This photo released by Brazil's air force shows an aerial view of an oil slick suspected to be from Air France's Flight 447. (Brazilian Air Force/Associated Press)

Investigators are looking into whether instruments used to calculate appropriate airspeed malfunctioned on an Air France jetliner that disappeared over the Atlantic Ocean.

A malfunction with the sensors that collect data used to calculate airspeed and altitude could have caused the plane to be travelling too fast or slow as it entered turbulence caused by thunderstorms, officials told the Associated Press.

The investigation is reportedly checking whether an external probe that measures air pressure might have iced over. The probe feeds data to onboard computers.

Experts say that jetliners need to be flying at just the right speed when encountering violent weather. If they are flying too fast, they run the risk of breaking apart. Too slow, and they could lose control.

Brazilian search planes have spotted debris from Air France Flight 447 about 650 kilometres northeast of the archipelago of Fernando de Noronha. (CBC)

Meteorologists said the Air France jet entered an unusual storm with160 km/h updrafts that acted as a vacuum, sucking water up from the ocean. The incredibly moist air rushed up to the plane's high altitude, where it quickly froze in -40 C temperatures.

The updrafts also would have created dangerous turbulence.

Meanwhile, Brazil's navy issued a statement Thursday saying that wreckage recovered by a helicopter crew earlier in the day was not from the plane.

The clock is ticking on finding debris before it spreads out, sinks or disappears, said French military spokesman Christophe Prazuck.

"That's the priority now, the next step will be to look for the black boxes," he said.

A U.S. naval plane has also joined the search effort and two deep-water submersibles — believed key to finding the voice and data recorders — are not expected to arrive until next week.

The Pourquoi Pas, a French sea research vessel carrying manned and unmanned submarines, is heading from the Azores and will be in the search zone by June 12, Prazuck said. The equipment includes the Nautile, a mini-sub used to explore the undersea wreckage of the Titanic, according to French marine institute Ifremer.

Carrying 216 passengers

The Air France Airbus A330 was en route from Rio de Janeiro to Paris with 216 passengers and 12 crew on board when it disappeared from radar screens Sunday night as it flew through stormy weather.

A Brazilian air force pilot flies during search operations for debris from Air France's Flight 447. (Johnson Barros, Brazilian Air Force/Associated Press)

Meanwhile, soldiers at Fernando de Noronha's airport — where any recovered human remains would be taken — unloaded body bags and a refrigerator truck from a military plane on Thursday.

No bodies or body parts have yet been reported.

Brazilian searchers on Wednesday had spotted a large oil slick and debris, including a seven-metre chunk of plane and a seat.

Air France officials have informed the passengers' families they should abandon any hope of finding survivors, a grief counsellor working with the relatives said Thursday.

Guilliaume Denoix de Saint-Marc said Air France CEO Pierre-Henri Gourgeon spoke to the families during a private meeting on Wednesday in Paris.

"What is clear is that there was no landing. There's no chance the escape slides came out," said Denoix de Saint-Marc.

Among the passengers were vacationers, newlyweds, a group of business people returning from a trip they had been sent on as a reward for their work, an 11-year-old boy returning to England alone, and Brad Clemes, 49, a Coca-Cola executive who was born and raised in Guelph, Ont., and lived in Belgium.

The debris zone is spread over more than 230 kilometres, located about 640 kilometres northeast of the Fernando de Noronha islands. The ocean floor where the debris has been spotted drops as low as 7,000 metres below sea level.

Automatic messages failed to show

The French Accident Investigation Agency said Thursday that automatic messages received from the plane failed to show how fast it was going. But the agency warned against "hasty interpretation or speculation" about the crash.

Nelson Marinho holds a photo of his son, Nelson Marinho, who was a passenger on Air France Flight 447, in Rio de Janeiro on Wednesday. (Patricia Santos/Associated Press)

Investigators trying to determine what happened are relying on those automated messages sent by the plane as it ran into the storms.

An investigator who spoke to The Associated Press on condition of anonymity detailed a series of failures that ended with the plane's systems shutting down, suggesting it broke apart in the sky.

The pilot sent a manual signal late Sunday saying he was flying through an area of black, electrically charged cumulonimbus clouds that come with violent winds and lightning.

Ten minutes later, automatic messages indicated a series of problems: the autopilot had disengaged, a key computer system switched to alternative power and controls needed to keep the plane stable had been damaged. An alarm sounded indicating the deterioration of flight systems.

The final automatic message, 14 minutes after the pilot's first message, signalled a loss of cabin pressure and complete electrical failure.

A report filed by a pilot of a Spanish airliner flying in the same area as the Air France flight said he saw a bright flash of white light that plunged to the ocean.

"Suddenly, off in the distance, we observed a strong and bright flash of white light that took a downward and vertical trajectory and vanished in six seconds," said the report by the Air Comet pilot.

Patrick Smith, a U.S. airline pilot and aviation analyst, said it appears as if the plane experienced electrical failure.

"What jumps out at me is the reported failure of both the primary and standby instruments," Smith said. "From that point, the plane basically becomes unflyable.

"If they lost control and started spiralling down into a storm cell, the plane would begin disintegrating, the engines and wings would start coming off, the cabin would begin falling apart."

Debris can indicate breakup pattern: expert

John Cox, the former head of safety with the Air Line Pilots Association (ALPA), said a large field of debris indicates the plane could have come apart in flight and that the location of the various parts of wreckage could help show the sequence of the breakup.

"They need to get very sensitive hydrophones [underwater microphones] down and then scour the area around the debris fields," he said. "That will begin to tell them what parts of the airplane came down where."

Cox said the roughly 20-kilometre oil slick spotted on the ocean surface doesn't necessarily indicate the plane broke up when it hit the water, as suggested by Brazilian Defence Minister Nelson Jobim.

"The airplane carries fuel in a variety of places … that may mean only … the centre fuel tanks or one wing fuel tank was there," he said.

He praised French investigators as some of the best in the world, saying he hopes the flight voice and data recorders will be located.

"They’ll localize the area, and then bit by bit I think they'll find at least some of the airplane, and I think they’ll find the recorders," he said.

Cox said investigators will examine all possible theories.

"As an investigator, the worst thing you can do is come up with a theory and fall in love with it in an attempt to prove it. You have to let the evidence lead you where it leads you," he said.

With files from The Associated Press