Southwest Airlines Boeing 737 7H4 incident
Boeing 737 7H4 · Incident: powerplant system/component malfunction/failure en route
From San Francisco International Airport (SFO) to an unrecorded destination
Event
- NTSB case
- ENG11IA011
- Event typeAccident: someone killed or seriously injured, or the aircraft substantially damaged (NTSB definition). Incident: an occurrence that affected or could have affected safety, short of that. Glossary
- Incident
- InvestigationHow far the investigation had got when the record was published: preliminary, ongoing, final or completed. Unknown where the source does not say. Glossary
- Completed
- API-reported fatalities
- 0 · all aircraft and ground
- Ground fatalities
- Unknown
- OperationWhat kind of flying it was, grouped by the rules it flew under: airline (US Part 121), air taxi and commuter (Part 135), general aviation (Part 91 and similar), military or government. Glossary
- Airline
- WeatherVisual meteorological conditions (VMC): good enough to fly by looking outside. Instrument conditions (IMC): cloud or low visibility, flying by instruments. Glossary
- Not recorded
Boeing 737 7H4
- Aircraft type
- Boeing 737
- Category
- Airplane
- RegistrationThe aircraft's tail number, such as N12345 or G-ABCD. Registrations are reissued, so the same one years apart can be a different aircraft. Glossary
- N248WN
- Operator
- Southwest Airlines
- Onboard fatalities
- Unknown
- Route
- From San Francisco International Airport (SFO), San Francisco, CATo not recorded
- Aircraft age
- Not recorded
- Flight rulesThe regulations the flight operated under: in the US, Part 91 (general aviation), Part 121 (airlines), Part 135 (air taxi and commuter) and others; for flights abroad, the NTSB's coarser commercial or non-commercial code. Glossary
- Part 121: Air Carrier
- Phase of flightThe stage of the flight when things started to go wrong: standing, taxi, takeoff, initial climb, en route, maneuvering, approach or landing. Glossary
- En route · climb to cruise
- Defining eventThe single coded event the NTSB judges best describes what happened (records from 2008 on). Older records name the first occurrence in the sequence instead. Glossary
- Powerplant system/component malfunction/failure
- DamageDestroyed: beyond practical repair. Substantial: damage that affects the structure, performance or handling and normally needs major repair. Minor: less than that. Glossary
- None
Cause areas
- Aircraft › Aircraft power plant
- Personnel issues › Task performance
Approximate · Coordinates from the NTSB case API, which marks them as estimated or does not say; no uncertainty radius is established.
NTSB narrative
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.
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.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record ENG11IA011
- Event ID
- 20110105X33913
- Case number
- ENG11IA011
- Dataset
- full-current
- Source SHA-256
- 5cf380f0061817c0331a6b2d8cc7e0ee3a79bea469a1001dc5c10e56f35f5ab3
Source notes (4)
- Unreviewed is an editorial label, not an investigation status. API-sourced is not report-checked or human-reviewed. Explicit event totals are used without summing aircraft injury tables; onboard allocation is withheld. Unknown values remain unknown. The operation category is mapped from the NTSB-reported FAR part and has not been reviewed.
- Filled from the NTSB case API where the bulk record had no value: local date, investigation status, coordinates, damage. Values present in the bulk record are kept.
- The date is the local date; the NTSB stores the UTC date 2010-12-25.
- API snapshot SHA-256: 09d5182875689316b280fc6072951ffacd53490614e62263bba8fd63d9b2bec7; retrieved 2026-09-15T15:40:02.258Z.