Incident
Shortly after takeoff, the aircraft experienced a fire in the number 3 engine. The onboard fire bottles were activated and successfully extinguished the fire. The flight crew returned to the departure airport and landed without further incident.
Investigation
Post-incident examination determined that a secondary fuel line had pulled out of a B-nut ferrule fitting at the fuel flow divider manifold. This disconnection allowed fuel to discharge into the engine cowling area, where it ignited.
Prior Occurrences
The FAA Service Difficulty Report (SDR) database contained records of five prior events in which either a primary or secondary fuel line had pulled out of the B-nut ferrule fitting. A factory modification to the fuel lines was being implemented on a fleet-wide basis, but it had not yet been accomplished on this particular aircraft.
Conclusion
The investigation identified the disconnection of the secondary fuel line at the B-nut ferrule fitting as the source of the fuel leak that led to the engine fire. The flight crew's prompt action and the effectiveness of the fire suppression system prevented further escalation.