Avgas or Mogas? Our Perspective on Operating Solo Engines
The question of the “right” fuel for Solo engines has been the subject of ongoing debate for many years. The reason for this article is a recent recommendation by an aircraft manufacturer suggesting Avgas as the preferred fuel. We would like to provide our technical assessment from the perspective of engine overhaul, maintenance experience, and practical flight operations.
First, we would like to clarify that we make no statement regarding the compatibility of specific fuels with fuel tanks, fuel bladders, fuel lines, seals, or any other components used by individual aircraft manufacturers. We do not evaluate the design or material selection of other manufacturers. Our discussion is limited exclusively to the effects of different fuels on the engine itself.
The Major Disadvantage of Avgas: Lead
Avgas 100LL still contains tetraethyl lead. Although the lead content has been reduced compared to older aviation fuels, lead remains an undesirable component in modern aircraft engines that are approved for unleaded fuels.
Based on our many years of experience with Solo and Rotax engines, virtually every engine that has been operated predominantly on Avgas over an extended period shows significant lead deposits. These deposits form on spark plugs, in combustion chambers, on pistons, and particularly within piston ring grooves. The consequences may include sticking piston rings, poorer combustion characteristics, increased maintenance requirements, and, in the long term, greater engine wear.
We consider this effect particularly critical in the two-stroke engines commonly used in gliding operations, such as Solo and Rotax engines. In a two-stroke engine, the fuel-air mixture first passes through the crankcase, where it also provides lubrication for all crankshaft and connecting rod bearings. The lead contained in Avgas can accumulate in these areas as well. As a result, deposits are not confined to the combustion chamber but also reach the bearing surfaces that are essential to engine longevity.
During engine teardown inspections, we regularly observe lead deposits in the crankcase area as well as on bearings and lubrication surfaces. In our experience, these deposits can adversely affect lubrication conditions and contribute to premature wear and bearing failures. For engines that were specifically designed and approved for operation on unleaded fuel, the continuous use of a leaded fuel therefore appears difficult to justify.
Anyone who regularly overhauls Solo engines and investigates engine failures will notice the same pattern repeatedly: engines operated on Avgas for many years commonly exhibit substantial lead contamination, while comparable engines predominantly operated on Mogas generally show significantly cleaner internal components.
Not Everything Relevant to the Fuel System Is Beneficial for the Engine
Potential issues associated with ethanol-containing fuels are often cited as an argument in favor of Avgas. It is true that ethanol can create challenges in certain older fuel systems or in components that were not designed for ethanol exposure.
However, this does not automatically mean that Avgas is the better choice for the engine.
If a particular fuel system is sensitive to ethanol, this issue should be evaluated from the perspective of the fuel system itself. It should not automatically lead to a general recommendation for leaded fuel in the engine. From an engine standpoint, lead remains an undesirable operating substance.
Long-Term Positive Experience with Unleaded Automotive Fuel
For decades, numerous DG and LS sailplanes have successfully operated on unleaded gasoline in accordance with the applicable flight and maintenance manuals.
Where permitted by both the aircraft and engine manufacturer, Mogas (preferably with 0% ethanol content) should be used. Alternatively, premium unleaded gasoline containing up to 5% ethanol (E5) is also suitable. Practical experience has shown that reliable engine operation is achievable when fuel is stored properly and manufacturer recommendations are followed.
Environmental Considerations Matter Too
Another aspect worth considering is environmental impact. Avgas 100LL remains one of the very few leaded fuels still in widespread use worldwide.
While leaded fuels disappeared from road transportation decades ago, parts of the aviation industry continue to emit lead intentionally. Wherever technically feasible and approved by the manufacturer, the use of unleaded fuels therefore appears both environmentally responsible and consistent with modern standards.
Our Conclusion
Avgas is an approved and readily available fuel, and it certainly has its place where no suitable unleaded alternative is available.
However, for engines that are specifically approved for unleaded fuels, we do not consider Avgas to be the optimal choice. In our view, the well-known issue of lead deposits, the additional maintenance burden, and the environmental impact all argue against making Avgas the default recommendation.
For this reason, we generally recommend the use of a suitable manufacturer-approved unleaded fuel in Solo and Rotax engines. In our opinion, Avgas should remain the alternative fuel option, not the preferred standard fuel.