Accident Overview
On August 23, 2013, at approximately 1600 Alaska daylight time, an amphibious float-equipped Cessna U206F was substantially damaged during a forced landing following a loss of engine power near Takotna, Alaska. The aircraft, registered to Johnson Services LLC and operated by Yute Air Taxi, was conducting a repositioning flight under Title 14 Code of Federal Regulations Part 135. Visual meteorological conditions prevailed, and a visual flight rules flight plan was filed.
The cross-country flight originated from McGrath, Alaska, about 15 minutes before the accident, with an intended destination of the Innoko National Wildlife Refuge. The airline transport rated pilot, the sole occupant, reported no injuries.
Pilot Report
The pilot stated that approximately 15 minutes into the flight, the engine suddenly lost power without any warning, vibration, or unusual sounds. He heard a loud clicking sound consistent with propeller RPM, and the propeller appeared to spin easily with minimal drag and no vibration. Attempts to restart the engine were unsuccessful, prompting a forced landing to a heavily wooded area with the landing gear retracted. During the landing, the aircraft struck multiple trees and came to rest upright.
Post-Accident Examination
A post-accident inspection by the pilot revealed structural damage to the left and right wings. The aircraft was recovered to McGrath approximately one month later. The engine was removed and sent to Continental Motors Inc. in Mobile, Alabama, for further examination.
On March 11, 2014, representatives from Continental Motors Inc. and Yute Air Taxi examined the engine under NTSB supervision. The engine appeared intact, with all six cylinders and accessories attached. Disassembly revealed no preexisting mechanical malfunction in the magnetos, ignition harness, spark plugs, fuel pump, fuel manifold valve, fuel metering system, alternator, or starter.
The oil pump was intact with normal operational signatures and light scoring. The oil filter contained a significant amount of metal flakes and slivers.
All six cylinders, pistons, valves, and rocker arms were intact and undamaged. The crankshaft was found fractured at the number two main bearing journal (third crankshaft cheek). Main bearing journals and connecting rod journals showed no abnormal wear or lubrication distress.
Number one and three main bearings exhibited contamination imbedded in their surface layers. Number two main bearings showed lubrication distress, thermal smearing, fragmentation, and extrusion. Number four and five bearings had normal signatures but with some contamination and wear into the copper layer.
The number two connecting rod was bent along its I-beam section and showed mechanical damage. Its bearings exhibited lubrication distress and thermal smearing. Other connecting rods and bearings were normal.
The camshaft, gears, and crankcase showed minimal damage, though the number one, two, and three main bearing supports had lock slot elongation and bearing shift signatures.
Metallurgical examination of the crankshaft by Continental Motors Inc. and the NTSB Materials Laboratory revealed a fatigue fracture with multiple initiation points in the fillet radius between main journal 2 and the adjacent cheek. Ratchet marks and beach marks were observed. No inclusions or microstructural anomalies were found, and the core hardness was within specifications.
Maintenance History
Engine logbooks showed that the engine was overhauled/remanufactured at the Continental Motors factory on May 23, 2001, and installed on the accident aircraft on January 31, 2002. Cylinders were replaced at various times: number two and four on July 9, 2010 (1,400.2 hours since overhaul); number six on May 17, 2011 (1,435.9 hours since major overhaul). The most recent maintenance was an oil and filter change on June 6, 2013, at 1,554.7 hours since major overhaul. At the time of the accident, the engine had approximately 1,620 hours total time since major overhaul.