N29137Cessna U206C · 1968

On June 9, 2014, about 1305 Alaska daylight time, a Cessna U206C airplane, N29137, sustained substantial damage during a forced landing, following a loss of engine power near Chitina, Alaska. The airplane was operated by Copper Valley Air Service, Glennallen, Alaska, as a visual flight rules (VFR) scheduled commuter flight under the provisions of 14 Code of Federal Regulations (CFR) 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, Alaska, at 1235 destined for McCarthy, Alaska.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) 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 wings and horizontal stabilizer.

The engine was removed from the airframe and sent to the facilities of Continental Motors, Inc. (CMI) Mobile, Alabama, for further examination. The engine was examined on February 24, 2015, by representatives from CMI under the supervision of the NTSB IIC, along with another NTSB investigator. A visual inspection revealed that the right crankcase half was cracked, beginning at the number one cylinder mount 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.

A CMI metallurgist, with permission from the NTSB IIC, 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.

The closest weather reporting facility was Gulkana Airport, about 58 miles northwest of the accident site. At 1253, a weather observation from Gulkana Airport was reporting, in part: wind from 160 degrees, at 9 knots; visibility, 10 statute miles; clouds and sky condition, 7,000 feet overcast, temperature, 57 degrees F; dew point 41 degrees F; altimeter, 29.91 inHG.