Diverging flight paths in sustainable aviation law

The United Kingdom and the European Union have now finalised separate legal frameworks to promote Sustainable Aviation Fuel (SAF) adoption, the UK’s Renewable Transport Fuel Obligations (Sustainable Aviation Fuel) Order 2024 (the UK SAF Mandate) and the EU’s ReFuelEU Aviation Regulation (ReFuelEU). Both came into force on 1 January 2025, and while they share the overarching goal of reducing aviation emissions, their design, scope and compliance mechanisms differ significantly. These divergences create a complex operating environment for fuel suppliers, airlines, airport operators and investors active across both jurisdictions.

Jurisdictional Scope and Applicability

The first key distinction lies in who is caught by each regime. The UK SAF Mandate applies primarily to aviation fuel suppliers operating in the UK who own or supply at least 15.9 terajoules of jet fuel annually. Airlines and airports are not directly bound by obligations under this framework.

ReFuelEU, by contrast, casts a wider regulatory net. It applies not only to fuel suppliers but also to aircraft operators conducting more than 500 commercial passenger flights departing from EU or EEA airports each year, as well as to airports handling over 800,000 passengers or 100,000 tonnes of freight. The EU regulation thus creates interlocking obligations across the aviation ecosystem, fuel suppliers must meet blending mandates, while airlines and airports are required to ensure uptake and logistical readiness.

The geographical reach also diverges. The UK regime is limited to fuel supplied for use within the UK, while ReFuelEU governs flights departing from any EU or EEA airport, including those operated by non-EU carriers. For multinational operators, this distinction can mean the difference between compliance under one system and parallel obligations under two.

Defining SAF

Both regimes are premised on the central role of SAF as a “drop-in” fuel, capable of being blended with conventional jet kerosene and used in existing aircraft engines without modification. Unlike emerging alternatives such as hydrogen or battery-electric propulsion, SAF can integrate seamlessly into current infrastructure, making it the linchpin of near-term decarbonisation strategies.

However, the UK and EU differ in how they define and categorise SAF.

Under the UK SAF Mandate, SAF is grouped into three categories according to production pathway. The first and most established class, HEFA (Hydroprocessed Esters and Fatty Acids), covers fuels derived from waste oils and fats such as used cooking oil, vegetable oil and animal fats. These currently dominate global SAF production. The second category encompasses non-HEFA fuels produced from waste streams including biomass and municipal solid waste. The third, Power-to-Liquid (PtL) or synthetic SAF, also known as eSAF, covers fuels made from renewable electricity and captured carbon, which are considered the most sustainable long-term option due to their low lifecycle emissions and independence from biological feedstocks.

ReFuelEU adopts a broader but more granular approach. It recognises traditional biofuels comparable to HEFA but provides greater incentives for “advanced biofuels” made from residues, algae and biowaste. It also includes “recycled carbon aviation fuels,” derived from non-renewable waste such as industrial off-gases, provided they deliver at least 70 per cent lifecycle greenhouse gas (GHG) savings. The EU goes further still by introducing distinct categories for low-carbon synthetic fuels and renewable hydrogen-based eSAF, both of which are subject to a dedicated sub-mandate.

Crucially, ReFuelEU prohibits crop-based biofuels except for a narrow subset of advanced feedstocks, reinforcing the EU’s stance against fuels associated with land-use change or deforestation. The UK has not imposed a comparable restriction, allowing greater flexibility in feedstock sources.

The UK SAF Mandate

The UK’s regime forms part of a wider legislative package designed to accelerate the domestic SAF market. It imposes two main obligations on aviation fuel suppliers: a general SAF blending requirement and a separate PtL sub-mandate.

From 2025, suppliers must ensure that SAF constitutes 2 per cent of the total fossil jet fuel supplied in the UK. This obligation rises to 10 per cent by 2030 and 22 per cent by 2040. The PtL sub-mandate takes effect later, in 2028, starting at 0.2 per cent and rising to 0.5 per cent in 2030 and 3.5 per cent in 2040. These targets, though modest compared to the EU’s, are underpinned by a structured compliance and certification system.

Suppliers must register with the Department for Transport (DfT) within 28 days of supplying fuel for aviation use. Compliance can be demonstrated either through the acquisition of SAF Certificates, each representing 34 megajoules of eligible fuel, or by paying a statutory “buy-out” price. Certificates are issued in three types: HEFA SAF Certificates, Main SAF Certificates for non-HEFA biofuels, and PtL Certificates for synthetic fuels. Suppliers can carry forward up to 25 per cent of certificates to the following year or transfer them to another party, though each certificate retains its label and associated cap.

The buy-out mechanism acts as a safety valve for suppliers unable to secure sufficient SAF. The buy-out price is set at £0.137 per megajoule for the main obligation and £0.145 for PtL, equating roughly to £5,880 and £6,250 per tonne respectively. Unlike ReFuelEU, which imposes penalties, the UK approach allows a monetary alternative to physical compliance, potentially weakening the policy’s coercive effect but providing short-term market stability.

To ensure quality and sustainability, all fuels must be verified by an independent third-party auditor from a DfT-approved list. Although there is currently no cap on HEFA content, limits will gradually tighten, from 92 per cent in 2027 to 74.7 per cent in 2030 and 42 per cent in 2040.

The Government also intends to introduce a revenue certainty mechanism through the forthcoming SAF Bill, which received its first reading in the House of Lords in October 2025. The Bill aims to underwrite investment in domestic production facilities, addressing industry concerns that capital may otherwise flow to cheaper biofuel projects at the expense of eSAF development.

The ReFuelEU Regulation

ReFuelEU forms part of the EU’s “Fit for 55” legislative package, which seeks to cut net GHG emissions by 55 per cent by 2030. It positions SAF as the most effective near-term tool for aviation decarbonisation, establishing binding obligations on fuel suppliers, airlines and airports alike.

Fuel suppliers must ensure that a minimum of 2 per cent of all aviation fuel supplied in 2025 qualifies as SAF. This figure escalates to 6 per cent in 2030, 20 per cent in 2035, and 70 per cent by 2050, making it one of the most ambitious regulatory trajectories globally. Within this total, a synthetic fuels sub-mandate requires at least 1.2 per cent eSAF content by 2030, increasing to 35 per cent by mid-century.

Unlike the UK’s buy-out mechanism, ReFuelEU enforces compliance through financial penalties. Member States are responsible for setting specific penalties but must ensure they are “effective, proportionate and dissuasive.” The Regulation stipulates a minimum 3

2penalty equal to at least twice the price difference between SAF and conventional jet fuel, with suppliers also required to make up any shortfall in subsequent reporting periods. The Commission’s indicative calculations suggest penalties of around €2,700 per tonne for standard SAF and nearly €14,000 per tonne for eSAF.

Aircraft operators also bear obligations. To prevent “tankering”, the practice of carrying excess fuel from airports in jurisdictions without SAF mandates, ReFuelEU requires airlines to refuel at least 90 per cent of their flight fuel requirements at the departure airport. Temporary exemptions may be granted by national authorities for specific routes, but otherwise the rule applies universally to flights departing from EU or EEA airports. Operators must also submit annual compliance reports by 31 March.

Airports and their managing bodies are likewise drawn into the regulatory framework. They must ensure that sufficient refuelling infrastructure is available to facilitate SAF delivery and use. Airports meeting the passenger or freight thresholds are automatically within scope, while smaller airports may opt in voluntarily.

A defining feature of ReFuelEU is its strict sustainability filter. In addition to the crop-based biofuel ban, only SAF achieving a minimum 70 per cent lifecycle GHG reduction qualifies towards compliance. This threshold will tighten over time, reinforcing the EU’s commitment to high-integrity emissions accounting.

Comparative Analysis

The UK model prioritises flexibility and market liquidity. The use of tradable certificates and a buy-out price reflects a pragmatic approach designed to encourage early participation without imposing excessive compliance risk. It also aligns with the UK’s broader decarbonisation framework under the Renewable Transport Fuel Obligation (RTFO), which favours tradable credit mechanisms.

The EU framework, conversely, leans toward compulsion. By embedding penalties and binding refuelling rules, it seeks to guarantee physical SAF uptake rather than financial equivalence. Its prohibitions on crop-based fuels and its robust GHG thresholds set a higher sustainability bar, albeit at the cost of market simplicity.

For multinational suppliers and carriers, these differences present practical challenges. Companies operating across both markets must track separate certification standards, reporting timelines and verification protocols. A supplier compliant in the UK through certificate buy-outs could still face heavy penalties in the EU for failing to meet physical blending quotas. Similarly, an airline’s fuelling strategy that complies with UK law could breach EU tankering restrictions.

The Investment Challenge

Despite regulatory momentum, both regimes face a common obstacle: supply. Industry analysts estimate that meeting a global 5 per cent SAF share by 2030 would require a fifteen-fold increase in production from 2024 levels. Current projects fall far short of that trajectory.

The cost disparity between conventional kerosene and SAF remains prohibitive. HEFA-based SAF costs between two and seven times more than fossil jet fuel, while PtL and eSAF can be up to eight times more expensive. Airlines are reluctant to enter long-term offtake contracts at such prices, fearing that costs will fall before contracts expire, rendering their commitments uncompetitive.

Technology readiness also lags behind policy ambition. No large-scale eSAF facilities have yet reached Final Investment Decision (FID) in Europe, and in 2025 Shell cancelled plans for a major Rotterdam plant. High capital expenditure, uncertain feedstock supply and limited policy certainty continue to deter investors.

The UK’s proposed revenue certainty mechanism, alongside initiatives such as the Advanced Fuels Fund and Project Skypower, may provide a partial solution by de-risking early investment. The EU is likewise exploring similar support measures under its Innovation Fund. However, unless large-scale production capacity materialises, both mandates risk becoming compliance exercises built on paper rather than physical decarbonisation.

Outlook for 2025 and Beyond

The year 2025 marks the formal launch of both regimes, but also the beginning of a reality check. Widespread political and industry support for SAF has not yet translated into sufficient investment or supply. The early compliance periods are likely to be characterised by limited SAF availability, heavy reliance on HEFA feedstocks, and, in the UK, extensive use of buy-out payments.

In the coming years, the interplay between the SAF mandates, emissions trading systems, and carbon offsetting schemes will determine whether the aviation sector can meaningfully decarbonise without eroding competitiveness. For legal practitioners advising clients in aviation, energy or finance, understanding these frameworks will be critical. SAF obligations now intersect with environmental reporting, cross-border trade, competition law and infrastructure investment, demanding a multidisciplinary approach.