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4 Answers

True course compared to VOR radials

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General Aviation

I always wondered this one. A good example of my question is the BVR VOR east of Denver Colorado. Why is it that if I plot a true course on my TAC and apply the 8.5 degrees E magnetic variation, it does not exactly parallel the VOR 360 radial. I picked BVR because it is very close to the 8.5 E isogonic line on the chart. Another way stated, why isnt the VOR arrow on the compass rose on the TAC 10degrees off from the nearest lat/long lines instead of 8d30m (8.5 degrees) as showed for magnetic variation.

4 Answers



  1. Nathan Parker on Apr 18, 2011

    According to the AFD, that VOR has 10E declination built into the signal.  VORs are only adjusted every few years to match the actual variation in the area, so there may be discrepancies.
     
    BYERS N39°45.95 W103°55.68 NOTAM FILE DEN. DENVER(H) VORW/DME 113.5 BVR Chan 82 269° 34.6 NM to Denver Intl. 5252/10E. H–5A, L–10F

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  2. Richard Hages on Apr 18, 2011

    Nathan is right. What happens is that when the VOR is commisioned its north is set to the current magnetic north. And we know that magnetic north changes. The entry on the bottom of Nathans answer is from the A/FD. For every navaid it will indicate the amount of variation for that specific navaid, in this instance 10 degrees east. So over time as magnetic north shifts, the navaid remains the same. This leaves you with true north, magnetic north, and each VOR will have it’s own north. This is a really common mistake for new pilots to make when plotting a VOR course. Don’t tune in your true or magnetic course into the OBS, make sure that you either use the compass rose off of the map for the VOR or calculate the course based on the variation in the A/FD. Hope that helps
    Check out my aviation blog The Art of Flying. Thanks!

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  3. Matthew Beyer on Apr 18, 2011

    These are great answers and it makes sense. I looked at airnav.com and it made a reference to 1995 with 10E. It took me a few to realize that the navaids are mixed in with the airports in the AFD, I see it now for this one. Thanks again for the great answers.

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  4. Best Answer


    John D. Collins on Apr 19, 2011

    I agree with most of what has been said on this subject and offer the following comments.  The original variation when the VOR was installed or readjusted is called the declination.  When airways are determined along with any approaches, departure procedures, or waypoints (intersections) are determined, the associated radials are based on the declination.  Unless the VOR has the declination updated, the radials will be at the same bearing with respect to the original assigned values.  That means that the radials will continue to define the same path over the ground or spot above the surface of the earth, even though the magnetic variation may have changed.  The surveyed airways or waypoints will still require the OBS to be set to the original values, even if the actual magnetic bearing has changed due to the variation change.  If you select a random bearing that isn’t associated with an airway or waypoint, you need to remember to plot the bearing on a chart using the original declination or your position is not likely to match the calculated location.

     

    Nathan suggests that the declination is update every few years.  My experience is that it may be a decade or more, as changing the VOR declination is a big deal.  Every airway, approach, departure or arrival procedure, and intersection in which the VOR plays a role needs to be changed, the charts need to be updated, databases need to be updated, etc.  All these changes have to be flight tested and verified.  I have seen a single VOR affect dozens or even as many as 100 changes.  The point I am making is that updating the declination is not a trivial matter.

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