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AF447 inquiry grapples with stall-warning enigma

French investigators' attempts to unravel the reasons why Air France flight AF447's crew failed to recognise the Airbus A330's stall situation are seeking the possible inclusion of visual alarms to accompany aural warnings.

Seconds after pitot icing - and the loss of airspeed information - led the aircraft's autopilot to disconnect, the pilot gave a sudden nose-up input that caused the stall warning to trigger.

Investigation agency BEA says analysis of 17 events occurring in similar conditions to those affecting AF447 illustrated that crews found the stall warning "surprising" and "frequently mentioned their doubts" over its relevance.

"These judgements may be explained by the lack of awareness of the margins in relation to the trigger threshold of the stall warning and by not knowing the triggering conditions of the warning," the BEA says.

Although AF447's non-flying pilot may have noticed the brief warning, it adds, he might not have been able to put it in context - possibly because he was unaware of the flying pilot's nose-up response, the relative proximity of the limits of the flight envelope, and the switch to alternate control law which removed angle-of-attack protections.

More mystifying was the apparent failure to react to the prolonged second stall warning, lasting for 54s, generated as the aircraft climbed out of its assigned 35,000ft (10,600m) cruise level towards 38,000ft.

But in the 46s between autopilot disconnection and this second stall warning, another alert - the "C-chord" altitude horn - had sounded almost continuously for 34s.

"In an aural environment that was already saturated by the C-chord warning, the possibility that the crew did not identify the stall warning cannot be ruled out," the BEA states.

It cites cognitive research suggesting that visual, rather than auditory, information is prioritised by pilots coping with high workloads.

"Piloting, calling heavily on visual activity, could lead pilots to a type of auditory insensitivity to the appearance of aural warnings that are rare and in contradiction with cockpit information," the analysis says.

Although some of the flying pilot's actions appear consistent with approach-to-stall procedures, the BEA cites evidence that the pilot might have interpreted certain cues - such as buffeting and aerodynamic noise - as an indication of overspeed rather than stall.

Not only had the pilot reduced thrust shortly before the prolonged stall warning, but he also later mentioned having an "impression" of speed, and there was an attempt to extend the speedbrakes.

The BEA also states that the flight director was advising a nose-up attitude, and this "may have confirmed the [pilot's] belief that the stall warning was not relevant".

"In its current form, recognising the stall warning, even associated with buffet, supposes that the crew accords a minimum level of 'legitimacy' to it," it notes.

The BEA also says the pilot might not have been fully aware of the A330's switch to alternate law, and perhaps "embraced the common belief" that the aircraft could not stall.

While the master warning indicator light would have supported the aural alert in signalling the onset of the stall, the BEA is recommending that the European Aviation Safety Agency determines the conditions in which a dedicated visual indication should be made mandatory.

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