Definition
A sealed, curved glass tube partially filled with liquid and containing a small ball, used as the inclinometer portion of a turn-and-slip indicator or turn coordinator. The ball moves freely along the curve in response to the combined forces of gravity and centrifugal force acting on the aircraft, indicating whether a turn is properly coordinated.
Plain English
A small bent glass tube with fluid and a ball inside, mounted on the instrument panel. The ball rolls left or right depending on whether your turn is balanced. If the ball is centered, the turn is coordinated. If it slides to one side, the turn is slipping or skidding.
Context Anchor
Seen on turn-and-slip indicators and turn coordinators when checking whether a turn is coordinated.
Why Pilots Care
Reveals slips or skids so the pilot can adjust rudder and aileron inputs for coordinated, efficient flight and safer handling.
Analogy
It works a little like a small level: the ball’s position gives you a simple visual clue about whether things are balanced.
Intuition Check
Do not read this as just any curved tube made of glass. In this FAA context, it means the liquid-filled tube in the turn instrument that holds the ball used to judge turn coordination.
Example Sentence 1
During the steep turn, the ball in the curved glass tube slid to the outside, so the pilot added inside rudder to recenter it.
Example Sentence 2
The ball slid to the outside of the curved glass tube, indicating the airplane was in a skid.