Other Applications of the 1 in 60 Rule — Page 214, Lesson 192

We’re moving on to a new part of the book now: the other applications of the 1 in 60 rule. So far you’ve used it for track errors in the plan view, but now we’re going to tilt that view sideways and use it for glide slopes, and then for radio aids like VOR and DME. Let’s start with height on a glide slope. The idea is to use the same track error technique, but look at the situation in side elevation instead of plan. For convenience, we call the glide slope angle Z°. You only need to understand how to apply the formula, not how it’s derived. Here’s the key approximation. Suppose the range is 1 nautical mile. That’s 6080 feet, but for the 1 in 60 rule we call it 6000 feet. That introduces only about 1% error, which is acceptable. Now watch the pattern. If Z equals 1°, then when the range is 60 feet, the height is 1 foot. When the range is 600 feet, the height is 10 feet. When the range is 6000 feet, the height is 100 feet. If Z equals 2°, then at 60 feet range the height is 2 feet, at 600 feet it’s 20 feet, and at 6000 feet it’s 200 feet. That gives us the rule: to make good a glide slope of Z°, your rate of descent should be 100 times Z feet per nautical mile. So a 2.5° glide slope is 250 feet per nautical mile, a 3° glide slope is 300 feet per nautical mile, a 3.5° glide slope is 350 feet per nautical mile, and a 5.5° glide slope is 550 feet per nautical mile. Let’s apply it. Example 1: on a 3° glide slope at 4 nautical miles from touchdown. Height in feet equals 3 times 100 times 4, which is 1200 feet. Example 2: on a 5.5° glide slope at 3 miles from touchdown. Height equals 5.5 times 100 times 3, which is 1650 feet. A note on the real world: 3° glide slopes are the most common. 2.5° glide slopes are often found at military airfields operating high speed jets. Glide slopes greater than 3° are normally found when airfields are located near high terrain or high buildings. London City has an extremely steep glide slope of 5.5°. In all problems, glide slopes will be given. Now, rate of descent, or ROD. The rate of descent required to maintain a glide slope at a given speed can be estimated using the 1:60 rule. The simple problem is to calculate the height of the aircraft when it is one minute from touchdown. After all, the aircraft has to lose this height in one minute. The range at one minute is given by ground speed divided by 60. So let’s tie it together. You’ve got the height per nautical mile from the first rule, and now you’re converting that into a rate of descent by figuring out how many nautical miles you cover in one minute, which is ground speed divided by 60. That gives you the feet per minute you need to descend.