History of Flight
On August 6, 2008, at approximately 1540 Pacific daylight time, Delta Air Lines flight 624, a Boeing 757-232 (registration N666DN), experienced an uncontained release of high pressure turbine material from the right engine, a Pratt & Whitney PW2037, as power was increased for takeoff at McCarran International Airport, Las Vegas, Nevada. The captain reported hearing a loud bang at the start of the takeoff roll, observing a loss of power from the right engine, and illumination of the right engine fire warning. The crew stopped the airplane on the runway and shut down the right engine. They declared an emergency and requested airport fire department personnel to inspect the engine. The fire warning light extinguished upon engine shutdown, so no fire bottle was discharged by the crew. Fire department personnel observed a hole in the bottom of the right engine nacelle and a glow inside; they discharged a fire bottle through open pressure relief doors. After confirming no fire, the airplane taxied back to the gate, and all 2 pilots, 4 flight attendants, and 166 passengers deplaned normally without injury. The airplane was operating under 14 CFR Part 121 as a scheduled flight from Las Vegas to John F. Kennedy International Airport, New York.
Engine Disassembly and Examination
The engine was removed and shipped to Delta's Technical Operations Center in Atlanta, Georgia, for disassembly and examination. A hole was found through the flanges between the high and low pressure turbine cases, in line with the 2nd stage turbine rotor at the bottom of the engine. Disassembly revealed four adjacent 2nd stage turbine blades missing from the hub. One blade was completely missing; the other three had only the bottom serration of the blade root remaining. The remaining blades were mostly intact but exhibited rub marks, heat discoloration, nicks, and dents. The 2nd stage turbine hub had four adjacent blade retaining lugs fractured between the inner and middle serrations. Fracture surfaces showed dark, elliptical flat areas radiating from the front acute corner. A fluorescent penetrant inspection (FPI) of the hub revealed 59 crack indications on 35 of the 60 blade retaining lugs.
2nd Stage Turbine Hub History
According to Delta's maintenance records, the 2nd stage turbine hub (part number 1B6002J, serial number DKLBA86546) had accumulated 29,097.5 hours and 12,549 cycles since new, with 10,880.7 hours and 4,392 cycles since last heavy maintenance. The PW2000 Engine Manual specifies a life limit of 15,000 cycles for this hub. Records show the hub underwent an overhaul between December 10 and 22, 2004, which included a four-step chemical descaling process, grit blasting with 500 grit aluminum oxide, steam cleaning, oven drying, FPI, and dimensional inspection.
Research and Testing
Dimensional inspection of the hub at P&W's East Hartford facility using a coordinate measuring machine (CMM) indicated all diametrical features conformed to manual requirements. However, inspection of blade retaining lug serrations using CMM and optical comparator revealed profiles 0.0092 inches below minimum tolerance, meaning missing material. Metallurgical examinations at P&W and the Safety Board's Materials Laboratory determined the four dark, elliptical fracture surfaces were consistent with fatigue cracks at elevated temperatures, originating from multiple origins along the root radius between the inner and middle serrations. Similar smaller fatigue cracks were found on other lugs. The hub's blade slot serrations showed a wavy surface pattern most prominent at the forward edge. No material anomalies were found.
A review of Delta's cleaning process for PW2037 2nd stage turbine hubs revealed that a more aggressive cleaning method had been adopted. The engine manual originally required blasting with plastic bead media (SPOP 19), but as hubs accumulated time, cleaning became more difficult. An alternate procedure (SPOP 10) using 500 grit aluminum oxide dry grit blasting was authorized and eventually became standard. P&W subsequently revised the manual to delete SPOP 19 in favor of SPOP 10. SPOP 10 procedures include a caution to avoid dwelling in one area for more than two seconds. Testing by P&W on test panels showed minimal material removal when following procedures, but significant removal if the nozzle was held too close, allowed to dwell, or held at an angle of about 45 degrees.
Other PW2037 2nd Stage Turbine Hubs
In June 2007, Delta found another PW2037 2nd stage turbine hub (operated by a U.S.-registered Part 121 carrier) with multiple cracks in the blade retaining lugs. The FAA was notified by P&W under 14 CFR 21.3. Although found 14 months prior to the Las Vegas incident, P&W had not determined the cause of those cracks. A dimensional inspection of that hub found deviations in blade slot serration profiles similar to those on the Delta hub. Metallurgical examination revealed fatigue cracks very similar to those on the fractured hub. Maintenance records showed that hub underwent multiple grit blasting cycles before acceptance.