History of Flight
On December 27, 2013, at about 1055 Pacific standard time, a Stinson 108-1 airplane, registration N8848K, experienced a loss of engine power. The pilot conducted a forced landing in an open field near Lodi, California. The owner-pilot operated the airplane under 14 CFR Part 91 as a personal flight. The pilot and one passenger were not injured. The airplane sustained substantial damage to its wings and fuselage after striking a berm and nosing over during the landing rollout.
The flight had departed Kingdon Airpark (O20) in Lodi at about 1030, bound for Modesto City-County Airport-Harry Sham Field (MOD) in Modesto, California. Visual meteorological conditions prevailed, and no flight plan was filed.
According to a responding deputy from the San Joaquin Sheriff's Department, the pilot reported that shortly after takeoff the engine began to sputter. The pilot turned back toward the departure airport and attempted to restart the engine. After switching fuel tanks, the engine regained full power momentarily, then quit completely. The pilot executed a forced landing to an open field that appeared clear of obstructions. However, during the landing rollout, the airplane struck a berm and nosed over, coming to rest inverted.
A Federal Aviation Administration (FAA) inspector who responded to the accident site noted that the airplane traveled about 400 yards before impacting a ditch. The FAA inspector also reported that the pilot had recently installed new shoulder harness-type seatbelts.
Examination
The engine was examined on January 10, 2014, at Plain Parts in Pleasant Grove, California. Visual examination revealed that the right side of the engine crankcase near the accessory section had separated. After removing spark plugs, rocker box covers, and the top of the engine case, investigators manually manipulated the propeller back and forth. Full rotation could not be achieved due to crankshaft damage, but visible movement of the crankshaft was noted at the rear of the engine. Investigators reported that the crankshaft and camshaft near the accessory section had broken, and several bearings were 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 sent to the NTSB metallurgical laboratory 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 number one and number two connecting rod journals. The fracture face was relatively flat with a thumbnail-shaped pattern consistent with fatigue. The fatigue crack initiated at the crankshaft surface, with the primary fatigue crack propagating through 79 percent of the web cross-section. A second fatigue crack propagated from the opposite side of the fractured crankshaft web, comprising about 5 percent of the web fracture surface. The remaining fracture surface exhibited features consistent with overstress. The primary fatigue crack originated from the surface of the crankshaft near the edge of a darkened area on the number two connecting rod journal. The fracture features of the camshaft were consistent with overstress. The specialist's 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.