Cessna U206F N35952

Summary

On 24 August 2013 at about 00:00 local time, a Cessna U206F registered N35952, operated by Yute Air Taxi, was involved in an accident near Tokotna, Alaska, United States. One person was on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

24 August 2013 at 00:00

Classification

Accident

Location

Tokotna, Alaska, United States

Coordinates

61.9928, -156.0297

Aircraft

Cessna U206F

Registration

N35952

Category

Airplane

Engines

1

Operator

Yute Air Taxi

Operating rule

Part 135: Air Taxi & Commuter

Phase of flight

Not recorded

Route

McGrath → Innoko

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

ANC13LA085

Nobody was injured.

A total loss of engine power due to the failure of the crankshaft, which resulted from a mechanic’s application of insufficient torque on the cylinder through bolts.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

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.

Quoted verbatim from the NTSB record.