History of the Flight
The aircraft departed Jersey's Runway 26 at 0817 UTC, with the co-pilot as pilot flying. After leveling at 2,000 ft at 200 KIAS, ATC instructed a right turn to heading 340°. The crew felt a jolt accompanied by a left roll, corrected by the autopilot. A second jolt disengaged the autopilot, triggering a continuous cavalry charge warning. The co-pilot silenced it and took manual control. She found it difficult to continue the right turn as the aircraft pushed left, requiring significant right control input. The commander requested delaying vectors from ATC. Slowing to 180 KIAS reduced abnormal forces. The commander attempted control from his side and experienced the same forces, describing the aircraft as fully controllable but needing substantial right input. The co-pilot resumed flying. The commander noted normal flight deck indications and neutral trim positions. He reported a slight control problem to ATC and requested left turns for an ILS approach. After using aileron trim, the co-pilot noted improvement. The crew prepared for a manually flown ILS. At around 1,500 ft, they exited cloud, but flight director modes failed. The co-pilot flew raw data ILS, with the commander making callouts. The aircraft landed at 0831 UTC and taxied normally. After shutdown, controls felt normal.
Investigation Findings
Post-flight testing could not identify any fault causing the event. The investigation revealed two minor issues related to autopilot computer maintenance and testing. The operator took safety action to address these. The flight data recorder showed two sharp left rolls (5° then 10°) while in a right turn, each preceded by rapid aileron movements in the correct sense. No turbulence was indicated. The autopilot automatically disengaged after the second roll.
Aircraft and Systems
The BAe ATP (SE-MAO) was manufactured in 1989 and had accumulated 28,164 cycles. It was converted to cargo configuration in 2007. The autopilot system includes two independent computers. Disengagement can occur for various reasons, including computer failure or invalid signals. The last autopilot fault report was in June 2019 and was rectified.
Meteorological Conditions
At 0820 UTC, Guernsey reported wind 200°/9 kt, visibility >10 km, broken cloud at 900 ft, showers in vicinity, temperature 18°C, QNH 1010 hPa.
Recorded Data
The FDR and CVR were successfully downloaded. The FDR recorded aileron positions and attitude from AHRU No 1. Aileron data is relative and not calibrated. No abnormal acceleration fluctuations were noted.