PIA TRAINING CENTRE (PTC) Module 13 -AEROPLANE AERODYNAMICS, STRUCTURES AND SYSTEMS

Category – B2 Sub Module 13.13 - Fuel Systems

MODULE 13
Sub Module 13.13

FUEL SYSTEMS (ATA 28)

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Category – B2 Sub Module 13.13 - Fuel Systems

Contents

SYSTEM LAY-OUT ---------------------------------------------------------------- 3

FUEL TANKS ----------------------------------------------------------------------- 5
SUPPLY SYSTEMS ---------------------------------------------------------------- 8
DUMPING, VENTING AND DRAINING -------------------------------------- 10
CROSS-FEED AND TRANSFER ------------------------------------------------- 13
INDICATIONS AND WARNINGS ---------------------------------------------- 19
REFUELLING AND DEFUELLING ---------------------------------------------- 31
LONGITUDINAL BALANCE FUEL SYSTEMS --------------------------------- 37

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Category – B2 Sub Module 13.13 - Fuel Systems

Fuel System

Aircraft fuel systems vary in complexity from the extremely are equal to the outboard tanks. On some newer aircraft models
simple systems found in small, single-engine airplanes to the a feature is built in to the fuel management system, which will
complex systems in large jet transports. Regardless of the type transfer fuel from tank to tank automatically to control the
of aircraft, all fuel systems share many of the same common shifting of the center of gravity.
components. Every system has one or more fuel tanks, tubing
Improper management of the fuel system has caused more
to carry the fuel from the tank(s) to the engine(s), valves to
control the flow of fuel, provisions for trapping and the removal aircraft accidents than failures of any other single system.
Engine failure will occur if all of the fuel in the tanks has been
of water and contaminants, and a method for indicating the fuel
burned, but engines will also stop if an empty tank is selected to
quantity. Although fuel systems in modern aircraft are designed
supply fuel to the engines, even though there is fuel in the other
with the safety and reliability aspects in mind, it is largely
tanks.
dependent upon the proper inspection and maintenance of
these systems.
Contamination in the fuel may clog strainers or filters and shut
All powered aircraft, whether rotary or fixed wing, depend upon off the flow of fuel to the engines. Contamination may take
many forms, including solid particles, water, ice and bacterial
a continuous, uninterrupted flow of uncontaminated fuel under
growth. Water that condenses in partially filled tanks will stop
all operating conditions for the engines to run. The weight of the
the engine when it flows into the metering system. Water in
fuel constitutes a large percentage of the aircraft's total weight.
turbine-powered aircraft is a special problem, as the more
This may range from about 10% of the gross weight of small
personal airplanes, to more than 40% for jet aircraft used on viscous jet fuel will hold water entrained in such tiny particles
that it does not easily settle out. When the fuel temperature
long haul flights.
drops at high altitude, the water may form ice crystals that can
The weight of the fuel requires that the structure be strong freeze and clog the fuel filters and shut off the flow of fuel. Many
enough to carry it in all flight conditions. The aircraft designer jet engines have fuel heaters to prevent ice formation on the
locates the fuel tanks so that the decreasing weight from fuel fuel filters and fuel metering system components.
consumption will not cause an imbalance condition of the
aircraft due to the shifting of the aircrafts center of gravity. To The type or grade of aircraft fuel must be carefully matched to
reduce stresses on the airframe and improve structural life, the engine. It is the responsibility of the ground engineer and
many jet transports have fuel management procedures that the pilot in command to verify before a flight is started that the
specify how the fuel is to be used from the various tanks. For aircraft is adequately supplied with the proper fuel. The person
example, a Boeing 747 will first use the fuel in the center wing doing the refueling can assist by being vigilant for problems with
tank, followed by fuel in the inboard tanks until their quantities fuel quality and type.

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Category – B2 Sub Module 13.13 - Fuel Systems

SYSTEM LAY-OUT

Light Aircraft Fuel Systems

Light aircraft fuel systems usually consist of two, either rigid or

flexible fuel tanks which are normally housed in each of the
wing structures. The fuel from the wing tanks is then fed
through to a selector valve/shut off valve which is located within
the cabin.

The ‘Selector valve’ is primarily the control which also separates

the ‘aircraft fuel system’ from the ‘engine fuel system’ and
therefore may also be termed the ‘LP cock’.
After the selector valve the fuel is fed through a filter and a fuel
pump called a ‘Booster Pump’ to the engine

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Category – B2 Sub Module 13.13 - Fuel Systems

Fig 1

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Category – B2 Sub Module 13.13 - Fuel Systems

FUEL TANKS

Fuel tanks normally fall into three categories of construction:

 Rigid
 Flexible
 Integral

Rigid tanks

These are normally made from metal or plastic material, they

are fitted internally where space permits. Flexible fuel tanks
have an advantage over rigid tanks, because they can be
shaped and fitted into odd shaped spaces where rigid tanks
cannot be fitted. In general, flexible tanks are lighter and easier
to handle and store than rigid tanks. Integral fuel tanks are of
rigid construction because they are part of the airframe structure.
They are not independent items like the other tanks.

Whatever the construction method, fuel tanks should be shaped

so that almost all the fuel is available to the engine. Awkward
pockets which prevent fuel from leaving the tank are
undesirable and are avoided if possible.

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Flexible Fuel Tanks

Flexible fuel tanks may be constructed with thin and very

flexible walls (called bag tanks) or they may be made of thicker
less flexible material. These tanks are made in shapes to fit
particular spaces in the aircraft structure and their flexibility
enables the tanks to be folded and inserted through a small
aperture, which would not allow a rigid tank of similar capacity
to be fitted. Because flexible tanks can be made in shapes to
suit most of the space available, a greater fuel capacity is made
available to a particular aircraft when flexible tanks are used.
Some aircraft fuel systems are designed to include rigid, flexible
and external fuel tanks so that the greatest possible fuel load is
carried.

Flexible fuel tanks are resilient, like an inner tube and because
they are resilient, the tanks can withstand a considerable
amount of distortion or shock loading. If a flexible tank is not
completely full it is unlikely to burst on a crash impact

Fig 4 Flexible Fuel Tank

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Category – B2 Sub Module 13.13 - Fuel Systems

Integral Fuel Tanks

An integral fuel tank is a fuel-proofed space in the aircraft

structure which is filled with fuel and provided with the
appropriate fittings and connections for fuel feed, fuel transfer,
air lines, vents and fuel pumps required at that particular
position in the fuel system. Connections and fittings cause few
problems, but sealing and fuel proofing the aircraft structure is
the vital element, which decides the success of the integral tank.
An integral wing tank is usually an area of a main plane
between the front and rear spars and bounded by the external
skin, which covers the wing structure. The tank area is sealed
and fuel proofed during assembly. Special sealants are used
under controlled conditions and the skin attachments, structures,
rivets and bolts are assembled whilst the sealant is wet. Dry
assemblies cannot be adequately sealed afterwards.

Fig 5 Integral Tank Sealing

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Category – B2 Sub Module 13.13 - Fuel Systems

Tank Numbering SUPPLY SYSTEMS

Aircraft manufacturers number fuel tanks, in which case the ENGINE FUEL FEED
philosophy will be from left to right, nose to tail
Design Requirements of an Aircraft Fuel Feed System

On an aircraft, a fuel system should be designed to comply with

many requirements as laid down in Joint Airworthiness
Requirements. An example of these requirements is as follows:

1. Each fuel system should be constructed and arranged to

ensure a flow of fuel at a rate and pressure to ensure
proper functioning of the engine for each likely operating
condition.

2. The fuel system must allow the supply of fuel to each

engine through a system independent of the system
supplying fuel to any other engine.

3. The system design should be such that it is not possible

for any pump to draw fuel from two or more tank
simultaneously unless means are provided to prevent
the introduction of air into the system.

4. If fuel can be pumped from one tank to another in flight,

the fuel tank vents and transfer system must be
designed so that no structural failure can occur because
of over-filling.
Fig 7 Fuel Tank Layout
5. Integral tanks must have facilities for interior inspection
and repair.

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Category – B2 Sub Module 13.13 - Fuel Systems

6. Fuel tanks must be designed, located and installed so 12. Each fuel line must be designed, installed and
that no fuel is released in or near the engines in supported to prevent excessive vibration and allow a
sufficient quantities to start a fire in otherwise survivable reasonable degree of deformation and stretching without
crash conditions. leakage.

7. Pressure cross-feed lines passing through crew,

passenger or cargo compartments shall either be
enclosed in a fuel and vapour proof enclosure, ventilated
and drained to the outside, OR consist of a pipe without
fittings and routed or protected against accidental
damage.

8. The system shall incorporate means to prevent the

collection of water and dirt or the deposition of ice or
other substances from satisfactory functioning of the
system.

9. Lines, which can be isolated from the system by means

of valves or fuel cocks, shall incorporate provision for the
relief of excess pressure due to expansion of the fuel.

10. Each fuel tank filler connection must be marked with

type of fuel and be provided with a bonding point and
drain discharging excess fuel.

11. There must be a fuel strainer at each fuel tank outlet

or for the booster pump(s).

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Category – B2 Sub Module 13.13 - Fuel Systems

DUMPING, VENTING AND DRAINING

Dumping (Jettison)

Fuel jettison systems are fitted to a number of large commercial

aircraft to allow the jettisoning of fuel in an emergency thus
reducing weight so as to prevent structural damage when
landing.

Fuel jettison systems are often fitted after the installation of a

center tank, because of the extra fuel weight.

The system illustrated is from a wide-bodied twin fitted with

multi tanks and booster pumps. The jettison pipe is branched
off the feed pipe between the inner tank fuel pump and the inner
tank shut off valve.

A check valve is installed to separate the outer tanks during

Fig 18 Jettison System
jettisoning. The function of this check valve is to prevent the
dumping of the outer tanks fuel. The jettison pipe runs inside
the wing tanks through the ribs into the outer tanks, where the
jettison valves are installed. These valves are fitted to the
bottom of the tank.

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Category – B2 Sub Module 13.13 - Fuel Systems

Fuel Vent System

A ram air intake maintains a slight positive pressure in the vent

system, thus decreasing fuel vaporization, and preventing
negative pressures in the tank through changes in aircraft
attitude and fuel usage (Refer Fig 22). In some aircraft, the vent
system also prevents the building up of dangerous pressures in
the tanks during refueling, should the automatic cut-off fail, by
dumping excess fuel. Generally, there are two vent pipes in
each tank, the inboard vent is open-ended, but the outboard
vent is fitted with a float valve, the purpose of which is to
minimize fuel transfer both between tanks and into the
vent/surge tank during changes of aircraft attitude. Fuel, which
is spilled into the venting system, collects in the vent/surge tank.
On some aircraft the vent/surge tank drains under gravity into
the main tanks, but on some types of aircraft an automatic
pumping system is used

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Fig 22

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Category – B2 Sub Module 13.13 - Fuel Systems

CROSS-FEED AND TRANSFER

Cross-feed valves and transfer valves enable the transfer of fuel

from one tank to another or from any fuel tank to any engine.
Some transfer valves will have an automatic function, others will
require more crew input

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Fig 26

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Two Man Crew

Providing the mode selector is set to “AUTO”, the transfer The normal position is AUTO. In this position the valves are
valves (fuel cells) will automatically function. The valves will be open when the tank is full and automatically shut when the tank
signaled to open by means of low level sensors in the inner cell. is empty.
Both valves will open when either tank quantity falls to the
appropriate level. Once open they will be latched open. The This is indicated by magnetic indicators, showing: -
valves are automatically closed at the next refuel operation.
The ECAM/EFIS systems page (Fuel) will display valve
 Green in-line, when the valves are open.
operation, there is no direct control from the overhead panel.
 Green cross-line, when the valves are shut.
The ‘X’ feed valves would normally be closed in flight. To open
the valve the push button switch on the overhead panel would  Amber, during the transit or when there is a failure.
have to be pushed on ‘OPEN’. A light indicates position.
The inner tank and outer tank shut off valves are each
Three Man Crew controlled by a rotary selector.

Older aircraft will have a fuel control panel on the flight They are marked by an engraved line to show the selected
engineer’s panel. This example is taken from a wide-bodied position.
twin. No inflight transfer from tank to tank is possible, but fuel
can be fed from any tank to any engine. The valves in this 1. In-line for open.
instance are not called “TRANSFER VALVES” but “TANK
SHUT OFF VALVES”. 2. Cross-line for closed.

Each center tank shut-off valve is controlled by a three-position

selector: ON, AUTO and SHUT. Inside of the rotary selector are disagreement lights. They
illuminate during the transit of the valve. The light extinguishes
when the valve has reached the selected position, but it will
remain on when the valve has a different position to the switch.

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Fig 27 Fuel Control Panel

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Category – B2 Sub Module 13.13 - Fuel Systems
Cross Feed

There is a fifth rotary selector situated just between the other

four.
This is the selector for the cross-feed valve.

As this valve is provided with two electrical motors, the selector

has two different engravings marked I and II.

In-line ‘I’ means the valve is opened by motor ‘I’ and in-line ‘II’
means that the valve is opened by motor ‘II’. Cross-line means
the valve is closed. The knob also includes a disagreement light.
The function is identical to the lights of the other selectors.

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Fig 28 Cross Feed

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INDICATIONS AND WARNINGS

The type of indicators and warnings will be dependent on the

technology level of the aircraft (analogue or glass cockpit). The
first example is the indication and warnings for a wide-bodied
twin (analogue) with a three-man crew.

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Fig 29 Fuel Control Panel

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The second example is from the same aircraft modified to a

glass cockpit and two man crew operation

Fig 30Fuel Control Panel – Glass Cockpit

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ENG and APU LP Valves Annunciators: Indicate the position

of the LP Valves.

X Feed Pushbutton Switch: Controls the position of the

Cross-feed Valve.

Wing Tank Isol Valves Pushbutton Switch: There is a

guarded pushbutton switch for each Wing Tank Isolation Valve.

Pump Pushbutton Switch: There is a pushbutton switch for

each pump. With all pushbuttons pressed in fuel feed sequence
operates automatically.

Trim TK Isol Valve Pushbutton Switch: Guarded pushbutton

switch manually overrides the Auto Mode or CGCC Control of
the Trim TK Isol Valve.

Trim TK Pumps Pushbutton Switch: There is a pushbutton

switch for each pump, both pushbuttons pressed in Fuel
Transfer will be controlled automatically.

Auto Mode Pushbutton Switch: Guarded pushbutton switch

manually overrides CGCC control of Trim TK Pumps and
Transfer Valves.

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Fig31
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Category – B2 Sub Module 13.13 - Fuel Systems

Fuel Level Sensing

A modern aircraft will use thermistors to send signals through

amplifiers to actuate warnings, sequencing, etc. Older aircraft
may use float switches as shown in the following diagram

Fig 32
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Overflow Sensing
Float operated switches are of a magnetic type, similar to the
one shown above and are designed to isolate the electrical If during refueling the high level shut off system fails, fuel enters
mechanism from the fuel tank for safety reasons. Upward the adjacent vent tank and washes around the overflow sensor.
movement of the float brings the armature closer to the magnet
and at a predetermined fuel level, it has sufficient influence to This is indicated by the amber FULL light on the refuel panel.
attract the magnet, which results in operation of the micro Low Level Sensing
switch. As the fuel level and the float fall, the attraction of the
armature is eventually overcome by the combined forces of the Low level sensing is divided into:
counterweight and the micro switch spring and the
counterweight falls, changing the micro switch circuit.  outer tank low level and
 inner/center tank low level sensing.
Whether they are float switches or thermistors, their functions
are as follows. If the outer tank LO LVL sensor is exposed to air, the
associated amber LO LVL light comes on.
 High level sensing.
 Overflow sensing. The inner/center tank low level sensing have only in the AUTO
 Low level sensing. MODE a function (ref. fuel pump control).
 Under full level sensing.
Calibration Sensing (Fuel Trim only)
Level sensing for calibration (Fuel Trim only).
Calibration sensors are installed in center tanks, inner tanks and
High Level Sensing trim tank. They give a signal at a predetermined filling level in
the trim tank for accuracy test of the fuel quantity indication
High level sensing is installed to prevent an overfilling of the fuel during refueling. For the trim tank the calibration sensor
tanks. When the fuel washes around the respective sensor, the: switching level is corrected by the stabilizer position.
associate refuel/defuel valve closes.
Under Full Level Sensing
blue FULL light on the fueling panel comes on.
When the fuel quantity drops in either outer tank below a certain
The high-level signal from the inner and outer tanks could be level, the maximum flight speed (VMO) becomes reduced in
used for computation purposes in the fuel quantity computer, order to protect the wing structure. The sensor signals are sent
when refueling in AUTO MODE. to the ADC (Air Data Computer).

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Fuel Quantity System Measurement and Indication Principle of Capacitance Gauging

The system has the following tasks A capacitor is an electrical device which stores electrical charge.
The amount of charge it can hold depends upon three physical
 Measuring of the fuel quantity in the tanks properties of the capacitor itself, namely:
 Indicating of the fuel quantity on
 The fuel quantity indicator  The surface area of the plates.
 The pre-selector  The size of the gap between the plates.
 The ECAM system fuel page  The insulating material (dielectric) between the plates.
 ECAM/EFIS.
 Controlling of automatic refueling In a fuel tank “capacitor stack” two of the above are fixed, ie. the
 Fuel quantity messaging to the flight management area of the plates and the gap between them. The only variable
computer. is the dielectric which, in a fuel tank, is either fuel or air or both.
The amount of charge held in the capacitor, when the tank is full,
The system comprises: will be of a preset value. As the fuel level falls, the dielectric will
gradually change to air and the amount of charge stored will
 fuel quantity computer. reduce. This change in capacitance is sensed by a signal
 capacitance probes. conditioner and the change in fuel level is thus sensed.
 capacitance index compensator.
 cadensicon sensor.
 attitude sensor.
 THS position detector.
 associated indicator in the flight compartment.

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Fuel Quantity Indicating System

Each tank has installed a group of probes arranged so that a

minimum of one probe is immersed at all times, the number of
probes will vary from aircraft to aircraft. The following example
is from a wide-bodied twin fitted with a fuel trim system.

The number of probes is:

6 in each outer tank.
6 in each inner tank.
4 in the center tank.

The probes of each group are wired in parallel and connected to

a summing adapter, located on the wing rear spar. The probe
level signals are sent to the fuel quantity computer

Fig 33 Wing Capacitance Probe Installation

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Capacitance Index Compensator

One compensator is installed in each tank to the lowest located

capacitance probe.

Separated wiring for these units is routed to the fuel quantity

computer.

The purpose of the index compensator is to sense the different

types of fuels, additives, etc. and make correction signals for
accurate fuel readings.

Fig 34 Capacitance Index Compensator - Installation

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Fig 35
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Measurement THS Position Detector

The signals from the capacitance probes in each tank are sent Senses the THS position steady and gives its signal to the fuel
via adapters to the fuel quantity computer. The computer quantity computer for correction of trim tank fuel measurement.
calculates the fuel quantity. To increase the measuring
accuracy, further signals enter the computation: Fuel Quantity Indicator

The fuel quantity of the tanks is normally displayed in 10 kg

Capacitance index compensator, balances different fuel types.
steps. Power supply and the indication signals are delivered by
Condensicon sensor: the fuel quantity computer. To avoid transmission errors, the
Senses while refueling indicator sends feedback signals to the computer. The indicator
is also used for test purposes. In the test mode, the indicator
displays different number codes
 The Density

 Dielectric constant of running fuel.

Attitude Sensor:

Senses on ground and in flight the attitude of the aircraft to

the:

 Roll axis (longitudinal)


 Pitch axis (lateral)

 The attitude signal computation depends on the
AIR/GRND signal (wing bending direction).

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REFUELLING AND DEFUELLING

Refueling a small aircraft is no more complex than filling the
Refueling family car. One limitation is that on some aircraft it is not
possible to fly the aircraft with all the seats occupied with full
As you will be aware, as any liquid flows through a pipeline, it baggage allowance, when the tanks are full. This means that if
will produce Static Electricity. If this static electricity were the aircraft is to be flown fully loaded, it may be necessary to re-
allowed to discharge in the presence of aviation fuel vapour, an fuel to less than full, to keep the aircraft within its weight limits.
explosion would result, with possible catastrophic results. To As the aircraft become more complex, the refueling exercise
therefore minimize the explosion risks, the following guidelines has to be carried out with more care. If the aircraft is small but
must be followed. has say, two tanks in each wing, and the fuel load is to be three
Safety Precautions: quarters full; then it may be the rule for that aircraft that the
inner tanks have to be filled to the top first and the remainder
 Use correct grade of fuel (Av-gas, Av-tur, Av-tag). put into the outer tanks. This puts less bending load on to the
 No smoking within 15m. wing spars.
 No metal studded or tipped footwear. When we get to larger aircraft, there are several further
 Correct bonding of Aircraft and Bowser. problems to consider. Not only must the aircraft be filled laterally
in the correct order but, if the aircraft has the fin, tail plane and
 Correct positioning of Bowser. rear fuselage tanks mentioned earlier, it must be refueled in the
 No vehicles or Ground Equipment under the aircraft. correct order longitudinally as well to ensure the aircraft stability
is maintained.
 Maintenance activity kept to a minimum.
 No replenishment of LOX. Modern large aircraft utilize pressure refueling, which has
replaced open line refueling on most aircraft with high fuel
 No transmitting of Radar capacities. The time taken to fill a Boeing 747 through a normal
 Aircraft & Bowser not to be left unattended. hose and nozzle system would take hours. With pressure
refueling, a large diameter hose is rigidly connected to a
 Check and remedy fuel spillage or leakage. coupling in the aircraft and fuel under pressure of about 40 psi
is pumped into the aircraft tanks. To assist this operation, most
 Appropriate Fire Appliance readily available.
aircraft can have the total fuel load pre-set at the point of
 The electrical state of the Aircraft must not change while connection so that the aircraft stops the refueling at the correct
connected to the Bowser. time. The illustrations show the location and layout of a typical,
Boeing 777, refueling panel.
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 PIA TRAINING CENTRE (PTC) Module 13 -AEROPLANE AERODYNAMICS, STRUCTURES AND SYSTEMS
Category – B2 Sub Module 13.13 - Fuel Systems

l Boeing Refueling Panel

Fig 36
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Category – B2 Sub Module 13.13 - Fuel Systems

Pressure Refuel – Functional Description

Fuel flows from the refuel adapters into the refuel/jettison

manifold. When the refuel valves open, fuel flows from the
manifold into the fuel tanks. A flow tube at the end of each
refuel valve decreases the exit force of the fuel. The flow tube
also puts the fuel in different parts of the tank.

Fig 37 Refuel System Layout

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 PIA TRAINING CENTRE (PTC) Module 13 -AEROPLANE AERODYNAMICS, STRUCTURES AND SYSTEMS
Category – B2 Sub Module 13.13 - Fuel Systems
Fig 38 Refuel Manifold Drain Valve

As each tank reaches full, the high-level sensor signals the

refuel valve to close to stop fuel flow. When all refuel flow
ceases, fuel that is left in the refuel/jettison manifold goes
through the manifold drain valves and into the main tanks. The
manifold has two vacuum relief valves. These valves permit air
into the manifold when the fuel leaves via the manifold drain
valves.

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 PIA TRAINING CENTRE (PTC) Module 13 -AEROPLANE AERODYNAMICS, STRUCTURES AND SYSTEMS
Category – B2 Sub Module 13.13 - Fuel Systems

If a refuel system failure prevents the refuel valves from closing,

fuel goes into the surge tanks. If the fuel gets to the level of the
surge tank float switches, the switch closes, and all refuel
valves are closed.

Surge
Fig 39
Tank
Float
Switch
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Category – B2 Sub Module 13.13 - Fuel Systems

Defueling

Defueling a pressure type fuel system is almost the reverse of

the refueling procedure. A de-fuel bowser would be connected
to the single fuel point coupling, and using a combination of
both the bowser’s suction pump and the aircraft’s own fuel
supply booster pumps, selected tanks can have their contents
returned to the bowser.

Defuel System Layout

Fig 40
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Category – B2 Sub Module 13.13 - Fuel Systems

LONGITUDINAL BALANCE FUEL SYSTEMS

The weight of the fuel is a large percentage of an aircraft’s total

weight, and the balance of the aircraft in flight changes as the
fuel is used. These conditions add to the complexity of the
design of an aircraft fuel system. In small aircraft the fuel tank or
tanks are located near the center of gravity so the balance
changes very little as the fuel is used. In large aircraft, fuel tanks
are installed in every available location and fuel valves allow the
flight engineer to keep the aircraft balanced by scheduling the
use of the fuel from the various tanks. High performance military
jets and more modern civil aircraft will use a fully automatic fuel
scheduling system to reduce the workload on the flight crew.

In supersonic flight the aerodynamic center of pressure moves

aft, thus changing the longitudinal stability. This is compensated
in the Concord by moving the center of gravity by shifting fuel as
necessary between the fuel tanks in the fin and the wings as
shown in the previous diagram, the front and rear tanks are
Trim tanks, and the center section contains the main tanks

Fig 41

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Category – B2 Sub Module 13.13 - Fuel Systems

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 PIA TRAINING CENTRE (PTC) Module 13 -AEROPLANE AERODYNAMICS, STRUCTURES AND SYSTEMS
Category – B2 Sub Module 13.13 - Fuel Systems

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