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I noticed KBFI's class delta overlaps SeaTac's bravo by 500 to 700 feet. Does anyone know which tower the overlapping airspace belongs to? Does it classify as Bravo airspace and do you need to hear the magic "cleared into the bravo" phrase before entering the overlapping areas?

Bonus question: There is a tiny sliver of class D airspace surrounding the surface B of SeaTac on the western, southern and southeastern parts of the bravo that is surface. Anyone know if this belongs to SeaTac because there is no other field in the center of the area and if so, why on earth does a bravo airport also have a tiny section of class D surrounding it?

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This is a great question. For those who are unfamiliar, and in areas where airspace is complex, it’s sometimes useful to look at the VFR Flyway Chart. This is the one on the back of FAA Terminal Area Charts.

Seattle VFR Flyway Planning Chart

The dashed blue outlines surround the crazy cutouts that are Seattle’s Class D airspaces.

Why does KBFI stick into the KSEA Class B?

Class D airports, or at least those that don’t exist entirely within more restrictive airspace, will always have a ceiling. This is because there will be sections that are not contained within the Class B (or C) areas. For those areas, we need to know how high the Class D airspace goes.

If there is Class B and Class D airspace in the same place, which is it?

This one is in the AIM, 3-1-3:

Hierarchy of Overlapping Airspace Designations

a. When overlapping airspace designations apply to the same airspace, the operating rules associated with the more restrictive airspace designation apply.

So, if you’re over the I-90 bridge at 1900 feet, you’re in KBFI Class D. If you’re at 2100 feet, you’re in KSEA Class B.

Why does KSEA have weird class D chunks?

This one is easier - because Class B doesn’t always go to the surface (only at the core), but there is still a need to filter out traffic from the immediate airport environs. Because two-way communication is required, pilots will need to talk to the tower before getting too close (~5SM) to the airport.

Another factor is that Class D centers on the airport; Class B is centered on the SEA VOR, and they are not coincident. This is so that the rings could (when it was circular) be defined with DME.

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  • $\begingroup$ Thanks. That FAR/AIM is what I was after. In your example you cite Seattle as being Class C when it's actually B (which is obvious from the map coloring). Also I'm not sure that your explanation about the thin sliver of Delta around the SeaTac bravo is 100% correct. I've never seen any Bravo that doesn't go to the surface (do you know of an example?) and why not just have SeaTac extend the surface bravo? Thanks again. $\endgroup$ – seattle272SP Sep 29 '14 at 6:29
  • $\begingroup$ Whoops, typo. And the B does go to the surface - just not for a 5 mile radius. Seattle has pretty unique topography and it also has been updated to use the new non-round B design. $\endgroup$ – egid Sep 29 '14 at 14:39
  • $\begingroup$ Oh, and because the B is centered on the VOR while the D is centered on the airport. $\endgroup$ – egid Sep 29 '14 at 14:42
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    $\begingroup$ Your image has perished. :( Any chance you still have it lying around and can replace it? $\endgroup$ – reirab Apr 16 '15 at 2:43
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The SeaTac Class D space is below the SeaTac Bravo.

Its purpose is basically that if you are getting close to the Bravo, SeaTac wants to be in communication with you, and know that you're aware of your proximity to the Bravo. But they probably won't issue you a Transponder Code or give you vectors (unless traffic is near by).

Its just gives them confidence that you're aware of where you are and what you're doing before you end up in front a 767 on short-final. :)

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Bonus question: There is a tiny sliver of class D airspace surrounding the surface B of SeaTac on the western, southern and southeastern parts of the bravo that is surface. Anyone know if this belongs to SeaTac because there is no other field in the center of the area and if so, why on earth does a bravo airport also have a tiny section of class D surrounding it?

In the FAA's "Airspace Designations and Reporting Points" document, this airspace is specifically described as "Class D airspace... extending upward from the surface and designated as an extension to a Class B surface area." (page D-164). It is the only example of this kind of airspace in the entire US national airspace system.

There is also one single example of "Class D airspace... extending upward from the surface and designated as an extension to a Class C surface area." It is at Biggs Army Air Field at El Paso, TX (KBIF) (page D-165).

The purpose of the Class D "extension" at SeaTac is to require approaching pilots to communicate with the control tower, without imposing the requirement for an actual clearance to enter the airspace, and without imposing the other requirements associated with Class B airspace. Yes, it does "belong" to SeaTac.

The Class D "extension" at KBIF is more of a "hybrid" situation-- KBIF has its own control tower but is actually located in the Class C surface area of El Paso International (KELP). The Class D "extension" is intended to facilitate operations at KBIF, and abuts the Class C surface area of KELP.

On a related note, Los Angeles International Airport (KLAX) has Class D airspace associated with it that appears to be functionally equivalent to the Class D airspace "extensions" at SeaTac, but it is not described as an "extension" in the "Airspace Designations and Reporting Points" document. Instead, it is described on pages D-148 through D-149, in the main section on Class D airspace, section 5000, whose description (page D-1) includes the phrase "Each Class D airspace area designated for an airport in this subpart contains at least one primary airport around which the airspace is designated." Each piece of Class D airspace at KLAX is described as part of a ring of Class D airspace that completely encircles KLAX. There are two such rings in total, one extending to 2700' AGL and one extending to 2500' AGL. Most of the Class D airspace in each ring is invisible and non-functional because it coincides with Class B airspace, leaving just two protruding "tabs" of functional Class D airspace that serve essentially the same purpose as the Class D "extensions" at SeaTac.

Page number references are to the FAA's "Airspace Designations and Reporting Points" document, current edition Order JO 7400.11D.

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  • $\begingroup$ @StephenS - see edited answer-- $\endgroup$ – quiet flyer Mar 17 at 18:47
  • $\begingroup$ If any other cases like LAX exist, it would be useful to add them to this answer. $\endgroup$ – quiet flyer Mar 17 at 18:57
  • $\begingroup$ The second-to-last paragraph answer of this answer could serve as the foundation for both a question, and an answer, specific to LAX. $\endgroup$ – quiet flyer Mar 17 at 21:34
  • $\begingroup$ A related ASE question could ask WHY the Class D airspace at LAX has been designated in a different manner than the Class D airspace at SeaTac. It may, or may not, have something to do with how far the D areas project from the inner Class B surface area; at LAX the distance appears to be very short. $\endgroup$ – quiet flyer Mar 18 at 12:26

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