On July 4, 2000, about 1910 Alaska daylight time, a wheel equipped Cessna 207A airplane, N73100, sustained minor damage during an emergency landing at the Deadhorse Airport, Deadhorse, Alaska. The airplane was being operated as a visual flight rules (VFR) on-demand cargo/U.S. mail flight under Title 14, CFR Part 135, when the accident occurred. The airplane was registered to, and operated by, Cape Smyth Air Service, Inc., Barrow, Alaska. The solo certificated airline transport pilot was not injured. Visual meteorological conditions prevailed, and VFR company flight following procedures were in effect. The flight originated at the Deadhorse Airport, Deadhorse, about 1900, and was en route to Nuiqsut, Alaska.

During a telephone conversation with the National Transportation Safety Board investigator-in-charge on July 10, the pilot reported that about 10 minutes after departure, while in cruise, level flight, he noted a light sheen of oil forming on the windshield, and he elected to return to the Deadhorse Airport. He said that by the time he was within two miles of the airport he was having difficulty seeing though the windshield due to a heavy accumulation of oil. The pilot reported that while on approach to runway 4, the propeller rpm increased for about 20 seconds, followed by the propeller detaching from the engine. He said that he was able to glide the airplane to the runway, and land without further incident.

The propeller and engine crankshaft flange were located, and sent to the NTSB metallurgical laboratory for examination.

The airplane was equipped with a Teledyne Continental Motors (TCM) IO-520-F engine.

The engine had accrued a total time in service of 9.0 hours, and was installed in the incident airplane on July 1, 2000, 3 days before the incident. The engine maintenance records note that the engine was "rebuilt/zero timed on September 15, 1999 by Teledyne Continental Motors in accordance with an FAA approved Quality Control System." Assembly records indicate that a rebuilt crankshaft, serial number A239610N, was installed during the rebuild.

A Safety Board metallurgist examined the fractured crankshaft flange, and reported that his initial visual examination revealed that about one half of the fracture was on a flat plane and contained crack arrest positions, features indicative of fatigue cracking. He added that the primary fatigue initiation appeared to be from a very large number of individual initiation sites in the center of the fatigue region.

A representative from TCM's in-house analytical laboratory reported that previous crankshaft flange failures of this nature were the direct result of a propeller blade strike during engine operation. However, the operator's director of maintenance reported that no propeller blade strikes occurred with the incident engine within the 9.0 hours of operation, or 3 days, since the incident engine was installed.

The representative from TCM's in-house analytical laboratory reported that during a portion of the rebuilding process, the crankshaft is plated with tin that serves as a corrosion inhibitor. He speculated that if a crack were present prior to being rebuilt, tin would be present within the fracture surface. The Safety Board metallurgist that examined the fractured crankshaft flange reported that there was no detectable tin found within the fracture surface. The Safety Board metallurgist added that if a crack was present at the time of the tin application, a preexisting crack might not have been sufficiently open to allow penetration of the tin.

The Safety Board released the fractured crankshaft to the operator's director of maintenance on October 11, 2000. The Safety Board retained no other airplane or engine components.

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