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Kerosene, used as a fuel for lighting, was also used as a fuel for turbine engines. As the design of these engines was relatively insensitive to fuel properties, kerosene was chosen particularly because of its availability, considering that every drop of petrol was needed for the war effort.

After the Second World War, the U.S. AIR FORCE started to use "wide cut" fuel, which is, essentially, a hydrocarbon mixture including petrol and kerosene. Again, this option was taken due to availability.

However, this product compared to kerosene has a series of disadvantages, such as its high volatility, causing:

Great losses due to evaporation at high altitudes.
Great risk of fire during handling on the ground.

For these reasons, in the 1970s the U.S. Air Force inverted the process, converting from JP4 (wide cut) to JP8 (a kerosene-type fuel). The U.S. Navy began to use the high flashpoint kerosene-type fuel JP5 for its aircraft in the 1950s.

When the commercial jet industry was developed in the 1950s, kerosene was chosen for having the best combination of properties. Jet B (wide cut) is still used in some areas, such as Canada and Alaska, due to its suitability for cold climates, but Jet A and Jet A-1 are predominant in the rest of the world.

In the U.S., Jet A (with a freezing point of -40°C) is used because of its price and availability. In the rest of the world, Jet A-1 is used, as it has a freezing point of -47 °C, which makes it suitable for long international flights, especially on polar routes and during the winter.

Specifications

There are two organisations that take on the role of implementing and maintaning the specifications of commercial jet fuel for aircraft turbines: the ASTM (American Society for Testing and Materials) and the MOD (United Kingdom Ministry of Defence).

The specifications issued by these two organisations are very similar. Other countries have their own specifications for jet fuel, which are very similar or identical to those issued by the ASTM and the MOD. In the CIS and part of Eastern Europe, the Gost standard has been adopted for jet fuel specifications.

The main specifications are:

ASTM D-1655: The Standard Specification for Aviation Turbine Fuels. Including the specifications for the three commercial jet fuels: Jet A, Jet A-1 and Jet B.
Defence Standard 91-91: United Kingdom Ministry of Defence maintains this specification (formally entitled DERD 2494) for Jet A-1.
CGSB-3.22: These are specifications from Canada which cover the Jet B used in parts of Canada and Alaska.
GOST 10227: Russian specifications for the light kerosene-type fuel, TS-1, used in the CIS and part of Eastern Europe.
Joint Checklist: This is a combination of the strictest requirements of the ASTM D-1655 and the DEF STAND 91-91 in one document: Aviation Fuel Quality Requirements for Jointly Operated System.
IATA: International Air Transport Association publishes the document Guidance Material for Aviation Turbine Fuels Specifications. It contains specifications for four types of fuel for aircraft turbines: Jet A; Jet A-1 and TS-1 and the "wide cut" Jet B.

Jet A meets the requirements of the ASTM, Jet A-1 meets those of the Joint Checklist, TS-1 those of the Russian GOST and Jet B those of the Canadian CGSB.

Performance of Jet A-1

The primary function of jet fuel is to provide the aircraft with power, the key factors being its energy content and combustion quality.

Other important performance properties are:

stability
lubricity
fluidity
volatility
non-corrosivity and cleanliness

As well as providing the energy, the fuel is also used as a hydraulic fluid in the engine control systems and as a coolant for certain fuel system components.

Energy content

The turbine generates power by the converting the chemical energy stored in the fuel into a combination of mechanical energy and heat.

This property can be measured from the Heat of Combustion, which is the heat released when a given quantity of fuel is burnt under specific conditions. Its value will depend upon the type of hydrocarbons that make up the fuel and it can be predicted from the density, as this also varies according to the fuel's chemical composition.

Its values can be expressed volumetrically (energy per unit of volume) or gravimetrically (energy per unit of weight).

In general, less dense jet fuels have a lower gravimetric energy content, whilst denser jet fuels have a higher volumetric energy content.

A jet fuel with a high volumetric energy content increases the energy that can be stored in the aircraft's fuel tanks, thus providing a greater flight range.

Combustion characteristics

In an aircraft turbine, small carbonaceous particles are formed early in the combustion process. These particles continue to burn as they pass through the flame and are completely consumed. However, these particles become incandescent under certain pressure and temperature conditions within the turbine's combustion section, causing infrared radiation to be absorbed by the combustor walls. This increases the heat received by heat transfer from the combustion gases, which can lead to premature cracks or engine failures.

If these carbonaceous particles are not completely consumed by the flame, they can collide with the turbine blades and stators, causing erosion. The carbonaceous particles are also responsible for the visible smoke emitted by some turbines. Smoke formation is determined mainly by engine design and operating conditions, although for a given design, fuel compostion can influence the emissions.

Stability

There are factors that can cause the quality of the jet fuel to deteriorate, such as time (storage stability) and exposure to high temperatures in the engine (thermal stability).

Jet fuel instability is caused by certain chemical reactions which involve the oxidation of certain components. These products remain dissolved in the fuel but may attack and shorten the life of elastomers in the fuel system.

In addition, as a result of these chemical reactions, gums and solid particulates can be formed, which may clog fuel filters or even be deposited in fuel pipes, restricting the flow.

Thermal stability is one of the most important jet fuel properties because the fuel serves as a heat exchange medium in the engine and airframe. Engine problems caused by changes in thermal stability only become evident after hundreds or thousands of hours of operation.

For that reason, jet fuel is subjected to tests under severe conditions, in order to be able to see a measurable effect in a reasonable period of time.

Lubricity

Aircraft turbines are designed to work with jet fuels within a certain viscosity range, in which the jet fuel provides adequate hydrodynamic lubrication.

There are certain chemical compounds that form part of the composition of jet fuel which have lubricant properties. These compounds contain nitrogen, sulphur and oxygen.

Fluidity

Physical properties, such as Viscosity and Freezing Point are used to characterise the fluidity of jet fuel.

Viscosity: Jet fuel at high pressure is injected into the combustion section of the turbine through nozzles. There, the liquid fuel is transformed into very small droplets in the form of a spray, which evaporate quickly as they mix with air. The droplet size is influenced by the viscosity of the fuel. If it is too high, the engine can be difficult to relight in flight.

This also forces the fuel pump to work harder to maintain a constant fuel flow rate.

Freezing point: The primary criterion for fuel system performance is pumpability, the ability to move fuel from the fuel tank to the engine, and it depends upon the fluidity of the fuel and the design of the fuel system. Jet fuel generally remains pumpable at 4 to 15 °C below its freezing point.

Volatility

This is the jet fuel's tendency to vaporize and it is characterised by two physical properties: Vapour Pressure and Distillation Profile.

Volatility is important because a fuel must vaporize before it can burn. However, too high a volatility can result in evaporative losses or fuel system vapour lock.

Corrosion

Jet fuel should not corrode any of the materials with which it comes into contact during its distribution and use. Therefore, engine manufacturers and those involved in producing the fuel system conduct strict fuel compatibility tests before approving a material for fuel system use. Jet fuel contains potentially corrosive compounds, such as mercaptans and organic acids, although these are limited by the specifications.

Cleanliness

Fuel cleanliness means the absence of solid particulates and water.

Particulates, such as dirt, rust, etc., can clog fuel filters and increase fuel pump wear.

Water, in addition to not burning, freezes at high altitudes and the resulting ice may impede fuel flow. It may also facilitate the corrosion of some metals and the growth of microorganisms. There are other products that can affect the purity of the fuel, such as surfactants, mixes, anilines, microbes, etc.

Safety properties

Jet fuel can be hazardous if not handled properly, as it is easy to ignite and it burns rapidly. There are two properties related to its handling that are used to determine the characteristics of jet fuel.

Flash point: This is an indication of the risk of ignition associated with the fuel and measures the product's tendency to form an inflammable mixture with air under controlled conditions.

This parameter is used in safety and transport regulations to classify material as inflammable or combustible and to define, in accordance with this, the safety measures that should be considered during its distribution or storage.

Electrical conductivity: When jet fuel moves through a pipe, pump, valve, or fine filter, static electrical charges are generated. The rate at which the static charge dissipates is proportional to the liquid's ability to conduct electricity.

When the charge accumulated in the fuel exceeds the ionization potential of the air above the liquid, it can be discharged as a spark. The energy of the spark can initiate an explosion if the liquid is inflammable and the composition of vapor and air is in the inflammable range.

In order to prevent the accumlation of charge, a static dissipating additive is used.