Sustainable aviation fuel (SAF): what it is and why it matters

Odds are you have flown on it without knowing. Sustainable aviation fuel, usually shortened to SAF, is jet fuel that does the same job as the fuel refined from crude oil, but it is made from very different starting materials. It is one of the few tools the airline industry has to cut its carbon footprint without waiting decades for brand new kinds of aircraft. Here is what it is, in plain terms, and why it keeps coming up.

What SAF actually is

Ordinary jet fuel, called Jet A or Jet A-1, is a kerosene refined from crude oil that was pumped out of the ground. SAF is chemically similar, so it powers the same engines, but it starts from feedstocks that are not fossil crude. The idea is to get the energy an aircraft needs from sources that put far less new carbon into the atmosphere over their full life.

Where it comes from

SAF can be produced from a range of raw materials, and the exact source matters a lot for how clean it ends up being. Common ones include:

• Used cooking oil and waste animal fats left over from food production.
• Farm and forestry leftovers, like corn stalks, husks, and wood residue.
• Municipal solid waste, meaning the everyday household trash that would otherwise go to landfill.
• Purpose-grown energy crops that are not competing with food.
• Carbon captured from the air or from industry, then combined with hydrogen to build fuel from scratch. These are sometimes called e-fuels or power-to-liquid.

A drop-in fuel, used in a blend

Here is the part that makes SAF practical today: it is what engineers call a drop-in fuel. That means it is close enough to regular jet fuel that airlines can use it in existing planes, existing engines, and existing airport fuel pipes with no changes at all. There is nothing to redesign and nothing new to install on the aircraft.

For now, SAF is not used on its own. Safety rules require it to be blended with conventional jet fuel, and the current certified limit is up to about half and half. Most real-world flights use far less than that. Work is underway to certify engines and fuel systems for one hundred percent SAF, but the everyday reality right now is a blend.

Why it cuts emissions

A fair question is: if you still burn it, how is it cleaner? The answer is in the full life cycle, not just the moment of combustion. Burning SAF does release carbon dioxide out the back of the engine. But the plants or waste it came from absorbed carbon while growing, or would have released greenhouse gases anyway if left to rot in a landfill. When you add up the whole journey, from feedstock to flight, SAF can cut lifecycle emissions by a large margin compared to fossil jet fuel, up to around eighty percent in the best cases, depending on the feedstock and how it is produced.

The two big hurdles: supply and cost

If SAF is a drop-in fuel that cuts emissions, why is not every flight running on it? Two stubborn hurdles.

• Supply. The world simply does not make much of it yet. SAF is still well under one percent of all the jet fuel burned globally. There is not enough waste oil and other feedstock in existence today to fuel the entire industry, so production has to scale up enormously.
• Cost. Because it is made in small batches from limited feedstocks, SAF costs several times more than ordinary jet fuel. That price gap is the main thing slowing airlines down, and it is why government incentives and mandates are being used to close it.

SAF is not a magic fix, and no serious person claims it is. But it is real, it works in the planes flying today, and it is one of the clearest near-term ways to make each trip a little lighter on the planet. As production grows and costs fall, the blend in your tank should slowly climb. Curious how long your next trip actually takes, gate to gate? You can check any route in the FlightBeat calculator.

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