Localizer Approaches

Localizer approaches

What's a Localizer approach?

In a nutshell, a Localizer approach is an ILS approach without the Glide Slope. And, because there is no glide slope information provided, the Localizer approach is considered to be a non-precision approach.

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

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

Most every ILS approach plate is also a Localizer approach plate.

Here is the ILS approach plate for Wichita, Kansas

A detail of the "Landing Minimums" section:

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

? That shows that if the "RAIL" is out of service you can still descend to the same vertical minimum of 340 feet, but that your visibility minimum increases to a RVR of 4,000 feet or 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 lead in to the runway. Seen from an approach vantage they appear to be a single white ball of fire leading to the touch down zone.

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

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

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

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

MDA stands for Minimum Descent Altitude. Unlike an approach with a Glide Slope, you descend to the minimum altitude as stated on 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, or execute a Missed Approach. On an approach with a Glide Slope the lowest that you can descend, and the Missed Approach point, are one and the same- 200 feet above the ground. On a non-precision approach (one without a glide slope), you descend, look for the runway, and continue until you reach the MAP, Missed Approach Point.

In a perfect world, you would reach the MDA and the MAP at the same time. In a less than perfect world, and on a bad day flying, you would reach the MAP before you reached your MDA, thus probably blowing the approach because you didn't get down as low as permitted to see the runway. Knowing this, you descend to your MDA before you reach the MAP, affording yourself the maximum opportunity to see the runway.

Enough of this MDA and MAP, I'm starting 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? That represents the Localizer approach. Notice how it differs from the ILS at the Outer Marker.

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

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

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

OK, so you cross the OM at 2,700 feet and start your descent toward 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 Localizer frequency. When you tune in the Localizer in your nav. radio the DME information will be displayed.) If you see the runway environment before the DME counts down to 1.4, you land. If you reach the 1.4 MAP and don't see the runway, it's Missed Approach Time.

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

Notice that the ILS approach crosses the MM at 220 feet and continues just a little farther until it reaches the DH at 200 feet. Then the line becomes an arrow, representing the MAP. On the Localizer portion, 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 anything is 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 a CD

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

October 8, 2000

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