Flight-director logic queried after A321 incident

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French investigators looking into an Airbus A321 stall-protection incident near Paris have again questioned the logic and response of flight directors to unusual circumstances.

The Air France aircraft’s autopilot had been attempting to maintain an altitude of 4,000ft with only idle thrust, after the autothrust had been disengaged by the captain.

With airspeed declining – unnoticed by the crew – the aircraft pitched up and began to lose height. The flight director responded by indicating a pitch-up order was necessary to regain the selected altitude.

The aircraft’s attitude triggered the stall-protection system. But while the autopilot disengaged automatically, the flight director remained active.

French investigation authority BEA notes that the captain made a pitch-up input to his sidestick, lasting more than 10s, in order not to lose altitude.

This input meant that the stall-protection system remained active, whereas the normal response is to exit the situation as quickly as possible by pushing the sidestick forward to reduce the angle-of-attack.

BEA says it “did not determine” whether the captain had been following the flight director when he made the pitch-up command.

Although the flight director’s orders were consistent with the captain’s intention not to lose altitude, and with the selected mode and functional logic of the aircraft, they were not suited for the low-airspeed situation.

“When the [autopilot] disengages inadvertently – as in this case, on the basis of high angle-of-attack – the relevance of keeping the [flight directors] on should be studied,” says BEA.

Flight-director logic emerged as a concern in the wake of the fatal stall of an Air France A330 over the South Atlantic in June 2009.

The pilot of the aircraft had maintained pitch-up inputs to the sidestick instead of countering the high angle-of-attack with a pitch-down input.

BEA, which investigated the crash, suggested at the time that the pilot might have trusted the flight director to the exclusion of other instruments, and recommended a re-evaluation of flight director logic and display during a stall.

As a result of the A321 incident, which occurred on 20 July 2012, the inquiry is again proposing a reconsideration of the instrument’s logic, so that it “disappears or displays appropriate orders” should the autopilot inadvertently disengage.