French investigators have laboured to explain the nose-up inputs made by the pilot of Air France flight AF447 which led the Airbus A330 to climb and subsequently enter a fatal stall.
The aircraft's autopilot disconnected as the A330 entered a region of turbulence, and the flying pilot made "abrupt and excessive" nose-up sidestick inputs, says the BEA in its final report into the June 2009 accident over the South Atlantic.
"The excessive nature of the [pilot's] inputs can be explained by the startle effect and the emotional shock at the autopilot disconnection," it states, adding that the problem was "amplified" by a lack of training for the delicate handling required for high-altitude flight.
It says the initial nose-up reaction may have been associated with a desire to regain the cruise level of 35,000ft after the aircraft lost about 300ft in height.
But while this early input can be "fairly easily understood", the BEA says the "same is not true" for the subsequent persistence of nose-up input which caused the A330 to climb rapidly to 38,000ft - an altitude which increased the risk of aerodynamic stall.
The BEA has excluded explanations such as an unsuitable seat position. But it says other possibilities include the crew's attention being focused on speed or roll, misperception of aircraft behaviour in the turbulence, "saturation" of mental resources, or the "attraction" of clear, less-turbulence sky at higher altitude.
"Whether the [flying pilot's] nose-up inputs were deliberate or not, there was no verbal expression of this to the [other pilot]," says the BEA.
Despite the onset of buffet and stall warnings at the higher altitude, the crew "never understood that they were stalling".
Failure to take the stall warning into account could have been due to low exposure to stall phenomena, possible misinterpretation of the buffet as overspeed, or flight-director indications which may have "led the pilots to believe that their actions were appropriate, even though they were not", the BEA believes.
Airbus says the BEA has "spared no effort" to understand the accident sequence fully.
Air France insists its crew "acted in line with the information provided" by instruments, and the pilots' perception of the jet's behaviour, notably given the "triggering and stoppage" of the stall warnings.