What does a heavy-lined blue box surrounding a NAVAID frequency indicate?

The presence of a heavy-lined blue box surrounding a NAVAID frequency indicates that both standard Flight Service Station (FSS) frequencies are available at all altitudes without the risk of terrain interference. This information is crucial for pilots, as it assures them that they can communicate reliably with the FSS regardless of the altitude at which they are operating.

Standard FSS frequencies help pilots in flight planning, position reporting, and receiving timely weather updates as well as any other flight-related assistance. The design element of the heavy-lined blue box serves as a clear visual cue in aeronautical charts, allowing for quick and easy identification for pilots navigating and planning their routes effectively.

This understanding of the blue box's significance is vital for safe and efficient flight operations, ensuring that pilots can access necessary information without concerns regarding terrain-related disruptions to their communication with flight services.