Sustainable fuel
7 min read

General aviation’s less-than-stellar environmental record has always been the elephant in the pilot’s lounge: everybody knows it’s there, but nobody wants to talk about it. While the industry has repeatedly fought and won the battle against air traffic control privatization, it has never really admitted there’s a battle to be fought when it comes to emissions. Perhaps surprisingly, that strategy of playing dumb has worked remarkably well over the years—at a time when many large companies are announcing ever more dramatic environmental pledges, GA airplanes continue to burn leaded avgas like they always have.

Some of that is due to size. The entire aviation world, from trans-Atlantic airlines to small flight schools, represents less than two percent of worldwide greenhouse emissions, far behind other sectors like road transport or steel production. And at least the airlines have made significant progress over the last three decades: per-passenger emissions have dropped by half, offsetting some of the strong growth in international air travel.

Unfortunately, that’s not enough anymore. For many environmental activists, politicians, and even public company CEOs, the goal is not to limit the damage but to achieve net zero emissions. This campaign is starting to achieve real results: the European Union is expected to ban internal combustion engines in new cars by 2035. So whether you think climate change is an existential threat or a hoax doesn’t really matter—it is a serious issue for the aviation industry, at least from a public relations standpoint. And right now we have an image problem.

Sustainable fuel

Get ready for the “greenwashing” of the airline world.

Trade groups are starting to notice: at the recent European Business Aviation Conference, hosted by NBAA, sustainable aviation was topic number one. Erik Lindbergh delivered a keynote of sorts, promoting his vision for an environmentally aware aviation industry, and he has some serious support behind his project. Many other speakers offered a similar message.

A plan for jets

One reason for all the excitement is that the turbine world finally has a story to tell: Sustainable Aviation Fuel (SAF). This uses materials like cooking oil and wood waste to reduce total carbon emissions by up to 80% compared to traditional Jet A. SAF has received a surge of investment recently: energy giant BP is ramping up production, engine maker Williams has flight tested its FJ44 engine on 100% SAF, and FBO chain Signature has made it available at a growing number of locations. Even United Airlines, in announcing its deal with Boom Supersonic, felt compelled to mention it is “The world’s first purchase agreement for net-zero carbon supersonic aircraft.” Such a sentence would have been completely nonsensical just a few years ago.

SAF is gaining fans mostly because there are no other realistic alternatives for airlines and business jets: there simply aren’t any powerplants that can provide the power and efficiency of a jet engine. For all the artists’ renderings on tech websites, Delta isn’t going to fly 300 passengers to Europe with batteries and solar panels. SAF is also (hopefully) a drop-in replacement for Jet A, so it doesn’t require a completely new supply chain.

As exciting as this sounds to many airline PR departments, it’s important to keep some perspective. IATA, the global airline organization, is hoping to use 26 million gallons of SAF in 2021. This sounds impressive but accounts for less than 0.5% of global jet fuel use. The price will also undoubtedly be higher than Jet A (some are lobbying for a tax break to help). Still, there is at least a plan with real targets and real airlines using the new fuel.

What about light airplanes?

At the other end of the aviation spectrum, electric motors are getting most of the attention. While the hype has consistently run ahead of reality, look past the Silicon Valley “flying car” startups and you’ll find a few companies making real progress with electric airplanes, notably Bye Aerospace and Pipistrel. Both are focused on a realistic goal: building a traditional two-seat airplane that can fly a one-hour training flight (no eVTOL science fiction here). That mission seems to be achievable with current battery and motor technology, and I expect to see some larger flight schools adopt these airplanes in the next five years.

Bye eFlyer

Can this technology scale up to 6-seat airplanes?

That means flight schools, business jets, and airlines all have a plan to tackle emissions, at least on paper. But there’s one category of airplane that is conspicuously absent from this list: high performance piston airplanes, the ones with four to six seats that cruise at 150+ knots and can fly 300+ nautical miles. These may be fewer in number than training airplanes, but they fly many more hours per year and fulfill some very important jobs in aviation.

What’s a Cirrus, Bonanza, or Seneca owner to do? There are plenty of possible scenarios, but none of them look very promising right now:

  • Turbine engines (burning SAF) could be scaled down to work in light airplanes, boosting performance and efficiency. It sounds appealing, but the economics of this idea seem fatally flawed. Multiple projects have been launched and abandoned over the decades.
  • Larger electric airplanes may come some day—Bye has floated plans for a King Air size electric twin—but this seems a long way off. Current battery technology simply cannot power an eight seat airplane that flies at 200+ knots with any kind of range.
  • How about hydrogen fuel cells? Some companies are true believers, including ZeroAvia, but progress has been erratic and their suggested timeline of delivering a 10-20 seat airplane by 2024 seems wildly optimistic.
  • Diesel engines could be a workable option, and theoretically they could burn SAF. But with the minor exception of Diamond’s Austro engine program, diesel engines have failed to find a market in general aviation.

Unfortunately none of these ideas have the easy appeal of SAF, which requires neither new engines nor new ground infrastructure. Most of the ideas above require huge certification efforts, and even if one of these ideas gets certified it will take many years before the typical GA pilot sees one at their airport. The average piston airplane is over 40 years old, so replacing the entire fleet is a truly daunting task (the car market is not a fair comparison here). Maybe someone will make a business out of re-engining old Cessnas, but such an upgrade would probably cost more than the airplane itself.

Unleaded avgas—yesterday’s problem


Nothing to see here…

In the meantime, the chances of finding an unleaded avgas solution will only get worse. The FAA’s Piston Aviation Fuels Initiative has been a disaster so far, and it shouldn’t be surprising—energy companies have little incentive to develop a new fuel, since avgas accounts for less than 0.1% of transportation fuel consumption in the US. If electric airplanes become a reality, that number will get even smaller.

Worse yet, the current business climate is openly hostile to new fossil fuel investment. ExxonMobil recently lost a high profile shareholder vote with activist investors who want the oil giant to take climate change much more seriously. BP and Shell—oil and gas companies, mind you—are targeting net-zero emissions by 2050. For an executive at a public energy company, spending time and money on unleaded avgas is a laughably bad idea. Not surprisingly, Shell’s unleaded avgas project looks dead and the latest update from Phillips suggests it is in hospice care. Wouldn’t it be ironic if the greening of aviation makes unleaded avgas harder to achieve?

It appears some parts of GA will be caught in the middle. Expect to see airlines boasting about their carbon neutrality, and large flight schools showing off their “zero emission” trainers. If the typical high performance piston airplane is still burning leaded avgas, we won’t fly under the activists’ radar for long. Even if we do, our dependance on a single supplier for lead additive puts us in a precarious situation.

Does that mean every Bonanza will be grounded? Hardly. I’m a strong believer in the “muddling through” theory of history: inertia is a powerful force, so even when it feels like a trend can’t continue, it often does. General aviation can and will survive as a niche activity, with an environmental exemption in much the same way antique cars can avoid emissions testing—”we’re too small to matter!” But that’s not a very inspirational vision. Are we willing to admit that GA will always be a small group of antique airplanes? Shouldn’t we dream bigger?

John Zimmerman
22 replies
  1. Lee
    Lee says:

    I don’t care what activists think, it is a non concern. Fight back and keep the gas coming! We don’t need alarmists.

    • Michael
      Michael says:

      Don’t look now Lee but you are the activist, fighting the fight of a dying industry that has done incalculable damage to the environment. There is near-universal consensus by people who know what they are talking about – not “alarmists” – that it is concerning indeed. How you can post this comment at a time when scores of people are dying as Canada sets temperature records day-after-day is beyond belief.

      • Jason
        Jason says:

        Will start of by saying that climate change is real. It has been for millions of years and will always be a constant in this equitation. How we contribute to it might move the needle, but Big tech and Big media are fully on the bandwagon, so here we go! Never mind the fact how China, India, along with other emerging economies try to help. Many in the scientific community do not really believe China’s effort to curb their greenhouse gases. Also, the push for electric to charge planes, trains and autos will need a realistic sources of energy, and that will be modern nuclear power. Yet the same folks that are total buy-ins to environmental change don’t want to hear anything about nuke power. But hey, we got everyone to wear a mask, we can get everyone to buy into just about anything with the right marketing.

      • Realist
        Realist says:

        Y’all need to stop with this emissions nonsense! Y’all have no idea how much fossil fuel is used by the worlds military’s collectively on the water, in the air and land based. Fighters, cargo and bombers, for example, are constantly being refueled by tankers which hold hundreds of thousands of pounds of fuel. The training of military pilots and then the hundreds if not millions of hours it takes each year to maintain those pilots to a superior level to be current and qualified. Are you guys really kidding me?? And the environmentalists are worried about GA?? Y’all are crazy and need medication if you think that there is a problem. BTW, ain’t no way on GOD’s Green Earth are you going to get China to agree to any of this nonsense! Go read about how big the Chinese military will be in 10 years. You people make me laugh….tree huggers!

        • Duane Mader
          Duane Mader says:

          US coal plants- 252 and decreasing
          Mainland China- 1082 and increasing. Plus, China is building coal fired power plants all over the world with it’s “Belt & Road” plan for domination.
          Need to send “how dare you” girl over there.

        • Gary Keller
          Gary Keller says:

          Some of us environmentalists do not care if China agrees to anything. We just want them to stop sending all their students to learn to fly in the US while they poison our children with leaded emissions.

  2. David Megginson
    David Megginson says:

    Well put. There’s no magic wand that will fix all these challenges, but simply hiding our heads in the sand and pretending the world isn’t changing around us will make G.A. even more irrelevant than we already are.

    And as we become less relevant, it will be harder and harder to keep the billions of dollars’ worth of prime land set aside for our local airports out of the hands of eager developers. So even if we succeed in keeping our leaded gas and high-performance airplanes just as they are now, we could easily end up with nowhere to land them.

  3. Daniel Pauls
    Daniel Pauls says:

    Along with your fully-electric aircraft will be the fully-automated flight control system (which is actually easier to implement and closer to full-on reality). A human might be along for the ride, and they might be allowed to manipulated the controls under the watchful “eye” of the computers, but in 20 years the only way you will be able to actually “fly” an aircraft is in one of our current “antiques”. Expect to be restricted to bubbles of airspace set aside for recreation and above sparsely-populated areas so we don’t get in the way of the “real” planes and pose a threat to people on the ground. Of course, ground transportation will likely follow a similar course. This would be more of a first-world situation, I’m sure other parts of the world would remain as they are today.

  4. Rick Bennett
    Rick Bennett says:

    I have my doubts about the potential acceptance of any training aircraft by a flight school that does not have the capability to be used to at least complete a private pilot license, let alone an instrument rating….
    An aircraft that can only fly for one hour will not have the reliable capability to even do a private.
    I pulled out my old log and took a look at my first 65 flights….. only 13 of those flights were less than 1.1 hours (20%). Acknowledging that those are based upon the hobbs…. Which include on ground warm-up, etc… and adding another 20%, that would still only cover a total of 40%.
    What flight school can afford to buy aircraft with which they know a student will be unable to finish even their private? Then also expect a student during their primary training to also have to pay to transition to a second more capable aircraft?
    I think the one hour mark is too short. A more effective target would be more like >2. And we haven’t even started to consider an instrument rating yet….

    • Stephen Phoenix
      Stephen Phoenix says:

      We could have an Electric Airplane Only limitation on the license. It would be pretty limiting right now, but at least allow all the training to take place in the same airplane.

      • Carl
        Carl says:

        Revitalizing GA needs to have a $50k, 4 seat, 2500# MTOW, 4 hr cruise @ 120 kts with reserves, and refueling time of <30 min. Wait, we have such an aircraft, and GA is still in decline. Maybe partially because that new C-172 costs not $50k, but more than the average new home.

  5. Stephen Phoenix
    Stephen Phoenix says:

    I think that the FAA made a big mistake when they went to the unleaded fuel initiative. They could have easily just marked a date at which leaded fuel could no longer be sold. Then let the market sort it out. The solutions probably would have involved changes to fuel tank venting systems and in many cases some engine mods or changes. But the change would have occurred and we would have all been better off for it. Now, they have spent a bunch of money and have nothing to show for it.

    • Kevin
      Kevin says:

      Absolutely. Sure, put in some extensions for some AC and exemptions for others, but please, at least draw a line in the sand and get the market to do its magic.

      Moreso, declare Avgas completely dead as of such-and-such a date and watch diesel bloom. The author says there is no market for diesels? – that’s because there’s no market movement without incentives. He is correct, we have a perfectly viable technology, today. Declare avgas dead and watch diesels running on SAF suddenly get more available and cheaper.

  6. Duane Mader
    Duane Mader says:

    AOPA recently did an article that stated jet fuel is 50 times as energy dense as the most advanced battery technology. Even if an electric motor is doubly efficient, that is currently (pun intended) too high of an obstacle to overcome for most operations. I’m all for progress in this field, just being realistic.

  7. James Geer
    James Geer says:

    Follow the money. GA will die a slow death as it becomes too expensive. There will be small fights over policy from time to time. The big hit will be the cost of energy.

  8. Mike Hanna
    Mike Hanna says:

    Contrary to some mind sets these days, one of (if not the only) condition affecting everything is that there are just too many people! And, there are very few with the ability and guts to really teach these millions and impart the truth about living, life and the responsibilities of each and everyone of us is out there. I’ve been in G/A, the airlines and Pt. 135 opns for over 60 decades; it’s a complex issue and preaching about one view or the other is not going to save the earth! I don’t have an answer either, so I suppose many of you will find my comments irrelevant and will throw me under the (electric) bus. I was invited to comment, as all of you were, so that’s my take.

  9. Robert Germann
    Robert Germann says:

    Happy Fourth of July 2021, excellent article and even better comments (feedback)
    Keep them coming on this immediate issue

  10. John Sperling
    John Sperling says:

    Here’s my two (or three) cents: Aviation’s 2% contribution to global greenhouse gas emissions is only part of the picture, which includes 2.5% of CO2 emissions, but does not include a 3.5% contribution to radiative forcing (warming). 2/3 of this warming contribution comes from contrails rather than CO2 emissions. This isn’t to let GA off the hook, but just to point out that the majority of aviation’s contribution to climate breakdown is from jet aircraft flying at high altitudes. Also, within the transportation category, aviation’s contribution is a much larger 12%. 2) Hydrogen has immense potential to power larger, commercial aircraft. It has an energy density 3 times that of kerosene. One of the main issues is that hydrogen requires more storage space than jet fuel, which is not by any means an insurmountable obstacle. 3) Sustainable aviation fuel is not the final solution. There is simply not enough arable land in the world to grow enough food AND supply global aviation’s requirements for biofuel. SAF is at best a stop-gap measure to keep emissions in check while new aircraft and powerplants are developed; however, climate breakdown does not lend itself to either/or solutions. We need an “all of the above” approach.

  11. Jeff S
    Jeff S says:

    Binning response on the usual head-in-the-sand comments, consider that 100LL is now responsible for 1/2 of the airborne lead pollution in the USA. Even in its liquid form, 100LL rates above limits for qualification as a hazardous substance by several multiples. Will anyone care to argue that lead is good for you?

    100LL is going away and creating a “100-rated” alternative without lead or potentially worse additives at a reasonable price is likely impossible. The need for high octane comes entirely from traditional a/c engines’ use of large cylinders with relatively high compression. Reducing compression (or manifold pressure limits) is an easy fix, but at the cost of power and efficiency. A matching increase in displacement, as being done with newer Lycoming & Continental engines can be done to keep the power output the same, but still at the cost of efficiency. Double the rpm, halve the displacement and the problems running lower octane fuel disappear, especially if using a modern electronic engine management system.

    Technological solutions (modern engines) exist, but magical ones (100UL, electric) don’t and neither will be cheap. Unless, that is, one simply doesn’t buy an airplane requiring 100LL in the first place, which may be the wisest path in the mid-term.

    • Jason
      Jason says:

      According to the EPA, since the 1980’s the US has reduced lead output in the air by 98% via regulations that we can agree are sensible. So you are suggesting aviation continues with a 1% of the remaining 2% with your over half remark? Not to mention, GA has been dying a slow death since I have been flying since the late 1980’s. Our air is certainly not as bad as alarmist want to make it. Especially when compared to China, India and other emerging cultures. Yet we want to stymie our own aviation activity over lead numbers lower than they have ever been??? I won’t go to the level of flaming as what Realist said the “mental” part, but I would say it is more an issue with folks understanding math. Good grief.

  12. Jack Morris
    Jack Morris says:

    Replacing magnetos with electronic computer controlled ignition could allow the use of mogas in piston GA. But the govt in the form of the FAA is the largest obstacle to this happening even though the govt is pushing environmentalism. Really, how difficult should it be to retrofit dual electronic ignition on an aircraft engine?
    The bigger picture concerns energy sources and generation. If you look at energy density from a practical standpoint, the environmental movement is not providing realistic solutions to the problem. Windmills and solar panels are not energy dense enough to replace fossil fuel for electric generation. And your ordinary 1.5 V alkaline flashlight battery is more energy dense than the best rechargeable battery. And by weight or volume, batteries are woefully less energy dense than kerosene or gasoline. Considering just electric generation, you would need to put windmills on a area the size of Italy in order to provide enough electric power to meet the energy demands of the USA. The only realistic solution is nuclear, where the energy density is orders of magnitude higher than solar or wind.
    The lack of acceptance of this solution convinces me that the environmentalists and politicians (no surprise) are not being honest with the public. In addition to hydro, if we adopted the newer fail-safe, minimal waste, nuclear designs to replace the older designs and phase out coal and gas electric generation, the net reduction in CO2 would allow fossil fuels to continue to be used for airlines and GA (and fuel efficient autos). The scientists and engineers who developed atomic energy gave us a zero carbon emission solution to our future energy needs. Solar cells and windmills just don’t cut it.


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