Definition
A flight instrument component, usually a curved glass tube containing a ball in fluid, that shows whether the airplane is in coordinated flight by indicating sideways (lateral) acceleration. The ball is centered when the forces of the turn are balanced; it moves toward the inside or outside of the turn when the rudder and bank are not coordinated.
Plain English
It is the small ball you see at the bottom of the turn coordinator or turn-and-slip indicator. If the ball is in the middle, the turn is smooth and balanced. If the ball slides off to one side, you are skidding or slipping and need to adjust the rudder.
Context Anchor
You see the inclinometer on the instrument panel, especially when checking whether a turn is coordinated during maneuvering, stall practice, or instrument flying.
Derivation
From Latin inclinare, meaning "to lean." The instrument literally shows whether the airplane is leaning the right way for the turn it is making.
Why Pilots Care
Helps maintain coordinated flight for efficiency and safety; an uncentered ball signals a slip or skid that wastes energy and can lead to loss of control.
Analogy
Think of a small marble in a curved tray on the dashboard of your car. Take a smooth, balanced corner and the marble stays in the middle. Take a rough corner where you turn the wheel without leaning into it, and the marble rolls to the side. The inclinometer ball does the same job in the airplane.
Intuition Check
Do not read inclinometer as a bank-angle instrument. It does not simply tell you how much the wings are tilted; it tells you whether the airplane is slipping or skidding sideways.
Example Sentence 1
As she rolled into the turn, she added rudder pressure until the inclinometer ball was centered.
Example Sentence 2
An uncentered inclinometer during a steep turn indicated a skid that required immediate correction.