> > 70% of 100LL aviation fuel is used by the 30% of the aircraft in the general aviation fleet that cannot use any of the existing alternatives
A large part of this problem, AFAIU, is that "cannot use" can mean lack of type certification. IOW, as a practical matter many (most?) pre-existing engines could use new, unleaded fuels, but engine certifications haven't yet been updated by the FAA. Only leaded-fuel is legally permitted in most old engine types, and old engines dominate in general aviation.
See, for example, this blurb further down in that Wikipedia article,
> In 2022, Paul Bertorelli of AVweb reported that the FAA is dragging its feet on broadly certifying G100UL, delaying approval of the fuel for more engines and spending over $80 million on EAGLE to re-start a search for an unleaded fuel when G100UL has been under evaluation for over 10 years.
where G100UL is (I think) the only alternative replacement fuel actually approved so far, certified just last year, and only for one particular family of engines.
A large part of this problem, AFAIU, is that "cannot use" can mean lack of type certification. IOW, as a practical matter many (most?) pre-existing engines could use new, unleaded fuels, but engine certifications haven't yet been updated by the FAA. Only leaded-fuel is legally permitted in most old engine types, and old engines dominate in general aviation.
See, for example, this blurb further down in that Wikipedia article,
> In 2022, Paul Bertorelli of AVweb reported that the FAA is dragging its feet on broadly certifying G100UL, delaying approval of the fuel for more engines and spending over $80 million on EAGLE to re-start a search for an unleaded fuel when G100UL has been under evaluation for over 10 years.
where G100UL is (I think) the only alternative replacement fuel actually approved so far, certified just last year, and only for one particular family of engines.