Sustainable Aviation Fuel Explained: What It Is and Why It Matters

Sustainable Aviation Fuel, or SAF, has become the aviation industry's primary near-term answer to decarbonisation. Unlike hydrogen or battery-electric propulsion, which remain decades away from powering long-haul flight, SAF works in today's aircraft, today's engines and today's fuel infrastructure. That single fact — compatibility without redesign — is why governments, airlines and fuel producers have converged on it as the credible path to lower aviation emissions between now and 2050.

What Is Sustainable Aviation Fuel?

SAF is a liquid fuel that meets the same technical specification as conventional jet kerosene but is produced from renewable or waste-derived feedstocks rather than crude oil. It can be made from used cooking oil, agricultural residues, municipal waste, forestry residues, or gases such as landfill methane and biogas that are converted into synthetic liquid fuels. Because SAF is chemically almost identical to fossil jet fuel, it delivers the same energy density and performs the same way in flight — the difference lies entirely in where the carbon in the fuel came from.

On a life-cycle basis, SAF typically cuts greenhouse gas emissions by 70–90% compared with fossil jet fuel, depending on the feedstock and production route used. It does not eliminate emissions from combustion — burning SAF still produces CO2 at the point of use — but because that carbon was recently absorbed from the atmosphere (by plants, or captured from waste streams that would otherwise decompose and release methane), the net addition to atmospheric carbon is substantially lower than pumping new fossil carbon out of the ground.

Why Aviation Cannot Simply Electrify

The case for SAF becomes clearer once you consider aviation's physics. Batteries store roughly 40–50 times less energy per kilogram than jet fuel. A battery pack heavy enough to power a long-haul aircraft across the Atlantic would weigh far more than the aircraft could ever lift, let alone leave room for passengers or cargo. Battery-electric aircraft are being developed, but they are realistically confined to short-hop regional routes carrying a handful of passengers — not the wide-body jets that carry the bulk of global passenger-kilometres and, more importantly, the bulk of aviation's emissions.

Hydrogen faces a related problem. Liquid hydrogen requires cryogenic storage at minus 253°C, needs roughly four times the volume of jet fuel for the same energy content, and would require substantial redesign of aircraft, airports and refuelling infrastructure. Airbus and others have hydrogen aircraft concepts on the drawing board, but credible entry into service is not expected before the mid-2030s at the earliest, and only for a subset of the fleet.

That leaves liquid drop-in fuels as the only lever available for the vast majority of flights over the next fifteen to twenty years. SAF is not a stopgap while better technology arrives — for long-haul aviation specifically, it is likely to remain the primary decarbonisation tool for a long time to come.

Drop-In Fuels: The Key to Compatibility

The term "drop-in fuel" is central to understanding why SAF has scaled faster than any alternative aviation energy source. A drop-in fuel can be blended with, or fully substitute for, conventional jet fuel without any modification to the aircraft, the engine, or the airport fuelling infrastructure. Every certified SAF production pathway must meet ASTM International's D7566 specification, which governs the chemical and performance properties a synthetic aviation fuel must have to be mixed safely with standard Jet A-1.

In practice, most SAF today is blended with conventional kerosene at ratios up to 50%, and refuelling trucks, pipelines and aircraft fuel systems handle the blended fuel exactly as they would pure fossil jet fuel. This is what allows an airline to introduce SAF gradually, at any airport with existing infrastructure, without waiting for a new generation of aircraft. It is also why SAF can be introduced into the fuel supply chain upstream — blended at the refinery or terminal — rather than requiring airline-by-airline or aircraft-by-aircraft changes.

How Mandates Are Driving Adoption

SAF remains considerably more expensive to produce than fossil jet fuel, and left purely to market forces, the volumes required to meaningfully cut aviation's emissions would take far longer to materialise. Governments have responded with blending mandates: regulatory requirements that fuel suppliers include a rising minimum percentage of SAF in the aviation fuel they supply, regardless of the cost premium.

The UK's SAF mandate, which came into force in January 2025, requires aviation fuel suppliers to ensure at least 2% of the fuel supplied to UK aircraft is SAF, rising on a defined trajectory to 10% by 2030 and 22% by 2040. The European Union's ReFuelEU Aviation regulation follows a similar path, starting at 2% in 2025 and rising to 70% by 2050. These mandates create guaranteed demand, which in turn gives fuel producers the confidence to commit capital to new production facilities — something that would be far harder to justify against SAF's currently high cost relative to fossil kerosene alone.

Feedstocks and Production Routes at a Glance

Several distinct production routes can turn waste and biomass into SAF, and no single route dominates. Hydroprocessed Esters and Fatty Acids (HEFA) technology, which converts used cooking oil and animal fats into jet fuel, is currently the most commercially mature and accounts for the large majority of SAF produced today. Alcohol-to-Jet routes convert ethanol or other alcohols, often derived from crops or agricultural waste, into jet fuel. Fischer-Tropsch synthesis builds synthetic fuel from a carbon monoxide and hydrogen mixture known as syngas, which can itself be derived from biomass gasification or from waste gases including landfill methane and biogas. Each route has different feedstock constraints — HEFA is limited by the finite global supply of waste oils and fats, while gas-based routes can draw on a wider and more geographically distributed set of waste streams.

The Road Ahead

SAF production remains a small fraction of global jet fuel demand today, and scaling it to the volumes mandates require by 2030 and 2040 will depend on new refineries, sustained policy support, and feedstock supply chains that do not yet exist at the necessary scale. But the direction of travel is now set by regulation in the UK, the EU and a growing list of other jurisdictions. For an industry with few decarbonisation options, SAF is not one choice among many — for the foreseeable future, it is close to the only one available at scale.