Difference in radial degree, SkyVector and Navigraph


I noticed that if I draw a line between two points on the map in SkyVector and do the same in Navigraph, the radial is different. How come? It is even off by 5 degrees.

The line from MMM vortac to MLF vortac in the USA. Navigraph tells me it is 21 degrees, but Skyvector is 16 degrees. In the MSFS, when I actually point my Nav radio to it, actually the one of Skyvector seems to be correct.

How come?

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In Navigraph, it is giving the actual magnetic heading between the two VORs. Skyvector is giving the VOR radials. They are not the same thing.

It is important to understand that the heading shown for a Victor or Jet airway on a low or high altitude chart is not (necessarily) the actual compass couse you fly between the two VORs.

The VOR radial is what you set into the course selector (OBS) of your nav indicator in order to fly the depicted radial path over the ground with a centered CDI needle.

VOR radials are referenced in degrees clockwise from the “zero” degree radial of that specific VOR

The VOR’s zero degree radial is aligned to magnetic north at the time the VOR is first put into service. Once that is done, the alignment of the zero degree “north” radial is almost never changed again. However, as time goes by, the actual location of the magnetic North Pole changes. In the western US, the change is quite significant from year-to-year.

Both MMM and MLF VORs had their magnetic north zero degree radial set in the year 1965, at which time the magnetic variation was 16 degrees east. Now, in the year 2023, the actual variation at both VORs is 11 degrees east. In other words, the magnetic North Pole has drifted 5 degrees to the west of where it was in 1965 - which was 58 years ago.

As it happens, the majority of VORs in the western US are based on the magnetic variation in the year 1965.

Neither a VOR itself nor a Nav receiver “knows” (or cares) where the actual magnetic North Pole is actually located at a given time. If departing from MMM and flying to MLF on either the low altitude airway V21, or the high altitude airway J9-107, as long as you set your Nav receiver course selector to 16 degrees, and keep the CDI needle centered, the aircraft will track the ground path of those airways perfectly. However your actual magnetic compass heading will be 21 degrees (assuming there is no wind).

In the year 1965, the actual magnetic compass heading would indeed have been 16 degrees, but not in the year 2023, because of the westward drift of the magnetic North Pole.

Thanks for the long, but very clear answer! That makes sense and I now understand why. :slight_smile:

Is there a way to set Navigraph to give the actual VOR to VOR heading?
I mean when flying actually VOR to VOR the old school way, you would draw a line on the map and read the outbound and inbound VOR headings.
Or doesn’t it makes sense to do it, since VOR’s will be obsolete in the future.

The “old school” way would be to use a plotter on a sectional which would give you the TRUE course between the two VORs. (Referenced to the nearest line of longitude on the chart). You would then add or subtract the CURRENT magnetic variation to get the magnetic heading. If you did this on a sectional for a line drawn between MMM and MLF you would get a heading of 21 degrees.

However, to track the outbound airway from MMM, you would set your Nav indicator to the published VOR radial of 16 degrees. As long as you “do whatever you have to do” keep the CDI needle centered, the aircraft’s ground track will match the depicted airway.

As a practical matter, your compass heading would only be 21 degrees if there was no wind. With any amount of crosswind, you would have to take a wind correction angle left or right to keep the needle centered, so your compass heading might be greater or lesser than 21 degrees. If you happened to have a strong left crosswind, your compass heading might (coincidentally) be 16 degrees.

There will always be a significant difference between the published radials between two VORs in the western US vs. the actual magnetic course between the same VORs - primarily because most western VORs are based on the variation that was current in the year 1965, and there has been a significant drift in location of the magnetic North Pole in the past 5 decades.

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