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Wild VOR Hunting

Air Badger Guest

Ok gentlemen, lets talk Turkey! Or at least let's talk VORs.

Inspired by my recent adventure in navigation (chronicled elsewhere in these very pages) but equally frustrated by my inability to make sense out of the VOR needle, I decided to try a little experiment to find out whether my problem was truly down to my being a bit thick when it comes to using a compass.

Having saved my last flight at Gatwick, I decided to start from there and fly to Bournemouth via the VORs at Mayfield and Goodwood.

After passing Mayfield, I set Nav 1 to Goodwood and rotated the OBS dial to 90 degrees while I continued to fly “roughly” toward it on a South West course.

My grand theory was that if I understand the basics of this correctly, the VOR needle should drift slowly toward the center as I fly on and if I turn West at the point it reaches the center I should then be flying Goodwoods 90 degree radial! Right?

Then (again if I understand the premise) if I stay on that course I will exit via the 270 degree radial which takes me more or less to Bournemouth….. Huzzah!

Well things started well enough with the needle starting its inward journey at about 26 miles out. I kept an eye on it and at 17 miles it centered and with a grin of triumph I turned West.

The needle held steady and I really thought I’d cracked it! 15 miles, 10 miles, it still looks good, but then at 5.5 miles out it swings violently to the left. Well I ignore this because I’m pretty sure I’m doing it right and I know that when you get close the needle can get wild, so you shouldn’t chase it.

After (presumably) crossing over Goodwood, I rotated the OBS to 270 degrees, expecting to see it at least make an attempt to center again as I continued West. But frankly it didn’t really make much of an effort, taking a good 11 miles to get as far toward the center as it was ever going to. (At a guess I’d say about 20 degrees off… but we Badgers are not renowned for our measurement guesses.)

So here’s the thing then………

Assuming I got the first bit right and the wild swing at 5.5 miles out was to be expected… then what’s the point? I mean all that the needle managed to do while it was centered was point me at the VOR and frankly I could have done just as good a job of that by following my nose and the map!

I can see that entering and exiting of specific radials would be an excellent thing, but what did I do wrong on the way out? And how far out can I expect to be able to intercept a radial in any case?

Not sure I’m ever going to get along with this VOR thing! I think I might be better taking the car.

Badger

PS: On the plus side, while parking at Bournemouth I found and used my first fuel station!! 🙂

8 Responses

Pro Member Chief Captain
CRJCapt Chief Captain

I got lost in the story...I hope this helps.
http://www.navfltsm.addr.com/vor-nav.htm
http://stoenworks.com/Tutorials/Understanding%20Vors.html

The purpose of the VOR system is to go TO or FROM a VOR only. It can also fix your position but basically it designed to go from one station to another. It must be used with the heading indicator, it won't point you in the correct direction. The VOR only senses your location and the location of the selected course, nothing to do with heading. You have to figure that by using correct VOR navigation procedures. The reason it's better than just following a compass heading is that it automatically corrects for unknown winds that would blow you off course if following a compass heading. Dead Reckoning navigation(following a compass course) also requires checkpoints on the ground to keep track of course alignment and progress along course. Winds different than forecast, sparse terrain or at night, this can be difficult. The VOR gives continuous indication that you are on the correct course and station passage gives a positive indication of location. The use of cross radials of another VOR or the use of DME information makes the VOR system even better.

Air Badger Guest

Thank you for the links CRJCapt 😀

Sorry you got lost in the story. Why use one word when you can use a whole paragraph I say! Embarassed

Still I was hoping that such a detailed description might lead to some of the the clever folk here at "Fly Away" momentarily releasing their grips on their joysticks, pointing at the screen and saying... "THAT's what he's doing wrong! It's obvious!"

I'll sit down with a cuppa and make my way through the links though. Perhaps one of them will open my eyes to that elusive light of wisdom, which I've managed to miss in the other bits and bobs I've read about it.

Actually I found myself having a sudden thought about this last night anyway and it suddenly occurs to me that perhaps the problem is the way I perceive the VOR signal in my head.

In the example above I dialed in the east radial on the OBR, because I figured that from a birds eye view, even though I was flying west, the east radial was the radial closest to me.

But perhaps I am over thinking this... I want to exit the VOR to the west, so perhaps I should have ignored the east radial in favour of simply rotating to 270 degrees in the first place!

I see another experiment coming up later on 😀

Thank's again for the help

Badger

Pro Member Chief Captain
CRJCapt Chief Captain

Maybe this will help also. ➡ http://www.luizmonteiro.com/Learning_VOR_Sim.htm

Air Badger Guest

Thank's again. But I think I more or less have my head around it now.

I've even experimented with setting NAV2 and the second VOR dial to a different station, so joining a second flight path at an exact point off the first.
(I thought that up myself I did... I'm a pilot I am!)

Tell me this though, at what range can I effectively intersect with a VOR radial? Is it the range at which the NAV radio can pick up the signal? (Which seems to be about 195 miles.)

Badger

Pro Member Chief Captain
CRJCapt Chief Captain

Real world, the VOR has three different classes with different service volumes(usable range) for different altitudes. The Low altitude VOR has a guaranteed service volume from 1,000 feet AGL to 18,000 ft. AGL of 40 nautical miles. That's the minimum, maximum depends on terrain and your altitude(it's line of sight). If you're getting a good signal without a warning flag, it's good.

Air Badger Guest

Thanks CRJ. Sorry if I'm being a bit thick but I'm not sure I'm clear on the question. In FS9 (as opposed to real life) are you saying that the higher I go the further the range? Is there no rule of thumb?

Badger

Pro Member Chief Captain
CRJCapt Chief Captain

Air Badger wrote:

Thanks CRJ. Sorry if I'm being a bit thick but I'm not sure I'm clear on the question. In FS9 (as opposed to real life) are you saying that the higher I go the further the range? Is there no rule of thumb?

Flight Simulator
In FS9,(using DAY, APE and MXQ VOR's)
High Altitude VOR- Max of about 195 nm.
Low Altitude VOR - Max of about 60 nm.
Terminal VOR- Max of about 37 nm.

The effects of altitude of VOR range seem to be simulated in FS using a basic formula at the low altitude side. Once you're at an altitude which gives a reliable signal, an increase in altitude does not seem to increase range.The maximum range is shorter at low altitudes, this simulates the loss of signal due to the curvature of the earth. The difference in range based on type of VOR, simulates difference in transmitter power.

Real world
To a point, higher you go the longer the distance you can receive a VOR signal. This is because the signal is line of sight. The curvature of the earth blocks the signal at low altitude and long distances. Different classes of VOR have different transmitting power resulting in longer range for the High altitude VOR and shortest for the Terminal VOR. There's probably is a basic rule that uses the altitude of the aircraft and the known curvature of the earth to determine when line of sight will be lost but I've never seen it. The service volumes below are guaranteed areas you should receive a reliable signal, you may receive a good signal outside of these volumes. The maximum range varies for each VOR, influenced by factors* and normally not calculated. The warning flag on the VOR will display when maximum range or loss of signal occurs.
*Factors that influence VOR range:
1. Elevation of terrain the VOR is located.
2. Altitude of aircraft above the terrain.
3. High terrain or buildings between station and aircraft
4. Class of VOR(signal strength)

Losing a VOR signal in good weather(VMC) is not a big problem. You would just maintain your present heading until receiving the next station. If there is no other station, you would revert to Dead Reckoning navigation(Time, Speed, Distance).
High Altitude VOR

Low altitude VOR

There is also a Terminal VOR that has a service volume of 25 nm up to 12,000 feet, normally used only for VOR approaches or short range navigation near a airport.

Air Badger Guest

Thanks for going to all that effort! 😀 I'll definitely try not to ask any more VOR questions now I'm sure that you've covered it all between links and explanations.

Of course I'm not making any promises!
😂

Badger

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