The FAA's Advisory Circular as well as the NTSB and other investigation authorities have recommended incorporating high-altitude operations into initial and recurrent training curriculums. The Airplane Upset Recovery Training Aid Team recommends line oriented flight training (LOFT) to familiarize crews with high-altitude slowdowns and approach to stall. Crews should always recover at the first indication of an impending stall.
However, as already noted, most simulators don't have the proper mathematical models to replicate the aircraft's high-altitude stall behavior. In addition, simulators need to be programmed with high-altitude atmospheric characteristics so that pilots can learn to recognize signs of a high-altitude upset and rehearse recovery techniques.
Icing has contributed to nine business jet stall accidents, five of which involved aircraft equipped with pneumatic deicing boots, and three of those had deployed the boots during approach but still had residual ice on them. Ice contamination can result in stall onset at a lower AOA and an increase in stall speeds without activation of the stall warning system.
In the wake of a non-fatal ATR-42 accident at Lubbock Airport, Texas, in 2009, the NTSB noted that in icing condition the activation angle of the stick pusher should be reduced or the system's benefit in reducing AOA prior to stall and during recovery efforts is lost. The Board concluded that a lower stick-pusher activation AOA would enhance safety in icing conditions and provide stall protection before an uncommanded roll develops during stall. Accordingly, it is recommending an evaluation of transport-category airplanes equipped with stick pushers to ensure their activation at an AOA that will provide adequate stall protection in the presence of airframe ice. According to Daniel Meier Jr., aviation safety inspector, flight operations, FAA headquarters, “A stall caused by icing is extremely hazardous because you cannot conserve altitude by maintaining attitude. Adhering to the standard of minimum altitude loss ingrained in training has resulted in pilots failing to recover from ice-related stalls and upsets that have resulted in altitude losses in excess of 5,000 ft.”
One of the important misconceptions that the new FAA Advisory Circular serves to dispel is that evaluation criteria for a recovery from a stall or approach-to-stall do not mandate a predetermined value for altitude loss.
The University of Illinois-Urbana's aeronautical engineering department conducted a research project to determine the effect of residual and inter-cycle ice accretions on airfoil performance. The study found that inter-cycle ice reduced the maximum lift coefficients about 60% from 1.8 (clean) to 0.7 (iced) and stall angles were reduced from 17 deg. (clean) to 9 deg. when iced. The effect of the small ridge-like features was local boundary layer separation on the airfoil's upper surface, particularly at higher AOA.
The U.S. NTSB has recommended “additional research to identify realistic ice accumulations, to include inter-cycle and residual ice accumulations . . . and to determine the effects of criticality of such ice accumulations; further, the information developed through such research should be incorporated into aircraft-certification requirements and pilot training programs at all levels . . .”
The NTSB has also recommended that manufacturers of turbine-powered airplanes be required “to provide minimum maneuvering airspeed information for all airplane configurations, phases and conditions of flight [icing and non-icing conditions]; minimum airspeeds also should take into consideration the effects of various types, amounts and locations of ice accumulations, including thin amounts of very rough ice.”
For the Advisory Circular to be effective, pilots need to be properly trained in the use of anti-icing and deicing devices, and know the proper speed and configuration to fly in potential icing conditions, especially when slowing to configure for landing, and through touchdown. Additionally, these speeds must be established with adequate stall margin, even when ice remains on critical portions of the aircraft, and there must be a proper recovery procedure to employ at the first indication of loss of control.
ASRS No. 964029 (July 2011) highlighted the conundrum that can result from deferred maintenance on a component, as permitted by an MEL. “During preflight, we found the stick pusher was not working properly,” the pilot reported. “During the stall protection test, the stick pusher would make a loud grinding sound. The sound could be heard as far back as the emergency exit row, as well as being felt in the floorboards till at least row three. It had been written up a few days prior, but the corrective action was “Ops check good.” Maintenance Control tried to get the contract mechanics to defer the stick shakers. We determined that the worst possible outcome would be the stick-pusher servo seizing, which would prevent the control column from being pulled aft, an essential part of controlled flight.”