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 cruising at 2,000 ft, the aircraft experienced two sudden left-wing rolls. The first roll was approximately 5 degrees, followed by a second, sharper 10-degree roll. This second movement triggered the automatic disengagement of the No 2 autopilot, accompanied by a continuous audible warning.
Following the disengagement, the flight crew found that the aircraft was difficult to maneuver, specifically noting that the control wheels required significantly more effort to turn to the right. The crew requested vectors from Air Traffic Control to manage the situation and slowed the aircraft to 180 KIAS, which helped reduce the abnormal control forces. During the subsequent approach to Guernsey, the flight director modes failed, forcing the crew to rely on raw data to complete the landing. The aircraft landed safely without any injuries to the two crew members and sustained no damage.
The investigation
Investigators examined the aircraft's flight data and cockpit voice recordings, which confirmed that the ailerons moved in a manner consistent with the observed rolls. Extensive mechanical and functional testing of the aileron control system, including the balance tabs and trim, revealed no mechanical obstructions or faults. The control cables and servos were found to be operating within normal parameters.
During the inspection of the avionics, investigators discovered localized corrosion and debris on the printed circuit boards of the No 2 autopilot computer. The manufacturer suggested this corrosion might have been caused by residual flux from a previous repair. Additionally, the investigation found that an aileron servo had exceeded its recommended overhaul interval because its operating hours had been incorrectly reset to zero during a prior maintenance event. While the corrosion was noted, no direct link could be established between the degraded circuit boards and the specific flight disturbances.
Findings
- The aircraft experienced uncommanded rolls and autopilot disengagement, but the specific trigger could not be identified as no faults were found during testing.
- The crew experienced increased resistance in the right-hand control input, though the cause of this force remained undetermined.
- The aileron servo had been operated beyond its 4,500-hour overhaul limit due to an error in recording component hours.
- Localized corrosion was present on the No 2 autopilot computer's circuit boards, likely due to trapped flux from a past repair.