A fully power‐operated control system, shown in fig. 1 applied to the clcvons of a tailless aircraft, includes a hydraulic actuator 65, 70 which is rendered irreversible under air loads transmitted from the control surface by arranging that in the neutral position of the control valve 64 the actuator is pre‐loaded by balanced hydraulic forces acting on opposite sides of the actuator piston. The valve 64 is mounted on the actuator cylinder 65, and is operated by a spring‐loaded rod 61 moved by a pilot‐actuated arm 60 through a control cable 12. In the neutral position the valve 64 permits the entry of liquid at half the normal working pressure to both ends of the cylinder 65, thus locking the piston and control surface, movement of the rod 61 operating the valve to permit liquid at full pressure to flow to one end only of the cylinder 65 to move the surface. The follow‐up movement of the casing of the valve 64 consequent upon the movement of the surface returns the valve to the neutral position to lock the surface in its displaced position. The rod 61 is spring‐loaded in both directions to ensure that only a safe load is applied to the valve in the event of the pilot's rate of control movement exceeding the rate of response of the actuator.
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