Primary finding
Probable cause
The loss of engine power due to connecting rod bolt overload failure. The rough, uneven terrain in the emergency landing area was a factor.
Investigator assessment
Analysis narrative
While in cruise flight, the engine suddenly made a loud sound, began to shake, then lost power. The pilot initiated a forced landing to an open field. The airplane touched down, bounced and became airborne before touching down the second time. The nose gear collapsed and the airplane slid to a stop. Post crash examination revealed holes in the crankcase. A teardown of the engine found that the number three and four connecting rod cap bolts separated. A metallurgical examination of the fractured bolts revealed features characteristic of a progressive overstress. The metallurgical specialist reported that the separation most likely occurred due to an inadequately torqued nut or loss of prestress over a period of time.
Source record
Factual narrative
On July 12, 1995, at 1900 Pacific daylight time, a Cessna 172P, N52139, collided with the terrain after the engine experienced a loss of power and the pilot initiated a forced landing to Vashon Island, Vashon, Washington. Visual meteorological conditions prevailed at the time and no flight plan was filed. The airplane was substantially damaged and the flight instructor and private pilot were not injured. The flight had originated from Seattle, Washington, on July 12, 1995, as an instructional flight. During telephone interviews and subsequent written statements, both pilots reported that while in cruise flight at 2,200 feet, the engine made a loud sound, began shaking, then quit. The flight instructor took over the controls while the private pilot secured the cockpit. The flight instructor stated that he initiated a forced landing to an open field. When the airplane touched down, it bounced and became airborne. When the airplane touched down the second time, the nosed gear collapsed and the airplane slid to a stop. Post crash inspection of the engine revealed holes in the top of the crankcase near the number three and four cylinders. After the engine was disassembled, it was found that the number three and four connecting rod cap bolts separated. Both connecting rod assemblies were sent to the National Transportation Safety Board, Materials Laboratory for a metallurgical examination. The specialist reported that "Stereo microscope examination of the fractured bolts revealed features characteristic of a progressive overstress." The specialist further reported that separation of the bolts most likely occurred due to an inadequately torqued nut or loss of prestress over a period of time. The aircraft maintenance logbooks indicate that the number four cylinder and connecting rod assembly had been worked on and signed off on 10/29/93, and had accumulated a total time of 1,066 hours at the time of the accident. The number two and three cylinders and connecting rod assemblies had been worked on and signed off on 6/8/94, and had accumulated a total time of 770 hours at the time of the accident. -