What happened
On 24 July 2009, a Cyclone AX2000 microlight, registration G-MZJR, was conducting a private flight from Clench Common, Wiltshire, toward Headcorn, Kent. The pilot, intending to cross the English Channel to France, had diverted to Popham Airfield to reassess the weather due to forecast rain showers. While flying toward Biggin Hill, the aircraft's engine suddenly ceased operation.
Following the engine failure, the pilot attempted a restart without success and identified a small field for an emergency landing. The aircraft descended through heavy rain showers, which limited the available landing options due to the undulating terrain. The pilot managed to maintain control and approached a rectangular field, but the landing was complicated by a 10 to 15 mph tailwind and a wet grass surface. The aircraft touched down 400 feet into the 500-foot-long field and failed to stop before striking a substantial wooden and rail fence. The impact caused the pilot to sustain serious injuries, while the passenger suffered minor injuries.
The investigation
Investigators examined the HKS 700E V3 engine to determine the cause of the sudden stoppage. A strip examination revealed that the engine had seized because the head of an exhaust valve in the number one cylinder had separated from its stem. This failure caused significant internal damage to the piston.
Metallurgical analysis of the failed valve stem showed that the fracture was caused by fatigue originating from thermally-generated corrosion. The investigation also noted that the remaining exhaust valve showed signs of surface cracking, suggesting it was also at risk of similar failure. Furthermore, the investigation reviewed the engine's maintenance history, noting that the valves had exceeded the manufacturer's recommended service life of 300 hours or five years.