What happened
On 14 December 2017, a BAE Systems (Operations) Ltd ATP, registration SE-MHF, was performing a commercial cargo flight arriving at East Midlands Airport. While conducting an ILS approach to Runway 2/7, the co-pilot attempted to manually disconnect the autopilot between 1,000 and 800 feet radio altitude. Despite multiple attempts using the control wheel disconnect button and the trim switch, the autopilot remained engaged.
A communication breakdown occurred when the co-pilot's verbalized difficulty was misinterpreted by the commander as a confirmation of successful disconnection. As the aircraft descended toward the 500-foot stability threshold, the commander noticed the aircraft was not properly configured and prompted the co-pilot to select landing flaps. At this point, the co-pilot again reported the inability to disengage the system. After the commander also failed to disconnect the unit, a go-around was initiated at approximately 230 feet above airfield level.
To regain manual control during the climb, the co-pilot utilized the synchronization (sync) switch, which de-energized the elevator and aileron clutches, allowing for a manual pitch-up attitude. The aircraft subsequently completed a manual landing without further incident.
The investigation
The AAIB examined flight data recorder (FDR) and cockpit voice recorder (CVR) information. The FDR revealed that while the crew believed the autopilot disconnected during the climb, it actually disengaged at approximately 425 feet above airfield level during the approach. Extensive testing of the aircraft's avionics and autopilot computers by the manufacturer and the AAIB found no technical defects or abnormalities in the autopilot units.
The investigation also looked into human factors, noting that the co-pilot had been awake for over 18 hours and reported feeling fatigued, which may have impacted communication and situational awareness. Additionally, the investigation noted that the aircraft's autopilot disconnect system provides no specific feedback if the system is already disengaged, which can lead to uncertainty.
Findings
- Miscommunication between the crew regarding the actual status of the autopilot engagement.
- Fatigue may have contributed to reduced alertness and ineffective intra-crew communication.
- The autopilot disengaged during the approach phase, rather than during the go-around as perceived by the crew.
- The lack of specific feedback when attempting to disconnect an already disengaged autopilot can cause confusion.
Safety action
Following this incident and two previous similar occurrences, the manufacturer has committed to reviewing the Emergency Checklist to determine if new procedures should be added to address situations where the autopilot fails to disengage.