Considering a very long route (equal to the max ferry range of an aircraft[1]).

Would it be more fuel efficient to fly multiple legs or a single one ?

  • Multiple legs would allow you to use less fuel, decreasing weight and fuel consumption
  • Single leg would dispense you having to go through multiples takeoffs and time increased at low, high drag altitude

How does theses balance ?

I'm interested into any real world aircraft[!].

[1]: Results might differ for each models so you can pick any you like.

  • $\begingroup$ Are you asking for airlines, or general aviation, or both? Are you wanting to factor in revenue or just the fuel and costs? $\endgroup$ – slookabill Mar 12 '16 at 0:32
  • $\begingroup$ @slookabill Both are equally interesting. I suspect GA planes to lean more on the multiples hops tho, since they usually fly quite low anyway. I'm mostly interested into the fuel cost, tho a bigger picture is also welcome. $\endgroup$ – Antzi Mar 12 '16 at 0:39
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    $\begingroup$ What does efficiency mean in this context? Least time? Least fuel consumption? Most profitable for an airline? $\endgroup$ – Zach Lipton Mar 12 '16 at 2:15
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    $\begingroup$ @Antzi: Personally, the reason I (and I suspect many GA pilots) tend to break longer trips into multiple hops has nothing to do with fuel. It's bathroom breaks. $\endgroup$ – jamesqf Mar 12 '16 at 4:15
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    $\begingroup$ Just a nitpick: Drag is lower at lower altitude due to the higher Reynolds number, at least when the aircraft flies at the same lift coefficient. $\endgroup$ – Peter Kämpf Mar 14 '16 at 4:09

If the total distance is long enough, flying it in several trips is more fuel efficient. In this answer I have used Breguet's equation to calculate the fuel required to fly a fully loaded A320-200 over 2000 km and 5700 km. The first trip needs 5 tons of fuel while the second needs 18.1 tons. This means the longer trip needs 27% more fuel per km of distance because more fuel is transported and the aircraft is heavier over most of the trip.

However, flight through lower, denser air is less efficient in terms of distance flown per unit of fuel burnt, so you need to find an optimum between flying mostly near the ground and flying with a heavier aircraft. On the other hand, the lighter aircraft will climb much faster, so the time spent at low altitude is lower per climb segment in case of the staged legs.

In the end, if the total distance is big enough to allow for several hops while still only a fraction of the total flight time is spent at lower altitude, the overall fuel consumption is easily lower.

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In a broad sense, if the flight distance is nearly equal to the max range of the aircraft, the route with shorter legs is prefereable from the point of fuel economy. The reason is simple- more the fuel you carry more the aircraft you have more weight, which requires more fuel.

All other things being equal, the fuel caried is payload- one can see it as the aircraft carrying extra payload or as the fuel eating into the regular payload. Either way, the extra fuel carried impacts on the fuel effeciency of the aircraft negatively.

  • According to this document, payload burns fuel at the rate of 2.5-5 % of its weight in fuel per flight hour. For the longest non-stop routes in service, the travel time is in the upwards of 15 hours. Taking a value of 3% fuel burn, this means that a significant portion of fuel consumption is actually to carry the fuel itself. This is a significant issue, especially when the oil prices are high.

  • The increased take off weight will result in more thrust requirement, TO length and time to climb (or more thrust again), adversely affecting the fuel consumption, while increasing engine wear.

  • While it is true that the longer distance flights spend more time in altitudes with less drag, it is questionable whether that translates into any significant real world benifits. For example, the latest 777 ULR version has a higher aspect ratio to reduce drag, requiring the folding of wings.

  • Of course, fuel use is only a part of the story. The other significant factor is time- most of the ultra long range flights are targeted towards business flyers, for whom the addition of more than 25% due to shorter legs may be unacceptable. In the end, the deciding factor may be the percentage of seats filled per flight. When SA cancelled the Singapore-Newark flight, one analyst noted:

“The plane burns a lot of fuel but carries very few passengers,” says Siyi Lim, an analyst at OCBC Investment Research in Singapore. “It didn’t make sense to continue.”

  • The ultra long distance flights also require additional facilities- extra crew for rotaion, extra food items carried etc. In general, the straight long distance routes have higher fares compared to the ones with stops in between.
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  • $\begingroup$ The answers to this question seem to contradict this. I think your SA example is not really what the OP was asking. Any flight that doesn't have enough passengers to pay for the fuel burn is likely subject to cancellation. I think he's asking with all other variables removed is it more fuel efficient to stop and fill up. Not saying you're wrong, it just seems to conflict with what they're saying on that other answer. $\endgroup$ – TomMcW Mar 12 '16 at 18:58
  • $\begingroup$ @TomMcW I don't think it's a contradiction - the other answer is talking at the opposite end of the spectrum (50km flights) whereas this answer applies to the 18 hour trips. I think the Singapore-Newark flight is a good example but the quote alone here doesn't provide enough context. That flight took so much fuel that it could only carry about 100 passengers, so it would require fewer empty seats to become unprofitable compared to regular flights. $\endgroup$ – Ben Mar 13 '16 at 0:44
  • $\begingroup$ I agree with you for GA flights that stay below 10,000 ft. The extra weight of fuel in the early part of the flight is probably more significant than the fuel burn for landing/takeoff sequence. But I'm skeptical about this assessment for bigger aircraft. I ran some numbers through an online flight sim calculator and got a way different result. I'll post it as an answer. Feel free to read it and point out what I missed. I'm open to criticism $\endgroup$ – TomMcW Mar 14 '16 at 2:42
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    $\begingroup$ @Ben That flight took so much fuel that it could only carry about 100 passengers Not so according to Airbus website. They say the A340-500 that was on the route is "capable of flying 9,000 nautical miles with a maximum passenger payload," of 375 pax. They were originally flying it with a 181-pax, two-class layout. They couldn't fill that so they reconfigured for 100 pax at a higher price. The nonstop failed for supply/ demand reasons. Much easier to fill a 2-leg flight $\endgroup$ – TomMcW Mar 15 '16 at 3:55
  • $\begingroup$ @TomMcW Ah you're right there, and I now distinctly remember reading an announcement when they got rid of the economy class seats (for the second time, IIRC). Still, such an ultra-long haul flight has tight economics to begin with because as you're so heavy, you do use more fuel on an hourly basis - aviation.stackexchange.com/questions/931/…. $\endgroup$ – Ben Mar 17 '16 at 8:32

For GA aircraft that stay at lower attitudes carrying the extra fuel for a direct flight certainly amounts to more fuel burn. But It seems to me that, in the case of large airliners, the process of landing then taking off would burn more fuel than just flying past. So I ran some numbers through a flight sim fuel calculator to see what I got.

I used the calculator at Simbrief.com. I have no idea of the real-world accuracy of it, but it took a lot of variables into account.

I used a their 777-200ER model for a flight from LAX to JFK with 250 passengers. BOS was alternate. I routed it over the MCI VORTAC (which a great circle route comes within a few NM of anyway).

It calculated trip fuel at 54,878 lbs.

I then ran it with a stop at MCI. I used the same SID out of LAX and the same STAR into JFK.

It calculated the LAX-MCI leg fuel at 32,280 lbs, and the MCI-JFK leg at 28,676 for a total trip of 60,956.

Stopping at MCI took 6,078 lbs. more fuel,which is about 11%.

The farther off the route the stop is the bigger difference that would be. The reason non-stop flights generally cost more than connecting flights obviously has more to do with economics, passenger logistics and convenience than fuel cost.

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    $\begingroup$ That's an interesting software, I will have a go myself later on! But your example isn't answering the question -- the OP is asking about a flight equal to the maximum range of the aircraft. LAX-JFK is pretty short for a 777. In your example yes, stopping at MCI would add 11% of total fuel, but if you did LAX-SIN (for example, and assuming that software is realistic), a stop over somewhere in between would add a lot less, and would maybe even save fuel. The reason is because you're burning extra fuel just to carry more fuel, whereas you could do two legs at a lighter weight. $\endgroup$ – Ben Mar 15 '16 at 3:07
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    $\begingroup$ Consider this: I don't know if you are into Formula 1 racing, but drivers can choose to have a pit-stop once, twice, or even three times during a race. It might not make sense to pit more than the bare minimum, right? Well, by pitting more often, they don't carry as much fuel, and therefore are lighter and have quicker lap times. This strategy won't work if the race is only 10 laps long, but if it is long enough you could win the race. It's similar with flying. Figuring out at what point 'long enough' is in order to make the stop beneficial requires airline data and many smart minds. $\endgroup$ – Ben Mar 15 '16 at 3:12
  • $\begingroup$ @Ben That makes sense. I just used LAX-JFK because I happened to know MCI was right in the middle. Took quite a while to crunch the numbers. If I get a chance I'll try a longer route. And, like you said, I have no idea how accurate that site is. It's made for flight sims. I don't have access to any real-world planning software and I couldn't find good reliable numbers to do my own calculation. $\endgroup$ – TomMcW Mar 15 '16 at 3:26

One factor I haven't seen mentioned is that many GA pilots like to fly with excessive reserve fuel (far beyond FAA requirements) as a safety margin. In addition some operators keep the tanks full in order to avoid condensation* in aircraft which tend to sit for long periods. For those reasons it is not unusual for a GA flight to carry significantly more fuel than it needs on any given leg. If you're going to carry a full tank anyway, it will certainly be more efficient to make the trip without stopping.

Airlines are obviously critical of waste related to carrying excess fuel and have policies in place intended to ensure that no more than the minimum required fuel is carried on any leg. The closer that works out to being true, the more relevant this question becomes. But knowing my share of airline pilots, I also find that the more disgruntled they are, the less inclined they are to pinch pennies for the boss. i.e. The benefits gained from flying shorter legs will likely be partially negated by carrying excess fuel anyway.

* Regardless of whether the condensation theory has merit, storing the tanks full is commonly taught as good practice.

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  • $\begingroup$ If the aircraft is stored with tanks full, would that fuel be siphoned out in case the pilot wants to fly a short leg ? $\endgroup$ – Antzi Apr 28 '16 at 9:35
  • $\begingroup$ That's an option but highly impractical. First you have to decide what to do with the removed fuel (store it or toss it). If you decide to store it then you probably won't want to put it back in the plane unless you have a really good filter to run it through. And Avgas is expensive, so no one wants to toss it. It would probably end up in a lawn mower. $\endgroup$ – hemp Apr 28 '16 at 9:48

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