The Long, Twisted And Slightly Ridiculous Story Of Avgas (Part 2)


For decades, the general aviation industry has struggled with finding a replacement for leaded avgas without success. The biggest driver of this failure is that there’s no reason to do so because the industry has been given an exemption to continue using lead. In this second of a two-part series, AVweb’s Paul Bertorelli explains how industry inertia and bureaucratic foot dragging killed efforts to eliminate lead from aviation fuel. See the first part of the series here:


  1. Ultimately there will be only two affordable fuels for piston aircraft: diesel (kerosene, Jet-A) and ethanol-free mogas. Both are produced now in vast quantities for vehicles. When we have an adult again in the WH, I predict we’ll be fracking everywhere and $1/gallon mogas. Or maybe the Feds will be ignored altogether and these sovereign states will go their own way and do what is right.

    • Care to make a bet on that $1/gallon prediction? I don’t bet much but I’ll take that one in a heartbeat.

  2. We need a side-by-side comparison article by a fuels expert on the qualities of 100LL vs UL94. There are a lot of reasons why people like 100LL, and we are familiar with them (not all involving octane/anti-knock.) UL94 could be produced in massive quantities to serve ALL the different user groups requiring E-free and somewhat higher octane. A cooperation between the Marina associations, landscaping community, racing communities, antique auto groups, Airboat community, and aviation, could result in economy-of-scale far surpassing the present market for 100LL.

  3. Hey Paul, Great series and much needed. Clear, concise with meaningful supporting details backed up by research and references. Couple of topics to include in part 3 (or part 6, it’s a complicated topic) TEL and “lubrication” of valve seats and guides and comparison of octane numbers and ratings and performance of Mogas, UL94 and Avgas. Maybe with mention of differences in octane numbers that international travelers will see as well.

    • Good request, Thomas! Add in the vapor pressure/vapor lock properties for high altitude flying. With GA in the USA being what it is, we especially need to know how long Mogas and UL94 can sit in the tanks of our aircraft without flying, and what are the variables regarding how humidity affects those periods of inactivity that relate to safety in a big way.

      • D B.

        Mogas is good for about four months; after that, I would remove it from my airplane and run it in something that has an electronic engine control that can cope with fuel variation, like a modern automobile.

        UL94 is basically 100LL without the TEL. Therefore they have the same longevity, as TEL does nothing for that aspect (it’s an octane booster, not a vapor suppressor or preservative.) It is about one year to be completely safe, but two to three years and still meeting specifications is not uncommon.

        Humidity has a greater effect on your cam and other engine parts than it does on fuel, but after several years of exposure, fuel can suffer. My point is worry about short term humidity effects on your engine, not on your fuel, which you should be using up as noted above.

        Hope this helps,

      • I can see there’s quite a bit of confusion here.

        First of all, UL94 is not mogas. It is an approved aviation fuel under ASTM D7547. Many, although not all, aircraft engines that don’t require 100-octane are approved to use it. Its aging characteristics are similar to 100LL, which is to say it practically last forever.

        As for lead and lubrication, this is a persistent myth. Lead was never considered a lubricant and it still isn’t. There has been some discussion that lead prevents valves from microwelding to the valve seats, the reasoning being that leaded fuels have lower flame front speeds than unleaded fuels. Higher burn rates. so the theory goes, of unleaded fuels caused the microwelding.

        Hardened valve seats have eliminated this problem. And the FAA did some wear testing of unleaded fuels and found no valve seat recession issues or microwelding.

    • Thomas,

      TEL doesn’t really lubricate guides or valve seats. It is thought to provide some “cushioning” effects but hardened seats provide the same protection. And all aircraft engines have hardened seats. The lead in 100LL is four times the lead that was present in leaded 80-87 octane aviation gas that many engines were designed to run on, hence, lead fouling is a real problem on engines which do not require 100LL. The lead also makes the oil dirtier, and as it reacts to the combustion process can leave deposits of lead salts on the valve guides, which is the major cause of stuck valves in GA airplanes.

      The octane ratings of Mogas are expressed as (RON+MON)/2=AKI; or the average of Research Octane Number (RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions) and Motor Octane Number (MON is determined at 900 rpm engine speed instead of the 600 rpm for RON and is lower than RON)
      and AKI is the Anti Knock Index. So 91 RON in Germany is equal to 87 AKI in the USA. With an 8 – 12 Octane Number difference between RON and MON.

      Hope this helps,

  4. Great series, Paul, and thanks for taking a topic that has been as clear as mud, and making it as clear as pond water (no…that’s really a complement!). With America, in general, seemingly unable to solve any important problems anymore, it’s difficult to imagine an unleaded avgas solution during whatever’s left of my lifetime. On the other hand, if the avgas industry can achieve said solution, maybe there’s hope for the rest of America as well(?). Meanwhile, I plan to devote whatever grey matter I have left to worrying about other things.