What is indicated by the red line on the airspeed indicator?

The red line on the airspeed indicator signifies the never exceed speed, commonly referred to as Vne. This is a critical airspeed that pilots must not exceed under any circumstances because doing so can lead to loss of control or structural failure of the aircraft. The Vne value is determined by the aircraft manufacturer and is based on the design limits of the aircraft.

Understanding this speed is crucial because it establishes a hard limit. Any speed above this line indicates excessive airspeed which can lead to dangerous situations, such as aerodynamic failures or structural damage, particularly in turbulence or if abrupt control inputs are made. This is why pilots are trained to respect the red radial mark and ensure that the aircraft remains well below this threshold during operations.

The other options do indicate important airspeeds as well but serve different purposes within different contexts of flight: the indicated normal operating speed range or stall speed would not be marked with a red line; instead, they correspond to other segments on the airspeed indicator, each designed to denote safe operational parameters within those specific phases of flight.