What happened
During takeoff, a 14 CFR Part 121 international cargo flight experienced an under-cowl fire in its number four engine. In response to the fire, the crew shut down the affected engine, activated the onboard fire extinguishing system, and performed an emergency return to the airport.
The investigation
Post-flight inspections of the engine revealed that the entire outboard engine cowl was missing and the engine had sustained extensive fire damage. Investigators identified that a fuel tube leading to the oil cooler had separated from the coupler attached to the oil cooler, creating a gap of approximately one-half inch. This fuel tube, which has a diameter of about 1.25 inches and operates at a working pressure of roughly 200 psi, utilizes a heat-exchange principle with high-volume fuel flow.
Findings
The investigation determined that the separation of the fuel tube assembly occurred at the brazed sleeve joint between the ferrule and the fuel tube. Detailed examination and X-ray mapping revealed that the brazed joint failed to meet the minimum requirements for filler material adhesion as specified by Aerospace Material Specification (AMS) 2664F for silver brazing.