What does the blue radial line on an airspeed indicator represent?

The blue radial line on an airspeed indicator represents the best rate of climb speed (often referred to as VY in aviation terminology) for an aircraft when one engine is inoperative. This speed is critical for pilots to know, especially when flying multi-engine aircraft, as it helps to achieve the best altitude gain within a specified time frame under the conditions of reduced power.

This speed allows the pilot to maximize performance and altitude gain, which is essential in scenarios where the aircraft has lost an engine. Operating at or above this speed ensures that the aircraft maintains a safe margin above the stall speed, thereby enabling optimal climb performance and safety during critical flight phases after an engine failure.

In terms of understanding flight safety and performance management, maintaining the best rate of climb speed is crucial for ensuring that the aircraft can safely ascend despite the loss of power from an engine.