Tech Talks: Introduction to Jet Fuel
Jet fuel is a type of aviation fuel designed for use in aircraft powered by gas-turbine engines. It is clear to straw-colored in appearance. The most commonly used fuels for commercial aviation are Jet A and Jet A-1 which are produced to standardized international specifications. The only other jet fuel commonly used in civilian turbine engine powered aviation is Jet B which is used for its enhanced cold-weather performance.
Jet fuel is a mixture of a large number of different hydrocarbons. The range of their sizes (molecular weights or carbon numbers) is restricted by the requirements for the product; such as, the freeze point or smoke point. Kerosene-type jet fuel (including Jet A and Jet A-1) has a carbon number distribution between about 8 and 16 carbon atoms per molecule; wide-cut or naphtha-type jet fuel (including Jet B) has between about 5 and 15 carbon atoms per molecule.
Differences between Jet A and Jet A-1
Jet A specification fuel has been used in the United States since the 1950s and is only available in the United States (and a few select locations such as Gander Airport in Newfoundland), whereas Jet A-1 is the standard specification fuel used in the rest of the world.
Both Jet A and Jet A-1 have a flash point higher than 38°C (100°F), with an auto-ignition temperature of 210°C (410°F). The primary differences between Jet A and Jet A-1 are the higher freezing point of Jet A (−40°C vs. −47°C for Jet A-1), and the mandatory requirement for the addition of an anti-static additive to Jet A-1. Typical physical properties for Jet A and Jet A-1. Jet A-1 fuel must meet the requirements for a number of different specifications; specifically:
- DEF STAN 91-91 (Jet A-1)
- ASTM specification D1655 (Jet A-1)
- IATA Guidance Material (Kerosene Type)
- NATO Code F-35
DEF STAN 91-91 and the ASTM D 1655 are the specifications most often quoted for Jet A-1, while the specification for Jet A is limited to ASTM D1655 (Jet A).
Typical physical properties for Jet A-1 / Jet A fuel:
|JET A-1||JET A|
|Freeze point||-47ºC (-53ºF)||-40ºC (-40ºF)|
|Density at 15ºC||0.804 KG/L||0.820 KG/L|
|Specific energy||43.15 MJ/KG||43.02 MJ/KG|
Jet B is a fuel in the naphthakerosene region that is used for its enhanced cold-weather performance. However, Jet B's lighter composition makes it more dangerous to handle. For this reason it is rarely used, except in very cold climates. It is a blend of approximately 30% kerosene and 70% gasoline and is known as wide-cut fuel. It has a very low freezing point of -60° Celsius and a low flash point as well. It is primarily used in US and some military aircraft.
The DEF STAN 91-91 (UK) and ASTM D1655 (international) specifications allow for certain additives to be added to jet fuel, including:
- Antioxidants to prevent gumming, usually based on alkylated phenols, e.g., AO- 30, AO-31, or AO-37;
- Antistatic agents, to dissipate static electricity and prevent sparking; Stadis 450, with dinonylnaphthylsulfonic acid (DINNSA) as the active ingredient, is an example.
- Corrosion inhibitors, e.g., DCI-4A used for civilian and military fuels, and DCI-6A used for military fuels;
- Fuel system icing inhibitor (FSII) agents, e.g., Di- EGME; FSII is often mixed at the point-of-sale so that users with heated fuel lines do not have to pay the extra expense.
- Biocides are to remediate microbial (i.e., bacterial and fungal) growth present in aircraft fuel systems. Currently, two biocides are approved for use by most aircraft and turbine engine original equipment manufacturers (OEMs); Kathon FP1.5 Microbiocide and Biobor JF.
- Metal deactivator can be added to remediate the deleterious effects of trace metals on the thermal stability of the fuel.
Military jet fuels
There are more military grades of jet fuel than commercial grades and military organizations around the world use a different classification system. The US military prefixes its grades with the initials “JP” which stands for “Jet Propellant”. Some military grades are almost identical to their civilian counterparts and differ only by the amounts of a few additives; Jet A-1 is similar to JP-8, Jet B is similar to JP-4.
Other military fuels are highly specialized products and are developed for very specific applications. JP-5 fuel is fairly common, and was introduced to reduce the risk of fire on aircraft carriers (JP-5 has a higher flash point—a minimum of 60°C). Other fuels were specific to one type of aircraft. JP-6 was developed specifically for the XB-70 Valkyrie and JP-7 for the SR-71 Blackbird. Both these fuels were engineered to have a high flash point to better cope with the heat and stresses of high speed supersonic flight. One aircraft-specific jet fuel still in use by the United States Air Force is JPTS, which was developed in 1956 for the Lockheed U-2 spy plane.
Synthetic jet fuel and Jet biofuels
Most of the future challenges in the jet fuel arena will come from alternative fuel sources and their impact is already being felt. A significant effort is under way to certify Fischer–Tropsch (FT) synthetic fuels for use in U.S. and international aviation fleets. This effort is being led by an industry coalition known as the Commercial Aviation Alternative Fuels Initiative (CAAFI), also supported by a parallel initiative under way in the U.S. Air Force, to certify FT fuel for use in all aviation platforms. The U.S. Air Force has a stated goal of certifying its entire fleet for use with FT synthetic fuel blends by 2012 and there are programs under way to certify Hydrogenated Renewable Jet (HRJ) biofuels as early as 2013.
Synthetic jet fuels show a reduction in pollutants such as SOx, NOx, particulate matter, and hydrocarbon emissions. It is envisaged that usage of synthetic jet fuels will increase air quality around airports which will be particularly advantageous at inner city airports. The air transport industry is responsible for 2 percent of man-made carbon dioxide emitted. Boeing estimates that biofuels could reduce flight-related greenhouse gas emissions by 60 to 80 percent.
Most of these projects involve blending synthetic or biofuels jet with conventional petroleum based jet fuel in various ratios. The area where we tend to be most challenged is that of specifications and the applicability of existing test methods to these new hybrid fuels. For example, do the precision statements and lower detection limits still apply? Even sampling requirements are changing. Only a week ago we received instruction that only stainless-steel sampling equipment is now to be used.
The next few years will see major changes to the inspection and testing of jet fuel, including sampling and handling. It has always been a critical product but it is also becoming a specialist product and we will need to develop specialist that are knowledgeable and equal to the product. Hang on – there is turbulence up ahead and it’s likely to be a bumpy ride.