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

The red line on the airspeed indicator represents the never exceed speed, which is the maximum speed an aircraft should never exceed in any operation. Flying above this speed can lead to structural damage or failure of the aircraft due to aerodynamic limitations. The never exceed speed is a critical safety parameter that pilots must observe to maintain the integrity of the aircraft during flight maneuvers, particularly in situations involving turbulence or abrupt control inputs.

Other markings on the airspeed indicator have their own significance; for example, normal operating speeds and flap operating speeds are typically indicated by different color coding or lines, which are essential for safe flight operations but differ from the red line's critical warning. It's vital for pilots to understand these indications to ensure safe and efficient flight operations.