Investigators have found that an EasyJet Switzerland crew inadvertently allowed an Airbus A319 to descend at excessive speed, before a sudden sidestick input threw several cabin crew to the floor.
One cabin crew member sustained a serious injury during the 20 July 2014 incident.
The A319, bound for Basel, had been cleared to descend to 24,000ft and had been operating in 'open descent' mode, which idles the engines and uses pitch to prioritise a target airspeed – in this case M0.76.
Swiss investigation authority SUST says the aircraft accelerated to 315kt and was further cleared to descend to 18,000ft.
While the descent mode was switched to 'vertical speed', with a setting of 2,500ft/min, the target speed remained at M0.76. As the aircraft descended the engine thrust increased in order to achieve this target.
The airspeed reached 345kt, close to the maximum operating speed of 350kt, as the A319 approached clouds at around 20,000ft.
SUST says the captain had been "pre-occupied" by the presence of the cloud formation, and the first officer – concerned about the possibility of exceeding speed limits in turbulence – called for a speed check.
Although the captain reduced the target speed to M0.54, the aircraft momentarily reached 349kt. The captain disengaged the autopilot, says the inquiry, and "instinctively and abruptly" pulled on the sidestick.
The overspeed warning sounded and the A319's pitch transitioned from 2.5° nose-down to 2° nose-up, subjecting the aircraft to a force of 2.33g.
While the passengers were unaffected, three of the four cabin crew were thrown to the floor during this manoeuvre, one of whom suffered a serious ankle injury.
The captain proceeded to enter several changes to the target speed before it was fixed at 275kt and the aircraft continued its descent normally.
SUST says the A319 (HB-JZQ) was undamaged.
Investigators could not identify any "extraordinary factors" which might have increased the pilots' workload during the descent. The inquiry cites inappropriate management of the descent mode, and has attributed the late detection of the proximity to maximum operating limits to a "lack of diligence" in visually monitoring crucial flight parameters.