thumb|upright=1.14|An [[Grumman American AA-1|American Aviation AA-1 Yankee being refueled with 100LL avgas]]
Avgas (aviation gasoline, also known as aviation spirit in British English) is an aviation fuel used in aircraft with spark-ignited internal combustion engines. Avgas is distinguished from conventional gasoline (petrol) used in motor vehicles, which is termed mogas (motor gasoline) in an aviation context. 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 (spark-knock). There are ongoing efforts to reduce or eliminate the use of lead in aviation gasoline.
Kerosene-based jet fuel is formulated to suit the requirements of turbine engines which have no octane requirement and operate over a much wider flight envelope than piston engines. Kerosene is also used by most diesel piston engines developed for aviation use, such as those by SMA Engines, Austro Engine, and Thielert.
Properties
The main petroleum component used in blending avgas is alkylate, which is a mixture of various isooctanes. Some refineries also use reformate. All grades of avgas that meet CAN 2–3, 25-M82 have a density of at . (6 lb/U.S. gal is commonly used in America for weight and balance computation.) Density increases to at , and decreases by about 0.1% per increase in temperature.
Avgas has an emission coefficient (or factor) of of CO<sub>2</sub><!-- find replacement link for 18.355 figure --> or about 3.07 units of weight produced per unit weight of fuel used. Avgas is less volatile, with a Reid vapor pressure range of 5.5 to 7 psi, than automotive gasoline, with a range of 8 to 14 psi. A minimum limit ensures adequate volatility for engine starting. The upper limits are related to atmospheric pressure at sea level, 14.7 psi, for motor vehicles and ambient pressure at 22,000 ft, 6.25 psi, for aircraft. The lower avgas volatility reduces the chance of vapor lock in fuel lines at altitudes up to 22,000 ft.
The particular mixtures in use today are the same as when they were first developed in the 1940s, and were used in airline and military aero engines with high levels of supercharging; notably the Rolls-Royce Merlin engine used in the Spitfire and Hurricane fighters, Mosquito fighter-bomber and Lancaster heavy bomber (the Merlin II and later versions required 100-octane fuel), as well as the liquid-cooled Allison V-1710 engine, and air-cooled radial engines from Pratt & Whitney, Wright, and other manufacturers on both sides of the Atlantic. The high octane ratings were traditionally achieved by the addition of tetraethyllead, a highly toxic substance that was phased out of automotive use in most countries in the late 20th century.
Historically, many post-WWII developed, low-powered 4- and 6-cylinder piston aircraft engines were designed to use leaded fuels; an unleaded replacement fuel is being developed and certified for these engines. Some reciprocating-engine aircraft still require leaded fuels, but some do not, and some can burn unleaded gasoline if a special oil additive is used.
Consumption
The annual US usage of avgas was in 2008, and was approximately 0.14% of the motor gasoline consumption. From 1983 through 2008, US usage of avgas declined consistently by approximately each year. As of 2024, the annual US usage of avgas was , most of which contained lead, and 170,000 aircraft in the US used leaded avgas.
Grades
Grades of avgas are identified by two numbers associated with its
Motor Octane Number (MON). The first number indicates the octane rating of the fuel tested to "aviation lean" standards, which is similar to the anti-knock index or "pump rating" given to automotive gasoline in the US. The second number indicates the octane rating of the fuel tested to the "aviation rich" standard, which tries to simulate a supercharged condition with a rich mixture, elevated temperatures, and a high manifold pressure. For example, 100/130 avgas has an octane rating of 100 at the lean settings usually used for cruising and 130 at the rich settings used for take-off and other full-power conditions.
Antiknock agents such as tetraethyl lead (TEL) help to control detonation and provide lubrication. One gram of TEL contains 640.6 milligrams of lead.
{| class="wikitable sortable"
|+ Table of aviation fuel grades
|-
! scope="col" | Grade
! scope="col" | Colour (Dye)
! scope="col" | Lead (Pb) content maximum (g/L)
! scope="col" class="unsortable" | Additives
! scope="col" class="unsortable" | Uses
! scope="col" class="unsortable"| Availability
|-
| 80/87<br />
| red + a little blue
| 0.14
| TEL
| It was used in engines with low compression ratio.
| Phased out in late 20th century.
|-
| 82UL
| purple<br />
| 0
| ASTM D6227; similar to automobile gasoline
|
| , 82UL is not being produced.
|-
| 85UL
| none
| 0
| oxygenate-free
| piston-engine ultralight aircraft. Motor Octane Number min 85. Research Octane Number min 95.
|
|-
| 91/96
| brown
| TEL
| Made particularly for military use.
| phased out in 1960s
|-
| 91/96UL
| none
| 0
| ethanol-free, antioxidant and antistatic additives; ASTM D7547
| In 1991, Hjelmco Oil introduced 91/96UL (also meeting leaded grade 91/98 standard ASTM D910) in Sweden. The fuel can be used in more than 90% of the piston aircraft fleet worldwide.
| In 2010, the European Aviation Safety Agency (EASA) cleared this fuel for aircraft engines whose manufacturer has approved it, based on 20 years of trouble-free operations.
|-
| B91/115
| green<br />
| 1.60
| TEL; see standard
| Formulated for Shvetsov ASh-62 and Ivchenko AI-14 – nine-cylinder, air-cooled, radial engines.
| The Commonwealth of Independent States, produced exclusively by Warter (Poland)
|-
| UL94
| purple
| 0
| n/a
|
| American unleaded fuel developed by Swift Fuels LLC.
|-
| 100/130<br />
| green<br />
| 1.12
| TEL
| Mostly replaced by 100LL.
| Southern Australia, Chile
|-
| 100LL<br />
| blue
| 0.56
| TEL;
| Most commonly used aviation gasoline.
| Common in North America and western Europe
|-
| 100VLL
| blue
| 0.45
| TEL; max
| "Very low lead" substitute for 100LL
|
|-
| G100UL
| green
| 0
| Aromatic compounds such as xylene or mesitylene
| Composed primarily of aviation alkylate.
| American unleaded fuel developed by General Aviation Modifications, Inc. Fleet-wide approved in 2022. Not certified by ASTM.
|-
| UL102
| none
| 0
| n/a
| Blend of 83% mesitylene, 17% isopentane
| American unleaded fuel developed by Swift Fuels LLC. Limited quantities produced for testing.
|-
| 115/145<br />
| purple<br />
| 1.29
| TEL, historically xylidine
| The largest, boost-supercharged radial engines needing this fuel's anti-detonation properties.
| Limited batches are produced for special events such as air races. Reintroduced by Warter Aviation 2015.
|}
100LL (blue)
thumb|right|Taking a fuel sample from an under-wing drain using a GATS Jar fuel sampler. The blue dye indicates that this fuel is 100LL.
100LL (pronounced "one hundred low lead") may contain a maximum of one-half the tetraethyllead allowed in 100/130 (green) avgas.
Some of the lower-powered () 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.
Many general aviation aircraft engines were designed to run on 80/87 octane, roughly the standard (as unleaded fuel only, with the "{R+M}/2" 87 octane rating) for North American automobiles today. Direct conversions to run on automotive fuel are fairly common, by supplemental type certificate (STC). However, the alloys used in aviation engine construction are chosen for their durability and synergistic relationship with the protective features of lead, and engine wear in the valves is a potential problem on automotive gasoline conversions.
Fortunately, significant history of engines converted to mogas has shown that very few engine problems are caused by automotive gasoline. 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. Automotive gasoline can vaporize in fuel lines, causing a vapor lock (a bubble in the line) or fuel pump cavitation, thereby starving the engine of fuel. This does not constitute an insurmountable obstacle, but merely requires examination of the fuel system, ensuring adequate shielding from high temperatures and maintaining sufficient pressure in the fuel lines. This is the main reason why both the specific engine model as well as the aircraft in which it is installed must be supplementally certified for the conversion. A good example of this is the Piper Cherokee with high-compression engines. 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.
Vapor lock typically occurs in fuel systems where a mechanically-driven fuel pump mounted on the engine draws fuel from a tank mounted lower than the pump. The reduced pressure in the line can cause the more volatile components in automotive gasoline to flash into vapor, forming bubbles in the fuel line and interrupting fuel flow. If an electric boost pump is mounted in the fuel tank to push fuel toward the engine, as is common practice in fuel-injected automobiles, the fuel pressure in the lines is maintained above ambient pressure, preventing bubble formation. Likewise, if the fuel tank is mounted above the engine and fuel flows primarily due to gravity, as in a high-wing airplane, vapor lock cannot occur, using either aviation or automotive fuels. Fuel-injected engines in automobiles also usually have a "fuel return" line to send unused fuel back to the tank, which has the benefit of equalizing the fuel's temperature throughout the system, further reducing the chance of vapor lock developing.
In addition to vapor locking potential, automotive gasoline does not have the same quality tracking as aviation gasoline. To help solve this problem, the specification for an aviation fuel known as 82UL was developed as essentially automotive gasoline with additional quality tracking and restrictions on permissible additives. This fuel is not currently in production and no refiners have committed to producing it.
and most are specified by ASTM D910 or other standards.
{| class="wikitable sortable"
|+ Table of aviation fuel dyes
|-
! scope="col" | Dye (nominal colour)
! scope="col" | chemical
|-
|blue
|alkyl derivatives of 1,4-diaminoanthraquinone, like Oil Blue A and Oil Blue 35
|-
|yellow
|p-diethylaminoazobenzene or 1,3-benzenediol, 2,4-bis [(alkylphenyl)azo-]
|-
|red
|alkyl derivatives of azobenzene-4-azo-2-naphthol
|-
|orange
|benzene-azo-2-napthol
|}
Phase-out of leaded aviation gasoline
The 100LL phase-out has been called "one of modern GA's most pressing problems", because 70% of 100LL aviation fuel is used by the 30% of the aircraft in the general aviation fleet that cannot use any of the existing alternatives.
There are three fundamental issues in using unleaded fuels without serious modification of the airframe/engine:
- The fuel must have a high enough octane rating (and meet other specifications) to replace leaded fuels,
- The engine must be certified to use the fuel, and
- The airframe must also be certified to use the fuel.
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.
In a February 2008 interview, TCM president Rhett Ross indicated belief that the aviation industry will be "forced out" of using 100LL in the near future, leaving automotive fuel and jet fuel as the only alternatives. In May 2010, TCM announced that they had licensed development of the SMA SR305 diesel engine.
In November 2008, National Air Transportation Association's 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.
By May 2012, the US Federal Aviation Administration (FAA Unleaded Avgas Transition rulemaking committee) had put together a plan in conjunction with industry to replace leaded avgas with an unleaded alternative within 11 years. Given the progress already made on 100SF and G100UL, the replacement time might be shorter than that 2023 estimate. Each candidate fuel must meet a checklist of 12 fuel specification parameters and 4 distribution and storage parameters. The FAA has requested a maximum of US$60M to fund the administration of the changeover. In July 2014, nine companies and consortiums submitted proposals to the Piston Aviation Fuels Initiative (PAFI) to assess fuels without tetraethyl lead. Phase one testing is performed at the William J. Hughes Technical Center for a FAA approved industry replacement by 2018.
In July 2021, the first commercially-produced unleaded avgas, GAMI's G100UL, was approved by the Federal Aviation Administration through a Supplemental Type Certificate.
Lycoming Engines provides a list of engines and fuels that are compatible with unleaded fuel. However, all of their engines require that an oil additive be used when unleaded fuel is used: "When using the unleaded fuels identified in Table 1, Lycoming oil additive P/N LW-16702, or an equivalent finished product such as Aeroshell 15W-50, must be used."
In , TCM changed its policy. 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".
New unleaded fuel grades
91UL (or UL91)
Hjelmco Oil first introduced unleaded Avgas grades in Europe in 2003, after its success with 80UL. This grade of Avgas is manufactured to meet ASTM D7547. This fuel is also usable in any aircraft in Europe or the United Kingdom where the engine is certified to use it, whether or not the airframe is certified to do so, too.
93UL (Ethanol-free 93AKI automotive gasoline)
The firm Airworthy AutoGas tested an ethanol-free 93 anti-knock index (AKI) premium auto gas on a Lycoming O-360-A4M in 2013. The fuel is certified under Lycoming Service Instruction 1070 and ASTM D4814.
UL94 (formerly 94UL)
Unleaded 94 Motor octane fuel (UL94) is essentially 100LL without the lead.
In March 2009, Teledyne Continental Motors (TCM) announced they had tested a 94UL fuel that might be the best replacement for 100LL. 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. Flight testing has been conducted in an IO-550-B powering a Beechcraft Bonanza and ground testing in Continental O-200, 240, O-470, and O-520 engines. In May 2010, TCM indicated that despite industry skepticism, they are proceeding with 94UL and that certification was expected in mid-2013.
In June 2010, Lycoming Engines indicated their opposition to 94UL. Company general manager Michael Kraft stated that aircraft owners do not realize how much performance would be lost with 94UL and characterized the decision to pursue 94UL as a mistake that could cost the aviation industry billions in lost business. Lycoming believes the industry should be pursuing 100UL instead. The Lycoming position is supported by aircraft type clubs representing owners of aircraft that would be unable to run on lower octane fuel. In June 2010, clubs such as the American Bonanza Society, the Malibu Mirage Owners and Pilots Association, and the Cirrus Owners and Pilots Association collectively formed the Clean 100 Octane Coalition to represent them on this issue and push for unleaded 100 octane avgas.
In November 2015, UL94 was added as a secondary grade of unleaded aviation gasoline to ASTM D7547, which is the specification that governs UL91 unleaded avgas. UL91 is currently being sold in Europe. UL94 meets all of the same specification property limits as 100LL with the exception of a lower motor octane number (94.0 minimum for UL94 vs. 99.6 minimum for 100LL) and a decreased maximum lead content. UL94 is an unleaded fuel, but as with all ASTM International unleaded gasoline specifications, a de minimis amount of unintentionally added lead is permitted.
Since May 2016, UL94, now a product of Swift Fuels, is available for sale at dozens of airports in the United States. Swift Fuels has an agreement for distribution in Europe.
UL94 is not intended to be a full replacement for 100LL, but rather is designed to be a drop-in replacement for aircraft with lower-octane-rated engines, such as those that are approved for operation on Grade 80 avgas (or lower), UL91, or mogas. It is estimated that up to 65% of the fleet of current general aviation piston-engine-powered aircraft can operate on UL94 with no modifications to either the engine or airframe. Some aircraft, however, do require a FAA-approved Supplemental Type Certificate (STC) to be purchased to allow for operation on UL94.
UL94 has a minimum Motor octane number (MON, which is the octane rating employed for grading aviation gasoline) of 94.0. 100LL has a minimum MON of 99.6. In August 2016, the FAA revised SAIB HQ-16-05 to include similar wording regarding the acceptability of using UL94 in aircraft and engines that are approved to operate with avgas that has a minimum Motor octane rating of 80 or lower, including Grade 80/87.
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.
On April 6, 2017, Lycoming Engines published Service Instruction 1070V, which adds UL94 as an approved grade of fuel for dozens of engine models, 60% of which are carbureted engines. Engines with displacements of 235, 320, 360, and 540 cubic inches make up almost 90% of the models approved for UL94. and patented several alternatives for non-alcohol based fuels which can be derived from biomass fermentation. Over the next several years, the company sought to build a pilot plant to produce enough fuel for larger-scale testing and submitted fuel to the FAA for testing.
In 2008, an article by technology writer and aviation enthusiast Robert X. Cringely attracted popular attention to the fuel, as also did a cross-country Swift-Fueled flight by the AOPA's Dave Hirschman. Swift Enterprises' claims that the fuel could eventually be manufactured much more cheaply than 100LL have been debated in the aviation press.
The FAA found Swift Fuel to have a motor octane number of 104.4, 96.3% of the energy per unit of mass, and 113% of the energy per unit of volume as 100LL, and to meet most of the ASTM D910 standard for leaded aviation fuel. Following tests in two Lycoming engines, the FAA concluded it performs better than 100LL in detonation testing and will provide a fuel saving of 8% per unit of volume, though it weighs more than 100LL. GC–FID testing showed the fuel to be made primarily of two components — one about 85% by weight and the other about 14% by weight. Soon afterward, AVweb reported that Continental had begun the process of certifying several of its engines to use the new fuel.
From 2009 through 2011, 100SF was approved as a test fuel by ASTM International, allowing the company to pursue certification testing. satisfactorily tested by the FAA, tested by Purdue University, and approved under ASTM specification D7719 for high-octane Grade UL102, allowing the company to test more economically in non-experimental aircraft.
In 2012, Swift Fuels LLC was formed to bring in oil and gas industry experience, scale up production and bring the fuel to market. By November 2013, the company had built its pilot plant and received approval to produce fuel in it. Its most recent patent, approved in 2013, describes methods by which the fuel can be produced from fermentable biomass.
The FAA scheduled UL102 for 2 years of phase 2 testing in its PAFI initiative beginning in the summer of 2016.
G100UL
In February 2010, General Aviation Modifications Inc. (GAMI) announced that it was in the process of developing a 100LL replacement to be called G100UL ("unleaded"). This fuel is made by blending existing refinery products and yields detonation margins comparable to 100LL. The new fuel is slightly more dense than 100LL, but has a 3.5% higher thermodynamic output. G100UL is compatible with 100LL and can be mixed with it in aircraft tanks for use.
In demonstrations held in July 2010, G100UL performed better than 100LL that just meets the minimum specification and performed as well as average production 100LL.
G100UL was approved by the Federal Aviation Administration by the issuance of a Supplemental Type Certificate at AirVenture in July 2021. The STC was initially only applicable to Lycoming-powered models of the Cessna 172. The company indicated that the retail cost was expected to be 0.60–0.85 US dollars per US gallon higher than 100LL.
In September 2022, in a surprise announcement, the FAA approved an STC for the use of the fuel for all piston-engined aircraft and engine combinations. In February 2023, GAMI began selling supplemental type certificates to allow aircraft owners to use the fuel when it becomes available. In April 2024, GAMI announced that 1 million gallons of G100UL had been produced. Fuel availability in the US was forecast for airports in California, Washington and Oregon by the middle of 2024 and the rest of the country by 2026.
Shell Unleaded 100-Octane Fuel
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. The fuel is alkylate-based with an additive package of aromatics. No information has yet been published with regard to its performance, producibility or price. Industry analysts have indicated that it will likely cost as much as or more than existing 100LL.
UL100E
In 2018, LyondellBasell and VP Racing Fuels, an established motorsport fuel manufacturer, announced its intention to participate in developing an unleaded fuel for piston-powered aircraft. As of December 2024, it has reached Phase 4 of testing, according to the FAA.
Environmental regulation
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. Children in residences or childcare facilities in close proximity to airports with moderate to high piston engine aircraft traffic are at especially high risk of high blood lead levels. The United States Environmental Protection Agency (EPA) has noted that exposure to even very low levels of lead contamination has been conclusively linked to loss of IQ in children's brain function tests, thus providing a high degree of motivation to eliminate lead and its compounds from the environment.