On a standard three-degree ILS at 200 feet above TDZE, you’re 3816 feet from touchdown. Put these pieces together and it’s clear that they orient and guide, but using them for measurement is interesting. Approaches with slopes of less than 2.75 degrees get an extra six bars to reach 3000 feet. The choice of 24 rows of lights 100 feet apart is no coincidence since most Cat-I ILS approaches require 2400 RVR or mile visibility. Overall, this bright road 2400 feet up to the threshold gives the pilot a visual indication of crab angle and helps prepare for the landing. Some runways have these side-lights continue past the threshold, recessed into the runway itself, but those in-runway ones are white. It’s lighting before the green threshold bar. These are the red “side-lights” mentioned in the FARs, and they’re red to remind you this isn’t runway you’re looking at. Only the glideslope or VASI/PAPI give you descent angle guidance, but the shape of this box also helps. These essentially make a runway-shaped box in the 1000 feet before the runway. Two columns of red, three-bulb barrettes connect the far ends of the roll bar with the green threshold lights of the runway. This barrette is mainly for Cat-II or lower approaches. At 500 feet before the threshold, there’s a mini roll bar of just three extra lights on either side of white center row.
This is often called the “decision” or “roll” bar and both identifies where the last 1000 feet of the ALS starts, and provides some roll information for the pilot to visually keep the wings level in the transition to visual. The SFLs stop at a white bar 1000 feet from the threshold that’s 100 feet wide-visually about three times as wide as the other rows so far. There’s a lot more lighting in that last 1000 feet than just the rows of five white lights. But if they distract you, put on your best Elmer Fudd voice and ask Tower to “Kill da wabbit.” Their purpose is to help you acquire the ALS as a whole and orient to the runway as it comes out of the gloom. These are the sequenced flashing lights (SFL), sometimes called the rabbit. The flashes fire in sequence so it appears as a ball of light zipping down the ALS centerline, towards the runway, twice a second. These rows of five white lights continue to the threshold, but the flashing lights are removed in the last 1000 feet. There are typically 14 of these rows with the flashing light in the center. Each flashing light is in the middle of a narrow row of five white lights parallel to the threshold. Initially leading you from oblivion to the runway is a column of flashing lights extending from the runway center line. The extra descent to 100 feet can be helpful, but unless you’re CAT-II or better qualified, the red light part is pointless.The system with all the bells and whistles most common in the U.S. Tell the voice of that CFI in your head to stuff it on this one. (Note that in Canada and some other countries, the ceiling is controlling, not the visibility as in the U.S.)Īlso missing in this discussion is the CFI favorite about not descending lower than 100 feet above touchdown-zone elevation by the approach lights alone unless you also see the red side-row lights or terminating bars. By knowing which ALS to expect, you get a visual yardstick of that flight visibility. If you don’t also have the required flight visibility, you can’t go below DA/MDA. FAR 91.175 says in two places that it doesn’t matter what you can see. Even without knowing exactly what lights to expect, you can get some orientation and guidance-follow these lights to the pavement. ALSs all have some combination of lights, sometimes flashing, to get your attention and offer some direction. Know your lights and the ALS will better fill that need.Īnyone who has popped out of low clouds knows the runway isn’t always centered in the windscreen or even at your roll or pitch angle. We talked about this in general (“Making IMC Transitions,” October 2012 IFR), but an ALS performs three essential functions: orientation, measurement and guidance. It bridges the gap between instrument flight and visual flight. It’s nearly certain that you’ll first see the ALS, allowing you to descend to 100 feet AGL and search for something resembling a runway.īut the ALS deserves more respect than just a bunch of lights short of the threshold. As much as we drill instrument students on the 10 items from FAR 91.175 (c) you could see to go below DA/MDA, in the real world if we see anything that might be part of the runway, we land on it.
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