Let me set the scene for you — Page 189, Lesson 166

We're starting a brand-new chapter now, Chapter 10, and it's called "The 1 in 60 Rule." This is one of those beautiful, practical tools that pilots use all the time, because it lets you measure angles without ever pulling out a protractor. Let me set the scene for you. When you're flying, one or both of your hands are on the control yoke, and the other might be on the throttle some of the time. Either way, that doesn't leave you many spare hands to measure an angle on your map with a protractor. So over the years, pilots have had to find other ways to calculate angles quickly and easily. One of these is called the "1 in 60 rule." Here's the core idea. Imagine you have a line on a piece of paper exactly 60 mm long. Now raise a perpendicular at one end, exactly 10 mm high, and join them with a hypotenuse. That creates a triangle, and it creates an angle at the base, which we'll call angle z. If the adjacent side is 60 cm long and the opposite side is 10 cm long, then the angle z will be 10°. Similarly, if the adjacent is 60 cm long and the opposite is 5 cm high, z will be 5°. If the opposite is 8 cm high, z will be 8°, and so on — up to a maximum of about 20 degrees, when the theory starts to break down. So the rule of thumb is: for every 1 unit of offset over a 60-unit distance, you get 1 degree of angle. That's why it's called "1 in 60." Now let's bring this into the cockpit, because that's where it really shines. Suppose you're flying along a planned track of 100°(T) — that's 100 degrees true. You've flown 60 miles since your last fix, which was on track. You now pass over a clearly recognizable ground feature, and you note from your map that it is 4 miles right of track. Using the 1 in 60 rule, you can establish that your actual track flown — that's the Track Made Good, or TMG — was 4 degrees right of your planned track. So you have actually flown a TMG of 104°(T). You are now in a position to calculate some sort of correction, and you have not had to use a protractor to establish the angle. We will discuss how you make the appropriate correction in the next chapter. Let me make sure the terminology is crystal clear. "Track Made Good" is the actual path you've traced over the ground, as opposed to the track you planned. And the "(T)" after the degrees means "true" — that's the direction measured relative to true north, not magnetic north. That's a distinction you'll see constantly in navigation. So the whole beauty of this rule is that it turns a geometry problem into simple arithmetic. You flew 60 miles, you're 4 miles off, so you're 4 degrees off. No protractor, no trig tables, just a quick mental calculation. And the limit is important: it holds up to about 20 degrees, and beyond that the theory starts to break down, so don't try to stretch it further. That's the 1 in 60 rule in a nutshell. It's a tool you'll use again and again in navigation, and now you know exactly where it comes from and why it works.