Plain English Meaning
A red line painted on the face of a gauge that marks a hard limit you must not cross. If the needle reaches the red line, you are at the edge of safe operation; going past it is not allowed.
Full Definition
A red radial line on an aircraft instrument is a fixed marking that indicates a never-exceed limit or a minimum operating limit for the parameter that instrument displays. Crossing a red radial line means the aircraft, engine, or system is being operated outside its approved range, which can result in damage, loss of control, or structural failure.
Context Anchor
Seen on the airspeed indicator in multiengine airplanes, especially when studying or practicing engine-failure procedures.
Derivation
Radial means 'arranged like spokes from a center point.' On a round gauge, the markings run outward from the middle like spokes on a wheel, so a 'radial line' is one of those spoke-like marks. Red is the universal color for danger or prohibition, so a red radial line is a spoke-shaped mark warning of a do-not-cross limit.
Why Pilots Care
Exceeding a red radial line risks structural damage, loss of control, or engine failure.
Intuition Check
Do not read the red radial line as a general “danger speed” or an engine redline. In this context, it specifically marks the minimum control speed for a multiengine airplane with the critical engine failed.
Example Sentence 1
During the climb, the pilot kept an eye on the oil temperature gauge to make sure the needle stayed well below the red radial line.
Example Sentence 2
On the tachometer the red radial line showed the maximum RPM the engine could safely sustain.