What happened
The pilot reported that while conducting a swath run, the aircraft engine began to vibrate significantly. In response, the pilot reduced engine power but found that applying power again did not resolve the issue as the vibration persisted. Consequently, the pilot elected to conduct a forced landing. During the landing phase, the airplane encountered wet terrain which caused it to nosed over.
The investigation
A subsequent examination of the aircraft revealed that a bolt located in the propeller counterweight had broken. Investigators were unable to determine the specific source or cause of the break in the bolt.
Findings
The primary mechanical failure identified was the broken bolt within the propeller counterweight assembly. The inability to identify the origin of this fracture left the root cause undetermined, though it directly contributed to the engine vibration that necessitated the emergency landing.