Air Accidents Investigation Branch
Summary:
SE-LPS was on approach to Ronaldsway Airport, Isle of Man with the co-pilot as PF. As the aircraft approached the minimum descent altitude, the co-pilot attempted to disengage the autopilot. There was no audio tone to indicate the disengagement and the co-pilot felt there was resistance in the flying controls. Both pilots checked the cockpit indications which seemed to show that the autopilot had disengaged. The commander took control and also felt resistance in the flying controls. He pressed and held the synchronisation (syn) button on the control column which he felt released the controls and was able to land the aircraft normally.A definite cause could not be found for the autopilot not disengaging as designed. 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.
On 2 December 2021, another autopilot occurrence on an ATP, registration SE-MAJ, was reported to the AAIB. The results of this investigation are included in this report.
Safety action was taken by the CAA to include additional testing of the autopilot system as part of the continued airworthiness programme of 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.
Two Safety Recommendations have been made to the CAA regarding the use of magnetic tape recorders.