An pinion piece on why SAF is capped at a 50% blend, why India’s 2019 CORSIA objection was right, and why it no longer matters — the mandatory phase begins in eight months.
If you want to understand Sustainable Aviation Fuel in one sentence, borrow a picture from the Indian kitchen.
Crude oil is milk. Refine it and you get Jet A-1, the kerosene that has flown the world for seventy years — the butter of aviation. Now take that butter and clarify it further: strip the water, the milk solids, all the stubborn fractions that scorch and smoke. What you get is ghee. Purer. Cleaner. Burns hotter, stores longer, and — to anyone eating the paratha — utterly indistinguishable from the butter it came from.
SAF is the ghee. And the aircraft burning it cannot tell the difference.
This is the single most important fact about SAF, and it is the one most consistently missed in the policy conversation. A Boeing 777 at FL370 does not know whether the fuel crossing its high-pressure pump came from a Texan oilfield or a Pune sugar refinery. It does not need to know. The certification standard — ASTM D7566 — was written precisely so that once a blended batch has been tested, it is redesignated under ASTM D1655 as ordinary Jet A-1. Same freeze point. Same flash point. Same energy density. Same lubricity. The fuel that leaves the refinery gate is, by specification, the fuel the aircraft was always certified to burn.
For the passenger in 14A, nothing changes. For the atmosphere, everything does.
Why only half the ladle?
And yet — and this is where every SAF conversation eventually runs into the same wall — the current global standard permits a maximum blend of only fifty per cent. Pour in one drop more and the fuel is no longer certified. For algae-derived (HC-HEFA) and synthesised iso-paraffin (SIP) pathways, the ceiling is even lower: ten per cent. Why? If SAF is chemically equivalent kerosene, why the arbitrary fraction?
The answer sits not in the engine, but in the seals, hoses and fuel-system elastomers of the thirty thousand aircraft already in service.
Conventional Jet A-1 contains between eight and twenty-five per cent aromatics — complex ring-shaped hydrocarbons, the benzene family. Aromatics are the moody cousins of the paraffin molecules that actually release the energy. They contribute almost nothing to combustion efficiency. They are, in the cold accounting of propulsion, an inconvenience.
But they do one job that nothing else has yet replicated. They swell rubber.
Every aircraft fuel system carries a spiderweb of nitrile O-rings, elastomeric seals and flexible couplings. These seals are designed — at the certification level of the Boeing 737 Classic and the Airbus A320ceo, of the CFM56 and the V2500, of every fuel bowser on every apron from Bahrain to Bengaluru — to sit in contact with a fuel that contains aromatics. The aromatic molecules slip into the elastomer matrix and keep the rubber plump and pliant. Without them, the seals shrink. A shrunk seal leaks. A leak on the ground is a fire risk; a leak in flight is worse.
Most SAF pathways — HEFA, Fischer-Tropsch SPK, alcohol-to-jet — produce a fuel that is almost entirely paraffinic. Zero aromatics, or close to it. Chemically elegant. Operationally, for a legacy fleet, a problem. Industry testing indicates that aromatics content must not fall below roughly eight per cent for seals to remain dimensionally stable.
Run the arithmetic. Blend a paraffinic SAF with zero aromatics against a conventional Jet A-1 averaging around seventeen per cent aromatics, at a fifty-fifty ratio, and you arrive at 8.5 per cent — just above the safety line. Push to sixty per cent SAF and you drop below the threshold. That is the whole story. The fifty per cent cap is not a political concession or an environmental half-measure. It is a rubber problem.
The 50% cap is arithmetic, not policy: 17% aromatics in conventional Jet A-1, diluted 50:50 with zero-aromatic SAF, lands at 8.5% — just above the seal-integrity floor.
There are other considerations — lubricity, cold-flow behaviour at the minus-47°C freeze specification, thermal stability in a hot wing tank on a Doha afternoon — but aromatics are the binding constraint. Work is under way on bio-aromatics that would let paraffinic SAF self-supply the compound it needs, and on new elastomer formulations for clean-sheet aircraft that never needed the aromatics in the first place. Both Airbus and Boeing have flown single-engine, hundred-per-cent SAF test flights. The cap will eventually rise. But for the fleet flying tonight, fifty per cent is the honest limit of current engineering.
The money question
Ghee costs more than butter. SAF costs more than Jet A-1 — currently two to five times more, depending on feedstock, pathway and geography. That premium is not a conspiracy, it is a consequence of scale: the global SAF industry produces a rounding error against the three hundred billion litres of jet fuel the world burns each year. Supply is constrained, feedstock logistics are immature, and capital is still being deployed cautiously.
In a competitive commodity market, a twice-priced substitute cannot sell itself. Airlines will not voluntarily pour more expensive fuel into wings already operating on razor-thin margins. Which is why the entire climate calculus of aviation rests not on SAF’s chemistry, but on SAF’s economics. And economics, in aviation, follows regulation.
Enter CORSIA.
The scheme India argued against, then had to accept
The Carbon Offsetting and Reduction Scheme for International Aviation was adopted at the ICAO Assembly in 2016. Its architecture is straightforward, if uncomfortable: international aviation emissions are capped at eighty-five per cent of 2019 levels, and any airline growing beyond that line must either (a) burn eligible SAF, or (b) buy carbon offsets to cover the excess. Monitoring begins for everyone. Offsetting begins in phases.
The scheme was always going to roll out in two stages — a voluntary phase from 2021 to 2026 in which states could opt in, and a mandatory phase from 2027 to 2035 in which they could not opt out. India, along with China and Russia, declined the voluntary phase. The argument was made cleanly and, frankly, correctly, at the 40th ICAO Assembly in 2019 and again on the floor of the Lok Sabha that December, when the Minister of State for Civil Aviation, in reply to Unstarred Question No. 4012, stated in terms that CORSIA’s design was “not fair to the airlines of developing states who have potential to grow much more after 2020 compared to the airlines of developed states.” India filed reservations with ICAO in both 2016 and 2019.
There was substance to that grievance. A scheme baselined on 2019 emissions locks in the historical advantage of mature aviation markets and penalises the growth that developing markets still need. An American carrier in 2019 was already flying close to its asymptote; an Indian carrier in 2019 was flying twenty per cent of what it projected to fly by 2035. The offsetting bill, calibrated to growth above the baseline, would therefore fall disproportionately on the newcomers.
And yet: the argument was lost. Not in the corridors of ICAO Montreal, but in the arithmetic of membership. CORSIA’s mandatory phase applies, from 1 January 2027, to every ICAO member state above a 0.5 per cent share of global international Revenue Tonne Kilometres. India crosses that threshold several times over. The reservation made India’s position clear. It did not exempt India from the scheme.
So here we stand, eight months out. On 1 January 2027, every Indian operator flying internationally — IndiGo, Air India, Air India Express, Akasa, SpiceJet — enters the mandatory phase. The monitoring has been in place since 2019. The bill lands in 2028 for the 2027 compliance year.
What the bill actually looks like
The DGCA’s own projection, presented at the ICAO CORSIA Seminar, shows Indian international CO₂ emissions growing at 7.8 per cent annually — from 6.28 million tonnes in 2022 to a projected 21.48 million tonnes by 2035. The CORSIA baseline for India, set at 85 per cent of 2019 emissions, is 7.17 MT. The gap between projected emissions and the baseline — the offset-eligible growth — compounds every year.
To put that in numbers an airline treasurer understands: IATA has projected industry-wide CORSIA compliance costs of $1.7 billion for 2026 alone, with cumulative costs through 2035 approaching ten billion dollars. Offset prices, currently around $22 per tonne of CO₂, are forecast by several market analysts to rise to $30–45 by 2030 as the mandatory phase tightens demand against a constrained supply of eligible credits. For the Indian industry, a back-of-the-envelope calculation on the DGCA’s own growth projections puts the cumulative offset liability through 2035 somewhere north of one and a half billion dollars — and that is the optimistic end.
Every tonne of CO₂ that an airline abates with SAF is a tonne it does not have to offset. The moment SAF’s all-in price crosses below the price of a CORSIA-eligible credit, SAF stops being a sustainability story and becomes a balance-sheet story.
What India should actually be doing
India’s indicative SAF blending targets — one per cent in 2027, two per cent in 2028, five per cent by 2030 — are modest by European standards (the ReFuelEU mandate hits six per cent by 2030 and seventy per cent by 2050) and extremely modest relative to what CORSIA arithmetic will demand. They are a floor, not a ceiling, and even the floor requires domestic SAF production capacity that India does not yet have.
The country has the feedstock. India generates used cooking oil, agricultural residue and municipal solid waste on a scale that could comfortably support a million-tonne SAF industry within a decade. What it lacks is the refinery infrastructure, the certification pathway throughput, and the offtake contracts that would de-risk investment. NABCB’s accreditation framework is in place; two Indian verification bodies are operational; the MoU with ISCC for CORSIA sustainability certification is signed. The scaffolding exists. The building does not.
If there is one argument I would press on Raisina Hill and in the corner offices of Bharat Petroleum, Indian Oil and Reliance, it is this: every litre of SAF India imports from 2027 is a litre of offset cost India is paying to somebody else’s refinery. Every litre India produces domestically is both a climate instrument and an import substitution. The same argument that justified the ethanol blending programme for petrol applies in its entirety to the aviation fuel stream. The numbers are, if anything, larger.
India’s CORSIA position was honourable in 2016 and defensible in 2019. In 2026, with the mandatory phase eight months away, it has to be operationally complete. That means scaled domestic SAF production, clear price-support mechanisms for the blending years of 2027 to 2030, and a sovereign position in the carbon offset market so that Indian airlines are not forced buyers in a sellers’ market dominated by credits from Brazilian forestry and Kenyan cookstove projects.
The alternative is to pay the bill in foreign exchange while our crops are burned in the fields of Punjab.
The closing thought
The seat-back safety card on every aircraft reminds passengers that in the event of a loss of cabin pressure, they should fit their own oxygen mask before helping others. CORSIA is the moment the mask has dropped. The voluntary phase was the announcement; the mandatory phase is the cabin descending through ten thousand feet.
India can breathe through this. The fuel is already certified. The engineering is already understood. The ghee is already on the stove.
What remains to be decided is whether we produce it ourselves, or send our money abroad to buy somebody else’s.
Capt. Amit Singh, FRAeS, is a Boeing 777 and Airbus A320 captain with over 18,000 flight hours. He is the founder of Safety Matters Foundation (safetymatters.co.in), hosts the mindFly Katha podcast, and writes on aviation safety, human factors and regulatory policy. Views expressed are personal.
Sources: ASTM D7566 / D1655 specifications; ICAO CORSIA SARPs (Annex 16 Vol. IV); DGCA India CORSIA Seminar presentation; Lok Sabha Unstarred Question No. 4012 (12 December 2019); IATA CORSIA compliance cost projections; Fastmarkets CORSIA market outlook (September 2025); ICAO CORSIA Fact Sheet (December 2025).
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