Engines, aircraft, and the rules and certification standards and procedures that govern them have evolved together since the 1930s on the seemingly firm foundation of leaded avgas, which forms part of the operating limitations on which the type certificates of aircraft and engines are based. Fuels and fuel systems have come to be perfectly matched to aircraft, and aircraft to them.
But avgas, which is the only leaded fuel still produced in the United States, is a boutique product — the amount of road fuel refined is 700 times greater — and it has long been obvious that its days were numbered. If a completely compatible “drop-in” replacement existed and it cost a buck a gallon less than 100LL, leaded fuel would have vanished of its own accord. That this hasn’t happened underscores the fact that fuel producers, engine and airframe manufacturers, and end users have no self-interested motive (the lead poisoning of themselves and their children being below the threshold of perception) to eliminate 100LL. The Environmental Protection Agency, however, has now made them an offer they can’t refuse.
