Calculating how much weight of fuel can seem a bit confusing, and certainly lethal if done incorrectly. In this article I will show you how to make an estimated calculation so that if you need to calculate the actual weight, you’ll have a rough idea to begin with.
The reason we have to calculate the weight of fuel instead of just taking an average figure per gallon or litre is that the volume of fuel changes depending on its temperature. If you fill a tank to the brim on a cool evening and the next morning the temperature rises as the sun beats down on the wing, the fuel will start to spill out of the vent. This is because the density of the fuel is greater when it is cold than hot as it expands in the heat. The density of water also changes with temperature but the amount is negligible, however it is at its densest at 4°C. When we talk about the specific gravity of fuel (or specific density), we are comparing it to a sample of water at 4°C. Water weighs 1 kilogram per decimetre (which is 10cm x 10cm x 10cm/ or 1 litre at 4°C). If we take a litre of fuel and it has a ‘specific gravity’ of 0.8, that means it will weigh 80% of the weight of water, so 0.8kg.
Calculating the weight of fuel in litres to kilograms is simple. In my head, if I added 78 litres of fuel to the tank, if it was water it would be 78 kilograms, and as it is fuel (hopefully not water!) we can multiply it by the Spg (in the UK in winter try using 0.8 and in the summer 0.70 as safe numbers—— it is in fact 0.71 at 15°c). 78kg X 0.8 = 62.4kg
UK gallons to pounds also has a similar hack to work out the weight, because 1 (UK!!) imperial gallon of water weighs 10 pounds.
So to add Imperial 45 gallons and the weight is 45 UK gals X 10(if it was water) X 0.8 (as it is fuel)= 360lbs. Be careful when using gallons because you fill up in the UK
Using one definition of the gallon and your aircraft manual is probably quoting US gallons.
To do this on the whizz wheel I align number 45 under Imperial Gallons, then look where it says SP.G (lbs not KGS) under 80. The number is 36, and we know to add the zero because a gallon of water is 10 lbs. (see picture)
Finally, basic conversions of pounds to kilograms on the whizz wheel can be done but the units are not labelled (I think they are not labelled so that we don’t calculate the weight of fuel as that of water by mistake?) If you look at the line showing SP.G in KGS on the CRP, you can see that if the SP.G continued past 90, the 100% mark is where KM-M-LTR are positioned. Similarly with pounds, if you go to 100% we have the Imperial gallon. Place 10 under KM-M-LTR and under IMP.GAL you can read 22. This means that 10Kgs is 22Lbs.
Another even easier way of converting KG to pounds is double and add 20%, Lbs to KG halve and minus 20%). So 10Kg is 20 (10×2) plus 2 (20% of 10) chichis 22 pounds.
Finally in this age of limitless information available on the internet, and complex calculations done by computers, you might see the CRP as a redundant tool. What I like about it is that it forces you to understand the calculation you are making and to think about the answer before you start.