No Quick, Easy Path to Unleaded Avgas

The Unleaded Avgas Transition Aviation Rulemaking Committee that was established by the FAA 18 months ago has made its recommendations, and there are no easy answers. Despite what you have heard or read, no unleaded avgas formula  is a direct replacement in every way for 100ll. The committee hopes the FAA will establish an 11-year pathway that will lead to an unleaded fuel that causes the least disruption in piston aircraft operation.

The committee’s fundamental finding is that there is no unleaded fuel–and it is highly unlikely that one can be created–that is a “drop in” replacement for leaded 100ll avgas. By “drop in” replacement the committee means a fuel that would replace 100ll with no change of any kind to aircraft operation or to the fuel supply network.

There is general agreement that lead must eventually be eliminated from avgas as it is the only remaining transportation fuel in the U.S. containing lead. The amount of lead used in all avgas in the U.S. is tiny, but it is still a target of potential litigation by environmental groups and even state and local governments.

Lead is still used in avgas to raise octane and thus allow high powered, hot running air- cooled aircraft piston engines to operate with a safe detonation margin. Even minor detonation is destructive and extremely dangerous in an aircraft engine so a replacement avgas must preserve that margin.

But there are many other unique characteristics of avgas that makes it suitable for piston aircraft including vapor pressure, storage life, cold temperature performance and compatibility with aircraft fuel system materials. Though a few existing fuel formulas may meet the minimum octane requirement, the committee found that no existing fuel can match every essential performance characteristic of 100ll.

Another huge problem the committee identified is that there are no realistic market forces to drive a transition to an unleaded fuel. Avgas volume is a microscopic segment of all transportation fuels, and sales volumes are shrinking, not growing. There are simply no financially sound reasons for fuel makers and suppliers to invest the many millions necessary to create an unleaded replacement.

There are also no FAA regulatory standards or testing methods established for avgas. Over the decades refiners developed higher performing fuels and engine makers designed engines to operate on those fuels. The FAA merely blessed the standard that was created by the industry. In other words, the improved fuels came first, and engines followed. Now we’re trying to go the other way and find a new fuel that will perform in engines designed for leaded avgas.

With the absence of market forces—read profit motive–to create a new unleaded avgas the committee believes the FAA must intervene. The committee recommends that the FAA establish a centralized facility to test candidate fuels to determine which is the optimum replacement. And then the FAA must create a streamlined certification process that will provide a method for the 200,000 plus piston powered aircraft to be approved to use a replacement fuel.

If the committee’s recommendations are adopted as many as 10 candidate fuels will be tested to determine which fuel would have the least impact on piston aircraft operation. The committee determined that testing candidate fuels will take five years once the program begins.

Once an optimum unleaded avgas replacement is indentified through extensive testing, it will be up to the market to determine how quickly the new fuel could be adopted. A key feature of any new fuel is that it be compatible with 100ll so that the piston fleet can continue to operate while fuel suppliers eventually fill the avgas network.

The committee believes a transition to unleaded fuel will take 11 years once the testing phase begins and the total cost will be around $71 million with the government investing $57.5 million and industry $13.5 million. It’s possible, even likely, that a majority of piston aircraft owners will need to make at least some modification to their aircraft to use the new fuel, even if those modifications are as minor as new certification documentation. But it’s also possible some engine/airframe combinations will need hardware changes, fuel system modifications, or even reduction in performance or weight carrying capability.

The entire process is fiendishly complicated because every aspect of piston aircraft operation will be impacted beginning with the fuel manufacturer, through the delivery and storage system, and finally to the aircraft operator. You have read and heard that direct 100ll replacements already exist and are just waiting for FAA approval. But that simply isn’t true. The transition away from leaded fuel will be many years long, expensive and we must be ready for the unexpected complications.

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25 Responses to No Quick, Easy Path to Unleaded Avgas

  1. Thomas Boyle says:

    To summarize:

    The FAA established a committee on replacing 100LL.
    The committee set a requirement that any replacement be an exact, identical duplicate of 100LL, with no lead in it (knowing in advance that this is close to specifying that we need an identical drop-in replacement for an Lycoming IO 360, with no fuel injection, but with no change of any kind to the aircraft’s operation).
    The committee does not consider the possibility that much of the market could be served right now by available alternatives and/or near substitutes that could be available relatively quickly.
    The committee declares that no such identical replacement exists (ignoring the existence of near substitutes).
    Noting that 100LL itself emerged through market forces and was not a creation of the FAA, the committee said the FAA will now need to regulate any proposed replacement. Thus, just in case any market participant wants to introduce a near substitute, they are on notice that the FAA will make that prohibitively expensive, and refuse any substitute that is not identical, thereby ensuring that there will be no market alternative.
    Having ruled out all plausible ways for the market to solve the problem, the committee says there are no market incentives that would lead to a replacement and says the FAA will have to step in and get a budget for the next 11 years to think about it.

    Did I get that about right?

    Talk about “here to help”!

  2. Mac says:

    Yes, the committee got it right. The market has offered no solution to leaded avgas. The lead issue has been looming for decades but there are no market forces sufficiently strong to create a lead-free replacement fuel. And with avgas sales volume dropping every year any market force that exits grows weaker.
    Mac Mc

    • Andrew Grant says:


      I have read all 99 pages of this report, and most of the Appendices. While the stated goal is fine, someone has censored all references to (A) the extensive list of STC’s obtained by EAA and others for many engines and aircraft to run on 82 UL = 87 auto gas, (B) the available combustion control tools developed by the auto industry with controlled fuel injection, variable solid state ignition, detonation detection and avoidance, variable valve timing, which are the very tools now used by both Continental and Lycoming to offer new aero-engines using mogas, (C) the fact that we already have a growing fleet of mogas-using aircraft, who have found suppliers willing to install mogas tanks and pumps, no problem, (D) the fact that even in the US, and especially elsewhere, Jet-A burning aero-diesels have responded to the very market forces the committee denies, (E) gas turbine suppliers are introducing smaller turboprops because they can see the opportunity. The proposed engine and aircraft testing is OK – but would be far quicker if performed by industry – 11 years must be a joke, even in DC. But I sent a letter to AOPA Pilot a couple of months ago, saying much of the above, and that our behavior in hanging onto 100LL is hurting our ability to recruit new pilots – they don’t want to fly carbureted engines with WW2 fuel and WW1 ignition systems, they think we are nuts! Needless to say, AOPA didn’t print it, must be the same censor that screwed up the AvGas report. Pilot since 1965, but I support Progress! Andrew Grant, 15G, Ohio

  3. Thomas Boyle says:

    On the contrary, the market has tried several times and is trying now: there are two fuel alternatives, one available right now and one in testing. Most of the fleet is already certificated (or could be) on a fuel other than 100LL, and it’s not clear that any of the fleet actually functionally needs the stuff (although some engines might need modifications to burn an alternative fuel).

    G100UL is a near-equivalent being developed by a market-driven player.

    Mogas is an alternative that works for much of the fleet, and there are market players trying to bring it back to life. In fact, many aircraft prefer it and burn 100LL only because the “big boys” insist on 100LL, which is why two or three generations of pilots have learned to check for lead fouling on the plugs of airplanes that were never intended to use leaded fuel (I wouldn’t be surprised if “lead poisoning” had killed a few people). Sure, the “big boys” might need to adjust to burning 91 octane, but the “small boys” have been having to adapt to 100LL for decades. Such is life.

    Continental has been looking at using 91 octane premium mogas in engines formerly designated as requiring 100LL.

    I understand there are market-based alternatives that could be used for other engine types too, allowing them to burn premium mogas if they modify the pistons and/or ignition timing, and maybe update the performance section of the POH. Frankly, the fact that any engines are still designed and sold new for 100LL is a bit of a problem. Given that a ban on lead is foreseeable, an engine manufacturer who isn’t certifying their engines with premium mogas may be looking at the risk of customer lawsuits if the lead ever does get banned.

    None of these is “a fuel that would replace 100ll with no change of any kind to aircraft operation or to the fuel supply network”.

    But they’re all feasible or potentially feasible market-based solutions to the problem of a ban on lead, if it arises.

  4. Thomas Boyle says:

    As a further note, I know that GAMI (producer of several formulas of G100UL) has been complaining that the FAA keeps essentially inventing reasons why their fuels are not equivalent to 100LL – despite the fact that it has no actual specification for 100LL and that there are multiple, very different, variants of 100LL in existence.

    This is consistent with bureaucratic incentives I’ve mentioned before: don’t take a risk, just say “no”. The bureaucrat who approves an unleaded fuel takes the chance that something will go wrong, ending his career. The bureaucrat who says “no”, takes no such risk.

  5. Eck! says:

    I still lament the loss of 80/87, as that was what my C150 liked. Autogas was equally good until the added alcohol (not sure that will not work, but the Peterson STC disqualifies it). While it runs on 100LL I found over the years cylinder and valve issues that were not present running 80/87 or autogas.

    There is a large segment of the fleet that can burn anything close to 80/87 and they
    have to run 100ll which is _not_ good for the engine! For those that can and there are many why not a 80/87 replacement!


  6. Bill Berson says:

    Any way to absorb lead from the exhaust with some large exhaust scrubber?
    The lead could be recycled and reused.

    I know this sounds crazy, but all ideas should be considered.

  7. Guido Bisogno says:

    What’s so disappointing is that the amount of lead introduced in the environment by avgas is so ridiculously low and yet we still have to ‘fight’ this battle rather than step to the plate and bring facts to show the impact is not worth the effort and certainly the money that will be spent to make this infintesismal improvement in the atmosphere……

  8. Dick Frederick says:

    I seem to recall that the Sun Oil company added lead at the pump some years back. If Unleaded Av Gas was produced in its present form, I believe that we would get something like 91 octane. For the few piston aircraft that need a higher octane rating, lead could be mixed at the pump. This would solve the leaded fuel distribution problem (lead contamination of transportation equipment), and should be easily workable. A truly insignificant amount of lead would be added to the environment.

    Obviously this is not politically feasible but I just wanted to point out that adult solutions are possible.

  9. Gerard Blake says:

    Mac, Swift Fuel 100SF seems to be a direct replacement for 100LL.

  10. Mac says:

    Hi Gerard,
    The key words are “seems to be”. Swift fuel may in fact be the closest to a direct replacement for 100ll, but it does not match every characteristic of 100ll exactly. And exact matters in aviation fuel. Swift may prove to be the least disruptive replacement, but so far, testing is not complete on it to the level necessary for fleet wide certification. Nor is testing complete for any other possible fuel. There are hopes and even claims that this fuel or that will replace 100ll with no change in performance or other characteristics but nobody knows for sure. I wish the committee was wrong about how long it will take to complete fuel testing, but if they are wrong, it’s almost certainly in being too optimistic. Everything in aviation always takes longer than anyone expects.
    Mac Mc

    • Thomas Boyle says:


      From what I’ve read, there is NO “exact” replacement for 100LL – and that includes 100LL! There actually isn’t any such thing as 100LL. There are several formulations of it (especially when you consider other countries), and all of them are “100LL” but none of them is an “exact” replacement for the others. Instead, they’re all “close enough”.

      It seems that a big part of the problem is that the FAA has never had to define “close enough” before and doesn’t know how, and is afraid that any definition it offers will turn out to have a (bad) loophole.

      There’s literally no way to know if Swift Fuel is a “close enough” substitute, because “close enough” isn’t defined. We can’t even tell when 100LL is close enough, it seems. It has simply never been a real problem.

      Maybe the thing to do is run Swift Fuel through some flight testing (something like is done in environmental testing of new airliners) and just get on with it. Either it works, and it’s close enough; or a problem comes up, and it’s not. Shouldn’t take 11 years; maybe 1.

  11. Art says:

    If it will take years and millions of dollars to solve this problem, then encourage the authorities to promote engines that run on jet-a, or better yet, diesel.
    Both of these fuels are available, and engines that use them are proven to be reliable at the high power levels needed for an aircraft powerplant.

    • Engine manufacturers do not seem interested in building Diesel engines to replace the 150-200hp avgas engines that are so prevalent in the fleet. And the elimnation of avgas without availability of alcohol-free unleaded auto gas would effectively ground many aircraft with smaller engines (A-65s & O-200s) and even a large chunk of the LSA fleet equipped with Rotax engines. The PC emphasis on ethanol in auto gas is a major issue.

      • Borneo Pilot says:

        Actually it’s the 150-200hp engines where you CAN get a diesel replacement. The Diamond Twin Star uses a 170hp AE300 turbo diesel from Austro Engine, and I know there is an STC for a diesel 182 (that would be a slightly bigger engine, can’t remember the details). It’s the big bore, 300hp range that does NOT currently have any options in diesel engines, and it is these very engines (big bore Continentals) that need 100LL the most. Supposedly Continental has been working on a 300hp diesel for years, but we’ve yet to see anything. I know the company I fly for would replace our TSIO-520′s at the next engine change, even if the engine cost substantially more, because of the increased savings in fuel burn/fuel cost.

      • Jeff Boatright says:

        “PC emphasis” ?

        Hardly. Try “Agri-business political donors”.

        Ethanol exists to placate Big Corn. Has nothing to do with environmentalists other than providing cover to hide behind.

    • Mac says:

      Hi Art,
      A jet-A piston engine would be an excellent solution, but there are many problems, most importantly the power to weight ratio of a diesel compared to a spark ignition gasoline engine. Lighter weight diesel engines are being perfected for automotive use, but the duty cycle and operating conditions for a car are different than for an airplane. What works in cars hasn’t worked out well in airplanes over the years, but we can still hope.
      Mac Mc

  12. Richard says:

    In the automotive market the introduction of the new generation of clean diesel engines (which require ultra-low sulfur fuel) has been hampered by government regulation, specifically Federal action to allow each of the states to regulate emissions. This creates an economic model that is of questionable viability for the manufacturers. Then there is the matter of ethanol, specifically corn based ethanol. It is not “green”. Neither is it sustainable and worst of all it requires more energy to make it than it generates. Don’t forget about the damage it causes to engines.

    It would appear that the alternatives are to continue using the existing fuels as long as possible, pointing out the miniscule quantities involved or prepare a transition period where the engines will simply have to be adapted to the available fuels. In the automotive world the transition to unleaded gas required different valve seats and valves as well as changes in compression ratios all of which could be accomplished at major overhaul intervals where a lot of parts get changed out anyway. The adoption of modern fuel injection and ignition systems in new engines could regain much of the lost by allowing greater compression ratios and more precise fuel management.

    In the automotive world, higher compression ratios with existing fuels have been accomplished by the use of direct fuel injection (rather than port type fuel injection) and more precised ignition combined with improved combustion chamber design. Porsche, for example, was able to actually increase horsepower and fuel economy in their engines when adopting these technologies.

    Until such time as the price of turbo shaft engines is dramatically reduced I simply do not see them as a viable option for small general aviation aircraft.

    Perhaps the best option is simply to hope for benign neglect of the situation by the regulatory authorities who, presumably, are occupied with bigger problems.

  13. Michael Kobb says:

    I would have really appreciated at least a summary of how the current GAMI and Swift fuels fail to satisfy the drop-in requirement. In what area do they not suceed? It’s very hard to evaluate the prospects for a solution with so little actual information.

  14. Mac says:

    Michael, you ask the questions we all want answers to. The testing necessary to approve any 100ll replacement hasn’t been done. The issues are huge. Let’s say a fuel performs perfectly in a large range of engines in a test cell, or even in flight. But what about all of the possible engine-airframe combinations? Do the fuel tank materials in airplane X disolve with the new fuel? That’s a possibility. What about the materials used in fuel lines and hoses? What about vapor pressure? Will the fuel feed and vapor return lines on airplane X behave the same as those on airplane Y even with the same engine installed? Every one of those issues is vital to keeping the engine running, no matter what the octane of the fuel. And testing enough airframe-engine combinations to gain confidence will take a long time.
    Of course, anybody can seek approval to use a fuel in a specific airframe/engine combination. That is how the auto fuel STCs were granted. But then somebody has to make the fuel. Somebody has to convince FBOs to stock it. And somebody has to insure the liability of the fuel maker/retailer. The liability of a fuel maker/retailer is absolute. Loading an unapproved avgas into an airplane would be as big a mistake for a retailer/FBO as would be pumping jet A into a piston airplane.
    We all want a quick and easy path to leaded avgas replacement but none exists. And there simply isn’t enough volume in the refiner/distributor/FBO market for avgas to support more than one fuel on a widespread national level.
    Mac Mc

  15. Michael Kobb says:

    Mac, in your article you wrote: “Despite what you have heard or read, no unleaded avgas formula is a direct replacement in every way for 100ll.”

    Meanwhile, I read your comment just above to say, “Not enough testing has been done of any proposed unleaded avgas to verify its suitability as a replacement.”

    Those are two different statements, aren’t they? One is affirmatively stating that none of the proposed fuels are good enough. The other says that none of the proposed fuels has been proven.

    I think anybody can understand that more testing is required before we’re all comfortable with the new fuels, and perhaps problems will be found or changes will have to be made. But the first statement sounds like it’s back to the drawing board.

  16. Chris Koch says:

    I think the aviation gasket and seal manufacturers should be only using modern materials that are unaffected by bio-fuels and be forced (preferably by market forces) to no longer sell rubber or neoprene products. Filter manufacturers have long known how to remove water from fuels so the venerable gascolator should be updated. Fuel tanks have to deal with corrosive water and bio-agents.
    Lets face it, the typical owners of 65 – 180 hp. air cooled engines run them way past the manufacturer’s suggested 12 year/2000 hour TBO (including me). These owners can all use ordinary automotive unleaded fuel without ethanol for decades to come.
    I find that most owners of higher horsepower (and higher compression ratio) air cooled engines are in the income bracket (or are professionals) sufficient to maintain and upgrade their engines at TBO to work with adanced bio-fuels.
    In my mind, the real fuel solution is Jet-A with an anti-anaerobic/aerobic additive. Modern diesels run fine at lower compression ratios (for example, Mazda’s Diesel engine with direct injection and low 14:1 compression ratio, part of its “SkyActiv” series (see:
    I doubt we will ever see “one fuel answer for all aviation engines”. There will have to be separate experimental, LSA (and non-professional GA) as well as professional piston GA fuel solution categories in order to move ahead with the no-lead solutions.

  17. JC says:

    Sorry Mac –

    Been reading you for years, but your take on this one’s WAY out in left-center field.

    First – Lets determine who’s really in the front left seat on this decade-old issue. It’s either Gami, or Swift, or both. Everyone else is just b#tching about the work these guys are doing. What’s the value-add in that? Someone once said, “Lead, follow, or get out of the way” – this clearly puts the word-processor-drivers solidly in the “get out of the way” category.

    Second – How about we write we write a column praising Swift and Gami and encouraging the government and the alphabet soup “also-rans” to silence the word processing automatons, and let the solution developers run the show! Write about Gami’s and Swift’s progress – not the government’s lack of same. These companies have solutions that need to be productionalized. Continually writing about the most recent of an untold numbers of unproductive juice and doughnut sessions with the government doesn’t do anything to move the ball forward – Gami and Swift are doing what needs to be done – no one else is at this point.

    Third – It’s time to take the gloves off on this issue and get to a solution. Major kudos to both Gami and Swift for pushing the enormously difficult task forward regardless of all the postulating by the countless non-contributors out there. Mac – these guys have actually BUILT SOMETHING, a task unbelievably more complex than sitting back kvetching about what could have/might have been. Gami and Swift aren’t comfortably ensconced behind a PC throwing stones at other people’s work- they’re builting solutions that writers and bureaucrats refuse to acknowledge. FWIW – I’d risk it all and start another career to work for organizations with the kahoona’s of these guys. Where’s the write up heaping praise on them for their progress?

    Forth – Who’s helping move the ball forward for Gami and Swift? Who’s putting planes in the air burning these fuels and collecting statistics? Who’s working on legal clearances/coverage for volunteers to help prove/disprove their solutions? Who’s driving campaigns to get these guys additional funding to shove the solution over the finish line and end this charade? It’s d@mn for sure not AOPA, FAA, EAA, Flying Magazine or any other organization that continues to print countless column inches of drivel about what’s not being done! Where do I sign up to volunteer myself and my Comanche as a test vehicle? Who’s in charge of that effort? Where’s the write up about that? Who do I need to speak with to help with the solution rather than sitting back wringing my hands waiting on another DECADE to slide by? Who’s running that effort? Why isn’t the aviation press holding them up as part of the solution?

    Fifth – Distance yourself from all the other drivel scribes and meeting goers who’s only interest is making a career out of 100LL replacement. You’re better than that. The alphabet soup organizations used to LEAD THE GOVERNMENT TO SOLUTIONS – not the other way around. What happened to the “damn the torpedoes – full speed ahead” attitude that built these organizations? There are endless nay-sayers on every staff publishing “nothing yet, but we’re continuing to sit through endless meetings” garbage and calling it progress! No one want to read that nonsense.

    Mac – Where’s the beef?! For God sake man you have the bully pulpit – LEAD!

    Atlanta, GA

  18. Mac says:

    Sorry, j.c., but I’m in no position to lead on behalf of GAMI or Swift. I have no idea if their fuels will be the optimum replacement or not. We can let the market decide, but market forces are not present. Those companies, or any other, will need to somehow certify their fuels in each engine/airframe combination. There is no mechanism to do that except on an individual basis as the auto fuel STCs were done. So that means the FAA and industry must jointly develop a system to do what has not done before. Will that take time? I think so. As a said, I wish there was a better solution but this time all piston airplane owners have a stake which means we are in this together for better, or more likely, worse.
    Mac Mc

  19. Tom says:

    Seems like 97% politics to me.

    The FAA had no trouble grounding countless “fat ultralights” and refusing to allow them into E-LSA passed thier arbitrary deadline. They legally made thousands of aircraft junk that can’t be certified except for under a very restrictive experimental exhibition. I know, I have two under ex exhibition and it’s a pain. It’s the largest scrap of flyable aircraft since WWII.

    But for the couple hundred airplanes that actually need 100LL, they will hold up the entire industry. Interesting. Most of those operators would do better with a turbine anyway. Look, my Rotaxes don’t like lead anyway, bring out the AVGAS 91 UL.

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