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.

Long after the petroleum industry settled upon the accepted formulation of avgas, it was codified in an American Society for Testing and Materials standard, D-910, which is now the touchstone for candidate replacement fuels. There is no guarantee, however, that D-910 includes every fuel property that might affect aircraft operations. A candidate replacement for 100LL must therefore be thoroughly tested, both in ground test cells and in flight, to verify its compatibility and safety. Even before that can be done, however, test protocols have to be devised that will satisfy all the players in this game — engine and airframe manufacturers, fuel producers and, finally and most importantly, end users, who will be the first to file lawsuits if something goes wrong.

For example, suppose the selected replacement fuel were similar to 100LL in all respects except that its vapor pressure were somewhat higher. Various avenues are open to deal with this problem. Limits can be imposed on flight altitudes and temperatures; fuel pumps can be submerged in the tanks of low-wing aircraft, so that fuel remains under pressure on its way to the engine; or flight tests can be performed on each aircraft and engine type to determine whether a vapor pressure higher than the baseline 100LL standard is acceptable. The challenge of approving auto fuel for some airplanes was similar; it was handled by type-specific Supple-mental Type Certificates.

The same applies to detonation. If the detonation resistance of a replacement fuel were somewhat lower than that of 100LL, some high-performance engines might require modified operating limitations, or knock sensors (already common in automobiles) and automated devices for regulating mixture or power. Those systems would, of course, have to become part of the engines’ type certificates.

Even if a replacement fuel were in every way a lead-free twin of 100LL, claims would certainly arise over accidents or mechanical problems that occur shortly after a user adopts the new fuel. Problems may be random, but it’s natural for an owner to blame them on whatever he changed most recently. We saw this pattern when 100LL replaced 80/87 (because previously lead-free engines, habitually run too rich, started getting lead fouling in their plugs) and when 100LL replaced 100/130 (because valve problems were blamed on the loss of some of the lubricating property of lead).

Backing away from the specific complications of the transition and looking ahead several years, an ominous factor is the lack of a “market-driven reason to move to a replacement fuel” mentioned in the 2012 FAA report. The small and ever-diminishing size of the avgas market; the fact that the average age of the “legacy” avgas-burning fleet is measured in decades; competition from jet and diesel fuels and alternative engine types — all these factors militate against large capital investments in a new fuel. Even the FAA is pinching its pennies; its 2014 budget request for unleaded avgas R&D is an infinitesimal $5.6 million. NASA has not gotten involved; the problem is too mundane.

The large majority of general aviation aircraft — including most homebuilts and singles with engines of less than 250 horsepower — comprise what the FAA calls the “transparent fleet” that can use an existing lead-free fuel without modification. In Europe, these have already received regulatory approval for lead-free 91-octane. Aircraft with high-compression and turbocharged engines, however, including most larger twins, might require modifications to run on an eventual replacement. Though there are far fewer of these aircraft, they use the majority of the fuel. Most FBOs have tankage for only one nonturbine fuel, however, and at many locations the larger fuel demands of the nontransparent fleet may drive out the lower-octane alternative. Already today we see larger FBOs at larger airports discouraging smaller aircraft by high fuel prices. Perhaps the future will find aircraft naturally segregating themselves at airports supplying one fuel type or the other, but not both. The economics of replacement may be complex and far-reaching, well beyond the mere per-gallon cost of the new fuel.

The elimination of 100LL represents a critical challenge for general aviation. It could produce profound consequences, or pass practically unnoticed. It could lead to an industry-wide renaissance or to a number of funerals or to neither — at this point, no one can say.

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Peter Garrison taught himself to use a slide rule and tin snips, built an airplane in his backyard, and flew it to Japan. He began contributing to FLYING in 1968, and he continues to share his columns, "Technicalities" and "Aftermath," with FLYING readers.

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