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I am wondering which information of fuel amount is the significant one. Because all light aircraft have fuel quantity indicators scaled in litres or gallons, so in a unit of volume. But I heard that in airliners and other bigger planes have fuel quantity displayed in kg or lbs. So which unit is "correct" and a running engine at constant power setting needs constant amount of fuel in kg or litres? Please help me out with that and please explain also the importance of changing fuel density with temperature (and maybe with some other factors?).

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    $\begingroup$ Strongly related to On what basis is the weight unit of the FQIS selected by airlines of various countries?. Mass is potential energy, what means for the crew is energy to run the engine, not volume. $\endgroup$
    – mins
    Nov 21, 2021 at 15:18
  • $\begingroup$ Both. Mass is used for weight and balance and total BTUs in the fuel for range. Volume (based mostly on temp) is used to see if it will fit into the volume of the tanks and to pay for it. $\endgroup$
    – dawg
    Nov 22, 2021 at 19:35

3 Answers 3

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Mass is more useful, but volume is easier to measure.

Mass is needed for weight-and-balance calculation, and the energy stored in the fuel is also proportional to mass, so when the engine is set for cruise parameters, it will burn roughly the same mass per unit of time every time, making it more useful for range calculation too.

However there is nothing that could directly measure mass of the fuel, while the volume can be measured by a simple float in the tank. Therefore

  • For GA planes you have indicator in volume, and assuming the standard density is usually good enough.
  • Transport aircraft have advanced sensors that can estimate the density as well and indicate fuel mass.

See also How does a fuel density sensor work? (densitometer, cadensicon) and How is fuel mass measured in airliners?

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    $\begingroup$ Volume limits how much fuel a given tank can hold; on a warm day with warm fuel, "full tanks" gives you less fuel - i.e. less mass and therefore less range - than you'd have with cold fuel. $\endgroup$
    – Ralph J
    Nov 21, 2021 at 20:13
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    $\begingroup$ @RalphJ, yes. However in larger aircraft you always load the fuel you need for the trip plus appropriate reserves, so you almost never reach the volume limit. $\endgroup$
    – Jan Hudec
    Nov 22, 2021 at 7:54
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Kilo’s are important. Energy contents is given in kJ/kg, so the weight of the fuel on board determines the range of the aircraft. For commercial aviation, this precision is imperative.

Liters are used in cars and small aeroplanes because they are easier to measure. But on warm days a warm liter of fuel weighs less than a cold liter - volume measurement introduces the variability of expansion.

It is possible to directly measure the kg/sec of fuel that is taken on board, it is simply that the mass fuel rate meters are more expensive than the volume ones.

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    $\begingroup$ this. Mass determines how much fuel you have. Volume in liters or gallons is just a convenient way of measuring it but is not nearly as accurate. Density change due to temperature can skew the measurement by several percent. For example, 40C avgas is a full 10.5% more volume than -40C avgas of the same mass. $\endgroup$
    – PcMan
    Nov 21, 2021 at 20:00
  • $\begingroup$ Or more precisely: how much energy you have... both weight and volume are perfectly valid when measuring how much of a substance one has. Energy is of importance in aviation. $\endgroup$
    – Jpe61
    Nov 21, 2021 at 23:51
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    $\begingroup$ @Jpe61 True, and that is why weight is the gold standard. Volume expands and retracts as a function of temperature, weight remains constant. $\endgroup$
    – Koyovis
    Nov 22, 2021 at 1:21
  • $\begingroup$ @Jpe61 "both weight and volume are perfectly valid when measuring how much of a substance one has" is a false statement. $\endgroup$
    – PcMan
    Nov 22, 2021 at 15:40
  • $\begingroup$ Lol. Go troll somewhere else. $\endgroup$
    – Jpe61
    Nov 22, 2021 at 20:50
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Its all about the ease of measuring out the required fuel.

Before departure the pilot will need to calculate his take-off weight. eg if you have a plane with a max take-off weight of 300,000kgs.. the weight of the fuel, the plane and the load cannot exceed this. Assuming the weight of the plane and load is 250,000kgs.. this means that there is space for 50,000kgs. If this is not enough then load has to be removed to allow the weight of the fuel required. Sounds complex but not really when you do it everyday.

The problem shows up when you have to pump the fuel into the tanks. While it may be possible to weigh the bowser (tanker) and derive the fuel uplifted by reweighing it after the plane has been fuelled, its a bit cumbersome. Also for the amount of fuel that a widebody can uplift (170,000kgs on a good day for a B747-400) it is pretty impractical. Also most airports use hydrant systems for refuelling where there is a fuelling port in the ground at the parking stand and all the fuelling truck does is meter and pump it into the plane.

Measuring fluids however is much easier and has been in use for a long time. How it works is as follows.

  1. The Specific gravity (SG) on the day needs to be known. The SG is the ratio of the weight of the liquid compared to the weight of 1litre of pure H2O which is also 1kg. For this example lets use an SG of 0.779. In this case one litre of fuel will equal 0.779kg. The density varies with temperature and the fueller will usually check it before fuelling using a density meter/hydrometer (like used to check battery acid).

  2. Assuming the pilot requires 50,000kg of fuel. To convert 50,000kgs into litres, we need to divide the weight by the SG (50,000 / 0.779). this equals 64,185 which is the amount in litres. Most airliners have a fuelling panel where you just set the amount required in kgs (or Lbs) but the calculation behind the automation is the same. (note:A real life situation will have some fuel already in the tanks)

  3. Upon completion of fuelling, the fueller will typically handover a fuelling receipt with the uplift amount in kgs. Here the pilot or engineer will convert this amount into kgs and factor in the fuel which was already in tanks. The final number is what the pilots have planned for and expect for the flight.

  4. It seems pretty hit and miss and in real life the numbers are not really accurate. It is common to have a few hundred kilos more on an aircraft like the 744.

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