Definition
A mechanical spring made from a length of round or square wire wound into a helical (spiral) shape, used to absorb and release energy through compression, extension, or torsion. In aircraft, coil springs are commonly found in landing gear shock absorption, valves, control mechanisms, and various assemblies where a controlled, repeatable resisting force is required.
Plain English
A piece of metal wire wound into a spiral that pushes back when you squeeze it or pulls back when you stretch it. It stores energy when you press on it and releases that energy when the pressure is removed.
Context Anchor
Seen in aircraft maintenance discussions for parts that need to return to position, hold tension, or absorb motion.
Derivation
Coil' comes from the Old French 'coillir', meaning 'to gather' or 'collect together' -- think of gathering wire into loops. 'Spring' comes from Old English 'springan', meaning 'to leap or burst forth', describing how the device snaps back to its original shape. Together: a gathered, looped wire that springs back.
Why Pilots Care
Coil springs absorb landing shocks and maintain tension or return force in flight-critical components, preventing binding or loss of control authority.
Analogy
A simple example is the spring inside a click pen. When you press it, the spring stores force; when you let go, it pushes back.
Intuition Check
Do not confuse coil spring with an electrical coil. Here, coil describes the wound shape of the metal spring, not an electrical part.
Example Sentence 1
The mechanic replaced a broken coil spring in the nose gear shock strut during the 100-hour inspection.
Example Sentence 2
A broken coil spring in the throttle linkage caused the engine to stick at partial power during the approach.