[fjasper]

I don't know all the details, but they've been working on a lead-free fuel for aviation use for a long time. It's apparently a difficult technical problem, due to the various requirements of avgas.

It has to last practically forever in storage (which pretty much knocks ethanol out of the picture), has to be extremely resistant to detonation (running in some large, highly stressed, turbocharged, air-cooled engines), has to be combatible with the materials in the fuel systems in the existing fleet (airplanes aren't replaced at anywhere near the rate cars are-a significant proportion of the light aircraft fleet (the ones burning gasoline instead of jet fuel) are from the '50s, '60s, and '70s). It has to work when it's extremely cold, and when it's extremely hot.

Incidentally, the first item on the list is why avgas makes the perfect winterization fuel for machinery without catalytic converters. Put avgas in your snowblower at the end of winter, and the next fall, it'll start as though it's got brand new gas in it.

100 octane low-lead (100LL), the standard aviation gasoline, has way more lead in it than leaded car gas ever did, from what I understand, and fuel and engine design are closely optimized to each other.

Airplane engines routinely run at 100% power (wide open throttle, at maximum rpm) for minutes at a time, then 75% power for hours at a time, so there's not as much wiggle room for little differences in parameters.

In addition, safety data takes into account the power output of the engines to determine if you're able to load a certain amount of cargo into the airplane and take off from a certain runway safely. As a result, any reduction in power output due to fuel reformulation is basically unworkable, since you'd have no way to determine if the flight could be made safely on the alternative fuel, short of complete recalculation of the performance data. It's not that it can't be done, it just all costs money, sometimes a lot of money.

The low power aircraft engines that were designed to run on 80/87 octane gas can usually be signed off to run on unleaded car gas, but the the higher performance engines, basically almost any engine that's used for commercial or business operation, needs the high octane juice.