History of Flight
On June 14, 2010, at approximately 1945 central daylight time, a Boeing DC9-51, registration N671MC, operated by Delta Air Lines, experienced an uncontained, nacelle-contained 1st stage fan blade separation in the No. 1 (left) engine during a ground run for maintenance. The engine was a Pratt & Whitney JT8D-17. Three aviation maintenance technicians (AMTs) were involved: two in the cockpit and one observing on the ground. They were performing a trim run on the No. 2 (right) engine, advancing its throttle to about 1.85 EPR before returning it to idle. The AMTs then advanced the No. 1 engine's throttle; at about 1.7 EPR, they heard a noise described as a "boom" and felt a vibration. They immediately retarded the No. 1 engine to idle, but the vibration continued, so they shut down both engines. The ground observer saw flames and sparks from the front and rear of the No. 1 engine. After exiting the airplane, the AMTs noted damage to the engine and airframe. At the time, the airplane was operated by AMTs for maintenance reasons with no intent for flight as defined by 49 CFR Section 830.2.
Injuries and Damage
None of the three AMTs were injured. The aircraft sustained numerous dents and several holes in the left side of the fuselage adjacent to and forward of the No. 1 engine's inlet. No other damage was reported, and there was no fire.
Aircraft and Engine Information
The aircraft was a Boeing DC9-51, serial number 47660, with 69,397 hours accumulated since new. The No. 1 engine, Pratt & Whitney JT8D-17, serial number 696690, had accumulated 61,173 hours and 50,034 cycles since new, with 3,053 hours and 2,685 cycles since last overhaul. The fractured 1st stage fan blade had been overhauled prior to installation.
Examination and Testing
On-scene examination revealed one 1st stage fan blade completely missing from the front compressor hub blade slot. Several pieces were recovered from the engine's fan duct, but not the entire blade. The blade root and shank section showed fracture along the suction side at the bedding flank fillet radius, and the dovetail lobe was missing. A piece of the airfoil with midspan shrouds had the shrouds broken off; only one shroud was recovered. The fracture surfaces exhibited flat, elliptical features.
The fractured blade pieces were sent to the NTSB Materials Laboratory. Metallurgical examination of the blade root revealed fatigue progressions along the fracture face originating at the forward end along the bedding flank-to-root shank fillet radius. The origin area was smeared by a post-fracture impact. The pressure side of the dovetail showed radial smearing and gouging.
Examination of the midspan shroud and airfoil piece indicated the separated shroud was from the suction side, with fatigue progressions originating at the shroud-to-airfoil fillet radius, indicative of reverse bending fracture. No well-defined fatigue striations were identified by SEM.
The fractured blade and other fan blades were sent to Pratt & Whitney for visual and dimensional inspection of the leading edge radius. The specified leading edge radius is 0.009 inch; blade No. 1 measured 0.035 inch. According to Pratt & Whitney, a 0.035 inch radius is typical for about 11,000 hours of operation, and blunt leading edges can induce a flutter mode leading to fatigue fractures in the blades' root section.