<P> The complexity and weight of mechanical flight control systems increase considerably with the size and performance of the aircraft . Hydraulically powered control surfaces help to overcome these limitations . With hydraulic flight control systems, the aircraft's size and performance are limited by economics rather than a pilot's muscular strength . At first, only - partially boosted systems were used in which the pilot could still feel some of the aerodynamic loads on the control surfaces (feedback). </P> <P> A hydro - mechanical flight control system has two parts: </P> <Ul> <Li> The mechanical circuit, which links the cockpit controls with the hydraulic circuits . Like the mechanical flight control system, it consists of rods, cables, pulleys, and sometimes chains . </Li> <Li> The hydraulic circuit, which has hydraulic pumps, reservoirs, filters, pipes, valves and actuators . The actuators are powered by the hydraulic pressure generated by the pumps in the hydraulic circuit . The actuators convert hydraulic pressure into control surface movements . The electro - hydraulic servo valves control the movement of the actuators . </Li> </Ul> <Li> The mechanical circuit, which links the cockpit controls with the hydraulic circuits . Like the mechanical flight control system, it consists of rods, cables, pulleys, and sometimes chains . </Li>

How do the controls of an airplane work