Accident Overview
On June 9, 2014, about 1305 Alaska daylight time, a Cessna U206C airplane, registration N29137, sustained substantial damage during a forced landing near Chitina, Alaska. The airplane was operated by Copper Valley Air Service of Glennallen, Alaska, as a visual flight rules (VFR) scheduled commuter flight under 14 Code of Federal Regulations Part 135. The certificated airline transport pilot and sole passenger were not injured. Visual meteorological conditions prevailed, and company flight following procedures were in effect. The flight departed Gulkana Airport, Glennallen, at 1235, destined for McCarthy, Alaska.
Pilot Statement and Landing
During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge on June 10, the pilot stated that he was in level cruise flight when he heard a loud "bang," followed by a quieter "bang," prior to a total loss of engine power. He made a forced landing to an area of densely populated black spruce trees. During the forced landing, the airplane nosed over, sustaining substantial damage to the wings and horizontal stabilizer.
Engine Examination
The engine was removed from the airframe and sent to the facilities of Continental Motors, Inc. (CMI) in Mobile, Alabama, for further examination. The engine was examined on February 24, 2015, by representatives from CMI under the supervision of the NTSB investigator-in-charge and another NTSB investigator. A visual inspection revealed that the right crankcase half was cracked, beginning at the number one cylinder mount and continuing upward and slightly forward about 3 inches. The examination revealed that most of the engine's major internal damage was associated with the number one piston assembly. The number one piston was fractured at the piston bore, releasing the number one rod and piston pin assembly. 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. There were no signs of lubrication distress.
Metallurgical Findings
A CMI metallurgist, with permission from the NTSB investigator-in-charge, examined the number one connecting rod retaining bolts. The metallurgical examination revealed beach marks on the fractured surfaces of both connecting rod bolts consistent with fatigue fractures that initiated at the surface.
Weather Conditions
The closest weather reporting facility was Gulkana Airport, about 58 miles northwest of the accident site. At 1253, a weather observation from Gulkana Airport reported: wind from 160 degrees at 9 knots; visibility, 10 statute miles; clouds and sky condition, 7,000 feet overcast; temperature, 57 degrees Fahrenheit; dew point, 41 degrees Fahrenheit; altimeter, 29.91 inches of mercury.