Accident Summary
While en route to the destination airport, the pilot heard a loud bang from the engine, described as sounding like something shattering internally. The engine subsequently experienced a total loss of power. The pilot maneuvered the airplane to a pasture, where it collided with a fence during the landing roll.
Engine Inspection and Teardown
According to the engine logbooks, the last annual inspection had been performed about 6 months and 124 hours before the accident. The left magneto had been replaced about 3 months and 54 hours prior. A teardown inspection of the engine revealed that the left idler gear was damaged, with several teeth missing. One tooth was found wedged between the forward face of the gear and the crankcase.
Metallurgical Examination
A detailed metallurgical examination of the left idler gear determined that it failed due to fatigue. Fatigue features were observed at two locations, both having initiated in the radius between a gear tooth flank and the root. The gears that were engaged with the idler gear were intact and mostly undamaged. The fracture surface at one fatigue location was much more reflective than the other. The reflective surface, indicative of rubbing motion of one fracture surface against another, indicated a misalignment of the meshing gears. The fracture surface at the other location was duller, indicating little or no rubbing, consistent with the fracture occurring after the first fracture with the rubbed surface. The second fracture propagated until the material below the gear teeth could no longer support operational stresses, and the portion of the gear located between the two fatigue fractures departed the left idler gear.
Probable Cause
The fatigue was probably initiated by abnormal contact between the left idler gear and the other mating gears. The relatively recent installation of the left magneto gear was suspect as the likely source of the initiating stressors.