Fuels used in aircraft engines (turbine and piston powered) must conform to strict requirements, to give optimum engine performance, economy, safety and overhaul life. Fuels are classed under two headings, kerosene type fuel (AVTUR) and wide cut gasoline (AVGAS).
- AVGAS: Aviation gasoline, for use in conventional piston engines with ignition systems.
- AVTUR: Aviation kerosene, for use in gas turbine engines and the new diesel engines that are being developed and licensed for aircraft use.
Both fuels are distillates of mineral oil, and have different properties. Aviation gasoline is a mixture of lighter hydrocarbons and manufactured to produce a fuel with a specific gravity or SG of 0.72 at 15°C, whereas Aviation Kerosene is a mixture of heavier hydrocarbons and produced with an SG range of 0.75 to 0.84.
All fuels possess the following properties to different degrees:
- Volatility: is the measure of a fuels tendency to change state from a liquid to a vapour.
- Vapour Pressure: is the term used to indicate the ambient pressure at which the fuel will vaporise. A high vapour pressure indicates that the fuel will vaporise at high atmospheric pressure.
- Flash Point: is the lowest temperature at which there are sufficient fuel vapours, due to the fuel's volatility, above the liquid to ignite. This will result in combustion that will burn all the vapours seen as a flash; there will not be a sustained flame.
- Fire Point: this is the lowest temperature at which the fuel can sustain combustion through vaporisation.
- Auto Ignition Temperature: is the temperature at which a fuel will spontaneously ignite, without the presence of an ignition source.
- Flammability Limits: excluding mono fuels there are upper and lower flammability limits, these define the percentage by volume of fuel vapour to air within which combustion can take place. If there is too much vapour or too little vapour combustion is not supported [hence flash point].
- Fluidity: viscosity is the resistance to flow, the term fluidity is a measure of a fuel's ability to flow. As the fuels are made up of a mixture of hydrocarbons they react differently and the fuel starts to thicken as the temperature drops, increasing the fluid's viscosity.
- Freezing Point: as aviation fuels are a mixture of hydrocarbons, they do not become a solid homogenous mass at one temperature like water, but individual hydrocarbons will form ice crystals. The freezing point is taken as the point at which the last ice crystal disappears from the fuel as it warms up.
- Electrical Conductivity: all materials can effectively be classed as either conductors or non-conductors. The conductivity of fuels is normally expressed in conductivity units or CUs. 1CU is equal to 10-12 ohms, pure hydrocarbons fuels are essentially non-conductors whereas distilled water is a good conductor.
Aviation Gasoline - AVGAS
The fuel that is used for spark ignition piston engines is a refined petroleum distillate, and is one of the hydrocarbon families that consists of approximately 85% carbon and 15% hydrogen. When mixed with air and then burnt, the carbon and hydrogen combine with the oxygen in the air to form carbon dioxide and water vapour.
Aviation Gasoline is different from motor vehicle fuel since it is subject to a more rigid control of quality assurance; it also has a much higher resistance to detonation. On piston engine aircraft it is important that the correct type of fuel is used, since using the incorrect type of fuel can lead to low engine performance, detonation and engine failure.
The specifications for aviation gasoline were developed for the earlier carburetted aero engines. They have been developed into three grades as engine power has increased, which are dyed different colours for identification however the basic specifications remain the same.
Aviation gasoline having a low density [compared to kerosene] is volatile and has a high vapour pressure. Gasoline will easily vaporise [evaporate] at sea level on an ISA standard day. As the ambient pressure decreases the volatility of the fuel increases and it is said to be ‘boiling off' for example at 10,000 feet the ambient pressure has dropped by approximately 31% of the sea level value.
High ambient temperatures will also increase the volatility of the fuel. Therefore if an aircraft has been parked in high ambient temperatures [heat soaked] takes off from and climbs rapidly to altitude the boil off will be greater than the same aircraft taking off from the same airport when it has not been heat soaked.
The actual amount lost through this process is small as it is the lightest hydrocarbons that vaporise at higher gas pressures, but it is an indication to another problem, called vapour locking. If the fuel in a pipe or component is hot enough or at a sufficiently low pressure as to be able to vaporise a bubble of vapour can form preventing the flow of fuel.
This can be tackled in two ways, by eliminating places within the fuel system where such vapour locking can occur and increasing the pressure within the fuel system to a value greater than the fuel's vapour pressure.
The flash point of Avgas is approximately -40°C, thus the fire point is within the normal range of ambient atmospheric conditions. The flammability limits are between 1.2 % volume to 7.0 % volume. The freezing point of Avgas has to be -58°C or lower.
This has several effects on the fuel; firstly it makes starting easier and at low ambient temperatures reduces the likelihood of running problems, however due to the high vapour pressure Avgas can easily vapour lock as temperatures increase or ambient pressure decreases.
The evaporation rate of a fluid is controlled not only by temperature and pressure but also by the surface area of a fluid in contact with the air. In the process of evaporating fluids draw in heat from the surrounding area, as you will remember from the last time your skin was swabbed with surgical spirit before an injection. In a carburettor's venturi where the ambient pressure and temperature decrease as the velocity increases, the mist of fuel that enters the venturi is atomised. The evaporation decreases the temperature in the surrounding structure further and any moisture that is carried in the air stream will freeze on contact with the carburettor. The most likely item for the moisture to touch is the throttle plate; this condition is termed carburettor icing, see carburettor and fuel icing later in these notes for more detail.
Whereas pure hydrocarbon is a complete insulator AVGAS as a blend of hydrocarbons has insulating properties with a CU ranging between 1 to 10 when compared with distilled water, which has a value of approximately 10 million.
A problem occurs where fuel is being pumped through pipes and filters with dissimilar materials that it can pick up a static electrical charge faster than the charge can dissipate to the surrounding structure. If the fuel is being pumped from one container to another it is possible for the charge to spark out of the liquid and ignite the fuel air vapour above its surface. This hazard is increased if the fuel splashes or forms a mist. To overcome this chemical compounds called static dissipaters can be added to the fuel before delivery to an aircraft.
There are three grades of aviation gasoline, these are:
- 80 grade
- 100/130 grade
- 100 LL grade
80 grade aviation gasoline is coloured red for identification purposes. It is for use in low powered (low compression ratio) normally aspirated (carburetted) aero engines. The grade number gives its octane rating, which is an indication of its resistance to detonation.
100/130 grade fuel is dyed green for identification and is for use in higher powered (compression) engines as it is more resistant to detonation. To improve the fuel's resistance to detonation liquid lead, which is tetraethyl lead, is added. This of course has an environmental impact so 100LL grade was introduced.
100 LL Grade
100LL is dyed blue for identification and has the same octane rating as 100 grade but has a Lower Lead content and has become the standard fuel grade for high powered engines (injected, turbo charged or super charged engines).
Mogas (Automobile Fuel)
This fuel has a lower vapour pressure than AVGAS. Therefore it tends to cause vapour locks in pipelines at high temperature and altitudes. Carburettor engines using this fuel are more susceptible to carburettor fuel icing. Also it has a low lead content, which can lead to detonation and pre-ignition. Before using MOGAS consideration should be taken as to all its disadvantages.
This is a measure of the fuel's resistance to detonation, the higher the octane number, the higher will be its resistance. The aircraft flight manual or equivalent will state the minimum octane rating. A fuel with an octane number lower than that recommended should never be used. As we have seen fuel octane ratings are colour coded (eg, 100LL BLUE).
Where aviation gasoline is stored and has an interface with a change of air the light hydrocarbons will vaporise. The fuel can oxidise and two of the bi-products of this oxidation are soluble gums and insoluble black particulates. The tetraethyl lead will oxidise into an insoluble white mass. The particulates and gums can block fuel filters or clog up carburettor jets and narrow bore pipes.
Aviation Kerosene - AVTUR
Aviation turbine fuel was developed from paraffin for use in gas turbine engines. This fuel has a greater density than Avgas and a lower vapour pressure [about 0.14 Psi] so is less volatile. This, whilst reducing the likelihood of vapour locks occurring and the loss of fluid via boil off, requires the fuel to be conditioned to ensure that it will flow and vaporise when sprayed through special injectors at high pressure into the combustion chamber of the engine.
Having a low vapour pressure results in the fuel having a flash point and fire point that is higher than gasoline [a minimum of 38°C] but it also results in AVTUR having a higher freezing point. As with gasoline the turbine fuels are a mixture of hydrocarbons each with its own freezing point. As kerosene cools its viscosity increases, this makes it harder to pump and results in a decrease in fuel flow.
When the fuel is cooled to its freezing point the hydrocarbons with the highest freezing point turn into waxy crystals. Further cooling will progressively increase the wax content as successive hydrocarbons freeze. This results in an increase in viscosity, however kerosene remains pumpable to approx 6°C below its freezing point. Continued cooling will convert kerosene into slush and eventually become a semi solid waxy block.
As it has to be pressurised prior to atomisation in the combustion chamber the fuel has to pass through a high-pressure pump and fuel-metering system, these are covered in the gas turbine engine notes but are mentioned here to explain why turbine fuel must have lubricant properties. The fuel has to be chemically stable, however if it is stored over a long period in conditions where oxidation can occur then soluble gums and insoluble particulates form which will clog filter units and pipes.
There are four commercial grades of aviation turbine fuel available these are:
- Jet B
- Jet A
- Jet A1
This was the original American aviation turbine fuel it is what is termed a ‘wide cut', ‘wide range' or ‘wide distillate' as it is manufactured from mixing 70% gasoline with 30% kerosene. This has several very important effects, which are:
- A high vapour pressure [2.6 Psi]
- A low flash point [below 0°C]
- A low fire point [below 0°C]
- A low freezing point
- A lower SG
- Reduced lubricity
- Greater fuel loss through evaporation
As can be seen from the first three points Jet B is a greater fire risk which reduces crash survivability, its use will increase the wear in the fuel components and if it is added to an aircraft that normally uses Jet A or Jet A 1 will require the fuel control system to be adjusted as its mass per unit of weight is less. Jet B is not used as a commercial fuel except in areas such as Alaska or Canada where its lower freezing quality is useful in arctic conditions.
This is a light kerosene fuel used in the countries of the CIS and has similar properties as Jet B.
Was developed for use in long haul high altitude commercial airliners this was achieved by lowering the freezing point to -47°C and is considered to be the preferred aviation kerosene fuel for commercial aviation worldwide. Jet A-1 has a flash point of 38°C and a specific gravity at 15.5°C of 0.807.
Was the next generation of turbine fuel this is termed 'straight' kerosene as there is no blending with gasoline. The flash point for this fuel is 38°C and its freezing point is -40°C. This fuel is normally used for domestic flights within the USA, as it is cheaper to produce than the current world standard commercial gas turbine fuel Jet A1 and is not readily available.
Unlike aviation gasoline turbine fuels are not dyed and can vary in appearance from water white to straw yellow in colour.
Low freezing points are essential due to the high altitudes modern helicopters fly at. Most fuels contain additives to combat the problem of fuel icing. Should the temperature fall to that where fuel icing is present, then ice crystals or a gel can form blocking filters and components.
Fuel should be examined on a regular basis for signs of contamination listed below. This is achieved by taking a sample of fuel from the fuel drain points situated at the bottom of each fuel tank, fuel filter and where applicable, cross feed lines.
- Globules of water
- More than a trace of sediment
- Positive reaction to water finding paste, paper or chemical detector
As aviation fuel is produced it passes through a ‘drying' process that removes any water from the finished product, however fuel is hydroscopic and will absorb moisture from the ambient atmosphere. As with the air warm fuel will support more moisture than cold fuel and as warm air is more humid there is more water in the air for the fuel to absorb.
Water that is absorbed into a fuel from the water vapour in the atmosphere has been broken down into minute particles that when they pass into the fuel due to their size are supported by the fuel, this is termed ‘dissolved water' or referred to as water in suspension. This water cannot be removed from the fuel without special equipment and provides the water for fuel icing in the carburettor induction system above the throttle plate.
Dissolved water that has precipitated out as the fuel cools [saturation point], or water that has entered the fuel on mass will, due to its greater mass, collect at the bottom of the tank, and is termed ‘free water.' One of the mechanisms by which free water enters fuel is seen by the condensation that forms on the outside of a glass containing an iced drink or a chilled can taken from a refrigerator on a hot day.
As fuels, especially Avgas, are volatile they absorb heat from the fuel tank structure in the process of their evaporation, thus any moisture in the airspace above the fuel will condense out and either run down the tank walls through the fuel or drip into the fuel where it will settle out at the bottom of the tank. Keeping the fuel tanks full of fuel during periods when the aircraft is not been used will reduce the chance of condensation and limit the amount of water that can build up in the tank. There is always an interchange of moisture laden air into the tank system, therefore from a water contamination perspective it always pays to leave the aircraft with full fuel tanks, to minimise the air gap above the fuel.
An airborne fungus called cladisporium resinae can contaminate fuel tanks. The fungus exists in the fuel / water interface living in the fuel and deriving food from the hydrocarbon fuel. The fungus which takes the form of blanket weed [as found in a pond] traps water within its mass of strands and can therefore survive for a prolonged period of time after the free water has been drained off until it builds up again. The fungus can block fuel filters and due to the trapped water content freeze, blocking fuel pipes and filter units, also due to the water content where it remains in contact with alloy fuel tank structure it can cause corrosion.
Fungus is considered to be the particular problem for aircraft that use kerosene and have integral tanks, although in reality the fungus can affect gasoline burning aircraft. To prevent infestation a biocide can be mixed with the fuel, this will also control small, established amounts of the fungus but for large scale infestations the fuel system would have to be stripped and cleaned.
Water Sediment Checks
As detailed above, water in the fuel system can literally be a killer, and therefore has to be guarded against. The pilot's main weapon in this is vigilance by carrying out water sediment checks before flight and by uplifting fuel from recognised suppliers who have a large volume turnover. This should ensure that if a problem exists it is spotted before flight and that the supply is unlikely to be contaminated with either water or fungus as it will not remain in the fuel suppliers depot very long.
For the water sediment check a sample of fuel is drawn off in turn from the lowest point of each tank and the fuel filter(s) in to a clear walled container. This is then observed and held up to a good source of light, for the clear and bright test. Refer to diagram 10.2 below and the following text.
In this the term clear refers to checking the fluid for traces of sediment, particulates and other solid matter.
In this the term bright refers to checking the fluid for traces of free water and dissolved water, which can also be termed ‘entrained water.' When held up to the light if the fluid has a hazy appearance it can be due to the light refracting and reflecting as it passes from fuel into water and back into the fuel again. If the haziness clears from the bottom upwards then it was air that has become entrained into the fuel as the sample was taken. If the haziness remains or slowly starts to clear from the top of the sample downwards then this is entrained water.
Free water will settle to the bottom of the sample and the interface between the two fluids will be seen as a meniscus.
The person carrying out this check has to guard against drawing a perfect sample of free water as some aviation kerosene has the appearance of water, and free water will be tainted with the smell of kerosene.
To guard against this a small volume of water is added to the container before a sample is taken and its level is marked off on the container, then the sample is taken.
- if the meniscus disappears then the sample is water
- if the meniscus appears at a point higher than the mark there is free water in the fuel sample
- if the meniscus coincides with the mark it is a pure fuel sample
Where sediment or water is found in the fuel samples the affected tank(s) filter(s) must be drained off until a pure fuel sample is taken.
Un-usable Fuel, Standpipe and Sump
Diagram detailing: Sump, Water Drain, Standpipe, Finger Strainer and Vent.
To prevent the risk of engine failure due to water or contaminants passing through the fuel line, the fuel supply to the engine is taken from above the bottom of the tank. In diagram it is shown being drawn off via a standpipe, this leaves a volume of fuel that cannot be used by the engine and is termed un-usable fuel. The aircraft's pilot's manual will specify this volume and its mass. To prevent any large item of debris that would block the fuel line if it entered it, a coarse mesh [gauze] filter referred to as a ‘finger strainer' is fitted over the inlet of the standpipe.
Each fuel tank must have a low point, which is termed a ‘sump' to which any free water can drain down and can be drained from via a water drain valve. In diagram above the water drain valve is shown fitted in a distinct sump as per a rigid tank. For integral tanks the sump will be formed by the dihedral of the wing and the height of the take off pipe above the bottom of the tank.
Water Drain Valves
Water drain taps are fitted at the lowest point of the fuel tank and in filter units to allow any free water to be drained off when the pilot carries out the water sediment checks prior to flight. The water drain cock is sprung loaded to close and has to be pushed upwards to open, pilots must ensure that after operating the water drain tap that it seats correctly and stops the flow of fuel.
The primary purpose of lubrication is to reduce friction that is created between the moving parts of the engine. By good lubrication then engine wear can be substantially reduced therefore prolonging engine life. Friction is reduced by placing a film of oil between the moving parts, which covers the surfaces preventing contact of the metal surfaces. Therefore movement is between the layers of oil, and not the metal surfaces.
Oil also seals between moving parts, an example of which is by applying an oil film on the cylinder walls and piston that forms a seal in the cylinder preventing gas leaks from the combustion chamber. Also oil protects against shocks between engine components such as the crankshaft, connecting rods and valve operating mechanism by applying a film of oil that cushions the shocks.
Another important function of the oil as it circulates around the engine is to absorb heat from the internal engine components. This heat is removed by passing the oil through an oil cooler; where air passing through the oil cooler absorbs heat from the oil. As the oil circulates it collects contamination in the form of dirt, dust and carbon that have been introduced into the engine by the atmosphere and the combustion process. Finally the oil must provide protection from corrosion of the internal metal parts by ensuring a film of oil on them.
Lubricating Oil Types
There are various types of oils available to fulfil the requirements of engine operation.
Straight Mineral Oil; or straight oil as it is commonly called, is normally used after maintenance or when running in a new engine where it is used for the first 50 hours of engine life. This type of oil can cause sludge to form that may result in clogging of oil ways and filters. This oil can be used for engines that do not require ashless dispersant oil.
Ashless Dispersant Oil; contains a dispersant that holds contamination in suspension therefore preventing the formation of sludge that can occur with the straight mineral oil and is deposited safely in the filter rather than the engine. This oil cannot be mixed with straight mineral oil, therefore it is essential that a check be carried out to ascertain what type of oil is being used in an engine.
Synthetic Oil; It is superior to the other oils mentioned above in all aspects but due to expense and limited service experience, few piston engine manufacturers approve it. It is, however, used exclusively in gas turbine engines.
Oil grades are determined according to their Viscosity, where viscosity is defined as the fluid friction, or the resistance to flow and is very important in engine operation. High viscosity oil is thick so therefore flows slowly, whilst low viscosity oil is thin and will flow freely. Therefore the oil is required to maintain viscosity in order to withstand high bearing pressures and temperatures. Mixing or using the wrong type of oil will often result in engine deterioration and possible failure.
The viscosity of an oil is affected by changes in temperature where temperature increase will thin the oil allowing it to flow more freely i.e. lower its viscosity and vice versa. Therefore at high ambient temperatures high viscosity oil is used and at low ambient temperatures low viscosity oil is used. The oil selected for use will therefore depend on the average ambient temperature and it is essential that the correct oil grade is used for efficient lubrication.
Oils are grades by numbers that indicate their viscosity, where the higher the number the higher the viscosity, the slower the oil flows and vice versa. The numbers are obtained by using the Saybolt Universal Viscometer where a measured amount of oil at a particular temperature is timed as it flows through a calibrated orifice. If it takes 20 seconds to flow through the viscometer then it is given the grade SAE 20, where SAE stands for the Society of Automotive Engineers. For commercial aviation using a number double that of the SAE number i.e. SAE 20 equals commercial aviation grade 40 identifies the grades.
The letter W may be used when grading oil, when the W is after the number this indicates that the oil is satisfactory oil for winter use e.g.40W. Alternatively a W before the number indicates that it is an ashless dispersant oil e.g. W80.
Oils are now produced to meet the requirement of more than one grade. Take the case of a multi-grade oil of SAE 20W/40W, it possesses a viscosity within both the SAE 20W range -18°C and the SAE 40W range at 99° therefore giving a wider performance range and near constant viscosity than single grade oils
Fuel Tank Types
Flexible tanks are made from a fuel resistant rubberised fabric, which has the advantage of being lighter than the comparable rigid tank. They can be manufactured to fit into areas where it would not be practicable to produce and fit a rigid tank, see diagram above. Being flexible [to an extent] they are more crash worthy than either rigid or integral tanks provided that they do not get punctured.
Some tanks have an external layer of high-density closed cell foam that swells when in contact with fuel so self-seals [reduce the size of the leak]. While modern aircraft can incorporate flexible fuel tanks fitted internally, they are no longer the main fuel storage system as they have the following disadvantages.
- They have to be clipped or tied to the surrounding structure to preventing them collapsing as the fuel is used.
- The areas in which they are fitted have to be lined with tape to prevent any sharp edges puncturing them.
- Once used with fuel the tank must not be allowed to dry out as it can split and leak.
- There is no guarantee that the bottom of the tank is flat, it can be rucked causing ridges that trap water.
- The fuel from a leaking flexible tank can run down the internal structure before showing on the exterior of the aircraft.
- Over a period of time these tanks can become porous so have to have a finite life.