On May 17, 2001, about 2100 Alaska daylight time, a wheel-equipped Cessna 207 airplane, N917AC, sustained minor damage during a forced landing, about 5 miles south of Quinhagak, Alaska. The airplane was being operated as a visual flight rules (VFR) positioning flight under Title 14, CFR Part 91, when the incident occurred. The airplane was registered to, and operated by, Arctic Circle Air Service, Inc., Fairbanks, Alaska. The certificated commercial pilot, and the one company mechanic aboard, were not injured. Visual meteorological conditions prevailed, and VFR company flight following procedures were in effect. The flight originated at the Platinum Airport, Platinum, Alaska, about 2045, and was en route to Bethel, Alaska.

During a telephone conversation with the National Transportation Safety Board investigator-in-charge on May 22, the pilot reported that earlier on the day of the incident, during a Title 14, CFR Part 135 passenger flight, he had difficulty controlling the propeller RPM, so he elected to make a precautionary landing at the Platinum Airport. He added that after calling the operator's maintenance and operations base in Bethel, the operator elected to dispatch an additional airplane from Bethel, to Platinum, in order to transport a company mechanic, along with a new propeller governor. The passengers were then transported back to Bethel in the second airplane. The incident pilot related that after the mechanic replaced the propeller governor, during an engine run-up, the airplane's engine and governor appeared to be operating normally, so they began their flight to Bethel. He said that about 10 minutes after departure, while in cruise flight, he noted a light sheen of oil forming on the windshield, and he elected to make a precautionary landing on a remote beach. As he retarded the throttle to start the descent, 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 beach, and land. During the landing flare, the tail skid contacted the beach, and sustained minor damage.

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

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

A review of the engine maintenance logbook revealed that on May 18, 1998, the incident engine was removed from another company airplane, and shipped to TCM, in Mobile, Alabama. The engine logbook notation indicated that the reason for removal was: "...for compliance with CSB 99-3A."

On June 3, 1999, TCM made the following log book entry: "Received engine for crankshaft inspection. Engine was disassembled to comply with AD 99-09-17 and CSB-3A. The following parts were replaced: Crankshaft, all main bearings, all connecting rod bearings, collar, and new gaskets and seal. Engine was assembled in accordance with the appropriate production procedures. Engine was tested in accordance with the applicable standard acceptance test. Engine is acceptable to return to service. INSTALLED CRANKSHAFT SERIAL NUMBER - C580." According to a representative from TCM's in-house analytical laboratory the crankshaft was a remanufactured part that reportedly was ground undersize, and re-nitrided at TCM.

The engine had accrued a total time in service of 347.1 hours, and was installed in the incident airplane on April 2, 2001, or 94.3 hours before the crankshaft separation.

The engine was removed from the incident airplane, and shipped to TCM's in-house analytical laboratory. On June 27, 2001, in the presence of a National Transportation Safety Board Investigator, the engine was disassembled. The investigator reported that the an internal examination of the engine revealed that the engine was visually unremarkable except for the absent propeller shaft, and flange. He noted a significant amount of heat discoloration and modest circumferential scoring around the propeller shaft, and propeller shaft bearings. He added that the oil transfer collar assembly, which is located between the two propeller shaft bearings, was fractured in one location. The NTSB investigator remarked that the heat discoloration was limited to the two most forward bearings, indicating a localized, loss of lubrication.

A Safety Board metallurgist examined the fractured crankshaft flange, and reported that his initial visual examination revealed a fatigue fracture, originating from a heat check (excessive temperature) site.

A representative from TCM's in-house analytical laboratory reported that previous crankshaft flange failures of this nature were the direct result of operations being conducted in very cold temperature conditions, coupled with improper engine preheating prior to engine operation. However, a review of historical meteorological data in the area where the incident airplane was operating, revealed that between April 2, 2001, and May 17, 2001, the temperature was about 30 degrees F.

The Safety Board released the fractured crankshaft flange and engine to the operator's president on December 5, 2001. The engine was released to the operator while still in storage, at TCM's analytical laboratory in Mobile, Alabama. The Safety Board retained no other airplane or engine components.

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