FSX-MS VOR versus GPS discrepancy

[JonP01 104]

Now that I am doing longer flights with greater distances between waypoints I am noticing an annoying discrepancy between the GPS route shown on any flight plan I create (which I assume is based on great circles) and navigating from VOR to VOR. Since I tend to fly older aircraft without flight management gadgetry, most of my navigation is using NBD and VOR.

So if there is more than, say, 50 miles or so between VORs, there is a significant variation between the route I would take to a given VOR on a given radial as opposed to the magenta line on the flight plan.

Normally I wouldn't be too worried about this, except with the ATC, it has a limited tolerance of route "deviation" and will re-vector me. Since I don't think this is due to any sort of actual bugs in FSX or magnetic deviation issues, I am assuming it has something to do with the GPS route being based on great circles (regardless of the distance between - or type of - waypoint) and the way the sim measures the radial distances between VORs.

Not being a pilot I tried to research all of this - to not very much avail - other than to find a video for an ATPL course that describes the calculations required to determine conversion angle between a great circle and rhumb line. But whilst this theoretically would enable me to make a course adjustment at any given point in time, you'd basically have to keep re-calculating these adjustments all the time. And it doesn't seem right to me that your average pilot navigating via VOR would be sitting there with a scientific calculator making re-calculations all the time in order save maybe at best a minute travelling time on a flight for pleasure. They would surely simply just dial in the radial on the VOR and stay straight on the line, making small adjustments along the way so as to stay on the line.

So unless I am missing something very obvious here, it seems like I have to work around this "problem", variously by:

1. Making re-calculations all the time and adjusting the course selection accordingly until the variance basically becomes nothing as I approach the VOR.

2. Just ignore ATC altogether and fly "VFR" so that I can then follow the VOR radial without ATC re-vectoring me.

3. Have small distances between wyapoints so the rhumb to great circle variation is small enough such that ATC don't care.

4. Cheat by opening up the GPS and making the course compensation based on that (but that is cheating and they did not have GPS in the era of aircraft I usually fly).

 

So I am wondering how other people get around this. Am especially interested in real world pilots and what they do. For example, is FSX even correct in that there should be a variance between the GPS route and the very same VOR route?

Thanks

[SAS443 1,723]

5 hours ago, JonP01 said:

Am especially interested in real world pilots and what they do. For example, is FSX even correct in that there should be a variance between the GPS route and the very same VOR route

Start by ask yourself: Is navigating by VORs a means of flying a rhumb line (loxodrome) or great circle (orthodrome)?

At a glance a VOR radial seem to be loxodromic, but it is in fact orthodromic. Radio waves in its basic form travel in straight lines (not accounting various bending errors, esp for NDB's). Dont believe it? Compare initial TT and final TT of a flown VOR radial of say 100 NM. They will not be the same (although the difference is not great either).

My empirical reply would be that it should not matter. Magenta FMS line is great circle and by keeping the OBI centered from/to a VOR, your flightpath should virtually be the same as the suggested FMS route.

Edited January 4, 2022 by SAS443

[JonP01 104]

Yes, it should be but it isn't. You can try a flight from Baltimore for example, to KORD using the VOR / IFR option for the flight plan in FSX. The track when keeping the OBI centred can be up to massive 6 nm different to the GPS magenta line (eventually becoming zero as it approaches the cone of confusion), so something is wrong somewhere (in the sim I mean). And I have tried this with three aircraft and they all behave the same way. In real life I agree - there should not be any differences.

[JonP01 104]

Doing some further research and it appears the issue is the magnetic variation assigned to the VOR itself in FSX is different to the magnetic variation defined in FSX at that same coordinate. I will have to do further research to see how fixable it is but it seems the problem was recognised even before FSX and just carried through to FSX.

I would not regard it as such a big issue if it did not cause issues when flying IFR (due to the problems it causes with ATC continually trying to vector me off the VOR radial). I'd be happy with either an ATC "fix" or a magnetic deviation "fix" given it is in the end just a game.

Edited January 4, 2022 by JonP01

[Bob Scott 7,721]

Are you using the updated magdec.bgl file from Herve Sors?  The FMS data from Navigraph/Aerosoft for most of the advanced add-ons is based on the updated r/w magnetic declination data reflected in that file, as are the databases in the various flavors of Garmins.

The ATC "fix" would be getting a better ATC program--Radar Contact v4 is a very good ATC solution, and was released as freeware years ago.  It is downloadable from the AVSIM library.  It uses FSUIPC to interface with the sim, so it works with every ESP-based sim from FS98 through P3Dv5.  It also allows you to set the crosstrack error threshold that ATC will allow without intervening.

A VOR station has a fixed mag deviation set in its calibration that is not necessarily the same as the current mag deviation at its location.  What is set is what's used to TERPS and flight check the radials forming approaches and airways, and changing that setting requires adjusting those radials and a new TERPS analysis and flight check, so the setting can and does vary somewhat from the actual mag declination over time (IIRC VORs are flight checked every two years or so).

The published course between VORs on airways, especially high airways that can have legs of up to 260nm (130nm max from each VOR) often changes at the designated changeover point because the two VORs have different mag declination settings.  That said, the crosstrack distance between a great circle direct track and a VOR-to-VOR airway, even at the max airway interval of 260nm, should not be enough to cause ATC to notice, at least not until you get to the high latitudes close to the magnetic poles where magnetic course changes more dramatically over short distances.

 

[JRBarrett 3,075]

6 minutes ago, Bob Scott said:

A VOR station has a fixed mag deviation set in its calibration that is not necessarily the same as the current mag deviation at its location.  What is set is what's used to TERPS and flight check the radials forming approaches and airways, and changing that setting requires adjusting those radials and a new TERPS analysis and flight check, so the setting can and does vary somewhat from the actual mag declination over time (IIRC VORs are flight checked every two years or

Exactly this. A VOR’s radials do not (necessarily) indicate their actual magnetic heading. They indicate the offset from that particular VOR’s zero degree radial, and that radial points to where magnetic north used to be (at the time the VOR was last calibrated). That may be quite different than where magnetic north is today. The radials do not represent the course to fly - they represent the OBS setting to use to track the radial over the ground with a centered needle.

Most of the VORS in the US state of New Mexico are based on variation from 1965. ABQ VOR for instance has a zero-degree radial that is offset almost 5 degrees from current magnetic north.

[JonP01 104]

Thanks for the responses all - very helpful indeed. A lot of effort put into them so thanks so much from this non-pilot. Yes, this all stems from my ignorance that a VOR station was simply set and forget even in the real world. I never even considered the concept of ongoing calibration until I started reading into it all.

I think I have probably seen one of the "worst" offenders in the stock FSX where if you fly from Baltimore to Chicago using the standard flight planner VOR option, there is one slightly longer leg (from memory 146 odd miles) where clearly the calibration of that station is way off as you would need to fly a good 9 miles west of the magenta to have the OBS centred when you start that leg. This is probably the reason I am only really getting annoyed by this now, since I am now officially retired and have much more time for serious, longer distance flying, also having built a reasonably up to date machine last year to enable me to buy more serious add-ons. Before then it was shorter flights less than an hour, lower altitudes and often VFR just with an ocassional IFR flight thrown in so I did not lose my basic proficiency (or whatever degree of proficiency the stock FSX conveys). And most of the time the distance between waypoints was very small which did not reveal the extent of the problem other than being "mysteriously" out of alignment with the GPS (but usually less than a mile).

As for the magnetic deviation file, I did look into it. From what I understand (I may be wrong), that update will "align" the magnetic deviations closer to what they are today in the real world but I am not sure it is a universal fix for calibration issues that cause a variance in the VOR tracking though it will probably help in some cases. That said, there doe snot seem any harm in using it as it does not break any existing functionality from what I can tell.

There is of course also a NAVAID update but from what I understand it "breaks" the stock flight planner and you have to disable all the airways otherwise it falls over. There was also a mention it effects the AI but I did not see any comment into what it does. But I think breaking the standard flight planner for me at least is probably just swapping one issue for another. But maybe down the track (when I plan to go to either P3D or FS2020) I will also move over to the more sophisticated options.

Finally, I would just be interested to see if anyone can setup a "stock" flight from KBWI to either KDTW or KORD using the VOR to VOR / IFR option. It should include that ~140 mile leg I mentioned previously. If you then strictly follow the OBI and course setting on that leg, the ATC should continually ask you to turn right as you are well and truly outside the "tolerance" zone. If it doesn't then I must be doing something wrong to make things even worse.