Picture a tanker at Heathrow filling a plane. Some of what goes in is made from used cooking oil and animal fat, not from oil drilled out of the ground. It is refined until it is so close to jet fuel that the engine cannot tell. People call it sustainable aviation fuel, SAF for short.
Plant takes carbon from the air→Fuel is made from the plant→Plane burns it→Carbon goes back to the air
Here is the odd part. SAF burns to the same carbon dioxide out of the exhaust. The difference is where the carbon came from. Oil was underground for millions of years; burning it adds carbon the air did not have. A plant pulled its carbon out of the air last year; burning it puts that back.
CO2 outOrdinary fuel
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CO2 outSAF
A plane runs on SAF. What comes out of the engine?
Nothing. SAF is clean.
Not yet. SAF is still a fuel, and fuel burns to carbon dioxide. The saving is not at the exhaust. It is in where the carbon came from.
Carbon dioxide, about as much as from ordinary fuel.
Right. The engine burns carbon either way. What is different is that the SAF's carbon was in the air recently, so the count over the fuel's whole life is lower.
Water vapour only.
Not yet. Burning any carbon fuel makes carbon dioxide and water both. SAF makes both too; the exhaust is not where it differs.
Jet fuel100 carbon, jet fuel = 100
SAF, whole life30 carbon, jet fuel = 100
So the saving is counted over the fuel's whole life, from field to exhaust. People call that the lifecycle. Heathrow's own figure is that SAF can cut lifecycle carbon by 70 in 100 or more; this lesson uses 70. The UK rule only accepts fuel that cuts at least 40, and refuses fuel made from food crops.
Move the control through the share of SAF in Heathrow's fuel. Each SAF tonne saves 70 in 100 of its carbon over its life.
SAF share2 in 100
Flight carbon cut1.4 in 100
SAF share: 2 in 100
2 in 100 is the UK's rule for 2025. It cuts flight carbon by 1.4 in 100.
SAF share3.1 in 100
Flight carbon cut2.2 in 100
SAF share: 3.1 in 100
3.1 in 100 is what Heathrow's airlines actually used in 2025, ahead of the rule. 2.2 in 100 saved.
SAF share5.6 in 100
Flight carbon cut3.9 in 100
SAF share: 5.6 in 100
5.6 is Heathrow's target for 2026: 3.9 saved.
SAF share10 in 100
Flight carbon cut7 in 100
SAF share: 10 in 100
10 in 100 is the UK rule for 2030: 7 saved.
SAF share11 in 100
Flight carbon cut7.7 in 100
SAF share: 11 in 100
11 is Heathrow's own 2030 goal: 7.7 saved, which is the up to 7 in its plan.
SAF share22 in 100
Flight carbon cut15.4 in 100
SAF share: 22 in 100
22 in 100 is the UK rule for 2040: 15.4 saved. Still not half.
To cut flight carbon by 7 in 100 with SAF, what share of the fuel must be SAF?
70 in 100.
Not yet. Seventy in 100 SAF would cut nearly half of all flight carbon. The 70 is the saving per tonne, not the share needed.
About 10 in 100. Each tonne saves only 70 in 100, so you need more SAF than the cut you want.
Right. Ten in 100 of the fuel, times 70 in 100 saved per tonne, is 7 in 100 of all flight carbon. The share needed is always bigger than the cut.
7 in 100. One for one.
Not yet. One for one would need SAF to save 100 in 100 of its carbon. It saves about 70, so 7 in 100 SAF cuts 4.9, not 7.
Move the control.
19.4 million tonnesThe footprint
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621,428 tonnesSAF saving, kept beside it
Heathrow does not yet count the SAF saving inside its footprint. It reports it separately: 621,428 tonnes saved in 2025. The reason is honest. The world's counting rules have not settled who gets to claim a SAF tonne, the airport, the airline or the fuel seller. Until they do, the number sits beside the count, not in it.
Each is a claim about SAF. Is it true at the exhaust, or true over the fuel's whole life?
One card at a time. Tap the pile it belongs to.
Card 1 of 6
Makes the same carbon dioxide as jet fuel
Cuts carbon by about 70 in 100
Needs no change to the plane
Depends on what it was made from
Makes less soot
Only counts if the field and the factory are counted too
Makes the same carbon dioxide as jet fuel → At the exhaust
Not yet. At the exhaust. The engine burns carbon to carbon dioxide whatever the fuel's past.
Cuts carbon by about 70 in 100 → Over the whole life
Not yet. Over the whole life. The saving is that the carbon was taken from the air recently, not drilled; nothing at the engine changes.
Needs no change to the plane → At the exhaust
Not yet. At the exhaust, and in the tank. It is made to match jet fuel so closely that the engine cannot tell.
Depends on what it was made from → Over the whole life
Not yet. Over the whole life. Fuel from waste oil saves a lot; fuel from food crops can save little or nothing, which is why the UK rule refuses it.
Makes less soot → At the exhaust
Not yet. At the exhaust. SAF contains less of the stuff that makes soot and sulphur, so the engine's exhaust is cleaner locally, a separate gain from carbon.
Only counts if the field and the factory are counted too → Over the whole life
Not yet. Over the whole life. A lifecycle count follows the fuel from field to exhaust; leave out the field and there is nothing to save.
3At the exhaust
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3Whole life
Right. Three facts about the exhaust, three about the fuel's life. The carbon saving lives entirely in the second pile, which is why it is counted differently and claimed carefully.
Used oil and fat→Runs out→Fuel from waste or electricity→Factories not yet built
The hard limit is supply. SAF today is mostly used cooking oil and animal fat, and the world has only so much of either. The UK rule caps how much of the 2030 target can come from them, so the rest must come from fuels made from waste or electricity, in factories that mostly do not exist yet.
100 in 100 SAF
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Net zero?
An airline advertises a flight as net zero because it bought SAF for 100 in 100 of the fuel. A passenger looks at the exhaust and sees smoke. Who is right?
Both, about different things. The exhaust makes carbon; the lifecycle count is about 70 in 100 lower, not zero.
Right. Full SAF still burns to carbon dioxide at the engine. Over its life it saves about 70 in 100. Zero is not what the count says, even at 100 in 100 SAF.
The airline. Full SAF means no carbon.
Not yet. The saving per tonne is about 70 in 100, not 100. A flight on full SAF is a big cut, not a zero, and the exhaust shows why.
The passenger. SAF makes no difference.
Not yet. The exhaust looks the same, and the lifecycle count is not. The difference is where the carbon came from, which the passenger cannot see from the window.
Lesson complete
SAF burns the same in the engine. Its carbon was taken from the air recently, so the count is lower.