Accident Overview
A pilot who had intended to practice instrument procedures in visual meteorological conditions reported having no memory of the accident flight. A witness observed the airplane on a "modified right base" to the airport and shortly after heard the sound of the airplane impacting a lake.
Engine Examination
Post-accident examination of the engine revealed that the magneto idler gear support pin and gasket were dislodged from the two attachment studs. The lock washers and both nuts for the attachment studs were not located, and the threads on the studs were unremarkable. When the right magneto was removed, the idler gear was observed to be disengaged from the magneto drive gears. No additional mechanical anomalies were found with the engine.
Maintenance History
The last annual inspection was performed approximately 77 hours prior to the accident. The entry for the annual inspection in the engine logbook included that both magnetos had been removed during the inspection and were "reinstalled...and timed to specs."
Conclusion
The engine's magneto idler gear support pin and gasket were dislodged, resulting in the idler gear disengaging from the drive gears. The nuts and lock washers for the attachment studs were missing. The engine exhibited no other mechanical anomalies.