What happened
While climbing through 11,500 feet MSL, the aircraft experienced an uncontained failure of the number 2 engine's low pressure turbine section. Following the shutdown of the affected engine, the crew performed a fuel dump of 147,000 pounds. The flight then returned to Los Angeles, where the aircraft completed an uneventful landing.
The investigation
Post-accident inspections of the aircraft identified a two-foot hole in the number 2 engine cowl turbine area at the 9 o'clock position. Investigators also noted minor dents on the pylon, the underside of the left wing, and the number 1 engine cowl. Additionally, several fan blades on the number 1 engine exhibited nicks and leading edge damage.
Upon disassembly of the engine, investigators found that the oil pressure line to the number 4 bearing had fractured. This fracture resulted in evidence of oil spray and fire signatures on the 6th stage low pressure turbine disk. Measurements of this disk showed growth of approximately 1.06 inches over the maximum allowed by the blueprint, a condition that caused all 6th stage blades to be liberated from the disk.
Metallurgical examination of the fractured tube revealed a fatigue crack extending through roughly 25 percent of the tube's circumference. The origins of this crack were traced to spiral scratches located on the inside diameter of the tube. Maintenance and overhaul documentation from Northwest Airlines and Pratt & Whitney regarding the cleaning of the number 4 bearing internal pressure tube assembly specifies an oven baking procedure and includes cautionary notes prohibiting the use of reamers or wire brushes for internal cleaning.