History of Flight
On December 24, 2010, a Southwest Airlines Boeing 737-7H4, registration N248WN, powered by two CFM56-7B24 turbofan engines, experienced an in-flight fire after departing from San Francisco International Airport (SFO), California. The flight crew reported hearing a light pop and noted an engine fire warning on the No. 2 (right-hand) engine. After the No. 2 engine fire handle was pulled, the engine fire warning light switched off. The crew performed the engine fire, severe / damage / separation / seizure checklist from the quick reference handbook (QRH) and diverted to Oakland International Airport (OAK), California, where a successful single-engine landing was performed. The airplane taxied to the gate, and all passengers deplaned. No injuries were reported. The flight was a regularly scheduled 14 CFR Part 121 domestic flight from SFO to Los Angeles (LAX).
Engine Examination
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. The engine was removed and shipped to the GE Aviation Services Carter Field facility in Fort Worth, Texas, where an external evaluation and test cell runs were conducted. At high engine power settings, fuel was found leaking from between the fuel manifold and fuel manifold cover. The fuel manifold cover is rectangular with four holes for retaining bolts that secure it to the fuel manifold housing. Examination revealed that the lower, most forward bolt was missing, and the remaining three bolts were insufficiently torqued. A shim check at the missing bolt location measured a gap of 0.008 inch, exceeding the specified 0.001 inch or less. The breakaway torques of the remaining three bolts ranged from 0 to 30 inch-pounds, while the installation torque required by Service Bulletin (SB) 73-0054 is 62-68 inch-pounds. The cover was removed, exposing torn packing material (two flaps totaling approximately 0.600 inch). The parking bracket mounting hole where the missing bolt was located showed scoring consistent with previous bolt presence. The part numbers of the installed bolts matched the parts catalog.
Flight Data Analysis
Analysis of data from the flight data recorder (FDR) showed an increase in fuel flow to the right engine with no corresponding increase in exhaust gas temperature (EGT) or N1 (low-pressure rotor speed). This is consistent with an external fuel leak, as an internal increase in fuel flow would have caused EGT and N1 to rise.
Service Bulletin History
Service Bulletin 73-0054 was issued to remove the burner staging valve (BSV) from the CFM56-7B24 engine, which had been integrated to reduce flameout risk at low speeds. The BSV internal valve is removed, but the housing is retained, and a new fuel manifold cover and parking bracket are installed with four bolts. The fuel manifold cover includes an integral seal. The engine's maintenance records show that SB 73-0054 was complied with by GE Aviation, Engine Services (Celma) on December 17, 2008, during the last shop visit. Photos from the final inspection at GE Celma showed all four bolts in place, but it could not be determined if they were properly torqued. The engine had accumulated 6,845 hours since that shop visit, with no subsequent maintenance in that area per Southwest records.
Investigation Findings
To determine if the loose bolt condition resulted from design or manufacturing issues, detailed examinations were performed:
- The fuel manifold cover and fuel manifold were dimensionally checked with a coordinate measuring machine (CMM). Flatness was within normal limits (0.00102 inch for manifold, 0.00127 inch for cover). No faults affecting fitting or clamping were found.
- The integral packing elastomer was torn and extruded at the missing bolt location. Microscopic examination indicated the tear was consistent with an unseated seal subjected to pressurized fuel.
- Scanning electron microscope (SEM) evaluation of the three remaining bolt threads found no abnormalities.
- Threaded inserts in the fuel manifold were sectioned and showed no abnormalities.
- The self-locking features (Vespel collars) inside the threaded inserts were in normal condition.
- No mechanical abnormalities were identified in any parts; the loose condition was attributed to human error during assembly.
- A sample of 20 Southwest Airlines engines was examined, and all 80 bolts had breakaway torques above the acceptable 62 inch-pounds.
- A field review by CFM revealed that 467 installation kits for SB 73-0054 had been shipped and installed since 2001 with no technical problems reported. The highest-time installation had 15,000 hours in service.