Page 48 Par 7-4
a. 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.
b. 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.
c. On an aircraft equipped with in-flight deicing systems, there will at all times be residual or some stage of intercycle ice on the wings.
d. Airspeed in cruise can have a significant effect on the nature of an icing encounter.
An aircraft that cruises at a fast airspeed will increase the rate of ice accumulation. However, if the airspeed is sufficiently fast, 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.
e. 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.
f. Again, care should be exercised when using an autopilot in icing conditions, while in cruise, just as in other phases of flight. 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. 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 non-boosted controls, a situation may develop in which autopilot servo-control power is exceeded. The autopilot disconnects abruptly, and the pilot is suddenly confronted by an unexpected control deflection.
g. 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.
h. 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.
i. Care should be exercised 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. Nacelle and engine anti-ice systems should be activated if the presence of ice crystals is suspected.
7-5. DESCENT.
a. 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. If configuration changes are made during this phase of flight, they should be made with care in icing conditions, noting the behavior of the airplane. See the discussion in the preceding chapter.
c. When leveling off, especially with the autopilot engaged, ensure that sufficient power is applied to maintain proper airspeed.
7-6. HOLDING.
a. 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. Caution concerning the use of the autopilot, as described above, is also applicable to holding during or after flight in icing conditions.
c. 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. Consult the AFM for use of flaps. Many AFMs prohibit use of flaps for extended periods in icing conditions.
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Page 50 Par 7-7
7-7. APPROACH AND LANDING.
a. 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. Flaps and slats should be deployed in stages, carefully noting the aircraft’s behavior at each stage.
(2) 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.
(3) Additionally, before beginning the approach, deicing boots should be cycled 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. 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, credit card, etc.)
c. 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 airplane being flown at a low altitude.
d. 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. The ice contamination may lead to wing stall, ice-contaminated tailplane stall (ICTS), or roll upset.
e. Wing stall and roll upset may occur in all phases of flight. However, available statistics indicate that ICTS rarely occurs until approach and landing. Many AFMs will have a limitation on the maximum flap use approved in icing conditions due to ICTS susceptibility. If your airplane does not have such a limitation, and it was certificated prior to 2000, it may be best to perform a no-flap landing at a higher than normal approach speed if the airplane has accumulated ice on the wings and tailplane. However, because of the higher approach speed, longer runways may be needed for this procedure.
f. During the landing flare, carry higher than normal power if there is ice on the airplane. Many non-fatal icing accidents have been attributed to stall during the flare.
7-8. WING STALL.
a. 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. An increased landing speed will mean a longer landing roll; so if possible, the pilot may want to consider a longer runway for increased rollout distances.
c. This discussion of ice-contaminated wing stall is considerably shorter than the discussion of ICTS that follows. This is not because the latter is more frequent. Statistics indicate that incidents or accidents due to ice-contaminated wing stall are the more common result of flight in icing conditions. However, pilots have extensive training in recognizing and coping with wing stalls, but little or no training for tailplane stalls, which are very unusual on most designs when the tailplane is clean. It is also important to recognize that although pilots have extensive training in recognizing and coping with stalls of clean wings, the stall of an ice-contaminated wing will often be markedly different and may call for aggressive pitch down inputs quite unlike those used in the case of a clean wing stall. The stall characteristics of an aircraft with ice-contaminated wings may be markedly degraded, and serious roll control problems are not unusual.
d. As explained in Chapter 4, Icing Effects, Protection, and Detection, 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.
7-9. ICE-CONTAMINATED TAILPLANE STALL (ICTS).
a. The basic aerodynamics of ICTS was described briefly in Chapter 4, Icing Effects, Protection, and Detection. ICTS occurs when a tailplane with accumulated ice is placed at a sufficiently negative AOA and stalls.
b. This angle would not be expected to be reached without at least partial deployment of the flaps. There are few, if any, known incidents of ICTS in cruise (when flaps would not ordinarily be deployed). However, when the flaps are deployed, tailplane ice, which previously had little effect other than a minor contribution to drag, now can put the tailplane at or dangerously close to stall.
c. 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. The initial deployment of the flaps should be only partial. Vibration or buffeting that follow deployment is much more likely to be due to incipient tailplane stall than wing stall if there was no vibration buffet before deployment. The reason is that after deploying the flaps, the wing will be at a less positive angle, and so farther from stall, while the tailplane will be at a more negative angle, and so closer to stall. There are a number of specific cues associated with ICTS to which a Par 7-8 Page 51
Page 52 Par 7-9 pilot should be sensitive, particularly during this phase of flight. Most of these cues are less readily detected with the autopilot engaged.
• Elevator control pulsing, oscillations, or vibrations,
• Abnormal nose-down trim change,
• Any other unusual or abnormal pitch anomalies (possibly resulting in pilot-induced oscillations),
• Reduction or loss of elevator effectiveness,
• Sudden change in elevator force (control would move nose down if unrestrained), and
• Sudden uncommanded nose-down pitch.
NOTE: The pilot should observe the following guidelines for action if these cues are encountered:
• Flaps tend to alter the airflow reaching the tailplane and should be retracted immediately to the previous setting, and the appropriate nose-up elevator pressure should be applied.
• Airspeed should be increased appropriately for the reduced flap extension setting.
• Sufficient power should be applied for the airplane’s configuration and conditions. (High engine power settings may adversely affect the response to ICTS conditions at high airspeed in some aircraft designs. Manufacturer’s recommendations in the AFM or POH should be observed regarding power settings.)
• Nose-down pitch changes should be made slowly, even in gusting conditions, if circumstances allow.
• If a pneumatic deicing system is used, the system should be cycled several times to clear the tailplane of ice.
d. Note that some of the measures for recovery from ICTS are the opposite of those for recovery from wing stall. Thus distinguishing between the two is very important. If for any reason there is a large or rough ice accretion on both the wing and tailplane (perhaps because of ice protection system failure), manage approach and landing with great care.
e. Deployment of flaps permits the aircraft to be flown with wings at a less positive attack, decreasing the probability of wing stall. However, the AOA at the tailplane is more negative, putting it closer to stall. Similarly, at any particular flap setting, 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 with respect to use of the flaps and to airspeed.
f. 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.
g. 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.
h. When ICTS or wing stall is a possibility, uncoordinated flight such as side or forward slips should be avoided 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.
i. Landing with a tailwind component may result in more abrupt nose-down control inputs and should be avoided if possible. If an airplane 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 needed for this procedure.
7-10. ROLL UPSETS.
a. Roll upsets caused by ice accumulations forward of the ailerons also are 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 increasing airspeed or extending wing flaps to the first setting if at or below VFE (maximum flap extension speed.) If in a turn, the wings should be rolled 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.
NOTE: These procedures are similar to those for wing stall recovery, and in some respects opposite from those for recovery from the ICTS.
b. 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 7-9 Page 53 (and 54)
Par 8-1 Page 55 CHAPTER 8. SUMMARY
a. 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 two other pilot guides, AC 120-58 and AC 135-17. This Pilot Guide focuses on ice that forms in flight, and should be useful to all pilots of fixed-wing aircraft. 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.
b. 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.
c. 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.
d. 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 any airspeed minimums to be observed during or after flight in icing conditions. Airspeed loss can be very rapid, and without an autothrottle, the autopilot will not maintain airspeed. Monitor airspeed and do not rely on the airplane’s 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 condition.
Appendix 1 APPENDIX 1. AIRCRAFT ACCIDENTS RELATED TO ICING
1. American Eagle ATR-72 (Roselawn, Indiana).
a. Of the recent air carrier accidents, the one with arguably the most significant implications regarding in-flight icing is the October 31, 1994, crash of an ATR-72 turbopropeller transport aircraft. The aircraft was on a flight from Indianapolis, Indiana, to Chicago’s O’Hare International Airport, flying with the autopilot engaged and in a holding pattern, descending to 8,000 feet through supercooled clouds and SLD. Later analysis by the NTSB estimated that the supercooled drops in the area ranged between 0.1 mm and 2 mm in size.
b. Before the aircraft entered the hold, its engine RPM was increased to 86 percent as called for in the ATR-72’s AFM for flight in icing conditions (specified as true air temperature (TAT) of less than 7°C in the presence of visible moisture). As the aircraft began holding, the flaps were extended to 15 degrees to lower the aircraft’s AOA, and the engine RPM was reduced to 77 percent, presumably because the crew determined they were no longer flying in icing conditions. After holding for over half an hour, the aircraft was cleared to descend to 8,000 feet, and the crew retracted the flaps to avoid a flap overspeed warning.
c. According to the NTSB, the encounter with the icing conditions in the hold resulted in a ridge of ice accreting aft of the aircraft’s wing deicing boots and in front of the aircraft’s unpowered ailerons. As the aircraft descended to its cleared altitude, its AOA increased and the airflow began to separate in the area of the right aileron. This resulted in a sudden and unexpected aileron hinge reversal that exceeded the autopilot’s ability to control the aircraft, and it disconnected. This left the flightcrew in a full right-wing-down position within a quarter of a second, which was followed by a series of unsuccessful attempts to correct the aircraft’s attitude, resulting in a descent that at times reached 24,000 feet per minute and precipitated the structural failure of the aircraft’s elevators. The aircraft then impacted a soybean field at high speed resulting in the deaths of all 68 passengers and crew.
d. The NTSB’s investigation resulted in several findings, but ultimately, the most important regarding the effects of icing conditions on aircraft was the degree to which the conditions affected a properly certificated aircraft and the limited information available to the flightcrew with respect to the severity of the conditions they were experiencing.
2. Air Florida B-737 (Washington, District of Columbia).
a. Although this accident is well known because it illustrates the dangers of taking off without proper ground deicing, for the purposes of this AC, the accident offers another lesson with respect to icing and its hazards. On January 13, 1982, almost an hour after the B-737 had been deiced; it took off from Washington’s National Airport in light-to-moderate snowfall with temperatures below freezing. In addition to snow and ice forming on the aircraft fuselage during the extended taxi, ice had collected in the compressor inlet pressure probes of the two engines, apparently because the flightcrew neglected to turn on the engine anti-icing system. These inlets are used in conjunction with exhaust pressure to determine engine thrust settings for display in the cockpit as an Exhaust Pressure Ratio (EPR). As one of its functions, the engine’s anti-icing Page 1
Appendix 1 system is designed to maintain a flow of heated air over the compressor inlet to prevent the formation of ice on these probes.
b. As the flightcrew increased the engine throttles to takeoff thrust, the engines actually were developing less thrust than was indicated on the EPR gauges. This thrust deficit, combined with the accumulation of snow and ice on the wings, resulted in a stall immediately after rotation, with the aircraft climbing only to 200 or 300 feet before stalling and colliding with the 14th Street bridge.
c. While this accident, like almost all others, was caused by a combination of factors, it serves as an example to pilots that icing hazards affect not only aircraft aerodynamics but also aircraft instrumentation. In fact, studies conducted after the accident indicated that an immediate combined application of full power and lowered aircraft attitude theoretically could have allowed a safe recovery in a situation similar to this accident.
3. Northwest Orient B-727 (Thiells, New York).
a. Because this accident was a ferry flight and only the flightcrew of three was killed, it has not gained the notoriety of other airline crashes. However, it is significant to the subject matter of this AC, because it is an example of how a well-trained, professional flightcrew neglected to use the anti-icing equipment available on a modern aircraft and then became confused and disoriented by the inconsistent instrument indications that resulted.
b. On December 1, 1974, the aircraft in question was slated to conduct a nighttime ferry flight from New York’s John F. Kennedy International Airport to Buffalo, New York, to position for a return charter flight. After a normal takeoff, the aircraft was climbing to its assigned altitude of 31,000 feet when ATC received a series of radio transmissions from the flightcrew in which they declared an emergency and stated that they were out of control and in a stall. The aircraft descended rapidly, broke up in flight, and crashed in a forest near Bear Mountain, New York.
c. NTSB investigators examining the aircraft wreckage the next morning discovered that the two pitot head heater switches were in the “off” position. An analysis of the aircraft flight data recorder and cockpit voice recorder (CVR) revealed that the before-takeoff checklist was followed incorrectly by the first officer, resulting in the pitot heat being turned off instead of on as required by the checklist.
d. Investigators determined that as the aircraft was climbing through 16,000 feet at 305 knots and at a climb rate of 2,500 feet per minute, airspeed and vertical speed indicators began to climb, without any change to the engine power settings, to a point that actually exceeded the aircraft’s capability. This created confusion among the flightcrew, but the CVR revealed that they attributed the aircraft’s performance to the aircraft’s light load. As the aircraft reached 23,000 feet, the mach overspeed warning came on and the crew continued to pull on their yokes to trade speed for altitude. This resulted in an excessive AOA followed by a stall.
The flightcrew became disoriented and confused by the series of alarms and contradictory indications, resulting in a spiral dive from which they could not recover. The encounter with icing conditions was further worsened because the B-727 has tail-mounted pitot tubes for the Page 2
Appendix 1 elevator feel system that also were blocked with ice. The feel system modulates the amount of force the pilots have to exert on the elevator controls with changes in speed. Because the pitot tubes were covered with ice, smaller movements of the controls resulted in higher control movements and eventually higher vertical acceleration forces on the aircraft.
e. While the loss of a pitot static system is something that all instrument-rated pilots are trained to cope with, the insidious nature of such system malfunctions can result in confusion because the instruments often provide erroneous readings without necessarily indicating a system failure. Pilots must remain vigilant for such unusual indications from their instruments and instead rely on attitude flying.
4. Mitsubishi MU-2B-36 (Malad City, Idaho).
a. This accident demonstrates the importance of ensuring that ice protection systems are operational when flying into an environment where the possibility of encountering icing conditions exists. It also highlights how critical airspeed awareness is, especially while flying in icing conditions.
b. On January 15, 1996, a Mitsubishi MU-2 departed Salt Lake City, Utah, and climbed to 16,000 feet on a part 91 IFR flight to Pocatello, Idaho. The airplane encountered structural icing conditions while in cruise, and according to radar data, began slowing from a cruise speed of about 190 knots while slightly deviating from heading and altitude. The airspeed decreased to about 100 knots and the flightcrew declared an unspecified emergency; then radio contact was lost. The airplane began a right turn, then entered a steep descent and crashed. All aboard the airplane were killed because of the impact. The pilot of a Beech 1900 about 12 minutes in trail of the MU-2 stated that he encountered moderate rime ice at 16,000 feet. The Beech pilot activated the pneumatic boots three times and descended to 12,000 feet to exit the icing conditions. The MU-2 AFM warned that during flight in icing conditions, stall warning devices may not be accurate and should not be relied on, and, to minimize ice accumulation, pilots should maintain a minimum cruise speed of 180 knots or exit the icing conditions. An investigation determined that the captain of the MU-2 was aware of deficiencies in the timer for the pneumatic boots, along with other maintenance deficiencies. Although icing conditions were forecast in the destination area, no icing was forecast for the en route portion of the flight.
c. The NTSB determined that the probable cause of the accident was continued flight by the flightcrew into icing conditions with known faulty pneumatic equipment, structural (airframe) ice, and failure of the flightcrew to maintain adequate airspeed, which resulted in the loss of aircraft control and collision with terrain. The en route icing condition, which was not forecast, was also cited as a factor relating to the accident.
5. Piper PA-34-200T (Des Moines, Iowa).
a. The circumstances surrounding this accident demonstrate the dangers associated with tailplane icing and why proper use of deicing equipment is critical to flying in icing conditions.
b. On January 9, 1996, a Piper PA-34 departed Sioux Center, IA, bound for Des Moines, IA (DSM). In his written statement, the pilot stated that he flew a normal ILS approach with 10 degrees of flaps extended. Upon breaking out of the clouds, he checked for ice with a Page 3
Appendix 1 flashlight, and observed the windshield and the left inboard wing, to be free of ice. He elected to fly the approach fast, and with 25 degrees of flaps.
c. Upon crossing the runway threshold at DSM, the pilot stated that he lowered the flaps to 25 degrees, and that the airplane responded by pitching down. The pilot then stated that he immediately released the flaps and added power, but said the airplane was “basically uncontrollable at this point.” He then reduced power and lowered the flaps before striking the runway on its centerline and sliding 1,000 feet before coming to a stop. The accident resulted in serious injury to the pilot, who was the sole occupant.
d. Examination of the wreckage revealed heavy impact damage to the airplane’s forward fuselage, engines, and wings. Approximately one-half inch of rime ice was observed adhering to the leading edges of the left and right horizontal stabilizers and along the leading edge of the vertical stabilizer.
e. The DSM airport’s ATIS, at 2256 EST, reported that a pilot in a Cessna 210 experienced light to mixed icing, 8 miles northeast of the airport, between 2,500 and 3,700 feet MSL.
f. The NTSB determined the probable cause of the accident was the pilot’s failure to use the airplane’s deicing system, which resulted in an accumulation of empennage (tail assembly of an airplane) ice and a tailplane stall. Factors relating to this accident were the icing conditions and the pilot’s intentional flight into those known conditions.
6. Embraer EMB-120 (West Palm Beach, Florida).
a. This accident is another example of a stall in icing conditions, suspected to be SLD.
The NTSB was unable to determine if the stall warning system activated prior to stall. This accident also illustrates the rapid loss of airspeed that can occur in icing conditions and the need for pilots to monitor airspeed closely in icing conditions, even with autopilot engaged.
b. On March 19, 2001, Embraer EMB-120, operated by Comair Airlines, Inc., was en route from Nassau International Airport, Bahamas, to Orlando International Airport, Florida.
While in cruise flight at 17,000 feet, the aircraft encountered icing conditions, and departed controlled flight, descending to an altitude of about 10,000 feet. The pilots recovered control of the airplane and diverted to West Palm Beach, Florida, where they landed without further incident. Though none of the 28 persons on board was injured, the airplane sustained substantial damage to the elevators and the horizontal stabilizer.
c. In post accident interviews, the captain stated that while in visual conditions, the airplane flew normally. Flight data recorder (FDR) data indicate that about 7 minutes before the upset occurred, the airplane was at about 17,000 feet, with the autopilot engaged and airspeed stabilized near 200 knots. The data indicates the airspeed slowed to 185 knots over a several minute period as the autopilot began trimming airplane nose-up (ANU) to maintain altitude. The airspeed then decreased to about 137 knots over the next 3 minutes. The airplane continued to maintain a constant altitude as the autopilot trimmed the airplane from about 0 to about 7 degrees ANU. The first officer stated that immediately before the upset occurred, he switched the leading-edge deicing system inflation cycles switch from “light” to “heavy” and the propeller Page 4
Appendix 1 deicing system cycles switch from “norm” to “cold” because he saw “more ice accumulation than he had ever seen” on the wing and spinner. FDR data indicate that when torque indications for both engines were about 55 percent and the airspeed was about 141 knots, the autopilot was disengaged. The airplane then pitched down and rolled about 80 degrees to the left, then rolled back to near level. During the next 20 seconds, engine torque increased to about 98 percent on both engines, the airplane rolled about 110 degrees to the left, returned to level flight, rolled about 130 degrees to the right, returned to level flight, then rolled 360 degrees to the right before returning to near wings level, with torque on both engines stabilized at about 22 percent. The airplane’s behavior during the upset is consistent with an ice-induced stall event.
d. The first officer stated in post accident interviews that the stick shaker and aural stall warning, which is part of the airplane’s stall warning/protection system, activated but did not indicate whether it was before, during, or after the upset. The Safety Board’s investigation could not precisely determine whether or when the stick shaker and aural stall warning activated.
e. Meteorological data at the time of the accident indicated that Comair flight may have encountered an area of icing conducive to the formation of super-cooled large droplets (SLD).
The EMB-120 is not certified for flight in SLD conditions. FDR data indicate that airspeed had decreased to only about 137 knots before control of the airplane became difficult and altitude was no longer maintained. However, according to the Embraer EMB-120 AFM performance section, the airplane stalling speed is about 115 knots calibrated airspeed for an airplane at the accident airplane’s approximate gross weight at the time of the event (23,800 pounds).
f. The National Transportation Safety Board determined the probable cause(s) of this accident as follows:
• The failure of the flightcrew to maintain airspeed during an encounter with severe icing conditions.
• This resulted in an inadvertent stall, loss of control, and structural damage to the airplane.
7. Raytheon (Beech) BE-90 (Rawlins, Wyoming).
a. This accident is another example of a stall in icing conditions. The need for extra vigilance by pilots near a destination airport reporting precipitation (snow) near freezing is highlighted in this accident. The ice accretions on the airplane suggest the conditions may have been worse than forecast and exceeded part 25, appendix C criteria.
b. On January 11, 2005, a BE-90, operating as an air ambulance was flying from Steamboat Springs, Colorado (SBS), to Rawlins Municipal Airport/Harvey Field (RWL).
Approaching RWL, the pilot initiated a right turn outbound to maneuver for the final approach course of the VOR/GPS approach to runway 22. On the inbound course to the airport, the airplane impacted mountainous terrain, approximately 2.5 nautical miles east-northeast of the airport. The airplane, configured for landing, struck the terrain wings level, in a 45-degree nose-down dive, consistent with impact following an aerodynamic stall. There were three fatalities and one serious injury.