Engine Malfunction
Approximately 30 minutes into the flight, the engine began to miss, backfire heavily, and sputter. The number 1 cylinder temperature "shot off the gauge," and a red warning light illuminated. The engine subsequently lost power, and the airplane began to descend.
Forced Landing
The pilot executed a forced landing on a highway. During the landing roll, the airplane struck a truck parked on the shoulder, shearing off its left wing. The airplane then overturned and skidded approximately 300 feet before coming to a halt.
Post-Accident Examination
Engine disassembly revealed that the magneto idler gear, which drives the dual magneto, was missing eight teeth. Three of the teeth were displaced from the gear and exhibited signatures consistent with mechanical overload, while the remaining five teeth had been ground off. The respective idler gear shaft bores at the back of the crankcase and at the accessory housing were missing approximately one-third of their circumferential material. The fracture surfaces exhibited signatures consistent with overload. The overload forces appeared to have been applied inboard, evidenced by the symmetrical displacement of the bore material at each end of the subject idler gear shaft.