The pilot reported that, shortly after takeoff, the Stinson 108’s engine began to sputter and that it eventually lost all power. He was unable to return to the airport and initiated a forced landing to an open field near Lodi, Calif.
During the landing, the plane hit a berm, nosed over, and then came to rest inverted.
A post-accident examination of the engine revealed that the crankshaft and camshaft had broken adjacent to the accessory section.
A metallurgical examination of the crankshaft fracture surfaces revealed a thumbnail-shaped pattern consistent with fatigue that had initiated at the crankshaft surface near the edge of the No. 2 connecting rod journal. It is possible that the fatigue initiated due to the No. 2 connecting rod bearing shifting during operation. The camshaft fracture features were consistent with overstress.
Maintenance records indicated that the last annual inspection was completed about nine months before the accident and that, at that time, the engine had accumulated about 250 hours since major overhaul.
The NTSB determined the probable cause as the fatigue failure of the crankshaft, which resulted in a total loss of engine power and subsequent forced landing.
NTSB Identification: WPR14LA079
This December 2013 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.