Pilots have been forewarned of the imminent demise of leaded avgas for many, many years and the blue fuel is still plentiful. So it’s easy to dismiss talk of its looming disappearance now as alarmist rhetoric.
But regulatory and market forces are combining in powerful new ways that may someday force 100LL into extinction. And general aviation companies, including engine and airframe manufacturers, electronics, and petroleum firms, are taking steps to prepare new products designed to allow piston airplanes to keep flying into the future.
“After 20-plus years of research, no silver bullet replacement fuel has yet been found to satisfy the needs of the entire general aviation fleet,” said Rob Hackman, AOPA’s senior director of regulatory affairs. “So a second option that appears more likely would include some sort of engine modification for a portion of the fleet in addition to an unleaded fuel. It remains to be seen whether a bolt-on, FAA-certified, aftermarket solution will become available at a realistic price. AOPA members are the end user, and we’re going to continue to make sure their interests are protected. Whatever the industry agrees upon as a replacement to 100LL, it’s got to be safe and legal for the entire fleet to use and allow as much time as needed for the industry to transition.”
The regulatory threats to aviation fuel currently come from two primary sources: a lawsuit filed by the Friends of the Earth that compels the Environmental Protection Agency to collect more data on possible damage caused by lead emissions; and possible changes to the federal Clean Air Act that would lower the ambient lead level in the atmosphere.
Long-term exposure to high levels of lead has been shown to cause or contribute to a variety of serious medical conditions. But lead levels dropped substantially after cars switched to unleaded fuels in the 1980s, and avgas emissions are still well within allowable levels.
Any regulatory changes that affect the future availability of avgas are likely to take place gradually, however.
The greater, and more immediate, potential dangers to GA fuel supply are economic.
Avgas comes from the cream of the refining crop, and it must be segregated from unleaded fuels throughout its production and transportation processes. Avgas is costly to produce and cumbersome to deliver—and its market share is dwindling.
In the 1960s when many airliners were still propelled by piston engines, leaded avgas accounted for about 10 percent of the total U.S. gasoline supply. Now, avgas accounts for less than 1 percent, and that percentage is shrinking as demand for auto fuel rises.
U.S. refineries produce about 250 million gallons of avgas annually. That may sound like a lot, but it’s less than a single day’s output of auto fuel.
In the 1960s, FBOs commonly sold two or even three kinds of leaded avgas: one dyed red for low-compression engines, another blue for most others, and a super-high-octane purple fuel for the most powerful piston engines.
Now, airport fuel storage facilities are built to accommodate just one kind of fuel for piston aircraft—100LL, the blue kind. Although some airports may still have facilities to handle unleaded auto fuel, it’s unlikely that any new facilities will be built to accommodate more than one avgas.
“Given today’s infrastructure and economics, the industry can’t support two fuels for piston aircraft engines,” said Hackman.
No new refineries have been built in the United States for more than 30 years, and existing ones are operating at or near capacity. Fat profit margins for refineries producing auto fuel, jet fuel, and heating oil make their owners less inclined to go through the contortions necessary to produce leaded avgas.
And since handling leaded fuel is a cumbersome process and the future of avgas is uncertain, fuel distribution firms are increasingly reluctant to bear the expense of replacing or updating aging infrastructure dedicated to the handling and distribution of avgas.
The economics of the piston aircraft fleet come down to two numbers: 70 and 30. Seventy percent of the aircraft engines today could fly just fine on unleaded fuel, and unleaded fuel STCs are currently available for most of them. Cherokees, Skyhawks, Cubs, Champs, and many others use low-compression engines that could most likely make the switch to unleaded fuel with the stroke of a pen. But those airplanes only account for about 30 percent of current avgas sales.
The flip side of the equation is that 30 percent of today’s piston fleet would require significant modifications to switch to a lower-octane, unleaded fuel. And those include high-compression and most turbocharged aircraft such as Cirrus SR22s, Bonanzas, Cessna 350s and 400s, Piper Navajos, and other high-performance airplanes that account for 70 percent of current avgas sales. Such business aircraft fly more hours, consume more fuel per hour, and require high-octane fuel to meet their range, altitude, and payload targets.
A fuel’s octane rating is a measure of its resistance to destructive detonation. The higher the octane, the higher the temperatures a fuel can tolerate, and the result is more power and higher overall aircraft performance.
Adding lead to any fuel is a proven, reliable way to increase octane. And some historians credit the broad availability of leaded, highly refined aviation fuel for boosting allied aircraft performance in World War II. An ample supply of better fuel increased range, altitude, and payload for allied fighters and bombers and provided significant advantages in combat.
Traditionally, lead was also used to lubricate internal engine valves and components and extend engine life. But today most aircraft engines don’t need lead for lubrication—only octane-boosting performance.
Pilots have looked longingly to gas stations for a relatively inexpensive fuel to power their piston airplanes. But car gas has always had limited utility for aircraft use—and its problems, from an aviation perspective, usually outweigh any cost advantage.
For starters, the quality, purity, and octane levels for car gas vary widely. Additives made for certain seasons and geographic areas can cause huge fluctuations in internal vapor pressures and lead to “vapor lock,” a dangerous condition that can completely shut down aircraft engines.
Ethanol is a nearly universal additive in today’s car gas—and it bonds with water that can form ice that clogs fuel lines at low temperatures. Ethanol also harms hoses, gaskets, and some fuel tanks as well as certain aircraft engine accessories. Also, automobile gas doesn’t store well, and fuel that has been sitting for weeks or months can cause catastrophic problems in airplane engines.
Aircraft engine manufacturers and petroleum companies are experimenting with a variety of fuels including 95UL that’s essentially today’s avgas minus the lead. The octane level drops with the loss of lead, and refiners can take some additional steps in the production process to gain some of the octane level back but cannot get back to the levels when lead is used. Early evidence suggests the vast majority of existing engines could burn unleaded aviation fuel safely and reliably with a minimal performance penalty—but most high-compression engines and those with turbocharged piston engines may require sophisticated (and as-yet non-FAA certified nor commercially available) electronic controls and sensors.
Bio fuels offer the tantalizing prospect of meeting existing specifications for 100LL while eliminating lead and reducing the environmental impact of petroleum production and use. Swift Enterprises is testing a synthetic fuel it says will have a higher octane rating and better performance than today’s avgas—and it could be produced domestically. Swift is a small, start-up firm with 11 employees, however, and it’s a long way from being able to produce new fuel in industrial quantities.
“We’re a step above petroleum in that our fuel provides a five-to-15-percent increase in range,” said Todd Burrough, Swift’s chief operating officer. “We want our fuel to be a drop-in replacement for avgas.”
Burrough says Swift has sent its fuel to the FAA and independent labs for testing and verification.
The two dominant piston engine manufacturers, Continental and Lycoming, are pursuing a broad array of strategies for an unleaded future. Both are exploring electronic controls that would allow their current and future engines to run on unleaded fuel, as well as new, purpose-built diesel engines designed exclusively for aircraft.
“There are a number of forces that will influence the future availability of avgas—and we want our customers to have as many options as possible,” said Ian Walsh, Lycoming’s executive vice president and general manager. “At some point in the next 10 years, there has to be an approved, affordable, unleaded avgas. One-hundred low lead is going to be around for a certain period of time, but eventually it’s going to dwindle away—and its price is going to move as a result.”
Walsh said Lycoming will push to ensure any new, unleaded aviation fuel is compatible with existing engines. But the company also has been working for years on a diesel broadly based on its existing engine designs. And sophisticated electronic controls could allow future engines to use a variety of fuels.
“Technological advances have made it plausible to think about aviation engines that could burn a range of fuels,” Walsh said. “The ability to use a range of fuels is a bridge.”
Electronic controls would adjust each engine’s power output depending on the nature of the fuel, and pilots would have to monitor octane levels and engine output to determine aircraft performance.
Mac Little, a spokesman for Continental, said the company is performing a sophisticated series of tests exploring the long-term use of unleaded fuels in its current engines.
“We’re starting with our big-bore, turbocharged engines, which are the most demanding,” he said. “So far, the results have been very intriguing—but it’s going to take a lot more research.”
Continental has explored diesel aircraft engines and came close to certifying and producing one for the GA market in the 1990s, but the company ultimately decided against it.
That may change, however, now that German manufacturer Thielert pioneered the market. Diamond bought large quantities of Thielert engines and Cessna planned to offer them in its ubiquitous 172s before Thielert declared bankruptcy this year after allegations of financial mismanagement.
“Thielert proved that diesel aircraft engines are viable as a global product,” Little said.
There’s a sense of urgency at Continental, he said, about finding the right technical solutions for operating piston engines on unleaded fuel.
“We know we can’t wait,” he said. “We as an industry are going to define a spec for a new unleaded aviation fuel and move forward.”
The engine manufacturers as well as aftermarket firms such as General Aviation Modifications Inc. (GAMI) have spent years developing digital electronic controls designed to enable the existing piston fleet to shift to unleaded fuel. But the challenge of finding a single, reliable, certifiable and cost-effective solution has been immense.
Others have explored a new generation of small, lightweight, turbine engines for general aviation use. But while turbines have the advantages of running on widely available jet fuel and can operate far longer between overhauls, they also carry the disadvantages of high acquisition costs and poor efficiency at the lower altitudes where most non-pressurized GA airplanes fly.
AOPA’s Hackman said the association plans to resist any moves that would reduce safety or require large numbers of costly new engines for existing airplanes.
AOPA has been in close touch with the General Aviation Manufacturers Association (GAMA) as well as government and industry leaders to build consensus and technical specifications for a future unleaded aviation fuel.
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