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Cessnaflyer

VOR-toVOR question about radial accurracy

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vor-vor.gifThis is a dying art I know but I am curious about the error I see when navigating from VOR to VOR. As you see in the picture, I'm tracking BZM in my primary VOR and GSO in my secondary on a 264 radial. Since I'm about half way, I had just switched from flying GSO in my primary until I was sure I'm getting a good signal from BZM. When I switched, as you can see, I got full left deflection because the 264 radial of GSO is obviously not the exact one that intersects with BZM. Though I know I'm going to lose precision lifting the vector from the chart and then again when I try to dial that in each of the VORs but why am I off a full 11 degrees?One of the advantages of VOR is they don't accumulate error over distance. If I'm slightly off the radial, by say 2 degrees, then I should still only be off that 2 degrees thirty miles outbound that radial. Likewise, if I'm off a couple of degrees on the inbound radial, same thing. Variance should not be an issue since, in this case, there's only a single degrees difference.So, why off 11 degrees? It seems the introduced errors should only account for four or five degrees tops.Thanks for any explanation.-mark

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Well like you mentioned you are going to get an error lifting it from the chart when there is no airway present. Also you have to account for magnetic variation because the VOR's are aligned with magnetic north. It is normal to have these differences. A route I am used to flying is V2 between ELN and MWH in central Washington. On those airways you will notice there is a difference in 4 degrees and it is only a total distance of 48 miles. If there was no charted airway and you were eyeballing the radials from the chart there is the ability to get it 10 degrees off, I've seen students do it many times.

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Thanks for your reply Chris.I understand what you're saying; especially the inaccuracy when determining the radial when there is no airway. However, I see just as much error when I do fly an airway. For example, Flying V155 (RDU-LVL), the published radial is 041 for the 71nm trip. I still get 10-12 degrees off or more by switch-over. Mag variance doesn't seem to be much of a factor since there's only one degree there (9W-8W). It's still a mystery to me that I'm always off that much.Of course, I can cheat (and do) as I can see myself veering off when the lead VOR wakes up which I have tuned in my secondary VOR. It's never a big deal to get lined up pretty well by the time I make the switch but I wish I understood better how I'm getting off that much so I don't have to cheat. Especially with an airway, you would think if I held that radial pretty well or corrected to it before I make the switch that the only error should be variance, instrument error, the ability to read the VOR, and precession. I do ALL I can to correct all of that before making the switch. Can all that really add up to 11 degrees over 62 miles (30 miles really)? Maybe so. Could the problem be the compass? I've often wondered about that. I don't always read the compass as good as the HI and even after correcting for precession, I'm not always sure my heading is as accurate as it should be.Nice to hear you say these differences are normal but I would like to improve where I can.Regards-mark

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What are the dates on your charts? If you are using older or newer charts then the flight sim database that could also be a cause to your problems.

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>What are the dates on your charts?Good point. Some are as old as ten years (variance has moved a lot since then I know). Some are fairly recent. I've been using skyVector.com a lot lately which are pretty well up-to-date I would assume. I'm sure that would introduce some error.Well, if we're down to this level then I suppose I'm not doing anything drastically wrong. :) I expected you to say "Oh, you need to ...". Since you haven't done that, I guess it's just a matter of practice and being extra careful about the plotting. I probably should start creating flight plans in FSX and be sure to use their vectors instead of plotting my own. I haven't been in much a habit of doing that. I just assumed the numbers would not be that far off. I'll certainly start paying much more attention to that.Thank you for taking the time to address my perhaps rather silly questions.-mark

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Nope nothing drastically wrong. Seems like you have a good handle on how the flight planning works.A little added bit of information. In the air the VOR receivers can have an error up to 6 degrees and on the ground +/- 4 degrees so you can see even with what you think being 'on course' you could be up to 6 degrees off legally and still be technically 'on course'. At a distance of 60 miles away from the VOR that can be an error up to 6 miles from your intended course (Each VOR radial at 60 miles is 1 mile in width). Oh and one thing I forgot to add. I believe the sky vector website scans in the charts. When the North and South sides of the map are lined up they may not have been done exactly and this sometimes introduces errors when you cross this seam.

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