Paris: French pilots on Wednesday suspended co-operation with an inquiry into the 2009 crash of an Air France airliner as a dispute over the causes of the disaster opened deep wounds in France`s aeronautics industry.
The SNPL airline pilots union declared the boycott after it emerged that crash investigators had removed a recommendation about one of the Airbus A330`s systems from an interim report last week, focusing instead on possible pilot error.
The BEA air crash investigation agency said its final report may make a recommendation on the A330 stall alarms but said it also needed to analyse why pilots had ignored the crucial alert for almost a minute without making the "appropriate" responses.
The ongoing inquiry into what caused flight AF447 from Rio to Paris to slam into the Atlantic Ocean, killing all 228 people on board, has pitched France`s flagship carrier and its pilots against planemaker Airbus and crash investigators.
The outcome may have legal implications for dozens of potential compensation claims on both sides of the Atlantic.
The BEA said last week that crew had failed to respond to repeated stall warnings and listed a series of actions that experts said went against the handbook for dealing with a sudden loss of lift.
Its 10 recommendations included better training for pilots to fly aircraft manually, particularly at high altitudes.
On Wednesday, the BEA confirmed French press reports that it had removed a recommendation on the stall alarm from a draft report because more work needed to be done on the subject.
In a statement, it said this would be handled in tandem with a study of human behaviour under stress which is expected to dominate the next phase of its two-year inquiry, with a final report embracing both topics due to be issued later this year.
Pilots however said the agency should reveal its thoughts on the design of Airbus cockpit systems sooner.
"Why ignore in the official report the recommendation on the stall alarm? Were other significant modifications made to the report?" the SNPL said in a statement, noting that BEA`s reputation had been "seriously shaken."
An association for families of crash victims said the recommendation`s removal had undermined the credibility of the investigation and called for its immediate publication.
Stall warning trap?
The BEA says it is too early to say why the A330 crashed on June 01, 2009, but they have more or less pieced together how.
The autopilot switched off after the aircraft`s speed sensors became blocked with ice at high altitude and "fly by wire" features -- designed to keep the plane within safe limits even when being hand-flown -- were also disengaged.
Under manual control, the A330 rose and then appeared to enter a stall, plunging from 38,000 feet with alarms sounding.
Air France said on Friday its pilots must have been confused because the stall alarm kept going on and off. According to information from the black box flight recorder, the plane`s stall alarm sounded 11 times.
Pilots have complained of a stall warning "trap" due to a design feature under which the computers stop calculating the stall once the speed drops below 60 knots, well below normal.
The BEA recommendation would have asked authorities to study whether the alarm should be modified, two sources said.
Airbus refused to enter the fray, but people familiar with the Toulouse-based firm accused the French national airline and its pilots of trying to sink the inquiry by diverting attention from evidence set out in the black box tapes.
"Their only solution is to blow up the investigation," said one source, asking not to be identified.
The stall alarm controversy marks the third wave of speculation about the cause of the crash, which focused firstly on possibly faulty speed sensors, then pilot error and now the basic parameters that determine whether a plane stays aloft.
When an aircraft enters an aerodynamic stall, air stops flowing correctly under its wings and it can no longer fly.
Investigators want to establish not only whether pilots reacted wrongly to the stall warning but also how the USD 200 million aircraft came to be in such a hazardous condition.