The Navigation Computer - Distance, Speed, Time and Conversions — Page 94, Lesson 95

I want to walk you through a couple of worked examples on the navigation computer that deal with fuel calculations, and then we'll move into distance conversions. Let's start with the first fuel example. Your fuel flow is 130 imperial gallons per hour, and you have 380 gallons left above your reserves. The question is: what is your Safe Endurance? Safe Endurance means the time you can safely fly on the fuel you have, not counting your reserve fuel. Here's how you set it up on the navigation computer. First, set the black triangle — that's the '60' index mark on the inner scale — against 130 on the outer scale. The black triangle represents 60 minutes, so by aligning it with 130 gallons per hour, you've set the rate. Next, align the cursor with 380 on the outer scale — that's your available fuel quantity. Then read off the answer on the inner scale, which gives you 175.5. But 175.5 what? You use the yellow ring — that's the minutes-to-hours conversion scale around the inner scale — to see that 175.5 minutes is 2 hours and 55.5 minutes. So your Safe Endurance is 2 hours and 55.5 minutes. Now the second fuel example. You have used 3700 litres of fuel in the last 1 hour and 45 minutes. What is your fuel consumption rate? First, find 3700 on the outer scale and align the cursor with it. Then bring 1 hour 45 minutes — which is 105 minutes — on the minutes scale into alignment with the cursor. If you need to, use the yellow scale to convert hours and minutes to minutes. Once that's set, read off the consumption rate against the black '60' triangle. The answer you read is 21.1. But a common-sense check tells you that this must actually be 2110 kilograms per hour — the decimal place is determined by the magnitude of the numbers. For reference, the electronic calculator solution gives 2114.29, so the navigation computer gives you a very close approximation. Now let's move to distance conversions. The navigation computer can also handle conversions for distance, volume, and weight. For distance conversions, around the edge of the outer scale you'll find various index marks with red labels. There are three of these for distance conversions. The key one we're using here is the 'km-m-ltr' index — that stands for kilometres, miles, and litres — and the 'naut m' index for nautical miles. Here's the relationship you need to remember. The distance from the Equator to the pole is 10,000 kilometres, and it's also 90 degrees of latitude times 60 minutes per degree, which is 5400 nautical miles. So 10,000 kilometres equals 5400 nautical miles, which means 10 kilometres equals 5.4 nautical miles. That's why on the outer scale, the km-m-ltr index is at 10, and the naut m index is at 54 — or 5.4, depending on where you read the decimal. Let's work the example: convert 148 kilometres to statute miles and to nautical miles. You align 148 on the inner scale with the km-m-ltr red index on the outer scale. Once that's set, you can read the equivalent in statute miles and nautical miles from the appropriate index marks. The figures show you exactly how this alignment looks on the computer.