VOR Station Passage Discussion

During an instrument lesson with one of my students a couple weeks back, he asked me an interesting question while shooting the LOC BC RWY 23 approach into KHUF.

If you look at this approach, obviously it is a localizer back course approach, but his question was concerning the final approach fix. When you look very close, you will notice that the FAF is defined by the VOR station passage. The VOR happens to sit off away from the back course of the localizer, slightly to the left about 1/8 of a mile or so.

In the 172s we fly, we have dual VOR receivers. So to set up for this approach we must set our CDI 1 to track the localizer, obviously. But to assure that we can identify the FAF, we must set our CDI 2 to get the TO/FROM flip, which identifies station passage, and the FAF. At this point, we start our time, and descend to our MDA of 980 ft.

So anyway, my student's question was this. "It doesn't matter what I have the #2 CDI set on, because I'll get the TO/FROM flip no matter what, right?" I then told him no, and that it had to be set up on the final approach course, because you'll get a good clean and fast flip that way. But then I thought, wait a minute, is that right?

That night I went home on got on my trusty little tool, Tim's Air Navigation Simulator, to experiment with this matter. I hadn't studied how this worked in a while, so it was time for a good refresher. Turns out I was right, you do need to set the #2 CDI to match your final approach course, but here is why.

I learned that whatever course you have your CDI set up to, the TO/FROM flip occurs when you pass an imaginary line perpendicular to your dialed in course. For instance, here is our setup on this approach.

You can see, we have our avionics set up to simulate the LOC BC RWY 23 approach that we were practicing that day. We have the CDI 1 representing the localizer back course final approach course, and the VOR (FAF) in the CDI 2 just adjacent to the approach course. The lines indicate the radial we have tuned in.

This next picture will show some drawing by me indicating with this setup on the avionics, where you will get to and from indications on the #2 CDI. The solid lines indicate the radials we have tuned in, but the dashed line indicates where we will get a TO/FROM flip. As you can see, even if you are slightly off course, you will still get a flip. In fact you will get a flip anywhere you cross this dashed line, even if you are not on the dialed in radial.

However, let's say that we weren't paying attention to what we had our CDI 2 set on, and we left it on say a 320. Let's take a look at what that scenario would look like.

You can see that you would never get a flip on this setup. If you tracked the localizer all the way in, with a 320 set up on #2 CDI, you would have a FROM indication all the way in. Let's say you had #2 set up on a 140, and you happened to drift off course before you hit the FAF. You would actually get a TO/FROM flip way before you crossed the VOR.

Needless to say this would be very dangerous. You would actually start your descent way before hitting the FAF. They publish those altitudes for a reason, and if you descended before hitting the FAF, you could have some lovely radio towers, or buildings waiting for your arrival. :)

Bottom line is this. Any time you are identifying the FAF by an off course VOR, make sure you have the the published final approach course dialed in on all avionics so you get a TO/FROM flip at the right spot.