What happened
On 9 April 2021, a BAe ATP, registration SE-LPS, was conducting a cargo flight from East Midlands Airport to Ronaldsway Airport, Isle of Man. During the final stages of the ILS approach to Runway 26, the co-pilot attempted to disengage the autopilot (AP2) as the aircraft approached minimum descent altitude.
Although the cockpit displays indicated that the autopilot had disconnected, no audible warning tone was heard. Furthermore, the co-pilot encountered heavy resistance when attempting to maneuver the aircraft. The commander took control and experienced similar resistance in the flight controls. To resolve the issue, the commander utilized the synchronisation (syn) button on his control column, which released the tension and allowed the aircraft to be landed safely. Following the landing, the aircraft's audio warning unit began emitting continuous, uncancelable alerts.
The investigation
The AAIB investigation focused on the cause of the control resistance and the anomalous audio warnings. Investigators examined the aircraft's flight data, cockpit voice recordings, and physical components.
Testing of the audio warning unit revealed corrosion on the electrical connector and internal printed circuit boards. This was linked to moisture ingress through the cockpit's direct vision windows, a known issue for this aircraft type. Regarding the autopilot, the investigation could not definitively replicate the control resistance. However, the manufacturer performed a theoretical analysis of the autopilot engagement logic. This analysis suggested that an intermittent failure in the autopilot disconnect switch or its debounce circuit could potentially cause the autopilot computer to disengage while the servo solenoids remained energized, resulting in high physical forces at the control column.
Findings
- The primary cause of the control resistance remains unconfirmed, though a theoretical failure of the autopilot disconnect switch or debounce circuit was identified as a possible scenario where servos remain engaged despite a computer disengagement.
- Corrosion and moisture ingress through the direct vision windows likely caused the extended duration of altitude alerts and the continuous autopilot disconnect warnings.
- The commander's familiarity with a previous AAIB report regarding a similar incident allowed for a rapid, effective response using the synchronisation button.
- The aircraft's configuration regarding the synchronisation button functionality differed from the standard due to an unrecorded service bulletin embodiment.