What happened
Approximately 30 minutes into the flight, the engine began to miss, backfire heavily, and sputter. During this period, the number one cylinder temperature increased rapidly, exceeding the limits of the gauge and triggering a red warning light. As the engine lost power, the aircraft began a descent.
The pilot performed a forced landing on a highway. During the landing roll, the Cessna struck a truck parked on the shoulder, which resulted in the left wing being sheared off. Following the impact, the aircraft overturned and skidded 300 feet before coming to a halt. There were no fatalities reported.
The investigation
An examination of the engine disassembly revealed that the magneto idler gear, which drives the dual magneto, was missing eight teeth. Investigators found that three of these teeth had been displaced from the gear and showed signs consistent with mechanical overload. The remaining five teeth had been ground off.
Additionally, the idler gear shaft bores located at the back of the crankcase and at the accessory housing were missing approximately one-third of their circumferential material. The fracture surfaces on these bores exhibited signatures of mechanical overload. The evidence suggested that the overload forces were applied inboard, as indicated by the symmetrical displacement of the bore material at both ends of the idler gear shaft.