ANC95LA058
ANC95LA058

On May 25, 1995, about 1000 Alaska daylight time, a wheel equipped Cessna 206, N19BR, crashed during an emergency forced landing, about 49 miles west of Bethel, Alaska. The airplane was being operated as flight number 40, a visual flight rules (VFR) cross-country cargo flight to Chevak, Alaska, under Title 14 CFR Part 135 when the accident occurred. The airplane, registered to and operated by Arctic Circle Air, Fairbanks, Alaska, received substantial damage. The certificated commercial pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed. Company VFR flight following procedures were in effect. The flight originated at the Bethel airport about 0940.

The operator reported that the pilot was in cruise flight about 600 feet above the ground when the engine suddenly quit. The pilot performed an emergency landing in a stream bed. The airplane received damage to the nose gear, firewall, right wing, and left main gear. Maintenance personnel from the operator responded to the scene and reported that the engine accessories normally driven by the crankshaft, did not rotate when the propeller was turned by hand.

Examination of the maintenance records indicated that the engine, an IO-520-F, serial number 286286-R, had accrued 1,154 hours since it was rebuilt by the manufacturer on July 23, 1992. The engine was installed in the accident airplane on September 3, 1992. During an annual inspection on October 11, 1993, the piston rings were replaced on the number 2,4, and 6 cylinders after the engine had accrued 507.7 hours since being rebuilt. On December 7, 1994, the number 6 cylinder was replaced at 915.1 hours of operation. At 1004.0 hours of operation on January 16, 1995, the engine's oil pump was serviced due to low oil pressure and the engine oil galleries were cleaned by blowing them out with compressed air.

On May 9, 1995, during a 100 hour inspection, metal was found in the engine oil filter and the number 4 cylinder was replaced. The engine had accrued 1,140.2 hours since the rebuild.

On October 25, 1995, a teardown examination of the engine revealed that the crankshaft was fractured at the number 3 short cheek, between the number 2 rod bearing and the number 2 main bearing. The engine contained numerous fragments from the number 2 main bearing. The number 2 bearing saddle was severely worn and deformed. The remaining bearing saddles appeared undamaged. The crankshaft, serial number B174, was the same crankshaft number that was entered on the manufacturer's quality assurance inspection record sheet when the engine was assembled.

On September 29, 1992, the manufacturer issued a mandatory service bulletin number M92-16. The bulletin indicated that the manufacturer began utilizing vacuum arc remelt (VAR) steel in the forging of crankshafts in 1978. This material was one that produced a forging with the smallest amount of impurities in the finished product. The bulletin required the replacement of airmelt (non-VAR) crankshafts at the next overhaul or whenever the crankshaft was removed or made accessible for any reason. Non VAR crankshafts were not to be re-used in any other application. Crankshafts made from VAR steel were identified with the raised letters VAR forged into the crankshaft on the number 3,7, or 9 cheek. The service bulletin indicated that rebuilt IO-520-F engines with serial number 286230-R and higher, had been manufactured with a VAR crankshaft and were not affected by the bulletin, unless the engine's crankshaft had been replaced.

The crankshaft was examined by the National Transportation Safety Boards's Materials Laboratory on December 8, 1995. The examination revealed that the raised VAR forging was not present on the crankshaft. A heat code of "H94" was stamped on the number 9 crankshaft cheek. According to the manufacturer, the heat code and lack of a VAR stamp indicated that the crankshaft was an airmelt shaft. The manufacturer also indicated that the use of VAR crankshafts was initiated to address subsurface failures.

The metallurgical examination revealed markings indicating that a fatigue crack emanated from multiple origins at the forward radius of the number 2 rod journal. The initial fatigue origin area contained mechanical smearing damage that obliterated the original fracture features. The surface of the crankshaft's number 2 main bearing journal contained severe circumferential gouge damage. The surface of the remaining crankshaft rod and main journals contained light scoring without evidence of heat damage. Fragments of the number 2 main bearing exhibited severe wear and deformation damage. Etching of the crankshaft revealed a layer of nitrided surface about 0.04 inches deep that was within the manufacturer's specified range for the nitride case depth. A microhardness test of the crankshaft produced an average hardness above the minimum specified hardness for the shaft.

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