In 2021, I flew from Chicago to Washington D.C. on a United Airlines flight billed as the first with an engine running exclusively on sustainable aviation fuel, or SAF for short. The flight was meant to draw attention to the possibility of SAF, in this case made from recycled cooking oil, to cut aviation emissions—and a reminder that the technology works today and can be used on existing planes. [time-brightcove not-tgx=”true”] SAF production has grown substantially since then, and yet airlines still use it for only a small fraction of their fuel.