Accident Sequence
During a base leg approach to an airport, the airplane's engine lost power. The pilot executed a forced landing to a swampy area short of the runway.
Examination Findings
Post-accident examination of the engine disclosed that within the accessory section, the idler gear assembly—normally responsible for transferring power from the crankshaft to the left magneto and the camshaft—had been displaced. The mounting shaft was also displaced, and its hole in the crankcase was elongated. The upper portions of both screws retaining the shaft mounting plate were missing. The lower screw appeared to have sheared, while the remnant of the upper screw exhibited mechanical damage. The crankcase surface around the upper screw hole showed fretting. The idler gear itself had missing and damaged gear teeth. Remnants of the shaft mounting plate retaining screws and the screw lockplate were found at the bottom of the crankcase.
Maintenance History
The engine's operating time was below the manufacturer's recommended time between overhaul (TBO). However, the manufacturer also recommended an overhaul if the TBO was not reached within the previous 12 years. The accident engine had not been overhauled in over 30 years. The specific sequence of the failure could not be determined. Although an excessive amount of time had passed since the previous overhaul, no correlation could be established between the lack of overhaul and the displacement of the idler gear assembly.