What happened
On 8 October 2016, a Fournier RF 47 was conducting a cross-country flight when the engine began to lose power and sputter. The pilot also noted an unusual smell of oil. Seeking a safe area for an emergency landing, the pilot selected a ploughed field in Oordegem, Belgium. During the final approach, the engine ceased operation entirely.
Upon touching down in the soft terrain, the aircraft rolled for approximately 10 to 15 meters before briefly becoming airborne again. During the second touchdown, the nose landing gear sank into the soft ground, causing the strut to fail and the aircraft to flip over. The pilot escaped the wreckage with slight injuries, though the aircraft sustained significant damage to the engine, landing gear, and airframe.
The investigation
Investigators examined the engine and the maintenance history of the aircraft. It was noted that the engine had recently undergone repairs to replace exhaust valves. The investigation focused on the mechanical state of the valve train and the technical documentation available to the pilot-owner.
Technical analysis of the engine revealed that the intake valve of cylinder number two had been pulled into the combustion chamber. This was caused by the failure of the valve keys. Furthermore, investigators found evidence of abnormal contact between the valve rotators and the rocker arms, which had led to rapid wear of the valve keys.