Overview
• Operator: United Airlines
• Aircraft Model: Boeing 737-9 MAX
• Registration: N37516
• Flight Number: UA-2376
• Route: Fort Lauderdale, FL (FLL) to Newark, NJ (EWR), USA
• Date: 28 June 2023
• Occupants: 179 passengers, 5 crew
Incident Summary
United Airlines flight UA-2376, a Boeing 737-9 MAX, landed safely on Newark Liberty International Airport’s Runway 22L and was taxiing to the apron when the crew received a fire indication for the left-hand engine (LEAP-1B). The crew shut down the engine and discharged one fire bottle, extinguishing the indication.
No visible smoke or flames were observed. The aircraft was towed to the gate, where passengers disembarked normally.
Subsequent maintenance inspection revealed evidence of a fuel leak from the No. 1 (left) engine. Further teardown and analysis determined that the fuel nozzle b-nut connections were not properly torqued, leading to fuel leakage and ignition.
The NTSB released its final report on 30 January 2025, concluding that improper maintenance procedures were the probable cause of the fire.
Sequence of Events
1. Landing and Taxi
• The aircraft landed without incident on Runway 22L at Newark Liberty International Airport.
• While taxiing, the crew received a fire warning for the left-hand engine (LEAP-1B).
2. Crew Response and Fire Suppression
• The crew shut down the left engine and discharged one fire bottle.
• The fire indication extinguished, and no visible flames or smoke were observed.
3. Aircraft Towed to Gate
• Given the absence of visible fire, the aircraft was towed to the gate.
• Passengers disembarked normally, and no injuries were reported.
4. Maintenance Inspection and Findings
• United Airlines maintenance personnel identified fuel leakage from the left engine.
• The thrust reverser doors were opened, revealing heat damage and soot on the engine case and external surfaces.
Investigation and NTSB Findings
The NTSB released its final report on 30 January 2025, concluding the probable cause of the engine fire was:
“A No. 1 (left) engine fire due to a fuel leak that originated from the fuel nozzle 17 b-nut connection that was not properly torqued in accordance with the aircraft maintenance manual procedure.”
Technical Analysis
• Fuel Nozzle Malfunction
• During the engine teardown, investigators found that the b-nut connections on fuel nozzles 17 and 18 had zero torque and could be loosened with no resistance.
• The fuel leaked from nozzle 17’s pilot secondary inlet b-nut, ignited upon contact with hot surfaces, and resulted in an undercowl fire.
• Maintenance Error
• The improper torquing of the fuel nozzle b-nut connections was identified as the root cause of the incident.
• Examination at the GE Aerospace Failure Analysis Laboratory found no manufacturing defects in the fuel nozzles that would have prevented normal operation if torqued correctly.
• CFM LEAP Series Engine Issues
• The investigation noted that fuel nozzle coking has led to early and repetitive fuel nozzle replacements across the CFM LEAP engine fleet.
• As of October 2024, United Airlines has replaced 443 fuel nozzle sets on their CFM LEAP fleet due to performance concerns.
Aircraft and Fleet Status
• The affected Boeing 737 MAX 9 (N37516) was removed from service for further inspections and repairs.
• United Airlines and CFM International are reviewing maintenance procedures to ensure proper torquing of fuel nozzle b-nuts during replacements.
• The FAA and NTSB have issued safety recommendations for stricter compliance with maintenance manual torque specifications to prevent similar incidents.
Safety and Operational Considerations
• Proper Maintenance Procedures
• This incident underscores the critical importance of following aircraft maintenance manual (AMM) torque specifications during component replacements.
• Airlines and maintenance personnel must double-check torquing of fuel nozzle b-nuts to prevent leaks and potential fires.
• CFM LEAP Engine Fuel Nozzle Replacements
• The CFM LEAP series has experienced repetitive fuel nozzle replacements due to coking issues.
• Airlines operating Boeing 737 MAX and other CFM LEAP-powered aircraft should be vigilant about fuel system integrity to prevent leaks.
• Fire Detection and Suppression System Effectiveness
• The fire bottle successfully suppressed the engine fire, preventing further escalation.
• The crew followed correct emergency procedures, shutting down the engine and safely taxiing to the gate.
Disclaimer
This report is based on publicly available information as of 30 January 2025. While every effort has been made to ensure accuracy, the completeness of the details cannot be guaranteed. If you are the rightful owner of any referenced materials and wish them removed, please email takedown@cockpitking.com.
