The aircraft was carrying out a cargo flight from East Midlands Airport to Guernsey Airport. As the aircraft commenced its descent from FL180, the ball in the slip indicator moved out to the left as normal and the pilot under training attempted to trim it back into the centre. He was unable to do so, and the autopilot disconnected automatically, causing a significant left bank and a nose-down attitude. The commander took control, closed the power levers and returned the aircraft to a safe flightpath. He had difficulty moving the flight controls and could not advance the power levers, believing both to have frozen due to ice. As the aircraft descended, the flight controls and power levers returned to normal and a safe landing was carried out.
It is possible that the initial control upset was the result of the crew applying aileron trim instead of rudder trim whilst attempting to correct the yaw. Although the cause of the stuck power controls could not be established definitively, it is possible that the left power lever was restricted because of wear in the roll-over lever locking mechanism, although this would not explain the locking of the right power lever reported by the pilots.
Action was taken by the manufacturer to improve the effectiveness of both an existing Service Bulletin, relating to wear in the locking mechanism, and an electronic Service Information Leaflet, relating to the purging of moisture from engine control cables.
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Published 6 December 2018