Page last updated at 16:39 GMT, Friday, 5 June 2009 17:39 UK

The tortuous search for Air France crash clues

By Tom Symonds
BBC transport correspondent

Brazilian air force searches for Air France jet
The region where the plane went missing is notorious for storms

For French air accident investigators, trying to extract meaning from the shreds of information about the last minutes of Flight AF 447 must be excruciating.

They have neither the flight data recorder nor the cockpit voice recorder. Both are thousands of metres below the surface of the Atlantic.

Instead they have a series of messages sent out over a satellite network called ACARS.

It is a system not designed for crash investigation but for airline engineers to monitor developing faults on planes, and even problems like over-flowing toilets or on-board sickness.

Nevertheless, the messages have told the investigators that speed sensors on the aircraft were out of kilter with each other.

The Airbus information circular says "there was an inconsistency between the different measured airspeeds", and goes on to advise operators of the company's various aircraft what to do in a similar situation - including using other technologies, such as GPS, to monitor the plane's speed.

Messages leaked

The ACARS messages themselves have not been released but an unverified list has been leaked. The BBC cannot vouch for its accuracy.

What is clear is that the crew, who should have been able to see some of these warnings on their cockpit displays, would have been assailed by demands for action from the aircraft's systems

Among the list is one line: "Nav ADR disagree".

This suggests a possible problem with the plane's Air Data Reference (ADR) systems, which use air pressure sensors to measure the plane's speed.

If one ADR disagrees with the others, this fault message can be generated.

In previous crashes the sensors, or pitots, have got iced up in severe weather, despite being electrically heated.

Yet at high altitude, the plane's safe speed margins are narrower. Push the limits, and pilots can end up flying in what they call "coffin corner".

Other messages seem to show a string of systems either failing or giving warnings of failure.

Over the course of a few minutes, the autopilot switched off. The plane seems to have begun flying in what is called an "alternate flight law".

Fly-by-wire aircraft like the Airbus A330 actively "protect" pilots and passengers by preventing large or unstable manoeuvres, such as turning too hard.

It is possible, in an alternate flight law, for some of that protection to be lost. It may be the plane was no longer being prevented from suffering damage from turbulence, high speed or rapid changes of pitch.

But investigations always have a fundamental problem when analysing this sort of information.

Was one of these faults the cause of the crash, or the effect of whatever caused it?

Emergency checklists

What is clear is that the crew, who should have been able to see some of these warnings on their cockpit displays, would have been assailed by demands for action from the aircraft's systems.

Any fault requires the pilots to consult emergency checklists to try to diagnose what has gone wrong, and then take quick decisions to recover the situation.

It is possible that within a few minutes the plane, in severe turbulence, was losing height and out of control, with the crew battling to deal with multiple failures.

But the biggest mystery is why the aircraft seems to have ended up flying through what meteorologists believe were storm cloud formations towering more than 15,200m into the air.

The volatile Intertropical Convergence Zone along the equator is navigated safely by hundreds of aircraft every day.

They have a weather radar showing the locations of storms and Lufthansa has described how one of its crews, in the area at the time, steered a course through the turbulent skies without any problems.

None of the fault messages leaked so far have suggested the radar system itself failed.

But without the aircraft's recorders, investigators are working with one hand tied behind their backs, while the world waits for answers.

Print Sponsor

The BBC is not responsible for the content of external internet sites

Has China's housing bubble burst?
How the world's oldest clove tree defied an empire
Why Royal Ballet principal Sergei Polunin quit


Americas Africa Europe Middle East South Asia Asia Pacific