On February 11, 1999, approximately 1850 mountain standard time, a British Aerospace BAe 146-200A, N608AW, received minor damage when the number 4 engine experienced an uncontained failure during climb to cruise near Aspen, Colorado. The 2 airline transport pilots, 2 flight attendants, and 84 passengers were not injured. The airplane was being operated by Air Wisconsin under Title 14 CFR Part 121. Visual meteorological conditions prevailed for this scheduled domestic passenger flight which originated from Aspen, Colorado, 13 minutes before the incident. An IFR flight plan had been filed with a destination of Denver, Colorado.

The captain said that they were climbing through 14,700 feet msl when they heard a loud noise and strong vibration from the number 4 engine. He shut down the engine, and pulled the fire handle. A passenger reported seeing flames come out the tailpipe. The pilot reported that they received no indications of an engine fire in the cockpit. He discharged a second fire bottle in the number 4 engine, and then declared an emergency. The captain reported completing all the check list items, and that the remaining flight to Denver was uneventful.

Postincident examination of the engine revealed that the number 4 turbine disc had all its blades separated just above the blade platform except 1, which separated just below the blade platform. According to Air Wisconsin maintenance records, the number 4 turbine disc assembly was installed 22.5 hours or 22 cycles prior to this event. The manufacturer of the turbine disc said that the wheel was new; however, all 60 of the blades had seen previous use.

Metallurgical evaluation by the National Transportation Safety Board's (NTSB) Materials Laboratory found a fatigue crack in the blade that separated below the blade platform. The laboratory further noted that the fatigue crack originated from a fretting [relative movement between metals] damage mark, located at the fir-tree portion (see attached metallurgical reports). The laboratory said that "fretting damage areas reduce the fatigue properties of a material, and are sources of fatigue cracking."

The manufacturer performed an additional examination of the number 4 turbine wheel and blade root. They reported that 9 blade retention pins, which were used to prevent blade radial movement, did not meet the requirement of no more than 0.005 inch clearance between pen head and wheel. They further reported that blade stress concentration was induced by the wheel broach slots sharp edges. Their engineering staff determined that the failure was due to subsequent "high-cycle fatigue."

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