(Reuters) - A French airliner plunged out of control for four minutes before crashing into the Atlantic in 2009, investigators said, in a report raising questions about how crew handled a "stall alarm" blaring out in the cabin.
Information gleaned from black boxes, and recovered almost two years after the disaster killed 228 people, confirmed that speed readings in the Airbus cockpit had gone haywire, believed to be linked to the icing of speed sensors outside the jet.
As Air France pilots fought for control, the doomed A330 dropped 38,000 feet, rolling left to right, its engines flat out but its wings unable to grab enough air to keep flying.
The plane crashed on June 1, 2009, en route from Rio de Janeiro to Paris
Aviation industry sources told Reuters that this action went against the normal procedures which call for the nose to be lowered in response to an alert that the plane was about to lose lift or, in technical parlance, 'stall'.
This type of aerodynamic stall is nothing to do with a stall in the engines, both of which kept working as crew requested.
"A stall is the moment at which a plane stops flying and starts falling," said David Learmount, operations and safety editor at the British aviation publication Flight International.
A top aircraft industry safety consultant said the standard guidance in the Airbus pilot manual called in this event for the pilot to lower the nose by pushing the control stick forward.
"The BEA is now going to have to analyze and get to bottom of how crew handled this event," said Paul Hayes, safety director at Ascend Aviation, a UK-based aviation consultancy.
"The big question in my mind is why did the pilot flying (the aircraft) appear to continue to pull the nose up," he said.
French investigators said the emergency began with the autopilot disengaging itself two and a half hours into the flight and the junior pilot, who had been in control at take-off, picked up manually and saying "I have control."
The autopilot appears to have responded to a loss of reliable airspeed information. This was accompanied moments later by the disembodied voice of a recorded "stall" alert.
It is what happened next that is likely to fuel most theories on what preceded the crash, but Air France and its main pilots union insisted faulty speed probes were the root cause.
In a passage likely to attract particular scrutiny, the BEA said the pilot "maintained" the nose-up command despite fresh stall warnings 46 seconds into the four-minute emergency.
"The inputs made by the pilot flying were mainly nose-up," the report added.
The Airbus jet climbed 3,000 feet to 38,000 feet despite the crew having decided earlier against a climb, and then began a dramatic descent, with the youngest pilot handing control to the second most senior pilot a minute before impact.
The captain returned after "several attempts" to call him back to the cockpit but was not at the controls in the final moments, according to information gleaned from black boxes.
By the time the 58-year-old returned, just over a minute into the emergency, the aircraft was in serious trouble: plunging at 10,000 feet a minute with its nose pointing up 15 degrees and at too high an angle to the air to recapture lift.
The BEA did not provide extracts of the transcript for the last minute before the jet hit the water with its nose up.
It promised a fuller interim report which could say more about the causes of the crash in July.
SLIGHT TURBULENCE AHEAD
Relatives of victims had waited long for the report.
"It's very emotional to see the unrolling minute by minute or second by second at some points of what happened," said John Clemes, vice president of the families' support group.
"You automatically think of your family member and how they were living through this. It's the events that caused the deaths of 228 people so it's traumatic and moving."
The BEA report put to rest speculation that the pilots recklessly flew into the center of an equatorial storm cell.
Pilots had decided calmly to alter course slightly to avoid turbulence shortly before the crisis. But the pilot did tell flight attendants to prepare for a "little bit of turbulence."
"In two minutes we should enter an area where it'll move about more than at the moment; you should watch out," he told cabin staff. "I'll call you back as soon as we're out of it."
Air France
The crew's response to stall warnings contrasts with advice to pilots contained in an Airbus training seminar in October last year, according to a document obtained by Reuters.
In large red capital letters, the slide presentation says that in the event of a stall warning, pilots should "APPLY NOSE DOWN PITCH CONTROL TO REDUCE AOA (ANGLE OF ATTACK)."
Two aviation industry sources said the drill in force at the time of the accident was to apply full thrust and reduce the pitch attitude of the aircraft, which means lowering the nose.
Later guidance calls for pilots to push the nose down and adjust thrust as necessary, they said, asking not to be named.
Despite the apparent anomaly, aviation experts said it was early and most probably far-fetched to blame the miscommands -- so basic one compared it to hitting the accelerator instead of the brake when facing a car collision -- on a conscious error.
"One of the weird things about this is that the aircraft was definitely stalled, because the crew had had a stall warning, but they were not doing anything to recover from the stall," Learmount said. "It was almost as if they didn't know the aircraft was stalled, because they could have recovered."
The report and a more detailed follow-up are eagerly awaited by lawyers representing victims' families, but cannot be used in many courts. A separate French criminal probe is also under way.
(Editing by Ralph Boulton)
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