The Navigation Computer - Distance, Speed, Time and Conversions — Page 103, Lesson 100

I want to walk you through a practical fuel conversion technique on the navigation computer. We're going to convert a volume of fuel into pounds weight and then into kilograms, using specific gravity — and I'll show you exactly how the scales work. Let's start with the pounds weight conversion. Imagine we already have the inner scale of the computer set up from a previous step — we're not touching it now. We move the cursor to 0.80 on the scale labelled "Sp.G lbs". That's the specific gravity scale for pounds. This scale runs from 70 to 95, which represents specific gravities from 0.70 up to 0.95. So 0.80 sits right in the middle of that range. Now, with the cursor at 0.80 on the Sp.G lbs scale, we read off the value on the inner scale. That gives us 20.0. But we need to think about the order of magnitude — the actual size of the answer. We're dealing with 250 imperial gallons of fuel, and the specific gravity is 0.8. So we expect an answer around 2000 pounds. And in this case, the answer comes out exactly to 2000 lb. Now let's do the conversion to kilograms. Again, we do not move the inner scale — it stays exactly where it is. This time, we move the cursor to 0.80 on the "Sp.G kgs" scale. That's the specific gravity scale for kilograms. This scale is marked from 70 to 90, representing specific gravities from 0.70 to 0.90. So 0.80 is again right in the middle. With the cursor at 0.80 on the Sp.G kgs scale, we read off the inner scale and get 9.08. Now we estimate the order of magnitude. There are a couple of ways to do this. One way: 1135 litres times 0.8 gives about 900 kilograms. Another way: we know 2000 pounds is just under 1000 kilograms. So the answer of 908 kilograms makes perfect sense. Now, the book gives you a set of practice tables to complete. These are tabulations where you fill in missing values. The first table deals with groundspeed in knots, distance in nautical miles, and time in minutes — you'll be given two of the three and need to find the third. The second table covers fuel flow, time, and fuel used, with various units like litres per hour, kilograms per hour, imperial gallons, US gallons, and pounds per hour. The third table asks you to convert between nautical miles, statute miles, and kilometres. The fourth table converts metres to feet. And the fifth table is a fuel conversion table where you work with litres, imperial gallons, US gallons, specific gravity, kilograms, and pounds — all in one grid. These are the book's practice questions — let's try them one at a time.