Pilot Debrief

CESSNA U206F near Tokotna, AK — 2013-08-24

Final reportANC13LA085
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Date
2013-08-24
Location
Tokotna, AK, USA
Aircraft
CESSNA U206F
Registration
N35952
Category
Airplane
Highest injury
None
Fatalities
0
Phase of flight
Climb

Probable cause

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.

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.

Analysis

Primary failure mode
Engine power loss
First missed decision gate
Pilot could have conducted a precautionary landing after engine power loss.

NTSB coding

Evidence available

  • Photos
  • 14 docket documents
View NTSB final reportView NTSB docket

Docket documents14

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