Shedding More Light on Those Approach Lights, Part 2
Are You Ready for Your Close-up?
An airport's approach lights are a pilot's welcome-home sign. Aside from being pretty, they serve the important function of making the transition from IMC to VMC easier and safer. Additionally, the approach lighting system helps pilots make better estimates of in-flight visibility.
As an airplane approaches DH while on glideslope, the decision bar appears just ahead and a little above the nose as shown by Figure 1. Eventually, the decision bar slips below the airplane's nose and out of direct sight as you continue below DH.
Remembering last week's discussion, at DH your airplane is typically .4 to .7 nautical miles from the runway threshold (that's 2,400 feet to 4,200 feet). Since the decision bar is 1,000 feet from the threshold, how can it appear to slip below the airplane's nose at DH? Geometry is the answer.
If, at DH, the airplane is several thousand feet from the threshold and the decision bar is only 1,000 feet from the threshold, then the decision bar must appear somewhere out in front of the airplane as shown in Figure 2. If we consider an average pilot, having an average sitting height, flying an average airplane, configured in an average way, and everything else in the universe (and all parallel universes) being average, then the decision bar should appear just above the cowling when the airplane is at DH. Of course, as you continue down on glideslope, you'll eventually sense the decision bar directly underneath you when you're approximately at 100 feet above the TDZE. (Remember, these numerical values are all approximations, but they're accurate enough for the assumptions we're making.)
The decision bar's location provides you with an advantage when transitioning from IMC to VMC at DH. Abandoning those little cockpit dials and transitioning to visual cues at DH isn't necessarily an easy task. As you look up to see the runway or approach light environment, you must, by default, take your eyes off the attitude indicator. If it's dark, rainy or snowy, (or the kids in back have their hands over your eyes), you may have difficulty identifying the lateral (bank) cues necessary to keep the airplane flying straight. The decision bar's horizontal spread of bright white lights acts like an external attitude indicator, providing you with visual bank references as you seek other runway cues (Figure 3).
Now, how would you feel about being at DH, searching for runway cues and being simultaneously blinded by flashing strobe lights? Aside from the night blindness this offers, feeling your pupils slam shut in the face of these bullet-like, mini-novas isn't much fun. That's why sequence flashing lights (strobe-type lights) go up to, but not beyond, the decision bar.
Take a look at Figure 4. Notice that the sequence flashing lights stop at the decision bar. If you're on the approach, the sequence flashing lights (along with the decision bar) should begin to pass below the cowling at DH. Therefore, they are less likely to blind you if they're hidden beneath the airplane. They can, however, still be an annoyance. If you want to stop the strobe action before you arrive at DH, simply ask the controller to kill the rabbit. He or she should immediately shut them off with this command. Please don't use a hybrid variation of this statement like, snare the hare or bag the bunny. If you do, you might find a holding pattern in your future, or you may find that the controllers have a lot of rabbits to kill before they get to yours.
As an aside, some airports have RAIL (Runway Alignment Indicator Lights) instead of SF (sequence flashing) lights. Sequence flashing lights are imbedded in the approach lighting system, RAIL are not (Figure 5). RAIL systems usually consist of five flashing lights installed before the beginning of a 1,400 foot MALS (Medium Intensity Approach Light System). With RAIL installed, the MALS becomes a MALSR (Medium Intensity Approach Light System with RAIL).
If you see a lighting system acronym with the letter "F" at the end instead of "R," then the sequenced flashing lights are imbedded in the approach light system and go all the way to the decision bar (Figure 6). A MALSF (Medium Intensity Approach Light System with sequenced Flashing lights) is an example of a lighting system having 1,400 feet of lights and RAIL ending at the decision bar (Figure 6).
Approach lights are very important. You should know what type is available before you begin any approach. This is another way of saying that you should know what type of lighting system you'll expect to see at DH. I've seen put-put golf courses lit up in such a way that a pilot might mistake the course for an airport's approach lighting system. Imagine how embarrassing this would be on an NTSB accident report: Airplane came to final stop when it hit the big clown's head. This is why Jeppesen placed a picture of the approach lighting system on the briefing strip of their newer approach charts (Figure 7). Use this picture to determine what the approach lighting system looks like before you begin the approach.
Aside from attitude information, the decision bar also allows you to make more refined in-flight visibility estimates. Figure 8 shows an airplane at DH with a middle marker at .6 nm (3,600 feet) from the threshold. If DH coincides with glideslope interception of the middle marker, then the airplane is approximately 3,600 feet (horizontally) from the threshold.
In this example, let's assume you can see the approach lights at DH. More specifically, you can clearly see the decision bar just over the nose of the airplane. What is your estimated flight visibility? It's approximately 2,600 feet (3,600 feet minus the 1,000 feet distance of the decision bar from the threshold).
In this case you can't see the runway but you can see the decision bar. Therefore, you know you have 2,600 feet visibility (appx. the 1/2 mile needed for this ILS approach) and you can obviously see the approach lights. Since you meet the legal requirements, you may continue below DH as we discussed last week. Of course, I know you'll always use good judgment in making this assumption! Descending below DH in these conditions may be legal, but I'm not suggesting that it's always safe.
Suppose you're on approach to an airport having a middle marker that is .5 nm (3,000 feet) from the runway threshold? At DH, if you can see up to but not beyond the decision bar, then you have approximately 2,000 feet visibility. That's less than the 1/2 mile visibility required for this particular approach. Therefore, you'd need to see at least an extra 600 feet beyond the decision bar to have the required visibility (or 2,600 feet). On a MALSR, the light bars are spaced at 200 foot intervals (Figure 9). On an ALSF 1 or 2, the light bars are spaced at 100 foot intervals (Figure 9). In the above example, with MALSR at this airport, you'd need to see at least three light bars (3 x 200 feet = 600 feet total) beyond the decision bar to have the required 1/2 mile flight visibility.
At this point you're probably thinking, "How am I supposed to count light bars when I'm hurling toward the airport at over a hundred miles per hour while strapped in several thousand pounds of sheet metal?" The answer is: You're not. No one is asking you to count light bars at DH. You're simply making a quick perceptual observation. You're looking to determine if you can see at least two light bars beyond the decision bar. No counting here. Besides, you need to give your visual system a little credit. Try this experiment. Close your eyes, turn your head, open your eyes and shut them quickly. In that short interval, you made a fairly detailed picture of the items in your room, didn't you? When I try in my uncle Fred's living room, in just a fraction of a second, I can recall seeing a Chevy engine block, three pistons, a spear, a 55 gallon drum of oil, a chicken coup and an armadillo. Surely, without any fuss, you can determine if you see one, two or three light bars beyond the decision bar, can't you?
OK, time for that instructor-to-pilot talk again. I don't recommend that you go fishing for the runway below decision height, even if the situation is legal. If the middle marker is .5 nm or less from the threshold, it's not wise to go below DH unless you can see the landing threshold or beyond. Let's face it, these minimums are for folks who are very good at doing this, folks who do it all the time -- professional pilots. I share this information with you because I don't believe we help anyone by keeping knowledge from them. Use this info wisely. How?
Let's suppose you shot an approach to minimums and someone asked you to prove you had the required flight visibility for the approach (I'm assuming, of course, that you did). You're not likely to have anyone ask this, but it's a meaningful exercise nevertheless. All you need to do is recite a few of the sentences from this and last week's article. Once your inquisitor hears things like decision bar, MALSR, red terminating bars or distances of middle markers to runways, he or she will be less likely to think you broke the rules. He'll go looking for someone else who doesn't know these things. Once again, knowledge makes you safer (and wiser).
Several years ago I had an airline pilot in the audience when I presented this information. After the presentation he commented that when he's at DH, if he sees the lights, he keeps right on going. In one sense he's not doing anything unsafe. After all, Part 121 operators can't begin an approach unless the RVR is at or above that shown on the approach chart. Therefore, even before they begin an approach, the visibility in the touchdown zone is at or above minimums. The problem here is that the visibility in the touchdown zone isn't necessarily the visibility between DH and the runway threshold (I call this area [my coinage] the approach light zone).
Remember, touchdown RVR (Runway Visual Range) is an electronic visibility estimate for the runway's touchdown zone (the first 3,000 feet). RVR doesn't measure visibility in the approach light zone. In fact, the point at which a decision is made to continue below DH can be 4,200 feet (.7 nm) or more from where the RVR measurement begins. That's why the regulations require all pilots (regardless of what regulation they operate under: Part 91, Part 121 Part 135, etc.) to have the required flight visibility shown on the approach chart before leaving DH or MDA.
The 1980 preamble to FAR 91.175 speaks to this point very clearly. In relation to commercial operators, the FAA stated, "If an RVR report is currently available.... It does not relieve or take precedence over the pilot's responsibility below MDA or DH to ensure that the required flight visibility exists. Once a pilot has passed the final approach fix, no provision... supersedes the pilot's responsibility to assess visual references below the MDA or DH. Thus even though a report of RVR may indicate that the weather is above minimums and the RVR reports take precedence over other weather reports... for initiating an approach, when below MDA or DH the pilot must, in his judgment, determine that the actual weather conditions are at least equal to the prescribed minimums to continue an approach." Of course, RVR isn't controlling for Part 91 operators. It is, however, an important consideration for commercial operators.
In next week's segment we'll cover how to estimate flight visibility on nonprecision approaches as well as the traps typically associated with these approaches. Stay tuned.
For more information on this subject, see "Light Up Your Night: A Guide to Airport Lighting Systems."
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