Definition
A small black ball suspended in a curved, fluid-filled glass tube mounted at the bottom of the turn coordinator (or turn-and-slip indicator), used to show whether a turn is coordinated. The ball's position reflects the balance between the horizontal component of lift and the centrifugal force acting on the aircraft in a turn. When centered, the turn is coordinated; when displaced toward the inside of the turn, the aircraft is slipping; when displaced toward the outside, it is skidding.
Plain English
A little ball in a curved tube on the instrument panel that tells you whether your turn is smooth and balanced. If the ball is in the middle, the turn is just right. If it slides off to one side, the rudder and bank aren't matched up.
Context Anchor
Seen on the turn coordinator during instrument flying, especially while practicing turns, holding patterns, and recovery from unusual attitudes.
Derivation
“Coordination” means making separate actions work together. In this term, it refers to the ailerons and rudder working together so the airplane turns without sliding sideways. “Ball” refers to the actual small ball in the instrument.
Why Pilots Care
An uncoordinated turn wastes performance, feels uncomfortable to passengers, and at low airspeeds can lead to a skid that develops into a spin. Keeping the ball centered with rudder pressure is one of the most basic and safety-critical habits a pilot builds.
Grounding Statement
If the ball is not centered, the airplane is not moving through the turn as cleanly as it could.
Intuition Check
Do not read “coordination” here as teamwork between people. In this instrument, coordination means the airplane’s controls are working together so the aircraft is not sliding sideways in the turn.
Example Sentence 1
As she rolled into the standard-rate turn, she noticed the coordination ball had slid to the right and added right rudder to center it.
Example Sentence 2
When the coordination ball moved to the right, the pilot adjusted rudder pressure to bring it back to the center.