Concept Of Aviation Fuelling - SKengineers

 

AVIATION FUELLING -

An aviation fuel truck -

At some airports, underground fuel pipes allow refuelling without the need for tank trucks. Trucks carry the necessary hoses and pumping equipment, but no fuel.

Aviation fuels are petroleum-based fuels, or petroleum and synthetic fuel blends, used to power aircraft. They have more stringent requirements than fuels used for ground use, such as heating and road transport, and contain additives to enhance or maintain properties important to fuel performance or handling. They are kerosene-based (JP-8 and Jet A-1) for gas turbine-powered aircraft. Piston-engine aircraft use gasoline and those with diesel engines may use jet fuel (kerosene). By 2012 all aircraft operated by the U.S. Air Force had been certified to use a 50-50 blend of kerosene and synthetic fuel derived from coal or natural gas as a way of stabilizing the cost of fuel.

Specific energy is an important criterion in selecting fuel for an aircraft. The much higher energy storage capability of hydrocarbon fuels compared to batteries has so far prevented electric aircraft using electric batteries as the main propulsion energy store becoming viable for even small personal aircraft.

As aviation moves into the renewables era, hydrogen-powered aircraft might enter common use. These are zero-emission for CO2 but not for NOx. Cryogenic hydrogen can be used as a liquid at temperatures below 20 K. Gaseous hydrogen involves pressurized tanks at 250–350 bar.[3] With materials available in the 2020s, the mass of tanks strong enough to withstand this kind of high pressure will greatly outweigh the hydrogen fuel itself, largely negating the weight to energy advantage of hydrogen fuel over hydrocarbon fuels. Hydrogen has a severe volumetric disadvantage relative to hydrocarbon fuels, but future blended wing body aircraft designs might be able to accommodate this extra volume without greatly expanding the wetted area.

Types of aviation fuel -

Conventional aviation fuels -

Jet fuel -

Ground fuelling of a MIG-29 from a URAL tanker (2011).

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

Jet-A powers modern commercial airliners and is a mix of extremely refined kerosene and burns at temperatures at or above 49 °C (120 °F). Kerosene-based fuel has a much higher flash point than gasoline-based fuel, meaning that it requires significantly higher temperature to ignite. It is a high-quality fuel; if it fails the purity and other quality tests for use on jet aircraft, it is sold to ground-based users with less demanding requirements, such as railroads.

Avgas -

Avgas (aviation gasoline) is used by small aircraft, light helicopters and vintage piston-engined aircraft. Its formulation is distinct from the conventional gasoline (UK: petrol, or "aviation spirit" in this context) used in motor vehicles which is commonly called mogas or auto-gas in aviation context. Although it comes in many different grades, its octane rating is higher than that for "regular" motor gasoline.

Emerging aviation fuels -

Biofuels -

Alternatives to conventional fossil-based aviation fuels, new fuels made via the biomass to liquid method (like sustainable aviation fuel) and certain straight vegetable oils can also be used.

Fuels such as sustainable aviation fuel have the advantage that few or no modifications are necessary on the aircraft itself, provided that the fuel characteristics meet specifications for lubricity and density as well as adequately swelling elastomer seals in current aircraft fuel systems. Sustainable aviation fuel and blends of fossil and sustainably-sourced alternative fuels yield lower emissions of particle sand GHGs. They are, however, not being used heavily, because they still face political, technological, and economic barriers, such as currently being more expensive than conventionally produced aviation fuels by a wide margin.

Compressed natural gas and liquified natural gas -

Compressed natural gas (CNG) and liquified natural gas (LNG) are fuel feedstocks that aircraft may use in the future. Studies have been done on the feasibility of using natural gas and include the "SUGAR Freeze" aircraft under NASA's N+4 Advanced Concept Development program (made by Boeing's Subsonic Ultra Green Aircraft Research (SUGAR) team). The Tupolev Tu-155 was an alternative fuel testbed which was fuelled on LNG. The low specific energy of natural gas even in liquid form compared to conventional fuels gives it a distinct disadvantage for flight applications.

Production of aviation fuel -

The production of aviation fuel falls into two categories: fuel suitable for turbine engines and fuel suitable for spark-ignition piston engines. There are international specifications for each.

 Jet fuel is a gas turbine fuel used in propeller and jet aircraft and helicopters. It has a low viscosity at low temperature, has limited ranges of density and calorific value, burns cleanly, and remains chemically stable when heated to high temperature.

Aviation gasoline, often referred to as "avgas" or 100-LL (low-lead), is a highly refined form of gasoline for aircraft, with an emphasis on purity, anti-knock characteristics and minimization of spark plug fouling. Avgas must meet performance guidelines for both the rich mixture condition required for take-off power settings and the leaner mixtures used during cruise to reduce fuel consumption. Aviation fuel can be used as CNG fuel.

Avgas is sold in much lower volume than jet fuel, but to many more individual aircraft operators; whereas jet fuel is sold in high volume to large aircraft operators, such as airlines and military.

Why do Planes use Kerosene?

 Planes use kerosene for five key reasons:

it has a low freezing point

it has a low viscosity

it’s highly flammable

it’s affordable

it's considered an all-round safe choice

Low Freezing Point

One of the primary reasons as to why jets rely on kerosene is due to its low freezing point. Aviation kerosene has a freezing point of -47 °C.

Planes fly at extremely high altitudes, which means they spend a lot of air-time in sub-zero temperatures. As a result, planes need to use fuel with a low freezing point – like kerosene - so the fuel functions properly without solidifying during the flight.

Highly Flammable -

Kerosene is highly flammable, more so than diesel, which gives it the explosive burning qualities required for take-off. In fact, diesel’s lack of flammability would not generate enough initial power to get the plane off the ground, ruling it out as an option.

Gasoline is also highly flammable, but its energy pay back is poor and fuel consumption too rapid, which is more inefficient. This means a plane would have to carry a greater volume of fuel for the same journey duration.

Low Viscosity -

Aviation kerosene is less viscous than gasoline when used during flight, making it the preferred choice for jet crafts. Liquids with a high viscosity are thick, sticky and gluey – this is not an ideal property for jet fuel!

Kerosene maintains a low viscosity during flights thanks to its low freezing point. This means it will keep the plane running as it should and won’t clog up the engine.

Affordable -

Kerosene is much cheaper than gasoline, making it a more affordable option for airlines. Flights are an expensive operation, so its important for airlines to use a cheaper source of fuel without compromising safety.

Safe -

Safety is a crucial factor for all airlines, and aviation kerosene is an extremely safe source of fuel.

Firstly, kerosene has a higher flash point than gasoline, meaning its unlikely to cause unplanned combustion.

Secondly, kerosene has a lower freezing point, so it won't thicken up and clog the engine when the plane is moving through extremely low temperatures.

Thirdly, aviation kerosene has additives to enhance its safety and reduce the risk of unplanned hazards. These additives include anticorrosive agents, anti-static chemicals and de-icing agents.

An aeroplane being refuelled with aviation kerosene.

Is Aviation Kerosene and Jet Fuel the Same Thing?

 Jet Fuel is most commonly referred to as Jet A1, but its technical name is actually Avtur (Aviation Turbine Fuel). Avtur is designed for use in both turbo-jet and turbo-prop aircraft, which basically accounts for all modern planes of any real size. Avtur can rightfully be mixed up with is standard grade kerosene, because that is what it is, and, in the UK at least, this is the grade of fuel that is also used for heating oil in boilers and home-range cooking stoves!

Aviation Fuel Facts -

A “standard” 747 in mid-flight will burn approximately 4 litres of fuel every second which, based on a cruising speed of 565 mph, means consumption of 25 litres per mile (giving a fuel economy range of 0.18 miles per gallon!).

A 747 has the capacity to carry a staggering 220,000 litres and this only gives it a distance range of around 8,500 miles - not enough to get to Australia from London non-stop.

A 747 with a full tank of fuel, adds an incredible 175 tonnes to the plane’s payload.

Total consumption of jet fuel in the UK is an impressive 15bn litres per annum, but an incredible 55% of this volume goes through Heathrow – that’s over 22m litres per day.

All of Heathrow’s jet fuel comes in by pipeline - either direct from refineries (Immingham, Fawley and Stan-low can all pump product directly into Heathrow’s fuel farm) or via import pipelines from the Thames estuary (over 60% of UK jet fuel is now imported).

Once it arrives at Heathrow, the Avtur is stored in the airport tank before being further distributed via its 80 miles of pipelines. This incredible underground “hydrant” system runs the length and breadth of the airport and has multiple fuelling points to allow the airport’s mobile bowsers to connect up and fuel planes across the whole Heathrow complex.

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 (30.81 MJ/litre).

Kerosene type BP Jet A-1, 43.15 MJ/kg, density at 15 °C is 804 kg/m3 (34.69 MJ/litre).

Kerosene type BP Jet TS-1 (for lower temperatures), 43.2 MJ/kg, density at 15 °C is 787 kg/m3 (34.00 MJ/litre).

Density -

In performance calculations, airliner manufacturers use a density of jet fuel around 6.7 lb/USgal or 0.8 kg/l.

Specific cases are -

Bombardier Aerospace: The Challenger Multi-role Aircraft is a special mission variant of the Bombardier Challenger 650 business jet platform. Bombardier bases performance on the use of fuel with an average lower heating value of 18,550 BTU/lb (43.147 MJ/kg) and a density of 0.809 kg/l (6.75 lb/US gal).

Embraer: In its airport planning manual for the E195 uses an adopted fuel density of 0.811 kg/l (6.77 lb/US gal).

Chemical composition -

Aviation fuels consist of blends of over two thousand chemicals, primarily hydrocarbons (paraffins, olefins, napthalenes, and aromatics), additives such as antioxidants and metal deactivators, biocides, static reducers, icing inhibitors, corrosion inhibitors, and impurities. Principal components include n-heptane and isooctane. Like other fuels, aviation fuel for spark-ignited piston engines are described by their octane rating.

Alcohol, alcohol mixtures, and other alternative fuels may be used experimentally, but alcohol is not permitted in any certified aviation fuel specification. In Brazil, the Embraer Ipanema EMB-202A is a version of the Ipanema agricultural aircraft with a modified Lycoming IO-540-K1J5 engine so as to be able to run on ethanol. Other aircraft engines that were modified to run on 100% ethanol were several other types of Lycoming engines (including the Lycoming 235N2C, and Lycoming IO-320and certain Rotax engines.

In use -

Aviation fuel storage tanks at George Bush Intercontinental Airport, Houston, Texas

Aviation fuel generally arrives at the airport via pipeline systems, such as the CEPS. It is then pumped over and dispensed from a tanker or bowser. The fuel is then driven up to parked aircraft and helicopters. Some airports have pumps similar to filling stations to which aircraft must taxi. Some airports have permanent piping to parking areas for large aircraft.

Aviation fuel is transferred to an aircraft via one of two methods: over-wing or underwing.

Over-wing -

Refuelling a HK36-TTC Super Dimona

Over-wing fuelling is used on smaller planes, helicopters, and all piston-engine aircraft. Over-wing fuelling is similar to car fuelling — one or more fuel ports are opened and fuel is pumped in with a conventional pump.

Underwing -

Most widebody aircraft use a double single-point

Underwing fuelling, also called single-point refuelling or pressure refuelling where not dependent on gravity, is used on larger aircraft and for jet fuel exclusively.

For pressure refuelling, a high-pressure hose is attached and fuel is pumped in at 275 kPa (40 psi) and a maximum of 310 kPa (45 psi) for most commercial aircraft. Pressure for military aircraft, especially fighters, ranges up to 415 kPa (60 psi). Air being displaced in the tanks is usually vented overboard through a single vent on the aircraft. Because 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. An early use of pressure refuelling was on the de Havilland Comet and Sud Aviation Caravelle. Larger aircraft allow for two or more attachment points; however, this is still referred to as single-point refuelling, as either attachment point can refuel all of the tanks. Multiple attachments allow for a faster flowrate.

Misfuelling -

Because of the danger of confusing the fuel types, precautions are taken to distinguish between avgas and jet fuel beyond clearly marking all containers, vehicles, and piping. The aperture on fuel tanks of aircraft requiring avgas cannot be greater than 60 millimetres in diameter. Avgas is often dyed and is dispensed from nozzles with a diameter of 40 mm (49 mm in the United States).

Jet fuel is clear to straw-colored, and is dispensed from a special nozzle called a J spout or duckbill that has a rectangular opening larger than 60 mm diagonally, 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.

Safety precautions -

British Airways Airbus A321 being refuelled

Any fuelling operation can be very dangerous, and aviation operations have characteristics which must be accommodated. As an aircraft flies through the air, it can accumulate static electricity. If this is not dissipated before fuelling, an electric arc could occur and ignite fuel vapours. To prevent this, aircraft are electrically bonded to the fuelling apparatus before fuelling begins, and are not disconnected until after fuelling is complete. Some regions require the aircraft and/or fuel truck to be grounded too. Pressure fuelling systems incorporate a dead man's switch to preclude unmonitored operation.

Aviation fuel can cause severe environmental damage; all fuelling vehicles must carry equipment to control fuel spills. Fire extinguishers must be present at any fuelling operation. 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.

Avgas is the only remaining lead-containing transportation fuel. Lead in avgas prevents damaging engine knock, or detonation, that can result in a sudden engine failure.