The Navigation Computer - Distance, Speed, Time and Conversions — Page 94, Lesson 99

I want to walk you through a practical example of using the navigation computer for fuel conversions. This is a skill you'll use regularly in flight planning and fuel management. Let's start with the problem: we need to convert 300 US gallons into four different units. First, imperial gallons. Second, litres. Third, pounds weight. And fourth, kilograms — but for the weight conversions, we need a specific gravity value, which in this case is 0.80. The first step is always the same: set what you know. On your navigation computer, you align 300 on the inner scale against the "U.S. GAL" index on the outer scale. That's your starting point — you're telling the computer that 300 US gallons is your known quantity. Now, without moving the scales, you read off against the "IMP. GAL" index. That gives you 250 Imperial gallons. So 300 US gallons equals 250 Imperial gallons. Remember, a US gallon is smaller than an Imperial gallon, so the number of Imperial gallons is lower. Next, still with the same setting, read off against the litres index. That gives you 1135 litres. So 300 US gallons is also 1135 litres. Now, here's where it gets interesting. For the weight conversions — pounds and kilograms — we need to use the specific gravity scales. Look at the two scales I'm referring to on the computer. These are the specific gravity, or SG, scales. Specific gravity is the ratio of the density of a substance to the density of water. For aviation fuel, a typical specific gravity might be 0.80, as in this example. To convert to pounds weight, you align the specific gravity value on the appropriate scale. With the specific gravity set to 0.80, you can then read off the weight in pounds against the volume you already have set. Similarly, for kilograms, you use the same specific gravity setting and read off against the kilograms scale. So, to summarise the process: you set 300 US gallons against the US gallon index. Then you read off against the Imperial gallon index for 250 Imperial gallons, against the litres index for 1135 litres, and then using the specific gravity scales set to 0.80, you can read off the weight in pounds and kilograms.