LOS ANGELES, CA, USA
N628US
Boeing 747-251B
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.
On January 5, 1996, about 1200 hours Pacific standard time, a Boeing 747-251B, N628US, sustained an uncontained failure of the number 2 engine turbine section during the en route climb phase of flight near Los Angeles, California. The aircraft was operated by Northwest Airlines, Inc., as their flight number 1, a regularly scheduled non-stop international passenger flight from Los Angeles to Narita Airport, Tokyo, Japan. Visual meteorological conditions prevailed at the time and an IFR flight plan was filed for the flight. The aircraft sustained minor damage to the numbers 1 and 2 engine cowls, and the fan stage of the number 1 engine. None of the 366 passengers and 19 crew onboard were injured. The flight originated from Los Angeles International Airport about 1145 on the morning of the accident. The captain reported that while climbing through 11,500 feet msl the airplane began to shake and the low rotor (N1) rpm on the number 2 engine surged to 90 percent, then dropped to 40 percent, as the exhaust gas temperature rose to the redline limit. The glareshield red fire warning light illuminated and the fire bell sounded; however, the red light in the number 2 fire handle did not illuminate. The second officer noticed that the number 2 engine nacelle temperature indication also reached its upper limit. The engine was secured in accordance with the "Engine fire, Severe Damage, or Separation Checklist" and the fire bottle was discharged. Following engine shutdown, the number 2 nacelle temperature decreased. The crew dumped 147,000 pounds of fuel and returned to Los Angeles, where an uneventful landing was made at 1300. The crew did not declare an emergency and no attempt was made to restart the number 2 engine. Postaccident inspection of the aircraft revealed a 2-foot hole in the number 2 engine cowl turbine area at the 9 o'clock position. No evidence of fire outside the engine case was observed. 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. The engine was removed from the aircraft and shipped to Northwest's engine overhaul facility where a disassembly and inspection was conducted on January 16 under the supervision of a Safety Board Powerplant specialist. The complete Powerplant Group Chairman's report is appended to this document. 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 revealed disk growth about 1.06 inches over blueprint maximum. No 6th stage blades, including roots, were found in the disk. The fractured oil pressure tube was removed from the engine and submitted to the Safety Board's Materials Laboratory for metallurgical analysis. The complete metallurgical report is appended to this document. According to the report, fatigue striations were found indicative of multiple fatigue crack planes from multiple origins along the inside diameter of the tube, with propagation mainly through the thickness of the tube wall. Spiral scratches and crazed cracking were found on the tube's inside wall. The fatigue fracture planes observed followed the scratches and crazed cracking lines in the central fatigue initiation region. Northwest Airlines and Pratt & Whitney maintenance/overhaul documents relating to the overhaul and cleaning of the number 4 bearing internal pressure tube assembly were examined. Cautionary notes state that wire brushes or reamers are not to be used for internal cleaning of the tubes. The referenced documents are appended to this report and list the approved cleaning methods.
The fatigue fracture of the number 4 bearing oil pressure tube due to an improper maintenance cleaning procedure by company maintenance personnel.
Source: NTSB Aviation Accident Database
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