What does the blue radial line on the airspeed indicator of a light multiengine airplane indicate?

The blue radial line on the airspeed indicator of a light multiengine airplane is significant because it denotes the single-engine best rate of climb speed, often referred to as Vyse. This is the speed at which the aircraft can achieve the greatest altitude in the shortest amount of time when one engine fails. Understanding the importance of this speed is crucial for safe multiengine flying, as it helps the pilot manage climb performance effectively during an engine failure.

Maintaining Vyse after an engine failure is essential for ensuring that the aircraft can gain the best climb rate, which is particularly important for clearing obstacles or achieving safe altitude. It provides performance data critical to making informed decisions in emergency scenarios.