What happened
During departure from a rural village, the pilot experienced a partial loss of engine power. At an altitude of approximately 600 feet agl, the pilot reported hearing a loud "clunk-clunk" sound accompanied by engine vibration.
In response to the engine issues, the pilot reduced power and executed a turn downwind to return to the same runway used for departure. However, while performing the turn from base to final approach, the engine continued to lose power. The pilot was unable to maintain enough altitude to reach the runway, resulting in a forced landing in deep snow. The pilot was not injured.
The investigation
A postaccident inspection of the engine revealed a fractured connecting rod bolt. Subsequent metallurgical examination performed at the NTSB laboratory determined that the failure of the rod bolt was due to fatigue failure.
Records indicated that the engine had been rebuilt approximately 1,642 service hours before the accident. The manufacturer's recommendation for an engine overhaul is 2000 hours.