File photo of AOA probes on an Airbus A330 (photo: ATSB)
The European Aviation Safety Agency (EASA) issued an Emergency Airworthiness Directive for several Airbus models, detailing emergency procedures in the case of undue activation of Alpha Protection.
An occurrence was reported where an Airbus A321 encountered a blockage of two Angle Of Attack (AOA) probes during climb, leading to activation of the Alpha Protection (Alpha Prot) while the Mach number increased. The flight crew managed to regain full control and the flight landed uneventfully.
When Alpha Prot is activated due to blocked AOA probes, the flight control laws order a continuous nose down pitch rate that, in a worst case scenario, cannot be stopped with backward sidestick inputs, even in the full backward position. If the Mach number increases during a nose down order, the AOA value of the Alpha Prot will continue to decrease. As a result, the flight control laws will continue to order a nose down pitch rate, even if the speed is above minimum selectable speed, known as VLS.
This condition, if not corrected, could result in loss of control of the aircraft.
This systems is installed on Airbus A318, A319, A320, A321, A330 and A340 aircraft. To address this unsafe condition, Airbus have developed a specific Aircraft Flight Manual (AFM) procedures. The Airworthiness Directive requires amendment of the applicable AFM.
This is considered to be an interim action and further AD action may follow.