Definition
The small ball, located inside a curved tube at the bottom of the turn coordinator (or turn-and-slip indicator), that shows whether the airplane is in coordinated flight. When the ball is centered, the airplane is coordinated. When the ball is displaced to one side, it indicates a slip or a skid, depending on the relationship between the bank and the rate of turn.
Plain English
A little ball in a curved glass tube on the instrument panel that tells the pilot whether the airplane is turning smoothly. If the ball sits in the middle, the turn is balanced. If it slides to one side, the airplane is sliding sideways through the air and the pilot needs to correct it with rudder.
Context Anchor
Seen on the instrument panel as part of the turn coordinator or turn-and-slip instrument, especially when checking coordinated flight during turns.
Derivation
Slip and skid both describe sideways motion. A slip is when the airplane slides toward the inside of the turn (not enough rudder). A skid is when the airplane slides toward the outside of the turn (too much rudder). The instrument is named for the two conditions it detects.
Why Pilots Care
Coordinated flight reduces drag, improves fuel efficiency, and lowers the chance of an unintentional stall or spin entry.
Analogy
Think of a marble in a curved tray on a car dashboard. Take a smooth, banked turn and the marble stays in the middle. Turn too sharply without leaning, and the marble slides outward. Lean too much without turning enough, and it slides inward. The slip/skid ball works the same way.
Grounding Statement
If the ball is centered, the forces in the turn feel balanced; if the ball moves left or right, the airplane needs correction.
Intuition Check
Do not read slip and skid as casual everyday words meaning only a slide on the ground. Here they mean two different kinds of uncoordinated flight shown by the ball moving away from center.
Example Sentence 1
Rolling into the turn, the pilot added a touch of right rudder to keep the slip/skid indicator centered.
Example Sentence 2
A skid developed when the pilot over-banked without adding rudder, pushing the ball to the outside of the turn.