Frustration is building within Airbus over Air France's and cockpit union SNPL's public claims that the stall alarm on flight AF447 activated in a manner which would have confused the A330's crew.

While the airframer has formally said little about the latest findings by the French Bureau d'Enquetes et d'Analyses, a clear rift has emerged over the human factor aspect of the accident.

"The situation was not ambiguous and the stall was obvious," said a source at the airframer, pointing out the 54s continuous vocal alarm which sounded as the aircraft climbed to about 37,500ft and the flying pilot kept his sidestick nose-up.

As the stall worsened, the aircraft's airspeed bled away and it began to descend, the angle of attack increasing to 41.5° before the airspeed fell below 60kt and the angle of attack became "non-computed data" - an invalidation which shut off the stall alarm.

In the 75s following the shut-off, however, the stall warning sounded another eight times as the angle of attack data briefly became valid again, each time at values from 38-43°, as the airspeed fluctuated with the aircraft's attitude.

Four of those eight warnings occurred while the flying pilot was holding the sidestick fully back, with no indication from the flight-data recorder of a relaxation in response to the alarm.

Another two instances occurred while the pilot made inputs "apparently not related to the alarm", said the source, while the other two occurred when the pilot's actions were "possibly consistent" with the stall alarm switching off and on.

"We are not in a situation where the crew has a systematic and consistent reaction to the stall warning," the source added.

"It is crucial that a crew reacts immediately to the stall warning, in order never to get into such a situation. Airliners are not to be flown at such angles of attack - what happened beyond [the initial continuous alarm cut-off] is clearly far out of the flight envelope. None of our test pilots ever went into such a situation."

Seven months before the loss of AF447, Air France issued a warning to crews over the possibility of anemometric anomalies on the A330 and A340 fleet.

Six incidents had been reported by crews characterised by loss of speed indications at high altitudes, in potential icing conditions and turbulence, during cruise at M0.80-0.82.

Among other symptoms listed by Air France were disconnection of the autopilot, a switch to alternate law, and the possible sounding of the stall warning. Crews had not reported any sensation of overspeed or approach to stall, despite the stall alarm.

Pilots were told to remain attentive to the possibility of problems and, in the event of taking manual control of the aircraft, to proceed with small corrections.

Air France accelerated replacement of pitot probes, a programme which had started five days before the 1 June 2009 accident, following the crash.

It has introduced new simulator sessions focused on unreliable airspeed procedures, with a focus on high-altitude flight in alternate law and training in modified stall responses. The carrier has also amended its internal operational rules governing relief duty and put in place a new cockpit decision-making structure.

Source: Air Transport Intelligence news