Understanding Vors

Understanding VOR's,VORTAC's and How To Use Them

by Hal Stoen

first release: 2 December, 1998

INTRODUCTION

The practical aspects of understanding and using the VOR areso simple that it is confusing. When I used to flight instructI would use a variety of ways to explain how it all worked until"click" and the light bulb went on. Hopefully I willbe able to accomplish the same in written form. So if it all seems"Greek" to you, please plow on and I'll do my best tofind the switch to that bulb.

JUST WHAT IS A VOR, VORTAC ETC.?

The VOR system is the backbone of air navigation in the UnitedStates and most other countries. You may have seen them from theair, while driving through the country or at an airport. Theyusually are round buildings, about thirty feet in diameter witha cone sticking out the top. Many are painted in a red and whitecheckerboard pattern.

VOR is an acronym for Very high frequency Omni Range. VORTACis the same with TAC standing for TACAN, a military designationfor it's distance information on a VOR signal. VOR/DME is a VORwith Distance Measuring Equipment co-located at the site.

When you tune in a VOR the DME will automatically display thedistance to or from the station. Notice however that you mustselect which VOR signal you want the DME to display: Nav 1 orNav 2. There is no other action required on the pilots part toreceive distance information. In X-Plane all VOR's have DME.

In addition there are various classes of VOR's that relateto the distance the signal is usable. The lowest powered of theseis the TVOR (Terminal VOR) and the highest powered are those usedfor the high altitude structure- above FL180. This class is notedon charts with an "H". This is an over simplification,but will suffice for this discussion.

HOW DO THEY WORK?

The VOR radiates an omnidirectional signal- in other wordsit goes out in all directions.
This, versus an ILS signal which is "beamed" in a verynarrow sector. The VOR signal is electrically phased so that thereceived signal is different in various parts of the 360 degreecircle. For simplicity's sake in this discussion I will referto the VOR as if it had 360 radials. In reality the signal isinfinitely variable.

LET'S GET ORIENTATED!

Try picturing yourself standing in a large expansive area.It's dark, and you don't know where you are. But you have a friendin this dark expanse that will use a navigational signal. Yourfriend will fire off a strobe light and then turn on a flashlightthat is pointed right at due magnetic North. Your friend slowlyrotates in place with his flashlight taking six minutes to completea full circle. At that time he fires off the strobe light andstarts all over again.

You see the strobe go off and start counting......at 45 secondsinto your count the flashlight sweeps by. Now you know where youare relative to your friend in that dark room- 45/360 from dueNorth, or 045 degrees. And, that is only place that youcan see that flashlight sweep by 45 seconds after the strobe.

That is a really simple way to think of how directional informationworks in VOR navigation. Like spokes in a wheel the "radials"of a VOR radiate out from the center of the station providingazimuth information to the pilot in his aircraft. Each radialis referred to by its direction of emanation from the VOR. Inother words the radial going out straight East is the 090 degreeradial. The one pointed straight South is the 180 degree radialand so on.

If you are due Northeast of a VOR you are on the 045 degreeradial. It does not matter which direction you are headed. Ifyou are pointed Northeast you are on the 045 degree radial. Ifyou are pointed Southwest you are still on the 045 degree radial.The VOR receiver in the aircraft has absolutely no idea of theaircraft heading- and does not care. The information is displayedas if you were in a saucer-shaped aircraft, without a front ora rear.

There is a potentially confusing word used in VOR discussionsand that is "bearing". Bearing is a relative word. Forexample the right wing tip is 90 degrees to your right as yousit in the front office, the left wing tip is 90 degrees to yourleft. The use of the word was carried over from sailing days intothe aviation lexicon. It is used in VOR work to describe someof the controls of the display, for instance the Omni BearingSelector- the OBS. The OBS was given its name from the beginningand although it may be confusing if taken literally, just acceptit as a term.

THE DISPLAYS

There are several choices of displays in X-Plane, in this discussionwe will use the ones in the Cessna 172. These displays consistof two different types, the HSI and the VOR head. In VOR navigationthey both provide the pilot with the same information, just ina slightly different format.
The HSI:

(For complete information on this display see the HSI.)

The VOR head:

ORIENTATION AND OVER-VIEW:

This is how X-Plane represents VOR's. For this discussion wewill use the Riverside, California VOR. First, notice the frequency-112.4. If you want to use this VOR for navigation that is thefrequency you tune in on your navigation receiver. In addition,notice that the compass rose is orientated with the 000/360 radialpointed at magnetic North. Also note that the "ticks"for the four cardinal points of the compass (000, 090, 180 and270) are slightly larger than the 10 degree "ticks".

So let's look at an overhead view of an airplane that is out there,in that dark room, with a pilot that is trying to figure out justwhere he is in relationship to the VOR. It may help if you canimagine yourself in a saucer shaped aircraft rather that a conventionalone with a "front" and a "rear".

In the above situation if you turn the OBS it will center in twolocations: 090 degrees and 270 degrees. When the CDI is centeredat 090 degrees the TO/FROM indicator will read "FROM".This makes sense when you think about it as you are indeed onthe "090 degree radial from the VOR". In addition,if you were to fly a heading of 090 degrees and track the CDIyou would be flying from the VOR.

When the CDI is centered at 270 degrees the TO/FROM indicatorwill read "TO". If you were to fly a heading of 270degrees and track the CDI you would be flying to the VOR.

In the above example no matter what heading you fly, or whatyou have the OBS turned to, one fact remains: you are locatedon the 090 degree radial.

In the above situation if you turn the OBS it will center in twolocations: 090 degrees and 270 degrees. When the CDI is centeredat 270 degrees the TO/FROM indicator will read FROM. This makessense when you think about it as you are indeed on the 270 degreeradial from the VOR. In addition, if you were to fly aheading of 270 degrees and track the CDI you would be flying fromthe VOR.

When the CDI is centered at 090 degrees the TO/FROM indicatorwill read TO. If you were to fly a heading of 090 degrees andtrack the CDI you would be flying to the VOR.

In the above example no matter what heading you fly, or whatyou have the OBS turned to, one fact remains: you are locatedon the 270 degree radial.

In the above situation if you have the OBS set to 090 the TO/FROMindicator will read FROM. The CDI will be to the right of centerindicating that the 090 degree radial is to your right assumingthat you are flying a 090 degree heading, or a close approximation.Actually, the crossover point for the CDI to read reverse informationis 90 degrees on either side of your OBS selection- in this caseit would be 000 and 180.

In the above situation if you have the OBS set to 270 the TO/FROMindicator will read TO. The CDI will be to the left of centerindicating that the 270 degree radial is to your left assumingthat you are flying a 270 degree heading, or a close approximation.Actually, the crossover point for the CDI to read reverse informationis the same as above, 90 degrees on either side of your OBS selection-000 and 180.

So, in navigating with VORís the practice to keep inmind is that the OBS heading and the aircraft heading should bein the same sector. If you are flying East (090) you would tunethe OBS to 090 degrees and track the CDI from the VOR.

Let me put the cart before the horse for just a minute andthen we will come back to VOR orientation.

VOR NAVIGATION, part 1:

In the above example you are flying from West to East usinga single VOR for navigation. When you are at "A" theOBS would be set to 090 degrees and you would be East bound onthe 270 degree radial. The CDI would be centered and the TO/FROMindicator would read TO. If you drifted off course to the Norththe CDI would be to the right of center indicating that your courseis to the right. If you drifted off course to the South the oppositewould apply. You have the OBS set to 090 degrees because thatis the course you are flying and the direction you are headed.

As you cross the VOR at B several things happen. The CDI willusually peg out to either the far left or right of the indicator.This happens because all of the azimuth information is concentratedinto such a small area that it is virtually impossible to keepthe CDI centered over the VOR. This area is also cryptically knowas "the zone of confusion". So don't go chasing theneedle as you get close to the VOR- the heading that got you therewill work for you as you cross over.

O.K., now you have crossed B and are flying towards C. Whathappens? Nothing. Your course is still East bound so the OBS settingremains at 090. As you cross over the VOR the TO/FROM indicatorwill change from TO to FROM. Also, before you crossed the VORyou were navigating to the VOR inbound on the 270 degree radialand now you are navigating from the VOR outbound on the 090 degreeradial. And, just as before, if you drift off course to the Norththe CDI will be to the right of center to indicate that your desiredcourse is to the right, and the opposite applies if you driftto the South of course.

Now someone is saying "Hmm, what if I flew from pointA to point B and set the OBS to 270 degrees instead of 090 degrees?"Outside of confusing the living heck out of yourself the answeris "nothing". Can you do it? Sure, but all course indicationsfrom the CDI would be opposite readings. Navigation can be difficultenough as it is without going out of your way to confuse yourself.

BACK TO ORIENTATION :

Going back to basic orientation now, letís see if thisall makes sense to you.

If you are at point A and turn the OBS to 090 degrees whatwill the display be? Turn yourself to a heading of 090. The 090degree radial is off to your right, and that is where the CDIwould be to indicate that your selected course, 090, is to theright. What about the TO/FROM indicator? If you were to draw aline 90 degrees on either side of the selected course (180 and000) you would find that you are on the 090 degree side from theVOR and therefor the flag indicator would read FROM.

If you are at point A and turn the OBS to 180 degrees whatwill the display be? Turn yourself to a heading of 180. The 180degree radial is off to your right, and that is where the CDIwould be to indicate that your selected course, 180, is to theright. What about the TO/FROM indicator? If you were to draw aline 90 degrees on either side of the selected course (270 and090) you would find that you are on the 000 degree side from theVOR and therefor the flag indicator would read TO.

If you are at point B and turn the OBS to 270 degrees whatwill the display be? Turn yourself to a heading of 270. The 270degree radial is off to your right, and that is where the CDIwould be to indicate that your selected course, 270, is to theright. What about the TO/FROM indicator? If you were to draw aline 90 degrees on either side of the selected course (180 and000) you would find that you are on the 090 degree side from theVOR and therefor the flag indicator would read TO.

If you are at point C and turn the OBS to 180 degrees whatwill the display be? Turn yourself to a heading of 180. The 180degree radial is off to your left, and that is where the CDI wouldbe to indicate that your selected course, 180, is to the left.What about the TO/FROM indicator? If you were to draw a line 90degrees on either side of the selected course (090 and 270) youwould find that you are on the 180 degree side from the VOR andtherefor the flag indicator would read FROM.

If you are at point D and turn the OBS to 180 degrees whatwill the display be? Turn yourself to a heading of 180. The 180degree radial is off to your left, and that is where the CDI wouldbe to indicate that your selected course, 180, is to the left.What about the TO/FROM indicator? If you were to draw a line 90degrees on either side of the selected course (090 and 270) youwould find that you are on the 000 degree side from the VOR andtherefor the flag indicator would read TO.

VOR NAVIGATION, part 2:

In the above illustration you are at the Crystal Airport andwish to fly to the Oxford Airport using VOR navigation. Throughclever measuring on your part you know that the Farmington VORis exactly 090 degrees from Crystal, the Magnolia VOR is 135 degreesfrom the Farmington VOR and that the Oxford airport is 180 degreesfrom the Magnolia VOR.

Before takeoff at the Crystal Airport you tune in the FarmingtonVOR on your navigation receiver and set your OBS to 090 degrees.Airborne, you listen to the VOR and verify that you have the correctstation. As long as you are not flying on a designated airwayit is perfectly acceptable to recenter the CDI if necessary. Flyto the Farmington VOR and when the TO/FROM indicator flips toFROM turn to a new heading of 145 degrees to intercept to 135degree radial outbound. Next turn the OBS to 135 degrees and interceptthe radial. The TO/FROM indicator will indicate FROM.

If the Magnolia VOR was not too distant from Farmington anda good signal was received it would be perfectly acceptable toeliminate tracking outbound from Farmington and instead tune inMagnolia and fly directly to it. For illustration purposes thoughlet's say that Magnolia is too far to receive a decent signalwhen you are at Farmington. Track outbound from Farmington onthe 135 degree radial until you are halfway to the Magnolia VOR.At this point Tune in the Magnolia VOR, identify, center the CDIwith the OBS and fly to Magnolia. The TO/FROM indicator will readTO.

In actual practice, as long as you have two VOR receivers onboard,you would tune Nav-2 to Farmington and Nav-1 to Magnolia. Nearthe halfway point you would verify the Magnolia VOR on Nav-1 andnote that the CDI was near center with 135 degrees set in thewindow. At the halfway point you would select Magnolia with Nav-2.

Upon reaching the Magnolia VOR turn the OBS to select the 180degree radial. Turn to intercept the radial and track it untilyou reach the Oxford airport.

FLYING AN ESTABLISHED AIRWAY:

As I stated at the beginning this is not meant to be an IFRprimer. For complete and thorough IFR lessons see Andrew AyerísIFR Tutorial. Having said that, let's take a brief peek at howan airway looks and how you would fly it.

In the above illustration the established route between theFarmington and Magnolia VOR's is Victor 168. It is 100 nauticalmiles between the stations. For some reason unknown to us thepoint where we switch VOR's to navigate on is not at the halfwaypoint of 50 miles but is instead at 40 miles from Farmington and60 miles from Magnolia. This may be because Farmington is a lowpowered VOR, obstructions that degrade the signal quality, etc.

Flying from East to West you will be tracking outbound on the271 degree radial from the Magnolia VOR and inbound on the 089degree radial to the Farmington VOR. Inasmuch as this is an establishedairway, Victor 168, you would fly these radials if you were operatingIFR.

Whoa, let's back up a second here. Is that correct? Outboundon the 271 radial, and inbound on the 089? Shouldn'tthat be "Outbound on the 271 and inbound on the 091"?Well, Grasshopper, you would think so. And, that would be truewere it not for the Magnetic North Pole. There are lines of magneticvariation across our globe, and compass readings must be "adjusted"as your angle from the Magnetic North Pole increases or decreases.This is most notable when flying East/West routes, and less sowhen flying North/South routes.

If you were operating VFR you are under no obligation to flythe established airway. In fact, many pilots fly the airways witha several mile "offset" to decrease the chances of conflictingtraffic.

Flying from Magnolia you would set your outbound course to271 degrees and the TO/FROM indicator will read FROM. When 60miles out from Magnolia you would change over to the FarmingtonVOR for navigation and set the course indicator to 269 degrees,the reciprocal of 089. The TO/FROM indicator will read TO. Onnavigation charts the radial is always the course shown.

USING THE VOR AS AN AID IN AN APPROACH:

Once again I invoke the "read the IFR tutorial clause".This illustration is just to help you get orientated in usingthe VOR as an approach aid.

Note that the 114 degree radial from the HUT VOR is used as alead-in indicator. If you were flying the approach you might beassigned a DME arc by ARTC. As you fly the arc, based in thiscase off of the HUT VOR, you would have your Nav 2 receiver tunedto the ILS frequency of 110.1 and the course indicator set to131 degrees (your Nav 2 display is an HSI and this is how youset up a back course on that type of display). Your Nav 1 receiverwould be tuned to the HUT VOR and the course indicator would beset to 114 degrees. The TO/FROM indicator would read FROM, assumingyou are in the Southeast quadrent. As the CDI on Nav 1 startsto center (from left to right) you would be preparing to turnto 311 degrees and intercept the localizer back course approach.

Note also that the IAF (Initial Approach Fix) STORG is basedon the 079 degree radial from the HUT VOR. At some point whileintercepting and tracking the localizer you would reset the courseindicator on your Nav 1 display from 114 degrees to 079 degrees.

As you approach STORG on the approach the CDI will be displacedto the left side of the display and then begin moving towardscenter. The TO/FROM indicator would read FROM. When the CDI reachesthe exact center point of your display you are at STORG and wouldbegin your descent as appropriate.

SUMMATION:

I hope that I was able to find that "switch" thatmade understanding VOR's click in your mind. Once you understandhow they operate and are used in aviation navigation they trulydo become easy to utilize.

As always I request that if you find any errors in this presentation,or if I didn't make things clear enough, that you please contactme so that I may make corrections.

This tutorial is available on aCD

This tutorial, along with additional content, is availableon a CD. Click here formore information.

Hal Stoen

revised: 10/06/00

clickto return to the index