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In early July 2025, the federal spending law known as the One Big Beautiful Bill Act significantly reduced funding for efforts to create renewable or sustainable fuels that can power aircraft over long distances while reducing aviation’s environmental impact.
Aviation contributed about 2.5% of global carbon emissions in 2023. Reducing these emissions is challenging because there are few alternatives for large, portable fuel with enough energy density. For example, electric batteries powerful enough to fly an international flight would be much larger and heavier than current airplane fuel tanks.
One potential solution, which I work on as an aerospace engineer, involves a category of fuels called “sustainable aviation fuel” (SAF). Unlike conventional jet fuel derived from petroleum, SAF is produced from renewable and waste resources such as used cooking oil, agricultural leftovers, algae, sewage, and trash. These fuels are similar enough to standard jet fuels that they can work in existing aircraft without major modifications.
Prior to Donald Trump’s second term as president, the U.S. government aimed to produce 3 billion gallons of this type of fuel by 2030 and enough to power every U.S. commercial jet flight by 2050. However, achieving these goals will require significant effort.
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A military jet flying with a refueling hose connected to it is an illustration of how such fuels can be used in practice.
### A Range of Source Materials
The earliest efforts to create sustainable aviation fuels relied on food crops like corn or soybean oil. While readily available, growing these materials competed with food production.
Next-generation biofuels use non-food sources such as algae or agricultural waste, which don’t compete with food supplies. If processed efficiently, they also have the potential to emit less carbon because algae absorb CO2 during growth and using agricultural waste avoids its decomposition into greenhouse gases.
These biofuels are harder to produce and more expensive due to new technologies and lack of logistics for collecting, transporting, and processing large quantities of source material.
Some researchers are developing biofuels with genetically modified bacteria that convert specific raw materials into usable fuels. In one method, algae produce sugars or oils which are fed to engineered bacteria to create ethanol, butanol, or alkanes. Another effort involves photosynthetic microbes like cyanobacteria converting sunlight and CO2 directly into fuel.
All these approaches aim to create sustainable, carbon-neutral alternatives to fossil fuels. However, most of this technology remains in labs, not yet available at airports.
### Blends Are Being Tested
The U.S. Federal Aviation Administration allows up to 50% sustainable aviation fuel mixed with conventional jet fuel. The exact percentage depends on how the fuel was made and its compatibility with existing aircraft systems.
Two major hurdles are cost and supply. Sustainable fuels are more expensive, varying by process and raw material. For instance, in 2024, Jet-A wholesale averaged US$2.34 per gallon, while one type of sustainable fuel wholesaled at about $5.20 per gallon.
The federal budget enacted in July 2025 reduced government subsidies, effectively raising the cost of producing these fuels. In 2025, global production is expected to be less than 1% of worldwide aviation fuel demand.
### Planes Can Use These Fuels
Companies like General Electric and Rolls-Royce have shown that their jet engines can run on sustainable fuels. However, these fuels can have slightly different density and energy content from standard jet fuel, affecting aircraft weight distribution and flight range.
Other parts of the aircraft also need to be compatible, such as those storing, pumping, and maintaining fuel balance. This includes valves, pipes, and rubber seals. As a visiting professor at Boeing in 2024, I learned that manufacturers are working closely with suppliers to ensure sustainable fuels can be safely integrated into every part of the aircraft.
### Good Promise, With Work Ahead
Sustainable aviation fuels offer a promising way to reduce air travel’s carbon footprint without major redesigns. They can significantly cut CO2 emissions from current planes, helping mitigate climate change.
The work requires research and investment from governments, manufacturers, and airlines worldwide. While the U.S. may not be directly involved, sustainable fuels will become more common in the future, potentially powering greener flights.
