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Radials vs Courses

Don Wood Guest

Over the past months, there seems to be a fair amount of confusion in a variety of posts of the word "radial" as it relates to VOR navigiation. If those of you who understand it will forgive a little basic instruction, please let me try and define VOR's, radials, courses, headings, and how they differ.

VOR stands for Very high frequency Omnidirectional Range. Physically, it consists of a ground transmitter using a specified VHF radio frequency, an airborne reciever, and a cockpit instrument that provides a visual signal of the information transmitted by the ground station.

I'm not going to discuss how it physically works but a ground VOR stations transmits a set of radio signals that define each degree of arc, starting with magnetic north and continuing clockwise until the full circle is completed. Each one of these degrees of arc is termed a "radial". If you visualize a clock face with 360 spokes radiating from it, equally spaced apart, it may be easier to understand.

You begin with magnetic north, which is 360 degrees, the first spoke is 1 degree, the second 2 degrees, etc, all the way around to 360. Each of these spokes is a radial with the same number as the degrees of arc it represents and is often expressed as Rnnn where R = radial and nnn = the number of degrees. Thus, R360, R001, R002.

Course is the direction you fly the aircraft to go where you want to go. The confusion many people seem to have is equating the direction they need to fly with the radial of the VOR. Let's assume you are directly southeast from a specific VOR and your flight plan calls for you to fly directly to it. To fly directly to a VOR from the southeast, you need to fly a course of 315 degrees (northwest). However, while you are flying that course to the VOR, you are not on R315. Since southeast is 135 degrees from magnetic north, you are flying on the 135 radial on a course of 315 degrees. This is important to understand because ATC flight plans and vectors are often expressed as "intercept the 135 radial of ABC VOR, direct ABC" which means, continue in the previously cleared direction until you reach the 135 radial then turn toward the VOR and fly a course of 315 until reaching the VOR.

I have often responded to threads, using the word "reciprocal" when talking about VOR radials and courses. In navigation, reciprocal is the heading or radial directly opposite the number in question. For example magnetic south (R180) is the reciprocal of magnetic north (R360). To calculate reciprocals you add or subtract 200 from the number in question then subtract or add 20 to the result. In my example 360-200+20 = 180. In the example in the paragraph above 315 is the recirocal of 135 (135+200-20 = 315).

Heading is the direction you point the nose of the aircraft to achieve the desired course. In zero wind or with a direct head wind or tail wind, heading and course will be the same. However, if you have any cross wind component, you will need to fly a heading that corrects for wind drift to remain on the desired course. That is where the cockpit indicator comes into play. If you are flying as instructed, and you notice that the indicator needle is moving off center, you need to turn the aircraft sufficiently toward the course for the needle to return to and remain centered. The amount of turn necessary decreases as the distance from the VOR decreases.

I hope this information has been useful to some of you.

Pro Member Chief Captain
liam (Liono) Chief Captain

Once again Don Wood explains things so clearly. I reckon this should be a sticky. Who's with me
👍

Pro Member Chief Captain
Manuel Agustin Clausse (Agus0404) Chief Captain

Me!

This was a well done explanation. Good job Don Wood! 👏

Pro Member First Officer
Michael_H First Officer

Ditto from me.

Thanks again Don for taking the time to post that. Most ariticulate.... 👍

Pro Member First Officer
JTH First Officer

Good explanation, but one thing I'm not 100% clear on though is say if my flight plan calls for me to head 300 degrees after takeoff to get to my first VOR. Then it says from there, go 200 degrees until you reach the next VOR.

In that example, when I tune the first VOR after takeoff, what do I set the OBS to? 300 degrees (the heading that will take me to the VOR) or 200 degrees (the heading that will take me away from the VOR)? And in that case how will I know when to turn - is it when the to/from indicator changes to "from"?

Many thanks in advance for anyone who helps to clear this up. Don't know what it is with VOR but it's always been a bit of a headache 😕

Don Wood Guest

JTH: Whether you are inbound or outbound from a VOR, you set your OBS (Omni Bearing Selector) to the course that you need to fly. While you are inbound, the From/To indicator will read "to". Once you have passed the station, the OBS remains on the course you want to fly and the indicator reads "from".

Often, you have to change VOR's along an airway. To keep it simple, let's assume that a segment of airway V25 is a straight line between VOR's ABC and XYZ with a course of 200 degrees. Leaving ABC, your OBS would be set to 200 and you would fly outbound on that course and on that VOR until about midway along the segment. Once you are sure you were receiving the signal from XYZ, you would change your primary VOR to that frequency and continue to fly inbound. Often VOR's are located such that you can receive the signal from the second one long before you lose the signal from the first. Many pilots will fly on the closest VOR, however, once I begin receiving a reliable signal from the second one, I switch my navigation to that one. It is purely a personal preference and I know of no regulation that requires you to use the closest one as long as both can be received.

If a course change is required, you simply set the OBS to the new course and make the turn as you pass over the station. Station passage is indicated by the change of indicater "to" to "from".

If an intersection is defined by the radial of a second VOR, once you reach that intersection, then you set the OBS to the new course, making sure that you are navigating on the VOR for that radial, make your turn, and continue inbound. For example MERRY intersection is defined by the crossing of ABC R200 and XYZ R310 and the airway follows those two radials. You would fly outbound on R200 with your OBS set to 200 and your VOR set to ABC. Your second VOR would be set to the XYZ VOR with 310 set in the OBS. Once the needle for the second VOR centers, you would turn to course 130 (the reciprocal of 310), set the primary VOR to XYZ VOR, set the OBS to 130, center the needle, and fly that course inbound.

I hope this helps.

Pro Member First Officer
HardLanding First Officer

Yes, thanks, VOR is so beautiful because it's really so simple; it's just not always easy to put everything into practice.

I don't know if Don Wood is any relation to the Charles Wood who authored the navigation tutorials in the Content section, but for myself, I can't praise those highly enough. They're clear and not the least bit dry; funny and very well written. If anyone else is just starting out the way I am and is trying to get a handle on navigation, I don't see how you could miss with those.

HL

Pro Member Chief Captain
jarred_01 Chief Captain

Thanks very much Don! 👍

And I'm for a sticky as well.

Pro Member Trainee
roger123 Trainee

Thank you Don. Very well presented.
A sticky is a good idea
🙂

Pro Member First Officer
HardLanding First Officer

The 172SP has two Nav radios and each of those has operating and standby frequencies, which lets a lot of the work get done before I'm in the air.

HL

Pro Member First Officer
JTH First Officer

Thanks for that Don. I think I get it but the one thing that keeps confusing me is why even set the OBS in the first place?

I mean if the to/from indicator tells you when you've passed a VOR, couldn't you just turn to a new heading with your heading indicator and not use the OBS at all? So why key in the OBS heading? I think that this could be what's making me struggle a bit to grasp VOR, thanks in advance for all your patience.

Don Wood Guest

You have to use the OBS to set your course. When you navigate on a VOR for a flight plan, you are trying to arrive at the VOR on a specific course and depart it on a specific course. Setting the OBS allows the cockpit intrument indicator to display whether or not you are on course and, if not, in which direction you must fly to get back on course.

The OBS is the device that sets the VOR receiver to receive the ground-based signals and interpret on which radial you currenly are located.

Pro Member First Officer
JTH First Officer

Don Wood, here's what I did today. I took off using VOR to VOR navigation. I tuned the first VOR and immediately after takeoff followed the heading of it. I then tuned my OBS to the heading I wanted to go in once I passed it. The needle on the OBS stayed stuck to the left until I came really close to the VOR and then it pretty quickly slid over to hard right. Now my question is should I follow it or wait for the to/from indicator to change shortly after that? When I check my flight analysis I find that I didn't exactly get the right heading so as a result I was too far to one side of the VOR and the to/from changed when I was 3 or 4nm out. As a result the next VOR was even further out, etc.

Hope I'm making sense and thanks again for any help... JTH 🙂

Pro Member First Officer
HardLanding First Officer

JTH wrote:

why even set the OBS in the first place? I mean if the to/from indicator tells you when you've passed a VOR, couldn't you just turn to a new heading with your heading indicator and not use the OBS at all?

You could fly the heading from the VOR, and if everything went perfectly you would get the same result as using the OBS. But with only the compass, a wind could put you on the wrong track even while the compass indicated a correct course. Also, once off course, say due to a moment of inattention or whatever, with only the compass it would be difficult to know which heading to fly as a correction. With VOR, you need only get the needle recentered to be assured of reaching your next planned VOR. VORs are landmarks. As long as you are within radio range, you can always fly to a point whose position you definitely know. (If I've understood VOR correctly.)

HL

Don Wood Guest

JTH: The reason you are off course approaching the VOR is because once you set the course to the next VOR, you have no way of knowing where you currently are if you are only relying on VOR information for navigation. HardLanding was correct in saying that the only way you could reliably fly to a VOR from a known position without using the OBS is if you had absolutely zero wind drift and made absolutely no error in piloting technique. Both of these conditions are so unlikely as to be next to impossible.

The correct technique is to set your receiver to the frequency of the VOR you want to fly to, set the OBS to the desired course, center the needle, then fly to that VOR keeping the needle centered. Once you reach the VOR, turn to the course you want to follow outbound, reset the OBS to the desired course, center the needle, and continue your flight keeping the needle centered. It's important to use that sequence because if you fly over the VOR and reset the radio before making your turn, you will be much farther off course than if you turn then reset.

The reason you experienced needle fluctuation close to the VOR is because a very small amount of course error results in greater needle deflection the closer you get to the transmitter. The reason for this can be visualized by thinking again about the clock face with the 360 spokes radiating from it. At the clock face, those spokes are very close together. The farther you get from it, the wider distance there is between each adjacent spoke. Since your airborne receiver is measuring deviation from the selected spoke (radial), the closer you get and are off course, the wider the deviation is going to be. 50 miles from a transmitter, if you are a mile off course, the needle will barely be deflected at all. Within a mile of the transmitter, if you are a mile off course, the needle will be fully deflected and you will have no reliable data to tell you where you are.

VOR navigation is only one technique to get from here to there in VFR conditions. However, if you are flying IFR and do not have GPS, LORAN, doppler navigation, or something similar, VOR navigation is the most reliable method for navigation. It pays to always use the correct technique navigating on VOR's so you are in the habit of doing so when the VOR is the only method you have to determine where you are. Many light aircraft, including mine, do not have GPS and VOR navigation is the method of choice when flying IFR or when there are insufficient terrain features to be able to navigate by observation,

Try flying across the midwest sometime using only visual navigation. Since every corn or wheat field looks like every other and there are no mountains and very few cities, rivers or lakes, once you get away from a major highway, you can get lost very quickly without VOR or some other electronic navigation device. Even in the sim, it will be very challenging. If you want a real test of your pilotage skills, pick an airport around Denver to depart, do a VFR flight plan to Wichita, turn off your VOR, ADF, and other electronic navigation devices, and see how long you are able to fly before you have no idea at all where you are. (no fair pulling up the sim map-you won't have that in a real light aircraft).

Don Wood Guest

To answer HardLandings' question, I am not related to Charles Wood. In fact, my real last name is not Wood. It is a screen name I use to avoid typing a whole lot of extra letters.

Pro Member Chief Captain
Matthew Shope (mypilot) Chief Captain

Don Wood wrote:

Over the past months, there seems to be a fair amount of confusion in a variety of posts of the word "radial" as it relates to VOR navigiation. If those of you who understand it will forgive a little basic instruction, please let me try and define VOR's, radials, courses, headings, and how they differ.

VOR stands for Very high frequency Omnidirectional Range. Physically, it consists of a ground transmitter using a specified VHF radio frequency, an airborne reciever, and a cockpit instrument that provides a visual signal of the information transmitted by the ground station.

I'm not going to discuss how it physically works but a ground VOR stations transmits a set of radio signals that define each degree of arc, starting with magnetic north and continuing clockwise until the full circle is completed. Each one of these degrees of arc is termed a "radial". If you visualize a clock face with 360 spokes radiating from it, equally spaced apart, it may be easier to understand.

You begin with magnetic north, which is 360 degrees, the first spoke is 1 degree, the second 2 degrees, etc, all the way around to 360. Each of these spokes is a radial with the same number as the degrees of arc it represents and is often expressed as Rnnn where R = radial and nnn = the number of degrees. Thus, R360, R001, R002.

Course is the direction you fly the aircraft to go where you want to go. The confusion many people seem to have is equating the direction they need to fly with the radial of the VOR. Let's assume you are directly southeast from a specific VOR and your flight plan calls for you to fly directly to it. To fly directly to a VOR from the southeast, you need to fly a course of 315 degrees (northwest). However, while you are flying that course to the VOR, you are not on R315. Since southeast is 135 degrees from magnetic north, you are flying on the 135 radial on a course of 315 degrees. This is important to understand because ATC flight plans and vectors are often expressed as "intercept the 135 radial of ABC VOR, direct ABC" which means, continue in the previously cleared direction until you reach the 135 radial then turn toward the VOR and fly a course of 315 until reaching the VOR.

I have often responded to threads, using the word "reciprocal" when talking about VOR radials and courses. In navigation, reciprocal is the heading or radial directly opposite the number in question. For example magnetic south (R180) is the reciprocal of magnetic north (R360). To calculate reciprocals you add or subtract 200 from the number in question then subtract or add 20 to the result. In my example 360-200+20 = 180. In the example in the paragraph above 315 is the recirocal of 135 (135+200-20 = 315).

Heading is the direction you point the nose of the aircraft to achieve the desired course. In zero wind or with a direct head wind or tail wind, heading and course will be the same. However, if you have any cross wind component, you will need to fly a heading that corrects for wind drift to remain on the desired course. That is where the cockpit indicator comes into play. If you are flying as instructed, and you notice that the indicator needle is moving off center, you need to turn the aircraft sufficiently toward the course for the needle to return to and remain centered. The amount of turn necessary decreases as the distance from the VOR decreases.

I hope this information has been useful to some of you.

😀 😀 😀 What would we do without you!

Pro Member First Officer
JTH First Officer

Thank you Don Wood, I can at last now use VOR!

I made a successful VFR "VOR to VOR" flight today in my Cessna 172. Should I stick to VFR if I want to navigate by VOR? When I try an IFR "VOR to VOR" flight there is no challenge, as ATC just gives me all my headings and keeps nagging me to go back on my assigned heading if I veer off it and do my own thing (I am using JustFlight Traffic 2005 if that makes a different). Also, will VOR navigation end when I "graduate" to the bigger jets? Do 737 pilots etc. only now use GPS or is there still some room for VOR navigation?

Thanks once again for your great help 😂

Don Wood Guest

JTH: A way to practive VOR navigation in FS is to fly on a VFR flight plan rather than an IFR plan. In that case, ATC gives you no instructions at all except when you call for takeoff or landing clearance.

Most heavier aircraft and quite a few lighter ones now have GPS navigation systems. However, even if my aircraft had a GPS, which it does not, I would still use VOR navigation as a backup since, if your GPS receiver fails, you still need to know where you are and how to get where you are going.

I cannot tell you that every commercial aircraft now has GPS. I suspect most do but there may be some who do not. I recall that when US Secretary of the Interior Ron Brown was killed in the crash of a USAF transport a few years ago, they were navigating on an ADF since the aircraft did not have GPS, Loran, or interial navigation systems and there were no usable VOR's in mountainous area they were in. However, as I stated above, even if the aircraft has GPS, unless it is a triple redundant system, I would still use VOR's as a backup in case the GPS system failed.

Pro Member First Officer
JTH First Officer

Okay, in that case would you tune all the VOR stations along the way even if the GPS was working, or when you say backup do you mean you would just be ready to start using VOR if the GPS failed?

By the way, what is ADF anyway? I think it's the only gauge in the 172 I don't know the function of. Thanks once again (I owe you one after all this help 😂 ).

Don Wood Guest

JTH: If I had GPS, I would still use VOR's throughout my flight to keep track of my location as a backup in the event the GPS failed. Any prudent IFR pilot uses all the information available to him/her to keep track of their location. In my case, I use VORs for primary navigation and ADFs for backup. I have lately started carrying a portable GPS for backup but that is not approved by FAA for primary navigation.

ADF is an older navigation device and it stands for Automatic Direction Finding. It is also sometimes known as an NDB or non-directional beacon. It is composed of a ground station that transmits a beacon signal on a specified frequency, an airborne radio receiver, and a cockpit instrument that has a needle that points in the direction of the ground station when the signal is being received. Unlike a VOR, it gives you no precise information about your exact location. It simply gives you a position you can point to and arrive at.

ADF's are still used for primary en-route navigation in a few areas of the US where VOR coverage does not exist. That condition is more common in less developed areas of the world but there are still a few places in the US that need them. ADF's are also components in many VOR based IFR approaches and there are still a few IFR approaches where the ADF is the primary navigation instrument.

To help understand, think back on my explanation of VORs with the 360 radiating spokes. The ADF has no such spokes. It simply sends the same radio signal in all directions. You can fly to it but that instrument provides no other information about where you are relative to the ADF ground station.

Pro Member First Officer
HardLanding First Officer

As long as we're on the subject....

I know someone who spends their summers on Sardinia--lucky person--so I tried a VOR routed flight from Cagliari, in the south, to Olbia, in the north. I dialed in the frequencies given to me by the flight planner but I had a heck of a time picking up the VORs once in the air. I found, online, an airnav type site with listings of the VORs, but there were no remarks for them detailing the VOR limitations or unusable areas. This kind of information is easy to get for U.S. VORs, on AirNav for example, but is it hit and miss for other ones around the world? It's not in FS9.1 somewhere, is it?

HL

Pro Member First Officer
JTH First Officer

Thanks for the explanation of ADF. Do you dial in the ADF frequency to the same radio as you tune VOR? Also where would I acquire the ADF frequencies if I wanted to use one for navigation?

Finally, could you explain VOR intersections to me and under what circumstances I would use them? That's the last piece of the navigation puzzle that I need to understand, thanks a lot 😂

Don Wood Guest

ADF's (NDB's) are depicted on sectionals, wide area charts, IFR enroute charts, and on IFR departure/arrival/approach charts where they exist.

The airborne ADF and VOR radio receivers are separate and one cannot receive the other. Another use for the ADF receiver is to dial in AM commercial broadcast stations. I don't fly in Mexico anymore but, when I did, there were fairly large areas where neither VOR or ADF navigation was available. I used the ADF to tune to AM stations in towns along my route and the needle would point me in that direction.

Some intersections are points on defined airways and are used for navigation. They are points where specified radials from two different VOR's intersect. Sometimes they are used just to help define position along a long route segment. Often they are used to designate a point where a change in heading must be made to remain on the airway.

They are also sometimes used as defined points on an IFR arrival, departure, or approach.

Where an intersection exits, it will be defined on the chart with indicators of the VORs on which each radial is based, the frequency for that VOR, and the specific radial that forms the intersection,

For instance, if you download the ILS Rwy 20 approach for Cedar City, UT, from Airnav, you will note that BERYL intersection is the missed approach holding point for that approach. You will also see BERYL is formed by the intersection of the Milford VOR 197 radial and the Cedar City VOR 278 radial.

Pro Member First Officer
JTH First Officer

Great, thanks for all your help 😉

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