Initial findings from the crashed Air France Airbus A330's flight recorders have left investigators trying to understand why the crew maintained nose-up inputs despite entering a fatal stall.
Few clues have emerged as to why the pilots of flight AF447 failed to rescue the twinjet as it descended rapidly towards the Atlantic Ocean after losing lift at 38,000ft (11,600m).
Even though the captain had left the cockpit, leaving the relief co-pilots in charge, about 8min before the emergency developed, he returned shortly after the onset of the stall. At this point the aircraft was at 35,000ft and still 3min before impact.
France's BEA investigation agency has yet to determine why this time and altitude window was apparently not sufficient for the crew to arrest the A330's descent.
The more senior of the two co-pilots had 4,479h on A330/A340s - more than the captain, although all the captain's hours had been in command - while the other co-pilot had 807h on type.
As the captain prepared to leave the cockpit to take a routine rest break, the aircraft was flying through a turbulent cloud layer at 35,000ft at Mach 0.82. But a shallow temperature gradient outside the aircraft meant the aircraft - fully laden with 216 passengers and 12 crew - was limited as to how far it could climb.
The aircraft was heading for the TASIL waypoint, which marks the boundary between Brazilian and Senegalese oceanic airspace, but the flying co-pilot also mentioned that the jet had "failed" to log on to the Senegalese air traffic centre in Dakar.
About 6min after the captain left the cockpit, the crew was expecting more turbulence and the non-flying pilot suggested a deviation to the left of the flightpath. The aircraft adjusted course by 12° to the left and, as turbulence continued to increase, the crew chose to slow the aircraft to Mach 0.8 - a normal practice when penetrating rough air in order to keep the airspeed from slipping outside design limits.
The crisis began to unfold about 2min later when the A330's autopilot and autothrust disengaged. While the BEA has not specifically stated the reason, there have been several instances of pitot icing causing a similar disengagement on A330/A340s, as a result of the aircraft being unable to obtain consistent airspeed data from the pitot system.
Air France openly stated: "It appearsthat the initial problem was the failure of the speed probes which led to the disconnection of the autopilot at the loss of the associated piloting protection systems."
After the disengagement AF447 switched, as designed, from its "normal" to its degraded "alternate" flight control law. But this critically meant it lost the angle-of-attack envelope protection present in "normal" law.
As the pilot moved the sidestick left, to counter a roll to the right, he also made a nose-up input that prompted two stall warnings - an indication that the aircraft had exceeded a critical angle-of-attack threshold.
The primary flight display on the captain's side showed a "sharp fall" in speed from 275kt (510km/h) to 60kt, said the BEA, and the aircraft's angle of attack "increased progressively" beyond 10°.
While the jet had been cruising at 35,000ft it began to climb, with a vertical speed of 7,000ft/min (35.56m/s), heading towards 38,000ft, despite the crew's being aware that the aircraft would have difficulty sustaining flight at higher altitude because of the temperature gradient and the slower cruise speed.
BEA said the pilot made nose-down inputs as well as inputs for left and right roll. The vertical speed fell back to 700ft/min, the displayed speed "increased sharply" to 215kt, and the angle of attack reduced to 4°.
In a sign that the situation was starting to confuse the crew, the non-flying pilot "tried several times" to call the captain back to the cockpit, said the BEA.
As he called for the captain, there was another stall warning. Air France's A330 stall procedure at the time required setting take-off/go-around thrust and reducing pitch attitude. But while the AF447 crew set the thrust levers to the correct position - and the engines always responded correctly - the flying pilot "maintained nose-up inputs".
The aircraft's angle of attack increased and its trimmable horizontal stabiliser - which automatically trims in "alternate" law - increased from a 3° nose-up position to 13° nose-up.
Having reached a maximum altitude of 38,000ft, with its angle of attack at 16°, the aircraft lost lift completely and began to fall, nose-up.
Just 90s after the autopilot had disengaged, as the jet descended through 35,000ft at close to full power, the captain re-entered the cockpit. At this point the A330's speeds were rendered invalid by the aircraft systems and the stall warning cut off. The angle of attack had reached more than 40°, and the jet was rolling up to 40° left and right, although its pitch did not exceed 15°.
AF447's trimmable horizontal stabiliser remained in the nose-up position it had previously attained. The A330's logic allows the aircraft to enter an "abnormal attitude" law if the angle of attack exceeds 30°, which would have inhibited the auto-trim and forced the crew to use manual trim to adjust the stabiliser.
Failure to realise the need for manual trim - which is rarely required - has been cited in a number of stall events, including the loss of an Airbus A320 over the Mediterranean Sea near Perpignan in November 2008.
But Flight International understands that preliminary data shows AF447 did not enter "abnormal attitude" law, because other criteria were not accepted by the flight control computer. The aircraft remained in "alternate" law and that the auto-trim would have been available to the crew.
"The [flying pilot] made an input on the sidestick to the left and nose-up stops, which lasted about 30s," said the BEA, further confirming that - despite the stall condition - his inputs were "mainly nose-up".
The first signs of action to recover the stall came just 20s after the captain returned to the cockpit, said the investigators.
AF447's thrust was reduced to "idle", with the engines delivering 55% of N1, and the pilot started making pitch-down inputs. The angle of attack reduced, the speeds became valid and the stall warning restarted.
The angle of attack nevertheless stayed above 35° and the pilot acknowledged the aircraft's deteriorating altitude as it continued to descend rapidly towards 10,000ft.
Some 40s before impact, said the BEA, both pilots made "simultaneous inputs" on the sidesticks before the flying pilot surrendered control to the other. At a descent rate of more than 10,900ft/min, and a ground speed of 107kt, and having turned more than 180° to the right during its descent, the aircraft hit the water nose-up and disintegrated. There were no survivors.
Airbus has not issued any urgent guidance to operators following the release of the preliminary accident sequence by the BEA.
Source: Flight International