VOR

[PompeyPaul]

Any freebie programs \ websites or info that explains how this crazy device works ? I don't really understand it and whenever I answer a question on it, it appears to be more guess work than anything.

Although the answer always seems simple after somebody told me.

I.e. if I am on a 90degree radial FROM a VOR, what bearing do I fly (assuming no wind) to arrive at the for ? It's the inverse which is 180 degrees. But what happens after I fly over the VOR ? Do I have a 90 TO on the VOR now ? Do I have 180 FROM ?

How on earth do I read the VOR component itself to see what it's telling me ? I can't even read the degrees ?

Any freebie programs, or web pages that go into it are gratefully received.

[Andy_RR]

MS Flight Simulator is great for fiddling with and learning your RNAV stuff.

[hobbit1983]

Try this page http://www.visi.com/~mim/nav/

Provides graphical representation in real time of two VORS/ADFs. Takes a little getting used to but is quite useful.

MSFS is also quite good for this kind of thing, and isn't too expensive these days either.

[Ultralights]

if you are on the 090 Radial FROM the VOR, then your heading would be 090 Deg..

if you were flying TO the VOR on the 090 radial, your heading will be 270 deg, but you will still be on the 090 radial, until you cross the station and then begin flying FROM the station on the 270 Radial.

[BackPacker]

PompeyPaul,

Think of the VOR beacon as two separate beacons:
- One that does a very short "beep" exactly on the top of the second.
- Another that does a continuous "beep" but sweeps a narrow beam through 360 degrees in one second, pointing to magnetic north exactly on the top of the second.

The VOR receiver you have in your airplane measures the time difference between the two beeps. If the time difference is, for instance, a quarter of a second, you are 90 degrees magnetic from the VOR. If it's half a second, you are 180 degrees magnetic from the VOR. And the accuracy is theoretically infinite. If you've got equipment calibrated to 0.01 degree, and the VOR is also calibrated to that precision, you could fly a 123.45 radial if you wanted to. But such accuracy is generally not needed.

(Yes, this is technically not entirely accurate, but it works for now.)

The VOR equipment in the plane doesn't show these bearings as such, but shows it through the OBS display. Now the design of the OBS display is a little unexpected for a first-time user, but the reasoning behind it is that a pilot does not need to know his present position. He already knows it. Rather, he wants to fly a certain track or intercept a certain track. So the pilot sets whatever he wants to achieve on the OBS, and then the OBS tells him what he needs to do.

And with a bit of fiddling you can still let the OBS tell you where you are in reference to the VOR.

How to fly directly TO a VOR: Fiddle the OBS ring so that the flag shows TO, and the needle is in the middle. Then fly the magnetic course which is on top of the OBS. (Compensating for wind of course.)

How to fly directly FROM a VOR: Fiddle the OBS ring so that the flag shows FROM, and the needle is in the middle. Fly the magnetic course again.

How to intercept a course TO a VOR: Set the course to intercept on the OBS ring. If the needle is left of the center, fly 30-45 degrees left of the course to be intercepted until the needle centers. If the needle is right, fly 30-45 degrees right.

Intercepting a course FROM a VOR: Same thing. Set the course to intercept. Fly left of that course if the needle is left, fly right of that course if the needle is right.

How to determine your position (bearing) FROM a VOR? Fiddle the OBS ring so that the needle is centered and the flag shows FROM. Note the number at the top of the OBS ring. Take your map and draw a line FROM the VOR with this magnetic bearing. Your position will be along this line. (You need two VORs, or a VOR/DME to get your position.)

You can also fiddle until you have the TO flag and read off the bearing. But then you've got to draw a line from your aircrafts position TO the VOR along that bearing. Useful if you know where your aircraft is and the position of the VOR is unknown... :-) If you have a TO flag with the needle centered, add or subtract 180 degrees and you've got the bearing FROM the VOR TO your aircraft. Or just rotate the OBS through 180 degrees to get a FROM flag with the needle centered.

Ref your specific questions:

1. You are on the "90 degree radial and the flag shows FROM" (and the needle is centered I presume). This simply means that if you fly 90 degrees, then you are flying 90 degrees away FROM the VOR. So to fly directly TO the VOR you need to fly 270 degrees (90+180).

Note however, that if you just turn the aircraft around to 270 deg but leave the OBS ring as it is (90 degrees), it will show a FROM indicator (because 90 degrees would take you away FROM the VOR) and the needle will work backwards. This is called "reverse sensing". Don't do it. Always set the OBS ring to the magnetic course you want to fly (when following a VOR radial) or the magnetic track (radial) you are intercepting. Then TO and FROM only mean that the VOR is in front or behind you.

2. If you are flying a certain radial TO a VOR, then the TO flag will show. If you get close, you get into the "cone of confusion" for a minute or so where the flag will show an error (striped red/white most likely) and then the FROM flag will show. This means that you've passed the VOR and are now flying away FROM it.

MSFS works great for this, as mentioned. But if all you want is to understand VORs, then go to 3000 feet in the vicinity of a VOR and put it in "slew" mode. All of a sudden the laws of aerodynamics no longer apply, and you can hover, fly backwards, rotate on a pinpoint, you name it.

Just remember - the VOR does not tell you your present position. Rather, the VOR is something that you set your intentions in (via the OBS ring) and the OBS then tells you what you need to do to achive that.

[PompeyPaul]

Thank you all, especially backpacker, I think I get it now!

[PompeyPaul]

Just took one of these freebie quizes http://www.exams4pilots.org/

Passed all of the VOR questions :)

[BackPacker]

You're very welcome. Just remember that the CAA exam is about not-so-real-life situations, for instance what the VOR will show when you're in a particular sector. So from the VOR display you've got to work out in what sector you are. Normally, in real life, you use the VOR the other way around. You know what sector you're in, you just need to intercept & follow a certain radial.

[foxmoth]

This is quite a good one to play with
http://www.relia.net/~george/aviation/sim/
Whoops, just edited to say, yes it is the same as the other one:ooh:

[IO540]

I tear my hair out when I read most descriptions of how a VOR works. Not to say any of them are wrong (the actual technical ones are usually wrong but that doesn't matter). As an electronics engineer I even once did a design of a VOR receiver, many years ago. But I suppose if you are going to write a book called "VOR and ADF", and you want to sell it in the common pilot shops, then it needs to have a price tag allowing a decent trade margin, so it needs at least 200 pages...

For a simple explanation, I like to think of a VOR as a lighthouse, but one working in radio waves and not with visible light.

With one twist on the real lighthouse though: it has different colour glass windows fitted on different bearings, so if you see e.g. red light you know you are somewhere around say 230 degree bearing from it.

That's what a VOR gives you: a rotating beam, whose instantaneous bearing is encoded onto the radio signal. The aircraft receiver decodes the information and presents the bearing you are sitting on. Very simple!

The difficult bit is that the receiver does't give you the bearing literally e.g. "230 degrees". (You can get that but it costs extra) VORs go back to the 1950s, before transistors so everything had to be simple, and very importantly they were designed for flying long cross country legs. So the receiver is designed to facilitate the flying of long legs, and the presentation thus consists of the bar that moves left to right, and the two flags. This is what confuses people. The contentional VOR receiver presentation is good for long legs, and a real pain for more practical stuff like position fixing.

Everything which the receiver presents (i.e. the bar deflection and the flag settings) is derived solely from the received signal (the radial) and from the position of the OBS dial. The rule used is simple enough but almost impossible to describe without a diagram.

In the intended use (flying long legs) it's very simple to use. The problem comes when trying to sit the IR exams; in those they throw at you trick questions about which quadrant etc you are in. There are various clever ways of working those out. But AFAIK this stuff is never used in real flying.

If you fly around with one of those ICOM handheld radios which receive a VOR and display the radial in plain degrees, you realise it's actually totally trivial and obvious. All the confusion comes from trying to understand the standard receiver instrument.

[GroundBound]

A quick Google for "VOR navigation" will give you heaps of information.

Here's another simulator (http://www.luizmonteiro.com/Learning_VOR_Sim.htm)

And an another explanation (http://www.navfltsm.addr.com/vor-nav.htm)

GB

[neilia]

Paul,

The best way to approach VOR questions is to always draw a picture. And the basic concept you need to clear up in your head is that the radial you are ON is completely independent of the heading you are flying.

To take the example you gave: "If I am on a 90degree radial FROM a VOR, what bearing do I fly (assuming no wind) to arrive at the VOR?What happens after I fly over the VOR? Do I have a 90 TO on the VOR now? Do I have 180 FROM?"

To deal with this, draw a picture, which is basically a compass rose. The VOR itself is the dot in the middle, the 90 radial is a line out from the VOR at 90 deg, the 270 radial is a line out from the VOR at 270 deg etc etc. Now place your aeroplane ON the 90 deg radial. It should be immediately obvious that the heading you have to fly to get TO the VOR is 270 deg. Imagine now you've overflown the VOR. Place your aeroplane on the picture again - it's on the 270 radial now. What heading would you now have to fly to get TO the VOR? 90 deg. What heading would you have to fly to get away FROM the VOR? 270 deg.

TO and FROM are as simple as that - what HEADING must I fly to get TO the VOR? and what HEADING must I fly to get away FROM the VOR?

Hope that helps! It's certainly torture coming to grips with it, but once the penny drops you'll wonder what you ever found confusing ;)