"I hate this" - student pilot tuning the heading bug instead of the course selector
What is a VOR?
A VOR, or "Very High Frequency Omnidirectional Range", is a ground-based navigational facility that emits radio frequencies to provide radial-based navigational guidance. Aircraft possess a VOR antenna (typically shaped like a "V" on a general aviation airplane's tail) to receive the radio signals. There are many different types: VOR, VOR/DME, VORTAC, etc. Each has a specific functionality (e.g., radial and distance information). The term VOR is generically used to group all of these facilities, although it's actually a station that only offers radial information.
What do we mean by radials? Well, the signals that the station emits modulate and are phased. Each phase relates the aircraft's bearing from the VOR. In essence, imagine radials emitting outbound from the VOR station. The airplane is capable of tuning a specific radial and navigating it by a course deviation indicator (CDI). A CDI is an instrument that allows you to tune a radial and show you whether that radial (or "course") is to your left or to your right. There are many different types, but the most generic is shown below. In the below image, my course is off to my left. That is depicted by the vertical white line (literally the "deviation indicator") moved to the left. Each dot or line of deviation is 2 degrees. So I am currently to the right of my desired course. More to follow on this instrument later in the article.
An important element to understand is that a VOR will provide you with position information only, not direction. For that you need to determine your heading by looking at your compass or on your horizontal situation indicator (HSI) where the CDI is overlayed on heading information.
Aircraft position with respect to a VOR is specified by the radial you are on. This implies that the radial that you are on is not necessarily the radial you are navigating. But let's start slow. In the image below, three airplanes (they are circles but use your imagination, please) are flying (independently of direction of flight, that's why I chose circles and not airplane icons...) around a VOR. The blue airplane is currently ON the 060 radial (denoted R-060). The green airplane is on R-290, and the red airplane is on R-210.
Navigating by VOR
This is where things get complicated. But feat not! I will help you. Student pilots often believe that VOR radials span outbound only. That is untrue. The radials actually go through the station, and are tunable on both sides. For example, the red airplane above could be tuned to either R-210 or R-030 (the reciprocal). However, it is located on R-210.
To help us understand where we actually are, the CDI instrument provides us with "TO" or "FROM" indications. The CDI will literally have a triangle icon which will flip up or down to indicate TO or FROM. The CDI instrument below shows the TO flag circled in yellow.
If you observe the CDI image above, the outer rim is the course selection, not heading. As such, the instrument will not provide you with direction information - purely position (see, I told you). That flag helps us understand our position and what radial we are navigating.
If I am heading in the general direction of the course I tune, the TO flag indicates that you are flying towards the station, and the FROM flag indicates that you are flying away from the station. So let's assume that I am already navigating using a VOR (i.e., we are not lost trying to figure out our position). What I'm trying to say is that to navigate a radial,
You should always tune the RADIAL that generically matches your HEADING.
If you are flying north towards a VOR, you should dial R-360, even though you may physically be south of the station on the 180 radial.
In the images below, the VOR is tuned to a course of 006 degrees (R-006). Assuming I am navigating the VOR correctly, I am heading northbound (006 is north). The flag shows a TO indication, because I am indeed navigating towards the VOR. The airplane position is therefore the green circle. If my position were the red circle, the CDI would indicate a FROM flag (and I would still be tuned to R-006).
If I were to tune R-186 (so I spin the course so that it reads 186) while navigating from the south to the station (I'm the green circle), the CDI thinks that I am now navigating southbound, and therefore the flag would switch to FROM. In that scenario, I am using the VOR backwards , and I would therefore observe reverse sensing. If I'm left of course, the CDI moves to the right and not to the left.
Homing a VOR
So far, we assumed that we were already navigating a radial. That is, I am in the correct direction, on the correct radial, etc. Now, let's assume I don't know where I am and I wish to use a VOR to find my position. Well, you can "home" a VOR to identify your position along a radial. If you use two different VORs, you can actually find your exact position by the intersection of two radials.
How do you home a VOR? You spin the course until your CDI centers and the flag indicates TO. The course you have dialed is the radial that you need to fly to get to the VOR and the tail of the CDI (reciprocal of your course) is the radial you are on. Below are two CDIs that have been homed. The one on left (blue) shows that to get to the VOR you need to fly the 070 radial. Currently the airplane is on the 250 radial. That is the important bit! All I need to do is trace a line out of the VOR on 250 and I am somewhere on that line. The CDI on the right shows that I need to fly a course of 200 to fly to the station. However, I am on the 020 radial. So if I trace 020 out of the VOR, I am somewhere on that radial.
Now let's assume these are two geographically close VORs. Let's trace the "tails" (the bottom of the CDI) and find our positions. The red VOR is CHS (Charleston) while the blue VOR is CRE (Grand Strand). All I did was trace on the sectional the tails of the CDI (circled for you on the CDIs). Where they intercept is where I am located. So I'm not lost anymore.
Most questions on your FAA written are designed to trick you and understand how to read the CDI independently from heading and in abnormal situations. This article is not about those weird situations. It's just about nominal navigation. Truth be told, you will normally navigate VORs correctly in a planned fashion.
to demonstrate this, let's build a quick VOR-based route and see what radials we should tune to correctly navigate them. In reality, VOR signals are not always strong, so for this example, let's assume that halfway between two VORs I switch from the first to the next one to get a stronger signal.
Let's start from the top right corner at CRE. To navigate southwest to the CHS VOR, I need to tune the R-240 in my CDI (240 is southwest). The flag will show FROM and I will physically be on that R-240. Halfway through the leg, I will tune to the CHS VOR. I will tune the R-240 with a TO flag. I will physically be over the R-060. Ge it?
At the end of the day, when you are confused on what radial you should tune, SET THE COURSE (I.E., RADIAL) TO THE ONE THAT CLOSEST MATCHES YOUR HEADING. So if I'm going from CHS to CAE, I am heading northwest bound. The options are either the 310 or 130 radial. 310 is northwest, while 130 is southeast. So dial 310.
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