Definition
In a turn-and-slip indicator, restraining springs are calibrated springs attached to the gimbal of the rate gyro that resist its precession. The springs allow the gyro to tilt only a specific amount for a given rate of yaw, so the resulting deflection of the needle corresponds directly to the aircraft's rate of turn.
Plain English
Small springs inside the turn indicator that hold the spinning gyro back, so it only tips a little for a slow turn and more for a faster turn. That controlled tipping is what moves the needle.
Context Anchor
Seen in descriptions of how the turn-and-slip indicator senses and displays a turn.
Derivation
Restraining' comes from the Latin restringere, meaning to hold back or limit. The springs literally hold the gyro back from tipping freely, which is exactly what makes the instrument readable as a rate.
Why Pilots Care
They ensure the needle shows actual turn rate rather than swinging freely or staying deflected after the turn ends.
Intuition Check
Restraining springs do not restrain the airplane. They restrain movement inside the instrument so the pointer gives a controlled indication.
Example Sentence 1
The restraining springs in the turn-and-slip indicator are calibrated so that a one-needle-width deflection equals a standard-rate turn of three degrees per second.
Example Sentence 2
After rolling out of the turn the restraining springs return the needle smoothly to the center mark.