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

I want to walk you through a really practical application of the 1 in 60 rule — one you'll use every time you fly an instrument approach. This is about calculating your rate of descent on a glide slope. Let's start with the core idea. The 1 in 60 rule tells us that if your ground speed is 60 knots, your range — the distance you cover along the ground in one minute — is one nautical mile. On a standard 3° glide slope, that one mile of forward travel corresponds to a height loss of 300 feet. So, at 60 knots ground speed on a 3° glide slope, your rate of descent — ROD — needs to be 300 feet per minute. Now, if your ground speed doubles to 120 knots, your range in one minute becomes two miles. On that same 3° glide slope, two miles of forward travel means you lose 600 feet of height. So your required rate of descent becomes 600 feet per minute. From this pattern, we get a very handy rule of thumb for a 3° glide slope only: Rate of Descent in feet per minute equals 5 times your ground speed in knots. So ROD = 5 × ground speed. That's your quick mental calculation for a standard 3° approach. But what if the glide slope is something other than 3°? The easiest method is to first solve for a 3° glide slope using that 5 × ground speed formula, and then factor the answer by the actual glide slope angle divided by 3°. In other words, you multiply your 3° ROD by (actual glide slope ÷ 3°). Let me show you with an example. Suppose you need to maintain a 4° glide slope at a ground speed of 100 knots. First, for a 3° glide slope, your ROD would be 5 × 100 = 500 feet per minute. Then, for the actual 4° glide slope, you take that 500 and multiply by 4/3. That gives you 667 feet per minute — call it 670 for practical purposes. Now, there's another situation you'll face: changing your speed while already on the glide slope. The rules are straightforward. To maintain the glide slope, if you decrease ground speed, you must decrease your rate of descent. If you increase ground speed, you must increase your rate of descent. For a 3° glide slope only, the amount of change in ROD is simply 5 times the change in speed. So Change in ROD = 5 × change in speed. And remember, the same caveat applies for glide slopes other than 3° — you'd factor that change accordingly. Let's work through two examples from the book. Example 1: You're on an ILS approach to London Heathrow using a 3° glide slope. Your ground speed is 140 knots. What rate of descent do you need? Since it's a 3° glide slope, you use the 5 × ground speed estimate. ROD = 5 × 140 = 700 feet per minute. Example 2: You're approaching London City Airport on a glide slope of 5.5° at a ground speed of 120 knots. What rate of descent is required? First, find the 3° ROD: 5 × 120 = 600 feet per minute. Then factor by the actual glide slope over 3°, so 600 × 5.5/3. That gives you 1100 feet per minute. So in summary, for any glide slope, the process is: calculate ROD for 3° using 5 × ground speed, then multiply by (actual glide slope ÷ 3°). For speed changes on a 3° slope, the ROD changes by 5 times the speed change. These are practical, cockpit-ready calculations you'll use on approach.