Fitting with torsion box produced from a plastic material reinforced with carbon fiber, comprising a torsion box (2) with side walls and individual members for attaching the box to respective frames of the tail fuselage (1), along with a stiffening element (4) which joins the side walls (2a, 2b) and which comprises individual end parts (4a, 4b) and a central part (4c); where each end part (4a, 4b) comprises first sections (4d, 4e) which are extended parallel to the side walls (2a, 2b) to both sides of the central part (4c), second sections (4f, 4g) which are extended towards the corresponding side wall (2a, 2b) and third sections (4h, 4i) each one backing onto and attached to the side wall (2a, 2b); the first sections (4e, 4d) of the end parts (4a, 4b) and the side walls (2a, 2b) of the torsion box include individual primary passage holes (5a, 5b, 5c, 5d) aligned in order to create the first fastening.