Leaded fuel is considered one of the greatest environmental failures in modern human history. Adding tetraethyl lead to gasoline reduced knock in internal combustion engines, which was widely considered a good thing. It was only later that the deleterious health effects came into view, by which point there was a massive fleet of lead-dependent automobiles and an industry reluctant to change. Still, the tide turned, and over the last 50 years, unleaded fuel has become the norm for automotive use across the world.And yet, there remains a hold out—a world where engines still burn leaded fuels and spray their noxious fumes across the countryside. In the aviation sector, leaded fuel remains a normal part of everyday operations to this day amidst concerted efforts to eliminate it for good.“Low” LeadLeaded gasoline is a thing of the past in the automotive world, but remains a standard fuel for piston-engined aircraft to this day. Credit: Ahunt, public domainPiston-engined aircraft do not typically run on the same fuels as automobiles. Instead, they burn aviation gasoline, or Avgas, which comes in specific grades and is designed to suit the needs of aircraft engines, by being less volatile and more suitable for high-performance applications.The most common grade is 100LL (low lead), which is used widely across North America and Western Europe. Despite the moniker, the fuel contains 0.56 grams/litre of tetraethyl lead (TEL), somewhat higher than many leaded automotive fuels used in the 20th century. As with ground-based applications, the additive is used to provide a measure of valvetrain protection by offering cooling and preventing microwelds between contacting parts. It also provides an easy increase to the fuel’s effective octane rating. The latter is particularly useful in aviation contexts where engines run under high load conditions for extended periods of time, and where performance is critical.Other grades of aviation fuel are also in regular use in various parts of the world, many of which still contain significant levels of TEL as well. It’s worth noting that turbine-based aviation engines are not relevant to this issue, as they burn kerosene-based fuels which are lead-free.100LL fuel is dyed blue for easy identification on the flight line. It’s one of the most widely used fuels in piston-engined aircraft. Credit: Ahunt, public domainThe basic makeup of aviation gasoline was largely decided by the mid-1940s, a period in which fuels were heavily developed to suit the needs of then-cutting-edge piston military aircraft. At the time, knock resistance was key to enabling supercharged aircraft engines to achieve higher power levels, a point of key military interest during World War II. Tetraethyl lead was an easy way to achieve this, and this requirement also led to development of technologies like water-methanol injection.Unfortunately, burning leaded fuel effectively sprayed significant amounts of lead into the environment. This lead to elevated blood lead levels in the population, causing premature deaths, neurological damage, and negatively impacting development in children. This is perhaps somewhat galling given that the inventor of TEL, Thomas Midgley Jr., himself suffered significant health effects from the compound. Many workers would also die during early efforts to produce industrial amounts of TEL in the 1920s. It’s one of many examples from the 20th century of industrial will prevailing in spite of obvious severe health risks from a dangerous but otherwise useful chemical.Despite early knowledge of the dangers, it took some time for the negative impacts of TEL to become readily apparent on a wide scale. Japan lead the charge with a leaded fuel ban for automotive use in 1986, with other developed countries following suit in years to come. It would take decades for the last domino to fall, with Algeria finally outlawing the fuel in 2021.As per the MSDS, 100LL fuel is not good for humans or the environment. Credit: Shell MSDSHowever, the aviation world has not been so quick to abandon lead. Much of the reasoning behind this comes down to practicality. Aviation piston engines simply require high octane fuel and TEL has proven one of the easiest ways to achieve a high rating. 100LL, for example, has a MON rating of 100, which is quite high compared to even premium gasoline used in automotive applications.Engines designed to run on TEL often rely on the additive to prevent excessive valve wear, too, so running these engines on non-leaded fuels can significantly increase wear. This would be an expensive inconvenience in an automotive application, but when the engine is what’s keeping you in the sky, it’s less desirable to risk a failure by running a cleaner fuel.In 2019, the FAA estimated that there were 167,000 aircraft in the United States that relied on 100LL avgas, and 230,000 worldwide. The agency had asked in 2014 for industry proposals to make a transition towards unleaded fuels for internal combustion applications.However, testing revealed issues with proposed alternatives, and was eventually halted in 2018. The FAA has since provided a draft plan in 2026 that lays out the timeline to phase out leaded aviation fuel for good. The intent is to end the use of 100LL fuel in the United States by 2030, excepting Alaska, which will phase out the fuel in 2032. The intention is to take an incremental approach, giving the industry time to develop and certify unleaded replacement fuels—with G100UL, 100R, and UL100E all candidates for FAA approval.Real-world use of these fuels will then be monitored for compatibility and safety and to determine if further support or changes are required to manage the transition away from 100LL. For now, the timelines are still subject to change, particularly in Alaska, where piston-engined aircraft are particularly vital for transport and logistics are harder to manage. However, it marks a very real commitment to ending the use of leaded aviation for good, at least in the United States. If the FAA does manage to pull off this feat, it should be readily achievable for other countries around the world.Ultimately, leaded aviation fuels aren’t causing the same level of damage to humanity and the environment as leaded automotive fuels, purely by virtue of their more limited use. Still, it’s never ideal to be spraying lead into the environment, and the health risks are always going to be elevated for those near general aviation airports or under flightpaths of piston-engined aircraft. It’s positive that there is a real commitment to end the use of these fuels, but much work remains to be done to end the reign of tetraethyl lead for good.Featured Image: “Tetraethyl Lead” by [David Brodbeck]