Definition
The ball indicator is the lower portion of a turn-and-slip indicator (or turn coordinator), consisting of a small black ball sealed inside a curved, fluid-filled glass tube. It shows whether the airplane is in coordinated flight by displaying the relationship between centrifugal force and gravity acting on the ball during a turn. When the ball is centered, the turn is coordinated. When the ball is off-center, the turn is either slipping (ball toward the inside of the turn) or skidding (ball toward the outside of the turn), and rudder input is required to recenter it.
Plain English
A small ball in a curved tube that tells the pilot whether the airplane is turning smoothly. If the ball is in the middle, the turn is balanced. If it slides to one side, the pilot needs to press the rudder pedal on that same side to bring the ball back to the center.
Context Anchor
Seen during turns, especially while checking the turn coordinator or turn-and-slip instrument in the cockpit.
Why Pilots Care
Proper use prevents uncoordinated flight that wastes energy and can contribute to spins or stalls in certain conditions.
Analogy
Think of the ball like a marble in a curved bowl on the dashboard of a car. In a smooth, well-banked turn the marble stays at the bottom of the bowl. If the car turns too sharply for the bank, the marble slides outward; if it banks too steeply for the turn, the marble slides inward. The ball in the cockpit works the same way.
Intuition Check
The ball indicator is not a bank-angle indicator. It does not show how much the wings are tilted; it shows whether the airplane is sliding sideways during the turn.
Example Sentence 1
During the steep turn, the instructor reminded the student to glance at the ball indicator and add a little right rudder to keep the ball centered.
Example Sentence 2
A ball indicator deflected to the right means the airplane is skidding through the turn.