Saturday, May 13, 2017

Cessna U206C Super Skywagon, N29137: Accident occurred June 09, 2014 in Chitina, Alaska

Aviation Accident Final Report - National Transportation Safety Board: https://app.ntsb.gov/pdf

Investigation Docket - National Transportation Safety Board: https://dms.ntsb.gov/pubdms

Aviation Accident Data Summary - National Transportation Safety Board:  https://app.ntsb.gov/pdf

http://registry.faa.gov/N29137

Additional Participating Entities:  
Federal Aviation Administration / Flight Standards District Office; Anchorage, Alaska
Continental Motors; Mobile, Alabama

Copper Valley Air Service; Glennallen, Alaska

NTSB Identification: ANC14LA040
Scheduled 14 CFR Part 135: Air Taxi & Commuter
Accident occurred Monday, June 09, 2014 in Chitina, AK
Probable Cause Approval Date: 05/01/2017
Aircraft: CESSNA U206C, registration: N29137
Injuries: 2 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

While in level cruise on the scheduled passenger flight, the airline transport pilot heard a loud “bang,” followed by a quieter “bang” and a total loss of engine power. He conducted a forced landing to a densely-wooded area, during which the airplane nosed over, resulting in substantial damage to the wings and horizontal stabilizer. A visual examination of the engine revealed a crack in the crankcase. A subsequent examination of the No. 1 piston assembly revealed fatigue fractures in both connecting rod retaining bolts. Additional internal damage to the engine was found to be consistent with damage caused by the release of the connecting rod from the crankcase.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
The failure of the engine’s No. 1 piston connecting rod retaining bolts due to fatigue, which resulted in a total loss of engine power during cruise flight. 

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.

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