ANIAK, AK, USA
N70076
Cessna 207
During cruise flight, the pilot noted a loss in engine RPM and oil pressure at the same time. He performed an emergency landing on a frozen lake. During the landing roll, the right wingtip struck a spruce tree, and the airplane was substantially damaged. During an examination, the accessory section of the engine did not rotate, when the propeller was turned by hand. A teardown of the engine revealed that the crankshaft was fractured at the number 3 short cheek, between the number 2 rod bearing and the number 2 main bearing. The engine contained numerous fragments from the number 2 main bearing. The number 2 bearing saddle was severely worn and deformed. Other bearing saddles exhibited scoring. A metallurgical examination of the crankshaft revealed a fatigue failure at the aft radius of the number 2 main bearing journal adjacent to heavy rubbing marks from the number 2 main bearing.
On November 18, 1995, about 1330 Alaska standard time, a wheel equipped Cessna 207, N70076, collided with a spruce tree during a forced landing, about 10 miles southeast of Aniak, Alaska. The airplane was being operated as a visual flight rules (VFR) cross-country positioning flight under Title 14 CFR Part 91 when the accident occurred. The airplane, operated by Mark Air Express, Anchorage, Alaska, sustained substantial damage. The certificated airline transport pilot and the sole passenger were not injured. Visual meteorological conditions prevailed. VFR company flight following procedures were in effect. The flight originated at the Bethel airport, Bethel, Alaska, about 1227. The operator reported that the pilot noted a simultaneous loss in engine RPM and oil pressure and he performed an emergency landing on a frozen lake. During the landing roll, the right wingtip struck a spruce tree. The airplane received damage to the right wing outboard wing ribs and the right aileron. A company maintenance team reported that the accessory section of the engine did not rotate when the propeller was turned by hand. The wing damage was confirmed on November 30, 1995, after the engine was replaced and the airplane was flown to Anchorage. Examination of the maintenance records indicated that the engine, an IO-520-F13B, serial number 286336-R, was rebuilt/zero timed by the manufacturer on November 2, 1992. The engine then accrued 1,692.8 hours of operation. It was overhauled on October 31, 1994, and then installed in the accident airplane on November 17, 1994. At the time of the accident, the engine had accrued 933.1 hours since being overhauled. On January 18, 1996, a teardown examination of the engine revealed that the crankshaft, serial number 880744, was fractured at the number 3 short cheek, between the number 2 rod bearing and the number 2 main bearing. The engine contained numerous fragments from the number 2 main bearing. The number 2 bearing saddle was severely worn and deformed. Other bearing saddles exhibited scoring. The right side of the engine case exhibited gouging and impact marks adjacent to the number one cylinder. The spark plugs exhibited a gray, dry appearance. The number one piston skirt was deformed and jammed against the engine case. A segment of the number two piston skirt was broken circumferentially at the bottom of the piston. The number one piston connecting rod was damaged and gouged at the lower end of the rod, adjacent to the crankshaft. The oil pump housing exhibited scoring of the walls. The camshaft exhibited impact marks at the number two cylinder, 180 degrees apart between the intake and exhaust cam lobes. The camshaft was bent about 10 degrees at the same point. The cam lobe for the number 2 cylinder exhibited a step in one-half of the lobe, adjacent to the intake valve lifter. The number 2 cylinder intake valve lifter exhibited pitting of the lifter to cam lobe mating surface. All other valve lifters were undamaged. The crankshaft, camshaft, and valve lifter were examined by the National Transportation Safety Boards's Materials Laboratory on July 19, 1996. The metallurgical examination of the crankshaft revealed a series of rachet marks indicating multiple fatigue initiations from the exterior surface of the aft radius of the number 2 main journal. Examination of the radius from which the fatigue cracking initiated revealed heavy rubbing from contact with the number 2 main bearing. Examination of the number 2 bearing revealed flattening and deformation of the bearing material. One edge of the bearing was lipped over and shaped in the radius between the number 2 main journal and crankcheek. Examination of the camshaft revealed excessive wear of the number 2 cylinder intake valve lobe. The mating surface of the number 2 cylinder intake valve lifter was peened and contained spalling damage.
fatigue failure of the engine crankshaft at the number 2 main bearing journal, where rubbing contact had occurred with the number 2 main bearing. A factor relating to the accident was: the lack of suitable terrain for an emergency landing.
Source: NTSB Aviation Accident Database
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