VOR Navigation Basics for Aviators

Let's admit it. VOR navigation isn't the most user friendly way of getting around, but once you understand a few basics and some recipes on how to use the darned system, it's really not that bad.

I am writing this blog post with the student pilot in mind, but hopefully the private pilot who never really understood VOR navigation will get something out of this posting.

There are two ways to work with VORs: the concept and the actual use. The concept is something you do with simulators or at home with pencil and paper. A very good and free simulator can be found here and I affectionately call it Tim's Airnav. Actual use of the system as I describe below means understand what you want to do, then follow the recipe for that item. If you are in the cockpit and working on the conceptual part, you are in the wrong place. You should be home at the kitchen table. The cockpit is a great place to make sure you can use the VORs, but not a place to understand how they work. The degree to which this is true is proportional to turbulence and how fast the weather is closing in.

There are three things you will normally do to navigate with VORs:

1. Intercept a radial and fly inbound (inbound means fly towards the VOR)
2. Intercept a radial and fly outbound (outbound means to fly away from the VOR)
3. Go direct from your current position to a VOR

Number 3 is really used once the radials mentioned in 1 and 2 are intercepted or it can be a "go direct to the Puddytat VORTEC" directive from your instructor or air traffic control).

Now, before I go any further, some concepts we must agree on or we will not get anywhere and I will be talking into the cone of confusion or perhaps addressing the zone of ambiguity.

A single VOR station is a group of antennas on the ground that fire at different times and emit radio signals during each transmit cycle. Your cockpit VOR receiver receives these signals and allows you to determine on which of 360 radials your aircraft is located. A radial is an electronic "line of position" (navigators say L O P). The radials are numbered 000 to 359. The 000 can also be referred to as the 360 degree radial. A technician turns a knob at the ground station and adjusts the ground track of the block of radials so that the 000 (360) radial is pointing at magnetic north for that particular spot on the earth. Remember that the angle of magnetic north from geographic north varies from point to point on the surface of the earth - that's why it's called "variation". Having radials reference magnetic points on the compass simplifies the life of the pilot because his references to direction are made via magnetic references (compass, magnetic flux detection, etc.) just as runway numbering simplifies our life because they are numbered by magnetic direction.

Next concept, the radial. Think of the VOR transmitter as a dot on the ground from which 360 lines emanate. These lines are called radials. Each line is numbered 000-359. Each line is a radial. Whoops, I repeat myself, but I did it intentionally. A radial always comes FROM (note the emphasis on "from") the VOR. For example, the 090 radial (magnetic direction is always expressed in 3 digits with leading zero(es) if necessary) radiates from the VOR and makes a ground track towards magnetic east. It doesn't matter if I jump on the 090 degree radial and I fly towards the VOR or I fly away from the VOR, I am still on the 090 radial. Let's say that I fly towards the VOR on the 090. In other words, I am flying magnetic west (towards 270). One mile away from the VOR I am still on the 090 and I am still flying west. A little later we are right on top of the VOR. At this point all the radials kind of come together - it's big mess and it's called the "Cone of Confusion". However, if I ignored the drunken CDI on my omnibearing selector (OBS) display, it will eventually settle down as I keep heading west and if all were executed just right, I have crossed the VOR, left the 090 degree radial and now I am flying outbound on the 270 degree radial heading away from the VOR and into the sunset.

Again, all radials have a name and they have an origin at at the VOR - they emanate "FROM" the ground based VOR transmitter.

Now the part that messes up the beginner and a lot of experienced folks, the notion of the "TO" and the "FROM" flag on the omnibearing selector display. If you turn the dial (omnibearing selector) and you center the CDI and there is a "FROM" indication then the value you read at the top of protractor is the radial on which you are positioned. If you center the CDI and a "TO" indication is shown in the TO/FROM window, then you are on the radial indicated on the bottom of the protractor. Confused? If so, don't feel alone. It's not so bad once you understand the reason for this and it revolves around the fact that humans aren't very good flying with reverse sensing. To illustrate, suppose you want to fly on the 090 degree radial towards our VORTAC and cross the ground station and keep on going into the sunset on the 270 degree radial. Let us fast forward into the future and say you have passed the VOR and you have 270 dialed on top of the OBS protractor scale. Let's go even a step further and you notice the CDI is exactly centered which means you are a great VOR navigator because your aircraft is located right on the 270 degree radial. Now let's suppose you parallel the 270, but drift southward. This would cause the CDI to drift to the right side of the display. It's telling you that the line you tuned in (270 degree) is on your right and you aren't on it anymore. You need to turn the aircraft a few degrees right to re-intercept the 270 degree radial. If the needle drifts to the left of the display while the aircraft parallels the 270 radial, you have drifted north and you must turn the aircraft to the left to re-intercept the 270 radial. Pretty neat. Desired radial on your left, needle on your left, turn left. And ditto for right except the radial is on your right, the needle is on your right and you must turn right. Get it? In both cases you turn into the needle to correct. How much turn? - hold on I will cover that later.

Once you understand this you will know what to do to keep on a radial when you are flying outbound from the ground station: dial in the radial number on the top of the OBS display, keep the needle in the center. If needle moves left, turn left a little; if the needle goes right, turn right a little. Now the catch which is conceptually troubling for most. The catch is that what happens if you are flying inbound towards the ground station on the 090 radial and you dialed in 090 on the top of the protractor? Well if the needle is centered, you are on the 090 radial and the TO/FROM flag will indicate "FROM". Now let's suppose that your aircraft drifts north. Which way does the needle go? Where do you want it to go? You want it to go to the left of center to tell you to turn the AC left to re-intercept the 090 radial. Does it? Answer: emphatic NO! Why? Because when you have a radial value dialed in and you are on that radial, 090 in this case, the system assumes that you are heading away from the ground based station so the deviation indicator shows that the line you desire is either right underneath you, to your right if you drifted north or to your left if you drifted south of the 090 radial. This introduces a very important concept: the VOR receiver in your aircraft knows nothing about the heading of your aircraft. It's is only telling you where you are in relation to a radial. If you are north of the 090 hanging by a sky hook and you spin the aircraft on its cable, the CDI will be to the right of center, assuming that you dialed in 090 on the OBS, regardless of where the aircraft is pointing at any moment as you pirouette in the sky. If you are on the 090 radial it will be centered as your aircraft makes its 360 rotation on the skyhook cable swivel. If you are to the south it's the same as north except the needle is to the left of center as you do your swivel trick.

In short, there is an implicit assumption that you are always going outbound on a given radial and that the CDI shows your deviation from the radial based on the assumed outbound direction of the aircraft. But we are NOT going outbound we are going inbound to the ground station on the 090! Well if you put 090 on the top of the OBS display you will have to be straight jacketed when you land because the needle will always be going the wrong direction relative to your deviation away from the 090 radial! For example, if the aircraft drifts north, while you are heading west, the needle moves to the right side! If you turn right to intercept - Ouch! If you keep your current position, but magically move the aircraft 180 degrees so you are going away from the VOR, then the needle is telling you the correct direction to turn to re-intercept the 090 radial. The fact that it goes the wrong way when you go inbound on the 090 with a "FROM" indication is called reverse sensing. The manufacturers of the receiver system said, hey we can help pilots with this problem (I made this up, but it works even if it's not true). If they are going inbound on the radial, we can let them dial in the reciprocal (radial + 180) which is 270 degrees (090 + 180) and give them proper sensing. Now things become ambiguous for the pilot if he gets a centered needle after dialing in 090 while on the 090 radial and he gets a centered needle if he dials in 270! Solution: put a TO/FROM flag on the display to tell the pilot whether or not he has the "real" radial or he has dialed in the reciprocal. Problem solved. So, if I am on the 090 radial and I am inbound to the VOR, I dial in 270, see a "TO" indication, and the sensing of the CDI needle is reversed so that if I am north of the 090, the CDI is to the left and so on.

Ok, now that we have established that let's talk about the 3 recipe items. Again, in order to execute one of the three recipes, you have to understand what you are trying to accomplish. For example, do you want to intercept a radial and go to the VOR station or do you want to intercept and fly away from the VOR station? With that in mind the recipes follows.

Here we go. I have assumed you have tuned and ID'ed the VOR by its Morse code. If you do not understand anything I have said above this point, just memorize the three recipes following.

Intercept a radial and fly in toward the ground station:

1. Dial in the desired radial on the bottom of the OBS protractor (a "TO" indication appears)
2. Note direction of needle: left or right of center
3. Decide on angle to use to intercept (45 degree intercepts are common, let's use it here)
4. Moving eyes from center of OBS protractor 45 degrees towards the CDI needle, note value
5. Turn your airplane to that value
6. Wait for needle to sweep towards center, as it does, turn towards value on top of OBS
7. You are now flying towards the ground station

Intercept a radial and fly in away from the ground station:

1. Dial in the desired radial on the TOP of the OBS protractor (a "FROM" indication appears)
2. Note direction of needle: left or right of center
3. Decide on angle to use to intercept (45 degree intercepts are common, let's use it here)
4. Moving eyes from center of OBS protractor 45 degrees towards the CDI needle, note value
5. Turn your airplane to that value
6. Wait for needle to sweep towards center, as it does, turn towards value on top of OBS
7. You are now flying away from the ground station

Intercept a radial and fly towards the ground station:

1. Turn dial, look for a "TO" indication
2. While the "TO" indication is present, center needle
3. Read value from top of OBS protractor
4. Turn aircraft to a magnet value equal to the value on top of the OBS protractor
5. Make appropriate corrections to keep the CDI needle in the center
6. You are on your way to the VOR

Memorize, memorize and memorize these 3 recipes, especially if you are a student pilot - works every time.

Here is an example for intercepting a radial and going inbound. You desire to intercept the 090 radial and go inbound. You are somewhere east'ish of the ground station. You dial 090 on the bottom of the OBS (that puts 270 on top - no math for you!). If you are north of the 090 radial the CDI will be towards the left of the display. Look at the top, it says 270. Count 4 big lines and a small line towards the needle across the protractor arc which yields 225. Turn your airplane to 225 degrees magnetic to obtain a 45 degree intercept. The needle will swing towards the center as you approach the line. As it does, turn your aircraft towards 270 degrees magnetic and keep the needle in the center.

The recipe is brainless: what do you want to do? Intercept and go outbound or inbound? Dial in the radial on the top or bottom of the display depending on whether you are going outbound or inbound respectively, obtain intercept value on the needle side of the display and turn to that direction to intercept then go inbound or outbound depending on what you wanted to do in the first place. The direct-to case is trivial: TO indication and needle in center then turn aircraft to value on top of the display and work to keep the needle centered as you cross the ground station.

You do have to have an idea roughly where you are located relative to the VOR for this to work. However, for practical purposes you will not be asked to, say, intercept the 090 radial and go inbound if you are west of the ground station. There are other tricks to fool a pilot that deal with zones of ambiguity and cones of confusion, understand why this is, but it's not likely you will encounter this on a check ride.