Definition
The small black or white sphere inside a curved, liquid-filled glass tube on the turn-and-slip indicator (or turn coordinator), which moves left or right to show whether the aircraft is in coordinated flight. When the ball is centered between its reference marks, the forces acting on the aircraft are balanced; when it is displaced to one side, the aircraft is slipping or skidding and rudder input is required to recenter it.
Plain English
A small ball that sits in a curved glass tube on one of the cockpit instruments. If it stays in the middle, you are flying smoothly. If it slides to one side, you need to press the rudder pedal on that same side to bring it back to the middle.
Context Anchor
Seen on the turn-and-slip or turn coordinator instrument during instrument flying, especially while checking whether turns are coordinated.
Why Pilots Care
Centered ball confirms coordinated flight, which is more efficient, reduces drag, and lowers the risk of loss of control in a turn.
Analogy
It is similar to a small level: if the ball moves away from center, it is showing that the airplane is not balanced sideways.
Grounding Statement
Picture the ball sliding left or right when the airplane is being forced sideways instead of moving cleanly through the air.
Intuition Check
Do not read the ball as the main turn-rate or bank-angle display. It does not tell you how fast you are turning; it tells you whether the turn is coordinated or whether the airplane is slipping or skidding.
Example Sentence 1
During the climbing turn, the ball drifted to the left, so the student added left rudder until it was centered again.
Example Sentence 2
During the steep turn, the ball moved to the outside of the turn, indicating a skid that required left rudder correction.