Sylvie Willis
Aviation gasoline, also known as Avgas, is a specialized fuel used in aircraft with spark-ignited internal combustion engines. The most common formulation of Avgas supplied in the US is 100 Low Lead (100LL), which has a maximum lead concentration of 2.12 gPb/gal. 100LL is a lower lead version of Aviation Gasoline 100 (containing a maximum of 0.56g of lead per litre). While leaded gasoline was phased out in 1996 with the passage of the Clean Air Act, it still fuels around 170,000 piston-engine airplanes and helicopters. This has led to concerns about the environmental impact of aviation and the potential health risks associated with lead exposure, particularly in areas near airports.
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What You'll Learn
- Aviation Gasoline 100LL has a maximum lead concentration of 2.12 gPb/gal
- Tetraethyl lead (TEL) boosts octane rating, preventing engine knocking
- No operationally safe, suitable unleaded replacement has been found
- LL is used by 30% of aircraft in the general aviation fleet
- Leaded aviation fuel is the largest remaining source of lead emissions in the US
Aviation Gasoline 100LL has a maximum lead concentration of 2.12 gPb/gal
Aviation Gasoline 100LL, also known as Avgas, is a type of aviation fuel used in aircraft with spark-ignited internal combustion engines. It has a maximum lead concentration of 2.12 gPb/gal, which is lower than that of Aviation Gasoline 100 (0.56g lead/litre Max). Aviation Gasoline 100LL is commonly used in piston engine-powered private, commercial, and military training aircraft.
The use of leaded aviation fuel has been a topic of concern due to its environmental impact and potential health hazards. Lead emissions from aviation fuel have been identified as "the largest remaining aggregate source of lead emissions to air in the U.S." by the Environmental Protection Agency (EPA). The presence of lead in the fuel can result in areas near airports being exposed to high levels of lead particles, which can have detrimental effects on human health, particularly on children's brains and nervous systems.
Despite the efforts to reduce or eliminate lead in aviation gasoline, it continues to be used due to operational safety concerns associated with lower octane levels in unleaded fuels. Tetraethyl lead (TEL), the additive in leaded aviation fuel, significantly increases the octane rating, preventing engine knocking and potential engine failure. However, the absence of TEL in unleaded alternatives may result in insufficient octane levels for certain engines, especially during high-power settings like takeoff and climb.
The transition to unleaded aviation fuel has also been challenging due to the lack of a widely available 100-octane unleaded replacement. While alternatives such as G100UL and UL94 exist, they may not meet the octane requirements of high-performance engines or require additional certifications and infrastructure changes. Additionally, the aviation industry faces bureaucratic challenges and the need to ensure that any new fuels are thoroughly tested and certified for safety.
Nevertheless, there is ongoing progress towards reducing the reliance on leaded aviation fuel. The Federal Aviation Administration (FAA) has established initiatives to develop safe unleaded replacement aviation gasoline, and companies like ExxonMobil are offering Aviation Gasoline 100LL as a lower lead option for aircraft engines. The phase-out of 100LL is expected, with alternatives such as G100UL and UL94 gaining traction, but it remains a complex process due to the diverse needs of the aviation industry.
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Tetraethyl lead (TEL) boosts octane rating, preventing engine knocking
Avgas (aviation gasoline) is a specialized fuel used in aircraft with spark-ignited internal combustion engines. The most commonly used grades of avgas still contain tetraethyl lead (TEL), a toxic lead-containing additive used to aid in lubrication of the engine, increase octane rating, and prevent engine knocking (spark-knock). TEL has been used as a fuel additive since the 1920s, with its discovery as an effective knock inhibitor in 1921.
TEL boosts the octane rating of aviation fuel, which is crucial in preventing engine knocking. Engine knocking, also known as pinging or pinking, occurs when the fuel-air mixture ignites before the flame front reaches it due to the compression by the piston and slight unevenness in the mixture. This results in additional pressure waves in the cylinder, leading to the typical metallic pinging noise. By increasing the octane rating, TEL raises the temperature and pressure at which auto-ignition occurs, reducing the likelihood of early ignition and preventing engine knocking.
The use of TEL in aviation fuel has been a subject of concern due to its environmental impact and health hazards. Efforts have been made to phase out leaded aviation fuel and find alternative fuels that meet the octane requirements without the use of TEL. However, no operationally safe and suitable replacement has been found to meet the needs of all piston engine aircraft. The phase-out of 100LL aviation fuel, which has a maximum lead concentration of 2.12 gPb/gal, has been called "one of modern GA's most pressing problems" due to the significant number of aircraft that rely on it.
While the automotive industry has transitioned to unleaded gasoline and alternative antiknock agents, the aviation industry continues to face challenges in finding suitable replacements for TEL that can ensure the safe operation of aircraft. The Federal Aviation Administration (FAA) and other organizations have established initiatives to develop and transition to unleaded aviation gasoline, but the process has been slow and complex.
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No operationally safe, suitable unleaded replacement has been found
Avgas (aviation gasoline) is a specialised fuel used in aircraft with spark-ignited internal combustion engines. The most commonly used grades of avgas contain tetraethyl lead, a toxic additive that serves several purposes, including lubrication of the engine, increasing octane rating, and preventing engine knocking. While there have been efforts to reduce or eliminate the use of lead in avgas, it has proven challenging to find a suitable unleaded replacement.
The use of leaded fuels raises operational safety concerns. Tetraethyl lead (TEL) is added to avgas to increase the octane levels, and without it, some engines may experience failure. As a result, TEL has not been banned from avgas, despite the environmental and health concerns associated with lead use.
Aircraft manufacturers, the petroleum industry, and aviation authorities like the Federal Aviation Administration (FAA) have collaborated for over a decade to find alternative fuels that meet the octane requirements of piston engine aircraft without the use of TEL. However, no operationally safe and suitable unleaded replacement has been found to meet the needs of the entire piston engine aircraft fleet. The challenge lies in developing a fuel that can match the performance and safety characteristics of 100LL while also complying with the complex marketplace, technological, and economic demands.
Several initiatives have been established to address this issue, including the Aviation Rulemaking Committee (ARC) involving the EPA and industry stakeholders, as well as the Piston Aviation Fuels Initiative (PAFI), a joint government-industry effort to evaluate and authorise the use of high-octane unleaded fuels. In 2012, the FAA also issued the Unleaded Avgas Transition (UAT) Action Plan, which integrates various activities, including seeking private-sector companies to develop new unleaded aviation gasoline formulations. Despite these efforts, the complexities of developing a suitable unleaded replacement for 100LL have proven to be a significant challenge.
While there are unleaded Avgas fuels available, such as UL91 and UL94, they may not be compatible with all aircraft, and the process of approving supplemental type certificates for new fuels can be slow. Additionally, the demand for 100LL remains high, with approximately 167,000 aircraft in the United States relying on this fuel for safe operation. The transition to an unleaded alternative will require infrastructure adjustments, such as adding new tanks or dedicated fuel trucks, and addressing challenges related to misfuelling and insurance.
As of 2024, a drop-in unleaded replacement for 100LL is expected to be approved in late 2025. The only fuel that has progressed to the final stage of testing is UL100E from LyondellBasell/VP Racing. This fuel has successfully completed a 350-hour engine durability test, and if approved, it could be a significant step towards reducing lead emissions and improving operational safety in the aviation industry.
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100LL is used by 30% of aircraft in the general aviation fleet
100LL, or 100 low lead, is a type of aviation fuel that contains tetraethyl lead (TEL), a toxic additive that increases the octane rating of the fuel and prevents engine knocking. TEL also acts as a lubricant and aids in the lubrication of the engine. While the use of TEL has been phased out in automotive gasoline since the 1970s, it is still present in aviation fuel due to the high octane requirements of aircraft engines.
The general aviation fleet comprises more than 200,000 aircraft worldwide, with 167,000 of those in the United States. Of these, it is estimated that 30% rely on 100LL fuel and cannot use any existing alternatives. This is because these aircraft require high-octane fuel to prevent damaging engine detonation that can result in sudden engine failure. While unleaded options like UL91 and UL94 are available and can be used by some aircraft without modifications, they are not suitable for all aircraft, particularly those with higher octane requirements.
The aviation industry is actively working towards a transition to unleaded fuel, with companies like GAMI developing unleaded 100-octane fuel. However, finding a suitable replacement that meets the octane requirements of all aircraft without the use of TEL has proven challenging. The Federal Aviation Administration (FAA) has established initiatives to develop safe unleaded replacement aviation gasoline, including the Aviation Rulemaking Committee (ARC) and the Piston Aviation Fuels Initiative (PAFI).
The phase-out of 100LL is considered one of the most pressing issues in general aviation, as it threatens the freedom to fly and the economic impact of the industry. Additionally, the environmental impact of leaded aviation fuel is expected to be a significant concern in the coming years. While progress is being made, the transition to unleaded fuel must be done safely and carefully to ensure the continued operation of the affected 30% of aircraft.
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Leaded aviation fuel is the largest remaining source of lead emissions in the US
Leaded aviation fuel, also known as avgas, is used in aircraft with spark-ignited internal combustion engines. It is a specialized fuel that is added to piston engines to boost their octane levels and prevent engine knocking. Avgas is the only remaining transportation fuel in the United States that contains tetraethyl lead (TEL), a toxic additive. While the automotive industry once accounted for the majority of airborne lead, it has since phased out the use of TEL, leaving aviation as the largest remaining source of lead emissions in the US.
TEL is an organic compound that, in small quantities, boosts octane levels in aviation fuel. The use of TEL in automobile gas was largely phased out by 1986, resulting in significant reductions in lead emissions from cars. However, TEL has not been banned from avgas because no operationally safe alternative has been found that meets the needs of all piston engine aircraft. Aircraft manufacturers, the petroleum industry, and the Federal Aviation Administration (FAA) have been working for over a decade to find a suitable replacement.
The FAA has established initiatives to develop an unleaded replacement for avgas, including the Aviation Rulemaking Committee (ARC) and the Piston Aviation Fuels Initiative (PAFI). In 2012, the FAA also issued the Unleaded Avgas Transition (UAT) Action Plan, which involves private-sector companies that have applied for Supplemental Type Certificates to operate with new, unleaded aviation gasoline formulations. Despite these efforts, the transition to unleaded avgas has been challenging due to the unique requirements of piston engine aircraft.
The most common formulation of avgas supplied in the US is "100 Low Lead" (100LL), which has a maximum lead concentration of 2.12 gPb/gal. Approximately 167,000 aircraft in the US rely on 100LL for safe operation, and it is used by 70% of the aircraft in the general aviation fleet. While there are unleaded Avgas fuels available, the FAA and aircraft manufacturers have been slow to approve their use due to safety concerns and the need to ensure compatibility with existing aircraft engines and airframes.
The continued use of leaded aviation fuel has raised concerns among residents living near airports, as areas surrounding these airports are often inundated with tiny lead particles. Lead has been proven to have detrimental effects on children's brains and nervous systems, and the Environmental Protection Agency (EPA) has initiated actions to reduce lead emissions from aviation sources. However, the process of finding a suitable replacement for leaded aviation fuel that meets safety and performance requirements has been ongoing for many years.
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Frequently asked questions
Lead (tetraethyl lead, TEL) is added to aviation fuel to raise the octane rating significantly, preventing detonation (aka knocking). Without it, these engines could suffer catastrophic failures, especially in high-power settings like takeoff and climb.
Aviation Gasoline 100LL is a lower-lead version of Aviation Gasoline 100 (0.56g lead/litre max). It has a maximum lead concentration of 2.12 gPb/gal.
Unleaded avgas alternatives, like G100UL and UL94, are emerging, but they either lack the octane required for the highest-performance engines or require additional certification and fuel distribution infrastructure.
