What happened
During the cruise phase of flight, the engine developed a massive oil leak, which subsequently caused the engine to lose power. The pilot performed a forced landing at the nearest airport. During the approach and touchdown, the pilot was unable to clearly see the runway because oil had covered the windshield. This lack of visibility resulted in a hard landing, which caused the landing gear to collapse upon impact.
The investigation
A post-accident examination of the engine identified that an improper length set-screw had been used to retain the propeller governor idler-gear shaft. Because the screw was too short, it was impossible to properly use staking to secure the screw against the shaft. This allowed the screw to back off, causing the normally stationary shaft to rotate. As the shaft rotated, it cut through an access plug on the front of the engine, creating the path for the oil to drain overboard.
Investigation findings suggest the screw was likely installed during a recent sudden-stoppage inspection when the idler gears were replaced. Lycoming Service Instruction 1343 provides the specific requirements for determining the correct length and installation method for this set-screw.