Control System

A control system links the flight controls of an aircraft (and some spacecraft) with its control sur­faces. An aircraft’s control surfaces are the ailerons, elevators, and rudder.

Cables and Hydraulics

The simplest control system uses cables. When the pilot moves one of the controls in the cockpit, the control pulls a cable. The cable is threaded through the plane to a control surface in a wing or the tail. Moving a control pulls a cable, and the cable moves a control surface.

Early airplanes used a simple control system of this kind. Today, only the smallest and slowest aircraft are con­trolled with cables. Bigger and faster air­craft are harder to control with muscle power. When the pilot tries to move the controls, the control surfaces resist because of the greater force of the air pushing back against them.

The biggest and fastest aircraft, including most airliners, have mechani­cal muscles that are much more powerful
than any pilot’s. An airplane’s engines drive pumps that force an oily liquid through pipes. The pipes go from the pumps to machinery in the wings and tail that moves the control surfaces. The flow of oil through the pipes is con­trolled by valves. The valves work like faucets-opening a valve lets oil flow through it, while closing it stops the flow of oil. Moving the flight controls in the cockpit opens or closes valves and sends the oil to the actuators, the machines that move the control surfaces.

This sort of control system, operated by liquid in pipes, is called an hydraulic control system. Aircraft usually have three or even four separate hydraulic control systems. If one fails, there are always more to take its place.