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&#x27;d thought).  One caveat is that is an upper-bound number, not all 100LL fuel has the maximum quantity of lead additive.  It&#x27;s fine if it reaches 100 octane on a smaller amount.  It&#x27;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.<p>Most of the fleet (~70%) would be OK switching to 94-octane unleaded.  The biggest problem is that Congress&#x2F;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&#x27;re talking about 0.1% of the fuel market.  Congress&#x2F;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.<p>Also, due to the nature of general aviation this is a very distributed problem.  Most general aviation occurs at relatively small airfields&#x2F;airstrips and there&#x27;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&#x2F;fuel fees designed to discourage refueling, etc).  So you&#x27;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.<p>That leaves the 30% of aircraft that can&#x27;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&#x27;s already dying due to a cost spiral.<p>General aviation is a feeder for commercial aviation, and that&#x27;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 &gt;$100k through a university (which is not commercially viable due to the ~$20k average salary of an entry-level regional pilot).<p>General aviation is also a neat cultural touchstone that I am sad to see shrivelling into nothing :(