excessive flow is sustained.
Sludge. A heavy contaminant that forms in an aircraft
engine lubricating oil because of oxidation and chemical
decomposition of the oil.
Sludge plugs. Spool-shaped sheet metal plugs installed in
the hollow throws of some engine crankshafts.
Slug. The unit of mass equal to that which experiences an
acceleration of one foot per second, per second when a force
of one pound acts on it. It is equal to 32.174 pounds, or
14.5939 kilograms, of mass. Also called a G-pound.
SOAP. Spectrometric oil analysis program. An oil analysis
program in which a sample of oil is burned in an electric
arc and an analysis is made of the wavelength composition
of the resulting light. Each chemical element in the oil,
when burned, produces light containing a unique band of
frequencies. A computer analyzes the amount of each band
of frequencies and prints out the number of parts of the
element per million parts of the entire sample. SOAP can
predict engine problems by warning the engine operator of
an uncharacteristic increase of any elements in the oil.
Sound suppressor. The airframe component that replaces
the turbine engine tail pipe. It reduces the distance the sounds
made by the exhaust gases propagate by converting low-
frequency vibrations.
Specific gravity. The ratio of the density of a material to the
density of pure water.
Specific weight. The ratio of the weight of an aircraft engine
to the brake horsepower it develops.
Spline. Parallel slots cut in the periphery of a shaft, parallel to
its length. Matching slots, cut into the hub or wheel that fits
on the shaft, lock the shaft into the device to transmit torque.
G-26Sprag clutch. A freewheeling, nonreversible clutch that allows
torque to be applied to a driven unit in one direction only.
Springback. A condition in the rigging of an aircraft engine
control in which the stop at the engine is reached before the
stop in the flight deck. The flight deck control moves slightly
after the stop in the engine is reached, and when it is released,
it springs back slightly.
Spur-gear pump. A form of constant-displacement fluid
pump that uses two meshing spur-gears mounted in a close
fitting housing. Fluid is taken into the housing where it fills
the space between the teeth of the gears and is carried around
the housing as the gears rotate. On the discharge side of the
pump, the teeth of the two gears mesh, and the fluid is forced
out of the pump.
Squat switch. An electrical switch actuated by the landing
gear scissors on the oleo strut. When no weight is on the
landing gear, the oleo piston is extended and the switch is in
one position; but when weight is on the gear, the oleo strut
compresses and the switch changes its position.
Squealer tip (compressor blade tip). See profile tip.
Squeeze film bearings. Another name for oil-damped
bearings. See oil-damped bearings.
Stage length. The distance between landing points in airline
operation.
Stage of a compressor. One disc of rotor blades and the
following set of stator vanes in an axial-flow compressor.
Staggered timing. Ignition timing that causes the spark plug
nearest the exhaust valve to fire a few degrees of crankshaft
rotation before the spark plug nearest the intake valve.
Standard day conditions. Conditions that have been decided
upon by the ICAO for comparing all aircraft and engine
performance. The most basic standard day conditions are:
temperature, 15 ºC or 59 ºF; altitude, mean sea level; pressure,
29.92 inches of mercury.
Standard J-1. A World War I training airplane powered by
a Curtiss OX-5 engine.
Standpipe. A pipe which protrudes upward from the base
of an oil tank and through which oil used for normal engine
lubrication is drawn. In the event of a catastrophic leak when
all oil available to the engine-driven pump is lost overboard,
enough oil is available from an outlet below the standpipe to feather the propeller.
Starter-generator. A single-component starter and generator
used on many smaller gas-turbine engines. It is used to start
the engine, and when the engine is running, its circuitry is
shifted so that it acts as a generator.
Static pressure. The pressure of an unmoving fluid.
Static rpm. The number of revolutions per minute an aircraft
engine can produce when the aircraft is not moving.
Steam cooling. A method of liquid cooling in which the
coolant, normally water, is allowed to absorb enough heat
that it boils. The steam gives up its heat when it condenses
back into a liquid.
Stellite. A nonferrous alloy of cobalt, chromium, and tungsten.
Stellite is hard, water resistant, and corrosion resistant, and it
does not soften until its temperature is extremely high. Stellite
is welded to the faces of many reciprocating engine exhaust
valves that operate at very high temperatures.
Stepping motor. A precision electric motor whose output
shaft position is changed in steps by pulses from the control
device. Stepping motors can make high-torque changes in
small angular increments to their output shaft.
Stoichiometric mixture. The air/fuel mixture ratio that,
when burned, leaves no uncombined oxygen nor any free
carbon. It releases the maximum amount of heat, and
therefore produces the highest exhaust gas temperature. A
stoichiometric mixture of gasoline and air contains 15 pounds
of air for 1 pound of gasoline.
Straight-through combustor. A combustor in a gas turbine
engine through which the air from the compressor to the
turbine flows in an essentially straight line.
Stratosphere. The upper part of the Earth’s atmosphere. The
stratosphere extends upward from the tropopause, which is
approximately 36,000 feet above the surface of the Earth, to
approximately 85,000 feet. The temperature of the air in the
stratosphere remains constant at -56.5 ºC (-69.7 ºF).
Stress. A force within an object that tries to prevent an outside
force from changing its shape.
Stroboscopic tachometer. A tachometer used to measure
the speed of any rotating device without physical contact.
A highly accurate variable-frequency oscillator triggers a
high-intensity strobe light. When the lamp is flashing at the
same frequency the device is rotating, the device appears to
G-27stand still.
Stroke. The distance the piston moves inside the cylinder.
Sump (aircraft engine component). A low point in an
aircraft engine in which lubricating oil collects and is stored
or transferred to an external oil tank. A removable sump
attached to the bottom of the crankcase of a reciprocating
engine is often called an oil pan.
Sump (fuel tank component). A low point in an aircraft fuel
tank in which water and other contaminants collect and are
held until they can be drained out.
Supercharged engine. A reciprocating engine that uses a
mechanically driven compressor to increase the air pressure
before it enters the engine cylinders.
Supercharger. An air compressor used to increase the
pressure of the air being taken into the cylinders of a
reciprocating engine.
Surface roughness. The condition of the surface of a
reciprocating engine cylinder wall that has been honed to
make it hold lubricating oil. Surface roughness is measured
in micro-inches rms.
Surge. A condition of unstable airflow, through the
compressor of a gas turbine engine, in which the compressor
blades have an excessive angle of attack. Surge usually affects
an entire stage of compression.
Synthetic oil. Oil made by chemical synthesis of a mineral,
animal, or vegetable base. Synthetic oils have appropriate
additives that give them such characteristics as low volatility,
low pour point, high viscosity index, good lubricating
qualities, low coke and lacquer formation, and low foaming.
T
Tachometer. An instrument that measures the rotational
speed of an object.
TAI. Thermal anti-ice. A system used to prevent the
formation of ice on an aircraft by flowing heated air inside
the structure.
Tail pipe. The portion of the exhaust system of a gas turbine
engine through which the gases leave. The tail pipe is often
called the exhaust duct, or exhaust pipe.
TBO. Time between overhauls. A time period specified by
the manufacturer of an aircraft engine as the maximum length of time an engine should be operated between overhauls
without normal wear causing parts of the engine to be worn
beyond safe limits. TBO depends upon proper operation and
maintenance in accordance with the engine manufacturer’s
recommendations. The overhaul of an engine when it
reaches its TBO hours is not mandatory, except for certain
commercial operators that have the requirement written into
their operations manual.
TDC. Top dead center. The position of a piston in a
reciprocating engine when the piston is at the top of its stroke
and the wrist pin, crankpin, and center of the crankshaft are
all in line.
TEL. Tetraethyl lead.
Test club. A wide-blade, short-diameter propeller used on a
reciprocating engine when it is run in a test cell. A test club
applies a specific load to the engine and forces the maximum
amount of air through the engine cooling fins.
Thermal efficiency. The ratio of the amount of useful work
produced by a heat engine, to the amount of work that could
be done by all of the heat energy available in the fuel burned.
Thermal expansion coefficient. A number that relates to
the change in the physical dimensions of a material as the
temperature of the material changes. The thermal expansion
coefficient of aluminum is approximately twice that of steel.
Thermal shock. The sudden change in engine operating
temperature that occurs when engine power is suddenly
reduced at the same time the airspeed, thus the cooling, is
increased. Thermal shock occurs when an aircraft is required
to rapidly descend to a lower altitude.
Thermistor. A semiconductor material whose electrical
resistance varies with its temperature.
Thermocouple. A device used to generate an electrical current.
A thermocouple is made of two dissimilar metal wires whose
ends are welded together to form a loop. A voltage exists in
the loop proportional to the difference in temperature of the
junctions at which the wires are joined. The amount of current
flowing in the loop is determined by the types of metals used
for the wires, the temperature difference between the junctions,
and the resistance of the wires.
Thermosetting resin. A plastic resin that, once it has been
hardened by heat, cannot be softened by heating again.
Thermostatic valve. A temperature-sensitive valve that
controls the temperature of oil in an aircraft engine. When
G-28the oil is cold, the valve shifts and directs the oil through
the oil cooler.
Thermoswitch. An electrical switch that closes a circuit
when it is exposed to a specified high temperature.
Three-dimensional cam. A drum-shaped cam in a hydro-
mechanical fuel control whose outer surface is ground so that
followers riding on the surface, as the cam is moved up and
down and rotated, can move mechanical linkages to control
the fuel according to a preprogrammed schedule.
Throttle. The control in an aircraft that regulates the power
or thrust the pilot wants the engine to produce.
Throw (crankshaft design). See offset throw.
Thrust horsepower. The horsepower equivalent of the thrust
produced by a turbojet engine. Thrust horsepower is found by
multiplying the net thrust of the engine, measured in pounds,
by the speed of the aircraft, measured in miles per hour, and
then dividing this by 375.
Thrust. The aerodynamic force produced by a propeller or
turbojet engine as it forces a mass of air to the rear, behind
the aircraft. A propeller produces its thrust by accelerating
a large mass of air by a relatively small amount. A turbojet
engine produces its thrust by accelerating a smaller mass of
air by a much larger amount.
Time-Rite indicator. A patented piston-position indicator
used to find the position of the piston in the cylinder of a
reciprocating engine. The body of the Time-Rite indicator
screws into a spark plug hole, and as the piston moves
outward in the cylinder, it contacts the arm of the indicator. A
pointer contacted by the arm moves across a calibrated scale
to show the location of the piston in degrees of crankshaft
rotation before top center.
Timing light. An indicator light used when timing magnetos
to an engine to indicate when the breaker points open. Some
timing lights incorporate an oscillator or buzzer that changes
its pitch when the points open.
TIT. Turbine inlet temperature. The temperature of the
gases from the combustion section of a gas turbine engine
as they enter the turbine inlet guide vanes or the first stage
of the turbine.
Toggle. A T-shaped handle fitted onto the end of a cable
used to engage a simple starter with an overrunning clutch.
Top overhaul. An overhaul of the cylinders of an aircraft engine. The valves, pistons, and cylinders are overhauled,
but the crankcase is not opened.
Torque. A force that produces or tries to produce rotation.
Total pressure. The pressure a column of moving fluid
would have if it were stopped from its motion. Total pressure
is the sum of dynamic pressure and static pressure.
Total temperature. The temperature of moving fluid that
has been stopped from its motion. Total temperature is the
sum of static temperature and the temperature rise caused by
the ram effect as the fluid was stopped.
Townend ring. A type of ring cowling used over a single-row
radial engine. The cross section of the ring is in the form of an
airfoil that produces enough forward thrust to compensate for
the cooling drag of the engine. In the United States, townend
rings are often called speed rings.
Track. The path followed by a blade segment of a propeller
or helicopter rotor in one rotation.
Tractor engine. An engine installed with the propeller facing
the front of the aircraft. Thrust produced by the propeller
mounted on a tractor engine pulls the aircraft through the air.
tractor propeller. A propeller mounted on an airplane in such
a way that its thrust pulls the aircraft.
Trailing edge. The thin edge at the rear of a propeller blade.
Transducer. A device that changes energy from one form
to another. Commonly used transducers change mechanical
movement or pressures into electrical signals.
Transformer. An electrical component used to change the
voltage and current in an AC circuit.
Transonic range. Flight at Mach numbers between 0.8
and 1.2. In this range, some air passing over the aircraft is
subsonic, and some is supersonic.
Trend monitoring. A system for comparing engine
performance parameters with a baseline of these same
parameters established when the engine was new or newly
overhauled. Parameters such as EGT, rpm, fuel flow, and oil
consumption are monitored on every flight, and the baseline
is plotted. Any deviation from a normal increase or decrease
warns the technician of an impending problem.
Tricresyl phosphate (TCP). A colorless, combustible
compound, (CH 3C6H4O)3PO, that is used as a plasticizer in
aircraft dope and an additive in gasoline and lubricating oil.
G-29TCP aids in scavenging lead deposits left in the cylinders
when leaded fuel is burned.
TSFC. Thrust specific fuel consumption. A measure of
efficiency of a turbojet or turbofan engine. It is a measure
of the number of pounds of fuel burned per hour for each
pound of thrust produced.
Turbine. A wheel fitted with vanes, or buckets, radiating
outward from its circumference. The reactive or aerodynamic
force caused by the fluid flowing through the vanes is
converted into mechanical power that spins the shaft on
which the wheel is mounted.
Turbine engine. See gas turbine engine.
Turbine inlet guide vanes. A series of stator vanes
immediately ahead of the first-stage turbine. The function of
the inlet guide vanes is to divert the hot gases in the proper
direction to enter the turbine, and to provide a series of
convergent ducts which increase the velocity of the gases.
Turbine nozzle. Another name for turbine inlet guide vanes.
Turbocharger. An exhaust-driven air compressor used to
increase the power of a reciprocating engine. A turbocharger
uses a small radial inflow turbine in the exhaust system to
drive a centrifugal-type air compressor on the turbine shaft.
The compressed air is directed into the engine cylinders to
increase power.
Turbo-compound engine. A reciprocating engine that has
power recovery turbines in its exhaust system. The power
extracted from the exhaust by these turbines is directed into
the engine crankshaft through a fluid coupling.
Turbofan engine. A type of gas turbine engine that has a
set of lengthened blades on the low-pressure compressor
or low-pressure turbine. Air moved by these special blades
bypasses the core engine and produces between 30% and
75% of the total thrust.
Turbojet engine. A gas turbine engine that produces thrust by
accelerating the air flowing through it. A minimum of energy
is extracted by the turbine, with the majority used to produce
an exhaust velocity much greater than the inlet velocity. The
amount of thrust produced by the engine is determined by the
amount the air is accelerated as it flows through the engine.
Turboprop engine. A turbine engine in which several stages
of turbines are used to extract as much energy as possible. The
turbines drive reduction gears which in turn drive a propeller.Turboshaft engine. A turbine engine in which several stages
of turbines are used to extract as much energy as possible.
The turbines drive shafts which are used to drive helicopter
rotors, generators, or pumps.
Turbosupercharger. A centrifugal air compressor driven by
exhaust gases flowing through a turbine. The compressed air
is used to increase the power produced by a reciprocating
engine at altitude.
Two-spool engine. See dual-spool gas turbine engine.
Two-stroke cycle. A constant-volume cycle of energy
transformation that completes its operating cycle in two
strikes of the piston, one up and one down. When the piston
moves up, fuel is pulled into the crankcase, and at the same
time the air/fuel mixture inside the cylinder is compressed.
When the piston is near the top of its stroke, a spark plug
ignites the compressed air/fuel mixture, and the burning and
expanding gases force the piston down. Near the bottom of
the stroke, the piston uncovers an exhaust port and the burned
gases leave the cylinder. When the piston moves further
down, it uncovers the intake port, and a fresh charge of fuel
and air are forced from the crankcase into the cylinder.
U
UDF engine. Un-ducted Fan™. The trade name registered
by General Electric for a type of ultra-high-bypass turbofan
engine that drives one or more wide-blade propellers that have
between eight and twelve blades. These blades, which are not
enclosed in a duct or shroud, are very thin, have wide chords,
and are highly swept back in a scimitar shape that enables them
to power airplanes flying in the speed range near Mach 0.8.
UHB (ultra-high-bypass) engine. A turbine that drives a pair
of ducted or un-ducted contrarotating propellers which have
eight to 12 variable-pitch blades. These blades are very thin,
have wide chords, and are swept back with a scimitar shape
that allows them to power airplanes flying in the speed range
of Mach 0.8. The blades are made of advanced composites
for high strength and light weight. USH engines may be of
either the tractor or pusher type, and have a bypass ratio in
excess of 30:1.
Underspeed condition. A speed condition in which the
engine is turning at an RPM lower than that for which the
propeller governor is set.
Unidirectional fibers. Fibers in a piece of composite material
arranged so that they sustain loads in only one direction.
Updraft carburetor. A carburetor that mounts on the bottom
G-30of a reciprocating engine. Air entering the engine flows
upward through the carburetor.
Upper-deck pressure. The absolute pressure of air at the
inlet to the fuel metering system of a turbocharged engine.
Upper-deck pressure is the same as the turbocharger
discharge pressure.
V
V-blocks. A fixture that allows a shaft to be centered and
rotated to measure any out-of-round condition.
V-engine. A form of reciprocating engine in which the
cylinders are arranged in two banks. The banks are separated
by an angle of between 45º and 90º. Pistons in two cylinders,
one in each bank, are connected to each throw of the
crankshaft.
Valence electrons. Electrons in the outer shell, or ring,
around the nucleus of an atom. It is the valence electrons
that give an atom its electrical characteristics and are the
electrons that may be pulled loose from an atom to cause
electrical current.
Valve overlap. The portion of the operating cycle of a four-
stroke-cycle reciprocating engine during which both the intake
and exhaust valves are off of their seats at the same time.
Vapor lock. A condition of fuel starvation that can occur
in a reciprocating engine fuel system. If the fuel in the line
between the tank and carburetor is heated enough for the
fuel to vaporize, a bubble will form in the line. If the vapor
pressure of the bubble is high enough, it will block the fuel
and keep it from flowing to the engine.
Vapor pressure. The amount of pressure needed above a
liquid to prevent it from evaporating.
Vaporize. The changing of a liquid into a vapor.
Vectored-thrust engine. A turbojet or turbofan engine with
the fan and/or exhaust nozzles mounted in such a way that
they may be rotated in flight to produce forward, vertically
upward, or rearward thrust.
Velocity. A vector quantity that expresses both the speed
an object is moving and the direction in which it is moving.
Velocity turbine. A turbine driven by forces produced by the
velocity, rather than the pressure, of gases flowing through
the vanes.Venture. A specially shaped restrictor in a tube designed to
speed up the flow of fluid passing through it. According to
Bernoulli’s principal, any time the flow of fluid speeds up
without losing or gaining any energy from the outside, the
pressure of the fluid decreases.
Vernier coupling. A timing coupling used with base-
mounted magnetos. The vernier coupling allows the timing
to be adjusted in increments of considerably less than one
degree.
Vertical tape instrument. A tall rectangular instrument that
displays the quantity of the parameter being measured by a
movable strip of colored tape. The presentation resembles a
vertical bar graph.
Vibration loop. A loop in a rigid fluid line used to prevent
vibration from concentrating stresses that could cause the
line to break.
VIFF. Vectoring in forward flight. A method of enhancing
the maneuverability of an airplane by vectoring the exhaust
gases and/or fan-discharge air to produce thrust components
not parallel to the longitudinal axis of the aircraft.
Viscosimeter. An instrument used to measure the viscosity
of a liquid. The time required for a given volume of liquid at
a specified temperature to flow through a calibrated orifice
is used to indicate the viscosity of the liquid.
Viscosity. The resistance of a fluid to flow. Viscosity is the
stiffness of the fluid, or its internal friction.
Viscosity index (VI). A measure of change in viscosity of an
oil as it changes temperature. The higher the viscosity index,
the less the viscosity changes.
Viscosity index improver. An additive used to produce a
multi-viscosity lubricating oil. The polymer additive expands
as temperature increases and contracts as temperature
decreases. VI improvers cause viscosity to increase as oil
heats and decrease as it cools.
Volatile memory. Computer memory that is lost when the
power to the computer is turned off.
Volatility. The characteristic of a liquid that relates to its
ability to vaporize or change into a gas.
Volumetric efficiency. The ratio of the volume of the charge
of the fuel and air inside the cylinder of a reciprocating engine
to the total physical volume of the cylinder.
G-31Von Ohain, Dr. Hans Pabst. The designer and developer
of the first turbojet engine to power an airplane. His HeS3b
engine was built in Germany by the Heinkel Company and
it flew in a Heinkel He178 airplane on August 27, 1939.
Vortex. A whirling mass of air that sucks everything near it
toward its center.
Vortex dissipator. A high-velocity stream of compressor
bleed air blown from a nozzle into an area where vortices
are likely to form. V ortex dissipaters destroy the vortices that
would otherwise suck debris from the ground into engines
mounted in pods that are low to the ground.
W
Wake. The high-velocity stream of turbulent air behind an
operating aircraft engine.
Wankel engine. See rotating combustion (RC) engine.
Waste gate. A controllable butterfly valve in the exhaust pipe
of a reciprocating engine equipped with an exhaust-driven
turbocharger. When the waste gate is open, exhaust gases
leave the engine through the exhaust pipe, and when it is
closed, they leave through the turbine.
Watt. The basic unit of power in the metric system. One watt
is the amount of power needed to do one joule (0.7376 foot-
pound of work) in one second. One watt is 1⁄746 horsepower.
Wet-sump engine. An engine that carries its lubricating oil
supply in a reservoir that is part of the engine itself.
Wet-sump lubrication system. A lubrication system in
which the oil supply is carried within the engine itself. Return
oil drains into the oil reservoir by gravity.
Whittle, Sir Frank. The British Royal Air Force flying
officer who in 1929 filed a patent application for a turbojet
engine. Whittle’s engine first flew in a Gloster E.28 on May
15, 1941. The first jet flight in America was made on October
2, 1942, in a Bell XP-59A that was powered by two Whittle-
type General Electric I-A engines.
Windmilling propeller. A propeller that is rotated by air
flowing over the blades rather than powered by the engine.
Work. The product of a force times the distance the force
is moved.
Worm gear. A helical gear mounted on a shaft. The worm
meshes with a spur gear whose teeth are cut at an angle to its face. A worm gear is an irreversible mechanism. The rotation
of the shaft, on which the worm gear locks the spur gear so
its shaft cannot be rotated.
Wrist pin. The hardened steel pin that attaches a piston to
the small end of a connecting rod.
Y
Yaw. Rotation of an aircraft about its vertical axis.
Z
Zero-lash valve lifter. A hydraulic valve lifter that maintains
zero clearance in the valve actuating mechanism.
I-1
Index
A
Abrasion ....................................................................... 10-3
Accelerating System .................................................... 2-13
Accessory Section ........................................................ 1-38
accessory case ........................................................... 1-38
gear train ................................................................... 1-38
Accessory Zone Cooling .............................................. 6-39
Adjusting Idle Mixtures ............................................... 2-29
Aerodynamic Factors ..................................................... 7-4
Aerodynamic twisting force ........................................... 7-9
Aeromax Aviation 100 (IFB) Aircraft Engine ........... 11-11
Afterburning/Thrust Augmentation ............................. 3-27
Afterfiring .................................................................. 10-31
Airblast Nozzles ........................................................... 2-45
air bleed ........................................................................ 2-11
Air Entrance ................................................................. 1-37
air filter ........................................................................... 3-1
Airflow Controls .......................................................... 6-10
Airflow Section ............................................................ 2-21
airframe fuel system ....................................................... 2-2
Air Oil Coolers ............................................................. 6-27
Air pressure against the propeller piston ....................... 7-9
air scoop ......................................................................... 3-1
air throttle assembly ..................................................... 2-26
Air Turbine Starters
air path ...................................................................... 5-14
axial flow turbine ...................................................... 5-12
control assembly ....................................................... 5-15
drive coupling ........................................................... 5-12
output shaft assembly ............................................... 5-12
pressure-regulating and shutoff valve ...................... 5-14
ring gear housing ...................................................... 5-13
rotor switch actuator ................................................. 5-12
sprag clutch assembly ............................................... 5-12
transmission housing ................................................ 5-13
turbine housing ......................................................... 5-13
Air Turbine Starter Troubleshooting Guide ................. 5-16
Aluminium Propeller Inspection .................................. 7-21
Aluminum Wire Terminals .......................................... 4-63
Ambient Conditions ................................................... 10-58
American wire gauge (AWG ....................................... 4-48
Analyzer Safety Precautions ...................................... 10-61
Anti-Icing Systems ....................................................... 7-17
APU Fire Detection and Extinguishing System ........... 9-17
APU Fire Warning .................................................... 9-18
Fire Bottle Discharge ................................................ 9-18
Area of a Circle ............................................................ 1-28Atmospheric air .............................................................. 1-1
augmentors ................................................................... 3-22
Augmentors .................................................................. 6-33
Authorized Personnel That Meet FAA Regulations ....11-3
Autofeathering System ................................................. 7-20
Automatic Mixture Control (AMC) ............................. 2-18
Auxiliary Ignition Starting Units ................................. 4-17
B
Backfiring .................................................................. 10-30
baffles ........................................................................... 6-33
Balancing Procedure .................................................... 7-26
ball bearings ................................................................. 1-53
Ball Bearings ................................................................ 1-22
Basic Engine Operating Principles ............................ 10-29
Basic Fuel System .......................................................... 2-2
Bearings ....................................................................... 1-22
Bellmouth Compressor Inlets ...................................... 3-19
Bendix/Precision Fuel-Injection System ...................... 2-21
Bend Radii ................................................................... 4-55
Bernoulli’s principle .................................................... 1-58
blade angle ............................................................. 7-2, 7-5
Blade Tracking ............................................................. 7-22
Boeing 777 Aircraft Fire Detection and Extinguishing
System .......................................................................... 9-13
Boeing 777 Fire Extinguisher System ...................... 9-14
Fire Extinguisher Containers ................................ 9-14
Continuous Fault Monitoring ................................... 9-13
Engine Fire Operation .............................................. 9-17
Engine Fire Switches ................................................ 9-16
Fire Detection ........................................................... 9-13
Nacelle Temperature Recording ............................... 9-13
Overheat Detection ................................................... 9-13
Single/Dual Loop Operation .................................... 9-14
Squib ......................................................................... 9-16
System Test .............................................................. 9-14
Bonding ........................................................................ 4-65
Brake Horsepower ....................................................... 1-31
Brayton cycle ............................................................... 1-58
Breaker Point Inspection .............................................. 4-39
Breather Pressurizing System ...................................... 6-29
Breather Subsystems .................................................... 6-30
Brinelling ..................................................................... 10-3
brush block ................................................................... 7-18
Built-In Engine Timing Reference Marks ................... 4-26
Burning ........................................................................ 10-3
Burnishing .................................................................... 10-3
I-2C
Cam Rings .................................................................... 1-18
Capacitance .................................................................... 4-8
Capacitor Discharge Exciter Unit ................................ 4-44
Carbon Fouling of Spark Plugs .................................... 4-33
carbon seal ................................................................... 1-53
Carburetion Principles ................................................... 2-5
Application of Venturi Principle to Carburetor .......... 2-5
Venturi Principles ....................................................... 2-5
Carburetor Adjustment and Checks ........................... 11-24
carburetor air filter ......................................................... 3-1
Carburetor Air Temperature (CAT) Indicator ........... 10-21
carburetor fuel strainer ................................................. 2-16
Carburetor Icing ............................................................. 2-8
Fuel evaporation ice ................................................... 2-8
Impact ice ................................................................... 2-9
Throttle ice ................................................................. 2-9
Carburetor Induction System ......................................... 3-1
Induction System Filtering ......................................... 3-4
Induction System Icing ............................................... 3-3
Induction System Inspection and Maintenance .......... 3-4
Induction System Troubleshooting ............................ 3-5
Carburetor Removal ..................................................... 2-27
Carburetor Synchronization ....................................... 11-17
Carburetor Systems ........................................................ 2-7
accelerating system ..................................................... 2-7
idle cutoff system ....................................................... 2-7
idling system .............................................................. 2-7
main metering system ................................................. 2-7
mixture control system ............................................... 2-7
power enrichment system ........................................... 2-8
Carburetor Types ........................................................... 2-8
float-type carburetor ................................................... 2-8
pressure-type carburetor ............................................. 2-8
Centrifugal force ............................................................ 7-3
Centrifugal twisting force ...................................... 7-3, 7-9
Centrifugal twisting moment ......................................... 7-8
Chafing ......................................................................... 10-3
Changing Oil ................................................................ 6-17
Assembly of and Installation of Oil Filters .............. 6-18
Draining Oil .............................................................. 6-17
Oil and Filter Change and Screen Cleaning ............. 6-17
Oil Filter Removal Canister Type Housing .............. 6-17
Oil Filter/Screen Content Inspection ........................ 6-17
Troubleshooting Oil Systems ................................... 6-18
Charging the Propeller Air Dome ................................ 7-27
Checking Alignment .................................................... 10-9
Checking and Adjusting Propeller Blade Angles ........ 7-23
Checking Ignition Induction Vibrator Systems ........... 4-33
Checking Takeoff Thrust ........................................... 10-57
Checking the Carburetor Actuation ........................... 11-19Checking the Friction Torque in Free Rotation ......... 11-21
Checking the Internal Timing of a Magneto ................ 4-29
Checking the Propeller Gearbox ................................ 11-21
Checks and Adjustments After Engine Runup and
Operation ...................................................................... 8-13
Chipping ....................................................................... 10-3
Circuit Protection Devices ........................................... 4-70
Cleaning ....................................................................... 10-5
Cleaning Propeller Blades ............................................ 7-27
Cleaning the Oil Tank ................................................ 11-20
Clearances .................................................................. 10-52
Cloud Point .................................................................... 6-3
Cold Cylinder Check .................................................. 10-43
Combination Splash and Pressure Lubrication .............. 6-4
Combustion Drain Valves ............................................ 2-46
Combustion Process ................................................... 10-29
Combustion Section ..................................................... 1-43
Combustion Section Inspection ................................. 10-47
Compactness .................................................................. 1-3
Complete fuel system ................................................... 2-31
Components ................................................................... 9-1
Composite Propeller Inspection ................................... 7-21
Compression .............................................................. 10-31
Compression Ratio ....................................................... 1-29
Compressor Inlet Screens ............................................ 3-18
compressors .................................................................. 1-38
Axial-Flow Compressor ........................................... 1-40
rotor ....................................................................... 1-41
disk-type rotor ................................................... 1-41
drum-type rotor .................................................. 1-41
stator ..................................................................... 1-40
Centrifugal-Flow Compressors ................................ 1-38
compressor manifold .................................. 1-38, 1-39
diffuser ........................................................ 1-38, 1-39
impeller ................................................................. 1-38
Compressor Section ..................................................... 1-38
Conductor Insulation .................................................... 4-52
Dielectric strength .................................................... 4-52
Insulation resistance ................................................. 4-52
Conduit ......................................................................... 4-69
Connecting Rods .................................................. 1-9, 10-5
Connecting Terminal Lugs to Terminal Blocks ........... 4-64
Connector Identification .............................................. 4-68
Connectors ................................................................... 4-67
Constant-Speed Feathering Propeller .......................... 7-15
Constant-Speed Propellers ........................................... 7-14
constant speed unit (CSU) ........................................... 7-32
Containers ...................................................................... 9-8
Continental/TCM Fuel-Injection System ..................... 2-25
Continuity Check of Aircraft EGT Circuit ................ 10-63
Continuous ignition ...................................................... 4-43
I-3Continuous-Loop Detector Systems .............................. 9-3
Automatic Self-Interrogation ..................................... 9-6
Combination Fire and Overheat Warning .................. 9-5
Dual-Loop Systems .................................................... 9-6
Fault Indication ........................................................... 9-6
Fenwall Continuous-Loop System ............................. 9-3
Fire Detection Control Unit (Fire Detection Card) ....9-6
Kidde Continuous-Loop System ................................ 9-4
Sensing Element ......................................................... 9-5
Support Tube-Mounted Sensing Elements ................. 9-6
System Test ................................................................ 9-5
Temperature Trend Indication .................................... 9-5
Controlling or Monitoring the Electrical Load ............ 4-70
Convergent-Divergent Exhaust Nozzle ....................... 3-26
Convergent Exhaust Nozzle ......................................... 3-26
Cooling System ............................................................ 11-6
Cooling System of Rotax 447 UL SCDI ...................... 11-4
Cooling System of the Rotax 582 UL DCDI ............... 11-4
Copper Wire Terminals ................................................ 4-62
Corrosion ...................................................................... 10-3
cowl flaps ..................................................................... 6-33
Cowling ........................................................................ 6-33
Crack ............................................................................ 10-3
Crankshaft .......................................................... 10-5, 10-9
Crankshaft Balance ........................................................ 1-8
Crankshafts .................................................................... 1-7
Crimping Tools ............................................................ 4-62
Cut ................................................................................ 10-3
Cutting Wire and Cable ............................................... 4-60
Cycling timers .............................................................. 7-19
Cylindar Removal ...................................................... 10-41
Cylinder Assembly Reconditioning ........................... 10-11
Cylinder Baffle and Deflector System Inspection ....... 6-37
Cylinder Barrel ............................................................. 10-7
Cylinder Barrels ........................................................... 1-15
cylinder bases ............................................................... 6-34
Cylinder Compression Tests ...................................... 10-39
Cylinder Grinding and Honing .................................. 10-17
Cylinder Head .............................................................. 10-4
Cylinder Heads ............................................................. 1-14
Cylinder Head Temperature Indicator ....................... 10-23
Cylinder Installation ................................................... 10-42
Cylinder Numbering .................................................... 1-15
Cylinder Replacement ................................................ 10-41
Cylinders ...................................................................... 1-14
Cylinder Temperature Indicating Systems ................... 6-37
D
Daily Maintenance Checks ........................................ 11-21
DC Generators ............................................................. 4-12
Compound Wound DC Generators .......................... 4-13DC Generator Maintenance ...................................... 4-13
Generator Ratings ..................................................... 4-13
Parallel (Shunt) Wound DC Generators ................... 4-12
Series Wound DC Generators .................................. 4-12
Decarbonizing .............................................................. 10-5
decarbonizing solution ................................................. 10-6
Degreasing ................................................................... 10-5
deice boot ..................................................................... 7-19
Deicing Systems ........................................................... 7-18
Density altitude .............................................................. 3-6
Dent .............................................................................. 10-3
Deoiler .......................................................................... 6-28
Detonation .................................................................. 10-29
Dielectric Inspection .................................................... 4-41
differential pressure controller ..................................... 3-13
Differential Pressure Tester ....................................... 10-39
Diffuser ........................................................................ 1-43
Dimensional Inspection ............................................... 10-7
Direct Cranking Electric Starter ..................................... 5-2
Direct Cranking Electric Starting System for Large
Reciprocating Engines ................................................ 5-3
motor assembly ....................................................... 5-3
starter motor ............................................................ 5-3
Direct Cranking Electric Starting System for Small
Aircraft ....................................................................... 5-6
Direct Drive VW Engines .......................................... 11-12
Disassembly ................................................................. 10-2
Discharge Indicators ...................................................... 9-9
discharge nozzle ............................................................. 2-6
Discharge Valves ........................................................... 9-8
Disconnection of Engine Controls ................................. 8-7
Disconnection of Lines .................................................. 8-8
Divided-Entrance Duct ................................................ 3-17
dome assembly ............................................................. 7-35
Draining the Engine ....................................................... 8-6
Dry Sump Lubrication System Operation .................... 6-11
Dry Sump Oil Systems ................................................... 6-4
Duplex Fuel Nozzle ..................................................... 2-44
Durability ....................................................................... 1-2
Dye Penetrant Inspection ............................................. 10-6
Dynamic Balancing ...................................................... 7-26
Dynamic Dampers ......................................................... 1-8
E
Economizer System ..................................................... 2-13
Eddy Current Inspection .............................................. 10-7
EEC system .................................................................. 2-36
Efficiencies .................................................................. 1-33
E-gap ............................................................................ 4-29
EGT Circuit Error ...................................................... 10-65
Electrical Disconnects .................................................... 8-6
I-4Electrical Equipment Installation ................................. 4-70
Electrical Load Limits .................................................. 4-70
Electrical Wiring Installation ....................................... 4-54
Electric Starting Systems and Starter Generator Starting
System .......................................................................... 5-10
Electric System .................................................. 11-5, 11-8
Electronic Control Unit (ECU) .......................... 4-14, 4-15
Emergency Splicing Repairs ........................................ 4-63
Engine and Engine Compartment Inspection ............ 11-23
Engine Checks ........................................................... 10-57
Engine Cooling Systems .............................................. 6-32
Engine Cowling ........................................................... 6-36
Engine Fire Detection Systems ...................................... 9-2
Engine Fire Extinguishing System ................................. 9-7
Engine Fuel System Components ................................ 2-41
Engine General Requirements ..................................... 11-1
Engine Inspection Charts ........................................... 11-25
Engine Instruments ........................................ 10-20 , 10-24
Engine Mounts ............................................................. 8-16
Mounts for Reciprocating Engines ........................... 8-16
Mounts for Turbofan Engines .................................. 8-17
Turbine Vibration Isolation Engine Mounts ............ 8-17
Engine Noise Supression ............................................. 3-29
Engine Oil Pressure Indicator .................................... 10-55
Engine Oil Temperature Indicator ............................. 10-55
Engine Operational Problems ........................................ 8-2
Engine or Component Lifespan Exceeded ..................... 8-1
Engine Preservation ................................................... 11-23
Engine Preservation and Return to Service ................. 8-19
Engine Pressure Ratio Indicator ................................. 10-54
Engines ........................................................................... 1-3
Inline Engines ............................................................. 1-3
Opposed engines ......................................................... 1-4
O-Type Engines .......................................................... 1-4
Radial Engines ............................................................ 1-4
V-Type Engines .......................................................... 1-4
Engine Shipping Containers ........................................ 8-21
Engine Shutdown ....................................................... 10-58
Engine Speed Varies With Flight Attitude (Airspeed) 7-30
Engine Starting ........................................................... 10-25
Engine Stopping ......................................................... 10-28
Engine Thermocouples Out of Calibration ................ 10-64
Engine Troubleshooting ............................................. 10-35
Engine Warm-up ........................................................ 10-25
Erosion ......................................................................... 10-3
Exhaust Gas Temperature Indicator (EGT) ............... 10-55
exhaust leak .................................................................. 3-23
Exhaust Manifold and Stack Failures .......................... 3-24
Exhaust Section ................................................ 1-51, 10-54
Exhaust cone ............................................................. 1-51
exhaust nozzle .......................................................... 1-52insulation blanket ..................................................... 1-52
tailpipe ...................................................................... 1-52
Exhaust System .......................................................... 11-24
Exhaust System Inspection .......................................... 3-22
Exhaust System Repairs ............................................... 3-24
Exhaust Systems With Turbocharger ........................... 3-24
F
Factors Affecting Engine Operation .......................... 10-31
Factors Affecting Selection of Conductor Material .....4-50
Factors Affecting the Selection of Wire Size .............. 4-48
FADEC for an Auxiliary Power Unit .......................... 2-36
FADEC Fuel Control Propulsion Engine .................... 2-37
FADEC Fuel Control Systems ..................................... 2-36
FADEC System Description ........................................ 4-14
Failure To Feather or Feathers Slowly ......................... 7-30
Feathering .................................................................... 7-16
Feathering Operation ................................................... 7-37
Fire Detection System Maintenance ............................ 9-10
Fire Detection System Troubleshooting ...................... 9-12
Fire Extinguisher System Maintenance Practices. ....... 9-12
Fire Extinguishing Agents ............................................. 9-7
Fire Switch ..................................................................... 9-9
Fire Zones ...................................................................... 9-7
Firing Order ................................................................. 1-16
Fittings ......................................................................... 2-32
Fixed-Pitch Wooden Propellers ................................... 7-12
Flaking ......................................................................... 10-3
Flash Point and Fire Point .............................................. 6-3
Float Chamber Mechanism System ............................. 2-10
Float-Type Carburetors .................................................. 2-9
Flow Divider ...................................................... 2-23, 2-45
Fork-and-Blade Rod Assembly .................................... 1-11
Fretting ......................................................................... 10-3
Friction ........................................................................... 6-1
Rolling friction ........................................................... 6-1
wiping friction ............................................................ 6-1
Friction and Brake Mean Effective Pressures .............. 1-32
Friction Horsepower .................................................... 1-32
Fuel/Air Control Unit ................................................... 2-25
Fuel/Air Mixtures ........................................................... 2-3
Fuel Control Assembly ................................................ 2-26
Fuel Control Maintenance ............................................ 2-40
Fuel Control Unit ......................................................... 2-17
idle needle valve ....................................................... 2-17
manual mixture control ............................................ 2-17
power enrichment valve ........................................... 2-18
regulator fill valve .................................................... 2-18
fuel control unit (FCU) ................................................ 7-33
fuel counter .................................................................. 2-46
Fuel Discharge Nozzle ................................................. 2-27
I-5Fuel Discharge Nozzles ............................................... 2-24
Fuel Economy ................................................................ 1-2
Fuel Filters ................................................................... 2-42
Fuel Flow Meter ......................................................... 10-22
Fuel Heater ................................................................... 2-42
Fuel-Injection Pump ..................................................... 2-25
Fuel-Injection Systems ................................................. 2-21
Fuel Injector ................................................................. 2-21
Fuel Manifold Valve .................................................... 2-26
Fuel Metering ............................................................. 10-31
Fuel Metering Devices for Reciprocating Engines ........ 2-2
Fuel Metering Section .................................................. 2-23
fuel metering system ...................................................... 2-2
Fuel Nozzle and Support Assemblies ........................ 10-50
Fuel Oil Coolers ........................................................... 6-27
Fuel/Oil Mixing Procedure .......................................... 11-5
Fuel Pressure and Oil Pressure Check ....................... 10-27
fuel pressure gauge ...................................................... 2-24
Fuel Pressure Gauge .................................................... 2-32
Fuel Pressure Indicator .............................................. 10-21
Fuel Pressurizing and Dump Valves ............................ 2-45
Fuel Quantity Gauges .................................................. 2-32
Fuel Spray Nozzles and Fuel Manifolds ...................... 2-43
Fuel System ........................................................ 11-5, 11-6
Fuel System Inspection and Maintenance .................... 2-31
Fuel System Operation ................................................. 2-40
Fuel System Requirements ............................................ 2-1
Fuel Tanks .................................................................... 2-32
Functional Check of Aircraft EGT Circuit ................ 10-63
Functions of Engine Oil ................................................. 6-1
G
Galling .......................................................................... 10-3
Gap Erosion of Spark Plugs ......................................... 4-35
gas turbine engine ........................................................ 3-16
Gas Turbine Engine Bearings and Seals ...................... 1-52
Gas Turbine Engine Performance ................................ 1-59
Gas Turbine Engines .................................................... 1-36
Turbofan engines ...................................................... 1-36
turbojet ..................................................................... 1-36
turboprop engine ....................................................... 1-36
turboshaft engine ...................................................... 1-37
Gas Turbine Engine Starters .......................................... 5-8
General Maintenance Practices for the Light-Sport Jabiru
Engines ....................................................................... 11-23
General Maintenance Practices on Light-Sport Rotax
Engines ....................................................................... 11-15
General Overhaul Procedures ...................................... 10-1
General Procedures for Engine Removal and Installation ..
Gouging ........................................................................ 10-3
govenor .......................................................................... 1-6Governor Mechanism ..................................................... 7-9
Governor System Operation ........................................ 7-11
Graphite Fouling of Spark Plugs .................................. 4-35
Great Plains Aircraft Volkswagen (VW)Conversions 11-14
Grooving ...................................................................... 10-3
Ground Check ............................................................ 10-26
Grounding .................................................................... 4-65
Ground Operation Engine Fire ................................... 10-57
H
Hamilton Standard Hydromatic Propellers .................. 7-34
Harness Testing ............................................................ 4-42
Hartzell Constant-Speed .............................................. 7-14
Head Bolts .................................................................. 11-24
High-Tension Ignition Harness Faults ......................... 4-41
High-Tension Magneto E-Gap Setting (Bench Timing) .....
High-Tension Magneto System Theory of Operation ...4-2
High-Tension Retard Breaker Vibrator ....................... 4-21
HKS 700T Engine ........................................................ 11-9
Hoisting and Mounting the Engine for Installation ....... 8-9
Connections and Adjustments .................................. 8-10
Horsepower .................................................................. 1-28
Hub ............................................................................... 7-29
hub assembly ................................................................ 7-35
Hunting and Surging .................................................... 7-30
Hydraulic Lock .......................................................... 10-25
Hydraulic valve lifters .................................................. 1-21
Hydraulic Valve Tappets/Lifters .................................. 1-20
hydromatic propeller .................................................... 7-35
hydromatic propeller feathering installation ................ 7-37
I
Ice Control Systems ..................................................... 7-17
Identifying Wire and Cable .......................................... 4-53
Idle Mixture ............................................................... 10-33
Idle Speed Adjustment ............................................... 11-19
Idle Speed and Idle Mixture Checks .......................... 10-28
Idling System ............................................................... 2-12
Igniter Plugs ................................................................. 4-45
Ignition Harness ............................................................. 4-7
Ignition Harness Maintenance ..................................... 4-41
Ignition Switch Check .................................................. 4-31
Ignition Switches ........................................................... 4-8
Impulse Coupling ......................................................... 4-20
Inclusion ....................................................................... 10-3
Indicated Horsepower .................................................. 1-30
Induction Manifold .................................................... 10-33
Inertia Starters ................................................................ 5-1
Inline engine cylinders ................................................. 1-15
Inspecting the Magnetic Plug ..................................... 11-21
I-6Inspection and Depreservation of Accessories .............. 8-5
Inspection and Repair of Combustion Chambers ......10-49
Inspection and Replacement of Powerplant External Units
and Systems ................................................................... 8-5
Inspection Process ........................................................ 10-2
Installation .................................................................... 4-55
Installation and Testing .............................................. 10-19
Installation of Cable Clamps ........................................ 4-57
Installation of Carburetor ............................................. 2-29
Installation of Connectors ............................................ 4-69
Internal Muffler Failures .............................................. 3-24
J
Jabiru 2200 Aircraft Engine ....................................... 11-11
Jabiru Light-Sport Engines ........................................ 11-10
K
Knuckle Pins ................................................................ 1-10
L
Lacing and Tying Wire Bundles .................................. 4-58
Double-Cord Lacing ................................................. 4-59
Lacing Branch-Offs .................................................. 4-59
Single-Cord Lacing .................................................. 4-58
Tying ........................................................................ 4-59
Large springs .................................................................. 7-9
Lead Fouling of Spark Plugs ........................................ 4-34
Leak Testing ............................................................... 10-17
Light-Sport Aircraft Engines ....................................... 11-3
Lines ............................................................................. 2-32
lock-pitch solenoid valve ............................................. 7-33
Low-Tension Magneto System .................................... 4-11
Low-Tension Retard Breaker Vibrator ........................ 4-23
Low-Voltage Harness .................................................. 4-14
Lubrication System ............................... 11-7, 11-19 , 11-24
Lubrication System Breather Systems (Vents) ............ 6-25
Lubrication System Check Valve ................................ 6-26
Lubrication System Instrumentation ............................ 6-25
Lubrication System Maintenance Practices ................. 6-13
Lubrication System Requirements ................................. 6-4
Lubrication Systems ..................................................... 11-4
Lubrication System Thermostatic Bypass Valves ....... 6-27
Lycoming 0-233 Series Light-Sport Aircraft Engine 11-15
M
Magnetic Chip Detectors ............................................. 6-28
Magnetic Circuit ............................................................ 4-2
Magnetic Particle Inspection ........................................ 10-7
magneto .......................................................................... 4-7
Magneto and Distributor Venting .................................. 4-7
Magneto-Ignition System Operating Principles ............. 4-1
Magneto-Ignition Timing Devices ............................... 4-26Magneto Mounting Systems ........................................ 4-10
Main Fuel Pumps (Engine Driven) .............................. 2-41
Main Line Strainers ...................................................... 2-32
Main Metering System ................................................. 2-10
Maintenance and Inspection of Ignition Leads ............ 4-31
Maintenance Schedule Procedures and Maintenance
Checklist .................................................................... 11-16
Manifold and Augmentor Exhaust Assembly .............. 3-22
Manifold Pressure Indicator ....................................... 10-23
Marking Materials for Combustion Section Parts .....10-49
Mechanical Efficiency ................................................. 1-35
Mechanical failures ........................................................ 9-1
Metal Fixed-Pitch Propellers ....................................... 7-13
Metal Particles in the Oil ............................................... 8-2
Metal Propeller Inspection ........................................... 7-21
Tachometer inspection ............................................. 7-21
Mixture Control System ............................................... 2-12
Mounting lugs ................................................................ 1-6
Muffler and Heat Exchanger Failures .......................... 3-23
N
Naturally Aspirated Induction Systems ......................... 3-1
Newton’s third law of motion ........................................ 1-1
Nick .............................................................................. 10-3
Nonfeathering .............................................................. 7-14
O
Oil Change ................................................................. 11-20
Oil Cooler ............................................................. 6-9, 6-14
Oil Cooler Flow Control Valve ...................................... 6-9
oil filter bypass valve ..................................................... 6-7
oil filter check valve ....................................................... 6-6
Oil Filters ....................................................................... 6-7
canister housing filter ................................................. 6-7
Cuno oil filter ............................................................. 6-7
spin-on filters .............................................................. 6-7
Oil Fouling of Spark Plugs .......................................... 4-34
Oil Injection Lubrication of Rotax 503 UL DCDE, 582 UL
DCDI, and 582 UL DCDI ............................................ 11-4
Oil Jets ......................................................................... 6-25
Oil Level Check ......................................................... 11-19
Oil Pressure Gauge ........................................................ 6-8
dual-type oil pressure gauge ....................................... 6-8
Oil Pressure Indicator ................................................ 10-22
Oil Pressure Regulating Valve ............................. 6-7, 6-24
Oil Pressure Relief Valve ................................... 6-16, 6-24
Oil Pump .............................................................. 6-6, 6-22
gear-type oil pump .................................................... 6-22
gerotor pump ............................................................ 6-23
Oil Tank ....................................................................... 6-21
Oil Tanks .............................................................. 6-4, 6-13
I-7Oil Temperature Bulbs ................................................. 6-15
Oil Temperature Indicator .................................. 6-9, 10-22
One or More Inoperative Thermocouples in Engine
Parallel Harness ......................................................... 10-64
On-Speed Condition ..................................................... 7-11
Operating Cycles .......................................................... 1-25
Diesel Cycle ............................................................. 1-27
Four-Stroke Cycle .................................................... 1-25
Compression Stroke .............................................. 1-27
Exhaust Stroke ...................................................... 1-27
Intake Stroke ......................................................... 1-26
Power Stroke ......................................................... 1-27
Rotary Cycle ............................................................. 1-27
Two-Stroke Cycle ..................................................... 1-27
Operating Flexibility ...................................................... 1-3
Operational Effect of Valve Clearance ...................... 10-33
Opposed Light-Sport, Experimental, and Certified Engines
Optical Fire Detection Systems ..................................... 9-3
Principle of Operation ................................................ 9-3
Optimizing Engine Running ...................................... 11-19
Other Disconnections ..................................................... 8-8
Overspeed Condition ................................................... 7-11
P
Peening ......................................................................... 10-3
Performing Ignition System Checks ............................ 4-31
Personnel Authorized to Perform Inspection and
Maintenance on Light-sport Engines ........................... 11-2
Piston ................................................................ 10-5, 10-11
Piston Construction ...................................................... 1-12
Piston Displacement ..................................................... 1-28
Piston Pin ..................................................................... 10-5
Piston Pins .................................................................. 10-11
Piston Position Indicators ............................................ 4-27
Piston Repairs ............................................................ 10-17
Piston Rings ................................................................. 1-13
Compression Ring .................................................... 1-13
Oil Control Rings ..................................................... 1-13
Oil Scraper Ring ....................................................... 1-13
Piston Ring Construction ......................................... 1-13
Pistons .......................................................................... 1-11
Pitting ........................................................................... 10-3
Plain Bearings .............................................................. 1-22
Plain-Type Connecting Rods ....................................... 1-11
PLA potentiometer ....................................................... 2-36
Pneumatic Synchronization ....................................... 11-18
Pneumatic Thermal Fire Detection ................................ 9-3
Pour Point ....................................................................... 6-3
Power ............................................................................. 1-2
Power Check .............................................................. 10-27
PowerLink Ignition System ............................... 4-14, 4-16Powerplant Electrical Systems ..................................... 4-47
Powerplant Selection ..................................................... 1-3
powerplant vibration .................................................... 7-22
Pratt and Whittney PT6 Hartzell Propeller System .....7-31
Pre-flight Checks ....................................................... 11-22
Pre-ignition ................................................................ 10-30
Pre-Oiling ................................................................... 10-25
Preparation of Engine for Ground and Flight Testing .8-12
Fuel System Bleeding ............................................... 8-13
Pre-Oiling ................................................................. 8-12
Preparation of Engines for Installation .......................... 8-3
Preparing the Engine for Removal ................................. 8-6
Preservation and Depreservation of Gas Turbine Engines .
Preservation and Storage of Engines ........................... 8-18
Corrosion-Preventive Compounds ........................... 8-18
heavy compound ................................................... 8-18
light mixture .......................................................... 8-18
Corrosion-Preventive Materials ............................... 8-18
Dehydrating Agents .................................................. 8-18
silica gel ................................................................ 8-18
pressure .......................................................................... 3-1
Pressure and Scavenge Oil Screens ............................. 6-15
Pressure Indication ......................................................... 9-8
Pressure Injection Carburetors ..................................... 2-14
Pressure Subsystem ...................................................... 6-29
Pressure System ........................................................... 6-28
Pressure Warning Signal .............................................. 2-33
Primary Electrical Circuit .............................................. 4-3
Principles of Engine Lubrication ................................... 6-1
Principles of Operation ................................................ 7-36
Propeller Aerodynamic Process ..................................... 7-2
Propeller Auxiliary Systems ........................................ 7-17
Propeller Balancing ...................................................... 7-24
propeller blade ............................................................... 7-3
Propeller blade flyweights ............................................. 7-9
Propeller Check ............................................................ 8-13
Propeller Controls and Instruments ............................... 7-5
propeller control system ............................................... 7-31
propeller governor .......................................................... 7-8
Propeller-governor oil on the propeller piston side ....... 7-9
Propeller Inspection and Maintenance ......................... 7-20
Propeller Location .......................................................... 7-5
Pusher Propellers ........................................................ 7-6
Tractor Propeller ......................................................... 7-5
Propeller Lubrication ................................................... 7-27
Propeller Overhaul ....................................................... 7-29
propeller pitch ................................................................ 7-2
Propeller Pitch Check ................................................ 10-27
Propeller Principles ........................................................ 7-1
Propeller Reduction Gearing ........................................ 1-23
I-8Propeller Removal and Installation .............................. 7-26
Propellers ............................................................... 7-1, 7-6
Constant-Speed Propellers ......................................... 7-7
Controllable-Pitch Propeller ....................................... 7-6
Feathering Propellers .................................................. 7-8
Fixed-Pitch Propeller .................................................. 7-6
Ground-Adjustable Propeller ..................................... 7-7
Reverse-Pitch Propellers ............................................ 7-8
Test Club Propeller ..................................................... 7-6
Propeller Shafts ............................................................ 1-23
Propellers Used on General Aviation Aircraft ............. 7-12
Propeller Synchronization and Synchrophasing .......... 7-19
Propeller Vibration ....................................................... 7-22
Prop Reassembly .......................................................... 7-29
Propulsive Efficiency ................................................... 1-36
Protection Against Chafing .......................................... 4-56
Protection Against High Temperature ......................... 4-56
Protection Against Solvents and Fluids ....................... 4-56
Protection of Wires in Wheel Well Area ..................... 4-57
Pumps ........................................................................... 2-32
Push Rod ...................................................................... 1-21
Q
QECA Buildup Method for Changing of Engines ......... 8-3
R
Radial Engine Exhaust Collector Ring System ........... 3-22
Ram Recovery .............................................................. 1-60
Reassembly ................................................................ 10-19
Receiving Inspection .................................................... 10-2
Reciprocating Engine Cooling System Maintenance ..6-34
Reciprocating Engine Cooling Systems ....................... 6-32
Reciprocating Engine Exhaust System Maintenance
Practices ....................................................................... 3-22
Reciprocating Engine Exhaust Systems ....................... 3-20
collector system ........................................................ 3-21
short stack system ..................................................... 3-21
Reciprocating Engine Ignition System Maintenance and
Inspection ..................................................................... 4-26
Reciprocating Engine Ignition Systems ......................... 4-1
Reciprocating Engine Induction Systems ...................... 3-1
Reciprocating Engine Lubrication Systems ................... 6-4
Reciprocating Engine Operating Principles ................. 1-25
Reciprocating Engine Operation ................................ 10-24
Reciprocating Engine Overhaul ................................... 10-1
Major Overhaul and Major Repairs .......................... 10-1
Top Overhaul ............................................................ 10-1
Reciprocating Engine Power and Efficiencies ............. 1-27
Reciprocating Engines ................................................... 1-5
Accessory Gear Trains ............................................... 1-7
Accessory Section ...................................................... 1-7Crankcase Sections ..................................................... 1-5
Design and Construction ............................................ 1-5
Reciprocating Engine Starting System Maintenance
Practices ......................................................................... 5-6
Reciprocating Engine Starting Systems ......................... 5-1
Reciprocating Helicopter Engine and QECA .............. 8-15
Installation, Rigging, and Adjustment of Helicopter
QECA ....................................................................... 8-16
Removal of Helicopter QECA ................................. 8-15
Testing the Engine installation ................................. 8-16
Reduction Gear Assembly ........................................... 7-31
Refacing Valve Seats ................................................. 10-13
Regulator Section ......................................................... 2-22
Regulator Unit .............................................................. 2-16
Relays ........................................................................... 4-71
Reliability ....................................................................... 1-2
Removal and Installation of an Opposed-Type Engine 8-13
Removal, Maintenance, and Installation of Ignition System
Components ................................................................. 4-46
Igniter Plugs ............................................................. 4-47
Ignition System Leads .............................................. 4-46
Removal of Reciprocating Engines ............................... 8-1
Removing Hard Carbon ............................................... 10-5
Removing the Engine ..................................................... 8-8
Repair and Replacement ............................................ 10-10
Replacement of Ignition Harness ................................. 4-33
Requirements and Characteristics of Reciprocating Engine
Lubricants ...................................................................... 6-2
Resistance and Insulation Check ............................... 10-64
Resistance of Circuit Out of Tolerance ...................... 10-65
Revmaster R-2300 Engine ......................................... 11-12
Rigging Carburetor Controls ........................................ 2-29
Rigging, Inspections, and Adjustments ....................... 8-13
Adjusting the Fuel Control ....................................... 8-14
Rigging Power Controls ........................................... 8-13
Rocker Arms ................................................................ 1-21
Roller Bearings ............................................................ 1-22
Rotax 447 UL (SCDI) .................................................. 11-3
Rotax 503 UL (DCDI) ................................................. 11-3
Rotax 503 UL DCDI .................................................... 11-4
Rotax 582 UL DCDI .................................................... 11-4
Rotax 912/914 .............................................................. 11-5
Routing ......................................................................... 4-55
Routing Precautions ..................................................... 4-57
S
Scavenge System ......................................................... 6-28
Scoring ......................................................................... 10-3
Scratches ...................................................................... 10-3
Secondary Electrical Circuit .......................................... 4-6
Selector Valves ............................................................ 2-32
Sender ........................................................................ 11-25
I-9Series Wound DC Generators ...................................... 4-12
Servicing Propellers ..................................................... 7-27
Setting the Propeller Governor .................................... 7-41
Shorts to Ground/Shorts Between Leads ................... 10-65
Simplex Fuel Nozzle .................................................... 2-44
Single and Dual High-Tension System Magnetos ....... 4-10
Single Capacitor Discharge Ignition (SCDI) ............... 11-3
Single-row radial engine cylinders .............................. 1-16
Single-Row Radial Engines ......................................... 1-16
Slack in Wiring Bundles .............................................. 4-55
Solderless Terminals and Splices ................................. 4-61
Solid Lifters/Tappets .................................................... 1-20
Spark Plug Inspection and Maintenance ...................... 4-33
Spark Plug Inspection Prior to Installation .................. 4-37
Spark Plug Installation ................................................. 4-38
Spark Plug Lead Installation ........................................ 4-38
Spark Plug Reconditioning Service ............................. 4-36
Spark Plug Removal .................................................... 4-35
Spark Plugs ...................................................... 4-24, 11-24
electrodes .................................................................. 4-24
insulator .................................................................... 4-24
outer shell ................................................................. 4-24
Spectrometric Oil Analysis Engine Inspection Program 8-2
Spectrometric Oil Analysis Program ........................... 6-20
parts per million (PPM) ............................................ 6-20
Spliced Connections in Wire Bundles ......................... 4-54
Splicing Copper Wires Using Preinsulated Wires ....... 4-63
Splicing with Solder and Potting Compound ............... 4-63
Stain ............................................................................. 10-3
Static Balancing ........................................................... 7-24
Stripping Wire and Cable ............................................. 4-61
Stromberg PS Carburetor
Accelerating Pump ................................................... 2-21
Manual Mixture Control ........................................... 2-21
Structural Inspection .................................................... 10-6
Sudden Reduction in Speed ........................................... 8-1
Sudden Stoppage ............................................................ 8-1
Supercharged Induction Systems ................................... 3-5
Absolute Pressure Controller .................................... 3-15
Ground-Boosted Turbosupercharger System ............. 3-9
Internally Driven Superchargers ................................ 3-6
Normalizer Turbocharger ........................................... 3-9
Sloped Controller ..................................................... 3-15
Turbocharger Controllers and System Descriptions 3-14
Turbocharger System Troubleshooting .................... 3-15
Turbosuperchargers .................................................... 3-7
compressor assembly .............................................. 3-7
exhaust gas turbine assembly .................................. 3-7
Typical Turbosupercharger System .......................... 3-10
Variable Absolute Pressure Controller (VAPC) ......3-14
Surge Protection Valves ............................................... 6-10Switches ....................................................................... 4-71
swivel outlet fitting ...................................................... 6-21
T
Tachometer .................................................... 10-55 , 11-25
Tachometer Check ..................................................... 10-64
Tachometer Indicator ................................................. 10-23
Tappet Assembly ......................................................... 1-20
cylindrical tappet ...................................................... 1-20
tappet ball ................................................................. 1-20
tappet roller ............................................................... 1-20
tappet spring ............................................................. 1-20
Teledyne Continental 0-200 Engine .......................... 11-15
Test Cell Requirements .............................................. 10-20
Thermal Discharge Indicator (Red Disk) ....................... 9-9
Thermal Efficiency ...................................................... 1-33
Thermal Switch System ................................................. 9-2
Thermocouple Systems .................................................. 9-2
Throttle Body ............................................................... 2-15
Thrust ........................................................................... 1-57
Thrust Reversers .......................................................... 3-27
Thrust Vectoring .......................................................... 3-29
timer or cycling unit ..................................................... 7-18
Timing Disks ................................................................ 4-27
Timing Lights ............................................................... 4-28
Timing the High-tension Magneto to the Engine ........ 4-30
Torquemeter ............................................................... 10-24
Torquemeter (Turboprop Engines) ............................ 10-55
total engine fuel and control system ............................ 2-34
filter bypass valve ..................................................... 2-34
flow divider and drain valve assembly ..................... 2-36
fuel manifold assembly ............................................ 2-36
hydromechanical fuel control assembly ................... 2-34
vane fuel pump assembly ......................................... 2-34
Troubleshooting Aircraft Tachometer System ........... 10-65
Troubleshooting and Abnormal Operation ................ 11-22
Abnormal Operating ............................................... 11-23
Exceeding of Maximum Admissible Cylinder Head
Temperature ........................................................ 11-23
Exceeding of Maximum Admissible Exhaust Gas
Temperature ........................................................ 11-23
Exceeding the Maximum Admissible Engine Speed ...
Troubleshooting ..................................................... 11-23
Engine Keeps Running With Ignition OFF ........ 11-23
Knocking Under Load ........................................ 11-23
Troubleshooting a Starter Generator Starting System .5-12
Troubleshooting EGT System .................................... 10-64
Troubleshooting Propellers .......................................... 7-30
Troubleshooting Small Aircraft Starting Systems ......... 5-7
Troubleshooting Turbine Engines .............................. 10-59
Turbine Blade Inspection ........................................... 10-50
I-10Turbine Blade Replacement Procedure ..................... 10-51
Turbine Disk Inspection ............................................. 10-50
Turbine Engine Analyzer Uses .................................. 10-59
Turbine Engine Condition Monitoring Programs .......... 8-2
Turbine Engine Cooling ............................................... 6-38
Turbine Engine Emissions ........................................... 3-31
Turbine Engine Exhaust Nozzles ................................. 3-24
Turbine Engine Fuel System—General Requirements 2-33
Turbine Engine Ground Fire Protection ........................ 9-8
Turbine Engine Ignition Systems ................................. 4-43
Turbine Engine Inlet Systems ...................................... 3-16
Turbine Engine Instruments ....................................... 10-54
Turbine Engine Insulation Blankets ............................. 6-40
Turbine Engine Lubricants .......................................... 6-18
Bulk stability ............................................................ 6-19
Cleanliness ............................................................... 6-19
Compatibility ............................................................ 6-19
Load-carrying ability ................................................ 6-18
Seal Wear ................................................................. 6-19
Vapor phase deposits ................................................ 6-18
Turbine Engine Lubrication Systems ........................... 6-20
Turbine Engine Maintenance ..................................... 10-44
Blending and Replacement ..................................... 10-47
Causes of Blade Damage ........................................ 10-45
Compressor Section ................................................ 10-45
Inspection and Cleaning ......................................... 10-45
Turbine Engine Oil System Maintenance .................... 6-31
Turbine Engine Operating Principles ........................... 1-56
Turbine Engine Operation .......................................... 10-57
Turbine Engine Wet-Sump Lubrication System .......... 6-30
reservoir .................................................................... 6-31
magnetic drain plug .............................................. 6-31
Provision ............................................................... 6-31
sight gauge ............................................................ 6-31
vent or breather ..................................................... 6-31
Turbine Fuel Controls .................................................. 2-33
FADEC-type contro ................................................. 2-33
hydromechanical/electronic fuel control ........ 2-33, 2-34
Hydromechanical Fuel Control ................................ 2-34
Turbine Ignition System Inspection and Maintenance 4-45
Check System Operation .......................................... 4-46
Repair ....................................................................... 4-46
Turbine Lubrication System Components ................... 6-21
Turbine Nozzle Inlet Guide Vane Inspection ............ 10-52
Turbine Oil Filters ........................................................ 6-23
Turbine Section ............................................................ 1-46
disk rim ..................................................................... 1-48
rotor element ............................................................. 1-47
Turbine blades .......................................................... 1-49
turbine disk ............................................................... 1-47
turbine inlet nozzle assembly ................................... 1-47Turbocharger and Control System ............................... 11-8
Turbofan Engines ......................................................... 1-55
Turboprop and Turboshaft Compressor Inlets ............. 3-19
Turbo-Propeller Assembly ........................................... 7-31
turbopropeller (turboprop) engine ................................ 1-54
accessory drive housing assembly ............................ 1-55
gearbox assembly ..................................................... 1-55
power section assembly ............................................ 1-55
Turboprop Engines and Propeller Control Systems .....7-30
Turboprop exhaust nozzles .......................................... 3-25
Turboprop Operation ................................................. 10-59
Troubleshooting Procedures for Turboprop Engines ......
Turboprop Powerplant Removal and Installation ........ 8-15
Turboshaft Engines ...................................................... 1-55
Twisting Wires ............................................................. 4-54
Two-Cycle Engine ....................................................... 11-3
Two Cylinder Rotax Engine ........................................ 11-3
Two-Stroke Engines ..................................................... 11-4
Two-Way Check Valve .................................................. 9-9
Typical Dry-Sump Pressure Regulated Turbine
Lubrication System ...................................................... 6-28
Typical Dry-Sump Variable Pressure Lubrication System .
Typical Injection Carburetor ........................................ 2-15
U
Ultrasonic Inspection ................................................... 10-7
Pulse-Echo ................................................................ 10-7
Resonance ................................................................. 10-7
Through Transmission .............................................. 10-7
Underspeed Condition ................................................... 7-9
Unfeathering ................................................................ 7-16
Unfeathering Operation ............................................... 7-38
Universal Propeller Protractor ..................................... 7-23
Upsetting ...................................................................... 10-3
usually .......................................................................... 10-3
V
Valve Construction
valve head ................................................................. 1-17
valve stem ................................................................. 1-17
valve duration ............................................................... 1-18
Valve Lapping ............................................................ 10-17
valve lift ....................................................................... 1-18
Valve Operating Mechanism ....................................... 1-18
Valve Reconditioning ................................................ 10-14
Valves .............................................................. 1-16, 10-11
Valve Springs ................................................... 1-21, 10-11
Valve Train .................................................................. 10-5
Vapor Lock .................................................................... 2-1
vapor vent system ........................................................ 2-16
I-11Variable-Geometry Duct .............................................. 3-17
Viscosity ........................................................................ 6-2
Viscosity Index .............................................................. 6-2
Visual Inspection ......................................................... 10-3
Voltage Drop in Aircraft Wire and Cable .................... 4-50
voltage drop (IR drop) ................................................. 4-50
Volumetric Efficiency .................................................. 1-35
V-type engine cylinders ............................................... 1-15
W
Warning Systems ......................................................... 9-10
Water Injection System ................................................ 2-40
Water Injection Systems for Reciprocating Engines ...2-33
Wear Metals and Additives .......................................... 6-20
Aluminum ................................................................. 6-20
Chromium ................................................................. 6-20
Copper ...................................................................... 6-20
Iron ........................................................................... 6-20
Lead .......................................................................... 6-20
Molybdenum ............................................................ 6-20
Nickel ....................................................................... 6-20
Phosphorous ............................................................. 6-20
Silver ........................................................................ 6-20
Tin ............................................................................ 6-20
Titanium ................................................................... 6-20
Weight ............................................................................ 1-2
Wet-Sump Lubrication System Operation ................... 6-13
Wire Groups and Bundles ............................................ 4-54
Wire Size ...................................................................... 4-48
Wood Propeller Inspection .......................................... 7-21
Work ............................................................................ 1-27
X
X-ray ............................................................................ 10-7
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.