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Following a discussion on travel.SE, I wonder: would flying more slowly actually save fuel?

I suppose that it would save fuel on a volume of fuel per flight hour basis, but the flight would end up being longer, so would the total flight actually save fuel?

I’m under the impression that there is actually an optimal air speed, and diverging from that in either direction would actually use more fuel.

Which one is true?

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    $\begingroup$ I don't know about flying but driving slower does save fuel. At 120kph my car runs at 6.5L/100km but at 100kph it's 5.5L/100km. If I'm not in a hurry, I'd rather save 1L of gas instead of saving 10min of time. $\endgroup$ Sep 29, 2023 at 5:57
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    $\begingroup$ @user3528438 Your fuel efficiency may be better at 100kph compared to 120kph, but there still is an optimum. According to Wikipedia that should be between 56kph and 80kph for an internal combustion car. Below that, your efficiency will get worse again. $\endgroup$
    – Bianfable
    Sep 29, 2023 at 6:07
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    $\begingroup$ If fuel consumption would be proportional (i.e.: linear) to speed, then you would not save any fuel, as suspected. $\endgroup$
    – U. Windl
    Sep 29, 2023 at 8:31
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    $\begingroup$ Related: Fuel economy in aircraft. Actually SAS tried to fly at reduced speed, but only for 2 years. $\endgroup$
    – mins
    Sep 29, 2023 at 9:21
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    $\begingroup$ They already do compared to my car doing 80mph when the best thing to do is 45. A 747's max speed is 20mph over what it's 'econ' is : "econ. 907 km/h (490 kn; 564 mph), max. 939 km/h (507 kn; 583 mph)" - that's half of my error range, but at ten fold to start. They ain't messing around. $\endgroup$
    – Mazura
    Sep 29, 2023 at 16:19

2 Answers 2

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Yes it saves fuel, unless you get really slow (well below cruise speed - more later.)

Since drag (and thus power required) increases with the square of velocity it will save fuel for the total flight. A simple example even if not truly realistic: going twice as fast generates four times the drag and thus four times the fuel burn (ignoring small changes in engine efficiency) and it gets you there in half the time, so it would still cost twice the fuel burn for the flight. And as a side note, drag is proportional to air density. So flying higher reduces drag and fuel burn for the same speed.

You are correct. There is an optimal air speed. It's called Max Endurance speed. It is at the point where total drag (induced and parasitic) is at a minimum. It is significantly slower than normal cruise speeds. Going slower than that will increase power required and fuel burn even as you go slower. This is 'operating on the backside of the power curve.'

But for maximum range, that occurs slightly higher than minimum power speed at the speed for best L/D. Going slower than the max range speed will result in more fuel over a fixed distance.

One caveat is that if you have a head wind or tail wind, the fuel burn for a given distance will change.

And to make things more interesting; the airlines look at total costs for the flight, not just fuel burn. Crew costs are also significant. In the performance computations within the FMS there is an input called 'Cost Index.' Each airline calculates the index (offline) and provides it to the crew for input. It basically provides an adjustment factor based on the relative costs of fuel vs. crew. With high relative fuel costs, the Cost Index will calculate a slower speed to save fuel. If fuel is cheap relative to the crew, the CI will calculate a higher speed to reduce the crew costs (since they get paid by the hour.)

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    $\begingroup$ Not only crew costs but maintenance costs (things that have to be done after x flight hours) and capital costs (getting an extra flight per day out of their $100 million airplane). $\endgroup$
    – user71659
    Sep 29, 2023 at 0:18
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    $\begingroup$ Parasitic drag power is (to a first approximation) the cube of airspeed, but drag force and parasitic energy per distance is the square of airspeed. I think induced drag power is proportional to 1/speed (cost per distance 1/speed^2). For ground vehicles you have parasitic drag, dry friction (power ~ speed, energy/distance constant), but no induced drag. On a bicycle air friction is much more of a limit than dry friction. $\endgroup$ Sep 29, 2023 at 5:52
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    $\begingroup$ Not just airlines, but other operators as well - cargo companies and civil surveillance (e.g. powerline/pipeline monitoring or aerial photography) also care about total flight cost (perhaps also constrained by visual/weather conditions for survey flights). $\endgroup$ Sep 29, 2023 at 8:01
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    $\begingroup$ @mins While I see your argument, the swing in block times based on cost index isn't enough to get an extra flight per day out of the a/c. And it would be contingent on the CI not changing. Realistically, the CI may swing the block times by 5-10%. The schedules often have more pad than that to cover weather or other delays. If there is passenger demand the airlines will make the flight available. If they can't add a flight due to airport limits, they can change aircraft to one with more capacity. $\endgroup$
    – Gerry
    Sep 30, 2023 at 18:15
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    $\begingroup$ @Gerry It's not a simple linear matter. Flights need to arrive at a hub with a minimum time to allow connections. Airports have curfews. For these cases, a matter of minutes can make a flight possible. Demand is also not flat; people dislike early morning or late night flights, so shaving 15 minutes off the 5 previous flights changes a lower-demand 8 PM flight into a just-off-work 6:45 flight. $\endgroup$
    – user71659
    Sep 30, 2023 at 20:57
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There is an optimal speed for whatever you are trying to do -- for example, and optimal speed to cover the most distance per unit of fuel is probably what you are thinking of.

However, if you were flying a reconnaissance aircraft, you might care about staying in the air for as long (time) as possible, and there is an optimal speed for that too.

Most transport aircraft are flown at their best range speed. We sometimes fly these aircraft a little faster than that -- because people also value their time and it doesn't cost a lot more fuel, but can save significant time.

This is all considering the aircraft fixed. Another option would be to put ourselves in the role of the aircraft designer.

We could design the aircraft differently -- such that the new aircraft's best range speed was slower than the old aircraft. Such an aircraft might have better fuel economy than the old aircraft.

However, airlines are highly incentivized to reduce fuel burn. Better fuel economy is one of the only reasons they will replace a fleet of aircraft with a newer model. Consequently, modern airliners are highly optimized to reduce fuelburn. Improving on them is very challenging.

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    $\begingroup$ Most transport aircraft are flown at their best range speed. - Gerry's answer disagrees, saying they usually they fly somewhat faster, to save on crew costs and maintenance per flight hours costs, etc. while only burning a bit more fuel than at optimal-range cruise speed. "Near" would be a better word than "at", since my understanding is that most passenger flights (and probably cargo) could save a bit of gas by flying a bit slower. $\endgroup$ Sep 29, 2023 at 15:33
  • $\begingroup$ Re: aircraft design. I seem to recall some older transport planes having a somewhat higher typical cruise speed, like maybe MD-80? But en.wikipedia.org/wiki/Boeing_737 shows newer 737 models having a higher cruise speed than older 737, now Mach 0.785, up from 0.745. And Boeing 777 has a Mach 0.84 typical cruise speed, according to google. If you're willing to sacrifice speed, you can use larger wings, but that's a problem at airports. (Or your plane has to be smaller.) $\endgroup$ Sep 29, 2023 at 15:56
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    $\begingroup$ @PeterCordes I think the highest design cruise speed for a bit transport was the 747. Newer aircraft have been designed to slightly slower speeds to save fuel. Business jets on the other hand are playing a game of one-upsmanship, each pushing a tiny bit faster than the last aircraft so their rich owners can have bragging rights. $\endgroup$ Sep 29, 2023 at 17:06
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    $\begingroup$ @PeterCordes I used to tell my students -- all aircraft are span constrained. If you don't have a span constraint, you just don't know what it is yet. $\endgroup$ Sep 29, 2023 at 17:06
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    $\begingroup$ @reirab Could be. My main point was that cruise Mach for these aircraft peaked in 1960 or 1970 and that aircraft introduced since then have actually been designed to reign that back in. $\endgroup$ Oct 2, 2023 at 15:41

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