Then by — Page 84, Lesson 89

I want to walk you through how the navigation computer handles fuel flow, distance, speed, and time problems. Let's start with the fuel flow example. We've already seen how to set up a rate per hour using the black triangle at 60 on the inner scale. Now, by putting 60 minutes on the inner scale against 2000 on the outer scale, we are actually dividing 2000 by 60. If you look at the red arrow in the figure, you are getting the rate per minute — which comes out to 33.33 kilograms per minute. So the black triangle at 60 represents one hour, and when you align it with a quantity on the outer scale, you're setting up a rate per hour. But if you align the 60-minute mark directly against that same quantity, you're dividing to get the rate per minute. Now let's move into distance, speed, and time problems. The rule is simple: on the navigation computer, the outer scale is always distance or speed, and the inner scale is always time. Keep that firmly in mind. Let's work through the first example. How far do you go in 48 minutes at 120 knots ground speed? First, set what you know. You bring the black '60' triangle against 120 knots on the outer scale. That sets your ground speed. Then you find 48 minutes on the minutes scale — that's the inner scale. Read off the answer on the outer scale: 96 nautical miles. Next example: in 30 minutes you have covered 90 nautical miles over the ground. What is your ground speed? Again, set what you know on the computer. Set 30 minutes on the inner scale against 90 nautical miles on the outer scale. Now read off the ground speed against the '60' black triangle. The answer you see is 18.0. But is that 18 knots? At 18 knots you would stall. At 1800 knots you would strip the wings off. Common sense tells you this must be 180 knots — which is the right answer. So always check your order of magnitude. Final example: your ground speed is 220 knots and you have 57 nautical miles to go. How long will it take? As before, set what you know. Move the black '60' triangle against 220 knots. Now bring the cursor round to 57 nautical miles on the outer scale — because it is a distance. Read off the time on the inner scale. The answer is 15.5. Are we expecting 15 minutes or 150 minutes? 57 nautical miles is about a quarter of 220, so about a quarter of an hour is obviously the right answer. It is 15.5 minutes. So the pattern is always the same: set what you know, then read off what you need. The black triangle at 60 on the inner scale is your key for speed. The outer scale holds distances and speeds. The inner scale holds time in minutes. And always, always check your order of magnitude with common sense.