On September 6, 1995, at 0918 central daylight time (cdt), an Aerospatiale ATR 72, N414WF, operated by Simmon's Airlines, Incorporated of Dallas, Texas, as American Eagle Flight number 4118, and piloted by an airline transport rated pilot, received minor damage when its number one engine exhaust section and aft nacelle was involved in a fire. The flight crew shut the engine down and extinguished the fire, and returned the airplane to its departure airport. Visual meteorological conditions prevailed at the time of the incident. The 14 CFR Part 121 flight was operating on an IFR flight plan. No injuries were reported by the 4 flight crew members and 14 passengers. The flight departed Chicago, Illinois, at 0854 cdt.

The on-scene investigation revealed the number one engine's lower cowl interior had heat discoloration under the turbine support case (TSC) drain fitting. Heat discoloration was also noted on the TSC's lower half. Additional heat discoloration was observed on the bell mouth area of the nacelle exhaust trough and associated fire loop hardware. Sooting was observed on the nacelle heat shrouds and tailpipe areas. The TSC drain line fitting had tongue shaped soot patterns originating from its edges.

The engine's drain system manifold (manifold) is located forward of the TSC assembly on the bottom of the engine. Inspection of the manifold found that it would not drain excess fuel overboard as it was designed to do. The exit drain line from the manifold to nacelle drain mast was 99 percent blocked. Removal of the blockage revealed it to be a carbon-like material. The blockage was at the end of the drain line attached to the mast fixture. See drawing appended to this report.

The TSC drain line fitting was removed for inspection. A copper crush gasket (gasket) was found on the fitting's nipple end. The gasket interior size was about 40 percent larger than the nipple size. The gasket was not centered on the exhaust section nipple opening. An offset pattern shaped like the gasket surrounded this opening. The gasket interior and exterior edges were burned and deformed.

Examination of the fuel nozzle manifold found number 7 and 8 fuel nozzle transfer tubes had leaking "O" rings. According to the Federal Aviation Administration Principal Maintenance Inspector (PMI) associated with the investigation, the manufacturer's recommended procedure had not been followed by company maintenance personnel.

The procedure specifies that the "O" rings be installed onto the transfer tube. The transfer tube is installed into the manifold. It is then removed and the "O" rings examined for damage. If no damage is found, the transfer tube can be installed with the "O" rings. Two of the "O" rings for number 7 and 8 nozzles were cut and had scratches in them. Six other "O" rings were not seated properly. This type of condition creates an internal leak that is bypassed to the manifold's drain line.

Airplane maintenance records showed both engines on N414WF had their fuel nozzles changed on September 5 and 6, 1995. Flight 4118 was the first flight of the day after the maintenance had been performed. There is no requirement to check the nacelle drain mast line after a fuel nozzle change. The drain line inspection had been accomplished about 100 hours before the nozzle change. This inspection was done within the manufacturer's recommended 300 hour interval.

The drain manifold's fuel nozzle drain line and TSC drain line are positioned directly opposite each other. Leaking fuel is normally ported out the nacelle mast drain attached to the system drain manifold.

The fuel manifold transfer tube locking plates for the number 7 and 8 nozzle assemblies were found improperly installed. According to the PMI, this increases the fuel leak flow rate. Fuel pressure within the manifold lines is approximately 930 pounds per square inch at takeoff. Refer to the maintenance manual excerpt appended to this report for correct installation procedures.

On August 24, 1994, Pratt and Whitney issued a Service Information Letter (SIL) that addressed an improved "O" ring. The SIL states, in part, that some operators had found "O" rings that did not conform to Pratt and Whitney specifications. Pratt and Whitney offered the operators an even exchange of the old stock numbered "O" rings for the newer type (AS3209 and ST3367 respectively). Pratt and Whitney issued Service Bulletin number 21364 on September 16, 1994. The bulletin strongly recommended the use of the new "O" ring. The current Pratt and Whitney illustrated parts' catalog shows the part numbers for both "O" rings. Copies of these documents are appended to this report.

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