Final answer:
Blue hash marks on a VFR sectional chart represent the boundaries of Class D controlled airspace surrounding airports, requiring pilots to acknowledge certain flight rules and communication procedures.
Step-by-step explanation:
On a VFR (Visual Flight Rules) sectional aviation chart, blue hash marks indicate the presence of an airspace with specific flight rules, commonly known as controlled airspace. The blue has marks often surround an airport to represent Class D airspace, which is airspace where air traffic control provides services to pilots, but communication is not always mandatory for VFR flights, unlike Class B and Class C airspaces which have solid blue or magenta lines, respectively. When pilots see these blue hash marks, they need to be aware of the potential for increased radio communication and traffic, and adhere to the airspace's operational requirements.
These markings are essential for pilots to understand as they plan their flights as it helps maintain safety and proper traffic flow in areas where aircraft are operating in close proximity.