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The rudder is a primary flight control surface which controls rotation about the vertical axis of an aircraft. This movement is referred to as "yaw". The rudder is a movable surface that is mounted on the trailing edge of the vertical stabilizer or fin. Unlike a boat, the rudder is not used to steer the aircraft; rather, it is used to overcome adverse yaw induced by turning or, in the case of a multi-engine aircraft, by engine failure and also allows the aircraft to be intentionally slipped when required.
In most aircraft, the rudder is controlled through the flight deck rudder pedals which are linked mechanically to the rudder. Deflection of a rudder pedal causes a corresponding rudder deflection in the same direction; that is, pushing the left rudder pedal will result in a rudder deflection to the left. This, in turn, causes the rotation about the vertical axis moving the aircraft nose to the left. In large or high speed aircraft, hydraulic actuators are often used to help overcome mechanical and aerodynamic loads on the rudder surface.
Rudder effectiveness increases with aircraft speed. Thus, at slow speed, large rudder input may be required to achieve the desired results. Smaller rudder movement is required at higher speeds and, in many more sophisticated aircraft, rudder travel is automatically limited when the aircraft is flown above Manoeuvring Speed to prevent deflection angles that could potentially result in structural damage to the aircraft.