Why Some Planes Still Run On Leaded Gas And Why That Might Finally Come To An End


In the past couple of months, there have been revolutionary moves in aviation that will make flying a bit healthier for us all. There is finally a drop-in replacement for leaded gasoline that every general aviation plane can run on right now. And the Environmental Protection Agency has published its long-awaited Proposed Endangerment Finding on the health effects of leaded aviation gasoline. Finally, after decades of waiting, aviation gasoline is finally going the way of leaded car gasoline.

Back in August 2021, fuel stations in Algeria finally stopped dispensing leaded gasoline. The nation was the last in the world to fuel cars with leaded gas. Here in the United States, the EPA banned leaded gasoline in 1996, and unleaded fuel was already widely available as early as 1975. With Algeria catching up, it means that no matter where you are in the world, you can rest easy knowing that the car in front of you isn’t spewing out lead.

Since then, there’s been one key place where you could still be exposed to lead emitted from a vehicle, and it’s right above your head. According to the Federal Aviation Administration, there are approximately 167,000 aircraft in the United States that run on leaded avgas. That number bumps up to 230,000 aircraft if you include the rest of the world.

These aircraft include everything from trainer Cessnas to small commercial aircraft and they burn 150 to 175 million gallons of leaded fuel every year. There are different grades of avgas, and the most popular one that you’ll find in the United States is 100LL, or 100 low-lead.

Why Planes Still Burn Leaded Fuel

Mercedes Streeter

The EPA says that while airborne lead emissions have dropped 99 percent since 1980, airplanes remain the largest source of remaining airborne lead emissions. Breathing in lead emissions can have detrimental impacts to the human body ranging from nervous system, cardiovascular system, and immune system damage to learning problems and a lower IQ in children. Lead is also absorbed by plants and the ground. There is no known safe level of lead exposure.

These effects have been known for a long time. Yet, when leaded fuel got banned in 1996, the general aviation fleet got an exemption. Why? The FAA says that it comes down to aircraft engines requiring high octane fuel to reduce knocking, and Tetraethyl Lead is an effective way of boosting octane:

Octane is a measure of the performance of a fuel as it burns in an engine combustion chamber. It is a measure of a gasoline’s ability to resist detonation, or “knock”. Octane is important to the safe operation of an aircraft or automobile engine. High compression, high displacement engines, such as those found in many high performance, piston engine aircraft, require high octane fuels so that detonation, which is the uncontrolled ignition of the fuel in the combustion chamber, does not damage pistons and other engine components and result in engine failure. High performance engines allow an aircraft to operate at increased speeds and with more payload, but these engines require higher octane avgas. Operating aircraft or automotive piston engines on fuels with lower octane than they require may result in damage from knock, but it is generally safe to operate piston engines on fuels of a higher octane rating than their minimum requirement. In other words, it is safe to go up in octane, but not down.

[Tetraethyl Lead] is an organic compound that contains lead and, in small quantities, is very effective in boosting octane. The ban of TEL in automobile gas was phased in over a number of years and was largely completed by 1986 and resulted in significant reductions of lead emissions to the environment. TEL was has not yet been banned for use in avgas, because no operationally safe alternative is currently available.

And lead isn’t just bad for the human body, it can be bad for engines. If you don’t use a proper engine operating technique, lead can build up and foul spark plugs.

There has been an effort to replace leaded avgas for decades, and there has been some development. Some aircraft can run Mogas, which is gasoline meant for cars. However, this fuel doesn’t meet the specifications for many aircraft engines.

Mercedes Streeter

And as AVWeb’s Paul Bertorelli mentions in his excellent history on leaded avgas, Mogas can have ethanol in it. Ethanol is hydroscopic, which means that it collects water and thus, can result in phase separation. Sucking up watery fuel in your motorcycle is annoying, but ultimately, there aren’t thousands of feet between you and the ground like you’d have in a plane. So running gas meant for a car isn’t the perfect solution.

In Bertorelli’s history, he also notes that some companies have offered hardware modifications to allow aircraft to run unleaded fuel. However, these have often been expensive and haven’t found market penetration. Thus, the search for the universal solution continued.

If you have the time, watch Bertorelli’s video and its sequel, they’re really good:

What Regulators Want To Do About Leaded Avgas

In 2011, the FAA started an unleaded avgas transition aviation rulemaking committee. It resulted in the 2014 Piston Aviation Fuel Initiative, combining the world’s fuel producers and the aviation industry to find a universal solution out of leaded avgas. A major sticking point in the FAA’s effort was to find a single fuel that can replace leaded fuel without modifying the aircraft.

However, as Bertorelli and a Popular Science report point out, the world’s fuel producers seem to have little motivation to spend development money on something like this. Avgas is one-tenth of one percent of what the fuel companies sell for cars. And while avgas has a good profit margin, PAFI asks the fuel producers to invest into a fuel with dwindling sales over decades. Of course, keep in mind that leaded fuel is still legal, so nothing is forcing a company to develop unleaded Avgas. The FAA established a deadline of 2018 for PAFI partners to find a 100LL replacement, yet that never happened.

An Unleaded Fuel For Every Plane


Thankfully, that hasn’t stopped some companies from pushing forward, anyway. Swift Fuels has developed its UL94, an unleaded fuel that could be used in more than half of general aviation aircraft without modification. And on September 1, the FAA approved supplemental type certificates (STCs) for a drop-in fuel that can finally solve this lead problem.

General Aviation Modifications Inc. has certified G100UL, a 100-octane unleaded avgas that can be used in all piston general aviation aircraft without modification. The decades-long dream of eliminating lead is finally getting realized. This fuel has been in development for more than 12 years and GAMI’s first certificate for the fuel were approved in 2021. As AVWeb reports, the certification of this fuel has been a bit controversial, with the FAA delaying approval of the fuel more than once.

Unleaded fuel for every general aviation aircraft is finally here. And getting it is pretty easy. GAMI says that you’ll be able to buy a supplemental type certificate from its website, then you’ll be good to go to fuel up on GAMI’s unleaded fuel wherever you find it. But don’t expect to buy it at your local airport today. GAMI says that it needs to ramp up production and set up distribution. This could take a while and for some time, GAMI admits that its fuel may cost more than 100LL. But the goal is to get the price down to parity.

The EPA Proposes That Lead Emissions Are Bad For You (Obviously)

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Mercedes Streeter

At the same time, the EPA has published a proposed version its very long-awaited endangerment finding. Environmental groups, physicians, and governments have been asking for an endangerment finding since 2006, and have even accused the regulator of dragging its feet on researching the effects of airborne lead. But finally, a proposed endangerment finding has been published to the Federal Register, and it doesn’t reveal anything surprising:

In this action, the Administrator is proposing to find that lead air pollution may reasonably be anticipated to endanger the public health and welfare within the meaning of section 231(a) of the Clean Air Act. The Administrator is also proposing to find that engine emissions of lead from certain aircraft cause or contribute to the lead air pollution that may reasonably be anticipated to endanger public health and welfare under section 231(a) of the Clean Air Act.

The EPA covers a lot in its 98-page document, and there are a few things worth noting in it. One is the EPA’s estimate that since 1930, America’s general aviation fleet has burned 38.6 billion gallons of fuel, and have emitted 113,000 tons of lead into the air. Those numbers exclude emissions from military aircraft. The most recent data quoted by EPA estimates that 470 tons of lead were emitted by planes in 2017. That data also goes on to note that aircraft emissions for 2017 made up for 70 percent of the total U.S. lead inventory.

The EPA’s proposed endangerment finding quotes a number of studies. These studies have found that airborne lead can contaminate vegetables, and thus offer a pathway of exposure to people. Meanwhile, other studies noted by the EPA have found that levels of airborne lead are higher near airports, and that the blood lead levels of children who live close to airports are higher.

What Happens Next

The important part about this is that when the proposed endangerment finding becomes finalized, it becomes fuel for the EPA to phase out leaded avgas:

The proposed findings in this action, if finalized, would not themselves apply new requirements to entities other than the EPA and the Federal Aviation Administration (FAA). Specifically, if the EPA issues final findings that lead emissions from covered aircraft engines cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare, then the EPA would, under section 231 of the Clean Air Act, promulgate aircraft engine emission standards for that air pollutant.

It’s unclear how long it’ll take to see a ban on leaded avgas, but it is clear that leaded avgas is now living on borrowed time. Two airports in California have already banned leaded fuel. With GAMI’s drop-in replacement for leaded fuel and this proposed endangerment finding, regulators have the tools that they need to force lead out once and for all. Hopefully, soon enough, pilots can still fly their planes and nobody will breathe in lead emissions from those aircraft.

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48 Responses

  1. I’m still trying to understand how these engines require leaded gasoline, Mercedes’ citation of why small aircraft require leaded gas doesn’t hold water, at least as I understand it.

    I looked up the compression on a Lycoming O-360; a textbook example of a general aviation motor, they power the Cessna 172. They make up to 220HP, though the average is around 180HP out of the 360 cubic inch 6 cylinder motor. That’s not a lot of power, but when one considers that is only ~2700 RPM, that’s somewhat impressive!

    The specs also list a compression of 8.5:1… That’s so low that I’m fairly certain nothing that is naturally aspirated (besides maybe a Harley) has been sold for the roads in the US since the 80s with that low of a compression ratio.

    Is it because the planes are running a crap load of advance at takeoff (not actually sure on that one, just a guess) on WOT with less airflow because they’re moving slowly, thus the lead is preventing knock at that precise moment, otherwise the lead is unnecessary?

    8.5:1 is pretty low compression for an ICE motor, and granted, while it is air cooled motor, and also knowing that the compression is lower to keep the power pulses from destroying the propeller, why is the lead actually needed?

    Any aeronautical power train engineers around that could explain?

    1. The EPA is just proposing an endangerment finding. It has to get published in the Federal Register, go through a comment period, then get published as a final finding, then it goes to rule making to determine a phase out date.

      10-15 years sounds about right for the whole process. Hopefully GAMI ramps up production faster than that and by the time the final rule making happens, it is a non-issue since everyone switched already.

      The EPA has been talking about making an endangerment finding for decades, and has basically just been waiting for an alternative fuel to be available. G100UL isn’t at the pumps yet, but should be soon.

    2. The government is too busy trying to outlaw ICE autos to consider that people are flying around in 1960’s fuel.

      Gas is bad! But we won’t close the loophole that makes SUVs and trucks cheaper than making coupse and sedans and paying fines for lower fleet averages!

      I’m feeling so cynical today.

  2. I’m surprised this goes so unreported on. I’ve mentioned leaded airplane fuel a few times and people are always insistent that I must be wrong until they pull it up on their phones.

    I live in the footprint of a major airport and was originally intending to do food farming in the backyard area but… the more I think about it, indoor hydroponics may be the way to go…

    1. “the more I think about it, indoor hydroponics may be the way to go…”

      Outdoor hydroponics should be fine too (and much cheaper). Just wash your produce.

      You may also be able send out soil samples to your local university ag extension for testing and see if you have a problem at all.

  3. Since this is a direct replacement with no engine modifications needed, does this mean it would be an option for anyone still running an old car that hasn’t been rebuilt with hardened valve seats?

    You might think, that after a 25 year phase in period that started 50 years ago and ended 25 years ago, there wouldn’t be many around, but, talking to old guys at car shows, it seems there actually are a lot people out there who are still putting a lead substitute additive in the tank with each fill up.

    Always seemed like that would be a really annoying hassle to me, if this means those folks could buy gas that’s already blended to work straight from the pump, I could seem them jumping on it.

    1. Once some people are set in their ways there will be no changing it ever. Their car hasn’t blown up with the additive, so they will continue to use it.

      It probably won’t even make the top 10 list of unnecessary things some old car guys insist on doing.

    2. This fuel won’t help with the valve seat issue – that occurs because lead acts as a lubricant between the valve and the seat. No lead means no lubricant.

      It could be a good fit for those older muscle cars with high compression engines that need high octane but there are already no lead racing fuels available for that niche.

  4. Slightly off topic, but has leaded gas always been sold alongside unleaded? Not sure about other automakers, and I could be wrong here, but I never heard of VW requiring leaded gas in any of their air-cooled models. My Beetle is a 72 and never called for it.

    1. Lead was used for lubricating exhaust valve seats on old cast iron heads. Or they didn’t actually have seats, just valve cut outs in the soft iron of the head. VWs used aluminum heads, so they had to use hardened seats in them to begin with. Then in the 70s US automakers started putting in hardened seats in their cast iron heads for unleaded use.

  5. If this really is a perfect 100% drop-in replacement for 100LL, I’m surprised the FAA is requiring planes get an an STC to run it. Can they not just declare 100LL and G100UL to be equivalent, and be done with it? I guess I don’t understand how the FAA works. There’s probably some dumb bureaucratic reason why they can’t do it that way.

    I’n also curious about the environmental and health impact of G100UL itself. It’s pretty hard do do worse than lead, but there must be some kind of additive in there to give G100UL the necessary anti-knock properties, and there’s no reason to believe that it’s some foraged, artisanal, hand-crafted blend of fairy farts and unicorn tears. I can’t find an MSDS for it and (surprise surprise) toxicity is not addressed in GAMI’s Frequently Asked Questions press release. It’s a proprietary blend of something-or-other. What is it?

  6. I’m not really surprised that it took so long. They needed a literal silver bullet to eliminate the excuse to run leaded, and you read the portion of sales this fuel makes up. On top of that, the quality of fuel in a plane is so critical. It will be some time before this stuff is readily available, and people will still not trust it. I am just thankful that autos went cold turkey with lead long before my kids were born. Avgas is just the last ~1% to tackle. Of course there are plenty of other environmental pollutants to worry about still, so this is kind of a drop in the bucket unfortunately…

    1. Seriously, all vehicles that are primarily used as grocery getters and daily commuters should be held the same fuel economy standards. Holding light trucks to a lesser requirement made sense when light trucks were mostly sold as work vehicles, but, today, a “light truck” usually means a taller, heavier, hatchback or wagon that just gets worse economy than a hatchback or wagon

      1. Late to the party on this but my Kia Soul is titled as a truck/van in my province. Don’t know if it is a Canada thing, just Ontario or the same in the states. I know why Kia is happy with this, but there is no way it is a truck or van.

  7. About time! Though I guess a big takeaway I get from the article is that this will still take a long time to replace leaded avgas. I’d think especially in the smaller and rural airports (like the one I see out my windows).

    As I was reading I kept thinking about lead levels close to these small airports, and then near the end “other studies noted by the EPA have found that levels of airborne lead are higher near airports.” I wonder how much higher the concentrations are, and what contributing factors (climate, population density, vegetation, humidity, etc., etc.) may make it better or worse.

        1. The last one I saw (Summer of last year) was definitely piston-engined.

          I do wonder, however, how many farmers still use crop dusters versus a sprayer attachment on the John Deere. I’m sure flying reduces time tremendously, but at the cost of either hiring someone to do it or getting a pilot’s license and maintaining a plane.

          1. “I’m sure flying reduces time tremendously, but at the cost of either hiring someone to do it or getting a pilot’s license and maintaining a plane.”

            I’d guess in the past a lot of those duster pilots were ex military so they were already licenced and trained for that kind of flying. Especially those who sprayed Agent Orange. Dunno how much the maintenance cost, I imagine that depended a lot on the vintage of the plane.

          2. I’d imagine it has to do with the size of their farm. Most commercial farms in my area of New England are tiny in the grand scheme of farms, and the last time I’ve seen a crop duster here must have been at least a decade ago.

            It’ll be interesting to see if drone dusters become more of a thing for smaller-scale agriculture, since they have the time saving benefits of a traditional crop duster for very little training and money. Investment, maintenance and “fuel” costs on an electric drone are basically negligible, you can map the aircraft to fly itself around your field to the point where the pilot is a glorified lookout, and having taken the Part 107 exam I’m confident that anyone remotely competent enough to work on a farm could pass it while half asleep, since the test mostly comprises questions about maps and weather.

          1. I just recently moved from farm country in North Dakota and crop dusters are quite a common sight. You’re correct that just about everyone was using turbine powered Air Tractors, I can count on one hand the number of piston powered spray planes I encountered, and only ever worked in one. Those were either being operated by a farmer spraying his own fields, or someone learning the business to move up into the turbine powered planes. From what I gather it just made better business sense to operate the big turbine planes, and they are BIG if you ever get a chance to view one up close it’s surprising.

    1. The nice thing about GAMI’s G100UL is that it is totally mixable with 100LL. Just dump it in the same holding tank and after a few loads, the holding tank will be 100% unleaded fuel. The only infrastructure change needed is the 100LL signage needs to be swapped for G100UL.

      The plane needs a paperwork only STC to satisfy the FAA, but no hardware changes are needed on the aircraft side.

        1. The existing mogas STCs are ‘cheap’ in aviation terms ($1.50 per hp), at least for the ones that don’t need additional fuel pumps installed. GAMI has indicated the pricing will be similar, and no hardware changes are needed.

          1. I’d gladly spend $300 to get to stop using lead so long as the price delta stays reasonable, not only is it better for everyone but my spark plugs will last longer and engine will run cleaner, possibly lowering maintenance costs. Also, not having to directly handle leaded fuel is a positive, between dipping the tanks to verify fuel quantity and sumping them to ensure there’s no water it’s almost garanteed that you’re getting some on you. If they can stick to the .60-.85 cent cost difference that’s reasonable for a lot of GA operators to swallow, even guys like me who aren’t made out of money.

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