A French navy plane joins the search for debris from Air France 447
The Air France jet which went missing over the Atlantic sent 24 error messages minutes before it crashed, French investigators say.
Investigators also said the plane's autopilot was not on, though they do not know if it had been switched off or was not working.
Weather experts said there was no evidence storms the plane encountered were "exceptional" for the season.
The Airbus A330 vanished en-route from Rio de Janeiro to Paris on Monday.
Officials do not know what triggered the plane's problems, but it was flying through an area of thunder storms and turbulence.
Speaking at a press conference in Paris, the director of France's air accident investigation agency, Paul-Louis Arslanian, said a total of 24 error messages were received in the final moments of Air France 447, as its systems shut down one by one.
But he said it was impossible to tell from the plane's signals why the autopilot was not on.
Faulty speed meters
Mr Arslanian confirmed that the missing jet had had a problem calculating its speed, adding that it was a recurring problem on the A330s and that Airbus was undertaking a replacement programme.
SEARCH FOR FLIGHT AF 447
1 June: Contact lost with plane over mid-Atlantic
2 June: First debris spotted from the air includes an airline seat. Brazilian defence minister says debris is from missing plane
3 June: More debris spotted, including a 7m-wide chunk of metal. Fuel slick seen on ocean surface
4 June: Buoys and pallet recovered from ocean said to be from plane. Officials later retract statement
"We have seen a certain number of these types of faults on the A330," Mr Arslanian said. "There is a programme of replacement, of improvement."
But he insisted the planes were safe in the meantime.
The deputy head of the French weather service, Alain Ratier, said the weather pattern was normal at the time that Flight 447 disappeared.
"According to the analysis of the infrared images, there is nothing to suggest that there was a cluster of thunderstorms of exceptional intensity," Mr Ratier said.
"Certainly there was powerful cumulo-nimbus [storm clouds], but these are found frequently in this area and in normal climatic conditions," he added.
Hopes of locating the plane have been frustrated so far.
Brazilian search teams first said on Tuesday they may have spotted debris from the plane. But material recovered from the sea on Thursday turned out to be unrelated to the Air France jet.
Efforts are now focusing on the two sonar beacons - or "pingers" - attached to the flight's data recorders boxes, the BBC's Hugh Schofield reports from Paris.
But at the news conference, a spokesman for France's accident investigation bureau said there was no guarantee the beacons were still attached to the flight recorders.
He said, given the likely force of the impact of a crash, they could easily have become separated.
On Friday, French Defence Minister Herve Morin said a French submarine was being sent to join in the search since it had sonar equipment that could help locate the airliner's flight data recorders.
The US is also sending specialised listening equipment.
Without the data recorders, investigators are unlikely to determine the cause of the crash, our correspondent says.