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Having read about Rutan's Voyager and more recently about Virgin Atlantic GlobalFlyer I wondered how would some modern long haul airliner such as a cargo Boeing777 (or other airliner) perform at this specific longest range flight attempt?

Assuming take off at MTOW, and carrying as a payload no passengers but only usable fuel (tanks in cabin connected to existing fuel lines) and pilots.

What would roughly be the achievable range and/or how would one proceed to estimate this range?

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    $\begingroup$ Several years ago, I chatted with a pilot who'd recently qualified on the 777. He spoke very highly of it, saying that the only problem with the airplane is that "it can hold way too much gas." The scenario given here would elevate that objection to a higher dimension! Interesting question nevertheless. $\endgroup$
    – Ralph J
    Sep 27, 2023 at 21:59
  • $\begingroup$ Flying slower will improve range. How much so is moot. || I tried to enthusiastically ask CGPT4 to provide an order of increase with decreasing speed. It utterly failed to do so. $\endgroup$ Sep 29, 2023 at 2:03

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First, go to the Boeing Airport Planning documents website. This includes a wealth of information about most Boeing commercial aircraft. Choose the document that matches the aircraft you're interested in. I'll choose the 777F document because you mentioned a cargo 777.

Scroll through to PDF page 45. That is a payload-range diagram for the aircraft. 777F Payload Range Diagram

This chart shows the aircraft's capabilities in terms of payload, range, and fuel. The X-axis represents the range. The Y axis represents the amount of payload in terms of OEW (operational empty weight) plus Payload. OEW is used because different airlines outfit their aircraft in slightly different ways, so their starting point (OEW) is different. The 777F's OEW is about 318000lb.

The horizontal line across the top of the payload-range diagram depicts the maximum zero fuel weight (MZFW). This is typically a structural limit for maximum landing weight. You can't land heavier than this without risking breaking the gear.

The first corner of the payload-range diagram represents the maximum gross weight at brake release. In this case, 766,000 lb. I.e. 448,000 lb of payload and fuel. The upper-left corner of this line is the maximum payload choice for that gross weight -- 229000 lb payload, 219000 fuel.

Following along that line, we remove a pound of payload and replace it with a pound of fuel (to maintain constant brake release gross weight). As we do this, the aircraft can reach longer range -- same weight, but more fuel. This continues until the fuel tanks are full. I.e. we reach the second corner on the chart-- 320860 lb fuel.

At that point, we can continue reducing the payload to increase range -- but since we can't add more fuel, we don't get as much of a benefit.

If you wanted to add more fuel tanks in the cargo bays of the 777F, you could do so. The empty tanks would add to the OEW. So instead of starting from 318,000 you might start a few thousand pounds heavier.

To estimate the best achievable range, you want to extrapolate the line from the first corner on the payload-range diagram (the one at constant brake release gross weight). Extrapolate that line out down and to the right until you reach the OEW of your modified aircraft with more tanks. Performing highly accurate eyeball extrapolation, I estimate 13,000 nautical miles.

The slope (m) of this corner of the payload-range diagram represents an important fundamental aircraft performance quantity. We just have to invert it and change the sign...

The slope is (very approximately)

$m=rise / run = -100000 lb / 3000 nm$

$m=-\frac{1}{SR}$

or

$SR=-\frac{1}{m}$

Where $SR$ is the specific range of the aircraft. So about 30nm/1000lb fuel. This is a measure of fuel efficiency similar to MPG for a car.

If you look at the term in the front of the Breguet Range Equation, I think you will see the Specific Range there.

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    $\begingroup$ I may have misunderstood, but doesn't this chart/answer depend on a weight limit FOR LANDING which we could/probably would ignore for a situation like this? $\endgroup$
    – MikeB
    Sep 28, 2023 at 9:14
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    $\begingroup$ MZFW isn't about landing; your max landing weight limitation covers those concerns. Max zero fuel weight is essentially a "max fuselage weight" for the case that the wing tanks are mostly empty and you have the greatest bending moment on the wings. That bending, not the gear, is what MZFW protects: even flying short hops (minimal fuel required), you can't necessarily load iron weights into the fuselage right up to max landing weight. More an issue on some planes than others. $\endgroup$
    – Ralph J
    Sep 28, 2023 at 13:46
  • $\begingroup$ @MikeB There are many constraints that limit the performance of an aircraft. This diagram tries to depict them all. The only one that has anything to do with landing is the horizontal line across the top. On this aircraft it is MZFW -- on some aircraft is is Maximum Landing Weight. It always represents the maximum payload possible. Rest assured, the chart summarizes a full mission. $\endgroup$ Sep 28, 2023 at 16:00
  • $\begingroup$ thanks for this very good explanation of such a chart, i guess 13,000 nm is already amazing, shouldn't the extrapolated part of the curve vary its slope since it drops "payload" while flying, (at some point in flight)? $\endgroup$
    – user721108
    Sep 28, 2023 at 16:38
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    $\begingroup$ Yes, that corner is concave. If you print it out and sight along the line, you'll see the curvature. The specific range of the aircraft varies in flight. The slope of this line represents an average over the entire mission. All of these missions (along that line) start at the same gross weight -- but they end at different gross weights, so the average changes. $\endgroup$ Sep 28, 2023 at 17:38
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First cut numbers, working from Wikipedia's B-777 page, and using the 777-200LR, since that seems to be the longest-ranged of the models:

  • Max Takeoff Weight (aircraft + passengers + cargo + fuel) = 766,000 lbs
  • Operating empty weight (just the aircraft & crew) = 320,000 lbs
  • Max Fuel, conventionally configured: 321,000 lbs (approximately)
  • Leaves non-fuel weight available, with full tanks, of 125,000 lbs (i.e. 766 - 320 - 321 = 125)

So that's how much more fuel weight you could carry in the new tanks without the aircraft being overweight. Which is less than half-again the existing fuel load, so we aren't going to be getting dramatic 3X increases in range.

The published range for the 777-200LR is 8,555 NM, which is stated as almost enough to connect any possible pair of airports in the world. Multiplying that times the ratio of notional fuel capacity to existing fuel capacity (i.e. 446,000 / 321,000) gives a result of 11,886 NM. Not near Voyager's 26,366 NM flight.

Disclaimers:

This leaves out a lot of factors. How much OEW you lose by stripping out seating, galleys, and so forth, and how much you add by adding fuel tanks & pumps, is one.

Also, the distance you fly per pound of fuel when an airplane is full of gas and heavy, is significantly less than the distance you fly on a pound of fuel in a light aircraft. So while the 8,555 nm range figure is presumably starting at a max-weight takeoff (the same as the notional flight), it ends heavier than the hypothetical flight -- so the "extra" gas effectively gets used more efficiently, since it's used when the aircraft is lighter than it would be in revenue service. (This dynamic is captured in Rob McDonald's answer, and his 13,000 NM range seems like about the right increase here.)

Also, the published range builds in some worst case, or 90% worst case assumptions about winds and reserves -- if you say you can fly from X to Y, you also want to be able to fly from Y to X, even when the winds are strong (i.e. a headwind on one of the two legs). If you're planning a record-setting flight, you just fly with the winds, and where they're best for you. And you'll spend the additional last hours of your flight at a maximally efficient (high) altitude because you're so light. Which probably is where you'd enjoy strong tailwinds as well. So all of that would improve on the number above by quite a bit, since you're not trying to fly in both directions between your two airports routinely, but only once when conditions are best.

Nevertheless, unless you get waivers for a far higher takeoff weight than what's published (which the performance on a cold day at Edwards AFB would probably support), you're still far, far short of being able to do what Voyager did.

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    $\begingroup$ Setting aside for a moment the specific question on this page, as a non-aviator I had absolutely no idea, did not realize, the fuel in an airliner when you "top it off" is roughly the same as the airliner!!! $\endgroup$
    – Fattie
    Sep 28, 2023 at 9:03
  • $\begingroup$ There's a reason those two planes look like they do. They're essentially gliders that don't depend on thermals. $\endgroup$ Sep 28, 2023 at 9:51
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    $\begingroup$ @Fattie: The amount of fuel in a modern airliner is crazy. A SpaceX Falcon 9 rocket has roughly the same fuel capacity as a Boeing 747. It takes several large tank trucks to refuel a single one of them. Kind of puts into perspective how much fossil fuel we burn every day, because thousands of big planes fly all around the globe every day and yet aviation only contributes ~3% of the world’s CO2 emissions. $\endgroup$
    – Michael
    Sep 28, 2023 at 10:35
  • $\begingroup$ @JörgWMittag what differientiate those three is mostly flight speed i think, the airliner is some sort of a glider too, meant to fly much faster $\endgroup$
    – user721108
    Sep 28, 2023 at 16:04
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    $\begingroup$ @PeterCordes I suspect that it would be lots of engineering analysis to raise the MTOW. What could get approved for a one-time record attempt, no passengers, new brakes, miles of runway, etc could probably be lots higher than what they'd get (or try for) for regular commercial service. So could they get it raised? My hunch is, yes, by a good bit. Enough to approach 3X the published range? Doubtful. $\endgroup$
    – Ralph J
    Sep 29, 2023 at 3:41

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