Avgas

Additional recommended knowledge Essential Laboratory Skills Guide Daily Visual Balance Check Daily Sensitivity Test Contents 1 Avgas properties and varieties 1.1 Gasoline 2 Avgas compared to other fuels 3 Properties 4 References

Avgas is a high-octane aviation fuel used for aircraft and racing cars. Avgas is a portmanteau for aviation gasoline, as distinguished from mogas (motor gasoline), which is the everyday gasoline used in cars. Avgas is used in aircraft that use piston or Wankel engines; gas turbines can operate on avgas, but typically do not. Turbine and diesel engines are designed to use kerosene-based jet fuel.

Avgas properties and varieties

Avgas has a lower volatility than mogas (i.e. it does not evaporate as quickly), which can be important for high-altitude use and higher temperatures. The particular mixtures in use today are the same as when they were first developed in the 1950s and 1960s, and therefore the high-octane ratings are achieved by the addition of tetra-ethyl lead (TEL), a highly toxic substance that was phased out for car use in most countries in the 1980s. The main petroleum component used in blending avgas is alkylate, which is essentially a mixture of various isooctanes, and some refineries also use some reformate.

Avgas is currently available in several grades with differing maximum lead concentrations. Since TEL is a rather expensive additive, a minimum amount of it is typically added to the fuel to bring it up to the required octane rating so actual concentrations are often lower than the maximum.

Jet fuel is not avgas. It is similar to kerosene and is used in turbine engines. In Europe, environmental and cost considerations have led to increasing numbers of aircraft being fitted with highly fuel-efficient diesel engines; these too run on jet fuel. Civilian aircraft use Jet-A, Jet-A1 or in severely cold climates Jet-B. There are other classification systems for military turbine and diesel fuel. See Jet fuel.

Gasoline

Gasoline used for aviation fuel generally has two numbers associated with its octane rating. Examples of this include the (now almost completely unavailable) 80/87 avgas, and the 100/130 avgas. The first number indicates the octane rating of the fuel tested to "aviation lean" standards, which is similar to the Motor Octane Number (MON) rating given to automotive gasoline. 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.

100LL, spoken as "100 low lead", contains a lead based anti-knock compound but less than the "highly-leaded" 100/130 avgas it effectively replaced. Most piston aircraft engines require 100LL but it is scheduled to be phased out in the United States because of the lead toxicity. An alternative fuel has not yet been developed for these engines. While there are similar engines that burn non-leaded fuels aircraft are often purchased with engines that use 100LL because many airports only have 100LL. 100LL contains a maximum of 2 grams of lead per US gallon, or maximum 0.56 grams/litre and is the most commonly available and used aviation gasoline.

82UL is an unleaded fuel similar to automobile gasoline but without additives. It may be used in aircraft that have a Supplemental Type Certificate for the use of automobile gasoline with an aviation lean octane rating (MON) of 82 or less or an antiknock index of 87 or less. It may not be used in engines that require 100LL. See Octane Rating. The FAA highly recommends installing placards stating the use of 82UL is or is not approved on those airplanes that specify unleaded autogas (mogas) as an approved fuel^[1].

Gasoline (MOGAS) may be used in aircraft that have a Supplemental Type Certificate for automotive gasoline. 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 of this include the popular Cessna 172 or Piper Cherokee with the 150 hp variant of the Lycoming O-320. Some aircraft engines were originally certified using a 91/96 avgas and have STC's available to run "premium" 91 anti-knock index automotive gasoline. Examples of this include some Cherokee's with the 160 hp Lycoming O-320 or 180 hp O-360 or the Cessna 152 with the O-235.

Avgas 80/87 has the lowest lead content at a maximum of 0.5 grams lead per U.S. gallon, and is only used in low compression ratio engines.

Avgas 100/130 is a higher octane grade aviation gasoline, containing a maximum of 4 grams of lead per US gallon, maximum 1.12 grams/litre. 100LL "low lead" was designed to replace avgas 100/130.

In the past other grades were also available, particularly for military use, such as avgas 115/145 and 91/96. Note that the octanes of avgas cannot be directly compared to those of mogas, as a different test engine and method is used to determine the octane. The first (lower) number is the lean mixture rating, the second (higher) number is the rich mixture rating. For mogas, the octane rating is typically expressed in the U.S. as an anti-knock index (known as "pump rating"), which is the average of the octane rating based on the research and motor test method ((R+M)/2).

Fuel dyes aid pilots in identifying the proper fuel in their aircraft. 80/87 is red, 100/130 is green, 115/145 is purple (leading to the U.S. Naval aviation slang term "grape juice" for avgas, standardized use of purple paint on aviation fueling systems shipboard in the U.S. Navy, and the distinctive purple shirts worn by flight deck refueling personnel "grapes" aboard U.S. aircraft carriers) and 100LL is blue, while jet fuel, JET A1, is clear or straw, being undyed. Untaxed diesel fuel for off-road use is also dyed red.

The annual U.S. usage of avgas was 236 million gallons (893 million liters) in 2006.^[2]

Avgas compared to other fuels

Many general aviation aircraft engines were designed to run on 80/87 octane, roughly the standard for automobiles today. Direct conversions to run on automotive fuel are fairly common and applied via the supplemental type certificate (STC) process. However, the alloys used in aviation engine construction are rather outdated, and engine wear in the valves is a potential problem on automotive gasoline conversions. Fortunately, significant history of mogas-converted engines has shown that very few engine problems are actually caused by automotive gasoline. A larger problem stems from the 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), 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 160 hp or 180 hp 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 Cessna high-wing airplane, vapor lock cannot occur, using either aviation or automotive fuels.

In addition to vapor locking potential, automotive gasoline does not have the same quality tracking as aviation gasoline. To help solve this problem, an aviation fuel known as 82UL has recently been introduced. This fuel is essentially automotive gasoline that has additional quality tracking and restrictions on permissible additives.

The main consumers of avgas at present (mid-2000s) are in North America, Australia, Brazil, and Africa (mainly South Africa). Care must be taken by small airplane pilots to select airports with avgas on flight planning. For example, U.S. and Japanese recreational pilots ship and depot avgas before flying into Siberia. Shrinking availability of avgas drives usage of small airplane engines that can use jet fuel.

In Europe, avgas prices are so high that there have been a number of efforts to convert the industry to diesel instead, which is common, inexpensive and has a number of advantages for aviation use. However, avgas remains the most common fuel in Europe as well.

Properties

Avgas has a density of 6.02 lb/US gallon at 15 °C, or 0.72 kg/l, and this density is commonly used for weight and balance computation. Density increases to 6.40 lb/US gallon at -40 °C, and decreases by about 0.5% per 5 °C increase in temperature.^[3]

Avgas has an emission coefficient (or factor) of 18.355 pounds CO₂ per US gallon,^[4][5] or about 3.05 units of weight CO₂ produced per unit weight of fuel used.

References