The NTSB published its final report on the 2023 incident.
A United 737 MAX 9 aircraft (Photo: AirlineGeeks | William Derrickson)
The National Transportation Safety Board has issued its final report about the June 28, 2023, engine fire on a United Boeing 737 MAX 9. The fire occurred during taxi after the jet landed in Newark, New Jersey. The incident occurred as United flight 2376 was arriving from Fort Lauderdale, Florida.
The flight crew reported receiving a No. 1 engine fire warning during taxi. They proceeded to shut down the engine and discharged one fire bottle, which extinguished the fire.
No visible smoke or flames were observed externally, but maintenance personnel later found evidence of a fuel leak and heat damage to engine components.
According to the NTSB’s report, the fire originated in the No. 1 (left) engine due to a fuel leak from an improperly torqued fuel nozzle connection. Specifically, the fuel nozzle 17 pilot secondary inlet b-nut was found to have zero torque, allowing fuel to escape and ignite on contact with hot engine surfaces.
Examination of the engine revealed signs of an undercowl fire. Importantly, there was no engine case burn-through and the fire was fully contained, investigators stated.
During a teardown inspection, investigators discovered that not only the fuel nozzle 17 b-nut, but also the nozzle 18 b-nut had zero torque and could be removed without resistance. This suggests improper installation during previous maintenance.
The NTSB determined the probable cause of the incident to be “A No. 1 (left) engine fire due 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.”
The fuel nozzles on the affected engine were replaced by airline mechanics on Jan. 12, 2023. After the replacement, the aircraft had accumulated 1,705 flight hours until the fire.
While not directly related to this incident, the report noted an ongoing issue with fuel nozzle coking in CFM LEAP series engines. This problem has necessitated frequent early replacements of fuel nozzles across the fleet.
United reported completing 443 fuel nozzle set replacements on its CFM LEAP engines as of October 2024.
In response to the incident, the carrier has revised its maintenance procedures for fuel nozzle replacement. The new process includes additional steps to ensure proper installation and multiple leak checks, including a pressurized nitrogen test and visual inspections during engine wet motor and part-power operations.
The incident aircraft, registered as N37516, sustained minor damage. None of the 184 individuals on board – including 179 passengers and five crew members – were injured.
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