Par 5-6 Page 44
b. Ice Accumulation. Airplanes are vulnerable to ice accumulation during the initial climbout in icing conditions because lower speeds often translate into a higher Angle of Attack (AOA). This exposes the underside of the airplane and its wings to the icing conditions and allows ice to accumulate further aft than it would in cruise flight. At rotation and climbout, 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 forms may be out of the pilot’s view and go undetected. Extreme 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. CRUISE. An aircraft that is certificated for flight in icing conditions will be able to cope with most icing encounters provided that its ice protection systems 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 aircraft 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 some unprotected areas that will collect ice.
Although ice in such areas should not compromise 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 Vs in a clean configuration as a minimum (Vs 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 from 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 intercycle 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 many aircraft will expose larger 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 extreme 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 immediately. On an aircraft equipped with in-flight deicing systems, there will at all times be residual or intercycle ice on the wings.
Par 5-7 Page 45
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 compressibility effects may melt some of the ice and prevent accumulation in those areas. Generally, only very high performance aircraft can attain such speeds. During the flight, periodically verify that all anti-icing and/or deicing systems are working. During the en route portion of the flight, have an exit plan that is regularly reevaluated as necessary.
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 made with care. This is because ice on the aircraft that had little effect in cruise may have a much different and potentially more hazardous effect in other configurations. Remember that for normal cruise configurations and speeds, both the wing and tailplane are ordinarily at moderate AOA, making wing or tailplane stall unlikely. After configuration changes and in maneuvering flight, wings or tailplane (especially after flap deployment) may be at more extreme AOA, and even residual or intercycle ice may cause a stall to occur at a less extreme angle than on a clean aircraft.
e. Autopilot. When the autopilot is engaged, it can mask changes in handling characteristics due to aerodynamic 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 aircraft that relies on aerodynamic balance for trim, the autopilot may mask control anomalies that would otherwise be detected at an early stage. If the aircraft has nonboosted controls, a situation may 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 airplane when operating in icing conditions. If this is not desirable because of cockpit workload levels, pilots should monitor the autopilot closely for abnormal trim, trim rate, or airplane attitude. As ice accretes on aircraft without autothrottles, the autopilot will attempt to hold altitude without regard for airspeed, leading to a potential stall situation. Unless authorized in the AFM, the preferential 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 critical that the pilot monitor airspeed to assure that at least the minimum flight speed for the configuration and environmental conditions is maintained.
There have been events in which the airspeed loss from cruise to stall occurred in a matter 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 airframe and ice detectors may not show any indications of an icing environment. This can occur at very low ambient temperatures and high altitudes. The pilot
Par 5-7 Page 46 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 systems because an increase in thrust may be required to provide sufficient bleed air.
This increased thrust may reduce the descent rate of high-performance aircraft whose high-lift attributes already make descents lengthy without the use of aerodynamic 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 Chapter 4.)
c. Sufficient Power. When leveling off, especially with the autopilot engaged, ensure that sufficient power is applied to maintain proper airspeed.
5-9. HOLDING.
a. Ice Accumulation. During holding, an airplane may be more vulnerable to ice accumulation because of the slower speeds and lower altitudes during this phase of flight.
b. Autopilot. Caution concerning the use of the autopilot, as described above, is also applicable to holding during or after flight in icing conditions.
c. Configuration Changes. If configuration changes (such as deployment 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 original 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 LANDING.
a. Sudden Movements. During or after flight in icing conditions, when configuring the airplane for landing, the pilot should be alert for sudden aircraft movements. Often ice is picked up in cruise, when the aircraft’s wing and the tailplane are likely at a moderate AOA, making a relatively ice-tolerant configuration. If effects in cruise are minor, the pilot may feel comfortable that the aircraft can handle the ice it has acquired.
(1) Extension of landing gear may create excessive amounts of drag when coupled with ice. The pilot 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 anomalies 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 normal performance. 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 may be forward visibility. Windshield anti-icing and deicing systems can be overwhelmed by some icing encounters or may malfunction. Pilots have been known to look out side windows or, on small GA aircraft, attempt to remove ice accumulations with some type of tool (e.g., plotter or credit card).
c. Workload. Pilot workload can be heavy during the approach and landing phase.
Autopilots help to reduce this load. The advantages of a reduced workload must 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 reveal 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 contamination may lead to wing stall, Ice-Contaminated Tailplane Stall (ICTS), or roll upset.
e. Stall and Roll. Wing stall and roll upset may occur in all phases of flight. However, available statistics indicate that ICTS rarely occurs until flaps are fully extended on susceptible airplanes. Some AFMs have a limitation on the maximum 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 increase in airspeed. If the runway is
Par 5-10 Page 48 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 leading 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 contaminated with ice, will ordinarily stall at a lower AOA, and thus at a higher airspeed. Even small amounts of ice, particularly if rough, may have some 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 ice. 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 from 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 formations in front of them on the wing.
5-12. ICTS.
a. General. The basic aerodynamics of ICTS were described briefly in Chapter 3. ICTS occurs when a tailplane with accumulated 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 previously 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. Deployment of flaps permits the aircraft to be flown with wings at a less positive AOA, decreasing the probability of wing stall, but the AOA at the tailplane is more negative making it more susceptible to icing accumulation. Lower speeds put the aircraft closer
Par 5-12 Page 49 (and 50) to wing stall and higher speeds put it closer to tailplane stall. Thus, there is a restricted operating window that varies by aircraft with respect 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 maximum flap limitation in icing, it is usually because of ICTS susceptibility. A wing stall would be more common than an ICTS if the flap limitation 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, if 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 UPSETS.
a. Prevention. Roll upsets caused by ice accumulations forward of the ailerons are also possible during an icing encounter, particularly in SLD conditions. During the slow speeds associated with approach and landing, such control anomalies can become increasingly problematic. Pilots can remedy 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 them unless it can be determined that the upper surface of the airfoil is clear of ice. Retracting the flaps will increase the AOA at a given airspeed.
• Verify that the wing ice protection is functioning normally and symmetrically through visual observation of each wing. If there is a malfunction, follow the manufacturer’s instructions.
CAUTION: These procedures are similar to those for wing stall recovery, and in some respects opposite from those for recovery from the ICTS.
b. Proper Procedure. 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 extremely important that the pilot maintain awareness of all possibilities during or following flight in icing.
Par 6-1 Page 51 (and 52) CHAPTER 6. SUMMARY
6-1. GENERAL. Ice-contaminated aircraft have been involved in many accidents. Takeoff accidents have usually been due to failure to deice or anti-ice critical surfaces 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, even 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 certificated for flight in icing conditions can safely operate in the conditions for which the aircraft was evaluated during the certification process, but should never become complacent about icing. Even short encounters with small amounts of rough icing can be very hazardous.
6-4. GUIDANCE. The pilot should be familiar with all information in the AFM or POH concerning flight in icing conditions and follow it carefully. Of particular importance 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 system 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 should be familiar with any information in the AFM or POH relating to these conditions, including aircraft-specific cues for recognizing these hazardous conditions.
Appendix 1 Page 1 (and 2) APPENDIX 1. RECOMMENDED READING
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 may contain other useful and valid information. For more detailed information, pilots are referred to the current editions of the following U.S. Government publications:
a. Aeronautical Information 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 Aircraft Fuel Anti-icing Additives.
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 Problems.
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, Automated Weather Observing Systems (AWOS) for Non-Federal Applications.
l. National Transportation Safety Board (NTSB) Aircraft Accident Database.
m. P-8740-24, Winter Flying Tips — FAA Accident Prevention Program Publication.
Appendix 2 APPENDIX 2. ICING CHECKLISTS The following checklists contain icing-specific items that should be considered before operating in possible icing conditions. The checklists are intended to supplement pilot information. These checklists should not replace or supersede Airplane Flight Manual (AFM) or pilot’s operating handbook (POH).
1. Piston Aircraft.
a. Preflight.
(1) Always obtain a thorough preflight weather briefing. Evaluate cloud types, bases, and tops; types of precipitation; freezing levels; and pilot reports.
(2) Pack additional items 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 climb or descend to warmer 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 systems to check for proper operation.
(9) Clear any accumulated ice or snow from brakes and wheel fairings.
(10) Check controls externally for ice/snow binding.
b. Taxi/Takeoff/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 manufacturer, cycle landing gear to clear snow or slush from wheel wells.
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 immediately. 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 formation and regularly 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 performance or handling degradation that may be an indicator of ice accumulation.
(9) If using an autopilot, if workload permits, periodically disengage and manually fly the aircraft to identify handling changes caused by airframe icing.
c. Approach and Landing.
(1) Be prepared for unexpected attitude 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. Turbopropeller Aircraft.
a. Preflight.
NOTE: Professional flightcrews flying complex, high-performance aircraft should always refer to the AFM or POH and company guidance materials as the authority for procedures for flight into icing conditions.
(1) For ground deicing operations, refer to company 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 precipitation; freezing levels; and pilot reports.
(3) Preflight icing inspections of the aircraft in ground icing conditions are essential.
Tactile inspections are mandatory for some aircraft and are very valuable for detecting clear ice.
Appendix 2 By physically touching the surface, any fine contaminants not easily visible can be detected.
Refer to AFM or POH to determine if a tactile inspection is mandatory for your aircraft.
(4) Clean all ice, frost, and snow off of the 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, cycle anti-icing and deicing systems to check for proper operation.
(8) Clear any accumulated ice or snow from brakes and wheel fairings.
(9) Inspect the engine inlets of turbine engines and remove any accumulated ice from the nacelle inlet as well as around the nacelle drain hole and around the fan blades.
b. Taxi/Takeoff/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 accumulated ice while taxiing, per the AFM.
(4) After takeoff, if recommended by the manufacturer, 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 systems.
A rule of thumb is that anti-ice systems should be activated at the first sign of visible moisture with air temperatures some margin above freezing. Deicing systems should be activated at the first sign of ice accretion.
(6) Power settings with bleed air on should be set according to the POH or AFM reference section.
(7) Use visual cues to identify ice formation and regularly check for ice accumulation behind protected areas on the aircraft.
(8) Stay alert for ice formations on wings that may cause control problems.
(9) If there is a need to use wing-deicing systems, there is a possibility that the tail may be accumulating ice as well.