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 &quot;low-lead&quot;).  Also, general aviation only consumes about 0.1% of the transportation fuel supply, so we&#x27;re talking about a minuscule fraction of the fuel consumed.<p>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&#x27;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.<p>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.<p><a href="http:&#x2F;&#x2F;www.gami.com&#x2F;g100ul&#x2F;avgas0211.pdf" rel="nofollow">http:&#x2F;&#x2F;www.gami.com&#x2F;g100ul&#x2F;avgas0211.pdf</a><p><a href="http:&#x2F;&#x2F;www.aci-na.org&#x2F;static&#x2F;entransit&#x2F;smallairports2011conf&#x2F;WDerosier.pdf" rel="nofollow">http:&#x2F;&#x2F;www.aci-na.org&#x2F;static&#x2F;entransit&#x2F;smallairports2011conf...</a><p>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&#x27;s pretty normal for your average single-engine aircraft to reach a lifespan of 40-50 years, and there&#x27;s still plenty of aircraft that are pushing 70 or 80 years old.<p>As such it&#x27;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.<p>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&#x27;re talking about &lt;0.1% of the transportation fuel supply so it&#x27;s just not that big a deal.