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On a typical airliner (let's say A320 / B737 / or similar), comparing a plane full of passengers vs one half full, how much fuel an airline can save on a trip when the plane is only half full?

(This is assuming both cases will accomplish the same mission, distance, time, etc.)

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    $\begingroup$ Related (maybe duplicate): How much does it cost extra to fly with one additional passenger? $\endgroup$
    – Bianfable
    Commented Apr 16 at 6:05
  • $\begingroup$ define "save". sure, you use a smaller total amount of fuel, but you still need the fuel to move the same size aircraft, so the fuel per passenger goes really high, making your efficiency worse. $\endgroup$
    – Federico
    Commented Apr 16 at 7:16
  • $\begingroup$ @Federico I see your point, but i mean just the fuel alone, just that. How many pounds of fuel they would have to purchase in one case vs the other. $\endgroup$
    – Gabe
    Commented Apr 16 at 12:49
  • $\begingroup$ Highly related, but with 80% instead of 50% payload: How can I calculate the fuel consumption of an Airbus 320-200 at various loads? $\endgroup$ Commented Apr 16 at 19:18
  • $\begingroup$ Be aware sometimes airlines bring more fuel than necessary, because it costs less to carry the extra-weight than to buy the same quantity at the destination airport (tankering). $\endgroup$
    – mins
    Commented Apr 18 at 18:00

2 Answers 2

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The most can be saved with short range planes on long trips. When passenger load is around 10% (as it is for very long range airliners), reducing that to 5% will not make much of a difference. But we can do better than that.

If you want a good estimate, the venerable Breguet equation will already give good results. First we need to know how much of a difference halving the payload will make: The average mass of a passenger plus baggage for short-range trips is 100 kg.

If we use an A320 with 180 passengers on a short trip of 1000 km as the basis, the zero-fuel mass would be OEW + 18 t = 60.6 t in the 100% case and 9 tons less in the 50% case. Allow for 2 t of fuel reserves and plug that into the Breguet equation, using a fuel burn of $b_f$ = 0.000018 kg/Ns and a speed of Mach 0.78, which equates to $v$ = 262 m/s in 11.000 m altitude: $$m_1 = m_2 \cdot e^{\frac{R\cdot g\cdot b_f}{v\cdot L/D}}$$ The L/D should be 18 and for the range we use R = 1000.000 m. $m_1$ is take-off mass and $m_2$ landing mass. This gives a fuel need of 2387.5 kg for the 100% case and 2044 kg for the 50% case. To be realistic, you would need to include different fuel flow during climb and descent, but the real value shouldn't be too far from the results we get from the Breguet equation. Halving passenger load only saves 343 kg or 3.8% of the reduction in payload on a short trip.

Now we repeat the calculation for the maximum range of the A320 of 5,700 km. For the 100% load factor case, take-off mass becomes 77.5 tons, which is just shy of the 78 tons MTOW of the A320. Covering 5700 km with 180 passengers will consume 14887.5 kg of fuel while doing the same with only 90 passengers will consume 12747 kg or 85.6%. This is 2140.5 kg less or 23.77 kg per passenger.

If we now do the same calculation for an extreme range like 12.000 km in a Boeing 777 or an Airbus A340, we cannot fill all seats because so much fuel needs to be taken onboard.

What can be saved, therefore, varies between 4% and 15% of the reduction in payload, details depending on passenger mass fraction and range.

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The answer is heavily dependent on the exact airliner. As a general rule, the practical answer is "not enough".

Reducing the airliner's weight saves fuel during takeoff (slightly), climb (linearly) and cruise (linearly). But it doesn't save fuel during taxi and landing, which comprise a meaningful proportion of fuel burn for short flights.

According to this NASA report, incremental fuel burn ranges between 0.002 and 0.007 gallons per lbs per hour. Tossing out the outliers, 0.004 appears to be the middle value. For 100 pax at 200 lbs each, that's 80 lbs/hr. For a 737 or A320 burning 800-1000 lbs/hr, this is 8-10% of their overall fuel burn.

In conclusion: flying with only half the passengers takes about 10% less fuel.

Large airliner operators - A320 and up that can take containers, and especially A300 and up that take AKEs - will try to fill the remaining weight limit up with cargo if possible.

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  • $\begingroup$ Thanks Therac. Why it wouldn't save fuel during taxi? Don't you need less power(hence less fuel burn) to move an half full airplane trough the taxiway than a full airplane?. Or the difference would be just too minimal? $\endgroup$
    – Gabe
    Commented Apr 16 at 12:40
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    $\begingroup$ @Gabe When the engines run at idle, most of the fuel is spent just spinning them, not doing any useful work. Taxiing has an efficiency of less than 1%. $\endgroup$
    – Therac
    Commented Apr 16 at 14:04

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