EASA Airworthiness Directives Publishing Tool
An A330 aeroplane, equipped with Angle of Attack (AoA) sensors with conic plates installed, recently experienced blockage of all sensors during climb, leading to autopilot disconnection and activation of the alpha protection (Alpha Prot) when Mach number was increased.
Based on the results of the subsequent analysis, it is suspected that these conic plates may have contributed to the event. Investigations are on-going to determine what caused the blockage of these AoA sensors.
Blockage of two or three AoA sensors at the same angle may cause the Alpha 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 side stick deflection, even in the full backward position.
This condition, if not corrected, could result in reduced control of the aeroplane.
condition on Airbus A330 and A340 aeroplanes.
AoA conic plates of similar design are also installed on A320 family aeroplanes, and installation of these AoA sensor conic plates was required by EASA AD 2012-0236, making reference to Airbus SB A320-34-1521 for in-service modification. This requirement has now been removed with revision 1 of that AD.
To address this condition on A320 family aeroplanes, Airbus developed an “AOA Blocked” emergency procedure, published as a temporary revision (TR) of the Airplane Flight Manual (AFM), to ensure that flight crews, in case of AoA sensors blockage, apply the applicable emergency procedure.
For the reasons described above, this Emergency AD requires insertion into the AFM of the Airbus TR.
This AD is considered to be an interim measure and further AD action may follow.
With multiple frozen/blocked AOA probes and an increasing mach no, the flyby wire system might trigger alphaprot mode. As the system thinks the aircraft is stalling the flybywire system induces a constant nose-down input to the aircraft.
The speed indication however will be normal as the pitot-probes are functioning normal.
The nose-down input is not fedback via the sidestick, nor on the screen. And pilot full nose-up input might not be able to counter the nose-down input of the alpha-prot system.
So the brilliance of engineering instructs us in a procedure to turn off 2 ADR's in order to revert the aircraft to Alternate law, in which no alpha prot exist. Off course first you have to realise what is going on, which could be difficult as there is no feedback from the system to the pilot.
But yes you read it correctly, the system is able to completely override pilot input despite being completely wrong. And the only way to recover from uncontrolable situation is to switch of ADR's.
The procedure:
At any time, if the aircraft goes to an unmanageable pitch down attitude despite continuous deflection of the sidestick in the full backward position (in case the flight crew missed the below symptoms or delayed the application of one of the below procedures):
Keep one ADR ON. Turn OFF two ADRs.
And before anyone asks, yes I fly Airbus A320 family. (not 737 as my name might suggest)