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On January 10, 1999, about 1330 Alaska standard time, a wheel equipped Cessna 207 airplane, N6312H, sustained substantial damage during a forced landing, about 1 mile southeast of the Quinhagak airport, Quinhagak, 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, Village Aviation, Inc., Anchorage, Alaska. The solo commercial pilot received minor injuries. Visual meteorological conditions prevailed, and VFR company flight following procedures were in effect. The flight originated at the Bethel Airport, Bethel, Alaska, about 1300, and was en route to Quinhagak.
During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge on January 11, the pilot reported that during the initial descent to the Quinhagak airport the engine began to run rough, and lose power. He said that he switched fuel tanks in an effort to restore engine power, and noted that all engine oil pressure had been lost. He noted that the roughness improved momentarily, followed by a severe engine vibration, and complete loss of engine power. The pilot selected a forced landing area on snow and tree-covered terrain.
The airplane sustained substantial damage to the landing gear, fuselage, and wings.
The airplane was equipped with a Teledyne Continental Motors (TCM) IO-520-F engine.
The pilot held a commercial pilot certificate with airplane single-engine land, single-engine sea, and multiengine land ratings. The most recent second-class medical certificate was issued to the pilot on March 30, 1998, and contained the limitation that the pilot must wear corrective lens.
The pilot was hired by Village Aviation on August 6, 1997. On September 10, 1998, the pilot completed his most recent part 135 check ride in accordance with FAR 135.293 and 135.299.
The airplane had accumulated a total time in service of 10,286 hours. The most recent 100 hour inspection was accomplished on December 16, 1998, 75.2 hours before the accident.
The engine had accrued a total time in service of 175.0 hours, and was installed in the accident airplane on November 6, 1998. The engine maintenance records note that the engine was "rebuilt/zero timed on 7/14/98 by Teledyne Continental Motors in accordance with an FAA approved Quality Control System." Assembly records indicate that a "new" crankshaft, serial number F1098505N, was installed during the rebuild.
TEST AND RESEARCH
On February 25, 1999, an engine tear down and inspection was conducted at Alaskan Aircraft Engines, Inc., Anchorage, Alaska. The inspection revealed that the crankshaft of the TCM 520 engine had sustained a complete fracture of the #2 cheek between the outboard edge of one hanger blade, and the outboard edge of the opposing hanger blade. The fracture surfaces displayed postfailure circumferential scoring.
The failed crankshaft was sent to the National Transportation Safety Board's Materials Laboratory for examination. A Safety Board metallurgist reported that the separated crankshaft displayed evidence of fatigue cracking that emanated from the base of the hanger blade of the #2 cheek. He noted that the fatigue origin sites on both mating surfaces were obliterated by rubbing damage, which precluded the metallurgical investigation from determining the exact fatigue origin area.
During the course of the investigation, the engine manufacture disclosed that they had experienced six similar failures of crankshaft cheeks involving recently remanufactured 520 and 550 series engines. These crankshafts were manufactured between March and June 1998, and had accumulated between 85 and 175 hours in service.
Because the six other events did not result in accidents, the NTSB was not involved in the investigation of any of the other crankshaft failures.
The NTSB investigator-in-charge, and a senior metallurgist from the NTSB's Washington D.C. Materials Laboratory, traveled to the TCM production facility located in Mobile, Alabama. An extensive review of the manufacturing process revealed that a hydraulic bushing installation tool, used to install the counterweight bushings on the blade, was producing a shallow tool mark on the base of the hanger counterweight blade. A metallurgical examination revealed that on some crankshafts, a crack in the nitride layer that originated from the shallow tool mark, would in some cases ultimately lead to the fatigue failure of the crankshaft cheek.
On April 22, 1999, the Federal Aviation Administration's Atlanta Certification Office, issued an emergency airworthiness directive (AD) requiring:
All TCM 470, 520, and 550 engine crankshafts manufactured in 1998, with less than 300 hours in service, will be inspected in accordance with the TCM critical service bulletin (CSB) within 10 hours of operation. This AD affects about 3,000 TCM engines. The CSB incorporates the use of an ultrasonic inspection process that will be performed only by TCM personnel, or those trained by TCM to perform this special ultrasonic inspection.
Supplements A and B were not completed for this report.
The Safety Board released the fractured crankshaft to the owner's representative on May 28, 1999. No other airplane components were retained by the Safety Board.