ILS Back-Course Approaches

Localizer Back Course approaches

What does "Localizer Back-Course" mean?

It means confusion.

OK, that's cheap humor- sorry.

How about this: "A Localizer Back Course" is just like a regular Localizer Approach except that Left is Right, and Right is Left. In addition, it may, or may not, have a Glide Slope. If it has a Glide Slope, it is called a "Localizer Back Course with Glide Slope".

Hmm, I think I liked the "confusion definition" better.

"Humor" aside, what is a Localizer Back Course?

In a conventional Localizer, the signal radiates outwards from the approach end of the runway, along the extended center line of the runway. Usually, a by-product of this is a signal that radiates in the reverse direction. Some installations take advantage of this "spurious signal" and use it for an instrument approach.

Wait a minute. Tell me again what a "Localizer Approach" is

A Localizer Aproach is a regular ILS without the Glide Slope. Because there is no "up/down" information given a Localizer Approach is referred to as a "non-precision approach". However, just to muck up the waters, there are Back Course approaches that include a Glide Slope. They're kind of rare, but they're out there.

If you are not clear on this, please read the tutorial "Localizer Approaches" in the Tutorial Section.

OK, but what's with this "Left is Right and Right is Left" business when you talk about the "Back Course"?

When you fly a "conventional" Localizer Approach, or any approach for that matter, a needle deflection to the left means that your desired course is to the left, and that you as the pilot must correct the plane's heading to the left to recapture the approach course. The opposite applies for right deflection of course.

However, in a Localizer Back Course, if the needle deflects to the left it means that your desired course is to the right, and that you must correct in the opposite direction to recapture the desired approach course. In other words, left is right and right is left.

Perhaps they should have named it the "Localizer Backwards Approach".

Huh?

Let's take a look at a Localizer Approach.

Picture the Localizer signal also "propagating" in the opposite direction, back down the runway.

Notice the "barbed" section on the Localizer Front Course? That's the "right side" of the Localizer. The "clear" section is the "left side" of the Localizer. When you are in the "barbed side", your course indicator (HSI, VOR head) will be on the left side of the instrument, indicating that the desired course is to the left. The opposite, naturally, applies to the left side of the Localizer Course.

In "the old days", VOR instrument displays had painted markings. The Right side of the display had a blue arc, while the Left side had a yellow arc, representing the Right and Left sides of the Localizer.

You may come across these displays in older aircraft.

Now, notice on the drawing above that shows the Front and Back portions of the Localizer that the "barbed" side and the "solid" side of the Localizer are both on the right side, as viewed from Localizer Front Course end of the runway. The "clear" side is on the left side. Hold this thought, it's important for what's to follow.

Let's show some indicators in various locations on the Localizer Approach using an HSI

Here is how your HSI display will look at various points on the Localizer Front Course, runway 23.

Position 1:Position 2: Position 3:

And here's how the old fashioned VOR head would look:

Position 1: Position 2: Position 3:

In all cases, the CDI (Course Deviation Indicator) indicates where your desired course is, and which way you have to go to get back on course.

Now, let's see what the HSI display looks like over on the Back Course side. Note that the Course Selector is now set to the reciprocal of runway 23, runway 5, which would now be your landing runway.

(note: there was an error in this graphic prior to 4/16/01. It is now corrected. My thanks to Tom Wilson for pointing this out.)

Position 4: Position 5: Position 6:

And here's how the old fashioned VOR head would look:

Position 4: Position 5: Position 6:

Whoa, that's confusing. So, on the Back Coure, Left is Right, and Right is Left.

Why is this so?

Because the displays, HSI and VOR head, don't care which way the "pointed end of the airplane" is facing. They could be placed in a flying saucer, it doesn't matter.

And, as far as the VOR head is concerned, it doesn't matter what you have the CDI set to. If you were in position "6", in the above example, you could twirl that knob all around the "dial". The indication would not change.

Here's where the "old timers" with their old VOR head display, the one shown above with the "yellow and blue sectors", had an "edge". Notice that in all of the displays showing the VOR head, that the needle is always showing which side of the Localizer the instrument is in? No matter if you are on the Front Course, or the Back Course, the needle is in the correct "zone". An old joke was that these pilots wore a pair of colored gloves when flying Back Course Approaches. They put a blue one on the right hand, and a yellow one on the left. If the VOR head display showed the needle to the left, in the "blue zone", they could look at the blue glove on their right hand and know that they had to go to the right to get back on course. True? I don't know, but it's a good story.

Well, my airplane doesn't have those cute little blue and yellow "zones" on the VOR head

No, it doesn't. So, if you're flying a Back Course Approach with a VOR head only, just remember that left is right and vice versa.

And now, drum roll please, the secret to end this confusion

When flying a Back Course, simply turn the HSI Course Selector to the Front Course heading.

What? It's that simple?

Yep, that's all there is to it. Let's look at the approach diagram again, this time turning the Course Selector on the HSI to the Front Course heading, 055 degrees.

Position 4: Position 5: Position 6:

Now the indications are correct. Left is left, and right is right.

Wait a minute, you couldn't do this with the VOR head, how come you can do it with the HSI?

Because of the way the HSI is made. When you rotate the Course Selector knob, the one down there on the lower left corner, the entire center part of the HSI rotates. When that happens, the CDI needle always stays on the "correct" side. Why did they design it that way? So that you could make a Back Course Approach without "opposite indications". Well, there are some other navigation display benefits too.

OK, now that you know the "secrets" of the Back Course, let's fly an approach.

NOTE: It assumed that the you have read the tutorial on ILS Approaches. We will not cover the approach in great detail, only those things that are distinctive to the Back Course.

Here is a Back Course Approach into the old Denver Stapelton Airport:

There it is in plain English. This is a Back Course Approach. And, it has DME. Because there is no Glide Slope, there has to be a way for you, the pilot, to know where you are on the approach. In could be radials from a VOR, in this case it's DME fixes.

Ah, there it is. The information that you need to set your HSI. See that "FRONT CRS 351"? The reason that it is there is so you can set your HSI to the Front Course heading so that you will get "correct" Left / Right indications during the approach.

There probably is a "remnant" of the Glide Slope for the ILS runway 35 approach drifting around in the airspace.

Here is the step-down profile. Notice that the first DME fix is referenced off of the Denver VOR, and that all of the following ones are from the Localizer. Also, notice that Maltese Cross at the NDB? That indicates that this is the Final Approach Fix, where you start your Final Approach Segment of the approach.

When flying this approach you would tune your ADF to Thornton, 281Kc. This will aid in orientation, and give you a display of your passage over Thornton.

Approach Control would have you at 9,500 feet when you are established on the 15 mile arc off of the Denver VORTAC. Once you intercept the Localizer, and are established on it, you would step down to 9,000 feet. At the 13.0 DME fix you would descend out of 9,000 for 7,000. At Thornton, or the 5.7 mile DME fix, you would descend out of 7,000 for the MDA, Minimum Descent Altitude, of 5,580 feet. The Missed Approach Point is at .8 DME.

And all this time, Left would be Left, and Right would be Right.

Localizer Back Course Approaches? Hey, piece of cake!

This ends the tutorial on Localizer Back Courses. If you have any suggestions, corrections, or comments, please contact me. Thank you.

This tutorial is available on a CD

This tutorial, along with additional content, is available on a CD. Click here for more information.

Hal Stoen