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

Let's get straight into the calculations you'll actually fly with on an ILS approach. I want to walk you through the 1:60 rule and how we use it to predict two things: the height you should be at on the glide path, and the rate of descent you need to hold. First, the setup. You're flying a 3° glide path, you're 4 NM from touchdown, and your groundspeed is 150 knots. The 1:60 rule is our quick mental check. The idea behind it is that at 60 units of range, a 1° angle subtends roughly 1 unit of height. That's where the "1:60" name comes from. Let's find the height first. The formula is: Height equals Glide Path Angle times Range, divided by 60, all multiplied by 6076 feet. That 6076 is the number of feet in one nautical mile. So for our example: 3° times 4 NM, divided by 60, times 6076 feet. That gives us 1215 feet. But we can simplify this. Notice that 6076 divided by 60 is roughly 100. So the simplified version is: Height equals Glide Path Angle times Range times 100. For our numbers: 3 times 4 times 100 equals 1200 feet. Now, a word of caution. The trigonometric solution, using accurate values, gives a height of 1274 feet. So the simple 1:60 formula underestimates the height by about 74 feet. That's fine — we're using this as a check for gross errors, not for precise navigation. It's a sanity check. Now for the rate of descent, the ROD. The logic is: find the change of height per nautical mile, then multiply that by your speed in nautical miles per minute. The formula is: ROD equals Glide Path Angle times 1, divided by 60, times 6076 feet, times Ground Speed divided by 60, in feet per minute. Again, we simplify. The 6076 over 60 becomes 100, so we get: ROD equals Glide Path Angle times 100, times Ground Speed over 60. For a 3° glide path, that's 3 times 100, which is 300, times Ground Speed over 60. And 300 over 60 simplifies to 5. So for a 3° glide path, ROD equals 5 times Ground Speed, in feet per minute. For our example: 5 times 150 knots gives 750 feet per minute. Again, this is an approximation. The trigonometric value is 796 fpm, so we slightly underestimate. But again, it's a check. One critical note: this simplified formula, ROD equals 5 times Ground Speed, is only valid for a 3° glide path. If you're flying any other glide path angle, you calculate for a 3° glide path first, then divide by 3 and multiply by your actual glide path angle. Or, of course, you can use your Navigation Computer. So to recap: height check is Glide Path Angle times Range times 100, and for a 3° path, ROD is 5 times Ground Speed. Both are approximations that underestimate slightly, but they're your in-cockpit sanity checks for gross errors.