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Say I thought of a very easy and inexpensive way to reduce the takeoff weight of a commercial airplane (body, fuel and passengers) by 1%. How much would this mean to the industry? Would it make much of a difference, or would the savings be insignificant enough to not really be worth anyone's time?

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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Nov 9 '17 at 6:25
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There is an easy way to get an idea of what magnitude the savings are: The Breguet equation can answer this. You first calculate a reference aircraft and then start all over with one that weighs 1% less. The difference in fuel consumption shows how much fuel is saved per flight.

Our reference design is the A320 which has an L/D of 18 and we let it fly a trip of R = 2,500,000 m. $m_1$ is take-off mass and $m_2$ landing mass. Allow for 3 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 landing mass $m_2$ is the operational empty mass OEW + 18 t payload + 3 t of fuel reserves = 63.6 t. The take-off mass $m_1$ is 10% higher, so the difference in fuel consumption is 1% of that trip fuel of 6,239 kg or 62 kg. That is not much for a single trip.

But now consider that this one A320 will fly 50,000 trips before it retires. Over its lifetime the 1% in mass reduction will save 3,119 metric tons of fuel. This fills 29.4 DOT-111 railway tank cars!

And there are about 12,000 airliners of 150 seats and more and easily twice that number if all regional jets are included, flying 3.8 billion passengers and 53 million tons of freight per year on 35.4 million flights. If we make the heroic assumption that our A320 flight is typical, this translates into global fuel savings of about 2.2 million tons of fuel per year if only 1% of mass could be saved on every airplane.

Another way to approach this is the average fuel efficiency of modern airliners which consume between 3 and 3.5 liters of fuel per 100 passenger kilometers. Using ICAO's number of 6.6 trillion passenger kilometers in 2015, this would equate to 174 million tons of aviation fuel used per year for revenue passenger transport, assuming a mean density of kerosene of 0.81 kg/l. The same source gives an average load factor of 80%, so one fifth of seats is empty; the efficiency number above is for a full airplane, however. Therefore, our estimate needs to be increased to 217 million tons.

Saving only 1% of this would reduce fuel consumption by 2.17 million tons per year, which comes pretty close to the first estimate. This is enough to fill more than 41 Panamax oil tankers (the maximum size for the old Panama canal) or 18 Neopanamax oil tankers.

Or, to look at this from a different angle, with the current price of kerosene of 1.76 USD per gallon, the saved fuel equates to 1.25 billion USD if everyone paid US prices for Jet A-1 fuel. Compared to the average net profit of all airlines per year between 2007 and 2017, which amounts to 13 billion USD, this is almost 10% of yearly profits.

Now this doesn’t look so insignificant any more, does it?

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    $\begingroup$ I disagree in the sense that such figures are more misleading. Big numbers blow out our human minds, we can't put them into perspective intuitively. The really important figure is still: 1%. Whether it's important or significant is another matter, but it's much easier (and more valid!) to grasp it. $\endgroup$ – Zeus Nov 7 '17 at 3:59
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    $\begingroup$ Hm, it actually still doesn't seem like that much to me as a laymen. I feel like airplanes must waste far more fuel than this just due to logistical inefficiencies/preventable delays/etc. $\endgroup$ – Mehrdad Nov 7 '17 at 7:24
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    $\begingroup$ @Zeus: May I offer a different view? The question is "How much would this mean to the industry", and I offered the perspective for the whole industry. Ask anyone in airline operations how significant 1% fuel savings are and their answer would sound like Koyovis. If your mind is blown by big numbers, maybe try to put them into perspective (like I did with the railway car comparison). Or would you prefer an answer along the line of "the numbers are too big to contemplate, so you don't get an honest answer"? $\endgroup$ – Peter Kämpf Nov 7 '17 at 8:00
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    $\begingroup$ No, I'm just amused by the fact both answers used a really complex equation to get that 1% reduction number. Note that indexmundi.com/energy/?product=jet-fuel says 5 million barrels/day of all jet fuel consumption. 1% of that is then 50 thousand/day, times 365 is 18 million barrels. At 42 galleons per barrel, and your 1.76$/galleon, that gives us 1.33 billion USD. However, this was consumption for all uses; some subset of that is used by commercial airlines. Still, this second derviation validates yours! $\endgroup$ – Yakk Nov 7 '17 at 19:54
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    $\begingroup$ @PeterKämpf I think that the last full paragraph of your answer, where you compare cost of the fuel to yearly profits, is really the most meaningful assessment of how much it matters to the industry, and you might consider focusing more on that than on how many tankers and so on. Those absolute numbers/sizes are interesting, and yeah, they're really big, but everything about the whole industry is really big, so they don't give as strong a sense of how much it matters to the industry. $\endgroup$ – Cascabel Nov 7 '17 at 20:03
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One % of takeoff weight is huge. On a B747 with MTOW of 300 tons, that is a weight saving of 3,000 kg, weight that needs not be lugged around for the couple of decades that the aircraft will be flying.

At the aircraft factory where I used to work, there were Idea Boxes everywhere where one could deposit suggestions for weight savings, every kg saved adds to improvement of fuel economy. How much depends on range and a whole lot of other factors, and can be quantified for specific aircraft.

1%. Yuuge.

If we make an estimate for a B747-400 flying maximum range of 12,700 km according to slide 7/14 out of this presentation.

enter image description here

MTOW is given as 400,000 kg, fuel weight as 175,000 kg, this makes $W_{final}$ = MTOW - $W_{fuel}$ = 225,000 kg. With 1% saving, MTOW is reduced to 396,000 kg. Range stays the same if $\frac{W_{init}}{W_{final}}$ stays the same. So the new $W_{final}$ becomes:

$$ W_{final} = \frac{396,000}{400,000}\cdot 225,000 = 222,750 kg$$

$W_{final}$ is again MTOW - $W_{fuel}$ =>

$$W_{fuel} = MTOW - W_{final} = 396,000 - 222,750 = 173,250 kg$$

Final fuel saving is 175,000 - 173,250 = 1,750 kg

For one long distance, maximum range flight, a reduction of 1% in TO weight saves 1,750 kg of fuel. Every percent of weight savings equates to a percent of fuel saved. Every trip, for 2 - 3 decades.

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  • $\begingroup$ MTOW of 300 tons? Try 438 tons. $\endgroup$ – Carlo Felicione Nov 7 '17 at 4:45
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    $\begingroup$ There you go, even Yuger. $\endgroup$ – Koyovis Nov 7 '17 at 4:50
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    $\begingroup$ You take savings from any source you can. Taxiing is necessary anyway, regardless of the MTOW savings. Would you not want to save 62 kg of fuel every trip for 20 years, because you need to spend fuel taxiing? $\endgroup$ – Koyovis Nov 7 '17 at 7:54
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    $\begingroup$ Again, reasonably complex mathematics that generates the conclusion "reduce aircraft mass by 1% and save 1% fuel". Which is the kind of conclusion you'd be surprised if it wasn't true. $\endgroup$ – Yakk Nov 7 '17 at 16:26
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    $\begingroup$ @Yakk I don't think I'd upvote a question which made the assumption that it was true without justifying it. So many things don't scale the same in science, and planes are quite complicated balanced pieces of machinery. I think it's very important to demonstrate a justification for assuming they do. $\endgroup$ – Cort Ammon Nov 8 '17 at 1:04

protected by Federico Mar 12 '18 at 13:49

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