All active pilots are familiar with the rules for making contact with ATC before entering Class D airspace: two-way communications must be established before entering the designated airspace (FAR 91.129)—simple enough. But when does a pilot enter Class D airspace? The answer is not always as straightforward as it appears.
Aeronautical Information Manual section 3-2-5 states the following about Class D airspace: “Generally, that airspace from the surface to 2,500 feet above the airport elevation (charted in MSL) surrounding those airports that have an operational control tower. The configuration of each Class D airspace area is individually tailored and when instrument procedures are published, the airspace will normally be designed to contain the procedures.” (Emphasis added.)
First, notice the caveat: generally. In other words, while Class D airspace typically extends from the surface of the airport to 2,500 feet agl, that is not always the case. Rarely is a Class D ceiling exactly 2,500 feet above the airport elevation—sometimes it is hundreds of feet less, sometimes more. Second, and more importantly, the radius of Class D airspace is individually tailored by airport, and often contains extensions to accommodate approach procedures. Take Republic Airport in Farmingdale, New York, as an example: The airspace contains an irregular shape on the north side of the airport in order to accommodate instrument approaches. This renders the radius of the airspace on the north side of the airport significantly greater than 4 nautical miles—the distance many pilots typically expect and often find.
"Big deal," you might say, "I’m a conscientious pilot and review charts closely before each flight in accordance with the requirements of FAR 91.103, which states. 'Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight.'" Unfortunately, however, unusual configurations can be easy to miss unless you are looking for them. This is especially true in congested airspace. Just glance at the Class D airspace in the greater Los Angeles area to get a sense of how easy it is to overlook irregular configurations in a crowded urban area. I’m not sure how one would describe the airspace at Fullerton Municipal Airport or Jack Northrop Field/Hawthorne Municipal Airport—but it is anything but a nice, round cylinder.
Moreover, if you rely on information generated by a moving map on a multifunction display, be aware that some of the older systems demarcate all Class D airspace with a standard 4-nm ring. Just last year, the FAA brought an enforcement action against a pilot who relied on an MFD map (with current data no less) that inaccurately indicated a 4-nm ring around a Class D airport, when in fact, the airport in question contained a 2-nm extension. The take away—make sure that you understand the limitations of your GPS data, and always consult the sectional itself, whether in paper or electronic format.
At the end of the day, it is up to pilots to ensure that they have accurate information about the airspace in which they intend to fly around. The next time you plan to fly into Class D airspace, be sure to look closely at the airspace configuration—you might find something unexpected.