Incident Overview
An aircraft suffered an uncontained left engine failure during the initial climb after takeoff. The flight crew returned to the departure airport and executed an uneventful landing. The engine was subsequently removed and sent to the repair facility that had performed major repairs on it 76.9 flight hours prior.
Teardown Examination
During the teardown examination of the engine, investigators found that the No. 5 bearing inner race retaining nut had its oil supply holes plugged. This condition was attributed to the maintenance procedures applied during the most recent repair.
Maintenance Procedure Deviation
According to the Pratt and Whitney Standard Practices Manual, the process for plating silver over steel requires that the No. 5 bearing race be grit blasted to remove old plating. The specified method, Service Process Operations Procedure (SPOP) 10, calls for the use of PMC 3052-9 aluminum oxide grit. The SPOP consumable materials list identifies PMC 3052-9 as 500 aluminum oxide grit.
However, during the actual processing of the nut, maintenance personnel used 120 grit aluminum oxide for grit blasting. This was contrary to the Pratt and Whitney recommendation, which specifies using 200 to 500 grit aluminum oxide media or glass beads. Additionally, the procedure required that the nut be cleaned and flushed after grit blasting to remove any residual debris.
The use of coarser grit than recommended likely resulted in debris that plugged the oil supply holes, leading to the engine failure.