Final moments of Air France 447 revealed
That is one of the findings of France's BEA aviation safety agency which has analysed the black box flight recorder for the 2009 accident .
"We have no valid indications," one pilot of the Rio to Paris flight was quoted as saying as the aircraft dropped towards the sea, three minutes before it hit the water.
The 58-year-old captain, who had left the cockpit to take a routine rest, returned but did not retake control of the plane.
"There was an inconsistency between the speeds displayed on the left side and the integrated standby instrument system," BEA said in a statement following its study of the recovered flight data recorders.
"This lasted for less than one minute."
According to the chronology of the flight provided by BEA, the two co-pilots decided at two hours and eight minutes into the flight to turn slightly to the left to avoid an area of turbulence.
Two minutes later the autopilot disengaged, the instruments began showing that the speed had slowed dramatically and the engine stall warning began to sound.
"During the following seconds, all of the recorded speeds became invalid and the stall warning stopped," said the BAE preliminary report.
"So, we've lost the speeds," it quoted the second of the two co-pilots as saying. The responses by the co-pilot flying the aircraft "were mainly nose-up" and "the airplane climbed to 38,000ft".
BEA said: "The descent lasted 3 min 30, during which the airplane remained stalled. The engines were operating and always responded to crew commands."
The last data on the recorder showed that plane's nose was up at a sharp angle as it plunged at 10,912ft (3,300m) per minute.
BEA director Jean-Paul Troadec said: "These are so far just observations, not an understanding of the events."
A fuller report into the cause of the tragedy is expected to be released in a few months time.
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