Key things here:
The gas used in the small planes with piston-driven propellers has lead in it. Yeah, the same stuff that was banned from automotive fuel because it causes brain damage in kids.
That lead in the fuel airplanes burn? It ends up in kids blood
There’s a new fuel mix starting to be available which doesn’t contain lead, and which works for all airplanes. It’s available in some places in the US, but not all of them yet.
The FAA has a plan to make going lead-free a requirement by 2031…but right now there isn’t anything stopping airport owners (often local governments) from making the switch happen faster. Only thing which might stop this is that the Republican version of the FAA reauthorization bill contains a requirement that airports keep on selling leaded fuel forever, so it’s also important to stop that from becoming law.
This makes the most sense by far. I think it has a lot to do with we just don’t put the money towards research and development we did in the 40’s and 50’s. Sure we still R&D but no one is dreaming up completely new ideas from the ground up and getting funding.
The most promising short-term approach is switching from spark ignition to compression ignition: Jet fuel is very comparable to diesel fuel, and doesn’t have the same problems with lead that avgas has. It has a higher energy density, providing lower fuel burn and longer range. And it is significantly cheaper than 100LL, let alone 100UL.
Europe has a significant number of diesel-engined general aviation aircraft burning Jet-A. Several manufacturers have received supplementary type certificates to swap out gasoline engines for diesel in many of the most popular GA aircraft.
Longer term, electric and hydrogen are being researched, but it will be a long time before they are viable options in the GA fleet.