Enroute L-4 VOR Victor Airway Question

[C-JBCS]

Hello, I have a question about about Victor airways and their radials off from a VOR... I was wondering if the radials for the Victor airways were shown as magnetic or true on the low enroute charts?

I'm looking at the Oceanside OCN VOR and the V23 airway on the L-4 chart. The VOR compass rose indicates the V23 airway is on the 301 radial outbound to KELPS...

I was tinkering around with Garmin's GNS simulator (installed via the RXP product). I was trying to program the TEC route from KCRQ to KSNA into the GNS simulator. I can't program the V23 airway so what I tried was to program OCN direct, then all of the intersections along the V23 airway to SLI...

What I found was that the desired track from OCN to KELPS was 304 degrees magnetic on the GPS and not 301 radial as shown on the L-4 chart... The 304 desired track continued on down the way to the next intersection after KELPS.

I know it's rather trivial but I was curious what reason might be for the difference...

As always, thanks for everyone's time here on PE.

Jay

[Peter Grey]

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.

A couple of questions people have on flying this.

Using a VOR what course do I dial in? 301 (the VOR Radial)
Using a GPS what course do I dial in? 304 (what the GPS says)
What heading do I fly (assuming no wind, or to base wind correction angles off of)? 304

Hopefully this answers your question.

[gavink42]

If you're up for a quick read, Jeppessen explains it in good detail:

http://www.jeppesen.com/download/aopa/may01aopa6.pdf

The short version is that GPS courses are computed differently from how the airway magnetic courses are. The greater the distance between the points (east to west), the greater the difference you will see in magnetic headings between Victor airways and GPS.

There is also a possibility of several degrees of airway course error, due to the constant change in local magnetic variation. Like runway numbers, the VORs aren't realigned until there is a several degrees of change (years).

So, if you're navigating by GPS, fly the GPS heading. If by VOR, fly the radial (assuming no wind).

Both will get you to exactly the same place on the map!


[Edit] What Peter said!

[Scott Medeiros]

VORs transmit radials on a magnetic course. We know that the earth's magnetic North Pole is constantly moving, so VORs need to be calibrated to compensate for this. According to airnav, OCN has a variation of 15E which was last calculated in 1965. Just as runway numbers sometimes change due to magnetic change, the magnetic course needs to be adjusted and new charts printed. You will still be flying on the correct airway because you are actually flying a 301 radial based off the VOR location and it's calibrated north (360 radial). So even though the numbers are different the location and course you are flying will be the same.

[C-JBCS]

Awesome! Thank you guys very much. I tell ya, I've learned oodles more in 1 month on PE than in years of flight siming off line... Really cool stuff!

Thanks again

Jay

[Ryan B]

I've got a question that's on topic...

If my equipment suffix is /G but I filed an airway, can I use the GPS to enter the points along the airway and follow the CDI or do I HAVE to use the VOR for navigation?

[C-JBCS]

That's a good question, I didn't think of that. This morning I flew the TEC from CRQ to SNA, but I flew the gps intersection to intersection as I mentioned above. I didn't encounter any real issue, but that being said, I don't know if we're able to do so legally... Would also like to hear about this one.

Cheers

[Pieces]

If my equipment suffix is /G but I filed an airway, can I use the GPS to enter the points along the airway and follow the CDI or do I HAVE to use the VOR for navigation?

You can navigate point to point with the GPS. As Peter explained above, you'll end up following the same course. It is good practice to have the correct VOR tuned in as a backup if the GPS fails for some reason.

[C-JBCS]

Great to know, thanks!