NTSB Identification: ENG10IA037
14 CFR Unknown
Incident occurred Monday, June 14, 2010 in Minneapolis, MN
Probable Cause Approval Date: 09/20/2013
Aircraft: DOUGLAS DC9, registration: N671MC
NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft incident report.
On June 4, 2010, a Delta Air Lines Boeing DC9-51, N671MC, had a case-uncontained, nacelle-contained 1st stage fan blade separation in the No. 1 (left) engine, A Pratt & Whitney (P&W) JT8D-17 turbofan, during a ground run for maintenance at the Minneapolis-St. Paul International Airport. Delta Air Lines maintenance personnel were conducting a trim run on the No. 2 (right) engine. After they had checked the No. 2 engine's power, they advanced the No. 1 engine's throttle to check its power when at about 1.7 EPR [engine pressure ratio], they heard a boom and felt a vibration. They shut down the engines and upon exiting the airplane, noticed the No. 1 engine's fan blades were damaged. One 1st stage fan blade was completely missing from the front compressor front hub's blade slot. Several pieces of the missing 1st stage fan blade were recovered from the engine's fan duct. The metallurgical examination of the fan blade pieces revealed a fatigue fracture on the blade's suction-side blade root bedding flank. In addition, there were fatigue fractures on the fractured fan blade's suction-side midspan shroud from the upper and lower surfaces of the shroud. A dimensional inspection of the fractured fan blade's leading edge revealed the radius was 0.035 inch in comparison to the required 0.009 inch. According to P&W, the leading edges of the 1st stage fan blades erode during normal operation. However, P&W also advised that the amount of wear observed on the separated blade would be associated with approximately 11,000 flight cycles rather than the engine's 2,685 flight cycles since the last overhaul, during which the 1st stage fan blades were overhauled. According to P&W, a worn and blunt leading edge can cause a flutter mode that can lead to increased stresses in the fan blade and cause fatigue fractures in the root and airfoil. The erosion of the fan blade leading edge was likely the result of faulty restoration of the leading edge profile or failure to restore the leading edge in a timely manner.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: The failure of the facility that overhauled the 1st stage fan blade to properly restore the leading edge, which caused a flutter mode resulting in a fatigue crack and fracture of the fan blade. Full narrative available
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