Aviation Fuel

Aviation fuel is essential for powering aircraft and enabling flight. There are several different types of aviation fuel used in civil and military aviation today. The most common aviation fuels are aviation gasoline (avgas) and jet fuel. Each has different properties and uses. This article provides an in-depth look at the various aviation fuels, their makeup, properties, production, and usage in aircraft engines.

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Avgas

Avgas (aviation gasoline) is used to power piston engine aircraft. These include small planes for private and commercial use as well as some military planes. There are several different grades of avgas available.

Avgas Grades

The main grades of avgas are:

• 100LL (100-octane, low lead) – This is the most common grade of avgas used today. It provides the highest octane rating and is required for high-performance piston aircraft engines. The “LL” refers to the low lead content compared to previous avgas formulations.
• 100 – A higher lead formulation not in widespread use. Provides a marginally higher octane rating than 100LL.
• 82UL (82-octane, unleaded) – A newer unleaded avgas that can be used in some lower-compression piston engines as a replacement for 100LL. Not suitable for all aircraft.
• 80/87 – Lower octane avgas varieties that were phased out in favor of 100LL.

The octane rating is a key factor for determining the anti-knock properties of avgas for use in piston aircraft engines. Higher octane fuels like 100LL allow higher compression ratios and prevent detonation (knocking) in the engine.

Composition

100LL avgas is composed of:

• Tetraethyllead (TEL) – Lead-based antiknock additive, provides octane boost. Makes up ~0.56% of 100LL.
• Toluene – Aromatic hydrocarbon, improves octane rating.
• Xylenes – Aromatic hydrocarbons in the xylene isomers family. Improve octane.
• Ethylbenzene – Another aromatic hydrocarbon.
• Isopentane – Branched-chain alkane hydrocarbon. Improves cold starting.
• n-Pentane – Straight chain alkane hydrocarbon. Improves volatility.
• Aviation alkylates – Branched-chain paraffinic hydrocarbons created from refining light olefins. Improves combustion performance.

The exact composition can vary between suppliers. Components like toluene and xylenes provide high octane levels, while the branched alkane components improve volatility and cold flow properties important for aviation use. Tetraethyllead has been crucial for providing detonation protection for high-compression piston engines, but poses environmental concerns due to lead content.

Production

Avgas is produced by specialized refineries and fuel suppliers that blend various hydrocarbon components together along with tetraethyllead. Strict quality control standards regulated by ASTM International must be met. Avgas production relies on components like alkylates that are not found in standard motor gasoline, so it must be produced separately.

With the phase-out of leaded automotive gasoline, avgas is now the sole remaining transportation fuel to contain TEL. Declining avgas demand has led to restructuring of refinery and distribution networks. Avgas production can also be impacted by the availability of certain specialty components like alkylates.

Usage in Aircraft

The main users of avgas are piston-engine aircraft in the general aviation industry. This includes:

• Private planes
• Charter and sightseeing aircraft
• Crop dusters and aerial applicators
• Flight trainers
• Recreational planes

Many military planes also utilize avgas, such as trainer planes, reconnaissance and transport aircraft. Avgas allows these piston engines to achieve the power and performance necessary for flight.

Higher horsepower engines typically require 100LL, while lower-compression engines can use lower octane grades. Some aircraft even require a minimum octane rating higher than 100LL provides.

The introduction of unleaded avgas grades like 82UL provides an alternative for aircraft able to operate without tetraethyllead. However 100LL remains the predominant choice today due to its universal suitability for all piston engines.

Jet Fuel

Jet fuel powers turbine engine aircraft, including commercial airliners and many military planes. There are several grades of jet fuel, each with specific applications.

Jet Fuel Types

The principle types of jet fuel are:

• Jet A – A civilian kerosene-based jet fuel commonly used on commercial airliners. Has a relatively low freeze point suitable for operations across a wide climate range.
• Jet A-1 – A variant of Jet A with an even lower freeze point for operations in extremely cold climates. Used mostly in North America and Asia.
• Jet B – A naphtha-kerosene blend suitable for extremely cold climates. No longer widely used.
• JP-8 – A military jet fuel similar to Jet A-1 with additional corrosion inhibitors and anti-icing additives. The most common fuel for US military aircraft.
• JP-5 – A high flash point kerosene-based jet fuel used primarily on US Navy aircraft carriers to mitigate fire hazard.
• JP-4 – A historic gasoline-kerosene blend phased out in favor of purer kerosene fuels. Had better cold weather performance but was more volatile.

Jet fuels are composed primarily of various hydrocarbon components made from crude oil distillation. They must meet stringent specifications for fuel properties set by organizations like ASTM International.

Composition

Jet fuels are composed of a range of hydrocarbon compounds in the C9-C16 range. These include:

• Alkanes (paraffins) – Saturated straight and branched chain hydrocarbons. Provide the main energy content.
• Cycloalkanes (naphthenes) – Saturated cyclic hydrocarbons. Improve combustion performance.
• Aromatic hydrocarbons – Unsaturated cyclic hydrocarbons based on the benzene ring. Boost fuel stability.
• Alkylbenzenes – Branched chain aromatic hydrocarbons. Increase energy density.

The exact composition can vary between jet fuel types and suppliers. Additives may also be included such as antioxidants and metal deactivators. Jet fuels have a higher freezing point than gasoline or diesel, so maintaining performance at low temperatures is a key property.

Production

Jet fuel is produced by refining crude oil to obtain the right blend of hydrocarbon components. This involves separation processes like fractional distillation and extraction. Additional downstream processes can improve fuel characteristics through methods like isomerization and alkylation.

Stringent quality control standards regulated by ASTM must be met. Parameters like freezing point, flash point, viscosity, and sulfur content are tightly controlled. Aviation fuel handling procedures must also prevent contamination.

Major jet fuel producers include specialized refineries operated by major oil companies like ExxonMobil, Chevron, and Shell. Airports may also have local jet fuel pumping and storage facilities.

Usage in Aircraft

Jet fuels are used to power turbine jet engines on a huge range of military and civilian aircraft:

• Commercial airliners (Boeing, Airbus, etc.)
• Business jets (Gulfstream, Bombardier, etc.)
• Military fighters and bombers
• Turboprop regional planes
• Helicopters
• Private jets

The grade of jet fuel selected depends on factors like flight altitude, climate, and requirements for fuel anti-icing or flash point. Most commercial flights use Jet A/Jet A-1, while the US military predominantly uses JP-8 fuel. Some aircraft like supersonic fighters may need specialized jet fuel formulations.

Jet fuel is stored in fuel tanks in the wings or fuselage and pumped to the engines on demand. The heat energy from combusting jet fuel creates the high-energy gases that spin the engine turbines and provide thrust. Aviation turbine engines require extremely clean fuel to avoid any contamination or coking.

Alternative Aviation Fuels

With growing interest in sustainability, research is ongoing into alternative drop-in replacement fuels that can supplement or replace conventional aviation gas and jet fuel:

• Biofuels – Sustainable aviation fuels derived from biological sources like plant oils, animal fats, municipal waste, and algae. Still in development but some are approved for use when blended with standard jet fuel.
• Synthetic paraffinic kerosene (SPK) – Jet fuel made from gasification of sources like coal, natural gas, or biomass followed by Fischer-Tropsch synthesis to hydrocarbons. Offers very low sulfur and aromatic content.
• Power-to-liquids fuel – Jet fuel synthesis using carbon dioxide and water as raw materials and renewable energy sources. Still in conceptual phase.
• Liquefied natural gas (LNG) – An alternative aircraft fuel burned directly in modified gas turbine engines. Requires bulkier fuel storage.

The main drivers for these alternatives are to reduce aviation’s carbon footprint and dependence on crude oil. But petroleum-based fuels still dominate commercial aviation due to availability and cost advantages. Sustainable options need more development to be economically viable.

Key Aviation Fuel Properties

Aviation fuels must meet strict specifications for their chemical and physical properties dictated by standards bodies like ASTM International. Some key aviation fuel properties include:

Octane rating – Resistance to knocking from premature ignition. Crucial for defining the performance limits of piston aviation engines using avgas. Higher octane allows higher compression ratios.

Energy density – The amount of energy stored per unit volume. Jet fuels maximize this to extend aircraft range.

Freeze point – The temperature at which fuel starts to solidify. Must be low enough to prevent freezing at high altitudes.

Flash point – Minimum temperature when fuel vapors can ignite. Higher flash points improve safety, especially for fuel storage.

Fluidity – Ability to flow easily, especially at low temperatures. Important for pumpability and engine injection.

Corrosion – Tendency to corrode metal surfaces. Corrosion inhibitors are added to help prevent component damage.

Stability – Resistance to oxidation and breakdown during long-term storage and use. Leads to varnishing issues.

Volatility – Measure of how readily fuel evaporates. A balance is required to allow easy vaporization but prevent excessive volatility.

Sulfur content – Sulfur can lead to acid formation. Jet fuel sulfur levels are strictly limited to prevent engine damage.

Aviation Fuel Handling

Careful standardized procedures are required for the safe transportation, storage and handling of all aviation fuels. Contamination is the biggest concern. Some key aspects include:

• Specialized aviation-rated fuel trucks, rail cars, pipelines and airport fuel farms just for aviation fuel service. Separated from other fuels.
• Extensive filtering down to fine micron levels at all distribution points to catch contaminants.
• Strict quality control testing regimes to verify fuel purity and performance characteristics.
• Segregated storage tanks and plumbing for each fuel grade to prevent mixing.
• Special additive injection systems in airport fuel farms.
• Specific gravity checks to identify any fuel mixing.
• Hydrant pits and sealed-nozzle refueling trucks to avoid exposure during aircraft refueling.
• Personnel training for fueling procedures like bonding and grounding, filtration, documentation, labeling, sampling, and water draining.

Such handling practices are essential to deliver clean and dry fuel with the stringent properties needed for aircraft. Any deviations can lead to major performance or safety consequences.

Current Trends

Some current trends affecting aviation fuel include:

• Shift from leaded avgas – Efforts to transition piston aviation away from leaded 100LL avgas due to environmental concerns. Alternatives like unleaded 82UL face barriers to widespread adoption.
• Rise of biojet fuels – Continued work to develop economically viable and sustainable bio-derived jet fuel sources to reduce emissions. Limited availability so far.
• Improved refining efficiency – Emerging methods like chromatography to more finely tune the hydrocarbon content of jet fuel for optimal energy density and cold flow.
• New additives – Additives to further enhance jet fuel stability, thermal breakdown resistance, coking prevention and low-temperature flow.
• Impact of blended wing aircraft– Potential to achieve significantly higher fuel efficiency, but may require new fuel technology to manage heat distribution in structural fuel tanks.
• Hydrogen aspiration – Conceptual research into using hydrogen fuel cells or combustion in future hybrid or hydrogen aircraft designs to achieve zero emissions.

So while petroleum fuels still dominate aviation, there is much interest in new fuel solutions to enable cleaner and more efficient air transportation. But any changes will need to be balanced with the paramount importance of flight safety.

FAQ

What is the difference between aviation gasoline and jet fuel?

Aviation gasoline (avgas) is used in piston-engine aircraft and is similar in composition to automotive gasoline. Jet fuel is used in turbine jet engines and is a kerosene-type fuel. Avgas provides high octane for knock resistance while jet fuel offers better energy density.

Why was tetraethyllead added to aviation gasoline?

Tetraethyllead (TEL) was added to early aviation gasoline as an antiknock agent to improve octane ratings, allowing higher performance piston engines. Alternative octane boosters have drawbacks that made TEL the preferred choice historically despite toxicity concerns.

How soon are alternatives to leaded avgas expected?

Some options like unleaded 82UL avgas are already usable replacements in certain lower-compression piston engines. But finding an economical drop-in replacement that meets all the performance requirements of 100LL avgas across the entire piston fleet remains a challenge.

What gives jet fuel its freezing point and cold temperature performance?

Jet fuel is refined and blended to contain mostly non-cyclic and branched alkanes that resist freezing. Additives can further enhance cold flow properties. This allows jet engines to function even at the subzero temperatures encountered at cruising altitudes.

Why don’t most passenger airplanes use diesel or turbine engines?

The high power-to-weight ratio, efficiency at altitude, smooth operation and reliability of turbine jet engines makes them overwhelmingly favored for commercial aviation use. Diesel or piston engines cannot provide the performance required.

How much fuel does a large passenger jet consume on a typical flight?

A mid-size jet like a Boeing 737 can burn around 4-5 gallons (15-20 liters) per mile traveled. Longer flights are more efficient. On a 500 mile flight it may consume 5000-10000 gallons of jet fuel in total. Bigger planes like 747s use proportionally more.

How often are aircraft fuel filters changed?

Onboard fuel filters have manufacturer recommended service intervals that depend on hours flown or elapsed time. Small airplane fuel filters may be changed every 50 hours. Airlines typically change jet fuel filters about 2-3 times per year. Contamination issues can dictate more frequent replacement.

What fuel property is most responsible for engine breakdown and coking?

Fuel thermal stability and resistance to oxidation is most critical. Oxidized fuels leave harmful varnish deposits that choke filters and lead to coking on hot engine components. Additives help improve stability. Proper fuel handling and storage limits degradation.

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