Whoa. Wait a minute. Stop the presses! Wasn’t unleaded automobile gasoline (so-called “pump gas”) approved for use in some engine/airframe combinations years ago, at least for the smaller piston engines, e.g., the Lycoming O-360 installed in thousands of Cessna 172s and other light planes?

Well, yes, unleaded auto gas was, and the Experimental Aircraft Association (EAA) earned several STCs allowing it to be burned safely in low-compression aircraft engines like the O-360. Owner/operators were delighted, not only saving money over having to use more expensive 100LL avgas (the only avgas grade now available) but also being able to fly their aircraft without emitting lead into the atmosphere. That is, until Congress ruled that ethanol had to be added to auto gasoline to lower CO2 emissions (and because the corn-growing industry, which produces ethanol, has a pretty powerful lobby).

“It is not a straight trade off, as pump gas contains blends of ethanol for oxygenation,” Doug Macnair, EAA vice president, government relations, told B&CA. “The EAA’s approvals of pump gas are based on auto gas without ethanol, which was mandated by Congress to be blended with auto gas. So our STC program has become less viable because the untainted gas is rarely available.” Consumers can find it if they search hard enough for the occasional gas station or FBO that will carry ethanol-free auto gas, also referred to as “mogas.” Like 100LL, it is considered a specialty fuel and so is much more expensive, mirroring the price of avgas. Thus, any economic advantage it might have presented to aircraft owners in the pre-ethanol days has disappeared. Nevertheless, readers interested in tracking mogas locations can find a directory frequently updated by users at www.autofuelstc.com/mogas_locations.phtml.

In order to make auto gas widely available as an aviation fuel, as some EAA members have pressed the organization to pursue, “you’d have to change the will of Congress to allow the ethanol to be removed,” Macnair explained, “which is unlikely, and second, we would be asking for aviation use of an auto product, and we do not have the influence to change that, as it is seen as being for cars only. We also would have to convince the refiners to make it, and they would refuse due to the small size of the market. So economic and political forces stand against the use of auto gas. It’s in an environment where the fuel is just not available.”

The issue is moot, anyway, Macnair insisted, as “It is the view of the EAA that the future of avgas has to be one that encompasses the entire piston-powered aircraft fleet — that has to be the foundation of any discussion about avgas. You can’t have those discussions about splintering out octane levels in isolation. There are those who think auto gas is the solution, but there has to be a solution that applies to the whole fleet [i.e., the 30 to 40% of the piston fleet that must have 100-octane fuel to prevent detonation in its high-compression engines.] The low-octane portion of the fleet does not require the high-octane fuels that we are seeking and which come at a price, and that’s what drives the persistent discussion about auto gas — it’s an economical issue.”

So the major limitation of auto gas is that it doesn’t meet the needs of the entire fleet. “And the aviation fuels marketplace is so small we cannot be in the position of splintering it into multiple grades,” Macnair said, adding that “the numbers don’t support multiple grades. It is such a small market the economies aren’t there. You won’t see $2.60 a gallon auto gas on an airport, and when you turn that into an aviation grade with a separate distribution system, it isn’t the same fuel you buy at the corner gas station any more. So our concern is that we have to find a fuel that can serve the entire fleet, is economical, and readily available. That is our bottom line and the principle toward which we have driven the FAA’s PAFI [Piston Aviation Fuels Initiative] and why the fuels being evaluated are high-octane solutions with the potential to be broadly producible.”

Avgas Versus Auto Gas

How compatible are avgas and auto gasoline? The answer is in their respective octane rating systems.

Octane in avgas is expressed as the “motor octane number” (MON), while octane in auto gas is expressed by the “anti-knock index” (AKI). Now, aviation fuel with a 100 MON, if converted to the AKI scale, would come out to about 108 octane. The key is that aviation octane allows much higher engine performance and quality (thanks to higher standards in refining, storage and frequent testing) than auto fuel. B&CA