Yep, most of the small piston-engine aircraft are designed to run on 100-octane fuel. The only practical way to get the octane that high is leaded gasoline. However, the lead levels are lower than they used to be (avgas is typically 100LL for "low-lead"). Also, general aviation only consumes about 0.1% of the transportation fuel supply, so we're talking about a minuscule fraction of the fuel consumed.
More modern aircraft can often get by with 94-octane premium unleaded. However, they cannot tolerate ethanol in their fuel, and due to congressional mandates for ethanol blends it's become increasingly difficult and expensive to source ethanol-free premium gasoline. Paradoxically, this pushes general aviation back to 100LL, which can easily be sourced without ethanol.
Modern aircraft also have a trend towards diesel engines (often adapted from automotive diesels) that can be run on Jet A-1 fuel. These are actually very promising because they have good performance-to-fuel economy. See the Diamond DA62 for an example.
The root problem is that the general aviation fleet turns over incredibly slowly due to intensive maintenance requirements imposed by the FAA (eg annual total overhauls). It's pretty normal for your average single-engine aircraft to reach a lifespan of 40-50 years, and there's still plenty of aircraft that are pushing 70 or 80 years old.
As such it's very difficult to achieve a phase-out of a fuel type within timeframes that people consider reasonable. Like I said, the shift towards 94-octane premium unleaded and diesels is already happening, but it will probably take another 20-30 years for the fleet to naturally rotate to engines that support alternative fuels.
FAA oversight requirements (parts sourcing and mechanic labor) make everything airplane-related insanely expensive. You could mandate engine replacements, but this would be so expensive it would basically total 50% of the general-aviation fleet in an instant. At the end of the day we're talking about <0.1% of the transportation fuel supply so it's just not that big a deal.
I'm not in a panic about the lead in aviation fuel, but the relatively tiny amount of fuel used isn't really the thing you want to consider, you want to look at the absolute harm done by the use.
I do realize that in this case the harm is likely extremely difficult to even estimate.
The total US lead emission from general aviation is about 500 tons per year, which in 2002 was about 45% of total lead emissions (which is higher than I'd thought). One caveat is that is an upper-bound number, not all 100LL fuel has the maximum quantity of lead additive. It's fine if it reaches 100 octane on a smaller amount. It's also a historic number: the industry is shifting towards 100 Very Low Lead type with a 20% lower TEL limit. And general aviation has been in decline for about two decades due to spiralling cost. So that number is probably on the high side.
Most of the fleet (~70%) would be OK switching to 94-octane unleaded. The biggest problem is that Congress/states have made it extremely difficult to source this type of fuel, because the overwhelming majority of fuel is produced for automotive usage. Ethanol and other mandated additive blends are unacceptable for aviation usage, and we're talking about 0.1% of the fuel market. Congress/states have put up barriers that impede a shift and they need to do something to help that overcome those barriers, not just ban 100LL.
Also, due to the nature of general aviation this is a very distributed problem. Most general aviation occurs at relatively small airfields/airstrips and there's many factors that make it difficult for GA pilots to use large airports (limited access to those airspace types, high traffic levels from commercial aviation, high ramp/fuel fees designed to discourage refueling, etc). So you've got to get 92UL available at a large number of small sites. Most small airports will carry at most two types of fuel, 100LL and Jet A1, and many will only carry 100LL. Most airports are running on super thin margins and adding 92UL would mean not carrying 100LL.
That leaves the 30% of aircraft that can't use 92UL pretty high-and-dry. Some can be modified, some will need engine replacements, a lot will be out of luck. Unfortunately they are likely concentrated at the affordable end of the fleet, and that translates into more cost increases in a sector that's already dying due to a cost spiral.
General aviation is a feeder for commercial aviation, and that's going to become an even tougher and more expensive path to follow. Going commercial requires a large number of flight hours which are typically accrued by getting an instructor license and training casual pilots - so cost increases that reduce casual pilot time percolate very directly into a reduced supply of commercial-eligible pilots. The only other route is ex-military pilots, or paying >$100k through a university (which is not commercially viable due to the ~$20k average salary of an entry-level regional pilot).
General aviation is also a neat cultural touchstone that I am sad to see shrivelling into nothing :(
Re being a commercial feeder, that's like lots of things, if individuals start refusing to bear the cost of obtaining hours, airlines will be required to make changes. They might increase pay for pilots, lobby for changes to the licensing process or provide alternative ways to get those flying hours (I'm not sure how you subsidize hours without perverse incentives, but I guess you could protect investment using loans that had forgiveness for service).
You've pointed out some purposes that GA fulfills, but you haven't addressed what harm those lead emissions are actually doing. It could be minimal. Or, it could be pretty severe when you look at the aggregate impact. At a structural, societal level, poisoning people to avoid higher direct costs for air transport is likely not a good trade-off, so it is necessary to look at both sides of it.
A few nits I'd pick. Even though 70% of the fleet could burn 94UL, that represents well under half of the gallons burned. Turbocharged engines (often in twin engine airplanes) fly far more hours than the average 7.5:1 compression piston single. Yes, flight school aircraft on the low end fly a lot, but most piston singles average under 75 hours per year and often at 8-9 gph. Turbo singles, IO550s (Cirrus and Bonanza), and turbo or IO550 twins are typically burning 1.5x that per engine and flying 2-3x the hours.
I think most of the engines that can't be easily modified to run on 94UL are concentrated on the expensive end of the fleet. Most 182s/172s/Cherokees/G-cats, etc can either already run today under the Petersen STC or could be easily modified. It's the Bonanza, Cirrus, Golden Eagle, P-Navajo, etc that need 100LL (or GAMI or similar) and can't run on 94UL. Those tend to be way more expensive than the lower end of the fleet.
More modern aircraft can often get by with 94-octane premium unleaded. However, they cannot tolerate ethanol in their fuel, and due to congressional mandates for ethanol blends it's become increasingly difficult and expensive to source ethanol-free premium gasoline. Paradoxically, this pushes general aviation back to 100LL, which can easily be sourced without ethanol.
Modern aircraft also have a trend towards diesel engines (often adapted from automotive diesels) that can be run on Jet A-1 fuel. These are actually very promising because they have good performance-to-fuel economy. See the Diamond DA62 for an example.
http://www.gami.com/g100ul/avgas0211.pdf
http://www.aci-na.org/static/entransit/smallairports2011conf...
The root problem is that the general aviation fleet turns over incredibly slowly due to intensive maintenance requirements imposed by the FAA (eg annual total overhauls). It's pretty normal for your average single-engine aircraft to reach a lifespan of 40-50 years, and there's still plenty of aircraft that are pushing 70 or 80 years old.
As such it's very difficult to achieve a phase-out of a fuel type within timeframes that people consider reasonable. Like I said, the shift towards 94-octane premium unleaded and diesels is already happening, but it will probably take another 20-30 years for the fleet to naturally rotate to engines that support alternative fuels.
FAA oversight requirements (parts sourcing and mechanic labor) make everything airplane-related insanely expensive. You could mandate engine replacements, but this would be so expensive it would basically total 50% of the general-aviation fleet in an instant. At the end of the day we're talking about <0.1% of the transportation fuel supply so it's just not that big a deal.