VOR Nav - Turn/Twist/Interpret/Track

When you are given the task of intercepting a course to or from the VOR you are likely heading in an arbitrary direction. Is that direction helping or hurting you? Who knows? But if you first turn to a heading that matches the desired course you will at worst be paralleling the course you need to fly, and will be going in the right general direction.

All too often I see people simply continue blindly on whatever heading they are on, often causing them to get further and further away from the desired course and thereby making it take longer to eventually intercept the desired course. This is most striking if they are tasked to pick up a new course after VOR station passage. This is a point where the radials are VERY close together (they come to a point at the VOR station itself, obviously).

How do you know the correct direction? The heading you take depends upon whether you are being asked to fly TO the VOR station, or FROM the VOR station. There are a number of ways to state a VOR navigation task. For instance :

Note, this first step of turning is only to help with getting the pilot initially oriented and to avoid flying further from the desired course. If the pilot can identify the desired course quickly enough (i.e. if they understand and can execute the Twist and Interpret steps quickly enough) it isn’t strictly needed.

Now that the aircraft is headed in the right general direction, setting up for the desired course can happen. I suggest doing this in two steps (though, the initial step isn’t strictly necessary, but it helps orient the user). The task set before us is to fly a 300° course to the VOR starting from an arbitrary location on an arbitrary radial.

First twisting the OBS to center the needle with the TO/FROM indication as desired. So, if you are told the intercept the 120° bearing to the VOR, twist the OBS to center the needle with a "TO" showing on the CDI. This gets the OBS initially oriented and informs you as to which radial you happen to currently be on. So lets do that first.

We see that we are on a radial that is directly south of the VOR, though that is not the course we desire. But we have a TO indication, and a centered needle. The first orientating twist is done.

Now for the second step. We twist the OBS to the desired course. In this case we twist until we have 300° selected. This causes the needle to offset as that selects a radial which we are not currently on.

The result looks like what is seen in the previous figure.. Now all that is left is to interpret what we see.

This is where people go astray. They look at their current heading, often tilt their heads looking at the CDI, and try and figure out if they need to turn left or right. It is confusing. It also doesn’t work very well! But interpreting the indications is not hard, as long as you understand what you are seeing.

Given the indication above, we can divide the CDI into two hemispheres. Since we have TO indication the top half of the CDI tells you ALL HEADINGS that would take you closer to the VOR (closer, but not necessarily directly to the VOR). The bottom hemisphere is a collection of headings that would take you further from the VOR. This helps because the task given is to fly "to the VOR on a 300° course".

We now have one final thing to figure out. How to get to that pesky course. Is it right? Is it left? STOP THAT! Forget left and right. You are a pilot. You want a a heading. So here is the secret.

Looking at that top half of the CDI you see possible headings ranging from roughly 210° to 030°, and now just need to figure out what will work. In that top hemisphere the quadrant indicated by the needle has the collection of headings that will achieve your goal.

Looking at the shaded area in this image. It is in the upper hemisphere (TO the VOR) and is in the quadrant on the side of the needle deflection (to the course). Therefore in this shaded quadrant are headings that will take you both toward the VOR and toward your course. These headings are between roughly 300° and 030°. Which do you pick? It depends upon how quickly you wish to intercept your desired course. A 305° heading will slooooowly creep you closer to your course because the intercept angle is so slight. Maybe so slowly, in fact, that you’ll pass the VOR before you intercept. A 030° heading will have you flying an intercept angle of about 90 degrees;, which is very aggressive. Unless you significantly lead the turn to your desired ultimate heading you’ll quickly fly right through your desired course and have to re-intercept.

Normally picking a heading somewhere in the middle is a good choice, particularly if you don’t have a full deflection of the needle. When doing this does it matter what heading you are CURRENTLY ON? NO! The way you are pointed as you make the determination of which way to head to get to your course is utterly irrelevant. Why do I obsessively talk about how your current heading doesn’t matter, nor does it play into any decision you make about how to get to your course? It is because that is a source of regular confusion on the part of fledgling pilots. So I really want to hammer it home.

Now we need to track the desired course. This is mostly a matter of turning to the proper heading (in the above example that would be 300°), then correcting for wind. As you get close to your course the needle will move toward the circle in the center of the horizontal row of dots on the CDI, often referred to as the donut. As it gets close to the donut start your turn to the desired heading. You will need to lead the turn a bit otherwise you’ll fly through your course and have to re-intercept. Once intercepted, you are on the desired course line.

You know the correct no-wind heading, it is the one that is the direct bearing to or from the VOR. You can read that right off the face of the CDI. But we rarely have a no-wind condition, so wind corrections are required. You can start by flying that heading, however, and watch the needle.

If the needle moves outside the center donut, tweak your course. The needle can wiggle a bit, mostly because this is an analog system and the detecting of subtle phase changes in the signal can vary from moment to moment. Therefore when you are outside the donut make a course correcting, often of only a few degrees, then give it time to change indication.

I all too often see people just randomly selecting headings to try and figure out what will hold the desired course. However, there is a better strategy that can be used to zero in on the heading that will result in successfully tracking the desired course.

If you get pushed off the course, identify a heading that will re-intercept. As you re-intercept you have two pieces of useful information. First, you know that the simplistic no-wind heading won’t maintain the course. Second, you know that the intercept heading is good for intercepting, but if held will simply fly you through the course. You need something in-between the two.

So as you center your needle pick the heading exactly in-between. Then give it time so you can see how well that newly selected heading holds the course. There are three possibilities. One, it might be perfect, and the needle stays in the donut. Two, it might not be enough and you get blown off the course just as you did earlier. Three, it might be too much, and you once again fly through the course.

In the example above the bearing direct to the VOR was 300°. For the sake of discussion lets say that flying a 300° heading failed to hold the course. As you look at the CDI to determine a heading to re-intercept, you select a heading of 280° (so a slight turn to the left). That heading end up working to bring you back to your course, and re-center your needle. What now?

As the needle centers turn to another heading selected to, you hope, track the desired course. You know that 300° didn’t hold the course, and that 280° intercepted the course (and if maintained would continue to fly through the desired course), so picking something in the middle might result in you selecting a heading of 290°, that being a heading right between the two you used earlier.

Obviously, if 290° holds the course you are good. In either of the other two cases you have to repeat the re-intercept exercise. But, this time the deviation should be less. Pick a heading that will re-intercept, and upon intercepting pick a new heading between the intercept heading and the previous heading that failed to hold the course. Any software geeks out there will recognize this as a "binary search".

For any of this to work in a reliable manner, though, you must have heading discipline. By that I mean, when you pick a heading you must hold it, and not let your heading wander. If your heading is wandering you can’t know if the reason you are off course is because of wind, or because you can’t keep your nose from wandering.

There is one sad fact, and that is winds change and after you have found the perfect heading …​. the wind changes and you have to do it all over again. Welcome to flying!