Cessna 172 N116SV accident near Port Orange, Florida

Probable cause

The separation of the nut from the stud and the fracture of the bolt that secured the crankshaft idler gear to the engine crankcase, which resulted in a total loss of engine power due to the inability of the left crankshaft idler gear assembly to drive of the camshaft.

NTSB analysis

After an uneventful 1-hour flight, the flight instructor and student pilot were returning to the airplane’s home base. The flight instructor pointed out to the student that the airplane had descended below the assigned altitude, and the student replied, “we have no power.” The flight instructor took over the flight controls and attempted to restore power. Because that effort was unsuccessful, the flight instructor pitched the airplane to achieve the best glide speed. After realizing that the airplane would not reach the nearest airport, he performed a forced landing in a field, and the airplane nosed over during the landing roll.

Postaccident examination of the engine revealed that the camshaft was not being driven by the left crankshaft idler gear assembly because it was out of position. The nut that secured the idler gear to a stud at the left crankcase was separated, and a bolt that also secured the idler gear to the left crankcase was fractured. Extensive damage was noted to the left crankcase in the area that supported the idler gear and the area into which the securing bolt was threaded, indicating movement of the crankshaft idler gear shaft for an extended period of time. The available evidence for this investigation did not allow a determination regarding the reason for the separation of the nut from the stud and the fracture of the bolt that secured the idler gear to the crankcase.

The engine was last overhauled about 11 years and 3,488 hours before the accident. The owner/operator of the airplane at the time of the accident stated that, during the time that he owned the airplane, maintenance personnel had not cut open the oil filter to inspect the filter element during each oil change. Photographic evidence showed the presence of non-ferrous particles in the oil filter element. It is likely that these particles were from the damaged left crankcase. If maintenance personnel had cut open the oil filter and inspected the filter element during each oil change (the most recent of which was during the last inspection of the engine 3 months before the accident), it is possible that they would have detected the presence of non-ferrous particles in the filter element and taken action to identify the source.

Text from the NTSB. Open the NTSB docket for ERA21LA388 for the full record.