iFly GPS Forum

Hi Walter,

Since your post, and , as you may see from other posts I put in this thread, from two or more careful tests with ATC, and comparing what iFly shows as airspace boundary perimeter on both vector mode and sectional mode, it appears to me that it is virtually certain that the iFly display is showing me as outside airspace by nearly a mile. In the case specifically of KMFR I will go so far as to say "IS certain."
And that it's information is internally inconsistent with its graphic display (e.g., showing about 4.3 nm on digital readout to airport center when perimeter (which is by implication showing 5 nm.)
(Re the internal inconsistency, I could repeat the experiment an could send a screen capture showing aircraft visually distinctly outside perimeter while simultaneously iFly shrew distance to airport as well inside 5nm.)
And ATC stands by their distance reading mentioning (a) they calibrate their radar each day and (b) reminding that in event of issues or incident FAA or NTSB would use the radar distance .

It is of course true that it is prudent for all pilots to not cut margins to minimum (not fly within, say, half a mile of and airspace if intent is to be clear of it, and iFly does have the disclaimer of only intended for situational awareness...... but I do suggest this finding of an apparent systematic error on the "wrong side" (it would be far less concern if the display erred by showing pilot he/she was a mile INSIDE airspace while the were I fact one mike clear) needs a closer look.

If this design flaw is outside of iFly's turf (for example if the airspace boundaries that come from a detailed collection of lat/lon points provided by the FAA are off but close to a mile and causing the false "I see I'm safely outside the perimeter" error and doing do not just on iFly but other inflight displays that, IMO needs to be dog down into.

Seems to me that it is a matter of time and statistics and luck of the draw that some pilot will rely on the visual perimeter airspace moving map display (having been told and developed confidence that horizontal position is easily good to 100 feet horizontally) and get in minor, moderate, or serious trouble. Could even come back to rest on adventure pilot's desk?

So my question/request is, might you agree this warrants being treated as a potentially serious bug or design flaw, and dig into it further?

Cordially and curiously,

Alex

One potential cause for this is the lack of transponder, specifically its Mode 5/C (altitude encoder).

To the best of my knowledge, the FAA uses 2D primary radar. However, radar does not measure ground range to the target (i.e. the distance from the radar to the point above which the plane is flying), it measures slant range. Without knowing the plane's altitude, the software processing the radar inputs into a track to be displayed to the ATC might be making all kind of assumption as to the plane's actual altitude, thus moving its track to where it might think it is (making assumptions as to its altitude).

My suggestion is to check what iFly tells you vs what an independent GPS tells you, e.g. a Garmin 430 installed in a plane. You can also check what your location is on a map (iFly vs real world) when you're on the ground - iFly will display the same thing whether or not you're in a plane... My experience is that iFly is very accurate, but I have not tried it "head-to-head" vs a 430.

CD said, 'It would not, even if substantial, explain why iFly would have an _internal_ inconsistency in that distance to the airport on its digital readout would show 4.3 nm (ncidentally in full AGREEMENT with the 4.3 nm ATC is seeing me at) while when I am directly on the moving map perimeter of the airspace which should be marked on the screen as 5nm out from the airport."

If you're saying the circle drawn on the map representing the class D region is ~4.3 nm in radius, then that's a mapping issue independent of aircraft position measurement.

Is that what you're saying?