Aviation fuel

Aviation fuel

Aviation fuel is a specialized type of petroleum-based fuel used to power aircraft. It is generally of a higher quality than fuels used in less critical applications such as heating or road transport, and often contains additives to reduce the risk of icing or explosion due to high temperatures, amongst other properties.

Most aviation fuels available for aircraft are kinds of petroleum spirit used in engines with spark plugs i.e. piston engines and Wankel rotaries or fuel for jet turbine engines which is also used in diesel aircraft engines. Alcohol, alcohol mixtures and other alternative fuels may be used experimentally but are not generally available.

Avgas is sold in much lower volumes, but to many more individual aircraft, whereas Jet fuel is sold in high volumes to large aircraft operated typically by airlines, military and large corporate aircraft.

The Convention on International Civil Aviation, which came into effect in 1947, exempted air fuels from tax. Australia and the USA oppose a worldwide levy on aviation fuel, but a number of other countries have expressed interest.

Avgas
Avgas is a high-octane fuel used for aircraft and racing cars. The term avgas is a portmanteau for aviation gasoline, as distinguished from mogas (motor gasoline), which is the everyday petroleum spirit used in cars. Avgas is typically used in aircraft that use reciprocating or wankel engines.

Jet fuel
Jet fuel is a clear to straw colored fuel, based on either an unleaded paraffin oil (Jet A-1), or a naphtha-kerosene blend (Jet B). It is similar to diesel fuel, and can be used in either compression ignition engines or turbine engines.

In use
Aviation fuel is often dispensed from a tanker or bowser which is driven up to parked aircraft and helicopters. Some airports have pumps similar to filling stations that aircraft must taxi up to. Some airports also have permanent piping to parking areas for large aircraft.

Regardless of the method, aviation fuel is transferred to an aircraft via one of two methods: overwing and underwing. Overwing fuelling is used on smaller planes, helicopters, and all piston-engine aircraft. Overwing fuelling is similar to car fuelling — one or more fuel ports are opened and fuel is pumped in with a conventional pump. Underwing fuelling, also called single-point, is used on larger aircraft and for jet fuel exclusively. For single-point fuelling, a high-pressure hose is attached and fuel is pumped in at 40 PSI and a max of 45 PSI. Anything higher needs to be stopped for it can cause damage to the wings. Since there is only one attachment point, fuel distribution between tanks is either automated or it is controlled from a control panel at the fuelling point or in the cockpit. As well, a dead man's switch is used to control fuel flow.

Because of the danger of confusing the fuel types, a number of precautions are taken to distinguish between AvGas and Jet Fuel beyond clearly marking all containers, vehicles, and piping. AvGas is treated with either a red, green, or blue dye, and is dispensed from nozzles with a diameter of 40 millimeters (49 millimeters in the USA). The aperture on fuel tanks of piston-engined aircraft cannot be greater than 60 millimetres in diameter. Jet Fuel is clear to straw in colour, and is dispensed from a special nozzle called a "J spout" that has a rectangular opening larger than 60 millimetres in diameter so as not to fit into AvGas ports. However, some jet and turbine aircraft, such as some models of the Astar helicopter, have a fuelling port too small for the J spout and thus require a smaller nozzle to be installed in order to be refuelled efficiently.

Energy content
The net energy content for aviation fuels depends on their composition. Some typical values are:
BP Avgas 80, 44.65 MJ/kg, density at 15 C is 690 kg/m3
Kerosene type BP Jet A-1, 43.15 MJ/kg, density at 15 C is 804 kg/m3
Kerosene type BP Jet TS-1, (for lower temperatures) 43.2 MJ/kg, density at 15 C is 787 kg/m3

Chemical composition
Aviation fuels consist of blends of over a thousand chemicals, primarily Hydrocarbons (paraffins, olefins, naphthenes, and aromatics) as well as additives such as antioxidants and metal deactivators, and impurities. Principal components include n-octane and isooctane. Like other fuels, blends of Aviation fuel used in piston engined aircraft are often described by their Octane rating.

Safety precautions
Any fuelling operation can be very dangerous, and aviation fuelling has a number of unique characteristics which must be accommodated. As an aircraft flies through the air, it can accumulate a charge of static electricity. If this is not dissipated before fuelling, an electric arc can occur which may ignite fuel vapours. To prevent this, aircraft are electrically bonded to the fuelling apparatus before fuelling begins, and are not disconnected until fuelling is complete. Some regions require that the aircraft and/or fuel truck be grounded as well.

Aviation fuel can cause severe environmental damage, and all fuelling vehicles must carry equipment to control fuel spills. In addition, fire extinguishers must be present at any fuelling operation, and airport firefighting forces are specially trained and equipped to handle aviation fuel fires and spills. Aviation fuel must be checked daily and before every flight for contaminants such as water or dirt.

Many airlines now require that safety belts be left unfastened should passengers be aboard when refueling happens.