Inquiries into the loss of Air France flight AF447 have yet to explain fully why the pilots failed to avert the Airbus A330's fatal stall, despite its onset being characterised by buffet and the activation of a stall warning.
But the AF447 investigation has said that the two pilots had not received any high-altitude training for unreliable airspeed procedures and manual aircraft handling. It is formally recommending review of training and check programmes, and crucially the mandatory creation of specific exercises for manual handling, including stall recovery.
French investigation agency Bureau d'Enquetes et d'Analyses has released the findings after more detailed analysis of the flight recorders retrieved from the wreck of the A330.
The autopilot had disconnected shortly before the stall, and the BEA has said, for the first time, that this was "likely as a result of the obstruction of the pitot probes in an ice crystal environment".
Less than a minute after this disconnection the aircraft was "outside its flight envelope", said the BEA, following manual inputs which were mainly nose-up.
The A330 entered a climb that took it out of its assigned altitude of 35,000ft towards 37,500ft. Its stall warning sounded, and the aircraft experienced buffet, but neither pilot referred to the warning - which sounded intermittently but, at one point, triggered continuously for 54s - nor did they formally identify the stall situation.
Shortly after the stall warning, however, the flying pilot applied take-off/go-around thrust and made nose-up inputs.
The aircraft continued to pitch up and its angle of attack increased. It entered a stall and descended rapidly, the crew failing to recover before impact with the sea.
BEA has determined that the movement of the trimmable horizontal stabiliser - which moved to fully nose-up during the accident - and that of the elevators was "consistent with the pilot's inputs" throughout the flight.
No emergency message was transmitted by the crew.
BEA has made 10 new safety recommendations in its latest update on the investigation, including an advisory that authorities should assess the requirement for angle-of-attack indicators in the cockpit. AF447's angle of attack was not displayed directly to the pilots.
Air France said the latest findings showed a "combination of multiple improbable factors led to the disaster in less than four minutes" including the icing of the pitot probes, loss of associated control-law protection, and roll movements.
"It should be noted that the misleading stopping and starting of the stall warning alarm, contradicting the actual state of the aircraft, greatly contributed to the crew's difficulty in analysing the situation," it said. "At this stage, there is no reason to question the crew's technical skills."