I normally answer questions I receive. But in this column, I will be answering a question that every pilot and general aviation aircraft owner should be asking: “What do I need to do to my aircraft before the transition to unleaded avgas and 100LL is gone forever?”
Some of the more pessimistic experts would probably tell you to sell your airplane while it still has some value. I am more optimistic and think that with some work — and a little luck — you and your aircraft should be fine.
I know that the FAA, the EPA, and all of the other industry experts have claimed throughout this whole process that the transition to an unleaded 100 octane fuel will be an invisible change with no noticeable difference in performance or operational conditions. If you believe that, I have some beachfront property just east of Miami I will sell you cheap.
Since you will be operating your aircraft engine on a fuel it was not designed to run on, the first concern will be exhaust valve recession.
The biggest concern here is with new valves and seats in a new or overhauled engine.
With leaded fuels, the lead improves the sealing with new parts which, in turn, improves the heat transfer and reduces leakage. This reduces the valve face and seat temperature and reduces the seat wear and erosion.
The key here is if you are having a cylinder or cylinders replaced and will be starting out on only unleaded avgas, it is critical that hardened exhaust valve seats are installed. Both Lycoming and Continental offer them, but there may be some old seats still in the system, so check and make sure.
If you have an older orphan brand engine like a Franklin, P&W, Wright or other, your job is slightly more difficult. You have maybe five years until the 2030 self-imposed deadline for GA to transition to unleaded fuel — or as long as it takes the powers that be to finalize whatever specifications or requirements for the new unleaded fuel or fuels they come up with.
I assume there are owner groups that can work together to find a supplier for hardened seats for the various makes and models of orphan aircraft that are still being flown and get them approved. Without hardened seats, the life expectancy of these aircraft will be very short.
Another component of living with only unleaded fuels is exhaust valve/seat operating temperature.
In most engines, especially carburetor models, not all cylinders operate at the same air/fuel ratio in all conditions.
When you lean out the engine until you get some roughness, at this point the leanest cylinder is so lean that it misfires or at least stops producing normal power, while the other cylinders still produce normal power. You then richen the mixture until the engine smooths out.
At this point the individual mixtures of each cylinder vary with one or more probably being at or near peak exhaust valve temperature. This is going to increase the probability of exhaust valve seat erosion and recession.
So what can be done?
I recommend that every pilot consider purchasing a multi-point EGT (Exhaust Gas Temperature) or even a CHT (Cylinder Head Temperature) system. When flying, monitor the temperatures and ensure that all are below peak.
I know a lot of pilots have single point EGT systems, but the hottest cylinders can change with varying conditions. Upgrading to a multi-point system and being sure it is properly calibrated is good insurance that all cylinders will be in a safe range.
Another point of concern is anti-knock performance.
When we introduced 100 Low Lead avgas, we received knock complaints from many customers, especially from pilots operating big radial engines. Most had to reduce the allowed boost pressure for the engine during takeoff when going from 100/130 high lead to 100/130 low lead fuels.
One concern here is that the new unleaded products may not perform as well as 100LL. Also if more than one fuel is approved, they may not all perform equally, so there may need to be a different spec for each fuel.
There are many other minor points that I don’t have the space to cover, but the one big point you need to be aware of is seal and fuel system compatibility.
This is a big concern for the experts coming up with the ASTM specification and one that has no clear answer.
If you look at all of the different fuel system seals and components used since the Wright brothers’ first flight, it is mind boggling. And we can’t find fresh samples of each component to run tests on. So, there may be leaks, needle and seat problems, or a score of other problems to deal with.
When buying a used airplane, they always tell you “buyer beware.” With the new fuels, it will be everyone beware.
A note: Gene McNeely, who flew from 1994 to 2017 with the Aeroshell Aerobatic Team (now the Titan Aerobatic Team), passed away July 5, 2025. He was a great pilot and a good friend and will be missed.
It was bad enough in the mid 1970’s when 80/87 gas was discontinued and smaller engines had to drink 100LL. At our flight school, O-320 engines started with stuck valves and bent pushrods, and our O-200 engines burned intake valves and seats with expensive regularity. Our O-235 engines fouled plugs with lead, requiring 25 hour cleanings, and some piston rings choked with lead and carbon to the point of compression readings below 40 PSI, and a few fractured pistons! With 80/87, none of the above ever occurred!
Now, the push is lead free gas, and Lord knows how much money has been wasted doing it! What would this change do to our Lycoming and Continental engines, not to mention many other makes? NOBODY knows!
Since the consumption of avgas is but ONE THIRD of ONE PERCENT of all gasoline consumption, this push to again change our fuel is ridiculous!
Is there an additive that can be used with the New fuels ? What do we use to stop our engines from being damaged?
Until the manufacturer of Lycosaurus engines decides to be a part of the solution, this discussion will go nowhere useful. There is only so much magic that can be done on the fuel side in isolation without a willingness from engine manufacturers to also be a part of the solution. And so far I HAVE NOT SEEN THAT HAPPENING. Forget it.
I got tired of this discussion and decided to move into the future by building myself an airplane with an aviation engine that runs mogas (100LL not recommended). At the price of Premium gas at the station, compared to 100LL, I could not be happier. Even if an unleaded solution is found, are you willing to pay premium to buy it if you don’t even need it? What if they stop making it and we go back to square one? I much rather have an engine that can run what millions of cars run today, which ensure I will always be able to get it forever at a reasonable price.
And for those who still want to argue that European aircraft engines can get away with running mogas because they are water cooled, the ULPower engines run mogas, up to 180hp and are aircooled with a 2000 hr TBO. I don’t know how good or bad these engines are (I don’t own one) but it seems that the technology is viable.
I am sure that if the US engine manufacturers decided to also move into the future, they could fix the valve recession issue and then we would be done with this problem (at least for most of the GA fleet). And for those that still want to run high powered Lycosaurus feel free to pay for the boutique fuels. But I ain’t going subsidize you 🙂
Plain as day, “Get the small guys out of the air”
Since you will be operating your aircraft engine on a fuel it was not designed to run on, the first concern will be exhaust valve recession. This is with out a doubt a lie about unleaded fuel. The FAA has fully vetted all unleaded fuels for general aviation aircraft.
Actually, most GA aircraft engines were designed to run on 80/87 or 91/96 octane gas — NOT 100LL or 100/130.
Therefore, Big Iron, You lose
Yes, the type certificate for my GO-300, E-298, states ‘ aviation gasoline 80/87, minimum grade.’
So, a higher octane fuel is acceptable, but 80’87 had very low TEL, about 1/4th that of 100LL.
I have the auto gas stc but cannot use it in CA since all auto fuel has ethanol.
“The FAA has fully vetted all unleaded fuels for general aviation aircraft.” Are you positive about this statement Phillip?? I have two engines to take care of and they’re $100k a piece to overhaul..
I may stick with the advice of the guy who spent 33 years at Shell Oil.
When did manufacturers start putting hardened valve seats in their mew cylinders? Also, what testing is being done for compatability with fuel systems? There are rubber and neoprene hoses, composite tanks, various tank sealers with all kinds of compounds that may not do well with new formulations. Is there any assurance of compatability with any of these materials? Or are we on our own to see what ends up in the dumps or clogging up the fuel systems?
I would add that any fuel lacking formal engine manufacturer acceptance or approval is an alteration to the original type certificate and should be used with caution and constant monitoring.
Ben, thank you for tribute to Gene McNeely. He will be missed.
All three unleaded fuels represent an alteration to the original type certificate. It’s not clear the OEMs bring much insight to the approval process, based on Lycoming’s presentation at OSH.
GAMI’s G100UL is 100/160 compared to 100LL’s 100/130. The two ether fuels, from Swift and Lyondell, use a trick to ‘meet’ the rich octane rating, and both companies say their fuels will require the top 15% of engines to change, somehow… but they’re not pursuing those approvals. The ether fuels have lower energy content than 100LL, while GAMI’s fuel has higher energy content than 100LL. (Ether fuels fewer miles per gallon, GAMI fuel more miles per gallon)
Choose wisely!