VORs & Victor airways in FAA sectionals

[Zonkor]

Here's a question that nobody had an answer for so far, including several experienced FAA instructors and an FAA examiner.

I noticed a discrepancy when comparing the following magnetic tracks of Victor airways in FAA sectional charts:

a. The magnetic track of Victor airways as written in the chart.
b. The magnetic track obtained by measuring the angle between the compass rose and true North with a protractor.

Often these values are not consistent and in some cases deviate notably, i.e., by several degrees! Also, in some cases two nearby VORs deviate considerably in their variation: Their compass roses are not aligned and inconsistent with local variation, despite being between the same two isogonic lines.

Does anybody have an explanation for this?

Overall, this is a pity, since you'd obtain different magnetic track when planning a flight by measuring with a protractor, or by taking the track obtained from the VOR rose or the indicated magnetic Victor track for granted.

Also, what's the FAA policy for updating the magnetic orientation of VORs and Victor airways (if possible with reference)?

[englishal]

My guess would be that Victor Airways were defined with reference to a VOR radial. Over time the magnetic variation changes so although the 270R from VOR might still be the 270R from VOR, it might actually be 266M these days, bearing in mind these V airways may have been around for decades.

Otherwise you'd have to either re-do all the V airways on the chart or "rotate" every VOR with a Variation change....

Purely a guess though!

[BEagle]

VORs are aligned using 'station declination', a magnetic variation value which applied at the date of commissioning and is often updated periodically, whereas magnetic variation changes continuously.

Hence a VOR's idea of 'magnetic' north will often differ from the actual magnetic north - until the discrepancy is sufficient large for the VOR to require re-alignment, at which time a NOTAM will be issued.

[Zonkor]

BEagle: So if two nearby VORs deviate by several degrees (I've seen 3-5 degrees difference at least), then their local variation differs by that much?

Not that I don't believe you, but I find this kind of surprising, since the bracketing isogonic lines should specify the regional variation to 1-degree accuracy. BEagle, do you have any background material or reference on this?

Furthermore, what's the best practice for doing flight planning along Victor airways?

1. Use the magnetic track specified for the airway on the sectional?
2. Measure the magnetic track of the airway with a protractor?

[Zonkor]

englishal: I'm with you, but then the magnetic tracks of the Victor airways specified on the sectionals would be (indeed are?) all inaccurate and worthless for precise flight planning, unless you actually use the measured magnetic tracks.

I'm wondering how many pilots are aware of this peculiarity ...

[MarkerInbound]

ATC is expecting you to fly the published radiails. Figuring a wind correction is almost useless, are you going to fly the heading you figured or the heading that keeps the needle centered. Granted it gives you a starting point when you turn outbound. But at 10 or 20 miles from the station the difference of a degrees is only a 600 or 1200 foot offset respectively. If you're watching the compass so much you don't have a one or two degree heading error you're not looking outside enough.

[MarkerInbound]

I'll see if I can track down a "Flight Check" pilot and see if he can get any input fcrom the folks in the back.

[Obi_Wan]

Thanks BEagle.

I'd assumed that the radio kit I fly with wasn't that reliable as flying the same VOR hasn't always got me to the same point.

There's been much talk about retiring the VORs so I also had an element of, "is their maintenance still being addressed if they're on their way out".

Old Ben

[darkroomsource]

BEagle: So if two nearby VORs deviate by several degrees (I've seen 3-5 degrees difference at least), then their local variation differs by that much?

Not quite, variation occurs constantly, but the VOR isn't "adjusted" except when it's out of alignment by a significant amount. So, two "nearby" VORs, one installed in 1950, is off in one direction by 2 degrees, the other VOR, installed in 1990 is off in the opposite direction by 2 degrees. Both are within acceptable differences, however between them the difference is 4 degrees. If they were both installed at the same time, they'd probably both be off by the same amount. Unless, of course, one was "adjusted" because it was .1 degree over the limit while the other was still just barely within the limit.


There is one other aspect to why there are differences between the "stated" angle and the "measured" angle, and that has to do with the surface of the earth being a sphere, and when you flatten it to make a map, the lines aren't straight, but when a map is drawn, the lines are straight. This can mean that when you measure an angle on a map, you get a slightly different value than if you were to put a compass on the ground and measure it.

[India Four Two]

There is one other aspect to why there are differences between the "stated" angle and the "measured" angle

darkroomsource,

I doubt that there is a significant error due to map projection. I'm not sure about UK maps, but the aeronautical VFR and IFR charts, that I am aware of, use the Lambert Conformal Conic projection. This has a useful property that straight lines on the map approximate great circles, unlike plotting charts and nautical charts, which use the Mercator projection. On Mercator charts, a straight line is a rhumb line.

[englishal]

Flying V airways in the old days, I'd just set up the radial on the CDI and keep the needle centered. These days I'll bung it into the G1000 as "V8" and follow the Magenta

[MarkerInbound]

Here you go.

What the writer is seeing is normal, and is not generally a problem.

There is magvar and then there is magvar. Magnetic variation at its most basic is simply the difference between true north and magnetic north. With facilities, it gets more complicated, because actual magnetic variation moves constantly as the magnetic poles move. Procedures cannot be updated constantly to reflect exact magnetic variation at any given time, so facilities are assigned a magnetic variation value, or reference north angle, based on a five year epoch magvar value for the area (Determined by NOAA, NGS and NOS.), and is not changed until it differs from the epoch year value by by several degrees (Used to be 3 degrees; might still be but I don't know what's current.). In some cases, facility reference north can vary from chart calculated magvar by several degrees because it's a big job to update magvar on facilites, especially some facilities that support dozens of procedures. It's a compromise between use of available resources and keeping facility magvars reasonably close to actual values. This does not present a problem in practice, since airplanes are protected when using VORs by selecting the published radial, and accurately navigating the presented course.

Here is a relevant excerpt from the 8260.19E:
http://www.faa.gov/documentLibrary/m...r/8260.19E.pdf

Start at Page 67 (Chapter 5, Section 2)

[Zonkor]

That explains a lot, MarkerInbound, thanks for the reference!

Surprisingly intricate (not to say dirty) business, this VOR / Victor airway navigation! But as you say, MarkerInbound, probably not too much of an issue for practical purposes. However, not all VOR navigation is done via Victor airways but arbitrary tracks to / from a VOR. Thus it would be good to outline the correct procedure to obtain the correct magnetic heading for VOR navigation.

So I guess the the appropriate method to obtain a magnetic heading from a Victor airway is to take its stated magnetic heading on the sectional [1]. In the case of an arbitrary course to / from the VOR, the most appropriate magnetic track seems to come from the intersection of the track line with the VOR rose (which is a somewhat inaccurate measurement due to the small geometric scale on the chart), as opposed to to measuring the true track with a protractor and apply variation to magnetic.

BTW, there's an interesting side effect to the misalignment of VORs: One airway connecting two unaligned VORs would have two magnetic headings (i.e., one for each direction) that are not exactly reciprocal as a consequence.

[englishal]

I wouldn't get too hung up on it. When tracking VOR to VOR or VOR to SOMEWHERE I don't think I have ever calculated a magnetic heading to steer. I simply look at the chart and see which radial I need. Going VOR to VOR when I hit VOR A I want to head outbound on Radial ABC, set the CDI and then changeover to VOR B and track inbound on Radial DEF (which is actually Radial ABC but TO the VOR). If I wanted to fix my position from two VORs then I'd use the RADIALS and the rose on the chart, centre the needles, draw my lines and that is it. If I want to go from VOR to SOMEWHERE I'll draw a line from the VOR and see which radial SOMEWHERE lies on, then head outbound on that radial doing whatever it takes to keep the needle centered.

As mentioned the Mag track of a victor airway is irrelevant in practical terms. You want to centre the needles on the desired radial and keep them centered. Ground tracks and hence charts are referenced True North so magnetic track is not relevant. On an airway you will have wind correction so you might be O/B on the 270R but actually have a heading of 255M.

[MarkerInbound]

You'll see that a bunch where the outbound course from one VOR does not line up with the inbound course to the next. Not sure if it's still in use but there used to be an airway coming up from Florida where the outbound radial would have you fly an even altitude and the inbound radial would require an odd altitude. There was a NOTAM for that airway saying fly north at even altitudes and south at odd altitudes.

Remember the service volume for most VORs down low is only 40 miles. I'd guess most Victor are less than 100 miles from VOR to VOR. The airway is the greater of 4 miles either side of the centerline or that airspace inside lines diverging 4.5 degrees either side of the centerline. The point where where the 4.5 degrees exceeds 4 miles occurs at 51 miles from the station. One degree of error will give you one mile of offset at 60 mile downrange so if you're 4 degrees off you're still on the airway. I'm not sure if that's where the plus or minus 4 degrees for the VOR check comes from.

Granted if you're trying to get to Uncle Bob's International Cow Pasture Airport on the ABC 090 at 90 miles, you might have to make a few wide circles to find it.

[Zonkor]

MarkerInbound, englishal:

I agree with both of you, but it appears we are talking about different things. My main point is:

Problem
If you are flying a VOR radial that you measured from an FAA sectional chart with a protractor (as many do and are taught ground school) then you'll be actually flying on a different track than you're thinking. This is due to the VOR variation discrepancy mentioned by MarkerInbound above, which seems to result in a deviation of several (three?) degrees, resulting in a small but unnecessary systematic navigation error.

Solution
The (only?) proper way to avoid this issue is to read the radial from the VOR rose. (Since this rose is rather small on the chart, it's not easy to get an accurate reading.)

I hope I made myself clearer now.

[stevelup]

Without using either a GPS or radio navigation, there's no way you could possibly maintain a heading with any degree of accuracy over any kind of appreciable distance - so it's irrelevant!

I'm pretty sure your solution is back to front by the way - surely the numbers on the VOR rose relate to the actual VOR radials. If you want to fly an accurate magnetic track, your protractor method will be closer?

[englishal]

Zonkor,

You are making a problem out of nothing! You either...a) Measure which radial you want from the VOR rose, and then set that radial and fly it by keeping the needle centred or b) Measure from the chart the TRUE ground track you want, then calculate in Variation, Deviation and wind correction to give you a MAGNETIC heading to fly from the VOR and ignore the VOR radials completely.

a) is easier, so that is what I would do Incidentally if you put your protractor on the VOR rose then each degree is 1 radial and hence it is just as easy to read as the true ground track from the chart references....

Of course these days it is even easier. Direct Enter enter, put the aircraft heading marker on the magenta and keep it there

[darkroomsource]

Without using either a GPS or radio navigation, there's no way you could possibly maintain a heading with any degree of accuracy over any kind of appreciable distance - so it's irrelevant!

Tell that to Charles Lindbergh

[stevelup]

With respect, Zonkor isn't Charles Lindbergh and I'm sure Lindbergh would have been using celestial navigation alongside the dead reckoning!