For many years airplanes have been the only transportation vehicles still allowed to use leaded fuel in the United States. Even NASCAR transitioned to unleaded gasoline for its stock car races a couple years ago in a symbolic move of going green. In aviation we have had a solid defense for leaded fuel because there has been no other option. Many high-powered piston engines can’t operate to their full power potential without the knock retarding presence of tetraethyl lead in the fuel. Aviation did its part in the three-decade-old effort to ban lead from motor fuels by creating 100 low lead octane avgas, which contains a fraction of the lead used in the original 100 octane formula. But there is still lead in 100LL and even the comparatively tiny amount of lead exhausted by all piston airplanes is too much for society to accept indefinitely.
There are several unleaded gasoline formulas that can power the majority of aircraft piston engines because the biggest number of engines are the low-powered naturally aspirated type that power the huge fleets of Skyhawks, Cherokees and other light singles. However, the actual use of avgas is stood on its head by the fact that the majority of the fuel is burned by a minority of high-powered airplanes that fly more and consume more fuel than the lighter airplanes. Since FBOs are realistically able to supply only one version of avgas, a 100LL replacement needs to serve the entire piston fleet, or at least come as close as possible to that goal.
