Definition
The red line on an airspeed indicator marks VNE, the never-exceed speed of the aircraft. Flight at any speed above this line is prohibited under any circumstances because it can cause structural damage or failure.
Plain English
It is the top speed limit for the airplane. The needle should never go past the red line, no matter what.
Context Anchor
Seen on the airspeed indicator, usually at the high-speed end of the scale, especially during descents or any situation where airspeed is increasing.
Derivation
VNE comes from the aviation convention of using V-speeds, where 'V' stands for velocity (Latin velocitas, meaning speed). 'NE' stands for 'never exceed.' Knowing this makes the meaning easy to remember: the velocity you never exceed.
Why Pilots Care
Exceeding the red line can lead to flutter, structural failure, or loss of control.
Intuition Check
Do not think of the red line as a suggested caution area. It is the limit: at or above this mark, you must reduce airspeed and not allow the airplane to go faster.
Example Sentence 1
During the descent, the pilot reduced power and added drag to keep the airspeed well below the red line.
Example Sentence 2
In turbulence, staying well below the red line (VNE) is essential for safety.