iFly GPS Forum

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ComputerDoc wrote:

"...If they ARE using 5 nautical miles as their radius, the results my experiment gets makes sense.... as the sectional is most likely using 5 statute miles as the radius.
Maybe I'll phone them and ask. ..."


Vaughn,
You wrote, in part --
"That's what I asked a few days ago! Somebody's confusion between statute and nautical miles is still the only explanation that makes sense to me. The numbers match perfectly."

Vaughn, My numbers too match perfectly with the theory (now confirmed) that the tower is using a 5nm radius.

Apologies for posting this text in more than one reply, but it appears that the last two posters at least didn't see this from me yesterday:
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I now believe that 99% of this disparity can be explained as follows:
Apparently the iFly Sectional diagram shows the radius of the MFR airspace as 5 statute miles.
As far as I know both the iFly and the sectional (from which iFly gets the data) are officially correct in doing that.
I just called the MFR tower and spoke to a controller there and he said they use 5 NAUTICAL MILES (which is 5.75 statute miles) as their radius.
That would perfectly explain why they are seeing me as about 3/4th of a mile inside their airspace when my moving map iFly is showing I am just at their perimeter.

If this resolves it (and I think it does) the REMAINING questions are
1) should they be using 5nm radius (I am not sure I want to go down that path of discussion with them)?
2) are other control towers doing this?

Thanks to Jim's and Walter's comments that led me realize that possible explaination, and thus to pick up the phone and ask them if they used 5nm for their radius.

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Anyone game to phone THEIR local tower and ask if they use statute or nautical miles as the radius of their airspace?

Alex

Here is the FAA's official goo on how one is supposed to define a Class D airspace, including the rules and formulas for determining it's radius:
https://www.faa.gov/air_traffic/publications/atpubs/AIR/air1702.html#3doI71270Chri

Scanning through this doc...Class D can be all sort of sizes, and there is no mention of "5 miles radius" (statute or nautical) being a default. I suspect many airports just work out to be about 5 miles, so that became a rule of thumb. I further suspect that the controller in the original post simply doesn't realize that the rule of thumb is statue, not nautical. He should probably just be gently corrected...and I would simply ask him to measure his Class D on a TAC chart as evidence of his error.

-Walter

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Walter Boyd
President, Adventure Pilot