Sorry for posting this so late. I just found out about it today via a news alert.
From Bloomberg News:
The Air France Flight 447 crash inquiry is reviewing pilot instructions issued by Airbus SAS for dealing with instrument failures of the kind implicated in the accident, according to the lead investigator.
France’s BEA air-accident investigation bureau is examining the directive to climb in response to the loss of airspeed data, Alain Bouillard said in an interview. Air France said it has restricted use of the procedure in thin air at high altitudes on concern that it may increase the risk of a mid-air stall.
The emergency maneuver “can lead to a reduction in speed” when carried out at cruising level, Air France safety chief Etienne Lichtenberger said in an interview. “The risk of a low- speed stall is significant at high altitude, so it’s not a good idea to reduce speed.” Airbus said it stands by the guidance.
The switch leaves Air France at odds with the drill still applied by other airlines. In its preliminary findings, the BEA blamed erroneous airspeed data for system failures logged by automated transmissions from the A330 airliner en route to Paris from Rio de Janeiro, minutes before it plunged into the mid- Atlantic on June 1, 2009, killing all 228 people on board.
The Airbus maneuver instructs pilots to climb at a five- degree pitch attitude — the aircraft’s angle above horizontal — when airspeed readings become unreliable anywhere above 10,000 feet (3,048 meters). Only later in the procedure are they told to check whether it’s safe to level off.
‘Hard to Fathom’
When cruising at or above 35,000 feet, Flight 447’s last known altitude, pulling up the nose and climbing is an inappropriate response to speed-sensor failures, according to pilots and independent experts.
“It’s hard to fathom why they would suggest that,” said Hans Weber, president of Tecop International Inc., an aviation consulting firm based in San Diego, who has given safety advise to the U.S. Federal Aviation Administration and companies including Airbus parent European Aeronautic, Defence & Space Co.
“If you’re at high altitude and you carry on climbing at five degrees for too long you will lose control of the aircraft,” Weber, a physicist by training, said in an interview. “It’s what pilots call the coffin corner — you’re quickly running out of lift in the thinner air.”
The BEA is “looking at the pertinence of these procedures” and may suggest a review even if the plane’s flight recorders, which might indicate what caused the crash, aren’t found, Bouillard said in the June 18 telephone interview. “It’s one line of inquiry — but it’s still too early to say whether anything needs to be improved or changed.”
‘Piloting Sense’
Cedric Maniez, a pilot who flies A330s for Air France, said knowing when to follow the Airbus drill was a “matter of good piloting sense.”
When airspeed data is lost at high altitude, “you don’t touch the pitch attitude, you just try to keep it level with constant thrust,” Maniez said. “Unfortunately there’s no way of knowing what happened aboard Flight 447 or to what extent the unsuitability of this emergency maneuver might have played a role.
Three search operations have failed to recover the black- box flight recorders.
Jeremie Teahan, a spokesman for the European Aviation Safety Agency, which certified the maneuver and reviewed it again after the crash, said the authority has “not found any issues with the Airbus procedures for the time being.”
Airbus spokesman Stefan Schaffrath said in an e-mail that “strict adherence to these approved procedures remains the best way to manage unreliable airspeed situations.”
New Instructions
Four days after the crash, Air France gave its pilots new instructions that contradict the Airbus procedure for coping with airspeed-data loss.When the problem occurs at safe cruising altitude, pilots should “maintain the same pitch attitude and engine thrust,” according to the June 5, 2009, memo signed by Lichtenburger and three other executives. Crews should then troubleshoot “without carrying out the emergency maneuver.”
Most pilots realize that there is no need to climb when already at cruising altitude, Lichtenberger said. Air France issued the memo because it nonetheless “felt there was a risk that pilots might follow the Airbus procedure to the letter.”
Air France also raised the issue with Airbus and EASA officials after its own tests showed that maintaining the five- degree configuration could slow an A330 from 270 knots to 230 knots in about two minutes, Lichtenberger said. “That means you’re getting closer to stalling speed.”
Stall Warnings
With an estimated mass of 205 metric tons at the time of the crash, the Airbus A330 would have had a stalling speed of about 170 knots, data from the manufacturer show.
According to the final radio transmissions, the failure of Flight 447’s airspeed sensors, or Pitot tubes, caused the autopilot to shut down about four minutes before a rapid loss of altitude, recorded in the final message. Debris analysis and post-mortems of the 50 bodies show that the plane hit the water belly-first in a near-vertical plunge, investigators say.
The BEA has documented 13 other cases of high-altitude airspeed-data loss, of which nine resulted in stall warnings. Some of the crews — all of which managed to overcome the problem — had begun and then abandoned the emergency climb maneuver when the alarm sounded, Bouillard said.
The BEA has also called for further study of atmospheric ice crystals that may be capable of disabling Pitot tubes for longer periods and at higher altitudes than previously thought possible.
“When Airbus wrote the instructions they were probably of a mind that the emergency would occur well below cruising altitude,” Tecop’s Weber said. “There tends to be an assumption that the chances of encountering real atmospheric problems are very much reduced at high altitudes.”
