NTSB Identification: DEN99IA038.
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Scheduled 14 CFR (D.B.A. AIR WISCONSIN )
Incident occurred Thursday, February 11, 1999 in ASPEN, CO
Probable Cause Approval Date: 06/22/2000
Aircraft: British Aerospace BAE 146-200A, registration: N608AW
Injuries: 88 Uninjured.
NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.
The airplane experienced an uncontained number 4 engine failure during climb to cruise altitude. 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 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. A high-cycle fatigue crack was found in the one blade which had separated below the blade platform. The metallurgist noted that the fatigue crack originated from a fretting damage mark located at the fir-tree portion. The manufacturer 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.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: The inadequate maintenance overhaul of the number four turbine disc by the manufacturer. A factor was the turbine blade's failure due to high cycle fatigue fracture. Full narrative available
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