UK investigators have determined that an Aurigny Air Services ATR 72-500 suffered a serious in-flight upset after its crew vainly attempted to climb out of a region of icing instead of following the correct checklist procedures.
It had entered a frontal weather system over the English Channel, featuring moderate icing conditions, and the accretion of ice reduced the turboprop's performance to the point where it reached its operational ceiling.
The eventual upset involved excessive pitch and roll, with fluctuating airspeed and loss of altitude, before the crew regained control.
Although anti-ice systems were engaged during the initial climb out of Guernsey, the crew received a 'degraded performance' warning as the turboprop passed 11,000ft on the way to its cleared altitude of 17,000ft.
Aware that the aircraft was picking up ice, the captain mentioned the quick-reference handbook checklist in response to the caution message but did not carry it out. Nor did the crew conduct the 'severe icing' procedure.
The captain initially increased the target airspeed to 175kt – which was 10kt above the minimum icing speed – as required, and the ATR's rate of climb fell to 25ft/min.
With the aircraft effectively in level flight, says the UK Air Accidents Investigation Branch, the captain believed the target airspeed could be safely reduced to 165kt.
In response to this, the aircraft increased pitch and started to climb again, and the captain remarked about seeing whether it could "get above" the clouds.
But after a further similar target speed increase, and reduction, the crew realised that the aircraft had insufficient climb performance to reach its cleared altitude. The pilots asked to level off at 13,000ft and air traffic control approved the request, directing the aircraft to a waypoint identified as NORRY in the southern UK.
This required the aircraft to turn, to achieve a 10° change in heading. But the combination of ice accretion, inappropriate speed, and an active 'high bank' mode – which allowed banks of up to 27° – resulted in a sudden loss of control.
The aircraft initially rolled 32° left before, with the autopilot disengaging, rolling right to 38°, and then left again to 73°. The ATR pitched to 16° nose-down.
Its captain ordered the first officer to carry out upset-recovery actions, which included extending the flaps.
As the aircraft recovered the pitch reversed from nose-down to 19° nose-up and the airspeed fluctuated between 123kt and 190kt. The ATR, which had descended about 1,000ft, was returned to level flight at 13,000ft.
The first officer transmitted a distress call during the recovery and the crew opted to return to Guernsey where it landed without further incident. None of the 65 occupants was injured.
Examination of the aircraft found no faults with the ice-detection, anti-ice or de-icing systems, or with the aircraft performance monitoring system, although the flaps had been subjected to overspeed during the recovery.
Investigators point out that, had the pilots carried out the 'degraded performance' procedure, they would have been directed to the 'severe icing' checklist. The procedures would have required disengaging the autopilot and maintaining a minimum speed of 175kt.
"The crew did not observe these actions," says the inquiry into the 21 December 2016 incident. "Consequently, departure from controlled flight was more likely because the aircraft was flown slower than required."
By leaving the autopilot engaged, it adds, the crew "would not have been aware" of any handling indications pointing to an imminent loss of control.
Investigators acknowledge that the crew had been pursuing a "well-intentioned" aim of climbing out of the icing conditions, but that this led directly to the upset. It says stressful situation can lead to poor decision-making.
"In such circumstances it may be necessary to abandon the immediate goal and pursue an alternative, safer course of action," it adds, "even if that course of action is perceived as taking the aircraft further away from the desired state."
Investigators identified the aircraft as G-COBO, the same Aurigny ATR which had been involved in another icing-related control incident at Manchester nine months earlier.