A directional control surface, usually mounted on an aircraft's vertical stabilizer, which causes an airplane to yaw (and, sometimes, roll) to the left or right when deflected.

The rudder of an aircraft is one of the aircraft's ; usually mounted on the trailing edge of the , it enables the aircraft to yaw (turn about the axis passing vertically [in the aircraft's frame of reference] through the aircraft's center of gravity) to the left or right relative to the airflow around the aircraft.

The rudder is used very little during normal flight, unlike the and . There are three main situations in which significant rudder inputs are used, all of them during less-than-perfect circumstances:

  1. When landing an aircraft in a crosswind, the rudder is deflected into the wind to allow the aircraft to line up its longitudinal axis with that of the runway without being blown sideways by the wind; for instance, if the wind is blowing across the runway from left to right, the rudder is deflected to the left to compensate.
  2. If a multiengine aircraft suffers a failure of an engine located off-center, this produces a thrust imbalance that will tend to yaw the aircraft towards the side with the failed engine. Deflecting the rudder to the side opposite the failed engine counteracts this and allows the aircraft to fly straight despite the asymmetry in thrust; for instance, if an engine mounted on the left wing fails, the rudder is deflected to the right to compensate.
  3. When the aircraft is on the ground, it cannot turn itself by banking, as the ground would get in the way. Therefore, the rudder is used if the aircraft has to turn while moving at high speed during ground operations; for instance, if an aircraft on its takeoff run has to dodge an obstruction on the runway that either wasn't there or wasn't visible when it began taking off, or if one of an aircraft's wheel bogies collapses during a landing and causes the aircraft to veer to one side. (Low-speed turns are instead accomplished by steering the aircraft's nosewheel/tailwheel, and/or by differential .)

See Wikipedia for more information.