What happened
The aircraft was operating in cruise flight conditions at an altitude of 3,000 feet when the engine began to run roughly and vibrate noticeably. The mechanical failure progressed rapidly, leading to a complete loss of power. In response to the emergency, the pilot executed a forced landing on rough terrain located below the aircraft's position. During the descent and impact sequence, the airplane struck trees before coming to rest.
The investigation
Following the accident, investigators examined the engine to determine the root cause of the failure. Technical analysis revealed that the No. 1 cylinder exhaust valve had failed specifically at the valve stem due to fatigue. Further inspection of the component showed that the outside diameter of the valve stem exhibited excessive wear, which likely contributed to the structural integrity loss.
Findings
The mechanical examination confirmed that the engine failure was caused by a fatigue fracture of the No. 1 cylinder exhaust valve stem. The presence of excessive wear on the valve stem's outside diameter was identified as a significant factor in the component's failure.