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In this lesson, we will examine the properties of the various

fuels used in aircraft engines.

Aircraft piston engines use gasoline and gas turbine engines

kerosene.

Both of these fuels are produced from crude oil.

In simple terms, the required fuel is extracted from the crude

oil by a distillation process.

The oil is boiled in a furnace and then the oil vapour is

cooled in a distillation column.

The various hydrocarbons in the oil all have different boiling

points, so they condense out of the vapour and are tapped off at

different points in the column.

The heavy oils condense out first and are collected near the

bottom.

The process then continues up through the tower, with kerosene

and gasoline, which are relatively light and volatile,

being taken out near the top.

The specification from ideal fuel for either a gas turbine

engine or a piston engine include the following main

requirements.

It should flow easily under all operating conditions.

It should have complete combustion under all conditions.

It needs to have a high calorific value.

The calorific value is a measure of the amount of heat released

during its combustion.

It is measured in British thermal units per pound or

kilojoules per kilogram.

It should be non corrosive.

There should be no damage to the engine from combustion

byproducts.

The fuel should present a low fire hazard.

Engines should start easily and the fuel must be able to

lubricate the moving parts of the fuel pumps and other

components in the system.

In practice, the cost of satisfying all these criteria is

prohibitive and therefore compromises have to be made.

Piston engined aircraft use gasoline fuels grouped under the

title Aviation Gasoline or Avgas.

Aviation gasoline is manufactured to conform with the

exacting specifications that are issued in the United Kingdom by

the UK Aviation Fuels Committee on behalf of the Defence

Material Standardisation Committee.

The specification number for gasoline is Defence Standard or

DEF Stan 91-90.

Aviation gasoline is graded by octane rating.

The octane rating of a fuel is a measure of the fuel's resistance

to detonation when subjected to high temperature and pressure.

A fuel's octane rating is checked in a test engine with

the fuel air mixture weak and with it rich.

This gives the fuel 2 octane ratings.

The weak figure is used in the fuel's name.

So for instance, Avgas 100 is a 100 octane fuel when the mixture

is weak, but a 130 octane when the mixture is rich.

These two octane ratings are known together as the fuel's

performance number or index.

So avgas 100 has a performance index of one hundred 130.

High performance engines with high compression ratios require

high octane fuel.

Engines with lower compression ratios can use lower octane

fuel.

The grades of aviation gasoline currently authorized and

available are avgas 100 and avgas 100.

LL Afgas has a freezing point in the region of -58°C and a

flashpoint, which is the temperature at which the fuel

vapour will ignite in contact with a flame of -40°C or colder.

It has a specific gravity of 0.72 at 15°C.

Specific gravity is important as fuel is delivered by volume, but

pilots need to know its mass or weight as the energy available

from the fuel is dependent on its mass, not its volume.

Afgas 100 is coloured green and has a performance number of one

hundred 130.

Avgas 100 LL has the same performance number as Avgas 100,

but it has a lower lead content.

It is coloured blue.

MO gas or motor gasoline can sometimes be used in certain

airframe engine combinations, but only under the conditions

specified in Civil Aviation Authority Airworthiness notices

#9898A98B and 98C.

Because of the higher volatility and water content of MO gas,

carburettor icing and vapour locking are much more likely to

occur if this fuel is used.

The subjects of carburetor icing and vapour locking are covered

in detail in the Piston Engine Series of Lessons.

Information and advice on the use of mogas can be found in the

CAA Safety Sense leaflet #4B.

Diesel piston engines use AVTA.

This fuel is described in the Gas Turbine Engine Fuel section.

Gas turbine engineed aircraft use kerosene fuels.

These are heavier and less volatile than gasoline fuels.

They are placed between gasoline and diesel fuel in the

distillation process.

The two main types of gas turbine fuel in use in civilian

aircraft are aviation turbine fuel or AVATAR, and aviation

turbine gasoline or AVTAG.

As with aviation gasoline in the United Kingdom, the

specifications for these fuels are issued by the UK Aviation

Fuels Committee on behalf of the Defence Material Standardization

Committee.

There are two grades of AVATAR in use.

Jet A1 is used in Europe and most of the rest of the world,

whilst Jet A is used in the United States of America.

Jet A1 is a kerosene type fuel with a nominal specific gravity

of 0.8 at 15°C.

It has a flashpoint of 38.7°C and a waxing point of -50°C.

Waxing is explained later in the lesson.

Jet A is a similar type of fuel to Jet A1, but it has a waxing

point of -40°C.

Gas turbine fuels are not dyed for identification.

They retain their natural colour, which can range between

what is termed as water clear to a straw yellow.

Avtag is known in Civil Aviation as Jet B.

It is not generally used in civilian aircraft.

It is a gasoline and kerosene mix with a nominal specific

gravity of 0.77 at 15°C.

It has a flashpoint of -20° and a waxing point of -60°C.

Because of the very low waxing point, it may be found in use at

a small number of civil airfields in very cold climates.

If a fuel sample is placed in a clear glass container and

swirled vigorously and the sample appears cloudy or hazy,

then there could be one of two reasons for this.

If the cloudiness appears to rise quite rapidly towards the

top of the sample, then air is present.

But if the cloud falls quite slowly towards the bottom of the

sample, then water is present in the fuel.

Having said that, a cloudy appearance usually indicates the

presence of water.

A certain amount of water is present in all fuel.

The water which is dissolved in the fuel gives rise to a number

of fuel system problems.

As an aircraft climbs to altitude, the fuel is cooled and

the amount of dissolved water it can hold is reduced.

Water droplets form, and as the temperature is further reduced,

the droplets turn to ice crystals which can block fuel

system components such as filters.

A microbiological fungus called Cladosporium resinae is present

in all turbine fuels.

This fungus grows rapidly in the presence of water to form long

green filaments, which can block fuel system components.

The waste products of the fungus are corrosive, especially to

fuel tank sealing substances.

The inclusion of a fuel system icing inhibitor or FSII in the

fuel will help to overcome these problems.

The inhibitor in common use inhibits the ability of the

water in the fuel to freeze and acts as a pesticide which

******* the growth of the fungus in the fuel.

Jet A1 has FSII, but Jet A does not.

As we have already said, water is always present in fuel.

The amount will vary according to the efficiency of the

manufacturer's quality control and the preventative measures

taken during storage and transfer.

Further measures can be taken to minimize water accretion once

the fuel has been transferred to the aircraft tanks.

If the fuel can be allowed to settle after replenishment, then

the water droplets, being heavier than the fuel, will fall

to the bottom of the tank can then be drained off through the

water drain valve, which is situated in a sump in the lowest

part of the fuel tank.

Once the fuel is in the aircraft tanks, the main source of water

contamination is the atmosphere that remains within the tank.

If the tanks are topped up to full, then the atmosphere is

excluded together with the moisture it contains, thus

minimizing the likelihood that fuel will be contaminated.

Caution is required here.

Filling up the tanks may prove an embarrassment later if the

ambient temperature rises as the fuel in the tanks will expand

and there is a danger it may spill out of the vent system.

Filling the fuel tanks may also incur a performance penalty as

the aircraft may be too heavy to take off with the required

passenger or cargo load and some fuel may have to be removed as a

further precaution in order to prevent any ice crystals

blocking fuel filters and jets, the fuel may be passed through a

fuel heater located on the engine which uses hot air from

the engine compressor to remove any ice crystals which may have

formed in the fuel when it's cooler than 0°C.

Some systems utilize a fuel cooled oil cooler as well as or

instead of a fuel heater.

The fuel cooled oil cooler uses the hot engine oil to warm the

fuel and in doing so produces the added benefit of cooling the

oil.

A number of additives may be blended into the fuel either at

the refinery or the airfield to improve the operating ability of

the fuel.

A lubricity agent is added to the fuel to reduce wear in

moving parts of the fuel system Components.

Static dissipator additives partially eliminate the hazards

of static electricity generated by the movement of fuel through

modern fuel transfer systems.

This would otherwise be a particular problem with the high

flow rates generated during refuelling and de fuelling.

Corrosion inhibitors protect ferrous metals in fuel handling

systems such as pipelines and storage tanks from corrosion.

Certain of these corrosion inhibitors improve the

lubricating qualities of some gas turbine engine fuels.

Metal deactivators suppress the tendency which some metals,

particularly copper, have to cause the fuel to oxidize.

We are now going to talk about fuel waxing.

This is something that happens to fuel when it is extremely

cold.

Do not confuse this with the water in the fuel freezing.

Gas turbine fuel consists of many hydrocarbons, each with its

own freezing point, so it doesn't become as solid at one

temperature in the same way that water does.

As the temperature of the fuel drops, the heavy hydrocarbons

which have the highest freezing point freeze 1st and form wax

crystals.

As the temperature drops further, the hydrocarbons with

slightly lower freezing points start to solidify, and so on as

the temperature continues to fall.

The waxing point of a fuel is the temperature at which wax

crystals begin to form.

Jet A1 has a waxing point of -50°C, so waxing is only a

problem for aircraft operating for prolonged periods at high

altitude.

In very low temperatures, the crystals can clog the fuel

filter and interfere with the operation of the engine fuel

control unit.

The effects of waxing can be minimized by the refinery

keeping the heavy hydrocarbons in the fuel to a low level.

The temperature at which a liquid boils will reduce as the

pressure on its surface reduces.

As an aircraft climbs, the pressure on the surface of the

fuel reduces, and with that reduction comes an increased

likelihood that the fuel will boil and form vapour in the

pipelines.

The vapour locks that this effect causes can cut off the

fuel supply to the engine, with the result that the engine will

stop.

Fuel booster pumps fitted inside the tanks can help to overcome

this problem by pressurizing the fuel in the pipelines from the

tank to the engine, pushing fuel towards the engine rather than

engine driven pumps, sucking fuel from the tanks and further

reducing the pressure.

Specific gravity, or SG is defined as the ratio of the

density of a given solid or liquid substance to the density

of water.

Or to put it more simply, it is the weight or mass of the liquid

divided by the weight or mass of an equal volume of water.

So in the example shown here, we have one gallon of fuel in the

left container and it weighs 8 lbs.

In the right container there is an equal volume 1 gallon of

water and it weighs 10 lbs.

The specific gravity of the fuel is then 8 / 10 or 0.8.

Specific gravity has no dimensions.

The higher the specific gravity of a fuel, the greater will be

the mass or weight of fuel for a given volume, and conversely,

the lower the specific gravity, the less will be the mass or

weight for any given volume.

The specific gravity of fuel varies inversely with its

temperature, so depending on the storage conditions, the specific

gravity of the fuel supplied to an aircraft may be different

from the standard value.

On modern aircraft fuel systems this usually makes little

difference as fuel in the aircraft tanks is measured in

mass or weight rather than volume, with the fuel quantity

measuring system compensating for changes in fuel specific

gravity.

However, if a high fuel load is required, it may not be possible

to get sufficient volume of fuel on board for the required mass

if the specific gravity of the fuel is low.

That is the end of the lesson.

The tables listing the characteristics of the various

fuel types are shown on screen.

Remember that ice crystals formed from water in the fuel

are prevented from blocking the fuel filter by heating the fuel

upstream of the filter using either hot air or oil.