Class D airspace lateral limit/radius
Asked by: Skyfox 16505 views Airspace, General Aviation
As I've been revising my aviation study documents, one thing I came across that I didn't know the source of is the "typical" radius of class D airspace around an airport being 4.4 NM/5 SM. It's something that's been in my head since I started getting into aviation over 20 years ago. However, the only written source I can find with that number for a radius is my old private pilot ground school notebook, so it's something the instructor said when talking about class D. Since that number is not stated in the text book for that class I don't know where the instructor got that number. The only other print sources I could find that listed any sort of typical/normal lateral limits of class D are my Gleim IFR written exam test prep book (1996 edition) and the course book for the King instrument rating video series (also from around 1996), and they both specify 4 NM, not 4.4. On the videotape they say the same thing but note that the only reason we're being told it's 4 NM is because that's what's asked on the test (which explains why it's in both of those books). I'm figuring that perhaps the 4 NM mentioned by those two sources and why that's the answer on the test is related to the 4 NM radius for the speed limit in 91.117 b.
The only source I can find that supports the 4.4 NM typical radius around a class D airport is from CFInotebook.net which states, "Class D areas are tailored to the area, but 5 SM (4.4 NM) standard radius surrounding those airports that have an operational control tower." However, I can't find what source the author of that page would have gotten that number from.
Every other source I can find lists nothing for a normal/typical class D airspace radius, and at most says each one is specifically designed for the local requirements. And that's understandable, since a plotter on a chart shows a range of different sizes and FAA order 7400.11A. Some airports do measure 4.4 NM in their class D airspace radius but that doesn't seem to be the majority.
So my question is, is there any source in FAA publications or text books that specifically states a 4.4 NM radius as being somewhat typical for class D airspace? (Not including any class D or E extensions for approaches, of course.) Or is this something I should take out of my study documents as too irrelevant to bother with?
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