What does an X inside a flag on an IFR low altitude chart indicate?

An X inside a flag on an IFR low altitude chart signifies the Minimum Crossing Altitude (MCA). This indicates the lowest altitude at which an aircraft must cross a specific fix or point on the route. The MCA is crucial for ensuring that aircraft maintain safe clearance above obstacles, terrain, or other air traffic. It helps in providing a controlled and safe transition through certain segments of airspace, especially where there are elevation changes or other factors that could affect vertical navigation. Understanding this concept is vital for pilots in planning their routes and maintaining compliance with air traffic regulations.