On a United flight today from Chicago to Washington, D.C., for the first time ever with passengers aboard, one engine will be filled with 100% sustainable aviation fuel instead of fossil fuels. It isn’t the first time the airline has used greener fuel—each day some flights from Los Angeles International Airport use sustainable aviation fuel mixed with standard jet fuel. But current regulations limit the potential blend to no more than 50%. The airline now wants to show that it’s possible to rely entirely on lower-emissions fuel.
“This is really a major milestone demonstrating that the future should be flying sustainably,” says Lauren Riley, managing director of global environmental affairs and sustainability at United Airlines. “Today, at least at United Airlines, 98% of the greenhouse gas emissions across our entire operations are from consuming jet fuel as we fly our planes. And transitioning to using alternative fuels, like sustainable aviation fuel, is really, really important because it pulls down the carbon dioxide emissions from flying.”
The fuel on today’s flight, from a company called World Energy, is made from fats, cooking oils, and grease, and using it results in 80% fewer emissions than regular jet fuel. It’s blended with synthesized compounds called aromatics—normally made from fossil fuels, but in this case derived from plant sugars. When the first sustainable aviation fuels were produced, they needed to be blended with petroleum-based fuels to have the right chemical composition to meet the industry standard. But that’s no longer the case because of the new plant-based aromatics.
“Producers have made a lot of progress in the quality and the technical alignment between alternative fuels and conventional fuels, that we don’t need that blending requirement anymore for the fuel to fly,” Riley says. “It’s exactly the same between the two types of jet fuels.”
In a test flight without passengers in October, United demonstrated that the fuel performed exactly like typical fuel. ASTM, the organization that sets standards for various industries, currently requires that airlines using sustainable aviation fuel use the 50% blend; the Federal Aviation Administration requires that airlines meet the ASTM fuel standard. (Special permission was given for the demonstration flights, and required that one engine be filled with regular jet fuel, even though it wouldn’t be used.) United argues that it’s time for the standard to be updated so that airlines have the option of flying solely on alternative fuel.
Still, even if 100% sustainable-aviation-fuel flights are approved now, airlines will still face another challenge—there isn’t enough supply. The amount of the sustainable fuel that United has available is “far less than 0.1% of our fuel supply, and we’re the market leader,” Riley says. “So there’s just not enough right now, and that’s unacceptable.” The airline, among others, is pushing for new federal policy that would give a tax credit for alternative fuels, with the biggest incentive for producers that help reduce emissions most. Riley says this would give a long-term signal to producers to ramp up production.
The next generation of sustainable aviation fuels are likely to be synthetic fuels rather than biofuels—such as jet fuel made from CO2 sucked from the air. Some flights will also likely run on electricity or hydrogen relatively soon, but for the foreseeable future, that will only happen on shorter routes.
“The reality is, at least from where the technology is right now, hydrogen and electric will be most useful in our shorter-distance flying,” Riley says. “So commuter regional distances, not long haul. As we look at long haul, we’re going to need an alternative fuel that’s liquid for the energy density so that we can travel those distances.” Today’s flight shows that it’s possible.