Definition
A rigid bar or link that constrains a moving part to travel along an arc of fixed radius about a pivot point. In aircraft structures and mechanisms, a radius bar is used to guide the motion of a component — such as a landing gear leg, control surface linkage, or flap track follower — so that it swings on a precise, repeatable path rather than moving freely.
Plain English
A stiff bar that forces a moving part to travel along a curved path of a fixed distance from a pivot, like the hand of a clock sweeping a circle.
Context Anchor
Seen in landing gear maintenance, inspection, and repair descriptions, especially where the gear must move but still stay correctly aligned.
Derivation
Radius comes from the Latin radius meaning 'spoke of a wheel' or 'ray.' A spoke holds the rim of a wheel at a fixed distance from the hub — exactly what a radius bar does for the part it guides.
Why Pilots Care
If a radius bar is bent, loose, or damaged, the landing gear may not stay properly aligned, which can affect ground handling and increase stress on the gear during takeoff, landing, or taxi.
Analogy
Think of a compass used for drawing circles: the metal arm holds the pencil at a fixed distance from the centre point, so the pencil can only trace a perfect arc. A radius bar does the same job for a moving aircraft part.
Intuition Check
Do not read “radius bar” as a measurement of turn radius or distance. In this context, it is a physical bar in the landing gear that controls how a wheel or gear part is held and moves.
Example Sentence 1
The mechanic inspected the radius bar on the main landing gear for cracks and wear at its pivot points.
Example Sentence 2
After forming the new bracket, the technician checked it with a radius bar before installation.