Cessna 150L N6622G accident near Trenton, Michigan

Probable cause

The loss of preload torque on a connecting rod nut and bolt, which precipitated a separation of the connecting rod from the engine's crankshaft and resulted in the total loss of engine power.

NTSB analysis

The pilot had been conducting a banner towing operation for about 55 minutes when he felt a heavy vibration and heard a “loud bang.” The airplane's engine subsequently lost power, and the pilot executed a forced landing. During the forced landing, the nose landing gear collapsed when it contacted "heavy snow and unimproved terrain," resulting in substantial damage to the firewall. A postaccident examination of the engine revealed that one of the connecting rods had separated from the crankshaft. Metallurgical examination determined that one of the two connecting rod bolts had failed in overstress. The second connecting rod bolt was deformed but otherwise intact; its associated nut had separated from the bolt and was undamaged. The lack of damage to one of the connecting rod nuts in conjunction with the overstress failure of the opposing bolt was consistent with a loss of installation torque on the intact nut. The engine had accumulated 2,836 hours since overhaul. The operator did not supply engine overhaul maintenance records but provided a statement indicating that the installed bolts and nuts were new at the engine cylinder's last maintenance (an engine manufacturer service bulletin instructs that connecting rod bolts and nuts be replaced any time they are removed). However, due to the lack of maintenance records and the number of hours since last overhaul, the investigation could not conclusively attribute the loss of preload torque to the overhaul operation.

Text from the NTSB. Open the NTSB docket for CEN11LA182 for the full record.