An actuator for a flight control surface of an aircraft, including: a fixed annulus for fixing to the fuselage, an rotatable annulus with respect to the fixed annulus about an axis of rotation, an output ring which can be fixed to the control surface, and a coupling mechanism having a disc with translational mobility with respect to the mobile annulus in a direction parallel to the axis of rotation, the disc able to move between an engaged position, and a disengaged position, in which the disc is disengaged from the mobile annulus to disconnect the output ring from the mobile annulus. A retaining piece keeps the disc in the engaged position, and a rupture member activates to break the retaining piece to allow the disc to move into the disengaged position.