NTSB Identification: LAX07LA268.
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Accident occurred Friday, August 24, 2007 in Lopez Island, WA
Probable Cause Approval Date: 03/31/2008
Aircraft: Piper PA-28-181, registration: N2939J
Injuries: 2 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

While en route to the destination airport, the pilot heard the engine generate a loud "bang," as though something had shattered internally. The engine subsequently experienced a total loss of power. The pilot maneuvered the airplane to a pasture where the airplane collided with a fence during the landing roll. According to the engine logbooks, the last engine annual inspection had been performed about 6 months and 124 hours prior to the accident. The left magneto had been replaced about 3 months and 54 hours prior to the accident. A teardown inspection of the engine disclosed that the left idler gear was damaged with several teeth missing and one tooth wedged between the forward face of the gear and crankcase. A detailed metallurgical examination of the left idler gear revealed that it failed due to fatigue. Fatigue features were observed at two locations, and both had 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 on the left idler gear was much more reflective than the other. A reflective surface, indicative of a rubbing motion of one fracture surface against another, indicates a misalignment of the meshing gears to produce the motion. The fracture surface at the other location was duller, indicating that little or no rubbing occurred, 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 the stresses of operation and the portion of the gear located between the two fatigue fractures departed the left idler gear. The fatigue was probably initiated by abnormal contact between the left idler gear and the other mating gears, with the relatively recent installation of the left magneto gear being suspect as the likely source of the initiating stressors.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

A loss of engine power due to the fatigue failure of the left idler gear. Inadequate maintenance installation of the recently installed left magneto was also causal.

Full narrative available

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