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

I want to walk you through some more conversion techniques on the navigation computer. We've already looked at converting between nautical miles, statute miles, and kilometres. Now let's build on that. Let's start with a quick recap of the conversion method. When you set a distance on the inner scale against the nautical miles index on the outer scale, you read the answer off the inner scale. For example, if you set a value and move the cursor to the "naut m" index, you read the answer on the inner scale — say 8.0, which you interpret as 80 nautical miles. Then, to get the answer in statute miles, you move the cursor to the "stat m" index and read off the inner scale again — in that example, you'd get 92 statute miles. Here's a critical point I want to emphasise: always read the answer off the inner scale. A very common mistake is to read these conversion answers off the outer scale instead. If you do that, you'll wonder why the answers always seem the same — for instance, 54 nautical miles and 62.5 statute miles would appear identical, which is obviously wrong. The outer scale gives you the same number because it's the fixed reference; the inner scale is where the converted value actually changes. Now let's do a new example involving metres, yards, and feet. Suppose we want to convert 256 metres to yards and feet. First, set what you know. On the outer scale, locate the "m" index — that's the metres index, marked alongside the number 10. Opposite that index, set 256 on the inner scale. But remember, on the inner scale you position the digits 25.6, because the computer uses a sliding decimal point — you interpret the magnitude yourself. So you set 25.6 on the inner scale against the metres index on the outer scale. Now, to read off yards: find the "yards" index on the outer scale. Opposite that index, read the inner scale — you'll see 28.0, which you interpret as 280 yards. Then, for feet: find the "feet" index on the outer scale. Opposite that, read the inner scale — you get 84.0, which is 840 feet. So the complete conversion is: 256 metres equals 280 yards equals 840 feet. Now let's move to volume conversions. The navigation computer also handles volume conversions between litres, imperial gallons, and US gallons. Each has its own index mark on the outer scale: "litres", "imp.gal", and "u.s. gal". Let's work an example: convert 136 litres to imperial gallons and to US gallons. As always, set what you know. Put 136 against the litres index on the outer scale. That means you align the inner scale value of 136 with the litres index mark. Then, to read off imperial gallons, look at the "imp.gal" index on the outer scale — opposite it on the inner scale you read 30. So 136 litres equals 30 imperial gallons. Next, for US gallons: keeping the same setting, look at the "u.s. gal" index on the outer scale. Opposite it on the inner scale you read 36. So 136 litres also equals 36 US gallons. Here's a useful memory aid: there are about 4½ litres to one imperial gallon, and there are 5 imperial gallons to 6 US gallons. Keeping these rough ratios in mind helps you check that your answer is in the right order of magnitude. Now let's look at volume-to-weight conversions. The navigation computer also has a facility to convert volume — in litres or gallons — to weight in kilograms or pounds, provided you know the specific gravity of the liquid, typically fuel. The indices on the computer have been spaced so that you can start with a value in one set of units and read off the answer in the other units directly. Let me give you a quick refresher on specific gravity. 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 exactly one kilogram. In the imperial system, one imperial gallon of water weighs exactly 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. Why does this matter in aviation? The calorific value of fuel — the energy it releases when burned — is related to its mass, not its volume. That means 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 can be expressed as range in nautical air miles, or possibly as endurance in time. For example, 200 litres of fuel at a specific gravity of 0.80 weighs 160 kilograms. If you instead use fuel with a specific gravity of 0.75, you would need 213 litres to cover the same distance, because the lower-density fuel has less mass per litre. So the navigation computer's volume-to-weight conversion lets you work out exactly how much fuel mass you have, which is what really determines your aircraft's range and endurance.