Air France Flight 447’s pilots were probably bewildered by erratic instrument readings and may have done the opposite of what was needed to keep the jet from crashing into the Atlantic in 2009, safety specialists said.
One pilot responded to stall alarms by angling the Airbus SAS A330’s nose higher, according to a preliminary report from the French air-accident investigation office. Pilots are trained to avert stalls, which occur when an aircraft slows enough that its wings lose lift, by dropping the nose to increase speed.
Earlier transmissions from the jet had shown that airspeed sensors had failed, presenting pilots with a sharp drop in speed readings after the flight entered ice clouds. Instruments showed the speed tumbled from 275 knots to 60 knots, the investigation office, the Bureau d’Enquetes et d’Analyses, said yesterday. The stall warning sounded three times.
“I don’t think there’s any question the reaction wasn’t what we would like it to be, what it should be -- you keep the nose down,” said John Cox, a former pilot now with Safety Operating Systems LLC in Washington.
The preliminary report showed pilots scrambling to prevent disaster in the three and a half minutes the jet fell toward the ocean at a speed of 180 feet (55 meters) a second. The least-experienced of the three pilots was flying the aircraft until less than a minute before recordings stopped, with the captain present though not at the controls. The crash killed all 228 people aboard.
‘All Over Map’
In repeatedly raising the nose, the pilots may have thought they were reacting properly because the speed readings were “all over the map” and turbulence from a storm might have tricked them into feeling they were going too fast, said Richard Healing, a former U.S. National Transportation Safety Board member who is now a consultant in Alexandria, Virginia.
“We don’t have the full picture of what their flight displays showed them and the full transcript of their conversations,” Healing said. “All we know is that the information wasn’t reliable, and that a lot of warnings were going off and it was probably very, very confusing.”
Because the accident happened at night during foul weather and in the middle of the ocean, the pilots didn’t have the benefit of seeing a horizon or using landmarks to help gauge their speed, orientation and direction the nose was pointing, Healing said.
‘What Way Is Up’
“It’s the only thing that would make any kind of sense -- that they’d gotten spatially disoriented, they don’t know what way is up and they don’t fully understand what the airplane is doing,” said Bill Waldock, a professor at Embry-Riddle Aeronautical University who has taught accident investigation for 25 years.
The plane’s two flight recorders were recovered from 3,900 meters (12,800 feet) beneath the Atlantic and returned to Paris this month, two years after the jet disappeared on June 1, 2009.
Yesterday’s report doesn’t indicate whether the pilots realized they were in a stall, or if Captain Marc Dubois, 58, ever attempted to regain control of the plane. Dubois was away from the cockpit when the autopilot disengaged four hours into the flight.
Dubois had logged a total of 11,000 flying hours in his airline career, compared with 6,500 hours by the first co-pilot and 2,900 by the second.
The data and cockpit voice recording suggest the pilots never realized that the plane had stalled, BEA Chief Investigator Alain Bouillard said in an interview.
“They hear the stall alarm but show no signs of having recognized it,” he said. “At no point is the word ‘stall’ ever mentioned.”
Even as the plane plunged from 38,000 feet to 10,000 feet, “they make this observation without seeming to understand that they are in a stall,” Bouillard said.
The BEA’s report gave no indication about the tone in the cockpit, with only a few reference to exchanges about who was in charge or the co-pilot saying early on “So, we’ve lost the speeds.” The third member of the crew was given control in the last minute before impact.
No comments by pilots in the last minute were published by the BEA. The office said that the plane’s so-called angle of attack, which defines the angle between air flow and the longitudinal axis of the aircraft, always remained above 35 degrees during the descent.
“What happened is pretty straightforward,” Waldock said. “It’s just a question of why.”
The report concluded that the aircraft remained stalled during its descent, and that the engines were operational and responded to crew commands throughout.
On June 4, 2009, Airbus told airlines to remind pilots how to respond to inconsistent speed readings and two months later recommended A330 and A340 operators switch from Thales SA pitot tubes to Goodrich Corp. versions. Records in Europe and the U.S. document dozens of incidents where the probes failed and pilots retained control. Airbus said last week that it had no additional recommendations to operators of the A330 aircraft.
The preliminary findings from the black-box data haven’t established any conclusions about the accident’s causes or led to any recommendations. An interim report is due in mid-July.