History of Flight
On October 10, 2011, at about 3:04 eastern daylight time, a Delta Air Lines Boeing 757-200, registration N553NW, powered by two Pratt & Whitney PW2037(M) turbofan engines, was conducting takeoff roll at Hartsfield-Jackson Atlanta International Airport, Atlanta, Georgia. The flight crew reported that at about 60 knots indicated airspeed, they heard a loud bang and observed left engine fire indications. The captain aborted the takeoff and shut down the engine. The airplane was taxied back to the gate without further incident. The flight was operating under 14 CFR Part 121 as domestic passenger Flight 2210 from Atlanta to Dallas/Fort Worth International Airport. No injuries were reported. Minor damage occurred to the airplane.
Damage to the Aircraft
The leading edge and underside of the left wing had numerous dents and small impact marks. Both halves of the left engine core cowl, hinge-mounted on the pylon, were severely distorted. The core cowl halves could be rotated beyond their normal range of travel until they contacted the left wing. The wing damage location was consistent with having been struck by the core cowl halves during the rupture event.
Diffuser Case Rupture
The engine diffuser case, high pressure turbine (HPT) case, and nozzle guide vane (NGV) support joint (M-flange) fractured at a bolt hole located at 12 o'clock. A longitudinal fracture extended from that bolt hole about 15 inches forward into the diffuser case and about 2.5 inches aft into the HPT case. A 270° circumferential fracture in the diffuser case about 15 inches forward of the M-flange, and a 120° circumferential fracture in the HPT case about 2.5 inches aft of the M-flange were present. An approximately 15-inch-wide section of the diffuser case, with attached fractured segments of the HPT case and NGV support, had unwrapped from the right side of the engine. The diffuser case, NGV support, and HPT case were shipped to the NTSB materials lab for further investigation.
Materials Investigation
Metallurgical examination of fracture surfaces revealed low cycle fatigue cracks initiated in all three components at the M-flange bolt hole, propagating across the flange surface inboard (inner strap) and outboard (outer strap) of the bolt hole. Scanning electron microscopy found discernible fatigue striation marks on the diffuser case and HPT case fracture surfaces. Analysis determined the first fatigue crack initiated in the HPT case, followed by crack initiation in the diffuser case. Cracks propagated through the HPT case M-flange outer and inner straps over approximately 7,305 and 7,384 cycles, respectively; cracks in the diffuser case M-flange propagated through the outer and inner straps over approximately 2,471 and 5,159 cycles, respectively.
Continued Airworthiness
Continued airworthiness of PW2000 diffuser and HPT cases is ensured by periodic fluorescent penetrant inspection (FPI). FPI is a surface crack detection method using a penetrating fluid with a fluorescent suspension that enters crack separations by capillary action; a black light is used to visually detect cracks. Diffuser and HPT cases undergo FPI when completely disassembled per manufacturer recommendation, with no cracks permitted. According to Delta Air Lines maintenance records, the diffuser case and HPT case were last exposed for inspection during overhaul at Pratt & Whitney's Cheshire Engine Center (P&W CEC) in August 2008, 3,479 cycles before the failure. A review of P&W CEC shop records found that both cases underwent FPI during this shop visit and were returned to service with no cracks found.
Crack Growth Study Findings
A crack growth study found that an approximate 0.46-inch crack was present in the HPT case M-flange inner strap and case wall, an approximate 0.26-inch crack was present in the diffuser case M-flange and case wall, and that both diffuser and HPT case M-flange outer straps were cracked through when the cases were overhaul inspected at P&W CEC. The study also showed that the first fatigue crack initiated in the HPT case, followed by crack initiation in the diffuser case. The P&W CEC repair station was closed in August 2011. The reason for the ineffective inspection was not determined but was narrowed to chronological and operator-based scenarios. A suspect population of 50 engines was identified. The FAA published Airworthiness Directive (AD) 2014-05-32, requiring eddy current inspection of M-flanges for PW2037, PW2037D, PW2037M, PW2040, PW2040D, PW2043, PW2143, PW2240, PW2337, PW2643, and F117-PW-100 engines in the suspect population within 100 flight cycles or 30 days. No additional cracks were found.
Corrective Actions
AD 2014-05-32 also requires that, effective May 5, 2014, high-sensitivity FPI penetrant be used when accomplishing FPI of the diffuser case aft flange (M-flange) and HPT case forward flange (M-flange) for the listed engine models. Additionally, the AD makes performance of diffuser and HPT case M-flange high-sensitivity FPI at every piece part opportunity an airworthiness limitation for those engines.