Cessna U206F accident near Tokotna, Alaska, August 24, 2013

The NTSB's narrative final · quoted from the NTSB record

The pilot reported that, during cruise flight, the engine suddenly lost power without any warning, vibration, or unusual sounds. The pilot stated that he heard a loud clicking sound that was consistent with the propeller rotating and that the propeller seemed to be spinning easily with minimal drag and no vibration. Despite the pilot's attempts to restore engine power, the engine would not restart, so he initiated a forced landing to a heavily wooded area with the landing gear retracted. During the landing, the airplane struck multiple trees and then came to rest upright. Examination of the engine revealed that the crankshaft was fractured near the No. 2 main bearing journal. Metallurgical examination of the crankshaft fracture revealed evidence of fatigue. In addition, the crankcase exhibited varying degrees of fretting near the Nos. 1, 2, and 3 main bearing crankcase parting surfaces. The Nos. 1, 2, and 3 main bearing supports exhibited lock-slot elongation and bearing shift signatures, and the No. 2 intermediate main bearing was fragmented and extruded from the bearing support. The evidence is consistent with the application of insufficient torque on the cylinder through bolts. It is likely that, when the No. 2 main bearing extruded from the bearing support, the crankshaft lost support, and the fatigue fractures subsequently initiated. Engine logbook entries indicated that, about 219 flight hours before the accident, a mechanic replaced the Nos. 2 and 4 cylinders, which would have required the removal and installation of hardware on the Nos. 2 and 4 cylinder through bolts. It is likely that the mechanic applied insufficient torque to the through bolts after replacing the Nos. 2 and 4 cylinders.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.