Autothrottle mix-up behind near-identical A320 incidents

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French investigators are citing misunderstanding of cockpit automation for two similar incidents, both involving Air France Airbus A320s on approach to Tel Aviv, within five days of one another.

The inquiry into the events indicates that the crews did not fully appreciate the need to follow the flight director indications during an approach made in “open descent” mode on the type.

This mode prioritises airspeed over altitude. The autothrottle maintains idle thrust while the flight director indicates the necessary action to achieve the target airspeed.

French investigation authority BEA says the first incident occurred on 3 April last year, during a visual area navigation approach to Tel Aviv runway 26 – an unusual procedure for short-haul Air France crews.

The A320 (F-HEPE) had been operating in “open descent” mode, with idle thrust, and had decelerated to an approach speed of 138kt.

Just before the final turn the crew disengaged the autopilot but left the autothrust and flight director active. But during the turn the flying pilot gave a pitch-up command for about 10s, contradicting the flight director which had signalled that pitch-down input was needed.

BEA says the reason for the pilot’s command is unexplained. But the aircraft pitched up and, as a result, the airspeed fell to 122kt – some 16kt below the approach target – and a speed warning sounded.

The pilot started to abort the approach and set go-around thrust, but without informing the monitoring pilot, who countered momentarily with a contrary nose-down input – although this had no effect because the button which transfers side-stick control was not pressed.

Protection systems on the A320 activated, locking the thrust at go-around levels. The airspeed increased and, although the pilot retarded the thrust levers, this had no effect because the system logic maintains the thrust lock until the crew disengages the autothrottle.

The A320 continued to climb, accelerating to 223kt, taking it beyond the speed limitation for its flap configuration and generating an overspeed warning. It also overshot the 2,200ft height threshold for a missed approach.

Autothrust disengaged when the crew set the thrust levers to idle, and the thrust lock was released. The pilots re-engaged autothrust and carried out a second approach without further incident.

BEA says the crew believed the autothrottle would adjust its thrust to maintain airspeed, and that the pilot’s pitch-up input indicates that the crew “did not identify the risks” of failing to follow the flight director in “open descent” mode.

Five days later, on 8 April, a second A320 (F-GKXO) experienced a similar incident, says BEA, as the crew used “open descent” mode while conducting the same approach. As with the previous event, the autopilot had been switched off but the autothrottle and flight director were engaged.

“The [flying pilot] did not follow the orders of the [flight director],” says BEA, adding that the airspeed was not monitored. Although a speed warning sounded the aircraft’s protection systems did not activate.

BEA acknowledges the difficulties presented by the unusual procedure in place at Tel Aviv, and says that simulator training specifically for this approach was introduced in March this year.

But it also points out that the inquiry into the 3 April event revealed a “misunderstanding” regarding the operation of the autothrottle and that the same lack of comprehension showed up in the subsequent incident five days later.