The ill-fated Air France flight 447, which crashed into the Atlantic ocean on June 1, 2009, en route from Rio De Janeiro to Paris, killing all 228 people onboard, had plunged at nearly 11,000 feet per minute during the final few minutes even as its pilots struggled to regain control following failure of speed sensors, French authorities revealed, Friday.
The 2 black boxes or data recorders recovered from the crash site in early May have given the investigators an insight to the dying moments of flight 447.
France's Bureau of Investigation and Analysis (BEA) said, Friday, that the aircraft ran into severe thunderstorm and the engine stalled, prompting the pilots to respond by pulling the nose up.
The aircraft climbed to 38,000 ft when the stall warning was triggered and the airplane stalled, the BEA report said.
The report said speed sensors also failed as speed displayed on the left primary flight display was inconsistent with those on the integrated standby instrument system (ISIS).
The aircraft rolled in air before stalling. The stall warning system sounded thrice.
The two co-pilots and the captain attempted to pull up the aircraft but failed. Flight 447 began its rapid, nightmarish descent into the ocean at 10,912 feet per minute.
The pilots also lost contact with air traffic controller due to heavy turbulence.
After 3 minutes and 30 seconds, the aircraft crashed into the ocean, killing all 228 people onboard.
The black boxes also enabled the investigators to listen to the communications that went on inside the aircraft.
The investigators said that four hours and six minutes into the flight, one co-pilot, referred to as PF, told the cabin crew to expect heavy turbulence. In two minutes we should enter an area where it'll move about a bit more than at the moment, you should watch out... I'll call you back as soon as we're out of it, the co-pilot said. Turbulence, brought on by icy, windy conditions, was typical in that flight route.
Four minutes later, the autopilot and auto-thrust disengaged and the co-pilot said I have the controls. Around that time the stall warning sounded twice in a row.
The second co-pilot, referred to as PNF, said So, we've lost the speeds. A second later, the stall warnings sounded again.
At about that time, the speed displayed on the left side increased sharply. The aircraft was flying at an altitude of about 37,500 feet.
Sensing trouble, the co-pilot PNF called Capt. Marc Dubois, who was on a routine rest break, back to the cockpit.
At around 4 hours 11 minutes and 40 seconds into the flight, the captain re-entered the cockpit. The aircraft was then flying at an altitude of 38,000 feet.
Soon thereafter, both the co-pilots said they couldn't receive any speed indications. The stall warning also stopped.
About a minute later, co-pilot PF said the aircraft is going to arrive at level one hundred (10,000 feet). It meant the rapid descent had begun.
The last recorded measurement shows the plane plummeting at 10,912 feet per minute.
All recordings stopped at 4 hours 14 minutes and 28 seconds into the flight.
Investigators suspect the speed sensors, known as pitot tubes or probes, malfunctioned because of ice formation at high altitude. Failure of pitots led to a series of automation failures, disconnects, warnings and alarms for Flight 447.
Since the accident, Air France has replaced the pitots on its Airbus fleet with a newer model.
The report did not say whether the pitot tubes or the temporary absence of Dubois from the cockpit had any impact on the quickly unfolding events. Though Dubois returned to the cockpit, it seems he did not take back the controls.
Aviation experts say Air France pilots were trained to get out of a stall but adverse weather conditions, darkness, and a multitude of automated alarm signals may have overwhelmed the flight crew.
Only after long and detailed investigative work will the causes of the accident be determined and safety recommendations issued, this being the main mission of the BEA. The latter will be included in the final report, BEA said.
A full report on the incident is not expected before 2012.