Airbus Airworthiness Directive (AD)

Lufthansa A321 near Bilbao on Nov 5th 2014, loss of 4000 feet of altitude
         
         Incident Facts
         Date of Incident 05.11.2014
         Classification Incident
         Airline Lufthansa
         Aircraft Type Airbus A-321

         AircraftRegistration D-AIDP
         Aircraft Photos of D-AIDP

         A321

A Lufthansa Airbus A321-200, registration D-AIDP performing flight
LH-1829 from Bilbao, SP (Spain) to Munich (Germany) with 109 people on
board, was climbing through FL310 out of Bilbao about 15 minutes into
the flight at 07:03Z, when the aircraft on autopilot unexpectedly
lowered the nose and entered a descent reaching 4000 fpm rate of
descent. The flight crew was able to stop the descent at FL270 and
continued the flight at FL270, later climbing to FL280, and landed
safely in Munich about 110 minutes after the occurrence.

   The French BEA reported in their weekly bulletin that the occurrence
was rated a serious incident and is being investigated by Germany's BFU.

   The occurrence aircraft remained on the ground in Munich for 75 hours
before resuming service on Nov 8th.

   The Aviation Herald learned that the loss of altitude had been caused
by two angle of attack sensors having frozen in their positions during
climb at an angle, that caused the fly-by-wire protection (computer) to
surmise (incorrectly) that the aircraft entered a stall while it climbed
through FL310. The Alpha Protection (computer) activated - forcing the
aircraft to pitch down, which could not be corrected even by full back
stick input. The crew eventually disconnected the related Air Data Units
(pull circuit breakers) and was able to recover the aircraft.

   Following the occurrence EASA released emergency airworthiness
directive 2014-0266-E_1 stating:

   An occurrence was reported where an Airbus A321 aeroplane 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 (computer) 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.( computer is programmed to OVERRIDE
pilot inputs). (airplane is
   screaming towards the ground and the pilot cannot stop this !!) 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(computer) 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(by pulling circuit breakers), could
result in loss of control of the aeroplane. ( possible resultant contact
with ground )

   The EASA requires as immediate emergency action that the flight crew
operating manuals must be amended with a procedure to keep only one Air
Data Reference Unit operative and turning the other two off in following
cases:

   - the aircraft goes into a continuous nose down pitch movement that
can not be stopped by full backward stick deflection ( pilot inputs are
disregarded by the computer).
   - the Alpha Max (red) strip completely hides the Alpha Prot strip
(black/amber) without increase in load factor
   - the Alpha Prot strip rapidly changes by more than 30 knots during
flight maneouvers with increase in load factor while autopilot is on and
speedbrakes are retracted.

http://www.aeroinside.com/item/4946/lufthansa-a321-near-bilbao-on-nov-5th-2014-loss-of-4000-feet-of-altitude