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.