Abstract: A fluid-pressure-operable actuator system, for operating an aerodynamic control surface, including an actuator comprising a casing, piston and output member. Annular sealing means carried by the piston sealingly engages with the casing while annular sealing means disposed in an aperture through which the output member extends sealingly engages that member. Bearing means circumferentially flank the sealing means of the piston and/or the aperture and include parts of readily- and locally-deformable material, respectively in relatively-slidable load-bearing engagement with the casing and/or the output member. In the event that damage is sustained by the surface of the actuator engaged by the sealing means the bearing means is sufficiently deformed as to permit relative movement of the casing and output member to continue under the power of at least one other actuator associated with said control surface.