Definition
Movements of the rudder away from its neutral (centered) position, produced by the pilot pressing the left or right rudder pedal. Each press pivots the rudder to one side of the vertical stabilizer, creating a sideways aerodynamic force at the tail that yaws the airplane's nose left or right. The amount of deflection refers to how far the rudder is moved from center, and is controlled by how much pressure the pilot applies to the pedal.
Plain English
How far the rudder is pushed left or right by the pilot's feet on the pedals. A small push gives a small movement; a firm push gives a larger one. This swings the nose of the airplane in the direction of the pedal that is being pressed.
Context Anchor
Encountered during the takeoff roll, when the pilot is using the rudder pedals to keep the airplane tracking straight down the runway.
Derivation
Rudder comes from the Old English rother, meaning a steering oar used on boats. Deflection comes from the Latin deflectere, meaning to bend or turn aside. Together the phrase describes turning the steering surface aside from its centered position.
Why Pilots Care
Correct rudder deflections keep the airplane on the runway centerline; inadequate or excessive inputs cause runway excursions or loss of directional control.
Intuition Check
Rudder deflection does not mean the rudder is damaged or bent. It means the pilot has moved the rudder left or right from its neutral position.
Example Sentence 1
During the takeoff roll, the pilot used small rudder deflections to keep the nose tracking straight down the centerline.
Example Sentence 2
A sudden crosswind gust required larger right rudder deflections to prevent the nose from weather-vaning left.