Why this is a club finance question
Our analysis of what a tow really costs makes the point that fuel is under half of it — which is true, and is the correct correction to the common club belief that fuel is the whole story. But under half of a large number is still the largest single variable line, and it is the one that responds immediately to an external change nobody at your club controls.
A club running a Pawnee through a busy season burns a great deal of avgas. If the price per gallon moves, or the fuel at your field changes, or your tug needs work to accept what is available, that lands directly on your cost per launch and therefore on the tow fee you have to charge. This is a budget item with a four-year horizon, which means it is a committee item now.
Where the transition stands
As at September 2026:
- The FAA released its final transition plan in July 2026, aiming to move the piston fleet off 100LL by 2030.
- 100LL remains the dominant fuel — of roughly 222,000 registered piston aircraft, around 175,000 are actively flying, and the great majority still run on it.
- GAMI's G100UL is approved under an Approved Model List supplemental type certificate covering nearly all spark-ignition engines and airframes, and is on sale at a small number of airports in California, Oklahoma and Mississippi.
- Swift Fuels' 100R is approved by STC for roughly 1,200 engines and 1,600 airframes and has rejoined the FAA's Piston Aviation Fuels Initiative.
- VP Racing Aviation and LyondellBasell's UL100 is in PAFI testing, with initial fleet authorisation targeted for 2027.
The practical translation: approvals are running ahead of availability. For most clubs the constraint over the next few years will not be whether your tug is permitted to burn an unleaded fuel, but whether anyone within reach is selling one.
Your engine is probably the easy part
The engineering difficulty in replacing 100LL has always been concentrated in high-compression, high-output engines, where detonation margin is tight and octane is doing real work. Those engines are the reason the problem took decades to solve.
A towplane is generally the other case. The aircraft doing this job — Pawnees, Cubs, Citabrias, Huskys, Bird Dogs and their relatives — mostly carry lower-compression Lycoming and Continental engines of a generation that predates the 100-octane requirement altogether. Many of them were designed around fuels that no longer exist and are, in octane terms, undemanding.
That is not permission to fill your tank with anything. Approval is by specific engine and airframe combination, under a specific STC, and your mechanic and the STC holder are the authority on your aircraft. It is a reason to expect that when you go and check, the answer will be straightforward.
The fuel system is the real question
This is where a club should actually direct its attention.
Reports have emerged around unleaded fuels and compatibility with certain older materials — nitrile O-rings and some fuel tank sealants in particular — with the general recommendation being to replace older nitrile components with silicone or fluoropolymer equivalents where practical.
Now consider a club towplane. It is frequently fifty or sixty years old. Its fuel system contains hoses, seals, gaskets and sealant of uncertain age installed by people who are no longer available to ask. It has spent its life on one fuel. It is maintained, often very well, on a budget.
That is exactly the population where a materials-compatibility question deserves a real answer rather than an assumption. The useful move is not to worry about it; it is to put it on the next annual's list as a specific question to your mechanic: what in this fuel system is original, what is it made of, and what would you change before we switch fuels? Doing that at an inspection you were paying for anyway costs almost nothing. Doing it reactively, after a leak, in July, costs a season.
The argument that runs your way
Think about what a towplane's day actually is. Full power to 3,000 feet. Throttle closed. A long descent at low power. Land, taxi, do it again — thirty or forty times, with the engine spending a large fraction of its running hours at idle and never getting the sustained cruise that keeps a leaded engine clean. As duty cycles for lead fouling go, it is close to the worst one you could design.
Every club that operates a tug knows the consequences without necessarily connecting them to lead: plugs that foul and get cleaned, valve deposits, oil that goes black quickly and gets changed on a short interval because of what is in it.
Removing the lead removes the mechanism. Operators on unleaded fuels report significantly cleaner spark plugs and the prospect of extended oil change intervals in the absence of lead sludge. If that holds, the aircraft that benefits most is the one with the worst fouling duty cycle — which is the towplane.
So the honest expectation for a club is not simply "fuel gets more expensive". It is a higher price per gallon set against lower maintenance cost per hour, and the net could plausibly go either way. Nobody can tell you which for your operation, because it depends on your fuel price, your annual hours, and how much you currently spend on plugs, oil and top-end work. What we can tell you is that modelling only the fuel price will give you the wrong answer.
What this does to your tow fee
Two inputs to the launch cost calculator will move: the fuel price per gallon, and — over a longer horizon — the maintenance and engine reserve figures.
Ten minutes of committee homework: run your current numbers, then run them again with fuel 15% higher and again at 30% higher, and look at what happens to your cost per launch. What you get is a sensitivity, which is worth far more than a forecast.
If a 30% fuel increase moves your cost per launch by less than the rounding error on your current tow fee, you can stop worrying about this and go and look at your fixed costs instead — which, for most clubs, is where the money actually is.
What to do now
- Find out what your field will sell. Your FBO or airport manager has a view and a timeline, and they are more likely to have thought about it than you are. Ask this year.
- Ask your mechanic the fuel-system question at the next annual. What is original, what is it made of, what would you change first. Written down.
- Check what STC coverage exists for your exact engine and airframe. Not the type in general — your combination.
- Run the sensitivity, not the forecast. Know what a 30% fuel move does to your cost per launch, so that when it happens you already have the answer.
Where this page goes stale
Deliberately said out loud, because this is the fastest-moving subject on this site.
The regulatory position, the approvals, the availability and the prices are all in motion, and this page describes them as at September 2026. The structural arguments — that towplane engines are an easy case, that old fuel systems are the real question, that the tug's duty cycle is unusually punished by lead — should age well. The specific fuels, dates and coverage figures will not.
If you are reading this well after that date and it still says September 2026, treat the general reasoning as useful and every specific fact as needing a check. The FAA's unleaded avgas pages and EAGLE are the current sources, and if you find this out of date we would rather you told us than worked around it.