What happened
During cruise flight, the pilot reported hearing a distinct 'clunk' sound, followed immediately by a decrease in engine revolutions per minute (rpm). The reduction in engine noise and rpm persisted as the situation deteriorated. Unable to maintain altitude or power, the pilot executed a forced landing on tundra terrain. Upon impact, the airplane nosed over onto its back.
The investigation
A postaccident inspection of the aircraft revealed significant internal engine damage. Specifically, the exhaust valve stem of the number two cylinder had separated from its assembly. This detached component was subsequently ingested into the cylinder, causing catastrophic failure. Maintenance records indicated the engine had accumulated 4,555 hours since new and 1,089 hours since its last overhaul.
Findings
The primary mechanical failure involved the exhaust valve stem of the number two cylinder separating during operation. This component failure led to ingestion within the cylinder, resulting in total engine power loss. The pilot was forced to land on unprepared terrain, causing the aircraft to overturn.