Defective sensors which fed information on air speed to onboard systems were one possible factor in the loss of an Air France Airbus A330-200 on 1 June 2009.
Alain Bouillard, who is leading the investigation into the accident for the French accident investigation agency BEA, said today that the sensors, called Pitot tubes, were not the only factor in the accident.
"We can say that the pitot is strongly suspected of causing the incoherent speed readings. It is one of the factors but not the only one. It is an element, not the cause," Bouillard told The Times.
He was speaking today at a press conference outside of Paris. Flight AF447 was travelling from Rio de Janeiro to Paris, with 228 people on board, when it hit the water.
"The plane was not destroyed while it was in flight," he said. "It seems to have hit the surface of the water in level attitude and with a strong vertical acceleration." The Airbus appears to have been intact before contact with water.
A probable cause of the accident may never be known. Bouillard said: "We are far from determining the cause of the accident ... We are still hopeful of finding the black boxes."
He confirmed that the Airbus A330-200 was unable to fly on autopilot at the time of the crash. This was because the autopilot was not receiving speed, wind or direction information, he said.
"These tell us that the plane has to be, in this case, directed by the pilot," he said. He did not immediately say if the pilots were in control of Air France 447.
The plane's systems sent 24 automatic data messages to an Airbus maintenance centre in France in the minutes before the links went dead. Bouillard said: "These alerts don't mean the plane was unflyable, just that it had to be on manual pilot."
According to Aviation Week the list of data messages indicated a sequence of failures of computer systems which received information on air speed from the aircraft's pitot tubes. Investigators have suggested that the pitot-static sensors might have been blocked. This could have led to onboard systems feeding contradictory information to the pilots.
The messages included alerts to show malfunctions of the first primary and secondary flight control computers, said Aviation Week.
The few facts to emerge so far map the pattern of two much earlier fatal accidents in which computerized Boeing 757s - Aeroperu flight 603 and Birgenair flight 301 - went into the sea in 1996 after the pilots became confused by contradictory information and warnings.
In those accidents, blocked pitot system sensors caused the in-flight systems to go haywire, telling the pilots they were flying too fast and too slow. Both aircraft were intact before entering the sea. The autopilots had disengaged.
In the three accidents, 487 people lost their lives.
The Times's correspondent in France said the impression he had from the press conference was that the Air France Airbus might have been in a stall or recovering from one.
If true, this would again echo what happened in the moments before the loss of Aeroperu 603 and Birgenair 301. In both of those accidents the pilots were in a stall or trying to recover from one.
Advances in technology have improved air safety. But the two earlier accidents show how reliant pilots have become on their automated flight systems, and how helpless they can be when there multiple electronic failures.