N1704U

On March 6, 2013, about 1730 Alaska standard time, a Cessna 207 airplane, N1704U, sustained substantial damage during a forced landing on snow-covered tundra about 16 miles northeast of Bethel, Alaska. The airplane was operated as Flight 595, by Yute Air Alaska, Anchorage, Alaska, as a visual flight rules, scheduled passenger/cargo flight under the provisions of 14 Code of Federal Regulations Part 135. The airline transport pilot and the two passengers were not injured. Visual meteorological conditions prevailed at the time of the accident, and the airplane was operating on a company VFR flight plan. The flight had departed Bethel about 1718, en route to Kalskag, Alaska.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge on March 6, the director of operations for Yute Air Alaska reported that the flight originally departed Bethel destined for Kalskag. The pilot reported that about 15 minutes after departure, the engine made a "very loud and unusual whining" noise, which was followed by a partial loss of engine power. He reported the problem to his dispatcher via the company radio frequency, and began a turn toward a frozen pond for an emergency landing. During the turn, all engine power was lost. He then selected a closer area of snow-covered terrain, with small trees, as a forced landing site. Just prior to touchdown, the left stabilizer struck the tops of the trees, resulting in substantial damage to the horizontal stabilizer. After touchdown, the nose tire and fork broke free of the airplane.

The airplane was equipped with a Continental Motors IO-520 series engine. A preliminary examination of the engine by company maintenance personnel revealed a crack in the crankcase.

The airplane was recovered to a company facility where the engine was removed and shipped to Alaska Aircraft Engines, Anchorage for further examination. The engine was subsequently crated and shipped to Continental Motors, Inc. Analytical Department, Mobile, Alabama for disassembly and inspection.

On May 15, 2013 under the supervision of an FAA District Office MIDO Safety Inspector, the engine was disassembled and examined at Continental Motors.

The examination revealed that most of the engine's major internal damage was associated with the number one piston assembly.

According to a report prepared by the manufacturer's investigator, the number one piston was fractured at the piston pin bore, releasing the number one rod and piston pin assembly at the small end. The unsupported number one rod and piston pin assembly caused damage to the rear section of the crankcase and camshaft. The number one rod had released from the crankshaft, and exhibited extreme mechanical damage. The number one rod was found in the oil sump. There were no signs of lubrication distress. Portions of the piston skirt were recovered from the oil sump and inspected in the CMI lab. The number one cylinder, piston, rod, and sump contents were retained in bonded storage.

The number one piston assembly, connecting rod, and associated pieces were shipped to the National Transportation Safety Board Materials Laboratory, Washington, DC.

An examination of the number one piston assembly at the NTSB laboratory on February 12, 2014, revealed fatigue fractures in both connecting rod retaining bolts. Damage to the piston and wrist pin were consistent with damage following the release of the connecting rod from the crankshaft. A copy of the NTSB laboratory report is in the public docket for this case.