ILS Back-Course Approaches

Localizer Back Courseapproaches

What does "LocalizerBack-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 isRight, and Right is Left. In addition, it may, or may not, havea Glide Slope. If it has a Glide Slope, it is called a "LocalizerBack 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 radiatesoutwards from the approach end of the runway, along the extendedcenter line of the runway. Usually, a by-product of this is asignal that radiates in the reverse direction. Some installationstake advantage of this "spurious signal" and use itfor an instrument approach.

Wait a minute. Tell meagain what a "Localizer Approach" is

A Localizer Aproach is a regular ILS withoutthe Glide Slope. Because there is no "up/down" informationgiven a Localizer Approach is referred to as a "non-precisionapproach". However, just to muck up the waters, there areBack Course approaches that include a Glide Slope. They're kindof rare, but they're out there.

If you are not clear on this, please read thetutorial "LocalizerApproaches" in the Tutorial Section.

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

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

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

Perhaps they should have named it the "LocalizerBackwards 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 theLocalizer Front Course? That's the "right side" of theLocalizer. 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 sideof the instrument, indicating that the desired course is to theleft. The opposite, naturally, applies to the left side of theLocalizer Course.

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

Youmay come across these displays in older aircraft.

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

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

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

Position 1:Position 2: Position 3:

And here's how the old fashioned VOR head wouldlook:

Position 1:Position 2: Position 3:

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

Now, let's see what the HSI display looks likeover on the Back Course side. Note that the Course Selector isnow set to the reciprocal of runway 23, runway 5, which wouldnow 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 thisout.)

Position 4: Position5: Position6:

And here's how the old fashioned VOR head wouldlook:

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'tcare which way the "pointed end of the airplane" isfacing. They could be placed in a flying saucer, it doesn't matter.

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

Here's where the "old timers" withtheir old VOR head display, the one shown above with the "yellowand blue sectors", had an "edge". Notice that inall of the displays showing the VOR head, that the needle is alwaysshowing which side of the Localizer the instrument is in? No matterif you are on the Front Course, or the Back Course, the needleis in the correct "zone". An old joke was that thesepilots 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 theleft. If the VOR head display showed the needle to the left, inthe "blue zone", they could look at the blue glove ontheir right hand and know that they had to go to the right toget back on course. True? I don't know, but it's a good story.

Well, my airplane doesn'thave those cute little blue and yellow "zones" on theVOR head

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

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

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

What? It's that simple?

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

Position 4: Position 5: Position 6:

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

Wait a minute, you couldn'tdo this with the VOR head, how come you can do it with the HSI?

Because of the way the HSI is made. When yourotate the Course Selector knob, the one down there on the lowerleft 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 makea 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 thetutorial on ILS Approaches.We will not cover the approach in great detail, only those thingsthat are distinctive to the Back Course.

Here is a Back Course Approach into the oldDenver Stapelton Airport:

Thereit is in plain English. This is a Back Course Approach. And, ithas DME. Because there is no Glide Slope, there has to be a wayfor you, the pilot, to know where you are on the approach. Incould 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. Seethat "FRONT CRS 351"? The reason that it is there isso you can set your HSI to the Front Course heading so that youwill get "correct" Left / Right indications during theapproach.

Thereprobably is a "remnant" of the Glide Slope for the ILSrunway 35 approach drifting around in the airspace.

Hereis the step-down profile. Notice that the first DME fix is referencedoff of the Denver VOR, and that all of the following ones arefrom the Localizer. Also, notice that Maltese Cross at the NDB?That indicates that this is the Final Approach Fix, where youstart your Final Approach Segment of the approach.

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

Approach Control would have you at 9,500 feetwhen you are established on the 15 mile arc off of the DenverVORTAC. Once you intercept the Localizer, and are establishedon it, you would step down to 9,000 feet. At the 13.0 DME fixyou would descend out of 9,000 for 7,000. At Thornton, or the5.7 mile DME fix, you would descend out of 7,000 for the MDA,Minimum Descent Altitude, of 5,580 feet. The Missed Approach Pointis at .8 DME.

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

Localizer Back Course Approaches? Hey, pieceof cake!

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

This tutorial is available on aCD

This tutorial, along with additional content, is availableon a CD. Click here formore information.

Hal Stoen