Decision Height: Another Look at Minimums

Mr. DeMille thinks you're ready for your close up. No, not the famed director; I'm speaking of the controller who just gave you a clearance for the ILS. How close to the runway can you go? How do you measure flight visibility at the missed approach point? Did you know you can actually have two decision altitudes on an ILS? Here's how it works.

FAR 91.175 is very specific about when you may descend below Decision Height (DH) or Minimum Descent Altitude (MDA). You must meet two requirements before leaving DH or MDA. First, you're required to have the minimum flight visibility as shown in the published instrument approach procedure. Second, you must have at least one of ten individual references for the intended landing runway and it must be distinctly visible and identifiable to you (the pilot). Let's examine the runway references first.

I've broken the required runway references down into two categories: those on the approach side of the runway threshold and those on the landing side of the threshold (Figure 1). Nine of the first ten runway references are located on the landing side of the runway threshold. These items are:

1. The threshold.
2. The threshold markings.
3. The threshold lights.
4. The REIL lights.
5. The VASI.
6. The touchdown zone or touchdown zone markings.
7. The touchdown zone lights.
8. The runway or runway markings.
9. The runway lights.

The tenth item is located on the approach side of the runway threshold:

10. The approach light system (with a few additional qualifications I'll discuss shortly).

OK, here's the point. To go below DH or MDA, in addition to flight visibility, you must have at least one of the first nine items in sight or, if one of these are not visible to you, the approach lights must be in sight. Have you even wondered why you can use the approach lights in lieu of the first nine items to go below DH or MDA? There's a good reason for this.

Glideslopes have variable vertical angles and marker beacons vary in their distance from the runway. This causes the DH point to vary its horizontal distance from the runway threshold as shown in Figure 2. It's possible to be at DH on an ILS approach and be at a distance from the runway threshold that's greater than your required minimum flight visibility. Think about that for a second! You can have the minimum flight visibility for that approach and still not be able to see the runway threshold! Here's an example of how this might happen.

Figure 3 shows four examples of DH's and their distance from the runway threshold. We'll assume that DH is located at the point where the middle marker and the glideslope intersect. Thus, if you're on the glideslope, the middle marker should activate when you're at or near DH. This isn't always the case but it's a close enough approximation for our example. The number directly underneath (or near) the middle marker is the distance in nautical miles to the runway threshold. This information allows you to estimate your flight visibility when you're at DH.

For example, Figure 4 shows the middle marker at a distance of .4 nm (nautical miles) from the threshold. A distance of .4 nm equals 2,400 feet (1 nm is approximately 6,000 feet. Thus .4 x 6,000 = 2,400). Flight visibility is measured in statute miles with 1/2 mile equivalent to 2,400 feet. Therefore, if you're at DH and can see the runway or beyond, you know two things: You've have the required flight visibility (we'll assume 1/2 mile is required here) and one of the 10 items necessary for descent below DH. I wish the math were always this easy.

Suppose you're on an ILS approach where the middle marker is located at a distance of .5 nm, .6 nm, or .7 nm from the threshold as shown in Figure 3B, C & D. If 1/2 mile visibility is still required for the approach, then you may not be able to see the runway threshold at DH on any of these approaches. This is where the approach lights come in handy as a descent aid.

Let's assume the middle marker is .6 nm (3,600 feet) from the threshold as shown in Figure 5. At DH, if you have 1/2 mile flight visibility (2,400 feet), you still can't see the threshold. If the approach lights are visible, however, then you can use them to descend.

The regulations allow you to use the approach lights for descent under the assumption that you'll eventually pick up one of the first nine runway reference items on the landing side of the threshold. But, just in case you don't, here's where a limit is placed on using the approach lights for descent.

You can descend using the approach lights as a reference, but you cannot go less than 100 feet above the touchdown zone elevation unless you have at least one of the first nine items on the landing side of the threshold. (The touchdown zone elevation "TDZE" is the highest part of the first 3,000 feet of the runway. This value is always shown on approach charts. See Figure 6.)

If you think about it, under certain conditions you actually have two heights or positions where you make a decision on an ILS. The first one if the official DH. At this point, if you have the required flight visibility and can see approach lights, you may continue your descent. When you reach 100 feet above the TDZE (the second point), you can't go any lower unless you have one of the first nine runway reference items. There is one exception to this.

The regulations state that you may not descend below 100 feet above the TDZE using the approach lights as a reference unless the red terminating bars or red side row bars are also distinctly visible and identifiable. There are only a few approach lighting systems with red terminating or red side row bars. The ALSF-1 and the ALSF-2 are the most common as shown in Figure 7.

At 100 feet above the TDZE, if the red terminating or the red side row bars are distinctly visible and identifiable to you, you may continue your descent. Why is this allowed? First, notice that the red terminating bars are right next to the runway threshold lights. In other words, if you can pick out the red terminating bars then you know the landing threshold and threshold lights are nearby. Therefore, the red lights help you find the threshold lights as well as the runway. Additionally, red offers good contrast to the other lights at close distances.

The red side row bars are even more sophisticated than red terminating bars. They extend outward from the threshold approximately 1,000 feet. The wide horizontal display of red light bars provides pilots with the thing they need most when close to the ground ? over-the-cowling pitch and bank information when making the transition from IMC to VMC. At 100 feet above TDZE these red lights look like a prairie fire and are easy to see, even under very low visibility conditions. That's why the red side row bars (ALFS-2 systems) are common at airports having Category ll instrument approaches.

It's important to know that not all approach lighting systems have red terminating or red side row bars. In fact, the most common approach lighting system, the MALS (Medium intensity Approach Lighting System), doesn't have them as shown in Figure 8. So, if you end up at 100 feet above the TDZE and can't see one of the first nine runway references, a missed approach is necessary. This is one reason why it's very important to know what type of approach lighting is available at the airport before you shoot the approach. There are many other reasons too, but they are beyond the scope of our present discussion.

Let's remember that even though we can descend using the approach lights as a reference, we must have and maintain the required flight visibility all the way to touchdown. I share this information with you not because I want you making approaches under low visibility conditions without seeing the runway. You're big boys and girls, so you decide what's best for you within the limits of the FARs. I share this with you because the knowledge increases your confidence! It makes aviation seem less of a mystery when you know the reason behind the rules.

Some of you are feeling a little uncomfortable right now with the concept of two decision points, aren't you? You're probably saying, "Hey Rod, how am I supposed to tell when I'm 100 feet above the TDZE? After all, I'm so close to the ground that, with one sneeze, I'll hear the wheels make the touchdown sound: 'Ert, ert, ert.'" Good question grasshopper! Here's another bit of insight for you.

Notice that all standard approach lighting systems in the USA have something known as a decision bar, as shown in Figure 9. The decision bar is a horizontal light bar reference located 1,000 feet from the runway threshold. Would you believe me if I told you that you can tell your approximate altitude above the TDZE by identifying where the decision bar is in relation to your airplane? You can. Here's how.

The electronic glide slope "theoretically" intersects the runway approximately 1,000 feet beyond the threshold (Figure 10). From this point, let's follow the glideslope backwards, in the opposite direction of the approach. You'll notice that the glideslope centerline angles upward so as to cross the threshold at a height of approximately 50 feet. Here's where that basic geometry you learned in school is useful. If we continue outward another 1,000 feet, we should find the glideslope centerline an additional 50 feet higher than it is at the threshold, or at 100 feet above the TDZE. What light bar do we find at a distance of 1,000 from the runway threshold? It's the decision bar, of course. Therefore, at 100 feet above the decision bar, we should find the glideslope centerline. Do you see where I'm going with this?

If you're on the glideslope, as you cross directly over the decision bar, you'll be at an approximate altitude of 100 feet above the TDZE. It's basic geometry. Euclid would be proud of you for using his stuff (so am I). There's no need to look at your altimeter to determine when you're at 100 feet above the TDZE. The decision bar is relatively easy to sense peripherally as it goes directly underneath the airplane. This is just another way the approach lighting system makes the pilot's job much easier.

Wait, there's more in Part 2, next week. In the meantime, remember that I'm not sharing this information because I want you to go out and shoot approaches to minimums while using the approach lights as a descent reference when you can't see the runway threshold. Yikes! That's not always smart (even if you do have a lot of life insurance). Use a little common sense here. After all, you'd never buy a TV from a man on a street corner who's out of breath, would you? Of course not. That's because you have common sense. Therefore, why shoot approaches to minimums if you're not up to it? But, if you are proficient, at least you know how to do it in accordance with the regulations.

There's a lot of creative intelligence that goes into the design of an instrument approach procedure and approach lighting system. Next week we'll take a look at the difference between RAIL and sequence flashing lights and we'll talk more about the decision bar. We'll also look at how this information can be useful in making non precision approaches at airports having approach lights. Much more to come! Tune in next week.