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 allowable value.
Plain English
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.
Context Anchor
Seen on cockpit instruments, especially the rpm gauge during engine run-up, takeoff, climb, and propeller control adjustments.
Derivation
Comes from the literal red-coloured line painted on the instrument face. The colour red has long been used to mean 'stop' or 'danger', so a red line on a gauge signals an absolute limit.
Why Pilots Care
Exceeding the red line can lead to engine or propeller damage.
Intuition Check
Red line does not mean a suggested setting or a general caution area. It means an operating limit; in this context, do not let the rpm go above it.
Example Sentence 1
During the climb, the pilot reduced manifold pressure to keep the gauge well below the red line.
Example Sentence 2
The tachometer red line at 2,700 RPM prevented overspeed during the descent.