UK investigators have determined that the pilots of an EasyJet Airbus A319 made dual control inputs during a go-around, failing to prevent the aircraft suffering a severe hard landing at Luton.
As the aircraft descended below 50ft its descent rate rose from 600ft/min to nearly 900ft/min, and both pilots initiated a go-around having sensed that the aircraft was sinking too rapidly in the final stages.
But the A319's captain did not use the sidestick takeover button, with the result that his pitch-up input was countered by a pitch-down input from the flying pilot - a captain under training.
Investigators suggest the captain-under-training suffered "momentary confusion" between the actions of his left and right hands, because he pushed forward on the sidestick and briefly pulled the throttle back to idle.
The captain pulled his own sidestick aft but failed to push the takeover button, with the result that the dual pitch inputs were summed and generated a net 7° nose-down input.
In its analysis of the incident the UK Air Accidents Investigation Branch says this dual-input interval lasted around 4s.
"The [captain] made a nose-up control input on the sidestick but did not use the takeover push-button to establish control," it states. "So the effect was limited to reducing the magnitude of the nose-down input made by the [flying pilot]."
This was not enough to avoid runway contact and the A319 touched down on all three landing-gear at the same time, with an impact of just under 3g. While there were no injuries among the 148 occupants, the three undercarriage struts exceeded their maximum loads and had to be replaced.
If the captain had used the takeover button, says the AAIB, the severe hard landing might have been prevented.
But it says the flying pilot's nose-down input, the opposite of that expected, probably took the captain by surprise. "The sidesticks move independently," says the AAIB. "So [the captain] would have had no knowledge of the inputs being made by the [flying pilot] until the flightpath of the aircraft changed."
It also cites previous Airbus incidents - involving Skyservice, Aer Lingus and Lufthansa - to illustrate that use of the takeover button is not "instinctive".
The A319 (G-EZFV) had been arriving from Faro on 14 February last year and was conducting a manually-flown instrument landing system approach to Luton's runway 26.
After the hard touchdown the captain took control of the aircraft, performed the go-around and landed on the same runway. Both pilots were given additional simulator training before being returned to line flying. The captain had logged 10,700h including 500h on type, while the captain-under-training had almost 4,000h with 672 on type.
EasyJet has used flight-data monitoring to analyse baulked-landing manoeuvres by its crews. The AAIB says a recent analysis by the airline, covering 67 events, found the procedure was generally well-flown with no significant dual inputs.