Will Air France mystery ever be explained?

Somewhere under the Atlantic, it is assumed, are two important pieces of equipment.

Investigators probing the loss of Air France flight 447 hope they may yet explain one of the most baffling air crashes of recent times.

Almost three weeks ago AF447 crashed into the South Atlantic off Brazil, killing all 228 people on board.

Some bodies have since been recovered, as well as around 400 plane parts.

Still missing, however, are the so-called "black boxes" which record flight data and conversation in the cockpit.

If they never turn up will the crash remain unexplained?

When the flight disappeared off radar screens in the early hours of 1 June, one-third of the way from Rio de Janeiro to Paris, speculation soon focused on potential problems with the Pitot tubes on the A330 - something which the manufacturers, Airbus, were already addressing.

Pitot tubes exist on virtually all aircraft and detect speed by registering variations in air pressure.

If a pilot is unsure how fast he is flying he risks losing control of his aircraft.

Last year Airbus decided Pitot tubes on the A330 might be prone to blockage.

On 27 April Air France had started a programme of replacement, now expedited.

The French crash inquiry is the responsibility of the BEA - the Bureau d`Enquetes et d`Analyses pour la Securite de l`Aviation Civile.

On Wednesday the head of the organisation, Paul-Louis Arslanian, described the AF447 inquiry as one of the toughest he has known.

"It is one of the worst contexts for an aviation investigation," he told a news conference in Paris.

Little evidence

The problem is that the plane went down at sea - a highly unusual event.

Fewer than one in four of the bodies of the passengers have been recovered, and it is unlcear what proportion of the plane the 400 recovered parts actually amount to.

Still missing are the all-important Cockpit Voice Recorder and the separate Flight Data Recorder.

If either turns up intact it will energise an inquiry which currently has little to go on.

For once the cliche is justified: finding the voice and data recorders is a race against time.

Each carries a locator beacon which kicks in on contact with water and has a range of 1,500m.

But transmission will cease after 30 days.

The ocean bed in the presumed crash area is difficult and deep (up to 4km in places): if neither recorder turns up by the end of June it seems unlikely either will be seen again.

So can a worthwhile inquiry proceed?

French journalist Pierre Sparaco, who has been following international aviation for 40 years, acknowledges that the lack of the recorders is problematic, but insists it is not the end of the story.

"For instance, in this case they`ve retrieved seats used by the flight attendants during take-off and landing," he explained.

"Those seats were empty so you can assume at the time the accident happened the attendants were busy serving meals or taking care of the passengers. So they weren`t expecting anything to happen, not even difficult weather conditions."

Mr Sparaco also says it is significant that doctors say none of the bodies found so far have been burnt, which he says rules out an explosion.

Also important is that none of the bodies found have been reported to have been wearing life-jackets.

"If the plane was going down the flight attendants would get passengers to put the jackets on. So you can start working a scenario from there."

Learning lessons

Todd Curtis, an airline safety analyst in Seattle who runs the respected website, Airsafe, also believes there is enough data available for a meaningful investigation.

"Even if there had been nothing recovered from the aircraft there would still be material to work with," he said.

For a start there should be the maintenance records of that particular aircraft as well as the records and other experiences related to the entire A330 fleet, he said.

"You could have researchers going through all that and seeing if there`s any pattern which relates to a problem.

"So the absence of the data recorders won`t stop the inquiry in its tracks."

The other thing investigators have to go on are the automated messages sent by the plane`s Communication and Reporting System.

There were 24 of these to Air France in the four minutes before the plane disappeared.

Normally, though, these would constitute only one aspect of the investigation - not its core.

When a fatal crash occurs we console ourselves that, however tragic the human loss, things may ultimately get better because practical lessons will be learnt.

With AF447 the worry is that any lesson will remain incomplete, unless the plane`s data recorders emerge intact from the depths of the Atlantic Ocean within a few days.

By Vincent Dowd, BBC