Air France is replacing instruments that help measure airspeed on all its medium and long-haul Airbus jets, according to a memo sent to pilots following the disappearance of one of the airline's jets over the Atlantic with 228 people on board.

Investigators have been focusing on incorrect speed readings as a potential reason for why the Airbus A330 went down on Sunday en route to Paris from Rio de Janeiro.

While the airline refused to comment on the memo, saying it was for pilots only, the Associated Press reported that Air France has been replacing instruments known as Pitot tubes.

French search teams scouring the Atlantic have so far found no wreckage and are urging "extreme prudence" about possible plane wreckage retrieved so far, France's transport minister said yesterday.

Dominique Bussereau said an earlier announcement by Brazilian officials that they had recovered debris from AF447 had turned out to be false, and that the priority was finding the "black box" flight recorders.

A French nuclear submarine has been sent to area to help locate the recorders, but hopes that bodies may be found are fading amid worsening weather conditions.

Airbus has sent an advisory note to all airlines using the A330 reminding them of how to handle the plane in extreme weather conditions similar to those thought to have been experienced by flight AF447.

Airbus said this did not imply that the doomed pilots did anything wrong or that a design fault was in any way responsible for the crash.

Air France's chief executive, Pierre-Henri Gourgeon, told family members of the missing passengers at a private meeting yesterday that the plane had disintegrated, either in the air or when it crashed into the ocean.

With the crucial flight recorders still missing, investigators were relying heavily on the plane's automated messages to help reconstruct what happened.

One theory is that outside probes that feed speed sensors may have iced over, giving incorrect information and causing the autopilot to direct the aircraft to fly too fast or too slow when it met turbulence.

The last message from the pilot was a manual signal at 11pm local time, in which he said he was flying through an area of black, electrically charged clouds with violent winds and lightning.

At 11.10pm, a series of problems began: the autopilot disengaged, a key computer system switched to alternative power, and controls needed to keep the plane stable were damaged.

The French defence minister, Hervé Morin, and the Pentagon have said there were no signs that terrorism was involved, but Morin yesterday declined to rule out the possibility.