Abstract: A leaf (8) can move between an open position and a closed position relative to an opening (3, 4, 6, 7). In the closed position, a front edge (12) of the leaf is engaged in a groove (11) of the front edge (3) of the opening. At the beginning of the opening procedure, a thrustor (24) pivots two arms (18a, 18b), the ends of which slide in grooves (21a and 21b) of the leaf, toward the outside of the opening. This disengages the rear edge of the leaf relative to the rear edge (4) of the opening. Then, a thrustor displaces a carriage (39) along a slideway (38), while, by way of a roller (47) following a track (51), an arm (41) articulated to the carriage (39) and to the leaf (8) disengages the front edge (12) in a controlled manner relative to the plane of the opening. The arm (41) drives the leaf (8 ) toward the rear, while the leaf (8) pivots slightly and slides relative to the axis (19a) defined by the ends of the two rear arms (18a and 18b).