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.
Plain English
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.
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.