VOR Accuracy for Instrument Training and Checkride

[azpilot]

-- Note - I am going to talk to my CFII about this, but I'm curious what everyone here has to say as well --

I'm chugging along through my instrument training. I'm up to about 30 hours of simulated instrument time. Hoping to wrap up the checkride by the end of October. One of the things my instructor and I have noted is that there is about a 4 degree offset between the VOR nav radio 1 and the GPS signal. The two VOR radios are sufficiently accurate to pass an airborne VOR check, which meets the requirements to use them for IFR flight.

However, based on the approaches we have flown, it's pretty obvious that the VOR#1 is 4 degrees off. One of the regular VOR approaches we do for practice out here is the VOR runway 5 into KCGZ. The IAF is the TFD VOR. After you cross the IAF, you fly the 048 radial to runway 5 at KCGZ. It is 7.8 NM from TFD to the missed approach point.

If I did my math right, a 4 degree error results in a lateral deviation of ~3300 feet. At the visual descent point, 6.4 NM from the VOR, the lateral error would be ~2700 feet. That's pretty far off.

So what do I do on a checkride??? There is a really good chance this will be the VOR approach we fly for my checkride. The VOR technically meets the requirements for use in an IFR environment. But if I'm doing the most common VOR approach out here in our practice area, and I have all the instruments lined up, I'm not going to be anywhere near the runway.

Like I said, I'm going to talk to my CFII about it, and I will approach this situation based on the feedback I get from him. But I know how much us pilots love to split hairs and talk through stuff like this, so I figured I'd ask what you all think.

Question:

How hard is it to 'adjust' the nav 1 vor and fix it to get rid of the 4 degree offset???

 

[Larry in TN]

One of the things my instructor and I have noted is that there is about a 4 degree offset between the VOR nav radio 1 and the GPS signal. The two VOR radios are sufficiently accurate to pass an airborne VOR check, which meets the requirements to use them for IFR flight.

Are you taking the VOR station's declination into account?

Have you checked the VORs against a VOT?

 

[azpilot]

Are you taking the VOR station's declination into account?

Have you checked the VORs against a VOT?

There are only two VOT's in Arizona. One is at KPHX (bravo airporit). The other is not close. So... No.

As far as VOR declination goes, I don't think that's what's going on. We've done this check against three different VOR's. PHX, IWA and TFD. We get the same 4 degree offset with each one.

Now, I know this isn't the "legal" VOR check, but as far as I'm concerned, it's what counts. If I fly the VOR 5 approach into KCGZ, and I track the VOR, I'm not anywhere near the runway. When I fly the GPS approach, I'm right on the runway centerline.

 

[Larry in TN]

There are only two VOT's in Arizona. One is at KPHX (bravo airport).

If it's your own airplane, I'd take it to PHX and do the check. PHX is easy and it's good practice for your radio work with your instrument rating. Avoid the big airline pushes and you'll be in and out quickly.

If it's a rental, I'd talk to the FBO or airplane owner about getting it fixed.

 

[MauleSkinner]

Specifically, how are you determining that the VORs are off?

How close are the two VORs on the dual check?

How about finding a ground checkpoint? Are there any of those nearby?

id also suggest finding a flyover ground checkpoint that you can accurately measure on a sectional chart.

if you’re going to talk to the owner to try and get something fixed (I.e., money spent), I’d have very solid, specific answers to all of those questions first.

As far as the checkride, I wouldn’t have a problem going into a checkride if the VORs are “legal” as best as you can determine. Just be able to justify your position to the examiner, and make sure you clearly use the VORs as your primary course guidance for the VOR approach. If, as an examiner, I saw you fly the approach with the GPS centered and the VOR off course, we’d have an interesting discussion.

 

[RussR]

There are only two VOT's in Arizona. One is at KPHX (bravo airporit). The other is not close. So... No.

Is one of the VOR Ground Checkpoints closer?

Alternately, you don't really need to be on the ground for a VOT to work (for it to count as your 30-day check I think you do, but not just to test it). You just need to be able to receive it. Now, they aren't real high-power stations, but still, tune in 109.0 next time you're near PHX and see what your VORs say.

 

[Sluggo63]

By all means, go get the VOT check at PHX. If you're getting your instrument ticket, PHX will be good practice and shouldn't deter you at all.

How old is your CFI? Did he fly in the pre-GPS days?

What I really think is happening is that you aren't really realizing how much "non" there is in "non-precision" approaches. Being less than a 1/2 NM displaced from the runway CL at 1.4 NM away wasn't unusual. I remember breaking out on a VOR approach and having to scan left and right to find the runway.

I think we're so used to being dead-on accurate these days with GPS, we lose sight of what it was like in the old days before GPS.

We used to see it even with controllers. Back in the 727, we didn't have GPS and we'd always hear from controllers how we weren't on the airway, we were 4 miles left of course. Well, no kidding... we're 80 NM from the VOR, one radial is 1.3 NM thick. With a 4* allowable error, we're still well within tolerances.

Just fly the approach with the VOR centered up and you'll be fine. (But still do the VOT, just in case)

 

[luvflyin]

Is one of the VOR Ground Checkpoints closer?

Alternately, you don't really need to be on the ground for a VOT to work (for it to count as your 30-day check I think you do, but not just to test it). You just need to be able to receive it. Now, they aren't real high-power stations, but still, tune in 109.0 next time you're near PHX and see what your VORs say.

Phoenix’s doesn’t have remarks about usability, but some do. They are noted as G facilities, ground. Not Airborne. I would be leary of putting much stock in the results if used incorrectly.

 

[Walboy]

You can check on the ground at KIWA.

If one of the Nav Radios is more accurate than the other, use it. Tell the DPE why. I think @MauleSkinner 's advice is good. I wouldn't worry about trying to fix something that's not broken.

Something to think about on a non precision approach. Where to find the airport when you breakout might not be anywhere close to being in front of the airplane. Given a wind correction and a VOR receiver with some error in it at a long distance from the station, the runway might be off to one side or the other 45 degrees or more.

It would be a good time to bring in personal minimums into the discussion with the examiner. Show good ADM. If you know the lateral error could be significant on a non precision approach, you might want to raise your personal minimums above the approach minimums.

It's also a good reason to study up on obstacle clearance on approaches. I think the fact that you have a significant error is a really, really good learning experience.

 

[azpilot]

As far as the checkride, I wouldn’t have a problem going into a checkride if the VORs are “legal” as best as you can determine. Just be able to justify your position to the examiner, and make sure you clearly use the VORs as your primary course guidance for the VOR approach. If, as an examiner, I saw you fly the approach with the GPS centered and the VOR off course, we’d have an interesting discussion.

Ya, this is really what I was getting at. If the VOR meets the legal requirements for IFR flight, I would need to demonstrate that to the examiner before we go flying. I was thinking I'd explain that I know the VOR's are legal, but close to the limit. I'd be upfront and explain that I know if we do the VOR 5 approach into KCGZ, I'll actually be 4 deg off. I'd think that explaining all of that up front would demonstrate a good understanding of the regs as well as my understanding of how the approach works. Great advice, thank you.

 

[azpilot]

Is one of the VOR Ground Checkpoints closer?

Alternately, you don't really need to be on the ground for a VOT to work (for it to count as your 30-day check I think you do, but not just to test it). You just need to be able to receive it. Now, they aren't real high-power stations, but still, tune in 109.0 next time you're near PHX and see what your VORs say.

That's an interesting idea. I'll definitely give that a shot and see what happens. If nothing else, I'm curious. And like you say, it doesn't count as the legal check, but I'd still be interested to see how close it is.

 

[azpilot]

What I really think is happening is that you aren't really realizing how much "non" there is in "non-precision" approaches. Being less than a 1/2 NM displaced from the runway CL at 1.4 NM away wasn't unusual. I remember breaking out on a VOR approach and having to scan left and right to find the runway.

I think we're so used to being dead-on accurate these days with GPS, we lose sight of what it was like in the old days before GPS.

Ok, this is really interesting feedback. Thank you for sharing.

 

[wilkersk]

If a one degree error equals a 1nm (6076') deviation at 60 DME, then a 3,300' lateral deviation would be something over 30 DME, wouldn't it?

How hard are you having to look to find the runway at minimums?

 

[Van Johnston]

I think a better explanation is that VOR radials are not routinely (and I'm using that word generously) adjusted for the drift in the magnetic pole and the change in declination. For example, a VOR close to me, CSI, is almost right on the 4 deg 30 sec isogonic line, but according to Airnav, it is aligned to 8 deg declination in 1980!

CSI
CENTER POINT VORTAC
KERRVILLE, TX
Location
Lat/Long: 29-55-20.476N / 099-12-52.149W (29.9223544/-99.2144858)
Elevation: 2079.3 ft.
Variation: 08E (1980)

Also, regarding the VOT on the ground, we found we can receive the VOT on the ground at SAT up to 30 miles away at VFR altitudes around here. And when we do the checks against the VOT both our VOR receivers have less error than any other check method.

 

[luvflyin]

Ya, this is really what I was getting at. If the VOR meets the legal requirements for IFR flight, I would need to demonstrate that to the examiner before we go flying. I was thinking I'd explain that I know the VOR's are legal, but close to the limit. I'd be upfront and explain that I know if we do the VOR 5 approach into KCGZ, I'll actually be 4 deg off. I'd think that explaining all of that up front would demonstrate a good understanding of the regs as well as my understanding of how the approach works. Great advice, thank you.

Be prepared to tell him how you checked your VOR and where to find VOR checkpoints and VOT’s

 

[RussR]

Phoenix’s doesn’t have remarks about usability, but some do. They are noted as G facilities, ground. Not Airborne. I would be leary of putting much stock in the results if used incorrectly.

I believe those restrictions are based on receivability, i.e. signal getting blocked by buildings. If I remember my VOR theory right, a VOT transmits one VOR signal with no phase difference in the modulation - therefore showing a 360 FROM indication no matter where you are. So as long as you can receive it, you should get the 360 FROM, and there should be no possibility of getting anything else. I think it would be a perfectly adequate test for the OP's purposes.

 

[RussR]

We used to see it even with controllers. Back in the 727, we didn't have GPS and we'd always hear from controllers how we weren't on the airway, we were 4 miles left of course. Well, no kidding... we're 80 NM from the VOR, one radial is 1.3 NM thick. With a 4* allowable error, we're still well within tolerances.

Fun fact, airways get wider after 51 nm from the VOR station. At 80 nm, the airway is 6.3 nm either side of centerline, so you were probably still good even if the controller didn't think so.

The 51 nm is based on the "system accuracy" of the VOR being established at 4.5 deg. (Which, notice, is slightly larger than the 4 degrees of error.) So if you're on the 090 radial, it assumed you're really just somewhere between the 085.5 and 094.5 radials. 4.5 degrees at 51 nm =4 nm (tan (4.5 deg) x 51 = 4), so at that distance the airway widens at 4.5 deg until the changeover point to the next VOR, at which point it starts getting narrower again.

 

[sarangan]

-- Note - I am going to talk to my CFII about this, but I'm curious what everyone here has to say as well --

I'm chugging along through my instrument training. I'm up to about 30 hours of simulated instrument time. Hoping to wrap up the checkride by the end of October. One of the things my instructor and I have noted is that there is about a 4 degree offset between the VOR nav radio 1 and the GPS signal. The two VOR radios are sufficiently accurate to pass an airborne VOR check, which meets the requirements to use them for IFR flight.

However, based on the approaches we have flown, it's pretty obvious that the VOR#1 is 4 degrees off. One of the regular VOR approaches we do for practice out here is the VOR runway 5 into KCGZ. The IAF is the TFD VOR. After you cross the IAF, you fly the 048 radial to runway 5 at KCGZ. It is 7.8 NM from TFD to the missed approach point.

If I did my math right, a 4 degree error results in a lateral deviation of ~3300 feet. At the visual descent point, 6.4 NM from the VOR, the lateral error would be ~2700 feet. That's pretty far off.

So what do I do on a checkride??? There is a really good chance this will be the VOR approach we fly for my checkride. The VOR technically meets the requirements for use in an IFR environment. But if I'm doing the most common VOR approach out here in our practice area, and I have all the instruments lined up, I'm not going to be anywhere near the runway.

Like I said, I'm going to talk to my CFII about it, and I will approach this situation based on the feedback I get from him. But I know how much us pilots love to split hairs and talk through stuff like this, so I figured I'd ask what you all think.

Question:

How hard is it to 'adjust' the nav 1 vor and fix it to get rid of the 4 degree offset???

At the MAP you will be laterally displaced by 3300 ft. But an instrument approach is designed to provide obstacle clearance, not gurantee that you can land. I think you are legally fine, especially for a checkride. Your calculations already show that you won't see the runway if the weather is at minimum. So in real life there is no point in even trying, even if it is safe.

 

[azpilot]

If a one degree error equals a 1nm (6076') deviation at 60 DME, then a 3,300' lateral deviation would be something over 30 DME, wouldn't it?

How hard are you having to look to find the runway at minimums?

I just did the trigonometry. ARCTAN (4 deg) x 7.8NM = .0697 * 7.8 = 0.5436 NM = 3303 feet

However, you're math above is also right. ARCTAN (1 deg) = .0175 - > .0175 * 60 = 1.05

The part you're missing is that I believe my VOR receiver to be reporting to me a position that is 4 degrees off. Using your example, of 3300' feet of lateral deviation at 30 miles is correct at 1 degree of error. However, at four degrees of error you need to divide the 30 DME by 4, which would be 7.5 miles. On the approach I'm discussing, the distance from the IAP to the missed approach point is 7.8 NM.

All the math checks out.

 

[Kritchlow]

Correct me if I’m wrong, but depending on the geometry…
GPS uses great circle headings. VOR no. I’ve seen 6+ degree discrepancies, but I’m not sure if that applies to your situation.

 

[luvflyin]

I believe those restrictions are based on receivability, i.e. signal getting blocked by buildings. If I remember my VOR theory right, a VOT transmits one VOR signal with no phase difference in the modulation - therefore showing a 360 FROM indication no matter where you are. So as long as you can receive it, you should get the 360 FROM, and there should be no possibility of getting anything else. I think it would be a perfectly adequate test for the OP's purposes.

Makes sense up to point. What I don’t get though is some of the very explicit restrictions some of them have. It doesn’t seem that ‘unreceivability’ could be narrowed down that precisely. And if reflections or whatever would not affect the VOT signal because of the no phase modulation thing, then why would they have a need to publish usability points? It would be like if you can receive it, it’s accurate. If you can’t, well then you aren’t going to get any indication at all.

 

[IK04]

Swap the #1 and #2 radios and use the #1 for the approaches. Ignore #2 and have a nice day!

(That's what I did!)

 

[Larry in TN]

Swap the #1 and #2 radios and use the #1 for the approaches. Ignore #2 and have a nice day!

It's not the radio that's being tested in a VOR check, it is the phase-shift calibration in the OBS.

 

[azpilot]

It's not the radio that's being tested in a VOR check, it is the phase-shift calibration in the OBS.

Is that phase-shift calibration different for Nav 1 and Nav 2???

 

[azpilot]

Correct me if I’m wrong, but depending on the geometry…
GPS uses great circle headings. VOR no. I’ve seen 6+ degree discrepancies, but I’m not sure if that applies to your situation.

I would think that for the relatively short distances I'm talking about this would be irrelevant, but I confess that I am not 100% certain about this.

 

[Van Johnston]

@azpilot I think you are still missing the point. The unstated assumption you are making is that the VOR is aligned with the magnetic field. That is an incorrect assumption. The VOR is aligned with what the magnetic field was when the VOR was last aligned. You can see this information in Airnav (and I assume they get their data from some authoritative source). In your area, look at TFD. Last aligned in 1985.
TFD
STANFIELD VORTAC
CASA GRANDE, AZ
Location
Lat/Long: 32-53-09.076N / 111-54-31.444W (32.8858544/-111.9087344)
Elevation: 1316.0 ft.
Variation: 12E (1985)

https://www.airnav.com/cgi-bin/navaid-info

flight plan TFD-SSO in your tablet. It will say fly heading 095. Then look at V94 on the sectional. It says fly radial 093.

I can’t cite a reference, but I THINK if you fly the specified radial on a VOR approach it will still put you where you want to be when you pop out.

 

[Larry in TN]

Is that phase-shift calibration different for Nav 1 and Nav 2???

All the radios do is receive and decode (from the carrier wave) the two signals from the VOR; the phased signal and the reference signal. Those two signals are sent to the CDI where the OBS knob controls the phase shifter which shifts the phase of the phased signal so that it will match up with the reference signal when the aircraft is on the selected radial. It is the calibration of the phase-shift from the OBS that is being tested.

 

[dmspilot]

I would think that for the relatively short distances I'm talking about this would be irrelevant, but I confess that I am not 100% certain about this.

What method have you used to determine that VOR#1 is off by 4 degrees?

 

[flyingron]

Is one of the VOR Ground Checkpoints closer?

Alternately, you don't really need to be on the ground for a VOT to work (for it to count as your 30-day check I think you do, but not just to test it). You just need to be able to receive it. Now, they aren't real high-power stations, but still, tune in 109.0 next time you're near PHX and see what your VORs say.

Yep, ground checkpoints and VOTs need to be done at the airport of intended departure for them to count. You can't do them at airports you don't intend to depart from (or perhaps you can't unintentionally depart from an airport, I don't know which).

 

[azpilot]

@azpilot I think you are still missing the point. The unstated assumption you are making is that the VOR is aligned with the magnetic field. That is an incorrect assumption. The VOR is aligned with what the magnetic field was when the VOR was last aligned. You can see this information in Airnav (and I assume they get their data from some authoritative source). In your area, look at TFD. Last aligned in 1985.
TFD
STANFIELD VORTAC
CASA GRANDE, AZ
Location
Lat/Long: 32-53-09.076N / 111-54-31.444W (32.8858544/-111.9087344)
Elevation: 1316.0 ft.
Variation: 12E (1985)

https://www.airnav.com/cgi-bin/navaid-info

flight plan TFD-SSO in your tablet. It will say fly heading 095. Then look at V94 on the sectional. It says fly radial 093.

I can’t cite a reference, but I THINK if you fly the specified radial on a VOR approach it will still put you where you want to be when you pop out.

Thank you very much for patiently re-explaining, I do get it now. It's crazy that it's been almost 30 years since that VOR was re-aligned. It's basically on the 10E isogonic line. So even if the avionics in the airplane are "perfect", you're still guaranteed to be off by two degrees. wow...

 

[luvflyin]

Makes sense up to point. What I don’t get though is some of the very explicit restrictions some of them have. It doesn’t seem that ‘unreceivability’ could be narrowed down that precisely. And if reflections or whatever would not affect the VOT signal because of the no phase modulation thing, then why would they have a need to publish usability points? It would be like if you can receive it, it’s accurate. If you can’t, well then you aren’t going to get any indication at all.

@RussR , here’s one from your neighborhood. It’s an A/G VOT, with limits on it’s airborne use. Why, I dunno.

 

[midlifeflyer]

All the radios do is receive and decode (from the carrier wave) the two signals from the VOR; the phased signal and the reference signal. Those two signals are sent to the CDI where the OBS knob controls the phase shifter which shifts the phase of the phased signal so that it will match up with the reference signal when the aircraft is on the selected radial. It is the calibration of the phase-shift from the OBS that is being tested.

...which makes sense. Otherwise, why do a dual VOR check if the both must have the same error?

 

[midlifeflyer]

I would think that for the relatively short distances I'm talking about this would be irrelevant, but I confess that I am not 100% certain about this.

The GPS/VOR discrepancy can exist even in the shorter distance of an instrument approach. I know of one time in which an applicant busted an instrument checkride because if it. The applicant wasn't aware of and didn't understand the discrepancy. Ended up getting fixated on the difference in the numbers instead of flying the VOR approach and royally messed it up.

 

[dmspilot]

I would think that for the relatively short distances I'm talking about this would be irrelevant, but I confess that I am not 100% certain about this.

It's not relevant because the correct radial is printed on the chart for the airway or approach segment. The error only matters if you're trying to convert a radial to or from a magnetic course yourself.

 

[Van Johnston]

Yep, ground checkpoints and VOTs need to be done at the airport of intended departure for them to count. You can't do them at airports you don't intend to depart from (or perhaps you can't unintentionally depart from an airport, I don't know which).

How does that jive with only having to check every 30 days? And not that many airports have either VOTs or ground check points.

 

[Sluggo63]

How does that jive with only having to check every 30 days? And not that many airports have either VOTs or ground check points.

Most people probably do the VOR-VOR check if they have two (that's what I do), or they do the airborne checkpoint, or the "prominent ground point" procedure... It's all outlined in the AIM 1-1-4.

 

[John Collins]

Correct me if I’m wrong, but depending on the geometry…
GPS uses great circle headings. VOR no. I’ve seen 6+ degree discrepancies, but I’m not sure if that applies to your situation.

VOR radials are along a great circle route. All a great circle route means is it is the shortest course between two points on the surface of a sphere. The VOR radio signals propagate along a straight line of sight between the VOR station and the VOR receiver (excepting reflections). When plotted on a chart, they appear curved, although for short distances, the curve is not easily discernable.

 

[Larry in TN]

It's basically on the 10E isogonic line. So even if the avionics in the airplane are "perfect", you're still guaranteed to be off by two degrees. wow...

I don't know your level of understand on this topic so I apologize if some of this is too basic. Pre-GPS, pilots tended to spend a lot more time learning about, and using, VHF navigation than they do today. My Instrument checkride, in 1986, was in a C-310 with two VORs and one NDB. Didn't even have DME. It was a different world.

When the VOR is built, or calibrated, the radials are the same as their magnetic orientation. Over time, the local magnetic variation changes but the radials stay the same. This produces a non-zero declination for the VOR as its radials no longer match exactly to their magnetic courses.

If a VOR approach was setup on the R-090 from the VOR then that radial will always take you exactly to the same point. Some years later, your GPS may tell you that your ground track from the VOR to the MAP on the approach is 085° on the R-090 but that still takes you to the exact same spot.

The GPS units know this (and you can usually find the VORs declination listed in the information the GPS provides about the NAVAID). If you build a course off a VOR that has a large declination you'll see that the course the GPS built does not exactly match the radial that you built.

Anyway, that's why I asked if you were taking the declination into account earlier in the thread.

(I've been using "declination" as a relative declination, not absolute. i.e. if the VOR is perfectly aligned the declination is zero. Declination is actually the different between True North and the VOR's R-360 but it seems easier to discuss (to me) as a relative reference. Just understand that the number you'll see published is referenced to True North and you'll have to compare that to the current magnetic variation to see the relative declination)

 

[John Collins]

Thank you very much for patiently re-explaining, I do get it now. It's crazy that it's been almost 30 years since that VOR was re-aligned. It's basically on the 10E isogonic line. So even if the avionics in the airplane are "perfect", you're still guaranteed to be off by two degrees. wow...

Not correct. The TFD VOR was aligned in 1985, 37 years ago when the magnetic variation was 12 east. The current magnetic variation is 10 east. In 1985, you would need to set your OBS to 264 to track outbound on V94 from TFD to POTER and the no wind course would be 264 degrees magnetic. In 2022, you still need to set your OBS to 264 and if your VOR was perfect and you kept the VOR CDI needle centered, you would fly over precisely the same patch of ground to get to POTER along V94, but your current no wind magnetic course would be 266 degrees (actually closer to 267 because of round off). In either case, keeping the CDI in the center would track along V94 and fly over the exact same path.

Most pilots have no clue that radials are not current magnetic courses and they question why the GPS, which shows a track using the current magnetic variation, differs from the charted value. Airway are along radials, not current magnetic tracks. The charted radial number on airways (or approaches) are based on what you need to set into the OBS on your VOR indicator to fly the path. Approaches and runways also are not current magnetic headings, but are based one a magnetic variation at a point in time called the magnetic epoch. So at my airport, KUZA has runways 2/20 which use the epoch year 1990 (32 years ago) for the magnetic variation when it was 5 West. Current magnetic variation is now 8 West. Approaches to runway 2 are charted with a final approach course of 016 degrees magnetic, but the GPS shows 018 degrees magnetic when you are on course.

 

[Van Johnston]

Most people probably do the VOR-VOR check if they have two (that's what I do), or they do the airborne checkpoint, or the "prominent ground point" procedure... It's all outlined in the AIM 1-1-4.

Understand. I was poking at the implication the ground check only counts for the flight departing that airport, and does not count for the 30 day check.

Reading FAR 91.1717(b), it does sound that way. Maybe just poor sentence structure that makes it confusing?