This undated picture taken at Houston's George Bush Intercontinental Airport shows the Air France Airbus 330-200 that crashed in the Atlantic. (Airteamimages.com/Associated Press)

The pilot of a doomed Air France flight was flying with neither the help of the autopilot nor information about the plane's speed and direction when it slammed into the Atlantic Ocean a month ago, French officials said Thursday.

Debris from Air France Flight 447 was found about 640 kilometres northeast of the archipelago of Fernando de Noronha. (CBC)

Officials with the French air accident investigation agency, BEA, presented their preliminary report into the crash of Air France Flight 447 at a press conference in Le Bourget, outside of Paris.

The Airbus 330 crashed while en route to Paris from Rio de Janeiro as it flew through stormy weather the night of May 31. There were no survivors. Of the 228 people on board, 51 bodies have been recovered amid the debris 640 kilometres northeast of Brazil's Fernando de Noronha islands.

Lead investigator Alain Bouillard said the plane is not thought to have broken up in the air but plunged steeply into the ocean at an extremely high speed, because no large pieces of the aircraft have yet been found.

The aircraft "was not destroyed in flight," he said. "The plane seems to have hit the surface of the water on its flight trajectory with a strong vertical acceleration."

'Incoherent messages'

The initial findings BEA presented were based on a series of 24 "incoherent messages" that were sent out by the plane just before contact was lost, Bouillard said.

A Brazilian navy diver checks a piece of debris from Air France Flight 447 in the Atlantic Ocean in June. (Brazilian navy/Reuters)

Among the messages are indications the plane's autopilot had failed, that the aircraft was no longer properly reporting wind function and the cockpit was no longer receiving proper information about the plane's direction or speed, officials said.

The failed autopilot controls would have left the pilot in control of the airplane as it flew through the stormy weather with failing systems, officials said.

"The messages tell us … our plane would have been directed by the pilot because the autopilot was no longer working," said Bouillard.

There were also indications that airspeed sensors called pitot tubes had malfunctioned.

Experts had suggested following the crash that the pitot tubes may have iced over, destabilizing the plane's control systems. Air France has since replaced the monitors on all its Airbus A330 and A340 aircraft.

But Bouillard said the pitot tubes are not believed be the sole factor of the crash. "It is an element but not the cause," he said.

Far from establishing cause

The investigation is proving to be one of the most challenging in aviation history, he said. "Today we are very far from establishing the causes of the accident."

The flight's voice and data recorders have not been found, and without recovering large pieces of the plane it is a challenge to conduct forensic tests.

Among the small pieces of debris that have been retrieved, there is no indication of fire or explosion, he said.

The search for the black boxes will continue until July 10, though the 30-day window for homing signal has passed.

The search for bodies has been called off.

Investigators said Thursday that life-vests found amid the wreckage were not inflated, further indicating a rapid catastrophe for the flight.

Families of the victims of Flight 447 had the opportunity to be briefed by BEA and Air France officials immediately prior to the press conference.

"The families are hoping to have all the facts, above all to be able to avoid this eventually happening again," said Christophe Guillot-Noel, head of an association for victims of the flight.

With files from The Associated Press