HISTORY OF FLIGHT On September 6, 2017, at about 0019 PDT, a Delta Air Lines Boeing 757-232, registration N686DA, equipped with two Pratt & Whitney PW2037 turbofan engines, experienced a No. 1 (left) engine undercowl fire during takeoff from McCarran International Airport (LAS), Las Vegas, Nevada. The flight crew reported a left engine fire indication and associated aural fire alert at rotation/initial climb. The crew completed the quick reference handbook (QRH) procedures, declared an emergency, shut down the left engine and discharged one of the fire bottles; the fire warning momentarily was cleared. They then initiated engine out procedures to return to LAS airport. During the downwind leg of the airplane's flight pattern, the fire warning indication re-illuminated, and the second fire bottle was discharged, which cleared the fire warning a second time. The airplane made an uneventful overweight landing at LAS and was met by aircraft rescue and firefighting (ARFF) on the runway. ARFF sprayed fire retardant into the engine and confirmed the fire was extinguished. The airplane was cleared to taxi to the gate under its own power. There were no passengers or crew injuries reported. The flight was being operated in accordance with 14 Code of Federal Regulations Part 121 and was a regularly scheduled flight from LAS to John F. Kennedy International Airport (JFK), Queens, New York. DAMAGE TO THE AIRPLANE There was no damage to the airplane structure. The left engine thrust reverser exhibited thermal damage and discoloration on all interior surfaces except for a localized area between the 1 and 3 o'clock positions where the insulation appeared wet and shiny. The core cowls were thermally damaged, deformed and missing material. TEST AND RESEARCH Engine Examination and Disassembly The No. 1 engine was removed from the airplane by Delta Air Lines (DAL) maintenance personnel at LAS and shipped to Delta TechOps- Atlanta, Georgia for examination and disassembly. Party members from DAL, Pratt & Whitney (P&W), Boeing, the Air Line Pilots Association (ALPA), the Federal Aviation Administration, and the National Transportation Safety Board met at Delta TechOps from September 26-28, 2017. The compressor fan blades were all present and complete. Three fan blades located at the 6 o'clock position exhibited midspan shroud shingling and the fan blade tips were dug into the fan case rub strip and displaced about one inch in the aft direction. The fan could not be rotated by hand until the three shingled fan blades were forced back into their normal position with a pry bar. After the fan blades were adjusted, the fan could be spun smoothly by hand with concurrent rotation of the low pressure turbine. All exterior engine surfaces aft of the fan exit case were sooted. The engine had thermal damage and dark discoloration most concentrated over the high pressure compressor (HPC) and diffuser cases on the lower half of the engine (3 to 9 o'clock positions). Fuel components including the stator vane actuator (SVA), fuel flow transmitter (FFT), and associated fuel lines were thermally damaged with melted and missing material. The thermal damage to these components resulted in fuel system leak points. A SVA hose and tube assembly was found separated and hanging freely from the engine. A leak check of the fuel manifolds and fuel nozzle assemblies was performed by porting shop air into the fuel flow divider valve in accordance with engine maintenance manual procedures. When pressurized air was applied, an air leak was detected at fuel nozzle #7, located at the 3 o'clock position. The nozzle was removed from the engine as an assembly by cutting the fuel manifold supply line, so the fuel nozzle b-nut remained torqued. Preliminary x-rays of the nozzle were taken at Delta TechOps. The x-rays indicated the fuel nozzle b-nut was not properly installed. The fuel nozzle assembly SVA hose and tube assembly that was found separated were packaged and shipped to the P&W Materials and Processes Engineering Lab in East Hartford, Connecticut for additional analysis. Materials Analysis Fuel nozzle #7 was x-rayed at the P&W Quality and Standard Laboratory in East Hartford, Connecticut. The x-ray images confirmed that the b-nut was tilted relative to the nozzle platform and the end of the b-nut was not centered with the nozzle fuel supply tube (cross threaded). There was one or fewer threads engaged between the nozzle and b-nut. The fuel nozzle assembly was sectioned. The o-ring on the fuel nozzle side was present and intact. The second o-ring located on the fuel manifold supply line side was damaged and o-ring fragments were recovered outside of the o-ring gland, near the conical washer seal. The SVA hose and tube assembly was examined to verify there were no anomalies in the SVA flexible hose to rigid tube connection joint. The SVA flexible hose section exhibited thermal damage including a missing fire sleeve and hose liner. The hose liner was separated at one end of the connection to the rigid tube, but other joint connection remained intact. Metallographic sections were taken of both the intact joint and the separated joint and there were no significant differences observed. ADDITIONAL INFORMATION Corrective Action Following the event, DAL voluntarily inspected all PW2000 engines in the Delta TechOps repair shop, test cell, and spares pool for proper fuel nozzle installation, with no additional installation error findings. They also reviewed shop records and identified two in service engines that were overhauled/repaired at approximately the same time as the incident engine. The fuel nozzles on these engines were inspected with no findings. In an effort to avoid future installation errors, the diffuser and combustor assembly work instruction card was updated to add an inspector sign off requirement during the pneumatic leak check step of the fuel system assembly.