SEA96LA073
SEA96LA073

On March 31, 1996, approximately 1730 mountain standard time, a Cessna 500, N727TK, registered to and operated by TKA Express as a 14 CFR Part 91 business flight, experienced an uncontained engine failure during the take off ground roll at the Salt Lake City International Airport, Salt Lake City, Utah. Visual meteorological conditions prevailed at the time and an instrument flight rules flight plan was filed. The airplane was substantially damaged, and the private and commercial pilots, and their five passengers were not injured. The flight was destined for Visalia, California.

In written statements prepared by both pilots, they reported that the engine start and all pre-take off checks were completed with no abnormalities noted. The airplane was taxied to runway 35 and held for approximately ten minutes before the tower controller cleared the flight for take off. The first pilot reported that he advanced the thrust levers through 80-81%, when a loud "boom" was heard and the airplane began to shake. The second pilot stated that he looked out the right side window and noticed that the main fan from the right engine had come free and was flying through the air. The second pilot then looked at the right engine gages and noticed that there were no indications. The first pilot reported that he immediately brought the thrust lever for the right engine to idle power. When he was making the turn onto the taxiway to clear the runway, the right engine "blew." The first pilot continued to taxi back to the fixed base operator (FBO) as the second pilot notified the control tower of the situation.

After the engines were shut down and the passengers were deplaned, both pilots inspected the right engine and found that it was smoldering. It was noted that debris from the engine had punctured the side of the pressure vessel and damaged the wing.

Inspection of the engine by a Federal Aviation Administration inspector from the Salt Lake City Flight Standards District Office discovered that the impeller had broken into two large pieces. The impeller was removed from the engine and sent to the National Transportation Safety Board Metallurgical Lab for inspection. During the inspection the metallurgist reported that "a fatigue crack emanated from multiple origins on the aft surface..." The metallurgist further stated that "...this fracture revealed that the immediate fatigue origin area contained mechanical damage that obliterated the original fracture features. However, just off but near the origin the fracture was undamaged and displayed clear evidence of fatigue striations..." Calculations made by the metallurgist to determine the number of striations, indicates that the fatigue crack propagated a total of 8,066 cycles from the origin to the terminus. The impeller has a limited life of 8,400 flight cycles, after which it must be replaced.

A representative from Pratt & Whitney Canada reported that the impeller is forged, then machined to final shape. After machining, the impeller is blue etch anodized to determine if the surfaces "contain metallurgical anomalies." Examination of the impeller indicated that the exposed surfaces did exhibit a blue tint, which is indicative of blue etch anodizing. Further examination revealed that the reduced balance tab area and the holes located on the flange were a gray color typical of the base metal without the blue etch anodizing. The metallurgist reported that "SEM examination of the blue tint surfaces revealed minute pin-hole-like features, whereas, the surface of the reduced balance tab area and holes on the flange contained features typical of a machined surface with no pin-hole-like features."

The Pratt & Whitney representative reported that this impeller was forged in 1975, and machined in 1976. The impeller was installed in the accident engine when the engine was manufactured. The aircraft flight log indicates, as of 3/29/96, total engine cycles of 5,134.

Maintenance records indicate that the aircraft has operated in both the United States and for several years in Mexico. The aircraft was returned to U.S. registry in 1995. The records available to determine the entire history of maintenance performed and the total number of cycles on this engine are incomplete. It was determined that at the time of the accident, the engine had accumulated approximately 3,400 hours since the last overhaul dated in 1980.

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