Definition
An older two-part turn instrument consisting of a vertical needle driven by a rate gyro that shows the rate and direction of turn, and a curved glass tube containing a ball in fluid that shows whether the turn is coordinated. The needle indicates how fast the aircraft is turning; the ball indicates whether the turn is being flown with the correct balance of bank and rudder.
Plain English
It is a simple cockpit instrument with two parts working side by side. One part, the needle, tells you which way you are turning and how quickly. The other part, the ball, tells you whether your feet and your hands are working together properly during the turn.
Context Anchor
Seen in instrument flying when learning turn indicators and checking whether a turn is smooth and coordinated.
Derivation
The name comes from the two visible parts of the older turn-and-slip indicator: the moving needle and the ball in the curved tube. The nickname helps because it points directly to what the pilot sees on the instrument face.
Why Pilots Care
It lets the pilot keep turns coordinated without outside visual references, reducing the chance of a slip or skid that could lead to control problems.
Grounding Statement
Picture a cockpit instrument with one pointer for turning and one small ball that moves when the airplane is not turning cleanly.
Intuition Check
Do not read this as two separate cockpit items. In this context, needle and ball means one instrument: the needle shows the turn, and the ball shows whether the turn is coordinated.
Example Sentence 1
In the older trainer, the student learned to keep the needle on one width of deflection for a standard rate turn while keeping the ball centered with rudder.
Example Sentence 2
A centered ball on the needle and ball showed the approach turn had no slip or skid.