Boeing 737-9 N37516 incident near Newark, New Jersey

Probable cause

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.

NTSB analysis

The No. 1 (left) engine fire was caused by a fuel leak that originated from an improperly torqued fuel nozzle 17 pilot secondary inlet b-nut. The leaked fuel ignited when it contacted hot engine case surfaces and resulted in an undercowl fire. Fuel nozzle b-nut torque measurements recorded during the engine teardown revealed that the pilot secondary inlet b-nuts of fuel nozzles 17 and 18 both had zero torque and backed off with no resistance. Four fuel nozzles (positions 4, 16, 17 and 18) were examined at the GE Aerospace Failure Analysis Laboratory in Evendale, Ohio and no anomalies were identified that would have precluded normal function if the b-nuts had been torqued in accordance with the aircraft maintenance manual.

Although, not directly causal to the incident engine fuel leak, CFM LEAP series engine fuel nozzle coking has driven early and repetitive fuel nozzle replacements throughout the fleet. According to United Airlines (UAL), they have completed 443 fuel nozzle set replacements on their CFM LEAP fleet as of October 2024.

Text from the NTSB. Open the NTSB docket for ENG23LA031 for the full record.