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I was looking at ifr low enroute chart and just figured out that KSEA and KLAX are depicted as class D. Aren't they Class B airports? why do they have D mark?

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Your question is interesting but the answer is not fully obvious without a bit of research. In looking at the KSEA IFR Enroute chart (L01 IFR enroute chart -third image below) it does indeed show that a portion of the the Seattle International Airport is within Class D airspace.SEATAC Terminal Area Chart The second image below shows an excerpt from FAA Order 7400.11E describing the Class D airspace as it relates to KSEA.enter image description here The third image is from the L01 IFR Enroute Chart for Seattle. enter image description here

The fourth image below shows part of the legend for an IFR Enroute Low Altitude chart.enter image description here

So, although some general information relating to the OP's question may be addressed by looking at the link shown above by @randomhead as it applies to LAX, the OP is asking a reasonable question which is fundamentally different (from the question addressed in the link mentioned above) and probably should be left open.

Further, it is a fairly unusual circumstance to have Class D areas attached to the primary airport upon which the Class B airspace is predicated, so a discussion relating to identifying the affected areas that are Class D versus Class B may be very informative.

Likely, the Seattle Class D airspace (dimensions shown above), is to handle go-arounds, (that turn and enter a downwind to return to the airport as opposed to returning to the TRACON to be vectored out for another full [longer] approach), helicopter traffic, traffic watch aircraft, etc. (being on or staying on the Control Tower's frequency, for example) .

If you do the research for KLAX, it's the same principle (but different dimensions for the Class D).

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  • $\begingroup$ By the way, the question referenced by the second link in randomhead's comment, which is also the first link in my comment, does address the Class D airspace at SeaTac specifically. So it appears the question is a duplicate, except for the fact that it is aimed specifically at the low altitude IFR enroute chart rather than the VFR sectional chart.Some of the information in this answer duplicates the information given in the answer to that question (and to other questions-- some answers to the questions pertaining to LAX also addressed the situation at SeaTac.) $\endgroup$ Apr 28 at 15:25
  • $\begingroup$ Informative answer all the same. $\endgroup$ Apr 28 at 15:29
  • $\begingroup$ @quietflyer - I don't disagree with your comment. But, the specific question asked (regarding the Low Alt IFR Enroute Chart) is not addressed by the info in the links above. So, I think in totality an answer that addresses the specific question asked will ( to be fully understandable and in reasonable context) overlap some info/answers in previous questions. Since ASE is meant to help people understand various aviation related issues, excluding good questions because they may be similar to other questions is denying the opportunity for a broader understanding of complex issues. $\endgroup$
    – 757toga
    Apr 28 at 15:43
  • $\begingroup$ OK, agreed, makes sense. $\endgroup$ Apr 28 at 15:46

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