NTSB Identification: LAX96IA087.
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Incident occurred Friday, January 05, 1996 in LOS ANGELES, CA
Probable Cause Approval Date: 04/10/1998
Aircraft: Boeing 747-251B, registration: N628US
Injuries: 385 Uninjured.

NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.

As the aircraft climbed through 11,500 feet msl the number 2 engine sustained an uncontained failure of the low pressure turbine section. Following engine shutdown, the crew dumped 147,000 pounds of fuel and returned to Los Angeles, for an uneventful landing. Postaccident inspection of the aircraft revealed a 2-foot hole in the number 2 engine cowl turbine area at the 9 o'clock position. Minor dents were noted on the number 1 engine cowl, pylon, and the underside of the left wing. Several fan blades from the number 1 engine were found with nicks and leading edge damage. Disassembly of the engine revealed that the oil pressure line to the number 4 bearing had fractured, with evidence of oil spray and fire signatures on the 6th stage low pressure turbine disk. Measurements of the 6th stage disk disclosed disk growth about 1.06 inches over blueprint maximum, which liberated all 6th stage blades from the disk. Metallurgical examination of the tube fracture revealed a fatigue crack through about 25 percent of the tube circumference, with origins at spiral scratches found on the inside diameter of the tube. Periodically the tube must be cleaned of coke build-up. Northwest Airlines and Pratt & Whitney maintenance/overhaul documents relating to the overhaul and cleaning of the number 4 bearing internal pressure tube assembly specify a an oven baking procedure. Cautionary notes state that wire brushes or reamers are not to be used for internal cleaning of the tubes.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

The fatigue fracture of the number 4 bearing oil pressure tube due to an improper maintenance cleaning procedure by company maintenance personnel.

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