V186 discrepancy

[flyingdrill]

Note, I haven't tried this in my parallel beta installation, but this is reported in a stable 10.45 on an iMac!

I use updated Navigraph data. That's the 3 main default data files, and the 430 files. Like most of us in PE, I am regularly flying the TEC routes, and V186 is a common part of many routes. Leaving Van Nuys (for example - it doesn't really matter how one joins V186), we should eventually intercept the 095 radial from the VNY VOR. That should put you in the middle of the airway! Well, it doesn't. No matter how carefully I set the OBSs on NAV1 and NAV2, I end up flying parallel to the airway to the left of the correct path (i.e. North of the airway). I can check that I'm on the 095 degrees radial, as well, by the readout on the Garmin 530 (if the plane has one!). I see my track is on the N side of the airway on the Garmin display and on my iPad (I use FlyQ EFB - but I'm sure it's the same in Foreflight!).

I'm not sure whether it's a Navigraph problem, so I suppose I ought to try it with X-Plane's default data, but I'm curious to know whether anyone else has noticed this.

[Matthias Geiss]

There was a similar question not too long ago: viewtopic.php?f=26&t=6282&p=42048

Also keep in mind that victor airways are not thin lines, but actually 8nm wide corridors (even wider for a given segment when the two VORs are more that 102nm away from each other). So there's quite a bit of a margin for errors because of inaccuracies, changes in magnetic variation etc.

[Flagada002]

It is defined as the 095 radial from VNY up to TIFNI intersection. TIFNI intersection marks the change over point to the Paradise VOR, from there V186 is defined on the 278 radial from that VOR.

As such, V186 is NOT a straight line, and putting it in the garmin, you should always include TIFNI in the garmin, foreflight flyQ, or any other EFBs, otherwise the gps will draw a straight line between the VORs, which will be south of the actual airway.

[flyingdrill]

I remember the thread Matthias mentions, and also that the airway is quite wide. When I did my RW Instrument rating, the Victor airways weren't even depicted on the GPSs available, and we just flew on the VOR radials as specified! There has indeed been a lot of magnetic variation since most of these VORs were aligned, but one flies on the published radials, and that may well not reflect the true course! Flagada002's point is, of course, correct. I knew that, and I have all the intersections in the Garmins, and switch to PDZ at the appropriate point. I usually fly the GPS course, and have the 530 (if there is one) tuned to the relevant VOR, so I get the radial and DME from that. Then I tune NAV2 to the VOR, and I can then have the backup!

I think that I noticed the discrepancy more than usual, yesterday, and that could have been due to a variety of X-Plane factors. Also, I was intersecting V186, and I do intersecting jobs (for practice) using the VORs (VLOC) instead of just turning at the magenta line. The needle just didn't center till I was N of the airway, nor did the 530 show I was on that 095 radial till I was N of the airway. That was really my point. If the data is correct, I should be on the airway......

Note that FAR 91.181 states that one should fly "along the centerline of that airway"! - unless otherwise authorized by ATC.

[Flagada002]

Interesting. I did notice that the plane was a bit off from the GPS route generally (I VLOC the CDI as well being that GPS'ing it is just too much fun -__- ). but usually the error is not more than 2-3 nm so I don't care. The GPS would have different DTKs as well to account for the magnetic variation changing and overall be more precise no matter the distance to the VOR.

I'll fly it soon to double check xD

[Matthias Geiss]

Note that FAR 91.181 states that one should fly "along the centerline of that airway"! - unless otherwise authorized by ATC.

Of course you shouldn't exploit the given tolerances and wander back and forth through the width of the airway when your nav equipment allows for a reasonable accuracy.

On a side note: The Navigraph data does not update X-Planes magnetic variation table, but only the "true north" values for VORs. So it's very likely that a given radial of a VOR doesn't perfectly match the magnetic track which your plane's nav equipment is showing. Here is some more info on this topic:
http://forums.x-pilot.com/forums/topic/ ... variation/

[flyingdrill]

Matthias:

Thank you for that link, which was extremely interesting. Also, the link in that one to another discussion in the .org forums. So, Navigraph doesn't update X-Planes variation table! I never thought of that.
I have noticed all of this behavior before, but it was just yesterday that I found it even more obvious than usual, because I had a late instruction to intercept the airway, and I was coming in at a much greater angle than normal.......

Flagada002: You will see it, but (of course) it doesn't really affect our enjoyment or practice. Just me being a bit pedantic/OCD-ish

[flyingdrill]

Well, here we are in the Western Expansion, and I again discover that Navigraph's variation figures for VORs are often incorrect.

I was just flying the Seattle5 departure and Helns arrival to Portland, and my position on my EFB and looking at my back-up VOR radials showed discrepancies. Sure enough, again, the variations for the VORs were not as the FAA swung them in whatever year. Airnav and other sites assist, but the new X-Plane navigation data updates on their site give correct figures, and I tested them. Wonderful. Now the courses (Navigraph's 430 GPS data) and the VOR courses coincide, if I swap in X-Plane's new earth_nav.dat file each month. I (and others) have complained to Navigraph, to no effect.

BTW, if Keith reads this, I apologize for my brain f--- earlier, on calling up Seattle clearance. Also, I had (who knows why?) something, somehow corrupted in my Malibu installation. My trim was constantly winding itself to full trim up. A quick reinstall, and all was well. Took 3 attempts to get going on the flight


LATEST EDIT ON JAN 4:

Navigraph apologised for missing all this, and agreed earlier this morning they had made a big mistake in the variation field! VORs should now be correct in the 1701 release of the navdata!