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On December 29, 2004, about 1741 Alaska standard time, a Boeing 747-123 airplane, N858FT, sustained minor damage resulting from an under-cowl fire in the number four engine during initial climb after takeoff from the Ted Stevens International Airport, Anchorage, Alaska. The airplane was being operated as Flight 8445, by Polar Air Cargo of Purchase, New York, as an instrument flight rules (IFR) non-scheduled international cargo flight under Title 14, CFR Part 121, when the incident occurred. The three flight crew members were not injured. Visual meteorological conditions prevailed, and an instrument flight plan was filed. The flight originated at the Ted Stevens International Airport, Anchorage, about 1735, and was bound for the Osan Air Base, Republic of South Korea.
During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on December 29, the FAA aviation safety inspector who examined the airplane said there was a fire in the number four engine compartment, and that the entire outboard cowl was missing. He said the crew reported a fire alarm indication in the engine during initial climb, and that they reported shutting down the engine and activating the fire extinguishing system. The airplane dumped fuel, returned to Anchorage, and landed without incident.
During an examination of the airplane by the IIC on December 30, the entire outboard cowl was missing. The inboard cowl had been removed by the maintenance crew. The inside of the inboard cowl had extensive fire damage. The engine also had extensive fire damage, with the most damage in the area of the fuel controller.
The number four engine was removed from the airplane and taken to a storage warehouse. During a visual inspection of the outboard section of the engine adjacent to the oil cooler, the IIC noticed a misaligned large diameter fuel tube (PW Part No. 772123). The fuel tube had separated from the coupler attaching it to the oil cooler, leaving a gap of about one-half inch. The oil cooler is co-mingled with the fuel flow transducer, and uses the heat-exchange principle to cool engine oil, exchanging heat with high quantity fuel flow. The fuel tube is about one and one-quarter inches in diameter, and has a working pressure of about 200 pounds per square inch. The connection at the oil cooler end of the tube assembly included a ferrule that had been brazed to the fuel tube about one inch from the end of the tube. An elastomeric seal (O-ring) was placed around the tube adjacent to the lip of the ferrule, and both were held in place by a nut that was threaded to an externally threaded fitting. Two safety wires were attached to the nut, one wire from the nut to the fitting, and the second from the nut to a metal tab welded onto the fuel pipe. The wire from the nut to the metal tab was broken at the tab. Separation of the fuel tube assembly connection occurred at the brazed sleeve joint between the ferrule and the fuel tube. The fuel tube was removed from the engine without disturbing the ferrule retaining nut, which was safety-wired to the fitting. The tube assembly was shipped to the NTSB Materials Laboratory in Washington, D. C.
The accident airplane is a Boeing 747-123 (cargo configuration), serial number 20109. At the time of the incident, the number four engine had logged 75,863 service hours, with 2,183 service hours since overhaul.
TEST AND RESEARCH
A detailed examination of the tube assembly was conducted, and revealed that the torque on the retaining nut was so low that, once the safety was removed, the nut was easily removed by hand. The safety wire, attached from the nut to the metal tab, had failed in a typical overstress mode. The ferrule had been brazed to the tube at a distance from the end of the tube significantly greater than that specified by the manufacturer. Visual inspection revealed that the brazing filler material adhered to the tube better than it did to the ferrule. X-ray mapping of both brazed surfaces confirmed that the filler mater adhered more readily to the tube than to the ferrule. Aerospace Material Specification (AMS) 2664F for silver brazing requires eighty percent of the mating surfaces of a joint be joined by filler metal. X-ray mapping suggested that an estimated fifty percent of the ferrule surface, and sixty percent of the tube surface, had been covered by filler material before separation.
The Pratt & Whitney JT9D-7A engine is a discontinued model, and according to the manufacturer, there are about 220 of this model engine still in service worldwide. The fuel pipe in question is no longer in production, and is not used on any other engine model.