Inquiries into the triggering of stall-protection on an Air France Airbus A321 have found that the crew failed to monitor the aircraft’s airspeed, after unnecessarily deactivating the autothrust.
French investigation authority BEA is recommending that the European Aviation Safety Agency mandates systems to warn crews of low-speed situations.
The aircraft had been descending towards Paris Charles de Gaulle with its autopilot and autothrust engaged, but the pilot tried to increase the descent rate by retarding the thrust levers to idle.
BEA points out that this action was “pointless” because the descent had been undertaken in ‘open descent’ mode, which already maintains idle thrust. But the consequences were significant because retarding the levers disengaged the autothrust while leaving the autopilot active.
Air France had warned crews, five months earlier, of the potential “trap” created by this mixed-mode operation. It creates the risk, notably as the aircraft levels off during a descent, that pilots could forget that airspeed is no longer being managed automatically.
Airbus considers this mixed-mode operation to be “inadvisable”, says BEA.
As the Air France A321 levelled at 4,000ft it switched into a mode which prioritised maintaining altitude. But the idle thrust was not sufficient to keep the aircraft at this level, and the autopilot commanded an increase in angle-of-attack.
While the captain, who was flying, had selected an airspeed of 220kt, this was not taken into account because the autothrust was not engaged. The A321’s airspeed bled away as the aircraft pitched up in a struggle to maintain its height.
Neither pilot noticed the situation because they had received a traffic notification and were distracted by searching outside for this other aircraft.
As the A321’s angle-of-attack continued to increase, its airspeed fell to 177kt – some 26kt lower than the minimum speed the autothrust would have permitted – and the aircraft’s automatic stall-protection system activated.
Although the captain then advanced the thrust levers, he only moved them to the ‘climb’ position rather than the higher ‘take-off’ setting. The aircraft, descending at 1,000ft/min with an airspeed of only 172kt, did not immediately recover – partly because the captain also made a pitch-up input on his sidestick.
The A321’s altitude dipped to 3,840ft before its thrust stabilised and its airspeed started to increase. It gained height and the crew subsequently landed without further incident.
BEA’s inquiry into the 20 July 2012 event says the captain had been “aware” of the autothrottle disengagement when he initially retarded the thrust levers to idle. But he did not remember calling out the action, and the first officer said he was not aware of the autothrottle deactivation.
“The [first officer’s] not having noticed the disengagement, he wasn’t able to question this decision,” it adds.
BEA says the pilots then failed to monitor the airspeed, both having become pre-occupied with scanning outside for traffic even though there had been no collision-avoidance alert. Over-estimating the risk from the traffic, it adds, led to the “detriment” of checking crucial flight parameters.
Not until the stall-protection system began to activate, and the autopilot disengaged, did the crew realise that the airspeed had fallen away. The inquiry says there was no aural warning to alert the pilots earlier, and is recommending to EASA that such low-speed warning systems be mandated.