Faults similar to those on an Air France Airbus that crashed
into the sea on 1 June were major factors in two little-noticed
fatal losses of passenger jets more than a decade ago.
Blocked sensors, known as pitot tubes, fed unreliable data to
the onboard systems, giving the pilots conflicting information on
the aircraft's speed, before
Birgenair Flight 301 and
Aeroperú Flight 603 crashed into the sea in 1996.
The loss of the two Boeing 757s caused the deaths of 259
people.
Investigators of the crashed Air France Airbus Flight 447 have
said that
blocked pitot sensors might have been a factor in the accident,
which led to the loss of 228 lives.
"What we know is that these pilots were confronted with serious
technical problems and erroneous indications of speed in the
cockpit," said Eric Derivry, a spokesman for Air France's biggest
pilots union, speaking to Bloomberg.com.
"Speed information is an element that's basic to piloting an
airplane. Airspeed readings are crucial for pilots to keep control
of the aircraft."
French investigators said after reviewing data transmitted by
the doomed plane in its last minutes that it sent 24 automated
error messages.
John Hansman, director of the International Center for Air
Transportation at the Massachusetts Institute of Technology, said
blocked pitot tubes can "give false readings that might lead the
plane's computerised systems to misfire". By measuring air
pressure, pitot tubes help the onboard systems calculate air
speed.
After the Air France crash, Air France and
Toulouse, France-based Airbus issued reminders to pilots to follow
procedures when measurements become unreliable.
The available information means that in all three accidents the
pilots received inconsistent information about their speed shortly
before the loss of the aircraft, possibly from malfunctioning
"pitot" tube sensors.
Also, in all three crashes, the auto-pilot disengaged.
It is not known if the inconsistent readings played any part in
the loss of the four year-old Air France Airbus 330-200, Flight
447. French and US authorities are still trying to find the
Airbus's black boxes - the flight data and cockpit voice recorders
- off the coast of Brazil. They may never be recovered.
On Flights 301 and 603 the onboard systems told the pilots they
were flying both too fast and too slow - and gave them audible
warnings to this effect. Flying too fast can cause the aircraft to
break up; too slow and it may fall out of the sky.
Computer Weekly has reconstructed the last moments of Flights
301 and 603 to show that conflicting information in the cockpit can
contribute to fatal air accidents.