VOR Question

Rob Schaffer

Rob Schaffer

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Nov 27, 2007
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Green Lane, PA
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CLR2TKF
Flying this morning, I was cruising around at 2500 and 3000 feet watching the sunrise on the horizon and tracking a VOR. With both NAV's tuned to the same VOR I was finding that with NAV1 centered on the needle, NAV2 was 2 dots left, Center NAV2, NAV1 was then two dots right.

IIRC, each dot is 2 1/2*? so I'm possibly 5* different on my two NAV receivers in an airborne check?

I tried a few different radials and VORs (MXE, PTW, ETX) and the readings were all the same. Last year before we had some work done on the plane, it was typically 1 1/2 dots exactly.

Can someone point me to the VOR Check requirements for Airborne and ground stations?

Any ground stations available not to far from KLOM (Wings Field) that anyone knows about?
 
AIM 1-1-4. Unaccustomed as I am to flaming this early in the morning, I am amazed/appalled that you have to ask...unless you are a student, in which case you get a free pass.

What you describe sounds perfectly normal to me. A major variable is the distance between your location and the different VORs. It is not unusual (for example) to be flying east on an airway with the OBS set to 90 with the needle centered and have the needle jump a couple of dots when you change over to the VOR you are flying toward.

Remember that one degree expands to equal one mile on the ground when you are 60 nm from the VOR...you can do a lot of maneuvering that far out without the needle even budging.

You will find a list of VOR receiver check sites in the back of the Airport/Facility Directory for your area.

Bob Gardner

Bob Gardner
 
Most CDI's are 2 degrees per dot, so a 2-dot difference is usually a 4-degree difference, and that's a "pass" under 91.171(c). BTW, while the AIM parrots the regs, 91.171 is the official list of legal methods to check your VOR system(s) for IFR use. That said, if they're always 4 degrees apart, it would be wise to take it to the avionics shop for better tuning, because it's right at the edge of failing the test and being unable to fly IFR legally.

And Rob's an instrument student, so he deserves a free pass since this isn't part of the PP curriculum.
 
bobmrg said:
AIM 1-1-4. Unaccustomed as I am to flaming this early in the morning, I am amazed/appalled that you have to ask...unless you are a student, in which case you get a free pass.
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Seeing as you don't know enough about the OP to even know whether he's a student or pilot (which is in his signature if you bothered to read), I am amazed/appalled that you are flaming him for asking. As Ron said, it's not in the private syllabus.

If that's how you treat students (or other pilots), I'm glad I haven't bought your book.
 
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Rob, there is a Ground check station over at PNE on the North west side of the field. IIRC its on Taxway Hotel.

And just as importantly lets all take a deeeepppp breath here.
 
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Ron Levy said:
Most CDI's are 2 degrees per dot, so a 2-dot difference is usually a 4-degree difference, and that's a "pass" under 91.171(c). BTW, while the AIM parrots the regs, 91.171 is the official list of legal methods to check your VOR system(s) for IFR use. That said, if they're always 4 degrees apart, it would be wise to take it to the avionics shop for better tuning, because it's right at the edge of failing the test and being unable to fly IFR legally.
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If you center the needles on both CDIs and compare the OBS radials you don't need to worry about how many degrees per dot (which isn't required to be exactly 2 BTW and often does vary from CDI to CDI). If you think about it, within reason the scale factor of the CDI is of far less significance than the OBS accuracy for real world IFR navigation because you normally strive to keep the needles centered.

Rob, I'm still unclear WRT the statements in your query. Did you mean that with one local VOR selected on both nav's there was a "two dot" difference but the difference between the two CDIs was considerably less when both nav receivers were tuned to a different VOR a bit further away? Or did you mean the same "two dot difference" existed regardless of which VOR was being received on both navs at the same time? Or was it something else entirely?
 
gismo said:
If you center the needles on both CDIs and compare the OBS radials you don't need to worry about how many degrees per dot (which isn't required to be exactly 2 BTW and often does vary from CDI to CDI). If you think about it, within reason the scale factor of the CDI is of far less significance than the OBS accuracy for real world IFR navigation because you normally strive to keep the needles centered.

Rob, I'm still unclear WRT the statements in your query. Did you mean that with one local VOR selected on both nav's there was a "two dot" difference but the difference between the two CDIs was considerably less when both nav receivers were tuned to a different VOR a bit further away? Or did you mean the same "two dot difference" existed regardless of which VOR was being received on both navs at the same time? Or was it something else entirely?
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same two dot difference, tried out at multiple VOR stations.

AdamZ, if you ever want to join me for a flight to PNE to do a ground VOR check, I'd appreciate it.

Thanks for the references. When I get home and have my AIM/FAR handy I will look further into them.
 
bobmrg said:
AIM 1-1-4. Unaccustomed as I am to flaming this early in the morning, I am amazed/appalled that you have to ask...unless you are a student, in which case you get a free pass.

What you describe sounds perfectly normal to me. A major variable is the distance between your location and the different VORs. It is not unusual (for example) to be flying east on an airway with the OBS set to 90 with the needle centered and have the needle jump a couple of dots when you change over to the VOR you are flying toward.

Remember that one degree expands to equal one mile on the ground when you are 60 nm from the VOR...you can do a lot of maneuvering that far out without the needle even budging.

You will find a list of VOR receiver check sites in the back of the Airport/Facility Directory for your area.

Bob Gardner

Bob Gardner
Click to expand...

Bob,

I'm confused. What does this have to do with the OP's situation? He had both VOR receivers tuned to the same VOR.
 
Ted DuPuis said:
Seeing as you don't know enough about the OP to even know whether he's a student or pilot (which is in his signature if you bothered to read), I am amazed/appalled that you are flaming him for asking. As Ron said, it's not in the private syllabus.

If that's how you treat students (or other pilots), I'm glad I haven't bought your book.
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Well, Rob's signature block says he is a PP WORKING on Instrument. (As of 3:30 PM CDT. Maybe he changed it between then and now.)

However, I won't give Rob a TOTAL pass in that he should have enough knowledge of the Regs/AIM to be able to know where to look. :D
 
Greg Bockelman said:
However, I won't give Rob a TOTAL pass in that he should have enough knowledge of the Regs/AIM to be able to know where to look. :D
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While I'd agree there, Bob's response was uncalled for, and not the way someone should act towards the audience he wants to sell his book to. For that and other statements, I won't buy it, period.
 
Rob Schaffer said:
Can someone point me to the VOR Check requirements for Airborne and ground stations?

Any ground stations available not to far from KLOM (Wings Field) that anyone knows about?
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Not that anyone on here needs to speak on Rob's behalf but Rob is one of the most dedicated pilots I have had the pleasure to fly with. Maybe dedicated to the point of being anal retentive...:skeptical:..It’s a compliment.

A guy asks a simple question without looking it up and he needs a flame suit......thinking out loud is getting dangerous. Hang in there Rob, my Pop always said whether they talk good or bad about you at least someone’s taking the time to think of you. :yesnod:

Ok...had my chuckle for the night, you are free to return to your regular program.



Sec. 91.171 - VOR equipment check for IFR operations.
(a) No person may operate a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment of that aircraft --
(1) Is maintained, checked, and inspected under an approved procedure; or
(2) Has been operationally checked within the preceding 30 days, and was found to be within the limits of the permissible indicated bearing error set forth in paragraph (b) or (c) of this section.
(b) Except as provided in paragraph (c) of this section, each person conducting a VOR check under paragraph (a)(2) of this section shall --
(1) Use, at the airport of intended departure, an FAA-operated or approved test signal or a test signal radiated by a certificated and appropriately rated radio repair station or, outside the United States, a test signal operated or approved by an appropriate authority to check the VOR equipment (the maximum permissible indicated bearing error is plus or minus 4 degrees); or
(2) Use, at the airport of intended departure, a point on the airport surface designated as a VOR system checkpoint by the Administrator, or, outside the United States, by an appropriate authority (the maximum permissible bearing error is plus or minus 4 degrees);
(3) If neither a test signal nor a designated checkpoint on the surface is available, use an airborne checkpoint designated by the Administrator or, outside the United States, by an appropriate authority (the maximum permissible bearing error is plus or minus 6 degrees); or
(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).
(c) If dual system VOR (units independent of each other except for the antenna) is installed in the aircraft, the person checking the equipment may check one system against the other in place of the check procedures specified in paragraph (b) of this section. Both systems shall be tuned to the same VOR ground facility and note the indicated bearings to that station. The maximum permissible variation between the two indicated bearings is 4 degrees. (d) Each person making the VOR operational check, as specified in paragraph (b) or (c) of this section, shall enter the date, place, bearing error, and sign the aircraft log or other record. In addition, if a test signal radiated by a repair station, as specified in paragraph (b)(1) of this section, is used, an entry must be made in the aircraft log or other record by the repair station certificate holder or the certificate holder's representative certifying to the bearing transmitted by the repair station for the check and the date of transmission.
 
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dmccormack said:
Rob -- here's a handy link so you never have to be without the latest CFR/AIM!

http://www.faa.gov/air_traffic/publications/ATpubs/AIM/

http://www.faa.gov/regulations_policies/faa_regulations/
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I'll have to download an app onto my iPhone. Thanks for the links.

I agree, I could have looked it up and maybe I should have to save this bickering, but others probably know now where to find it cause I asked.

Gary, thanks for the compliment, that's probably because 90% of the time my passenger(s) are my 5 year old daughters.
 
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I think the FAR/AIM app for an Iphone was $3.99 at the app store.
I just saw it the other day.
 
Rob, I hope you didn't take offense at my gentle poking. I hope the smiley made it clear that I was only partly serious.;)
 
Ted DuPuis said:
Seeing as you don't know enough about the OP to even know whether he's a student or pilot (which is in his signature if you bothered to read), I am amazed/appalled that you are flaming him for asking. As Ron said, it's not in the private syllabus.

If that's how you treat students (or other pilots), I'm glad I haven't bought your book.
Click to expand...

I deserved that...didn't read his signature.

Bob
 
LearDriver said:
It sounds like Rob is getting this figured out.

Bob on the other hand needs to cut down on the extra cups of coffee in the morning.
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No, bob needs a delay on his computer that will hold posts for an hour or so while he gets his wits about him. Too quick on the trigger. :(

Bob
 
kkoran said:
Bob,

I'm confused. What does this have to do with the OP's situation? He had both VOR receivers tuned to the same VOR.
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The farther away you are from the VOR, the sloppier the needle reaction. Even with the same VOR selected, the anomaly didn't surprise me. The receivers were different, not identical.

As Ron says, within specs but....

Bob
 
bobmrg said:
No, bob needs a delay on his computer that will hold posts for an hour or so while he gets his wits about him. Too quick on the trigger. :(

Bob
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We all have our moments. When we pop off at someone out of impatience or frustration, it's as frustrating after the fact for the sender, as it was in the moment for the receiver.

Just because Bob wrote a few books doesn't make him immune to being human. It just make people pay more attention to his actions. I'm sure if we were all subjected to the same scrutiny, we would fail to live up to our own expectations.

-JD
 
gismo said:
If you center the needles on both CDIs and compare the OBS radials you don't need to worry about how many degrees per dot (which isn't required to be exactly 2 BTW and often does vary from CDI to CDI). If you think about it, within reason the scale factor of the CDI is of far less significance than the OBS accuracy for real world IFR navigation because you normally strive to keep the needles centered.
Click to expand...
Yep, and as an instrument student (like me) Rob is going to need to know that the FAA considers using the number of dots discrepancy as the WRONG way to do a VOR check. There's a question on the instrument written where one of the answers about the allowed error is wrong ONLY because it's phrased in terms of the number of dots, instead of centering the needles and comparing the OBS readings.
 
It sounds like Rob is getting this figured out.

Bob on the other hand needs to cut down on the extra cups of coffee in the morning.
Click to expand...


No, bob needs a delay on his computer that will hold posts for an hour or so while he gets his wits about him. Too quick on the trigger. :(

Bob
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Bob, I'm certainly willing to give you a pass, here - I've never seen you be anything but consistently helpful, and one unfortunate post doesn't change that. :no:
 
and anyone who won't buy his books for ANY reason external to the books themselves is just doing themselves a disservice. :yes:
 
azure said:
Yep, and as an instrument student (like me) Rob is going to need to know that the FAA considers using the number of dots discrepancy as the WRONG way to do a VOR check. There's a question on the instrument written where one of the answers about the allowed error is wrong ONLY because it's phrased in terms of the number of dots, instead of centering the needles and comparing the OBS readings.
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Thanks for the clarification! I'm going to go over to a nearby airport on my next flight and do a VOR ground check.
 
FWIW, I always do air checks (comparing the two). It's the easiest thing to do while flying.

Of course, if both of your VORs are off, and off in the same directly then, well...
 
Ted DuPuis said:
FWIW, I always do air checks (comparing the two). It's the easiest thing to do while flying.
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It's also usually the easiest way to do it on the ground (if there's a VOR station you can receive on the ground) rather than taxiing around to wherever they put the ground check point (if they even have one). And since the check must be in the book before an IFR takeoff...
 
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