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Aircraft Fuel Systems By: Eric Spoor

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Page 1: Aircraft Fuel Systems

Aircraft Fuel Systems

By: Eric Spoor

Page 2: Aircraft Fuel Systems

Aircraft Fuel

•Reciprocating engine fuels•Turbine engine fuels

▫Jet A – which is Kerosene▫Jet B – a blend of kerosene and gasoline▫Jet A-1 – used for operation at extremely

low temperatures Jet A and Jet B are the most common

Page 3: Aircraft Fuel Systems

Fuel System Contamination

• The higher the viscosity of the fuel, the greater it’s ability to hold contaminants in suspension

• This is why jet fuels, which have a higher viscosity than av-gas, are also more susceptible to contamination than av-gas

• The main contaminants that reduce the quality of fuel are:▫ Other petroleum products▫ Water▫ Rust▫ Scale▫ Dirt

Page 4: Aircraft Fuel Systems

Water Contamination

•Water contamination in fuel can be in two forms:▫Dissolved in the fuel▫Entrained or suspended in the fuel

•Water in fuel can cause icing in the aircraft fuel system, usually in:▫Boost pump screens▫Low pressure filters

•Large amounts of water can cause engine stoppage

Page 5: Aircraft Fuel Systems

Microbial Growth

•Microbial Growth is produced by various forms of micro-organisms that live and multiply in water which is in jet fuel

•These micro-organisms form slime that can be red, brown, green, or black

•The organisms feed on hydrocarbons in the fuel but require water to multiply

•This buildup can:▫Interfere with fuel flow and quantity

indications▫Start electrolytic corrosive action

Page 6: Aircraft Fuel Systems

Contamination Detection

•Coarse fuel contamination can be detected visually

•Uncontaminated fuel should be:▫Clean▫Bright▫Contain no perceptible free water

Page 7: Aircraft Fuel Systems

Contamination Detection (cont.)•Clean means the absence of any readily

visible sediment or entrained water•Bright refers to the shiny appearance of

clean, dry fuel•Free water is indicated by a cloud, haze,

or water slug▫Water saturated in fuel is not always visible▫Perfectly clear water can contain as much as

three times the acceptable limit

Page 8: Aircraft Fuel Systems

Contamination Detection (cont.)•There is no accurate method of detecting

fuel entrained water when it is frozen•For this reason, it is important that fuel is

checked when the water is in a liquid state▫This should not be done following a flight at

altitude when the fuel would be below 32 degrees F

▫It is more effective to drain the fuel after the fuel has set undisturbed for a period of time, allowing the water to precipitate and settle to the drain point

Page 9: Aircraft Fuel Systems

Fuel Systems

•The purpose of an aircraft fuel system is to store and deliver the proper amount of clean fuel at the correct pressure to the engine

•Fuel systems should provide positive and reliable fuel flow through all phases of flight including:▫Changes in altitude▫Violent maneuvers▫Sudden acceleration and deceleration

Page 10: Aircraft Fuel Systems

Fuel Systems (cont.)

•Fuel systems should also continuously monitor system operation such as:▫Fuel pressure▫Fuel flow▫Warning signals▫Tank quantity

Page 11: Aircraft Fuel Systems

Types of Fuel Systems

•Fuel systems can be classified in two broad categories:▫Gravity-Feed Systems▫Pressure-Feed Systems

Page 12: Aircraft Fuel Systems

Gravity-Feed Systems

•Gravity-Feed Systems use only the force of gravity to push fuel to the engine fuel-control mechanism

•The bottom of the fuel tank must be high enough to provide adequate pressure to the fuel-control component▫This type of system is often used in high-

wing light aircraft

Page 13: Aircraft Fuel Systems

Pressure-Feed Systems

• Pressure-Feed Systems require the use of a fuel pump to provide fuel-pressure to the engine’s fuel-control component

• There are two main reasons these systems are necessary:▫ The fuel tanks are too low to provide enough pressure

from gravity▫ The fuel tanks are a great distance from the engine

• Also, most large aircraft with higher powered engines require a pressure system regardless of the fuel tank location because of the large volume of fuel used by the engines

Page 14: Aircraft Fuel Systems

Fuel System Components

• Pumps• Tanks• Lines• Valves• Fuel Flow-meters• Filters and Strainers

• Quantity Indicators• Warning Components• Fuel Drains• Heaters

Page 15: Aircraft Fuel Systems

Fuel Pumps

•Fuel pumps are used to move fuel through the system then gravity feed is insufficient

•There are three main functions of fuel pumps, they are to move fuel from:▫The tanks to the engines▫One tank to another▫The engine back to the tanks

Page 16: Aircraft Fuel Systems

Fuel-Pump Requirements

•Engine fuel systems require main pumps and in some systems emergency pumps

•These requirements depend on the type of engines installed on the aircraft

Page 17: Aircraft Fuel Systems

Reciprocating-Engine Fuel-Pump Requirements•Reciprocating-engines which are not

gravity-fed require:▫At least one main pump for each engine▫These pumps must be engine-driven▫The pump capacity must capable of

providing enough fuel flow for all operations

Page 18: Aircraft Fuel Systems

Turbine-Engine Fuel-Pump Requirements•Turbine-Engines require:

▫At least one main pump for each engine▫Main pump power supply must be

independent of all other main pump power supplies

▫Each positive-displacement main pump must be able to be bypassed

Page 19: Aircraft Fuel Systems

Turbine-Engine Fuel-Pump Requirements (cont.)•Turbine-engines also require emergency

pumps▫The emergency pump must be immediately

available to supply fuel to the engine in the event of a main pump failure

▫Emergency pump power supplies must be independent of that of the corresponding main pump

▫If both the emergency and main pumps operate continuously, there must be some means of alerting the flight crew of a failure of either pump

Page 20: Aircraft Fuel Systems

Fuel Pump Classification

•One way to classify fuel pumps is according to the pump’s function

•These classifications are:▫Boost Pump▫Scavenge Pump▫Cross-feed Pumps

Page 21: Aircraft Fuel Systems

Fuel Pump Classification

•Another way to classify fuel pumps is by their method of operation

•These pumps are:▫Vane-type

Variable-volume▫Centrifugal▫Ejector

Page 22: Aircraft Fuel Systems

Vane-Type Pumps

•Vane-type fuel pumps are the most common

•They use a rotor which turns vanes in a cylinder, the vanes act to push the fuel through the system

•Vane-type pumps can have from two to six vanes and they may be variable volume also

Page 23: Aircraft Fuel Systems

Centrifugal Pumps

•Centrifugal pumps are used to move fuel from one tank to another or from the fuel tank to the engine

•They are electrically driven and some may operate at different speeds

Page 24: Aircraft Fuel Systems

Ejector Pumps

•An ejector pump is normally used to scavenge fuel from remote areas

•These pumps have no moving parts they rely on return fuel from the engine to pump the fuel

•Ejector pumps work on the venturi principle

Page 25: Aircraft Fuel Systems

Fuel Tanks

•Fuel systems on different aircraft may use several types of fuel tanks

•The three basic types of fuel tanks used on aircraft are:▫Integral▫Rigid Removable▫Bladder

Page 26: Aircraft Fuel Systems

Integral Fuel Tanks

• Integral Fuel Tanks are commonly located in the aircraft’s wings or fuselage

• These tanks are ones that are built into the structure of the aircraft and generally can not be removed

• Integral Fuel Tanks are formed by the actual structure of the aircraft

• The seams are sealed, usually with synthetic rubber, to produce an area inside the aircraft structure which will contain the fuel

• This type of tank is used in some light high-performance aircraft and turbine-powered transports

Page 27: Aircraft Fuel Systems

Rigid Removable Fuel Tanks

•Rigid removable fuel tanks are often made of aluminum components that are welded together

•These tanks are installed in compartments specifically made for the tank

•The tanks may be held in place with padded straps

•This type of tank is often found on more expensive light aircraft and reciprocating-engine-powered transports

Page 28: Aircraft Fuel Systems

Bladder Type Fuel Tanks

•Bladder type fuel tanks are basically a reinforced rubberized bag

•These tanks are installed in compartments which support the weight of the fuel

•The tank is held in place with buttons or snaps on the bottom and sides of the tank

•This type of tank is usually found on light aircraft and some turboprop and turbine-powered aircraft

Page 29: Aircraft Fuel Systems

Fuel Lines

•Fuel lines on aircraft are either made of rigid metal tubing or flexible hose

•Most of the fuel lines are the rigid type which are usually made of aluminum alloys

•The flexible hose fuel lines are either made of synthetic rubber or Teflon

•The diameter of tubing used is decided by the engine’s fuel requirements

Page 30: Aircraft Fuel Systems

Valves

•Fuel selector valves are used in aircraft fuel systems to:▫Shut off fuel flow▫Cross-feed▫Transfer fuel

•Selector valves may be operated manually or electrically depending on the installation

Page 31: Aircraft Fuel Systems

Filters and Strainers

•Fuel is usually strained at three points in the system

•Through a finger or bootstrap strainer in the bottom of the fuel tank

•Through a master strainer which is usually located at the lowest point in the system

•Through a third strainer near the fuel control unit

Page 32: Aircraft Fuel Systems

Quantity Indicators

•Mechanical▫Inverted float gauge▫Rotating dial gauge▫Upright float gauge▫Sight-glass gauge

•Resistance•Capacitance

Page 33: Aircraft Fuel Systems

Fuel Subsystems

Some aircraft fuel subsystems allow for fuel:

•Jettison•Heating•Cross-Feeding

Page 34: Aircraft Fuel Systems

Fuel Jettison

•The fuel jettison system comprises a combination of fuel lines, valves, and pumps provided to dump fuel overboard during an in-flight emergency

•This will reduce the weight of the aircraft so an emergency landing is possible

Page 35: Aircraft Fuel Systems

Fuel Heating

•Fuel heating is necessary for turbine engines to thaw ice particles in the fuel that would otherwise clog the filters

•Fuel is routed through a heat exchanger that uses either engine oil or compressor bleed air to bring the fuel up to an acceptable temperature

Page 36: Aircraft Fuel Systems

Cross Feeding

•Cross feed systems allow the flow of fuel from any of the tanks to any of the engines

•Some reasons that this system might be used are:▫Engine failure▫Problem with one or more fuel tanks▫Redistribute fuel for weight and balance

purposes

Page 37: Aircraft Fuel Systems

MODIFICATIONS IN FUEL SYSTEM OF CESSNA 152• There are hundreds of modifications available for the

Cessna 152. The most frequently installed include:• Auto fuel STCs, which permit the use of automobile

fuel instead of the more expensive aviation fuel.• Auxiliary fuel tanks for larger capacity. • Belly fuel drain valves to drain fuel from the lowest

point in the fuel system. •  There are hundreds of modifications available for the

Cessna 152. The most frequently installed include:• Auto fuel STCs, which permit the use of automobile

fuel instead of the more expensive aviation fuel.• Auxiliary fuel tanks for larger capacity. • Belly fuel drain valves to drain fuel from the lowest

point in the fuel system. •  

Page 38: Aircraft Fuel Systems

FADEC (Full Authority Digital Electronic Control)• FADEC is the primary interface between the

engine and the aircraft. It is located on the fan case at 10:30 position; is 13.5 inch wide; 18.6 inch long; 4.35 inch high; and weighs only 27.5 lb. The FADEC contains two channels that are called “A” channel and “B” channel. Each time the engine starts, the alternate channel will automatically be selected. The channels are linked together by internal mating connector for crosslink data transmission. Much more is accomplished by this control than simply sending a signal to the fuel metering unit to establish a fuel flow to the nozzles.

Page 39: Aircraft Fuel Systems

INSPECTION, MAINTAINANCE AND REPAIR OF FUEL SYSTEMS•The proper and regular inspection of

aircraft fuel systems is critical to the safe operation of the aircraft. The failure of a component may result in an engine failure due to insufficient fuel being delivered or in a fire on the aircraft.

Page 40: Aircraft Fuel Systems

FUEL SYSTEM INSPECTIONS

•The preflight inspection of a fuel system includes checking the fuel tanks visually for quantity of fuel. This requires the removal of tanks caps and looking into fuel tanks. Sometimes a drip stick must be used in order to determine that the proper quantity of fuel is in the tanks.

•All fuel drains should be opened for a few seconds to allow any accumulated water or sediment to drain out of the system.

Page 41: Aircraft Fuel Systems

Typical instruction for such a check are as follows•completely drain the fuel tank, including

unusable fuel•put the amount of fuel in the tank that is

specified as unusable.•check the quantity guage to see it reads

zero.•add fuel to the tank in the increment of 10

gal [37.9 L] and check the quantity guage at each increment to see that it accurately indicates the correct amount of fuel in the tank within acceptable limits.

Page 42: Aircraft Fuel Systems

FUEL TANK INSPECTION AND REPAIRS• Gasoline has an identification dye added, and

this dye concentrates around fuel leaks, making them easy to spot.

• Turbine fuel leaks are harder to spot when they are fresh, but the collection of dirt on the leak and discoloration make the leaks evident in a short period of time.

• When inspecting rigid removable and bladder type fuel tank installations, fuel leaks appearing on the skin of the aircraft are cause to inspect the fuel tank.

Page 43: Aircraft Fuel Systems

TROUBLESHOOTING• 1. AIR CONTAMINATION• a) Enters through venting pipes. May be

contaminated by dust and microbiological spores. It also may contain large amounts of moisture.

• b) Air displaces fuel volume.• c) The oxygen present will increase the oxidation of

the fuel. This is the beginning of sludge formation resulting in unstable fuel which will trigger filter and nozzle plugging. (Better known as unscheduled downtime).

• HOW TO REMOVE• Do not store fuels for long periods of time in

partially empty tanks without fuel stabilizers

Page 44: Aircraft Fuel Systems

TROUBLESHOOTING• 2. WATER CONTAMINATION• a) Easiest to identify and most destructive.• b) Free water leads to corrosion and fuel degradation.• c) Provides fertile growing place for bugs.• d) Vents and seals that are poorly designed,

improperly installed, or faulty can allow water to enter.

• HOW TO REMOVE• If possible stick with paste for the best defense. Don’t

depend only on electronic monitoring systems.• Tanks must be cleaned properly and don’t allow water

to store in empty fuel tanks (when aircraft isn’t flying).

Page 45: Aircraft Fuel Systems

TROUBLESHOOTING• 3. FUEL CONTAMINATION• a) Fuel in storage will degrade to form insoluble materials. (Particulates &

Sludge)• b) Results include plugged filters, fouled injectors/nozzles, formation of

combustion system deposits, and accelerated corrosion.• c) As the fuel cools, water will condense and generate free water.• WAYS OF FUEL CONTAMINATION

▫ Fuel gets contaminated if the filler caps of the fuel tanks are not properly closed or tightened .

▫ Fuel also gets contaminated if there is some water present in fuel tanks or the water isn’t properly drained out from fuel tanks.

▫ Fuel contamination also occurs when the fuel supplied to the fuel tanks is already contaminated .

• HOW TO REMOVE• Fuel stability is dependent on several factors including crude sources, refinery

processes, base fuel components, base fuel blends and storage conditions. • Fuel must be checked for impurities before loading into the fuel tanks• Filler caps of the fuel tanks must be tightened or closed properly to avoid the

contamination.• Fuel tanks must be cleaned properly so that no water is present in the tanks to

cause contamination.

Page 46: Aircraft Fuel Systems

TROUBLESHOOTING• 4. FAULTY DESIGN• STORAGE TANK PROBLEM• a) Most have been poorly designed, system wide.• b) Improper placement of water.• c) Lack of water control causes internal corrosion and a complex

corrosive mechanism resulting in microbial activity.• d) It can be removed by designing of the fuel system by skilled labour• 5. MICROBES IN FUEL• a) Microbes cause problems because they promote corrosion, and

their physical presence leads to plugging of filters and injectors/nozzles.

• b) Controlling micro biologics is difficult because it is almost impossible to exclude water entirely from commercial hydrocarbon fuels.

• CONTROL OF THE MICROBES• a) Elimination of water is most effective step.• b) Keeping the system aerated.• c) Remove contaminated materials if possible.•