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.Nav Canada: Canada Flight Supplement, page A40. Nav Canada, November 23, 2006 Avgas has an emission coefficient (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 in fuel lines at altitudes up to 22,000 ft.Petroleum prehistoric to petrochemicals, G.A.Purdy 1957, Copp Clark Publishing Company, Vancouver, Toronto, Montreal, p.312 and 342
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.
In Europe, avgas remains the most common piston-engine fuel. 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.
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.
+ Table of aviation fuel grades Grade Colour (Dye) Lead (Pb) content maximum (g/L) Additives Uses Availability 80/87 red + a little blue 0.14 TEL It was used in engines with low compression ratio. Phased out in late 20th century. 82UL purple 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 0.15 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 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 green 1.12 TEL Mostly replaced by 100LL. Southern Australia, Chile 100LL 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. 100R 0 n/a American unleaded fuel developed by Swift Fuels LLC. Received in 2024 for Cessna 172 Skyhawk R and S. UL102 none 0 n/a Blend of 83% mesitylene and 17% isopentane American unleaded fuel developed by Swift Fuels LLC. Limited quantities produced for testing. 115/145 purple 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)
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.
Automotive gasoline
Automotive gasoline—known as mogas or autogas among aviators—that does not contain ethanol may be used in certified aircraft that have a Supplemental Type Certificate (STC) for automotive gasoline, as well as in experimental aircraft and ultralight aircraft. 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 "gasohol" 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 automotive gasoline. Examples include the popular Cessna 172 Skyhawk or Piper Cherokee with the variant of the Lycoming O-320.
Some aircraft engines were originally certified using a 91/96 avgas and have STCs available to run "premium" 91 anti-knock index (AKI) automotive gasoline. Examples include some Cherokees with the Lycoming O-320 or O-360, or the Cessna 152 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 averaged motor vehicle octane rating system as posted on gas station pumps. Sensitivity is roughly 8 to 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 have been designed to use 100 octane fuel and modifications are necessary in order to use lower-octane fuel.
Many general aviation aircraft engines were designed to run on 80/87 octane, roughly the standard (as unleaded fuel only, with the 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.