This August 2007 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Aircraft: Piper Cherokee.
Location: Lopez Island, Wash.
Aircraft damage: Substantial.
What reportedly happened: While en route to the destination airport, the pilot heard an explosion coming from the engine compartment, followed by a total loss of power. He maneuvered the plane to a pasture for an emergency landing. The airplane touched down normally, but during the landing roll the plane hit a fence.
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. Investigators determined that 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. According to the logbooks, the last engine annual inspection had been performed about six months and 124 flight hours before the accident. The left magneto had been replaced about three months and 54 hours before the accident.
Probable cause: A loss of engine power due to the fatigue failure of the left idler gear.
For more information: NTSB.gov