Instrument Landing System (ILS) — Page 163, Lesson 159

Right, let's get into the ILS calculations. This is where we start turning the glide path into practical numbers you can fly with. The core idea here is the 1:60 rule. It's a simple approximation that says, over a distance of 60 units, a one-unit change in height corresponds to about one degree of angle. We use it to predict two things on an ILS approach: the height you should be at a given range, and the rate of descent you need. Let's work through the example. An aircraft is at 4 NM from touchdown, flying a 3° glide path, at a groundspeed of 150 kt. We want the height and the rate of descent. First, height. The rule gives us a formula: Height = (Glide Path Angle × Range) / 60 × 6076 ft. That 6076 is the number of feet in a nautical mile. But we can simplify it. Since 6076 divided by 60 is roughly 100, the formula becomes Height = Glide Path Angle × Range × 100. So for our example: 3 × 4 × 100 = 1200 ft. That's the height we'd expect at 4 NM on a 3° glide path. Now, the honest truth: the trigonometric solution gives 1274 ft. So the 1:60 rule underestimates the height by about 74 feet. That's fine — we're using this as a check for gross errors, not for precise navigation. Now the rate of descent, or ROD. The logic is: find the change of height per NM, then multiply by the speed in NM per minute. The formula is ROD = (Glide Path Angle × 1) / 60 × 6076 ft × (Ground Speed / 60) feet per minute. Simplified, for a 3° glide path, this becomes ROD = 5 × Ground Speed feet per minute. So for 150 kt: 5 × 150 = 750 feet per minute. Again, this is an approximation. The trigonometric value is 796 fpm, so we slightly underestimate. But as a planning tool, it's excellent. One critical note: this is only valid for a 3° glide path. If you're flying a different glide path angle, calculate for 3° first, then divide by 3 and multiply by your actual glide path angle. Or just use your Navigation Computer. So there you have it — two quick mental checks: height equals glide path angle times range times 100, and rate of descent equals 5 times groundspeed, both for a standard 3° glide path.