Definition
On a turn-and-slip indicator, ball displacement is the movement of the inclinometer ball away from its centred position between the reference lines, indicating that the aircraft is in uncoordinated flight — either slipping or skidding through the turn. The direction the ball moves shows which rudder input is required to restore coordination.
Plain English
It's how far, and which way, the little black ball in the bottom of the turn indicator has rolled out of the middle. When it's not centred, the turn isn't coordinated, and the pilot needs to add rudder on the side the ball has rolled toward to bring it back.
Context Anchor
Seen on the turn-and-slip indicator during instrument flying, especially while checking whether straight-and-level flight or a turn is properly coordinated.
Derivation
Displacement' comes from the Latin dis- ('apart') and placere ('to place') — literally 'placed away from where it should be.' Here, it simply means the ball has moved away from its centred position.
Why Pilots Care
Uncorrected displacement means uncoordinated flight, which reduces efficiency, increases drag, and can lead to loss of control in critical phases such as stalls or engine failure.
Intuition Check
Do not read ball displacement as bank angle or turn rate. It is the ball’s sideways shift from center, showing whether the airplane is balanced or being pushed sideways.
Example Sentence 1
She noticed ball displacement to the left during the turn and added left rudder until it returned to centre.
Example Sentence 2
With the ball centered there is no displacement, confirming the turn is coordinated.