VOR Checks

[cale42]

Ok.. getting ready for my CFII checkride. While talking to the inspector I will be flying with he emphasized that of course the plane we would be in should have a current VOR equipment check and that he would not allow a ground based VOR check to count. The first time I had ever heard this theory was a week earlier when a buddy busted his CFII ride b/c the plane only had a dual VOR ground check. The inspector says the regulations support this, but the regs that I read do not specify whether it has to be in the air or on the ground for a dual VOR receiver check.

This would seem odd, b/c lets say I own my own plane, and I haven't flown in 5 weeks. I go to do a trip and it is 200 overcast. No biggie, we have an ILS on the field, I can get back in if necessary, it will be a demanding but safe flight. Now I go to do my VOR check, there is no VOT or ground check on the field, I can't reasonalby do a VOR check between departure and entering the clouds, just not possible at that phase of flight. So I'm now grounded??

What are peoples thoughts on this.. does anyone know of regulation supporting this, I would love to have the proper reg in hand going into this.

cale

 

[U-I pilot]

It is in Part 91.171(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 two indicated bearings is 4 degrees."

Paragraph b is the VOT, airbourne, or ground check point.

I think the examiner isnt saying you cant, just merely that he'd like to see something else demonstrated airborne....
Hope this helps
B

 

[midlifeflyer]

I've never heard an interpretation of 91.171(c) that requires it to be an airborne check. The regulation certainly doesn't say that. I have noticed, though, that all of the knowledge test questions that involve a dual VOR cross-check involve aircraft that are airborne.

Here's a thought. VOR is line of sight. If you are on the ground without a VOT or certified ground check point, what are you reading in the dual VORs? Chances are that it's not some distant VOR since you and it are both on the ground and terrain, building, and the curvature of the earth would prevent an accurate reading. Is

Or is it on the airport where you are? In that case, it's a pretty good bet that it's either a bad signal due to buildings or you're it's in the cone of ambiguity. Maybe the comment was practical rather than legal.

Any real radio gurus out there?

I would love to have the proper reg in hand going into this.

The =only= correct response to an examiner you disagree with during the checkride is "Yes, sir."

 

[cale42]

He actually used the term "not legal" when briefing me on what to expect and said the regs support this. I hadn't the balls to make him name a reg at that point.

 

[U-I pilot]

I've never heard an interpretation of 91.171(c) that requires it to be an airborne check.......

The =only= correct response to an examiner you disagree with during the checkride is "Yes, sir."

(emphasis added) Agreed.
What I would ask him then in this case is this:
"I've noticed the bearing error for ground checks is 4 degrees and airborne is 6 degrees. Why would this airborne check require 4 degrees?"
Good luck..let us know what you find in the end......

 

[pilotman2105]

14 CFR 91.171
...
(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.

The way that I read it says that:

1- You must use either a test signal (VOT) or ground based checkpoint.
2- If those are not available, then you can use an airborne checkpoint

I'd say that the examiner is blowing smoke or doesn't know the rules himself. Most definitely, the rules do not support what he is talking about.

 

[pilotman2105]

Midlifeflyer,

I'm a ham radio operator. We study AM and FM signals including how they're made up and how they propogate.

Regarding the VOR signals and whether they can be distorted, essentially any signal can be distorted for various reasons. Lower frequency signals (AM signals) are generally more prone to distortion and reflection off of the atmosphere whereas higher frequency signals (FM signals) are not quite as prone to the distortion. With that said, FM signals can still propogate (albeit not as readily) off of the atmosphere, but more often they are "bounced" off of solid objects (buildings, the moon, comets). This creates interference at the receiving end.

So to make a long story short, VOR signals may not be received accurately on the ground because of the buildings and other solid objects which may be bouncing signals around all over the place. My guess is that where checkpoints are established are places where this interference has been checked and found not to be an issue.

 

[midlifeflyer]

The way that I read it says that:

1- You must use either a test signal (VOT) or ground based checkpoint.
2- If those are not available, then you can use an airborne checkpoint

I'd add a third to this. Clearly the regulation describes a preference for definite ground checkpoints than for airborne ones. Understandable since a VOT or sitting still on a specific point on the ground and receiving a designated radial has far more accuracy that flying circles around a factory to see if you receive the 256 Radial.

But then there's

3- Instead of doing one or two, you may test VORs against each other.

That's the one we're dealing with in the original scenario. Can that be on the ground, or as the DE in this case thinks, should it/must it be in the air?

Thanks for your Your comment

So to make a long story short, VOR signals may not be received accurately on the ground because of the buildings and other solid objects which may be bouncing signals around all over the place. My guess is that where checkpoints are established are places where this interference has been checked and found not to be an issue.

That tends to support the theory that method 3 should be an airborne check since it doesn't have the protection of being s specific place where the interference has been checked.

 

[DaveJ]

I'm an electrical engineer (as well as a ham operator), so also know a little about radio propagation. Yes, radio signals can be reflected, refracted and interfere with each other to give inaccurate indications, but what you're really checking with the VOR ground check are that both VOR receivers agree with each other. Let's say that due to local building reflections, a signal from a distant VOR indicates 45 degrees on the ground, but 47 degrees in the air over the same spot. It really doesn't matter. Just so long as both VOR receivers (which are both receiving the exact same signal and should read the same) read within +/- 4 degrees of each other you're good to go.

 

[A Squared]

I'm an electrical engineer (as well as a ham operator), so also know a little about radio propagation. Yes, radio signals can be reflected, refracted and interfere with each other to give inaccurate indications, but what you're really checking with the VOR ground check are that both VOR receivers agree with each other. Let's say that due to local building reflections, a signal from a distant VOR indicates 45 degrees on the ground, but 47 degrees in the air over the same spot. It really doesn't matter. Just so long as both VOR receivers (which are both receiving the exact same signal and should read the same) read within +/- 4 degrees of each other you're good to go.

I was thinking along these exact same lines. Regardless of how much distortion, refraction and/or multipath the signal is affected by, if the signal at a given spot on the ground is coherent enough to produce a stable indication in a VOR receiver, the two receiver check is just as valid as it ever is. All the two receiver check does is answer the question: Given the existing signal at this point, whatever it is, do my two receivers interpret it and display it as the same radial, within 4 degrees.

That is all the two receiver check ever does, which is something that should be borne in mind. It's entirely possible to have 1 VOR receiver which is off by 10 degrees and the other off by 13 degrees yet they still pass the 2 receiver check.

As far as the examiner goes, I think that this is yet another case of a Fed making up and enforcing his own private FARs Unfortunately, that is a fairly common phenomenon.

 

[pilotjc]

I think what the examiner is referring to is the VOR's standard service volume. If you are below 1000' AGL there is a limitation on how far away from the VOR station you can receive an accurate signal. See AIM Para. 1-1-8 figures 1-1-4 and 1-1-5 for the distances from the VOR.

 

[A Squared]

I think what the examiner is referring to is the VOR's standard service volume. If you are below 1000' AGL there is a limitation on how far away from the VOR station you can receive an accurate signal. See AIM Para. 1-1-8 figures 1-1-4 and 1-1-5 for the distances from the VOR.

Yeah, but if you look at the charts, they suggest that at ground level you'd be within the standard service volume of a terminal VOR within 8 NM of the VOR and a standard VOR within 10 NM. That desn't seem to support the examiner's position at all unless the VOR is 15 miles from the airport.

 

[nosehair]

cale42, your initial post uses a phrase which is somewhat confusing: you say "dual vor ground check". The dual vor check in 91.171(c) is checking one vor against another. Either on the ground or in the air. Makes no difference. And the permissible variation between the two is 4 degrees. On the ground or in the air. The two airborne checks stated in (b)(3)&(4) is 6 degrees. But these are not dual vor checks as described in (c). Ground checks are described in (2) as a point on the ground designated by the Administrator. Not dual vor checks conducted on the ground.

Your post also states your information comes from a buddy who busted his cfii ride. This is not to denigrate or alienate, but to educate. Most times someone busts a ride, it is because they are confused with the facts, and maybe this is a possibility. Consider that.
Finally, if I were in your shoes again, I would ask the examiner, after he handed me the temporary cfii certificate, how to explain to students the purpose of paragrapg (c). Maybe there is somthing there we don't get.
But you have to have all of your facts in (a)(2) and (b) in concert, which is what I am guessing your friend did not have.