Definition
The small black or white sphere inside a curved, fluid-filled glass tube at the bottom of a turn coordinator or turn-and-slip indicator. It indicates whether the airplane is in coordinated flight by responding to the balance between centrifugal force and gravity acting on the aircraft during a turn. When the ball is centered, the turn is coordinated; when it is displaced left or right, the turn is uncoordinated (slipping or skidding).
Plain English
A little ball in a curved glass tube on the instrument panel that tells you if your turn is smooth and balanced. Centered ball means a clean turn. Off to one side means the airplane is slipping or skidding through the air.
Context Anchor
Seen on the turn coordinator during instrument scan, straight-and-level flight, and turn practice.
Derivation
Ball comes from older English and Germanic words for a round object. In this instrument, the word is literal: it refers to an actual small round ball that moves in a curved tube.
Why Pilots Care
Centered ball confirms coordinated flight, which improves efficiency, reduces drag, and prevents disorientation in instrument conditions.
Analogy
Think of a marble sitting in a curved bowl. Tilt the bowl and the marble rolls toward the low side. The ball in the instrument behaves the same way -- it rolls to whichever side gravity and turning forces are pulling the airplane.
Grounding Statement
Picture a small ball rolling left or right in a curved tube; your job is to keep it resting in the center while the airplane flies.
Intuition Check
Ball does not mean a target, a control, or the airplane’s path here. It means the small indicator ball in the turn coordinator that shows sideways slipping or skidding.
Example Sentence 1
During the steep turn, the instructor told the student to step on the ball to keep the turn coordinated.
Example Sentence 2
With the ball centered the airplane maintained coordinated flight through the heading change.