SEA98TA152
SEA98TA152

On July 29, 1998, at 1215 Pacific daylight time, a Cessna TU206G, N756YC, registered to a private owner and operated by the Bureau of Land Management as a public use aircraft, experienced a loss of engine power. During the forced landing on a road located approximately 33 miles southeast of Burns, Oregon, the airplane collided with a fence. Visual meteorological conditions prevailed at the time and a company flight plan was filed. The airplane was substantially damaged and the airline transport pilot received minor injuries. The passenger was not injured. The flight had originated from Vale, Oregon, about two hours prior to the accident.

In written statements from both the pilot and the passenger, they reported that the purpose of the flight was for fire reconnaissance. The pilot stated that after about two hours into the flight, the engine suddenly quit. The pilot attempted a restart without success. The pilot told the passenger to contact dispatch to inform them of the situation. The pilot stated that he began to look for a landing spot and chose a dirt road in a canyon for the emergency landing. The pilot stated that "everything looked good" until final approach when the airplane suddenly lost altitude and the airplane landed short of the road. The airplane touched down hard in sage brush. The airplane continued to roll and collided with a barbed wire fence, which turned the aircraft sideways. The aircraft came to rest perpendicular to the road.

The aircraft was recovered and transported to Boise, Idaho. The engine was inspected and torn down. During the process, it was determined that the crankshaft had failed. The crankshaft and main bearings were removed and sent to the National Transportation Safety Board Materials Laboratory for examination. The metallurgist reported that the crankshaft was broken through the crankcheek between the number two main bearing journal and the number two connecting rod journal. Severe post-fracture mechanical damage totally obliterated the aft fracture face and the fracture features on the periphery of the forward fracture face. The central portion of the fracture face was undamaged. The metallurgist used a binocular microscope which revealed that, "the features over the major portion of the fracture were typical of fatigue cracking that initiated from the aft radius of the No. 2 main bearing journal and propagated through more than 90 percent of the crankcheek before final separation."

The metallurgist noted that "the number two main journal contained slight circumferential scoring in the aft radius and in the cylindrical portion in the area located adjacent to the radius from which the fatigue cracking initiated." The journal was darkly discolored. Heat damage was evident on both fracture faces and on the number two rod-bearing journal adjacent to the fracture face.

A Scanning Electron Microscope (SEM) was used to confirm the fatigue fracture mechanism and locations of the origins of fatigue cracking.

A visual examination of the number two piston revealed that a portion of the piston skirt had broken off from the remainder of the piston. Magnified examination of the piston and separated pieces disclosed that the fracture contained features typical of fatigue cracking. The cracking initiated in the groove for an oil scraping ring from two general origin areas and propagated through approximately 3/4 of the piston wall thickness. The fracture features outside of the fatigue region were typical of an overstress separation.

Both connecting rod bolts were broken. Examination of the mating fracture faces were typical of tensile overload. Neither of the bolts contained any evidence of preexisting cracking.

Extensive damage was noted to the interior of the case in the general area of the number two cylinder. The markings were consistent with the ends of the separated crankshaft scraping on the walls.

Extensive circumferential scoring was noted in the number two and number one main bearing seats. The bearing seat for the number two main bearing was worn to the extent that no evidence of the lubricating groove was visible on the right half of the crankcase.

Maintenance records indicate that the engine was rebuilt by Teledyne Continental Motors in May 1987. The engine was installed in Cessna TU206G, N5203X. In June 1995, after approximately 489 hours since the rebuild, the engine was removed for a major overhaul by SP Aircraft, Boise, Idaho. The engine was then installed in N756YC. The engine remained in the aircraft, and the records indicate routine 100 hour and annual inspections. In September 1997, and approximately 1,145 hours since the major overhaul, a top overhaul was performed. Routine annual and 100 hour inspections continued up to the time of the accident. At the time of the accident, the engine had accumulated a total engine time of approximately 1,880 hours, with 1,391 hours since major overhaul. The manufacturer recommends time between overhauls of 1,400 hours.

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