The head of the French agency leading the investigation into the crash of Air France Flight 447, that killed 228 people, including Bristol pupil 11-year-old Alexander Bjoroy, 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, including Alexander Bjoroy, an 11-year-old pupil from Clifton College preparatory school in Bristol. It was Air France's deadliest plane crash and the world's worst commercial air accident since 2001.

At a briefing in Paris, the investigators said the Airbus A330's communication system transmitted 24 error messages ahead of the flight's disappearance.

Fourteen of those messages were sent within the space of one minute, from 3.10am BST to 3.11am BST.

The messages showed "inconsistencies" between measured velocities and indications of systems failures including the autothrust and autopilot, the investigators said.

But Mr Arslanian warned the error signals were "not designed for investigations" and only gave an indication as to the status of particular systems.

David Learmount, operations and safety editor of Flight International magazine, said: "The fact that the autopilot was not working is not the cause of this accident."

He said it was "not at all unusual" for the autopilot to cut out and added "it's not the cause, it's the symptom" of this accident.

"It's designed to cut out at any time that it receives conflicting messages, which we know it was," he said.

"It's designed to cut out rather than controlling the airplane in an unsafe way – it's not an intelligent system, it doesn't know which of the data are correct."

He said the pilots would have been alerted with a loud warning system when the autopilot disengaged.