Boeing 737 N248WN incident near San Francisco, California

Probable cause

The probable cause of the undercowl in-flight engine fire was the insufficient installation torque of the bolts that secure the fuel manifold cover to the fuel manifold. Engine vibrations and fuel pressure cycles caused the bolts to gradually loosen further until one bolt lost all its tightening torque and fell out. The internal fuel pressure then forced open the fuel manifold cover at the location of the missing bolt, causing a gap between the two mating surfaces which allowed fuel to push past the integral packing, resulting in a fuel leak onto the hot engine cases where it ignited resulting in a fire.

NTSB analysis

The CFM56-7B24 turbofan engine experienced an inflight fire after departure. The flight crew reported an engine fire warning, discharged a fire suppression bottle and diverted where a successful single engine landing was performed. The airplane taxied to the gate where all the passengers deplaned. There were no reported injuries. Preliminary examination of the engine revealed sooting on the engine cases from the compressor aft flange to approximately six inches beyond the turbine rear frame aft flange, but no significant thermal damage. Detailed examination of the engine revealed that the lower, most forward of the four bolts that secure the fuel manifold cover to the fuel manifold was missing while the other three had low torque values. Due to the missing attachment bolt and low torque on the three remaining bolts a gap between the two surfaces occurred that allowed fuel to leak. A review of photographs taken before the engine was released from the Celma overhaul shop reveals that all four bolts were present at that time, but based on the torque values during the investigation, they were likely not properly torqued at overhaul. These original low torque values allowed the bolts to back out over time until one eventually fell out. Pressurized fuel within the cavity was then able to pry the loose-fitting fuel manifold cover open at the location of the missing bolt and leak past its integral packing, creating the leak and subsequent in-flight fire. Examination of other 737 airplanes with CFM56-7B24 engines revealed no broader field problem.

Text from the NTSB. Open the NTSB docket for ENG11IA011 for the full record.