Definition
A fine, spiral-wound spring used in mechanical aircraft instruments to provide a small, calibrated restoring force against the movement of a pointer or mechanism. As the measured quantity changes, the hairspring opposes the deflection so the pointer settles at a position that accurately represents the input.
Plain English
A very thin, coiled spring inside an instrument that gently pulls the needle back, so it points to the right value instead of swinging freely.
Context Anchor
Seen in discussions of mechanical aircraft instruments, instrument repair, and how cockpit needles are controlled.
Derivation
Named for its appearance: the spring is so fine and thin it resembles a strand of hair coiled into a spiral.
Why Pilots Care
Maintains accurate, stable instrument readings by providing consistent tension without adding weight or friction that could affect performance in flight.
Analogy
It works like the tiny coiled spring inside a mechanical wristwatch -- small, delicate, and precisely tuned to control how a pointer moves.
Intuition Check
Do not read “hairspring” as a spring made of hair. In instruments, it means a very fine metal spring used to control movement.
Example Sentence 1
A weak hairspring in the airspeed indicator caused the needle to lag behind actual airspeed changes during the test flight.
Example Sentence 2
A distorted hairspring in the airspeed indicator can cause the needle to lag or stick during rapid changes in speed.