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I was having a conversation concerning airplanes and the calculated fuel. We were having a conversation about the Airbus A380. Based on this site:

http://www.modernairliners.com/airbus-a380/airbus-a380-specs/

It shows a capacity of 320,000 Litres. After some conversion (I am in the US)

320,000 Litres = 84,535 gallons

84,535 gallons = 710,000 lbs

Now that is based on water density. I've heard that jet fuel isn't exactly comparable to water? Is this true? I am trying to confirm because 700 thousand pounds of fuel sounds like a lot? Is there something I am missing? An average Olympic sized swimming pool carries about 660,430 gallons of water. How does this plane carry that much weight? And what would be the easiest way to explain this to a layman like myself?

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  • $\begingroup$ How does it carry that much weight? By creating even more lift, because the structure and the payload weigh a considerable amount as well. $\endgroup$
    – Koyovis
    Feb 20, 2018 at 6:28
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    $\begingroup$ Just a minor thing, but I would recommend to avoid using decimals that are „introduced“ by converting units. The tank capacity won’t be exactly that many litres (and if it is, it’s still not very relevant), so converting it to 84,535 gallons is perfectly fine and, both from a „perceived precision“ and a readability standpoint, in my mind, preferable. Hope you don’t mind my comment on a minor detail! $\endgroup$
    – Cpt Reynolds
    Feb 20, 2018 at 6:49
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    $\begingroup$ Adding to @CptReynolds' comment, you typically want to consider the number of significant digits. The number of significant digits in 320,000 can be anywhere between two and six, but never more than six, so any final result should never have more than six significant digits, and quite possibly no more than two. With rounding, this makes your final weight figure 710,000 lbs (because 705 rounds up to 710). The number of significant digits is easier to see in scientific notation; for example, if the capacity was given as 3.20 * 10^5 L, we'd have known immediately to use three significant digits. $\endgroup$
    – user
    Feb 20, 2018 at 9:12
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    $\begingroup$ @CptReynolds and michael-kjörling - Thanks guys for the advice, I updated the original question to do some rough rounding up. $\endgroup$
    – ronbravo
    Feb 20, 2018 at 15:50
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    $\begingroup$ It's the calculation and conversions I'm trying to wrap my head around. Because 85,000lbs of a liquid would seem to take up a lot of space and my friend and I were trying to figure out where all this liquid goes. $\endgroup$
    – ronbravo
    Feb 20, 2018 at 19:07

3 Answers 3

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Fuel doesn't have the same density as water. We're not planning a real flight, so a quick and dirty fuel density figure from Wikipedia tells us:

In performance calculations, airliner manufacturers use a density of jet fuel around 6.7 lb/USgal or 0.8 kg/l.

So if we take your figure of 320,000 litres (it's a bit unclear quite how much of that is actually usable for flight, but that's a subject for another question), that's about 560,000lbs of fuel or around 254,000kgs.

Which, indeed, is a lot. The fuel is spread throughout the wings (and the trim tank in the tail), which are designed to carry the weight. If you look at What is the weight budget of a fully loaded A380?, you'll see how the fuel capacity interacts with the other weight specifications for the aircraft:

The maximum fuel weight is the difference between maximum ramp weight and zero fuel weight (which is 562,000−361,000=201,000kg)

Max take-off weight is a bit less than maximum ramp weight, but we'll handwave over taxi fuel for simplicity. The point here is that if you can fill the aircraft with cargo, it would be too heavy if you also filled it with fuel. If you want to take a full load of fuel, you'd have to reduce your payload to keep the total weight low enough. Working out the weight/fuel/range trade-offs for a given flight is what dispatchers and load planners do, using specialized software.

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Others have already addressed the issue of calculating fuel weight, but I'd like to take a moment to discuss what you wrote in a comment to the question.

85,000lbs of a liquid would seem to take up a lot of space and my friend and I were trying to figure out where all this liquid goes.

One of the beauties of the metric system is that one liter is exactly equal to one cubic decimeter (the latter, of course, being a cube of 10x10x10 cm; in US terms, that's a shade under 4x4x4 inches). A liter, like a cubic decimeter, is a unit of volume, rather than a unit of mass.

320,000 liters thus corresponds directly to 320,000 dm3. In a more usable unit, this is 320 m3 because 1 m3 = 1,000 dm3.

Looking at Wikipedia, the wing area of the A380 is given as 845 m2.

If the entire wing area can be used for fuel tanks, which is not the case, and there are only fuel tanks in the wings, which is not the case, this means that the average height of the fuel tanks in the wings would need to be equal to 320 m3 divided by 845 m2, or about 0.38 meters; a little thicker than the long side of a piece of A4 or Letter paper. I haven't flown in an A380 recently, but I suspect that the wings are thicker than this. The fuel also isn't kept only in the wings; while I don't know about the A380's configuration specifically, it's common for large aircraft to have both wing tanks, a center tank within the fuselage, and a trim tank farther back in the fuselage.

Bottom line here, there's plenty of room for that amount of fuel.

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Fuel comes out of the bowser and is metered in Litres (at least where I come from!). An aircraft is more interested in how much that is in kgs. So the person in charge of the fuelling has to take into account the SG or specific gravity of the fuel on that day. This varies according to the temperature. In the tropics it is usually around 0.780. This means if the fuel has a SG of .780 it weighs 0.780 of 1 litre of of water @ 4deg C (water is used as the reference unit). As 1 Litre of water is equal to 1 kg, 1 Litre of Jetfuel at SG 0.780 is equal to 780grammes. So if the bowser has metered out 100,000Litres of Jetfuel at SG 0.780, it will add 78,000kgs to the weight of the aircraft.

One flight I did was at maximum range of the B747-400. The flight-planning was done in HKG and they needed to know the actual SG as this will give the actual maximum weight of the fual they could carry. If I'm remember correctly the 747-400 can carry something like 170,000kgs of fuel but if the density is low you might not be able to fill it max weight-wise. I believe we uplifted around 167,000kgs that day.

There is another formula for gallons to Lbs but I cant remember it as I've worked mostly with Metric.

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  • $\begingroup$ the formula for gallon in Lbs seems to be a simple conversion formula $\endgroup$
    – Manu H
    Feb 20, 2018 at 15:56
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    $\begingroup$ The multiplicative conversion factors that Boeing uses for English to Metric are 0.45359237 for pounds to kilograms and 3.78541180 for U.S. gallons to liters. Thus 2.20462262 for kilograms to pounds and 0.26417205 for liters to U.S. gallons. In the past it was common to use 2.2 to convert from kg to lb, but that was a poor practice because it could result in an aircraft departing when slightly over the max takeoff weight. $\endgroup$
    – Terry
    Feb 20, 2018 at 18:13
  • $\begingroup$ I asked for max fuel for the first three 747-400s in my weight & balance program, and it came up with a 163028 kg at 0.8028 kg/L (359415 lb at 6.7 lb/gal) for all three, so your memory of 167000 kg is reasonable. You might have had a higher density or larger tankage. $\endgroup$
    – Terry
    Feb 20, 2018 at 18:57

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