PilotEdge Forums

Posted: **Sun May 24, 2015 8:02 am**

Here's a question about heading/course when flying between VORs in ForeFlight, hopefully it's a simple answer

When planning a route between 2 VOR stations how do you find the radial to fly FROM the first station?

Assume that the two VORs are far enough apart that you need to fly FROM the first one, then switch halfway to flying TO the second one. Flying direct TO a VOR is easy, of course, but I can't seem to figure out the radial to fly for the first part of the leg.

ForeFlight gives you the magnetic course from waypoint to waypoint, but my understanding is that this course won't be the radial to fly, since VOR radials aren't corrected for magnetic variation? Is this accurate? Radials that define Victor airways are published on the charts, but when flying between 2 random stations, I'm not exactly sure what to do.

Of course, I could just fly the heading that ForeFlight gives me, but I'm trying to fly using standard instruments, and just use FF for planning and situational awareness.

Any guidance would be appreciated!!

Thanks.

Posted: **Sun May 24, 2015 8:54 am**

Hello Evan,

This is a very good question. To start I'm going to quote a previous post I made on a similar topic, this post will give the background information before I can answer your direct question:

Welcome to the world of "declination", which is sort of like "variation" and "deviation" that most pilots are familiar with.

As you most likely know true north and magnetic north aren't in the same place, for various technical reasons aviation uses magnetic north for most navigation purposes.

What you may or may not know is that the magnetic north pole moves at a slow rate, this means that over time the course between 2 points will change as the magnetic north pole changes.

The government remaps the lines of variation every 5 years or so and updates IFR/VFR charts. However, they do NOT reprogram the magnetic north of VORs (way too much time and money to do so). Additionally, your GPS uses current (in some cases more current then the government data) magnetic variation data to determine courses between points.

So this means the VOR and your GPS are using different "magnetic north pole" points, which causes these small course errors. The "assumed variation" of a VOR is called "declination".

Using your specific example:

The true course from OCN to KELPS (first point on V23) is 316. The declination of the OCN VOR is 15E (determined in 1965...), making for a mag course of 301, hence the use of the 301 radial from V23.

Now let's look at the modern day GPS. The actual current variation near the OCN VOR is 12E (determined from the section chart which uses 2010 data). If we take our true course of 316 (which doesn't change) and take out 12, we get 304, or what your GPS is using.

So that's what's going on.

So your question boils down to "how do I determine the difference between the actual magnetic course and the VOR declination applied magnetic course?"

This is a 3 step process:

1. Determine the magnetic course between the 2 VORs and the magnetic variation between them.
2. Look up the VOR declination for the outbound VOR. Do this by looking up the VOR in the A/FD (or on Airnav).
3. Determine the difference between the answers you get in part 1 and part 2 and apply it to the magnetic course to get the VOR radial. For example if the variation is 10E and the declination is 15E, the difference is 5E. If your magnetic course is 100 you would set the VOR radial to 095.

I hope that answers your question. Note that in most cases the difference between the 2 is very small and is negligible.

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