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I was looking into my carbon footprint related to flying and seem to have came across somewhat of a contradiction.

WIKIPEDIA says "mid-range is better"

https://en.wikipedia.org/wiki/Fuel_economy_in_aircraft shows the fuel efficiency as a function of distance stating that the optimum distance is somewhere in the mid-range. Specifically

Above a certain distance it becomes more fuel-efficient to make a halfway stop to refuel, despite the energy losses in descent and climb. For example, a Boeing 777-300 reaches that point at 3,000 nautical miles (5,600 km). It is more fuel-efficient to make a non-stop flight at less than this distance and to make a stop when covering a greater total distance

IATA Carbon Footprint Calculator: "longer is better"

I used https://www.iata.org/en/services/statistics/intelligence/co2-connect/ to calculate the carbon emissions as a function of flight distance. In this case I use Newark as the starting point and choose different target cities averaging between both directions and using a Boeing 777 as aircraft wherever possible.

Despite their claim to be accurate and based on real airline data, the IATA numbers could easily be explained with a very simple model of

$$C = 59.2kg + D \cdot 0.056 \frac{kg}{km}$$

where $C$ is the weight of the emitted Carbon and $D$ is the distance flown. For most cities the error of this model was less than 1%. It's a simple linear model meaning "starting and landing produces 59kg and then it's a constant 56 kg per 1000 km distance flown".

Based on this model, a longer flight will always be more efficient (in terms of emissions per distance) since you amortize the emissions for starting and landing over a longer distance.

For example, I checked EWR->DXB vs EWR->FRA->DXB. The total distance is almost the same but emissions for EWR->FRA->DXB come out to be 739kg vs 669kg for EWR->DXB.

This seems to contradict the Wikipedia article which indicates that EWR->FRA->DXB should be more fuel efficient.

Questions:

  1. Can someone reconcile the apparent contradiction?
  2. If I want to minimize my carbon footprint should I split up ultra long hauls (where reasonably possible) or not ?
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Breaking up long haul flights in shorter segments may reduce the carbon footprint, if the detour made is only small and the shorter flights are flown on aircraft that have comparable or better efficiency that the long haul flight.

Copied from this answer:

The uplift of fuel costs approximately 4% of its own weight per hour of flight. That means the fuel you consume in the 4th hour of flight has burned approximately 11.5% of its original weight, just to carry itself. The fuel burned in the 7th hour of flight has burned 25% of its original weight , just to be carried for the previous 6 hours. (which means you have to load 133% of the fuel used in the 7th hour)

So a 10% detour in distance, with an extra stop, does not increase the total fuel consumption much. It may even reduce the total fuel consumption of the flight.

That answer looked at breaking up a transatlantic flight into two segments with Iceland as a halfway stop. The longer the flight, the more positive the effect of a stop in between can have.

It seems that the IATA calculator does not take into account that it costs fuel to carry fuel. They have made a linear approximation to a nonlinear curve.

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In addition to the factors mentioned by @Ralph J, also realize that the airlines do not operate identical equipment on all routes.

In the US, there are three general categories.

The big-name airlines do not directly fly the shorter, regional flights to smaller cities. These are handled by subcontracted regional carriers operating regional jets (often ERJ's or CRJ's).

On domestic routes between larger cities, the airlines typically operate their own, somewhat larger equipment. Typically a B737 or A319/320/321.

On international routes, the airlines operate long-haul aircraft such as B777, B787, A330, A350.

The airlines have a direct financial incentive to minimize fuel burn -- fuel is a big contributor to their operating costs. Consequently, they work hard to match the right aircraft to every route -- taking into consideration range and payload capacity (matched with demand for that city-pair).

Even within a gross model, there are lots of differences. For example, there are four generations of 737's that have been produced (or are being produced). The first entered service in 1968 -- airlines typically get about 25 years of service out of a 737. Each generation typically has three models (small, medium, large) and sometimes they come in long-range variants, etc.

Each new generation comes with a new generation of engines that are substantially more fuel efficient than the one that came before.

Breaking a given trip into two shorter trips may switch you from a newer, more efficient aircraft with good load factor (many seats filled means better seat-mpg) to a less efficient aircraft with possibly less load factor.

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The IATA formula, at least as summarized, ignores the problem that increasingly longer range requires increasingly higher takeoff weights, and more fuel used during the first part of the flight to carry fuel that's needed for the latter part of the flight. At some point, the inefficiency of taking off extremely heavy and initially cruising at low altitudes (driven by the heavy weight) starts imposing its own penalties. The first 1,000 miles (or km) flown by an aircraft near its max gross weight burns far more fuel than the first 1,000 miles flown by a lighter aircraft, all else (i.e. aircraft type) being equal.

If you assume that the "last 1,000 miles" flown by both the super-long-haul aircraft and the medium-haul aircraft are both flown at the same (comparatively light) weight and at the same (high, fuel-efficient) altitudes, then both would have comparable fuel burn per mile for that segment. The problem is that the first 1,000 miles of the super-long-haul jet burns far more fuel per mile, and at some point those inefficiencies will overcome the benefit of "amortizing" the fixed fuel burn across a greater number of miles. (Also, that "fixed" burn for takeoff + initial climb isn't really fixed -- it also goes up as takeoff weight increases. The descent & landing would be comparable, since they're both at basically whatever weight is assumed for landing.)

As far as

If I want to minimize my carbon footprint should I split up ultra long hauls (where reasonably possible) or not ?

It doesn't matter. The airplanes are flying with you or without you, and the delta in burn between having +1 person on "this" flight vs "these" flights is too minuscule to contemplate.

Airlines may or may not actually care about CO2 emissions (they certainly want to look like they care, but in reality...), but they most certainly care about the cost of fuel, which is where the carbon comes from. If there was a flight that was burning so much fuel that they'd make more money by flying it as 2 segments vs 1, or 1 instead of 2, then they'd do that. The fact that both the nonstop EWR-DXB and the stopover in FRA exist, reflects market dynamics, the airlines' networks, & what's profitable, and neither is going to go away because those who work to compute their carbon footprint choose whichever one.

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