Serious incident Boeing 747-251B N628US,
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ASN Wikibase Occurrence # 357870
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Date:Friday 5 January 1996
Time:13:30 LT
Type:Silhouette image of generic B742 model; specific model in this crash may look slightly different    
Boeing 747-251B
Owner/operator:Northwest Airlines
Registration: N628US
MSN: 22389/442
Year of manufacture:1980
Engine model:P&W JT9D-7Q
Fatalities:Fatalities: 0 / Occupants: 385
Aircraft damage: Minor
Category:Serious incident
Location:Los Angeles, CA -   United States of America
Phase: En route
Departure airport:(KLAX)
Destination airport:Tokyo, OF (RJAA)
Investigating agency: NTSB
Confidence Rating: Accident investigation report completed and information captured
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.

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

Accident investigation:
Investigating agency: NTSB
Report number: LAX96IA087
Status: Investigation completed
Duration: 2 years and 3 months
Download report: Final report




Revision history:

14-Mar-2024 07:25 ASN Update Bot Added

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