The European Aviation Safety Agency (EASA) issued an emergency airworthiness directive (AD) to ensure that flight crews, in case of Angle Of Attack (AOA ) probes blockage on certain Airbus A330 and A340 models, apply the applicable emergency procedure.
The AD describes an event involving an Airbus A330 that experienced a blockage of all Angle Of Attack (AOA) probes during climb leading to autopilot disconnection and activation of the alpha protection (Alpha Prot) when Mach number increased.
Analysis showed that this aeroplane was equipped with AOA probes having conic plates, and it is suspected that these plates might have contributed to the event. Investigations are on-going to determine the root cause of this AOA probes blockage. The AOA conic plates can also be installed on A340 aeroplanes.
The conic plate AOA probes are used in favour of flat plates to improve the protection of AOA sensors from ice crystals.
The blockage of two or three AOA probes at the same angle may cause the Apha Prot of the normal law to activate.
Under normal flight conditions (in normal law), if the Alpha Prot activates and Mach number increases, the flight control laws order a pitch down of the aeroplane that the flight crew may not be able to counteract with a sidestick deflection, even in the full backward position.
This condition, if not corrected, could result in reduced control of the aeroplane.
To address this condition, Airbus developed a ‘Blocked AOA probes’ emergency procedure. The EASA AD AD requires an amendment of the Airplane Flight Manual (AFM) to ensure that flight crews, in case of AOA probes blockage, apply the applicable emergency procedure.
This AD is considered to be an interim measure and further AD action may follow.