Unleaded Avgas Put to the Test
A series of critical tests are set for this summer at FAA’s William J. Hughes Technical Center in Atlantic City. The objective: put a pair of unleaded avgas formulations through their paces, this as the general aviation industry gets set to transition from 100LL (100-octane low-lead).
One-hundred low lead is the last of a breed, beloved by much of GA — but destined, as the final transportation fuel to leach lead into the heavens, to vanish. One-hundred low lead wasn’t banished without a bit of a beef — at least initially.
“At one time, four or five years ago, there may have been panic in the streets. People were literally wearing buttons at Oshkosh and expressing their concerns,” about the switch, says Mike Kraft, senior vice president and general manager of Lycoming Engines. The reaction “got the FAA administrator to kick off the Unleaded AvGas Transition Aviation Rulemaking Committee, which got the ball rolling.”
Thus the Piston Aviation Fuel Initiative. FAA calls PAFI a government and industry partnership aimed at “facilitating the development and deployment of a new unleaded aviation fuel that can be readily available across the GA fleet.”
What’s happening at Hughes is actually Phase 2 testing of a pair of fuels, one developed by Shell, the other by Swift Fuels. They’re the survivors of a winnowing Phase 1 process, a process that attracted 17 initial fuels produced by a half-dozen companies. Seventeen contender fuels became four. FAA says those four were subject to “basic fit-for-purpose and chemical property laboratory evaluations, six rig tests, materials compatibility testing, engine testing, and a literature study which evaluated the chemical components of the fuels” for toxicity and environmental impact.
With the conclusion of Phase 2, FAA Administrator Michael Huerta contends the industry will be “on track to have [100-octane] unleaded aviation gasoline fully evaluated and ready to be authorized for use by the general aviation fleet in 2018.”
“The FAA is doing something that is unprecedented,” says Mark Voss, DER for engines certification and continuing airworthiness at Continental Motors. “They are seeking, at the end of the program, to be able to grant flight approval for the use of the two fuels in the fleet.”
The GA fleet is hardly homogeneous. And that’s the challenge. “It ranges from everything such as … recreational aircraft and engines to very large, high-performance commercial” flying machines and powerplants, says Walter Derosier, vice president of engineering and maintenance for the General Aviation Manufacturers Association. The ultimate aim is “to identify what is the closest we can get to what would be essentially a ‘drop-in’ fuel for the entire fleet” — a propellant that requires no modification of engine or airframe.
The operant phrase is “closest we can get.” “There have been attempts in the past to create a formulation that was just a perfect drop-in for 100 low-lead,” says Continental Motors’ Voss, “After numerous, numerous experiments and samples being produced the conclusion was finally reached that it did not look probable that there would be just the perfect drop-in.
“As a result of that PAFI took the approach of, ‘We know we can’t have perfect, so let’s look at all these fuels that are being offered by the various companies.' And let’s try to get two … then we’ll understand what the plusses and minuses are of each of those fuels.”
Some essentials remain the same, of course. In a written statement to AMT, Swift Fuels says, “all avgas must meet ASTM fuel performance criteria for the specification” — anti-knock, vapor pressure, combustion, volatility, fluidity, stability, and such.
Then there’s performance, not merely of the engine itself, but of airframe/powerplant combinations. Equally critical is mixability with low-lead avgas. “Our ‘all-hydrocarbon’ fuels (in this instance UL102) are 100 percent mixable with 100LL,” without compromising performance says Swift. That’s especially important during the transitional period from low-lead to no-lead formulations.
Phase Two Tests: Getting Down to Business
In Phase 2, FAA says they’ll test Shell and Swift fuels in a total of 15 engines — six of Lycoming manufacture; six of Continental. In addition, two radial engines will be tested using unleaded avgas, as will one other engine, a Rotax 912S. Ten airframes are slated for testing: three built by Piper, three by Cessna, and one each by Robinson, Cirrus, and Beech. There’s even a vintage North American Harvard/T6 set for evaluation. The idea is to cast as wide a net as practicable. Some perspective: approximately 167,000 GA aircraft in the United States today rely on 100-octane low lead.
It’s just this sort of targeted diversity that PAFI was built to handle. UL100 avgas “has to work for the radial engines too,” says Lycoming’s Mike Kraft. “There are a lot of DC-6s and variants running big radials up in the Pacific Northwest still around the world.” Radials are a “Big part of the PAFI program. It’s not just the current, mainstream engines in production today. It’s pretty much every engine that remains certificated and in an airworthy state that we have to be cognizant of.”
GAMA’s Derosier says Phase 1 tests have already pretty much determined that — as far as maintenance practices are concerned — no-lead fuel’s “impacts are somewhat limited to minimal. For the most part, [they] should be transparent for most aircraft.” Seals could be an exception. “More than likely,” says Mark Voss, there will be … "some instances in which some elastomers (O-rings, gaskets, and the like) will have to be changed … We’re also looking very closely at things like fuel tank sealants and bladders.”
Voss says the process is “committed to ferreting out” potential gremlins, such as the effect of fuel densities. With the advent of UL100 fuels “there’s a few percentage points shift in [liquid fuel] density.” Such a shift would have no impact on a fuel system that measures on a volumetric basis. “You’re pumping more fuel with a slightly lower heating value, and everything just balances out perfectly.”
But how about the effect on a carbureted engine, one where you need that differential pressure, where the Venturi effect lifts the fuel out of the flow bowl? “If it’s higher density fuel, we may find that there are some tiny, tiny shifts in the fuel/air ratio that’s delivered to the engine,” says Voss. “It draws fuel up out of the float bowl, and if the fuel is heavier one could potentially expect lower fuel flow rates for a given airflow — which may result in a lean mixture. These are things we need to find out.”
While Lycoming’s Kraft and others contend powerplant de-rating concerns have largely been addressed — I “don’t think there’s going to be any,” he says, he leaves open the possibility of new operational placards or changes to aircraft manuals. “I think what remains to be seen is how these fuels really operate at extremes of temperature,” he says. Whether piloting a Cessna 402 around the tropics, or bulldozing through the sky in a DC-6 in the Yukon, “We’ve got to be considerate of these guys.”
PAFI Phase 2 will address just that sort of issue. There’s no readily available button to push for instant answers in the conversion from leaded to no-leaded propulsion. What there is, is PAFI Phase 2. “It’s an engineering project,” says Mike Kraft, “and we’ve got to keep chipping away at it.”
