What happened
On 14 February 2018, a BAE Systems (Operations) Ltd ATP, registration SE-MHE, was conducting a commercial cargo flight from East Midlands to Guernsey. During the descent from FL1t80, the pilot under training attempted to correct a yaw imbalance. While attempting to stabilize the aircraft, the pilot inadvertently applied aileron trim instead of rudder trim. This action caused the trim caution light to illuminate and triggered an automatic autopilot disconnection.
Upon disconnection, the aircraft immediately entered a significant left bank of approximately 45 degrees and a nose-down attitude. The commander took control and managed to level the wings and restore a safe flight path. However, the crew encountered further difficulties as the flight controls and power levers appeared to be restricted. The commander believed the controls had frozen due to ice, and the crew was unable to advance the power levers to increase engine output. After approximately three minutes of flight, the power levers returned to normal operation, allowing the crew to complete a safe landing at Guernsey Airport.
The investigation
The investigation examined flight data from the aircraft's recorders and cockpit voice recordings. Investigators analyzed the sequence of events leading to the autopilot's automatic disengagement. The study of the flight controls focused on the physical state of the aileron, rudder, and pitch trim systems, as well as the engine control cables.
Regarding the power lever restriction, the investigation looked into the possibility of icing in the control cables. While the crew suspected ice, the investigation found that the roll-over levers were still functional, suggesting the control runs were not frozen. A subsequent inspection of the aircraft revealed that the left roll-over lever had excessive wear, which prevented it from locking properly and could have restricted the left power lever from advancing. However, a definitive cause for the reported jamming of the right power lever could not be established.