The US National Transportation Safety Board (NTSB) is asking US and European aviation regulators to modify the stick pusher activation angle of attack (AOA) on the ATR 42 regional turboprop so that the system will automatically activate at a lower AOA in icing conditions.
The recommendation is in response to the non-fatal 27 January 2009 crash of an Empire Airlines ATR 42 landing in Lubbock, Texas, in icing conditions.
While the NTSB in its final analysis of the accident in May 2011 concluded that poor airspeed management by the pilots led to the aerodynamic stall preceding the crash, a subsequent analysis by NTSB investigators highlighted potential issues with the stall prevention system on the ATR 42 and other aircraft.
Unlike the ATR 72 stall prevention system, which reduces both the stick shaker and stick pusher AOA when the ice protection system is activated, the ATR 42 system reduces only the stick shaker AOA in icing.
The NTSB notes that ATR was not required under the certification standards at the time to reduce the stick pusher AOA for icing conditions, "however NTSB has long recognised that the certification standards do not capture real-world icing conditions," the recommendation states.
The NTSB says a more timely stick pusher activation "would not have prevented ground contact" in this accident, but "a stick pusher design that accounts for and activates before the reduced stall AOA" in icing conditions would enhance the safety of flight".
Beyond the ATR 42, the NTSB is asking the FAA and the European Aviation Safety Agency (EASA) to "evaluate all US and European-certificated transport category aircraft equipped with stick pushers to ensure that the stick pusher activates at an [AOA] that will provide adequate stall protection in the presence of airframe ice accretions."