The UK Air Accidents Investigation Branch has published a report into an investigation into an incident involving a BAe ATP (SE-LPS) which occurred at Ronaldsway Airport on the Isle of Man on 9 April 2021. Both pilots were unharmed.
The serious incident occurred when the autopilot did not disengage on selection and the pilots encountered resistance in the flying controls. The cockpit displays indicated that the autopilot had disengaged.
The commander landed the aircraft after pressing the synchronisation (SYN) button, which cleared the resistance in the flying controls. The definitive cause could not be found, but the manufacturer responsible for the design of the autopilot identified a possible scenario where the autopilot servomotors could remain engaged after the autopilot disengaged. This would result in higher-than-normal forces at the cockpit controls.
Audio anomalies during the flight were probably caused by corrosion in the audio warning unit because of moisture ingress through the DV windows.
As a result of this investigation safety action was taken by the UK CAA to consider additional testing of the autopilot system as part of the continued airworthiness programme for the ATP. The operator took safety action to reconfigure their fleet so that either pilot could override either autopilot via the SYN button on their respective control wheel. The operator also initiated remedial action to try and prevent water ingress into the cockpit.
In addition, two Safety Recommendations have been made to the CAA regarding the use of magnetic tape recorders.
