Description

On 22 December 2008, a Boeing 757-200 powered by Pratt & Whitney PW2037 engines and being operated by Delta Air Lines on a scheduled passenger flight departing Las Vegas for New York JFK in normal day visibility experienced sudden failure of the right engine as take off thrust was set and the aircraft was stopped on the runway for fire services inspection. Fire service personnel observed a hole in the bottom of the right engine nacelle and saw a glow inside so they discharged a fire bottle into the nacelle through the open pressure relief doors. In the absence of any contrary indications, this action was considered to have extinguished any fire and the aircraft was then taxied back to the gate on the remaining serviceable engine for passenger disembarkation. None of the 263 occupants were injured but the affected engine suffered significant damage.

Investigation

An investigation was carried out by the National Transportation Safety Board (USA) (NTSB). The failed engine was found to have experienced an uncontained release of high pressure turbine material. Examination of the engine found a hole in both the high pressure and low pressure turbine case flanges at the bottom of the engine in line with the 2nd stage turbine rotor.

Engine disassembly found that four adjacent 2nd stage turbine blades were missing and that the 2nd stage turbine hub had four fractured blade retaining lugs. Metallurgical examination of these fractures established that they had been “due to fatigue at elevated temperatures from multiple origins along the blade slot serrations”. No material defects were found in the hub. Inspection of the hub's blades slots showed that they were missing material. Delta reported that during engine overhaul, they had been using a more aggressive grit blasting procedure than originally recommended in order to remove varnish-like scale from the hub so that it was clean enough to carry out the required fluorescent penetrant inspection. They also advised that the use of the more aggressive procedure had become so prevalent that they had made it part of the standard cleaning process rather than an alternate if the less aggressive blasting did not clean hubs sufficiently. It was noted that Pratt & Whitney had eventually revised the PW2000 Engine Manual to delete the original hub grit blasting procedure in favor of the more aggressive option. However, it was noted that the latter procedure contained a highlighted caution “Do not stay in one area for more than two seconds. Use continuous motion.” Grit blasting tests conducted by P&W during the investigation showed that if the appropriate grit blasting procedures were not adhered to that significant material loss could result.

Although the incident hub was found to have only been grit-blasted once, maintenance records for another PW2037 2nd stage hub that was also found to have multiple cracks in the blade slots showed that it had been grit blasted multiple times to clean the hub.

It was noted that in June 2007, Delta had found another PW2037 2nd stage turbine hub in overhaul that had multiple cracks in the blade retaining lugs and that Pratt & Whitney and the FAA had been aware of this finding. Despite this occurrence 14 months earlier, P&W had not determined the cause of the cracks although a re-inspection of it found abnormalities in the blade slot serration profiles and fatigue cracks in the hub similar to those on the hub from the Las Vegas event. A review of the maintenance records of the previously faulty hub showed that it had undergone multiple grit blasting cycles before the supervising inspector would accept the part for penetrant inspection.

The full NTSB Final Report of the Investigation: ENG08IA038 includes the following Probable Cause, determined by NTSB on 22 December 2010 as:

“The engine experienced an uncontained release of high pressure turbine material due to fatigue cracks that occurred on four consecutive blade retaining lugs on the 2nd stage turbine hub. The cracked lugs resulted in the release of four 2nd stage turbine blades that penetrated the engine's cases and nacelle. The lugs cracked due to loss of material and resultant increase in stress load on the blade slot serrations due to excessive grit blasting when the hub was being overhauled by Delta Air Lines.”

It was also noted that:

“Contributing to the uncontained incident was the FAA's and Pratt & Whitney's failure to act in a timely manner when another PW2037 2nd stage turbine hub was found to have multiple cracks in the blade retaining lugs.”

Further Reading

• Airworthiness
• Damage Tolerance
• Uncontained Engine Failure