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.
Air France Airbus - its last few minutes?
Aeroperu 603 crash
Birgenair 301 crash
Airbus crash: can a triple-redundant system give false
readings?