Accident Details
On March 6, 2013, at approximately 1730 Alaska standard time, a Cessna 207 airplane, registration N1704U, sustained substantial damage during a forced landing on snow-covered tundra about 16 miles northeast of Bethel, Alaska. The airplane was operating as Yute Air Alaska Flight 595, a scheduled passenger and cargo flight under visual flight rules and 14 Code of Federal Regulations Part 135. The airline transport pilot and two passengers were not injured. Visual meteorological conditions prevailed at the time, and the flight was operating on a company VFR flight plan. The flight departed Bethel at about 1718 en route to Kalskag, Alaska.
Approximately 15 minutes after departure, the pilot reported that the engine made a "very loud and unusual whining" noise, followed by a partial loss of engine power. The pilot notified the dispatcher via company radio and began a turn toward a frozen pond for an emergency landing. During the turn, all engine power was lost. The pilot then selected a closer area of snow-covered terrain with small trees as a forced landing site. Just before touchdown, the left stabilizer struck the tops of small trees, resulting in substantial damage to the horizontal stabilizer. After touchdown, the nose tire and fork separated from the airplane.
Engine Examination
The airplane was equipped with a Continental Motors IO-520 series engine. A preliminary examination by company maintenance personnel revealed a crack in the crankcase. The engine was subsequently removed and shipped to Alaska Aircraft Engines in Anchorage, then crated and sent to Continental Motors, Inc. Analytical Department in 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.
The examination revealed that most of the engine's major internal damage was associated with the number one piston assembly. According to a report 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. This 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; it 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.
Laboratory Findings
The number one piston assembly, connecting rod, and associated pieces were shipped to the National Transportation Safety Board (NTSB) Materials Laboratory in Washington, DC. An examination on February 12, 2014, revealed fatigue fractures in both connecting rod retaining bolts. Damage to the piston and wrist pin was consistent with damage following the release of the connecting rod from the crankshaft.