Definition
A small hand tool consisting of a set of thin metal blades, each cut with a precisely formed concave or convex curve of a known radius, used to check the radius of a rounded edge, fillet, or corner on a part by visually matching the blade against the curve.
Plain English
A pocket-sized set of curved metal templates. You hold each curved blade against a rounded edge until you find the one that matches it perfectly, and the marking on that blade tells you the size of the curve.
Context Anchor
Seen in aircraft maintenance, repair, inspection, sheet-metal work, and parts drawings when a required curve or bend size must be checked.
Derivation
Radius comes from Latin for the spoke of a wheel, the straight line from the center of a circle to its edge. A radius gauge measures the size of that curve.
Why Pilots Care
Maintenance technicians use radius gauges to verify that fillets, rounded edges, and machined curves match the manufacturer's specifications. A wrong radius in a stressed part can create a stress concentration point that leads to cracking.
Intuition Check
A radius gauge is not a cockpit gauge with a needle or display. It is usually a hand tool used by a mechanic to compare a part’s curve against a known curve size.
Example Sentence 1
The technician used a radius gauge to confirm the fillet on the engine mount matched the dimension called out in the maintenance manual.
Example Sentence 2
During the preflight inspection after a repair, the technician verified the control surface hinge radius using the appropriate gauge.