What happened
During the initial climb phase following takeoff, the aircraft suffered an uncontained left engine failure. Following the failure, the flight crew returned the aircraft to the airport, where they completed an uneventful landing.
The investigation
Following the incident, the engine was removed and sent to the repair facility that had performed major repairs only 76.9 flight hours earlier. A teardown examination of the engine revealed that the oil supply holes on the No. 5 bearing inner race retaining nut were plugged.
An analysis of the repair process showed a deviation from the Pratt and Whitney Standard Practices Manual regarding the silver plating procedure over steel. The manual and the Service Process Operations Procedure (SPOP) 10 required the No. 5 bearing race to be grit blasted using PMC 3052-9 aluminum oxide grit, which is a 500 grit material. However, investigators found that the nut had been processed using 120 grit aluminum oxide. This use of much coarser media was contrary to the manufacturer's recommendation to use 200 to 500 grit aluminum oxide or glass beads. Additionally, the required cleaning and flushing of the nut after blasting had not been properly addressed.