It is a pretty good bet to make if I say that most of us in the aviation industry rarely give much - if any - thought about the Jet Fuel that we fill our tanks with. Sure there are considerations of cleanliness (no water- FOD- bacteria or other contaminates)- temperature (if you fill on a cold day you will get more fuel in your tanks)- and price (close to half of a flight departments budget goes in payment for the fuel used). All of these ...

AvBuyer  |  01st September 2009
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There’s more to it than you would imagine.

It is a pretty good bet to make if I say that most of us in the aviation industry rarely give much - if any - thought about the Jet Fuel that we fill our tanks with. Sure there are considerations of cleanliness (no water- FOD- bacteria or other contaminates)- temperature (if you fill on a cold day you will get more fuel in your tanks)- and price (close to half of a flight departments budget goes in payment for the fuel used). All of these issues might flash cross your mind occasionally- but I am pretty certain that unless you are in the oil refining business yourself- the physical properties of Jet Fuel are relatively unknown to you- hence this article.

Jet Fuel is traditionally a refined hydrocarbon fuel that is tapped-off near the middle of the distillation tower vessel during the crude oil cracking process. It is vented-off for collection between the extraction temperatures that form lubricating oil- diesel- gasoline and naphtha. Each fuel product is collected based upon their distillate temperature. Jet Fuel; effectively Kerosene- which forms when crude oil is boiled to 1-112°F and the resulting vapor cools as it rises upwards through the tower- eventually reaching approximately 482°F. Gasoline/Petroleum Spirit (AVGAS)- by contrast- forms at approximately 194°F.

Most modern hydrocarbon fuels are engineered to meet a specific octane rating. This is achieved by adding ‘iso-octanes’ which have been formed by distilling a crude oil product derivative and passing it through a catalyst before collection. When this is blended into the fuel- the manufacturer is able to reach the desired number of carbon and hydrogen atoms structured appropriately to achieve the desired performance from the fuel being manufactured.

The coal-tar derived liquid: naphtha is also modified through the isomerization process using a catalyst to convert it from a straight-chain hydrocarbon into an aromatic hydrocarbon which is used as a blending component in Jet Fuel. The paraffin gas- Butane- is used as the main blending component in AVGAS.

Even though a Gas Turbine Aero Engine (GTAE) has a 'wide-cut' of fuel types that it will burn (I will explain this later)- Unblended Kerosene as a raw fuel- is not suitable for daily aviation use. Therefore several blending processes have to occur before the Kerosene can be branded and sold as Jet Fuel.

Aircraft engines fly in an ever changing environment that sees extremes in temperatures- pressures and air density. These varying environments will affect how a fuel performs in several different ways- as laid out below.

Low Temperature and Water Affect
The physical properties of Kerosene cause this type of fuel to 'cloy' or 'wax' once the ambient temperature drops to below -35°F. If the fuel temperature is allowed to remain below -60°F for days- it will eventually become 100% solid.

Also Kerosene products by chemical composition- have a natural affinity to water. This is because a Hydrocarbon fuel molecule is composed of a long chain of carbon atoms that are surrounded by- and bonded to a variable number of hydrogen atoms; the larger the number of hydrogen atoms- the more volatile and potent the fuel product. Basic Kerosene does not have a set molecular structure and is rather a compilation of hydrocarbon chains that are made up of between 12 and 15 carbon atoms.

The atomically agreed composition of a standard Kerosene molecule consists of 13 carbon atoms tied to 18 hydrogen atoms. In simple terms- more carbon atoms means greater stability; more hydrogen atoms means more potency. The bio-fuels that are being considered as a replacement for the non-renewable raw material based fuels usually have oxygen atoms (esters) tied into the composition of the fuel molecule- thus reducing the potency of the fuel- a problem that must be overcome if a bio-diesel product is to replace kerosene completely in the future.

Atomically fuel and water molecules have a natural affinity towards each other. Therefore often there is water suspended in Kerosene- which in-turn is a guarantee that once the fuel tanks of an aircraft become cold soaked at altitude- ice will naturally form in the fuel.

The anti-icing agents that are used in Jet Fuel include either- EGME (Ethylene Glycol Monomethyl Ether)- or DI-EGME (Diethylene Glycol Monomethyl Ether). The 'Prist' additive manufactured by CSD- Inc. in Conroe- Texas uses DI-EGME as an icing inhibitor. Prist in its concentrated form has a freezing point of -121°F.

The presence of water also promotes corrosion and micro-biological growth within the fuel tanks- filters and lines of the aircraft- if it is allowed to sit for any length of time.

High Ambient Temperature Affect
Jet Fuel is a hydrocarbon fuel that has a relatively low volatility and high flashpoint; volatility being the ease with which a fuel evaporates into the air and gasses-off as vapor. Its volatility does increase with heat. Jet Fuel is also classified by refiners like Conoco/Phillips as being 'moderately' flammable- as compared to AVGAS which it classifies as being 'seriously' flammable.

The flashpoint of Jet-A is >100°F- while AVGAS is <35°F. Depending on the formulation of the blend- Jet Fuel may boil at a relatively low temperature (as low as 80°F.) When this occurs it is possible for some of the additives to begin to strip themselves away from the chemical blend which could drastically alter the performance of the fuel.

Living Organism Affect
If any water is present in the Jet Fuel in your tanks- it is immediately possible that micro-biological organisms may take up residence in your tanks. If you let this fuel and water cocktail sit for an extended period in your tanks- especially during the summer- the millions of bacteria- yeasts and moulds that will collect- grow and feed off your fuel- will very quickly become a major problem for you. This is because your normally clear-amber coloured fuel will turn into a brown sludge which can block your fuel filters and nozzles - and worse it will eventually turn your alloy tank structure into dust regardless of the epoxy- elastomers- or plated coatings that may protect the tank structure in your aircraft.

Hammonds/Fabcorp- of Houston- Texas- manufacturers BioBor JF which is a biocide designed specifically to toxically treat all organisms that naturally thrive within Kerosene. This treatment should be utilized regularly; it is especially important to employ it when your aircraft is down for maintenance for an extended period.

Jet Fuel By Type
The various formulations of Jet Fuel over the last 70 years include: Military designated JP-1- JP-2- JP-3- JP-4- JP-5- JP-6- JP-7 and JP-8- and Commercially designated Jet-A- Jet-A1 and Jet-B. All are Kerosene blends. The designation letters ‘JP’ stand for ‘Jet Propellant’. Following is a brief look at each:

• JP-1: The standardized formulation for JP-1 was created in 1944 for the first jet aircraft. It was designed to operate at an extremely low freezing point of -76°F. There is only very limited availability of this fuel due to cost to formulate it.
• JP-2- -3 and -4: All are blended kerosenes that use naphtha derivatives as their cutting agents giving varying flash and freezing points. These are all obsolete fuels.
• JP-5 and -6: Both are kerosene fuels that have been blended with AVGAS. A derivative of JP-5 is still used primarily by the United States Navy- while a JP-6 derivative is still used by extremely high-altitude military jets.
• JP-7: Was specifically developed for the Lockheed SR-71.
• JP-8 (here in the US) and F-34 (in Europe): Now the universally required Jet Fuel used by the world's military forces. It is a blended Kerosene that has an antioxidant- metals deactivator and static electricity dissipater additives blended into it. However the effect of these additives also results in a much lower boiling point (>90°F) than regular or icing-inhibited Jet-A (>300°F).
• Jet-A: A Kerosene blend that has a flashpoint of 100°F and a freezing point of -40°F.
• Jet-A-1: The same as the ‘non dash-one’ blend- but with a lower freezing point which is -52.6°F.
• Jet-A FSII (Formulation Standard No. Two): The same as Jet-A- but also blended with Prist to lower its freezing point to the same temperature level standard as the Jet-B formulation.
• Jet-B: A commercial fuel that has similar properties to the JP-2 family as it is a naphtha/kerosene derivative that provides a lower freezing point of -58°F. Predominately used in the Canadian Arctic as well as other polar regions.

Going back to JP-8- the specific additives broken down into their respective compounds and actions are as follows:

• The antioxidant (butylhydroxytoluene) suppresses the tendency for organic living compounds to form in fuel tanks.
• The metal de-activator (an amalgam of disalicylidene and propanediamine) effectively reduces the formation of gummy deposits within fuel tanks- lessening the possibility of corrosion forming.
• The anti-static agent (dinonylnaphthylsulfonic acid) is slightly conductive- thus providing electrical continuity (bonding) between the tank structure and the fuel.

Some commercial blends of Jet Fuel are produced with the military mandated additives in them- thus providing a slight competitive edge in this relatively staid hydrocarbon fuels manufacturing business.

Before any portion of a new batch of Jet-A leaves the refinery; it is tested to ensure that it conforms to the design specification for each of the following parameters:

• Un-dissolved water
• Sediment
• Suspended water
• Sulfur content
• Freezing point
• Flashpoint
• Net heat of combustion (similar to a calorific value but it is measured in BTUs)
• Thermal stability
• Electrical conductivity

Other Fuel Usage
Many GTAE manufacturers have often touted claims that their engines could easily run on a ‘wide-cut’ of different fuels. There are cases of ground based GTAEs used in industrial applications that include electrical- gas- heat or power generation- that are successfully run on coal or wheat dust- natural gas- propane- petroleum spirit- diesel- cooking oil- and many other obscure fuels. These alternative fuels are appropriate for industrial use- but their use in GTAEs that provide propulsion in-flight would produce erratic- unreliable and probably in most cases- dangerous results.

Often the GTAE manufacturers’ maintenance manual will provide specific guidance to an aircraft operator who finds him/herself out of fuel at an airport that does not have any Jet Fuel available. In all cases there will be an hourly time limit specified (often 25 hours) where continued use of the alternative fuel will at the least require a disassembly inspection of the engine- and at the worst may result in the destruction of the subject GTAE.

The most widely accepted alternative fuel that can be used temporarily is AVGAS. There is a certain amount of risk that comes with its use in GTAEs. First there is the potential for vapor-lock to occur in some of the fuel lines during the initial phases of flight due to the volatility characteristics of this fuel. Secondly AVGAS is much more flammable than Jet Fuel- thus the threat from explosion or spontaneous combustion caused by static or an electrical fault is greatly elevated. Thirdly the Net Heat of Combustion of AVGAS is considerably higher than Jet Fuel and thus continued use will most likely cause thermal stress and heat damage to many of the combustion components within the GTAE’s hot section.

Lastly there is a 10th difference between the Specific Gravity (SpG) of Kerosene (0.82) versus AVGAS (0.72)- thus it will act differently when compressed and run through a fuel pump and fuel nozzle. The damaging effects may be similar to those associated with the increased BTU energy issue. In some cases you will be instructed to adjust the metering rate on the Fuel Control Unit of each GTAE to match the lower SpG.

Surprisingly the threat of fuel icing occurring is unlikely when AVGAS is used- because this reciprocating engine application fuels freezing point is -72.4°F. If you do find that you have to temporarily use AVGAS- I recommend that you still inject some Prist into each upload of AVGAS. This is because the Prist has been endowed with some additional lubricating compounds that might help to reduce additional wear caused by the highly aromatic AVGAS.

A constant problem found in many GTAEs - especially older ones - is caused by the sulphur content found in Kerosene. The next time that you have an opportunity to visit an overhaul facility- ask the people there about ‘sulphidation.’ They will tell you that sulphidation is commonly a condition where turbine blades are damaged by irregular and unusual corrosion that is derived from a reaction of the sulphur in the Jet Fuel with the ingested ambient intake air mixture.

Salt water which is high in Chlorine is the most reactive induction air that most readily encourages sulphidation to occur. Some formulations of Jet Fuel include blended additives that have been designed to strip as much of the sulphur out of the Kerosene as possible. The Kerosene blends that are derived from shale oil or coal- have the highest sulphur content- so be alert to this issue when you are flying outside of the USA- or you are spending a lot of your time operating Oceanically.

None of us really know what the future holds when it pertains to Jet Fuel. I am certainly confident that synthetic and bio-fuels derived from renewable sources shall gradually take over from the hydrocarbon fuels that we currently fly on today. The first ever GTAE design was run in its prototype form in 1937- fueled by pure hydrogen. That was only 72 years ago which is less than a normal healthy person’s life-time. Because of the new paradigm of the environmental ‘Green Culture’ that is focused squarely on the issue of carbon emissions- I dare say there is a good chance that we could eventually return to using pure hydrogen as the Jet Fuel of the future.


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