Amateur-built FISHER DAK N359JM accident near Bozeman, Montana

Probable cause

A loss of engine power due to shearing of the crankshaft gear cluster bolts. Contributing to the accident was the lack of regular overhaul periods as recommended by the engine manufacturer.

NTSB analysis

The pilot was departing on a local flight when the engine lost power; the pilot force-landed the airplane in a field. Post accident examination of the engine showed that the bolts securing the crankshaft gear cluster to the crankshaft were sheared. The crankshaft gear cluster is geared to the camshaft gear and rotates as the engine operates. Examination of the bolts showed that all four bolts had sheared between the crankshaft gear cluster and the attach point to the crankshaft. Two bolt heads, including a portion of the bolts’ lengths, were missing. The other portions of these bolts remained threaded to the crankshaft attach points. The two remaining bolt heads were still safety-wired and in place; however, they had sheared from the crankshaft. The lower threaded portions of all four bolts remained threaded at the crankshaft attach points. No pieces of the missing bolt heads or bolt portions were identified in the accessory case or the oil sump. Examination of the fracture surfaces on the threaded bolt portions showed that the surfaces of the sheared bolts with the missing heads appeared battered. The portions of the bolts still threaded in the crankshaft that belonged to the remaining bolt heads were not battered. The engine had not undergone a complete overhaul of the type that would have exposed the accessory gear section to inspection for over 35 years. According to Teledyne Continental Motors Service Instruction Letter 98-9A, the C-85 engine should be overhauled every 12 years or 1,800 hours.

Text from the NTSB. Open the NTSB docket for WPR10LA003 for the full record.