Air France Airbus pitot sensor linked to two fatal crashes

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.