Correct. The FAA is set up to be default quite conservative (small c... "Reluctant to change things without a lot of work"). This makes a lot of sense given what they do (an amortized cost to public health over decades is a lot less likely to get people fired than a private plane falling out of the sky into the middle of an elementary school because the engine failed mid-flight due to a new fuel changing the mean time between failure in an unexpected way), but it does mean that even when things are understood to be safe and proven safe, simple inertia can keep the FAA from certifying them until someone lights a damn fire under them.

Although on this specific topic, I almost wonder if you could make a case that the unleaded avgas is safer not for public health, but for the private pilot and therefore the public in terms of the FAA's main understanding of safety (IE don't let planes crash). How much is a pilot's reasoning capacity compromised by chronic lead poisoning due to the necessary handling of avgas and breathing in fumes that they must do in operation of their plane?

Who is forcing them to fly this plane? I'm going to have to side with the rights of the people to not have lead dumped into their air over the right of someone to fly their own plane.

Air taxis, fire suppression, medevac, flight training, search and rescue, geomapping, aerial application, police, etc.

Very few of these high compression, high HP planes are flown by people just "flying our own plane". Most of us fly small planes that can easily burn unleaded.

You can understand my confusion when GP specifically says 'private', that doesn't immediately call to mind fire, police, and medevac.

The rest don't sound like particularly good reasons to keep showering people with lead.

s/private/piston/

I thought we had an alternative gas that has been produced and works but is just not yet fully certified and tested?

that is the definition of "no legal alternative"

My bad - should have restated that - I thought it was certified for a lot of planes and airports but not all (that it can potentially be certified for) and that there is a lot of potential there as a fix

Don't most of the more popular aircraft have STCs that allow them to run on automotive gas?

It covers about 80% of engine models, but only about 30% of total fuel purchased for those engines per year. (The high power airplanes fly more hours per year, resulting in a large spread between “engines” and “gallons per year” eligible.)

1. The "MoGas STC" costs money per plane to buy

2. It only applies to low compression engines, which is a lot of engines, but which also rules out most airplanes made since the 70s. There's a few exceptions, but not significant in terms of manufactured numbers to matter. (the STC you're likely talking about is the one that let engines run on 80 octane)

If the MoGas STC mattered then pilots would have adopted it because rec fuel (ethanol free gasoline) is significantly cheaper than AvGas

1. The cost of the STC rounds to $0. I think when I started flying it used to be $1 per horsepower; it looks like it’s still under $1000, which represents no barrier.

#2 and the low availability of mogas at airports is the reason for a lack of adoption fleet wide.

re: #2... because... ? Because they aren't going to stock a fuel that only applies to a handful of airplanes. The demand for it is near nil.

Exactly. You can barely sustain a fuel farm on the fuel that services 100% of the piston GA fleet. It's incredibly difficult to make the economics work to add a second fuel farm that serves only 30% of the gasoline sold, cannibalizing sales from your other fuel farm.

That's the premise/promise of G100UL: it can serve all the spark-ignition piston aircraft.

Demand would be a lot higher if the leaded alternative weren't allowed.

No, it wouldn't be that much higher for mogas (typically a 91-ish octane unleaded, E0 (ethanol-free) gasoline).

If there were an unleaded 100-ish octane fuel legally available as a substitute, that would have demand if 100LL were banned. Over 70% of the avgas burned is burned in airplanes that are not eligible for the STC* to allow them to burn mogas (typically as a result of having lower worst-case detonation margins as a result of being turbo-charged, super-charged, high-compression, or some combination).

* - Supplemental Type Certificate - an airplane modification, in this case a mostly [entirely for most airframes] paperwork modification, to their original type certificate.

In addition to the above, the STC requires that automotive gas with 0% ethanol be used. In many parts of the country, that is extremely difficult to find. In my area, the only place to buy it is one farm co-op that is way out in the country. In some places, it is not available at all. Luckily there's a website to find it, but places that carry it have been getting fewer and fewer. So the MoGas STC is not a long-term 100LL workaround.

https://www.pure-gas.org/