The heavy little secret of a long flight
Here is something most passengers never think about: an airplane has to burn fuel just to carry its fuel. Every kilogram in the tanks has to be hauled down the runway, lifted to 38,000 feet, and dragged across an ocean before it is finally burned. So a jet setting off on a fourteen-hour haul is enormously heavy at the gate, and it spends the first few hours burning fuel simply to move the fuel it will need later. As the tanks empty the airplane gets lighter, the engines throttle back, and every remaining mile gets cheaper. It genuinely sips late in a flight and gulps early on.
Pilots, dispatchers, and planners have a name for it: fuel carries fuel. And it has a lovely side effect. A short hop never has to load all that extra weight, so it flies light the whole way and burns noticeably less per mile than the same aircraft would on a marathon. Distance and fuel are not a straight line. They curve.
How to read the chart
That curve is exactly what the chart above draws. The horizontal axis is the length of the trip; the vertical axis is how much fuel the airplane burns per mile on a trip of that length. Grab the dot and drag it.
- The flat dashed line is the lazy way to guess fuel: assume one average burn rate for every trip, no matter how long. It never moves.
- The glowing curve is the honest answer, the one that accounts for fuel carrying fuel. It dips lower the shorter the trip, because a shorter flight is lighter and every mile is cheaper. The two lines meet only at the aircraft's maximum range, the one trip length where a flat rate happens to be right.
- The shaded gap is not fuel the airplane saves; it is how much a flat rate over-counts per mile. The airplane always burns its real amount, and the gap is simply the flat estimate's error, widest on the short legs where the plane is lightest. The readouts show it as a percentage and in tonnes.
Now switch aircraft. A narrowbody like the A320neo barely dips, because it never carries a huge fuel fraction. Flip to the A350-900ULR and the curve plunges: an ultra-long-range jet can leave the gate carrying more fuel than payload, so a flat rate over-counts its shorter legs the most. The fuel carries fuel stat under the chart puts a number on it, the ratio between what the last mile costs and what the first mile costs.
Meet the Breguet range equation
None of this is new. A French engineer named Louis Charles Breguet worked out the mathematics more than a century ago, and the Breguet range equation has been the backbone of aircraft performance ever since. Every airline performance department, every manufacturer brochure, every real flight plan rests on it. In plain terms it says the airplane's weight fades with distance as it burns fuel, so the fuel a trip needs grows a little faster than the distance itself.
The single number that characterizes each airplane is its Breguet constant, CB, a distance that folds cruise speed, aerodynamic efficiency, and engine thirst into one figure. A higher CB means a cleaner, thriftier airplane whose fuel curve bends more gently. Pick any aircraft and the chart shows its CB and its fuel fraction live.
CB fuel fraction the industry-standard range equation
Why it matters for fuel and CO2
This is not just trivia for aircraft nerds. Estimate fuel with the lazy straight line and you overstate it on most real flights, and because carbon follows fuel directly, every kilogram of Jet A burned releases about 3.16 kilograms of CO2, you overstate the emissions too. Getting the curve right is the difference between a rough guess and a number you can trust. Great Circle Pro uses the Breguet method, the most accurate way to tie fuel to weight and distance, so the fuel and CO2 on every route reflect how the airplane truly flies rather than a convenient average.
You will find the same figures on every Flight Plan card and in the full carbon breakdown of the emissions dashboard. There is more on the method in how it works, and it is listed among the tool's features. No login, no install, live in your browser.
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