United Flight Uses 100 Percent SAF To Power Passenger Flight


The 100 or so passengers on United 2701 from O’Hare to Reagan National on Dec. 1 likely didn’t notice anything different but the airline says the planet breathed a tiny bit easier. The Boeing 737-8 flew the 612 miles with one engine burning 100 percent sustainable aviation fuel (SAF) made by World Energy. “Today’s SAF flight is not only a significant milestone for efforts to decarbonize our industry,” United CEO Scott Kirby said in a statement, “but when combined with the surge in commitments to produce and purchase alternative fuels, we’re demonstrating the scalable and impactful way companies can join together and play a role in addressing the biggest challenge of our lifetimes.”

Normally, airlines are allowed to carry passengers using a maximum of 50 percent SAF in a blend with JetA. On this flight, United was allowed to fill one tank with 100 percent SAF to run one of the engines. The flight was set up to show “there are no operational differences between the two and to set the stage for more scalable uses of SAF by all airlines in the future,” Kirby said. United’s math determined that by using half SAF, the flight contributed a net 75 percent less carbon to the environment than if it had been flown on pure JetA. 

Russ Niles
Russ Niles is Editor-in-Chief of AVweb. He has been a pilot for 30 years and joined AVweb 22 years ago. He and his wife Marni live in southern British Columbia where they also operate a small winery.

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  1. I was initially confused by United’s math – running one engine on SAF and one on conventional Jet-A couldn’t possibly reduce the flight’s carbon emissions by 75%. To be fair to United though, it’s a mis-quote; they actually said that the SAF engine contributed 75% less carbon, not the overall flight. Since the other engine is running normal Jet-A, I suspect the overall flight showed a 37.5% saving. It makes for an interesting observation though – and perhaps a nice honesty from United. SAF’s contribution is not zero. They still have to grow the crop, make the stuff, transport it, and deliver it to the aircraft, all of which have a carbon footprint.

  2. “Normally, airlines are ALLOWED to carry passengers using a maximum of 50 percent SAF in a blend with JetA. On this flight, United was ALLOWED to fill one tank with 100 percent SAF to run one of the engines. ”

    If the stuff is so safe, why didn’t the FAA “allow” United to fuel BOTH engines? Obviously, they were concerned about SOMETHING about SAF.

    Either the stuff IS safe, or it ISN’T–can’t have it BOTH WAYS. As for United, “Five yard penalty for “virtue signaling”–I’d be less inclined to fly them due to this kind of claptrap.

    • I’m not a pilot yet but have been around aviation most of my life. I’m trying to understand something here. I did a quick look up to see what exactly SAF was and it came back as chemically identical to the regular fuel which is a hydrocarbon. Typical jet fuel comes from oil which largely at one point was plant life. If you are correct Arthur, which I suspect you probably are, then how does this reduce the so called carbon foot print? I’m not a chemist or chemical engineer but I am an engineer so I have a good grasp on science in general. I’m wondering what I might be missing here though. In very simplistic terms, plants take in light, CO2 and H2O and give off O2 and C6H12O2 (oxygen and sugar). Animals breath the O2 and derive energy from the sugar and give off CO2. The oil in the ground was a plant that died and decayed and eventually became oil. If the process for man to do what occurred naturally, but over more time, takes more processing (guessing energy), then unless it’s nuclear (maybe solar, wind, water) then it sounds like it probably produces more carbon byproducts not less. Perhaps sustainable in that we shorten the life cycle of the oil production because nature’s way is very slow, which I’d think speeding that up could be a good thing, but how does it reduce carbon output? Not trying to be political, it’s an honest question from someone who is trying to think through the overall cycle but admittedly not an expert.

      • The problem as I see it is that all the processes that go into it’s production are labor and fuel intensive and do not scale well. If they did then we would already be using them.

      • First off, all fuel requires processing, regardless of the source. How much energy is involved depends on the process.

        As an initial comparison, oil that’s in the ground is likely to stay there, unless we pump it out and burn it. So that adds carbon to the atmosphere that wouldn’t otherwise be there. SAF, on the other hand, is produced from plants that have captured carbon from the atmosphere. So as a first approximation, that’s a net win as long as it takes less total energy to produce the fuel than the resulting content.

        However… What’s the real net gain? If you hadn’t grown the plants that you produced the SAF from, what would have happened on the land? If the local conditions allow anything to grow (like decent soil and adequate rain), *something* will grow on that land. Does it capture more or less carbon the atmosphere than your SAF crop? And what happens when it does? Does it rot (or get consumed in a wildfire) and release the carbon to the atmosphere, or does it get buried and trapped forever?

        Working out the real life cycle result is an accountant’s nightmare (and poses huge opportunities to anyone who wants to fudge the data).

        • Processing also means GROWING all the plants as well as then harvesting, collecting, fermenting, etc. You cannot just ignore all that as well as the huge ecological impact of plowing vast areas of land.

        • That helps Andy… It seems that part of it anyway is carbon going to plants now vs in the past. I was trying to keep what already seemed long from getting longer, but yes all the rest about land use and so on I had in my mind anyway. There’s also the question I rarely hear talked about on this topic regarding the CO2 that is stored in polar ice, the fact that we are still in a geologic ice age and heading out of it (as best as the geologists can tell) so how does that compare to the rest. Also, I’ve seen some data on water in the atmosphere being a much bigger impact than CO2 so how does that all fit in. Sadly not much of the big picture it seems being presented. Not saying someone who has this as a career won’t know more, but seems like a lot of over simplification in the press and by politicians for what is certainly a very complicated topic if addressed with an objective mindset. Thanks for that feedback.

          • BINGO, Steve. Burt Rutan has an excellent one hour video on YouTube on exactly this subject. I urge people here to google it and watch it.
            Water vapor is a much larger impact than most people realize. And … if somehow magically all CO2 were eliminated … what the hell would the trees ‘eat’ to produce oxygen. What bunk !

          • Steve, what is unbelievable are people who ignore the truly vast and complex system and only concentrate on a fraction of a trace gas. I think it’s been said that it’s like diagnosing a car but only looking at the right rear lug nuts.

    • Wow, I hadn’t realized that everyone here on AvWeb worked on this SAF project and had access to all of this data. Too bad your program managers didn’t listen to you when you told them that working on SAF was not worth it, because is sounds like you all seem to know better than all the scientists and engineers that were involved. I was not aware that they did not consider all the factors that you are bringing up. I suppose if United wanted to truly make a difference they should have hired everyone here who has an opinion instead of industry experts and engineers. I’m sure their stockholders would have been happier.

  3. Since the aircraft was labeled as experimental (sign prominently displayed by the entry door), I hope there were no paying passengers on the fight. I don’t think the FAA allows experimental aircraft to carry passengers for hire. My concern with SAF is whether they can scale up production to a level that makes any real difference in carbon emissions.

  4. I simply do not believe the total carbon balance (all inputs) produces much if any net reduction in carbon emissions. All the petroleum used in making the fertilizer, equipment, transportation, etc. must be counted in the total and these components are not carbon trivial.

    • Precisely, Dale. Not far east of the Oshkosh airport is a HUGE ethanol processing plant. It was once showcased by Greta VanSusteren (who was from Appleton). From corn plus huge amounts of energy, they make ethanol that goes into our gasolene. Here’s the problem … without subsidies, they couldn’t afford to do that and NOW … with energy costs rising … I wonder how long that’ll go on? Beyond that, when I started spending summers up there 20 years ago, there were large swaths of land that was not being used. Now, every piece of rural land all over Wisconsin is growing corn. The people raising beef are now competing for that product … which drives up the cost of beef. There are no free lunches. This SAF thing is nothing more than feel good stuff.

      If United made a hybrid electric airplane which used bicycle pedal type cranks under every seat to recharge the batteries in flight … THEN they’d get my attention. 🙂

      • I agree, this “carbon trading “ nonsense is nothing more than a back door tax scheme. Note that there is no government entity federal or other proposing to put all funds generated by these schemes toward further reduction of “carbon” emissions.

  5. “The airline says the planet breathed a tiny bit easier.”

    Poor grammar aside, that’s a great example of virtue trumping chemistry.
    And emotion displacing intellect.

  6. I wonder if the exhaust will smell like French fries like an 80’s VW diesel running on home brew procured from McDonalds waste cooking oil.