WPR14LA079

The pilot reported that, shortly after takeoff, the engine began to sputter and that it eventually lost all power. The pilot was unable to return to the airport and initiated a forced landing to an open field. During the landing, the airplane struck a berm, nosed over, and then came to rest inverted. A postaccident 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 9 months before the accident and that, at that time, the engine had accumulated about 250 hours since major overhaul.

HISTORY OF FLIGHT On December 27, 2013, about 1055 Pacific standard time, a Stinson 108-1 airplane, N8848K, experienced a loss of engine power, and the pilot made a forced landing in an open field near Lodi, California. The owner/pilot operated the airplane under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. The pilot and one passenger were not injured. The airplane sustained substantial damage to its wings and fuselage when it struck a berm and nosed over during the landing roll out. The airplane had departed from the Kingdon Airpark (O20), Lodi, about 1030, and was destined for the Modesto City-County Airport-Harry Sham Field (MOD), Modesto, California. Visual meteorological conditions prevailed for the local area flight, and no flight plan had been filed. According to a responding deputy from the San Joaquin Sheriff's Department, the pilot reported that shortly after takeoff the engine started to sputter. He turned back to the departure airport, and attempted to restart the engine. After switching fuel tanks, the engine regained full power, then quit completely. The pilot made a forced landing to an open field, which appeared to be free of obstructions. However, during the landing rollout, the airplane struck a berm and nosed over coming to rest inverted. According to a Federal Aviation Administration (FAA) inspector, who responded to the accident site, the airplane traveled about 400 yards before impacting the ditch. The FAA inspector reported that the pilot had just put in new shoulder harness type seatbelts in the airplane. The engine was examined at Plain Parts, Pleasant Grove, California, on January 10, 2014. A visual examination revealed that the right side of the engine crankcase near the accessory section had separated. The spark plugs, rocker box covers, and the top of the engine case were removed. The propeller was manually manipulated back and forth; however, full rotation could not be achieved due to damage to the crankshaft, but visible movement of the crankshaft was noted at the rear of the engine. Investigators reported that the crankshaft and cam shaft near the accessory section had broken, and several bearings had been damaged. The crankshaft with a portion of the number one connecting rod, crankshaft main journal bearing, crankshaft connecting rod journal bearing, and a camshaft were shipped to the National Transportation Safety Board (NTSB) metallurgical laboratory in Washington, DC, for further examination. TEST AND RESEARCH The NTSB metallurgical laboratory specialist documented the submitted components; the crankshaft was received in two fractured pieces. The fracture occurred through the web between the numbers one and two connecting rod journals. The fracture face was relatively flat with a thumbnail-shaped pattern, which was consistent with fatigue. The fatigue crack initiated at the crankshaft surface, with the primary fatigue crack propagating through 79 percent of the webcross-section. A second fatigue crack had propagated from the opposite side of the fractured crankshaft web and comprised about 5 percent of the web fracture surface. The remaining fracture surface had features consistent with overstress. The specialists reported that the primary fatigue crack propagated from the surface of the crankshaft near the edge of a darkened area on the number two connecting rod journal. According to the specialist, the fracture features of the camshaft were consistent with overstress. The specialists' report is attached to the public docket for this accident. Maintenance records indicated that during the last annual inspection dated March 12, 2013, the engine had accumulated about 250 hours since the major overhaul.