What happened
On 6 October 1999, a Rans S6-ESD microlight, registration G-MYLA, was performing a private flight from a grass runway at Cherry Tree Farm in Monewden, Suffolk. During the initial climb, the aircraft's engine suddenly ceased operation. Witnesses observed the aircraft climbing normally before the engine noise stopped abruptly. Following the power loss, the aircraft began to turn to the right and lose altitude, eventually impacting a ploughed field in a steep, nose-down attitude. The impact destroyed the aircraft and resulted in one fatality.
The investigation
Investigators examined the engine and found evidence of extensive, long-term overheating. The internal examination revealed that the piston rings had seized within their grooves, leading to significant combustion gas blow-by. This overheating was linked to a slack and heavily worn cooling fan belt, caused by corrosion on the pulley surfaces. Additionally, the aircraft's cylinder-head temperature gauge was found to be non-functional, meaning the pilot would have had no cockpit indication of the rising temperatures.
Regarding the impact, the investigation noted that the pilot's seat harness failed at the point where the webbing was stitched to the metal tongue. This failure was attributed to a hard object, likely a trouser belt buckle, which had abrading the webbing. The sudden loading during the crash caused the stitching to fail, which prevented the harness from properly restraining the pilot and led to the fatal head injury.
Findings
- The engine failure was caused by a piston seizure resulting from inadequate cooling.
- The cooling failure was due to a worn fan belt and corroded pulleys.
- The pilot was unable to monitor engine temperatures due to a faulty temperature gauge.
- The use of fuel with a significantly low octane rating may have contributed to the overheating.
- The fatal injury was a direct result of the seat harness failure caused by an abrasive object in the pilot's clothing.