Lead Is Still King

If not leaded fuel — then what?

May 1, 2002

Is the end near? Will that magic ingredient — tetraethyl lead (TEL) — soon become extinct? A number of companies are preparing for this eventuality — and are spending a lot of time and money developing solutions. AOPA, industry, and governmental groups would like the solution to be transparent, meaning that the entire general aviation fleet would be able to burn the new fuel without any modifications. Unfortunately, at the current time there is no transparent solution. In fact, some of the cures appear to be worse than the disease.

Additive solutions that are being tested are no less likely to raise environmental issues than tetraethyl lead, which is added to today's fuel (100LL) at a rate of 2 milliliters per gallon.

An ethanol-based solution, AGE-85, is intriguing because its advocates claim that it is readily available and will lessen the U.S. dependence on foreign oil.

Another solution suggests that a single unleaded fuel with an octane rating of 91/96 would do the trick. This fuel would be a transparent solution for 70 percent of the airplanes in today's fleet. The other 30 percent of the fleet — airplanes with engines that can't safely operate without 100-octane fuel — would have to install a sophisticated combustion profile sensor that would automatically adjust the engine ignition timing to prevent damage during high-power operations. While this sounds like a Catch-22 solution, it may be the least expensive fleet-wide fix.

There are also experts who tout diesel engines as being the solution since these engines would be able to utilize widely available jet fuel.

Tetraethyl lead

In the early 1920s it was discovered that adding tetraethyl lead to automobile gasoline suppressed the tendency of the fuel to "knock." Normal fuel-air combustion proceeds in an orderly fashion, with the spark plugs introducing ignition that produces a predictable, evenly progressing flame front within the combustion chamber. The resulting rise in pressure pushes the piston down in the cylinder.

Knocking is the uncontrolled explosion of portions of the fuel-air mixture during engine operation. Knock (sometimes called pinging in automobiles) is the little brother of full-fledged detonation, which can cause serious engine damage by creating pressures and temperatures that are beyond the cylinder design limits. TEL is added to avgas to increase the octane number and suppress detonation. And the stuff is amazingly powerful — less than one part per thousand in a gallon of avgas is all it takes to provide detonation protection in today's 100LL fuel.

Beginning in the early 1980s, the Environmental Protection Agency (EPA) began phasing out the use of TEL in auto fuel. So the magic bullet, TEL, is under threat, although aviation has a waiver from the EPA to continue to use fuel containing it.

The EPA claims that it has no plans to cut off access to TEL or tighten the handling requirements, but this possibility is regarded by AOPA, and other industry leaders, as a sharp sword that is suspended by a slender thread over the neck of a critical segment of general aviation.

While this possibility is worrisome, economics seems to be the real concern. In its annual report for the year 2000, Octel — the only company outside of Russia producing TEL — stated that the gross profit from the sale of TEL topped $129 million. A nice number, but the following sentence from the report tells the real story: "The company is seeking to optimize returns over the declining life of TEL, and has and will continue to downsize and restructure its operations consistent with declining demand." The demand has been declining since the 1970s and continues to shrink at a rate of 15 to 20 percent per year. This steady decline in demand is an ominous fact for the segment of general aviation that requires 100-octane fuel.

Also of concern is the fact that oil companies have to take special precautions in the transportation and handling of leaded avgas. These precautions are expensive, and bothersome, especially because of the small market for this fuel.

In light of these facts, a transition away from 100LL would be reasonable and prudent. There's only one problem — TEL-laced fuel works better than any of the fuels that are waiting in the wings to take its place. Not only does TEL provide detonation protection, it also prevents valve and valve seat erosion, especially when putting new cylinders into service. Auto gas STCs from Petersen Aviation and the Experimental Aircraft Association (EAA) recommend using leaded fuel during cylinder break-in operations.

According to Cesar Gonzales, who serves on the study group looking for a 100-octane solution, "Lead is still king."

Power and certification

"The more you stress lead the better it works," says Gonzales, when speaking about lead's ability to suppress detonation. "It tries to catch up with the severity of the stress."

The Coordinating Research Council (CRC) is a group of more than 50 experts from more than 30 companies, representing airframe, engine, and component manufacturers, as well as the FAA, AOPA, universities, and industry trade organizations. The panel has been working on a regular basis to exchange ideas and hopefully come up with a transparent solution to replace 100LL. This group met last December in Miami. Right now solutions that have been proposed are:

  • High-octane replacement fuels from Exxon, British Petroleum, and Texaco.
  • Cessna/Phillips ETBE-based "safety net" fuel.
  • AGE-85, an ethanol/petroleum mix.
  • 91/96 unleaded fuel. High-performance engines would have to install an STCed ignition timing control device to utilize this fuel.
  • Diesel engines.

The solutions

Texaco, Exxon, British Petroleum, and Cessna/Phillips fuel formulations get detonation protection by replacing TEL with other additives. There's only one problem — at least two of the additives used in these fuels have already been banned or restricted for environmental reasons. In addition, these additives don't pack the punch that TEL does when it comes to detonation protection, so they're blended into the fuels at much higher concentrations than TEL. Additives that are being tested are ethers (ETBE), aromatic amines, manganese-containing MMT, and ferrocine-based additives.

The fuel companies have been working on a no-lead solution for more than a decade. In addition to the testing done by the major oil companies, fuels testing research has been increased at the FAA's Research and Testing Center in Atlantic City, New Jersey, because of increased funding from the FAA that was pushed through Congress by AOPA.

The results, according to various experts involved in testing, have proven that the current no-lead replacement fuels work well under some conditions, but haven't yet proven to be effective at combating knocking during all operating conditions. If today's replacement fuels were pushed into service, some engines would have to be protected from destructive detonation by restricting the power output under certain conditions.

Since the additives available right now won't do the job, the petroleum-based advocates are working on increasing their capability to produce a superalkylate.

Alkylate is the primary component of avgas. Blend agents, such as TEL, are added to the alkylate base. The goal of the superalkylate programs is to produce a cleaner, higher octane alkylate. A higher octane alkylate will allow petroleum companies to cut back on the quantity of detonation protection additives. The development and increased production of this superalkylate, which is also seen as the most promising solution to government mandates that require cleaner-burning automotive fuels in the near future, are aviation's best hope for an affordable supply of unleaded 100-octane fuel. Superalkylate-based fuels are presently being tested at the FAA Technical Center in Atlantic City.


AGE-85 is alcohol (ethanol) mixed with petroleum blendstock and a small amount of biodiesel. The FAA has granted supplemental type certificates (STCs) that permit the use of AGE-85 in a number of production airplanes. More information on AGE-85 progress is available online ( www.texasskyways.com).

Proponents of AGE-85 state that their fuel would cost one-third less than the current 100LL. While general aviation's share of the fuels market is tiny (yearly consumption by all the airplanes in Canada and the United States doesn't equal the automotive fuel consumption in the United States for one day), any program that lessens the country's dependence on foreign oil seems doubly promising.

AGE-85's biggest problem, aside from the fact that range would decrease 10 to 20 percent because of alcohol's lower BTU content per gallon, is how to create a distribution system. AGE-85 is a nonpetroleum-based product trying to compete in a market dominated by oil companies.

91/96 unleaded fuel and PRISM

"We are the pioneers in unleaded avgas," says Lars Hjelmberg, owner of Hjelmco Oil. In 1991 Hjelmco Oil Company certified its 91/96 unleaded avgas in Sweden. "Our 91/96 is fully transparent and fully legal." In addition to eliminating TEL, thus calming environmental concerns, Hjelmberg says unleaded fuel extends time between overhauls. "The Swedish equivalent of the FAA limits TBO extensions to 150 percent of the manufacturer's recommended limits. In spite of our short flying season, we have healthy four-cylinder Lycoming engines that we are required to remove when they get 3,000 hours." This fuel, which is very similar to 100LL without the TEL, could be easily produced in the United States.

The 91/96 octane unleaded fuel would be a transparent switchover for 70 percent of the U.S. general aviation fleet. For the other 30 percent of the fleet, General Aviation Modifications Inc. (GAMI), the folks who brought precision-tuned fuel injection nozzles to GA, has been testing a magneto replacement device it calls a pressure-reactive intelligent spark management (PRISM) system. GAMI of Ada, Oklahoma, has a very sophisticated engine test cell on site. The 350-horsepower Lycoming TIO540-J2BD engine from the Piper Chieftain is recognized as the high-powered GA engine with the least knock margin. GAMI has been able to run this engine on a batch of unleaded 91/96-octane fuel shipped to them by Hjelmberg. The trick? Install a PRISM system. This system monitors the amount of combustion pressure and when the pressure peak occurs in each cylinder, it automatically adjusts the spark plug firing time to prevent detonation. The drawback is the initial acquisition cost of a PRISM system.

Hjelmberg estimates that 91/96 unleaded would cost 10 percent less than today's 100LL. Using these numbers, the cost of a PRISM system would be recaptured over time, especially in light of the fact that most people involved think that all of the current replacement fuels would cost more than today's 100LL fuel. But there are still roadblocks to this solution. No one knows how quickly the FAA will certify the PRISM system — or any other alternative, for that matter.

George Braly of GAMI says, "We're the only ones who have run a high-performance engine to rated power with our solution. We're happy about that. But what we think is the real solution to this problem is to make one grade of unleaded fuel. Unleaded 95 octane, which I'm assured could be produced, would provide an extra safety margin." Producing a 95-octane unleaded fuel is a more realistic goal than producing a 100-octane unleaded fuel.

Cessna/Phillips 'safety net' fuel

Cessna Aircraft Company, working with Phillips Oil, has created a safety net fuel that will work if the supply of 100LL is suspended before a suitable alternative is found. Detonation suppression is obtained through adding ethyl tertiary-butyl ether (ETBE). This fuel has drawbacks — aircraft range would drop by 10 to 15 percent, and the fuel has a distinctive odor. In addition, ETBE is related to MTBE, an octane-enhancing additive in auto fuel that has been banned in California because of environmental concerns. Gonzales says, "The hazards of using ETBE are several magnitudes of order less than using MTBE. People don't like the smell, which is harmless, but when faced with using it or walking, they fall in love with the stuff."

This safety net fuel is not a transparent alternative nor is it intended to be. But if 100LL were to disappear tomorrow, this fuel could be used in place of 100LL.

Diesel engines ("Diesel Dawn," November 2001 Pilot and "Future Flight: Horsepower of a Different Color," August 2000 Pilot) are considered by some to be a solution to the 100LL dilemma. But there are some big drawbacks. First, there aren't any diesel engines in production that will generate the 300 to 350 horsepower needed to replace today's high-horsepower avgas-fueled engines. This, coupled with unknown (but generally long) certification times and the high cost of retrofitting these engines to existing airplanes, eliminates them from vying for consideration as a transparent solution to this problem.

A big rock and a steep hill

There is a tremendous amount of work to be done toward a no-lead 100-octane solution. Industry experts who offered opinions all said that TEL works better than anything that's been developed to this point. Oil companies may find a nontoxic additive solution through increased research into alkylation. The FAA Testing Center is hard at work testing formulations of existing fuel components. If it weren't for the fact that TEL is toxic, simple market demand would probably lessen the supply bottleneck by bringing other suppliers to the forefront, but that's unlikely to happen.

Unleaded avgas would eliminate some chronic engine problems such as combustion chamber and valve guide deposits, internal engine corrosion, and lubricating oil contamination. When lead was removed from automobile fuel, the auto industry and auto driving public suffered through a few years as the industry improved the technology necessary to deal with the new fuel. Today, 20 years later, automobile engines are more tractable, last longer, and pollute far less than they once did.

Unlike the auto industry, the aviation industry is severely restricted from making rapid progressive changes, such as engine combustion chamber improvements, sophisticated ignition units, and fuel metering systems, by the costly and cumbersome FAA certification process.

In a number of the proposed solutions, FAA certification issues related to time and cost are huge unknowns. Will the FAA issue speedy approvals for the PRISM system?

It's a certainty that FAA approvals for the new fuels will not be incorporated into airplane type certificates by the original equipment manufacturers. The reason for this is economic — any change in a type certificate would also require changes to the owners manuals and pilot operating handbooks. This action could be construed as a reopening of the 18-year litigation window that was instituted during the General Aviation Revitalization Act (GARA) of 1994. No company is going to reopen that window. So approvals will have to be obtained through the STC process.

One oil company scientist, who requested anonymity, said, "I think anyone would be a fool to put out a fuel without fleet tests." Fleet tests take time and money.

The clock that is counting down on TEL isn't yet ticking very loudly, and that's fortunate because the challenges of finding a suitable replacement fuel, and putting that fuel into widespread use, will involve cooperation between scientists and bureaucrats. We still have some time to come up with a solution. We just don't know yet what the solution will look like, how soon we will need it, or how it will work.

E-mail the author at steve.ells@aopa.org.

More Than a Decade of Fuel Advocacy

In the early 1990s, AOPA was extensively involved in a lobbying effort to prevent the Environmental Protection Agency (EPA) from banning the use of leaded aviation gasoline. With the help and support of major petroleum producers AOPA was successful. However, general aviation was left as the nation's sole user of a leaded motor fuel.

Since the early 1990s, AOPA has taken a proactive role to ensure that general aviation can continue operating in the absence of 100LL avgas. I meet several times a year with representatives of the FAA, EPA, petroleum producers, researchers, manufacturers, and universities to find a replacement for 100LL that requires little or no modification to the existing general aviation piston fleet.

Certifying a solution

Whether the ultimate replacement for 100LL is a total fuel solution or a fuel/equipment solution, the FAA must certify each specific airframe and powerplant combination to use it. In an effort to streamline this eventual certification effort, AOPA is working with the FAA to establish guidelines for creating a fuel specification that will make certification for use of a new unleaded fuel faster and cheaper. When the need arises, AOPA will work with the FAA to make airframe and powerplant certification for use of any new fuel or equipment as encompassing as possible.

The FAA also plays a key role in the research and development of potential unleaded replacements for 100LL. AOPA actively supports this research. AOPA Legislative Affairs staffers annually work with congressional committees to secure much-needed funding for alternative fuel research. In recent years, AOPA's lobbying efforts have resulted in a substantial increase in funding for these critical FAA programs, allowing the FAA to expand testing capacity and test fuel performance in a much broader spectrum of GA piston powerplants.

Alternative fuel research efforts

AOPA continually works with petroleum producers, researchers, and other interested parties to review and develop current and future alternative fuel research programs. This cooperative research effort, conducted under the auspices of the Coordinating Research Council (CRC), shows the most promise for ultimately producing an unleaded replacement for 100LL that requires minimal modification to the existing piston fleet.

Industry fuel standards

FAA regulations require that any fuel used in an aircraft must meet a specific standard that dictates minimum performance and quality criteria. Aircraft and engines are individually certified to operate on fuel that meets this benchmark. AOPA currently participates in petroleum standards development in the American Society of Testing and Materials (ASTM). AOPA recently supported the reinstitution of 91-octane unleaded avgas into the ASTM avgas specification. When the need arises to develop a new specification for a high-octane unleaded replacement fuel, AOPA will be there to ensure that the new fuel meets a standard acceptable for FAA airframe and powerplant certification.

Until all technically and economically viable options are exhausted AOPA will continue to support efforts to develop an unleaded aviation fuel that requires little or no modification to the existing fleet. Development of proprietary candidate fuels and equipment modifications will continue, and such solutions will be available for use should we ultimately fail to find a transparent solution. When the time comes, AOPA will be ready to advocate for a sensible specification and expeditious FAA certification. — Andrew Werking, AOPA Associate Director, Regulatory and Certification Policy