What happened
On 14 February 2018, a BAE Systems (Operations) Ltd ATP, registration SE-MHD, was conducting a commercial cargo flight from East Midlands to Guernsey Airport. During the descent from FL180, the pilot under training attempted to correct a lateral imbalance. During this process, the autopilot disconnected automatically, resulting in a sudden 45-degree left bank and a nose-down pitch.
Upon taking control, the commander found the flight controls difficult to manipulate and discovered that both power levers were stuck at the flight idle position. Believing the controls had frozen due to ice, the crew declared a MAYDAY. As the aircraft descended, the flight controls and power levers eventually returned to normal operation, allowing the crew to complete a safe landing at Guernsey.
The investigation
The AAIB examined the aircraft and analyzed data from the cockpit voice and flight data recorders. The investigation focused on two primary issues: the cause of the initial autopilot disconnection and the reason the power levers were restricted.
Regarding the flight upset, investigators found that the aircraft's rudder position remained unchanged during the period the pilot attempted to trim the aircraft. This suggested that the pilot may have inadvertently operated the aileron trim instead of the rudder trim. Such an action would increase the load on the aileron servomotors, eventually triggering the autopilot's automatic disconnection safety circuit.
Regarding the engine controls, while the crew suspected icing, the investigation found no evidence of frozen cables. However, a subsequent examination of the aircraft revealed that the left roll-over lever locking mechanism had excessive wear, which prevented it from always locking in the forward position. This wear prevented the associated power lever from being advanced from flight idle. While this explained the left lever, the cause of the reported restriction on the right lever could not be definitively established.