Piper J3C-65 N77583 accident near Belfair, Washington

Probable cause

The total loss of engine power due to the fatigue failure of the crankshaft accessory drive gear.

NTSB analysis

The flight instructor reported that during an instructional flight, while approaching the airport to land, there was a loud noise followed by a total loss of engine power. The flight instructor took control of the airplane and initiated a forced landing to an open area. During the landing roll, the airplane impacted rough and rising terrain. The left wing was partially separated, and landing gear was collapsed.

Postaccident examination of the engine found the crankshaft accessory drive gear had failed. Metallurgical examination of the crankshaft and gear remnants revealed the fracture surfaces of two of the gear teeth had signatures of fatigue. Mechanical damage to the terminus of the observed fatigue cracks prevented determination of the initiating event that resulted in the eventual failure of the gear. The failed gear prevented both magnetos from operating, which resulted in a total loss of engine power due to a lack of ignition.

Text from the NTSB. Open the NTSB docket for WPR23LA215 for the full record.