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

Let's pick up with the fuel-consumption examples, because they show you exactly how to drive the navigation computer for real-world problems. I want to walk you through two worked examples, then we'll move into distance conversions. First example: your fuel flow is 130 imperial gallons per hour, and you have 380 gallons left above your reserves. What is your Safe Endurance? Safe Endurance is simply how long you can keep flying on the fuel you have, before you hit your reserve. Here's the procedure. Set the black triangle against 130 — that black triangle is the "60" index, and it represents the rate, the gallons per hour. Then align the cursor with 380 on the outer scale — that's your fuel quantity. Now read off the answer on the inner scale: 175.5. But 175.5 what? It's minutes. Use the yellow ring — that's the hours-and-minutes conversion scale — to see that 175.5 minutes is 2 hours and 55.5 minutes. So your Safe Endurance is 2 hours 55.5 minutes. That's the whole logic: rate against the black triangle, quantity on the outer scale, time on the inner scale. Second example, the reverse problem. You have used 3700 litres of fuel in the last 1 hour 45 minutes. What is your fuel consumption rate? Find 3700 on the outer scale and align the cursor. Then bring 1 hour 45 minutes — which is 105 minutes — on the minutes scale into alignment with the cursor. Use the yellow scale to convert hours and minutes to minutes if you need to; here, 1 hour 45 minutes is 105 minutes. Now read off the consumption against the black '60' triangle. The answer is 21.1. But a common-sense check shows this must be 2110 kg/hour — because the outer scale is a slide rule, so 21.1 on the scale actually represents 2110 when you account for the magnitude. The electronic calculator solution is 2114.29, so the computer gives you a very close approximation. That's the key point: the navigation computer is a slide rule, so you always have to place the decimal point yourself using common sense. Now let's move to distance conversions. The navigation computer can also handle distance, volume, and weight conversions. Taking distance first: around the edge of the outer scale are various index marks with red labels. Three of these are for distance conversions. Let's look at the example: convert 148 kilometres to statute miles and to nautical miles. The km-m-ltr index on the outer scale is at 10, and the naut m index is at 54, or 5.4. You should remember that the distance from the Equator to the pole is 10,000 km, and it's also 90 × 60, which is 5400 nautical miles. So 10,000 km equals 5400 NM, and therefore 10 km equals 5.4 NM. That's the relationship the index marks encode. To do the conversion, align 148 on the inner scale with the km-m-ltr index. Then read the corresponding values against the other indices to get your statute miles and nautical miles. So the core idea is: the black triangle is your rate index, the outer scale holds your quantity or distance, the inner scale holds your time or converted value, and the red indices on the outer scale are your conversion factors. Always place the decimal point yourself. That's the heart of using this computer for fuel and distance work.