The Navigation Computer - Distance, Speed, Time and Conversions — Page 94, Lesson 97

We're now into the conversions part of the navigation computer. We've already done distance conversions, so let's build on that. The first thing I want to walk you through is the volume conversions. On the navigation computer, you have index marks for litres, imperial gallons, and U.S. gallons. These are the volume conversion indices. The procedure is exactly the same as what we did for distance: you set what you know, and read off what you need. Let's do an example. Convert 136 litres to imperial and to U.S. gallons. As always, set what you know. Put 136 against the litres index. Then read off against the imp.gal index. That gives you 30 imperial gallons. Then read off against the u.s. gal index, and that gives you 36 U.S. gallons. Now, here's a critical point for you as a pilot. You need to remember the order of magnitude here, so you don't misread the scale. There are about 4½ litres to an imperial gallon, and there are 5 imperial gallons to 6 U.S. gallons. So if you get an answer that's wildly off from that relationship, you know you've made a mistake. 136 litres divided by 4.5 gives you about 30, which matches. And 30 imperial gallons times 6/5 gives you 36 U.S. gallons. That checks out. Now, there's a more advanced facility on the navigation computer: volume to weight conversion. This lets you convert volume in litres or gallons to weight in kilograms or pounds, but only if you know the specific gravity of the liquid—usually fuel. The indices are spaced so you can start with an argument in one set of units and read off the answer in the other set. Let me give you a quick revision on specific gravity, because this is fundamental. Specific gravity, also known as relative density, is the density of a substance compared with the density of water. In the metric system, one litre of water weighs one kilogram. In the imperial system, one imperial gallon of water weighs ten pounds. So if a fuel has a specific gravity of 0.80, then one litre of that fuel weighs 0.80 kilograms, and one imperial gallon weighs 8.0 pounds. You just multiply the water weight by the specific gravity. Why does this matter so much in aviation? Because the calorific value of fuel is related to its mass, not its volume. That's a key point. The energy you get from fuel depends on how much mass you have, not how many litres or gallons. So you need a greater volume of fuel at a lower specific gravity to give the same amount of energy as a smaller volume at a higher specific gravity. This energy may be expressed as range in nautical air miles, or possibly as endurance. Let me give you the example from the book. 200 litres at 0.80 specific gravity weighs 160 kilograms. That's 200 times 0.8. Now, if you use fuel at 0.75 specific gravity instead, you'll need 213 litres to cover the same distance. Why? Because you need the same mass—160 kilograms—to get the same energy. 160 divided by 0.75 gives you 213.3 litres. So you burn more volume of the lighter fuel to get the same range. This is a real operational consideration. When you're planning fuel, you're not just counting litres—you're counting mass, because that's what determines your range and endurance. The navigation computer lets you do that conversion quickly, but you have to understand the underlying physics: specific gravity is the bridge between volume and weight. So to summarise what we've covered: volume conversions between litres, imperial gallons, and U.S. gallons, with the key relationships of 4½ litres per imperial gallon and 5 imperial to 6 U.S. gallons. Then volume-to-weight conversion using specific gravity, where one litre of water is one kilogram and one imperial gallon of water is ten pounds. And the crucial aviation principle that fuel energy depends on mass, not volume, so lower specific gravity means you need more volume for the same range.