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FAR 91.181a states that aircraft on ATS routes shall be flown along the route centerline. I would like a clarification regarding VOR airways. I have a GPS and a VOR receiver, and usually when I fly VOR airways I set both to navigate the leg I'm on. This gives me two navigation needles and two CDI sources. Usually they both line up very well, especially when close to the VORs themselves. A few times when I've gotten far from the station I've noticed a discrepancy between the the VOR nav and the GPS. I assume this is due to inherent error with the VOR as the aircraft gets farther from the station. When this occurs, which navigation source should I use?

I understand VOR airways have 8nm width to account for VOR error, but again, according to FAR 91.181, pilots must fly along the centerline (as best they can anyway). So will the GPS get me closer to the centerline, or the VOR receiver?

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    $\begingroup$ Is your GPS certified for IFR navigation? And, are you operating under IFR? $\endgroup$ – Ralph J Feb 1 at 3:06
  • $\begingroup$ @RalphJ: look at the tag $\endgroup$ – Peter Duniho Feb 1 at 4:06
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    $\begingroup$ I don't rely on tags to be part of the question, @PeterDuniho, both should be put in the question explicitly. $\endgroup$ – GdD Feb 1 at 8:52
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will the GPS get me closer to the centerline, or the VOR receiver?

GPS is usually more precise than VOR. Even when selective availability was used this was true, and that program hasn't been in effect since 2000.

The GPS will also have approximately the same error along the entire airway. The VOR's error will increase with distance from the VOR station.

That said, IMHO there's a difference between "which is more precise" and "which one should I use". Airspace features such as airways increase the chances of a collision by concentrating air traffic. An airway isn't as bad as an intersection or airport, of course, and flying appropriate altitudes for your direction of flight mitigates this risk even more. But I would tend to want to avoid being right on the centerline.

Many GPS receivers (and other RNAV devices) allow you to offset your course, so you can fly the airway that way, with a randomly selected offset that remains within the airway†. Or just use the VOR, and let its natural lack of precision help you to avoid being smack where all the other air traffic that's flying exactly on the centerline is.

† Strictly speaking, one should request the deviation from ATC, to avoid running afoul of FAR 91.181, which requires flying the centerline…it is unlikely they would object, but if they do, then you will find out the safe way

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  • $\begingroup$ Interesting. Isn't flying offset from the centerline directly against FAR 91.181? Also, isn't the concept of airways all about making it easier for ATC to provide separation (as answered by this question aviation.stackexchange.com/questions/56614/…)? $\endgroup$ – asb1230 Feb 1 at 1:46
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    $\begingroup$ @asb1230: 91.181: I suppose technically, you should request the deviation from centerline from your controller, but I doubt in practice this is something anyone cares about. As long as you are within the 8 miles protected width of the airway, I would be surprised if any controller would complain, never mind would the FAA file an enforcement action. As far as what airways are "all about", clearly they are not all about making ATC's job easier. That's part of it of course, but VOR airways would be useful even if ATC didn't exist. $\endgroup$ – Peter Duniho Feb 1 at 2:03
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  • $\begingroup$ Great references! I am understanding that despite the "keep them together to keep them apart" idea of airways, GPS navigation can backfire and "keep them too together" so to speak. $\endgroup$ – asb1230 Feb 2 at 2:36

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