What happened
On 18 August 2020, a BAe ATP, registration SE-MAO, was conducting a commercial cargo flight from Jersey Airport to Guernsey Airport. While the aircraft was leveled at 2,000 feet and performing a right turn, the crew experienced two sudden, uncommanded rolls to the left. The second roll was accompanied by the automatic disengagement of the No 2 autopilot and a continuous audible warning.
Following the disengagement, the co-pilot took manual control, but found that the aircraft was significantly harder to maneuver to the right, describing a persistent tendency for the aircraft to push to the left. To manage the situation, the crew requested delaying vectors from Air Traffic Control and reduced the airspeed to 180 KIAS, which helped alleviate the abnormal control forces. During the subsequent approach to Guernsey, the flight director modes failed, forcing the crew to rely on raw data for the landing. The aircraft landed safely without any injuries to the two crew members.
The investigation
The investigation examined the aircraft's flight data recorder (FDR) and cockpit voice recorder (CVR), as well as the physical condition of the flight control systems and autopilot components. Investigators analyzed the FDR data, which confirmed that the ailerons moved rapidly in a manner consistent with the autopilot servos' capabilities just before the rolls occurred.
Upon inspection of the No 2 autopilot computer, investigators discovered localized corrosion on the printed circuit boards (PCBs) in the section responsible for the autopilot servo and Standby Control System (SCS) logic. The manufacturer suggested this corrosion might have been caused by residual flux from a previous repair. However, extensive testing of the servos and the autopilot computer following the incident failed to identify a specific mechanical or electrical fault that could definitively be linked to the uncommanded rolls or the increased control resistance.