The head of the French agency leading the investigation into the crash of Air France Flight 447 said today that signals from the plane before it disappeared showed its autopilot wasn't on.

Paul-Louis Arslanian said it was not clear if the autopilot had been switched off by the pilots or had stopped working because it received conflicting airspeed readings.

Plane manufacturer Airbus said the investigation found the flight received inconsistent readings from different instruments as it struggled in a massive thunderstorm.

Head of the investigation Alain Bouillard said: "We also saw messages that show the automatic pilot wasn't working."

Arslanian says investigators were searching a zone of several hundred square miles for the debris.

It was 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, Mr Arslanian said.

"We have no guarantee that the pinger is attached to the recorders," Arslanian 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 specialising 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 at its deepest point and 2,835 feet at its shallowest.

Water salinity and temperature could affect the distance that the beacon's signal could travel, Kerleguer said.

The Airbus A330 from Rio de Janeiro to Paris disappeared nearly four hours after takeoff on Sunday night, killing all 228 aboard. It was Air France's deadliest plane crash and the world's worst commercial air accident since 2001.