Definition
A gyroscopic flight instrument that displays two separate pieces of information: the rate at which the aircraft is yawing (turning) about its vertical axis, shown by a vertical needle, and the quality of the turn (whether it is coordinated, slipping, or skidding), shown by a ball in a curved, fluid-filled tube. The needle is driven by a rate gyro; the ball is a simple inclinometer that responds to gravity and inertia.
Plain English
An instrument with two parts. A needle shows how fast the aircraft is turning left or right. A ball below it shows whether the turn is being flown smoothly or whether the aircraft is sliding sideways through the air.
Context Anchor
Seen on the instrument panel during basic instrument flying, partial-panel flying, and training for coordinated turns.
Derivation
The name describes exactly what the two parts do. 'Turn' refers to the needle showing rate of turn. 'Slip' refers to the ball showing whether the aircraft is slipping or skidding. The informal name 'needle and ball' simply names the two moving parts the pilot watches.
Why Pilots Care
Helps the pilot maintain coordinated flight to hold altitude, avoid stalls, and prevent unusual attitudes in instrument conditions.
Analogy
It is a little like watching both a compass-like pointer and a marble in a curved track: the pointer shows the turn, and the marble shows whether the motion is balanced.
Intuition Check
Do not assume the needle shows bank angle. It shows rate of turn. Do not assume the ball shows whether the wings are level. It shows whether the airplane is coordinated or sliding sideways.
Example Sentence 1
After losing the attitude indicator, the pilot used the turn-and-slip indicator to hold a standard rate turn while keeping the ball centered with rudder.
Example Sentence 2
When the ball slid to the outside of the turn, the pilot applied opposite rudder until the needle and ball showed a coordinated turn again.