On 6/19/2026 at 10:26 AM, Litjan said:

The plane I fly (747-400) is getting old, and the IRS systems are not getting any updates anymore.

That’s genuinely surprising! 😮

IRL the mag HDG/TRK displayed on modern airliner ND/FMS is based on true HDG/TRK subtract magvar mostly based on interpretation of nearby NAVAID/Airport data, so normally 28day cycle update will take care of them.

Only when FMS fail or in place with no nearby navdata (eg. cross ocean) it will fall back to IRS stored data, which might be decades old.

1~2 degree difference to chart might also due to chart not update frequent enough, for example in ZSSS the runway heading shown on ND is 003 for almost a decade but only about 1~2 year ago the eAIP chart updated from 002 to 003. But magvar in China in general is few degree and the change rate is only a few minutes per year anyway...

It also might be due to great circle interpretation (on FMS) vs rhumb-line (on some charts), if the distance is long enough. In my company <=3 degree difference is consider acceptable.

What matter the most for IFR flight is actually the calibration(incidence) of VOR on the ground, any VOR is of course geography fixed, the magvar change doesn't effect it's beam, but to make the course align to mag bearing it'll need to be align manually, in XP11/12 it's the same data of FMS NavDataBase so it's also frequently updated (28 days if you subscribe for that). but in other platform like P3D you hardly update the "base" navaids (part of scenery), that will cause problem, I have once flown 737-200 in fs9 in the US and when switching from one VOR beam to another as arrival chart, I lost my position totally....

RNAV/FMS is totally coordination and true TRK based internally, only display mag when needed, so not much effect on slightly misaligned magvar anyway.

6 hours ago, C2615 said:

IRL the mag HDG/TRK displayed on modern airliner ND/FMS is based on true HDG/TRK subtract magvar

Interesting, so the only cockpit instrument that actually makes use of Earth magnetic field, is the stand-by compass?

8 hours ago, C2615 said:

What matter the most for IFR flight is actually the calibration(incidence) of VOR on the ground, any VOR is of course geography fixed, the magvar change doesn't effect it's beam, but to make the course align to mag bearing it'll need to be align manually,

8 hours ago, C2615 said:

What matter the most for IFR flight is actually the calibration(incidence) of VOR on the ground, any VOR is of course geography fixed, the magvar change doesn't effect it's beam, but to make the course align to mag bearing it'll need to be align manually,

Most VORs in the southwestern United States are based on magnetic variation from 1965, and are now 5 to 6 degrees off from the current location of the magnetic North Pole.

An example is the Albuquerque VOR (ABQ) which is calibrated to the 1965 variation of 13 degrees East. The actual variation at ABQ in 2026 is 8 degrees East. In other words, the 000 degree (north) radial of ABQ points at where the magnetic North Pole was in 1965, not where the pole is now.

This really doesn’t matter. What many pilots (even highly experienced ones) do not understand is that a published VOR radial does not (necessarily) represent the magnetic heading of that radial. It represents the course that must be set into the OBS or course selector of the nav indicator in order to track that radial with a centered CDI needle.

Going back to ABQ: airway V190 departs the VOR to the northeast with the radial marked as 052 degrees. If you set 052 degrees into the OBS and fly the radial outbound with a centered needle, your track over the ground will precisely follow the track published on IFR or VFR charts - but your heading indicator will read 048 degrees.

As a practical matter, is there is any amount of crosswind, the pilot will be using a left or right wind correction angle, so the heading indicator would not have read 052 degrees even back in 1965 when the radial really did correspond to a magnetic heading of 052 degrees.

The FAA typically does not change the orientation of the 000 degree radial of a VOR (once set), because doing so is not a trivial exercise, and because the variation of all other VORs to which airways connect would also have to be changed. The previously mentioned airway V190 connects to the Fort Union VOR (FTI) which is also based on 1965 variation of 13 degrees East. The outbound radial number is 233 degrees, but the actual magnetic heading of that radial in 2026 would be 228 degrees. The inbound course (with a centered CDI) if following V190 from ABQ would be 53 degrees on the OBS/course selector.

12 hours ago, JRBarrett said:

Most VORs in the southwestern United States are based on magnetic variation from 1965, and are now 5 to 6 degrees off from the current location of the magnetic North Pole.

An example is the Albuquerque VOR (ABQ) which is calibrated to the 1965 variation of 13 degrees East. The actual variation at ABQ in 2026 is 8 degrees East. In other words, the 000 degree (north) radial of ABQ points at where the magnetic North Pole was in 1965, not where the pole is now.

True, but in XP's navdatabase system (sort equivalent to ARINC429 database) there is only magvar data for given VOR, same with AFCAD/ADE file for P3D afaik.

I'm not sure how FMS will use that data IRL (if you fly on 052 course near that VOR in a modern airliner, what the TRK will read, vs the whiskey compass). And how XP will use these data (I'll give a test once back home)

In CAE Level-D simulator's database, magvar and incidence are separate line of input for a given VOR.

5 hours ago, C2615 said:

True, but in XP's navdatabase system (sort equivalent to ARINC429 database) there is only magvar data for given VOR, same with AFCAD/ADE file for P3D afaik.

I'm not sure how FMS will use that data IRL (if you fly on 052 course near that VOR in a modern airliner, what the TRK will read, vs the whiskey compass). And how XP will use these data (I'll give a test once back home)

Here's what the documentation says about that field:

"Slaved variation for VOR, i.e. direction of the 0 radial measured in true degrees"

So if the navdata is correct, X-Plane should correctly model the orientation of the zero radial.

OK, after reading FTOB again and get some test on 737MAX simulator, I think I got something wrong and mixed.

At least for modern Boeing, FMS does have 3 different magvar input, but the sequence is not what I said.

1st is IRU var, it's use for current HDG/TRK, including the one shown on ND and the active legs on LEGS page

2nd is FMC var, it's used for most of the LEGS page.

3rd is NavDB var, it only uses for legs after a NAVAID.

So the 3 var doesn't override each other, but applied on different things.

As my test, when I setup ABQ V190 FTI in CDU, the LEGS page shows ABQ 052> DULKE 058>CAYAL 058>RENCO 058>FTI , but on ND is shows a straight line. (btw it's also how it's shown on Skyvector's navlog)

After crossing ABQ, the legs page changes to 058>DULKE...., the TRK on ND keeps at 058, while to keep the VOR needle in center I still need to dial 052 on COURSE, and the tail of RMI pointer stays at 052.

Edited Tuesday at 07:07 AM4 days by C2615

Excellent posts in this thread!

I've learned a good deal of details, or remembered some I had already forgotten about...

8 hours ago, C2615 said:

True, but in XP's navdatabase system (sort equivalent to ARINC429 database) there is only magvar data for given VOR, same with AFCAD/ADE file for P3D afaik.

I'm not sure how FMS will use that data IRL (if you fly on 052 course near that VOR in a modern airliner, what the TRK will read, vs the whiskey compass). And how XP will use these data (I'll give a test once back home)

In CAE Level-D simulator's database, magvar and incidence are separate line of input for a given VOR.

In both X-Plane and MSFS the VOR information in the database is used to actually place the Navaids into the scenery. The database includes the VOR LAT/LON, frequency, elevation, class of service and variation. The data comes from official sources. VOR variation describes the orientation of the VOR’s 000 degree radial in relation to true north. Clockwise from true north if the variation is East, and counterclockwise if the variation is West.

In my example, both ABQ and FTI have their 000 degree radial set to 13 degrees clockwise from true north (East variation) based on 1965 variation data, which has never been changed in all the intervening years, even though the actual magnetic variation in that area is now closer to 8 degrees.

The procedure I described in my previous post applies if one was flying along the airway using an actual Nav receiver: which requires tuning the VOR’s frequency, setting the published radial course in the OBS, and tracking the radial outbound with a centered CDI needle.

It works differently when FMS navigation is being used. The VOR receiver is not used. The FMS sensed aircraft position in modern aircraft is based on GPS. In this case, VORs in a flight plan route are treated like any other GPS waypoint. If you enter a flight plan that includes a leg “ABQ DIRECT FTI”, the FMS will first calculate the true track between the two VORs using their published LAT/LON, and then apply current variation to give a magnetic heading. It is not necessary that the VORs are actually operational, or even for the aircraft to be equipped with a VHF nav receiver when FMS/GPS nav is being used.

In the case of ABQ direct FTI the true track between them is 65 degrees, and the magnetic course (based on current variation in that area of 8 degrees East) would be 57 degrees.

If an aircraft flew that leg using FMS in a zero wind situation, the aircraft heading (which is the direction the nose points) would also be 57 degrees. The heading would be greater or lesser than 57 degrees if there is any crosswind component, but the magnetic track over the ground would always remain 57 degrees.

If you were flying V190 using raw data from the VOR, with the CDI needle centered and the OBS set to 52 degrees, the aircraft heading would be 57 degrees.

From a technical standpoint, a VOR’s published radial does not (necessarily) indicate the current magnetic course outbound. The radial number actually describes how many degrees that particular radial is rotated clockwise from that specific VOR’s 000 degrees radial. As mentioned, once the VOR 000 degree radial is aligned to current magnetic north (on the date it is first commissioned), it is never changed or updated again, even though local magnetic variation may change significantly in intervening years.

It is becoming a moot point in any case, as more and more VORs are being decommissioned, and those that remain are being relegated to a secondary “backup” role in aircraft navigation in favor of GPS