Boeing 757 232 N686DA incident near Las Vegas, Nevada

Probable cause

A No. 1 (left) engine undercowl fire caused by a fuel nozzle installation error during engine overhaul at Delta TechOps. A fuel nozzle b-nut was cross threaded, which allowed fuel to leak on hot engine case surfaces and subsequently ignite.

NTSB analysis

The No. 1 engine fire was caused by an improperly installed #7 fuel nozzle located at the 3 o'clock position of the engine. The nozzle installation error resulted in a fuel leak that subsequently ignited on hot engine case surfaces. All the fuel nozzles had been replaced as part of engine overhaul at the Delta Tech Ops facility 53.7 flight hours/18 cycles prior to the incident flight.

The cross threaded condition was evident during visual examination, and an x-ray of the fuel nozzle assembly confirmed the fuel nozzle b-nut was not properly seated, with one or fewer threads engaged. The nozzle assembly featured two o-rings and a conical seal washer (Voi-Shan) that acted in conjunction with the torque applied to the fuel nozzle b-nut to provide a seal. The o-rings and seal washer temporarily compensated for the cross threaded condition, until one of the o-rings began to break apart.

The cross threaded b-nut should have been visually detected by the mechanic and inspector. Although, if not visually detected, the cross threaded b-nut may have given a proper torque indication during the required torque check and passed the pneumatic leak check step of the installation procedures.

Several fuel system components including the fuel flow transmitter and stator vane actuator are located near the #7 fuel nozzle. The fire thermally damaged the case housings of both of these components resulting in secondary fuel leaks.

The damage to the nacelle components including the fan cowl, thrust reverser, and core cowls was due to fire exposure. The area of thermal insulation on the right thrust reverser half that was minimally discolored with a wet and shiny appearance was located radially outboard of the #7 fuel nozzle. The fuel spray from the nozzle provided localized cooling where the fuel contacted the insulation and was too rich to ignite, preserving the fire insulation.

The fan blade shroud shingling and displacement noted on three fan blades was most likely a result of firefighting efforts. After the blades were dislodged from the rub strip, the fan spun smoothly.

Text from the NTSB. Open the NTSB docket for ENG17IA036 for the full record.