Cloudy Conditions on the Avgas Front
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.
A year and a half ago, the FAA published the findings of its Unleaded Avgas Transition Aviation Rulemaking Committee in a 100-page report with 162 pages of appendices. The report opens with a gloomy summary of its important conclusions. Here are two of them:
An unleaded replacement fuel that meets the needs of the entire fleet does not currently exist.
No market-driven reason exists to move to a replacement fuel due to the limited size of the AVGAS market, diminishing demand, specialty nature of AVGAS, safety, liability, and the investment expense involved in a comprehensive approval and deployment process.
The property of avgas that dominates efforts to find a replacement is its ability — for which we use the shorthand name of “octane” — to support certain cylinder temperatures and pressures without spontaneously igniting. The use of tetraethyl lead, or TEL, to prevent spontaneous ignition, better known as detonation, began in the 1920s. Since then, no convenient, inexpensive and effective competitor to TEL has been found.
Contrary to the belief of many people who put higher-octane fuel into their cars thinking that it has more “power,” octane has no bearing on the energy content of a fuel. Back when several grades of avgas were made, 80/87 was just as energetic as 100/145. It just happens that spark-ignition piston engines can harvest more of that latent energy if they have higher compression ratios, and those require a higher-octane fuel.
Besides detonation resistance, however, other characteristics of a replacement fuel are scarcely less important. One is environmental safety; there is no point replacing the lead in avgas with another powerful toxin. Another is materials compatibility. Some potential fuel components, like ethanol and alcohol, are hard on elastomers used in engine, propeller and fuel system seals. If they had to be used, every airplane might have to have some parts, like O-rings and pump diaphragms, replaced. Another is lubricity; gasoline has lubricating properties that some potential antiknock fuel components, like toluene, lack. Another is vapor pressure, which represents resistance to bubbling at low ambient pressure and high temperature. Avgas has a much higher resistance to bubble formation than, for instance, auto fuel does. Excessive bubble formation, in the form of vapor lock, can make an engine run roughly or quit altogether.
A replacement fuel also has to be “fungible” with 100LL — that is, capable of being mingled in storage tanks and in aircraft without ill effect — and it must not interact disagreeably with current aviation oils.
Finally, a new formulation must be able to be manufactured out of existing components in existing equipment and stored and transported in existing containers, because avgas replacement is not likely to attract heavy capital investment at any level, from the big refiners down to the small-airport FBOs.