What happened
The pilot reported that upon arrival at Fort Collins, while maintaining cruise altitude, the engine suddenly lost power. During the subsequent emergency descent maneuvering, the pilot lost sight of the airport and landed in a plowed field.
The investigation
A post-crash examination of the engine revealed that the crankshaft accessory drive gear had independently rotated from the crankshaft. The retaining dowel, part number STD-1065, was found sheared. Additionally, the retaining bolt was backed out five or six turns. Although the safety clip was in place and functional, it was not engaged.
Findings
The investigation determined that fuel exhaustion was not the cause, but rather a mechanical failure of the engine's internal components. The Cessna 172 suffered a critical separation due to the sheared dowel and unengaged safety clip.