What happened
While cruising at 3,500 feet, the pilot of the aircraft experienced a sudden loss of engine power, describing the sensation as if the engine were running out of fuel. In response, the pilot switched the fuel selector valve from the right main tank to the left main tank, applied carburetor heat, and ensured the mixture was set to full rich. After two unsuccessful attempts to restart the engine, the pilot identified a suitable forced landing area in a bean field to the left.
During the final approach for the landing, the pilot moved the fuel selector valve to the off position. Upon touchdown, the aircraft rolled approximately 50 feet before nosed over and came to rest inverted. The accident resulted in no fatalities.
The investigation
The aircraft was recovered and examined by the FAA, with structural damage noted on the upper fuselage and vertical stabilizer. Investigators found two gallons of fuel in the left wing tank and evidence of leakage at the site; traces of fuel were also found in the right main tank.
A differential compression test was performed on the engine. Upon removing the No. 2, No. 3, and No. 6 rocker covers and rotating the propeller by hand, investigators observed movement in the crankshaft and connecting rod but no movement in the camshaft during rotation. Further inspection of the accessory case revealed that the crankshaft gear (P/N 17725), which drives the camshaft gear, had failed at the two keyway slots and was broken into two pieces. A portion of one key had been dislodged and found within the engine breather.
Findings
Analysis of the fractured crankshaft gear surfaces indicated that a fatigue crack initiated in a corner of one of the keyways. This was attributed to a small radius that had been cut into the larger radius of the keyway, creating a point for concentrated operational stress. Additionally, the fracture in the second keyway showed evidence of an overload event.