What happened
On 14 December 2017, a BAE Systems (Operations) Ltd ATP, registration SE-MHF, was performing a cargo flight from Belfast Aldergrove to East Midlands Airport. During the ILS approach to Runway 27, the co-pilot attempted to manually disconnect the autopilot to transition to manual handling. Despite multiple attempts using the control wheel disconnect button and the trim switch, the autopilot remained engaged.
As the aircraft descended toward the 500 ft radio altitude stability threshold, the commander became concerned with the aircraft's configuration. After the co-pilot again reported the inability to disengage the system, the commander attempted to intervene but was also unsuccessful. To manage the situation, the co-fly used the synchronization (sync) switch to de-clutch the servos, allowing for manual pitch control. The crew subsequently initiated a go-around at approximately 230 ft above the airfield level.
The investigation
The investigation examined the flight data, cockpit voice recordings, and the aircraft's avionics systems. While the crew believed the autopilot remained engaged until the climb phase, flight data recorder analysis revealed the autopilot actually disengaged at approximately 425 ft above the airfield level during the approach.
Investigators also looked into the crew's communication and physiological state. It was noted that the co-pilot had been awake for over 18 hours, and the crew was likely operating at a low point of their circadian rhythm. Furthermore, the investigation reviewed the aircraft's autopilot hardware and software, as well as previous similar incidents involving the same aircraft type.
Findings
- Communication breakdown: A misunderstanding occurred between the pilots regarding the status of the autopilot; the co-pilot's verbalized difficulty was not clearly understood by the commander.
- Lack of feedback: The autopilot system provides no specific indication if a manual disconnect attempt is made while the system is already disengaged, which may have contributed to the crew's uncertainty.
- Fatigue: The crew's alertness and communication may have been compromised by fatigue and extended periods of wakefulness.
- No mechanical failure: No technical defects or abnormalities were found in the autopilot units or the aircraft's flight control systems during testing.
Safety action
Following this incident and two previous similar occurrences, the aircraft manufacturer initiated a review of the Emergency Checklist to determine if new procedures should be added to address scenarios where the autopilot fails to disengage.