TL;DR: Same engine, opposite decision. In the United States I run 100LL by default, and I have written about why. In South Africa, Australia and most of Europe, a 912 or 916 owner would find that choice odd: pump petrol is the fuel, and avgas is the compromise. Neither of us is wrong. The difference is what comes out of the pump and what the airport sells. Put every country on two axes, how much ethanol is in its premium petrol and whether its airports sell anything but leaded avgas, and the world sorts into four boxes. Europe lives in the good one. The United States can legally burn its E10 premium in a Rotax, but almost no airport sells it and 100LL is cheap and everywhere, so hardly anyone does.
This is Part 1 of a two-part series, The Last Leaded Fuel. Part 1 describes how Rotax owners around the world actually fuel their aircraft. Part 2 asks why the United States is still burning leaded fuel when the alternatives have existed for decades.
A Letter From the Other Side of the World
A few weeks ago I received an email from a reader in South Africa who flies a Sling High Wing behind a Rotax 916 iS. He had read the MOGAS posts, done his own homework, and wanted to compare notes. His fuel reality looked like this: 95 RON unleaded at every service station, no ethanol in it, avgas expensive and periodically unavailable, and pump fuel simply assumed to be what you put in a Rotax. (RON is the Research Octane Number, the figure on pumps almost everywhere outside North America. US pumps show AKI, the average of RON and the harsher Motor Octane Number, so 95 RON is roughly the 91 you see on an American premium pump.) He was not asking whether to use mogas; that was settled. He had bought a handheld octane tester, run the water-shake test for ethanol, pulled the refiner’s published specification, and was checking that his local 95 met the numbers in the Rotax manual.
What struck me was not his conclusion but how foreign the question itself was. A Pilot-Owner’s Guide: MOGAS vs 100LL is one of the most-read posts on this blog, and it was written from inside one country’s fuel market. Most Rotax-powered aircraft are not in that country. Rotax is an Austrian engine, built for European fuel, and the great majority of the fleet flies in Europe, Australia, South Africa, South America and Asia, where the petrol at the pump and the avgas at the airport are both different from what I see in Washington State. This post widens the lens.
What Rotax Actually Consumes
Every four-stroke Rotax from the 80-hp 912 UL to the 160-hp 916 iS was designed around European pump petrol. That is not a marketing line; it is where the engine grew up. The 912 entered service in 1989 in the microlight and motorglider categories, where avgas was taxed, scarce or both, and Rotax has specified automotive gasoline to the European EN 228 standard ever since. The current operating-fluids Service Instruction covers the whole family in one document, and the requirements by engine look like this:
| Engine | Minimum octane | E10 | Unleaded avgas (UL91 / UL94) | 100LL |
|---|---|---|---|---|
| 912 UL / A / F (80 hp) | RON 90 / AKI 87 | Approved | Approved | Approved, with penalties |
| 912 ULS / S (100 hp) | RON 95 / AKI 91 | Approved | Approved | Approved, with penalties |
| 914 UL / F (115 hp, turbo) | RON 95 / AKI 91 | Approved | Approved | Approved, with penalties |
| 912 iS / iSc (100 hp, injected) | RON 95 / AKI 91 | Approved | Approved | Approved, with penalties |
| 915 iS / iSc (141 hp, turbo) | RON 95 / MON 85 / AKI 91* | Approved | Approved | Approved, with penalties |
| 916 iS / iSc (160 hp, turbo) | RON 95 / MON 85 / AKI 91* | Approved | Approved | Approved, with penalties |
Three details in that table matter for everything that follows. First, the 80-hp 912 UL runs on 90 RON, which is regular petrol in most of the world; for the largest single Rotax population, the mogas question barely exists outside North America. Second, everything else needs 95 RON, which is ordinary premium in Europe, South Africa and Australia and 91 AKI premium in the US, and that is exactly where the ethanol problem bites. Third, the turbocharged engines (914, 915, 916) have the least knock margin and the hottest engine bays, which makes them the most sensitive to vapor pressure, the thread that runs through the vapor lock and 915 vs 916 posts.
Two more rules apply to all of them. Fuels above E10 have not been tested and are not permitted. Metal-containing fuels and lead-replacement petrol are not allowed. And Rotax is explicit that its E10 approval covers the engine: the airframe’s fuel system is the builder’s problem, which on an experimental aircraft means yours. The same logic applies to the other mogas-designed light engines, Jabiru and ULPower among them, but this post stays with Rotax because that is where the fleet and my experience are.
A Wall and a Screen
Once you look at fuel from the Rotax’s point of view rather than the Lycoming’s, two facts about a country decide almost everything. One of them is a wall. The other is a screen: a barrier you can push through, at a cost.
The wall is how much ethanol is in the premium petrol you can buy. Rotax’s limit is E10, and past it there is no approved pump fuel, full stop. Countries with no ethanol mandate sit at E0. Europe’s premium grades sit at E5 and are drifting. The United States sits exactly on the wall, at E10, with E15 pushing from behind. Brazil and India are over it.
The screen is whether the airport sells anything but 100LL. Rotax approves leaded avgas, but you pay to push through: 50-hour oil changes, a lead scavenger in every tank, lead in the gearbox and in the oil analysis, all of it covered in my earlier post on living with 100LL. You can live on the wrong side of the screen, and most of the world’s Rotax fleet outside Europe does. You cannot live past the wall.
Put the two together and the world sorts into four boxes. Less ethanol is to the right, unleaded avgas is up, and the top-right corner is where you are behind neither the wall nor the screen. Three of the boxes have people in them. One thing position does not show is what 100LL costs, so the chart fills in the one dot where leaded avgas is cheap and on every ramp. That dot is the United States, and the fill explains more than the position does.
What Ethanol Does to an Airplane
Rotax’s E10 approval answers one question: will the engine burn it. Everything below is about the rest of the airplane, and about what happens between fill-ups.
- Water and corrosion. Ethanol pulls moisture out of the air through a vented tank. E10 holds roughly half a percent water in solution; past that, the ethanol and water drop out together as a corrosive, non-combustible layer at the sump, and the petrol left above it has lost the octane the ethanol was contributing. Cold at altitude brings the saturation point down. EASA’s SIoBiA study called this the single greatest threat to light aircraft on ethanol blends. The damage follows the water, so an airplane that flies weekly rarely sees it and one that sits in a humid hangar does.
- Elastomers and plastics. Hoses, O-rings, pump diaphragms, sender floats and gaskets swell, harden or leach depending on the compound. The hoses are the big one, and on Roci they are SAE J30R9 throughout, the fluoroelastomer-lined grade built for ethanol-extended fuel, as I have described before. The smaller seals and O-rings are the builder’s checklist, and the reason Rotax points you to the airframe manufacturer.
- Composite tanks and sealants. Ethanol softens some polyester, vinylester and even epoxy resins and attacks some tank sealants, which is why many composite aircraft forbid it; Aeroprakt’s service bulletin limiting ethanol to 5% in its glass-fibre tanks is typical. The Sling’s wing tanks are riveted aluminum with sealed seams, which takes the resin problem off the table.
- Vapor pressure and vapor lock. Counterintuitively, low ethanol blends raise vapor pressure. Adding 10% ethanol lifts Reid vapor pressure by about one psi, which is why the EPA granted E10 its “one-pound waiver” in 1992. On top of seasonal blend swings and a turbocharged engine in a tight cowl, that margin matters. See Beating Vapor Lock.
- Range and instruments. Ethanol carries about a third less energy than gasoline, so E10 delivers roughly 3% less per gallon according to the EIA; the iS-series FADEC has no lambda sensor, so it cannot see ethanol, and Rotax’s E10 approval means the maps tolerate the mild leaning. Capacitive fuel senders read the fuel’s dielectric constant, about 2 for gasoline and 24 for ethanol, so a probe calibrated on avgas will over-read on blended fuel. The Sling’s senders are float type as delivered, which sidesteps the problem.
- Shelf life. All gasoline ages, but avgas is paraffinic, antioxidant-dosed and specified for storage, one of 100LL’s quiet, legitimate advantages. Ethanol makes pump fuel age faster and worse, by bringing in the water that drives phase separation and by hosting the acid-producing bacteria familiar to anyone who has owned a boat. Stabilizers help the oxidation problem, not the water problem. Rotax’s SI puts no number on fuel age; it simply recommends avgas for long storage.
The field check is older than any of this: pour a measured amount of water into a graduated tube, top it up with the fuel, shake, and let it settle. If the water line rises, the difference is ethanol that migrated out of the petrol. It tells you nothing about octane and nothing about metal additives, but it answers the one question a pump label in a strange country may not.
Top Right: Two Clean Fuels
Usable pump petrol and unleaded avgas: the top-right box, and only Europe is in it. Europe is where the Rotax comes from, and it is the proof that lead-free fuel for light aircraft is not a research problem. Sweden has sold unleaded avgas continuously since 1981 (see the sidebar). Total launched its own UL91 across French, Swiss, German and British airfields in 2011, and the UK’s Light Aircraft Association promoted it as the natural fuel for Rotax engines. In Germany, aero-club airfields have pumped Super Plus for decades; EASA’s 2010 biofuels study estimated that about half of all fuel burned by piston aircraft in Germany was ethanol-free mogas. The European light fleet did not wait for a drop-in 100-octane miracle. It used the two fuels it had.
Sweden Did This in 1981
In 1981 a small Swedish company, Hjelmco Oil, put an unleaded avgas into nationwide distribution; its 91/96UL has been in continuous use since 1991 at some seventy airports, priced below 100LL, and Lycoming listed it as an approved fuel in 1995. It is, in effect, the fuel the United States is only now rolling out as UL94. Why it never crossed the Atlantic is the subject of Part 2.
But Europe’s box is drifting. The E0 corner is shrinking as E10 becomes the default grade and the premium E5 grades quietly take on real ethanol. In Britain, Esso’s Supreme+ 99 was the last widely available ethanol-free petrol until the company moved it to up to 5% ethanol nationwide from September 2023; UK Rotax owners now choose between E5 at the pump and UL91 at the airfield at close to avgas prices, and the LAA does not approve E10 in any of its aircraft, engine approval or not. France’s ethanol-free SP98 stations have largely vanished. Germany still has E5 Super Plus on most forecourts, and Norway’s 98 is still sold as ethanol-free by several brands, but the direction is one way. Europe is still in the top-right box; the UK’s arrow on the chart shows the direction of travel.
Bottom Right: Mogas or Lead
Usable pump petrol, but the only avgas is leaded. This is the biggest box, and what the people in it actually burn depends on the one thing the axes do not show: what 100LL costs. Where it is expensive or unreliable, the petrol station wins. Where it is cheap and on every ramp, the pump option goes unused. Both populations live here, and they arrived by very different roads.
The Southern Hemisphere: E0 by Default
South Africa, Australia and New Zealand share a fuel market that would make a US Rotax owner weep: no effective ethanol mandate, 95 and 98 RON unleaded at every station, and avgas that is expensive at best and absent at worst. Nobody designed that for aviation. It is an accident of farm politics and refinery economics, and the light-aircraft fleet simply adapted to it.
Australia sells four petrol grades, and only one of them, E10, contains ethanol; it is labelled as such and sold alongside ethanol-free 91, 95 and 98, and the Rotax community there runs on 95 or 98 from the servo as a matter of course. New Zealand is the same picture. South Africa phased lead out of road fuel in 2006, has never put ethanol into its pumps, and sells 95 unleaded at every forecourt. Meanwhile its avgas supply is thin enough to break: when BP withdrew from aviation fuel in South Africa in 2023, George Airport ran dry with a day’s notice, and southern Africa has seen regional avgas shortages going back more than a decade. In that world, carrying jerry cans to the petrol station is not a hobbyist’s economy. It is how you keep flying.
Two lessons from the south travel well. The first is the water-shake test above, which is what the reader who prompted this post used, and which costs nothing. The second is the one he had not considered. South Africa’s petrol regulations list two kinds of unleaded as permitted grades: metal-free unleaded, and “metal-containing unleaded petrol” dosed with the manganese octane booster MMT at up to 18 milligrams per litre, each with its own mandatory pump label. Rotax’s operating-fluids instruction prohibits metal-containing fuels. In practice the South African oil industry has resisted the manganese grade and most of what is sold is metal-free, but the regulation is live and the label is the only way to know. A handheld octane tester will not tell you, and neither will the water test. In South Africa, “unleaded 95” is not enough; the label has to say metal-free.
One more footnote for the long view: South Africa has had a mandatory ethanol-blending regulation on the books since 2012, never implemented, and re-gazetted with a pricing framework in 2025. The box a country sits in is not permanent.
The United States: Same Box, Opposite Habit
Let me be direct, because the earlier posts were not: the United States has a mogas option. A Rotax can legally burn the 91 AKI premium at any American station, which puts the country in this box, next to South Africa and Australia. It is just a poor option, for three reasons that stack. The premium is E10, which is not under the Rotax limit but exactly on it, and every seasonal batch is a little different. Almost no airport sells it: of the roughly 3,500 US airports that pump fuel, the number offering mogas has hovered around 110 for more than a decade, about three percent, so the fuel is at the petrol station, not on the ramp. And 100LL is relatively cheap and everywhere, so nothing ever forced the question. That is the gold dot on the chart.
The Renewable Fuel Standard pushed ethanol into essentially the entire road-fuel pool; most finished gasoline sold in the US is E10, and ethanol-free premium survives as a specialty product for boats and lawn equipment, found through sites like Pure-Gas.org rather than at the corner station. American geography adds an engineering problem Europe mostly does not have: pump gasoline here is blended to a seasonal vapor-pressure schedule, winter blends run far more volatile than avgas, and we have hot, high airports in numbers that Germany does not. That is the mechanism behind the vapor-lock incidents I have written about before, where an operator running a blend of premium mogas and 100LL went back to straight avgas and is waiting on UL94.
Which brings me back to my own decision. After a season of flying I chose 100LL as my default, accepted the 50-hour oil changes, brought Decalin back, and started watching the lead in the oil analysis. I still think that was the right call. But it was the right call only inside this box, on this side of it. Move Roci to Johannesburg or Perth and it becomes the wrong call overnight: 95 RON ethanol-free at every pump, avgas at a premium and unreliable, and a 2,000-hour engine designed for exactly the fuel that is cheapest and nearest. The engine did not change. The country did.
And the American side of the box is moving. MOSAIC opens the door to a generation of heavier, faster Rotax-powered aircraft in a country whose airports are not built to fuel them. The exit to the top is open: UL94 today, 100R as it spreads. But the ethanol axis is moving at the same time, in the wrong direction: year-round E15 has been legal in eight Midwestern states since April 2025, and a permanent national E15 bill is pending in Congress. Add the two moves together and the dashed arrow on the chart does not point to the top-right, where Europe is. It points to the top-left, sometime after 2030: unleaded fuel at the airport, nothing approved at the pump.
Bottom Left: Lead or Nothing
Pump petrol past the Rotax limit, and only leaded avgas at the airport. Not every country outside the United States is friendlier to a Rotax. Some have embraced ethanol so completely that pump fuel is no longer an option for one at all.
Brazil has blended ethanol into petrol since the 1970s and raised the mandatory blend from 27% to 30% in 2025. There is no E10 at a Brazilian pump, and Rotax’s limit is E10. India went further and faster: it hit E10 in 2022, E20 in 2025, and from April 2026 E20 with a minimum of 95 RON is the only petrol legally sold, with no lower-ethanol grade and no aviation offramp. These are countries that are, by any biofuels measure, far ahead of the United States, and they are worse places to own a 916 than Kansas.
So what do Rotax owners there actually do? Three things. They burn avgas, which both countries have made a point of producing at home: Brazil refines its own, and Indian Oil began domestic 100LL production in 2022, replacing imports from Europe for a national fleet that uses only about three million litres a year, mostly at flight schools. They go around the problem, as Brazil did when the Embraer Ipanema crop-duster became the first production aircraft certified to run on ethanol in 2005, with a national programme behind it. And, in Brazil’s large experimental and ultralight community, many simply run the 912 on E27 and now E30 pump petrol, outside any manufacturer approval, with the fuel-system consequences that implies. The first two are legitimate answers. The third is the one that keeps Rotax instructors awake.
Why Is There Ethanol in My Petrol?
Four motives, in roughly historical order, and most countries have at least two of them.
Energy security. The original reason. Brazil launched its ethanol programme after the 1973 oil shock to replace imported gasoline with sugarcane; the US Renewable Fuel Standard was sold the same way after 2001; India’s E20 drive is explicitly about a crude import bill that dominates its trade deficit. Ethanol is the one liquid fuel a country can grow.
Farm policy in an energy costume. A blending mandate is a guaranteed, price-supported market for corn, sugarcane or grain, and the constituency it creates then defends it. That is why blend levels ratchet in only one direction: E5, E10, E15, E20, E27, E30.
Octane after lead. When lead left road fuel, refiners needed another octane source, and ethanol at roughly 108 RON is cheap and effective. It slid into fuel specifications even in places with no farm lobby. South Africa chose the other route to the same octane, manganese, which is why its petrol is ethanol-free but not always metal-free.
Climate and air quality. The modern justification: lower lifecycle carbon, cleaner combustion. The science is contested, land use and fertilizer in particular, and the EU has partly walked back crop-based biofuels. But it is the argument that keeps the mandates politically alive where the first two have faded.
Top Left: Unleaded Avgas, No Mogas
The fourth box, unleaded avgas at the airport but nothing approved at the pump, is empty today. Nobody has yet combined a national blend past E10 with an unleaded aviation grade on the ramp. Two kinds of country could fill it. Brazil or India would, the day an unleaded avgas reaches their flight schools, and for them the step is an unambiguous improvement: the lead goes, and the mogas was already gone. The United States would land here by a different road, losing E10 and gaining 100R in the same decade, and for a Rotax that is a trade, not a win. The engine designed for pump petrol would end up with exactly one approved fuel, sold only at airports, like a Lycoming. A clean Lycoming, but a Lycoming.
What This Means If You Fly a Rotax
Two clean fuels (Europe): know your country’s E0 and UL91 map, the EuroGA forums and the national association sites maintain them, and watch the E5 drift. The question for the next decade is whether UL91 pricing comes down as E0 petrol disappears.
Mogas or lead, southern side (South Africa, Australia, New Zealand): the fuel is at the pump. Read the label for ethanol and, in South Africa, for metal content. Buy from a high-volume station, and if the airplane is going to sit, think about avgas for the last fill.
Mogas or lead, American side (the United States): the pump option exists, and for most of us the reasons not to use it are sound. Swift’s airport map shows where UL94 and 100R are today. Until one of them reaches your ramp, 100LL with the maintenance discipline it demands is defensible, and it is not forever. Keep one eye on the E15 debate; it decides whether you leave this box by the top or by the side.
Lead or nothing (Brazil, India, and anywhere the mandate passes E10): the mogas option is already gone or going. Plan around UL91, avgas, or, if you are building, a fuel system you have deliberately made ethanol-tolerant end to end, with the understanding that the engine approval still stops at E10.
Next Time
Everywhere 100LL was scarce or taxed, light aviation solved its fuel problem a generation ago. The United States is the exception not because it lacked the technology but because 100LL was too easy. What that abundance did to the thirty-year search for an unleaded 100-octane fuel, and why a fuel the FAA approved for the whole fleet in 2022 still cannot be bought in Washington State in 2026, is the subject of Part 2.
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