History of Flight On June 28, 2023, about 1019 eastern daylight time (EDT), a Boeing 737 MAX-9, N37516, powered by two CFM International LEAP-1B28 turbofan engines, operated as United Airlines Flight 2376, experienced a No. 1 (left) engine fire during taxi-in after landing at Newark Liberty International Airport (EWR), Newark, New Jersey. The airplane was operated as a Title 14 Code of Federal Regulations Part 121 flight from Fort Lauderdale-Hollywood International Airport (FLL), Fort Lauderdale, Florida to EWR. According to the flight crew, during taxi-in, they received a No. 1 engine fire warning indication and audible bell. The flight crew shutdown the No. 1 engine, discharged one fire bottle, and the fire warning indication ceased. There was no visible smoke or fire emanating from the No. 1 engine, and the airplane was towed to the gate. Maintenance personnel observed evidence of a fuel leak under the No. 1 engine at the gate. The No. 1 engine thrust reverser doors were opened for visual inspection and heat damage and sooting were observed on the engine cases and external components. Damage to the Airplane The airplane was not damaged during the incident. Test and Research LEAP-1B28, Engine Serial Number, 60A467, Examination and Teardown The No. 2 engine was shipped to the GE Aerospace Lafayette Engine Facility in Lafayette, Indiana for examination and disassembly. Investigation party members from the CFM International, Boeing Commercial Airplanes, United Airlines, Air Line Pilots Association, International (ALPA), the Federal Aviation Administration (FAA), and the National Transportation Safety Board (NTSB) completed the examination from July 19-20, 2023. The left side of the engine exhibited multiple indications of undercowl fire including discoloration and sooting of the engine core from the 6 to 12 o’clock positions, between the high-pressure compressor rear case and the aft flange of the high-pressure turbine case. The discoloration was most concentrated from the 6 to 9 o’clock positions. There was no evidence of engine case burn through or radial uncontainment. The fuel manifolds and fuel nozzles were leak checked with pressurized nitrogen and leak detection fluid was sprayed on the fuel nozzle-to-fuel manifold b-nuts and fuel nozzle flange bolts. A leak was identified at the fuel nozzle 17 pilot secondary inlet b-nut connection. Torque measurements of the fuel nozzle b-nuts and flange bolts revealed that the secondary inlet b-nuts of fuel nozzles 17 and 18 had zero torque and backed off with no resistance according to the GE Aerospace mechanics. Figure 1- Fuel Nozzle 17 Leak Indication Fuel Nozzle Maintenance History The fuel nozzle set on the incident engine was replaced on-wing by UAL mechanics on January 12, 2023. The installation and inspection of the fuel nozzles was performed in accordance with Boeing 737 MAX-9 aircraft maintenance manual (AMM) and were marked completed by a mechanic and inspector. The airplane had accumulated 551 flight cycles / 1,705 flight hours between the time the fuel nozzles were replaced, and the incident engine fire occurred. Materials Analysis Four fuel nozzles (positions 4, 16, 17, 18) were shipped to the GE Aerospace Failure Analysis Laboratory in Evendale, Ohio for materials analysis. Fuel nozzles 17 and 18 were identified to have zero b-nut torque at the pilot secondary inlet during the teardown examination while fuel nozzles 4 and 16 were found to have b-nut torque that fell within specifications. Fuel nozzle 16 was located in the fire affected area, and fuel nozzle 4 was located outside the fire affected area. A visual and binocular examination of the four fuel nozzle mating faces and threads did not reveal any mechanical deformation or indications of cross threading. Rubber castings of the secondary manifold ferrules were made for each of the four fuel nozzles. The sealing surfaces of the four ferrules appeared slightly concave, consistent with plastic deformation from installation against the fuel nozzle bullnoses. Coking was observed around the mating surface of fuel nozzles 17 and 18 but was not present on nozzles 4 or 16. The analysis concluded that there was insufficient evidence to determine how long a fuel leak may have occurred from the fuel nozzles 17 and 18 pilot secondary inlet. CFM LEAP Fuel Nozzle Coking Summary and Event History CFM LEAP series engine fuel nozzle coking has driven early and repetitive fuel nozzle replacements throughout the fleet. Elevated temperatures in the engine hot section following shutdown, also known as thermal soak back, can heat residual fuel in the fuel nozzles to create coking. Excessive fuel nozzle coking can result in engine start issues, decreased performance, and premature engine wear including loss of thrust and in-flight shutdown. In certain cases, excessive fuel nozzle coking can also result in increased fuel flow through certain fuel nozzles leading to distress of the HPT static structures. Currently, operators need to monitor for fuel nozzle coking and must replace fuel nozzles when coking accumulation reaches unacceptable levels. Coking related issues are generally identified through on-board monitoring systems and/or through CFM Customer Notification Reports (CNR). The FAA published Airworthiness Directive (AD) 2020-06-01, effective April 16, 2020, to address the unsafe condition due to fuel nozzle coking. The AD requires a revision the Airworthiness Limitations Section (ALS) to perform either fuel nozzle condition monitoring every 125 flight cycles (FC) or repetitive borescope inspections of the HPT for signs of fuel nozzle streaking distress every 600 FC. According to UAL, they have completed 443 fuel nozzle set replacements on their CFM LEAP fleet as of October 2024. The replacements are primarily performed at UAL engine maintenance shops at CLE and Tampa International Airport (TPA), Tampa, Florida. Corrective Actions United Airlines UAL revised their AMM fuel nozzle set replacement procedures following the incident to incorporate additional steps and checks to verify proper installation. The revised procedure requires additional external hardware to be removed from the engine to ensure better access to the fuel nozzles and reduce the likelihood of false torque indications. A pressurized nitrogen leak check of the fuel manifold is performed after the fuel nozzles are torqued, prior to external hardware installation. Leak detection fluid is applied to all fuel nozzle connections and each nozzle is visually inspected for leak indications. After the fuel nozzle set replacement is complete, an engine wet motor is performed with the cowl doors open to visually inspect for fuel leaks. During the wet motor, the fuel boost pumps are turned on, the fuel spar valve is opened, and the engine starter is engaged without engine ignition. A part-power leak check is then performed where the engine is started and accelerated to approximately 70% N1 (low-pressure spool) speed to increase fuel pressure above idle speed and actuate fuel system components that are only active above idle conditions. The part-power leak check is performed with the cowl doors closed. The cowl doors are subsequently opened to visually inspect for leaks after engine shutdown. CFM International CFM International introduced a reverse bleed system on the LEAP-1A engine model in 2024 that is designed to address the root cause of fuel nozzle coking by incorporating a blower to reduce heat soak back following engine shutdown. The reverse bleed system is planned to be introduced onto the LEAP-1B engine model in 2025.