Incident Overview
While climbing through 11,500 feet mean sea level, the aircraft's number 2 engine suffered an uncontained failure of the low pressure turbine section. The crew shut down the engine, dumped 147,000 pounds of fuel, and returned to Los Angeles, where they made an uneventful landing.
Inspection Findings
Postaccident inspection of the aircraft revealed a 2-foot hole in the number 2 engine cowl at the turbine area, located at the 9 o'clock position. Minor dents were found on the number 1 engine cowl, pylon, and the underside of the left wing. Additionally, several fan blades from the number 1 engine exhibited nicks and leading edge damage.
Engine Disassembly
Disassembly of the number 2 engine disclosed that the oil pressure line to the number 4 bearing had fractured. Evidence of oil spray and fire signatures were present on the 6th stage low pressure turbine disk. Measurements of the 6th stage disk showed disk growth of approximately 1.06 inches over blueprint maximum, which liberated all 6th stage blades from the disk.
Metallurgical Analysis
Metallurgical examination of the fractured oil pressure tube revealed a fatigue crack extending through about 25 percent of the tube circumference. The crack origins were traced to spiral scratches found on the inside diameter of the tube.
Maintenance Background
The oil pressure tube to the number 4 bearing requires periodic cleaning to remove coke build-up. Northwest Airlines and Pratt & Whitney maintenance and overhaul documents for this tube assembly specify an oven baking procedure. Cautionary notes in these documents state that wire brushes or reamers are not to be used for internal cleaning of the tubes.