What does an altitude/flight level underlined in red indicate?

What does an altitude/flight level underlined in red indicate?

• A. Iafdof
• B. Marsa
• C. Jet route altitude stratum
• D. Lowest usable flight level with 30.28 setting

Answer: (A)

Red underlining of an altitude or flight level on en-route charts marks the lowest usable level for that route segment. This means you must not plan or fly below that value, because below it obstacle clearance, radar or navigational coverage, or other operational constraints cannot be guaranteed. You may fly at that level or above, unless ATC assigns a higher altitude or a different segment has its own restriction. This notation helps pilots confirm the minimum vertical limit you can rely on for safe navigation along that airway or jet route.

Red underlining of an altitude or flight level on en-route charts marks the lowest usable level for that route segment. This means you must not plan or fly below that value, because below it obstacle clearance, radar or navigational coverage, or other operational constraints cannot be guaranteed. You may fly at that level or above, unless ATC assigns a higher altitude or a different segment has its own restriction. This notation helps pilots confirm the minimum vertical limit you can rely on for safe navigation along that airway or jet route.