Definition
The movable vertical control surface hinged to the trailing edge of the vertical stabilizer (fin) at the tail of the airplane, used by the pilot to control yaw — the rotation of the airplane about its vertical axis. Deflecting the rudder surface left or right pushes the tail in the opposite direction, swinging the nose toward the direction of rudder input.
Plain English
The hinged flap at the back of the airplane's tail fin that the pilot moves with the foot pedals to swing the nose left or right.
Context Anchor
You may see this term in discussions of directional control during taxi, takeoff, landing, and ground roll, especially when the airplane must stay aligned with the runway.
Derivation
From the Old English 'rother,' meaning a steering oar used at the stern of a boat. The aviation rudder borrows directly from the boat rudder — a vertical surface at the rear that swings the craft left or right. Knowing the boat origin makes the airplane rudder's job easy to picture.
Why Pilots Care
Correct use prevents uncoordinated flight, adverse yaw during turns, and loss of directional control in crosswinds or engine-out situations.
Intuition Check
Do not think of the rudder surface as part of the landing gear or as a boat rudder in water. In an airplane, it is the movable tail panel that works in airflow to move the nose left or right.
Example Sentence 1
During the preflight walkaround, the pilot checked that the rudder surface moved freely from stop to stop with no binding.
Example Sentence 2
After an engine failure, the pilot used the rudder surface to counteract the yaw and maintain straight flight.