System for attaching a first shell (1) with a first protrusion (1.2) and a second shell (2) with a second protrusion (2.3), comprising a butt joint (3) configured to attach the first and second shell (1,2) in an attaching interface, the butt joint (3) comprising an external strap (4) faced outward of the aircraft (10) comprising an external hole (4.1) and an external surface (4.2), an internal strap (5) faced inward of the aircraft (10), comprising an internal hole (5.1), a bolt (6), configured to pass through the external hole (4.1) and the internal hole (5.1), a nut (7) configured to fix the bolt (6) to the external (4) and internal (5) straps, the first protrusion (1.3) and second protrusion (2.3) of each structural shell (1, 2) remaining fixed between said external (4) and internal (5) strap, and a void area (8) arranged around the bolt (6), the void area (8) being an area formed by the external (4) and internal (5) strap. The first (1) and second (2) shell comprise respectively a first recess (1.4) and a second recess (2.4) over the protrusions (1.3, 2.3), and the first and second protrusions (1.3, 2.3) being located between external (4) and internal (5) strap of the butt joint (3), said first and second protrusions (1.3, 2.3) also partially occupying the void area (8).