What does a solid blue line indicate on an En Route Low Altitude Chart?

A solid blue line on an En Route Low Altitude Chart indicates a Mode C area. Mode C, which refers to the transponder requirement for aircraft, is crucial for maintaining safety in busy airspace. When operating in a Mode C area, aircraft must be equipped with a functioning Mode C transponder that provides altitude information to air traffic control. This helps in maintaining vertical separation between aircraft and enhances overall situational awareness for both pilots and controllers.

The presence of a solid blue line signifies that all aircraft operating within that airspace are required to have this equipment, thus contributing to safer and more organized air traffic management, particularly in environments where there is a higher density of air traffic. Understanding this symbol is key for pilots when planning their flights and ensuring compliance with airspace requirements.