Plain English Meaning

A red mark on a gauge that shows the highest value you are ever allowed to reach. Going past it is not permitted under any condition.

Full Definition

A red radial line marked on a cockpit instrument indicating a never-exceed limit. Operating the engine, propeller, or airplane beyond a red line is prohibited and can cause damage, structural failure, or loss of control. On the airspeed indicator, the red line marks VNE (never-exceed speed); on engine instruments such as the tachometer, manifold pressure gauge, or oil temperature gauge, it marks the maximum value.