5-1. GENERAL. This chapter focuses primarily on how to safely fly an aircraft certificated for flight in icing conditions, what is expected regarding communications of icing conditions and when it is advisable to exit those conditions. The following is only a sampling 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 CFR parts 91, 121, 125, and 135 specify the responsibilities of flightcrews concerning flight in icing conditions. Pilots are advised to check the current regulations for revisions. An important distinction in each of these regulations is the restriction on flight into known or forecast conditions. Because of the limitations of icing forecasts, it is admittedly difficult for pilots to be certain whether the conditions in which they are flying actually will result in an icing encounter, and it is even more difficult to determine the severity of the possible encounter. Pilots can be caught inadvertently in icing conditions that exceed these legal limits. 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 applicable 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 forecast severe icing conditions unless one or more of the following apply:
(1) The aircraft has ice protection provisions that meet part 135 appendix A.
(2) The aircraft has ice protection provisions that meet the requirements for transport category airplane type certification.
CAUTION: Even airplanes approved for flight into known icing conditions should not fly into severe icing. Many AFM Limitations Sections require an immediate exit when these types of conditions are encountered.
CAUTION: Airplane certification for flight into known icing conditions does not include freezing drizzle and freezing rain. In fact, some airplanes are prohibited from flying into freezing drizzle or freezing rain, regardless of its intensity. These conditions are very dangerous and can cause ice to form behind the protected areas.
b. Communications with Air Traffic Control (ATC). When encountering icing, controllers will not know if an aircraft is certificated or equipped for icing, the severity of the conditions, or what anti-icing or deicing equipment 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 actions are being 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 some parts of the country, along with the intensity of radio communications in such areas, it is possible that a pilot who encounters icing will not receive a clearance in time to exit the conditions before safety is compromised. 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 attempt sustained flight in these conditions. Final authority and responsibility for the safety of a flight rests with the pilot in command (PIC).
(3) At any time, a pilot should not hesitate to reject a controller’s instructions 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, inconsistent with 50 percent to 60 percent margin above stall speed (Vs) as recommended.
5-3. AVAILABLE 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 numerous sources of meteorological information available to pilots while in flight. A principal source of this information is Flight Watch. Flight Watch is a Flight Service Station (FSS)-provided en route flight advisory service designed to provide, upon request, timely weather information pertinent to the type, route, and altitude of flight. Flight Watch is available from 6 AM to 10 PM local time at altitudes generally above 5,000 feet above ground level (AGL). For weather information at other times or at lower altitudes, contact an automated flight service station (AFSS) via radio on a nearby frequency outlet as depicted on aeronautical charts. The FSSs providing this service are listed in the Airport/Facility Directory.
b. Hazardous In-Flight Weather Advisory Service (HIWAS). The HIWAS provides continuous, recorded hazardous in-flight weather forecasts over selected Very High Frequency (VHF) Omnidirectional Range (VOR) outlets within the HIWAS broadcast area. This broadcast area is a geographical zone of responsibility, including one or more HIWAS outlet areas (defined as the area within a 150 nautical mile radius of the HIWAS outlet) assigned to an FSS for hazardous weather advisory broadcasting.
c. Transcribed Weather Broadcast (TWEB). The TWEB is a continuous recording of meteorological and aeronautical information broadcast on low/medium frequencies and VOR facilities for pilots. The TWEB is based on a route-of-flight concept that includes, among other information, adverse conditions, route forecasts, outlooks, and PIREPs that may contain useful icing-related information.
d. Air Carrier Dispatch. Air carrier flightcrews normally 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 freezing drizzle. It also lacks the ability to detect small, ice crystals that have little to no liquid water present that can be in heavy concentrations near convective weather systems.
f. Data Link/Satellite Radio. Pilots now have the option to subscribe to and receive near-real-time weather and airspace information via satellite 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). These services can provide textual or graphical METARs, Terminal Area Forecasts (TAF), SIGMETs, and AIRMETs, updated pilot reports, updated winds and temperatures aloft, information about severe thunderstorms, icing levels, and graphical displays of temporary flight restrictions. Such up-to-date information 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. PREFLIGHT. The first step in preparing for any flight is to obtain 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 determining routes of flight, make note of airports along the way and highlight them on the chart for easy identification in case an alternate is needed.
(2) When determining routes, consider the climb performance of the airplane and the route’s minimum altitude, particularly in mountainous terrain. Because the airplane’s climb performance will be degraded in icing conditions, consult the AFM for any degradation data.
(3) Determine icing exit strategies during preflight. Determine if climbing or descending will be viable options based on the planned route of flight which includes required altitudes to maintain clearance from terrain, airspace, published routes, departure procedures, arrival procedures and approaches.
(4) When choosing alternate airports, remember that if structural icing occurs, higher approach speeds and, consequently, additional runway length may be required for landing.
(5) Consider carrying a high intensity flashlight for use in locating 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 off.
(7) If the aircraft is loaded near maximum gross weight, climb performance 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 formation that may require extra power to maintain altitude or airspeed, increasing fuel consumption.
(9) When performing an aircraft preflight inspection, remove all frost, snow, and ice from the aircraft surfaces because even very small amounts may adversely affect the aerodynamic properties of a wing. Placing an aircraft in a heated hangar is a good method of removing frost, snow, and ice; however, a pilot should ensure the aircraft is dry before removing it from the hangar to prevent the moisture from refreezing on the surface.
b. Frost. Frost can form on an airplane sitting outside on a clear night when there is moisture present in the air as the airplane’s skin temperature falls below freezing due to radiation cooling.
(1) Certain airplanes may be more vulnerable to ice formation 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 removed 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 aircraft is airborne. Even though holdover times for freezing drizzle and light freezing rain exist, aircraft certificated for flight in icing conditions are not evaluated for flight in freezing drizzle or freezing rain.
(3) Check the AFM to see if the use of Type II, III, or IV fluids is approved. In some cases, there may be limitations on takeoff procedure or minimum outside air temperature. If fluids are not mentioned in the AFM, consult with airplane manufacturer.
(4) Ensure there is no ice that may interfere with control 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 systems, 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 assume that any contamination, even snow, will blow off during takeoff. Wet snow may not blow off, and there may 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 pretakeoff check of the anti-ice/deice systems in accordance with the AFM or POH prior to takeoff.
a. Speed. While taxiing in snow or ice, leave extra 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 normal 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 aircraft is equipped with wheel fairings, be aware that snow may accumulate in the wheel fairings and freeze during flight. Make sure that all controls 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 may help to prevent ice from forming on the windshield during flight.
5-6. TAKEOFF AND CLIMBOUT. Depending on the recommendations of the manufacturer, the POH, or the AFM, on small aircraft 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 may have accumulated during taxi and takeoff.
a. Preparation. Verify that the airspeed indicator is working properly and that the pitot heat is on. Because of ATC restrictions and other traffic, climbout may not always be expeditious.
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.
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 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.
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.
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.
(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 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.
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 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.