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Is the most fuel-efficient route between points A and B always along a great circle ?

Although somewhat counter-intuitive, this comment on an aviation blog seems to suggest that longer paths (in terms of geographic distance covered) can save fuel:

Oceanic Flights these days are not restricted to the old “defined great circle routes”. Modern computerised flight planning systems definately take weather into account, and go searching for the best winds (tailwinds) or least adverse (headwinds). Particularly in the case of westbound flights across the Indian Ocean, where headwinds are often ferocious, the tracks actually flight planned, and actually flown, are often a long way off “great circle” routes.

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On a flat plane, the shortest distance between any two points is a straight line, right? So if you were at A and wanted to get to B in as few steps as possible, you would walk a straight line.

But what if there's a moving sidewalk along that route, going the wrong way? Now is the path of fewest steps still a straight line? Probably not: by adding a few steps at the start and end, you can bypass the moving sidewalk and avoid the extra steps it would force you to take. The moving sidewalk essentially added more ground, so that the shortest path via a tape measure is not the shortest path by steps taken.

The idea is the same with headwinds. The shortest path as measured by the ground is not the same as the amount of air you fly though, because the headwind effectively add more air. Since the engine is working on the surrounding air, not the ground under you, the fuel consumption is based on how much air you fly through -- not how much ground that happens to cover.

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    $\begingroup$ great analogy ! $\endgroup$ – summerrain Dec 30 '18 at 1:46
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That depends how you define longer in aviation. As you can look at it geographically or on the clock.

The hard answer is no, the most fuel efficient route is the one that has the aircraft in the air for the shortest amount of time. If you can take advantage of a big tail wind 20 miles south of the geographically shortest route or at a slightly higher altitude you may burn a bit of fuel getting there but on long haul flights you can make up for the burn, and even gain quite a bit by taking advantage of the winds

Similarly headwinds can really kill your efficiency even over short distances.

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  • $\begingroup$ "Longer" is meant geographically in the question. I thought this was apparent from the context. How should I rephrase the question to make that clear ? $\endgroup$ – summerrain Dec 20 '18 at 2:10
  • $\begingroup$ I think its clear, i have edited a bit to clarify my answer. $\endgroup$ – Dave Dec 20 '18 at 2:41
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    $\begingroup$ @summerrain How about "Can a longer path in terms of geographic distance covered consume less fuel than a great circle?" As Niels said, the answer is yes. Also be aware that it's possible that the route that results in the smallest fuel burn is not the necessarily the route that takes the least time. That's true more often than not, but if to get the most favorable winds you had to stay low, you might burn more fuel. $\endgroup$ – Terry Dec 20 '18 at 2:47
  • $\begingroup$ @Terry never mind! You answered it! $\endgroup$ – FreeMan Dec 20 '18 at 14:50
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Yes, the shortest route by distance (great circle route) is very often not the most fuel efficient route to take.

An aircraft at a given speed, altitude and weight will burn a predictable amount of fuel per hour, regardless of the distance traveled. It follows that the less time in the air, the less fuel is burned (generally speaking, and if flying economically). If there is a tailwind, the aircraft will travel faster without using any extra fuel, needing less time to go from point A to B, ergo less fuel consumed. So a pilot or dispatcher will review the forecast winds, and if they are strong enough, will detour from the direct great circle route to take advantage of them (or get out of their way if it is a headwind).

Anecdotally, I would predict that almost every single long haul flight flying east/westwards will deviate from the GC route due to winds (as opposed to other route changes like airspace restrictions).

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    $\begingroup$ It may be nit picking, but the statement, "An aircraft at a given speed will burn a predictable amount of fuel per hour" might be more strictly stated "An aircraft at a given speed, altitude, and weight will burn a predictable amount of fuel per hour. $\endgroup$ – Terry Dec 20 '18 at 4:47
  • $\begingroup$ @Terry nit picking or not, it could have left the impression that flight planning is anything but complex, so I have edited that in! Thanks $\endgroup$ – Ben Dec 20 '18 at 5:55
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    $\begingroup$ A good example of this is the new software QANTAS has rolled out. smh.com.au/business/companies/… $\endgroup$ – tl8 Dec 20 '18 at 6:20
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    $\begingroup$ Scheduled operators might choose to fly the great circle route anyway, but at a different power level, to maintain the same arrival time. Arriving late could mean lots of missed connections, arriving early may result in not having a gate available, and either could conflict with noise abatement rules. $\endgroup$ – StephenS Dec 20 '18 at 16:47
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You begin with a great circle route as the default. Then you adjust it to account for the presence of either favorable or unfavorable winds.

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  • $\begingroup$ So your answer is YES ? $\endgroup$ – summerrain Dec 20 '18 at 2:01
  • $\begingroup$ yes. it depends on the winds present along the flight path. $\endgroup$ – niels nielsen Dec 20 '18 at 2:03
  • $\begingroup$ You need also to take into account the ETOPS capabilities. ETOPS allows to navigate remote from diversion airports. $\endgroup$ – mins Dec 30 '18 at 12:53

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