The hydraulic circuits are similar except that mechanical servo valves are replaced with electrically controlled servo valves. The autopilot is now part of the electronic controller. The commands are processed by an electronic controller. The cockpit controls now operate signal transducers which generate the appropriate commands. The fly-by-wire FCS eliminates the complexity, fragility and weight of the mechanical circuit of the hydromechanical FCS and replaces it with an electrical circuit. The autopilot is still equipment furnished by the buyer of an aicraft.Īs an autopilot's reliability improves, the next logical stage of FCS evolution was to totally remove the mechanical circuit, creating the fly-by-wire FCS. The cockpit controls must be fitted with transducers. Later autopilots can accept steering commands directly from the cockpit controls. The pilot utilizes switches on the autopilot for control. With the invention of the autopilot, it is possible to control an aircraft electrically. The pilots feel as if they are flying an aircraft with a mechanical FCS. They increase resistance with airspeed and vice-versa. Artificial feel devices are fitted to the mechnical circuit of the hydromechanical FCS to simulate this 'feel'. The aerodynamic forces are only felt by the actuators. The hydromechanical FCS lacks this 'feel'. This gives a tactile feedback of airspeed and aids flight safety. In the mechanical FCS, the aerodynamic forces on the control surfaces are transmitted through the mechanisms and can be felt by the pilot. These include the Antonov An-225, the Lockheed SR-71 and most aircraft in-between. This arrangement is found in most jet transports and high performance aircraft. The hydraulic circuit powers the actuators which then move the control surfaces. The pilot's movement of a control causes the mechanical circuit to open the matching servo valves in the hydraulic circuit. The servo valves control the movement of the actuators. The actuators convert hydraulic pressure into control surface movements. The actuators are powered by the hydraulic pressure generated by the pumps in the hydraulic circuit. The hydraulic circuit has hydraulic pumps, pipes, valves and actuators. Like the mechanical FCS, it is made of rods, cables, pulleys, and sometimes chains. The mechanical circuit links the cockpit controls with the hydraulic circuits. With hydraulic FCS aircraft size and performance are limited by economics rather than a pilot's strength. Hydraulic power overcomes these limitations. The complexity and weight of a mechanical FCS increases considerably with size and performance of the airplane. Hydromechanical FCS (powered flight control units(PFCU)) This arrangement was used in early piston-engined transport aircraft and can even be found in early jet transports such as the all mechanical Boeing 707. The mechanisms move these tabs, aerodynamic forces in turn move the control surfaces reducing the amount of mechanical forces needed. Servo tabs are small surfaces hinged to the control surfaces.

Some mechanical FCS use servo tabs that provide aerodynamic assistance to reduce complexity. This arrangement is found on bigger or higher performance propeller aircraft such as the Fokker 50. Since an increase in control surface area in bigger airplane leads to an exponential increase in forces needed to move them, complicated mechanical arrangements are used to extract maximum mechanical advantage in order to make the forces required bearable to the pilots. The FCS uses a collection of mechanical parts such as rods, cables, pulleys and sometimes chains to transmit the forces of the cockpit controls to the control surfaces. They were used in early aircraft and currently in small aeroplanes where the aerodynamic forces are not excessive. The mechanical FCS are the most basic designs. hydromechanical FCS (powered flight control units(PFCU)).

Flight control systems (FCS) are classified as follows:

Intelligent flight control system