The notice — reminding air crews worldwide what to do when speed indicators give conflicting read-outs — was sent to pilots of all Airbus airliners and not just of the A330, the model that crashed Monday, a spokesman said.

The alert came as French air safety investigators said automatic messages broadcast by the Air France jet just before it plunged into the Atlantic on Monday had shown the plane's systems were giving different readings.

"Airbus overnight sent a reminder to all the companies using its planes on the procedures to follow in the case of inconsistency in speeds measured," a spokesman for the French-based manufacturer told AFP.

With AF 447's black box flight recorders still missing, investigators are focusing on signals sent before the jet went down as it flew through a storm en route from Rio de Janeiro to Paris.

The plane has several devices that measure speed but the data sent by them differed, said a spokesperson from the Office of Inquiries and Analysis (BEA).

A statement from BEA confirmed the plane had run into "significant convective cells typical of the equatorial regions" — patches of fierce weather with strong updraughts and downdraughts.

Similar to Qantas nosedive?

In October 2008 a Qantas Airbus 330 went into a brief nosedive that injured passengers in a terrifying incident that was blamed on automatic systems overriding each other and radically altering the aircraft's course.

Airbus has said the systems that caused the malfunction on the Qantas jet were not the same models as those on flight AF 447 and therefore no comparison should be made.

French Defence Minister Herve Morin told reporters in Paris he had not ruled out an attack on the plane, although he had not heard of any threats or claims of responsibility being made.

"I've never ruled out terrorism," he said. "There's no element or evidence trail that would allow us to corroborate that, but the inquiry that is under way has never ruled that out."

"Time is against us"

As crash investigators continued to analyse data, salvage crews raced to find the wreckage and bodies of passengers in the Atlantic.

"Time is against us," admitted French transport minister Dominique Bussereau. "We must do everything we can to find the flight recorders and certainly enlarge the search zone."

Brazilian officials on Thursday raised hopes the plane had been found when they said a palette and two buoys plucked from the Atlantic by navy salvage crews were pieces of the plane.

Hours later they backtracked, saying the items were simply sea trash, probably from a ship, as was an oil patch first said to be a fuel slick from the doomed French jet.

Several Brazilian navy vessels, and French and Brazilian planes, were hunting for wreckage, including a seat and a big chunk of what appeared to be fuselage, sighted by air force jets.

The French government has sent investigators to Brazil to inspect any debris recovered from the zone, around 1000 kilometres off-shore, and take them back to France.

Speculation over what caused the accident has ranged from a massive, lightning-packed storm in the area at the time, to turbulence, to pilot error or a combination of factors.

No mayday call was received, just a series of automatic data transmissions signalling the plane's systems were shutting down one by one, after which it presumably broke up or went into a fatal dive.

Crash victims remembered

Memorial services were held Wednesday in Paris and Thursday in Rio for those on board the plane, which included 72 French and 58 Brazilians. No bodies have yet been spotted at sea.

Many relatives of the passengers attended, but others declined, refusing to give up hope that somehow, despite the evidence, their loved ones had survived.

Brazil's air force has invited Brazilian relatives to its centre of operations in the northeastern city of Recife to observe developments.

Some are to go to Fernando de Noronha, a Brazilian archipelago 400 kilometres into the Atlantic that serves as a base for the search and initial collection point for any debris or bodies that might be recovered.

Recife, on the Brazilian mainland, has prepared a morgue and debris inspection area for anything found.

Officials have said the air force may organize an overflight of the search zone for the families.

AFP