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

I want to walk you through the other main applications of the 1 in 60 rule. We've already seen how the rule works for track error in the plan view — now we're going to look at it from a side elevation perspective, which lets us work with glide slopes. The applications you may be examined on are: calculating height on a glide slope, calculating rate of descent to maintain a glide slope, calculating change of speed on a glide slope, and calculating distance off the required track using radio aids like VOR and DME. Let's start with height on a glide slope. To find a rule of thumb for this, we use the same track error technique, but we view the situation in side elevation instead of plan. The glide slope angle is referred to as Z° for convenience. You only need to understand how to apply the formula, not how it's derived. Look at Figure 12.1. Suppose the range is 1 nautical mile. One nautical mile is 6080 feet, but for the 1 in 60 rule we accept an approximation of 6000 feet. This introduces only about 1% error, which is perfectly acceptable for a rule of thumb. Now let's build the pattern. If Z equals 1°: 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 — that's one nautical mile — the height is 100 feet. If Z equals 2°: when the range is 60 feet, the height is 2 feet. When the range is 600 feet, the height is 20 feet. When the range is 6000 feet, the height is 200 feet. This 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 for a 2.5° glide slope, that's 250 feet per nautical mile. For a 3° glide slope, 300 feet per nautical mile. For a 3.5° glide slope, 350 feet per nautical mile. For a 5.5° glide slope, 550 feet per nautical mile. Let me show you how this works with examples. Example 1: on a 3° glide slope at 4 nautical miles from touchdown. Height in feet equals 3 times 100 times 4, which gives 1200 feet. Example 2: on a 5.5° glide slope at 3 miles from touchdown. Height equals 5.5 times 100 times 3, which gives 1650 feet. A note on typical glide slopes: 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 Airport has an extremely steep glide slope of 5.5°. In all problems, the glide slope will be given to you. Now let's move to 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 in 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.