MagDec File vs MagVar in ADE

I have replaced the default MagDec.bgl file in Scenery/Base/scenery with a more up to date file. When modifying an airport with ADE I always look at the MagVar listed in the Airport Reference Point (ARP).

First question...Are MagDec and MagVar interchangeable terms for the same data?

Second question...If I update the MagDec file with the most current one available, do I even need to worry about the ARP MagVar value?

Third question...I took a look at the stock KORD airport from FSX. The Mag Var value in the ARP was 0.00 degrees. I then took a look at the FAA airport diagram effective 11 Oct 18 and it listed the MagVar for KORD as 3.6 West as of January 2015 with an annual rate of change of 0.1 degrees West, which puts the MagVar this year at 3.9 degrees West. The Jeppeson Charts I have through my Navigraph subscription list a MagVar for KORD of 3.0 West as of 11 Oct 18 and does not give the date of the reading or an annual rate of I am assuming Jeppeson updates the MagVar data on their chart each cycle. But their value is indicating an eastward annual rate of change if the FAA value in 2015 was correct. How "great" is the impact on navigation of a 3.9 vs a 3.0 difference in the MagVar?

Fourth question...Should I be worried about the ARP MagVar when I redo an airport with ADE. In the past, using FAA charts, I have always changed the MagVar using the as of date and annual rate of change, which brings up why I was looking specifically at KORD and it's 0.00 MagVar. KORD is the first airport I have redone that listed a 0.00 MagVar in the ARP. All others were either a positive or negative number. I always assumed, like with FSX longitude, that negative was west and positive was east. Is that correct?



Resource contributor

1. Yes, same thing.

2. AFAIK the ARP value is not used in FS, only the value in the MagDec.bgl file. The mag var at an airport can be easily checked by going to that airport, going into slew mode and pressing Ctrl-Spacebar to orient your plane to true north (Space Bar in FS9). The current heading value is the mag var. When I edit an airport's ARP Mag Var (i.e. Airport Properties, the same thing) to 1 from 6.50 (this is KINT in FSX) and compile the airport BGL, the actual variation at the airport does not change (which is 8 according to FSX).

Heck, ADE doesn't even use the airport properties value to load the airport from an AD4 file - the file I saved with a mag var of 1.0 still loads oriented as if it had a mag var of 6.5.

3, I believe that 3.0 is the current value used by the FAA (refer to so I assume that's where Jeppesen gets their value from. Perhaps the FAA only changes this when it changes enough to reach the next integer (i.e. 4)?

4. Again, I do not think that FS uses the mag var value in an airport file. At least I've never seen it.

Hope this helps,
Thanks for the reply Tom,

Yes sir, I use both Airnav data and charts as well as some Jeppeson charts when I redo an airport. I decided, as a glutton for punishment, to bring the stock KORD up to the July 29, 2018 update in the O'Hare improvement plan, including a large dirt polygon underlying where the new 9C/27C will be in 2020. Even Ray Smith's excellent KORD is not current as of July 29, 2018. I have the airport side with accurate runways, taxiways, and numbering done based on the True North background image I have.

Now I am putting in the approaches in ADE Approach Mode using, mostly, the AirNav Chartage for the current AIRAC. Here's the deal, though. I see the MagVar listed as 3.0 as of 2010, but on the airport diagram I see this as of 2015...

So, by 2018 "standards" the MagVar for KORD should be 3.9 degrees west. That should generate when your aircraft, using the method you describe, is pointing true north, the magnetic heading should be approximately 356 degrees...correct? Because that is exactly what I see in FSX when I do that with the ARP MagVar set to 3.9 degrees. If what you say about FSX not using the MagVar in the ARP I should be able to set the MagVar to anything I want and if the MagDec.bgl is current I should still be heading 356 degrees, but I don't. With a MagVar of 0.0 in the ARP and pointed true north the heading is 360 degrees in the aircraft, so MagVar in the ARP must do something in FSX.

Here's the reason I am asking all these silly questions. I am trying to do, as I mentioned above, the current AIRAC approaches to KORD in ADE Approach Mode. I have checked and double checked. My runway true headings as listed in AirNav are correct. The ARP MagVar is set to 3.9 degrees to get a true heading of 270 for runway 10L and a magnetic heading of 274 degrees. The problem is that the ILS also has a true heading of 270, but the arrow doesn't match the magnetic heading of 274. Using a guideline from the tip of the feather to the notch in the arrow all the approach waypoints are skewed slightly north of the guideline that shows a true heading of 270 and a magnetic heading of 274. None of the approaches "look" straight in to me (and some shouldn't because the approach chart indicates they aren't, but even these are skewed farther north than they should). I even made sure I had the "pointy" end of the ILS Arrow dead center on the real world location of the ILS transmitter.

So, although the difference is not that great that is what makes me wonder if a current MagVar is conflicting with the "most current I have" MagDec.bgl file. I tried to look at it to see what is even in that file, but I can't get BGL2XML to open it.

Thanks again for the reply Tom,

The runways are all positioned to there True headings according to the latest AirNav set of charts and those don't change with the movement of the magnetic North so I'm not going to stress on it too much. I was just curious. I just drew "True" heading guidelines from the runways and lined the waypoints up on that guideline unless it was an offset approach.

Thanks for your time.

Hi Randy,

You've just discovered the MSFS implementation of the "magnetic world"...
Updating MagDec.bgl will only affect your own aircraft's magnetic readouts.
As you've already found out, your ILS is now off. That's because all navaids have their own magvar settings, which are still according to stock data.

Go to the website of Herve Sors.
There are updates for navaids for both FS9 and FSX, but be sure to read what is updated and what isn't.

There's also a FAQ.

Personally, I've never bothered with updating magvar for my FS9. I can live with "wrong" magnetic headings, all my flying is FMC or GPS guided, flying fix to fix.
And I have lots of AI flying around, and AI aircraft will still use approach code based on stock magvar.

Thanks Dirk,

I've known about Aerosors for a long time, but the only thing I get from his site is the MagDec.bgl file. I install his entire Nav Update package to a bogus FSX location in my documents folder, but then just pull the MagDec out and throw the rest of the data away. I save the old one that was in FSX/Scenery/Base/Scenery and move the new one in there. Why not the entire package so all my data is AIRAC current?

It is a known issue that his update can and usually does "break" the default FSX Flight Planner function. I use it to plot my flightpath and intended waypoints for use in celestial navigation with the freeware Bubble Sextant available at AVSIM. Once I have all my points in the default flight planner I print the path out on graph paper and then use it to plot my star readings and make course adjustments to get to the next Star Reading waypoint.

The Celestial Navigation Charts are still available on the web, probably for Maritime use, not aviation, but the Air Force still trains their navigators in Celestial Navigation. Even the FAA still allows "Training Flights" for it and will allow you 20nm either side of your intended track before giving you a vector to get back on course. I used it during a Friday Night Ops on VATSIM once to go from Denver to Twin Falls, Idaho in a Lockheed L10A Electra and my before I could take my final Star Shot I saw the lights of the airport in front of me.

I have used it to cross the vast Canadian North, recreate Amelia Earharts fateful flight, recreate the flights of Ernest Gann from his book "Fate is the Hunter", and several times back and forth across the Atlantic. Without the default Flight Planner I would not be able to plot out a flight path on the graph paper that is critical for know how to get back to where you should be.

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