The riddle of flight AF447

Ten days after flight AF447 vanished over the south Atlantic, the mystery of what happened to the Air France Airbus and the 228 people on board is starting to become clear. Perhaps.

All the indications are that the calamity was influenced - if not entirely caused - by a malfunction of the three tiny tubes in the nose of the plane which measured its speed.

Air France voiced its concerns to Airbus months ago about the reliability in icy conditions of the model of speed sensors, or "Pitot tubes", fitted to the A330-200 which crashed 400 miles north-east of Brazil.

The French national airline, after reassurances from Airbus, started in April an unhurried programme to replace the tubes with an improved version.

Air France, under pressure from its own pilots, yesterday ordered that at least two of the three Pitot tubes on all its 34 remaining A330 and A340 aircraft should be upgraded immediately.

On Monday, one of the smaller Air France pilots' unions told its members to refuse to take the controls of any Airbus A330 or A340 which had not had its speed sensors changed.

But how can the malfunction of something so simple and basic as a speed sensor explain how a hyper-modern aircraft, only four years old and packed with sophisticated control and communication equipment, could fall from the sky without broadcasting a mayday message?

The aircraft issued 24 automatic, emergency signals in the space of a few minutes in the early hours of Monday 1 June, indicating the collapse, one by one, of most of its electrical and computer systems. Can that cascade of mechanical disasters all be traced to a problem with speed sensors?

Initially, the aircraft was said to have flown into a severe storm and to have "probably" been crippled by a lightning strike.

At the weekend, the French meteorological office said that the weather on the night of May 31 and June 1, while turbulent, was fairly normal for the south Atlantic.

The Brazilian authorities added to the confusion early last week by announcing that they had found debris from the missing aircraft, and a large slick of aviation fuel.

The location of this wreckage was nearer to the Brazilian coast, and further south, than the last known position of the airbus had suggested the crash site would be.

Had the aircraft tried to turn back to Brazil? If so why had it broadcast no mayday message?

The presence of the aviation fuel also appeared to rule out the possibility of a mid-air explosion and, therefore, of a terrorist attack.

The shamefaced Brazilian authorities admitted on Friday that they had spoken too soon. The wreckage and oil they had found came from unknown ships, not from the Airbus.

Within a day, genuine pieces of Airbus wreckage - and by yesterday 24 bodies and part of the tailplane - had been recovered to the north-east, along the plane's scheduled flight path toward Senegal and onward to Charles de Gaulle airport in Paris.

French investigators now say that terrorism has not been ruled out, and was never ruled out, but "all the indications" remain that the Airbus suffered some kind of catastrophic equipment failure.

Airline disasters are meat and drink to conspiracy theorists. Several alternative explanations still exist for the Concorde disaster in Paris in 2000 and the terrorist bombing of a Pan Am jumbo over Lockerbie in 1988.

The confusion and misinformation surrounding Flight AF447 will inevitably lead to similarly fevered speculation.

French investigation sources said yesterday that such confusion was difficult to avoid.

"You have a three-way muddle here," one official said.

"There is the understandable desire of the media and public for a rapid explanation and the unfortunate fact that most of this aircraft has vanished into deep ocean. On top of that, there are extremely complex, technical questions involved which are difficult for even experts to grasp fully."

A French nuclear submarine, the Emeraude, and a naval vessel containing robot submarines should reach the crash site within the next two days.

Investigators hope that the electronic equipment aboard the nuclear submarine will allow it to pick up the radio signals from beacons attached to the aircraft's black boxes, or flight recorders, which could be up to 15,000 feet below the surface.

Without the records contained in those boxes, it might be impossible to be certain about what happened to Flight AF447.

For the time being, attention is on the aircraft's speed sensors.

In November last year, Air France issued a warning note to its pilots about a "significant number of incidents" and "anomalies" with the functioning of the Pitot tubes, or speed detection devices, on A330 and A340 aircraft.

The tubes, fitted in or behind the nose, measure dynamic air pressure against the fuselage so to indicate the speed to the human pilot and co-pilot but also, crucially to the automatic, computerised "fly-by-wire" systems which take over the piloting of modern aircraft.

The previous "incidents" on Air France Airbus craft, starting in May last year, suggested an earlier model of Pitot tubes fitted to some planes could seriously malfunction in icy conditions.

The problems included wrong, or fluctuating, indications of speed; the automatic switching off of the automatic pilot and even a false warning that a plane was about to fall from the sky.

In the early hours of June 1, when Flight AF447 was 400 miles out into the Atlantic, one of the first of the 24 automatic distress messages that it broadcast to Air France headquarters indicated that the speed of the aircraft was fluctuating wildly.

Soon after, the computerised "fly-by-wire" system was switched off - possibly automatically, possibly because the pilots were wrestling to save the plane. At that point its fate and that of its passengers seem to have been rapidly sealed.

Still, French investigators say that they are not fully convinced that the speed sensor problem is the main explanation for the disaster. The jigsaw puzzle is incomplete, they say.

However, senior French pilots point out that the correct speed is crucial to any aircraft's capacity to fly, especially in turbulent conditions.

"Speed is a fundamental value for piloting a plane," said Patrick Magisson, a member of the technical committee of the main French pilots' union, SNPL.

"It is speed which makes an aircraft fly. It is speed which determines its aerodynamic balance."

In other words, the wrong speed - either too slow or too fast - can be calamitous.

If false speed readings are proven to be the cause of the Atlantic crash, both Air France and Airbus will have awkward questions to answer.

- INDEPENDENT