How does the VOR To/From flag work?

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hangar3
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How does the VOR To/From flag work?

Post by hangar3 »

Hello!

But what is the algorithm to display the To or From flag? Is the VOR connected to the heading indicator? :?

I am familiar with how to use the VOR to intercept radials, and I understand the purpose of the To/From flag. I also understood that the VOR can identify which radial the plane is on by measuring the phase difference between an "omni" reference signal emitted in all directions, and a "directional" signal that is rotating around the radial. However, I can't seem to find any explanation of what makes one flag or the other come up from a technical standpoint.

Thanks!
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photofly
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Re: How does the VOR To/From flag work?

Post by photofly »

The needle centres when the phase difference between the two signals you described reaches either 0 or 180 degrees. The TO/FROM indicator indicates whether the phase difference is between about 95 and 265 degrees (it shows "TO") or between about 275 and 85 degrees (it shows "FROM"). Outside those ranges the indicator is typically 'flagged'.

Is the VOR connected to the heading indicator?
No. That's the biggest mistake anyone can make while learning to interpret a VOR. The VOR indication has nothing at all to do with your heading, or which direction you happen to be flying.
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trey kule
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Re: How does the VOR To/From flag work?

Post by trey kule »

50+ years of flying,,and the question of how the flag actually worked never entered my mind.
Learned something new today which justifies the 1000 hours I have spent on Avcanads
You guys probably know all sorts of interesting things about the avionics...
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photofly
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Re: How does the VOR To/From flag work?

Post by photofly »

1000 hours? We're all friends here.
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hangar3
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Re: How does the VOR To/From flag work?

Post by hangar3 »

Hey photofly!

I don't think I follow :( Here's what I understand:

Say you pick a radial, 030 and you set your VOR to it. You are on the top right side, heading 030, away from the VOR on the 030 radial.

1) You set 030 on the VOR, so it knows you want a 30 degree phase shift from the omni signal.
2) The plane will recieve the omni and directional signals with a phase shift of 30 degrees.
3) You will be showing as being on the radial.

For the flag to show "To" on the 030 radial, the plane would need to be actually recieving the 210 signal.
For the flag to show "From" on the 030 radial, the plane would need to be actually recieving the 030 signal.


So now suppose you are on the 030 radial on the top right and heading 030. If it shows "From" like I said above, it makes sense.

But if you are on the 030 radial on the top right of the picture and heading 210, with the 030 radial picked, you are still recieving the 030 signal, so it would think you are going "From," with my logic from above. :roll: For me it would only make sense if the VOR picked up the 210 signal.

I'm not sure if it's because I missed the fundamental functioning of the VOR to begin with or what.

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photofly
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Re: How does the VOR To/From flag work?

Post by photofly »

But if you are on the 030 radial on the top right of the picture and heading 210, with the 030 radial picked, you are still recieving the 030 signal, so it would think you are going "From," with my logic from above. :roll: For me it would only make sense if the VOR picked up the 210 signal.
It doesn't matter what your heading is, if you're located on the 030 radial and you have the 030 radial set on the OBS the flag will show "FROM" - the VOR receiver doesn't know or care about your aircraft heading - it's all about where you are and nothing about which way you're pointing.
I'm not sure if it's because I missed the fundamental functioning of the VOR to begin with or what.
It sounds like it. The words "TO" and "FROM" on the display - they don't mean "you are flying TO/FROM the beacon" - but you can understand them to mean "If I make my heading match the OBS then I'll be flying TO or FROM the beacon." Many VOR displays don't use the words TO/FROM - they have arrow-heads instead, which is less prone to misinterpretation.

There are lots of different ways to interpret a VOR display - it's worth studying some of the others.
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kevind
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Re: How does the VOR To/From flag work?

Post by kevind »

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Trematode
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Re: How does the VOR To/From flag work?

Post by Trematode »

The TO/FROM flag is entirely dependent on your OBS selection. For any given lateral position around the station, there are two values you can select that will center the CDI -- the radial you're currently on, and its reciprocal. The TO/FROM flag is how the instrument differentiates between the two.

The signal from the station doesn't change. The way the instrument is presenting that information to you does, based on what you set up with the OBS.

OBS stands for Omni-Bearing Selector -- it might behoove you to think of this in terms of "Bearing To" and "Bearing From The Station", because that's exactly what it's showing you. Remember bearing is not the same thing as heading.
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hangar3
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Re: How does the VOR To/From flag work?

Post by hangar3 »

Trematode wrote: Remember bearing is not the same thing as heading.
Got it! :mrgreen:

Thanks everyone, I understand now!
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tango308
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Re: How does the VOR To/From flag work?

Post by tango308 »

Good trick for anyone stumbling onto a VOR question on a TC exam, replace the aircraft position on the annex with a large dot. This way you wont be led to think of the aircraft heading when you go to solve the question.
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