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FUEL SYSTEMS

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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

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Fuel System Contamination The higher the viscosity of the fuel, the greater its 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

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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

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Microbial Growth Microbial Growth is produced by various forms of micro-

organisms that live and multiply in water which is in jetfuel

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

The organisms feed on hydrocarbons in the fuel butrequire water to multiply

This buildup can: Interfere with fuel flow and quantity indications

Start electrolytic corrosive action

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Contamination Detection

Coarse fuel contamination can be detected

visually

Uncontaminated fuel should be: Clean

Bright

Contain no perceptible free water

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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

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Contamination Detection (cont.)

There is no accurate method of detecting fuel entrainedwater 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 setundisturbed for a period of time, allowing the water toprecipitate and settle to the drain point

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Fuel Systems The purpose of an aircraft fuel system is to store and

deliver the proper amount of clean fuel at the correctpressure to the engine

Fuel systems should provide positive and reliable fuelflow through all phases of flight including: Changes in altitude

Violent maneuvers

Sudden acceleration and deceleration

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Fuel Systems (cont.)

Fuel systems should also continuously monitor

system operation such as:

Fuel pressure

Fuel flow

Warning signals

Tank quantity

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Types of Fuel Systems

Fuel systems can be classified in two broad

categories:

Gravity-Feed Systems

Pressure-Feed Systems

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Gravity-Feed Systems Gravity-Feed Systems use only the force of

gravity to push fuel to the engine fuel-controlmechanism

The bottom of the fuel tank must be high enoughto provide adequate pressure to the fuel-controlcomponent

This type of system is often used in high-wing lightaircraft

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Pressure-Feed Systems Pressure-Feed Systems require the use of a fuel pump to

provide fuel-pressure to the engines 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 apressure system regardless of the fuel tank location because ofthe large volume of fuel used by the engines

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Fuel System Components Pumps

Tanks

Lines

Valves

Fuel Flow-meters

Filters and Strainers

Quantity Indicators

Warning Components

Fuel Drains

Heaters

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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

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Fuel-Pump Requirements

Engine fuel systems require main pumps and in

some systems emergency pumps

These requirements depend on the type ofengines installed on the aircraft

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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

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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

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Turbine-Engine Fuel-Pump

Requirements (cont.) Turbine-engines also require emergency pumps

The emergency pump must be immediately available tosupply fuel to the engine in the event of a main pump failure

Emergency pump power supplies must be independent ofthat 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 afailure of either pump

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Fuel Pump Classification

One way to classify fuel pumps is according to

the pumps function

These classifications are: Boost Pump

Scavenge Pump

Cross-feed Pumps

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Fuel Pump Classification

Another way to classify fuel pumps is by their

method of operation

These pumps are: Vane-type

Variable-volume

Centrifugal

Ejector

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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 thesystem

Vane-type pumps can have from two to six

vanes and they may be variable volume also

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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

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Ejector Pumps

An ejector pump is normally used to scavenge

fuel from remote areas

These pumps have no moving parts they rely onreturn fuel from the engine to pump the fuel

Ejector pumps work on the venturi principle

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Fuel Tanks

Fuel systems on different aircraft may use

several types of fuel tanks

The three basic types of fuel tanks used onaircraft are:

Integral

Rigid Removable

Bladder

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Integral Fuel Tanks Integral Fuel Tanks are commonly located in the aircrafts wings

or fuselage

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

Integral Fuel Tanks are formed by the actual structure of theaircraft

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

This type of tank is used in some light high-performance aircraftand turbine-powered transports

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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 lightaircraft and reciprocating-engine-powered transports

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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

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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 engines

fuel requirements

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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

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Filters and Strainers Fuel is usually strained at three points in the system

Through a finger or bootstrap strainer in the bottom ofthe fuel tank

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

Through a third strainer near the fuel control unit

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Quantity Indicators

Mechanical

Inverted float gauge

Rotating dial gauge

Upright float gauge

Sight-glass gauge

Resistance

Capacitance

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Fuel Subsystems

Some aircraft fuel subsystems allow for fuel:

Jettison

Heating Cross-Feeding

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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

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Fuel Heating Fuel heating is necessary for turbine engines to

thaw ice particles in the fuel that would otherwiseclog the filters

Fuel is routed through a heat exchanger thatuses either engine oil or compressor bleed air tobring the fuel up to an acceptable temperature

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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

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FUEL TANKS

Aircraft fuel tanks come in a variety of types and

sizes.

Can be located almost anywhere in the aircraft

(wings, fuselage, tail).

Managing fuel distribution between tanks on

large aircraft can be very involved.

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BLADDER TANKS

Rubber bladders are used to store fuel. Usuallyin the wings.

Will deteriorate over time, but are easier to

replace than metal tanks. Black flecks may appear in strained fuel which

indicates deterioration.

Tend to deform over time which causes water,fuel, and sediment entrapment.

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BLADDER TANK DEFORMATION

Over time the bladder begins to deform and rise up between attach points.

This causes fuel, water, and sediment to collect in the valleys.

Which results in increased unusable fuel, inaccurate quantity readings,

possible contamination during aggressive attitudes.

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RIGID REMOVABLE TANKS

Welded aluminum tanks inserted into the aircraft.

Usually fuselage tanks.

A disadvantage of this type of tank is added weight.

An advantage is the ability to remove and repair.

The Selair C-172 fleet is equipped with this type of tank

with the exception of two airplanes:

OSQ- 50G integral tanks SPY-60G integral tanks

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INTEGRAL TANKS (WET WING)

Integral tanks are made by sealing off

compartments inside the wings.

They have the advantage of utilizing existing

aircraft structure to contain fuel, which reduces

weight.

Commonly found in large aircraft.

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EXTERNAL WING TANKS (TIP TANKS)

These fuel tanks are mounted externally.

Tip tanks at the end of the wingtips. (C-310)

Underwing tanks: no those arent bombs.(Lockheed Jetstar)

Tip tanks can have an aerodynamic advantage

as they act like winglets.

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FUEL TANK LAYOUT

Fuel tanks can be arranged in multiple tank designs.

Fuel can be used simultaneously from different tanks, orone at a time.

On large aircraft the order in which tanks are filled andburned off has an effect on weight and balance.

Some complex fuel systems have fuel burn schedules whichinvolve systematic burn off and transfer between tanks toensure limits are not exceeded.

In the case of wet wing aircraft outboard tanks are usuallyfilled first and emptied last, to ensure wing structuralintegrity. The fuel in the wings counteracts the forces ofweight.

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F l b i t i i ft

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Fuel burn in swept wing aircraft can

have a significant effect on C of G.

Involved fuel burn schedules

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COLLECTOR TANKS

Aircraft with long wings are subject to fuel starvation

due to sloshing.

This is guarded against by incorporating collector tanks

into the system. All fuel goes to the collector tank prior to reaching the

engine.

This smaller collector tank is always full of fuel whichabsorbs any interruptions in feed due to sloshing.

Collector tank prevents engine fuel

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Collector tank prevents engine fuel

starvation due to sloshing.

INTEGRAL WING TANK

COLLECTOR TANK

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FUEL PUMPS

High wing carbureted aircraft are usually gravity fed and dontneed fuel pumps. (C-172)

Fuel injected and low wing aircraft require a fuel pump to supplypositive pressure to the fuel metering system.

Fuel pumps are also used to transfer fuel between tanks andprovide crossfeed.

Fuel pumps are usually lubricated by the fuel itself and canoverheat if run dry.

These pumps are usually engine driven. Fuel is fed to the engine at a rate faster than it can be used, this

means return lines are necessary.

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CAVITATION

The formation of an air pocket (cavity) in the fuel.

If the fuels pressure becomes too low it will vaporize.

The pump creates a low pressure area as the fuel is

accelerated. Air pockets forming on the suction side ofthe pump can cause cavitation.

Fuel pumps are incapable of pumping a gas.

This can cause pump damage, and possibly aninterruption in flow.

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BOOST PUMPS (STANDBY PUMPS)

Boost pumps are used:

As a backup for the engine driven pump.

Crossfeed operation.

Priming.

Start operation.

Fuel transfer.

Provide positive pressure to the engine driven pump.

Usually on for take off and landing to guard against an enginefailure due to pump failure at a critical point.

Boost pumps are also used to provide positive feed pressure toengine driven pumps which helps prevent cavitation.

These pumps are usually electrically powered.

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MOTIVE FLOW PUMPS (JET PUMP)

These pumps are usually used for inter-tank

transfer.

They rely on venturi effect to create suction.

A electrically or engine driven pump provides

flow in the line, then a venturi creates suction.

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FUEL VALVES

Used to guide the flow of fuel within the system.

Fuel valves can be manual (C-172, B-95) or

electrically powered.

Check valves restrict flow to one direction.

Tank selector valves control which tank is to be

used.

Firewall shut-off valves prevent fuel from reaching

the engine. Used to secure engine in emergency

situations.

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FUEL HEATERS

Jet fuel is prone to ice crystal formation andcongealing.

Fuel heaters are incorporated to ensure the fuel

is warmed to optimum operating temperaturesbefore it reaches the engine.

This is usually accomplished by some form of

heat transfer. Ex. Running the fuel lines througha heat exchanger plumbed with warm oil lines.

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FUEL VENTS

As fuel is removed from a tank it must bereplaced with air or a vacuum will be created andfuel flow will stop.

The vacuum could possibly create tank collapse. Provides an escape for air in the case of thermal

expansion.

Vents must be heated or flush mounted, orrecessed to protect against icing conditions.

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DRAINS AND STRAINERS

Drains at the low points of a fuel system are

important to drain water which collects at the

bottom. To drain tanks for maintenance.

Strainers collect contaminants in the fuel to

ensure they are not ingested by the engine.

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MEASURING QUANTITY

Most light aircraft utilize floats to measure fuel quantity.

More sophisticate aircraft use capacitance type quantity

indicators.

Jet fuel volume changes significantly with temperature.

Mass will remain constant and can be measured by

electric probes or light sensing prisms.

The gauges of this sort of system usually indicate fuelquantity in pounds.

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DIPSTICKS

Dipping fuel tanks is common practice with light

aircraft.

The gauges tend to be inaccurate and dipping the

tanks often results in more accurate readings.

Most large aircraft have a manual method of

determining fuel load in the event of gauge failure

Magnetic measuring sticks are one method ofaccomplishing this.

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CROSSFEED

Crossfeed capabilities of a multi-engine fuel system areessential to ensure fuel on the failed engine side is availablefor use.

Crossfeed also enables the pilot to correct fuel imbalancesituations.

It is important to understand how the system works for yourspecific aircraft.

In some systems certain tanks may be unavailable duringcrossfeed.

Specific procedures may apply. (B-95 failed engine selectormust not be off)

The decision to crossfeed fuel after an engine failure shouldnot be taken lightly. If the engine failure was the result ofcontaminated fuel it could mean trouble for the operative

engine.

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C-172

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C-210