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January 25, 2011
By AOPA ePublishing staff
Stall warning systems on pre-2000 aircraft may not activate at the onset of a stall in icing conditions, the FAA told pilots Jan. 24 in a special airworthiness information bulletin (SAIB).
The SAIB results from a study of nonfatal icing-related incidents dating back 25 years, in which the FAA found evidence of stall events during flight in icing conditions when the pilot or passenger said the warning system did not activate. It gives guidance on recognizing the signs of an imminent stall and recommended procedures for flight in icing conditions.
In some of the accidents, pilots experienced a “shudder” or buffet but did not recognize it as an impending wing stall, attributing it instead to either the engine or propeller icing.
“Occurrences of buffet or shudder in icing conditions should be treated as an imminent wing stall,” the SAIB explains. “Recover by reducing angle of attack and slowly applying full power/torque.”
The SAIB applies to aircraft designed before 2000, when “a clear and unambiguous buffet was accepted for stall warning in icing conditions, even if the airplane was equipped with a stall warning system and a heated stall warning sensor.” Stall warning systems in aircraft designs certificated since 2000 activate at higher speeds in icing conditions because they are designed and tested with critical ice accretions along the entire span of the wing, it says.
AOPA supports FAA educational initiatives that bring attention to operational issues rather than trying to address the issues in certification or aircraft limitations alone. The SAIB provides helpful information to pilots without imposing costly and burdensome regulations.
The SAIB recommends pilots consult the airplane flight manual, pilot’s operating handbook, and placards for limitations and procedures in icing conditions; if none are specified, it provides some basic guidelines. For aircraft without a specified airspeed for flight in icing, it recommends increasing airspeed by at least 25 percent for every phase of flight, as long as it does not exceed the aircraft’s maximum airspeed limitations.
Pilots should monitor airspeed closely when the autopilot is engaged in icing conditions and disconnect the autopilot at least once every five minutes to ensure normal trim and handling, it adds: “The autopilot may mask dangerous airspeed losses.” Other recommendations include cycling deicing boots prior to configuring for approach, reducing power slowly in the flare, and requesting priority handling or declaring an emergency to exit severe icing conditions. Airplanes not certificated for flight in icing are not tested for “inadvertent” encounters, it warns; pilots should not believe the myth that “thicker” general aviation airfoils are more tolerant of ice accretion.
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