Localizer Approaches

Localizer approaches

What's a Localizer approach?

In a nutshell, a Localizer approach is an ILS approach withoutthe Glide Slope. And, because there is no glide slope informationprovided, the Localizer approach is considered to be a non-precisionapproach.

It is assumed that the reader has already read the tutorialon the ILS approach. If you have not done so, and are not familiarwith the ILS, it's components, and how to fly it, you may reviewthe ILS tutorial by clicking here.

In this discussion we will cover what is different about theLocalizer from the ILS.

Most every ILS approach plate is also a Localizer approachplate.

Here is the ILS approach platefor Wichita, Kansas

A detail of the "Landing Minimums" section:

See that ?The "GS out" means Glide Slope out of service. If theGlide Slope component of the ILS is out of service, then the approachreverts to a Localizer approach. The minimums increase becauseyou no longer have vertical guidance. In this case by 140 feet,from 200 feet to 340 feet. The visibility minimums do not changebecause that is a function of the Approach Lighting System. Seethat

?That shows that if the "RAIL" is out of service youcan still descend to the same vertical minimum of 340 feet, butthat your visibility minimum increases to a RVR of 4,000 feetor 3/4 of a mile.

Remind me again. What's this "RAIL","RVR", and "ALS" stuff?

RAIL is the sequential line of strobe lights that leadin to the runway. Seen from an approach vantage they appear tobe a single white ball of fire leading to the touch down zone.

RVR is an electronically derived visibility value thatis usually determined by a unit that is placed near the touchdown zone. The reading is transmitted to the Control Tower sothat the controllers can provide this critical information tothe pilot on the approach. This information differs from "reportedvisibility", which is what the "1/2, 3/4, and 1"in the Landing Minimums section are. Reported visibility is takenby a ground observer reference objects on the ground that areat a known distance from the observer. Obviously, the "machine"derived information is more accurate.

ALS stands for the Approach Lighting System, the arrayof lights that are in front of the runway.

OK, but I see that when you havethe full ILS, the minimum descent altitude is referred to as "DH",but on the Localizer part it says "MDA". What's up withthat?

DH stands for Decision Height. It only applies to approachesthat provide glide slope information, such as the ILS, MLS (MicrowaveLanding System), or PAR (Precision Approach Radar). It's a conceptthat's a little difficult to grab. Look at it this way: You'redescending on your approach by referencing the Glide Slope. Intheory, you could follow it right on down until you hit the ground.At some point, usually 200 feet above the ground on an ILS, youmust decide when to break off the approach or continue on visuallyto a landing. The height that you do that is the Decision Height.

MDA stands for Minimum Descent Altitude. Unlike an approachwith a Glide Slope, you descend to the minimum altitude as statedon the approach plate. Once there, you start looking for the runway.The lowest you can go is the MDA.

At some point you will have to see the runway and land, orexecute a Missed Approach. On an approach with a Glide Slope thelowest that you can descend, and the Missed Approach point, areone and the same- 200 feet above the ground. On a non-precisionapproach (one without a glide slope), you descend, look for therunway, and continue until you reach the MAP, Missed ApproachPoint.

In a perfect world, you would reach the MDA and the MAP atthe same time. In a less than perfect world, and on a bad dayflying, you would reach the MAP before you reached yourMDA, thus probably blowing the approach because you didn't getdown as low as permitted to see the runway. Knowing this, youdescend to your MDA before you reach the MAP, affordingyourself the maximum opportunity to see the runway.

Enough of this MDA and MAP, I'mstarting to get a headache. How do you shoot a Localizer Approach?

Let's go back to the Wichita plate again.

See that "dashed line" in the Profile View? Thatrepresents the Localizer approach. Notice how it differs fromthe ILS at the Outer Marker.

Whenshooting the ILS approach, you would be level at 2,700 feet untilyou intercepted the Glide Slope, just before arriving at the OM.Then you would descend following the Glide Slope. However, onthe Localizer approach, you would remain at 2,700 untilyou reach the OM. Then you would start your descent towardsthe MDA of 1,570 feet. Notice how the view shows the Localizerportion nice and even, right down to the MDA?

Well, in the "real world" probably not too likelyto happen. Most Localizer approaches would look more like this:

The"Great Bureaucracy of aviation" realizes this, and takesit into account when establishing procedures and minimums fornon-precision (no Glide Slope) approaches.

OK, so you cross the OM at 2,700 feet and start your descenttoward the MDA. You level at 1,570 feet and check your DME reading.The MAP is at 1.4 miles. (The DME is derived from the Localizerfrequency. When you tune in the Localizer in your nav. radio theDME information will be displayed.) If you see the runway environmentbefore the DME counts down to 1.4, you land. If you reach the1.4 MAP and don't see the runway, it's Missed Approach Time.

One last thing on ILS and Localizer differences. Look at theend of the approach presentation in the profile view. See theMAP's, the solid line for the ILS course, and the dashed for theLocalizer?

Noticethat the ILS approach crosses the MM at 220 feet and continuesjust a little farther until it reaches the DH at 200 feet. Thenthe line becomes an arrow, representing the MAP. On the Localizerportion, you're already as low as you can descend, 1,570 feet,so you are level when you reach the MAP, which is at 1.4 DME.

This ends the tutorial on Localizer approaches. If anythingis not presented in a clear fashion, or not to your satisfaction,or if there are any errors, please contact me

This tutorial is available on aCD

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

October 8, 2000

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