What is known about the crash of an Air France airbus on 1 June bears similarities with the little-noticed loss much earlier of two computer-controlled passenger jets. Those two crashes raised questions of whether the pilots or systems were really in control.
Nobody knows what caused an Airbus A330-200 to enter the Atlantic Ocean off the coast of Brazil on 1 June with the loss of 228 passengers.
If the black boxes of Flight AF447 are recovered – and a search is still underway - it may be months or years before a probable cause is known, if ever.
Some information is known already though, largely because of data sent by the aircraft’s onboard systems to Airbus Industries in France before the links went dead.
Airbus said this data showed that the pilots might have received conflicting information about their speed. There was a “divergence in airspeed measurement” by the onboard systems of the Air France aircraft. This is one of the matters being investigated, said Airbus.
Data to the onboard computers about air speed came from sensors called pitot tubes, at least one of which was due for replacement. French authorities have suggested that inconsistent air speed readings are not dangerous.
France’s chief crash investigator told journalists at a briefing near Paris that the failure of the air sensor to convey reliable speed data might have kicked off a chain of events that led to the deaths of all 228 people aboard.
The sensors had not been replaced with the improved units, said Paul-Louis Arslanian, head of France’s air-accident investigation agency, at a briefing outside Paris. “But that doesn’t mean that without them the plane was dangerous.”
Whether conflicting messages to pilots from onboard systems is potentially dangerous is a moot point.
Using evidence gathered by documentary film-makers, cockpit voice recordings and various reports, Computer Weekly has investigated two little-noticed air fatal crashes in which a blocked Pitot-static system set off a chain of events, leading to the pilots receiving bewildering and conflicting messages from the onboard systems.
Two fatal crashes of passenger aircraft show that it is dangerous for pilots to have conflicting information and warnings in the cockpit.
The recovered black boxes from two computerized commercial airliners that crashed in the sea in the 1990s show what it's like in the cockpit when the Pitot-static system is blocked. Investigators of the crash of an Air France Airbus in the sea off Brazil say that blocked Pitot tubes could have been a factor in the accident.
In each case the onboard systems told the pilots that they were are flying too fast and too slow. Visual warnings were backed up by audible warnings, and the shaking of their flying controls - the stick shaker.
If pilots fly too slowly the aircraft may fall out of the sky; if too fast the aircraft may break up.
Static systems include Pitot tubes and static ports which feed data on air speed to the onboard systems which display information to the pilot.
The auto-pilot also gets its information from the Pitot system.
The pilots in the aircraft did not know that the Pitot-static system was blocked. This was established only after the black boxes - the cockpit and flight data recorders were recovered from the sea.
Investigators of the loss of an Air France Airbus in the Atlantic Ocean on 1 June 2009 say that the pilots might have received conflicting information on their air speed from the aircraft's Pitot tubes.
Report from Sky News
There are similarities between the crash of Air France Airbus, Flight AF 447, and the losses of Flights 301 and 603, all of which claimed a total of 487 lives.
With the three crashed aircraft:
- There were concerns about the performance of the Pitot system
- There were inconsistencies in the cockpit air speed data
- The auto-pilot was off (in the two earlier crashes this because the auto-pilot switched itself off when reaching the limit of its authority and then passing control to the pilots)
That said, it is impossible to say what caused the loss of 288 lives on Flight AF 447.
Air crashes are usually the result of a chain of events. If the Pitot systems were blocked and caused conflicting air speed information to the pilots of the Air France Airbus that might have had nothing to do with the accident, or might have been only one link in the chain.
In the losses of Flights 301 and 603, the blocked Pitot systems were important factors, in part because of the confusion of the pilots.
This is what happened in the cockpit when the pilots of Flight 301 and 603 were faced, unknowingly, with blocked Pitot systems.
The information is taken mainly from the documentary series "Air Crash Investigation" on National Geographic Channel, and from the official accident reports, together with transcripts of the cockpit voice recordings.
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Pictures from Rex Features.