Other Applications of the 1 in 60 Rule — Page 214, Lesson 194

Right, let's pick this up with the 1 in 60 rule applied to glide slopes. We've already established the core idea — that at 60 units along the ground, the height changes by 1 unit for every 1° of angle. Now we're going to turn that into a practical, cockpit-ready formula for rate of descent. Let me walk you through the logic first. If your ground speed is 60 knots, then in one minute you cover one nautical mile. On a 3° glide slope, using the 1 in 60 rule, one mile of range corresponds to a height of 300 feet. So, to stay on that slope, you must lose 300 feet in that same minute — that's a rate of descent of 300 feet per minute. Now double the speed. If your ground speed is 120 knots, in one minute you cover two miles. On that same 3° slope, two miles of range means your height is 600 feet. So you must lose 600 feet per minute. Notice the pattern — the rate of descent scales directly with ground speed. That gives us the key formula, and I want you to remember it exactly: ROD (in feet per minute) = 5 × Ground speed (in knots) — and this is for a 3° glide slope only. Let's check it against our examples. At 60 knots, 5 × 60 = 300 feet per minute. At 120 knots, 5 × 120 = 600 feet per minute. It works perfectly. But what if the glide slope isn't 3°? The book gives you a clean method: solve for a 3° glide slope first, then factor the answer by the ratio of actual glide slope ÷ 3°. Let me show you with their example. You need to maintain a 4° glide slope at a ground speed of 100 knots. First, solve for 3°: ROD = 5 × 100 = 500 feet per minute. Then factor for 4°: 500 × 4/3 = 667 feet per minute — call it 670 for practical purposes. So the full calculation is ROD = 5 × ground speed × (actual glide slope ÷ 3°). Now, there's a second scenario the book covers: what happens when you change speed while already on a glide slope. The rules are straightforward. To maintain the glide slope, if you decrease ground speed, you must decrease ROD. If you increase ground speed, you must increase ROD. And to calculate the amount of that change: Change in ROD = 5 × change in speed — again, for 3° glide slopes only. And the caveat applies: if your slope isn't 3°, you still factor by actual slope ÷ 3°. Let's run their two examples. Example 1: you're on an ILS approach to London Heathrow on a 3° glide slope, ground speed 140 knots. Since it's 3°, use the simple estimate: ROD = 5 × 140 = 700 feet per minute. Example 2: you're approaching London City Airport on a 5.5° glide slope at 120 knots ground speed. Here we must factor. Start with the 3° value: 5 × 120 = 600 feet per minute. Then multiply by 5.5/3. That gives 600 × 5.5/3 = 1100 feet per minute. So the full expression is ROD = 5 × 120 × 5.5/3 = 1100 feet per minute. So the complete picture: for a 3° slope, it's simply 5 × ground speed. For any other angle, factor that result by actual slope ÷ 3°. And when speed changes, the ROD change is 5 × the speed change, with the same factoring if needed. That's the whole toolset for glide slope ROD calculations.