Definition
The indicated airspeed marked by a red radial line on the airspeed indicator, representing a never-exceed or never-go-below limit. In multi-engine engine-inoperative flight, this term most commonly refers to VMC, the minimum control speed with the critical engine inoperative, marked as a red radial line on the airspeed indicator. Operating below the red radial in an engine-out condition can result in loss of directional control.
Plain English
It is the airspeed shown by a red line on the airspeed indicator. In a twin-engine airplane with one engine failed, it marks the slowest speed at which the pilot can still keep the airplane straight using the rudder and ailerons. Slowing below that line risks losing control of the airplane.
Context Anchor
Seen on the airspeed indicator in multiengine airplanes and discussed during engine-inoperative training.
Derivation
Radial means 'arranged like spokes from a center.' The airspeed indicator's needle pivots from a central point, so any line drawn out from that center along a specific speed is called a radial line. The color red signals a limit not to be crossed.
Why Pilots Care
Flying slower than this speed with an engine out removes the ability to counteract yaw and can result in an unrecoverable roll.
Grounding Statement
Picture the red line on the airspeed indicator as the point where, with one engine failed, control can become the main problem.
Intuition Check
Radial does not mean a radio navigation radial here. It means a line drawn from the center of the airspeed indicator. Indicated does not mean the airplane's exact speed through the air. It means the speed shown on the cockpit instrument.
Example Sentence 1
After the right engine failed, the pilot lowered the nose to keep airspeed safely above the red radial on the airspeed indicator.
Example Sentence 2
The pilot noted that any further reduction below red radial indicated airspeed would cause the airplane to roll toward the dead engine.