Univair Aircraft Corporation 108-1 N8721K accident near Tuftonboro, New Hampshire

Probable cause

Maintenance personnel’s failure to remove circumferential gouges in the aft radius of the R2 rod journal on the crankshaft during its last engine overhaul, which resulted in the fatigue failure of the crankshaft and a subsequent total loss of engine power.

NTSB analysis

About 7 minutes after takeoff and while leveling off the airplane at 1,800 ft, the pilot leaned the fuel-to-air mixture, and the engine immediately began to run roughly. The more experienced right-seat pilot took control of the airplane and attempted to restore power, but his attempts were unsuccessful. The propeller continued to rotate briefly but then stopped. The pilot maneuvered the airplane for a forced landing in a field, and the airplane subsequently impacted a raised concrete culvert.

Postaccident examination of the engine revealed that the crankshaft was fractured. Metallurgical examination of the crankshaft revealed evidence of fatigue cracking that had originated from circumferential gouges at the aft radius of the R2 rod journal. A review of engine records revealed that, about 45 years before the accident, the crankshaft was replaced during an engine overhaul and that the main and connecting rod bearing journals were ground 0.010-inch undersize. About 7 years before the accident, the engine was overhauled again, and the crankshaft main and connecting rod bearing journals were polished. The engine was installed in the accident airplane about 3 years before the accident; the engine failure occurred about 115 hours after the engine's last overhaul. It is likely that, if maintenance personnel had detected and removed the circumferential gouges at the aft radius of the R2 rod journal during the engine's last overhaul, the fatigue cracking would not have started and the crankshaft would not have failed.

Text from the NTSB. Open the NTSB docket for ERA13LA327 for the full record.