The Feedstock Ceiling: Why Sustainable Aviation Fuel Cannot Carry Heathrow's Growth Case

Expansion lobbyists have a new answer to the climate question. A third runway, they argue, need not add to aviation's carbon burden because sustainable aviation fuel (SAF) will do the heavy lifting. The claim is repeated in glossy consultations, in ministerial statements and in the runway's own public relations. It deserves an audit, not a slogan.

A mandate is not a supply chain

The UK's SAF mandate, which took effect in January 2025, is a blending obligation. It requires fuel suppliers to ensure that a rising share of jet fuel is sustainable: 2% at the outset, 10% by 2030 and 22% by 2040, subject to review. That is a genuine policy advance, and it should not be dismissed. But a mandate creates demand for a product; it does not conjure the product into existence. The obligation falls on fuel suppliers, not on airlines, and it applies across the United Kingdom, not to Heathrow alone.

In 2026 the gap between obligation and delivery is the central question. Ministers can point to targets; residents are entitled to ask where the fuel will come from, in what volume, at what price and with what carbon saving.

The feedstock ceiling

Almost all SAF supplied today is HEFA — hydroprocessed esters and fatty acids — made from used cooking oil, tallow and similar waste fats. These are finite, globally traded commodities. The UK imports a large share of its used cooking oil, and competition from road transport and from other countries' blending mandates is intensifying. The EU has tightened its rules on crop-based biofuels and on feedstocks associated with deforestation. Waste-based supply cannot be expanded at will by political announcement.

Power-to-liquid e-kerosene, which could in principle scale without land constraints, remains pre-commercial. It is expected in the 2030s at the earliest, and it is expensive. Alcohol-to-jet and other routes face their own constraints of feedstock, capital and certification. The honest conclusion is that SAF supply will grow, but slowly, and from a small base.

The carbon accounting question

SAF's climate benefit depends on the feedstock, the production pathway and the counterfactual. Regulators use default lifecycle values. Those values can flatter a fuel if they assume that waste would otherwise be burned or landfilled, or if they ignore indirect land-use change. Book-and-claim systems, in which a certificate is traded separately from the physical fuel, raise further questions about double counting across the UK mandate, the EU's ReFuelEU Aviation scheme and CORSIA. A tonne of carbon saved should be counted once, and only once.

Residents do not need to become accountants to spot the problem. If the carbon case depends on assumptions that are not published, it is not yet a case. It is a claim.

The scale test

Here the arithmetic becomes uncomfortable. UK aviation consumes of the order of ten million tonnes of jet fuel a year. A 10% mandate by 2030 therefore implies around one million tonnes of SAF for the whole country. Heathrow's proposed expansion — a third runway and associated terminal capacity — would take the airport towards 140 million passengers and hundreds of thousands of extra flights. Even on conservative assumptions about fuel burn per passenger, the additional demand from a third runway would absorb a large fraction of the UK's entire mandated SAF supply. At a 22% blend in 2040 the numbers grow further.

In plain terms: the fuel that would be needed to make expansion 'green' is the same fuel that every other UK airport will also be claiming. A national mandate cannot be spent twice.

Who pays the premium?

SAF currently costs several times more than conventional jet fuel. The mandate includes a buy-out mechanism, which allows suppliers to pay a fixed price instead of blending. If that price is set too low relative to the true cost of SAF, it becomes a cheap way to comply on paper while delaying investment. The cost of the transition will ultimately land on passengers, on taxpayers through subsidy, or on the climate through delayed abatement. Residents near Heathrow are entitled to ask which.

Questions for campaigners

When the 'green growth' case is made in 2026, these are the questions that should be asked — politely, persistently and in public:

• What proportion of the UK's mandated SAF supply in 2030 and 2040 would be consumed at Heathrow under the expansion scenario, and what is the source of that figure?
• Which feedstocks underpin the projection, and what share is imported?
• What lifecycle values are used, and do they include indirect land-use change?
• How is double counting with EU and CORSIA schemes prevented?
• What is the mandate's buy-out price, and how does it compare with the delivered cost of SAF?
• Has the applicant published its own fuel-burn forecast, or only passenger numbers?
• What happens to the climate case if SAF supply grows more slowly than projected?
• Will the expansion be contingent on binding SAF supply milestones, or merely accompanied by aspirations?

Pause is not opposition to technology

None of this is an argument against SAF. It is an argument against using an immature supply chain as a licence for growth. A pause would allow the government to publish a proper feedstock strategy, to align the mandate with a national aviation emissions cap, and to test whether a third runway is compatible with the fuel that can actually be delivered. That is not anti-technology. It is arithmetic.