NTSB · Safety Recommendation
Safety Recommendation A-12-024
Recommendation
TO THE FEDERAL AVIATION ADMINISTRATION: Require that Avions de Transport Régional (ATR) 42-series airplanes operating in the United States incorporate a revised stick pusher activation angle of attack (AOA), such that the stick pusher activates before the stall AOA in the presence of airframe ice accretions.
Safety Issue and Context
On January 27, 2009, about 0437 central standard time, an Avions de Transport Regional (ATR) Aerospatiale Alenia ATR 42-320 (ATR 42), N902FX, operating as Empire Airlines flight 8284, was on an instrument approach when it crashed short of the runway at Lubbock Preston Smith International Airport, Lubbock, Texas. The captain sustained serious injuries, and the first officer sustained minor injuries. The airplane was substantially damaged. The airplane was registered to FedEx Corporation and operated by Empire Airlines, Inc., as a 14 Code of Federal Regulations (CFR) Part 121 supplemental cargo flight. The flight departed from Fort Worth Alliance Airport, Fort Worth, Texas, about 0313.
Instrument meteorological conditions prevailed, and an instrument flight rules flight plan was filed.
The National Transportation Safety Board (NTSB) determined that the probable cause of this accident was the flight crew's failure to monitor and maintain a minimum safe airspeed while executing an instrument approach in icing conditions, which resulted in an aerodynamic stall at low altitude.
Contributing to the accident were 1) the flight crew's failure to follow published standard operating procedures in response to a flap anomaly, 2) the captain's decision to continue with the unstabilized approach, 3) the flight crew's poor crew resource management, and 4) fatigue due to the time of day in which the accident occurred and a cumulative sleep debt, which likely impaired the captain's performance.
Additional review of the stall protection system in the accident airplane and the events leading to the stall in this accident prompted the NTSB to make the following recommendations.
Response History
February 10, 2015 · NTSB Correspondence · FAA
We note that, in icing conditions, the ATR-42’s stick pusher activation AOA is set in accordance with applicable requirements. However, our concern is that these requirements do not ensure that the stick pusher provides adequate stall protection. Unlike the stick shaker, the stick pusher’s activation AOA does not change when the ice protection system is engaged and, as a result, may not offer stall protection when the airplane encounters icing conditions. We believe that the unsafe condition that was identified and used as justification for reducing the stick shaker’s activation AOA in icing conditions could similarly be used to reduce that of the stick pusher in the same conditions.
We point out that several airplanes, including the ATR 72, lower the trigger AOA for both stick shaker and stick pusher in the presence of ice to preserve the AOA margin between stick shaker and stick pusher. We are aware that, for the ATR 42-series and other older design airplanes, other mitigations are in place to provide stall protection in icing conditions. However, we continue to believe that a stick pusher design that accounts for and activates before the reduced stall AOA in icing conditions would enhance the safety of the flight, especially when pilots are operating at low altitudes.
We do not agree that low-speed alerting constitutes an effective solution for the problem in icing conditions, although it may be an effective safety enhancement for preventing low airspeed loss of control. However, because you indicated that your actions in response to Safety Recommendations A-12-24 and -25 are complete, the recommendations are classified CLOSED—UNACCEPTABLE ACTION.