Fuel is a material used to produce heat or power by burning. The burning fuel can be used to generate power for lamps, heaters , stoves, lanterns and for running automobile and jet engines. Aviation fuel must meet the strict requirements for flying characteristics and are hence different from the fuel used in automobile engines.
Car Fuel vs Jet Fuel
An internal combustion engine is used to power a car where as a gas turbine engine is needed to power a jet. As mentioned above, they both would require some heat source by burning some sort of fuel to power the engine.
Where car and jet fuel differ is just the number of hydrogen and carbon atoms that are presenting the fuel. For example, car fuel ( gasoline) consists of hydrocarbons that contain anywhere from 7 to 11 carbon atoms with hydrogen molecules attached. Jet fuel, on the other hand, contains hydrocarbons more in the range of 12 to 15 carbon atoms, that is, kerosene.
Therefore, airplane fuel is derived from a much heavier chain of hydrocarbon molecules and the main reason for using it is safety. It takes a higher temperature to ignite kerosene as compared to gasoline. Kerosene is also easily transported and available throughout the world and can stay in the liquid form at lower temperatures which is very important as an airplane flies thousands of feet above the ground.
Understanding a Few Terms
- Octane Rating/Number: It is a measurement of the quality or performance of the gasoline used. When you go to a gas station to fill petrol , the octane ratings are depicted by regular, mid-grade or premium. The higher the octane rating / number, the better the fuel burns in the engine and hence high performance cars have a need for higher octane ( premium) fuel. Similarly, the gasoline used in airplanes will have a higher octane rating than the gasoline used in cars.
- Specific Gravity (SG) :Specific gravity also called relative density, ratio of the density of a substance to that of a standard substance.The usual standard substance used for comparing is water . Fuels have specific gravity less than water, which means that they float on water. This helps in identifying the presence of water in fuel during a fuel check .
- Waxing Point : Waxing point, as the name suggests are the low temperature at which heavy hydrocarbons that are used in making jet fuels as discussed above, turn into wax crystals which clog the fuel filter. This could lead to operation limitations in the fuel system.
- Flash Point: Flash point is the lowest temperature at which a liquid when exposed to an open flame will turn into vapour. Below the flash point, insufficient vapour is present to support combustion.
Types of Aviation Fuel
Aviation Fuel is divided into 4 categories:
- Aviation Gasoline (AVGAS)
- Jet Fuel (JET A and Jet A-1)
- Kerosene-Gasoline Mixture ( AVTAG OR JET B)
- Bio Kerosene
Aviation Gasoline (AVGAS)
- AVGAS is the type of aviation fuel used in small piston engine powered aircraft within the general aviation community. These aircraft are predominantly used by private pilots and flying clubs and for tasks such as flight training and crop dusting.
- The octane rating of the fuel is specified with the grade , for eg AVGAS 100 is a 100 octane fuel.
- AVGAS 100 LL and AVGAS 100 are the most commonly used grades of fuel .
- The AVGAS 100 LL (low lead) and the AVGAS 100 have a lean mixture octane rating of 100 while its rich mixture octane rating is 130 that allows high performance engines to operate safely.
- The way to differentiate between these two grades of fuel is by their colour . The AVGAS 100LL is BLUE in colour whereas AVGAS 100 is GREEN in colour.A good way to never forget how to differentiate is by noticing that the word Blue and AVGAS 100LL have the letter L.
- AVGAS 115 was a high-octane avgas mostly used in military combat aircraft in the late 1940s when there was demand for high power output. Now, most military aircraft use turbine engines.
- Gas turbine engine aircraft’s use kerosene fuels.The two types of kerosene fuels used in civilian gas turbine engine aircraft’s are Jet A and Jet A-1.
- Jet A is a kerosene type of fuel with a specific gravity of 0.8 at 15 degrees C. It has a flash point of 38.7 degrees C and and a freezing point of -40 degrees C.
- Jet A-1 is similar in comparison to Jet A but has a freezing point of -50 degrees C. It is normally only available in the U.S.A.
AVTAG Fuel (Jet B)
- AVTAG also called Jet B is a kerosene-gasoline mix type fuel with a nominal specific gravity of 0.77 at 15 degrees C.
- Jet B can be used as an alternative to Jet A but it has a wider range of flammability as it has a lower flash point and hence is not generally used in civilian aircraft’s .
- Jet fuel is a type of fossil fuel and eventually will become expensive in the long term.
- Biokerosene is a mixture of jet fuel and a biofuels which are being used since 2008. There have been approximately 1,50,000 flights that have used Biokerosene to power their aircraft.
- However these fuels are more expensive than jet fuels and this cost premium is a key barrier to their wider use.
The primary types of contamination are water, particulate and microbiological material. In addition, contamination can occur from other fuel grades and chemicals that may be in multi-product transport systems. The fuel may also be rendered off-specification by either under-dosing/overdosing of approved additives, using an incorrect additive or from product testing issues not limited to, but including, poor sampling, incorrect test procedures and uncalibrated laboratory equipment.
Jet fuels composition allows water to absorbed easily. The degree of water present depends mostly on the temperature of the fuel. When the temperature of the fuel is less, some of the water particles that are already present in the fuel draw out of the solution and accumulate at the lower points of the tank. When the fuel is warmer, it promotes the absorption of moisture from the atmosphere and suspension in the fuel.
Certain bacteria and fungi thrive on the presence of water where it interfaces with jet fuel. The growth of microbial organisms leads to the corrosion of aluminium, steel and rubbers components of the fuel system.
Particulate contamination can occur in a number of different ways. From dirt and sand getting in open ports to degradation of fuel system lines , particulates are constantly being introduced into the fuel system.
During our preflight , if we carry the fuel inspection in a correct manner then the identification of any fuel contamination is not difficult. If a fuel sample appears cloudy of hazy , it could be because of two reasons, the presence of air or water. If the cloudiness appears to be moving towards the top of the sample then air is present , if the cloudiness appears to be moving towards the bottom of the sample then water is present as water has a higher specific gravity than fuel.
During fuel sampling ,fuel is drained into a clean and clear container that is filled at least halfway. After collecting the fuel sample, observing it against light helps in identifying water or particulates that could be present. Swirling the sample around by creating a tornado shaped vortex is another way of identification of fuel contamination. The absence of water at the lower points of the tank automatically leads to the elimination of microbial growth. The addition of jet fuel additives can also eliminate the growth of bacteria and other microbes.
JET FUEL ADDITIVES
A number of additives maybe blended into fuel either at the refinery or the airfield to improve the operational ability of the fuel. The most popular ones are:
- Fuel System Icing Inhibitors (FSII) : A certain amount of water is present in fuels as discussed above. FSII contains an icing inhibitor and a fungal suppressant to deal with fuel contamination due to water and microbiological fungus that can block fuel system components.
- HITEC (Lubricity Agent) : A lubricity agent when added to fuel can reduce the wear in fuel system components.
- Static Dissipater: These additives partially eliminate the hazards of static electricity generated by the movement of fuel through modern high flow fuel transfer systems .
- Corrosion Inhibitors: The corrosion inhibitors protect ferrous materials in pipelines, storage tanks against corrosion.
Fact of the Week
Concorde was born out of two separate French and British projects which joined forces in 1962. This partnership led to the British Aircraft Corporation (later British Aerospace, and now BAe Systems) and Aerospatiale (now EADS) of France to build a total of 20 Concordes, and became the foundation stone for the formation of Airbus.
Concorde recorded its fastest journey from New York to London on January 1st 1983, taking 2 hours 56 minutes.For most people, flying on Concorde is an impossible dream, the stuff of lottery wins and holidays of a lifetime.For the lucky few, ‘The Bird’ is a convenient method of travel, the equivalent of a bus or a train to the rest of us.Fred Finn was on the first and last Concorde flights and holds the Guinness World Record for the most Concorde flights as a passenger!. Click the link in the link here to know more about this iconic jet https://www.heritageconcorde.com/concorde-information–facts.
This is to this weeks post. Thank you for reading and hopefully you gained some new knowledge about aviation from this post. If you did , please don’t forget to share it with fellow aviation lovers and don’t forget to comment down below for suggestions. Until next week, stay safe and stay healthy.