Aim Understand the principals of operation of the fuel
systems
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Objectives 1.Describe the fuel system and components 2.Describe
fuel types and properties 3.State operational aspects associated
with the fuel system
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1. Fuel System Fuel System Overview The fuel system is made up
of a number of individual components including: Fuel Tanks Fuel
quantity detectors Strainers/filters Tank selectors Fuel pumps
Primers
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1. Fuel System Fuel Tanks Tanks can be constructed of aluminium
alloy, synthetic rubber or stainless steel They are normally
fabricated to be an integral part of the wing or fuselage Some
aircraft utilize bladder type fuel tanks They must be designed to
distribute weight evenly Expansion space should be incorporated to
account for fuel expansion on hot days Vents are incorporated to
allow air to escape as fuel expands due to heat and allow air to
enter as fuel is drawn into the engine. Some aircraft use forward
facing vents to maintain positive pressure above the fuel Tanks may
be pressurised to reduce vapour formation at altitude, suction
relief valves will be incorporated to ensure fuel flow should the
pressurisation system fail
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Fuel Tanks Baffles are fitted inside tanks to increase strength
and prevent fuel surging Sumps and drains are used to allow
contaminants to settle and be removed from the tank A standpipe is
used to draw the fuel into the system allowing us to avoid drawing
in any contaminants in the sump, this leaves some unusable in the
tank below the standpipe level 1. Fuel System
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Fuel Quantity Detectors Float Type Most light general aviation
aircraft utilize a simple float. Here a metallic float sits on top
of the fuel, movement of the fuel is transmitted via mechanical
linkage or electrical signal to the cockpit These systems are
notoriously unreliable, you must dip the tanks to ensure the actual
fuel quantity is known before flight 1. Fuel System
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Fuel Quantity Detectors Capacitance Type Sophisticated general
aviation aircraft and large aircraft use capacitance type fuel
quantity detectors, these are infinitely more reliable than float
type however they do cost more A capacitor is used to store charge
by separating two conductive plates with a dielectric, the
dielectric is used to polarize the electric field The amount of
charge that can be stored varies with the size of the plates, the
distance between the plates and the dielectric constant Air and
fuel have different dielectric constants therefore allowing
different amounts of charge, a voltmeter is used to measure this
voltage and a fuel quantity is derived 1. Fuel System
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Strainers/Filters The fuel will usually be strained at three
points: The first is the tank outlet where the fuel will normally
pass through a wire gauze to remove larger foreign matter The
second will be placed at the lowest point in the system and will
also incorporate a drain The final strainer will be located at the
carburettor or fuel control unit, here the fuel will be filtered
through a fine gauze before entering the engine 1. Fuel System
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Tank Selector Selector valves allow individual tanks to be
selected to feed the engine, they will typically also include
facility to isolate the system They must be readily accessible and
provide sufficient feel so that they cannot be inadvertently
miss-selected inflight The valves must be positioned on the
opposite side of the firewall to the engine so if a fire occurs it
cannot spread to the tanks 1. Fuel System
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Engine Driven Fuel Pump The engine driven fuel pump operates
automatically and provides fuel under pressure to the carburettor
or fuel control unit The most common system is the vane type: The
engine drives an impellor which uses centrifugal force to increase
fuel pressure 1. Fuel System Constant fuel pressure is maintained
by using a pressure relief valve, the fuel pressure is determined
by a combination of a spring tension and air pressure on top of a
diaphragm Should the pump fail a bypass valve is incorporated into
the system
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Electric Fuel Pump Sometimes referred to as boost or auxiliary
pumps Operate on the same centrifugal principals as the engine
driven bump but are driven by a small electric motor Are designed
to provide 120% of the maximum demand of the engine Are used to:
Prime fuel injected engines Provide fuel at the correct pressure
during start Purge the fuel lines of any vapour and reduce the
possibility of vapour lock Provide a backup to the engine driven
fuel pump Will usually incorporate a bypass valve in case of pump
failure 1. Fuel System
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Primer For carburetted engines insufficient fuel is lifted out
of the metering jets for reliable starting, particularly on cold
days where vaporisation of the fuel in the intake manifold is poor
The primer delivers extra fuel directly from the strainer bowl into
the manifold of one or more cylinders The system will normally
utilize a simple hand pump operated from inside the cockpit 1. Fuel
System The primer must be locked when the engine is running to
prevent excess fuel entering the intake manifold
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Gravity Feed System Only found on high wing aircraft where a
head of fuel is maintained between the fuel tank and engine As the
level of fuel in the tank reduces the fuel pressure will reduce,
the system is designed to support a flow rate of at least 150% of
the maximum required during any stage of flight 1. Fuel System The
two major disadvantages of the system are: Changes in attitude,
particularly negative loading, reduces fuel flow Susceptible to
vapour lock due to the low pressure in the system
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Pump Feed System Can be found on low or high wing aircraft
Requires a fuel pump to pump fuel from the tank to the engine 1.
Fuel System
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AVGAS Aviation gasoline (AVGAS) is manufactured and managed
under more stringent quality control standards than ordinary motor
gasoline (MOGAS) In general terms: AVGAS contains more lead than
MOGAS giving it a higher octane AVGAS is less volatile than MOGAS
reducing the susceptibility of vapour lock AVGAS is stored and
maintained at higher standards to increase safety 2. Fuel Types and
Properties
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Octane Ratings and Performance Numbers The anti-detonation (or
anti-knock) properties of fuel are designated by either an octane
rating or performance number When the system was first introduced
it was believed a rating of 100 represented the limit to which the
anti-detonation properties of gasoline could be taken,
technological advances have seen gasoline with anti-detonation
properties greater than 100 therefore fuels are now assigned a
performance number 2. Fuel Types and Properties Fuel GradeColour
MOGAS 91Purple MOGAS 91Red as of end 2013 MOGAS 96Yellow AVGAS 100
LLBlue AVGAS 100/130Green AVGAS 115/145Purple AVTURPale Yellow A
naturally occurring group of hydro carbons with an octane rating of
100 are known as iso-octane, with the addition of lead the
anti-detonation properties can increase up to 45% giving a gasoline
performance number of 145 A dye additive is added to fuel to easily
identify the octane The common fuel grades can be seen here
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Fuel Contaminants It is the responsibility of the oil company
to ensure there are no contaminants in the fuel at time of
delivery, once delivered it is up to the operator The most common
contaminant is water, this can enter tanks through condensation or
poor seals In order to avoid condensation forming the tanks should
be filled to full overnight Other impurities include rust, sand,
micro organisms (Mould), etc 3. Operational Aspects
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Fuel Checks Fuel checks should be done in accordance with CAO
20.2 and company procedures Fuel drains are located at the lowest
points of the fuel system and contaminants should settle to these
points If contaminants are detected you should continue draining
until pure fuel is drained The most common contaminant is water and
will settle as a bubble in the bottom of the drain. You must ensure
the whole sample is not contaminated with water There are a number
of commercially available fuel testing papers and pastes that react
when water is present 3. Operational Aspects