Air France has formally submitted concerns over the stall warning system in the ill-fated Airbus A330-200 lost over the South Atlantic, after further clarity over the crash of flight AF447 prompted fierce defence of the aircraft's pilots.
The carrier said it had put its concerns to the European Aviation Safety Agency on 1 August, having condemned the "misleading stopping and starting" of the stall warning which, it said, was "contradicting the actual state of the aircraft".
French investigation agency Bureau d'Enquetes et d'Analyses withdrew a safety recommendation on the stall warning before publishing its latest interim report into the accident, stating it was "premature". But the decision generated suspicion, with French cockpit union SNPL demanding an explanation and threatening to withdraw assistance until it could be assured the BEA was not simply setting up the crew as scapegoats.
The BEA has determined that the crew had not been trained to handle a high-altitude loss of airspeed data and risk of stall, and a partial transcript detailed the confusion in the cockpit over the twinjet's behaviour.
Detailed recorder traces from AF447 show that the aircraft's elevators remained at pitch-up throughout the final 3min of the 1 June 2009 flight, never falling below 15°, in line with primarily nose-up inputs from the flying pilot - despite automated voice warnings that the jet was stalling.
Movements of the elevators and the trimmable horizontal stabiliser, which adjusted to the fully-up position in response to the A330's angle of attack, were "consistent with the pilot's inputs" throughout the flight, said the BEA.
While the stall alarm sounded continuously for 54s the captain, urgently called back from a rest break, re-entered the cockpit just as it ceased. The warnings then became intermittent, owing to A330 logic that cuts out the alarm if airspeeds become invalid.
The cockpit transcript shows the captain asked the crew what they were doing, to which the non-flying pilot responded: "What's happening? I don't know what's happening."
BEA said its recommendation focused on functioning of the stall warning in extreme angles of attack, one which is "never encountered in flight tests, or even considered". It added that a human factors working group will analyse why the stall warning on AF447 sounded continuously "without provoking any appropriate reaction from the crew".
While the BEA highlighted the crew's actions and training, both Air France and SNPL stressed the technical failings experienced by the A330 - notably the formal finding that icing probably caused an unreliable airspeed indication.
SNPL president Jean-Louis Barber said the pitot failure "constituted the trigger" and the pilots then faced a "delicate, unexpected" and "totally novel" situation.
It insists the design of the stall warning "misled" the pilots. "Each time they reacted appropriately the alarm triggered inside the cockpit, as though they were reacting wrongly. Conversely each time the pilots pitched up the aircraft, the alarm shut off, preventing a proper diagnosis of the situation."
SNPL's argument essentially suggests the on-off alarm might have incorrectly been interpreted by the crew as an indication that the A330 was alternating between being stalled and unstalled, when it was actually alternating between being stalled with valid airspeed data and stalled with invalid airspeed data. The recorder traces for the last 3min of flight show preliminary evidence of a correlation between the reduction in elevator deflection and the sounding of the stall alarm.
Air France said: "At this stage there is no reason to question the crew's technical skills." BEA has recommended introducing specific manual flying training for such situations.