BOEING 767-300FFEDERAL EXPRESS CORP

The incident engine, a General Electric (GE) CF6-80C2 turbofan, suffered an undercowl fire due to a high-pressure fuel leak originating from a bolted flange interface between a high-pressure fuel crossover tube and the hydromechanical unit (HMU) accessory gearbox adapter pad. The engine’s accessory heat shield surface temperatures and stagnant airflow near the fuel leak location were sufficient to cause ignition. Two threaded inserts in the adapter pad were found protruding and likely prevented the fuel tube from correctly seating on the adapter pad surface. Threaded inserts were not securely installed into the adapter pad, which allowed the crossover tube bolts to partially pull out. Multiple insert holes had stripped threads, linear scoring, counterclockwise material smearing, missing surface anodization, cross-threading, and concave hole profiles. Avio Aero, the adapter manufacturer, implemented an automated pre-tap operation that pre-machined threads into the adapter pad to ease installation of self-tapping steel inserts. In early 2023, this pre-tap operation was determined to be a deviation from the part’s drawing requirements. Neither GE Aerospace nor Avio Aero could find documentation supporting the decision to pre-tap the adapter holes or Avio Aero communicating to GE of their decision to deviate from the print requirement. Avio Aero may have decided to pre-tap the adapter holes because other parts were using this process.