ANC96TA095

The pilot was in cruise flight over mountainous terrain in a float equipped airplane. The engine suddenly lost power, and the pilot performed an emergency landing on tundra covered terrain. During the landing, the airplane floats dug into the tundra, and the airplane turned completely over. A postaccident examination of the engine revealed a fatigue failure of the crankshaft.

On July 4, 1996, about 1405 Alaska daylight time, a float equipped Cessna 185, N185CA, crashed during a forced landing, about 35 miles north of Fairbanks, Alaska. The airplane was being operated as a visual flight rules (VFR) local area government flight under Title 14 CFR Part 91 when the accident occurred. The airplane, registered to the U.S. Department of the Interior and operated by the U.S. Fish and Wildlife Service, was destroyed. The certificated commercial pilot, the sole occupant, received minor injuries. Visual meteorological conditions prevailed. A VFR flight plan was filed. The flight originated at the Fairbanks International airport, at 1300.

The pilot reported that he was in cruise flight at 5,500 feet mean sea level (msl) over mountainous terrain that was about 4,000 feet high. He indicated that he heard a slight change in the engine sound and then it suddenly lost power. The pilot made a radio "mayday" call and began emergency procedures in an attempt to restore engine power. None were successful and the engine propeller completely stopped. The pilot performed an emergency landing in a previously burned area of tundra and muskeg. During the landing, the airplane's right float assembly dug into the ground and the airplane nosed completely over, coming to rest upright on the damaged floats.

After recovery, the engine was examined on July 18, 1996, in Anchorage, Alaska, at the Department of Interior maintenance hanger. The examination revealed metal particles in the oil filter. Metal flakes and scoring were noted in the oil pump. Removal of the oil sump revealed pieces of metal and bearing material in the bottom of the sump. One of the number 2 connecting rod bearing cap bolts was sheared near the head of the bolt. The nut and cotter pin were still attached to the separated segment of bolt.

The crankshaft, serial number 1289007N, was separated at the number 3 crankshaft cheek between the number 2 main bearing and the number 2 cylinder rod bearing. The number 2 main bearing saddle on each half of the engine case was distorted and flattened. The bearing was distorted and flattened. Small portions along the edges of the bearing were broken and located in the oil sump.

Examination of the engine maintenance records revealed it was rebuilt by Teledyne Continental Motors, Mobile, Alabama, on December 11, 1990. The engine, an IO-520-D, serial number 282598-R, was installed in the accident airplane on April 18, 1991. At the time of the accident, the engine had accrued 1,449.3 hours of operation since the rebuild. On September 13, 1994, 1,040 hours since the engine was rebuilt and 409 hours before the accident, the number 1, 2, and 5 cylinders were replaced. The last annual inspection on the engine and airframe was conducted on June 7, 1996, 33 hours prior to the accident.

The crankshaft was submitted to the National Transportation Safety Board, Materials Laboratory, Washington, D.C., for examination. The laboratory reported the crankshaft exhibited features typical of a fatigue crack emanating from multiple origins along the aft radius of the number 2 main journal.