Tokotna, AK, USA
N35952
CESSNA U206F
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.
On August 23, 2013, about 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 airplane was registered to Johnson Services LLC, Fairbanks, Alaska, and operated by Yute Air Taxi, of Wasilla, Alaska, under the provisions of Title 14 Code of Federal Regulations Part 135. The airline transport rated pilot, the sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed and a visual flight rules flight plan was filed for the repositioning flight. The cross-country flight originated from McGrath, Alaska about 15 minutes prior to the accident with an intended destination of the Innoko National Wildlife Refuge.In a written statement to the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), the pilot reported that about 15 minutes into 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 revolutions per minute (RPM) and that the propeller seemed to be spinning easily with minimal drag and no vibration. Despite attempts by the pilot, the engine would not restart and he initiated a forced landing to a heavily wooded area with the landing gear in the retracted position. During the off airport landing, the airplane struck multiple trees and came to rest upright. A postaccident examination of the airplane by the pilot revealed that the left and right wings were structurally damaged. The airplane was subsequently recovered to McGrath about one month later. The engine was removed from the airframe and shipped to the facilities of Continental Motors Inc., Mobile, Alabama, for further examination. The engine was examined by representatives from Continental Motors Inc., and Yute Air Taxi under the supervision of the NTSB IIC on March 11, 2014. The engine visually appeared to be intact and all six cylinders remained attached to the engine crankcase. All of the engine accessories remained attached to the engine via their respective mounts. The engine was placed on a rotating engine stand and subsequently disassembled. Examination of the magnetos, ignition harness, top and bottom spark plugs, engine driven fuel pump, fuel manifold valve, fuel metering system, alternator, and starter revealed no evidence of any preexisting mechanical malfunction. The oil pump was intact and the internal cavity and gears exhibited normal operational signatures with light scoring noted. The oil filter was removed, cut open, and the internal filter element contained a significant amount of metal flakes/slivers. All six cylinders were removed from the engine and were found intact and undamaged. All intake and exhaust valves and rocker arms were intact, and undamaged. All six pistons were intact, undamaged, and exhibited normal operational signatures. The crankshaft was found fractured at the number two main bearing journal, or third crankshaft cheek. The connecting rod journals, main journals, and thrust surfaces were undamaged and exhibited no evidence of abnormal wear or lubrication distress. The crankshaft counterweight pins, plates and snap-rings were intact. The counterweights were undamaged and had unrestricted movement on the hanger blades. The gear bolts were tight and safety wired and the gear teeth were undamaged. The number one (rear) crankshaft main bearings exhibited contamination imbedded in the surface layer. The number two (intermediate) crankshaft main bearings exhibited lubrication distress and thermal smearing of the surface babbit down to the copper layer. The bearings were fragmented and extruded from the bearing support. The fragments of the bearing were located within the oil sump. The number three (intermediate) crankshaft main bearings exhibited contamination imbedded in the surface layer. The number four (intermediate or front) crankshaft main bearings exhibited normal operating and lubrication signatures. The bearings were intact and exhibited an insignificant amount of contamination and hard particle passage. The bearings exhibited contamination imbedded in the surface layer. The bearings exhibited wear into the copper layer. The number five (front) crankshaft main bearings exhibited normal operating and lubrication signatures. The bearings were intact and exhibited an insignificant amount of contamination and hard particle passage. The bearings exhibited contamination imbedded in the surface layer. The bearings exhibited wear into the copper layer. The number one connecting rod assembly was intact and undamaged. The connecting rod nuts and bolts were intact and secure. The number two connecting rod assembly was intact and exhibited mechanical damage on the rod cap and was bent along the I-beam section of the rod. The connecting rod nuts and bolts were intact and secure. The number one and two connecting rod bearings exhibited lubrication distress and thermal smearing of the surface babbit, exposing the copper layer. The number three, four, five, and six connecting rod assemblies were intact and undamaged. The connecting rod nuts and bolts were intact and secure. The number three, four, five, and six connecting rod bearings exhibited normal operating and lubrication signatures. The connecting rod bearings were intact and exhibited an insignificant amount of contamination and hard particle passage. There was no evidence of lubrication distress. The camshaft was intact. The camshaft lobes exhibited normal operating signatures. The camshaft cluster gear was intact and exhibited normal operating signatures. The gear bolts were tight and safety wired and the gear teeth were undamaged. The accessory gears had continuity. The teeth were undamaged and exhibited normal operating signatures. The crankcase exhibited minimal exterior damage. The cylinder bays were intact and undamaged. The oil galleys and passages in the left and right crankcase halves were intact, clear and unrestricted. The number one, two and three main bearing supports exhibited lock slot elongation and bearing shift signatures. The number one, two and three main bearing parting surfaces exhibited a rough surface texture and fretting signatures. The crankshaft was examined by a Continental Motors Inc., metallurgist with permission from the NTSB Materials Laboratory. The metallurgy examination revealed that the fracture surface on the aft section of the crankshaft was severely damaged. The fracture surface on the forward section of the crankshaft was relatively undamaged and exhibited beach marks and several ratchet lines, which were consistent with a fatigue fracture with multiple initiation points. No ladder cracks or post separation cracks were identified using magnetic particle inspection. The metallurgist stated that the fracture propagated in fatigue from the surface, given the lack of any subsurface indication of origin. The part was sectioned perpendicular to the fracture surface at the initiation area and the cross section was mounted and polished for metallography. No inclusions or microstructural anomalies have been identified. The core hardness of the crankshaft was measured and found to be within the manufacturers specifications. The crankshaft was shipped to the National Transportation Safety Board Materials Laboratory for further examination. A Senior Materials Engineer performed an examination and found that the crankshaft was fractured through the cheek at the aft side of main journal M2. Main journals 1, 2, and 3 (numbered from the aft end) showed distress and scoring damage on the journal surfaces and onto the fillet radii at the forward end of journal 1 and the aft ends of journals 2 and 3. The number 2 connecting rod journal also showed damage in the fillet radius and on the cheek face at the forward end of the journal. The forward mating face of the fracture was mostly obliterated by post-fracture damage. The fracture surface had mostly smooth features with curving crack arrest lines, features consistent with fatigue. Across most of the fracture surface, fatigue features emanated from an origin area located in the fillet radius between main journal 2 and the cheek at the aft side of the journal. Ratchet marks were observed, consistent with fatigue initiation from multiple origins. For further information, see the NTSB Materials Laboratory Report within the public docket for this accident. Review of the engine maintenance logbooks revealed that the engine was overhauled/remanufactured at the Continental Motors Factory on May 23, 2001 and installed on the accident airplane on January 31, 2002. The number two and four cylinders were removed and replaced on July 9, 2010, at 1,400.2 hours since overhaul. The number six cylinder was removed and replaced on May 17, 2011, at 1,435.9 hours since major overhaul. The most recent maintenance performed on the engine was an oil and oil filter change which was conducted on June 6, 2013 at an engine time of 1,554.7 hours since major overhaul. The operator reported that at the time of the accident, the engine had about 1,620 hours total time since major overhaul.
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.
Source: NTSB Aviation Accident Database
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