VOT and VOR checks

[AggieMike88]

Worked through the ground school lesson on these and understand the concepts. And have a question on the practical application in my situation.

Looking at the AFD that covers my area (http://aeronav.faa.gov/pdfs/sc_rear_10JAN2013.pdf page 434 and 435), I don't see a facility that directly relates to Denton and KDTO.

So if I wanted to conduct a check next time I fly, how do I do that? Fly to a location that is on this list?

 

[Warrior]

Worked through the ground school lesson on these and understand the concepts. And have a question on the practical application in my situation.

Looking at the AFD that covers my area (http://aeronav.faa.gov/pdfs/sc_rear_10JAN2013.pdf page 434 and 435), I don't see a facility that directly relates to Denton and KDTO.

So if I wanted to conduct a check next time I fly, how do I do that? Fly to a location that is on this list?

I am a 30 minute flight to a VOR reciever ground checkpoint and while it is a little bit of an effort I have done it a couple of times. The easier way is to use 2 vors in your plane to do the vor check if you happen to have 2 that work.

How far is Denton from a VOR receiver checkpoint?

 

[AggieMike88]

I am a 30 minute flight to a VOT checkpoint and while it is a little bit of an effort I have done it a couple of times. The easier way is to use 2 vors in your plane to do the vor check if you happen to have 2 that work.

Yeah, after, I posted my question, I viewed the "airborne 2 vor check that must be within 4* of each other" session.

 

[AggieMike88]

Kinda interesting though that with the density of aircraft around D/FW, that there are very few ground check locations.

 

[EppyGA]

I just dial in a VOR on my two nav receivers and check them against each other.

 

[poadeleted20]

And remember when figuring out the TO/FROM indications on a VOT check that Cessna makes a one eighty "TO", but not a one eighty "FROM".

 

[flyingron]

Remember you can only use it at the airport of intended departure. If you never intend to depart, it's unavailable to you

 

[poadeleted20]

Remember you can only use it at the airport of intended departure. If you never intend to depart, it's unavailable to you

Then why do they have airborne VOT check points (I know there are such in Texas and Oklahoma)?

Dallas Love Field ..................................​

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113.3 A/G Airborne, use within 10
NM radius of Dallas
Love field between​

2000 and 10000.

Oklahoma City (Will Rogers World).........​

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108.8 A/G Within 10 NM radius
between 3000 and
5000 VOT unusable on
Twy H and Rwy 17L–35R
N of Twy H–2 and Twy E
N of Twy E–2/E–3​

junction.

Yes, I know, that's what the reg says. But the FAA really doesn't care as long as you do it before you accept a clearance involving use of or requirement for VOR's.

 

[AggieMike88]

10 miles around KDAL is a busy bit of space.

Likely I'll just make us of either the Bowie or Bonham VOR's and do the "cross check" method.

 

[tmyers]

I have always done the airborn VOR checks. I am about to hook my GPS to the #2 VOR and will be pulling the physical #2 vor out of the airplane. I can still dial up a VOR on the GPS and it will give me the exact radial to. Can this be used in the airborn comparison?

 

[ja_user]

I have always done the airborn VOR checks. I am about to hook my GPS to the #2 VOR and will be pulling the physical #2 vor out of the airplane. I can still dial up a VOR on the GPS and it will give me the exact radial to. Can this be used in the airborn comparison?

No, it cannot.

 

[poadeleted20]

And the reason it cannot is that VOR radials do not necessarily match their actual magnetic bearing from the station. The operative issues are variation and declination.

 

[alfadog]

Worked through the ground school lesson on these and understand the concepts. And have a question on the practical application in my situation.

Looking at the AFD that covers my area (http://aeronav.faa.gov/pdfs/sc_rear_10JAN2013.pdf page 434 and 435), I don't see a facility that directly relates to Denton and KDTO.

So if I wanted to conduct a check next time I fly, how do I do that? Fly to a location that is on this list?

Why not do it over KADS at 2500'?

 

[tmyers]

And the reason it cannot is that VOR radials do not necessarily match their actual magnetic bearing from the station. The operative issues are variation and declination.

Sounds reasonable. I wouldn't think this would change if the GPS has a CDI as it will still be reading true?

 

[poadeleted20]

Sounds reasonable. I wouldn't think this would change if the GPS has a CDI as it will still be reading true?

It's not reading true bearing, it's reading mag bearing, but the situation would not change. As I said, the problem is that the GPS give you actual mag bearing regardless while the VOR only reads the radial, and the radial may not be on the same mag bearing that the radial's number suggests. Also, even if variation matches declination for that VOR and there are no bending or distortion faults in the radial, as you go farther from the VOR, the difference between actual mag bearing versus the perceived radial begins to be significant (think great circle).

 

[alfadog]

Sounds reasonable. I wouldn't think this would change if the GPS has a CDI as it will still be reading true?

The GPS will be dead accurate and will drive the CDI dead accurate. The VOR radials are not necessarily so accurate to real-world position but we are looking for agreement between the VOR receiver and transmitter, not dead accuracy.

 

[alfadog]

I see that my nearest airborne checkpoint is 70 nm away and the nearest ground check 120 nm so I will continue relying on the two receiver method.

 

[poadeleted20]

I see that my nearest airborne checkpoint is 70 nm away and the nearest ground check 120 nm so I will continue relying on the two receiver method.

As long as I have two working VOR's, that's all I ever use.

 

[DrMack]

I have only ever used the two VOR comparison or the airway method. It makes no sense to me that we can't use GPS to fix our position as a reference in lieu of a ground reference point for a bearing to a VOR. GPS can be used in lieu of DME for an instrument approach so why not for a VOR check?

 

[alfadog]

I have only ever used the two VOR comparison or the airway method. It makes no sense to me that we can't use GPS to fix our position as a reference in lieu of a ground reference point for a bearing to a VOR. GPS can be used in lieu of DME for an instrument approach so why not for a VOR check?

Because your GPS is clueless as to what VOR radial you are on.

 

[DrMack]

Because your GPS is clueless as to what VOR radial you are on.

The GPS would be used to provide a lat/lon position in the same way as the visual VOR RECEIVER CHECK points that are listed in the AFD. It is the responsibility of the pilot to then know where that point is on a chart and its bearing to a station.

 

[JHW]

The GPS will be dead accurate and will drive the CDI dead accurate. The VOR radials are not necessarily so accurate to real-world position but we are looking for agreement between the VOR receiver and transmitter, not dead accuracy.

it's not a problem of accuracy. It's that the GPS and VOR are not displaying the same thing. The only way you could consider using GPS to assist in a VOR check is to use the GPS to identify when you are over a prominent landmark that has a charted airway over it.

 

[flyingron]

You can use the GPS to navigate to a self created airborne point but you're still supposed to fix it over a prominent visual landmark on an airway.

 

[alfadog]

You can use the GPS to navigate to a self created airborne point but you're still supposed to fix it over a prominent visual landmark on an airway.

Is that an approved VOR check? I.e. I see this tower on the sectional that is directly under the airway, as drawn on the sectional, therefore I can check my VOR that way?

(4) If no check signal or point is available, while in flight--
(i) Select a VOR radial that lies along the centerline of an established VOR airway;
(ii) Select a prominent ground point along the selected radial preferably more than 20 nautical miles from the VOR ground facility and maneuver the aircraft directly over the point at a reasonably low altitude; and
(iii) Note the VOR bearing indicated by the receiver when over the ground point (the maximum permissible variation between the published radial and the indicated bearing is 6 degrees).

 

[alfadog]

it's not a problem of accuracy. It's that the GPS and VOR are not displaying the same thing.

That is exactly what I am saying.

The only way you could consider using GPS to assist in a VOR check is to use the GPS to identify when you are over a prominent landmark that has a charted airway over it.

See my response above to this idea.

(4) If no check signal or point is available, while in flight--
(i) Select a VOR radial that lies along the centerline of an established VOR airway;
(ii) Select a prominent ground point along the selected radial preferably more than 20 nautical miles from the VOR ground facility and maneuver the aircraft directly over the point at a reasonably low altitude; and
(iii) Note the VOR bearing indicated by the receiver when over the ground point (the maximum permissible variation between the published radial and the indicated bearing is 6 degrees).

 

[alfadog]

Answered my own question.

 

[alfadog]

Here is one for you. 14 CFR 91.171 allows use of a visual landmark. Obviously the entire section predates GPS and does not consider GPS. Would it not be just as effective to navigate to a known intersection on an airway by GPS and check your VOR there? Perhaps we will see that change.

 

[azure]

Although GPS-computed magnetic bearings can't be used to test the VOR and CDI, many GPSs have internal NAV receivers and digitally display the VOR radial, or even bearing TO or FROM. The Pilot's Guide for the GNS480 is ambiguous as to whether the displayed value is computed from GPS info in the manner of the "DME" distance, or is directly derived from the VOR signal. However, my NAV2 is a SL-30 which also displays the TO/FROM bearing digitally and this value always agrees to within a degree with the 480's. Since the SL-30 has no GPS information (and I've seen nothing to suggest that it has any internal database containing station declination information), I'm 99.9% certain that the 480's TO/FROM display is from the VOR signal. (Also, it always agrees with the (digitally displayed) OBS setting on my Sandel that centers the CDI.) Other GPS pilot guides may be more explicit.

Anyway I'm wondering if those digitally displayed radials can be used for the receiver check? Not that you don't also need to check that the CDI centers when the OBS is dialed to that setting, but the digital display makes it easier to log the actual bearing error.

 

[alfadog]

Although GPS-computed magnetic bearings can't be used to test the VOR and CDI, many GPSs have internal NAV receivers and digitally display the VOR radial, or even bearing TO or FROM. The Pilot's Guide for the GNS480 is ambiguous as to whether the displayed value is computed from GPS info in the manner of the "DME" distance, or is directly derived from the VOR signal.

If you have a CDI on your panel like a GI-106A then your CDI button is clearly toggling which device is driving the CDI. Or are you talking about an indication on the CDI page of the main screen?

edit: Even the on-screen CDI has a source indication:

Or are you talking about something you might find on a "Nearest" page? That would be GPS data, I am pretty sure.

 

[azure]

If you have a CDI on your panel like a GI-106A then your CDI button is clearly toggling which device is driving the CDI. Or are you talking about an indication on the CDI page of the main screen?

edit: Even the on-screen CDI has a source indication:

Or are you talking about something you might find on a "Nearest" page? That would be GPS data, I am pretty sure.

The field that can display bearing information has "vor" in that picture. If a Morse identifier was identified, those letters would show the decoded VOR identifier. There is a soft button that functions as a 3-way toggle between To/Fr/VOR Id. Note that this display is in the NAV window -- by itself that strongly suggests that the information is VOR-derived. However the distance there is derived from GPS info, since the 480 doesn't have a DME receiver as such.

Of course there is a CDI button to switch between devices -- I wasn't talking about that.

 

[alfadog]

The field that can display bearing information has "vor" in that picture. If a Morse identifier was identified, those letters would show the decoded VOR identifier. There is a soft button that functions as a 3-way toggle between To/Fr/VOR Id. Note that this display is in the NAV window -- by itself that strongly suggests that the information is VOR-derived. However the distance there is derived from GPS info, since the 480 doesn't have a DME receiver as such.

Of course there is a CDI button to switch between devices -- I wasn't talking about that.

Mmm. Well, I have no time with a 480 but I am pretty good at reading manuals.

I looked at both the Quick Start:
Press ID/TO/FR to view the bearing To/From the VOR

and the Pilot Guide:
ID/To/Fr
When the Active frequency is a VOR, you can view the current course To/Fr the VOR as well as the Identifier and distance. This value appears between the Active and Standby VOR frequencies and to the right. Press Menu/Enter and then each press of the ID/To/Fr key will toggle between the display of the ID (Identifier), To radial, or From radial values.

And I can see the ambiguity. Course and bearing would seem to indicate GPS data while radial would seem to indicate VOR data. I would not hazard a guess as to what is meant and were it me, I would seek a clarification from Garmin if it mattered enough to be concerned about.

 

[poadeleted20]

Here is one for you. 14 CFR 91.171 allows use of a visual landmark. Obviously the entire section predates GPS and does not consider GPS. Would it not be just as effective to navigate to a known intersection on an airway by GPS and check your VOR there? Perhaps we will see that change.

Perhaps it would. Why don't you write to AFS-400 and suggest that change to 91.171? If it would work, I'm sure they'd do it. If not, I'm equally sure they'll explain why it doesn't work.
http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs400/

 

[alfadog]

Perhaps it would. Why don't you write to AFS-400 and suggest that change to 91.171? If it would work, I'm sure they'd do it. If not, I'm equally sure they'll explain why it doesn't work.
http://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afs/afs400/

Perhaps I will. I did get the A/FD entry for KTMB changed recently but that was via an online change form.

 

[poadeleted20]

Perhaps I will. I did get the A/FD entry for KTMB changed recently but that was via an online change form.

You got your toes in the water then, now let's go for the knees.

 

[tmyers]

When doing an airborne check over a known point. How close do you need to be to this point to count it? Thinking the only way to be directly over something and know that you are is inverted and neither the Arrow or my insurance company likes that. Just looking for an approximation here. If I only had the RV-10 I could do the inverted check with no problem. Just thought I would beat Geico to the punch.

 

[JHW]

When doing an airborne check over a known point. How close do you need to be to this point to count it? Thinking the only way to be directly over something and know that you are is inverted and neither the Arrow or my insurance company likes that. Just looking for an approximation here. If I only had the RV-10 I could do the inverted check with no problem. Just thought I would beat Geico to the punch.

you'll notice that designated airborne checkpoints are far enough from the VOR that your best effort to fly over the checkpoint is plenty good enough

 

[alfadog]

When doing an airborne check over a known point. How close do you need to be to this point to count it? Thinking the only way to be directly over something and know that you are is inverted and neither the Arrow or my insurance company likes that. Just looking for an approximation here. If I only had the RV-10 I could do the inverted check with no problem. Just thought I would beat Geico to the punch.

When you select the point yourself, they recommend at least 20 miles out and relatively low altitude. The error from you offsetting a bit will be minor. A 200' offset represents a 1/10 degree error at 20 miles out.

 

[azure]

Mmm. Well, I have no time with a 480 but I am pretty good at reading manuals.

Well, I have one in my plane and I've been using it for nearly 3 years now. If the NAV receiver-driven CDI centered at an OBS value that was more than a degree different from the radial shown in the NAV window on the 480, I'm pretty sure I would notice that. And considering the differences between station declination among the VORs in this area, I expect it would have happened by now. (Plus, there is the fact that the displayed radial always agrees, to within a degree, with the radial shown on the SL-30 display.)

I just wish the manual were clearer about that point, so I could cite an authoritative source for what I was saying.

 

[alfadog]

Well, I have one in my plane and I've been using it for nearly 3 years now. If the NAV receiver-driven CDI centered at an OBS value that was more than a degree different from the radial shown in the NAV window on the 480, I'm pretty sure I would notice that. And considering the differences between station declination among the VORs in this area, I expect it would have happened by now.

I just wish the manual were clearer about that point, so I could cite an authoritative source for what I was saying.

And no, I wouldn't consider some random support person at Garmin to be authoritative. Garmin did not design the GPS, so I'm not confident that my query would be routed to someone who really knows the answer.

Well, as a good technical reader and a retired engineer, my gut feeling was that it is as you say. After all, the actual radial has more navigation value (identifying airways, fixes, etc.) than simply knowing the bearing to the VOR. And by "ask Garmin", I meant write to their engineering department and get a written response, not make a call to support. Or call the engineering dept. and try to work your way to someone knowledgeable. That said, you seem to have a handle on it.

 

[Jaybird180]

I see that my nearest airborne checkpoint is 70 nm away and the nearest ground check 120 nm so I will continue relying on the two receiver method.

How do I locate my nearet VOT? What reference do I use to lookup the information?