What happened
On 27 November 2018, a Diamond DA42, registration N648KM, was conducting a private IFR flight from Retford Gamston Airport to Weston Airport, Dublin. While cruising at FL100, the crew received a left engine fire warning. In response, the pilot reduced the left engine to idle and declared a PAN emergency. Following guidance from Scottish and Liverpool Air Traffic Control, the crew initiated a diversion toward Liverpool.
During the descent through instrument meteorological conditions (IMC), the pilot attempted to manage the high workload by re-engaging the autopilot at approximately 3,000 ft. Shortly after, the aircraft experienced a sudden jolt, causing the autopilot to disengage. The aircraft immediately pitched nose-down and entered a rapid descent. The pilot managed to recover control once the aircraft reached visual meteorological conditions (VMC) at roughly 800 ft agl. Following a suggestion from Liverpool ATC, the crew diverted to RAF Woodvale, where they landed safely on a runway that was otherwise out of use due to surface conditions.
The investigation
The investigation focused on the sequence of events leading to the loss of control and the factors contributing to the pilot's decision-making during the emergency. Investigators examined the aircraft's handling characteristics under asymmetric thrust and the impact of the autopilot's limitations. The investigation also looked into the environmental factors, including the high-stress nature of the engine fire warning and the challenges of flying in IMC with unbalanced engine power.
Findings
- The pilot was operating in a high-stress, high-workload environment caused by the engine fire warning and an unplanned diversion.
- The use of the autopilot during asymmetric flight was a critical factor, as the system lacks a yaw channel and cannot compensate for the effects of unbalanced thrust.
- The autopilot likely exceeded its operating parameters due to the asymmetric condition, leading to its disengagement and the subsequent loss of control.
- The aircraft entered a spiral dive condition characterized by rapid yaw and roll.
- The pilot's ability to recover the aircraft was aided by reaching VMC at a low altitude, allowing for visual reference.
- There was no evidence of an actual fire, despite the persistent warning light.