VOR Check Question (Sheppard Air)

[AggieMike88]

Question 1107 from the Instrument Ground Instructor Prep

When making an airborne VOR check, what is the maximum allowable tolerance between the two indicators of a dual VOR system (units independent of each other except the antenna)?

A) 6° between the two indicated radials of a VOR

B) 4° between the two indicated bearings of a VOR.

C) Plus or minus 4° when set to identical radials of a VOR​

Sheppard Air has the correct answer as (b), citing FAR 91.171(c)

"If dual system VOR (units independent of each other except for the antenna) is installed on the aircraft, the person checking the equipment my 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°"​

And Sheppard Air's explanation does include the comment that this question often confuses the test takers.

Is this regulation and question not talking about the common "dual CDI" setup that the majority of us have in our aircraft? If that is correct, can someone supply a photo or image of what this instrument looks like?

I accept the answer to the question to be correct (essentially the question is a rephrase of the regulation). But I'm stumbling on visualizing what this would look like on an instrument panel.

 

[AggieMike88]

I'm just going to memorize the question and answer pair and motor on...

But since this could come up in the future from a student, I'd like to know an illustrative answer that helps them grokk what this is in the real world.

 

[James331]

Like a dual RMI like this or electronic

Also 530s and the like will show the radial and station when a VOR is dialed in.

 

[aggie06]

I think it’s the same dual VOR setup you’re imagining, but there is a difference in setting the bearing TO and determining difference from the radial (FROM). Someone else much more knowledgeable will probably come by soon, though.

 

[AggieMike88]

Also 530s and the like will show the radial and station when a VOR is dialed in.

The 480 in my airplane also does this... and it's a feature I've come to really appreciate.

 

[AggieMike88]

I think I have a better image I can use... The CDI depiction of a single pane Aspen PFD.

IIRC, if the system is setup for it, you can have both VOR1 and VOR2 shown on the display.

But I don't think we see many aircraft with a "stand alone" instrument that does this depiction. Therefore the confusion between the 6° of a typical dual CDI setup and the 4° of the system this question is asking about.

 

[James331]

I think I have a better image I can use... The CDI depiction of a single pane Aspen PFD.

IIRC, if the system is setup for it, you can have both VOR1 and VOR2 shown on the display.

But I don't think we see many aircraft with a "stand alone" instrument that does this depiction. Therefore the confusion between the 6° of a typical dual CDI setup and the 4° of the system this question is asking about.

I was looking for the same picture, many of the EHSIs can show both VOR 1&2 at the same time, I know the sandels and king EFIS do, maybe as EHSIs start to replace vac DGs in the fleet this will become more common.

 

[AggieMike88]

I was looking for the same picture, many of the EHSIs can show both VOR 1&2 at the same time, I know the sandels and king EFIS do, maybe as EHSIs start to replace vac DGs in the fleet this will become more common.

I did find a good image.... from the IFR testing supplement, Figure 81

And it is an instrument that isn't often found in today's "small" aircraft.


And I would agree with them becoming more common as folks update to glass solutions such as Aspen, G5's and others.

 

[ja_user]

At best it’s poorly written

 

[Hawker800]

What’s a VOR?

 

[AggieMike88]

I'm not seeing the issue with the answers provided.

It's almost verbatim from the regulation.

Yes... It is. And I wasn't quibbling over that.....

My confusion was that I have never seen such an instrument in real life, therefore could not relate personal experience to the regulation.

But as you read further down in this thread, we got it figured out by seeing the image from the testing supplement and also the electronic displays such as the Sandel and Aspen units.

 

[AggieMike88]

What’s a VOR?

The thing the FAA likes to use to confuse the heck out of pilots taking written exams.

 

[Ravioli]

IIRC, if the system is setup for it, you can have both VOR1 and VOR2 shown on the display.

But I don't think we see many aircraft with a "stand alone" instrument that does this depiction.

Dynon does it. The HSI can show the VOR indications of my 430W and SL30 (1 antenna) as well as GPS simultaneously.

 

[Tantalum]

What’s a VOR?

#magentaline

 

[DutchRoller]

What’s a VOR?

A three letter abbreviation which includes one letter from another three letter abbreviation.

 

[vancinad]

My confusion was that I have never seen such an instrument in real life...

Could be a single instrument with two needles stacked, but the most common configuration is just two indicators, isn't it?

 

[dmspilot]

This whole thread is a trainwreck

Stop. No idea where this obsession with dual-RMI indicators came from, but the question has absolutely nothing to do with dual-RMI indicators. It's simply asking what the tolerance is allowed under 91.171(c) when checking one VOR against another VOR. I would guess 99% of the GA fleet used for instrument training has such a VOR setup.

Answer choice (c) is incorrect because it would imply that one VOR could be -4° and the other VOR could be +4° which means there is a differential of 8° between the two. Which is not allowed.

There is no "plus or minus" in the wording of a dual VOR check in 91.171(c). If a test question asks you about dual VOR check, any answer that includes the words "plus or minus" is wrong. Boom.

And the only time an error of 6° is allowable is for a designated airborne checkpoint or the made-up airborne checkpoint using an airway and prominent landmark (both of which are ways of testing a single VOR) under 91.171(b)(3) or 91.171(b)(4), respectively. People see "airborne" in the question and do not read it carefully are likely to incorrectly choose answer choice (a).

 

[dmspilot]

Yes... It is. And I wasn't quibbling over that.....

My confusion was that I have never seen such an instrument in real life, therefore could not relate personal experience to the regulation.

But as you read further down in this thread, we got it figured out by seeing the image from the testing supplement and also the electronic displays such as the Sandel and Aspen units.

You are still confused. I'm certain you've flown aircraft with dual VORs many times. See my above post.

 

[Lantraxco]

Yeah, you guys are thinking too new. Dual VOR's means two separate VOR's as it says, "Two units independent of each other except the antenna." You're getting stuck on the word "Dual" which in this instance just means "Not single" as in we added a hole in the panel and installed a second VOR and now we have dual VOR's. Back in my day we hand propped our VOR's because there only was one needle and dadgum it we liked it!

 

[midlifeflyer]

Every time some talks about lawyers parsing words like angels dancing on the head of a pin, it's going to bring up a memory of this thread.

 

[MD11Pilot]

AggieMike
Just took the IGI yesterday and had that question. I too used Shepherd and their explanation helped. Like many others, I jumped on the “airborne” statement but as they pointed out that was just a distraction. At my age, I flew with the dual needles in the past so that part wasn’t new. The one question I missed was the sign off required for a 30 day VOR check. One answer was wrong, the other two were partially correct but I chose the wrong one. Be sure and really understand what they are asking...hint, it had to do with a repair shop test signal....you cannot sign that one off fully. 91.171 (d) says that the person making the operational check 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 is used, an entry must be made in the log or other record by the repair station certificate holder certifying to the bearing transmitted for the check and the date of transmission.

I misread the answers and the one I chose omitted the bearing error. Cost me a 100. I also had never read the requirements for a repair stations statement.

I have a repair shop about 150 yards away from my hangar but haven’t asked them to do this.