A premixing, tangential entry fuel injector (10) for a gas turbine engine features a secondary fuel-air injection insert (40) positively secured to a centerbody shell (38) by a braze joint (98). A secondary fuel supply tube (42), positively secured to both a centerbody base (36) and to the insert (40), is curved in at least two dimensions. In an exemplary embodiment, the tube is coiled into a spiral shape covering a single 360.degree. cycle. During engine operation, the centerbody expands axially in response to elevated temperatures in the engine's interior, causing the insert (40) to be displaced away from the base (36). The curvature of the tube allows the tube to flex slightly to accommodate the displacement. Ideally the curvature of the tube is such that the tube's natural frequency is well above the maximum vibratory frequency that the tube will experience during engine operation.