The pilots of Air France flight AF447 from Rio to Paris lost valid speed data before the Airbus A330 stalled and began a three minute plunge into the Atlantic in June 2009 with 228 people aboard, France's BEA aviation safety agency said on Friday.
"We have no valid indications," one pilot said according to information from the recovered flight data recorders released by the BAE.
One of the instruments showed "a sharp fall" in airspeed as the aircraft was passing through a zone of turbulence and the autopilot disengaged and the engines stalled.
The captain, who had left the cockpit to take a rest, returned but did not retake control of the plane.
"There was an inconsistency between the speeds displayed on the left side and the integrated standby instrument system (ISIS). This lasted for less than one minute," the BEA said in a statement following analysis of the recovered flight data recorders.
Interim inquiry results, prior to the recovery of the "black box" data recorders earlier this month, had pointed to a problem with the jet's air speed probes -- known as Pitots -- which are thought to have iced up, but there was no definitive conclusion.
Since the accident, Air France has replaced the Pitots on its Airbus fleet with a newer model.
Pilot's unions and some of the victims' families have accused the airline of reacting too slowly to safety warnings, but both Airbus and Air France insist they reacted properly.
Both companies are both being probed for alleged manslaughter in connection with the crash.
Air France called for waiting for the BAE's further analysis of the data to determine a cause of the crash, although it said it appears "that the initial problem was the failure of the speed probes".
It praised the "three skilled pilots" who "demonstrated a totally professional attitude and were committed to carrying out their task to the very end and Air France wishes to pay tribute to them.
According to the chronology of the flight provided by the BEA, the two co-pilots who were flying the plane decided at two hours and eight minutes into the flight to turn slightly to the left to avoid a zone of turbulence.
Two minutes later the autopilot disengaged, the instruments began showing "a sharp fall" in airspeed and the engine stall warning began to sound.
Low airspeed data can cause the airplane's system to reject as invalid other readings, according to the BAE.
"So, we've lost the speeds," it quoted the second of the two co-pilot as saying.
After the captain returned "all of the recorded speeds became invalid and the stall warning stopped," said the BAE report.
The responses by the co-pilot flying the aircraft "were mainly nose-up" and "the airplane climbed to 38,000 feet."
The BAE said "the descent lasted three minutes 30 (seconds), during which the airplane remained stalled."
It said "the engines were operating and always responded to crew commands."
The last data on the recorder showed that the plane's nose was up at a sharp angle as it plunged at 10,912 feet (3,300 metres) per minute.
Furthermore, it judged that the "weight and balance of the airplane were within the operational limits."
The captain had left the cockpit less than 10 minutes before the plane encountered trouble, and had discussed with the co-pilots that they were flying into an area of increased turbulence.
Once the trouble began the co-pilots tried several times to contact the captain before he returned to the cockpit roughly one minute and thirty seconds after the engines stalled.
The captain did not retake the controls, with the co-pilots flying the plane until the end, the investigators said during a conference call.
The BAE report said "the composition of the crew was in accordance with the operator's procedures."
The captain would normally take a rest during the flight and leave the two co-pilots at the controls.