Tinker wrote: <p><div class="Quote"><em>ComputerDoc wrote:</em>Call yesterday evening. One controller on duty but not busy. Happy to chat.<br /> &#160;</div>
Good job. You found exactly what I have found repeatedly. They are good guys, happy to help, and they don't bite. Be sure to report back here after you've done your test run.
Did the experiment with ATC today at MFR (class-D at Medford Oregon ...on Klamath Falls sectional).
Very helpful and cooperative manager.
I flew as precisely as I could on the exact circular boundary of the airspace shown on the moving map sectional.
Little wind and no turbulance so I believe at all times I was ...... per the iFly moving map screen within 1/4 mile or less of being exactly on boundary per the iFly screen. ATC checked in with me at about 7 point in my circular flight. Virtually all of the time I was flying that perimeter ATC was reporting they showed me almost exactly 1 mile within their airspace.
Now here is what is real interesting:
I had iFly set to route me to that airport (MFR) so at all times it was also displaying its estimate of my distance to the airport (presumably to the center point).
And that number in nautical miles WAS very consistant with what ATC was telling me.
That is, for example, when ATC said I was one mile inside iFly was reporting I was 4.2 miles from the airport.... essentially what ATC was saying.
In short, iFly's distance was correct (per ATC) but the nominal 5 mile radius circle on the moving map, per both the GPS distance readout and per ATC appears as if it has a smaller radius.
My theory/guess that what we would find is that the GPS and ATC disagree about where the CENTER of the airspace is (but agree it is a 5 mile radius) appears incorrect. If that were the case ATC would have seen me having a error on one side where iFly would show be further out of their space than they reported and on the OTHER side iFly would have shown me closer IN that ATC saw me. That didn't happen. On all sides I was closer in than the moving map showed me.
The controller made a point of politely reminding me that they have their radar calibrated for distance accuracy every day, and that in the event of any kind of incident FAA would use the radar distance as the actual distance.
None of this changes the pragmatic reality that it's prudent to cut myself and the controller and other traffic extra slack and stay a litle further out than I think needed if wanting to be clear of airspace. But I admit to being a bit surprised to find I need to stay at _least_ one mile outside the marked perimeter on the moving map to even just squeek by.
What was surprising is the apparent internal ~1 mile inconsistancy between where iFly shows visually the airspace perimeter is and what it itself reports as distance to airport.
So the iFly question that comes up for me to iFly staff or anyone who can answer .... a matter of curiosity and perhaps practicality... is this:
When iFly pops up a airspace warning is it using the apparently too deep in perimeter to trigger on OR on the distance to center of airport that it can compute from?
Alex
P.S., My earlier reported impression that ATC radar can detect altitude of an aircraft with no transponder is incorrect --- at least at MFR class-D . ATC explicitly told me they do not get any altitude info off me if I have no transponder.
All the more reason to report to them when flying over their airspace as they would show me as inside.