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Converting volume (eg. US Gal.s to Litres) is easy. We are primarily interested fuel volume for endurance but must also consider all up weight of the aircraft. To find that we must consider not only the volume of fuel carried but its weight per volume (eg. Kg.s per litre). This is expressed as the Specific Gravity (S.G.) of the liquid. The reference for weight (a label for the force of gravity) of a liquid is water. 1 litre of pure water weighs exactly 1 Kilo. (This is a small part of how the metric system was devised.) Thus, S.G of water is expressed as 1.0. Some liquids are heavier than water per litre (eg. the sulphuric acid in a conventional lead/acid battery has an S.G. of 1.250 +, - ). Their S.G. will be over 1.0. Other liquids such as Avgas and Avtur are lighter than water per unit volume so their S.G. will be less than 1.0. A typical S.G. used for Avgas is 0.72 (giving 0.72 Kgs per litre) and 0.8 is the accepted S.G. for Avtur.To find out what your fuel load (Avgas or Avtur) weighs it is best to begin with litres ( because that's how we buy or load it these days) and convert remaining tank content volume(s) to litres (say having established that from a dipstick graduated in US gallons). When you have the required total for endurance plus reserve** in litres just multiply it by the quoted S.G. to obtain the weight of that volume. If this has to be converted into pounds for the weight and balance of an American aircraft then fine, that's easily done using the chart above.

OK, I've done all the work up to now, its your turn. You need to know how much fuel per hour the R22 (or whatever aircraft you are flying) consumes. (It is widely held that 9 USG is a reliable figure for an R22.) Use that figure and the chart above to find your own hourly rate of fuel burn in litres. Find out how much time your journey will take and add ** reserve. Plan how much fuel must be added at the pump, minus usable fuel already in the tank(s). Recheck your figures, convert the total fuel load straight into endurance. If the resulting endurance is **reserve longer than the flight time you have predicted then all well and good. Keep a mental note of fuel useage against time elapsed (Make sure you know what time you begin the flight!) Work out and note the time you must 'land by' without using reserve fuel. Use some common sense and you will never have to declare a fuel emergency (or risk death for yourself and others). There really is no excuse for that to happen.

Make this train of thought habitual;

"The Aircraft I will be flying burns _____ litres of fuel per hour." "I need _____ litres to make this trip PLUS margin for safety **, totalling _____ litres." "My fuel load will weigh _____* kilos or _____ * pounds." *Delete the inappropriate.
** In N.Z. fuel reserve must be 30 mins.(at cruise consumption) for fixed wing. For Heli.s 20 mins. or equal to flight time if less than 20 mins. eg.10 mins planned flight, 10 mins reserve.

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