What happened
On 29 April 1999, a Mooney M2/M20J, registration G-BIWP, departed Sherburn-in-Elmet, Yorkshire, for a private flight to Texel, Holland. The aircraft was carrying two crew members and two passengers. During the initial climb, the aircraft was observed by witnesses to be performing normally. However, while flying near Selby, the aircraft entered a cloud layer and subsequently experienced a significant reduction in groundspeed and climb rate.
Witnesses on the ground reported hearing the engine cutting out multiple times before observing the aircraft descending in a steep, nose-down attitude. The aircraft entered a spin and impacted the ground, resulting in four fatalities. An intense post-impact fire destroyed the aircraft structure.
The investigation
The AAIB investigation examined the wreckage, the engine, and the aircraft's autopilot system. While the engine showed no evidence of a sudden mechanical failure, investigators discovered significant wear on the inlet valve cam lobes, which likely reduced the engine's maximum power output. The investigation also identified a defect in the autopilot's pitch servo, specifically an intermittent contact in the 'nose down' torque switch.
Radar data confirmed a reduction in power and airspeed consistent with the witnesses' accounts. The investigation also looked into the cockpit environment, noting that the pilots were wearing noise-suppressing headsets and that the autopilot was likely engaged during the power loss.
Findings
- The primary cause of the accident was a loss of engine power, likely due to reduced volumetric efficiency caused by worn inlet valve cams.
- The autopilot system likely masked the initial symptoms of the power loss and the impending stall by maintaining a constant aircraft attitude.
- The autopilot's tendency to trim the aircraft into an aft position likely made a manual recovery from the stall extremely difficult.
- The pilots' use of noise-suppressing headsets may have prevented them from immediately recognizing the engine's irregular sound.
- The aircraft's descent into cloud cover likely led to spatial disorientation, making a recovery from the incipient spin unlikely.