Aviation Gasoline
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Avgas (aviation gasoline, also known as aviation spirit in the UK) is an
aviation fuel Aviation fuels are petroleum-based fuels, or petroleum and synthetic fuel blends, used to power aircraft. They have more stringent requirements than fuels used for ground use, such as heating and road transport, and contain additives to enhanc ...
used in aircraft with spark-ignited
internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combus ...
s. ''Avgas'' is distinguished from conventional
gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic co ...
(petrol) used in
motor vehicle A motor vehicle, also known as motorized vehicle or automotive vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on Track (rail transport), rails (such as trains or trams) and is used for the transportation of pe ...
s, which is termed ''mogas'' (motor gasoline) in an aviation context. Unlike motor gasoline, which has been formulated since the 1970s to allow the use of
platinum Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver". Platinu ...
-content
catalytic converter A catalytic converter is an exhaust emission control device that converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction. Catalytic converters are usually ...
s for pollution reduction, the most commonly used grades of avgas still contain
tetraethyllead Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb( C2H5)4. It is a fuel additive, first being mixed with gasoline beginning in the 1920s as a patented octane rating booster that al ...
(TEL), a toxic substance used to prevent
engine knocking In spark ignition internal combustion engines, knocking (also knock, detonation, spark knock, pinging or pinking) occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignite ...
(premature detonation). There are ongoing experiments aimed at eventually reducing or eliminating the use of TEL in aviation gasoline.
Kerosene Kerosene, paraffin, or lamp oil is a combustible hydrocarbon liquid which is derived from petroleum. It is widely used as a fuel in aviation as well as households. Its name derives from el, κηρός (''keros'') meaning "wax", and was regi ...
-based
jet fuel Jet fuel or aviation turbine fuel (ATF, also abbreviated avtur) is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is colorless to straw-colored in appearance. The most commonly used fuels for commercial a ...
is formulated to suit the requirements of
turbine A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating e ...
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 SMA Engines (Société de Motorisations Aéronautiques) is a French manufacturer of Diesel engines for light aircraft, located in Bourges. A subsidiary of Safran, SMA engine production is currently limited to one model, the SMA SR305-230, whic ...
,
Austro Engine Austro Engine is an Austrian manufacturer of aircraft engines based at Wiener Neustadt in Lower Austria. History The production site and headquarters was built in 2007 with an investment of EUR 13 Million. The company was founded by Diamond Air ...
, and
Thielert Technify Motors GmbH is an aircraft Diesel engine manufacturer based in Sankt Egidien, Germany. In July 2013, Chinese state-owned Aviation Industry Corporation of China (AVIC) acquired the aircraft Diesel engine designs and manufacturing assets ...
.


Properties

The main
petroleum Petroleum, also known as crude oil, or simply oil, is a naturally occurring yellowish-black liquid mixture of mainly hydrocarbons, and is found in geological formations. The name ''petroleum'' covers both naturally occurring unprocessed crud ...
component used in blending avgas is
alkylate An alkylation unit (alky) is one of the conversion processes used in petroleum refineries. It is used to convert isobutane and low-molecular-weight alkenes (primarily a mixture of propene and butene) into alkylate, a high octane gasoline componen ...
, which is a mixture of various isooctanes. Some refineries also use
reformate Catalytic reforming is a chemical process used to convert petroleum refinery naphthas distilled from crude oil (typically having low octane ratings) into high-octane liquid products called reformates, which are premium blending stocks for high-o ...
. All grades of avgas that meet CAN 2–3, 25-M82 have a
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of at . (6 lb/U.S. gal is commonly used in America for
weight and balance The center of gravity (CG) of an aircraft is the point over which the aircraft would balance. Its position is calculated after supporting the aircraft on at least two sets of weighing scales or load cells and noting the weight shown on each set ...
computation.) Density increases to at , and decreases by about 0.1% per increase in temperature.
Nav Canada Nav Canada (styled as NAV CANADA) is a privately run, not-for-profit corporation that owns and operates Canada's civil air navigation system (ANS). It was established in accordance with the ''Civil Air Navigation Services Commercialization Act' ...
: ''
Canada Flight Supplement The Canada Flight Supplement (CFS) (french: link=no, Supplément de vol Canada) is a joint civil/military publication and is a supplement of the Aeronautical Information Publication (AIP Canada). It is the nation's official airport directory. It ...
'', page A40. Nav Canada, November 23, 2006
Avgas has an
emission coefficient The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a transition from a high energy state to a lower energy state. The photon energy of t ...
(or factor) of of CO2 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 Vapor lock is a problem caused by liquid fuel changing state to gas while still in the fuel delivery system of gasoline-fueled internal combustion engines. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carbur ...
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 In an internal combustion engine, a supercharger compresses the intake gas, forcing more air into the engine in order to produce more power for a given displacement. The current categorisation is that a supercharger is a form of forced induc ...
; notably the
Rolls-Royce Merlin The Rolls-Royce Merlin is a British liquid-cooled V-12 piston aero engine of 27-litres (1,650  cu in) capacity. Rolls-Royce designed the engine and first ran it in 1933 as a private venture. Initially known as the PV-12, it was later ...
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 rating An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating. The higher the octane number, the more compression the fuel can withstand before detonating ...
s were traditionally achieved by the addition of
tetraethyllead Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb( C2H5)4. It is a fuel additive, first being mixed with gasoline beginning in the 1920s as a patented octane rating booster that al ...
, a highly toxic substance that was phased out of automotive use in most countries in the late 20th century. Leaded avgas is currently available in several grades with differing maximum lead concentrations. (Unleaded avgas is also available.) 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. Historically, many post-WWII developed, low-powered 4- and 6-cylinder piston aircraft engines were designed to use leaded fuels; a suitable unleaded replacement fuel has not yet been developed and certified for most of 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. Lycoming provides a list of engines and fuels that are compatible with them. According to their August 2017 chart, a number of their engines 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." Lycoming also notes that the octane rating of the fuel used must also meet the requirements stated in the fuel specification, otherwise engine damage may occur due to detonation. Meanwhile, Teledyne Continental Motors indicates (in document X30548R3 most recently revised in 2008) that leaded avgas is required in their engines: "Current aircraft engines feature valve gear components which are designed for compatibility with the leaded ASTM D910 fuels. In such fuels, the lead acts as a lubricant, coating the contact areas between the valve, guide, and seat. The use of unleaded auto fuels with engines designed for leaded fuels can result in excessive exhaust valve seat wear due to the lack of lead with cylinder performance deteriorating to unacceptable levels in under 10 hours."


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. In
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
, avgas remains the most common piston-engine fuel. However, prices are so high that there have been efforts to convert to
diesel fuel Diesel fuel , also called diesel oil, is any liquid fuel specifically designed for use in a diesel engine, a type of internal combustion engine in which fuel ignition takes place without a spark as a result of compression of the inlet air and t ...
, which is more readily available, less expensive, and has advantages for aviation use.


Grades

Many 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 An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating. The higher the octane number, the more compression the fuel can withstand before detonating. ...
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 agent An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating by raising the temperature and pressure at which auto-ignition occurs. The mixture known as gasoline or petrol, when used in high compr ...
s such as
tetraethyl lead Tetraethyllead (commonly styled tetraethyl lead), abbreviated TEL, is an organolead compound with the formula Pb( C2H5)4. It is a fuel additive, first being mixed with gasoline beginning in the 1920s as a patented octane rating booster that al ...
(TEL) help to control detonation and provide lubrication. One gram of TEL contains 640.6 milligrams of
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
.


100LL (blue)

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 The Rotax 912 is a horizontally-opposed four-cylinder, naturally aspirated, four-stroke aircraft engine with a reduction gearbox. It features liquid-cooled cylinder heads and air-cooled cylinders. Originally equipped with carburetors, late ...
.


Automotive gasoline

Automotive
gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic co ...
 – known as mogas or autogas among aviators – that does not contain ethanol may be used in
certified Certification is the provision by an independent body of written assurance (a certificate) that the product, service or system in question meets specific requirements. It is the formal attestation or confirmation of certain characteristics of a ...
aircraft that have a Supplemental Type Certificate for automotive gasoline, as well as in
experimental aircraft An experimental aircraft is an aircraft intended for testing new aerospace technologies and design concepts. The term ''research aircraft'' or ''testbed aircraft'', by contrast, generally denotes aircraft modified to perform scientific studies, ...
and
ultralight aircraft Ultralight aviation (called microlight aviation in some countries) is the flying of lightweight, 1- or 2-seat fixed-wing aircraft. Some countries differentiate between weight-shift control and conventional three-axis control aircraft with ailer ...
. Some oxygenates other than ethanol are approved, but these STC’s prohibit ethanol-laced gasolines. Ethanol-treated gasoline is susceptible to phase-separation which is very possible due to the altitude/temperature changes light airplanes undergo in ordinary flight. This ethanol-treated fuel can flood the fuel system with water which can cause in-flight engine failure. 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. Further, the ethanol can attack materials in aircraft construction which pre-date “gasahol” fuels. 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 An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating. The higher the octane number, the more compression the fuel can withstand before detonating. ...
automotive gasoline. Examples include the popular
Cessna 172 Skyhawk The Cessna 172 Skyhawk is an American four-seat, single-engine, high wing, fixed-wing aircraft made by the Cessna Aircraft Company.Piper Cherokee The Piper PA-28 Cherokee is a family of two-seat or four-seat light aircraft built by Piper Aircraft and designed for flight training, air taxi and personal use.Plane and Pilot: ''1978 Aircraft Directory'', pages 62–64. Werner & Werner Corp, ...
with the variant of the
Lycoming O-320 The Lycoming O-320 is a large family of naturally aspirated, air-cooled, four-cylinder, direct-drive engines produced by Lycoming Engines. They are commonly used on light aircraft such as the Cessna 172 and Piper Cherokee. Different variants ...
. Some aircraft engines were originally certified using a 91/96 avgas and have STCs available to run "premium" 91
anti-knock index An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating. The higher the octane number, the more compression the fuel can withstand before detonating. ...
(AKI) automotive gasoline. Examples include some Cherokees with the
Lycoming O-320 The Lycoming O-320 is a large family of naturally aspirated, air-cooled, four-cylinder, direct-drive engines produced by Lycoming Engines. They are commonly used on light aircraft such as the Cessna 172 and Piper Cherokee. Different variants ...
or O-360, or the
Cessna 152 The Cessna 152 is an American two-seat, fixed- tricycle-gear, general aviation airplane, used primarily for flight training and personal use. It was based on the earlier Cessna 150 incorporating a number of minor design changes and a slightly ...
with the O-235. The AKI rating of typical automotive fuel might not directly correspond to the 91/96 avgas used to certify engines, as motor vehicle pumps in the US use the so-called "(R + M)/2" averaged motor vehicle octane rating system as posted on gas station pumps. Sensitivity is roughly 8–10 points, meaning that a 91 AKI fuel might have a MON of as low as 86. 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. Automotive gasoline is not a fully viable replacement for avgas in many aircraft, because many high-performance and/or turbocharged airplane engines require 100 octane fuel and modifications are necessary in order to use lower-octane fuel. Many
general aviation General aviation (GA) is defined by the International Civil Aviation Organization (ICAO) as all civil aviation aircraft operations with the exception of commercial air transport or aerial work, which is defined as specialized aviation services ...
aircraft engine An aircraft engine, often referred to as an aero engine, is the power component of an aircraft propulsion system. Most aircraft engines are either piston engines or gas turbines, although a few have been rocket powered and in recent years many ...
s were designed to run on 80/87 octane, roughly the standard (as unleaded fuel only, with the "/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 Vapor lock is a problem caused by liquid fuel changing state to gas while still in the fuel delivery system of gasoline-fueled internal combustion engines. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carbur ...
(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 The Piper PA-28 Cherokee is a family of two-seat or four-seat light aircraft built by Piper Aircraft and designed for flight training, air taxi and personal use.Plane and Pilot: ''1978 Aircraft Directory'', pages 62–64. Werner & Werner Corp, ...
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.


Gasohol

Rotax allows up to 10% ethanol (similar to E10 fuel for cars) in the fuel for
Rotax 912 The Rotax 912 is a horizontally-opposed four-cylinder, naturally aspirated, four-stroke aircraft engine with a reduction gearbox. It features liquid-cooled cylinder heads and air-cooled cylinders. Originally equipped with carburetors, late ...
engines. Light sport aircraft that are specified by the manufacturer to tolerate alcohol in the fuel system can use up to 10% ethanol.


Fuel dyes

Fuel dyes Fuel dyes are dyes added to fuels, as in some countries it is required by law to dye a low-tax fuel to deter its use in applications intended for higher-taxed ones. Untaxed fuels are referred to as "dyed", while taxed ones are called "clear" or "w ...
aid ground crew and pilots in identifying and distinguishing the fuel grades and most are specified by ASTM D910 or other standards. Dyes for the fuel are required in some countries.


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. In February 2008,
Teledyne Continental Motors Continental Aerospace Technologies is an aircraft engine manufacturer located at the Brookley Aeroplex in Mobile, Alabama, United States. It was originally spun off from automobile engine manufacturer Continental Motors Company in 1929 and owne ...
(TCM) announced that the company is very concerned about future availability of 100LL, and as a result, they would develop a line of
diesel engine The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-call ...
s. In a February 2008 interview, TCM president
Rhett Ross Rhett is both a given name and a surname. Notable people with the name include: Given name * Rhett Akins (born 1969), American country singer and songwriter * Rhett Bernstein (born 1987), American soccer player * Rhett Biglands (born 1977), former ...
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 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 unleaded avgas, GAMI's G100UL, was approved by the
Federal Aviation Administration The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic m ...
through a Supplemental Type Certificate.


New unleaded fuel grades


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 Continental Aerospace Technologies is an aircraft engine manufacturer located at the Brookley Aeroplex in Mobile, Alabama, United States. It was originally spun off from automobile engine manufacturer Continental Motors Company in 1929 and owne ...
(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 The Beechcraft Bonanza is an American general aviation aircraft introduced in 1947 by Beech Aircraft Corporation of Wichita, Kansas. The six-seater, single-engined aircraft is still being produced by Beechcraft and has been in continuous prod ...
and ground testing in
Continental O-200 The Continental C90 and O-200 are a family of air-cooled, horizontally opposed, four-cylinder, direct-drive aircraft engines of 201 in³ (3.29 L) displacement, producing between 90 and 100 horsepower (67 and 75 kW).''Federal Aviation Admini ...
, 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 Lycoming Engines is a major American manufacturer of aircraft engines. With a factory in Williamsport, Pennsylvania, Lycoming produces a line of horizontally opposed, air-cooled, four, six and eight-cylinder engines including the only FAA-certi ...
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 club Aircraft type clubs are organizations that provide information and support to a single aircraft type or a group of aircraft types from the same manufacturer or family of aircraft.Canadian Owners and Pilots Association: ''The COPA Guide To Buying ...
s 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 ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
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 The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic m ...
-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 An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating. The higher the octane number, the more compression the fuel can withstand before detonating ...
employed for grading aviation gasoline) of 94.0. 100LL has a minimum MON of 99.6. 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. The minimum AKI of UL94, as sold by Swift Fuels, is 98.0. Concurrent with the addition of UL94 to ASTM D7547, the FAA published Special Airworthiness Information Bulletin (SAIB) HQ-16-05, which states that "UL94 meets the operating limitations or aircraft and engines approved to operate with grade UL91 avgas," meaning that "Grade UL94 avgas that meets specification D7547 is acceptable to use on those aircraft and engines that are approved to operate with ... grade UL91 avgas that meets specification D7547." 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.


UL102 (formerly 100SF Swift Fuel)

Swift Fuels, LLC has attained approval to produce fuel for testing at its pilot plant in Indiana. Composed of approximately 85%
mesitylene Mesitylene or 1,3,5-trimethylbenzene is a derivative of benzene with three methyl substituents positioned symmetrically around the ring. The other two isomeric trimethylbenzenes are 1,2,4-trimethylbenzene (pseudocumene) and 1,2,3-trimethylbenze ...
and 15%
isopentane Isopentane, also called methylbutane or 2-methylbutane, is a branched-chain saturated hydrocarbon (an alkane) with five carbon atoms, with formula or . Isopentane is an extremely volatile and extremely flammable liquid at room temperature and pr ...
, the fuel is reportedly scheduled for extensive testing by the FAA to receive certification under the new ASTM D7719 guideline for unleaded 100LL replacement fuels. The company eventually intends to produce the fuel from renewable biomass feedstocks, and aims to produce something competitive in price with 100LL and currently available alternative fuels. Swift Fuels suggests that the fuel, formerly referred to as 100SF, will be available for "high performance piston-powered aircraft" before 2020. John and Mary-Louise Rusek founded Swift Enterprises in 2001 to develop renewable fuels and hydrogen fuel cells. They began testing "Swift 142" in 2006 and patented several alternatives for non-alcohol based fuels which can be derived from biomass fermentation.The patent lists Mary-Louise Rusek and Jon Ziulkowski as inventors. 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 The Aircraft Owners and Pilots Association (AOPA) is a Frederick, Maryland-based American non-profit political organization that advocates for general aviation. AOPA's membership consists mainly of general aviation pilots in the United States ...
'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 An octane rating, or octane number, is a standard measure of a fuel's ability to withstand compression in an internal combustion engine without detonating. The higher the octane number, the more compression the fuel can withstand before detonating ...
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 A fid is a conical tool traditionally made of wood or bone. It is used to work with rope and canvas in marlinespike seamanship. A fid differs from a marlinspike in material and purposes. A marlinspike is used in working with wire rope, natural ...
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. reporting on Press Release From 2009 through 2011, 100SF was approved as a test fuel by
ASTM International ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
, 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 summer 2016.


G100UL

In February 2010,
General Aviation Modifications Inc. A general officer is an Officer (armed forces), officer of highest military ranks, high rank in the army, armies, and in some nations' air forces, space forces, and marines or naval infantry. In some usages the term "general officer" refers t ...
(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. The production economics of this new fuel have not been confirmed but it is anticipated that it will cost at least as much as 100LL. 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. GAMI's unleaded G100UL was approved by the
Federal Aviation Administration The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic m ...
by the issuance of a Supplemental Type Certificate at
AirVenture EAA AirVenture Oshkosh (formerly the EAA Annual Convention and Fly-In), or just Oshkosh, is an annual air show and gathering of aviation enthusiasts held each summer at Wittman Regional Airport and adjacent Pioneer Airport in Oshkosh, Wisconsin, ...
in July 2021. The STC is initially only applicable to Lycoming-powered models of the
Cessna 172 The Cessna 172 Skyhawk is an American four-seat, single-engine, high wing, fixed-wing aircraft made by the Cessna Aircraft Company.
, but it is intended that aircraft types will be quickly expanded. The company indicated that the retail cost is expected to be 0.60–0.85 US dollars per US gallon higher than 100LL. In 2022, Paul Bertorelli of AVweb reported that the FAA is dragging its feet on broadly certifying G100UL, delaying approval of the fuel for more engines and spending over $80 million on EAGLE to re-start a search for an unleaded fuel when G100UL has been under evaluation for over 10 years. 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.


Shell Unleaded 100-Octane Fuel

In December 2013
Shell Oil Shell plc is a British multinational oil and gas company headquartered in London, England. Shell is a public limited company with a primary listing on the London Stock Exchange (LSE) and secondary listings on Euronext Amsterdam and the New Yor ...
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 in its performance, producibility or price. Industry analysts have indicated that it will likely cost as much as or more than existing 100LL.


Environmental regulation

TEL found in leaded avgas and its combustion products are potent
neurotoxin Neurotoxins are toxins that are destructive to nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insultsSpencer 2000 that can adversely affect function in both developing and mature ner ...
s 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 The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...
(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. On November 16, 2007, the
environmental group The environmental movement (sometimes referred to as the ecology movement), also including conservation and green politics, is a diverse philosophical, social, and political movement for addressing environmental issues. Environmentalists advoc ...
Friends of the Earth Friends of the Earth International (FoEI) is an international network of environmental organizations in 73 countries. The organization was founded in 1969 in San Francisco by David Brower, Donald Aitken and Gary Soucie after Brower's split with ...
formally petitioned the EPA, asking them to regulate leaded avgas. The EPA responded with a notice of petition for rulemaking. The notice of petition stated: The public comment period on this petition closed on March 17, 2008. Under a federal court order to set a new standard by October 15, 2008, the EPA cut the acceptable limits for atmospheric lead from the previous standard of 1.5 µg/m3 to 0.15 µg/m3. This was the first change to the standard since 1978 and represents an order of magnitude reduction over previous levels. 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. 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. The EPA identified avgas as one of the most "significant sources of lead". At an EPA public consultation held in June 2008 on the new standards, Andy Cebula, the
Aircraft Owners and Pilots Association The Aircraft Owners and Pilots Association (AOPA) is a Frederick, Maryland-based American non-profit political organization that advocates for general aviation. AOPA's membership consists mainly of general aviation pilots in the United States ...
's executive vice president of government affairs, stated that
general aviation General aviation (GA) is defined by the International Civil Aviation Organization (ICAO) as all civil aviation aircraft operations with the exception of commercial air transport or aerial work, which is defined as specialized aviation services ...
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". In December 2008,
AOPA The Aircraft Owners and Pilots Association (AOPA) is a Frederick, Maryland-based American non-profit political organization that advocates for general aviation. AOPA's membership consists mainly of general aviation pilots in the United States ...
filed formal comments to the new EPA regulations. AOPA has asked the EPA to account for the cost and the safety issues involved with removing lead from avgas. 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". AOPA interprets the new regulations as not affecting general aviation as they are currently written. Publication in the USA
Federal Register The ''Federal Register'' (FR or sometimes Fed. Reg.) is the official journal of the federal government of the United States that contains government agency rules, proposed rules, and public notices. It is published every weekday, except on feder ...
of an Advance Notice of Proposed Rulemaking by the USA EPA occurred in April 2010. The EPA indicated: "This action will describe the lead inventory related to use of leaded avgas, air quality and exposure information, additional information the Agency is collecting related to the impact of lead emissions from piston-engine aircraft on air quality and will request comments on this information." Despite assertions in the media that leaded avgas will be eliminated in the US by 2017 at the latest date, the EPA confirmed in July 2010 that there is no phase-out date and that setting one would be an FAA responsibility as the EPA has no authority over avgas. 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. In April 2011 at
Sun 'n Fun Sun ’n Fun Aerospace Expo (officially styled SUN 'n FUN) is a nonprofit organization in Lakeland, Florida dedicated to the promotion of aviation education. It is best known for the annual week-long fly-in and airshow at Lakeland Linder Intern ...
, Pete Bunce, head of the
General Aviation Manufacturers Association The General Aviation Manufacturer's Association (GAMA) is the industry trade association representing general aviation (non-military & non-airliner) aircraft manufacturers and related enterprises, chiefly in the United States.General Aviation Manufa ...
(GAMA), and Craig Fuller, president and CEO of the
Aircraft Owners and Pilots Association The Aircraft Owners and Pilots Association (AOPA) is a Frederick, Maryland-based American non-profit political organization that advocates for general aviation. AOPA's membership consists mainly of general aviation pilots in the United States ...
, indicated that they both are confident that leaded avgas will not be eliminated until a suitable replacement is in place. "There is no reason to believe 100 low-lead will become unavailable in the foreseeable future," Fuller stated. Final results from EPA's lead modeling study at the Santa Monica Airport shows off-airport levels below current 150 ng/m3 and possible future 20 ng/m3 levels. Fifteen of 17 airports monitored during a year-long study in the US by the EPA have lead emissions well below the current National Ambient Air Quality Standard (NAAQS) for lead.


Other uses

Avgas is occasionally used in amateur auto racing cars as its octane rating is higher than automotive gasoline thus allowing the engines to run at higher compression ratios.


See also

* Relative CO2 emission from various fuels *
Antiknock agent An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating by raising the temperature and pressure at which auto-ignition occurs. The mixture known as gasoline or petrol, when used in high compr ...
s are used in fuels for very efficient engines run at high pressures and temperatures, such as historic military-grade aviation gasolines.


References


External links


ASTM D910
{{Aircraft piston engine components Aviation fuels