Definition
Two related gyroscopic flight instruments that show the rate at which the aircraft is turning and whether the turn is coordinated. The turn-and-slip indicator (older design) uses a vertical needle to show the rate of turn around the vertical axis, while the turn coordinator (newer design) uses a small aircraft symbol that banks to show both rate of roll and rate of turn. Both instruments include an inclinometer -- a curved tube containing a ball in fluid -- which indicates whether the rudder and aileron inputs are properly balanced during the turn.
Plain English
An instrument that tells you two things at once: how fast the aircraft is turning, and whether the turn is being flown smoothly with the right amount of rudder. The little ball at the bottom shows whether the turn is coordinated -- if the ball is centered, the turn is balanced; if it slides to one side, the rudder input is wrong.
Context Anchor
Seen on the instrument panel during instrument flying, especially when checking turn rate and keeping the ball centered in a turn.
Derivation
"Turn coordinator" describes its role -- it helps the pilot coordinate the turn (match bank with rudder). "Slip" refers to the aircraft sliding sideways through the air when the turn is uncoordinated, which is what the ball detects.
Why Pilots Care
Coordinated turns prevent unnecessary sideslip, reduce pilot workload, and help maintain control and orientation especially when visual references are lost.
Analogy
Think of the ball like a marble in a curved bowl on the dashboard of a car. If you take the corner smoothly, the marble stays centered. If you turn the wheel without leaning into the curve properly, the marble rolls to one side -- telling you the turn is sloppy.
Intuition Check
Do not treat this as a bank-angle instrument. It shows turn rate and turn balance; the ball being centered means the turn is coordinated, not that the wings are level.
Example Sentence 1
Entering the holding pattern, the pilot rolled into a standard-rate turn using the turn coordinator and kept the ball centered with rudder.
Example Sentence 2
The student practiced partial-panel flying using only the turn coordinator and altimeter after the attitude indicator failed.