Stinson 108 N6532M accident near Lampasas, Texas

Probable cause

Improper case hardening of the crankshaft timing gear, which resulted in the fatigue failure of the gear and a total loss of engine power.

NTSB analysis

While enroute during a cross-country flight, the engine lost total power and the pilot performed a forced landing to a bridge. During the landing, the airplane contacted a cable placed across the bridge and nosed over, which substantially damaged the rudder.

Examination of the engine revealed that three teeth of the crankshaft timing gear had separated, and approximately 60 to 90 percent of the remaining gear teeth had been deformed and flattened, which stopped rotation of the camshaft and resulted in a total loss of power. The liberated teeth exhibited evidence of fatigue cracks and subsequent overstress fractures. The crankshaft timing gear and liberated teeth did not meet the 50 HRC (513 HV) standard for case hardening. No evidence of the engine overheating was found, which eliminated the possibility of an altered case hardening of the crankshaft timing gear via engine heat.

A review of maintenance records revealed that the engine had been overhauled about 58 operating hours before the accident and all ferrous metal parts were non-destructive tested (NDT), including the crankshaft timing gear; the testing did not identify surface or subsurface cracks. An annual inspection of the engine was performed about 8.4 flight hours before the accident and revealed no anomalies. The accident crankshaft timing gear was likely the original gear to the engine.

Text from the NTSB. Open the NTSB docket for WPR23LA117 for the full record.