What does a flag with an "X" in it represent?

The presence of a flag with an "X" typically signifies a Minimum Crossing Altitude (MCA) on aeronautical charts. This is an altitude that must be achieved or maintained before crossing a specific point in the airspace, particularly on certain routes or approaches. It is essential for ensuring safe vertical separation between aircraft and preventing conflicts, especially near busy airspace or when transitioning between different air traffic control sectors.

In practical terms, when a pilot observes this indicator on their navigation chart, it informs them that, in order to comply with air traffic control procedures, they must reach this specified altitude prior to their crossing point. This altitude is crucial to maintain safety and ensures that all aircraft remain in their designated flight levels.

Understanding the significance of MCAs is vital for pilots, especially in instrument flight rules (IFR) conditions, where adherence to specific altitudes is necessary for safe and efficient flight operations.