NTSB Factual Report

HISTORY OF FLIGHT

On October 3, 2009, at 0948 mountain daylight time, N359JM, a Mateskon Fisher Dakota Hawk, lost engine power during cruise and the pilot force-landed the airplane in a field approximately 5 miles southwest of Gallatin Field, Bozeman, Montana. During the landing, the right wing clipped a tree. The private pilot, who was also the registered owner of the airplane, operated it under the provisions of 14 Code of Federal Regulations Part 91. The pilot sustained minor injuries and the airplane was substantially damaged. Visual meteorological conditions prevailed and no flight plan was filed for the local area flight.

The pilot reported that he had departed Gallatin Field about 10 minutes prior to the power loss. The power loss was sudden. He attempted to restart the engine but was unsuccessful. During his approach to a field, the right wing clipped a tree.

AIRPLANE HISTORY

The pilot had built the airplane over a period of three years. It had accrued approximately 141.4 hours. The pilot indicated that the engine, a Teledyne Continental Motors C-85-12, had been installed on another airplane in the 1980s. The previous engine owner was going to install it on another airplane but did not do so and the engine had been placed into storage. The pilot then purchased the engine for his airplane. The pilot did note that recently he had the piston rings changed due to a higher than normal oil burn. The pilot did not have any other problems with the engine.

Review of the maintenance records for the engine showed that the last engine overhaul was a field overhaul that occurred on December 4, 1973. The engine was removed from another airplane in 1987, at 1085.8 hours since major overhaul, and 4,807.8 hours total time. It was in storage until the pilot/builder of the accident aircraft purchased it. The last top overhaul occurred on June 1, 2006. A typical top overhaul does not involve inspection of the accessory gear section.

TESTS AND RESEARCH

The engine was examined following the accident. The valve covers were removed and the propeller was rotated. No valve movement was evident. Investigators then removed the magnetos and starter from the accessory case and noted that none of the accessory gears turned when the propeller was rotated. Further examination revealed that the crankshaft gear cluster had separated from the crankshaft.

The crankshaft gear cluster is geared to the camshaft gear and rotates as the engine operates. The crankshaft gear cluster is attached to the aft section of the crankshaft by four bolts. 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 in 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.

ADDITIONAL INFORMATION

According to TCM Service Instruction Letter 98-9A, the C-85 engine should be overhauled every 12 years or 1,800 hours.

NTSB 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.