PARIS -- Air France has finished replacing air speed monitors on all its long-haul Airbus aircraft even though the cause of the Flight 447 disaster remains a mystery, a pilots' union official said Monday.

The search for the A330's black boxes was reinforced Monday with a high-tech U.S. Navy device that began listening for pings in the Atlantic Ocean.

With the flight recorders still missing, the probe into the disaster has focused on the possibility external speed monitors iced over and gave false readings to the plane's computers.

Air France had begun replacing the sensors on its A330 and A340 jets before the accident, but had not yet changed them on the plane that was lost.

After pilot complaints, the airline pledged to speed up the switch and it has now equipped all planes with the new sensors, said Erick Derivry, a spokesman for the SNPL pilots' union, though he stressed that there is no hard evidence that Pitot problems caused the accident.

The first of two U.S. Navy Towed Pinger Locators was put to work on Monday, pulled slowly in a grid pattern by a Dutch ship contracted by the French government.

The second locator was expected to start operating within hours across the 2,000 square mile (5,180 square kilometer) search area, said U.S. Air Force Col. Willie Berges, commander of the American military forces supporting the search.

A French nuclear submarine is also being used to look for signs of the black boxes.

The pings emitted by the black boxes begin to fade after 30 days. The plane went down on May 31 while flying to Paris from Rio de Janeiro.

"We have a limited amount of time to cover the search area," Berges said, but added that the hunt will go on even if no pings are detected beyond the 30-day time frame.

Experts say the evidence uncovered so far points to at least a partial midair breakup of the plane, with no signs of an explosion or terrorist act.

At the Paris Air Show, Airbus CEO Tom Enders defended the A330s, saying it has "more than 16 million flight hours, more than 3 million flights, and this is so far one of the safest commercial aircraft built."

In Geneva, Brazil's President Luiz Inacio Lula da Silva said French President Nicolas Sarkozy assured him the French government would compensate the families of crash victims. The two leaders were attending the International Labor Conference.

"He (Sarkozy) said that they will take the responsibility to pay the indemnity for all the families that suffered with the crash, the Brazilian and French families and of other countries," Silva told reporters, speaking through a translator.

An Air France spokesman, however, said the airline and its insurance company -- not the government -- are handling compensation. The official, who declined to name the insurance company, spoke on condition of anonymity as he wasn't authorized to discuss the matter.

The French presidential palace in Paris, the French mission in Geneva and a spokesman for Silva in Brasilia declined to comment.

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Bradley Brooks reported from Rio de Janeiro. Associated Press writers Greg Keller and Emma Vandore in Le Bourget, France, Alan Clendenning in Sao Paulo, and Frank Jordans in Geneva contributed to this report.