NTSB Factual Report

On September 19, 2006, at 1140 central daylight time, a Stinson 108, N97651, registered to a private owner, operating as a 14 CFR Part 91 personal flight, had a total loss of engine power in cruise flight and made a forced landing to a bean field in the vicinity of Holladay, Tennessee. Visual meteorological conditions prevailed and no flight plan was filed. The airplane received substantial damage. The private pilot reported no injuries. The flight originated from Morristown, Tennessee, on September 19, 2006, at 1015 eastern daylight time.

The pilot stated he was in cruise flight at 3,500 feet when the engine lost power, "just like it was running out of fuel." The pilot immediately switched the fuel selector valve from the right main fuel tank to the left main fuel tank, applied carburetor heat, and checked that the mixture was full rich. He attempted two engine restarts which were unsuccessful. The pilot observed a forced landing area to his left and entered left traffic to a bean field. While on final approach the pilot moved the fuel selector valve to the off position. The airplane touched down, rolled about 50 feet and nosed over inverted.

Examination of the crash site revealed the airplane came to rest on a heading of 030-degrees magnetic. The airplane was turned over by recovery personnel and examined by the FAA. The upper fuselage and vertical stabilizer received structural damage. Two gallons of fuel was present in the left wing fuel tank and there was evidence of fuel leakage at the accident site. The right main fuel tank had traces of fuel and there was evidence of leakage at the crash site.

The engine assembly did not appear to have any external damage. The engine cowling was installed and the lower engine cowling was damaged. Nine quarts of oil was present in the engine and all lines and hoses were connected. An operational check was completed on the mixture and throttle and no anomalies were noted. The propeller assembly was examined and one propeller blade had a slight bend. The airplane was recovered to Jackson, Tennessee, for further examination. The fuel flow was checked from the wing root to the carburetor and no restrictions were noted. Fuel was added to the airplane and an engine start was initiated. The engine backfired, and rotated during the starting attempts as if there was no compression. The airframe and power plant mechanic did not have the correct adaptor required to conduct a compression test on all six cylinders. The upper spark plugs were removed and a thumb check compression test was completed while the propeller was rotated by hand. One cylinder had compression, and two other cylinders had very low compression. No compression was noted on the remaining three cylinders.

The airplane was transported to Salem, Indiana, for further examination by the FAA. A differential compression test was performed on the engine and verified the compression test preformed in Tennessee. The No. 2, No. 3, and No. 6 rocker covers were removed and the propeller was turned by hand. Crankshaft and connecting rod movement was observed; however no movement of the camshaft was observed during the crankshaft rotation. The accessory case was removed and revealed the crankshaft gear (P/N 17725) which drives the camshaft gear was found in two pieces. The crankshaft gear had failed at the two keyway slots. A small portion of one key was separated and dislodged from its location in the crankshaft and was found in the engine breather, with the major portion of the key remaining in the crankshaft. The other keyway was damaged. The crankshaft gear pieces and key pieces were forwarded to the NTSB Materials Laboratory Division in Washington, D.C. for further analysis. Examination of the components revealed that one of the fracture surfaces on the crankshaft gear displayed features consistent with fatigue initiating in a corner of one of the keyways. Further examination revealed a small radius had been cut into the larger radius of the keyway and created a location for operational stresses to concentrate. The fracture of the other keyway displayed features consistent with an overload event.

The airplane logbooks were returned to the registered owner on September 30, 2006. The components were released to Salem Aircraft Maintenance, Inc. Salem, Indiana, on April 18, 2007.

NTSB Probable Cause

A total loss of engine power in cruise flight due to the fatigue failure of the crankshaft gear resulting in a forced landing and subsequent nose over.