The French accident investigation agency, BEA, found the doomed plane received inconsistent airspeed readings by different instruments as it struggled in a massive thunderstorm on its flight from Rio de Janeiro to Paris with 228 people aboard.
No debris from the aircraft has been found and without the aircraft's black box recorders, aviation investigators have little information to help them determine what caused the crash.
Airbus had recommended to all its airline customers that they replace speed-measuring instruments known as Pitot tubes on the A330, the model that crashed, said Paul-Louis Arslanian, the head of the agency.
"They hadn't yet been replaced" on the plane that crashed, said Alain Bouillard, head of the French investigation. Air France declined immediate comment.
Arslanian cautioned that it is too early to draw conclusions about the role of Pitot tubes in the crash, saying Airbus had made the recommendation for "a number of reasons."
Investigators are relying on 24 messages the plane sent automatically during the last minutes of the flight to try to locate the wreckage.
The signals show the plane's autopilot was not on, officials said, but it was not clear if the autopilot had been switched off by the pilots or had stopped working because it received conflicting airspeed readings.
In Brazil, visibility and weather conditions improved Saturday in the area searchers are focusing on but debris earlier spotted on the ocean's surface may have sunk by now.
"Debris doesn't indefinitely float, and when it sinks we will not have the means of finding them," Air Force Brig. Gen. Ramon Cardoso told reporters late Friday.
Earlier, Cardoso insisted that the debris spotted — an airplane seat, a slick of kerosene and other pieces — was from the plane. But he confirmed that Brazilian searchers had yet to recovered any of the material.
He said searchers did not pursue the reports of debris — the first sighting was reported on Tuesday — because priority was given to the hunt for survivors or the remains of victims.
Meanwhile, a German government-owned satellite spotted debris in the Atlantic on Wednesday, a German Aerospace Center spokesman said, but he added it was unclear whether the material came from the plane.
BEA chief Arslanian said the crash of Flight 447 does not mean similar plane models are unsafe, he said, adding that he told family members not to worry about flying.
"My sister and her son are going to take an A330 next week," he told a news conference at the agency's headquarters, near Paris.
He says planes can be flown safely "with damaged systems."
The flight disappeared nearly four hours after takeoff, killing all on board. It was Air France's deadliest plane crash and the world's worst commercial air accident since 2001.
The investigation is increasingly focused on whether external instruments may have iced over, confusing speed sensors and leading computers to set the plane's speed too fast or slow — a potentially deadly mistake in severe turbulence.
An Air France memo to its pilots Friday about the crash said the airline is replacing the Pitot tubes on all its medium- and long-haul Airbus jets.
Pitot tubes protrude from the wing or fuselage of a plane and help measure the speed and angle of the flight, along with less vital information like outside air temperature.
They feed airspeed sensors and are heated to prevent icing.
A blocked or malfunctioning Pitot tube could cause an airspeed sensor to work incorrectly and cause the computer controlling the plane to accelerate or decelerate in a potentially dangerous fashion.
On Thursday, European plane maker Airbus sent an advisory to all operators of the A330 reminding them of how to handle the plane in conditions similar to those experienced by Flight 447.
Peter Goelz, a former managing director of the National Transportation Safety Board, said that advisory and the Air France memo about replacing flight-speed instruments "certainly raises questions about whether the Pitot tubes, which are critical to the pilot's understanding of what's going on, were operating effectively."
But questions about speed sensors are only one of many factors investigators are considering. Automatic transmissions from the plane showed a chain of computer system failures that indicate the plane broke apart in midair.
President Barack Obama said at a news conference with French President Nicolas Sarkozy Saturday that the United States had authorized all of the U.S. government's resources to help investigate the crash.
Arslanian said investigators are searching a zone of several hundred square miles (square kilometers) for the debris.
An intensive international effort so far has failed to recover any confirmed wreckage, and concern has grown about whether searchers were even looking in the right place.
It is vital to locate a beacon called a "pinger" that should be attached to the cockpit voice and data recorders, now presumed to be deep in the Atlantic, Arslanian said.
"We have no guarantee that the pinger is attached to the recorders," he said.
Holding up a pinger in the palm of his hand, he said: "This is what we are looking for in the middle of the Atlantic Ocean."
Investigators are trying to determine the location of the debris in the ocean based on the height and speed of the plane at the time the last message was received. Currents could also have scattered debris far along the ocean floor, he said.
"You see the complexity of the problem," he said.
Laurent Kerleguer, an engineer specialized in the ocean floor working with the investigation team, said the zone seen as the most likely site of the debris was 15,112 feet (4,606 meters) at its deepest point and 2,835 feet (864 meters) at its shallowest.
France is sending a submarine to the area to try to detect signals from the black boxes, said military spokesman Christophe Prazuck. The Emeraude will arrive next week, he said.
___ AP Writers Patrick McGroarty in Berlin and Bradley Brooks in Rio de Janeiro contributed to this report.