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AC 91-74B - Pilot Guide - Flight in Icing Conditions
Reproduced for study. Always verify against the official GACAR at gaca.gov.sa.
Advisory
U.S. Department
of Transportation
Federal Aviation Circular Administration
Subject: Pilot Guide: Flight in Icing Conditions Date: 10/8/15 AC No: 91-74B
Initiated by: AFS- 800 Change:
This advisory circular (AC) contains updated and additional info rmation for the pilots of
airplanes u nder Title 14 of the Code of Federal Regulations (14 CFR) parts 91, 121, 125,
and 135. The purpose of this AC is to provide pilots with a conve nient reference guide on the
principal factors related to flight in icing conditions and the location of additio nal inf ormation in
related publications. As a result of these updates and consolidating of information, AC 91-74A,
Pilot Guide: Flight in Icing Conditions, dated December 31, 2007, and AC 91-51A, Effect of
Icing on Aircraft Control and Airplane Deice and Anti- Ice Systems, dated July 19, 1996, are
cancelled. This AC does not authorize deviations from established co mpany procedures or
regulatory requirements.
John Barbagallo Deputy Director, Flight Standards Service
10/8/15 AC 91 -74B
CONTENTS
Paragraph Page
CHAPTER 1. INTRODUCTION
1-1. Purpose ..............................................................................................................................1
1-2. Cancellation ......................................................................................................................1
1-3. Definitions .........................................................................................................................1
1-4. Discussion .........................................................................................................................6
CHAPTER 2. ATMOSPHERIC CONDITIONS ASSOCIATED WITH I CING
2-1. Aircraft Icing Conditions ..................................................................................................9
2-2. Cloud Types and Aircraft Icing ......................................................................................10
2-3. Fronts ..............................................................................................................................12
2-4. Convective Weather and Ice Crystals .............................................................................14
CHAPTER 3. ICING EFFECTS, PRO TECTION, AND DETECTION
3-1. Forms of Icing .................................................................................................................15
3-2. General Effects of Icing on Airfoils ................................................................................17
3-3. Effects of Icing on Unprotected Wings ..........................................................................19
3-4. Deicing Systems ..............................................................................................................19
3-5. Anti-Icing Systems ..........................................................................................................21
3-6. Effects of Icing on Roll Control ......................................................................................22
3-7. Tailplane Icing ................................................................................................................22
3-8. Propeller Icing .................................................................................................................23
3-9. Antenna Icing ..................................................................................................................24
3-10. Cooling Inlet Icing ..........................................................................................................24
3-11. Effects of Icing on Critical Systems ...............................................................................24
3-12. Certification for Flight in Icing Conditions ....................................................................26
3-13. Airplanes Not Certificated for Icing ...............................................................................28
3-14. Maintenance Considerations ...........................................................................................28
3-15. Ice Detection ...................................................................................................................29
3-16. Visual Cues of SLD Conditions ......................................................................................30
CHAPTER 4. FLIGHT PLANNING
4-1. Preflight Planning Information .......................................................................................33
4-2. Icing Intensity .................................................................................................................35
4-3. PIREP Cautions ..............................................................................................................36
CHAPTER 5. ICING OPERATIONS
5-1. General ............................................................................................................................39
5-2. Regulations for Icing Operations ....................................................................................39
5-3. Available In -Flight Information ......................................................................................40
5-4. Preflight ...........................................................................................................................41
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5-5. Taxi .................................................................................................................................43
5-6. Takeoff and Climbout .....................................................................................................43
5-7. Cruise ..............................................................................................................................44
5-8. Descent ............................................................................................................................46
5-9. Holding ...........................................................................................................................46
5-10. Approach and Landing ....................................................................................................46
5-11. Wing Stall .......................................................................................................................48
5-12. ICTS ................................................................................................................................48
5-13. Roll Upsets ......................................................................................................................49
CHAPTER 6. SUMMARY
6-1. General ............................................................................................................................51
6-2. Avoidance .......................................................................................................................51
6-3. Vigilance .........................................................................................................................51
6-4. Guidance .........................................................................................................................51
APPENDIX 1. RECOMMENDED READING (2 pages) ...........................................................1
APPENDIX 2. ICING CHECKLISTS (6 pages) .........................................................................1
LIST OF FIGURES
Figure 2-1. Warm Front ...............................................................................................................13
Figure 2-2. Cold Front .................................................................................................................13
Figure 3-1. Clear Ice ....................................................................................................................15
Figure 3-2. Clear Ice Buildup with Horns ...................................................................................15
Figure 3-3. Rime Ice ....................................................................................................................16
Figure 3-4. Mixed Ice ..................................................................................................................16
Figure 3-5. Lift Curve ..................................................................................................................18
Figure 3-6. Drag Curve ................................................................................................................18
Figure 3-7. Wing Boot .................................................................................................................19
Figure 3-8. Tail Down Moment ...................................................................................................23
Figure 3-9. Pitchover Due to Tail Stall .......................................................................................23
Figure 3-10. Propellor Ice Accretion During an SLD Encounter ..................................................24
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CHAPTER 1. INTRODUCTION
1-1. PURPOSE . This advisory circular (AC) updates the previous version and contains essential
information concerning safe flight in icing conditions, what conditions a pilot should avoid, and
how to avoid or exit those conditions if encountered. The information provided is relevant to fixed -wing aircraft, including those operating under Title 14 of the Code of Federal Regulations
(14 CFR) parts 91, 121, 125, and 135. The general guidance provided here in no way substitutes
for airc raft-type-specific information in a par ticular Airplane Flight M anual (AFM) or pilot’s
operating handbook (POH). This material is no t regulatory, nor does it establish minimum
standards. Where the term “must” is used in this AC, it reflects actual regulatory requirements; where the term “should” is used, it reflects recommendations from the Federal Aviation Administration (FAA).
1-2. CANCEL LATION . AC 91 -74A, Pilot Guide: Flight in Icing Conditions, dated
December 31, 2007; and AC 91-51A, Effect of Icing on Aircraft Control and Airplane Deice and
Anti-Ice Systems, dated July 19, 1996, are canceled.
1-3. DEFINIT IONS.
a. Adiabatic Cooling. A process by which a parcel of air co ols. W hen a parcel of air is
lifted, pressure is reduced due to the elevation increase. This reduction in pressure causes the
parcel of air to expand in volume and, in turn, the parcel cools to maintain an energy balance
because no energy is ad ded to the parcel.
b. Airmen’s Meteorolo gical Inf ormation (AIRMET). In-flight weather advisories
concerning weather pheno mena of o perational i ntere st to all pilots and especially to pilots of
aircraft not approved for flight in icing conditions. An AIRMET concerns weather of lesser
severity than that covered by an advisory of significant m eteorological info rmation (SIGMET) or
a convective SIGMET. AIRMETs may include advisories of m oderate icing.
c. Automated S urface O bserving Sy stem ( ASOS). A suite of sensors that m easure,
colle ct, and dissemi nate weather data to help meteorologists, pilots, and flight dis patchers
prepare and monitor weather forecasts, plan flight routes, and provide necessary information for
correct takeoffs and landings. There are many differences between an ASOS and an All Weather Operations Specialist ( AWOS ) (see subparagraph 1-3d). It is important for pilots to understand
the strengths and limitations of the various configurations. The ASOS is comprised of a
standardized suite of weather sensors and is a product of a National W eather Service (NWS),
Depart ment of Defense (DOD), and FAA joint venture. One of AS OS’s most i mportant features
is its ability to dete ct precipitation, including intensity of rain, snow, and freezing rain. One
current ASOS limitation is its inability to simultaneously detect and report freezing drizzle, ice pellets, or any other freezing precipitation without human augmentation when other form s of
precipitation are present. A detailed description of ASOS’s capabilities can be found at the NW S
ASOS ho mepage: http://www.nws.noaa.gov/asos/index.htm l.
d. Automated Weather Observation System (A WOS). A suite of weather sensors t hat
are procured by the FAA or purchased by individuals, groups, airports, etc. It is important to note
that the absence of reported precipitation does not m ean that such co nditions do not exi st. The
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AWOS may not be configured to report this inform ation or have precipitation reporting
capability. A detailed description of AWOS’s capabilities can be found in the AIM.
e. Aviation Weather Service Program. Aviation weather ser vice provided by the NW S
and the FAA that colle cts and disse minates pertinent w eather information f or pilots, aircraft
operators, and air traffic control (A TC).
f. Center Weather Advisory (CWA). An unscheduled weather advisory issued by NW S
meteorologists for use by ATC in alerting pilots to existing or anticipated adverse weather
conditions within the next 2 hours. A CWA m ay modify a SIGMET.
g. Clear Ice. A glossy, clear, or translucent ice for med by the relatively sl ow freezing of
supercooled water drops. The ter ms “clea r” and “ glaze” have been used for es sentially the sa me
type of ice accretion, although som e reserve “clear” for thinner accretions, which lack horns and
conform to the airfoil. If the freezing becomes more rapid, clear ice will turn cloudy as small
bubbles of air become trapped in the ice. If the conditions persist, the ice would be classified as
mixed.
h. Convection. An at mospheric motion resulting in the transport and mi xing of
atmospheric properties.
i. Cumulus Clouds. Clouds in the form of detached dom es or towe rs that are usually well
defined. Cumulus clouds develop vertically in the form of rising mounds of which the bulging
upper part often resem bles a cauliflower; the sunlit parts of these clouds are mostly brilliant
white. Their bases may be r elatively dark and near ly horizo ntal.
j. Current Icing Product (CIP). A graphical planning product that combines sensor and
numerical model data to provide a thr ee-dim ensional diagnosis of the probability and severity of
icing, plus the potenti al for the presence of supercooled large drops (SLD). This product is
automatically produced with no human modification. More information can be found on the Aviation Weather Center (AWC) Aviation Digital Data Service (ADDS) Web site.
k. Forecast Icing Conditions. Environm ental conditions expected by an NWS or an
FAA -approved weather provider to be conducive to the formation of in- flight icing on aircraft.
l. Forecast Icing Product (FIP). The FIP examines num erical weather prediction model
output to calculate the probability and severity of icing conditions, plus SLD potential. This product is automatically produced with no human modification. More information can be found on the AWC ADDS Web site.
m. Free zing Driz zle. Drizzle is precipitation at ground level or aloft in the form of liquid
water drops that have diameters less than 0.5 mm and greater than 0.05 mm. Freezing drizzle is water t hat remains in a liquid form at air te mperatures less t han 0 °C (supercooled) and can
freeze upon contact with objects on the ground or in the air.
n. Free zing Rain. Rain is precipitation at ground level or aloft in the form of liquid water
drops which have diam eters greater than 0 .5 mm . Freezing rain is rain that exis ts at air
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temperatures less than 0 °C, rema ins in liq uid for m (superc ooled), and freezes upon contact with
objects on the ground or in the air.
o. Front. The boundary between two air m asses. A front can be clas sified as cold, warm,
occluded, or stationary.
(1) Cold Front. Any nonoccluded front that m oves in such a way that colder air
replaces war mer air .
(2) Warm Fr ont. Any nonoccluded front that move s in such a way that war mer air
replaces colder air.
(3) Occluded Front. The front formed by a cold front overtaking a warm front and
lifting the warm air above the E arth’s surface. An occlusion (or frontal occlusion) forms when an
air mass is trapped between two colder air masses and is forced to higher and higher levels.
(4) Stationary Front. A front that has little or no move ment because the op posing
forces of the two air masses are relatively balanced.
p. Hazardous Weather I nformation. Summ ary of SIGMET s, Convective SIGMETs,
urgent Pilot Weather Reports (PIREP), CWAs, AIRMETs, and any other significant weather
provided to pilots that might not be routinely provided in a standard format or report.
q. Ice Crystal s. Ice crystals, which are often in high concentrations near convective
weather systems and lower concentrations in stratus or cirrus clouds, can accrete within turbine
engines and cause power l oss when in high concentrations. Ice crystals are not typically detected
by either conventional i ce detectors or airborne radar, and typically do not accrete on exter nal
airfra me surfaces.
r. Icing Envelopes. Icing envelopes used for the certification of aircr aft for flight in icing
conditions specify atm ospheric icing co nditions in term s of altitu de, tem perat ure, Liquid Water
Content (L WC), and drop size represented by the Median Volum e Diam eter (MVD).
(The envelopes use the term mean effective diameter, but this equates to the MVD for the
instru mentation and assumptions curre nt at the ti me the envelopes were established .) There are
two classes of icing envelopes: continuous m aximum and interm ittent maximum. The continuous
maximum is for stratus -type clouds, and the inter mittent m aximum is for cu mulus- type clo uds.
s. Impingem ent. The striking and adherence of a water droplet on an aircraf t surface. T he
impingem ent rate is the r ate at which droplets of a given size collect on a particular surf ace. In
general, i mpinge ment rates are higher for lar ger drops and sm aller co mponents, such as a very
high frequency (VHF) or a Global Positioning System (GPS) antenna.
t. Known, Observed, or Detected Ice Accretion. Actual ice that is observed visually to
be on the aircraft by the flightcrew or identified by onboard sensors.
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u. Light Icing . The rate of ice accu mulation may create a problem if flight is prolonged in
this environment (over 1 hour). Requires occasio nal cycling of manual deicing syste ms1 to
minimi ze ice accretions on t he airfr ame. A representa tive accretion rate f or reference purposes is
¼ inch to 1 inch (0.6 to 2.5 cm) per hour2 on the outer wing.3
v. Liquid Water Content (LWC). The total mass of water in all the liquid cloud drops
within a unit volume of cloud. LW C is u sually discussed in term s of gra ms of water per cubic
meter of air (g/ m3).
w. Median Volume Diameter (MVD). The diam eter such th at half the liq uid water in a
region of cloud is contained in drops of a smaller diameter, and half in drops of a larger diameter.
x. Mixed Ice. Simultaneous appearance of rim e and clear ice or an ice for mation that has
the characteri stics of both rim e and clear ice.
y. Moderate I cing. The rate of ice accumulation re quires freq uent cycling of manual
deicing systems1 to minimi ze ice accretions on t he airfr ame. The rate of accumulation is such
that an ything more than a short encounter is potentially hazardous. A representa tive accretion
rate for reference purposes is 1 to 3 inches (2.5 to 7.5 cm) per hour4 on the outer wing.5
z. One-Minute Weather. The m ost recent 1 -minute update weather broadcast based on
ASOS/AWOS measurem ents and availabl e to a pilot from an uncontrolled airport A SOS/AWOS.
aa. Orographic Cloud. A cloud that usually results from air flowing upslope from terrain
and being cooled adiab atically.
bb. Outside Air Temperat ure (OAT). The m easured or indicated air te mperature outside
the aircraft that is uncorrected .
cc. Pilot Briefing. A service provided by a Flight Service Station (FSS) or other
FAA -approved provider that can assist pilots with flight planning. Briefing item s may include
weather infor mation, Notices to Airmen (NOTAM), m ilitary activities, flow control information,
and other items, as requested.
dd. Pilot Weather Repo rt (PIREP). A report from a pilot of meteorological phenom ena
usually transm itted in a presc ribed form at. The letters “UA” identify the m essage as a routine
PIREP while the letters “UUA” identify an urgent PIREP.
1 It is expected that deicing or anti -icing systems will be activated and operated continuously in the automatic mode,
if available, at the first sign of ice accumulation, or as directed in the AFM. Occasional and frequent cycling refers
to manually activated systems.
2 These rates can be measured by a suitable icing rate meter.
3 It is assumed that the aircraft is approved to fly in the cited icing conditions. Otherwise, immediate exit from any
of these intensity categories is required by regulations (14 CFR part 91 §§ 91.13(a) and 91.527, part 121 § 121.341,
part 125 § 125.221, and part 135 § 135.227).
4 See footnote 2.
5 See footnote 3.
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ee. Rime Ice. A rough, milky, opaque ice fo rmed by the instantaneous freezi ng of sm all,
supercooled water drops. It is generally rougher in appearance than clear ice.
ff. Runback Ice. Ice that for ms from the freezing or refreezing of water leaving protected
surfaces and running back to unprotected surfaces.
gg. Severe Icing. The rate of ice accu mulation is s uch that ice protection sy stems fail to
remove the accumulation of ice and accu mulation occurs in areas not normally prone to icing,
such as aft of protected surfaces and other areas identifi ed by the m anufacturer. A representative
accretion rate for reference purposes is more than 3 inches (7.5 cm ) per hour6 on the outer wing.
Imme diate ex it is r equired by many Airworthiness Directives (AD), flight manuals , and
operations under part 91, §§ 91.13(a) and 91.527; part 121, § 121.341; part 125, § 125.221; and
part 135, § 135.227.7
hh. Significa nt Meteorolo gical Inf ormation (SIGMET). Infor mation about in- flight
weather of operational significa nce to the safety of all aircr aft. SIGMETs may include severe
icing. (See CWA and AIRMET.)
ii. Stagnation Point. The point on a surface where the l ocal air velocity is zero. The region
of ma ximum icing collection efficiency is near this point.
jj. Stratus Clouds. Clouds that form layers with a uniform base. Stratus clouds can appear
in ragged patches and m ay produce drizzle, rain, or snow.
kk. Sublimati on. A process in which ice turns directly into water vapor without passing
through a liquid state.
ll. Supercooled Large Drops (SLD). Water drops with a diam eter greater than
50 mi crometers (0.05 mm) that exist in a liquid form at air temperatures below 0 °C. SLD
conditions include freezing drizzle dro ps and freezi ng raindrops.
mm. Telephone Information Briefing Service (TIBS). A telephone recording of
meteorological and/or aeronautical i nformation obtai ned by calling an FSS.
nn. Trace Icin g. Ice beco mes noticeable. The rate of ac cumulation is sligh tly greater t han
the rate of sublim ation. A representative accr etion rate for reference purposes is less than ¼ inch
(6 mm) per hour on the outer wing. Deicing/anti-icing equipment is not utilized unless
encountered for an extended period of time (over 1 hour).
oo. Weather Advisory. In standard aviation weather f orecast ter minology, a warning of
hazardous weather conditions not predicted in the forecast area that m ay affect air traffic
operations. These reports are prepared by the NWS.
6 See footnote number 2.
7 Severe icing is aircraft dependent, as are the other categories of icing intensity. Severe icing may occur at any ice
accumulation rate when the icing rate or ice accumulations exceed the tolerance of the aircraft.
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1-4. DISCUSSION. Aircra ft icing re mains a key avi ation sa fety iss ue. Accident data has sho wn
that pilots are (intentionally or inadvertently) flying aircraft not certificated for flight in icing
conditions into such conditions, often with fatal results. Additionally, several accidents have
involved aircraft that are certifi cated for flight in icing conditions. Such accident s are often the
result of pilot co mplacency, lack of situat ional awareness ( e.g., lack of awareness of loss of
airspeed), poor technique, poor understanding of the airplane’s limitations and perform ance in
icing conditions, misconceptions about certification of the airplane and systems for flight in
icing, or a misunderstanding of icing terminology.
a. Certification . Pilots must deter mine if the aircraf t to be flown is certificated for flight
in icing conditions. An aircraft that is certificated for instru ment flight rules (IF R) is not
necessarily certificated for flight in icing conditions. To deter mine whether an aircr aft is
certificated for flight in icing conditions, the A FM or specific POH must be consulted. It is
imperative that the pilot ensur e the air craft is certificated to fly in icing conditions and that the
appropriate deicin g/anti- icing eq uipm ent is installed and ope rational prior to operating in icing
conditions. It is also critical that the pilot un derstand and comply with the applicable lim itations
and procedures when operating in icing conditions.
b. Flight Planning . If an aircraft is not certificated for flight in icing condition s, each
flight should be planned carefully so that icing conditions are avoided. During a flight, the pilots
should monitor available weather information (see Chapter 5, Icing Operations, on in- flight
operations) and be awar e of conditions that m ight req uire a c hange of flight plan to avoid ic ing
conditions. In the event of an inadvertent icing encounter, the pilot should take appropriate ac tion
to exit the conditions immediately, coordinating with AT C as necessary, and declaring an
emergency. In a recent study ( American Institute of Aeronautics and Astronautics ( AIAA) 2006
82, “A Study of U.S. Inflight Icing Accidents and Incidents, 1978 to 2002”), conflicts with ATC were common when pilots take action to exit icing conditions after an inadvertent icing
encounter. Very often, this was because the pilot devi ated from an IFR clearance and fail ed to
declare an em ergency or otherwise clarify the situation with the c ontroller. In a subset of these
cases, the contr oller actually offered to decl are an emergency for the pilot, but the pilot declined.
In another subset, the frequency was too busy for communications, often because the controller
was ove rwhel med with traffic. A num ber of pilots expected an im mediate response from ATC
when they reported diff iculties after encountering ice and expected a blanket clearance to es cape
icing without fir st declaring a state of emergency. In many c ases, such assu mptions proved to be
not only false, but fatal.
c. Engine Upsets . This AC also includes info rmation about a recently identified icing
threat, high- altitude ice crystal in gestion into turbine engi nes. Turbine engine upsets have
occurred from ice accreting within the engine at altitudes up to 42,000 feet and tem peratures
colder than -45 °C (-50 °F). These high- altitude ice crystals in large concentrations, typical ly
found near convective weather systems, do not accrete on external airfram e surf aces and may not
be visible on current- technology airborne radar system s.
d. Pilot Certification . Many pilots of aircraft certifi cated to operate in icing conditions
have had num erous icing encounters in which the aircraft systems coped e ffectively with the
icing conditions, despite a substantial ice buildup in some cases . However, a pilot should not
relax his or her vigilance in icing con ditions because of such experiences. A thin ice accretion on
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critical surfaces that develops in a m atter of minutes can have d ramatic effects on stall speeds,
stability, and contr ol. Wind tunnel testing indicates that if such accretions are particularly rough,
they can have more adver se effects than larger accretions that are relatively smooth.
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CHAPTER 2. ATMOSPHERIC CONDITIONS ASSOCIATED WITH ICING
2-1. AIRCRA FT ICING CONDITIONS.
a. Supercooled Clouds . Nearly all aircraft icing occurs in supercooled clouds. Liquid
drops are present at outside air tem peratures (OAT) below 0 °C (32 °F) in these clouds. At OAT
close to 0 °C (32 °F), the cloud may consist entirely of such drops, with few or no ice particles
present. At decreasing temperatures, the probability increases that ice particles will exist in
significant numbers along with the liquid drops. In fact, as the ice water content increases, t he
Liquid W ater Content (LWC) tends to decr ease since the ice particles grow at the expense of the
water particles. At te mperatures below about -20 °C ( -4 °F), most clouds are made up entirely of
ice particl es.
b. Ice Accumulation. The gener al rule is th at the more ice par ticles and the fewer liquid
drops that are present, the less ice accumulation on the airframe. This is beca use the ice particl es
tend to bounce off an aircraft surface, while the supercooled drops freeze and adhere. As a result, ice accu mulation is often greates t at te mperat ures n ot too far below 0 °C (32 °F), where LWC
can be abundant . LWC is usually negligible at te mperatures below about - 20 °C (-4 °F).
c. Thermal Ice Protection . An excepti on to the gen eral rule just sta ted may be made for
surfaces heated by a ther mal ice protection system (or from compressibility at higher speeds ).
d. Runback Ice Accretion. Tests have shown that when outside air te mperatures (OAT)
are near freezing, the result is no ice accretion near t he stagnation point, but the freezing or
refreezing of water running back on the airfoil, causing runback ice accretions, possibly behind the protected areas. The formation of a ridge is possible. Pilots should be vigilant at OAT
between -5 °C (23 °F) and +2 °C (35 °F).
e. Drop Adherence. The greater the LWC of the cloud, the m ore rapidly ice accumulates
on aircraft sur faces. The size of the drops also is im portant. Larger drops have greater inertia and
are less influenced by the airflow around the aircraft than sma ller drops. The result is that larger
drops will adhere to m ore of the aircraft surface t han s maller drops.
f. Median Volume Diameter (MVD) . Every supercooled cloud contains a broad ra nge of
drops, starting from between 1 and 10 m icrometers ( millionth of a meter) and usually not
exceeding 50 m icrometers (by co mparison, the thickness of the average hum an hair is
approximately 100 m icrometers). A single drop size m ust be chosen as represent ative, and in
icing ter minology this is the MVD, the diam eter such tha t half the liq uid water is in sm aller
drops, and half in larger drops.
NOTE: Icing conditions can occur during operations in clouds with a
significant am ount of liquid water in drops with dia meters larger than
100 mi crometers. These conditions are referred to as freezing drizzle aloft in
cloud or Supercooled Large Drop ( SLD ) in cloud. Paragraph 3-16 discu sses
some cues developed for aircraft with unpowered controls and pneum atic
deicing bo ots, mainly relating to th e location of the airf rame ice, which the
flight crew can use in attempting to deter mine if such drops may be present in
a cloud.
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g. SLD Conditions. An aircraft does not always have to be in a cloud to encounter SLD as
conditions can exist in freezing precipitation below a cloud deck.
2-2. CLOUD T YPES AND AIRCRA FT ICING.
a. Formation. Air can rise because of many factors, inclu ding convection, orographic
lifting (i.e., air forced up a mountain), or lifting at a weather front. As the air rises, it expands and
cools adiab atically. If a parcel of air reaches its saturation point, the m oisture within the par cel
will condense and the resulting drops form a cloud. Cloud water drops are generally very sm all,
avera ging 20 m icrometers in dia meter, and are of such sm all m ass that t hey can be held aloft by
small air currents within clouds.
b. Extent of Icing. If rising air is moist ( i.e., water vapor is plentiful) and lifting is
vigorous, the result can be clouds with substantial LWC and, som etimes, large drops. The greater
the LWC , the m ore rapid the icing; and the l arger the drops, the greater the extent of icing. Tops
of clouds often contain the mos t liquid water and largest drops, because the drops that reach the
tops have undergone t he most lifting. If the te mperatures are cold enough at the tops (below or
around -15 °C (5 °F)), i ce par ticles will usu ally start to for m that tend to deplete the liquid water.
c. Hazardous Conditions. Several types of clouds and the hazardous aircraft ici ng
conditions that ma y be associated with them are discussed below.
(1) Stratus Clouds.
(a) Stratus clouds, som etimes called layer clouds, form a stratified layer that may
cover a wide area. The lifting pr ocesses that form them are usually gradual, and so they rarely
have exceptionally high liquid water contents. Icing layers in stratus clouds with a vertical
thickness in excess of 3,000 feet are rare, so a change of altitude of a few thousand feet m ay take
the aircraft out of icing.
(b) Lake -effect stratus clou ds are ex ception al in that they may have very high LWC
because of the moisture available when they fo rm over lakes. In the continental United States,
lake- effect stratus c louds are most common in the Great Lakes region, particularly in early
winter when cold northwesterly winds blow over the unfrozen lakes.
(c) Drizzle -size drops occasionally occur in stratus clouds, and pilots should always
be on the lookout for cues that mi ght indicate the presence of these drops (see paragraph 3-16 for
a list of cues developed for aircraft with unpowered controls and pneumatic deicing boots).
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(2) Cumulus Clouds.
(a) Cumulus clouds, which often form because of vi gorous convection, can have
high LWC . If an aircraft traver ses them, the icing can be rapid. Because they tend to be of
limited horizontal extent, it m ay be possible to avoid m any of them. Because of the vertical
develop ment of cu mulus clouds, icing conditions can be found in layers thousands of feet in
depth, but with much less horizontal development than in stratus clouds.
(b) This class of clouds includes the cu mulonimbus, or thunderstorm, clouds.
Updrafts in such clouds can be gr eat and result in very large LWC s. Thus, a large icing threat can
be added to the other excellent reasons to stay out of such clouds. The thunderhead anvil can
spread out from the core for several m iles and is co mposed m ainly of ice crystals. These cry stals
will not adhere to unheated surfaces when they hit, but they m ay melt on a heated surface, run
back, and refreeze. The ice content in the anvils can be h igh, and ingestion of the ice cr ystals has
resulted in uncommanded thrust reductions.
(3) Orographic Clouds, Wave Clouds, and Cirrus Clouds. (a) Orographic clouds form when moist air is lifted by flowing up the side of a
mountain. As the parcel of air is lifted, it cools and form s a cloud. Such clouds can contain a
large volume of water and , in som e cases, large drops.
(b) Wave clouds, recognized by their wavy tops, can have high LWC s. Continued
flight along a wave m ay result in airfra me icing.
(c) Cirrus clouds, found at very high, cold alt itudes, are co mposed entirely of ice
particles. Flight through these clouds should not result in structural icing, although the possibility
exists for runback icing from the refreezing of particles that melted on therm ally or
aerodynam ically heated surfaces.
d. Free zing Rain and Free zing Dr izzle.
(1) Freezing rain fo rms when rain bec omes supercooled by falling through a
subfreezing layer of air. Ordinarily, air temperatures dec rease with in creasing altitu de, but
freezing rain requires a te mperature inversion, which can occur when a war mer air mass overlies
a colder air mass. This situation can occur along a warm front, where a w arm air mass overruns a
cold air mass. When flying in freezing rain, norm ally there is warm air (above 0 °C (32 °F))
above.
(2) Freezing raindrops are defined as drops of 500 m icrometers (0.5 mm ) diameter or
larger. A typical diam eter is 2 mm, and the few that grow much larger than about 6 mm tend to
break up. Using 20 mi crometers (0.02 mm ) as a typical diameter for a cloud drop, the diam eters
of rain and cloud drops differ by a factor of approxi mately 100, and the volum e and mass differ
by a factor of about 1,000,000. Drop mass affects how far aft of the stagnation point (leading
edge surfaces) drops will strike the aircraft. Subsequently, f reezing rain will r esult in ic e form ing
in areas far aft of where it w ould normally f orm in icing conditions without freezing rain .
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(3) Drops of f reezing drizzle consist of supercooled liquid water drops that have
diameters smaller than 500 micrometers (0.5 mm) and gr eater than 50 micrometers (0.05 mm).
While smaller than freezing rain, drops of freezing drizzle are still larger than regular cloud
drops, and can fo rm through the sam e process. It consists of supercooled liquid water drops that
have diam eters s maller than 500 m icrometers (0.5 mm) and greater than 50 mi crometers
(0.05 mm). However, freezing drizzle is perhaps more commonly formed by a different process,
known as the collision- coalescence process. When some drops in a cloud grow to approxim ately
30 m icrometers in diam eter through condensation, they begin to settle, falling fast enough so that
they collide with some s maller drops. If the drops coalesce, the result is a larger drop, which now
has an even bet ter chance of capturing s maller drops. Under favorable conditions, this process
can produce drizzle-size drops in a supercooled cloud, usually near the top, where the larger
drops generally are found in any cloud. S tatistics vary, but som e studies have reported that
freezing drizzle aloft form s more t han 80 percent of the tim e by the collisio n-coalescence process
in nonconvecti ve clouds. Thus, in freezing drizzle, the pilot cannot assum e that a warm layer
(above 0 °C (32 °F)) exists above the aircraft. When drizzle drops are found within a supercooled
cloud, they can result in accretions that cause very rapid and dangerous stall speed and drag increases for s ome aircraft and roll control anom alies for others. These s ituations m ay be caused
by the roughness, shape, and extent of the accretion that forms. This is an instance of SLD icing
as discuss ed earlier in this paragraph.
2-3. FRONTS.
a. Formation. When air masses of differing te mperatures, pressures, or relative hu midity
meet, a front is formed. If the front moves so that warm er air replaces colder air, it is c alled a
warm front; if it moves so that colder air replaces warmer air, it is called a cold front. An
occluded front form s when an air mass is trapped between two colder air masses and is forced to
higher and higher levels. In all three cases, significa nt lifting occurs. If sufficient moisture and
subfreezing tem peratures are presen t, icing con ditions are created.
b. Warm and Cold Fronts. Along a warm front, the warm er air tends to slide gradually
over the cold front, forming stratus clouds conducive to icing (see Figure 2-1). In a cold front,
the cold air plows under the warm air, lifting it more rapidly and resulting in the formation of
cumulus clouds with high LWC if the li fted air is moist (s ee Figure 2-2). SLD in the form of
freezing rain and freezing drizzle are so metimes found near fronts, as explained above.
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FIGURE 2-1. WARM FRONT
FIGURE 2-2. COLD FRONT
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c. Navigating a Front. Because of the icing and other hazards associated with some
front s, exposure to icing conditions of varying severity is possible. When flying through a front,
the pilot should take the shortest route through the front, instead of flying along the front, to reduce
the time spent in potential icing conditions.
2-4. CONVECT IVE WEA THER AND ICE CRYS TALS.
a. Convective Weather Systems. Convective weather system s, especially th ose
associated with tro pical weather fr onts, can pump large quantities of moi sture to high altitu des
that fre ezes into ice crystals th at can rem ain alo ft. These i ce crystals can re main as a cloud well
after t he convecti ve system has decayed. Clouds and tem perat ures less than 10 °C are better
indicators of the possible presence of ice crystals when near convective weather.
b. Hazards . Above flight level ( FL) 250, clouds contain little liquid water and mostly
contain ice particles. These clouds with no liquid water have about 20 times less radar
reflectivity than rain drops, a nd therefore are difficult to detect. Airborne weather radar will
receive little to no returns at these altitudes unless it is tilted down to lower altitudes near or below the freezing level. Strong returns from the lower altitudes indicate the possibility of hail,
severe turbulence, or large quantities of ice crystals that could be encountered above and accrete inside turbine engines when overflying these areas. Large deposits may ultimately result in engine upset, engine damage from ice shedding, power loss, or engine shutdown.
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CHAPTER 3. ICING E FFECTS, PROTECTI ON, AND DETECTION
3-1. FORMS OF ICING. Aircraft icing in f light is usually clas sified as being either structural
icing or induction icing. Structural icing refers to the ice that for ms on aircraft surfaces and
components, and induction icing refers to ice in the engine’s induction system.
a. Structural Icing. Ice for ms on aircraft structures and surfaces when supercooled
droplets adhere to them and freeze. Sm all an d/or narrow objects are t he best collectors of drops
and ice up most rapidly. This is why a sm all protubera nce within sight of the pilot can be used as
an ice evidence probe. It will generally be one of t he first parts of the airplane on which an
appreciable amount of ice will for m. An aircraft’s tailplane will be a better collector than its
wings, because the tailplane presents a thinner surface to the airstream. The type of ice that form s
can be classi fied as clear, ri me, or mixed, based on the structure and appearance of the ice. The
type of ice th at form s varies depending on the atmospheric and flight conditions in which it
form s.
(1) Clear Ice. A glossy, transparent ice fo rmed by the relatively slow freezing of
supercooled water ( see Figure 3-1, Clear Ice ). The terms “clear” and “ glaze” have been used for
essentially the sa me type of ice accretion. This type of ice is denser, harder, and som etimes more
transparent than rim e ice. With lar ger accretion s, clear ice m ay fo rm “horns” (s ee Fi gure 3-2,
Clear Ice Buildup with Horns ). Te mperat ures close to the freezing point, large amounts of liquid
water, high aircraft velocities, and large drops are conducive to the form ation of clear ice.
FIGURE 3-1. CLEAR ICE
FIGURE 3-2. CLEAR ICE BUI LDUP WITH HORNS
(2) Rime Ice. A rough, m ilky, opaque ice fo rmed by the instantaneous or very rapid
freezing of supercooled drops as they strike the aircraft (see Figure 3-3, Rime Ice ). The rapid
freezing results in the fo rmation of air pockets in the ice, giving it an opaque and rough
appearance, making it porous and brittle. For larger accretions, ri me ice m ay form a strea mlined
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extension of the wing. Low temper atures, lesser a mounts of liquid water, low velocities, and
small drops favor formation of rim e ice.
FIGURE 3-3. RIME ICE
(3) Mixed Ice. Mixed ice is a co mbination of clear and rim e ice for med on the sam e
surface. Because there is a difference in rates of ice accumulation, ice that builds up quickly traps
air pockets that give the ice a cloudy appearance (see Figure 3 -4, Mixed Ice). Hence, mixed ice is
sometimes called cloudy ice. It is the locatio n, size, shape, and roughness of the ice that is most
important from an aerodynamic point of view. This is discussed in paragraph 3-2.
FIGURE 3-4. MIXED ICE
b. Induction Icing.
(1) Ice in the induction system can reduce the amount of air available for combustion.
The most common exam ple of recip rocating engine induction icing is carburetor ice. Most pilots
are fam iliar with this p heno menon, which occurs when moist air passes through a carburetor
venturi and is cooled. As a result of this proce ss, ice may fo rm on the venturi walls and throttle
plate, res tricting airflow to the engine. This m ay occur at tem peratures between 20 °F (-7 °C) and
70 °F (21 °C). The problem is remedied by applying carburetor heat, which uses the engine’s own exhaust as a heat source to melt the ice or prevent its for mation. Fuel -injected aircraft
engines usually are less vulnerable to icing, but still can be affected if the engine’s air source
beco mes blocked with ice. Manufacturers provide an alternat e air so urce that may be selected in
case t he norm al system malfunctions.
(2) In turbine- engine -powered aircraft, air that is drawn into the engines creates an area
of reduced pressure at the inlet, which lowers the tem perature below that of the surrounding air.
In marginal icing conditions (i.e., conditions where icing is possible), this reduction in
temperature m ay be suff icient to cause ice to form on the engine i nlet, di srupting t he air flow into
the engine. Another hazard occurs when ice breaks off and is ingested into a running engine,
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which can cause dam age to fan blades, engine co mpressor stall, or combustor fla meout. When
anti-icing s ystems are used, runback water also can refreeze on unprotected surfaces of the inlet
and, if excessive, reduce airflow into the engine or distort the airflow pattern in such a m anner as
to cause compressor or fan blades to vibrate, possibly damaging the engine. Another problem in
turbine engines is ice, particularly snow and ice cry stals accumulating on the engine probes used
to set power levels ( e.g., engine inlet temperature or Engine Pressure R atio (EPR) probes), which
can lead to erroneous readings of engine instru mentati on (e.g., Air Florida B -737 accident
National Transportation Safety Board ( NTSB ) accident report NTSB/AAR -82-08).
(3) Ice also may accu mulate on both the engine inlet section and on the firs t or second
stage of the engine’s low -pressure co mpressor stages. This normally is not a concern with
pitot- style engine air flow inlets ( i.e., straight-line-of-sight inlet design). However, on turboprop
engines that include an inlet section with sharp turns, ice can accu mulate in the aerod ynam ic
stagnation points at the bends in the inlet duct. If ice d oes accu mulate in these areas, it can shed
into the engine, possibly resulting in engine operational di fficultie s or total power loss.
There fore, with the se types of engine configurations, the use of anti- icing or deicing syste ms per
the Airplane Flight Manual ( AFM ) is very important. Supercooled water drops tend to form ice
on the turbine engine inlet, fan, and first few stages of the com pressor. Ice cr ystals, when present
in high concentrati ons, tend to form ice deeper in the turbine engine’s compressor section. Ic e
accretions can ulti mately shed and damage the co mpressor, or cause eng ine surge or fla meout.
These conditions are analyzed and tested during original engine airworthiness approvals. T hese
tests are conducted to de monstr ate the turbine engine’s tolerance to these conditions.
3-2. GENERAL EFF ECTS OF ICING ON AIRFOILS. Figure 3 -5, Lift Curve, and
Figure 3-6, Drag Curve, below, depict important information about the effects of ice
contamination on an airfoil. (For this AC, an airfoil is a cross-section of a wing or tailplane).
a. Stall. Figure 3 -5 shows how ice affects the lift coefficient for an airfoil. Note that the
Maximum Coefficient of Lift (CLmax) is significantly reduced by the ice, and the Angle of Attack (AOA) at which a stall occurs (the stall angle) is much lower with ice than without ice.
When slowing down and increasing the AOA for an approach, the pilot may find that ice on the wing that had little effect on lift in cruise now induces a stall at a higher AOA associated with a
lower airspeed. Even a thin layer of ice at the leading edge of a wing, especially if it is rough, can have a significant effect in increasing stall speed. This effect may be even larger if ice accretes behind areas normally protected.
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FIGURE 3-5. LIFT CURVE
b. Drag. Figure 3-6 sh ows how ice affects the drag coefficient of the air foil. Note that the
effect is significant e ven at very s mall AOAs.
(1) A significant reduction in CLm ax and a reduction in the AOA where stall occurs
can res ult from a relati vely s mall ice accretion. A reduction of CLm ax by 30 percent is not
unusual, and a large- horn ice accretion can result in redu ctions of 40 percent to 50 percent. Drag
tends to increase steadily as ice accretes. An air foil drag increase of 100 percent is n ot unusual,
and, for large -horn ice accretions, the increase can be 200 percent or even higher.
FIGURE 3-6. DRAG CURVE
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(2) Ice on an airfoil can have other effects not depicted in these curves. Even before
airfoil stall, there can be changes in the press ure over the air foil that may affect a control surface
at the trailing edge.
3-3. EFFECTS OF ICING ON UNPROTECTED WINGS. An aircraft with a completely
unprotected wing is unlikely to be certificated for flight in icing conditions, but may
inadvertently encounter icing co nditions. Since a cross- section of a wing is an airfoil, the
remarks above on airfoils apply to a wi ng with ice along its span. The ice causes an increase in
drag, which the pilo t detects as a loss in airspeed or an increase in the power required to maintain
the sam e airspeed. (The drag i ncrease is also due to ice on other parts of the aircraft). The longer
the encounter, the gr eater the drag increase; even with increased power, it may not be possible to
maintain airspeed. If the aircraft has relatively limited power (as is the case with many aircraft
with no ice protection), it may soon approach stall speed and a dangerous situation. A sim ilar
scenar io applies to aircraft that are certificated for flight in icing conditions if the wing ice
protection system fails in icing co nditions.
3-4. DEICING SYSTEMS. The operating philosophy behind deicing sy stems differ s from that
of anti- icing syste ms because deicing syste ms are activated after encountering icing conditions,
permitting a certain amount of ice accumulation .
a. Pneumatic Boots. Pneu matic boots, pictured in Figure 3-7, consist of rubber tubes
attached to critical aircraft sur faces, such as the leading edges of wings and horizontal and
vertical sta bilizers. T he tubes may be either chordwise or spanwise. The pneu matic boots are
collapsed during normal operations, with suction provided by a vacuum pump to avoid
disruption of airflow over the wings. W hen the system is activated in flight, a tim er-operated
valve s electively inflates all tubes or half of the tubes int ermittently to crack the ice and then
allow the air flow over the wings to blow off the broken ice.
FIGURE 3-7. WING BOOT
(1) Because ice is per mitted to accrete between cycles (called intercycle or residual ice)
the wing or the tailplane is never entirel y clean. Residual and intercycle ice is inherent in the use
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of any available deicing system, including pneu matic boots. Proper operation of the boots is
necessary to minimi ze the effect of this ice. The a mount of ice increases as airspeed or
temperature decreases. At airspeeds typical of s mall airplanes , it m ay take many boot cycles to
effectively shed layers of i ce. It may appear that the boots are not having any effect at all until
shedding occurs. In m any icing accidents and incidents, loss of airspeed and stall can occur in a
span of minutes. Any remaining ice accretion will increas e in the st all speed.
(2) A layer of ice that is rough at any thickness on a wing’s leading edge can have a
significant effect on aircraft per formance, stability, and control. Consequently, so me
manufacturers now advise that the boots be cycled as s oon as icing is encounter ed, rather than
waiting f or a pre scribed thickness to accrete. The FAA recom mends that the deicing system be
activated at the first indication of icing as activating the boots early and often never results in
having more ice on the wing than waiting for a late activation. It is essen tial that the pil ot consult
the AFM or pilot’s operating handbook ( POH ) (the POH must be co nsistent with the opera ting
limitations section of the AFM) for guidance on proper use of the system.
(3) At the AOA typical of cruise, this ice should have very little effect on lift.
An increase in stall speed beco mes more of a concern at higher AOAs character istic of approach
and landing beca use the aircraft is operating closer to CLmax . Thus, the pilot should consider
continuing activation of the deic ing system for a per iod af ter ex iting the icing conditions so that
the wing will be as clean as possi ble and any effect on stall speed minimized. If the pilot cannot
exit the icing conditions until late in the approach or significant icing appears to remain on the
wing after activating the system, an increase in the aircraft’s stall speed is a possibility a nd
adjustment of the approach speed may be appropriate. Consult the AFM or POH for guidance.
(4) A traditional concern in the operation of pneumatic boots has been ice bridging.
This is at tributed to the formation of a thin layer of ice which fo rms to the shape of an expanding
deicing boot without be ing fractured or shed during the ensuing tube deflation. As the deformed
ice hardens and accretes ad ditional ice, the b oot m ay be ineffective in shedding the bridge of ice.
Studies done in the late 1990s have established that there are few, if any, docum ented cases of
ice bridging on modern boot designs. In addition, several icing tunnel tests sponsored by the FAA since 1999 showed no ice bridging on modern boot designs. Known cases are confined to boots of designs dating back a half century or m ore.
b. Electroim pact System. The electroimpact system deices a s urface using pulses of
energy to produce rapid flexing movem ents of the air plane’s skin surface, which break the bond
of accumulated ice and the shattered ice is then carried away by the airflow. This system is the
least commonly used.
c. Electrothermal System. The electrother mal system deices a surf ace by heating the
surface to a temperature above freezing to break the bond of accum ulated ice. The shattered ice
is then carried away by the airflow. The surface is allowed to cool to allow ice to form, and the
heat is activated again to shed the ice, thus repeating the cycle. Such syste ms are common on
propellers and helicopter main rotors, and have been recently introduced on wing and tail leading edges.
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(1) Propellers are deiced using rubber boots w ith embedded heater wires to break the
adhesion of ice to the propeller blades. So metimes the bl ades are heated alternately in sections
due to lim its of available electrical power. The alternate s ections are heated sym metrically to
avoid an im balance of the propeller while secti ons of ice are being removed and dislodged from
the propeller by centrifugal force. Often, on aircraft that have such systems, the skin surrounding
the airframe is reinfo rced with doublers to strengthen the sk in where ice is most likely to be
flung from the propellers. However, the initial imbalance caused by ice accu mulation and the
loud noise created by ice shedding and hitting the airfra me can be unsettling to passen gers and
distracting to flightcrews.
(2) Intercycle and residual i ce can accrete on ai rplanes with ele ctrothermal deicing
systems. It is typical f or these systems also to produce runback ice behind a protected area.
Because other parts of t he aircraft, i nclud ing part of the span of the wing, are not protec ted from
ice, a d rag inc rease from those areas will still be prese nt. This is accounted for in the ic ing
certification process, and the pilot can fly the aircraft safe ly by following the operating
procedures in the AFM or POH. Residual ice and the ice that accu mulates between deicing
cycles can be expected to have so me effect on CLmax , but note that this eff ect is significant only
at higher AOA.
3-5. ANTI -ICING SYSTEMS. An anti- icing system is designed to keep a surface entirely free
of ice throughout an icing encounter. Anti-icing protection for wings is normally provided by
ducting hot bleed air from the engines into the inner surface of the wing’s leading edge or through an evaporative or running wet system. Anti -icing systems are designed to be turned on
prior to encountering icing conditions. Operating them as de icing equipment may result in a
system failure or damage.
a. Bleed Air Systems. Bleed air systems are used for larger areas of the aircraft, such as
engine nacelles and wing leading edges. Bleed air from turbine engines is the most common type of anti-icing protection for engine nacelles and wings of transport and business turbojets. Hot air is distributed to piccolo tubes, which consist of a perforated pipe installed directly behind the airplane’s skin. Such hot air systems are quite effective in preventing the formation of ice.
NOTE: One drawback of a hot air system is that tapping air from th e engine
to anti-ice large surfaces affects engine temperature limits that require
reduced power settings , which reduce the performance of the engine(s). This
may have a significant effect on clim b perfor mance (especially one en gine
inoperative cli mb perfo rmance in multiengin e turboprops, turbojets, and
turbofans) . This per form ance loss is the reason this system is not common on
smaller turbine powered airp lanes. Pilots should keep in mi nd that while
cruising or descending with anti -ice syste ms on, higher- than -norm al power
settings m ay be required to ensure sufficien t bleed air is being supplied to the
anti-ice syste m, and to prevent engine surges/stalls (see t he particular AFM
for the appropriate settings.) In addition, in some situations on som e
airplanes, the hot air system may not fully evaporate all impinging water drops, resulting in runback ice. This may occur with an inoperative engine,
and m ay be the reason your AFM requires a minimu m engine power setting
on descent.
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b. Evaporative/Running Wet Systems . Evaporative or running wet systems are newer
designs used on smaller jet aircraft, turbopropeller, and piston aircraft. These chem ical s ystems
apply a chem ical agent that lowers the freezing point of water found on aircraft surfaces and
decreases the frict ion coefficient of those surfaces to prevent ice from adhering to the surfaces.
Examples of such chem ical agents are iso propyl alcoh ol and ethylene glycol. This wing anti- ice
system may be running wet by d esign, form ing runback ice accretions. The effects of these
accretions ar e evaluated during cert ification, but only in 14 CFR part 25 appendix C icing
conditions.
NOTE: While an aircraft’s AFM or POH is the ulti mate authority on the
operation of anti-icing systems, a good rule of thumb is to activate anti- icing
syste ms at the first s igns of visible m oisture en countered during conditions
conducive to icing. This will prevent the buildup of any appreciable a mounts
of ice.
3-6. EFF ECTS OF ICING ON ROLL CONTRO L.
a. Ailerons . This paragraph is in effect a contin uation of the previous one, since ice on the
wings forward of the ailerons can affect roll control. The ailerons are generally close to the tip of
the wing, and generally a stall starts near the root of the wing and progresses outward. In this
way, the onset of stall does not inter fere wit h roll control of the ail erons. However, the tips are
usually thinner than the rest of the wing, and so they m ost efficiently collect ice. This can lead to
a par tial stall of the wings at the tips, which c an affect the ail erons and thus roll control.
b. Airfl ow. If ice accu mulates in a ridge aft of the boots, but forward of the ailerons,
possibly due to f light in SLD conditions, this c an affect the ai rflow and interfere with the proper
functio ning of the ailerons, even without a partial wing stall at the tip.
(1) This is the phenomenon that the NTSB found to be responsible for the accident of
an ATR -72 turboprope ller aircraft in Roselawn, Indiana in October , 1994. Flight test
inves tigations following the accide nt su ggested two ways in which the ailer ons mi ght be affected
by ice in front of them.
(2) One has been ter med “aileron snatch,” in which an i mbalance of forces at the aileron
is felt by the pilot of an aircraft without powered controls as a sudden change in the aileron control force. Provided the pilot is able to adjust for the unusual forces, the ailerons m ay still be
substantia lly effective when they are defl ected. The other is that ailerons may be affected in a
substantial degradation in control effectiveness, although without the need for excessive control forces.
3-7. TAILPLANE ICING.
a. Downward Lift. Most aircraft have a no se-down pitching mom ent from the wings
becau se the center of gravity (CG) is ahead of the center of lift. It is the r ole of the t ailplane to
counter act this mo ment by providing downward li ft (see Figure 3-8, Tail Down Moment). The
result of this configuration is that actions that move the wing away from stall, s uch as
deploym ent of flaps or increasing speed, may increase the negative AOA of the tail. With ice on
the tailplane, it m ay stall after deploy ment of flaps (see Figure 3-9, Pitchover Due to Tail Stall).
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FIGURE 3-8. TAIL D OWN MOMENT
b. Tailplane Stall. Since the tailplane is ordinarily thinner th an the wing, it is a more
efficient collector of ice. On most aircraft, the ta ilplane is not visible to the pilot, who therefore
cannot observe how well it has been cleared of ice by any deicing system. Thus, it is important
that the pilot be alert to the possibility of tailpla ne stall, particularly after full f lap deflection, on
airplanes not evaluated for susceptibility. A no-flap landing should be considered to avoid a tailplane stall, consistent with AFM procedures. Tailplane stall is discussed in detail in
Chapter 5, paragraph 5-12.
FIGURE 3-9. PITCHOVER DUE TO TAIL STALL
c. Testing and Analysis. On many transport turbojets, the tailplane has no ice protection.
However, the tailplanes on these aircraft are usually quite thick and therefore are a less efficient
collector of ice. Furthermore, these aircraft are s ubjected to extensi ve certification testing and
analysis to ensure t hat the tailpla ne will not be placed at such an extreme angle in ac tual
operations to experience a stall, even with a large ice accretio n.
3-8. PROPELLER ICING. Ice buildup on operating propeller blades reduces thrust for the
same aerodynam ic reasons t hat wings te nd to lose lift and increase drag when ice accu mulates on
them. The greatest quant ity of ice n ormally c ollects on the spinner and inner radius of the
propeller. However, in one case of suspected large drop icing during a flight test
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(see Figure 3-10, Propeller Ice Accretion During an SLD Encounter ), ice was experienced along
the entire span of the propeller blades. This resulted in a 50 knot loss of airspeed in 1 minute, 25
seconds. T here was little airfra me ice and no indication of propeller icing. As ice accretes on the
propeller blades increasing blade drag, the propeller governor of the constant speed propeller flattens the blade pitch to maintain revolutions per minute (RPM). In the cockpit, the pilot s ees
no change in RPM or torque.
FIGURE 3-10. PROPELLER ICE ACCRETION DURING AN S LD ENCOUNT ER
3-9. ANTENNA ICING. Because of thei r small size and s hape, antennas that do not lay flush
with the aircraft’s s kin tend to accu mulate ice ra pidly. Furthermore, they of ten are devoid of an
internal anti-icing or deicing capability for protection. During flight in icing conditions, ice accumulations on an antenna may cause it to begin to vibrate or cause radio signals to beco me
distorted. Besides the distraction caused by vibration (pilots who have experienced the vibration
describe it as a “howl”), it may cause dam age to the antenna. If a fr ozen antenna breaks off, it
can dam age other areas of the aircraft in addition to causing a communication or navigation
system failure.
3-10. COOLING INLET ICIN G. Some types of electronic eq uipm ent generate sig nificant
amounts of heat and require independent sources of cooling, which often use external air scoops. These cooling inlets are susceptible to icing and may or may not be heated as part of the icing
protecti on syste m on older airplanes . Pilots should c heck th eir airplan e’s AFM to determine if th e
cooling inlets are protected from ice.
3-11. EFF ECTS OF ICING ON CRIT ICAL SYSTEMS.
a. Pitot Tube. The pitot tube is partic ularly vulnera ble to icing because even light icing
can block the entry h ole of the pitot tube where r am air enter s the sy stem. This will a ffect the
airspeed in dicator and is the reason most air planes are equi pped with a pitot heating syste m.
The pitot heater usually consists of coiled wire heating ele ments wrapped around the air entry
tube. If the pitot tube becom es blocked, and its associated drain hole rem ains clear, ram air no
longer is able to enter the pitot sy stem . Air already in the s ystem will vent through the drain hole,
and the rem aining will drop to am bient (i.e., outs ide) p ressure. Under th ese circumstances, the
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airspeed indicator reading decreas es to z ero because the airspeed in dicator senses no difference
between ram and static air pressure. If the pitot tube, drain hole, and static system all beco me
blocked in f light chan ges in airspeed will not be indic ated, due to the trapped press ures.
However, if the static sy stem rem ains clear, the air speed indicator would display a higher
than-actual airspeed as the altitude increased . As altitude is decreased , the airspeed in dicator
would display a low er-than-actual airspeed.
b. Static Port. Many aircraft also have a heating system to protect the static ports to
ensure t he entir e pitot-static system is cle ar of ic e. If the st atic port becomes blocked, the
airsp eed in dicat or would still function; however, it would be inaccura te. At altitu des above where
the static port beca me blocked, the airspeed indicator would indicate a lower- than-actual
airsp eed. At lower altitu des, t he airspeed indicator would displa y a higher -than-actual airspeed.
The trapped air in the sta tic sy stem would cause the alti meter to rem ain at the altitude where the
blockage occurred. The vertical speed indicator would remain at zero. On som e aircraft, an
alternate static air source valve is used for em ergencies. If t he alternate so urce is vented inside
the airpla ne, where st atic pressu re is usually lower than outsi de static pressure, selecti on of the
alternate so urce m ay result in the following erroneous instrument indications:
(1) The alti meter reads higher than normal .
(2) The indicated airspeed r eads greater than norm al.
(3) The vertical speed indicator mom entarily shows a cli mb.
c. Stall Warning Systems.
(1) Stall w arning syste ms provide essent ial in formation to pilots. A loss of these
syste ms can exacer bate an already hazardous sit uation. These system s range from a sophisticated
stall warning vane to a sim ple airflow -activated stall warning switch. The stall warning vane
(also called an “AOA sensor” since it is a part of the stall warning system ) has a wedge- like
shape, has freedom to rotate about a horizontal axis, and is connected to a transducer that
converts the vane’s movem ents into ele ctrical sig nals trans mitted to the airplan e’s flight data
computer. Normally, the vane is heat ed electrically to prevent ice fo rmation. The transd ucer is
also heated to prevent moisture from condensing on it when the vane heater is ope rating. If the
vane collects ice, it m ay send erroneous signals to such equipm ent as stick shaker s or stall
warning devices. Aircraf t that use a stall horn connected to the stall warning switch may not give
any indication of stall if the stall ind icator opening or switch becom es frozen.
(2) Because co ntamination of the wing reduces lift, even an operational, ice- free stall
warning system may be ineffective because the w ing will stal l at a low er AOA due to ice on the
airfoil. Heated or unheated, if the wing is contaminated in any way, an AOA will become
unreliable. The stall on set would occur p rior to activation of stall warning devices leading to a
potential pitch or roll upset. It is im perative that pilots maintain airspeed and monitor AOA
closely when in icing conditions.
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d. Windshields.
(1) On high-perfo rmance aircraft that require co mplex windshields to protect against
bird strikes and withstand pressurization loads, the heating element often is a layer of conductive
film or thin wire strands through which electric current is run to heat the windshield and prevent
ice from form ing.
(2) Aircraft that operate at lower altitu des and lower speeds generally have other
syste ms of window anti- icing/dei cing. One system consists of an electrically heated plate
installed onto the airplane’s windshield to give t he pil ot a narrow band of clear visibility.
Another system uses a bar at the lower end of the windshield to spray deicing fluid onto it and
prevent ice from fo rming.
e. Engine Pressure Ratio ( EPR ) Probe ( Turbine Engines).
(1) Ice crystals can clog and freeze over turbi ne EPR probes as well, resulting in
unreliable and misleading power indications. These indica tions m ay lead a pilot to believe that an
engine is producing more or less power than it actually is, and m ay result in improper thr ottle
adjustments.
(2) There have been several instances where EPR probes becam e clogged with ice
crystals during climb or cruise ( e.g., the Air Florida B -737 accident, NTSB accident report
NTSB/AAR -82-08). Pilots of turbojet aircraft should calculate a back up N1 setting for
takeoff/go -around in icing conditions as a crosscheck for EPR. The activation of engine nacelle
anti-ice when flying in heavy clouds usually prevents ice blockage.
f. Outside Air Temperature ( OAT)/True Air Temperature ( TAT ) Probe.
(1) Ice crystals can clog and freeze over t he heated temperature probe on so me aircraft.
This tendency to freeze over appears to be sensitive to the locati on of the probe on the airfram e.
If the OAT/TAT probe freezes over, the indicated tem perat ure will erroneou sly rise to 0 ºC and
hold. In this s ituation, som e aircraf t syste ms will a lert the flightc rew that th ere is a disagreement
between various ambient temperature sensors, thus indicating the pr esence of ice crystals.
(2) Freezing of the TAT probe has been a precu rsor in many of the tur bine en gine
power loss events occurring in the area of convective weather system s.
3-12. CERTIFICATION FOR FLIGHT IN ICING CONDITIONS.
a. Current Icing Certification. Aircraft which are “certificated for flight in icing
conditions” by Amendment 25-121 or higher go through an extensive procedure i ntended to
ensure t hat they can safely operate thr ougho ut thos e icing con ditions encompassed by the icing
envelopes specified by the FAA. The current icing certification process includes extensive
analysis (done today with sophisticated computer modeling), tunnel te sting, dry -air testing,
testing behind an icing tanker, and flight in natur al icing co nditions. The objective is to verify
that the aircraft has functioning ice prot ection and to ensure that the aircraft will have acceptable
perform ance and handli ng qualities in all the en viron mental conditions covered by t he icing
envelopes for which the aircraft has been tested. For exam ple, certification includes testing and
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analysis to show that an aircraft can hold in significant icing conditions for up to 45 minutes.
Nonetheless, pilots of certif icated aircra ft should not be casual about operations in icing
conditions, particularly extended operations. It is always possible to encounter an unusua l
condition for which the aircraft has not been certificated, such as Liquid Water Content ( LWC )
outside the envelopes, which may be indicated by a very rapid rate of accumulation. T his can
result in runback and ice accumulation aft of protec ted surfaces.
b. Limits of Icing Certification. SLD may result in drops im pinging aft of protected
surfaces and cau sing ice accu mulation behind the protected area of leading edges. These surfaces
may be very effect ive ice collectors, and ice accu mulations m ay persist as long as the aircraft
remains in icing conditions. Note also that icing conditions can develop very quickly and m ay
not be imm ediately recognized. The effect on stall speed increase and drag may be large. This
can be very hazardous, p articularly on approach and landing. On November 4, 2014, part 25,
§ 25.1420 and part 25 appendix O became effective in order to address SLD certification for
new, transport category airplanes, but SLD has not yet been incorporated into the certification of other aircraft types .
NOTE: If an aircraft has certain Supplemental Type Certificate ( STC ) items
installed, these may affect the icing certification of the aircraft as defined by
the manufacturer, some of which might not operate correctly when exposed to icing or be certified for icing conditions.
c. How to Know Whether a S mall A irplane i s Certificated for I cing.
(1) The airplane was certificated to 14 CFR part 23, § 23.1419 at Am endment 23-14 or
later if your AFM or POH references “part 25 appendix C” icing conditions, or
“14 CFR § 23.1419” at Am endment 23-14 or later.
(2) The “Certification Basis” section of your airplane’s Type Certification Data Sheet
(TCDS) ma y reference “14 CFR § 23.1419” at Am endment 23-14 or higher, or “SFAR 23.” The
TCDS can be found in the FAA’s online Regulatory and Guidance Library (RGL) at
http://rgl.faa.go v.
(3) If there is only a m inimum equipm ent lis t (ME L) for icing co nditions in th e AFM or
POH, the certification basis of your airplane is prior to Am endment 23-14 (1973).
d. Icing Certification Has Changed Over the Years.
(1) Current part 23 icing r egulations have only been applied to new airplane designs
certificated since 2000. In these new designs, the stall warning system on an icing- certified
airplane is designed and tested with critical ice accretions along the entire span of the wing. In many new designs this results in the stall warning speed bia sed higher in icing conditions.
(2) Prior to 2000, a clear and unambiguous buffet was accepted for stall warning in
icing conditions, even if the airplane was equipped with a stall warning system and a hea ted stall
warning sensor.
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NOTE: If the certification basis is Amendment 23 -43 or higher, you can be
sure the stall warning systems functions in the icing conditions for which the
airplane is certified. If it is lower than Amendment 23 -43, do not rely on your
stall warning system in icing.
(3) Airplanes certified for flight in icing after 1994 have been tested for susceptibility to
Ice-Contaminated T ailplane S tall (ICTS). ICTS cannot occur if AFM limitations and procedures
are followed.
(4) Prior to 1973, there were no requirements to test part 23 airplanes in icing
conditions. Part 23 airplanes were approved for flight in light icing conditions , and moderate
icing for limited time, if they were properly equipped. Many of these airplanes remain in the fleet
today. The ice protection systems on these airplanes should be considered a means to help exit icing conditions.
e. How C ertification Relates to Operating Rules. Operation of an aircraft in known
icing is based upon when an aircraft was built and how that aircraft was certified during
manufacture. Manufacturers specify how the installed equipment in that aircraft is to be operated
in the POH and AFM within ce rtain conditions of limitation.
3-13. AIRPLANES NO T CERTIF ICATED FOR ICING.
a. Ice Protection. All aircraft a re required to have ice pro tection for their propulsion
syste ms in case of an inadvertent icing encounter, and nearly all aircraft have pitot heat and an
alternate source of static air.
b. Avoidance. Airplanes not certified for icing are not tested for inadvertent icing
encounters. Pilots of these airplanes must avoid icing conditions and immediately exit icing if
inadvertently encountered.
(1) In recent years these airplanes have been involved in more icing accidents than
icing -certified airplanes.
(2) Do not believe the myth that thicker General Aviation (GA) airfoils are tolerant of
ice accretion. Research has shown that even for small amounts of ice accretion, effects not
apparent while operating in the middle of the flight envelope may be noticeable when operating
at the edge of the flight envelope. The most common are an increase in stall speed (with a late or no warning) or the inability to climb at altitude.
c. Inadvertent Encounter. Some GA aircraft not certificated f or flight in icing conditions
have ice protecti on systems on their wings and tailplane, providing an additional safety margin ,
should an inadvertent encounter with icing occur. These are intended for e mergency use only.
(1) The FAA recommends that aircraft not certificated for flight in icing conditions exit
icing conditions as expeditiously as possible.
(2) The differences between these sy stem s and fully certified syste ms are significant.
Airplane perfo rmance is unknown, stall warning in icing con ditions most likely will n ot activate
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prior to stall, controls m ay jam due to ice accretion, and system features required for known icing
may not be present in these “non- hazard” system s.
3-14. MAINT ENANCE CO NSID ERATIONS. Some anti -icing and dei cing syste ms are
known to be very reliable, while others may require a lot of m aintenance to rem ain effective.
Pneu matic boots, for example, are known for thei r susce ptibility to damage from many sources
and should be inspected carefully. The rubber used for the boots is subject to degradation from
atmospheric pollution, which results in the rubber cracking and losing some of its elastic
properties. An ice adhesion inhibit or should be applied to pneu matic deicing bo ots in accordance
with the maintenance manual and is highly recomm ended . Testing in 2005 showed that the
proper application of ice adhesion inhibitors improved ice shedding at colder tem peratures and a
reduced amount of residual and intercycle ice. Any product that is not recommended by the airplane or boot manufacturer should be approved by the FAA. Other problem s are defects,
delaminations, or tears in the rubber caused by the i mpact of objects, such as foreign m atter
found on airport ramps. Pinholes or tears in pneum atic dei cing boots will draw in moistu re when
system vacuum is supplied, and subsequent freezing of this moisture can render the system
ineffective. Maintenance personnel should evaluate defects in the boots when they are found.
a. Preflight Inspection . Flightcrews should always examine their airplane’s anti -icing and
deicing equipment as part of the normal preflight inspection. A full check of anti- icing and
deicing equipment should be performed especially when flight i nto known or forecast icing is
expected as identified in the AFM procedures.
b. Equipment Deficiencies. Flightcrews should consult the airplane’s MEL for details on
what is permitted to be inoperable and what equip ment deficiencies constitute no -go items.
3-15. ICE DETE CTION.
a. Electronic.
(1) Many mode rn aircraft co me equipped with electronic ice d etectors. A common
in-flight ice detector consists of a probe that vibr ates at a specific frequency. When ice begins to
form on the probe, the frequency of the probe’s vibration will change because of the increased
mass of ice on the probe, and an indicator will light in the co ckpit. These detectors are acti vated
for a short ti me period, generally one minute, after which the probe is heated electrically to melt
the accreted ice. The pr ocess is then repeated. If the ai rcraft is flying in continued icing
conditions, ice will continue to form on the probe , and , for some aircraft, the light in the cockpit
will re main on.
(2) Pilots sho uld consult the ir AFM or POH to determ ine if their ice d etection system is
an advisory or pri mary system. The diff erence between the systems is the redundancy of the
system and testing required for certification. T he m ajority of airplanes have an advisory system,
which means the pilot is responsible for detecting ice and ensuring ice protection syste ms are
activated. This is true e ven for ice protection sy stems that are auto matically activated when the
ice detection s ystem detects ice.
(3) Currently, there are no electro nic detection syste ms that can reliably detect ice
crystals, although new systems are under developm ent.
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b. Visual. Strategically located or unprotected protuberances visible to the crew may also
serve as ice indicat ors. For example, windshield wipers, pod pylons, or landing lights can serve
as icing reference s because they tend to build up ice first, or m anufacturers may provide one for
this purpose. These ice detectors, referred to as “ice evidence probes,” are typically in plain vi ew
of the cock pit. If ice begins to accu mulate on such an ice detector, the flightcrew should assume
the rest of the aircraft also is accumulating ice a nd take appropriate action. These detectors o nly
serve t heir purpose if pilots include them in their scan during flight in potential icing conditions.
If possible, pilots should m onitor critical surfaces at te mperatu res near freezing, si nce ice m ay
form on critical surfaces prior to fo rming on visual ice indicators.
3-16. VISUAL CUES OF SLD CONDITIONS. If SLD is known to be present, m ost aircraft
with unpowered controls and airfra me deicing systems should request a ro ute or altitu de change
to exit the conditions. This action may be prudent for othe r aircraft as well. The cues listed below
were developed for aircraft with un powered controls and pneum atic deicing boots as indicative
of SLD conditions. Of most concern is the accreti on of ice in areas aft of where it would usually
be found. Such aft accretions could som etimes be the result of runback due to high liquid water
content rather than SLD. Excessive runback icing, however, may have effects sim ilar to SLD, so
similar pilot action may be appropriate. The cues are:
a. Wing. Ice may beco me visible on the upper or lower surface of the wing, aft of the
active part of the deicing/anti -icing system. If the wings are visible, pilots should monitor for ice
accretion aft of the protected area. Pilots should also look for irregular or jagged lines of ice or
for pieces of ice shedding off the airplane. During night operations, pilots should use adequate
illumination to observe all areas.
b. Propeller. The aft lim it of ice accumulation on a propeller spinner that is not hea ted
will reveal ice extending beyond norm al lim its, typically back to the blades.
c. Window s. Unheated p ortions of side windows m ay begin to accu mulate granular
dispersed ice crystals or a translucent or opaque coating over the entire window. This icing m ay
be acco mpanied by other ice patterns on the windows, such as ridges. These patterns m ay occur
from within a few seconds to half a minute after exposure to SLD conditions.
d. Engine Nacelles. Ice may form on engine nacelles behind the inlet lip.
(1) Airframe. Ice coverage may become unusually extensive, with visible ice fingers or
feathers on parts of the airfra me that norm ally would not be covered by ice. The aircraft’s
perform ance may degrade . Pilots should remain vigilant when icing conditions are present, and
any alteration of the aircraft’s perform ance should be monitored closely as a sign of icing on the
airplane.
CAUTION: Pilots should be vigila nt for t he ice accretions listed above when
the follo wing are observed: (1) visible r ain or drizzle at temperat ures below
+5 °C OAT, and/or (2) drops that splash or splatter on impact at temperatures below +5 °C OAT.
CAUTION: Vigilance for SLD ice accretions sh ould also be exercised when
flying into or over areas reporting pr ecipitation at the surface, such as rain,
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freezing rai n, sleet, ice pellets, drizzle, freezing drizzle, or snow, where
tempera tures are near freezing. However, pilots should be aware t hat SLD
could occur aloft without any SLD precipitation on the surface. Current
weather information can m iss SLD, so it is i mportant to know and watch for
cues on the airplane.
(2) While the pilot should be aware of these general cues, there m ay be specific cues that
are characteristic of SLD icing on particular aircraft types. The pilot shou ld consult the aircraft
AFM or POH for descriptions of any such cues.
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CHAPTER 4. FLIGHT PLANNING
NOTE: All pilot s, whether t hey are General Aviation (G A) or air carrier
pilots, are responsible for obtaining as m uch infor mation as possible about
all me teorological condit ions, including icing conditions, before departure.
Aviation m eteorologist s at the National Weather Service ( NWS) Aviation
Weather Center (AWC), local NWS Field Offices, major airlines, and private
companies prep are icing foreca sts and continue to improve upon their
accuracy. A review of the current edition of AC 00 -45, Aviation Weather
Services, is strongly recommended as pilots will need to understand and
apply C urrent Icing Products (CIP), Forecast Icing Products (FIP) and other
services to the flight planning and operations discussed in this AC.
4-1. PRE FLIGHT PLANNING IN FORMATION. Information concerning icing can be
obtained through several different sources. Pilot Weather Reports (PIREP) are generally the most
useful as they are factual weather reports for specific times and places. Area forecasts, Airmen’s
Meteorological Information (AIRMET) , and significant meteorological information (SIGMET)
provide general information on forecasted inflight icing. Winds aloft forecasts also provide information to determine the approximate freezing level. The graphical CIP and the FIP combined with text based forecasts, provide adequate information for flight planning when icing conditions may exist.
a. Location of Fronts. Fronts play an important part in the formation of icing conditions.
Pilots should be aware of a front’s location, type, speed, and direction of movem ent. Pilots
should try to keep a m ental picture of where the front is moving and look for indications of
frontal activity or frontal passage, such as a wind shift or temperatur e change.
b. Cloud Laye rs. Pilots can reasonably expect inflight icing when flying in clouds with
temperatures at or below 0 °C (+32 °F). Forecasts, weather reports, and PIREPs of the cloud bases and tops are essential when flight planning for aircraft that have not been certified for flight in icing co nditions.
c. Free zing Levels. It is critically important for pilots to obtain the freezing levels for the
areas in which they will be flying to be able to m ake educated d ecisions on how to exit ic ing
conditions if they are encountered. It is also important for pilots to know if there are any
temperature inver sions aloft that mi ght alter the normal relationship betw een altitu de and air
temperature. Pilots sho uld be aware of m ultiple freezing levels and th eir locatio ns. The NW S
Aviation Digita l Data Ser vice (A DDS) Web site at http://adds.aviationweather.gov/icing/
provides a graphical depiction of the freezing level.
d. AIRMET and SIGMET. An AIRMET is a weather advisory issued only to amend the
area forecast concerning weather phenomena which are of operational interest to all aircraft and potentially hazardous to aircraft having limited capability because of lack of equipment, instrumentation, or pilot qualifications. A SIGMET is a weather advisory issued concerning weather significant to the safety of all aircraft. Pilots should not rely on these alone for reasons
stated before as AIRMETs for icing will not be issued unless the affected area is large, and a severe icing PIREP will not automatically result in an icing SIGMET.
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• SIGMETs and Convective SIGMETs advise of weather that is pote ntially hazardous
for all aircra ft, such as se vere icing. A SIGMET for severe icing appli es to al l
aircraft, from sm all GA air craft to tr ansport jets (see also the discu ssion in
paragraph 4- 2).
• The ADDS Web site at http://adds.aviationweather.gov/airm ets provides a graphical
depiction of the area s covered by AIRMETS and SIGMETS.
e. PIREPs . Pilots should consult PIREPs, since AIRMETs and SIGMETs will not
necessarily be issued as previously discussed. PIREPs are of high value since they are actual
weather reports at specific places and times. However, PIREPs from high -speed aircraft may not
represent the actual icing conditions as the ram air temperature rise can mask the true icing
conditions. The simplicity and the susceptibility to icing of the small low -speed general aviation
airplanes provide the most accurate reports of inflight icing. An example that shows the
importance of studying PIREPs is National Transportation Safety Board ( NTSB ) accident
report ERA12FA115.
f. Aviation Routine Weather Reports ( METAR ). Pilots should be aware that surface
observations not augmented by a human observer (“AUTO” in METAR) cannot report freezing
rain (FZRA) or freezing drizzle (FZDZ) concurrently with other occurring precipitation. Since
supercooled water drops can occur simultaneously with other reported precipitation, pilots on icing -certified airplanes sh ould be vigilant for severe icing when approaching such an airport,
particularly if “snow” or “snow and mist” are reported at temperatures slightly below or above freezing. An example that show the limitations of automated surface observations is NTSB
accident report DEN05FA051.
g. Air Temperature and Pressure. Icing tends to be found in low- pressure areas and at
temperatures at or arou nd freezing. Pilots can reference the surface analysis charts to identify
areas of low pressure. Freezing levels can be determined from the winds aloft forecast.
h. Icing in Stratiform Clouds. Because the icing conditions in stratiform clouds often are
confined to a r elatively thin laye r, either climbing or descending m ay be effective in exiting the
icing conditions wi thin the clouds.
(1) A cli mb ma y take the aircraft into a colder section of cloud that consists exclus ively
of ice particles. These g enerally con stitute little threat of str uctur al icing because it is unlik ely
that the ice par ticles will ad here to un heated surface s.
(2) The cli mb also m ay take the aircraft out of the cloud altogether to an altitude where
the ice gradu ally will sublim ate or shed from the air frame depending on the conditions.
A descent may take t he airplane i nto air with te mper atures abo ve freezing, within or below the
cloud, where the ice can melt.
i. Icing in Cumulif orm Cl ouds. Hazardous icing conditions can occur in cumulus clouds,
which som etimes have very high liquid water content. Therefore, it is not advisable to fly
through a series of such clouds or to execute holds within them. However , because these clouds
normally do not extend very far horizontally, any icing encountered in such a cloud may be of limited duration; it may be possible to deviate around the cloud.
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j. Snow. In flight, dry snow is unlikely to pose a hazard with respect to icing; however,
wet snow may begin to adhere to aircraft surfaces. If wet snow does begin to stick, it should then
be treated as an icing encounter because ice may begin to form under this ac cumulation of snow.
No aircraft is evaluated in the icing -certification process for this rare situation. If it occurs, the
aircraft should exit the co nditions as quickly as pos sible and declare an emergency or contact air
traffic control (ATC ) as neces sary. Be aware that freezing drizzle can coexist with snow. If you
are flying into or over areas reporting snow, it is important to understand that the presence of
snow does not necessarily mean that icing conditions are not present. See following paragraph for further information.
k. Free zing Rain and Driz zle. If flying into an airport with no human augmentation of
automated weather, be alert for severe ice accretions due to FZRA or FZDZ that would not be
automatically detected, particularly if the reported temperature is near freezing and any precipitation (snow, mist, rain, drizzle) is being reported by the automated station .
l. Ice Pellets. Ice pellets by the mselves are not a h azard to the airfram e with respec t to
icing, but a ground observation of ice pell ets could indicate Supercooled Large Drops ( SLD )
aloft.
m. Alternati ves. When contem plating flight into possible icing conditions in an aircraft
approved for flight in icing conditions, a major consideration of preflight planning is to have alternative courses of action if conditions are worse than expe cted. These alternatives could be a
change in altitude, heading, airspeed, or an alternate airport with adequate runway length. It is
important to note that aircraft that are appr oved for instru ment flight rules (IFR) operations but
not certified for known icing cond itions were n ot tested d uring the certification process for
inadvertent icing encounters. Therefore, pilots in such aircraft should em phasize ice avoidance
during preflight planning and pay special attention to pla nning an alternate course of action in
case actual i cing is encountered.
4-2. ICING INTENSITY. To report the inte nsity of icing, such as in a PIREP, the following
descriptions are used ( see paragraph 1-3 f or complete de finitions):
a. Trace Icin g. Ice beco mes noticeable. The rate of accu mulation is slightly greater than
the rate of sublim ation. A repres entative accretion rate f or refere nce purposes is less than ¼ inch
(6 mm) per hour on the outer wing. The pilot should consider exiting the icing conditions before they beco me wor se.
b. Light Icing. The rate of ice accu mulation requires occasio nal cycling of manual deicing
syste ms to minim ize ice accretions on the airfram e. A representa tive accretion rate f or reference
purposes is ¼ inch to 1 inch (0.6 to 2.5 cm) per hour on the unprotected part of the outer wing. The pilot should consider exiting the condition.
c. Moderate I cing. The r ate of ice acc umulation requires frequent cycling of m anual
deicing syste ms to m inimize ice accreti ons on the airfram e. A representative accretion rate for
reference purposes is 1 to 3 inches (2.5 to 7.5 cm ) per hour on the unprotected part of the outer
wing. The pilot should consider exiting the condition as soon as possible.
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d. Severe Ici ng. The rate of ice accu mulation is s uch that ice protection sy stems fail to
remove the accu mulation of ice and ice accumul ates in locations n ot normally prone to icing,
such as areas aft of protected surfaces and any other are as identi fied by th e manufacturer.
A representative accretion rate for reference purposes is more than 3 inches (7.5 cm) per hour on
the unprotected part of the outer wing. By regulation, imme diate exit is required.
4-3. PIREP CAUTIONS. Although PIR EPs are excellent sources of information about in-flight
icing, there are situations when these reports can be m isleading.
a. No Recent PIREP. An aircraft encounters icing conditions in an area where there were
no recent icing PIREPs. There are several possi ble reasons for t his:
(1) No aircraft recently flew in the area.
(2) Some aircraft recently flew in the area but did not encounter the icing conditions.
This is a c ommon occurrence, especially if the ar ea has li mited air tra ffic. Icing conditions are
extrem ely varia ble in b oth space and time. A slight change in altitude or flight path or the passage
of just a few minutes can mean the difference between encount ering and not encountering icing.
There are many docu mented cases of aircraft flying through approxim ately the sam e area at
similar altitudes at ap proxi mately the sa me time with one aircraft experiencing substantial icing
and the other experiencing none.
b. No Icing Reported. An aircraft encountered icing, but the pilot did not report it. Pilot
workloads might prevent a pilot from making a report, particularly when making an approach.
An aircraft might also encounter icing conditions that are more serious than those are reported in any recent PIREPs in the given area. There are several possible reason s for this:
(1) Icing conditions are extrem ely variable in space and ti me, as previously noted.
PIREPs depend on the type and ice protection of the reporting aircraft. If the pilot’s aircraft is slower or has less ice protecti on than the reporting aircraft, it m ay experience more serious icing
than the reporting aircraft in the sa me exact meteorological conditions. For exam ple, a
Boeing 747 m ay report light icing when flying through conditions that would cause a Mooney to
experience severe icing.
(2) PIREPs are sub jective, depending on the pilot’ s observations, how the pilot operates
the ice protection on the aircraft, and the pilot’s experience level with in -flight icing.
For exam ple, there are docu mented cases of pilots reporting light ic ing conditions when ice was
accreti ng on an ice evidence probe at a rate of approximately 1 inch per mi nute. In addition,
observation and assessm ent of icing is more difficult at night.
(3) Although PIREPs from similar aircraft are most relevant to the pilot’s aircraft, dire ct
transl ation to the pilot’s aircraft may still present difficulties. In a ddition to pilot subjectivity,
other relevant ques tions are:
• Was the reporting aircr aft flying slowly, or climbing, at a high AOA (which is
conducive to accum ulation over a lar ger area of t he aircraft)?
• What kind of ice protection does the reporting aircraft have, and is it
functioning properly?
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(4) When icing conditions exist, reporting m ay alert other crews to maintain vigilance ,
and pilots are reminded that such reporting of meteorological hazards is a regulatory requirement
under 14 CFR part 91, § 91.183; part 121, § 121.561; and part 135, § 135.67. Flightcrews s hould
ensure that when subm itting a PIREP of observed icing conditions, they accurately state the
conditions and effects of the icing o bserved and report them in a timely fashion to make the
PIREP as useful as possible. Pilots are encouraged to provide in- flight icing observations as part
of the weather forecasting process as often as practical. The importance of PIREPs to provide additional input into NWS forecasting products cannot be stressed enough. A report of no icing in a particular PIREP can be just as useful as those reporting ice subsequently improving the quality of CIP/FIP.
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CHAPTER 5. ICING OPERATIONS
5-1. GENERAL . This chapter focuses prim arily on how to safely fly an air craft certificated for
flight in icing conditions, what is expected regarding communications of icing conditions and
when it is advisable to exit tho se conditions. The following is only a sam pling of icing -related
items to consider when planning a flight. Pilots should consult the aircraft ’s Airplane Flight
Manual ( AFM ) or pilot’s operating handbook ( POH ) for approved checklists and operations in
their particular aircraft.
5-2. REGULATIONS FOR ICING OPERATIONS. Title 14 C FR parts 91, 121, 125, and 135
specify the respo nsibilities of flightcrews c oncerning flight in icing conditions. Pilots are advised
to check the current regulations for revisions. An important distinction in each of thes e
regulatio ns is the restriction on flight into known or forecast conditions. Because of the
limitations of icing forecasts, it is ad mittedly difficult for pilots to be certain w hether the
conditions in which they are flying actually will result in an icing encounter, and it is even m ore
difficult to deter mine the severity of the possible encounter. Pilots can be caught inadvertently in
icing con ditions that ex ceed thes e legal lim its. General operating and flight rules for General
Aviation ( GA) aircraft are found in part 91, but not all rules within part 91 are applicable to all
GA aircraft. Part 91 , § 91.501 states that the rules in subpart F apply only to large and
turbojet-powered multiengine airplanes and fractional ownership program aircraft that are not
covered by parts 121, 125, 129, 135, and 137. Section 91.527, Operating in icing conditions, falls
within subpart F and thus is not applic able to all GA aircraft . For aircraft not covered by
subpart F, there are no specific icing regulations ; however, § 91.9 prohibits you from flying
without complying with the operating limitations in the POH or placards. For part 121 aircraft, refer to part 121 , §§ 121.341 and 121.629; for part 125 aircraft, refer to part 125 , § 125.221; and
for part 135 aircraft, refer to part 135 , § 135.227. The operating rules have the following
language on severe icing:
a. Severe Icing. No pilot may fly a nontransport category airplane type- certificated (TC)
after December 31, 1964 into known or f orecast severe icing conditions unless one or more of
the following apply:
(1) The aircraft has ice pro tection provisions that meet part 135 appendix A.
(2) The aircraft has ice protection provisions that m eet the requirements for transport
category airplane type certification.
CAUTION: Even airplanes approved for flight into know n icing conditions
should not fly into seve re icing. Many AFM Limi tations Sections require an
immediate exit wh en these types of conditions are encountered.
CAUTION: Airplane c ertification f or flig ht into kno wn icing conditions does
not include freezi ng dr izzle and freezing rain. In fact, some ai rplanes are
prohibited from fl ying into freez ing drizzle or free zing rain, regardles s of its
intensity. These conditions are very dangerous and can cause ice to form
behind the protected areas.
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b. Communi cations w ith Air Traffic Control ( ATC). When encountering icing,
controllers w ill not know if an aircraft is certificated or equipped for icing, the severity of the
conditions, or what anti-icing or deicing equip ment is installed on the aircraft. The pilot should
communicate to ATC the severity of the icing conditions, its effect on their aircraft and
continued operations, whether an alteration of the current course and altitude is required, and, if necessary, if an alternate destination is needed. If an aircraft that is not certificated for flight in
icing conditions inadvertently encounters ice, exit icing conditions as expeditiously as possible and declare an emergency to ATC. Inform the controller of what actio ns are bei ng taken by the
pilot to cope with the emergency and coordinate with ATC for additional instructions, altitudes,
or headings needed to resume safe flight operations.
(1) In the congested airspace that exists in som e parts of t he country, along with the
intensity of radio communications in such areas, it is possible that a pilot who encounters icing
will not rec eive a cl earan ce in ti me to exit the c onditions befor e safety is co mpro mised. In this
case, it is recommended that the pilot declare an emergency and exit the conditions as soon as
practical.
(2) If an aircraft certificated for flight in icing conditions encounters freezing rain or
freezing drizzle, advise ATC and do not attem pt sustained flight in these conditions. Final
authority and responsibility for the safety of a flight rests with the pilot in comm and (PIC) .
(3) At any time, a pilot should not hesitate to reject a contr oller’s instructio ns if, in the
judgment of a pilot, those instructions would result in an unsafe condition. Pilots should not accept an airspeed assigned by ATC that is inconsistent with their manuals or the airplane manufacturer recommended airspeed, or if there is no specific icing airspeed information provided in the manuals, incons istent with 50 percent to 60 percent margin above stall speed (V
s)
as recommended.
5-3. AVAI LABLE IN -FLIGHT INFORMATION. Available services and products are best
described in the current edition of AC 00 -45, Aviation Weather Services. Brief descriptions of
some of the basic services are provided below:
a. Flight Watch. There are nu merous sources of meteorological inform ation available to
pilots while in flight. A principal source of this in form ation is Flig ht W atch. Fli ght W atch is a
Flight Service Station ( FSS) -provided en route flight advisory serv ice designed to provide, upon
request, timely weather in formation pertinent to the type, route, a nd altitude of flight. Flight
Watch is available from 6 AM to 10 PM local tim e at altitud es generally above 5,000 feet above
ground level (AGL). For weather information at other tim es or at lower altitudes, contact an
automated flight service station ( AFSS ) via radio on a nearby frequency outlet as depicted on
aeronautical charts. The FSS s providing this service are listed in the Airport/Facility D irectory.
b. Hazardous In-F light Wea ther Advisory Service (HIWAS). The HIWAS provide s
continuous, recorded hazardous in- flight weat her forecasts over selected Very High Frequency
(VHF) Omnidirectional Range (VOR) o utlets within the HIWA S broadcast area. This broadcast
area is a geographical zone of responsibility, including one or m ore HIWAS outlet areas (defi ned
as the area within a 150 nautical mile radius of the HI WAS outlet) assigned to an FSS for
hazardous weather advisory broadcasting.
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c. Transcribed Weather Broadcast (TWEB) . The T WEB is a continuous recording of
meteoro logical and aeronautical infor mation broadcast on low/m edium frequencies and VOR
facilities f or pilots. The TWEB is based on a route-of- flight concept that includes, among other
information, adverse con ditions, route forecasts, outlooks, and PI REPs that may contain useful
icing -related infor mation.
d. Air Carrier Dispatch. Air carrier flightcrews nor mally can contact their dispatch
facilities on specified company frequencies or through their airplane ’s onboard Aircraft
Communications Addressing and Reporting System (ACARS). Dispatch can then relay icing
information, changes in front movement or speed, or recent icing PIREPs.
e. Weather Radar. Since airborne weather radar detects raindrops, pilots should avoid
cells painted on radar when the temperatures are at or near freezing. Airborne weather radar
cannot, however, detect drizzle-size drops or cloud-size drops, and therefore should not be relied
upon to detect icing in clouds or fr eezing drizzle. It also lacks the ability to detect s mall, ice
crystals that have little to no liquid water present that can be in heavy concent rations near
convective weather sy stems.
f. Data Link/Satellite Radio. Pilots now have the option to subscribe to and receive
near-real-time weather and airspace in form ation via satellit e to their panel -mounted or handheld
avionics which may include free products via Flight Information Service- Broadcast (FIS-B), a
component of Automatic Dependent Surveillance- Broadcast (ADS -B), transmitted to the cockpit
avionics (refer to the current edition of AC 00 -63, Use of Cockpit Displays of Digital Weather
and Aeronautical Information, Appendix 1). Thes e services can pr ovide textual or graphical
METARs, Terminal Area Forecasts ( TAF ), SIGMETs, and AIRMETs, updated pilot reports,
updated winds and tem peratures aloft, information about severe thunderstorms, icing levels, and
graphical d isplays of te mpor ary flight restriction s. Such up- to-date inform ation can assist pilots
in avoiding hazardous weather and identifying potentially hazardous areas of unforecast weather.
Pilots must understand that some of the products available to them may not include pertinent icing information . Pilots are advised to verify the products they are receiving include the
necessary information consistent with their aircraft capabilities and route of flight.
5-4. PRE FLIGHT. The first step in preparing for any flight is to obta in a thorough weather
briefing as previously discussed in Chapter 4 and 5 using products presented in AC 00-45.
a. Considerations. Operations in or around icing conditions require some additional
considerations:
(1) When deter mining routes of flight, make note of airports along the way and
highlig ht them on the chart f or easy identif ication in case an alternate is needed.
(2) When deter mining routes, consider the climb pe rformance of the airplane and the
route’s minimum altitu de, particularly in mountainous terrain. Be cause the air plane’s cli mb
perform ance will be degr aded in ici ng conditions, consult the AFM for any degradation data.
(3) Deter mine icing exit strategies dur ing preflight. Determ ine if climbing or
descending will be viable options based on the planned route of flight which includes required
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altitudes to maintain clearance from terrain, airspace, published routes, departure procedures, arrival procedures and approaches.
(4) When choosing alternate airports, reme mber that if structural icing occurs, higher
approach speeds and, consequently, additional runway length may be required for landing.
(5) Consider carrying a high intensity flashl ight for use in l ocati ng ice accumulation on
the aircraft at night or in low visibility conditions.
(6) Consider using a transceiver as a backup radio in case of a communications loss
caused by an antenna icing up and/or breaking of f.
(7) If the aircraf t is loaded near maximum gross weight, climb perform ance will be
degraded, possibly increasing time spent in icing conditions.
(8) Extra fuel may be necessary because of additional fuel needed to operate icing
systems or from excess drag or weight caused by ice fo rmation that may require extra power to
maintain altitude or airspeed , increasing fuel consu mption.
(9) When performi ng an aircraft preflight inspection, re move all frost, snow, and ice
from the aircraft surfaces because even v ery small amounts may adversely affect the
aerodynami c properties of a wing. Placing an aircr aft in a heated hangar is a good method of
removing f rost, snow, and ice; however, a pilot should ensure the aircraft is dry before removing
it from the hangar to prevent the m oisture from refreezing on the surface.
b. Frost. Frost can form on an airplane sitting outside on a clear nig ht when there is
moisture present in the air as the air plane’s skin temperature falls below f reezing due to radiation
cooling.
(1) Certain airplanes may be more vulnerable to ice for mation from cold-soaked fuel
than others depending on how the fuel tanks are arranged and how much fuel they contain (more information regarding icing certification and testing can be found in the current edition of
AC 20-147, Turbojet, Turboprop, Turboshaft, and Turbofan Engine Induction System Icing and
Ice Ingestion ). Most aircraft specify allowances for the amount of frost that may be present
underneath the wing. On the Ground, clear ice can form on the upper surfaces of the wing when cold-soaked fuel (due to aircraft prolonged operation at high altitude) remains in contact with the
fuel tanks’ upper surfaces after landing, and during time on the ground when the airplane is exposed to conditions of atmospheric moisture (for example, fog, precipitation, and condensation of humid air) at ambient temperatures above freezing. Atmospheric moisture, when in contact
with cold wing surfaces, may freeze. This can even occur if conditions remain above freezing and are not expected to be, or recognized as, icing conditions.
(2) Frost, snow, or ice also can be rem oved with freezing -point depressant fluids. Refer
to the current edition of AC 120-58, Pilot Guide: Large Aircraft Ground Deicing, or AC 135-17,
Pilot Guide : Small Aircraft Ground Deicing, for discussion of the proper use of the fluids and
protection provided under various environmental conditions as summarized in holdover timetables. Anti -icing fluids are designed so that most of the fluid will flow off the aircraft by the
time the aircraft reaches rotation speed; consequently, they provide no icing protection once the
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aircraft is airborne. Even though holdover tim es for freezing drizzle and light freezing rain e xist,
aircraft certificated for f light in icing conditions are not evaluated for flight in freezing drizzle or
freezi ng rain.
(3) Chec k the AFM to see if the use of Type II, III, or IV fluids is approved. In so me
cases, there m ay be li mitations on takeoff procedure or minimum outside air temperature. If
fluids are not m entio ned in the AFM, consult with airplane m anufacturer.
(4) Ensure there is no ice that m ay interfere with co ntrol surface movement, braking, or
steering. Check the pitot heat, pitot tube opening, and stall warning system. Check for proper
functioning of any anti-icing or deicing systems. For fluid system s, make sure you see fluid
along the entire leading panels. It may take several minutes to prime the fluid system,
particularly if it has not been used in a while. Do not assum e that any conta mination, even snow,
will blow off during takeoff. Wet snow may not blow off, and there m ay be a layer of ice under
the snow.
CAUTION: If undetected and still present during takeoff, ice is most likely to shed when the wings flex at takeoff rotation. Simultaneous ice shedding from both wings of an airplane with aft mounted engines has been known to result in ice ingestion damage and power loss in both engines during takeoff.
5-5. TAXI. Always perform a pre takeoff check of the anti -ice/deice systems in accordance with
the AFM or POH prior to ta keoff.
a. Speed. While taxiing in snow or ice, leave extr a space around your aircraft and taxi at a
slower rate. Be careful when braking to prevent the wheels from skidding.
b. Braking. When stopping, begin to brake earlier than norm al because the aircraft may
require more distance to stop. Leave additional space in front of the aircraft during an engine runup; the aircraft may begin to slide on ice. Carefully check the braking action of the aircraft to ensure that snow or ice is not building up on any of the components of the brake system.
c. Wheel Fairings. If the aircra ft is eq uipped with wheel fairings, be aware that snow m ay
accumulate in the wheel fairings and freeze during flight. Make sure that all con trols have full
range of motion, and, if applicable, check that the carburetor heat is working.
d. Defroster. If the aircraft is not equipped with windshield anti- icing or deicing system ,
turn the defroster on high and leave it on. This m ay help to prevent ice from forming on t he
windshield during flight.
5-6. TAKEOFF AND CL IMBOUT. Depending on the recommendations of the manufacturer,
the POH, or the AFM, on sm all air craft and on certain light aircraft, it may be advisable during
climbout to apply the brakes and cycle the landing gear to break loose any snow, slush, or ice
that m ay have accu mulated during taxi and takeoff.
a. Preparation. Verify that the airspeed ind icator is working properly and that the pito t
heat is on. Because of ATC restrictions and other traffic, climbout may not a lways be
expeditio us.
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b. Ice Accumulation. Airplanes are vulnerable to ice accumulation d uring t he initial
climbout in icing conditions because lower speeds often translate into a higher A ngle of Attack
(AOA). This exposes the underside of the airplane and it s wings to the icing conditions and
allows ice to accumulate further aft than it would in cruise flight. At rotation and climb out, some
aircraft occasionally are susceptible to stall warning horn activation in icing. Pilot awareness of
this hazard in his or her particular aircraft is important to maintain situational awareness.
c. Vigilance. Consequently, any ice that form s ma y be out of the pilot’s view and go
undetected. Extre me vigilance should be exercised while climbing with the autopilot engaged.
Climbing in Vertical Speed (VS) mode in icing conditions is highly discouraged.
d. Monitor Airspeed. When climbing with the autopilot engaged in the vertical speed
mode, ice accretion will result in a loss of climb performance. If the vertical speed is not reduced, the autopilot will maintain the rate until stall. It is critical that the pilot monitor airspeed
to assure that the aircraft maintains at least the minimum flight speed for the configuration and environmental conditions.
5-7. CRUI SE. An air craft that is certificated for flight in icing conditions will be able to c ope
with most icing encounters prov ided that its ice protection s ystems are operating properly and
that the exposure is not extended beyond their capabilities. However, if it is possible to exit the
icing conditions by a change in altitude or flightpath, this is certainly advisable (see Chapter 3,
paragraph 3-13) . During any icing encounter, the pilot should carefully monitor the behavior of
the air craft and know when to activate the airplane’s anti -ice and deicing systems. Unless
otherwise stated in the flight manual, the pilot should activate pneumatic boot deicing systems at
the first sign of ice accretion.
a. Flight Speed. The aircraft will have s ome unprotected areas that will collect ice.
Altho ugh ice in such areas should not compromi se the safety of flight, it may cause enough
increase in drag to require the pilot to apply more power to maintain flight speed. However, do not exceed any maximum airspeed limitations for your airplane. An additional margin of speed should be added to maintain at least 50 percent to 60 percent above V
s in a clean configuration as
a minimum (V s x 1.5 or 1.6). Airspeed and engine power settings should be closely monitored
during in- flight icing conditions , and the pilot should make certain adequate speed margins are
maintained without requiring excessive power. This requires special attention on airplanes not equipped with autothrottle systems. In many icing events airspeed decreased f rom cruise to stall
in less than 3 minutes. The pilot should treat occurrences of buffet or shudder in icing conditions as an imminent wing stall.
b. Residual or Intercycle Ice. Residual or interc ycle ice on deiced areas can have a
similar effect. Typically, adding power is the recommended action, since reduction in flight
speed is associated with an increase in AOA, which on m any aircraft will expose l arger
unprotected areas on the underside of the aircraft to the collection of ice. If for any reason (ice protection failure, improper use of protection system in extre me icing conditions, etc.) the point
is reached where it is no longer possible to maintain airspeed through addition of power, the pilot
should exit icing conditions imm ediately. On an air craft equ ipped with in-f light dei cing sy stems,
there will a t all tim es be residual or intercycle ice on the wings.
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c. Effect of Airspeed. Airspeed in cruise can have a significant effect on the nature of an
icing encounter as the rate of accumulation generally increases with airspeed . However, if the
airspeed is fast enough, surface heating due to co mpressibility effects may me lt some of the ice
and prevent accumulation in those areas. Generally, only very high perfor mance aircraft can
attain such speeds. D uring the flight, periodically verify that all a nti-icing and/or deicing syste ms
are working. During the en route portion of the flight, have an exit plan that is regularly
reevaluated as neces sary.
d. Awareness. Even if the encounter is short and the icing not heavy, the pilot should
exercise particular awareness of the behavior of the airplane. Configuration changes following cruise in icing conditions, such as spoiler/flap deployment, should be m ade with care. This is
because ice on the aircraft that had little effect in cruise m ay have a m uch different and
potentially more hazardous effect in other configurations. Remember that for norm al cruise
configurations and speeds, both the wing and tailplane are ordinarily at moderate AOA, making
wing or tailplane stall unlikely. After configur ation changes and in m aneuvering flight, wings or
tailplane (especially after flap deploy ment) may be at more extreme AOA, and even residual or
intercycle ice m ay cause a stall to occur at a less extrem e angle than on a clean aircraft.
e. Autopilot. When the autopilot is engaged, it can mask changes in handling
characteristics due to aero dynamic effects of icing that would be detected by the pilot if the
airplane were being hand flown. When the autopilot is disconnected, pilots should be aware that
additional forces may be needed to maintain the current flightpath or pitch attitude. In an aircr aft
that relies on aerodynami c balance for tri m, the autopilot m ay mask control anom alies that would
otherwise be detected at an early stage. If the aircraft has nonboosted control s, a situation m ay
develop in which autopilot servo-control power is exceeded. The autopilot disconnects abruptly, and the pilot may be suddenly confronted by an unexpected control deflection.
f. Limitations. Pilots may consider periodically disengaging the autopilot and hand flying
the air plane w hen operating in icing co nditions. If t his is not de sirable becau se of cockpit
workload levels, pilots should monitor the autopilot closely for abnormal tri m, trim rate, or
airplane attitude. As ice accretes on aircraft without autothrottles, the autopilot will atte mpt to
hold altitude without regard for airspeed, leading to a potential stall situation. Unless authorized
in the AFM , the prefe rential vertical mode of the autopilot is airspeed hold, while the least
desirable is vertical speed , which should not be used. Pilots should be prepared for the possibility
of unusual control forces and flight control displacements when disconnecting the autopilot, especially in severe icing conditions.
g. Airspeed Monitoring. It is c ritical that the pilot monitor air speed to ass ure th at at le ast
the m inimum flight speed for the configuration and environm ental con ditions is maintained.
There have been events in which the airsp eed lo ss from cruise to stall occ urred in a matt er of
minutes.
h. Weather Systems. The pilot should exercise care when operating turbine engine
powered aircraft in or around convective weather systems. Ice crystals can be accreting in the
engine even though the airfram e and ice detectors may not show any indications of an icing
environment. This can occur at very low ambient tem peratures and high altitu des. The pilot
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should activate nacelle and engine anti -ice systems if the presence of ice crystals is suspected
and follow the procedures outlined in the aircraft’s AFM as needed.
5-8. DESCENT.
a. Speed of Descent. Pilots should try to stay on top of a cloud layer as long as possible
before descending into the clouds. This may not be possible for an aircraft that uses bleed air for
anti-icing syste ms because an increas e in t hrust may be required to provide sufficient bl eed air.
This i ncreased thrust m ay reduce the descent rate of high-perfo rmance aircraft whose high -lift
attributes already make descents lengthy without the use of aerodynami c speed brakes or other
such devices. The result may be a gradual descent, extending the aircraft’s exposure to icing
conditions.
b. Configuration Changes. If the pilot makes configuration changes, he or she should
take into consideration any effects of icing conditions on the aircraft and make the necessary
adjustments . (See the discussion in C hapter 4.)
c. Sufficient Power. When leveling off, especially with the au topilot engaged, ensure that
sufficient power is applied to maintain proper airspeed.
5-9. HOLDING. a. Ice Accumulation. During holdi ng, an airplane may be more vu lnerable to ice
accumulation because of the slower speeds and low er alti tudes during this phase of flight.
b. Autopilot. Caution concerning the use of the autopilot, as descri bed ab ove, is also
applicable to holding during or after fli ght in icing conditions.
c. Configuration Changes. If configuration changes (such as deploym ent of flaps) are
made before or during the hold and after or during flight in icing conditions, the pilot should be
prepared for any unusual behavior of the airplane during or after the change. If the aircraft reacts
adversely to a change of configuration , the pilot should return the aircraft to its orig inal
configuration. See the discussion above.
d. Flaps. Consult the AFM for use of flaps. Many AFMs prohibit use of flaps for extended
periods in icing conditions.
5-10. APPROACH AND LA NDING.
a. Sudden Moveme nts. During or after flight in icing conditions, when configuring the
airplane for landing, the pilot should be alert for sudden aircraft movem ents. Often ice is picked
up in cruise, when the aircraft’s wing and the tailpla ne are lik ely at a m oderate A OA, making a
relatively ice-tolerant configurati on. If effects in cruise are minor, the pilot m ay feel com fortable
that the aircraft can handle t he ice it has acq uired.
(1) Extension of landing gear may create excessive amounts of drag when coupled with
ice. The p ilot should deploy flaps and slats in stages, carefully noting the aircraft’s behavior at
each stage.
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(2) For some aircraft, if ano malies occur, it is best not to increase the amount of flaps or
slats and perhaps even to retract them depending on how much the aircraft is deviating from
norm al perfor mance. For other aircraft, retracting the flaps or slats at this stage could lead to a
stall. Therefore, configuration changes in this situation should only be performed per
manufacturer guidance.
(3) Additionally, before beginning the approach, the pilot should cycle deicing boots
because they may increase stall speed and it is preferable not to use these systems while landing. Once on the runway, pilots should be prepared for possible loss of directional control caused by
ice buildup on landing gear.
b. Forward Visibility. Another concern during approach and landing m ay be forward
visibility. Windshield anti- icing and deicing syste ms can be overwhelmed by som e icing
encounters or m ay malfunction. Pilots have been known to look out side windows or, on sm all
GA aircraft, atte mpt to rem ove ice accu mulations with s ome type of tool (e.g., plotter or credit
card) .
c. Workload. Pilot wor kload can be h eavy during the approach and la nding phase.
Autopilots help to reduce this load. The advantages of a reduced workload m ust be balanced
against the risks associated with using an autopilot during or after flight in icing conditions. An
unexpected autopilot disconnect because of icing is especially hazardous in this phase of flight due to the pilot’s operation of the airplane at a low altitude.
d. Final Phases. Accident statistics reve al that the majority of icing- related accidents
occur in the final phases of flight. Contributing factors are configuration changes, low altitude, higher flightcrew workload, and reduced power settings. Loss of control of the airplane is often a factor. Ice contam ination may lead to wing stal l, Ice-Contam inated Tailplane Stall (ICTS), or roll
upset.
e. Stall and Roll. Wing stall and roll upset m ay occur in all phases of flight. However,
available statistics indicate that I CTS rarely occurs until flaps are fully extended on susceptible
airplanes. Some AFMs have a li mitation on the m aximum flap approved for use in icing
conditions due to ICTS susceptibility, and some airplanes have been shown not to be susceptible.
Airplanes certified for icing after 1994 were tested for susceptibility.
(1) If your airplane was certified for icing after 1994, adhere to AFM limitations and
procedures for flap use in icing, if any.
(2) If your airplane was certified for icing prior to 1994 and it is less than 19,000 lb.,
consider a partial - or no-flap landing.
f. Approach Airspeed. Unless your AFM , POH, or any placard has specified an airspeed
for flight in icing, increase approach airspeed by at least 25 percent above non -icing airspeed for
the applicable flap setting. Follow your AFM , POH, or any placard that has limitations or
procedures for reduced flaps in icing conditions. If not and your airplane weighs less than
19,000 lb., consider a reduced-flap landing if landing field distance permits. Increase your approach speed accordingly and expect an increased landing distance. Estimate that landing distance will increase 20 percent for each 10 percent increas e in airspeed. If the runway is
Par 5 -10 Page 47
10/8/15 AC 91 -74B
contaminated, this distance may be even greater ( refer to the current edition of AC 91 -79,
Mitigating the Risks of a Runway Overrun Upon Landing).
g. Decrease in Lift. During the landing flare, if ice is present on the wing’s l eading edges,
expect a loss and an unpredictable response in lift due to the added drag and contaminated
airflow. Carry higher -than- normal power if there is ice on the airplane and limit the flare. Many
icing accidents have been attributed to an induced stall during flare.
5-11. WING STALL.
a. Angle and Speed. The wing, when contam inated with ice, will ordinarily stall at a
lower AOA, and thus at a higher airspeed. Even sm all amounts of ice, particularly if rough, ma y
have so me effect. An increase in approach speeds may be advisable if any ice remains on the
wings. How much of an increase depends on both the aircraft type and the amount of i ce. The
pilot should consult the AFM or POH.
b. Landing. An increased landing speed will mean a longer landing roll. If possible, the
pilot may want to consider a longer runway for increased rollout distances.
c. Ice Contamination. It has been noted that some incidents or accidents have ice
contamination as a contributing or causal factor. Stall recovery training for pilots is based on recovery at the first indication of stall. Complying with the FAA template and/or manufacturer guidance for aircraft stall recovery will restore control in both pitch and roll for all aircraft. This includes recovery fr om possible abrupt roll- off caused by asymmetrical wing icing.
d. Uneven Accretion. As explained in Chapter 4, the accretion may be uneven between
the two wings; therefore, the outer part of a wing, which is ordinarily thinner and thus a better collector of ice, may stall first rather than last. The effectiveness of ailerons may be reduced due
to ice fo rmations in front of them on the wing.
5-12. ICTS.
a. General. The basic aerodynam ics of ICTS were describ ed briefly in Chapter 3. ICTS
occurs when a tailplane with accu mulated ice is placed at a sufficiently negative AOA. There are
no known incidents of ICTS in cruise (when flaps would not ordinarily be deployed) or with partial flaps. When the flaps are fully deployed, tailplane ice, which previou sly had little effect
other than a minor contribution to drag, may now be a contributing factor to a stall event.
b. Signs of Wing Stall. While preparing for the deployment of flaps after or during flight
in icing, the pilot should carefully assess the behavior of the aircraft for any buffet or other signs
of wing stall. Since most icing accidents have been attributed to wing stall, you should always suspect a wing stall with any vibration or buffet if you have not just deployed full flaps. On airplanes certified for icing prior to 2000, you may not get stall warning, only buffet, prior to
wing stall.
c. Flap Setting. Deploy ment of f laps per mits the air craft to be flown with wings at a less
positi ve AOA, decreasing the probability of wing stall, but the AOA at the ta ilplane is more
negative making it more susceptible to icing accumulation. Lower speeds put the aircraft closer
Par 5 -10 Page 48
10/8/15 AC 91 -74B
to wing stall and higher speeds pu t it clos er to tailplane stal l. Thus, there is a restricted operating
window that varies by aircraft with resp ect to use of the flaps and to airspeed to prevent ICTS.
Airplanes certified for icing after 1994 have been tested for ICTS susceptibility. On these
airplanes, following AFM limitations and procedures will preclude ICTS. Some airplanes have been shown not be susceptible to ICTS and do not need reduced- flap setting.
d. Guidance. The pilot should be familiar with any guidance provided in the AFM or
POH. If the AFM does have a m aximum flap limit ation in icing, it is usually because of ICTS
susceptibility. A wing stall would be more common than an ICT S if the flap lim itation was
followed. Increased power increases susceptibility to ICTS in some designs (depending on
configuration), but not in others. Again, the pilot should consult the AFM or POH.
e. Landing. When landing with an increased risk of a stall, the pilot should avoid
uncoordinated flight such as side or forward slips and, to the extent possible, crosswind landings should be restricted because of their adverse effect on pitch control and the possibility of reduced
directional control. Landing with a tailwind component may result in more abrupt nose-down control inputs and should be avoided if possible. For some aircraft designs, i f the aircraft has ice
on the wings and tail, the pilot may be wise to exercise limited or no deployment of flaps, which will likely result in a higher- than-normal approach speed. Because of the higher speed approach,
longer runways may be necessary for this procedure.
5-13. ROLL U PSETS.
a. Prevention. Roll upsets caused by ice accu mulations forwar d of the ailer ons are also
possible during an icing encounter, particul arly in S LD conditions. During t he slow speeds
associated with approach and landing, such control anoma lies can beco me increasi ngly
problematic. P ilots can re medy roll upsets using the following guidelines:
• Reduce the AOA by reducing the aircraft pitch . If in a turn, the pilot should roll the
wings level.
• Set the appropriate power and monitor the airspeed and AOA.
• If the flaps are extended, do not retract th em unless it can be deter mined that the
upper surface of the air foil is clear of ice. Retracting t he flaps will increase the
AOA at a given airspeed.
• Verify that the wing ice protection is functioning norm ally and symmetri cally
through visual obser vation of each wing. If there is a malfunction, follow the
manufacturer’s i nstructions.
CAUTION: These procedures are si milar to those for wi ng stall reco very,
and in some respects opposite from those for recovery from the ICTS.
b. Proper P rocedure. Application of the incorrect procedure during an event can
seriously compound the upset. Correct identification and application of the proper procedure is
imperative. It is extre mely important that the pilot maintain awareness o f all possibilities during
or following flight in icing.
Par 5 -12 Page 49 (and 50)
10/8/15 AC 91 -74B
CHAPTER 6. SUMMARY
6-1. GENERAL . Ice-contam inated aircraft have been involved in m any accidents. Takeoff
accident s have usually been due to failure to deice or anti- ice critical s urfaces properly on the
ground. Proper deicing and anti- icing procedures are addressed in the current editions of two
other pilot guides, AC 120-58 and AC 135-17. Any ice encountered in flight, e ven in trace
amounts, can be dangerous. This guidance should help educate pilots about the potential hazards
of in-flight icing, ways to avoid such hazards, and how to cope with potential hazards effectively.
6-2. AVOIDANCE . The pilot of an aircraft that is not certificated for flight in icing conditions
should avoid all icing conditions. This guide provides guidance on how to do this, and on how to exit icing conditions promptly and safely should they be inadvertently encountered.
6-3. VIGILANCE . The pilot of an aircraft that is cer tificated for flight in icing conditions can
safely operate in the conditions for which the aircraft was evaluated during the certification
process, but should never becom e complacent about icing. Even short encounters with sm all
amounts of rough icing can be very hazardous. 6-4. GUIDANCE . The pilot should be fami liar with all information in the AFM or POH
concerning flight in icing conditions and follow it carefully. Of particular im portance are proper
operation of ice protection systems and adherence to minimum airspeeds during or after flight in
icing conditions. Monitor airspeed, pitch attitude, and do not rely on the airplane’s autopilot or
stall warning s ystem in icing conditions. There are some icing conditions for which no aircraft is
evaluated in the certification process , such as SLD conditions within or below clouds, and flight
in these conditions can be very hazardous. The pilot sh ould be fam iliar with any info rmation in
the AFM or POH relating to these conditions, including aircraft- specific cues for recognizing
these hazardous conditions.
Par 6 -1 Page 51 (and 52)
10/8/15 AC 91 -74B
Appendix 1
APP ENDIX 1. RE COMMEND ED RE ADING
1. This advisory circular (AC) was developed as an easy -to-read resource on flight in icing
conditions. As of the date of publication, this AC contains the most current information
available. The suggested reading list that follows may not have been updated recently but m ay
contain other useful and valid information. For m ore detailed infor mation, pilots are referred to
the current editions of the following U.S. Governm ent publications:
a. Aeronau tical Infor mation Manual (AIM) .
b. AC 00 -6, Aviation Weather for Pilots and Flight Operations Personnel.
c. AC 00 -45, Aviation Weather Services.
d. AC 20 -29, Use of Aircra ft Fuel Anti- icing Ad ditives.
e. AC 20 -73, Aircraft Ice Protection.
f. AC 20 -113, Pilot Precautions and Procedures to be Taken in Preventing Aircraft
Reciprocating Engine Induction System and Fuel System Icing Problem s.
g. AC 20 -117, Hazards Following Ground Deicing and Ground Operations in Conditions
Conducive to Aircraft Icing.
h. AC 20 -149, Installation Guidance for Domestic Flight Information Service- Broadcast.
i. AC 23.1419-2, Certification of Part 23 Airplanes for Flight in Icing Conditions.
j. AC 91 -79, Mitigating the Risks of a Runway Overrun Upon Landing.
k. AC 150/5220-16, Autom ated Weather Observing Systems (AWOS) for Non -Federal
Applications.
l. National Transportation Safety Board ( NTSB ) Aircraft Accident Database .
m. P-8740-24, Wi nter Fly ing Tips — FAA Ac cident Preve ntion Program Publication.
Page 1 (and 2)
10/8/15 AC 91 -74B
Appendix 2
APP ENDIX 2. ICING CHECKL ISTS
The following checklists contain icing- specific ite ms that should be considered before operating
in possible icing conditions. The checklists are intended to supplement pilot information. These
checklists s hould not replace or su persede Airplane Flight Manual ( AFM ) or pilot’s operating
handbook ( POH).
1. Piston Aircraft.
a. Preflight.
(1) Always obtain a thorough pref light weather briefing. Evaluate cloud types, bases,
and tops; types of preci pitation; freezing levels ; and pilot r eports.
(2) Pack additional ite ms in your flight bag such as a large flashlight, spare fresh
batteries, and transceiver.
(3) During preflight planning, identify alternate airports along the route of flight to be
used if unscheduled weather is encountered. Choose airports with longer runways.
(4) Always know how to escape icing conditions (either cli mb or descend to warm er
areas, make a 180 degree turn, etc.).
(5) During the preflight inspection, clean all ice, frost, and snow off the aircraft in
accordance with the POH or AFM .
(6) Check that pitot heat (and static heat if installed) are operable.
(7) Check pitot/static openings, fuel drains, and stall warning sensors to ensure they are
not clogged with ice.
(8) In accordance with the POH or AFM , cycle any deicing and anti -icing syste ms to
check for proper operation.
(9) Clear any accu mulated ice or snow from brakes and wheel fairings.
(10) Check controls externally for ice/snow binding.
b. Taxi/Takeo ff/In-Flight.
(1) Use brakes carefully during taxi to prevent skidding.
(2) Ensure that carburetor heat or alternate air is working.
(3) Check controls for full range of motion.
(4) After takeoff, if recommended by the m anufacturer, cycle landing gear to clear
snow or slush from wheel wells.
Page 1
10/8/15 AC 91 -74B
Appendix 2
(5) During flight, monitor engine revolutions per minute (rpm). A drop in rpm or
manifold pressure may indicate induction ice. Apply carburetor heat or alternate air if required.
(6) If your aircraft is not certified for flight into icing conditions and icing is
encountered in flight, you should exit the conditions immed iately . Anti -icing systems are to be
activated prior to entering icing conditions, while deicing systems are normally activated at the
first sign of ice accretion. Refer to the AFM or POH for proper operation of anti- icing and
deicing systems .
(7) Use visual cues to identify ice for mation and reg ularly check for ice accumulation
behind protected areas on the aircraft. If ice forms on the wing, there is a possibility that the tail
may be accumulating ice as well.
(8) Stay alert for any perfor mance or handl ing degrad ation that may be an indicator of
ice accumulation.
(9) If using an autopilot, if workload permits, periodically disengage and m anually fly
the aircraft to identify han dling changes caused by airfram e icing.
c. Approach and Landing.
(1) Be prepared for unexpected att itude changes when changing the airplane’s
configuration. If the aircraft begins to roll or pitch unexpectedly, return to the previous configuration.
(2) In accordance with the POH or AFM, use a higher approach speed into the landing
when carrying an accumulation of ice. Use a longer runway if available.
(3) After touchdown, use brakes sparingly to prevent skidding or in case of ice buildup
in brakes.
2. Turbo propeller Aircraft.
a. Preflight.
NOTE: Profession al flightcrews flying complex, high- performance aircraft
should al ways refer to the AFM or POH and company guidance materials as
the authority for procedures for flight into ici ng condition s.
(1) For ground deicing operations, refer to co mpany manuals, AC 120-58, and
AC 135-17 for guidance.
(2) Always obtain a thorough preflight weather briefing. Look for cloud types, bases,
and tops; types of preci pitation; freezing levels; and pilot r eports.
(3) Preflight icing inspections of the aircraft in ground icing conditions are essential.
Tactile ins pectio ns are mandatory for som e aircraft and are very valuable for detecting clear ice.
Page 2
10/8/15 AC 91 -74B
Appendix 2
By physically touching the surface, any fine con taminants not easily visibl e can be detected.
Refer to AFM or POH to deter mine if a tact ile inspection is mandatory for your aircraft.
(4) Clean all ice, fr ost, and snow off of t he aircraft in accordance with the POH or
AFM .
(5) Check that pitot heat and static heat are operable.
(6) Check pitot/static openings, fuel drains, and stall warning sensors to ensure they are
not clogged with ice.
(7) In accordance with the POH or AFM, c ycle anti -icing and deicing system s to check
for proper operation.
(8) Clear any accu mulated ice or snow from brakes and wheel fairings.
(9) Inspect the engine i nlets of turbine engines and remove any accu mulated ice from
the nacelle inlet as well as aro und the nacelle drain hole and around the f an blades.
b. Taxi/Takeo ff/In-Flight.
(1) Use brakes carefully during taxi to prevent skidding.
(2) Check controls for full range of motion.
(3) Perform regular engine power run-ups to shed accu mulated ice while taxiing, per
the AFM .
(4) After takeoff, if recommended by the m anufacturer, cycle landing gear to clear
snow or slush from wheel wells.
(5) Refer to the AFM or POH for proper operation of anti- icing and deicing syste ms.
A rule of thum b is that anti- ice systems should be activate d at the first sign of visible moisture
with air te mperat ures s ome margin above freezi ng. Deicing syste ms should be activated at the
first sign of ice accretio n.
(6) Power settings with bleed air on should be set according to the POH or AFM
reference section.
(7) Use visual cues to i dentify ice for mation and reg ularly check for ice accumulation
behind protected areas on the aircraft.
(8) Stay alert for ice formations on wings that may cause control problem s.
(9) If there is a need to use w ing-deicing syste ms, there is a po ssibility that the tail m ay
be accumulating ice as well.
Page 3
10/8/15 AC 91 -74B
Appendix 2
(10) If using an autopilot, if workload permits, periodically disengage and m anually fly
the aircraft to identify handling changes cause d by ice. This is especially i mportant if operated in
slow flight or in a holding pattern.
(11) Use airspeed bug to monitor changes to airspeed.
c. Approach and Landing.
(1) Be prepared for unexpected attitude changes when changing the airplane’s
configuration. If the performance characteristics change sudd enly, return to the previous
configuration.
(2) Deter mine if freezing drizzle or freezing ra in are being rep orted and avoid flying
into these areas. A ground observation of ice pellets indicates possibly freezing drizzle or rain
aloft. A ground observation of any type of precipitation when tem peratures are near freezing may
indicate freezing precipitation aloft, so be vigilant for severe icing conditions.
(3) In accordance with the POH or AFM, use a higher approach speed into the landing
when carrying an accumulation of ice. Use a longer runway if available.
(4) In accordance with the POH or AFM, carry so me power on flare and flare slightly
faster than n ormal if carryi ng ice. Use a longer runway if available.
(5) Cycle boots just before final approach.
(6) After touchdown, use brakes sparingly to prevent skidding or in case of ice buildup
in brakes.
3. Turbojet Aircraft.
a. Preflight.
NOTE: Profession al flightcrews flying complex, high- performance aircraft
should al ways refer to the AFM or POH and company guidance materials as
the authority for procedures f or flight into icing condition s.
(1) Because t urbojet air planes have t he perform ance capabilities to fly arou nd or
quickly pass through areas where icing conditions are encountered in flight, icing will pose more
of a hazard during the takeoff phase. Therefore, particular attention sho uld be paid to ground
deicing.
(2) For ground deicing operations, refer to co mpany procedures, AC 120-58, and
AC 135-17 for guidance.
(3) Ensure that deicing fluids are not sprayed into engines, auxiliary power units (APU) ,
pitot inlets, probe openings, or static ports.
(4) Do not spray heated fluids onto cold windows.
Page 4
10/8/15 AC 91 -74B
Appendix 2
(5) Deicing fluid fu mes are a potential irritant. If the air craft is being sprayed with
passengers on board, close all outside vents.
(6) Always obtain a thorough preflight weather briefing. Look for cloud types, bases,
and tops; types of preci pitation; freezing levels; and pilot r eports.
(7) Preflight icing inspections of the aircraft in ground icing conditions are essential.
Tactile ins pectio ns are mandatory for som e aircraft and are very valuable for detecting clear ice.
By physically touching the surface, any fine con taminants not easily visibl e can be detected.
Refer to AFM or POH to deter mine if a tact ile inspection is mandatory for your aircraft.
(8) Ensure tha t all ice, frost, and snow is removed f rom the aircraft in accordance with
the POH or AFM .
(9) Ensure that heated flight information warning sensors, Angle of A ttack ( AOA),
pitot/static, etc., are operating properly.
(10) Check pitot/static openings, fuel drains, and stall warning sensors to ensure they are
not clogged with ice.
(11) In accordance with the POH or AFM, c ycle anti -icing and deicing system s to check
for proper operation.
(12) Clear any accu mulated ice or snow from brakes and wheel fairings.
(13) Inspect the e ngine i nlets of turbine engines and remove any accu mulated ice from
the nacelle inlet as well as around the nacelle drain hole and fan blades.
b.Taxi/Takeo ff/In-Flight.
(1) Ensure that controls ha ve full range of motion.
(2) Refer to the AFM or POH for proper operation of anti- icing and deicing syste ms.
A rule of thum b is that anti- ice system s should be activated at the first sign of visible moisture
with air te mperat ures s ome margin above freezi ng. Deicing syste ms should be activated at the
first sign of ice accretio n.
(3) Power settings with bleed air on should be set according to the POH or AFM
reference section.
(4) For turbine engines, perfo rm regular engine power run-ups to shed accu mulated ice
while taxiing, per the AFM.
(5) Use visual cues to i dentify ice for mation and reg ularly check for ice accumulation
behind protected areas on the aircraft.
(6) Stay alert f or ice for mations in front of control surfaces that may cause control
problems.
Page 5
10/8/15 AC 91 -74B
Appendix 2
(7) If there is a need to use wing -deicing syste ms, there is a pos sibility that the
empennage m ay be accumulating ice as well.
(8) If using an autopilot, if workload permits, periodically disengage and m anually fly
the aircraft to identify handling changes caused by ice. This is especially i mportant if operated in
slow flight or in a holding pattern.
c. Approach and Landing.
(1) Be prepared for unexpected attitude changes when changing the airplane’s
configuration. If the ai rcraft’s perfor mance characteristics change suddenly, return to the
previous configuration.
(2) Deter mine if freezing drizzle or freezing ra in are being rep orted and avoid fly ing
into these areas. A ground observation of ice pellets indicates possibly freezing drizzle or rain
aloft. A ground observation of any type of precipitation when tem peratures are near freezing may
indicate freezing precipitation aloft, so be vigilant for severe icing conditions.
(3) Cycle boots before final approach, if equipped.
(4) In accorda nce with t he POH or AFM, use a higher approach speed i nto the landing
when carr ying an accu mulation of ice. Use a longer runway if availabl e.
(5) Carry so me power on flare and flare slightly faster than n ormal if carryi ng ice. Use a
longer runway if available.
(6) After touchdown, use brakes sparingly to prevent skidding or in case of ice buildup
in brakes.
Page 6
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