Avgas
Unlike motor gasoline, which has been formulated without lead since the 1970s to allow the use of catalytic converters for pollution reduction, the most commonly used grades of avgas still contain tetraethyl lead, a toxic lead-containing additive used to aid in lubrication of the engine, increase octane rating, and prevent engine knocking (premature detonation).
Because tetraethyllead is a toxic additive, the minimum amount needed to bring the fuel to the required octane rating is used; actual concentrations are often lower than the permissible maximum.
High prices have encouraged efforts to convert to diesel engines burning jet fuel, which is more readily available, less expensive, and has advantages for aviation use.
[16][39] Some of the lower-powered (100–150 horsepower or 75–112 kilowatts) aviation engines that were developed in the late 1990s are designed to run on unleaded fuel and on 100LL, an example being the Rotax 912.
[citation needed] Ethanol-treated gasoline is susceptible to phase-separation which is very possible due to the altitude/temperature changes light airplanes undergo in ordinary flight.
[citation needed] Additionally, the phase-separated fuel can leave remaining portions that do not meet octane requirements due to the loss of the ethanol in the water-absorption process.
[citation needed] Most of these applicable aircraft have low-compression engines which were originally certified to run on 80/87 avgas and require only "regular" 87 anti-knock index automotive gasoline.
[citation needed] Some aircraft engines were originally certified using a 91/96 avgas and have STC's available to run "premium" 91 anti-knock index (AKI) automotive gasoline.
The extensive testing process required to obtain an STC for the engine/airframe combination helps ensure that, for those eligible aircraft, 91 AKI fuel provides sufficient detonation margin under normal conditions.
A larger problem stems from the higher and wider range of allowable vapor pressures found in automotive gasoline; this can pose some risk to aviation users if fuel system design considerations are not taken into account.
Only later versions of the airframe with different engine cowling and exhaust arrangements are applicable for the automotive fuel STC, and even then require fuel-system modifications.
[44][45][46] There are three fundamental issues in using unleaded fuels without serious modification of the airframe/engine: In February 2008, Teledyne Continental Motors (TCM) announced that the company is very concerned about future availability of 100LL, and as a result, they would develop a line of diesel engines.
[49][50][51] In November 2008, National Air Transportation Association president Jim Coyne indicated that the environmental impact of aviation is expected to be a big issue over the next few years and will result in the phasing out of 100LL because of its lead content.
[55] In July 2021, the first commercially-produced unleaded avgas, GAMI's G100UL, was approved by the Federal Aviation Administration through a Supplemental Type Certificate.
They have announced a formal application to the FAA to approve the use of UL91 and UL94 in selected engines, stating that "Continental considers 91UL and 94UL fuel as a transitional step in a long-term strategy to reach a more sustainable aviation".
[60] Many common Lycoming engines are certified to run on this particular grade of Avgas,[22] and Cessna has approved the use of this fuel in a large number of their piston fleet.
The firm Airworthy AutoGas tested an ethanol-free 93 anti-knock index (AKI) premium auto gas on a Lycoming O-360-A4M in 2013.
This 94UL meets the avgas specification including vapor pressure but has not been completely tested for detonation qualities in all Continental engines or under all conditions.
[16][60] AKI is the octane rating used to grade all U.S. automotive gasoline (typical values at the pump can include 87, 89, 91, and 93), and also the 93UL fuel from Airworthy AutoGas.
[77] The publication of the SAIB, especially the August 2016 revision, eliminated the need for many of the UL94 STCs being sold by Swift Fuels, as the majority of the aircraft on the STC's Approved Model List are type-certified to use 80-octane or lower avgas.
[82][83][84][85] In 2008, an article by technology writer and aviation enthusiast Robert X. Cringely attracted popular attention to the fuel,[86] as also did a cross-country Swift-Fueled flight by the AOPA's Dave Hirschman.
In February 2023, GAMI began selling supplemental type certificates to allow aircraft owners to use the fuel when it becomes available.
[110] In December 2013, Shell Oil announced that they had developed an unleaded 100 octane fuel and will submit it for FAA testing with certification expected within two to three years.
[113] TEL found in leaded avgas and its combustion products are potent neurotoxins that have been shown in scientific research to interfere with brain development in children.
[118]On November 16, 2007, the environmental group Friends of the Earth formally petitioned the EPA, asking them to regulate leaded avgas.
The new standard requires the 16,000 remaining USA sources of lead, which include lead smelting, airplane fuels, military installations, mining and metal smelting, iron and steel manufacturing, industrial boilers and process heaters, hazardous waste incineration, and production of batteries, to reduce their emissions by October 2011.
[117][118][120] The EPA's own studies have shown that to prevent a measurable decrease in IQ for children deemed most vulnerable, the standard needs to be set much lower, to 0.02 μg/m3.
[121][122] At an EPA public consultation held in June 2008 on the new standards, Andy Cebula, the Aircraft Owners and Pilots Association's executive vice president of government affairs, stated that general aviation plays a valuable role in the USA economy and any changes in lead standards that would change the current composition of avgas would have a "direct impact on the safety of flight and the very future of light aircraft in this country".
They cited that the aviation sector employs more than 1.3 million people in the US and has an economic direct and indirect effect that "exceeds $150 billion annually".
The FAA administrator stated that regulating lead in avgas is an EPA responsibility, resulting in widespread criticism of both organizations for causing confusion and delaying solutions.
