Unleaded Avgas: The SAF for Pistons?

Not quite. The first thing to understand is that unleaded (UL) avgas is not technically a form of “sustainable aviation fuel”. While unleaded avgas is more environmentally-friendly, SAF has important sourcing and carbon-reduction requirements that unleaded avgas does not meet.

Why can JetA be SAF and not AvGas?

Sustainable Aviation Fuel (SAF) is a “drop-in” fuel that meets all the same technical and safety requirements as fossil-based jet fuel. SAF usually exclusives references JetA but sometimes the distinction is used to call it sustainable aviation turbine fuel (SATF) or sustainable aviation jet fuel (SAJF)).

SAF contains the same hydrocarbons (and thus the same tailpipe emissions) as fossil-based kerosene, but the difference is that the hydrocarbons came from a more sustainable source. This results in a net reduction of emissions when compared to fossil jet fuel on a life cycle basis.

In order to be certified as SAF under CORSIA, the fuel must meet two sustainability criteria (and an additional ten for SAF produced after 2024). This requires the fuel to achieve at least a 10% carbon intensity reduction and to be sourced from sustainable feedstocks without high carbon land impacts.

Once blended, SAF is certified under ATSM D1655, the same standard as conventional jet fuel, meaning it has the same technical and safety characteristics as conventional fuel. Therefore, no modifications are needed for use in existing pipelines, fuel farms, trucks, tanks, or engines. Any engine certified to fly on JetA can use SAF.

Unfortunately, at least for today, the same SAF feedstocks cannot be used for producing avgas or gasoline, only kerosene. So, despite the environmental benefits of unleaded avgas, it is still sourced from petroleum-based sources and would not qualify as “SAF”.

What does unleaded mean? Why is there lead in the first place?

So how is unleaded avgas better if it isn’t sustainable aviation fuel? Unleaded avgas improves on the sustainability of avgas through the removal of lead – a proven local pollutant that can be dangerous. Lead is necessary within avgas in order to prevent detonation – the ‘explosion’ of the fuel within the piston cylinder instead of a smooth burning. This can cause loss of engine performance and potential damage to the engine.

Octane is the measure of the gasoline’s ability to resist that detonation. In higher compression or displacement engines, such as those in high-performance (piston) aircraft, higher octane fuels become more critical to prevent detonation. It is generally safe to operate piston aircraft on a higher than required octane level, so 100LL (100 octane, low-lead) fuel has become the industry standard for avgas.

Aside: Visit General Aviation News’ history of Avgas for a great recap on why 100LL became the industry standard among many different octanes and types of avgas.

Tetraethyl lead (TEL), is an organic compound that contains lead and is very effective at boosting octane. TEL had been banned in automotive gas since 1968 but had not yet been banned from aviation due to the lack of a suitable replacement, until now.

How can we replace lead in avgas?

General Aviation Modifications Inc (GAMI) has developed a new G100UL (100 octane, unleaded) avgas replacement by using mesitylene or other related aromatics as a substitute for the TEL. GAMI’s G100UL fuel enables the same octane-boosting performance of lead, without the harmful pollution of actually using lead.

Swift Technologies is also using mesitylene as a replacement for lead but is planning to source the aromatic from biomass through a proprietary process for their 100SF unleaded avgas. Both are anticipated to be less than $1 more per gallon.

Due to the fact that only lead is being replaced in the fuel, the carbon content and thus intensity of the fuel remains relatively unchanged (3.10kg of CO2 per kg of Avgas). Therefore, the only sustainability improvement of the fuel comes from the removal of lead.

How do we access unleaded avgas?

Today the only opportunity to use an unleaded fuel will require an STC from GAMI, but as of September 2022, was approved for all spark-ignition piston engine aircraft. The first STC approval for GAMI was awarded in July 2021, with 600 additional engines approved in October 2021. The final expansion means that the GAMI STC can be a fleet-wide solution and that we are more likely to see a rollout of G100UL production and distribution to more airports.

It may still take years before there are significant quantities available at local airports, but the push from local communities to remove the sale of leaded fuels (and corresponding threats to close) their local airports will accelerate the demand. The FAA approval of the GAMI STC means there is finally at least one fleet-wide solution, and potentially another to come from Swift Technologies.

Where can I find more information about G100UL?

GAMI has published a FAQ that addresses many concerns and provides more in-depth information on the fuel than covered here. It does not reflect the recent fleet-wide approval but can answer many other questions: G100UL FAQ.

The industry/governmental Eliminate Aviation Gasoline Lead Emissions (EAGLE) Initiative – which has set the goal of a lead-free future for U.S. piston-engine aircraft by the end of 2030 without adversely affecting operations – published additional FAQ guidance here:  EAGLE G100UL FAQ