What does a dashed blue circle around an airport indicate on a sectional aeronautical chart?

A dashed blue circle around an airport on a sectional aeronautical chart signifies the boundary of Class D airspace. This type of airspace is designated around airports that have an operating control tower and is typically found in areas where the potential for higher traffic exists. The dashed line indicates that, while there are specific entry requirements and procedures, the approach to and departure from the airspace can occur without the need for a clearance or specific instructions from air traffic control, as long as the pilot remains in communication with the tower.

Understanding the boundaries of Class D airspace is critical for pilots to maintain safe and efficient operation in congested areas, as well as to comply with regulations pertaining to flying in controlled airspace. Other types of airspace, such as Special VFR or Class B airspace, have different indicators and specific regulations governing their use, which do not apply in this context.