U.S. Department

of Transportation

Federal Aviation

Administration

 Advisory

 Circular

Subject: EFFECT OF ICING ON AIRCRAFT

CONTROL AND AIRPLANE DEICE AND

ANTI-ICE SYSTEMS

1. PURPOSE. This advisory circular (AC) provides

information for pilots regarding the hazards of aircraft

icing and the use of airplane deice and anti-ice systems.

2. CANCELLATION. AC 91-51, Airplane Deice and

Anti-ice Systems, dated September 15, 1977, is cancelled.

3. RELATED READING MATERIAL. The

information contained in this AC complements the

documents listed below.

a. Current editions of the following AC's may be

obtained at no cost by sending a written request to U.S.

Department of Transportation, Subsequent Distribution

Center, Ardmore East Business Center, 3341 Q 75th

Avenue, Landover, MD 20785:

(1) AC 20-117, Hazards Following Ground Deicing

and Ground Operations in Conditions Conducive to

Aircraft Icing.

(2) AC 135-16, Ground Deicing and Anti-icing

Training and Checking.

(3) AC 135-17, Pilot Guide, Small Aircraft Ground

Deicing.

b. Current editions of the publications below

may be purchased from: New Orders, Superintendent of

Documents, P.O. Box 371954, Pittsburgh, PA

15250-7954.

(1) AC 00-6, Aviation Weather.

(2) AC 00-45, Aviation Weather Services.

(3) AC 61-21, Flight Training Handbook.

Date: 7/17/96 AC No: 91-51A

Initiated by: AFS-820 Change:

(4) AC 61-23, Pilot's Handbook of Aeronautical

Knowledge.

4. BACKGROUND. A review of aircraft accident and

incident reports indicates that pilots may not be fully

aware of the effects of icing on aircraft control. The

review also indicates that pilots may be unaware of the

limitations of aircraft deice and anti-ice systems and the

conditions under which those systems are approved for

flight into icing conditions.

5. DISCUSSION. One of the hazards to flight is aircraft

icing. Pilots should be aware of the conditions conducive

to icing, the types of icing, the effects of icing on aircraft

control and performance, and the use and limitations of

aircraft deice and anti-ice equipment.

a. It is important that a pilot understand the conditions

which are conducive to icing. An understanding of these

conditions allows the pilot to evaluate the available

weather data and make an educated decision as to

whether an intended flight should be made. One of the

best sources of available weather data is pilot reports.

The Federal Aviation Administration (FAA) encourages

all pilots to report their flight conditions when warranted.

(1) For ice to form, there must be moisture present

in the air and the air must be cooled to a temperature of

0°C (32°F) or less. Aerodynamic cooling can lower the

temperature of an airfoil to 0°C even though the ambient

temperature is a few degrees warmer. However, when the

temperature reaches -40°C (-40°F) or less, it is generally

too cold for ice to form. Ice is identified as clear, rime, or
AC 91-51A 7/17/96

mixed. Rime ice forms if the droplets are small and freeze

immediately when contacting the aircraft surface. This type

of ice usually forms on areas such as the leading edges of

wings or struts. It has a somewhat rough looking appearance

and is a milky white color. Clear ice is usually formed from

larger water droplets or freezing rain that can spread over a

surface. This is the most dangerous type of ice since it is

clear, hard to see, and can change the shape of the airfoil.

Mixed ice is a mixture of clear ice and rime ice. It has the bad

characteristics of both types and can form rapidly. Ice

particles become imbedded in clear ice, building a very rough

accumulation.

(2) The following table lists the temperatures at which

the various types of ice will form.

Table 1: Temperature Ranges for Ice Formation

Outside Air Tem.

perature Range

Icing Type

0°C to -10°C Clear

-10°C to -15°C Mixed Clear and Rime

-15°C to -20°C Rime

b. There are two kinds of icing that are significant to aviation:

structural icing and induction icing. Structural icing refers to

the accumulation of ice on the exterior of the aircraft;

induction icing affects the powerplant operation. Significant

structural icing on an aircraft can cause aircraft control and

performance problems. The formation of structural icing

could create a situation from which the pilot might have

difficulty recovering and, in some instances, may not be able

to recover at all. To reduce the probability of ice buildup on

the unprotected areas of the aircraft, a pilot should maintain

at least the minimum airspeed for flight in sustained icing

conditions. This airspeed will be listed in the airplane flight

manual (AFM).

(1) Structural icing can block the pitot tube and static

ports and cause the breakage of antennas on the aircraft.

This can cause a pilot to lose or receive erroneous

indications from various instruments such as the airspeed

indicator and altimeter and can cause a loss of communications and radio

navigation capabilities.

(2) The most hazardous aspect of structural icing is its

aerodynamic effects. Ice can alter the shape of an airfoil. This

can cause control problems, change the angle of attack at

which the aircraft stalls, and cause the aircraft to stall at a

significantly higher airspeed. Ice can reduce the amount of

lift that an airfoil will produce and increase drag several fold.

Additionally, ice can partially block or limit control surfaces

which will limit or make control movements ineffective.

Also, if the extra weight caused by ice accumulation is too

great, the aircraft may not be able to become airborne and, if

in flight, the aircraft may not be able to maintain altitude. For

this reason, Title 14 of the Code of Federal Regulations (14

CFR) prohibits takeoff when snow, ice, or frost is adhering

to wings, propellers, or control surfaces of an aircraft. This

clean aircraft concept is essential to safe flight operations.

(3) Another hazard of structural icing is the possible

uncommanded and uncontrolled roll phenomenon referred

to as roll upset that is associated with severe in-flight icing.

Pilots flying airplanes certificated for flight in known icing

conditions should be aware that severe icing is a condition

that is outside of the airplane's certification icing envelope.

Roll upset may be caused by airflow separation (aerodynamic

stall) inducing self-deflection of the ailerons and loss of or

degraded roll handling characteristics. This phenomena can

result from severe icing conditions without the usual

symptoms of ice accumulation or a perceived aerodynamic

stall.

(4) The term "severe--icing" is associated with the rapid

growth rate of visible ice shapes most often produced in

conditions of high liquid water content and combinations of

other environmental and flight conditions. Severe icing is

often accompanied by aerodynamic performance degradation

such as high drag, aerodynamic buffet, and premature stall.

(5) In addition, ice associated with freezing rain or

freezing drizzle can accumulate on and beyond the limits of

an ice protection system. This kind of ice may produce the

familiar performance degradation; however it may be

potential' hazardous. Freezing rain and freezing drizzle

contain droplets larger than the criteria specified by

certification
7/17/96 AC 91-51A

requirements. Temperatures near freezing can produce

severe icing.

(6) Another hazard of structural icing is the

tailplane (empennage) stall. Sharp-edged surfaces are

more susceptible to collecting ice than large blunt

surfaces. For this reason, the tailplane may begin

accumulating ice before the wings and can accumulate ice

faster. Because the pilot cannot readily see the tailplane,

the pilot may be unaware of the situation until the stall

occurs. There have been reports of ice on the tailplane

without any visible ice on the wing. This can occur if the

tailplane has not or cannot be deiced.

(7) A tailplane stall occurs when, as with the wing,

the critical angle of attack is exceeded. Since the

horizontal stabilizer counters the natural nose down

tendency caused by the center of lift of the main wing,

the airplane will react by pitching down, sometimes

uncontrollably, when the tailplane is stalled. Application

of flaps can aggravate or initiate the stall. The pilot

should use caution when applying flaps during an

approach if there is the possibility of icing on the

tailplane.

(8) Perhaps the most important characteristic of a

tailplane stall is the relatively high airspeed at the onset

and, if it occurs, the suddenness and magnitude of the

nose down pitch. A stall is more likely to occur when the

flaps are approaching the fully extended position, after

nose down pitch and airspeed changes following flap

extension, or during flight through wind gusts.

c. Small aircraft engines commonly employ a carburetor fuel

system or a pressure fuel injection system to supply fuel

for combustion. Both types of induction systems hold

the potential for icing which can cause engine failure.

(1) The pilot should be aware that carburetor icing

can occur at temperatures between -7°C (20°F) and +21

°C (70°F) when there is visible moisture or high

humidity. This can occur in the carburetor because

vaporization of fuel, combined with the expansion of air

as it flows through the carburetor, causes sudden cooling,

sometimes by a significant amount within a fraction of a

second. Carburetor ice can be detected by a drop in rpm

in fixed pitch propeller airplanes and a drop in manifold

pressure in constant speed propeller airplanes. In both

types,

usually there will be a roughness in engine operation.

Some airplanes are equipped with carburetor heat for use

in both prevention and removal of ice. The pilot should

consult the AFM or the pilot's operating handbook for

the proper use of carburetor heat.

(2) Fuel injection systems are less susceptible to

icing than the carburetor system. Ice, which can partially

or totally block the air from entering the engine, forms

on the air intake of the engine. The usual indication of

icing in a fuel injection system is the same as in a

carburetor system. An alternate air source located inside

the engine cowling is used to provide air to the engine to

continue combustion. Usually, this source is operated

automatically and has a manual backup system that can

be used if the automatic system malfunctions.

d. Ice detection is very important in dealing with icing in a

timely manner. A careful preflight of the aircraft should

be conducted to ensure that all ice or frost is removed

before takeoff. This is especially true in larger aircraft

where ice is difficult to see in some locations. Also, it is

more difficult to detect ice during flight on such, areas as

the tail, which may be impossible to see. At night, aircraft

can be equipped with ice detection lights which will assist

in detecting ice. Being familiar with the airplane's

performance and flight characteristics will also help in

recognizing the possibility of ice. Ice buildup will require

more power to maintain cruise airspeed. Ice on the

tailplane can cause diminished nose up pitch control and

heavy elevator forces, and the aircraft may buffet if flaps

are applied. Ice on the rudder or ailerons can cause control

oscillations or vibra tions.

e. When operating in icing conditions on the ground or in

flight, a pilot must have knowledge of aircraft deicing and

anti-icing procedures. Deicing is a procedure in which

frost, ice, or snow is removed from the aircraft in order

to provide clean surfaces. Anti-icing is a process that

provides some protection against the formation of frost

or ice for a limited period of time. There are various

methods and systems which are used for deicing and

anti-icing. A pilot must be knowledgeable regarding the

systems and the procedures to be used on the specific

aircraft before operating in icing conditions.

(1) There are numerous methods which are capable

of removing ice from an aircraft surface. One
AC 91-51A 7/17/96

method is pneumatic boots. This system is commonly

used on smaller aircraft and usually provides ice removal

for the wing and tail section by inflating a rubber boot.

Ice can also be removed by a heat system or by a

chemical fluid. Deicing the propeller is usually done by

electrical heat, but it can also be done with a chemical

fluid.

(2) Anti-icing can be accomplished by using

chemical fluid or a heat source. Anti-ice systems are

activated before entering icing conditions to help prevent

the ice from adhering to the surface. These methods

provide protection for the wings, tail, propeller,

windshield, and other sections of the aircraft that need

protection.

Table 2: Icing Intensity, Accumulation, and Pilot Action

Intensity Airframe Accumulation Pilot Action

Trace Ice becomes perceptible. Rate of accumulation of Unless encountered for one hour or more, deicice is slightly greater than the rate of loss due ing/anti-icing equipment and/or heading or altito sublimation. tude change not required.

Light The rate of accumulation may create a problem if Deicing/anti-icing required occasionally to reflight in this environment for one hour. move/prevent accumulation or heading or altitude change required.

Moderate The rate of accumulation is such that even short Deicing/anti-icing required or heading or altitude

encounters become potentially hazardous. change required.

Severe The rate of accumulation is such that deicing/ Immediate heading or altitude change required.

anti-icing equipment fails to reduce or control

the hazard.

f. For an airplane to be approved for flight into icing

conditions, the airplane must be equipped with systems

which will adequately protect various components. There

are two regulatory references to ice protection: the

application to airplane type certification in 14 CFR parts

23 and 25 and the operating rules contained in 14 CFR

parts 91 and 135.

(1) With regard to ice protection, airplane type

certification is currently accomplished by meeting either

the requirements of § 23.1419 or § 25.1419. These rules

require an analysis to establish the adequacy of the ice

protection system for the various components of the

airplane based on the operational needs of that particular

aircraft. In addition, tests of the ice protection system

must be conducted to demonstrate that the airplane is

capable of operating safely in the continuous maximum

and intermittent maximum icing conditions, as described

in part 25, appendix C. The type certificate data sheet

(TCDS) gives the certification basis for the airplane and

lists the regulations with which the airplane has demonstrated compliance. Therefore, when an airplane

complies with one of the regulations which refers

to part 25, appendix C, the icing certification is indicated

on the TCDS and in the AFM. The AFM lists the

equipment required to be installed and operable. The

AFM or other approved material will also show

recommended procedures for the use of the equipment.

(2) The operating rules contained in § § 91.527 and

135.227 also permit flight into specified icing conditions

provided that the aircraft has functioning deice and/or

anti-ice equipment protecting specified areas of the

aircraft. There are aircraft with partial installations of

deicing and/or anti-icing equipment that do not meet the

certification or the operating regulatory requirements for

flight into icing conditions. Those installations are

approved because it has been demonstrated that the

equipment does not adversely affect the aircraft's

structure, systems, flight characteristics, or performance.

In such cases, the AFM or other approved material must

explain the appropriate operating procedures for the

partial deicing and/or anti-icing equipment and contain a

clear statement that the aircraft is not approved for flight

into known icing conditions.
7/17/96 AC 91-51A

(3) It is important for pilots to understand that an airplane

equipped with some types of deice and/ or anti-ice systems may

not be approved for flight into known icing conditions. To be

approved for such flight, the airplane must be specifically

certificated to operate in known icing conditions.

(4) Also, it is important to remember that the

certification standards provide protection for the

majority of atmospheric conditions encountered, but not

for freezing rain or freezing drizzle or for conditions

with a mixture of supercooled droplets and snow or ice

particles. Some airfoils are degraded by even a thin

accumulation of ice aft of the deicing boots which can

occur in freezing rain or freezing drizzle.

6. SUMMARY. It is extremely important that pilots

understand the dangers of aircraft icing. Even if an

airplane is equipped and certificated to operate in known

icing conditions, there are limitations. Flight into known

or potential icing situations without thorough knowledge

of icing and its effects and appropriate training and

experience in use of deice and anti-ice systems should be

avoided. It is important to know both the pilot's and the

airplane's limitations. Pilots should become familiar with

the types of weather associated with and conducive to

icing and understand how to detect ice forming on

the airplane. Pilots should know the adverse effects of

icing on aircraft systems, control, and performance. They

should also know how to respond to the situation if

accidentally caught in icing conditions. A knowledgeable

pilot is better prepared to make timely decisions and

promptly recognize the factors that can contribute to

aircraft icing accidents.

7. ADVISORY MATERIAL. The procedures and

techniques discussed in this AC are advisory in nature.

They are general guidance and should not be construed

as required operating practices. This AC also contains

numerous references to compliance with 14 CFR. The

regulations themselves are not advisory, and compliance

is required. Applicable operating limitations and

procedures contained in manufacturers' FAA-approved

flight manuals and other approved documents take

precedence over the information contained in this AC.

For specific guidance, pilots should consult the

appropriate FAA approved flight manual.

Page 5 (and 6)

William J. White

Deputy Director, Flight Standards Service
7/17/96

AC 91-51A

Appendix 1

APPENDIX 1.

This appendix is a summary of the cues that a pilot

should recognize and corrective actions that can be taken

if the aircraft encounters an uncommanded or

uncontrolled roll upset due to severe in-flight icing. It is

based on the FAA's investigation of airplane accidents

and incidents during or after flight in freezing rain or

freezing drizzle conditions causing severe in-flight icing.

The term "supercooled large droplets" (SLD) includes

freezing rain or freezing drizzle.

The most effective means to identify severe icing are cues

that can be seen, felt, or heard. The general information

provided in this appendix is intended to assist pilots in

identifying inadvertent encounters with SLD conditions.

The suggestions below are not intended to be used to

prolong flight in conditions which may be hazardous.

Because of the broad range of environmental conditions,

limited data available, and various airplane configurations,

pilots must use the manufacturer's airplane flight manual

(AFM) for specific guidance on individual types of

aircraft.

Warning: This document describes two types

of upset: roll upset and tailplane stall (pitch

upset). The procedures for recovery from one

are nearly opposite those for recovery from the

other. Application of the incorrect procedure

during an event can seriously compound the

event. Correct identification and application of

the proper procedure is imperative.

DETECTING SLD

Cues:

1. Ice visible on the upper or lower surface of the wing

aft of the active part of the deicing boots. It may be

helpful to look for irregular or jagged lines or pieces of

ice that are self-shedding. All areas to be observed need

adequate illumination for night operation.

2. The aft limit of ice accumulation on the propeller

spinner. Nonheated propeller spinners are useful devices

for sorting droplets by size. SLD icing will extend beyond

normal ice limits.

3. Granular dispersed ice crystals or total translucent or

opaque coverage of the unheated portions

ROLL UPSET

of the front or side windows. This may be accompanied

by other ice patterns on the windows such as ridges.

These patterns may occur within a few seconds to

one-half minute after exposure to SLD conditions.

4. Unusually extensive coverage of ice, visible ice

fingers, or ice feathers on parts of the airframe not

normally covered by ice.

Additional Cues Significant at Temperatures near

Freezing:

1. Visible rain (consisting of very large water droplets).

In reduced visibility conditions, select taxi/ landing lights

"On" occasionally. Rain may also be detected by the

sound of droplets impacting the aircraft.

2. Droplets splashing or splattering on impact with the

windshield. Droplets covered by icing certification

envelopes are so small that they are usually below the

threshold of detectability. The largest size of the drizzle

droplets covered is about the diameter of a 0.5mm pencil

lead.

3. Water droplets or rivulets streaming on heated or

unheated windows. The droplets or rivulets are an

indication of high liquid water content (LWC) of any

sized droplet.

4. Weather radar returns showing precipitation. Returns

showing precipitation suggest that increased vigilance for

all of the cues is warranted. Evaluation of the radar may

provide alternative routing possibilities.

PREVENTION/CORRECTION

Before Takeoff:

1. Know the pilot weather reports (PIREP) and the

forecast.

2. Know where the potential icing conditions are

located in relation to the planned route and which

altitudes and directions are likely to be warmer or colder.

About 25% of the cases of SLD are found in stratiform

clouds colder than 0°C at all levels with a layer of wind

shear at the cloud top. There need not be a warm melting

layer above.
AC 91-51A

Appendix 1

In-Flight:

1. Maintain awareness of the outside temperature.

Know where the freezing level static air temperature

(SAT) is located. Be especially alert for severe ice

formation at total air temperature (TAT) near 0°C or

warmer (when the SAT is 0°C or colder). Many icing

events have been reported at these temperatures.

a. SAT is what would be measured from a balloon,

and would be the temperatures given in a forecast.

b. TAT is measured by a probe having velocity with

respect to the air. Because of heating due to compression

upstream of the probe, the total temperature will be

warmer than the SAT. The difference is kinetic heating or

the so called "ramrise." There is less kinetic heating in

saturated air than in dry air because it takes less heat to

raise the same unit mass by one degree. TAT and SAT

are normally associated with air data systems.

2. Avoid exposure to SLD icing conditions, usually at

temperatures warmer than -10°C (+14°F) SAT but

possible at temperatures down to -18°C (-1°F) SAT. Be

alert for cues and symptoms of SLD at temperatures

down to -15°C (+5°F) SAT. Normally, temperature

decreases between approximately 1.5°C (2.7°F) for

saturated air to 2.75°C (5°F) for dry air with each 1,000

foot increase in altitude. In an inversion, temperature

may actually increase with altitude.

7/17/96

It may be hazardous to make rapid descents close to the

ground to avoid severe icing conditions.

4. When severe icing conditions exist, reporting may

assist other crews in maintaining vigilance. Submit a

PIREP of the observed icing conditions. It is important

not to understate the conditions or effects of the icing

observed.

Roll Control Anomaly:

Actions When Exposed to SLD Conditions:

1. Disengage the autopilot. Hand-fly the airplane. The

autopilot may mask important cues or may self -

disconnect and present unusual attitudes or control

conditions.

2. Advise air traffic control and promptly exit the

condition, using control inputs that are as smooth and

small as possible.

3. Change heading, altitude, or both to find an area that

is warmer than freezing, substantially colder than the

current ambient temperature, or clear of clouds. In colder

temperatures, there may still be ice that has not

completely shed adhering to the airfoil.

1. Reduce the angle of attack (AOA) by increasing

airspeed or extending wing flaps to the first setting if at

or below the flaps extend speed (VFE). If in a turn, roll

wings level.

2. Set appropriate power and monitor the airspeed/AOA. A controlled descent is a vastly better

alternative than an uncontrolled descent.

3. If flaps are extended, do not retract them unless it

can be determined that the upper surface of the airfoil is

clear of ice because retracting the flaps will increase the

AOA at a given airspeed.

4. Verify that wing ice protection is functioning

normally and symmetrically by visual observation of the

left and right wing. If not, follow manufacturer's

instructions.

SUMMARY

Roll upset may occur as a consequence of, or prior to, a

wing stall due to anomalous forces that cause the ailerons

to deflect or because the ailerons have lost effectiveness.

Deflection of ailerons or loss of aileron effectiveness may

be caused by ice accumulating in a sensitive area of the

wing aft of the deicing boots under unusual conditions

associated with SLD and, rarely, normal cloud droplets in

a very narrow temperature range near freezing.

Pilots can minimize the chance of a roll upset by being

sensitive to cues that identify severe icing conditions and

promptly exiting the severe icing conditions before

control or handling characteristics of the airplane are

degraded to a hazardous level.

It is important to review the AFM for aircraft typespecific information. Also, pilots should check any

available icing related bulletins from the airplane

manufacturer.
7/17/96

AC 91-S1A

Appendix 2

APPENDIX 2. SUSPECTED TAILPLANE STALL

This appendix is a summary of the symptoms a pilot

should recognize and corrective actions that can be taken

if the airplane encounters a tailplane stall. This appendix

applies only to airplanes having tailplane pitch control. It

is not applicable to aircraft with foreplane (canard) pitch

control.

On some airplane designs, if the horizontal tailplane is

inadequately cleared of ice, either by anti-ice/deice

system failure, failure to operate the system properly, or

by ice, snow, or frost left on critical sections of the airfoil,

a tailplane stall could occur. Generally, tailplane stall

would be encountered immediately after extension of the

trailing edge flaps to an intermediate position or, more

commonly, after extension from an intermediate position

to the full down position. Usually, tailplane stall (or

impending stall) can be identified by one or more of the

symptoms listed below occurring during or after flap

extension. The symptoms) may occur immediately or

after nose down pitch, airspeed changes, or power

increases following flap extension.

Warning: This document describes two types

of upset: roll upset and tailplane stall (pitch

upset). The procedures for recovery from one

are nearly opposite those for recovery from the

other. Application of the incorrect procedure

during an event can seriously compound the

event. Correct identification and application of

the proper procedure is imperative.

TAILPLANE STALL SYMPTOMS

1. Elevator control pulsing, oscillations, or vibrations*

2. Abnormal nose down trim change*

3. Any other unusual or abnormal pitch anomalies

(possibly resulting in pilot induced oscillations)*

4. Reduction or loss of elevator effectiveness*

5. Sudden change in elevator force (control would

move nose down if unrestrained)

6. Sudden uncommanded nose down pitch

*May not be detected by the pilot if the autopilot is engaged.

CORRECTIVE ACTIONS

If any of the above symptoms occur, the pilot should:

1. Immediately retract the flaps to the previous setting

and apply appropriate nose up elevator pressure.

2. Increase airspeed appropriately for the reduced flap

extension setting.

3. Apply sufficient power for aircraft configuration and

conditions. (High engine power settings may adversely

impact response to tailplane stall conditions at high

airspeed in some aircraft designs. Observe the

manufacturer's recommendations regarding power

settings.)

4. Make nose down pitch changes slowly, even in

gusting conditions, if circumstances allow.

5. If a pneumatic deicing system is used, operate the

system several times in an attempt to clear the tailplane

of ice.

Warning: Once a tailplane stall is encountered,

the stall condition tends to worsen with

increased airspeed and possibly may worsen

with increased power settings at the same flap

setting. Airspeed, at any flap setting, in excess

of the airplane manufactur er's

recommendations for the flight and

environmental conditions, accompanied by

uncleared ice contaminating the tailplane, may

result in a tailplane stall and uncommanded

pitch down from which recovery may not be

possible. A tailplane stall may occur at speeds

less than VF7E.

SUMMARY

Ice can form on the aircraft's tail at a greater rate than on

the wing and can exist on the tail when no ice is visible

on the wing. When ice is visible, do not allow ice

thickness to exceed the operating limits for deicing

system operation or the system may not shed the tail ice.

If the control symptoms listed above are detected or ice

accumulations on the tail are suspected, land with a lesser

flap extension setting and increase airspeed

commensurate with the lesser flap setting. Avoid

uncoordinated flight (side or forward slips) and, to the

extent possible, restrict crosswind landings because of

the possible adverse effect on
AC 91-51A

Appendix 2 7/17/96

pitch control and the possibility of reduced directional

control. Avoid landing with a tailwind component

because of the possibility of more abrupt nose down

control inputs. Increased landing distances must also be

considered because of increased airspeed at reduced flap

settings.

Warning: Freezing rain, freezing drizzle, and

mixed conditions (snow and/or ice particles

and liquid droplets) may result in

extreme ice buildup on and aft of protected

surfaces, possibly exceeding the capability of

the ice protection system. Freezing rain,

freezing drizzle, mixed conditions, and descent

into icing conditions in clouds from above

freezing temperatures may result in runback

ice forming beyond protected surfaces where it

cannot be shed and may seriously degrade

airplane performance and control.