United Airlines Claims It Flew an Aircraft Using 100% Sustainable Fuel—Did It?

Much depends on how you define sustainable.

United Airlines filling up with SAF on the ground at an airport

United Airlines

United Airlines recently flew what its press release says is "an unprecedented flight that will serve as a turning point in the industry’s effort to combat climate change: For the first time in aviation history, a commercial carrier will fly an aircraft full of passengers using 100% sustainable aviation fuel (SAF)."

The 737 Max 8 carrying 100 passengers flew from Chicago's O'Hare International Airport to Washington's Reagan National Airport, running one engine on 100% SAF and the other on conventional jet fuel to prove there are no operational differences. One could be pedantic and note this means that the flight did not fly on 100% SAF but only 50%, but we will leave that one there. United CEO Scott Kirby said in a statement:

“Today’s SAF flight is not only a significant milestone for efforts to decarbonize our industry, 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.”

The flight is powered by SAF from World Energy, which makes its biofuels from vegetable oils and beef tallow, and from Virent, a subsidiary of oil giant Marathon, whose President Dave Kettner says "Virent’s proprietary technology demonstrates that SAF can be 100% renewable and 100% compatible with our current aviation fleet and infrastructure." On the Virent site, Kettner notes that it is made from corn sugar. Instead of SAF, they call it "synthesized aromatic kerosene (SAK) – a critical component that made the 100% SAF possible."

Graphic of Virent's Bioforming Process


"Most SAF – typically made from used cooking oil or vegetable oil – has to be blended with petroleum products because SAF doesn’t have a component called “aromatics,” which is required to meet today’s jet fuel specifications. Virent’s SAK, made from renewable plant sugars, provides those aromatics."

In a previous post, "Can We Keep Flying on Sustainable Aviation Fuels," I noted most SAF was made from fats, oil, and grease (FOG), "but there is limited waste grease and oil out there, and only so much lard and beef tallow available, and there are competing uses for them, including food products, soap manufacture and being turned right back into pet food and animal feed in the United States. So while FOG is the easiest and most efficient alternative to petroleum-based aviation fuel, there are limits on how much of it is available. I have also wondered how happy vegans would be, knowing that they are flying on animal fat."

This is why there was pressure from the farm industry, what they were calling "farm to fly" to make aviation fuel from corn and soybeans, which is what Virent is apparently doing. I had previously worried that "given that 17 billion gallons of aviation fuel are burned in a normal year in the U.S., and that airplanes are getting more efficient, one could crunch the math and find that you could plant corn and soy fencerow to fencerow from coast to coast and make enough biofuel to keep planes in the air, but at what cost? "

Andy Singer cartoon about corn

Andy Singer

We do not know how much corn sugar is used to make Virent's product, and what proportion of the SAF in the plane was their stuff or the World Energy SAF. We do know that Andy Singer nailed it in his cartoon, and that growing corn for fuel takes a lot of energy and probably emits as much carbon dioxide as regular jet fuel.

Treehugger's Sami Grover interviewed Dan Rutherford, the program director for the International Council on Clean Transportation (ICCT), who told him that SAFs were important, albeit expensive, and would play have a role to play.

I contacted him to get his thoughts about this flight. He tells Treehugger:

"We're very concerned about the potential use of crop-based biofuels. They will be cheaper than the advanced fuels with lower lifecycle emissions, but have lots of existing uses (food, even ethanol) so diverting those to jet fuel is likely to trigger land-use impacts (e.g. tropical deforestation overseas)."

Rutherford pointed me to a New York Times article on President Joe Biden's biofuel targets that expressed the same concerns: "Growing crops for fuel also competes with food production and strains water resources, according to scientists. And making fuels from waste, like discarded cooking oil, presents a far simpler challenge: There just isn’t enough old cooking oil available.”

Or as I put it in my article, "there just aren't enough dead cows and there is not enough land to keep us all up in the air." But that won't stop them from trying. Rutherford tells Treehugger:

"Airlines, for their part, have mostly stayed away from crop-based biofuels but, if you push too hard too fast on targets, there's always the temptation to go corn and soy. By our estimate, Biden's 10% by 2030 target is likely too high to hit with good fuels. Europe's approach, which is for a 5% target by 2030 with strict quality criteria, looks better."

And neither of those numbers are close to the 50% reduction in carbon emissions that we have to hit by 2030 to have a hope of staying below 2.7 degrees Fahrenheit (1.5 degrees Celsius) of global heating.

United BOOM flight in air above the clouds


United is getting a lot of good press but in the end, it is hard to really claim this flight or this fuel is 100% sustainable. Or claim that the company is serious about sustainability: It just ordered 15 supersonic jets, which it promises will run on SAF. But again, I wondered, "Can there possibly be enough lard, beef tallow, and schmaltz to keep a fleet of SSTs up in the air? Or is it just wishful thinking and greenwashing, with them ending up dropping conventional fuel into the plane because there isn't enough SAF?"

In the end, we probably have to follow Rutherford's prescription: more efficient planes and less flying.