Minneapolis, MN, USA
N671MC
DOUGLAS DC9
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.
History of FlightOn June 14, 2010, about 1945 central daylight time, a Boeing DC9-51 airplane, N671MC, operated by Delta Air Lines, experienced a case uncontained, nacelle contained 1st stage fan blade separation in the No. 1 (left) engine, a Pratt & Whitney (P&W) JT8D-17, during a ground run for maintenance. The three aviation maintenance technicians (AMT), two in the cockpit and one observing on the ground, stated that they were accomplishing a trim run, a maintenance procedure to check the engine’s power, on the No. 2 (right) engine. The AMTs in the cockpit stated that they had advanced the No. 2 engine’s throttle to about 1.85 EPR [engine pressure ratio] and then returned it to idle. The AMTs stated that they then advanced the No. 1 engine’s throttle and when the engine was at about 1.7 EPR, they heard a noise, which one of the AMTs referred to as a “boom,” and felt a vibration. The AMTs stated that they immediately retarded the No. 1 engine’s throttle to idle and when the vibration continued, shut down both engines. The AMT observing on the ground stated that he thought the engine had experienced a compressor stall because he had seen flames and sparks come out of the front and rear of the engine. After the AMTs in the cockpit had exited the airplane, they noted the damage to the No. 1 engine as well damage to the airplane. At the time of the incident, the airplane was being operated by AMTs for maintenance reasons and there was no intent for flight as defined by 49 Code of Federal Regulations Section 830.2. Injuries to Persons There were two AMTs in the cockpit and one AMT observing on the ground. None of the three AMTs were injured. Damage to Airplane The airplane sustained numerous dents and several holes in the left side of the fuselage adjacent to and forward of the No. 1 engine’s inlet. Other Damage There was no other reported damage. Airplane Information The airplane was a Boeing DC9-51, N671MC, serial number (SN) 47660. According to Delta Air Lines maintenance records, at the time of the incident, the airplane had accumulated 69,397 hours since new. The No. 1 engine was a P&W JT8D-17 turbofan, SN 696690. According to Delta Air Lines maintenance records, at the time of the incident, the engine had accumulated 61,173 hours and 50,034 cycles since new. The engine had also accumulated 3,053 hours and 2,685 cycles since the last overhaul. The maintenance records also show that the fractured 1st stage fan blade had been overhauled prior to installation in the engine. Fire There was no fire damage. Tests and Research The on-scene examination of the engine revealed one 1st stage fan blade was completely missing from the front compressor front hub’s blade slot. Several pieces of the missing fan blade were recovered from within the engine’s fan duct. The recovered pieces did not account for the entire fan blade. The blade root and shank section that also had part of the airfoil was fractured along the suction side at the bedding flank fillet radius and the full length of the dovetail lobe was missing along with the pressure side blade root platform. The piece of the airfoil with the midspan shrouds had the shrouds broken off from both sides of the airfoil. Only one of the separated midspan shrouds was recovered. The fracture surface on that recovered piece of shroud had flat, elliptical shaped features from the upper and lower sides of the shroud and the corresponded to the fracture surface on the airfoil. The fractured 1st stage fan blade pieces were sent to the National Transportation Safety Board’s Materials Laboratory for metallurgical examination. The piece of the blade that included the lower part of the airfoil, blade root platform, and the blade root had the full length of the suction side dovetail lobe separated at a 45 degree angle. The examination of the blade root with binocular microscope and scanning electron microscope (SEM) revealed fatigue progressions along the fracture face that originated at the forward end along the bedding flank-to-root shank fillet radius. According to the laboratory report, the area of the origin was smeared by a post fracture impact. The visual examination of the pressure side of the blade root dovetail revealed radial smearing and gouging. The examination of the midspan shroud and the piece of the airfoil from where the midspan shrouds separated revealed based upon the comparative fracture surfaces the separated shroud was from the suction side. The examination of the shroud revealed the midspan shroud had separated at the shroud-to-airfoil fillet radius. Binocular microscope examination of the fracture surface revealed fatigue progressions that originated at the fillet radius on the upper and lower sides of the shroud and progressed radially inward and outward, respectively. According to the laboratory report, the fatigue progressions were mirror images and were indicative of a reverse bending fracture. The examination of the fracture surfaces in the SEM could not identify any well defined fatigue striations. The fractured fan blade as well as the other fan blades from the engine were sent to P&W to visually and dimensionally inspect the fab blades’ leading edge radius. According to P&W, the leading edges of the 1st stage fan blades will erode during normal engine operation and require restoration. The 1st stage fan blade’s leading edge radius should be 0.009 inch. The dimensional inspection of the leading edge of 1st stage fan blade No. 1 revealed the radius was 0.035 inch. According to P&W, a 0.035 inch radius on a 1st stage fan blade is typical for about 11,000 hours of operation. Also according to P&W, blunt leading edges on 1st stage fan blades can induce a flutter mode that can lead to fatigue fractures in the blades’ root section.
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.
Source: NTSB Aviation Accident Database
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